Note: Descriptions are shown in the official language in which they were submitted.
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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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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
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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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