Closed-loop Control of a Parallel-plate Microactuator beyond the Pull-in Limit

In this paper, we present a controller design for servoing the position of a parallel-plate electrostatic microactuator beyond its open-loop instability point. Controller design considers nonlinearities from both the parallel-plate actuator and the parallel-plate position sensor, to ensure robust stability within the feedback loop. Desired transient response is achieved by a pre-filter added in front of the feedback loop to shape the input command. The microactuator is characterized by static and dynamic measurements, with a spring constant of 0.17 N/m, mechanical resonant frequency of 12.4 kHz, and effective damping ratio from 0.55 to 0.35 for gaps between 2.3 to 2.65 μm. The minimum input-referred noise capacitance change is measured at a gap of 5.5 μm, corresponding to a minimum input-referred noise displacement of 0.2 nm/√Hz. Results of the servo test show excellent agreement with design specifications (Rise time < 2 ms, overshoot = 0, and settling time < 5 ms) for the intended application as a magnetic probe tip actuator for data storage. Actuator displacement servoed as far as 55% of the gap is measured, which surpasses the static pull-in limit of one-third of the gap.