Design Notes on Precision Phase Locked Speed Control for Dc Motors

There are a number of high volume applications for DC motors that require precision control of the motor’s speed. Phase locked loop techniques are well suited to provide this control by phase locking the motor to a stable and accurate reference frequency. In this paper, the small signal characteristics, and several large signal effects, of these loops are considered. Models are given for the loop with design equations for determining loop bandwidth and stability. Both voltage and current motor drive schemes are addressed. The design of a loop for a three phase brushless motor is presented. PHASE LOCKING GIVES PRECISION SPEED CONTROL The precise control of motor speed is a critical function in today’s disc drives. Other data storage equipment, including 9 track tape drives, precision recording equipment, and optical disc systems also require motor speed control. As the storage density requirements increase for these media, so does the precision required in controlling the speed of the media past the read/write mechanism. One of the best methods for achieving speed control of a motor is to employ a phase locked loop. With a phase locked loop, a motor’s speed is controlled by forcing it to track a reference frequency. The reference input to the phase locked loop can be derived from a precision crystal controlled source, or any frequency source with the required stability and accuracy. A block diagram of the phase locked loop is shown in Figure 1.