GB2107876A - Temperature compensation of strain gauges - Google Patents

Abstract

A strain gauge comprises a silicon layer 12 with doped areas R1, R2, R3 and R4 forming piezo-resistive devices connected as a bridge whose imbalance is a measure of strain. A second similar silicon layer with similar piezo-resistive devices R5, R6, R7 and R8, is pre-strained by a wedge 21 to a fixed extent. The two bridges R1 to R4 and R5 to R8 are connected in a common circuit so that the temperature response of bridge R5 to R8 compensates for variations with temperature in the response of bridge R1 to R4. <IMAGE>

Description

SPECIFICATION Strain gauges The invention relates to strain gauges. Some forms of strain gauge such as those based on piezo-resistive semiconductor devices exhibit desirable high sensitivity but undesirable sensitivity to temperature variations. With silicon strain gauges both the resistance and the gauge factor, that is the slope of the strain/resistance characteristic, vary with temperature. Attempts have been made to balance variations in one of these factors against variations in the other but this form of compensation is effective over only a narrow temperature range.

An object of the present invention is to provide effective compensation for variations in temperature in a strain gauge based on temperature sensitive strain sensing elements.

According to the present invention there is provided a strain gauge comprising a first carrier to be subjected to strain, a set of temperature sensitive strain sensing elements interconnected to form an electrical measuring bridge formed integrally with the carrier and arranged so that the degree of imbalance of the measuring bridge varies with the strain of the carrier, a second carrier and set of compensating bridge elements formed integrally on the second carrier and held in a state of strain and maintained at substantially the same temperature as the first carrier, and a temperature compensation circuit incorporating both bridges and such that variations with temperature in the imbalance of the second bridge are arranged to compensate for variations with temperature in a strain indicating signal derived from the first bridge.

Preferably the carriers are formed from a common piece of material to facilitate matching of temperature sensitivity in the two bridges. The carriers may be interconnected by an integral neck which unites both carriers and maintains substantial temperature equilibrium between them but transmits little or no strain from one bridge to the other.

Preferably the carriers are formed of silicon and the brdge elements are formed on the carriers by selectively doping parts of the silicon. Both carriers may be formed from a common slice from a silica crystal.

In a preferred electrical circuit, both bridges are connected in series with each other, and a current control element to a potential source, a feedback loop from the output of the second bridge to the current control element maintains a constant control output to the second bridge despite changes in its temperature, the controlled current through the measuring bridge thus influencing its output in such a way as to compensate for variations in temperature. The current control element is preferably a transistor.

The constant output from the compensating bridge may be obtained by connecting these outputs to the bases of two transistors, the emitters of which are held at a constant differential potential and the collector of one of which provides a control current to the base of the current control element resistor.

An embodiment of the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a schematic representation of a piezo-resistive electro-mechanical device which forms the basis of a strain gauge in accordance with the invention; and Figure 2 is an electrical circuit diagram of a complete strain gauge, incorporating parts of the device of Figure 1.

A piezo-resistive strain sensing device 11 incorporates a silicon layer 12 secured to a support substrate 13. The strain to be measured in this example is bending of the combined substrate and silicon layer, which must of course be connected to a structure to be strain gauged in a suitable way. The silicon layer 12 incorporates four doped areas R1, R2, R3 and R4. The doping is such as to produce four resistive elements, the resistance of which varies with strain applied to these elements. In this example, two of the elements, R1 and R3, are in such an orientation and position that they have a resistance change with strain such as to curve the whole device so as to bend two opposite edges 14 and 1 5 but to leave edges 15 and 1 6 straight.In this bending mode, which is the mode intended for normal use of the strain gauge, R2 and R4 have an orientation and position which gives an opposite change of resistance. The device thus far described could have its resistive elements incorporated in a bridge circuit and be used as a strain gauge. It would work effectively at constant temperature but due to the temperature sensitivity of doped silicon would be impractical for use in an environment of varying temperature.

To overcome this deficiency, a compensating device is provided. The compensating device 1 6 comprises a silicon layer 1 7 with the same properties as the silicon layer 12. This is achieved by producing the two silicon layers from the same slice off a silicon crystal and in this example the two layers remain joined together by an integral neck 18 which is part of the same silicon slice as the two layers 12 and 17. The silicon layer 17 has four doped areas R5, R6, R7 and R8 similar to the doped areas R1, R2, R3 and R4 of layer 12. As with the first mentioned doped layers, R5 and R7 are strained in one bending mode of the layer 17, while R6 and R8 remain substantially unstrained.

Part of the silicon layer 1 7 is separated from its substrate 1 9 and is strained permanently by insertion of a wedge 21 between the substrate and the silicon layer. As illustrated, the bending mode matches the bending mode in which strain is applied to the silicon layer 12 of the strain sensing device. This strain alters the resistance of the doped areas and conditions these doped areas to take on temperature response characteristics corresponding to those of a strain sensing device.

While the neck 1 8 is sufficiently strong to retain the tqo silicon layers 12 and 17 together as a single physical unit, its position in relation to the mode of applied strain is such that it transmits little or no strain from one silicon layer to the other.

To provide a complete strain gauge with temperature compensation, the doped areas R1 to R8 are incorporated in two bridge networks in the electrical circuit of Figure 2. A sensor bridge SB is made up of doped areas R1 to R4, with like doped areas such as R1, R3, forming opposite arms of the abridge. A compensating bridge CB is made up of doped areas R5 to R8 with like doped areas such as R5, R7, forming opposite arms.

The two bridges CB and SB are connected in series with each other and with a current control element to a potential source. The current control device is the emitter/collector path of a transistor TR 1, the base of which acts as a current control input.

An output signal representing the strain in the sensing device 11 is derived from external connections to the sensor bridge SB on the opposite diagonal from the supply to the bridge.

Variations in strain on the sensing device vary the imbalance of the bridge and give an output signal indicative of strain. Temperature compensation is achieved in a way which will be described subsequently.

The diagonals of the compensating bridge CB, other than the supply connections, are connected to the bases of two transistors TR2 and TR3. The emitters of the transistors TR2 and TR3 are inter connected by a constant voltage device V. The collector of TR2 is connected to the potential source. The collector of TR3 is connected to the base of TR 1 while the emitter of TR3 is connected through a resistor R9 to ground. The transistors TR2 and TR3 and associated components from a feedback loop to control the current through TR1 and bridges CB and SB as to maintain a constant differential potential between the connections of bridge CB to the bases of TR2 and TR3.In further detail, because the emitter of TR2 and TR3 are held at a constant differential potential, base current flows to maintain the same differential potential between the bases and this in turn controls collector current of, in particular, TR3.

The collector current of TR3 is the base current of TR1, so the current through TR1 to the two bridges is automatically adjusted to provide the required differential potential between the connections from the bridge CB to the bases of transistors TR2 and TR3.

This arrangement results in temperature compensation for the sensor bridge in the following manner. If the temperature of the sensor bridge varies, which would alter its output signal, the temperature of the compensating bridge varies similarly, tending to cause a variation in the potential between the bases of TR2 and TR3. The feedback loop sets the current control device TR 1 to reset the series current through the bridges to a value which gives the required potential at the bases of TR2 and TR3. This variation in current applies to the sensor bridge SB as well as the compensating bridge CB (ignoring the small base currents) so the current flow in the sensor bridge is adjusted to the same extent as the current adjustment in the compensation bridge which causes complete temperature compensation in the compensation bridge. This current adjustment also provides good temperature compensation in the sensor bridge over a wide range of temperatures.

The embodiment described is intended for use as a pressure transducer and a diaphragm area is indicated by dotted line D.

Claims (11)

Claims

1. A strain gauge comprising a first carrier to be subjected to strain, a set of temperature sensitive strain sensing elements interconnected to form an electrical measuring bridge formed integrally with the carrier and arranged so that the degree of imbalance of the measuring bridge varies with the strain of the carrier, a second carrier and set of compensating bridge elements formed integrally on the second carrier and held in a state of strain and maintained at substantially the same temperature as the first carrier, and a temperature compensation circuit incorporating both bridges and such that variations with temperature in the imbalance of the second bridge are arranged to compensate for variations with temperature in a strain indicating signal derived from the first bridge.

2. A strain gauge as claimed in claim 1 wherein the carriers are formed from a common piece of material to facilitate matching of temperature sensitivity to the two bridges.

3. A strain gauge as claimed in claim 1 wherein the carriers are interconnected by an integral neck which unites both carriers and maintains substantial temperature equilibrium between them but transmits little or no strain from one bridge to the other.

4. A strain gauge as claimed in any preceding claim wherein the carriers are formed of silicon and the bridge elements are formed on the carriers by selectively doping parts of the silicon.

5. A strain gauge as claimed in claim 4 wherein both bridge carriers are formed from a common slice from a silicon crystal.

6. A strain gauge as claimed in any preceding claim wherein both bridges are connected in series with each other and a current control element to a potential source and wherein a feedback loop from the output of the second bridge to the current control element maintains a constant control output at the second bridge despite changes in its temperature, the controlied current through the measuring bridge thus influencing its output in such as way as to compensate for variations in temperature.

7. A strain gauge as claimed in claim 6 wherein the current control element is a transistor.

8. A strain gauge as claimed in claim 7 wherein a constant output from the compensating bridge is obtained by connecting these outputs to the bases of two transistors, the collectors of which are held at a constant differential potential and the emitter of one of these transistors provides a control current to the base of the current control element resistor.

9. A strain gauge substantially as described with reference to and as illustrated by the accompanying drawings.

10. A pressure transducer having a strain gauge as claimed in any preceding claim arranged to provide an electrical signal which varies with pressure.

11. A pressure transducer as claimed in claim 10 wherein the first carrier acts as a pressure sensitive diaphragm.