ABSTRACT
Open-loop frequency response analysis of a control system or its components provides dynamic characteristics of the system. Corner frequencies reveal the time constants. A controller has a response curve for each combination of controller settings. However, this family of curves can be simplified into a single curve for each control mode because the integral action affects the low-frequency response, the derivative action affects the high-frequency response, and the proportional action is simply a horizontal line. Using data from the open-loop response curves, closed-loop response can be found from a Nichols diagram. The Nichols diagram, or phase margin plot, may be used to define system performance. It is a plot of magnitude versus phase. The frequency response method is both a test and a graphical technique to obtain information about the control loop. The instrument manufacturer can put these tests to good advantage in evaluating instruments.
