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
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
  • G01L19/00—Details of, or accessories for, apparatus for measuring steady or quasi-steady pressure of a fluent medium insofar as such details or accessories are not special to particular types of pressure gauges
  • G01L19/04—Means for compensating for effects of changes of temperature, i.e. other than electric compensation

Definitions

• the invention relates to temperature compensation for a transducer device, and more particularly to electronically co pen- sating for the temperature dependence of the deformation properties of the sensor element of the transducer.
• differential pressure trans ⁇ ducer For many kinds of transducer devices, such as differential pressure transmitters, it is necessary that the effects of tempera ⁇ ture be accounted for so that the pressure measurement itself is not temperature dependent.
• a metal diaphragm is sealed between two chambers which 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 bridge 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 coupiing of the E cores.
• the diaphragm displacement is sensed by the transducer system as a change in reluctance, which through the bridge may be dis ⁇ played or recorded as a pressure differential.
• the displacement of the diaphragm is ideally proportional to the pressure difference between the chambers of the transducer de ⁇ vice.
• 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.
• the present invention provides an improvement over the prior art transducers in that the teiroerature dependence of the sensor ele ⁇ ment, such as the deformation characteristic of the diaphragm in a differential 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 temperature 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 betv/een the reference voltage of the voltage source and the voltage at the output of the variable conductance ladder. As the current source increases, the output voltage of the amplifier 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.
• the invention provides several advantages not available with known compensated transducers. Most importantly, the temperature depen ⁇ dence of the sensor element itself is accounted for by a piece-wise linear approximation which can be made as accurate as necessary by providing a sufficient number of sequential conductance paths in the variable conductance ladder.
• the ladder consists of diodes and resistances, which are extremely accurate in
• ⁇ T T their operation.
• 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.
• RTD's resistor- temperature devices
• the present invention when used in conjunction with other state-of-the-art transducer equipment, should permit this kind of accuracy.
• a first pro ⁇ grammable resistance is provided to remove a fixed amount of current from the variable conductance ladder whereby the circuit may be cal ⁇ brated to provide a known output voltage at any reference temperature.
• a second programmable resistance may be provided between the amplifier output and the variable conductance ladder for 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.
• FIG. 1 diagra atically 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.
• FIG. 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.
• FIG. 1 diagramatically shows a transducer system including a reluctance resistance bridge 10 having coils 12, such..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 AC excitation to the bridge 10.
• Such a bridge and signal generator arrangement, or equivalents thereof for pur ⁇ poses of the invention are more fully described in several prior art references, including U.S. Patents 3,995,493 Differential Pres-
• the ac output voltage signal from the signal generator to the bridge typically has an amplitude that is independent of sensor temperature.
• the normalized bridge output 18 as a function of sensor temperature is shown. It may be seen that over a temperature range of 40°F to 250°F, the bridge output 18 can vary by as much as 50% for the same pressure differential applied across the sensor diaphragm. This temperature effect must be ac- curately compensated if transducer accuracy over this temperature range is to be maintained within a few percent.
• the present invention modi ⁇ fies the ac output of the signal generator 14 such that the tempera ⁇ ture compensation is made in the transducer activation or excitation signal, rather than in the transducer output signal 18.
• the amplitude of the ac output of the signal generator is reduced according to the temperature of the sensor such that the activation voltage varies inversely with the temperature dependence of the ma ⁇ terial in the sensor.
• Figure 3 shows the voltage output of the inventive circuit
• the normalized bri-- dge output shown in Figure 2 is a smooth curve having a smooth tran ⁇ sition in temperature through points Tl , T2, T3, T4,
• the compensa ⁇ tion curve shown in Figure 3 is piece-wise linear between Tl, T2, T3, and T4.
• the number of piece-wise linear approximations required to compensate the inherent temperature dependence of the sensor is determined by the degree of accuracy required and the curvature of the temperature dependence of the sensor. For the sensor behavior represented in Figure 2, it has been found that a three segment piece- wise linear approximation is sufficient.
• a temperature compensated voltage 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 output Io that is linear with temperature.
• Such a device is commer ⁇ cially available as, for example, LM-134 from the National Semicon- ductor 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 in temperature.
• a voltage source 20 preferably in the range of 5 to 10 volts, provides a base or reference voltage Vo corresponding to the base or reference output of the current source at the calibration temperature.
• the circuit is initially calibrated so that at 40°F and with a corresDonding source current of about 278 micro-amperes, ' the output voltage V is exactly equal to the source voltage Vo. This is done by proper choice of resistance R7, or by providing a programmable resistance P] which can be adjusted to force V ⁇ to equal Vo at the calibration temperature.
• the resistor R is connected with the variable conductance ladder 24, illustrated in the form of a resistive diode matrix Rgs R 3 , R 4 , R 5 and D-j , D2 impart D3, to form a Morton divider at 22.
• R-j is connected to the voltage source 20 and the positive input of the operational amplifier A], and the ladder is connected to the negative- input of the amplifier.
• the operational amplifier A] performs the following operation: where R ⁇ and P2 will be explained below.
• I] is the fractional cur ⁇ rent which passes through the variable conductance ladder 24.
• the V- has a constant slope between 40°F and 150°F.
• diode D becomes conducting and resistor R 3 is added to the circuit.
• voltage V ⁇ follows the linear relationship represented between the points T 2 and T .
• resistant R 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.
• each transducer may be individually adjusted to_ have the same reference conditions as the other transducers.
• all compensating circuits 16 can be adjusted to provide the same output voltage V ⁇ at 40°F.
• each compensating circuit 16 can be adapted so that its output voltage V ⁇ at the reference condition will match the nominal ac output voltage of the signal source 14.
• the output voltage V ⁇ at the reference condition e.g., 40°F
• P- the first programmable resistant P, con ⁇ nected to the variable conductance ladder 24.
• P- 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 40°F, for different current sources -26, can be offset to provide the same output voltage V ⁇ in each compensating cir ⁇ cuit at 40°F.
• Another adjustment which 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 variations can be accounted for by a second programmable resistance P 2 connected between the compensating circuit output V ⁇ and the variable conductance ladder 24 whereby the amplifier signal is fed back through the second programmable resis ⁇ tance P 2 - 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 Law 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.
• the use of 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.

Landscapes

• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Measuring Fluid Pressure (AREA)
• Indication And Recording Devices For Special Purposes And Tariff Metering Devices (AREA)

Priority Applications (4)

Applications Claiming Priority (2)

Publications (1)

Family

ID=22154463

Family Applications (1)

Country Status (4)

Cited By (2)

Families Citing this family (1)

Citations (5)

• 1980
  • 1980-08-04 JP JP56501613A 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/sv not_active IP Right Cessation

Patent Citations (5)

Also Published As

Similar Documents

Legal Events