R. Kumar, C. Compton, R. Short

Rahul is preparing to swap the bottom stage of ZM5 due to a misbehaving strain gauge (see alog90802 and others for details). To "prove to ourselves it's actually broken" and to document how it behaves when broken, Rahul and I decided to exercise the PZT for the ZM5 PSAM by stepping it up from 0V. With the strain gauge servo off, we used the offset slider to step the PZT to 10V, 20V, 50V, then 100V with a 10 second ramptime and watched the strain gauge signal (see first half of time on the attached ndscope screenshot). The strain gauge readback responded as expected until we stepped to 50V, where it jumped back down, and at 100V the readback completely flatlined at -10V. Thinking we maybe made too big of a step, we repeated this process but only doing steps of 10V at a time. Again when reaching 50V on the PZT, the strain gauge jumped, this time all the way down to -9V readback.

This behavior is consistent with our leading theory of a loose contact or broken solder joint somewhere internal to the PZT/strain gauge assembly. As the PZT changes voltage, the mechanical change being made perhaps pulls the lead for the strain gauge loose or otherwise breaks contact with the circuit. We have confirmed "it's definitely broken" and we will move forward with the replacement.