Multimode digital control of a suspended polysilicon microstructure

Digital control of a suspended 350 /spl mu/m/spl times/380 /spl mu/m/spl times/1.6 /spl mu/m-thick surface-micromachined polysilicon plate is demonstrated in three degrees of freedom, with application to multimode accelerometers, vibratory rate gyroscopes, and actively positioned micromirrors. Plate displacement about the 2.2 /spl mu/m nominal position above the substrate is measured with shielded capacitive sensors connected to CMOS buffer circuits fabricated adjacent to the microstructure. Four micromechanical sigma-delta loops are used to control eight electrostatic actuators that drive the plate vertically (z) and in out-of-plane rotation (/spl theta/ and /spl phi/). Resonant frequencies are 2.7 kHz for the /spl theta/ rotational mode and 3.7 kHz for both z and /spl 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 limits the plate slew rate to 0.83 mm/s in air. Position is controlled to within /spl plusmn/25 mm, being limited by quantization noise at the 50 kHz sampling rate.