This work describes the use of resonant circuits for electrostatic actuation of MEMS devices with low voltage drive electronics. Resonant drive also offers a solution for position sensing and generates a sense signal without the need for a separate sense capacitor. Moreover, an inherent force feedback mechanism limits the drive voltage as the drive capacitor becomes larger and stabilizes parallel-plate actuators beyond the pull-in point of the constant voltage actuation. Resonant drive technique addresses the main challenges in MEMS interface design by using the same circuit and capacitor for both actuation and position sensing.
Resonant drive exploits the passive amplification in series RLC tank circuits. At electrical resonance, capacitor voltage amplitude can be much higher than the amplitude of the signal driving the tank. The tank circuit consists of an inductor connected in series to the MEMS drive capacitor. Since the drive signal is passively amplified through the tank, resonant drive does not need high voltage electronics for actuation. To maximize the amplification, an oscillator circuit brings the tank to electrical resonance. The oscillator circuit also controls the signal amplitude and sets the MEMS position.
As the actuator gap closes, the capacitance increases, causing the resonance frequency of the tank to decrease. The oscillator circuit automatically follows the frequency shift. Thus, the oscillation frequency is an electrical signal that tracks the actuator position. This alleviates the need for a separate sense capacitor and electronics. The increase in the drive capacitor also decreases the quality factor of the tank and automatically reduces the capacitor voltage amplitude. This phenomenon establishes an inherent feedback loop that counteracts the positive feedback leading to pull-in. Thus, the resonant drive circuit stabilizes parallel-plate actuators without using additional control mechanisms.
Resonant drive is demonstrated using torsional MEMS mirrors that require 45V for 4.5° of rotation. The RLC tank consisting of the mirror drive capacitor and a 390μH inductor has a Q of 14.5 and drives the mirror to 4.5° of rotation by using only a 9V peak-to-peak drive signal. Drive and sense circuits use standard CMOS parts. The sense frequency resolution of the oscillator is 25HzRMS, which corresponds to 33μrad (1.9 milli-degree) of mirror rotation. The phase noise of the oscillator circuit limits the sense resolution. The resonant drive circuit also exhibits an extended range of motion by driving a parallel-plate actuator up to 50% of the gap before pull-in.
September 30, 2005
Cagdaser, B. (2005). Resonant Circuits for MEMS Interfaces. United States: University of California, Berkeley.