Wheatstone bridge with temperature gradient compensation

Details Wheatstone bridge with temperature gradient compensation Wheatstone bridge with temperature gradient compensation

1998-06-03

2000-08-08

Noori, Max

Measuring and testing

Fluid pressure gauge

With pressure and/or temperature compensation

73720, 324720, 324648, G01L 1904, G01L 904, G01R 2704, G01R 2708

Patent

active

060984643

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to an electrical circuit of "Wheatstone bridge" type and, more particularly, to such an electric circuit in which there is an automatic compensation for temperature differences existing between the branches of the bridge.
Electrical circuits of Wheatstone bridge type are well known. They are used in many different applications in order to produce an output voltage the variation of which indicates the existence and magnitude of an imbalance between the resistances in the branches of the bridge. Normally, circuits of this type are designed in order to detect imbalances due to the variation of a main parameter with respect to a reference value or due to phenomena which have a different effect on the respective resistances in the different branches of the circuit. Evidently, it is preferable that the behaviour of the circuit should not be affected by other parameters or phenomena such as, for example, ambient temperature.
In theory, in the case of a classic Wheatstone bridge such as is shown in FIG. 1, the output voltage U.sub.S, depends on the resistances R.sub.1 to R.sub.4 of the bridge, and on the supply voltage, U.sub.A, in accordance with equation (1) below: ##EQU1##
In practice, the resistances of the elements R.sub.1 to R.sub.4 vary as a function of the ambient temperature and of the temperature coefficient of resistance (TCR), .alpha., when the latter is not negligible. If there is an offset, .theta..sub.i, between the temperature of each element R.sub.i and a reference temperature, the theoretical equation (1) is transformed into the equation (2) below: ##EQU2## (In equation (2) and those that follow, the temperature coefficient of resistance, .alpha., is treated as if it were identical for the four elements of the Wheatstone bridge. In practice, it is relatively simple to select the bridge elements so that this simplification is approximately true.
In the case where the four elements R.sub.i of the Wheatstone bridge are all at the same temperature, .theta., and the supply voltage is fixed, assuming that .alpha..theta.<<1, equation (2) is simplified and becomes identical to equation (1). Thus, in such a case, the ambient temperature scarcely affects the behaviour of the Wheatstone bridge. (It is possible, however, that there will be a residual temperature effect on the bridge due, for example, to deformations of the mechanical support on which the differences are located).
In the case where the elements R.sub.i of the Wheatstone bridge are not all at the same temperature, equation (2) conserves certain terms, whose value depends on the local temperature of the elements R.sub.i. This effect erodes the precision of the measurement made using the circuit.
The principle of the Wheatstone bridge is used, amongst others, in the field of pressure sensors using strain gauges.
A typical construction of such a pressure sensor is shown diagrammatically in FIG. 2, in which FIG. 2(a) indicates the physical disposition of the strain gauges in the sensor and FIG. 2(b) shows the corresponding electrical circuit. In the sensor of FIG. 2(a), four strain gauges, J.sub.1 to J.sub.4, are disposed on a deformable membrane, two gauges J.sub.1, J.sub.2 are in the central region of the membrane and the other two gauges J.sub.3, J.sub.4 are towards the periphery of the membrane. The strain gauges each have the same electrical resistance, R, at a given temperature and reference pressure. Electrical connections (not shown) are provided between the gauges J.sub.1 to J.sub.4 and a power supply and output terminals, so as to form a Wheatstone bridge circuit such as shown in FIG. 2(b).
In such a pressure sensor, the membrane deforms by curving outwards under the effect of a pressure applied in the direction of the arrow shown in FIG. 2(a). The strain, due to traction, produced in the central portion of the membrane leads to an increase, +.DELTA.R, in resistance of the gauges J.sub.1 and J.sub.2, while compression strain is produced in the periphery of the membrane leading to a reduction, -.DELTA.R

REFERENCES:
patent: 3665756 (1972-05-01), Russell
patent: 4174639 (1979-11-01), Raven
patent: 4444056 (1984-04-01), Romo
patent: 4565097 (1986-01-01), Dimeff
patent: 4765188 (1988-08-01), Krechmery et al.
patent: 5375474 (1994-12-01), Moore, Sr.
patent: 5877423 (1999-03-01), Mottsson

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