Optimal DC motor design for constant voltage seek motion

Abstract

This paper analyzes the seek motion (load moved between two stationary positions) performance in terms of move time, power dissipation, and gear coupling ratio between the load and motor shafts, of mechanical systems driven by DC motors. The motors considered here are operated under constant voltage during acceleration, and then the voltage is reversed for deceleration. This analysis is applicable to any kind of DC motor if the electrical time constant is very short compared Ith the technical time constant and the move time. This scheme minimizes the total power dissipation when linear drivers are used. The load considered is rigid and frictionless. The motor size in terms of power rate [1]. (torque square divided by inertia) and power dissipation the motor-to-load coupling (gear ratio), and the back EMF effect are all considered. The move time expression is given in dimensionless form so that the performance under all kinds of load inertia and stroke lengths can be analyzed, and compared with each other. A contour plot of the normalized move time shows the seek speed limitation under any load and power dissipation and indicates the region for optimal design. The system designer can decide immediately what motor size, in terms of power rate, and gear ratio should be chosen to meet the specified move time with minimal power. The designer can also establish the move time specification under a given power limit. There is no iteration required in the process. An optimal DC voice coil motor (VCM) design based on the analysis result is then described (Figure 5). The load is a rotary actuator (rotary arm carrying at bearing sliders with mu gnetic read/write heads, and slider suspension mechanisms) for accessing both sides of a 3.5′ diameter magnetic hard disk. Test results indicated that the specification of 7 msec move time for the average length track seek was met while using minimum power. © 1990 IEEE