A study of the piezoresistive effect in thick-film resistors and its application to load transduction

Inexpensive, accurate, robust transducers have been sought for a long time, particularly for load sensing applications. Most attempts to produce such a device have suffered from significant trade offs, especially between cost and accuracy. The two main factors which contribute to the relatively high cost of loadcells are the cost of the primary sensor and the cost of machining the billet to reduce effects due to the eccentricity of loading. A new type of strain sensor based on the piezoresistive properties of thick-film resistors is described. Characterisation of the sensing element has revealed that the gauge factor is significantly higher than that of a metal foil strain gauge and the temperature coefficients are lower than those found for a semiconductor strain gauge. Results show how the gauge factor can be optimised by varying the production parameters. A loadcell based on thick-film strain gauges is also described. The design is such that the problems of eccentric loading are virtually eliminated. The technology also allows the associated electronics to be incorporated into the sensor housing.