The determination of the stability of dynamical feedback systems from open loop characteristics is of crucial importance in control system design, and its study has attracted considerable research effort during the past fifty years. Those stability criteria which have a simple graphical interpretation such as the Nyquist criterion have become popular with design engineers because extensions that include nonlinearities in the feedback loop are readily incorporated in the same graphical method via frequency domain methods such as the circle or Popov criteria.
Read more at http://ebookee.org/The-stability-of-input-output-dynamical-systems-Volume-168-Mathematics-in-Science-and-Engineering-by-C-J-Harris-HARRIS_1687797.html#oyiwd410GTJbCVIr.99