An interactive CAD tool to teach and learn Nyquist criterion

In his 1932 seminal paper “Regeneration Theory”, Harry Nyquist provided a rigorous set of measurable conditions by which to determine the stability of a feedback amplifier. Nyquist redefined feedback as a frequency-dependent phenomenon, and stability in terms of transient disturbances composed of different frequencies. Nyquists stabiliy criterion applies not only to feedback amplifiers, but to all feedback control systems. The contribution of Nyquist is now a fundamental part of current courses on Automatic Control that has been generalized in several different ways, to complex systems with many loops and to nonlinear systems. The paper describes an interactive CAD tool with the purpose to teach and learn Nyquist criterion. The tool has been developed using Sysquake, a Matlab-like language with fast execution and excellent facilities for interactive graphics, and is delivered as a stand-alone executable that is readily accessible to students and users in feedback control systems.