Electronic servo simulators

The paper presents an outline of a method which will allow automatic control systems to be studied experimentally by means of an electronic apparatus called a ?simulator,? which is constructed so as to have the same characteristic equation as the control system. This apparatus may be used to predict the response of the control system to the usual standard input signals such as step-function changes in the value of the input quantity or of its derivative, or to the more complex input signals encountered in actual use. The method is considered to be valuable in cases where these responses are difficult to calculate because of the complexity of the control system or because it contains non-linear elements. Simulators may be broadly classified into two categories, first ?slow? simulators in which the time scale is such that any transients occur slowly enough to be recorded by an operator reading a meter or on a recording instrument; and secondly ?fast? simulators in which transients are recorded with an oscillograph. Often this type is operated at a recurrence frequency of, say, 50 c/s and is then called a ?repetitive? simulator. A simulator for an r.p.c. servo mechanism stabilized by ?phase advance? is discussed in detail to illustrate the methods of design and results obtained with the apparatus are compared with those calculated from the characteristic equation. Electrical computation circuits which have been found useful are summarized in a table. It is shown how the apparatus can be modified for the study of servos which are stabilized by feedback methods. Two cases where the control system contains non-linear elements have been studied; first the case where the servo-motor torque is limited, and secondly where backlash exists in the gearing between servo motor and reset. Curves showing the effect of various amounts of torque limitation and backlash are given. Finally, repetitive simulators are discussed briefly and circuits for producing the input signals and osci- llograph time-base waveforms are described.