Two wire current transmitter responsive to a resistance sensor input signal

Abstract

Circuitry including a resistance sensor is excited by an external power source. A reference voltage is established, the voltage developed across the sensor resistance is compared at an amplifier input with a reference voltage signal and the voltage across a feedback resistor, and the total current drawn through the circuitry adjusts as a function of the sensor resistance to give a balanced amplifier input. Total current may be made a linear function of sensor resistance or a nonlinear function by proper selection of certain resistors.

I claim: 
     
       1. A measuring system comprising a pair of .Iadd.current .Iaddend.terminals, a source of direct current and a load, means to connect the source and load in series between said current terminals, means to provide a variable voltage signal which varies in magnitude as a function of a variable condition to be measured .Iadd.said variable voltage means including voltage reference source means for generating a reference voltage.Iaddend., a current adjusting controller means having a voltage responsive input coupled to said variable voltage means for receiving said variable voltage signal and adjusting the current through said terminals as a function of the magnitude of the variable voltage signal at said input, said controller means being connected across said terminals and energized solely from current supplied through said terminals, resistance means connected to said current terminals .Iadd.and said voltage reference source means .Iaddend.to provide a voltage feedback which is a function of .Iadd.said reference voltage and .Iaddend.total current flowing through said terminals, and means connecting said resistance means to said input of said controller means to provide the feedback voltage as determined by the current through said resistance means to tend to balance a change in the variable voltage signal at said input occurring due to a change in magnitude of said variable condition and to stabilize the adjusted current through said terminals at a new magnitude which is representative of a change in magnitude of said variable condition. 
     
     
       2. The measuring system of claim 1 wherein there is provided resistance network means for providing a variable voltage signal, said network means being connected to said terminals, energized solely through said terminals, and including a first resistor which varies in value as a function of the variable condition to be measured, and comprising at least portions of the first mentioned means to provide a variable voltage signal, the first mentioned resistance means and the means connecting the resistance means to said input. 
     
     
       3. The measuring system of claim 2 wherein said resistance network means includes additional resistance means, said means to connect said first mentioned resistance means to said input including means to connect said first resistor, said first mentioned resistance means and said additional resistance means in a bridge circuit and to provide bridge voltage output terminals that are connected to said voltage responsive .[.imput.]. .Iadd.input.Iaddend.. 
     
     
       4. The combination of claim 2 wherein the first mentioned resistance means comprises a second resistor and the resistance network means additionally comprises third, fourth, and fifth resistors, all of said resistors being connected in a closed series loop having the first resistor connected between the second and fourth resistors and the third resistor connected between the second and fifth resistors. 
     
     
       5. The combination of claim 4 .[.and a.]. .Iadd.wherein said .Iaddend.voltage reference .Iadd.source .Iaddend.means .[.to provide.]. .Iadd.provides .Iaddend.a reference voltage for said .Iadd.resistance .Iaddend.network means .Iadd.and is .Iaddend.connected between a junction intermediate said fourth and fifth resistors and a junction intermediate said second and third resistors. 
     
     
       6. The combination of claim 2 wherein the first mentioned resistance means comprises a second resistor and said resistance network means additionally comprises third, fourth, and fifth resistors, all of said .[.resistor.]. .Iadd.resistors .Iaddend.being connected in a closed series loop, said first resistor being connected between the second resistor and fourth resistor and the third resistor connected between the second and fifth resistors, and wherein the voltage reference .Iadd.source .Iaddend.means is connected for current conduction from the junction of the .[.third and second.]. .Iadd.fourth and fifth .Iaddend.resistors to the junction of the .[.fourth and fifth.]. .Iadd.second and third .Iaddend.resistors, the junction of the first and fourth resistors comprising a first voltage terminal and the junction of the fifth and third resistors comprising a second voltage terminal, and the junction of the first and second resistors being coupled to one of said first mentioned pair of terminals and the other of said first mentioned pair of terminals being connected to the junction of said .[.second and third.]. .Iadd.fourth and fifth .Iaddend.resistors. 
     
     
       7. The combination of claim 6 .[.and a.]. .Iadd.wherein said .Iaddend.voltage reference .Iadd.source .Iaddend.means .Iadd.is .Iaddend.connected to said resistance network means and to said first mentioned pair of current terminals to provide a reference voltage for said resistance network means, .[.including.]. .Iadd.and comprises .Iaddend.a voltage reference element connected to the junction of said fourth and fifth resistors and a sixth resistor connected between the voltage reference element and the junction of the third and the second resistors and wherein the sixth resistor is connected in series with the second resistor and the first mentioned pair of current terminals. 
     
     
       8. The combination of claim 7 wherein the voltage reference element is a Zener diode. 
     
     
       9. The combination of claim 2 wherein said resistance network means includes second, third and fourth resistors connected in a symmetrical bridge arrangement having one end of said first resistor connected to one end of said third resistor and one end of said second resistor being connected to one end of said fourth resistor, the other ends of said first and second resistors being connected to one of said terminals, a fifth resistor having one end connected to .[.the other end.]. .Iadd.said one .Iaddend.of said terminals, and a sixth resistor having one end connected to the junction of said second and fourth resistors.Iadd., .Iaddend.and .Iadd.said .Iaddend.voltage reference .Iadd.source .Iaddend.means for providing a reference voltage for said resistance network means .[.including.]. .Iadd.includes .Iaddend.a voltage reference element having one end connected to the junction of said third and fourth resistors and the other end connected to the other end of said fifth resistor, and the other end of said voltage reference element being connected to the other end of said sixth resistor. 
     
     
       10. A transmitter for measuring a variable voltage dependent upon the value of a variable parameter, first and second wires each having a first end portion and a second end portion, a transmitter connected across said second end portions, a measuring device, current source means for providing the sole energy supply to the transmitter and the current that serves to transmit a signal from the transmitter, said measuring device and current source means being connected in series across the first end portions of said wires, said transmitter including means .Iadd.for .Iaddend.providing a voltage that varies in response to the variation of the parameter to be measured, .Iadd.and including internal means for generating a reference voltage .Iaddend.amplifier means having an input portion coupled to said variable voltage providing means and an output portion for providing an output voltage at said output portion that is dependent on the magnitude of the variable voltage at said input portion, means coupled to the output portion and the second end portions of said wires for regulating the flow of current through the wires as a function of the output voltage including feedback resistor means coupled .Iadd.to said internal means for generating a reference voltage and .Iaddend.across the second end portions of the wires for generating a feedack voltage dependent upon .Iadd.said reference voltage and .Iaddend.the current flowing in said wires, and means for electrically connecting said feedback resistor means to the input portion of said amplifier means to tend to balance variations in magnitude of the variable voltage at said input portion. 
     
     
       11. The combination as specified in claim 10 wherein said means providing a voltage that varies in response to variation of the parameter to be measured comprises a resistance network including a first resistor which varies in resistance as a function of .[.a.]. .Iadd.the .Iaddend.parameter to be measured, said feedback resistor means comprising a second resistor in said resistance network, said resistance network further including third and fourth resistors, said first, second, third, and fourth resistors being connected in a symmetrical bridge arrangement having one end of said first resistor connected to one end of said third resistor to provide a first voltage terminal, and one end of said second resistor being connected to one end of said fourth resistor to provide a second voltage terminal, and wherein the other ends of said first and second resistors are connected to one of said wires and the other ends of said third and fourth resistors are connected together to the other of said wires, said feedback resistor means further including fifth and sixth resistors and .[.a.]. .Iadd.said .Iaddend.voltage reference means for providing a reference voltage for said resistance network including a voltage reference element having one end connected to the junction of said third and fourth resistors and the other end connected through said fifth resistor to .[.the other.]. .Iadd.said one .Iaddend.of said wires and through said sixth resistor to said second voltage terminal. 
     
     
       12. An electrical current transmitting system comprising a pair of current terminals adapted for connection to a load and power source that are .Iadd.connected .Iaddend.in series; a resistance network means .Iadd.including means for generating a reference voltage connected therein, said resistance network means .Iaddend.connected in series with said current terminals to be powered and excited solely therethrough for providing a first variable voltage signal which varies as a function of the magnitude of a physical condition to be measured; a feedback resistance means connected to said resistance network means for providing a second voltage signal which is a function of .Iadd.said reference voltage and .Iaddend.the current through said feedback resistance means; means to be powered solely through said terminals .Iadd.and .Iaddend.connected to the feedback resistance means and to the current terminals to establish a current flow path between said terminals and to provide a known portion of current flowing between said terminals through said feedback resistance means including an amplifier means having a voltage input portion and an output portion providing an output signal which is a function of the voltage at said voltage input portion, and current control and adjustment means coupled to said output portion to receive the output signal and to adjust the current in said current flow path between said terminals as a function of said output signal; and means to couple said resistance network means to said voltage input portion to provide a voltage at said input portion of said amplifier means which is a function of said first and second voltage signals. 
     
     
       13. The combination of claim 12 further characterized in that .[.a.]. .Iadd.said .Iaddend.voltage reference means is connected to said resistance network means in series with said feedback resistance means between said terminals to provide a reference voltage for said resistance network means. 
     
     
       14. The combination of claim 13 wherein said network means includes a first resistor, and said voltage reference means includes a fixed resistance and a semiconductor reference element in series with said fixed resistance, the series combination of said reference element and fixed resistance being connected across said first resistor so that an increase in excitation voltage across said first resistor is provided as a result of an increased current through said voltage reference means. 
     
     
       15. The combination of claim 12 wherein said amplifier means comprises a chopper driver having a driver output terminal; said means to couple comprising a modulator means having first and second input terminals connected to said resistance network means to sense voltages in said network means, a third input terminal capacitively connected to the .[.drive.]. .Iadd.driver .Iaddend.output terminal and a modulator output terminal; an operational amplifier having an input terminal connected to the modulator output terminal and having an output terminal; a demodulator having a first input terminal connected to the amplifier output terminal, a second input terminal capacitively connected to the driver output terminal and having an output terminal; and said current control and adjustment means comprising a current control stage having a first input terminal connected to the demodulator output terminal, a second current terminal connected to one of the transmitting system pair of current terminals and a third terminal means connected to pass current through portions of said resistance network including said feedback resistance means to the other of the transmitting system pair of current terminals. 
     
     
       16. A signal transmitter comprising a pair of wires, a load, current supply means for providing a load current through said load and wires connected in series with the load across first ends of said wires, a current transmitter connected across said wires at the other .[.end.]. .Iadd.ends .Iaddend.thereof, the current flowing through said wires serving as the sole power supply to the transmitter as well as serving at the load to represent the signal transmitted from said transmitter, said transmitter including an input circuit means connected to said two wires to provide a variable voltage which tends to change in response to changes in a variable condition constituting the information to be transmitted and voltage responsive current controller means connected to the two wires and the input circuit means for adjusting the current through said two wires in response to the voltage supplied by the input circuit means, .Iadd.said input circuit means including a voltage reference element connected therein, .Iaddend.feedback resistor means connected to said current controller means and in series with said wires to have at least a known portion of load current carried by said wires flowing therethrough to provide a feedback voltage dependent on .Iadd.the reference voltage produced by said voltage reference element and .Iaddend.the magnitude of said load current, and means connecting said feedback resistor means to the input circuit means to provide a feedback voltage in opposition to the variable voltage so that the input voltage to the current controller means is dependent upon the variable voltage and the feedback voltage whereby adjusted load current flowing through said wires and load is representative of the magnitude of said variable condition. 
     
     
       17. The transmitter of claim 16 wherein the current controller means comprises a chopper driver having a driver output, a modulator having input terminals connected to the input circuit means, means coupling the driver output to said modulator, an amplifier having an input and an output and having its input connected to the modulator output, a demodulator having an input connected to the amplifier output, means coupling the driver output to the demodulator, and a current control stage having a first input terminal connected to the demodulator output, a second input terminal connected to one of said wires and an output terminal connected to the other of said wires. 
     
     
       18. The signal transmitting system of claim 16 wherein said input circuit means comprises .[.a.]. .Iadd.said .Iaddend.resistance network means to provide a variable voltage including a first resistor which changes in value in response to a change in a variable condition and said feedback resistor means comprises a second resistor also coupled to said resistor network means, said resistance network means further including third, fourth and fifth resistors, all of said resistors being connected in a closed series loop having the first resistor connected between the second and fourth resistors and the third resistor connected between the second and fifth resistors, and a voltage reference element and a sixth resistor connected in series between the junction of said fourth and fifth resistors and the junction of said second and third resistors, said means to couple said input circuit to said two wires comprising means coupling one of said wires to the junction of the fourth and fifth resistors to carry a first portion of the current to said resistance network and means also coupling the junction of the voltage reference element and the sixth resistor to said one wire to carry a second portion of the current in the one wire to the resistance network, said last-mentioned junction being connected through portions of said resistance network to the other wire and hence to said current supply means. 
     
     
       19. .[.An.]. .Iadd.In combination an .Iaddend.electrical network comprising six resistors and a source of constant voltage connected together and including first and second signal output terminals, a current input terminal and a current output terminal and means providing a source of current to said current input terminal, wherein the six resistors designated R 1  through R 6  and the .Iadd.constant .Iaddend.voltage source designated V c  are connected to be equivalent to an arrangement having R 1 , R 2 , R 3 , .[.R 4 , R 5  .]. .Iadd.R 5 , R 4  .Iaddend.connected in a closed series loop in the order cited having the first and second signal output terminals between R 1  and R 4  and between R 2  and R 3  respectively, the current input terminal between R 1  and R 2 , and the current output terminal between R 4  and R 5 , and the source V c  serially connected with R 6  from a junction between R 1   and R 2  to a junction between R 3  and R 5 , and connecting means to deliver a variable current from said current source means to a junction between V c  and R 6  so that the total current designated I T  delivered to the network which results in zero voltage difference at the first and second signal output terminals is   I.sub.T =V.sub.c [R.sub.2 (R.sub.4 +R.sub.5)-R.sub.1 R.sub.3 ]/[R.sub.1 [R.sub.5 (R.sub.2 +R.sub.3)+R.sub.6 (R.sub.3 +R.sub.5)]-R.sub.2 R.sub.4 R.sub.6 ]     and wherein the resistor designated R 4  varies with respect to a varying measurand approximately as     R.sub.4 =R.sub.0 (1+αT-βT.sup.2)     where T is the measurand, R 0  is the value of R 4  at a zero reference value of the measurand, α and β are constants and the resistor R 6  is selected to satisfy the equation ##EQU2## so that the total current I T  is linearly proportional to the measurand.   
     
     
       20. An electrical current transmitting system comprising first means providing a first voltage input signal, second means providing second voltage input signal, a first electrical circuit that includes said first and second means, said first circuit also including feedback resistance means to provide a voltage feedback signal proportional to the total current flow in said first circuit, an amplifier connected to the first circuit and energized by the first circuit for providing an output responsive to voltage unbalance between the first and second input signals and said feedback signal, means connected to the amplifier are responsive to the output of said amplifier for adjusting the current flowing through the first circuit to increase the current through said first circuit, and at the same time increase the current through said feedback resistance means when said first voltage input signal increases with respect to said second voltage input signal and to decrease the current through said first circuit and at the same time decrease the current through said feedback resistance means when said first voltage input signal decreases with respect to said second voltage input signal and to thereby correspondingly increase or decrease the magnitude of the feedback signal as determined by the amount of the current is adjusted, and a voltage reference element means connected to said first circuit for passing a portion of the current flowing through the first circuit to maintain said second voltage which is provided by said second means at a reference value, means to connect said feedback resistance means in said first circuit to provide a change in the magnitude of the feedback signal determined by the amount of current is adjusted to modify the voltage magnitude of the first input signal to tend to reduce any voltage difference between the first and second input signals, said first circuit carrying the total current for the electrical transmitting system, which is representative of the magnitude of said variable condition. 
     
     
       21. The combination of claim 19 and resistance means connected in series with said voltage reference element means and said means for adjusting the current to provide a small positive adjustment signal to the second voltage when the current increases. 
     
     
       22. A measuring system comprising a direct current source, a load, a single pair of wires, said source, load and wires being connected in series so that load current which flows through said load also flows through said pair of wires, current adjustment means for controlling and adjusting the flow of current through said wires and said load, means to connect said current adjustment means in series with said wires, means .Iadd.including means for generating a fixed voltage .Iaddend.for providing first and second voltages which differ from each other in accordance with the magnitude of a variable condition, .Iadd.and depend upon said fixed voltage .Iaddend.voltage responsive means having an input connected to the last mentioned means to sense differences in said first and second .[.voltages.]. .Iadd.voltage.Iaddend., said voltages responsive means also being connected to said current adjustment means for providing a control signal controlling said current adjustment means to change the current through said wires and said load in accordance with the difference between said first and second voltages, said means to connect including resistance means to provide a feedback voltage that varies as a known function of the value of total current through said load, and means connecting said resistance means to said means for providing the first and second voltages to provide variations in said feedback voltage caused by changes in current through said wires and said load to the input of the voltage responsive means to tend to maintain the difference between said first and second voltages substantially constant so that the changed current flowing through said wires and load as adjusted by said current adjustment means is representative of the magnitude of said variable condition. 
     
     
       23. A current transmitting system having all electrical actuation comprising a circuit having an input terminal and an output terminal, a first DC voltage signal input means for receiving a first DC voltage signal, a second DC voltage signal input means for receiving a second DC voltage signal, amplifier and control means connected in said circuit to be powered solely through said input and output terminals and coupled to both of said input means for controlling the total current flow in said circuit in response to DC voltage signals provided by said first and second voltage signal input means, a DC resistance bridge means coupled to the output of the amplifier and control means and being coupled to said first and second signal input means for providing DC voltage signals to said first and second signal input means, .Iadd.voltage reference means for generating a fixed DC voltage and connected in said DC resistance bridge, .Iaddend.means connected to said bridge to change the voltage at one of the signal input means, a fixed feedback resistor means coupled in said bridge and to said amplifier and control means to be responsive to total current flow between said input and output terminals to provide a voltage signal in the bridge means due to the current flow through said feedback resistor means to reduce any difference between the DC signals at the voltage signal input means as the current between said input and output terminals changes, whereby the changed current flow between said input and output terminals represents the amount of change in the value of the voltage signal provided by said first signal input means with respect to the voltage signal provided by said second signal input means. .Iadd. 24. An arrangement for measuring a physical parameter comprising: first and second terminals;   current measuring means and a power supply connected in series between said first and second terminals;   a current regulator connected to said first terminal for regulating current flow through the series combination of said measuring means and said power supply in response to a voltage signal;   a resistance network comprising feedback means for generating a feedback signal and connected to said second terminal and to said regulator for generating said voltage signal as a function of said parameter and said feedback signal, said voltage signal being dependent upon the total current flow through said series combination; and   a voltage reference source connected to said network to establish a reference voltage across said network, wherein said total current flows through said network and said regulator and serves as the sole source of   
     
     
        energy for said network and said regulator. .Iaddend..Iadd. 25.  The measuring arrangement in accordance with claim 24 wherein: said network further comprises:   a first resistor having a resistance value which varies as a function of said parameter, and second, third, and fourth resistors; all of said resistors being connected into a bridge circuit with said first resistor connected to first ends of said second and third resistors and said fourth resistor connected to the other ends of said second and third resistors with the junction of said first and second resistors being connected to said second terminal;   said feedback means comprising fifth and sixth resistors connected in series between said second terminal and the junction of said second and fourth resistors; and   said voltage reference source connected for current conduction from the junction of said third and fourth resistors to the junction of said fifth and sixth resistors, the junction of said third and fourth resistors   
     
     
        connected to said regulator. .Iaddend..Iadd. 26.  The measuring arrangement in accordance with claim 24 wherein: said resistance network comprises first, second, third, and fourth resistors, and said feedback means comprises a fifth resistor, all of said resistors connected into a closed series loop, said first resistor being connected between said second and fourth resistors and said third resistor connected between said second and fifth resistors and said fourth and fifth resistors connected together to close the loop;   said voltage reference source being connected for current conduction from the junction of said fourth and fifth resistors to the junction of said second and third resistors;   the junction of said first and fourth resistors defining a first voltage terminal and the junction of said third and fifth resistors defining a second voltage terminal, said voltage terminals being connected to said regulator to provide said voltage signal; and   the junction of said first and second resistors being connected to said second terminal and the junction of said fourth and fifth resistors being   
     
     
        connected to receive current from said regulator. .Iaddend..Iadd. 27.  The measuring arrangement in accordance with claim 26 wherein said first resistor varies in resistance value as a function of said parameter. .Iaddend..Iadd. 28. The measuring arrangement in accordance with claim 26 further comprising a sixth resistor connected in series with said voltage reference source whereby said reference voltage is a function of said total current. .Iaddend.