Simultaneous determination of changes in resistance and temperature in bridge circuits

The new concept of evaluation for bridge circuits offers the possibility of simultaneous determination of the change in resistance and temperature without the use of additional temperature sensors.
An offset can be directly compensated, which pays off especially in areas, where high accuracy is required.

• High-precision output signals of bridge circuits without additional temperature sensor
• Simultaneous determination of changes in resistance and temperature / ΔR and ΔT
• Immediate determination and compensation of temperature influence
• No adjustment of bridge circuit required
• Suitable for quarter, half and full bridges
• Suitable for current or voltage sources

At Karlsruhe University of Applied Sciences, a concept for evaluating bridge circuits was developed which enables the simultaneous determination of changes in resistance and temperature. This novel concept does not need any additional temperature sensors, because the bridge circuit is also used as temperature sensor – an advantage that is particularly beneficial for high accuracy requirements (such as in sensor technology).

Bridge circuits (bridges) are often used to determine a change in resistance, which represents a quantity to be measured. Their working principle is simple. They are therefore used in numerous applications, e.g. to determine physical parameters (in sensors for pressure, force, mag­netic field via AMR/ GMR effect). As resistances and especially changes in resistance (respectively sensor effects) are temperature-dependent, it is necessary to know the temperature of the bridge in order to determine the measured variable as accurately as possible. The sup­pression of a possible offset of output voltages is also desirable.

There are various solutions for balancing and compensat­ing temperature-induced offsets in circuits. Measuring errors can be reduced by adding temperature sensors, characteristic curves or calculation methods. However, these always require additional effort. In addition, know­ledge of the temperature is important for condition monito­ring in some applications.

The concept, which is based on conventional bridge cir­cuits, allows the simultaneous calculation of resistance and temperature changes in a bridge circuit by combining dif­ferent data, such as temperature coefficients (TC) and volt­ages of the bridge circuit. The method used describes two variants, with (1) or without (2) an additional resistor R5 (see figure). A temperature sensor is not required. Both current and voltage sources can be used for supply, depending on certain conditions, such as resistance values or TC of individual resistors. Variants 1 and 2 are possible as full, half or quarter bridges. By means of a micro-con­troller, the pure change in resistance and also the temper­ature can be calculated simultaneously and accurately. An adjustment of the bridge circuit is not necessary.