CA1253968A

CA1253968A - Power metering system and method

Info

Publication number: CA1253968A
Application number: CA000460125A
Authority: CA
Inventors: Michael A. Robinton; Alan H. Starkie
Original assignee: ROBINTON PRODUCTS Inc
Current assignee: ROBINTON PRODUCTS Inc
Priority date: 1983-08-01
Filing date: 1984-07-31
Publication date: 1989-05-09
Also published as: DE3448183C2; SE8504815D0; DE3490349T1; GB2178177A; SE452516B; GB8620428D0; FR2555379A1; GB2178260B; SE8504816D0; SE8504816L; SE8504814D0; GB8507289D0; FR2555318A1; DE3448182C2; SE8504813D0; GB8620429D0; GB2154329B; FR2555318B1; SE453129B; WO1985000711A1

Abstract

ABSTRACT OF THE DISCLOSURE
Power Metering System And Method The invention relates generally to circuits for producing one or more signals proportional to power or to another selected measurement parameter, and more particularly to an improved power metering circuit and method which employs delta-minus-sigma modulation and an offset compensation system.

Description

Description Power Metering System And Method Technical Field The invention relates generally to circuits for producing one or more signals proportional to power or to another selected measurement parameter, and more particularly to an improved power metering circuit and method which employs delta-minus-sigma modulation and an offset compensation system.

Background Art Meters which accurately measure electric energy flowing on a line are an essential part of an electric utility system. The most common type of meter used by the electric utility industry today is the rotating disc meter, which in its basic form is accurate and reliable but provides only limited information to the utility about power usage. In addition to total power consumption in kilowatt-hours, utilities often need to measure other parameters to properly determine the cost of supplying certain loads. Some highly reactive loads, for example, are more expensive to supply ~ because they induce a current/voltage phase mismatch ;~ known as power factor. To determine power factor, utilities have devised certain standard power measurements. Two widely used measurements are VARS
(for reactive volt-amperes) and Q. Both are power measurements proportional to the product of line current and voltage, with the voltage phase lagging the current by 90 for VARS, and by 60 Eor Q. Together with overall power consumption, readings of VARS and Q
allow the utility to measure power factor for which a penalty is usually charged. Another parameter of interest to utilities is polarity, or the direction of ., ~' power flow, since some applications both consume power and feed power back into the distribution system.
There is a continuing need in the electric utility industry for metering systems which are able to measure such parameters as VARS, Q and polarity, in addition to total power consumption.
Various electronic metering systems have been designed or proposed to replace the rotating disc meter. Examples of prior art electronic metering systems include those shown in the following patents:
U.S. 3,875,508; U.S. 3,955,138 and U.S. 4,182,983.
Those systems all employ modulators which produce a pulse-width modulated signal proportional to either current or voltage, and then use time-division or markspace multiplication, which gates or reverses the polarity of the other signal, to obtain a product signal. The product signal pulses vary in amplitude with respect to one analog value (current or voltage), and varies in width with respect to the other analog value. A low pass filter extracts the DC component of the product signal, which is proportional to power consumption. It would be advantageous to have a highly accurate metering system which is electronic and which is capable of producing power readings continuously in both kilowatt-hours and VARS or Q, at the lowest possible cost. It would also be advantageous if such a metering system could separately measure net power flow in each direction. The metering system should also be substantially free of errors due to voltage offsets in the active circuit elements. The system should therefore more advantageously include an offset compensation system which can economically compensate ~ for errors in multiple amplifier elements.

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Summary of the Invention m e invention is used in conjunction with a svstem including means for monitoring current and voltage signals on the line. A first signal transducer produces a first analog signal proportional to one of the current and voltage signals and a second signal transducer produces a second analog signal prcportional to the other of the current and voltagP signals. A
mcdulator mcdLlates one of the analog signals to produce a first mcdhlated signal which is changeable between two levels at predetermined first clock intervals, such that the first mKdNlated signal has an average level over any sufficient interval proportional to the selected one of the analog signals. First multiplying means are provided to gate the other of the analog signals in response to changes in the level of the first modulated signal, multiplying the analog signals together, to produce a product signal which is proportional to the power transported on the line. A convertor then converts the product signal to a first output signal whic~ in the preferred embodiment is chang~able between two levels at predetermlned convertor clock intervals in a manner which is proportional to the product signal and to the power carried on the line.
Specifi~lly the invention relates to a ncdulator for converting an input signal to an cutput pulse train which varies between two levels and whic;h has an average level over time prqporticnal to the input signal, c~mprising: means for supplyin~ the signals to a q nnde, switch means for supplying a second signal to ~he sum~ing ncde selected from at least two different referen~e signals hav mg predetermined magnitudes, wherein the differenoe at any time between the input signal and the seoond signal is a difference signal, measurin~ m,eans for in,tegratin~ the differenoe signal at the summing ncde and for determinin7 when the integrated difference signal _ 3 _ MIS/lcm .~

`` ~25396~3 reaches a threshold level. This neasuring means includes an amplifier element and offset compensation means for substantially eliminating any offset err~r resultin~ frcm a voltage offset existing between the amplifier inputs of the amplifier element, the compensation means includinq a fi~st storage element connected to one amplifier input and means for transferring an offsetting voltage to the first storage element to compensate for the voltage offset. Ihe n~dulator further aomprises a cl~ck for producing clock pulses at predetermined clock intervals, a bistable circuit responsive to the measuring means and the clock and which produces a first output signal chan~eable at each clock pulse between first and second levels whenever the integrated difference signal has crossed the threshold level durinq a clock interval, the switch means bein~ responsive to the bistable circuit such that one of the reference 8ignals iS supplied to the summing node when the first output signal is at its first level and another reference signal is supplied to the summing node when the first autput signal is at its second level, wherein the average value of the reference signals over time balances the input signal at the summing node and the average level cver time of the first cutput signal is proportional to the input signal and wherein the measuring means includes a seoond storage element connected betwren the summing node and a ccmmon ground, the seoond storage element providing part of a passive inte~ration means for integrating the difference signal, the voltage on the s ~ node being the integrated difference signal. In a related aspect, the inNention relates to a ccowerter for producing a digital signal indicative of the magnitude and polarity of an analog input signal comprising: cloc~ means for producing a clock signal which occurs at clock : times, separated by predetermined cl w k intervals; modulating means for - 3a -MIS/lcm .:: ' 't ' ~
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producing a mcdNlated signal changeable between first and second levels at the clock times and with the time ratio of the n~dNlated signzl at the first level to the second level, referred to as a measured duty cycle value, being indicative of the magnitude of the input signal with respect to a reference level; and output mans for determd m nJ the polarity of the input signal with respect to the reference level by sampling the mcdulated signal at least once every clock interval, and for producing at least one digital signal indicative of magnitude and polaritv in response to the modulated signal and the polarity determination and wherein the output means assigns a first polarity to the input signal when the modulated signal is at the first level for at least two consecutive clock intervals and a second pol æity when the mrdNlated signal is at the second level for at least two consecutive clock intervals.
The preferred metering system employing the present invention includes a convertor which separately measures the power at each polarity on the line and includes digital means for changing the phase relationship between the analog signals to produoe a product signal prcportional to a - selected phase relationship power value, such as U~RS or Q. The system also includes an offset cc~pensation syst~m which corrects for voltage offsets in the various operational amplifiers of a metering syst~m to - 3b -M~S/lGm ",~

i2539.6~3 eliminate offset errors and provide high accuracy. The offset compensation system described will correct for voltage offset between the inputs of N amplifier elements. The offset compensation system includes N
offset storage elements which are connected respectively to one input of each of the amplifier elements Eor receiving a compensating voltage which substantially reduces the offset error at the other input of the amplifier element. Any difference between the compensating voltage and the voltage offset of the amplifier element is termed an error voltage, which appears at the other amplifier input. The system includes a nulling circuit which can be connected sequentially to each of the N amplifier elements and to the offset storage element associated therewith so that each of the amplifier elements sequentially becomes the selected amplifier element being offset compensated.
The nulling circuit first is connected to the other input of the selected amplifier element during an intermittent transfer period to determine the error voltage. Then the nulling circuit is connected to the offset storage element connected to the one input of the selected amplifier element during an intermittent charging period which follows the transfer period. The offset compensation system includes means for sequentially providing the transfer and charging period for each of the N amplifier elements such that offset errors in the metering system are substantially eliminated.

Brief Description of the Drawings FIG. l is a schematic block diagram of a metering system for measuring power on a line according to the present invention.

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iZ5396~3 FIG. 2 is a schematic circuit diagram of the first modulator portion of the metering system shown in FIG.
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FIG. 3 is a schematic circuit diagram of the first output convertor portion of the metering system shown in FIG. l.
FIG. 4 is a series of graphical illustrations, designated FIGS. 4a through 4g, showing certain selected internal and output signals produced during the operation of the metering system of FIGS. l through 3.
FIG. 5 is a series of graphical illustrations, designated FIGS. 5a through 5i, showing various internal and output signals produced by the convertor of FIG. 3, during the measurement of signals of different polarities.
FIG. 6 is a schematic circuit diagram of an alternative embodiment moæulator for use in the subject ; power metering system which provides a phase lead in the modulated output signal.
FIG. 7 is a series of graphical illustrations, -~ designated FIGS. 7a through 7g, showing certain selected internal and output signals produced by the FIG. 6 modulator.
FIG. 8 is a schematic block diagram of a metering system according to the present invention which includes apparatus for producing measurements of YARS
and Q.
FIG. 9 is a schematic diagram of a signal multiplier which can be used in the metering system of FIG. 8 and which includes digital circuitry for phase adjustment to enable the production of VARS and Q
measurements.
FIG. lO is a series of graphical illustrations, designated FIGS. lOa through lOh, showing certain '.'.'~
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selected internal and output signals produced in the FIG. 9 multiplier.
FIG. 11 is a schematic diagram showing further details of the digital phase adjustment circuitry of FIG. 9.
FIG. 12 is a series of graphical illustrations, designated FI~S. 12a through 12d, showing a selected phase adjustment of a modulated signal produced by the multiplier of FIG. 9.
FIG. 13 is a schematic diagram of one embodiment of a voltage offset compensation system for use in the present invention.
FIG. 14 is a graphical illustration of the change in error voltage produced by the FIG. 13 compensation system.
FIG. 15 is a schematic diagram of a voltage offset compensation system of the type shown in FIG. 13 for an entire power measuring system.
FIG. 16 is a timing diagram illustrating the operation of the offset compensation system of FIG. 15.
FIG. 17 is a schematic diagram of a second embodiment modulator for use in the FIG. 1 power measuring system.
FIG. 18 is a series of graphical illustrations, designated FIGS. 18a through 18e, showing various signals produced by the FIG. 17 modulator.
FIG. 19 shows the FIG. 17 modulator with an alternative embodiment voltage offset compensation system.
FIG. 20 is a timing diagram showing the control signals for operating the voltage offset compensation system of FIG. 19.
FIG. 21 is an alternative embodiment of a modulator for use with the FIG. 1 measuring system which includes voltage offset compensation circuitry.

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FIG. 22 is a timing diagram showing the control signals for operating the offset compensation system of FIG. 21.
FIG. 23 shows an alternative embodiment modulator and associated dual polarity output circuitry.
FIG. 24 is a series of graphical illustrations, designated FIGs. 24a through 24j, showing various signals produced by the FIG. 23 modulatcr and associated output circuitry.

Best Mode For Carr~ing Out The Invention Referring to FIG. l, the metering system of the present invention i5 schematically illustrated as a means for measuring the electrical power carried on a power line lO, from a source 12 to a load 14. The current on line lO is indicated generally as IL, and the voltage as VL. The system includes a signal monitoring and conditioning means such as transformers 16 and 18, for monitoring VL and IL, respectively.
Transformer 16, designated the first signal means, produces a first analog signal IA1 proportional to VL
on line 20. Transformer 18, designated the second signal means, produces a second analog signal IA2 proportional IL on line 22. A shunt resistance 24 is connected across the secondary winding of transformer 18, through which most of the current on line 22 flows.
Shunt resistance 24 provides a low impedance current path and may be selected to control the overall range of the current signal IA2 on line 22.
The metering system and method of the present invention operates to multiply together the first and second analog signals IA1 and IA2, carried respectively on lines 20 and 22, and then to convert the multiplied product signal to a suitable digital form. Broadly, this is accomplished by modulating one of the signals ~:~539~8 and then gating, or switching, the other of the signals to yield a composite or product signal having an average value proportional to power. It will be understood by those skilled in the art that either the current or voltage could be modulated, and the resultant modulated signal used to gate the other of the two analog signals, to produce the product signal.
Accordingly, the designation of the first and second analog signals as the voltage and current signals, respectively, could be reversed without altering the fundamental operation of the metering circuit shown in FIG. l. Similarly, the designations for the first and second signal monitors could likewise be reversed.
The metering system provides a multiplying means for multiplying signals IA1 and IA2 together to produce a product signal which is proportional to the power transported on the line. To provide the necessary multiplication, the voltage signal IA1 is first supplied to a first modulator circuit 30. Modulator 30 constitutes a modulator means for converting the analog voltage si~nal IA1 to a Eirst modulated signal, which is changeable between two levels at predetermined clock intervals. In accordance with the principles of delta-minus-sigma modulation, the first modulated ;~ 25 signal output has an average level over any sufficient interval which is proportional to the first analog signal supplied to modulator input 32.
Referring to FIG. 2, the analog (voltage) signal IA1, is supplied to a summing node 36, through an impedance 38. Modulator 30 includes modulator feedback means for producing a feedback signal IF, which is also supplied to summing node 36. IF is controlled by the modulator output, called the first modulated signal, which appears on line 34. One or the other of a pair ` 35 of reference sources V1+ and Vl- are alternately '~
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connected to summing node 36 through an impedance 40 in response to the level of the first modulated signal.
Feedback signal IF switches between the positive and negative reference sources in a manner which balances the first analog signal IA1 over time. Instantaneous differences between IF and the first analog signal result in a difference signal Idiff out of summing node 36. The instantaneous difference between the input and feedback signals, namely Idiff, is integrated and measured by a modulator measuring circuit 42.
Measuring circuit 42, includes an active integrator having a capacitor 44 as the feedback element of an inverting operational amplifier 46. The signal at amplifier output 48 ramps up or down, depending on the t5 polarity of Idiff. The integrated signal at 48 is compared against a modulator threshold level by a comparator 50, which goes high when the signal is above the modulator threshold level and low when the signal is below the modulator threshold level.
The output of comparator 50 is supplied to the D
input of a modulator bistable circuit 52. The Q output of bistable circuit 52 is the first modulated signal.
Bistable circuit 52 changes only at predetermined first clock intervals which are determined by an external clock. A suitable clock for this purpose is provided by a conventional oscillator 54 and frequency divider circuit 56, shown in FIGS. l and 2. For simplicity, the time interval between the pulses produced by frequency divider 56 will be referred to as the first clock. Bistable circuit 52 has a Q output as well as Q, with Q being the inverse of Q. Both the Q and Q
outputs are used to control feedback signal IF by operating a pair of switches 58 and 60, respectively.
Since Q and Q are understood to be the inverse of one another, only the Q output is referred to herein as the first modulated signal. It should be understood, ~..

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125396~3 --1 o--however, that both the Q and Q outputs contain the information represented by the term "first modulated signal", and line 34 designates the lines carrying both the Q and Q signals.
Because the first modulated signal is output through bistable circuit 52, the first modulated signal on line 34 is changeable between two levels at the predetermined first clock intervals. Although the level may not change at each clock interval, the modulator circuit provides that when the first modulated signal does change levels, such change occurs only at the predetermined first clock intervals, and at no other times. Changes between the high and low levels of the first modulated signal produce simultaneous switching of switches 58 and 60, and corresponding reversals in the polarity of feedback signal IF to summing node 36. As the integrated difference signal ramps either up or down across the threshold level of comparator 50, chanaes in the level of the output of the comparator are produced. At each clock interval, bistable circuit 52 determines whether the output of comparator 50 has changed, and if so, produces a corresponding change in the Q and Q outputs.
The magnitude of the analog input signal causes a ~5 direct proportional change in the amount of time the ; first modulated signal is at a given level.
Consequently, the first modulated signal has an average level or amplitude which lies either at or between its two levels and, over any sufficient interval, such average amplitude is porportional to the analog input signal.
As an example of the operation of modulator 30, if the input signal at input 32 is zero, the Q output of ~-~ bistable circuit 52 will be high exactly the same amount of time it is low, producing an average level exactly midway between the high and low levels of Q.
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If the input signal at input 32 has a positive value, the positive current into summing node 36 must be balanced by a larger negative current supplied to the summing node by negative reference Vl-, through switch 58. Consequently, Q will be low proportionately longer than it is high and switch 58 will be closed and switch 60 will be open a greater amount of time than vice versa. If the input signal is negative, the positive feedback reference will need to be supplied more of the time in order for IF to balance the input signal, and Q
will be high more than it is low. It is a feature of the modulator of the present invention that Q can remain high or low for however long it takes IF to balance the input signal at the summing node.
To proauce a current signal for multiplication with the modulated voltage signal, the system includes means for providing inverted and non-inverted representations of the line current IL. Referring to FIG. l, the current analog signal IA2 is ~irst supplied to a gain amplifier 70, after which the signal is supplied to a signal inverter circuit 72. The illustrated inverting circuit includes an operational amplifier 74 and gain setting resistors 76 and 78. The amplified signal IA2 is supplied to the inverting input

2~ of amplifier 74, which is configured to produce a gain of -l. The inverted signal is then supplied to one of two switches, which together form first gating means 80. The inverted signal goes to switch 82, and a second line 84 carries the non-inverted amplified signal, IA2, to switch 86. It will be understood that ; a suitable center tap transformer could be used in place of second transformer 18, in which case the signals to switches 82 and 86 could be supplied directly from the transformer.
The Q and Q outputs of modulator bistable circuit 52 are used to operate switches 82 and 86, to gate the :, '~, i~25~'3961 ~1 2--second analog signal IA2 in response to the first modulated signal. Since Q is the inverse of Q, switches 82 and 86 are switched in alternate fashion, such that the output of gating means 80, at 88, is an analog signal switched in a modulated manner between positive and negative polarities. Such a gating operation is qenerally termed time division or amplitude-markspace modulation. Switches 82 and 86 accomplish the multiplication of the two analog signals representing the current and voltage of the power carried on line lO. The resultant signal, termed a product signal, appears at first gate output 88, and is proportional to the power carried on power line lO.
As shown in FIG. l, the product signal output from the first gating means is supplied to a first convertor circuit 90. The convertor circuit converts the product signal to a first output signal, on line 92, which is changeable between the two levels at predetermined convertor clock intervals in a manner proportional to the product signal. Convertor 90 operates essentially as a low pass filter which extracts the DC component or average va ue of the product signal. The resultant first output signal is proportional to the power carried on line lO.
Referring to FIG. 3, convertor 90 is essentially a delta-minus-sigma modulator of a type similar to modulator 30, which is designed to produce separate modulated output signals proportional to each polarity of the input signal. To simplify the description, convertor 90 and its operation will initially be described with respect to a first polarity of operation. The components within box 94 encompass all the elements used in single polarity operation. In the following example it will be assumed that the product signal to be converted is predominantly positive, which will be assumed to correspond with power flow on line `~' :
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1253~8 10 from source 12 into load 14. As in the modulator 30, the input signal to convertor 90, designated Ip tproduct signal), is initially supplied to a summing node 96 through an impedance 95. A feedback means supplies a second signal I2 to the summing node from one Gf a plurality of reference sources. For positive polarity operation, the reference sources will alternate between a negative reference source 98 (VR-), supplied through a switch 100, and a ground connection 102, supplied through a switch 104. Since only positive values of the product signal are being considered, switching I2 between ground and a negative value will be sufficient to balance the product signal at summing node 96, over time.
As previously described for modulator 30, any difference between product signal Ip and I2 is a difference signal, which is supplied to a measuring circuit 106. The measuring circuit integrates the difference signal and compares the difference signal to a first threshold level. The preferred embodiment measuring circuit shown in FIG. 3 includes an active integrator 107 consisting of amplifier element 108, and a capacitor 110 as a feedbac~ element. The voltage at amplifier output 112 ramps upward or downward, depending on the polarity of the difference signal at ; summing node 96. The integrated difference signal at 112 is supplied to a first comparator 114, which has a threshold set at a selected first threshold level.
When the integrated difference signal at 112 is above the first threshold level, the output of comparator 114 is high. When the integrated difference signal is below the first threshold level, the output of comparator 114 is low.
The comparator output, termed a first control signal, is supplied to the D input of a bistable circuit 118, by way of line 116. The Q output of . ~

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bistable circuit 118 is changeable only at predetermined convertor clock intervals, which are preferably longer than the first clock intervals for modulator 30. The convertor clock intervals can be produced by adding a second frequency divider 120 to first clock 56. The time intervals between the pulses produced by frequency divider 120 will be referred to as convertor clock intervals, and the frequency divider will be referred to as the convertor clock. The Q
output of bistable circuit 118 is the first output signal, which controls switches 100 and 104 to determine the operation of the feedback system which supplies second signal I2 to summing node 96. Switch 104 is operated through a gate 122, which outputs a high signal to close the switch only when both inputs 124 and 126 are low. Gate 122, as shown, is a conventional negative AND gate. During periods of positive product signals, input 126 will remain low, as will be described below. Consequently, whenever Q is high, switch 100 is closed, connecting VR- to summing node 96, and when Q is low, switch 100 is open and switch 104 is closed.
The operation and method of the metering system of the present invention will now be described with reference to FIGS. 1-4. For simplicity, it will be ~ assumed that power on line 10 flows predominantly in - the positive direction. The voltage on line 10 is shown in FIG. 4a as a sinusoidal AC waveform. Current IL is shown in FIG. 4f as an increasing value, represented by curve 128. The first step is for transformers 16 and 18 to monitor the current and voltage signals and to produce analog signals IA1 and IA2, wbich are proportional to line voltage and current, respectively. One of the analog signals, - 35 voltage signal IA1 in the preferred embodiment, is then supplied first to first modulator 30. Fig. 4c shows ~ ~ .

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the integrated difference signal produced within modulator 30 by the delta-minus-sigma modulation technique described above. The integrated difference signal i9 supplied to me~suring circuit 42. Fig. 4b illustrates the first clock intervals produced by first clock 56. As can be seen, the slope of the integrated difference signal in FIG. 4c changes only at the predetermined clock intervals determined by the first clock signal. Since bistable circuit ~2 clocks on the leading edge of each upwardly moving pulse, the predetermined first clock intervals are shown to begin at the points identified as a, b, c, d, etc. in FIG.
4b. The integrated difference signal is then supplied to comparator 50. Line 130 in FIG. 4c represents the modulator threshold level in comparator 50. Note that the integrated difference signal reverses slope at the beginning of each clock interval after threshold 130 has been crossed. The output of comparator 50 is shown in FIG. 4d. Whenever the integrated difference signal is below threshold 130 the comparator output is low and when the integrated difference signal is above threshold 130 the comparator output is high. The comparator output is then supplied to the D input of bistable circuit 52, which produces the Q, or first modulated signal output, illustrated in FIG. 4e. The Q
output is the result of modulating the voltage signal and is changeable between two levels at the predetermined first clock intervals.
Because the bistable circuit is changeable only at the predetermined clock intervals shown in FIG. 4b, the changes in Q slightly lag changes in the comparator output shown in FIG. 4d. Depending on the level of accuracy required in the signal multiplication system, ~` it may be desirable to compensate for the slight lag in the modulated signal introduced by bistable circuit 52.
Such correction can be accomplished by inserting an RC

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network in line 20 to introduce a small phase lead in signal IA1 as it enters modulator input 32. Another technique would be to induce a slight lag in the current analog signal IA2. A third alternative, utilizing a delta-minus-sigma modulator having digital phase lead circuitry will be subsequently described.
The phase adjustment introduced, which will only be a fraction of one first clock interval, should be the average of the delay induced by the lag of Q relative to the comparator output.
FIG. 4f illustrates equal and opposite analog signals proportional to line current IL . Line 128 is representative of a growing current signal and line 129 is the inverse signal produced by inverter 72. The next step is to gate the current analog signal using gating means 80. The output of gating means 80 is the product signal, curve 131, shown in FIG. 4g. Curve 131 is generated by switching between signals 128 and 129, in response to the first modulated signal shown in FIG.
4e. The average level or DC component of curve 131 is represented by line 132 of FIG. 4g.
In the example given, power is assumed to be flowing predominantly in one direction, into load 14.
Consequently, the product signal 131 shown in FIG. 4g is predominantly of a positive polarity, represented by line 132. It will be assumed, for the purpose of describing the operation of convertor 90 below, that the product signal has a predominant and average value which is positive. Although the actual polarity of the product signal is a matter of design choice, the product signal will be predominantly of a first ~` polarity when power on line 10 is of a first polarity, with power flow in one direction, and will be predominantly of a second polarity when power on line 10 is of a second and opposite polarity, with the power flowing in the other direction.

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The next step is to convert the product signal Ip to a first output signal changeable between two levels at predetermined intervals in a manner proportional to I . Reference will be made to FIGS. 3, 4 and 5.
Product signal Ip, as shown in FIG. 4g, is supplied to convertor 90. Both Ip and second signal I2, are supplied to summing node 96, where the instantaneous difference is integrated in integrator 106. The time constant of integrator 106 is selected to be long in comparison to the switching frequency of first modulator 30. Convertor 90 can therefore act as a low pass filter, responding only to the DC component, or average value, of product signal Ip. For this reason, Ip is depicted in FIG. 5a as a smooth analog curve, although it will in fact vary in the manner shown in FIG. 4g. FIG. 5a shows only the average value of I .
The time scale of FIG. 5a is considerabl~ compressed, compared with the scale in FIG. 4g. For the purpose of illustration, it will be assumed that interval 134 of FIG. 5a is equivalent to the entire length of curve 132, shown in FIG. 4g. FIG. 5b shows the convertor clock intervals produced by clock 120.
Considering only positive power flow, illustrated between to and tl , in Fig. 5a., integrator 106 will output an integrated difference signal ~IDS) as shown in FIG. 5c. The integrated difference signal ramps up and down around the first threshold level TLl of comparator 114. The integrated difference signal (IDS) is supplied to comparator 114, where it is compared to first threshold level T~l. Comparator 114 outputs a control signal 133 on line 116 as shown in FIG. 5d.
The next signal generated is the first output signal shown in FIG. 5e, which is output through bistable circuit 118. Control signal 133 changes levels depending on the level of the integrated difference signal relative to threshold TLl. When IDS is above . ; '' ~`:
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TLl, signal 133 is high and when IDS is below TLl, signal 133 is low. The next step is to output the first output signal shown in FIG. 5e through first bistable circuit 118. The first output has an average level proportional to a first polarity of power on line 10 over any sufficient interval. It is changeable only at the predetermined convertor clock intervals illustrated as w, x, y and z in FIG. 5b.
Single polarity operation of convertor 90 involves switching the feedback signal I2 between first reference source 98 and a second reference source 102, depending on the level of the first output signal (FIG.
5e). Since the second reference source 102 is a ground connection, the portion of convertor 90 thus far described will not accommodate negative power flow on line 10. When power flow (Ip) goes negative, as it does between times tl and t2 in FIG. 5a, additional circuitry in convertor 90 is employed. Referring to FIG. 3, convertor 90 includes a second comparator 140 which receives the output of integrator 107.
Comparator 140 has a second threshold level TL2 which is different from the first threshold level of comparator 114. The threshold levels should be set far enough apart to accommodate the widest anticipated variations in the integrated difference signal output from integrator 107, without crossing the threshold levels of both comparators simultaneously. The integrated difference signal is supplied to the non-inverting input of comparator 114 and to the inverting input of comparator 140, so that their outputs will be of opposite polarity. The output of comparator 140 goes high when the integrated difference signal is below the second threshold level in comparator 140 and goes low when the integrated difference signal is above the second threshold level in comparator 140.

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input of a second bistable circuit 142. Second bistable circuit 142 outputs a second output signal from its Q output. The second output signal is at one of two levels, depending on the level of the integrated difference signal relative to the second threshold level at each of the convertor clock intervals. The second output signal is supplied to input 126 of negative AND gate 122 and to a switch 146 for connecting a third reference source VR+ to summing node 96. The feedback signal I2 is thus governed by the level of the second output signal, which has an average level proportional to the second polarity power carried on power line 10.
Second polarity operation of convertor 90 will be described with reference to FIGS. 3 and 5. After time t1 the direction of power flow reverses and product signal Ip begins drawing charge from summing node 96.
Referring to FIG. 5c, just prior to time t1 the integrated difference signal is descending, meaning that the negative reference source VR- is connected to the summing node through switch 100. At the clock pulse following the crossing of the ~irst threshold level TLl, switch 100 will open and switch 104 will ; 25 close, connecting the summing node to ground. Since the product signal Ip is negative after t1t the integrated difference signal will continue integrating downward until reaching the second threshold level TL2 of comparator 140, when its output 135 will go high (see FIG. 5g). At the next comparator clock interval ~ after convertor 140 goes high, the Q output of bistable - circuit 142 ~the second output signal) will go high, as shown in FIG. 5h. When the second output signal goes ~- high, a switch 148 connected to third reference source 14S (VR+) is closed. The third reference source supplies a positive current I2 to summing node 96 to "',;, .
:``' ' `

counterbalance the negative product signal Ip and drive IDS back across TL2. When TL2 is crossed, signal 135 again goes low, causing the second output signal to go low at the next clock interval. During second polarity operation the first output signal (FIG. 5e) remains low and, whenever the second output signal (FIG. 5h) is low, both inputs to gate 122 are low and its output goes high. When the output of gate 122 goes high, switch 104 is closed and the ground connection reference source 102 is connected to s~mming node 96.
When switch 104 is closed, IDS is allowed to recross TL2 in the other direction. During the interim between times tl and t2, when the power flow is negative, the integrated difference signal is maintained in the vicinity of second threshold level TL2.
The convertor 90 shown in FIG. 3 is provided with three different reference sources, the second of which is a connection to the common ground for the metering circuit. Because of the configuration of circuit elements, the ground connection is used whenever the integrated difference signal is in the region between the first and second thresholds TLl and TL2. It is not essential that the second reference source be a ground connection. Separate positive and negative reference sources could be used for each polarity of operation, if desired. In such a case, the first and second reference sources would be used to supply the second signal I2 to summing node 96 when the product signal I
is of a first polarity and separate third and fourth reference sources could then be used to supply the - second signal I2 to summing node 96 when product signal Ip is of the other polarity. In practice, the selection of the values for the reference sources is ~` governed by the necessity of maintaining the integrated difference signal in the vicinity of the threshold level of the comparator in use. The magnitudes and ~, ., ~.~
~:`
. . ~,... . . .
.

~L253968 polarities of the reference sources are otherwise entirely a matter of design choice.
Using reference sources in converter 90 which include at least one ground connection improves the overall accuracy of the modulated signals output.
While variations may occur in the positive and negative voltage reference sources, the ground connection remains fixed. If one or both of the positive and negative reference sources is above or below its correct value, an error will be at one level slightly longer or shorter than it should ~e, since during the time the voltage reference source supplies the feedback signal it will supply slightly too much or too little current. The closer the input signal is to ground (zero) the smaller will be the error. Equal and opposite reference sources, such as those used in the feedback system of modulator 30, have a greater potential for error if there is mismatch between the reference voltages Vl+ and Vl~. Since the feedback system of modulator 30 always switches between Vl+ and Vl-, any error resulting from a reference voltage mismatch will tend to cause the modulated output to be at one or the other level an incorrect amount of time regardless of the magnitude of the input signal. This does not present a problem in the case of modulator 30 ; because it is modulating the line voltage signal, which generally varies by only a small amount. Accuracy, therefore, need only be maintained over a narrow range.
Converter 90, however, requires greater accuracy because of the wide variations in the product signal ~ representing line power. For this reason, the - separation of the convertor operations between positive and negative polarities of power has distinct advantages. Since only one polarity is measured by each comparator, the reference sources can use a ground ~` connection to provide the feedback signal, improving , ~' .

~53968 overall convertor accuracy. The information provided about power flow of each polarity is also desirable since it provides additional data about the nature of the load and its power requirements.
The first and second output signals output on lines 92 and 144 from convertor 90 (see FIG. 1) are changeable between two levels at the convertor clock intervals. To provide a suitable digitized output in which pulse density is propor'ional to power flow, a system for converting the output signals to pulse trains is provided. Referring to FIGS. 1 and 5, the first and second output signals are supplied to respective first and second AND gates 150 and 152. A
second input to the AND gates is supplied from convertor clock 120. FIG. 5f shows the pulse train produced for first polarity power from AND gate 150.
The pulse train has a pulse density proportional to the magnitude of power flow in one direction on line 10.
Similarly, for power flow in the opposite direction, FIG. 5i shows a pulse train for second polarity power from AND gate 152. Various means are available for processing the first and second digital output signals shown, respectively, in FIGS. 5f and 5i. For example, it would be convenient to supply the digital signals to a counter means for counting the positive and negative polarity pulses. The counter could then output a display, or record total power consumption. Counter 154 is illustrative of such a display concept. If, in addition, a gate signal is supplied to counter 154, measurements of power in appropriate units, such as kilowatts, could be readily obtained. Separate ` readings of power flow in each direction could also be obtained.
As previously noted since bistable circuit 52 (FIG. 2) is changeable only at predetermined clock intervals, a slight lag is introduced in the modulated ~i~
~si ~., !' ~' ~2S396~

output signal. FIG. 6 shows a novel delta-minus-sigma modulator 30' having digital phase lead circuity to compensate for the phase lag. Like elements in the FIG. 2 and FIG. 6 modulators are designated by like reference numerals. It should be noted that such digital phase lead circuitry has applications other than power metering systems. Moreover, if desired, phase lead can be provided which is more than sufficient to compensate for the phase lag caused by the output bistable 52 of FIG. 2.
The modified modulator 30'of FIG. 6, like the modulator of FIG. 2, includes a bistable circuit 52 controlling a source of feedback current IF through switches 58 and 60. A summing node 36 receives the input signal IA1 through input resistor 38.
Instantaneous differences between the feedback and input signals are represented by Idiff and that difference signal is measured by measuring circuit 42.
The control signal output from comparator 50 is high when the integrated difference signal is above the threshold of the comparator and is low when the - integrated difference signal is below the threshold.
Modulator 30' differs from modulator 30 in FIG. 2 in that it includes a digital shifter between measuring ; 25 circuit 42 and bistable circuit 52. The digital shifter introduces a time delay in the control signal ~ output from comparator 50. In FIG. 6, the digital ; shifter is a bistable circuit 59, which receives the control signal output from the comparator at its D
input~ For the purposes of the example given below, bistable circuit 59 is clocked at the same rate as bistable circuit 52, but one-half clock interval out of phase.
Operation of the modulator shown in FIG. 6 to achieve phase lead in the modulated output signal will ~; be described with reference to FIG. 7. Input signal ' ,. . .

i25~968 ~

IA1 to modulator 30' is shown in FIG. 7a. The output of first clock 56 is shown in FIG. 7b. First clock 56 also supplies the signal to bistable circuit 59 through an inverter 57, and the second clock signal is shown in FIG. 7c. If IA1 is positive at clock pulse a and the Q
output of bistable circuit 52, shown in FIG. 7g, is initially high, IF will be positive into summing node 36. That will produce a positive Idiff which is supplied to the inverting input of integrating amplifier 46, causing the integrated difference signal at point 47 to initially ramp downward, at 21 of FIG.
7d. Line 22 in FIG. 7d represents the threshold of comparator 50. When the integrated difference signal crosses threshold 22, the control signal shown in FIG.
7e goes from high to low. Assuming bistable circuit 59 clocks on upwardly moving pulses a', b', c', d', e', etc., the output of bistable circuit 59 will go from high to low at clock pulse a'. The output o~ bistable circuit 59 (Q') is referred to herein as the delayed control signal, which is subsequently supplied to the D
input of bistable circuit 52. FIG. 7f shows the delayed control signal and FIG. 7g shows the Q output of bistable circuit 52. When Q' goes from high to low, the Q output of bistable circuit 52 will go from high to low at its next clock pulse b. The change in Q
opens switch 60 and closes switch 58, causing IF to go negative. The integrated difference signal then ramps upward, crossing comparator threshold 22 and again causing the control signal to go high. At clock pulse d' of the second clock, the Q' output of bistable circuit 59 again goes high. That causes the Q output of first bistable circuit 52 to go high at its subse~uent clock pulse e.
The process described above will continue, with the Q output of bistable circuit 52 providing the signals for controlling the feedback loop of the ` ~253968 --modulator. Assuming the time delay introduced by the digital shifter represented by bistable circuit 59 is not large enough to create instability in the feedback loop, modulator 30' will produce a modulated signal e~uivalent, but not identical to that output from modulator 30. By equivalence, what is meant is that the Q output of bistable circuit 52 will be a modulat~d signal changeable at predetermined first clock intervals in a manner proportional to the signal input to the modulator. The Q' output bistable 59 will lead the Q output of first bistable circuit 52 by an amount dependent on the differences in the clock signals supplied to the two bistable circuits. This lead occurs as a natural consequence of the fact that the Q
output of bistable circuit 52 will change only at the next clock pulse following a change in the Q' output of bistable 59. The Q' output thus is a true "leading"
signal to the Q output.
The output signal on line 34 will have a phase lead of one-half of a first clock inter~al, as compared with the Q and Q outputs of bistable circuit 52. Since the clock intervals supplied to both bistable circuit 59 and bis~able circuit 52 are the same, the delayed control signal output on lines 34 will be changeable at the same intervals as the Q and Q outputs of bistable circuit 52 and will otherwise resemble any other delta-minus-sigma modulated signal. The clock signal supplied to bistable circuit 59 in effect becomes the determining clock signal governing changes in the ; 30 output of the modulator. It would be possible to substitute another type of digital shifter, such as a multi-stage shlft register, for bistable circuit 59, if the delay introduced is not so long as to destabilize the feedback loop. The digital shifter used might also be clocked at a different rate than the first bistable circuit 52, although that would change the ~.

. ~ , . .

`` 12539~8 characteristics of the delayed control signal. If, for example, a multi-stage shift register clocked at a high rate were inserted in place of bistable circuit 59, it would delay the control signal by a selected number of short intervals. The output of such a shift register would be a delayed control signal which is changeable at the higher clock rate. A shift register could also be employed, having different stages clocked at different rates. In such a configuration, the longest clock interval used to clock any of the stages would determine the intervals at which the final delayed control signal would be changeable. Any system for delaying the control signal should include at least one bistable circuit clocked at discrete intervals in order that the modulated output of the modulator (the delayed control signal) will be changeable at those discrete intervals.
The phase lead produced in modulator 30' can be selected. Such selection is accomplished by adjusting the clock signals supplied to bistable circuits 52 and 59. Assuming a first clock signal producing pulses at first clock intervals is supplied to bistable circuit 52 and a second clock signal producing pulses at second clock intervals is supplied to the digital shifter (bistable circuit 59), and both first and second clock intervals are equal, the phase offset between the clock signals will determine the amount of lead in the modulator output. In the example discussed with respect to FIG. 7, the second clock was the inverse of the first clock and the total offset was one-half a clock interval. If the clock pulses supplied by the second clock to bistable circuit 59 were three-quarters ~ of a clock interval ahead of the pulses supplied to `~ bistable circuit 52, a phase lead of three-quarters of ; 35 a clock interval would be produced. It is the amount of delay between a change in the Q' output of bistable `Y~
;

.. . .

.253!~

circuit 59 and the Q output of bistable 52 which determines the amount of lead time in the signal output on lines 34.
The amount of phase lead that can be achieved by the FIG. 6 modulator is dependent on the degree of delay that can be introduced into the feedback loop of a delta-minus-sigma modulator without causing it to destabilize. It is known, however, that a delay of a fraction of a clock pulse in the manner described in the example above is functional and produces the phase lead in the modulated signal as described.
FIG. 8 shows a metering system according to a further embodiment of the present invention which provides additional output power measurement in either VARS or Q. As described in the background section above, VARS and Q represent power measurements in which a specified phase relationship is introduced between the current and voltage signals. VARS is obtained by multiplying current with a voltage signal which lags by 90 degrees; Q is obtained by multiplying current with a voltage signal which lags by 60 degrees. In the metering system of this embodiment of the present invention, VARS, Q or any other desired phase relationship power value can be readily obtained by delaying the output of modulator 30 by a selected amount~ The delay can be conveniently produced using time delay means such as a shift register, in the manner described below.
The Q output of modulator 30 of the Figure ~
embodiment is supplied both to gating means 80 and to a shift register 160. Shift register 160 delays the output of modulator 30 by a selected delay interval.
The amount of delay depends on the selected phase relationship power value desired (VARS or Q), and also on the frequency of the AC waveform being measured ~50 or 60 Hz). To simplify the circuit, only the Q output :

12539~

of modulator 30 is supplied to shift register 160. The time delayed output of the shift register is then supplied to an inverter 161, and both the inverted and non-inverted signals become the time delayed signal on line 162. As used herein, the term "time delayed signal" is used interchangeably with "phase modified signal" and it should be understood that the phase modification introduced is accomplished by means of a time delay introduced in the signal.
Further processing of the time delayed modulated signal is exactly the same as for the first modulated signal of the embodiment of FIG. l. The time delayed modulated signal is supplied to a second gating means 164, which includes a pair of switches 166 and 168 controlled by the time delayed modulated signal. The inverted and non-inverted current analog signal IA2 is supplied to switches 166 and 168. The phase modified modulated signal alternately closes switches 166 and 168 to multiply the current and voltage signals together and produce a second product signal, at 170.
The second product signal is then supplied to the input of a V~RS/Q convertor 172, which is exactly like convertor 90 shown in FIG. 3. VARS/Q convertor 172 outputs first and second output signals, depending on the polarity of the power on line 10, in exactly the same manner as convertor 90. The outputs of convertor 172 are first and second output signals changeable between two levels at the convertor clock intervals in ~; a manner proportional to the second product signal and to the selected phase relationship power value (VARS or Q, 50 or 60 Hz) of the power on line 10. Subsequent ~ processing of the first and second output signals from -~ VARS/Q convertor 172 is exactly the same as for the outputs from convertor 90 shown in FIG. l, including use of counter means suitable for outputting selected power values.
'~' ., ., . ' , ' :`~
:
:

~253968 A selector (not shown) can be provided for selecting either VARS or Q as the second output of the metering system. The selector will adjust shift register 160 to produce the voltage lag needed to generate the selected phase relationship, and simultaneously select an appropriate display.
The exemplary and novel digital phase selection technique depicted in FIG. 8 is not limited to power metering applications. The technique rnay be used in any signal multiplication applications where the phase relationship between input signals may be adjusted in order to measure selected phase relationship product values.
FIG. 9 depicts a multiplier similar to the multiplier used in the power metering system o~ FIG. 8.
Like elements are designated by like reference numeral signals. IA1 and IA2 are the signals to be multiplied together and are assumed to be periodic waveforms, not necessarily sinusoidal, having a predetermined phase relationship to one another. As with the FIG. 8 power metering system, multiplication is accomplished by the technique known as time-division or markspace multiplication, in which one of the signals IA2 is modulated and then used to gate or reverse the polarity of the other signal IA1 to obtain a product signal.
Signal IA2 is supplied to a gating means in both inverted and non-inverted forms. A conventional inverter 72 supplies the signal to switch 82. The non-inverted signal is supplied to switch 65. The modulated signal for controlling switches 65 and 66 is supplied to the gating means by way of line 34.
Modulator 30 of FIG. 9 is eguivalent in construction and operation to corresponding modulator 30 of FIGS. 1 and 2. In order to achieve a selected phase relationship between signal IA1 and IA2 ~ a digital shifter 160 is used which introduces a selected -~ .

12:~39 Fi~

delay in the output of modulator 30. Digital shifter 160 can take numerous forms, a simple version being illustrated in element 198 of FIG. 11. The operation of a shift register can be illustrated conveniently as a series stages made up of bistable circuits 200 through 204 connected so that the Q output of one bistable circuit is supplied to the D input of the adjacent bistable circuit. A clock signal supplied to each of the bistable circuits via line 196 causes each stage to be clocked simultaneously. A digital pulse on line 53 into shift register 198, either going from low to high or from high to low, will be delayed one input clock interval by each bistable circuit it passes through. For example, if the signal on line 53 goes from low to high, the Q output of bistable circuit 200 will go from low to high at the next clock pulse.
Because of inherent switching delays, when the Q output of bistable circuit 200 goes from low to high and that signal is supplied to the D input of bistable circuit 201, its Q output must wait for the succeeding clock pulse to go high. In this manner, digital signals can conveniently be delayed by any desired number of discreet intervals simply by providing enough delaying stages in the shift register. Customarily, shift registers are provided with a plurality of output lines 206 at which the signal can be extracted. The location of the pin determines the overall delay introduced, as a function of the clock frequency.
The digital time delay means 160 of the multiplier system shown in FIG. 9 is assumed to be a conventional shift register such as shift register 198 of FIG. 11.
~; The multiplier system calls for the introduction of a selected time adjustment in one of the signals to be multiplied using a digital shifter to introduce a delay which is a selected number of discrete intervals.
~; Shift register 198 is a suitable digital shifter for .:
`~
.

:

1~39~

producing such a delay. Referring now to FIG. 10, it will be assumed that signals IA1 and IA2 are to be multiplied together and that a 90 degree phase lag will be introduced into signal IA2. FIG. lOa shows an exemplary first input signal IA1 tVL) and FIG. lOg shows an exemplary second input signal IA2 to be multiplied together. FIG. lOb shows the clock signal provided by clock 56 and FIG. lOc shows the output signal of the integrator 42 which results from input signal IA1. FIG. lOd shows the resultant output of comparator 50. The output of modulator 30 is shown in FIG. lOe, and is carried on line 53 of FIGS. 9 and 11.
The clock signal from modulator clock 56 is supplied to shift register 198 via line 196. In the example given, the cloc~ intervals, shown in FIG. lOb, are twenty-four times the frequency of signal IA2. A 90 degree phase lag will therefor require a delay of six clock intervals. Assuming pin 206' of shift register 198 to be the sixth pin, signal IA2, modulated and delayed by 90 degrees, will thus have been delayed by a total of six clock intervals output from clock 56. The pin 206' output of shift register 198 is illustrated in FIG.
lOf. The delayed modulated signal shown in FIG. lOf is ; an exact reproduction of the modulated Q output of modulator ~0, shown in FIG. lOe, moved to the right six clock intervals.
Signal multiplication is accomplished by supplying the delayed modulated signal shown in FIG. lOf to the signal gating means, via line 34. Line 34 includes both inverted and non-inverted versions of the delayed modulated signal by supplying the signal to a conventional digital inverter 161. Signal IA1 is that shown in ~IG. lOg both in inverted and non-inverted forms. Multiplication is carried out by means of switches 82 and 86, which are opened and closed alternately from one another, switching point 88 of ~,:

' ~`

~Z5396~3 FIG. 9 between the non-inverted and inverted versions of signal IA1. The resultant signal is shown in FIG.
lOh. The signal in FIG. lOh can then be passed through a suitable low pass filter 90 to produce an averaged or 5 D.C. value, as shown with line 132 of FIG. lOh. Line 132 represents a product signal proportional to the product value of IA1 and IA2 with a phase lag of 90 degrees introduced in IA2. If, for example, signal IA1 was proportional to current carried on a power line and signal IA2 was proportional to line voltage, the product signal represented by line 132 of FIG. lOh would be proportional to VARS.
A particular advantage of using a delta-minus-sigma modulator such as modulator 30 in conjunction with the subject multiplier is that the modulated signal is changeable only at predetermined clock intervals. Digital time delay techniques necessarily divide an incoming signal into discrete units or intervals. The length or duration of ~hose intervals is a matter of design choice. Digital-type signals carry information at pulse edges, when the signal goes from low to high or high to low. A shift register made up of a series of bistable circuits will "look" for such pulse edges each time it is clocked. The higher the clock frequency, the more frequently is the incoming signal sampled for a pulse edge. Since the delay introduced in a signal at each stage of a shift register depends on the clock frequency, shift registers clocked at a high frequency require more stages to produce a given delay than shift registers clocked at a low frequency. Of course, clocking a shift register at a low frequency means that the incoming signal is sampled less often for pulse edges, and this can be a disadvantage if the location of the pulse edges is not known, as is the case with ` conventional pulse width modulated signals. Modulator :`'`'`
:~
~ :;
.

' `` i~253~6~

30 outputs a signal having pulse edges which occur only at predetermined clock intervals. By synchronizing the clock signals supplied to the modulator and to shift register 198, the shift register will "look" for pulse edges only at the times required. This means that fewer shift register stages are needed to introduce a given delay in a modulated signal than would be the case if the location of the pulse edges were not precisely known. In fact, in the example given above, 10 the shift register can be clocked at the same rate as modulator 30, with no loss of information whatsoever.
It is therefore possible to use an economical shift register, possessing relatively few stages, to produce a given delay in a delta-minus-sigma modulated signal, 15 which when a far larger shift register would be needed to produce a comparable delay in a signal having pulse edges at random locations. Even if a relatively high frequency shift register was employed to delay a substantially lower frequency randomly modulated 20 signal, some loss of information would occur whenever a pulse edge was not precisely synchroniæed with the shift register clock. No such loss of information occurs in the embodiment of the present invention described above since the modulator and shift register 25 are synchronized with one another and pulse edges are `~ therefore not displaced.
The clock intervals by which the shift register is clocked need not be exactly the same as the first clock intervals of the modulator 30. It is, however, 30 preferable that the shift register clock be synchronized with the modulator clock. To avoid loss of information, the shift register clock should operate at a frequency no lower than that of the modulator, but may operate at higher rates to achieve virtually any t 35 desired time delay. A convenient way of increasing the frequency of the shift register clock while maintaining '`
..,~

-.'~ l :
' ' :
;

-125;:~9 synchronization with the first clock intervals o~ the modulator is to use a frequency divider for the modulator clock. While in the example described above, the desired time delay in the modulated signal corresponded with an integer number of first clock intervals, that may not always be the case. In order to achieve additional flexibility in the selection of a time delay it may be desirable to include either a second shift register or additional stages within a single shift register which are clocked at a higher frequency and which therefore introduce incremental delays in the modulated signal. The shift register states within element 212 of FIG. 11 illustrates a technique for providing further selectivity in the digital time adjustment of the present invention. In this example, the delayed signal output from any selected stage of shift register 198 is supplied to a second group of shift register stages shown in FIG. 11 as a second shift register 212. A plurality of bistable circuits 216 make up shift register 212. The delayed signal from shift register 198 is supplied to the input 214 of shift register 102. A clock signal, via line 208 preferably having a higher frequency than first clock 56, is supplied to the bistable circuits which form shift register 212. The higher clock frequency can conveniently be provided by means of an oscillator 220 operating at a higher frequency than first clock 56. Through use of a suitable frequency divider 210, clock signals of different frequencies can be supplied to the various shift register stages, as ~ well as to modulator 30, if desired.
-~ As used herein, the term first clock intervals ; generally refer to the clock signals output from first clock 56 and second clock intervals will be those output from second clock 220. In addition, the shift register stages illustrated in Figure 11 can be thought ., ~. , ~L~253~

of either as a first shift register 198 and a second shift register 212, or a single shift register having a plurality of stages which are clocked at various selected freq~encies. Either through use of separate oscillators or a single oscillator with a frequency divider, the provision of different clock signals increases the flexibility of the digital shifting techniques used in the present invention. Delaying a signal with a shift register having a number of stages all clocked at the same rate allows a signal to be delayed by any number of discrete intervals, u2 to the maximum number of stages in the shift register. By providing addition~l stages clocked by a different clock signal, additional selected delay intervals can be provided. A signal can be passed through a first shift register and delayed a certain number of first intervals, and then passed through a second set of shift register stages and delayed an additional number of second intervals. Thus, a delay of virtually any desired whole and fractional increments of the first intervals can be provided. Similar flexibility in signal delays by digital means can be achieved by using a second c ock which operates at the same frequency as the first clock, but is offset in time by a selected amount. For example, if a signal is passed through a first shift register clocked at first intervals and then supplied to an additional stage clocked with the inverse of the first interval clock signal, an additional delay of one-half of a first clock interval will be introduced. Depending on the offset between the clock signals supplied to the first and second group of shift register states, almost any amount of delay can be introduced.
~` An example of the operation of the modulator and digital time delay means of FIGS. 9 and 11 is given in FIG. 12. Assuming a first clock signal supplied by ., .

lZ539~

clock 56 to be that shown in FIG. 12b and a second clock signal supplied by second clock 220 to be that shown in FIG. lOa, a modulated signal input to the shift register will be delayed in the manner described below. In this example, second clock 220 is exactly twice the frequency of first clock 56. If, for example, a delay in the modulated signal of two-and-a-half first clock intervals is desired, the shift register will be configured so that otuput pin 206'' is connected to the second shift register input 214. In that way, a modulated signal input via line 53 will pass through two first shift register stages 200 and 201 and into the first stage of second shift register 212, after which the signal is output at pin 218. The signal will be delayed two full first clock intervals and an additional second clock interval by such a system. Assuming a modulated signal as appears in FIG.
12c is input to the configuration described above~ the output at pin 218 will be the signal shown in FIG. 12d.
The delayed modulated signal shown in FIG. 12d is exactly the same as the modulated signal shown in FIG.
12c, delayed by two-and-a-half first clock intervals.
The digital shifting technique of the subject multiplier has the advantage inherent in digital electronics of being relatively drift-free and error-free. Furthermore, the time adjustment is made in a ; manner independent of the signal being adjusted. In ; other words, it is not dependent on the frequency of the signal being adjusted in time. The system shown in FIG. 9 allows for phase adjustment in the multiplication of two analog signals without the use of R.C. networks and their associated signal perturbations. If delta-minus-sigma modulation is employed in the multiplication, the size of the shift registers employed need not be prohibitively large, while yielding a high level of accuracy.
~' ; ~``'' :, '~

:~25396~

In order to achieve high accuracy from the subject power metering system of the present invention over a wide dynamic range, it is important that offset errors be eliminated from the active circuit elements. Offset errors of sufficient magnitude to adversely affect measurement accuracy are commonly found in low cost operational amplifiers. The term voltage offset is generally defined as the voltage difference between a pair of inputs to an active circuit element, such as an operational amplifier, when the output is zero. It is a mismatch between the amplifier inputs, and the metering system of the present invention incorporates offset compensation means which correct for such mismatch.
FIG. 13 illustrates a novel offset compensation system, as applied to a single amplifier. The basic theory of the offset compensation system involves use of a capacitor or other storage element which is ; connected to one input of the amplifier and then charged to a compensating voltage. It will be understood that other equivalent systems for storing and supplying a voltage to an amplifier input could be used in place of a capacitor. Operational amplifiers often have more than two inputs, and sometimes include one or more inputs specifically designed for offset compensation purposes. The present invention will work ` equally well to offs~et compensate amplifiers having additional inputs. ~hichever input is designated to receive a compensating voltage to correct for voltage

3~ offset will be the input to which the capacitor is connected. The system further includes means for charging the capacitor to an offsetting voltage which substantially cancels out the effect of the voltage offset another amplifier input. For simplicity, only amplifier 70 (FIG. l) is shown in FIG. 13, although the offset compensation means of the present invention can ~ ~253~

sequentially correct a plurality of amplifiers, as set forth below.
The amplifier offset compensation means as applied to amplifier 70 includes an offset storage element, such as capacitor C1, connected to a first selected input 181 of the amplifier. A nulling circuit 182, connected through switches to both the offset storage element and the second selected input 183 of amplifier 70, is also provided. Nulling circuit 182 includes a charging amplifier 184 connected to the second input of amplifier 70, throùgh a switch Al. The nulling circuit also includes a temporary storage element, capacitor 186, and a series of switches, B, D and E, which connect capacitor 186 in to charging amplifier 184 as described below. Additional switches G1 and Hl connect charging amplifier 184 in a charging circuit, which adjusts the voltage stored on capacitor Cl.
The line current signal IA2 is supplied to the inverting input 183 of amplifier 70, which is ideally a virtual earth. Any voltage offset in amplifier 70 will appear initially as a voltage at inverting input 183.
As capacitor Cl becomes charged, the voltage at inverting input 183 will decrease until a virtual earth condition is reached. The difference between the compensating voltage VCOmp on Cl and the actual voltage offset of amplifier 70 is termed an error voltage V rror. It is VerrOr which appears at input 183. It is the purpose of the offset compensation means of the present invention to reduce Verror to a minimum.
The offset compensation means includes control means for accomplishing the functions set forth in box ~ 190. Essentially, the control means operates switches ':
' :~2539~

verrOr is supplied to the non-inverting ~nput of charging amplifier 184, which is configured as a unity-gain amplifier. Switch B, which is closed during the transfer periods, makes a feedback connection between the output 192 of charging amplifier 184 and the non-inverting input 226. A first terminal 228 of temporary storage capacitor 186 is also connected to inverting input 226. Switch D, when closed, connects a second terminal 230 of capacitor 186 to ground. Thus, during the transfer period, VerrOr appears at amplifier output 192 and is stored on temporary storage capacitor 186, together with the voltage offset of charging amplifier 184 (Voffset-AmP 184 ) During a subsequent charging period, control means 190 opens switches Al, B and D and closes switches E, Gl and Hl. That serves to disconnect the secona termi-nal 230 of capacitor 186 from ground and connects it to amplifier output 192, in a second feedbac~ loop. The result is that a voltage -VerrOr appear~s at amplifier output 192. The internal offset of charging amplifier ( offset-Amp 184) is cancelled by the equal and opposite value of the component -VOffSet-Amp 184 is supplied to output 192 from capacitor 186. The clo-sure of switch Gl and the opening of switch Al during the charging period also supplies the voltage VCOmp on offset storage capacitor Cl to the non-inverting input of charging amplifier 184. With ~VerrOr at charging amplifier output 192 and VCOmp at its input (during the charging period), a current ~Ierror is set up through impedance 224 and switch Hl which adjusts VCOmp in the direction necessary to reduce VerrOr during the next transfer period.
FIG. 14 illustrates the operation of the offset : compensation means during startup conditions. Assuming g offset-Amp 70 represents the voltage offset between the inputs of amplifier 70, and the , ~ _ ,~

;,,~, ~' ~253968 charge on capacitor Cl (vcOmp) is initially zero, then verrO , auring the initial transfer period will equal Voffset Amp 70. During the subsequent charging period a voltage ~VerrOr will appear at amplifier output 192.
A current ~Ierror will then be supplied to capacitor 186, increasing the value of VCOmp. The voltage VcOmp on capacitor Cl will serve to substantially reduce the offset error of amplifier 7~ until the next transfer period. The values of resistor 224 and capacitor Cl are chosen to produce a current -Ierror which will not excessively change the voltage on capacitor Cl during any single charging period. Capacitor Cl will therefor not become charged to the full offsetting voltage during the first few transfer and charging cycles. As approaches (Voffset-Amp 70) error progressively smaller. Eventually, V O will approach a stable minimum value sufficient to correct for leakage currents and other transient signals present in the circuitry. At that point the offset errors will be virtually eliminated.
Subsequent transfer and charging periods can either follow immediately after previous transfer and charging periods, or be separated by a time delay. In the preferred embodiment, where additional amplifiers are being offset compensated using the same nulling circuit 182, the transfer and charging periods associated with any one amplifier are separated by predetermined time intervals. Referring to FIG. 14, the next transfer period shows a VerrOr which is smaller, as shown at 222. As before, VerrOr is first stored on capacitor 186 and then, during the following charging period, appears at charging amplifier output ~ 192 an ~VerrOr. During this charging period the `~ current -Ierror is added to the charge on capacitor Cl, further reducing the magnitude of VerrOr during the following transfer period. During subsequent cycles, i~

, . ~. .

i25~9~8 VCOmp on capacitor Cl will approach the actual voltage offset of amplifier 70, reducing VerrOr to approximately zero.
The offset compensation system described above with respect to amplifier 70 can similarly offset compensate a plurality of amplifier elements. FIG~ 15 shows the preferred embodiment of the offset compensation system used to provide offset compensation for five different amplifiers. The five amplifiers to be offset compensated by the compensatlng means of the metering system are as follows: current signal gain amplifier 70, current signal inverting amplifier 74, first modulator integrating amplifier 46, Watts output convertor integrating amplifier 108 and VARS/Q output convertor integrating amplifier 180. Each of the amplifiers is similar to gain amplifier 70 discussed with respect to FIG. 13 in that all have virtual earth inverting inputs to which a signal is supplied. Each of these amplifiers is provided with respective offset storage elements, capacitors ~1 through C5. The non-inverting inputs of the amplifiers are connected to the charging amplifier 184 of nulling circuit 182 through respective switches Al through A5, as shown in FIG. 15.
Pairs of switches equivalent to Gl and Hl of FIG. 13, namely Gl through G5 and Hl through H5, connect charging amplifier 184 to the respective offset storage capacitor of each amplifier.
; A single nulling circuit 182 will store the error voltage and charge the offset storage capacitor of each amplifier, by means of the sequence described below.
For clarity, the control circuitry for operating the various switches shown in FIG. 15 is omitted. A
conventional controller of any suitable type can be used to control the switches in accordance with the ; 35 timing diagram illustrated in FIG. 16. The controller ~first closes switches Al, B and D during an initial ~' i Z~;396~

transfer period for amp 70, then open switches Al, D
and B and closes switches E, Gl and Hl during a charging period. The controller then provides additional successive transfer and charging period for each of the other amplifiers being offset compensated.
~fter the charging period of amplifier 70 the transfer period of amplifier 74 begins, with the controller closing switches A2, D and B and then opening those switches and closing switches E, G2 and H2 during the subsequent charging period. For amplifier 46, switches A3, B and D are closed during the transfer period and switches E, G3 and H3 are closed during the charging period. For amplifier 108, switches A4, B and D are closed during the transfer period and switches E, G4 and H4 are closed during the charging period. Finally, for amplifier 180 switches A5, B and D are closed during the transfer period and switches E, G5 and H5 are closed during the charging period.
After a transfer and charging period has been completed for an amplifier, all the switches associated with the amplifier, namely switches A, G and H, are left open. The charge stored on the respective offset storage capacitors will remain until the controller sequence produces a new charging period associated with that capacitor. Although some charge decay will occur, errors due to voltage offset are substantially reduced for each of the amplifiers. The frequency of operation of the controller for opening and closing the switches associated with the offset compensation means is a matter of design choice, but can be substantially slower than the clocks associated with the metering system.
The disclosed offset compensation system can be used to correct for offset errors in any number of amplifier elements associated with a metering system.
A single nulling circuit like circuit 182 can be .
~`
~, ~ ',.
i, ;

~2539S~ --sequentially connected to up to N amplifier elements and to their associated storage elements during a sequence of transfer and charging periods. Such an offset compensation system is economical, and is ideally suited to the use of CMOS integrated circuitry where offset errors can present problems. Although described with respect to the metering system of the present invention, the offset compensation system can be applied equally well to other types of power metering circuits employing operational amplifiers.
Such a meteriny circuit might include, for example, any suitable means for multiplying the analog signals representing current and voltage, as well as any suitable converter or filter circuit for producing an output signal from the product signal. Assuming that the metering system employs up to N amplifier elements in its various components, the offset compensation system of the present invention can substantially eliminate offset errors in the manner described below.
The N amplifier elements, each will include plurality of inputs. A first selected input in any such amplifier is the input for receiving a compensating voltage to correct for voltage offset. N
offset storage elements, such as capacitors, are also provided. One of the N offset storage elements is connected to the first selected input of each of the N
amplifier elements. The offset storage elements receive compensating voltages which substantially reduce the offset error at another input of the amplifier element to which it is connected, the other input being designated the second selected input. Any ; difference between the compensating voltage on the offset storage element and the voltage offset of the amplifier element is an error voltage which appears at the second selected input of the amplifier element. A
~ nulling circuit such as circuit 182 is also provided '.
., i .:

lZS39 for the power metering system. The nulling circuit can be connected sequentially to each of the N amplifier elements and to the offset storage element associated therewith. In the description below, the amplifier element to which the nulling circuit is connected, including its associated storage element, is termed the selected amplifier element. In the same manner as the system described above, the nulling circuit is first connected to the second input of the selected amplifier element, during an intermittent transfer period. The nulling circuit is then connected to the offset storage element associated with the selected amplifier element during the intermittent charging period following the transfer period. A control system then connects the nulling circuit sequentially to the remaining of the N
amplifier elements to provide transfer and charging periods for each of the amplifier elements. The sequence is continuously repeated, whereby all of the amplifier elements are offset compensated and the offset errors in the metering system are substantially eliminated.
Through incorporation of the offset compensation means described above, the metering system of the present invention meters power to a high degree of accuracy over a wide dynamic range. The need for relatively high cost calibrated or error-free amplifiers is eliminated, which makes the metering system relatively inexpensive. The system provides continuous parallel readings of power in both Watts and in VARS or Q. Because the modulator output from modulator 30 is clocked precisely at the first clock intervals, it is possible to manipulate the signal with digital logic. A shift register can be conveniently employed to introduce the time delay necessary to provide an appropriate phase shift for the VARS and Q
; measurement. By simply selecting the appropriate stage ., :

~ :' ` i2539~a ~

in the shift register, the delay in the modulated signal can be adjusted to produce the desired output (VARS or Q, 50 or 60 Hz). The invention thus eliminates the need for tuned analog phase shifters to produce the desired voltage lag. Because the output of the modulator can be supplied both to a power and a VARS/Q converter, simultaneous readings can be produced with only a single modulator. The system further provides digital outputs for each polarity of power flow on the line. Maximum information is therefore provided to a high degree of accuracy in an efficient and economical manner.
FIG. 17 shows a portion of an alternative embodiment modulator which is somewhat simpler in construction than modulator 30 shown in FIG. 2. In this emobdiment, a capacitor 44 is connected between the summing node 36 and ground. Capacitor 44 serves as the modulator integrator. The inverting input of a comparator 50 is also connected to node 36 with the non-inverting input being connected to ground.
Comparator 50 develops a control signal in response to voltage changes on node 36 which is coupled to a bistable circuit 52. Circuit 52 is used to control a pair of switches which supply a feedback signal to node 36, as will be subsequently described.
FIG. 18 illustrates several of the signals produced by the FIG. 17 modulator. Input signal VL is represented in FIG. 18a. Of course, in A.C. power metering applications, VL will be sinusoidal.
Initially, switch 58 is assumed to be closed, and a negative reference current is applied to summing node 36 through resistor 40. The values of Vl- and resistor -~ 40 are chosen to produce a current IF which is large ~! relative to input signal IA1 . Idiff will therefore have a net negative value, with current being drawn ` from capacitor 44. Conse~uently, the integrated :'.
:~' ~,.

:;

::

lZ~396 difference voltage signal initially decreases, as shown in FIG. 18c.
Clock 56 outputs signal shown in FIG. 18b.
Bistable circuit 52 clocks on the leading edge of each upwardly-moving pulse. At clock pulse a, the integrated difference signal of FI~. 18c has not yet crossed the threshold of comparator 50, so Q remains low and Q high and the difference signal continues to integrate downward. Since the difference signal is supplied to the inverting input of comparator 50, when the signal crosses the threshold, the comparator output switches from low to high. The control signal shown in FIG. 18d represents the output of comparator 50.
Consequently, at clock pulse b, bistable circuit 52 will change states and Q will go from low to high.
When Q goes high Q goes low and switch 60 is closed and switch ~8 is open. A positive reference signal is then supplied to summing node 36, causing integrated Idiff to increase until the next clock pulse at c. Between clock pulses b and c, the integrated difference signal again crosses the threshold level of comparator 50, causing the first control signal to go low. Q then goes low at the next clock pulse, causing the reference signal supplied to summing node 36 to again go negative. As VL increases, the slope of the difference signal changes and its value decreases until again crossing the threshold level. Q remains low until detecting a change in the first control signal at clock pulse f. Q then goes high, again switching the reference signal from negative to positive.
The above-described circuit and method operates as a delta-minus-sigma convertor in which only the difference between the input and reference signals is integrated and measured. The circuit always maintains `(35 the integrated difference signal around the threshold ~level of comparator 50. The Q output of bistable ' ~

-- ~2s3s~a ~-circuit 52 is chosen as the first output signal, having an average level or amplitude over time which is proportional to the magnitude of VL.
FIG. 19 shows a modulator circuit as in FIG. 17 which incorporates an alternative offset compensation system. In this embodiment, comparator 50, which is an operational amplifier element, is provided with compensating means for substantially eliminating any offset error resulting from a voltage offset existing between the amplifier inputs 306 and 308. As described above, a voltage offset is generally defined as the voltage required between inputs of an amplifier to produce a zero output. Ideally, voltage offset is zero, but in most real-world operational amplifiers an 15 offset of unknown value is usually present. With the present invention, a first storage element, such as capacitor 302 is connected to one of the amplifier inputs, and an offsetting voltage substantially equal to the voltage offset of the amplifier is stored on the 20 storage element to compensate for the voltage offset.
In the example shown in FIG. l9, capacitor 302 is located in the electrical path between summing node 36 and inverting amplifier input 306. It should be understood that capacitor 302, like capacitor 44 and 25 the other storage elements used in the embodiments described below, represent one type of storage element which can be used, and that other types of circuit elements, such as registers with D to A convertors and the like, could be used for the various storage 30 elements of the present invention.
f The offset compensation system also includes a feedback loop 300, which is intermittently connected around amplifier 50, between inverting input 306 and the amplifier output through a switch, C. When Switch 35 C is closed, the voltage offset appears at a low impedance at input 306. In order to store the voltage .~
''^''`'`'`

~;
~,' ~5~9~8 produced by the feedback loop on capacitor 302, switches A and B are provided to disconnect one end of the capacitor from summing node 36 and connect it to common ground 305.
The means for controlling the offset compensation system shown in FIG. 19 is clock 56, and FIG. 20 illustrates the control function. sistable circuit 52 clocks at the leading edge of each clock cycle, as indicated by arrows 312. Each upward moving pulse represents one clock pulse. Just as the clock signal begins to go from low to high, switches B and C are off and switch A is on, meaning that the feedback loop around amplifier 50 is disconnected and capacitor 302 is connected to summing node 36. As soon as the clock pulse begi~s, switches B and C turn on and switch A
turns off, connecting the feedbac~ loop around amplifier and connecting one terminal of capacitor 302 to ground. During this period, called the nulling period, the voltage offset ~VOffSet f amplifier 50 appears at input 306. Since capacitor 302 is connected between input 306 and ground, the voltage +Voffset is stored on the capacitor. During the last half of each clock cycle, called the measuring period, switches B
and C again turn off and switch A turns on. With non-inverting input 308 tied to ground, the error atinverting input 306 is the negative value of the voltage offset -VOffSet. Consequently, the signal being compared to the threshold level by comparator 50, when A is closed and B and C are open, is the voltage at summing node 36, the integrated difference signal, P Voffset Plus ~Voffset The voltage offset of comparator 50 is therefore cancelled, and the error it would otherwise produce in the threshold measurement is :;
essentially eliminated.
Another embodiment of a modulator utilizing an offset compensation system is shown in FIG. 21. In this ' ,' ~253968 embodiment, measuring means 298 has first and second amplifier elements 328 and 336, respectively, which serve as comparators and are alternately connected between summing node 36 and bistable circuit 52. First 5 amp 328 is provided with a switchable feedback loop 324 connecting output 330 with inverting input 326 through switch D. A first storage element in the form of capacitor 316 is connected in the electrical path between summing node 36 and inverting input 320 through switch E. A path is provided between one terminal 318 of capacitor 316 and ground, through switch F. Second amplifier element 336 also includes a switchable feedback loop 332 connected between output 338 and inverting input 334 through switch G, and a StQrage element such as capacitor 320 is in the electrical path between inverting input 334 and summing node 36, through switch H. A path between one terminal 322 of capacitor 320 and ground is provided, through switch J.
The embodiment of FIG. 21 is designed to provide two parallel offset compensated comparator circuits for measuring the integrated difference signal at summing node 36. When switches E and R are closed, first amplifier element 328 supplies the first control signal L~ to bistable circuit 52 and when switches H and L are closed, second amplifier element 336 supplies the first control signal to bistable circuit 52. By closing switches E, G, J and X and opening switches D, F, R and ; L, first amplifier element 328 is in a measuring mode supplying the control signal to bistable circuit 52, ~; 30 and second amplifier element 336 is :Ln a nulling mode in which the voltage offset of amplifier element 336 is : stored on capacitor 320. Storage of VoffSet on capacitors 316 and 320 is accomplished in exactly the ` same manner as with amplifier element 50 and capacitor 302 in the embodiment of FIG. 19. By reversing all the ~: switches, i.e., closing switches D, F, R and L and ~:, "r~ - ' ' -2539~;B

opening switches E, G, J and K, amplifier 328 is in the nulling mode and amplifier 336 is in the measuring mode in which the integrated difference signal at summing node 36 is supplied to inverting input 334 through capacitor 320, compensating for the voltage offset of amplifier 336 and providing an error-free first control signal to the D input of bistable circuit 52.
One advantage of the embodiment shown in FIG. 21 over that shown in FIG. 19 is that one offset compensated amplifier is available at all times in its measuring mode. Fùrthermore, switching between measuring and nulling modes in the embodiment of FIG.
19 occurred at the clock frequency of clock 56. If the sampling frequency, as determined by the frequency of clock 56, is sufficiently high, the amplifier elements which serve as comparators will be unable to stabilize after each nulling period, and errors will be introduced. The embodiment of FIG. 21, which uses conventional control logic for operating switches D, E, F, G, H, J and L, represented by element 340, can be operated at a frequency different from clock 56. A
`~ conventional frequency divider can be used to reduce the frequency of the control operations, for example.
In order to insure that adequate time is provided for the amplifiers in the embodiment of FIG. 21 to stabilize after each nulling period, control logic 340, which serves as control means for operating switches D, E, F, G, H, J, K and L, extends the measuring period ; for each amplifier element to allow time for stabilization. FIG. 22 shows the timing diagram for ~ the operation of switches D, E, F, G~ H, J, R and L by ; control logic 340. Switches K and L, which connect the outputs of the first and second amplifier elements, respectively, to bistable circuit 52, are operated out of phase with one another. Switch K is on half the time, and off half the time and switch L being off when ~ .

1~ 5~ 9~ ~

K is on, and vice versa. In addition to controlling the switches which connect the amplifiers to bistable circuit 52, control logic 340 also controls the switches which determine the nulling and measuring periods of amplifiers 328 and 336. Switches D, E and F
serve to connect a feedback loop around amplifier ~28 and connect the one terminal 318 of capacitor 316 to ground, in exactly the same manner as the embodiment of FIG. 19. Switches G, H and J perform the same function for amplifier 336. As can be seen from FIG. 22, the amplifier nulling and measuring periods of each amplifier element are not of the same duration. The nulling period for first amplifier 328, for example, begins when switch K turns off and ends before switch K
again is turned on. Similarly, the nulling period of second amp 336 begins when switch L turns off and ends before switch L turns on again. Consequently, the nulling period of each amplifier is shorter than the measuring period by a predetermined interval. This is done to allow time for the amplifiers to stabilize before being connected to bistable circuit 52~
It should be noted that, in addition to allowing extra time for amplifier stabilization before connecting either the first or second amp to the bistable circuit, control logic 340 operates inherently slower than clock 56. As can be seen from FIG. 22, the clock signal, which is not drawn to scale, operates at a substantially higher frequency than any of the switches in FIG. 21. Control logic 340 preferably includes a frequency divider for this purpose. The embodiment shown in FIG. 21 can thus employ a ~` relatively high frequency clock, for example 10 RHz, to provide frequent sampling and relatively high resolution, while nulling and offset compensating the amplifier elements at a low enough frequency to minimize errors due to slow amplifier response.

"., :~. ..... .

~2539~3 The method of the present invention performed by the embodiment of FIG. 21 includes an additional step in the measuring step, for switching between the first and second amplifier elements, 328 and 336, respectively. The compensating step includes measuring with the first amplifier element and nulling the second amplifier element and then measuring with the second amplifier element and nulling the first amplifier element, in a continuous cycle, such that at least one of the offset compensated amplifier elements is connected to the summing node at all times. In the preferred embodiment of the method, the nulling periods and measuring periods are different, and preferably slower than the clock intervals. Further, the nulling periods are shorter than the measuring periods for each amplifier element, in accordance with the timing diagram of FIG. 22. The measuring period of one amplifier element is begun prior to ending the measuring period of the other amplifier element such that any errors due to slow comparator response of the first of the amplifier elements as it is initially switched from nulling to measuring is eliminated.
Operation of the embodiment of FIG. 21 will produce the results illustrated in FIG. 18. Assuming VL is as shown in FIG. 18a, the integrated difference signal appearing at summing node 36 will be that shown in FIG. 18c. Both the first control signal of FIG. 18d and Q output of FIG. 18e will be unaffected by the intermittent operation and cyclical nulling and measuring periods of amplifiers 328 and 336. The embodiment of FIG. 21 provides greater accuracy at higher clock frequencies, but is otherwise functionally identical to the embodiment of FIG. 19.
Nodulator 30 used in the Figure 1 metering system can be used in other applications where it is necessary to provide modulated output signals which are ..
, ~' t i25396~

indicative of the polarity of the input signal~
Referring now to Figure 23, an alternative embodiment modulator which provides such output signals may be seen. Input signal IA~ is supplied to summing node 36 5 through resistor 38. One of the two reference signals, which are preferably of equal magnitude and opposite polarity, are also supplied to the summing node through resistor 40. The reference voltages Vl- and Vl+ are connected to the summing node through a pair of 10 switches 58 and 60, respectively, which are controlled by the modulator output. Instantaneous differences between input current IA1 and feedback current IF at node 36 are supplied to an integrator, which produces an ascending or descending ramp voltage. The 15 integrated signal is then compared against a threshold level by a comparator 50, which outputs a control signal indicating whether the output of the integrator is above or below the threshold level. The output of comparator 50 is supplied to a bistable circuit, such 20 as flip-flop 52.
The bistable circuit changes states only at predetermined clock intervals, as determined by clock 56. When the integrated signal crosses the threshold level of comparator 50, the outputs of bistable circuit 25 52 reverse states at the next clock pulse. The Q
output of bistable circuit 52, which is the first modulated signal of the present invention, controls switch 60, which connects the positive reference voltage Vl+ to summing node 36. The Q output, which is 30 always the inverse of the Q output, operates switch 58, connecting the negative reference voltage Vl- to summing node 36. Switches 58 and 60 are always operated alternately, meaning that one or the other of the reference signals is always supplied to summing } 35 node 36.

, :
:~ `

12539~8 The Q output of bistable circuit 52 is connected to the D input of a second bistable circuit 53 and both recsive clock signals from the same clock 56. Because of gating delays, changes in the Q output of bistable circuit 53 will always follow changes in the Q output of bistable circuit 52 delayed by one clock pulse. An AND gate 350 is also provided to receive the Q outputs from both bistable circuits 52 and 53, as well as a clock signal from clock 56. The AND gate serves as means for outputing a first digital signal which is proportional to the magnitude of one polarity of the input signal.
FIG. 24 illustrates the operation of the above-described circuit elements. Assuminq, for illustrative purposes that the voltage waveform at node 32 supplied to the alternative embodiment modulator is as depicted in FIG. 24a, the signal is converted to a first modulated signal at the Q output of bistable circuit 52 in the manner described above. The Q output of bistable circuit 52 is assumed to be that shown in the waveform of FIG. 24d. The output of clock 56 is represented by the waveform of FIG. 24b. The output of second bistable circuit 53 is termed "delayed Q" and is depicted by the waveform of FIG. 24e. Delayed Q is substantially equal to Q, but delayed in time by one clock interval. The present invention calls for combining Q, Delayed Q and a clock signal at an AND
gate 350 (see FIG. 23).
~ Although not necessary in idealized circuits in ;~ 30 which component delays are nonexistent, for real-world components it is preferable to include an inverter 57 between clock 56 and AND gate 350. Inverter 57 inverts the clock signal to yield an inverted clock signal shown in FIG. 24c. The reason for supplying an ~ 35 inverted clock signal to the AND gate is because ; propagation delays in bistable circuits 52 and 53 will :
, ., ,~
~ .
'`'~'~'`

-- 12~i39~8 tend to cause their outputs to lag slightly behind the output of clock 56, and will produce short simultaneous "high" conditions in all three signals at the wrong time. The result of not inverting the clock is an extraneous spike output from AND gate 350, which would represent an error pulse. For this reason, inverter 57 is included in FIG. 23. The resultant waveform output by AND gate 350 is shown in FIG. 24f.
The FIG. 24f waveform is essentially a digital representation of the amount by which the time Q is high exceeds the time Q is low. In the example of FIG.
24, waveform 24f contains only two pulses, generated successively, and appearing at the right side of the illustration. Those two pulses roughly coincide with the region where the FIG. 24a input is most negative.
Preferably, the frequency of the clock will greatly exceed the variations of the analog input signal, to produce higher resolutions than that shown in FIG. 24.
The principle of operation is exactly the same, however. In essence, combining a delayed modulated signal with the original modulated signal at an AND
gate produces an output which will go high only when Q
remains high for at least two successive clock pulses.
The clock signal causes the AND gate output to be a - 25 pulse train, having pulses at intervals of not less ~- than the clock intervals of the clock signal. In the - example just described, the AND gate outputs pulsesonly when all signals supplied to it are high. If Q is low for two or more successive clock pulses, it will have no effect on the output of AND gate 350 since only high inputs are measured. Thus, the output of the AND
gate is a representation of the magnitude of only one polarity of the input signal. The output is, in ;~ effect, a half wave rectified signal, represented digitally.
~.
~", , .
; '' ;, ~, ~ .

In order to produce a digital output proportional to the other polarity of the input waveform, the alternative embodiment modulator utilizes the Q outputs of bistable circuits 52 and 53 as first and second inverse modulated signals, respectively. Assuming the same input signal and clocks as shown in FIG. 24, ~
will be as shown in the waveform of FIG. 24g. Bistable circuit 53 provides a delayed Q signal as shown in FIG.
24h. Both signals are supplied to a second AND gate 352 (FIG. 23), together with the inverted clock signal shown in FIG. 24c. The output of second AND gate 352 is shown in the waveform of FIG. 24i and is termed a second digital signal. The second AND gate serves as means for outputting a second digital signal which contains pulses in proportion to the amount of time by which one level of said first inverse modulated ~ignal exceeds the other level. When all three inputs to the AND gate are high, pulses are produced at intervals of not less than the clock intervals of clock 120. In the present example, the waveform 24i represents the ; positive polarity component of the input signal. As can be seen, the location of the pulses roughly corresponds with the regions where the FIG. 24a input signal is high. The FIG. 24i waveform provides a digital representation of the magnitude of the positive half wave components of input signal.
Referring back to FIG. 23, the present invention can be further employed to produce a digital signal proportional to the magnitude of the full waveform of input signal. This is accomplished by supplying the first digital signal output of AND gate 350 and second digital signal output of AND gate 352 to an OR gate 351, which serves as gate means for combining the digital signals and for outputting a summation digital signal shown in FIG. 24j. The FIG. 24j waveform is proportional to the magnitude of the full input signal, -~, :. .

` .,,~

"

iL:253968 including both polarities, which is termed herein "absolute magnitude". The outputs of AND gates 350 and 352 are coupled up and down inputs of an up/down counter 354 so that the number of positive and negative pulses can be compared over any selected time interval.

~.

`:
.~

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN
EXCLUSIVE PROPERTY OR PRIVILEGE IS CLAIMED ARE
DEFINED AS FOLLOWS:

1. A modulator for converting an input signal to an output pulse train which varies between two levels and which has an average level over time proportional to the input signal, comprising:
means for supplying said signals to a summing node, switch means for supplying a second signal to said summing node selected from at least two different reference signals having predetermined magnitudes, wherein the difference at any time between said input signal and said second signal is a difference signal, measuring means for integrating the difference signal at said summing node and for determining when the integrated difference signal reaches a threshold level, said measuring means including an amplifier element and offset compensation means for substantially eliminating any offset error resulting from a voltage offset existing between the amplifier inputs of said amplifier element, said compensation means including a first storage element connected to one said amplifier input and means for transferring an offsetting voltage to said first storage element to compensate for said voltage offset, a clock for producing clock pulses at predetermined clock intervals, a bistable circuit responsive to said measuring means and said clock and which produces a first output signal changeable at each said clock pulse between first and second levels whenever said integrated difference signal has crossed said threshold level during a clock interval, said switch means being responsive to said bistable circuit such that one of said reference signals is supplied to said summing node when said first output signal is at its first level and another reference signal is supplied to said summing node when said first output signal is at its second level, wherein the average value of the reference signals over time balances said input signal at said summing node and the average level over time of said first output signal is proportional to said input signal and wherein said measuring means includes a second storage element connected between said summing node and a common ground, said second storage element providing part of a passive integration means for integrating said difference signal, the voltage on said summing node being said integrated difference signal.

2. A modulator as in claim 1 in which said second signal is selected from two said reference signals which are substantially equal in magnitude and opposite in polarity.

3. A modulator as in claim 1 in which said amplifier element serves as a comparator for determining when a signal input to said amplifier element reaches a threshold level, said amplifier element having one input connected in an electrical path to said summing node and the output connected to said bistable circuit, said amplifier element outputting a first control signal which is high when said integrated difference signal is above said threshold level and low when said integrated difference signal is below said threshold level.

4. A modulator as in claim 3 in which said first storage element is in the electrical path between said summing node and the inverting input of said amplifier element, the non-inverting input being connected to said common ground, said offset compensation means including a feedback loop around said amplifier element between said inverting input of said amplifier element and the output thereof which is connected periodically during an intermittent nulling period such that any voltage offset appears at low impedance at said inverting input during said nulling period, together with means for simultaneously disconnecting one terminal of said first storage element from said summing node and connecting said one terminal to said common ground such that said voltage offset is stored on said first storage element during said nulling period, said nulling periods alternating with measuring periods in which said feedback loop around said amplifier element is disconnected and said one terminal of said first storage element is reconnected to said summing node whereby the voltage offset of said amplifier element is balanced by the voltage on said first storage element during the measuring period.

5. A modulator as in claim 4 including means for synchronizing said nulling and measuring periods with said clock pulses such that said nulling periods occur during a portion of each said clock interval and said measuring periods occur when said first output signal from said bistable circuit is changeable in response to said measuring means.

6. A modulator as in claim 1 in which said measuring means includes first and second amplifier elements which serve as comparators for determining when a signal input to said amplifier element reaches a threshold level, said amplifier elements each having one input connected in an electrical path to said summing node, together with offset compensation means which include a first storage element on which said offsetting voltage is stored connected in said electrical path to one input of each said amplifier element, the outputs of said first and second amplifier elements being connected to said bistable circuit alternately during respective first and second measuring periods in which the amplifier element connected to said bistable circuit outputs a first control signal which is high when said integrated difference signal is above said threshold level and low when said integrated difference signal is below said threshold level, and including means for charging the first storage element connected to said first amplifier element to said offsetting voltage during a first amplifier nulling period while said second amplifier element is connected to said bistable circuit and for changing the first storage element connected to said second amplifier element to said offsetting voltage during a second amplifier nulling period while said first amplifier element is connected to said bistable circuit, said first amplifier nulling periods occurring between first measuring periods and said second amplifier nulling periods occurring between second measuring periods.

7. A modulator as in claim 6 in which said one input of each said amplifier element connected in an electrical path to said summing node is the inverting input thereof, and said offset compensation means includes a switchable feedback loop around each said amplifier element which is selectively connected during each respective amplifier nulling period between the inverting input of the respective amplifier element and the output thereof such that any voltage offset appears at low impedance at said respective inverting input, and means for transferring during the nulling period of each said amplifier element the voltage offset of the amplifier element to the connected first storage element.

8. A modulator as in claim 6 in which said offset compensation means causes the nulling period of each said amplifier element to be shorter than the measuring period thereof, such that said first amplifier nulling period ends a predetermined interval before the measuring period of said first amplifier element begins and said second amplifier nulling period ends a predetermined interval before the measuring period of said first amplifier element begins so as to minimize errors due to slow comparator response.

9. A modulator as in claim 1 in which one terminal of said amplifier element is connected to said summing node and the other terminal of said amplifier element is connected to said first storage element on which said offsetting voltage is stored.

10. A modulator as in claim 9 in which said amplifier element is a first amplifier element of said measuring means, which also includes a second storage element connected in a negative feedback loop between the inverting input and output of said first amplifier element, said inverting input also being connected to said summing node such that said integrated difference signal appears across said second storage element, and including a comparator element connected between the output of said first amplifier element and said bistable circuit, said comparator element determining when said integrated difference signal on said second storage element reaches a threshold level.

11. A modulator as in claim 10 in which any difference between the offsetting voltage on said first storage element and the actual voltage offset of said first amplifier element is an error voltage which appears at the inverting input of said first amplifier element, said offset compensation means including means for intermittently connecting said inverting input of said first amplifier element to the non-inverting input of a second amplifier element during a transfer period, and including a temporary storage element having a first terminal connected during said transfer period to both the output and to the inverting input of said second amplifier element, and a second terminal connected to a common ground during said transfer period, said offset compensation means also providing a charging period during which said first storage element is connected to the non-inverting input of said second amplifier element and said output thereof is disconnected from said first and connected to said second terminal of said temporary storage element such that a negative voltage equal to said error voltage appears at the output of said second amplifier element, and means for connecting the output of said second amplifier element to said first storage element through an impedance during said charging period such that a current is supplied to said first storage element proportional to said error voltage to change the voltage on said first storage element in a direction which reduces said error voltage during the next transfer period.

12. A converter for producing a digital signal indicative of the magnitude and polarity of an analog input signal comprising:
clock means for producing a clock signal which occurs at clock times, separated by predetermined clock intervals;
modulating means for producing a modulated signal changeable between first and second levels at said clock times and with the time ratio of said modulated signal at said first level to said second level, referred to as a measured duty cycle value, being indicative of the magnitude of said input signal with respect to a reference level; and output means for determining the polarity of said input signal with respect to said reference level by sampling said modulated signal at least once every said clock interval, and for producing at least one said digital signal indicative of magnitude and polarity in response to said modulated signal and said polarity determination and wherein said output means assigns a first polarity to said input signal when said modulated signal is at said first level for at least two consecutive said clock intervals and a second polarity when said modulated signal is at said second level for at least two consecutive said clock intervals.

13. The converter of claim 12 wherein said output means includes delay means for delaying said modulated signal so as to produce a delayed modulated signal and gating means for producing said at least one digital signal in response to said modulated signal and said delayed modulated signal.

14. The converter of claim 13 wherein said delay means delays said modulated signal by one said clock interval.

15. The converter of claim 14 wherein said delay means includes a bistable circuit.

16. The converter of claim 15 wherein said bistable circuit is clocked in response to said clock signal and an input of said bistable circuit receives said modulated signal.

17. The converter of claim 13 wherein said output means outputs a first of said digital signals which is indicative of the magnitude of said input signals of a first polarity and a second of said digital signals which is indicative of the magnitude of said input signals of a second polarity.

18. The converter of claim 17 wherein said output means outputs a third of said digital signals which is indicative of the magnitude of both said first and said second polarity input signals.

19. The converter of claim 13 wherein said modulator means includes a second bistable circuit clocked in response to said clock signal having said modulated signal as an output thereof.

20. A converter for producing a digital signal indicative of the magnitude and polarity of an analog input signal comprising:
clock means for producing a clock signal which occurs at clock times, separated by predetermined clock intervals modulating means for producing a modulated signal changeable between first and second levels at said clock times and indicative of the magnitude of said input signal with respect to a reference level; and output means for determining the polarity of said input signal with respect to said reference level by combining said modulated signal with said modulated signal delayed one said clock interval and for producing at least one said digital signal indicative of polarity and amplitude in response to said modulated signal and said polarity determination.

21. The converter of claim 20 wherein the time ratio of said modulated signal at said first level to said second level, referred to as measured duty cycle, is indicative of said magnitude of said input signal.

22. The converter of claim 21 wherein said modulated signal has a reference duty cycle value when said input signal magnitude is zero and said output means assigns a first polarity to said input signal when said measured duty cycle value is greater than said reference duty cycle value and a second polarity when said measured duty cycle value is less than said reference duty cycle value.

23. A method of converting an analog input signal to a digital signal indicative of the magnitude and polarity of the input signal comprising the following steps:
producing a clock signal which occurs at clock times separated by predetermined clock intervals;
producing a modulated signal from said input signal changeable between first and second levels at said clock times, with the time ratio of said modulated signal at said first level to said second level, referred to as a measured duty cycle value, being indicative of the magnitude of said input signal with respect to a reference level;
determining the polarity of said input signal with respect to said reference level by sampling said modulated signal at least once every said clock interval wherein a first polarity is assigned to said input signal if said modulated signal is at said first level for at least two consecutive said clock intervals and a second polarity when said modulated signal is at said second level for at least two consecutive said clock intervals: and producing said digital output signal in response to said modulated signal and said polarity determination.

24. The method of claim 23 wherein said step of determining polarity includes the step of delaying said modulated signal one said clock interval.

25. A converter for producing a digital signal indicative of the magnitude and polarity of an analog input signal comprising:
clock means for producing a clock signal which occurs at clock times, separated by predetermined clock intervals;
modulating means for producing a modulated signal changeable between first and second levels at said clock times and with the time ratio of said modulated signal at said first level to said second level, referred to as a measured duty cycle value, being indicative of the magnitude of said input signal with respect to a reference level and wherein said modulated signal has a reference duty cycle value when said input signal magnitude is zero and said output means assigns a first polarity to said input signal when said measured duty cycle value is greater than said reference duty cycle value and a second polarity when said measured duty cycle value is less than said reference duty cycle value; and output means for determining the polarity of said input signal with respect to said reference level by sampling said modulated signal at least once every said clock interval, and for producing at lest one said digital signal indicative of magnitude and polarity in response to said modulated signal and said polarity determination.

26. The converter of claim 25 wherein said output means includes delay means for delaying said modulated signal so as to produce a delayed modulated signal and gating means for producing said at least one digitial signal in response to said modulated signal and said delayed modulated signal.

27. The converter of claim 26 wherein said delay means delays said modulated signal by one said clock interval.

28. The converter of claim 26 wherein said output means outputs a first of said digital signals which is indicative of the magnitude of said input signals of said first polarity and a second of said digital signals which is indicative of the magnitude of said input signals of said second polarity,

29, The converter of claim 26 wherein said modulator means includes a bistable circuit clocked in response to said clock signal having said modulated signal as an output thereof.

30. The converter of claim 27 wherein said delay means includes a bistable circuit.

31. The converter of claim 30 wherein said first bistable circuit is clocked in response to said clock signal and an input of said bistable circuit receives said modulated signal.

32. The converter of claim 28 wherein said output means outputs a third of said digital signals which is indicative of the magnitude of both said first and said second polarity input signals.