A capacitive sensing integrated circuit for detection of micromotor critical angles

The theory, design, and measured performance of an integrated circuit which enables closed-loop control of electrostatic micromotors is presented. The micromotor control integrated circuit (MCIC) consists of low-noise sense electronics designed to detect critical rotor angles to a resolution of 0.5 degrees (0.05 fF) at a 1-MHz sampling rate, and control logic which cycles the micromotor drive state during continuous rotation to maintain maximum torque, independent of loading, Noise due to MOSFET switches and amplifiers in the analog section is modeled and shown to be 32 mu V referred to the system input, i.e., about half the desired switching resolution. The MCIC was fabricated using a 2-mu m, n-well CMOS process and functions as expected, The noise probability density function was measured using MCIC's digital output for different values of input-to-ground capacitance in order to verify the noise model, Good agreement with theory was observed, although the comparator exhibited some offset and hysteresis.