Improve control system performance.pdf

This manuscript was the basis for the article “A Refresher Course in Control Theory” printed in Machine Design, September 9, 1999. Use Control Theory to Improve Servo Performance George Ellis Introduction For the people who install and commission motion systems, control theory can seem mysterious. It is true that some controls problems require great expertise, but thats not the normal case. Most engineers can address the majority of controls problems by knowing how to apply just a few principles. Of course, the engineer whos only goal is getting machinery operational, does not need to understand control theory at all. However, if the goal is to produce the best performing machines in your market, an understanding of applied control theory is necessary. Many engineers learn the basics of controls in college, but they dont learn how to apply them at work and the unused information is quickly forgotten. In this article we will try to revive that knowledge as we discuss a few basic principles. We will review Bode plots and the open loop method, a reliable and simple method that characterizes system stability. Bode Plots Bode plots provide abundant information about the stability and responsiveness in one picture. Many times, the step response displayed on an oscilloscope is used to measure the performance of control systems. Thats understandable because a step is measured with an oscilloscope (usually available). Bode plots provide much more information than a step-response. Bode plots show the phase and gain of a control system over a range of frequencies. Phase and gain describe the response of a control system to a sine wave as shown in Figure 1. For linear control systems, sine waves are unique in that sine wave inputs generate sine wave outputs. Phase describes the time shift between input and output. Gain describes amp