Servo capture system

Abstract

Settling problems of a closed loop servo system are substantially eliminated by switching the servo system into position mode operation only when there is zero position error and zero velocity error. The fast and clean settling is accomplished by digitizing the instantaneous amplitude of the position signal when the difference count has decremented to zero and using that digital value as the address of a square root look up table which provides for each instantaneous amplitude of the position signal a digital signal indicative of the instantaneous velocity required to effect rapid settling at the rest position. The digitized velocity signal is converted to an analog signal by a digital to analog converter for energization of the drive motor. The servo system tracks the instantaneous amplitude of the position signal until both position signal amplitude and velocity are zero, at which time the system is mechanically stopped and stable, and then the system is, for the first time, switched to conventional position mode operation. Prior to the time that the difference count is decremented to zero, the difference or track count is used as the address of the look up table to provide digitized velocity signals. There is a change in the scale of the digital to analog converter when the difference count becomes zero so that all of the velocity square root curve used for positioning when the difference count was above zero can be used when the difference count is zero.