Effects of electrostatic forces generated by the driving signal on capacitive sensing devices

In measuring the capacitance of a variable mechanical capacitor used in a capacitive mechanical sensor, an electrical driving signal is usually needed. The electrostatic forces caused by the driving signal on the mechanical capacitor may interfere with the measurement and the normal operation of the devices significantly. In this paper, quantitative analyses on the effects of driving signal are made for single-sided driving, double-sided driving and double-sided driving with voltage feedback (i.e., force-balanced measurement schemes). The effects caused by the driving signal are found to be: (1) the zero offset of the sensors for single-sided driving signal, (2) the change of the measurement sensitivity, and (3) the reduction of the critical measurand signal level causing the pull-in effect that hampers the normal. operation of the device. The levels of critical measurand signal for specific driving signal levels are found quantitatively. Based on the analyses, the conclusions are: (1) the level of driving signal can be selected by the compromise among the requirements on the sensitivity, the accuracy and the reliability of the sensors devices for a specific configuration, (2) the side effects of the driving signal can be minimized by using the testing scheme of double driving with voltage feedback. (C) 2000 Elsevier Science S.A. All rights reserved.