Measuring and testing – Fluid pressure gauge – With pressure and/or temperature compensation
Patent
1980-11-06
1982-10-26
Woodiel, Donald O.
Measuring and testing
Fluid pressure gauge
With pressure and/or temperature compensation
73766, 7386263, G01L 1904
Patent
active
043555373
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to temperature compensation for a transducer device, and more particularly to electronically compensating 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 effects of temperature be accounted for so that the pressure measurement itself is not temperature dependent. In one kind of differential pressure transducer, 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 generator. 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 through the bridge may be displayed or recorded as a pressure differential.
The displacement of the diaphragm is ideally proportional to the pressure difference between the chambers of the transducer device. 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 temperature of the diaphragm. This material property of the diaphragm must be compensated or accounted for if a high degree of transducer accuracy is required.
Particularly when used in nuclear power plants, differential pressure transducers should be accurate to within .+-.1% over a temperature range of about 40.degree. F. to 250.degree. F. This kind of accuracy is not obtainable with transducers currently available commercially. Although commercially available transducers are capable of compensating for temperature effects arising in the electronic circuit itself, i.e., in the diodes and transistors, it is believed that no satisfactory means have previously been found for specifically compensating 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 element, 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 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 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 between 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 dependence 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. In the preferred embodiment
REFERENCES:
patent: 3841150 (1974-10-01), Pearson
patent: 3995493 (1976-12-01), Nishihara
patent: 4000643 (1977-01-01), Pearson
patent: 4011758 (1977-03-01), Reenstra
patent: 4233848 (1980-11-01), Sato
Combustion Engineering Inc.
Ristas L. James
Woodiel Donald O.
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