The writing of closed-loop control system transfer functions by inspection

IN many closed-loop control systems, an analysis of the system discloses that it is possible for signal energy to traverse different paths and to be modified in a different manner in these various paths. This is in contrast to the very simplest of closed-loop systems, where the signal traverses transfer elements in cascade. The reasons for using multiple-loop systems are many. Improved linearization, noise reduction, and change in impedance level can be achieved by using auxiliary loops. It is also possible to synthesize certain transfer functions by using additional feed-back paths. A simple example of this, is the integral control which can be obtained by using a properly designed series field in a rotating amplifier (Rototrol). It is also possible, by using multiple loops to feed information from various parts of the system to control elements, in order to improve system performance. Also multiple loops might arise where closed-loop control systems are used as components in larger closed-loop systems. For example, in the use of control systems in flight control, the system is part of a larger system closed through the pilot himself.