Observations on the characteristics of thick film k₃₃ strain sensors fabricated on steel substratesThis paper is based on a poster presentation made at the IMAPS 2001 Symposium, Baltimore.

Abstract

Results are presented from a programme of research aimed at establishing the mechanisms behind the effects of fabrication parameter variation on the performance of thick film strain gauges on steel substrates. The research is aimed at describing the effect on the repeatability of the device characteristics due to different choices of materials, thicknesses of printed layers, firing regimes and geometry of the gauges. In particular the effects of load and temperature on the offset and gain characteristics of a variety of different sensor constructions have been explored. The sensors described here are of a type where the applied strain is parallel to the measured resistance path but orthogonal to the substrate (k33). It has been found that these devices exhibit different characteristics to conventional thick film strain gauges that can help explain the mechanisms affecting gain and offset changes caused by temperature fluctuations and mechanical deformation.

An explanation of thermal behaviour of thick film strain gauges

“…The results of the analysis of strain as a function of resistor thickness in the models employed here clearly show that relative levels of strain decrease with increasing resistor thickness. And it is acceptable to think that gauge factor does not vary much with resistor thicknesses due to the reasons previously noted in an earlier study [4,5]. Thus, it would be reasonable to expect that the thicker resistors would have more negative shaped TCRs than their thinner counterparts.…”

“…For example, T min is more likely to be located out of room temperature range when stainless steel is employed as the substrate, which may be due to the high TCE mismatch between the steel substrates and the thick film layer. This results in a nearly linear temperatureresistance curve achieved over the observed temperature range when thick film resistors are printed on steel substrates [5].…”

“…However it is reasonable to think that the results derived from this model are valid to show the trend in the case with ceramic substrates as well. The dimensions employed in the modelling were those typically adopted in most of the test work [5,7,8], especially with the K 33 devices described in the following section. Uniform temperature variation from 20˚C to 130˚C is the only load which is applied to the model.…”

“…An enormous amount of research has been undertaken to investigate the temperature characteristics of thick film strain gauges since the early reports of strain sensitivity in thick film resistors [1,2]. Previous work has shown that the resistance versus temperature curve is nearly parabolic [3], both for conventional X-plane strain gauges [4] and the novel Z-plane strain gauges [5].…”

On the zero offset stability of thick film strain gauges

“…Zero offsets of the tested devices with load cycles were obtained by using a purpose designed test jig that had been successfully employed in previous experiments (Zheng et al , 2001). This consisted of a steel frame supporting a reference load cell (Maywood U4000 series) onto which the substrate carrying the resistor test patterns could be placed.…”