Nyquist criterion

This graphical method, which was originally developed for the stability analysis of feedback amplifiers, is especially suitable for different control applications. With this method the closed-loop stability analysis is based on the locus of the open-loop frequency response . Since only knowledge of the frequency response is necessary, it is a versatile practical approach for the following cases:

a)

For many cases can be determined by series connection of elements whose parameters are known.

b)

Frequency responses of the loop elements determined by experiments or can be considered directly.

c)

Systems with dead time can be investigated.

d)

Using the frequency response characteristic of not only the stability analysis, but also the design of stable control systems can be easily performed.