CA1150525A - Temperature compensation for transducer components - Google Patents
Temperature compensation for transducer componentsInfo
- Publication number
- CA1150525A CA1150525A CA000381578A CA381578A CA1150525A CA 1150525 A CA1150525 A CA 1150525A CA 000381578 A CA000381578 A CA 000381578A CA 381578 A CA381578 A CA 381578A CA 1150525 A CA1150525 A CA 1150525A
- Authority
- CA
- Canada
- Prior art keywords
- output
- ladder
- current
- temperature
- voltage
- 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
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Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
- G01L9/00—Measuring steady of quasi-steady pressure of fluid or fluent solid material by electric or magnetic pressure-sensitive elements; Transmitting or indicating the displacement of mechanical pressure-sensitive elements, used to measure the steady or quasi-steady pressure of a fluid or fluent solid material, by electric or magnetic means
- G01L9/10—Measuring steady of quasi-steady pressure of fluid or fluent solid material by electric or magnetic pressure-sensitive elements; Transmitting or indicating the displacement of mechanical pressure-sensitive elements, used to measure the steady or quasi-steady pressure of a fluid or fluent solid material, by electric or magnetic means by making use of variations in inductance, i.e. electric circuits therefor
- G01L9/105—Measuring steady of quasi-steady pressure of fluid or fluent solid material by electric or magnetic pressure-sensitive elements; Transmitting or indicating the displacement of mechanical pressure-sensitive elements, used to measure the steady or quasi-steady pressure of a fluid or fluent solid material, by electric or magnetic means by making use of variations in inductance, i.e. electric circuits therefor with temperature compensating means
Abstract
TEMPERATURE COMPENSATION FOR TRANSDUCER COMPONENTS
ABSTRACT OF THE DISCLOSURE
A circuit for compensating the temperature dependence of the deformation properties of a pressure transducer sensor, having four basic parts: a current source having an output proportional to the sensor temperature, a constant voltage source, a Norton divider, and an operational amplifier. One branch of the Norton Divider is a vari-able conductance ladder having an output current which increases at a programmed rate as the current from the temperature dependent source increases. The programmed rate is based on the temperature-dependent characteristic of the transducer sensor. The two branches of the Nor-ton divider are connected as inputs to the operational amplifier. The operational amplifier provides the output of the compensating circuit, which is the difference between the reference voltage of the voltage source and the voltage at the output of the variable conductance lad-der. As the current source increases, the output voltage of the am-plifier is reduced such that the temperature dependence of the output voltage is a close approximation to the inverse of the temperature dependence of the sensor deformation characteristic.
ABSTRACT OF THE DISCLOSURE
A circuit for compensating the temperature dependence of the deformation properties of a pressure transducer sensor, having four basic parts: a current source having an output proportional to the sensor temperature, a constant voltage source, a Norton divider, and an operational amplifier. One branch of the Norton Divider is a vari-able conductance ladder having an output current which increases at a programmed rate as the current from the temperature dependent source increases. The programmed rate is based on the temperature-dependent characteristic of the transducer sensor. The two branches of the Nor-ton divider are connected as inputs to the operational amplifier. The operational amplifier provides the output of the compensating circuit, which is the difference between the reference voltage of the voltage source and the voltage at the output of the variable conductance lad-der. As the current source increases, the output voltage of the am-plifier is reduced such that the temperature dependence of the output voltage is a close approximation to the inverse of the temperature dependence of the sensor deformation characteristic.
Description
s TEMPERATURE COMPENSATION FOR TRANSDUCER COMPONENTS
BACKGROUND OF THE INVENTION
The invention relates to temperature compensation for a transducer device, and more particularly to electronically compen-sating for the temperature dependence of the deformation properties of the sensor element of the transducer.
For many kinds of transducer devices, such as differential pressure transmitters, it is necessary that the ef~ects of tempera-ture be accounted for so that the pressure measurement itself is not temperature dependent. In one kind o~ differential pressure trans-i ducer, a metal diaphragm is sealed between two chambers wh;ch are at different pressures. An electric coil, such as an "E core", is located in each chamber on either side of the diaphragm. The E cores form branches on a br1dge and are excited by a voltage signal gene-rator. The pressure differential acting on the diaphragm displaces the diaphragm and this displacement changes the magnetic coupling of the E cores. The diaphragm displacement is sensed by the transducer system as a change in reluctance, which throu~h the bridge may be dis-played or recorded as a ~ressur~ differential.
The displacement of the diaphragm is ideally proportional to the pressure difference between the chambers of the transducer de-vice. However, the stress and strain relationship of the diaphragm is temperature dependent, i.e., a given pressure differential will displace the diaphragm a different amount depending on the tempera-ture of the diaphragm. This material property of the diaphragm must be compensated or accounted for if a high degree of transducer ac-curacy is required.
Particularly when used in nuclear power plants, differen-` tial pressure transducers should be accurate to within + 1% over a .
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BACKGROUND OF THE INVENTION
The invention relates to temperature compensation for a transducer device, and more particularly to electronically compen-sating for the temperature dependence of the deformation properties of the sensor element of the transducer.
For many kinds of transducer devices, such as differential pressure transmitters, it is necessary that the ef~ects of tempera-ture be accounted for so that the pressure measurement itself is not temperature dependent. In one kind o~ differential pressure trans-i ducer, a metal diaphragm is sealed between two chambers wh;ch are at different pressures. An electric coil, such as an "E core", is located in each chamber on either side of the diaphragm. The E cores form branches on a br1dge and are excited by a voltage signal gene-rator. The pressure differential acting on the diaphragm displaces the diaphragm and this displacement changes the magnetic coupling of the E cores. The diaphragm displacement is sensed by the transducer system as a change in reluctance, which throu~h the bridge may be dis-played or recorded as a ~ressur~ differential.
The displacement of the diaphragm is ideally proportional to the pressure difference between the chambers of the transducer de-vice. However, the stress and strain relationship of the diaphragm is temperature dependent, i.e., a given pressure differential will displace the diaphragm a different amount depending on the tempera-ture of the diaphragm. This material property of the diaphragm must be compensated or accounted for if a high degree of transducer ac-curacy is required.
Particularly when used in nuclear power plants, differen-` tial pressure transducers should be accurate to within + 1% over a .
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-2-temperature range of about 40F to 250F. This kind o~ accuracy is not obtainable with transducers currently available commercially.
Although commercially available transducers are capable of compen-sating for temperature effects arising in the electronic circuit it-sel-F, i.e., in the diodes and ~ransistors, it is believed that no satisfactory means have previously been found for specifically com-pensating the temperature dependence of the material properties of the sensor itself.
SUMMARY OF THE INVENTION
The present invention provides an improvement over the prior art transducers in that the temperature dependence of the sensor ele-ment, such as the deformation characteristic of the diaphragm in a different;al pressure transducer, is specifically accounted for by providing a compensating circuit having an output voltage which varies inversely with the temperature dependence of the sensor element.
The compensating circuit comprises four basic parts: a cur-rent source having an output proportional to the sensor temperature, a constant voltage source, a Norton divider, and an operational am-plifier. One branch of the Norton divider is a variable conductance ladder having an output current which increases at a programmed rate as the current from the temper~ture dependent source increases. The ` programmed rate is based on the temperature-dependent characteristic - of the transducer sensor. The two branches of the Norton divider are connected as inputs to the operational amplifier. The operational amplifier provides the output of the compensating circuit, which is the difference between the reference voltage of the voltage source and the voltage at the outpu~ of the Yariable conductance ladder. As the current source increases, the output voltage of the amplifier is re-duced such that the temperature dependence of the output voltage is a close approximation to the inverse of the temperature dependence of ` the sensor deformation characteristic.
The invention provides several advantages not available with known compensated transducers. Most importantly, the temperature dependence of the sensor element itself is accounted for ~y a pie~e-~ice linear approximation which can be made as accurate as necessary by providing a sufficient number of se~uential conductance paths in the variable conductance ladder. In the preferred embodlment the ladder consists of diodes and resistances, which are extremely accurate in . .
--their operation. This is in contrast to temperature compensating devices used in the prior art, such as thermistors and resistor-temperature devices (RTD's), which cannot provide the accuracy of ~
1% over the temperature range desired for use, for example, in nuclear power plants. The present invention, when used in conjunction with other state-of-~he-art transducer equipment, should permit this kind of accuracy.
Another advantage is that field adjustments made on the preferred embodiment of the invention very easily accommodate the slight variations among transducers manufactured from the same speci-fication. In the preferred embodiment of the invention, a first pro-: grammable resistance is provided to remove a fixed amount of current from the variable conductance ladàer whereby the circuit may be cali-brated to provide a known output voltage at any reference temperature.
Also, a second programmable resistance may be provided between the amplifier output and the variable conductance ladder ~or the purpose of adjusting the gain on the piece-wise linear approximations provided by the ladder. This adjustment is needed, for example, to account for the slighty varying diaphragm thicknesses from transducer to trans-ducer.BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 diagramatically illustrates an electrical circuit embodying the invention.
` Figure 2 graphically illustrates the behavior of a trans-ducer system output as a function of sensor temperature, in the ab-sence of temperature compensation.
Figure 3 graphically illustrates the temperature compensated output of the inventive circuit, which is provided as an input to the signal generator of the transducer system.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Figure 1 diagramatically shows a transducer system including a reluctance resistance bridge 10 ~aving coils 12, suc~-as E cores, for measuring the pressure differential across a sensor element such as a diaphragm (not shown). This bridge is activated by a signal genera-tor 14 which provides an A~ excitation to the bridge 10. Such a bridgeand signal generator arrangement, or equivalents thereo~ for pur-poses of the invention, are more fully described in several prior art references, including U.S. Patents 3,995,493 Differential Pres-sure Transducer, and 4,011,758 Magnitostrictive Pressure Transducer. In theprior art, the ac output voltage signal from the signal generator to the bridge typically has an amplitude that is independent of sensor temperature.
Referring also to Figure 2, the normalized bridge output 18 as a func-tion of sensor temperature is shown. It may be seen that over a temperature range of 40F to 250F, the bridge output 18 can vary by as much as 50% for the same pressure differential applied across the sensor diaphragm. This tempera-ture effect must be accurately compensated if transducer accuracy over this tem-perature range is to be maintained within a few percent.
In the illustrated embodiment, the present invention modifies the ac output of the signal generator 14 such that the temperature compensation is made in the transducer activation or excitation signal, rather than in the transducer output signal 18. In effect, the amplitude of the ac output of the signal gen-erator is reduced according to the temperature of the sensor such that the acti-vation voltage varies inversely with the temperature dependence of the material in the sensor.
Figure 3 shows the voltage output of the inventive circuit, VF, as a function of sensor temperature; the output VT of the compensating circuit is in-put to the signal generator 14 of the transducer system. It may be seen by com-paring Figure 3 with Figure 2, that at a given temperature the product of thetwo normalized curves is 1.00, which in effect removes the temperature depen-dence from the normalized bridge output signal 18. Thus, the bridge output will be the same for the same pressure differential at any temperature between 40F
and 250F.
It should be appreciated that although the normalized bridge output shown in Figure 2 is a smooth curve having a smooth transltion in temperature through points Tl, T2, T3, T4, the compensation curve shown in Figure 3 is piece-wise linear between Tl, T2, T3, and T4. The number of piece-wise linear approxi-~ .
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mations required to compensate the inherent temperature dependence of the sensor is determined by the degree of accuracy required and the curvature of the tem-perature dependence of the sensor. For the sensor behavior represented in Fig-ure 2, it has been found that a three segment piece-wise linear approximation is sufficient. In connection with the fol-.
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lowing description of the inventive circuit, it should be appreciated that the number of legs on the variable conductance ladder, which pro-vides the piece-wise compensation, will depend on the judgment of the designer and the accuracy desired.
Referring again to to Figure 1, there is shown a temperature compensated voltage VT which is the output of the inventive compen-sating circuit 16 and an input to the signal generator 14. The vol-tages to be described in connection with the inventive compensating circuit 16 are relative to a common of the signal generator 14 and bridge portion 10 of the transducer system. The compensating cir-cuit includes a current source 26 maintained at substantially the same temperature as the sensor element (not shown) and having an out-put Io that is linear with temperature. Such a device is commerciallY
available as, for example,'LM-134 from the National Semiconductor Company or part'AD-590 from the Analog Devices Corporation. This current source 26 is preferably located as close as possible to the sensor element. A suitable current source provides one micro-ampere change in current per ~K change ;n temperature.
A vol~age source 20, preferably in the range of 5 to 10 volts, volts, provides a base or reference voltage Vo corresponding to the base or reference output of the current source at the calibration tem-perature. The circuit is initially calibrated so that at 40F and with a corresponding source current of about 278 micro-amperes, the output voltage VT is exactly equal to the source voltage Vo. This is done by proper choice of resistance R7, or by providing a programmable resistance Pl which can be adjusted to force VT to equal Vo at the calibration temperature.
The resistor Rl is connected with the variable conductance ladder 24, illustrated in the form of a resistive diode matrix R2, R3, R4, R5 and D1, D2, D3, to form a Norton divider at 22. Rl is con-nected to the voltage source 20 and the positive input of the opera-tional amplifier Al, and the ladder is connected to the negative input of the amplifier.
The operational amplifier A1 performs the following opera-tion:
T = Vo - Il x (R6 ~ P2) ` where R6 and P2 will be explained below. Il is the fractional current which passes through the variable conductance ladder 24.
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; At ~he calibration temperature, e.g., 40F, VT-Vo, Io pas-ses through Rl and R2, and Il = O as controlled by R7 and Pl. The diodes Dl, D2 and D3 are non-conducting. As the temperature of the sensor increases and the current source Io increases, an increasing amount of current flows through resistor R2, which is a constant frac-tion o~ the current Io. According to the rule of the operational amplifier as described above, as Il increases an ever increasing vol-tage is subtracted from Vo so that the compensating circuit output volta~e VT decreases as Io increases.
Referring now to Figures 1 and 3, the VT has a constant slope between 40F and 150F. As the voltage across R2 increases to about 0.7 volts, diode Dl becames conducting and resistor R3 is added to the circuit. As Io continues to increase, voltage VT follows the linear relationship represented between the points T2 and T3. Like-wise, when the voltage across D2 reaches about 0.7 volts, resistantR4 comes into operation and the sequence continues for as many legs of the ladder are necessary to satisfactorily model the temperature behavior of the sensor material. The variable conductance ladder 24 therefore has a current output that is piece-wise linear with increas-ing current Io from the current source 26. The piece-wise linearity is programmed into the ladder on the basis of the information known `` to the designer from Figure 2. This information is ideally obtained from measurements on the uncompensated transducer system 10, 14, but can also be satisfactorily estimated from published data on the ma-terial properties of the particular sensor material.
It may be appreciated that in a typical nuclear power plant : dozens of nominally identical differential pressure transducers may be required. It would be desirable that temperature compensation in each of the transducers be as uniform as poss;ble. The preferred embodi-ment of the present invention provides features that permit easy calibratian whereby each transducer may be individually adjusted to have the same reference conditions as the other transducers. For example, all compensating circuits 16 can be adjusted to prov;de the ;` same output voltage VT at 40F. Or, each compensating circuit 16 can be adapted so that its output voltage VT at the reference condition will match the nominal ac output voltage of the signal source 14. In effect, the output voltage VT at the reference conditirn, e.g., 40F, .
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can be raised Gr lowered by the first programmable resistant P1 con-` nected to the variable conductance ladder 2~. Pl removes a fixed - amount of current from the ladder independent of the strength of the current source Io. This way, individual differences in the amount of current produced at 40F, for different current sourGes 26, can be offset to provide the same output voltage VT in each compensating cir-cuit at 40F.
Another adjustment wh;ch can easily be made with the pre-ferred embodiment of the invention is a gain adjustment on the slope of the piece-wise linear segments shown in Figure 3. It should be ap-` preciated that a shipment of transducer systems may all have the same specifications on the diaphragm thickness, for example, but variations will in practice occur. These varia~ions can be accounted for by a second programmable resistance P2 connected between the compensating circuit output VT and the variable conductance ladder 24 whereby the amplifier signal is fed back through the second programmable resis-tance P2. This adjustment is a ratio adjustment in which each of the slopes shown in Figure 3 is adjusted by a constant factor.
The preferred embodiment of the invention has been described in which the temperature dependence of the stress/strain relationship of a metal diaphragm is to be electronically compensated. The inven- -tion may be used in any system wherein Hook's l.aw or an analogue thereof is the material property forming the basis of the desired mea-surement, but where compensation for the variability of the tempera-ture is desired. The details of providing specific values for the circuit devices disclosed in the preferred embodiment, or construct-ing an equivalent circuit, will be obvious to one ordinarily skilled in this art. Likewise, the use o~ the invention in a totally resis-tive transducer system wherein a dc signal generator may be employed, or in modifying the transducer system output rather than the input, will be evident to the ordinary practitioner.
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Although commercially available transducers are capable of compen-sating for temperature effects arising in the electronic circuit it-sel-F, i.e., in the diodes and ~ransistors, it is believed that no satisfactory means have previously been found for specifically com-pensating the temperature dependence of the material properties of the sensor itself.
SUMMARY OF THE INVENTION
The present invention provides an improvement over the prior art transducers in that the temperature dependence of the sensor ele-ment, such as the deformation characteristic of the diaphragm in a different;al pressure transducer, is specifically accounted for by providing a compensating circuit having an output voltage which varies inversely with the temperature dependence of the sensor element.
The compensating circuit comprises four basic parts: a cur-rent source having an output proportional to the sensor temperature, a constant voltage source, a Norton divider, and an operational am-plifier. One branch of the Norton divider is a variable conductance ladder having an output current which increases at a programmed rate as the current from the temper~ture dependent source increases. The ` programmed rate is based on the temperature-dependent characteristic - of the transducer sensor. The two branches of the Norton divider are connected as inputs to the operational amplifier. The operational amplifier provides the output of the compensating circuit, which is the difference between the reference voltage of the voltage source and the voltage at the outpu~ of the Yariable conductance ladder. As the current source increases, the output voltage of the amplifier is re-duced such that the temperature dependence of the output voltage is a close approximation to the inverse of the temperature dependence of ` the sensor deformation characteristic.
The invention provides several advantages not available with known compensated transducers. Most importantly, the temperature dependence of the sensor element itself is accounted for ~y a pie~e-~ice linear approximation which can be made as accurate as necessary by providing a sufficient number of se~uential conductance paths in the variable conductance ladder. In the preferred embodlment the ladder consists of diodes and resistances, which are extremely accurate in . .
--their operation. This is in contrast to temperature compensating devices used in the prior art, such as thermistors and resistor-temperature devices (RTD's), which cannot provide the accuracy of ~
1% over the temperature range desired for use, for example, in nuclear power plants. The present invention, when used in conjunction with other state-of-~he-art transducer equipment, should permit this kind of accuracy.
Another advantage is that field adjustments made on the preferred embodiment of the invention very easily accommodate the slight variations among transducers manufactured from the same speci-fication. In the preferred embodiment of the invention, a first pro-: grammable resistance is provided to remove a fixed amount of current from the variable conductance ladàer whereby the circuit may be cali-brated to provide a known output voltage at any reference temperature.
Also, a second programmable resistance may be provided between the amplifier output and the variable conductance ladder ~or the purpose of adjusting the gain on the piece-wise linear approximations provided by the ladder. This adjustment is needed, for example, to account for the slighty varying diaphragm thicknesses from transducer to trans-ducer.BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 diagramatically illustrates an electrical circuit embodying the invention.
` Figure 2 graphically illustrates the behavior of a trans-ducer system output as a function of sensor temperature, in the ab-sence of temperature compensation.
Figure 3 graphically illustrates the temperature compensated output of the inventive circuit, which is provided as an input to the signal generator of the transducer system.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Figure 1 diagramatically shows a transducer system including a reluctance resistance bridge 10 ~aving coils 12, suc~-as E cores, for measuring the pressure differential across a sensor element such as a diaphragm (not shown). This bridge is activated by a signal genera-tor 14 which provides an A~ excitation to the bridge 10. Such a bridgeand signal generator arrangement, or equivalents thereo~ for pur-poses of the invention, are more fully described in several prior art references, including U.S. Patents 3,995,493 Differential Pres-sure Transducer, and 4,011,758 Magnitostrictive Pressure Transducer. In theprior art, the ac output voltage signal from the signal generator to the bridge typically has an amplitude that is independent of sensor temperature.
Referring also to Figure 2, the normalized bridge output 18 as a func-tion of sensor temperature is shown. It may be seen that over a temperature range of 40F to 250F, the bridge output 18 can vary by as much as 50% for the same pressure differential applied across the sensor diaphragm. This tempera-ture effect must be accurately compensated if transducer accuracy over this tem-perature range is to be maintained within a few percent.
In the illustrated embodiment, the present invention modifies the ac output of the signal generator 14 such that the temperature compensation is made in the transducer activation or excitation signal, rather than in the transducer output signal 18. In effect, the amplitude of the ac output of the signal gen-erator is reduced according to the temperature of the sensor such that the acti-vation voltage varies inversely with the temperature dependence of the material in the sensor.
Figure 3 shows the voltage output of the inventive circuit, VF, as a function of sensor temperature; the output VT of the compensating circuit is in-put to the signal generator 14 of the transducer system. It may be seen by com-paring Figure 3 with Figure 2, that at a given temperature the product of thetwo normalized curves is 1.00, which in effect removes the temperature depen-dence from the normalized bridge output signal 18. Thus, the bridge output will be the same for the same pressure differential at any temperature between 40F
and 250F.
It should be appreciated that although the normalized bridge output shown in Figure 2 is a smooth curve having a smooth transltion in temperature through points Tl, T2, T3, T4, the compensation curve shown in Figure 3 is piece-wise linear between Tl, T2, T3, and T4. The number of piece-wise linear approxi-~ .
.
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.
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mations required to compensate the inherent temperature dependence of the sensor is determined by the degree of accuracy required and the curvature of the tem-perature dependence of the sensor. For the sensor behavior represented in Fig-ure 2, it has been found that a three segment piece-wise linear approximation is sufficient. In connection with the fol-.
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lowing description of the inventive circuit, it should be appreciated that the number of legs on the variable conductance ladder, which pro-vides the piece-wise compensation, will depend on the judgment of the designer and the accuracy desired.
Referring again to to Figure 1, there is shown a temperature compensated voltage VT which is the output of the inventive compen-sating circuit 16 and an input to the signal generator 14. The vol-tages to be described in connection with the inventive compensating circuit 16 are relative to a common of the signal generator 14 and bridge portion 10 of the transducer system. The compensating cir-cuit includes a current source 26 maintained at substantially the same temperature as the sensor element (not shown) and having an out-put Io that is linear with temperature. Such a device is commerciallY
available as, for example,'LM-134 from the National Semiconductor Company or part'AD-590 from the Analog Devices Corporation. This current source 26 is preferably located as close as possible to the sensor element. A suitable current source provides one micro-ampere change in current per ~K change ;n temperature.
A vol~age source 20, preferably in the range of 5 to 10 volts, volts, provides a base or reference voltage Vo corresponding to the base or reference output of the current source at the calibration tem-perature. The circuit is initially calibrated so that at 40F and with a corresponding source current of about 278 micro-amperes, the output voltage VT is exactly equal to the source voltage Vo. This is done by proper choice of resistance R7, or by providing a programmable resistance Pl which can be adjusted to force VT to equal Vo at the calibration temperature.
The resistor Rl is connected with the variable conductance ladder 24, illustrated in the form of a resistive diode matrix R2, R3, R4, R5 and D1, D2, D3, to form a Norton divider at 22. Rl is con-nected to the voltage source 20 and the positive input of the opera-tional amplifier Al, and the ladder is connected to the negative input of the amplifier.
The operational amplifier A1 performs the following opera-tion:
T = Vo - Il x (R6 ~ P2) ` where R6 and P2 will be explained below. Il is the fractional current which passes through the variable conductance ladder 24.
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; At ~he calibration temperature, e.g., 40F, VT-Vo, Io pas-ses through Rl and R2, and Il = O as controlled by R7 and Pl. The diodes Dl, D2 and D3 are non-conducting. As the temperature of the sensor increases and the current source Io increases, an increasing amount of current flows through resistor R2, which is a constant frac-tion o~ the current Io. According to the rule of the operational amplifier as described above, as Il increases an ever increasing vol-tage is subtracted from Vo so that the compensating circuit output volta~e VT decreases as Io increases.
Referring now to Figures 1 and 3, the VT has a constant slope between 40F and 150F. As the voltage across R2 increases to about 0.7 volts, diode Dl becames conducting and resistor R3 is added to the circuit. As Io continues to increase, voltage VT follows the linear relationship represented between the points T2 and T3. Like-wise, when the voltage across D2 reaches about 0.7 volts, resistantR4 comes into operation and the sequence continues for as many legs of the ladder are necessary to satisfactorily model the temperature behavior of the sensor material. The variable conductance ladder 24 therefore has a current output that is piece-wise linear with increas-ing current Io from the current source 26. The piece-wise linearity is programmed into the ladder on the basis of the information known `` to the designer from Figure 2. This information is ideally obtained from measurements on the uncompensated transducer system 10, 14, but can also be satisfactorily estimated from published data on the ma-terial properties of the particular sensor material.
It may be appreciated that in a typical nuclear power plant : dozens of nominally identical differential pressure transducers may be required. It would be desirable that temperature compensation in each of the transducers be as uniform as poss;ble. The preferred embodi-ment of the present invention provides features that permit easy calibratian whereby each transducer may be individually adjusted to have the same reference conditions as the other transducers. For example, all compensating circuits 16 can be adjusted to prov;de the ;` same output voltage VT at 40F. Or, each compensating circuit 16 can be adapted so that its output voltage VT at the reference condition will match the nominal ac output voltage of the signal source 14. In effect, the output voltage VT at the reference conditirn, e.g., 40F, .
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can be raised Gr lowered by the first programmable resistant P1 con-` nected to the variable conductance ladder 2~. Pl removes a fixed - amount of current from the ladder independent of the strength of the current source Io. This way, individual differences in the amount of current produced at 40F, for different current sourGes 26, can be offset to provide the same output voltage VT in each compensating cir-cuit at 40F.
Another adjustment wh;ch can easily be made with the pre-ferred embodiment of the invention is a gain adjustment on the slope of the piece-wise linear segments shown in Figure 3. It should be ap-` preciated that a shipment of transducer systems may all have the same specifications on the diaphragm thickness, for example, but variations will in practice occur. These varia~ions can be accounted for by a second programmable resistance P2 connected between the compensating circuit output VT and the variable conductance ladder 24 whereby the amplifier signal is fed back through the second programmable resis-tance P2. This adjustment is a ratio adjustment in which each of the slopes shown in Figure 3 is adjusted by a constant factor.
The preferred embodiment of the invention has been described in which the temperature dependence of the stress/strain relationship of a metal diaphragm is to be electronically compensated. The inven- -tion may be used in any system wherein Hook's l.aw or an analogue thereof is the material property forming the basis of the desired mea-surement, but where compensation for the variability of the tempera-ture is desired. The details of providing specific values for the circuit devices disclosed in the preferred embodiment, or construct-ing an equivalent circuit, will be obvious to one ordinarily skilled in this art. Likewise, the use o~ the invention in a totally resis-tive transducer system wherein a dc signal generator may be employed, or in modifying the transducer system output rather than the input, will be evident to the ordinary practitioner.
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Claims (8)
1. An electrical circuit for compensating the temperature dependence of a sensor element in a transducer system, the system having a signal generator for activating the sensor element, com-prising:
a current source maintained at substantially the same tem-perature as the sensor element and having an output that is linear with temperature;
a temperature-independent reference voltage source;
a variable conductance ladder connected as one branch of a Norton divider having a fractional input current from said current source and a current output that is piece-wise linear with increasing current from said current source;
an operational amplifier connected between the variable con-ductance ladder and the voltage source, and having an output which is the difference between the reference voltage and a voltage which is proportional to the fractional current passing through the variable conductance ladder;
whereby the output voltage of the amplifier varies inversely in accordance with the temperature dependent properties of the sensor.
a current source maintained at substantially the same tem-perature as the sensor element and having an output that is linear with temperature;
a temperature-independent reference voltage source;
a variable conductance ladder connected as one branch of a Norton divider having a fractional input current from said current source and a current output that is piece-wise linear with increasing current from said current source;
an operational amplifier connected between the variable con-ductance ladder and the voltage source, and having an output which is the difference between the reference voltage and a voltage which is proportional to the fractional current passing through the variable conductance ladder;
whereby the output voltage of the amplifier varies inversely in accordance with the temperature dependent properties of the sensor.
2. The compensating circuit of Claim 1 further comprising a first resistance connected to the variable conductant ladder for removing a fixed amount of current from the ladder independent of the current source.
3. The compensating circuit of Claim 1 further comprising a second resistance connected between the amplifier output and the lad-der whereby the amplifier signal is fed back through the second resis-tance.
4. The compensating circuit of Claim 1 further comprising a first programmable resistance connected to the variable conductance ladder for removing a fixed amount of current from the ladder inde-pendent of the strength of the current source.
5. The compensating circuit of Claims 1 or 4 further com-prising a second programmable resistance connected between the ampli-fier output and said ladder whereby the amplifier signal is fed back through the second programmable resistance.
6. The compensating circuit of Claim 1 wherein the variable conductance ladder comprises a resistive diode matrix.
7. The compensating circuit of Claim 3 wherein the Norton divider comprises resistance R1 connected at one end to the current source and at the other end to the voltage source and to one input of the amplifier, and a resistive diode matrix connected in parallel with R1 between the current source and the other input to the opera-tional amplifier.
8. The compensating circuit of Claim 1 wherein the ampli-fier output is connected to a signal generator for activating the sensor element.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| PCT/US1980/001000 WO1982000519A1 (en) | 1980-08-04 | 1980-08-04 | Temperature compensation for transducer components |
| USPCT/US80/01000 | 1980-08-04 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1150525A true CA1150525A (en) | 1983-07-26 |
Family
ID=22154463
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000381578A Expired CA1150525A (en) | 1980-08-04 | 1981-07-13 | Temperature compensation for transducer components |
Country Status (4)
| Country | Link |
|---|---|
| JP (1) | JPS57501043A (en) |
| CA (1) | CA1150525A (en) |
| SE (1) | SE439837B (en) |
| WO (1) | WO1982000519A1 (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN103048085A (en) * | 2011-10-13 | 2013-04-17 | 江苏恩泰传感器有限公司 | Temperature compensation system of pressure sensor and temperature compensation method thereof |
Families Citing this family (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE3817098A1 (en) * | 1988-05-19 | 1989-11-30 | Dieter Dipl Ing Bohn | Method for the electrical representation of a physical measured variable in the form of an impedance change |
| DE19951817A1 (en) * | 1999-10-27 | 2001-05-23 | Micronas Gmbh | Two-wire sensor arrangement |
Family Cites Families (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3841150A (en) * | 1973-11-02 | 1974-10-15 | Honeywell Inc | Strain gauge transducer signal conditioning circuitry |
| US4011758A (en) * | 1973-12-26 | 1977-03-15 | Texas Instruments Incorporated | Magnetostrictive pressure transducer |
| JPS587182B2 (en) * | 1974-03-08 | 1983-02-08 | 横河電機株式会社 | Saatsu Oudousouchi |
| US4000643A (en) * | 1976-03-29 | 1977-01-04 | Honeywell Inc. | Apparatus for producing a compensating voltage |
| US4233848A (en) * | 1978-01-06 | 1980-11-18 | Hitachi, Ltd. | Strain gauge pressure transducer apparatus having an improved impedance bridge |
-
1980
- 1980-08-04 JP JP50161380A patent/JPS57501043A/ja active Pending
- 1980-08-04 WO PCT/US1980/001000 patent/WO1982000519A1/en unknown
-
1981
- 1981-07-13 CA CA000381578A patent/CA1150525A/en not_active Expired
-
1982
- 1982-03-31 SE SE8202060A patent/SE439837B/en not_active IP Right Cessation
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN103048085A (en) * | 2011-10-13 | 2013-04-17 | 江苏恩泰传感器有限公司 | Temperature compensation system of pressure sensor and temperature compensation method thereof |
Also Published As
| Publication number | Publication date |
|---|---|
| SE8202060L (en) | 1982-03-31 |
| JPS57501043A (en) | 1982-06-10 |
| WO1982000519A1 (en) | 1982-02-18 |
| SE439837B (en) | 1985-07-01 |
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| Date | Code | Title | Description |
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| MKEX | Expiry |