Servo Control of a DC Brush Motor

The PIC17C42 microcontroller is an excellent choice for cost-effective servo control in embedded applications. Due to its Harvard architecture and RISC features, the PIC17C42 offers excellent computation speed needed for real-time closed loop servo control. This application note examines the use of the PIC17C42 as a DC brush motor servo controller. It is shown that a PID (Propor-tional, Integral, Differential) control calculation can be performed in less than 200 ms (@16 MHz) allowing control loop sample times in the 2 kHz range. Encoder rates up to 3 MHz are easily handled by the PIC17C42's high speed peripherals. Further, the on-chip peripherals allow an absolute minimum cost system to be constructed. Closed-loop servo motor control is usually handled by 16-bit, high-end microcontrollers and external logic. In an attempt to increase performance many applications are upgrading to DSPs (Digital Signal Processors). However, the very high performance of the PIC17C42 makes it pos-sible to implement these servo control applications at a significant reduction in overall system cost. The servo system discussed in this application note uses a PIC17C42 microcontroller, a programmable logic device (PLD), and a single-chip H-bridge driver. Such a system might be used as a positioning control-ler in a printer, plotter, or scanner.The low cost of imple-menting a servo control system using the PIC17C42 allows this system to compete favorably with stepper motor systems by offering a number of advantages: ? Increased Acceleration, Velocity ? Improved Efficiency ? Reduced Audible Noise ? True Disturbance Rejection