GB2057212A

GB2057212A - A/D conversion

Info

Publication number: GB2057212A
Application number: GB8024592A
Authority: GB
Original assignee: COMARK ELECTRONICS Ltd
Current assignee: COMARK ELECTRONICS Ltd
Priority date: 1979-08-08
Filing date: 1980-07-28
Publication date: 1981-03-25
Also published as: GB2057212B

Abstract

A signal processing system employs a voltage to frequency converter 2 connected in succession to voltage sources Vin, Vref, Vo by electronic switches I, II, III to provide gated numerical corresponding values N1, N2 and N3 which are stored in a microprocessor S. The value of the source Vin is then computed from these values N1, N2, N3 and from a stored number N4 equivalent to the true value of the source Vref by the relationship <IMAGE> <IMAGE>

Description

SPECIFICATION Electronic system for, and a method of, processing signals The present invention relates to an electronic system for, and to a method of, processing signals for measurement, monitoring or control purposes.

According to the present invention, an electronic system at least includes conversion means for converting an analogue input voltage into a corresponding digital signal, switching means adapted to selectively connect the conversion means to a voltage to be measured or monitored and to other reference voltages in a predetermined sequence and processing and control means for controlling the switching means and for processing the resultant sequence of digital signals to compute the numerical value of the voltage to be measured or monitored.

The present invention also provides an analogous method of processing signals which comprises selectively connecting an analogue voltageldigital signal converter to a voltage to be measured or monitored and to other reference voltages in a predetermined sequence and processing the resultant sequential digital signals from the converter to compute the value of the voltage to be measured or monitored.

Two or more reference voltages may be adopted and the computation preferable utilizes a polynomial based on assessments of the digital signals.

The processing of the signals to compute the voltage value may involve counting digital pulses for a set period while the converter is connected to each of the voltage sources, i.e., the voltage to be measured or monitored, and the reference voltages in turn, storing the corresponding numerical counts, at least temporarily, and performing a computation with the stored numerical counts, together with at least one further stored number representing the true value of one of the reference voltages. Conveniently, the other of the reference voltages is zero. This sequence may be repeated in a cyclic manner.

The invention can be utilized in a variety of different measurement and control applications. Typical applications of many are in the measurement and display of temperature, where the voltage to be measured is provided by a thermocouple or temperature sensor, and as a digital voltmeter.

The principle of the invention is depicted in Figures 1 and 2 of the accompanying drawings, wherein: Figure 1 is a graph showing idealized relationships between voltage, frequency and numerical counts and Figure 2 depicts, in schematic form, the essential parts of a system based on the invention.

Figure 1 depicts a linear relationship between voltage and frequency applicable to an ideal voltagel frequency converter. Two fixed known voltage values Vo (zero) and V ref provide frequencies of fo and fl.

These frequencies fo, fl are associated with numerical values N3, N2 respectively. A voltage to be measured Vin, shown at two levels above and below the reference voltage V ref, by way of illustration, produces a new frequencyf2 and associated numerical value Ni. The numerical values N1, N2 and N3 can be based on a count of the frequencies f2, fl and fo taken over a set time period.Assuming that a further value N4 is available which numerically represents the actual true value of the reference voltage V ref, the present invention can be realized by a computation based on the mathematical relationship: Vin = (N1 - N3) x N4----i) (N2 - N3) The aforesaid method of processing signals can thus utilize this specific relationship in performing the computation.Thus, a method of processing signals to produce a numerical value representing a voltage to bd measured or monitored comprises connecting an analogue voltage!digital signal converter to said voltage and to first and second fixed reference voltage sources in a predetermined sequence to provide numerical values : N1 proportional to the value of the voltage to be measured or monitored, N2 proportional to the first fixed reference voltage, N3 proportional to the second fixed reference voltage, conveniently zero Volts and computing a numerical value representing the voltage to be measured or monitored by the relationship (N1 - N3) x N4 (N2 - N3) where N4 is a number directly indicative or equivalent to the true value of the first reference voltage.

The values N1 to N3 may be dynamic values, while the value N4 is a fixed static value.

Figure 2 depicts in simplified form a measurement system utilizing these principles. The aforementioned voltages Vin, V ref and Vo are provided by appropriate sources. Conversion means in the form of an analogue to digital converter 20 (voltage frequency converter) has its input selectively connected to the voltage sources by way of switching means 21. A processing and control means 22, preferably including one or more microprocessors, controls the switching means 21 and operates the latter to connect the converter 20 to each of the sources in a pre-determined sequence. The appropriate numerical values N1 to N3 above are derived as digital counts from the sources and are stored in memories in the processing and control means 22. The numerical value N4 is also stored in a memory of the processing and control means 22.The computation based on the relationship 1 ) above is performed by the processing and control means 22, which produces an output (OUT) representing the value Vin. The processing and control means 22 may operate the switching means 21 in a cyclic manner to provide a continuous measurement or monitoring function.

According to a preferred embodiment of the invention, an electronic system thus includes conversion means, switching means and processing and control means as aforesaid and the processing and control means accepts numerical values: N1 proportional to the value of the voltage to be measured or monitored, N2 proportional to a first fixed reference voltage and N3 proportional to a second fixed reference voltage.

The processing and control means stores a number N4 directly equivalent to the value of the first reference voltage and computes a value representing the voltage to be measured or monitored by the relationship (NI - N3) x N4 (N2 - N3) The invention may be understood more readily and various other features and aspects of the invention may become apparent, from consideration of the following description.

An embodiment of the invention will now be described, by way of example only, with reference to Figure 3 of the accompanying drawings, which is a block schematic diagram of one system made in accordance with the invention. For convenience, in Figure 3 the chain-dotted lines 20, 21 and 22 are used to identify the equivalent parts of the system shown in Figure 2. As shown in Figure 3, a system serves to convert an analogue voltage source Vin into an equivalent digital output for display for measurement, monitoring or control purposes. Conversion means in the form of a device 2 operates to transform a voltage input B into digital pulses at output C. The train of pulses at output C have a repetition frequency dependent on the magnitude of the voltage at the input B.The voltage input B is derived from various sources, including the source Vin, and these sources are selectively switched and connected to the input B, as described hereinafter. The overall operation of the system is controlled by a microprocessor 5 appropriately programmed. The microprocessor 5 incorporates an oscillator 6 or is fed by an external oscillator 6, preferably a crystal oscillator.

The output C from the device 2 is fed through a gating device 3 which, in turn, feeds a counter 4. The counter 4 is used as a pre-scalenfor the microprocessor 5 and reduces the count rate to a convenient frequency forfeeding the flag input E of the microprocessor 5. The microprocessor 5 thus receives a carry-count signal E from the counter 4 and at the end of a counting period, defined by the gating time "on" period, the microprocessor 5 reads the input F, adds this to the count of signal E and stores the result in a memory. This counting arrangement is suitable for the components adopted in the system and specified hereinafter, but it is possible with other components to modify the counting arrangement.Thus, it is feasible to utilize a larger capacity external counter 4 and transfer the complete count at the end of the counting period to the microprocessor 5 via inputs F, thereby dispensing with the carry signal E. Conversely, with a microprocessor 5 capable of directly reading the originating count, the external counter 4 can be eliminated.

The microprocessor 5 reads, stores andíor processes the digital count signal representing the prevailing input voltage at B. The operation ithQgating device 3 is controlled by a divider/timer 7 which is, in turn, controlled by the microprocessor 5 with control signals G. The oscillator output is divided to a lower value by the microprocessor 5 and also by the external divideritimer 7. It is possible to have the entire divider chain internal to or external of the microprocessor 5. The arrangement is such that the gating device 3 is enabled, i.e. conductive, for a period of about 100m secs.. The gating period is not critical but a 1 OOm sec period is useful in rejecting 50 and 60 HZ mains supply interference.During the gating time on period the pulses from output C are counted by the counter 4. At the end of the on period, the gating device 3 is disabled, i.e., non-conductive, and the microprocessor 5, having previously read and counted input E, reads the input F, adds and stores the result and clears the counter 4 with a reset signal R ready for the next cycle.

The device 2 is driven in this example by an amplifier 1 but the amplifier 1 is however, not essential to the invention. The amplifier 1 hasa resistor feedback network and a bias voltage source 17 connected to a first input K. The ratio of the feedback resistors sets the gain of the amplifier 1 and the gain is set in conjunction with the bias voltage from source 17 to operate the device 2 over its linear operating range. The amplifier 1 has a second input A which is selectively connected by means of switches 1, II, lil to a respective one of three sources; namely the source Vin, a reference voltage V ref provided by a device 8 and a zero voltage Vo. The reference voltage V ref is selected to have a value reasonably suited to the maximum value of the voltage Vin and a convenient practical value for V ref is about 2/3 Of the maximum value of Vin.The device 8 providing the reference voltage V ref may be a simple zener diode or the like but a more versatile device providing a range of reference voltages can be adopted. The switches I, II and Ill are preferably electronic devices actuated by control signals from the microprocessor 5. It may be necessary to provide interface circuits between the switches I, II and Ill and the microprocessor 5. During operation, the following sequence of events take place.

The switch I is closed while the switches II and Ill are open, the gating device 3 is enabled for a first gating period and a digital count signal N1 proportional to the voltage Vin is read and stored in a first memory location in the microprocessor 5. The switch II is then closed while the switches I and Ill are open, the gating device 3 is enabled for a second gating period and a digital count signal N2 proportional to the voltage V ref is read and stored in a second memory location in the microprocessor 5. Finally, the switch Ill is closed while switches I and II are open, the gating device 3 is enabled for a third gating period and a digital count signal N3 proportional to zero voltage is read and stored in a third memory location in the microprocessor 5.The microprocessor 5 has permanent memory store in which a numerical digital count N4 equal to the true value of the reference voltage V ref is stored. The microprocessor 5 utilizes the stored numeral values N 1 to N4 to compute the value of the voltage Vin = N5 by the mathematical relationship N5 = (N1 - N3) x N4 (N2 - N3) A typical example is as follows: Vin = 50000 Fl V Vref = 42000 uV .'. N4 = 42000 N1 =28215 N2 = 24361 N3 = 4128 The measured value is then computed as 28215 - 4128 x 42000 = 50000 24361 -4128 This measured value N5 can be displayed as a digital display in microvolts.By repeating the sequence above, the voltage Vin can be continuously monitored and or displayed. The voltage Vin may, however, be derived from another parameter, e.g., temperature or pressure and in this case voltage Vin is provided by a suitable transducer, e.g. a cold junction temperature sensor. The displayed computed quantity may be in units directly commensurate with the sensed parameter, e.g., degrees celcius or millibars. The voltage Vin may be bi-polar since the computation can cope with negative and positive values. Any transition between negative and positive values in the voltage Vin will not give rise to errors.

In order to increase the frequency of the output from the converter device 2, thereby to permit a higher numerical values to be counted and stored, a phase-lock control loop can be provided between the converter device 2 and the oscillator, external or otherwise, of the processing and control means 22 as represented by the dotted line P in Figure 3. This phase-lock loop provision enables the system to compute the desired value with greater resolution.

Figure 3 also depicts one form of display arrangement with latches 9, 15 feeding decoder drivers 10, 16.

The latter have output lines connected to cathodes and anodes of four LED displays 11-14.

In a practical construction based on the Figure 3 system, the following components can be employed: Switches l, l l & Ill FET type Cm 4016 National Semicon ductors Ltd.

Amplifier 1 Type LM 308 National Semiconductors Ltd., with a feedback resistor ratio pro viding a gain of 150x.

Microprocessor 5 Intersil Type 6100 Oscillator 6 or Intel type 4048 Converter device 2 Burrbrown Type VFC32 Gating device 3 Fairchild Type 4106 Counter 4 Fairchild Type 4040 Divider Timer 7 Fairchild Type 4040 Provided that the period between successive counts is reasonably short, any drift in the amplifier 1 and the device 2 will not affect the overall system. Furthermore, by making the count period the same as one or more periods of a known symmetrical interference signal the effects of this interference can be eliminated.

The switching of the input to the device 2 may have a different sequence to that described and the computation would vary accordingly. The following alternative sequence, where 0 denotes switch open and 1 denotes switch closed, provides more input measurement and display updating; Switch Switch Switch Sequence I II Ill 1 1 0 0 2 0 1 0 3 1 0 0 4 0 0 1 (5 etc 1 0 0 (.... Repeat 2, 3,4 Another sequence which can be adopted is as follows: Switch Switch Switch Sequence I II Ill 1 0 1 0 2 0 0 1 3 1 0 0 4 1 0 0 5 1 0 0 6 1 0 0 7 1 0 0 (8 etc. 0 1 0 (.. Repeat 2-7 This seven step sequence is useful in a system where a rest period for computation is desirable.

Further switching sequences can be adopted for a variety of applications.

Claims

1. A method of processing signals which comprises selectively connecting an analogue voltage/digital signal converter to a voltage to be measured or monitored and to other reference voltage sources in a pre-determined sequence and processing the resultant sequential digital signals from the converter to compute the value of the voltage to be measured or monitored.

2. A method according to claim 1, wherein the processing of the signals comprises counting digital pulses for a set period while the converter is connected to each of the voltage sources and the voltage to be measured or monitored in turn, storing the corresponding numerical counts, at least temporarily, and performing a computation with the stored numerical counts together with at least one further stored number directly representing the true value of one of the reference voltages.

3. A method according to claim 1, wherein one of the reference voltages is zero.

4. A method according to claim 2, wherein another of the reference voltages is zero.

5. A method according to claim 1, wherein the converter is selectively connected to first and second fixed reference voltage sources and to the voltage to be measured or monitored to provide dynamic numerical values: N1 proportional to the value of the voltage to be measured or monitored, N2 proportional to the value of the first reference voltage and N3 proportional to the value of the second reference voltage; and the processing step utilizes the values N1, N2 and N3 together with a fixed number N4 representing the true value of the first reference voltage and computes the value of the voltage to be measured or monitored by the relationship: (N1- N3)x N4 (N2 - N3)

6.A method according to claim 2, wherein the converter is selectively connected to said one and a second fixed reference voltage source and to the voltage to be measured or monitored to provide numerical counts: N1 proportional to the value of the voltage to be measured or monitored, N2 proportional to the value of said one reference voltage, and N3 proportional to the value of said second reference voltage and the computation is based on the relationship (N1- N3)x N4 (N2 - N3) where N4 is said further stored number.

7. A method according to claim 6 or 7, wherein the second reference voltage is zero.

8. A method according to any one of claims 1 to 7, wherein the connecting and processing steps are repeated in a cyclic manner.

9. A method according to any one of the preceding claims and further comprising displaying the computed value of the voltage to be measured or monitored.

10. A method of processing signals substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.

11. An electronic system which includes conversion means for converting an analogue input voltage into a corresponding digital signal, switching means for selectively connecting the conversion means to a voltage to be measured or monitored and other reference voltage sources in a predetermined sequence and processing and control means for controlling the switching means and for processing the resultant sequence of digital signals to compute the numerical value of the voltage to be measured or monitored.

12. A system according to claim 11, wherein the processing means is operable to count digital pulses for a set period while the switching means is controlled to connect the converting means to each of the reference voltage sources and the voltage to be measured or monitored in turn to store the corresponding numerical counts, at least temporarily, and the computation utilizes the numerical counts and at least one further stored number which represents the true value of one of the reference voltages.

13. A system according to claim ii, wherein one of the reference voltages is zero.

14. A system according to claim 12, wherein another of the reference voltages is zero.

15. A system according to claim ii, wherein the switching means is controlled to selectively connect the conversion means to first and second fixed reference voltage sources and to the voltage to be measured or monitored to provide dynamic numerical values: N1 proportional to the value of the voltage to be measured or monitored, N2 proportional to the value of the first reference voltage and N3 proportional to the value of the second reference voltage; and the processing means stores a fixed number N4 representing the true value of the first reference voltage and computes the value of the voltage to be measured or monitored by the relationship: (N1- N3) x N4 (N2 - N3)

16.A system according to claim 12, wherein the switching means is controlled to connect the conversion means to said one and a second fixed reference voltage source and to the voltage to be measured or monitored to provide stored numerical counts: N1 proportional to the value of the voltage to be measured or monitored, N2 proportional to the value of said one reference voltage: and N3 proportional to the value of said second reference voltage and the computation is based on the relationship: (N1- N3)x N4 (N2 - N3) where N4 is said further stored number.

17. A system according to claim 15 or 16, wherein the second reference voltage is zero.

18. A system according to any one of claims 11 to 17, wherein the control means controls the switching and processing means to repeat the switching and computation steps in a cyclic manner.

19. A system according to any one of claims 11 to 18 and further comparing display means for displaying the computed value of the voltage to be measured or monitored.

20. A system according to any one of claims 11 to 19 in combination with a transducer which transforms a parameter into the voltage to be measured or monitored.

21. An electronic system according to any one of claims 11 to 19, wherein the processing and control means is constituted by or includes a microprocessor.

22. An electronic system according to any one of claims 11 to 21 and further comprising gating means controlled by the control means to gate the signals from the conversion means to the processing means.

23. An electronic system according to any one of claims 11 to 22, wherein the conversion means is controlled from the control means by a phase-lock loop.

24. An electronic system substantially as described with reference to, and as illustrated in the accompanying drawings.