CA1311032C - Two-wire telemetering system including power regulated transmitting device - Google Patents
Two-wire telemetering system including power regulated transmitting deviceInfo
- Publication number
- CA1311032C CA1311032C CA000614353A CA614353A CA1311032C CA 1311032 C CA1311032 C CA 1311032C CA 000614353 A CA000614353 A CA 000614353A CA 614353 A CA614353 A CA 614353A CA 1311032 C CA1311032 C CA 1311032C
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- CA
- Canada
- Prior art keywords
- power
- transmitting device
- voltage
- current
- line
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Fee Related
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Classifications
-
- G—PHYSICS
- G08—SIGNALLING
- G08C—TRANSMISSION SYSTEMS FOR MEASURED VALUES, CONTROL OR SIMILAR SIGNALS
- G08C19/00—Electric signal transmission systems
- G08C19/02—Electric signal transmission systems in which the signal transmitted is magnitude of current or voltage
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B3/00—Line transmission systems
- H04B3/54—Systems for transmission via power distribution lines
- H04B3/548—Systems for transmission via power distribution lines the power on the line being DC
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B2203/00—Indexing scheme relating to line transmission systems
- H04B2203/54—Aspects of powerline communications not already covered by H04B3/54 and its subgroups
- H04B2203/5429—Applications for powerline communications
- H04B2203/5433—Remote metering
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B2203/00—Indexing scheme relating to line transmission systems
- H04B2203/54—Aspects of powerline communications not already covered by H04B3/54 and its subgroups
- H04B2203/5429—Applications for powerline communications
- H04B2203/5458—Monitor sensor; Alarm systems
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B2203/00—Indexing scheme relating to line transmission systems
- H04B2203/54—Aspects of powerline communications not already covered by H04B3/54 and its subgroups
- H04B2203/5462—Systems for power line communications
- H04B2203/547—Systems for power line communications via DC power distribution
Abstract
TWO-WIRE TELEMETERING SYSTEM INCLUDING
POWER REGULATED TRANSMITTING DEVICE
Inventor: Stanley Chlebda, a citizen of the United States and resident of Huntingdon Valley, Pennsylvania ABSTRACT OF THE DISCLOSURE
A telemetering system in which a DC powered transmitting device responsive to a process or other variable yields a direct-current signal in accordance with the variable that is supplied over a two-wire line to a receiving station in which a DC voltage supply is connected to the line through a load resistor. Developed across this resistor is an output signal in a predetermined current range for operating an indicator or other instrument. The same line supplies operating power to the power input terminals of the transmitting device through a switching-type step down power regulator that yields constant power under high voltage-low current as well as under low voltage-high current conditions, thereby making more power available to the transmitting device and increasing its load drive capability.
POWER REGULATED TRANSMITTING DEVICE
Inventor: Stanley Chlebda, a citizen of the United States and resident of Huntingdon Valley, Pennsylvania ABSTRACT OF THE DISCLOSURE
A telemetering system in which a DC powered transmitting device responsive to a process or other variable yields a direct-current signal in accordance with the variable that is supplied over a two-wire line to a receiving station in which a DC voltage supply is connected to the line through a load resistor. Developed across this resistor is an output signal in a predetermined current range for operating an indicator or other instrument. The same line supplies operating power to the power input terminals of the transmitting device through a switching-type step down power regulator that yields constant power under high voltage-low current as well as under low voltage-high current conditions, thereby making more power available to the transmitting device and increasing its load drive capability.
Description
~ 31 t ~3~
BACKGROUND OF INVE~TION
Field of Invention:
This invention relates generally to a telemetering system in which a direct-current signal yielded by a transmitting device responsive to a variable of interest is conveyed over a two-wire line to a receiving station having a DC
power supply whose output i.s supplied to the transmitting device over the same line to provide operating power therefGr, and more particularly to a system of this type capable of supplying a relatively large amount of power to the transmitting device.
Status of Prior Art:
A two-wire telemetering system is particularly useful in an industrial process control loop in which a value sensed at a transmitting device by a thermocouple or other sensor of the process variable being metered is converted into a direct-current signal that is conveyed over a two-wire line to a remote receiving station for operating indicators, recorders, controllers or other instruments in the process control loop. Systems of this type are disclosed in the Herzl et al. patent 4,084,155, the Shauger patent 4,158,765, and the Sterling et al. patent 4,692,328.
One important advantage of a two-wire telemetering system is that the same wires serve not only to convey the current signal from the transmitting device to the station but also to conduct operating power from the receiving station to the transmitting device, thereby obviating the need for extra wires in remote control applications. Also, -2- ~
t3t t~32 a current output minimizes susceptibility of the system to voltage noise spikes and eliminates line drop problems.
For a process control telemetering system, American National Standard ANSI-MC 12.1 - 1975 and ISA-S 50.1, "Com-patibility of Analog Signals for Electronic Industrial process Instruments" specify that the standard output signal (of a transmitting device) shall be a current a range of 4 mA to 20 mADC [Section 3.2 of the Standard], and that the standard voltage signal (of the receiver) shall be 1 volt to 5 volt dc [Section 3.3.2 of the Standard]. These standards are generally accepted and practiced by the industrial process control industry.
It has been found that known telemetering systems of this type fail in some instances to supply adequate operating power for transmitting devices. For example, if the transmitting device is a differential-pressure (D-P) transducer operating in conjunction with a square root extractor, the power demands of the D-P transducer and the associated square root extractor are not satisfied at low input levels when the device operates in the usual 4 to 20 mAdc range. And if one wishes to include a micro-processor in a transmitting device, because of the existing constraints in power availability, this may not be possible.
It is known to effect linear regula~ion of the power supply voltage for the transmitting device in a two-wire telemetering system. But such linear regulators act to restrict power consumption of the transmitter control circuitry and to reduce its drive capability. Typically, a two-wire ::
), 2 ~, .
transmitter is specified with a minimum power supply operating voltage with stated conditions, a linear relationship being established between operating voltage and the resistance load drive capability.
I For example, operation at 12.5 volts and zero ohms may be specified as well as operation at 24 volts and 500 ohms. The internal transmitter control electronics is necessarily limited to a current consumption of less than j', 3.8 mA, for the operating range of the system is 4 to 20 ~ mA. Hence the higher the minimum operating voltage, the lower the load resistance must be.
By reason of such power limitations, it may become necessary in many instances to operate the telemetering system in a four-wire configuration rather than with a ¦ two-wire line. Thus additional wires are required to convey ¦ adequate operating power to the transmitting device, thereby sacrificing the important benefits of a two-wire system.
I For any two-wire loop, the transmitter control ¦ electronics power consumption is limited by the external l power supply voltage (the source of all power consumed ¦ in the loop) and the minimum operational current in the ~ loop (4 ma). For example, the power consumption of the I ¦ control electronics of all standard two-wire transmitters ll is limited to 4 ma~ V power supply.
1 If the control electronics inside the transmitter requires a voltage equal to the power supply voltage, then 4 ma is the maximum current which can be consumed. However, ., ~4 -t !~ ~ 2 in a situation where the control electronics requires less than the power supply voltage, then a higher current consumption can be achieved. The invention is addressed to this situation by providing more power to the transmitter control electronics S when the control electronics is implemented with circuitry requiring relatively low voltage with respect to the available voltage, rather than limiting the power consumption to 4 ma~ V low.
SUMMARY OF INVENTION
In view of the foregoing, the main object of this invention is to provide a two-wire telemetering system wherein the same line conducts a direct-current signal from a field station transmitting device to a receiving ~ station which supplies operating power at a substantially constant level to the transmitting device, the amount of operating power supplied being sufficient even at low signal levels to energize relatively complex transmitting devices such as magnetic flowmeters, field-mounted multiplexers and microprocessor-based transmitting transmitting devices.
1~1 1032 More particularly, an object o~ the present invention is to provide a two-wire telemetering system o~ the above type in which the power supplied to the transmitting device is regulated by a switching-type power regulator that delivers to the power input terminals of the device substantially constant power under high voltage-low current as well as under low voltage-high current conditions.
Also an object of the invention is to provide a two-wire telemetering system of the above type which is of relatively simple, low cost construction, yet operates efficiently and reliably.
Briefly stated, these objects are attained in a telemetering system in which a DC powered transmitting device responsive to a process or other variable yields a direct-current signal in accordance with the variable that is supplied over a two-wire line to a receiving station in which a DC voltage supply is connected to the line through a load resistor. Developed across this resistor is an output signal in a predetermined current range for operating an indicator or other instrument. The same line supplies operating power to the power input terminals of the transmitting devi-ce through a switching-type step down power regulator that yields constant power under high voltage-low current as well as under low voltage-high current conditions, thereby making more power available to the transmitting device and increasing its load drive capability.
'~' -5a-::`
~31 1~J32 More particularly, the present invention provides a two-wire tel~meterirlg system comprising: (a) a DC powered -t.ransmitting device operating at a predetermined voltage and having power input terminals connected to an internal electronic circuit, said device being responsive to a process variable metered by a sensor to yield at its output a direct-current signal in accordance with said variable in a predetermined current range; (b) a two-wire line, one end of which is connected to the output of said transmittiny device to convey said signal; (c) a receiving station remote from the transmitting device provided with a DC power supply having a voltage at least twice as high as said predetermined voltage and a load resistor in series therewith connected to the other end of the line to receive said signal which is applied to a receiver and to at the same time supply power from said DC
supply to said transmitting devi.ce; and (d) a switching-type step-down power regulator interposed between the power input terminals of the transmitting device and said one end of the line, the output voltage of the power regulator always being lower than the voltage applied thereto from the power supply through the line, the power regulator yielding constant power under high voltage-low current as well as under low voltage-high cuxrent conditions, thereby making more power available to the transmitking device and increasing its load drive capability.
5b ~31 ~32 BRIEF DESCRIPTION OF DRAWI~GS
For a better understanding of the invention as well as other objects and further features thereof, reference is made to the following detailed description to be read in conjunction with the accompanying drawings, wherein:
Fig. 1 is a schematic circuit diagram of a two-wire telemetering system in accordance with the invention;
Fig. 2 is a schematic circuit diagram of the basic elements of a step-down switching-type regulator included ~ 10 in the system; and ; Fig. 3 is a simplified circuit representing the general case in regard to the amount of power which is available when linear and when switching-type regulation is used in the system.
DETAILED DESCRIPTION OF INVENTION
The Telemeterinq SYstem:
Referring now to Fig. 1, there is shown in this figure the basic components of a two-wire telemetering system in accordance with the invention which includes a DC-powered transmitting device responsive to a process or other variable whose value is metered by a sensor 10.
The-transmitting device is linked by a two-wire line Wl and W2 to a remote receiving station in which a DC power supply represented by battery 11 i- connected in series with a load resistor 12 through which flows an output signal in the standard current range (i.e., 4 to 20 mAdc). The voltage developed across load resistor 12 is applied to a receiver 13 which may be a process variable indicator, a recorder, a controller and any other device appropriate to process control. The two wire line Wl and W2 are connected to the transmitter device at terminals T4 and T5.
.
'' :
' ~ 3 ~ ~ 032 I Sensor 10 may be a thermocouple, a differential-pressure transducer or any other field-mounted device for metering the process variable. This variable may be fluid flow rate, temperature or pressure to produce an analog signal proportional thereto. In the example shown, this analog signal is applied to the microprocessor-controlled circuit 14.
The output yielded by microprocessor-controlled circuit 14 which appears at terminal Tl is applied to one 10 , input of a differential amplifier lS to whose other input is applied a feedback voltage developed across a feedback resistor RFB. ~he output of amplifier 15 modulates an i output transistor 16 connected in series with resistor RFB between lines Wl and W2.
I Microprocessor-controlled circuit 14 is provided with power input terminals T2 and T3, terminal T3 being connected to line W2 through resistor RF~ at terminal T5.
Terminal T2 is connected to line Wl through a switching-type ,I power regulator 17 at terminal T4. Regulator 17 serves ;~ 20 ¦~ to supply relatively high power to the microprocessor-controlled ,I circuit 14 at a constant level, which remains substantially unchanged under high voltage-low current as well as low ¦ voltage-high current conditions.
ll The voltage between terminals 2 and 3 is V2_3, ~' while that between terminals 4 and 5 is V4_5. The current going into regulator 17 is designated Ips, and the current from regulator 17 into the microprocessor-controlled circuit 14 is designated IdC. Voltage V2_3 is determined by the . , ~31 1032 power supply sp~clfications of the componen~s used in microprocessor-controlled circuit 14. Typically, voltage V2 3 is 5Vdc.
The aim of the invention is to obtain an IdC current which is greater than I current at the regulated 5Vdc voltage in order to satisfy the current requirements of the microprocessor-controlled circuit 14. This goal is achieved by switching regulator 17 when V2 3 is lower than V4 5 as will later be explained.
Switching-type power regulators suitable for this purpose are disclosed in the following publications:
A. 1987 "Switchmode (A Designers Guide for Switching Power Supply Circuits and Components)" published by Motorola Corporation.
8. "Applications Handbook (1987-1988)" published by Unirode Corporation.
C. "Linear and Interface Circuit Applica~ions 1985" (Section C - Switching Power Design) published by Texas Instruments.
7a t~ ~
Regulator 17 includes a transistor switch 18 which switches on and off at a predetermined frequency. During the interval switch 18 is on, the input voltage is applied to the input of an LC filter formed by an inductor 19 and a capacitor 20, thereby causing the current to increase.
When the swi~ch is off, the energy stored in inductor lg maintains current flow to the load, circulating through a "catch" diode 21.
The regulator is monitored and controlled by a control circuit generally represented by block 22. This control circuit includes an oscillator driving a pulse width modulator, an error amplifier and a precision voltage reference. The error amplifier compares the input reference voltage with a sample of the voltage from the filter circuit.
As the load increases, the output voltage drops. The error amplifier senses this drop and causes the pulse-width modulator to remain on for a longer period of time, delivering wider control pulses to transistor switch 18.
~he width of the pulse determines how long the transistor switch permits current to flow and therefore how much current is yielded at the output. If the load decreases, narrower control pulses are delivered to the switching transistor until the output voltage remains at ; a constant value.
The SteP-Down Power Requlator:
Switch-type power regulators are available in three basic configurations:
(1) A step-down or "buck" regulator (2) A step-up or "boost" regulator (3) an inverting regulator .
1 3 1 ~ ~32 The present invention makes use of a step-down power regulator whose operation will now be explained in connection with Fig. 2. It will be seen that transistor switch 18 is placed in series with inductor 19 between the DC input Vl and the DC output V0, diode 21 being connected to the input side of the inductor and capacitor 20 to the output side thereof.
Transistor switch 18 in the buck circuit interrupts the DC input voltage to supply a variable width pulse to the simple averaging filter formed by inductor 21 and capacitor 2n. When switch 18 is closed, the input voltage is applied across this filter and current flows through inductor 21 to the load. When switch 18 is open, the energy stored in the field of the inductor maintains the current through the load.
In this buck circuit, peak switching current is proportional to the load current. The output voltage V0 is equal to the input voltage Vl times the duty cycle.
Hence the output voltage is always less than the input voltage in the step-down switching regulator.
Comparison of Regulators:
- A linear series or shunt regulator, because it functions in a continuous mode, will dissipate relatively large amounts of power. Typically, the efficiency of a linear regulator is less than 50%. And when the input-to-output voltage differential is large, the resultant efficiency then falls to well below 40~. In contradistinction, a switching-type power regulator, which uses the on-off cycle of a transistor switch to regulate power, has typical efficiencies running well over 60%.
_g_ ~' 1~
13~ ~032 There are three reasons why a switching-type regulator achieves much higher efficiencies than a linear regulator.
First, since the power transistor switch is turned either off or on, this results in either low current or low voltage during most of its operation. S~cond, good regulation is attainable over a wide range of input voltages, and third, high efficiency can be achieved over wide ranges in load current.
To further explain these distinctions, reference is now made to Fig. 3 in which element 23 represents the transmitter which is supplied with a voltage VT by battery 11 (24 volts) through a two-wire line Wl and W2 in series with a load resistor R.
The power dissipated (PD) by transmitter 23 is expressed by the following equations:
PD = 24 I - RI~ = V~I (Power Dissipated by Transmitter) @ I = 4ma, PD = 96mW - R (16 uW/ohm) = 0 _ R L 6K ohm ~ I = 20ma PD = 480 mW - R (400 uW/ohm) = oL R~ 1.2K ohm = VT = 24 -20 (10 3 ) R
- - For linear regulation the maximum power (PE) available for the electronics in the transmitter is:
PE = 4ma- VT ~ 20ma = 4ma [24 - 20 (10-3)-R ]
= 96 mW - (80 uW/ohm)~R
131 103~
For a switching-type regulator, the maximum power (PE) available for the electronics in the transmitter is:
PE ~ PDMIN = ~ L96 MW -R (16 uW/ohm~;
where C~ is the efficiency of the switching regulator PE = .75 ~96 mW - R (16 uW/ohm)~ @~ = 75%
In one practical embodiment, the required operating voltage for the electronic circuits of the transmitter included in the two-wire telemetering system is the +5Vdc.
As shown in Fig. 1, the current drawn from power supply 11 is represented as Ips, the current going through microprocessor 14 supplied with power by regulator 17 is represented as IDC, the current passing through signal output transistor 16 is represented as ICTL~ and the 4 to 20 mA current passing through resistor RFB is represented as IouT.
We shall now assume that instead of a switch-type regulator, use is made in Fig. 1 of a linear regulator.
With linear regulation applied to this system, the major c~nsideration is then that IDC, the current dissipated ~y the +5Vdc microprocessor 14 and the electronics associated therewith is always equal to or slightly less than Ips, the power supply current. In addition, since IpS + ICTL = IoUT
- (the transmitter output current which is in the 4 to 20 mA range), the intensity of IDC~ the curxent going through the electronic circuits in the transmitter cannot be permitted to exceed 4mA.
'',;
., . .
Typical design values for the linear regulation scheme are as follows:
IouT (minimum) = 3.8 ma ~ Input Conditions V+/- (minimum) = 12.5 volts~
IDC (maximum~ - 3.8 ma @ +5vdc Power available for control electronics = 19 MW
Maximum loop load resistance @ 24vdc and 20.8 ma (24 - 12.5) = 553 ohms 20.8 ma When however the system is in accordance with the invention and includes a switching-type regulator 17 as shown in Fig. 1 to supply constant power to the electronic circuitry of the transmitter the major consideration ls that minimal power be dissipated by the regulator so that the power supplied to the electronic circuitry is only slightly less than the power supplied by the power supply.
As a consequence the current IDC consumed by the transmitter electronics can be greater than the power supply current Ips.
Typical design values for the switching regulator scheme are as follows:
IoUT (minimum) = 3.8 ma Regulator Efficiency (minimum) = 75~ - Input Conditions Loop Resistance (maximum) = 550 ohms V +/- (minimum~ = 6.5 volts Power available for electronic circui~ry of transmitter:
a) IUT = 3.8 ma DC = 75 24-[3.8ma-(550~ + RFB)] 3.8 ma = 12.27 ma 5vDC
t31 ~0~2 RFB = 100 ohms,,V ~ input of power supply = 21.53 volts a) ~ IOUT = 20-8 ma ~C 75 ~24-[20.8ma (550n+ RFB)]~20.8ma = 32.70 ma RFB = 100 ohms, V input of power supply =
10.48 volts Thus a comparison of linear regulation with switching-type power regulation in accordance with the invention leads ; to the following conclusions:
I. The switching-type regulator in the two-wire telemetering system affords significantly more power for the electronic circuits of the transmitter with 550 ohms of load impedance powered by a 24Vdc power supply and operating over a current range of 3.8 to 20.8 mA.
The linear regulator produces 19 MW of power, whereas the switching-type regulator yields 61.35 MW, resulting in a 222% increase in available power.
, II. The switching-type regulator is capable of driving a significantly higher load resistance when the two-wire telemetering system is powered by a 24 volt power supply. A linear regulator with a minimum V+/- of 12.5V
, equals a 552 ohm load. A switching-type regulator with a minimum ' V+l/- of ~.5 V equals an 841 ohm load. This represents , a 50~ increase in drive capability.
' While there has been shown and described a preferred -'' 25 embodiment of a two-wire telemetering system including power regulated transmitting device in accordance with the invention, it will be appreciated that many changes , and modifications may be made therein without, however, departing from the essential spirit thereof.
,:
~' .,
BACKGROUND OF INVE~TION
Field of Invention:
This invention relates generally to a telemetering system in which a direct-current signal yielded by a transmitting device responsive to a variable of interest is conveyed over a two-wire line to a receiving station having a DC
power supply whose output i.s supplied to the transmitting device over the same line to provide operating power therefGr, and more particularly to a system of this type capable of supplying a relatively large amount of power to the transmitting device.
Status of Prior Art:
A two-wire telemetering system is particularly useful in an industrial process control loop in which a value sensed at a transmitting device by a thermocouple or other sensor of the process variable being metered is converted into a direct-current signal that is conveyed over a two-wire line to a remote receiving station for operating indicators, recorders, controllers or other instruments in the process control loop. Systems of this type are disclosed in the Herzl et al. patent 4,084,155, the Shauger patent 4,158,765, and the Sterling et al. patent 4,692,328.
One important advantage of a two-wire telemetering system is that the same wires serve not only to convey the current signal from the transmitting device to the station but also to conduct operating power from the receiving station to the transmitting device, thereby obviating the need for extra wires in remote control applications. Also, -2- ~
t3t t~32 a current output minimizes susceptibility of the system to voltage noise spikes and eliminates line drop problems.
For a process control telemetering system, American National Standard ANSI-MC 12.1 - 1975 and ISA-S 50.1, "Com-patibility of Analog Signals for Electronic Industrial process Instruments" specify that the standard output signal (of a transmitting device) shall be a current a range of 4 mA to 20 mADC [Section 3.2 of the Standard], and that the standard voltage signal (of the receiver) shall be 1 volt to 5 volt dc [Section 3.3.2 of the Standard]. These standards are generally accepted and practiced by the industrial process control industry.
It has been found that known telemetering systems of this type fail in some instances to supply adequate operating power for transmitting devices. For example, if the transmitting device is a differential-pressure (D-P) transducer operating in conjunction with a square root extractor, the power demands of the D-P transducer and the associated square root extractor are not satisfied at low input levels when the device operates in the usual 4 to 20 mAdc range. And if one wishes to include a micro-processor in a transmitting device, because of the existing constraints in power availability, this may not be possible.
It is known to effect linear regula~ion of the power supply voltage for the transmitting device in a two-wire telemetering system. But such linear regulators act to restrict power consumption of the transmitter control circuitry and to reduce its drive capability. Typically, a two-wire ::
), 2 ~, .
transmitter is specified with a minimum power supply operating voltage with stated conditions, a linear relationship being established between operating voltage and the resistance load drive capability.
I For example, operation at 12.5 volts and zero ohms may be specified as well as operation at 24 volts and 500 ohms. The internal transmitter control electronics is necessarily limited to a current consumption of less than j', 3.8 mA, for the operating range of the system is 4 to 20 ~ mA. Hence the higher the minimum operating voltage, the lower the load resistance must be.
By reason of such power limitations, it may become necessary in many instances to operate the telemetering system in a four-wire configuration rather than with a ¦ two-wire line. Thus additional wires are required to convey ¦ adequate operating power to the transmitting device, thereby sacrificing the important benefits of a two-wire system.
I For any two-wire loop, the transmitter control ¦ electronics power consumption is limited by the external l power supply voltage (the source of all power consumed ¦ in the loop) and the minimum operational current in the ~ loop (4 ma). For example, the power consumption of the I ¦ control electronics of all standard two-wire transmitters ll is limited to 4 ma~ V power supply.
1 If the control electronics inside the transmitter requires a voltage equal to the power supply voltage, then 4 ma is the maximum current which can be consumed. However, ., ~4 -t !~ ~ 2 in a situation where the control electronics requires less than the power supply voltage, then a higher current consumption can be achieved. The invention is addressed to this situation by providing more power to the transmitter control electronics S when the control electronics is implemented with circuitry requiring relatively low voltage with respect to the available voltage, rather than limiting the power consumption to 4 ma~ V low.
SUMMARY OF INVENTION
In view of the foregoing, the main object of this invention is to provide a two-wire telemetering system wherein the same line conducts a direct-current signal from a field station transmitting device to a receiving ~ station which supplies operating power at a substantially constant level to the transmitting device, the amount of operating power supplied being sufficient even at low signal levels to energize relatively complex transmitting devices such as magnetic flowmeters, field-mounted multiplexers and microprocessor-based transmitting transmitting devices.
1~1 1032 More particularly, an object o~ the present invention is to provide a two-wire telemetering system o~ the above type in which the power supplied to the transmitting device is regulated by a switching-type power regulator that delivers to the power input terminals of the device substantially constant power under high voltage-low current as well as under low voltage-high current conditions.
Also an object of the invention is to provide a two-wire telemetering system of the above type which is of relatively simple, low cost construction, yet operates efficiently and reliably.
Briefly stated, these objects are attained in a telemetering system in which a DC powered transmitting device responsive to a process or other variable yields a direct-current signal in accordance with the variable that is supplied over a two-wire line to a receiving station in which a DC voltage supply is connected to the line through a load resistor. Developed across this resistor is an output signal in a predetermined current range for operating an indicator or other instrument. The same line supplies operating power to the power input terminals of the transmitting devi-ce through a switching-type step down power regulator that yields constant power under high voltage-low current as well as under low voltage-high current conditions, thereby making more power available to the transmitting device and increasing its load drive capability.
'~' -5a-::`
~31 1~J32 More particularly, the present invention provides a two-wire tel~meterirlg system comprising: (a) a DC powered -t.ransmitting device operating at a predetermined voltage and having power input terminals connected to an internal electronic circuit, said device being responsive to a process variable metered by a sensor to yield at its output a direct-current signal in accordance with said variable in a predetermined current range; (b) a two-wire line, one end of which is connected to the output of said transmittiny device to convey said signal; (c) a receiving station remote from the transmitting device provided with a DC power supply having a voltage at least twice as high as said predetermined voltage and a load resistor in series therewith connected to the other end of the line to receive said signal which is applied to a receiver and to at the same time supply power from said DC
supply to said transmitting devi.ce; and (d) a switching-type step-down power regulator interposed between the power input terminals of the transmitting device and said one end of the line, the output voltage of the power regulator always being lower than the voltage applied thereto from the power supply through the line, the power regulator yielding constant power under high voltage-low current as well as under low voltage-high cuxrent conditions, thereby making more power available to the transmitking device and increasing its load drive capability.
5b ~31 ~32 BRIEF DESCRIPTION OF DRAWI~GS
For a better understanding of the invention as well as other objects and further features thereof, reference is made to the following detailed description to be read in conjunction with the accompanying drawings, wherein:
Fig. 1 is a schematic circuit diagram of a two-wire telemetering system in accordance with the invention;
Fig. 2 is a schematic circuit diagram of the basic elements of a step-down switching-type regulator included ~ 10 in the system; and ; Fig. 3 is a simplified circuit representing the general case in regard to the amount of power which is available when linear and when switching-type regulation is used in the system.
DETAILED DESCRIPTION OF INVENTION
The Telemeterinq SYstem:
Referring now to Fig. 1, there is shown in this figure the basic components of a two-wire telemetering system in accordance with the invention which includes a DC-powered transmitting device responsive to a process or other variable whose value is metered by a sensor 10.
The-transmitting device is linked by a two-wire line Wl and W2 to a remote receiving station in which a DC power supply represented by battery 11 i- connected in series with a load resistor 12 through which flows an output signal in the standard current range (i.e., 4 to 20 mAdc). The voltage developed across load resistor 12 is applied to a receiver 13 which may be a process variable indicator, a recorder, a controller and any other device appropriate to process control. The two wire line Wl and W2 are connected to the transmitter device at terminals T4 and T5.
.
'' :
' ~ 3 ~ ~ 032 I Sensor 10 may be a thermocouple, a differential-pressure transducer or any other field-mounted device for metering the process variable. This variable may be fluid flow rate, temperature or pressure to produce an analog signal proportional thereto. In the example shown, this analog signal is applied to the microprocessor-controlled circuit 14.
The output yielded by microprocessor-controlled circuit 14 which appears at terminal Tl is applied to one 10 , input of a differential amplifier lS to whose other input is applied a feedback voltage developed across a feedback resistor RFB. ~he output of amplifier 15 modulates an i output transistor 16 connected in series with resistor RFB between lines Wl and W2.
I Microprocessor-controlled circuit 14 is provided with power input terminals T2 and T3, terminal T3 being connected to line W2 through resistor RF~ at terminal T5.
Terminal T2 is connected to line Wl through a switching-type ,I power regulator 17 at terminal T4. Regulator 17 serves ;~ 20 ¦~ to supply relatively high power to the microprocessor-controlled ,I circuit 14 at a constant level, which remains substantially unchanged under high voltage-low current as well as low ¦ voltage-high current conditions.
ll The voltage between terminals 2 and 3 is V2_3, ~' while that between terminals 4 and 5 is V4_5. The current going into regulator 17 is designated Ips, and the current from regulator 17 into the microprocessor-controlled circuit 14 is designated IdC. Voltage V2_3 is determined by the . , ~31 1032 power supply sp~clfications of the componen~s used in microprocessor-controlled circuit 14. Typically, voltage V2 3 is 5Vdc.
The aim of the invention is to obtain an IdC current which is greater than I current at the regulated 5Vdc voltage in order to satisfy the current requirements of the microprocessor-controlled circuit 14. This goal is achieved by switching regulator 17 when V2 3 is lower than V4 5 as will later be explained.
Switching-type power regulators suitable for this purpose are disclosed in the following publications:
A. 1987 "Switchmode (A Designers Guide for Switching Power Supply Circuits and Components)" published by Motorola Corporation.
8. "Applications Handbook (1987-1988)" published by Unirode Corporation.
C. "Linear and Interface Circuit Applica~ions 1985" (Section C - Switching Power Design) published by Texas Instruments.
7a t~ ~
Regulator 17 includes a transistor switch 18 which switches on and off at a predetermined frequency. During the interval switch 18 is on, the input voltage is applied to the input of an LC filter formed by an inductor 19 and a capacitor 20, thereby causing the current to increase.
When the swi~ch is off, the energy stored in inductor lg maintains current flow to the load, circulating through a "catch" diode 21.
The regulator is monitored and controlled by a control circuit generally represented by block 22. This control circuit includes an oscillator driving a pulse width modulator, an error amplifier and a precision voltage reference. The error amplifier compares the input reference voltage with a sample of the voltage from the filter circuit.
As the load increases, the output voltage drops. The error amplifier senses this drop and causes the pulse-width modulator to remain on for a longer period of time, delivering wider control pulses to transistor switch 18.
~he width of the pulse determines how long the transistor switch permits current to flow and therefore how much current is yielded at the output. If the load decreases, narrower control pulses are delivered to the switching transistor until the output voltage remains at ; a constant value.
The SteP-Down Power Requlator:
Switch-type power regulators are available in three basic configurations:
(1) A step-down or "buck" regulator (2) A step-up or "boost" regulator (3) an inverting regulator .
1 3 1 ~ ~32 The present invention makes use of a step-down power regulator whose operation will now be explained in connection with Fig. 2. It will be seen that transistor switch 18 is placed in series with inductor 19 between the DC input Vl and the DC output V0, diode 21 being connected to the input side of the inductor and capacitor 20 to the output side thereof.
Transistor switch 18 in the buck circuit interrupts the DC input voltage to supply a variable width pulse to the simple averaging filter formed by inductor 21 and capacitor 2n. When switch 18 is closed, the input voltage is applied across this filter and current flows through inductor 21 to the load. When switch 18 is open, the energy stored in the field of the inductor maintains the current through the load.
In this buck circuit, peak switching current is proportional to the load current. The output voltage V0 is equal to the input voltage Vl times the duty cycle.
Hence the output voltage is always less than the input voltage in the step-down switching regulator.
Comparison of Regulators:
- A linear series or shunt regulator, because it functions in a continuous mode, will dissipate relatively large amounts of power. Typically, the efficiency of a linear regulator is less than 50%. And when the input-to-output voltage differential is large, the resultant efficiency then falls to well below 40~. In contradistinction, a switching-type power regulator, which uses the on-off cycle of a transistor switch to regulate power, has typical efficiencies running well over 60%.
_g_ ~' 1~
13~ ~032 There are three reasons why a switching-type regulator achieves much higher efficiencies than a linear regulator.
First, since the power transistor switch is turned either off or on, this results in either low current or low voltage during most of its operation. S~cond, good regulation is attainable over a wide range of input voltages, and third, high efficiency can be achieved over wide ranges in load current.
To further explain these distinctions, reference is now made to Fig. 3 in which element 23 represents the transmitter which is supplied with a voltage VT by battery 11 (24 volts) through a two-wire line Wl and W2 in series with a load resistor R.
The power dissipated (PD) by transmitter 23 is expressed by the following equations:
PD = 24 I - RI~ = V~I (Power Dissipated by Transmitter) @ I = 4ma, PD = 96mW - R (16 uW/ohm) = 0 _ R L 6K ohm ~ I = 20ma PD = 480 mW - R (400 uW/ohm) = oL R~ 1.2K ohm = VT = 24 -20 (10 3 ) R
- - For linear regulation the maximum power (PE) available for the electronics in the transmitter is:
PE = 4ma- VT ~ 20ma = 4ma [24 - 20 (10-3)-R ]
= 96 mW - (80 uW/ohm)~R
131 103~
For a switching-type regulator, the maximum power (PE) available for the electronics in the transmitter is:
PE ~ PDMIN = ~ L96 MW -R (16 uW/ohm~;
where C~ is the efficiency of the switching regulator PE = .75 ~96 mW - R (16 uW/ohm)~ @~ = 75%
In one practical embodiment, the required operating voltage for the electronic circuits of the transmitter included in the two-wire telemetering system is the +5Vdc.
As shown in Fig. 1, the current drawn from power supply 11 is represented as Ips, the current going through microprocessor 14 supplied with power by regulator 17 is represented as IDC, the current passing through signal output transistor 16 is represented as ICTL~ and the 4 to 20 mA current passing through resistor RFB is represented as IouT.
We shall now assume that instead of a switch-type regulator, use is made in Fig. 1 of a linear regulator.
With linear regulation applied to this system, the major c~nsideration is then that IDC, the current dissipated ~y the +5Vdc microprocessor 14 and the electronics associated therewith is always equal to or slightly less than Ips, the power supply current. In addition, since IpS + ICTL = IoUT
- (the transmitter output current which is in the 4 to 20 mA range), the intensity of IDC~ the curxent going through the electronic circuits in the transmitter cannot be permitted to exceed 4mA.
'',;
., . .
Typical design values for the linear regulation scheme are as follows:
IouT (minimum) = 3.8 ma ~ Input Conditions V+/- (minimum) = 12.5 volts~
IDC (maximum~ - 3.8 ma @ +5vdc Power available for control electronics = 19 MW
Maximum loop load resistance @ 24vdc and 20.8 ma (24 - 12.5) = 553 ohms 20.8 ma When however the system is in accordance with the invention and includes a switching-type regulator 17 as shown in Fig. 1 to supply constant power to the electronic circuitry of the transmitter the major consideration ls that minimal power be dissipated by the regulator so that the power supplied to the electronic circuitry is only slightly less than the power supplied by the power supply.
As a consequence the current IDC consumed by the transmitter electronics can be greater than the power supply current Ips.
Typical design values for the switching regulator scheme are as follows:
IoUT (minimum) = 3.8 ma Regulator Efficiency (minimum) = 75~ - Input Conditions Loop Resistance (maximum) = 550 ohms V +/- (minimum~ = 6.5 volts Power available for electronic circui~ry of transmitter:
a) IUT = 3.8 ma DC = 75 24-[3.8ma-(550~ + RFB)] 3.8 ma = 12.27 ma 5vDC
t31 ~0~2 RFB = 100 ohms,,V ~ input of power supply = 21.53 volts a) ~ IOUT = 20-8 ma ~C 75 ~24-[20.8ma (550n+ RFB)]~20.8ma = 32.70 ma RFB = 100 ohms, V input of power supply =
10.48 volts Thus a comparison of linear regulation with switching-type power regulation in accordance with the invention leads ; to the following conclusions:
I. The switching-type regulator in the two-wire telemetering system affords significantly more power for the electronic circuits of the transmitter with 550 ohms of load impedance powered by a 24Vdc power supply and operating over a current range of 3.8 to 20.8 mA.
The linear regulator produces 19 MW of power, whereas the switching-type regulator yields 61.35 MW, resulting in a 222% increase in available power.
, II. The switching-type regulator is capable of driving a significantly higher load resistance when the two-wire telemetering system is powered by a 24 volt power supply. A linear regulator with a minimum V+/- of 12.5V
, equals a 552 ohm load. A switching-type regulator with a minimum ' V+l/- of ~.5 V equals an 841 ohm load. This represents , a 50~ increase in drive capability.
' While there has been shown and described a preferred -'' 25 embodiment of a two-wire telemetering system including power regulated transmitting device in accordance with the invention, it will be appreciated that many changes , and modifications may be made therein without, however, departing from the essential spirit thereof.
,:
~' .,
Claims (4)
1. A two-wire telemetering system comprising: (a) a DC
powered transmitting device operating at a predetermined voltage and having power input terminals connected to an internal electronic circuit, said device being responsive to a process variable metered by a sensor to yield at its output a direct-current signal in accordance with said variable in a predetermined current range; (b) a two-wire line, one end of which is connected to the output of said transmitting device to convey said signal; (c) a receiving station remote from the transmitting device provided with a DC power supply having a voltage at least twice as high as said predetermined voltage and a load resistor in series therewith connected to the other end of the line to receive said signal which is applied to a receiver and to at the same time supply power from said DC
supply to said transmitting device; and (d) a switching-type step-down power regulator interposed between the power input terminals of the transmitting device and said one end of the line, the output voltage of the power regulator always being lower than the voltage applied thereto from the power supply through the line, the power regulator yielding constant power under high voltage-low current as well as under low voltage-high current conditions, thereby making more power available to the transmitting device and increasing its load drive capability.
powered transmitting device operating at a predetermined voltage and having power input terminals connected to an internal electronic circuit, said device being responsive to a process variable metered by a sensor to yield at its output a direct-current signal in accordance with said variable in a predetermined current range; (b) a two-wire line, one end of which is connected to the output of said transmitting device to convey said signal; (c) a receiving station remote from the transmitting device provided with a DC power supply having a voltage at least twice as high as said predetermined voltage and a load resistor in series therewith connected to the other end of the line to receive said signal which is applied to a receiver and to at the same time supply power from said DC
supply to said transmitting device; and (d) a switching-type step-down power regulator interposed between the power input terminals of the transmitting device and said one end of the line, the output voltage of the power regulator always being lower than the voltage applied thereto from the power supply through the line, the power regulator yielding constant power under high voltage-low current as well as under low voltage-high current conditions, thereby making more power available to the transmitting device and increasing its load drive capability.
2. A telemetering system as set forth in claim 1 in which the sensor produces an analog signal that is applied to a microprocessor-controlled circuit in the transmitting device whose output yields said power supply current.
3. A telemetering system as set forth in claim 2, wherein said power supply current is applied to an output transistor coupled to said one end of the line.
4. A telemetering system as set forth in claim 3, wherein said sensor is a differential-pressure transducer.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US33122989A | 1989-03-31 | 1989-03-31 | |
| US331229 | 1989-03-31 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1311032C true CA1311032C (en) | 1992-12-01 |
Family
ID=23293114
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000614353A Expired - Fee Related CA1311032C (en) | 1989-03-31 | 1989-09-28 | Two-wire telemetering system including power regulated transmitting device |
Country Status (4)
| Country | Link |
|---|---|
| CA (1) | CA1311032C (en) |
| DE (1) | DE3934007C2 (en) |
| FR (1) | FR2645308A1 (en) |
| GB (1) | GB2229897A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US6539794B1 (en) | 1994-02-18 | 2003-04-01 | Johanngeorg Otto | Arrangement for measuring the level of contents in a container |
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| DE487874T1 (en) * | 1990-11-30 | 1992-10-15 | Yokogawa Electric Corp., Musashino, Tokio/Tokyo, Jp | SIGNAL DESIGNER. |
| DE4121961A1 (en) * | 1991-06-28 | 1993-01-07 | Siemens Ag | CIRCUIT FOR THE ENERGY SUPPLY OF FIELD DEVICES |
| IT1254843B (en) * | 1992-03-25 | 1995-10-11 | Nuovo Pignone Spa | ROOM THERMOSTAT PERFECTED, PARTICULARLY SUITABLE FOR WALL BOILERS |
| US5416723A (en) * | 1993-03-03 | 1995-05-16 | Milltronics Ltd. | Loop powered process control transmitter |
| DE4307082A1 (en) * | 1993-03-06 | 1994-09-08 | Guetermann & Co | Method and apparatus for the electronic recording of quantities on vending machines |
| US5535243A (en) * | 1994-07-13 | 1996-07-09 | Rosemount Inc. | Power supply for field mounted transmitter |
| DE19723645B4 (en) * | 1997-06-05 | 2006-04-13 | Endress + Hauser Gmbh + Co. Kg | Arrangement for signal transmission between a donor site and a receiving site |
| US5959372A (en) * | 1997-07-21 | 1999-09-28 | Emerson Electric Co. | Power management circuit |
| DE19925943A1 (en) * | 1999-06-08 | 2000-12-21 | Krohne Messtechnik Kg | Circuit arrangement for data acquisition, transmission and evaluation |
| DE59913163D1 (en) * | 1999-10-07 | 2006-04-27 | Endress & Hauser Gmbh & Co Kg | High-performance double two-wire measuring system and measuring device |
| US6684340B1 (en) | 1999-10-07 | 2004-01-27 | Endress + Hauser Gmbh + Co. | Measuring instrument having two pairs of lines connected to two indentical pairs of terminals, via which signal current flows through one pair and supply current flows through the other pair |
| DE10034685B4 (en) * | 2000-07-17 | 2010-07-08 | Vega Grieshaber Kg | Energy saving |
| DE10258965B4 (en) * | 2002-12-16 | 2007-06-21 | Endress + Hauser Gmbh + Co. Kg | Device for optimizing the power balance of a sensor for the determination and / or monitoring of a physical process variable of a medium |
| EP1471641A1 (en) * | 2003-04-25 | 2004-10-27 | Siemens Aktiengesellschaft | Input control circuit for an electric device |
| US7187158B2 (en) | 2004-04-15 | 2007-03-06 | Rosemount, Inc. | Process device with switching power supply |
| DE102004021750A1 (en) * | 2004-04-30 | 2005-12-01 | Endress + Hauser Conducta Gesellschaft für Mess- und Regeltechnik mbH + Co. KG | Method for signaling alarm states on a field device of automation technology |
| DE102005009459A1 (en) * | 2005-01-26 | 2006-07-27 | Ranft, Georg, Dr. | Switching arrangement for output stage of analog transmitter, has series connection arranged between two connecting terminals, and point connected to input of power source and input of switching amplifier at one terminal |
| DE102007006503B4 (en) * | 2007-02-09 | 2009-02-19 | Werner Turck Gmbh & Co. Kg | signal Conditioners |
| DE102007021099A1 (en) | 2007-05-03 | 2008-11-13 | Endress + Hauser (Deutschland) Ag + Co. Kg | Method for commissioning and / or reconfiguring a programmable field meter |
| EP2045791B1 (en) | 2007-10-01 | 2014-01-15 | Siemens Aktiengesellschaft | An electronic device |
| DE102007058608A1 (en) | 2007-12-04 | 2009-06-10 | Endress + Hauser Flowtec Ag | Electric device |
| US7970063B2 (en) | 2008-03-10 | 2011-06-28 | Rosemount Inc. | Variable liftoff voltage process field device |
| DE102008022373A1 (en) | 2008-05-06 | 2009-11-12 | Endress + Hauser Flowtec Ag | Measuring device and method for monitoring a measuring device |
| CN102859852B (en) | 2010-04-19 | 2015-11-25 | 恩德斯+豪斯流量技术股份有限公司 | The drive circuit of measurement translator and the measuring system formed by this drive circuit |
| DE202010006553U1 (en) | 2010-05-06 | 2011-10-05 | Endress + Hauser Flowtec Ag | Electronic measuring device with an optocoupler |
| US8786128B2 (en) | 2010-05-11 | 2014-07-22 | Rosemount Inc. | Two-wire industrial process field device with power scavenging |
| DE102010030924A1 (en) | 2010-06-21 | 2011-12-22 | Endress + Hauser Flowtec Ag | Electronics housing for an electronic device or device formed therewith |
| DE102011076838A1 (en) | 2011-05-31 | 2012-12-06 | Endress + Hauser Flowtec Ag | Meter electronics for a meter device and meter device formed thereby |
| DE102013100799A1 (en) | 2012-12-21 | 2014-06-26 | Endress + Hauser Flowtec Ag | Converter circuit with a current interface and measuring device with such a converter circuit |
| DE102013109096A1 (en) | 2013-08-22 | 2015-02-26 | Endress + Hauser Flowtec Ag | Tamper-proof electronic device |
| DE102014100693A1 (en) * | 2014-01-22 | 2015-07-23 | Phoenix Contact Gmbh & Co. Kg | signal Conditioners |
| DE102014108107A1 (en) | 2014-06-10 | 2015-12-17 | Endress + Hauser Flowtec Ag | Coil arrangement and thus formed electromechanical switch or transmitter |
| DE102016114860A1 (en) | 2016-08-10 | 2018-02-15 | Endress + Hauser Flowtec Ag | Driver circuit and thus formed converter electronics or thus formed measuring system |
| DE102018126808A1 (en) * | 2018-10-26 | 2020-04-30 | Krohne Messtechnik Gmbh | Field measuring device |
| DE102022119145A1 (en) | 2022-07-29 | 2024-02-01 | Endress+Hauser Flowtec Ag | Connection circuit for a field device and field device |
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| DE2445337C2 (en) * | 1974-09-23 | 1986-05-15 | Philips Patentverwaltung Gmbh, 2000 Hamburg | Circuit arrangement for the transmission of electrical measured value signals |
| US4084155A (en) * | 1976-10-05 | 1978-04-11 | Fischer & Porter Co. | Two-wire transmitter with totalizing counter |
| US4158765A (en) * | 1977-11-08 | 1979-06-19 | Fischer & Porter Co. | Totalizer for two-wire transmitter |
| US4339750A (en) * | 1980-08-20 | 1982-07-13 | Rosemount Inc. | Low power transmitter |
| US4527896A (en) * | 1982-03-04 | 1985-07-09 | Mikron Instrument Company, Inc. | Infrared transducer-transmitter for non-contact temperature measurement |
| DE3615452A1 (en) * | 1986-05-07 | 1987-11-26 | Endress Hauser Gmbh Co | ARRANGEMENT FOR TRANSMITTING SIGNALS IN A MEASURING ARRANGEMENT |
-
1989
- 1989-09-28 CA CA000614353A patent/CA1311032C/en not_active Expired - Fee Related
- 1989-10-03 GB GB8922271A patent/GB2229897A/en not_active Withdrawn
- 1989-10-05 FR FR8913232A patent/FR2645308A1/en active Pending
- 1989-10-11 DE DE19893934007 patent/DE3934007C2/en not_active Expired - Lifetime
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US6539794B1 (en) | 1994-02-18 | 2003-04-01 | Johanngeorg Otto | Arrangement for measuring the level of contents in a container |
Also Published As
| Publication number | Publication date |
|---|---|
| GB2229897A (en) | 1990-10-03 |
| DE3934007A1 (en) | 1990-10-04 |
| GB8922271D0 (en) | 1989-11-15 |
| DE3934007C2 (en) | 1996-10-31 |
| FR2645308A1 (en) | 1990-10-05 |
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