A 200 °C Motor Control ASIC

Abstract

A custom application-specific integrated circuit (ASIC) has been designed for positional control of brushless DC or servo motors in high-temperature (>200 °C) environments. Applications would include valve and position control for aerospace and industrial systems. Patented high-temperature circuit design techniques facilitate hightemperature operation from a conventional, low-cost, 0.5-micron bulk CMOS foundry process. The ASIC is highly integrated to enable software- and processor-free local control of motor position, and uses external power MOSFETs for motor commutation. Motor position can be controlled in open- or closed-loop modes with an integrated rotational variable displacement transformer (RVDT) direct digital synthesis (DDS) waveform generator, rail-to-rail op-amp driver and demodulation circuit. The ASIC can accept both analog (0–10 V) or digital (SPI bus) position setpoint commands from an external controller. Motor position is indicated by both analog and digital output signals. The full-scale displacement of the controlled motor is programmable from 5 to 8 bits of resolution, permitting 32–256 positions of control. Safety features such as a 500-ms power-on delay, overtemperature and motor overcurrent detection, and control signal undervoltage lockout were included to minimize the need for external control. ASIC bench-test results confirmed circuit functionality at ambient temperatures up to 225 °C using room-temperature power MOSFETs and motor load. ASIC performance at the 8-bit level was demonstrated, although the clock oscillator frequency shifted by about 15% over the full temperature range. Control of the motor at 200 °C was also demonstrated, although moderate loss of motor holding torque was observed due to internal heat generation in the motor. The ASIC was combined with commercially-available off-the-shelf high-temperature components on a printed wiring board (PWB) to form a compact (4 × 3.5 inch) motor control demonstration system capable of prolonged operation at temperatures beyond 200 °C. Environmental and long-term testing of the PWB is planned to demonstrate system reliability.