experiments support the point of view that time delay can
decrease the damping with IOL. The original damping led to a
fast convergence time and small overshoots and IOL decreased
it, which took more than 0.6 s to reach the final state. The
PIESO increased the damping so that the number of overshoot
peaks was much less than IOL and it converges within 0.15 s.
Besides the time-delay effect, the measured initial gap is
not perfectly known, which cannot totally cancel the driven
force’s nonlinearity that also affects the feedback controller’s
performances.