MULTIMODE DIGITAL-CONTROL OF A SUSPENDED POLYSILICON MICROSTRUCTURE

Digital control of a suspended 360 mu m x 380 mu m x 1.6 mu m-thick su rface-micromachined polysilicon plate is demonstrated in three degrees of freedom, with application to multimode accelerometers, vibratory r ate gyroscopes, and actively positioned micromirrors, Plate displaceme nt about the 2.2 mu m nominal position above the substrate is measured with shielded capacitive sensors connected to CMOS buffer circuits fa bricated adjacent to the microstructure. Four micromechanical sigma-de lta loops are used to control eight electrostatic actuators that drive the plate vertically (z) and in out-of-plane rotation (theta and phi) . Resonant frequencies are 2.7 kHz for the theta rotational mode and 3 .7 kHz for both z and phi modes. The system is evaluated using a mixed mechanical/electromechanical/circuit simulation in SPICE, Closed-loop transient simulation of a 150-Hz square-wave position input signal is in good agreement with experimental results. Squeeze-film damping lim its the plate slew rate to 0.83 mm/s in air, Position is controlled to within +/-25 nm, being limited by quantization noise at the 50 kHz sa mpling rate.