Nyquist stability analysis

Nyquist analysis is a frequency response method for determining absolute and relative stability of closed-loop control systems. Most other methods, such as the Routh-Hurwitz criterion, can only determine absolute stability, and they require that the characteristic equation is a finite polynomial in s. Additionally, in the presence of delays in the system, these other methods only yield approximate information regarding system stability. In contrast, Nyquist analysis yields exact information about both absolute and relative stability of the system, even in the presence of delays.