  We explore the undamped response of step-voltage driven parallel plates and
fixed-fixed microbeams. We consider a third order correction in the parallel
plate system as a reduced order system to model mid-plane stretching in
fixed-fixed beams. In both cases, we employ energy methods to determine the
threshold values displacement and voltage. This is the point beyond which there
is no oscillatory motion. The dynamic pull-in parameters of the corresponding
microactuator model are identified as these critical values. We thoroughly
analyze the effects of mid-plane stretching in fixed-fixed microbeams.
Additionally, the phase portraits for these actuators are also thoroughly
examined. Empirical relations for the oscillation period/switching time for
these microactuators are also developed. These results correlate to a high
accuracy to previously published results.
