Question

The characteristic equation of a feedback control system is s^3+Ks^2+9s+18. When the system is marginally stable, the frequency of the sustained oscillation:

(a) 1

(b) 1.414

(c) 1.732

(d) 3

This question was posed to me by my school teacher while I was bunking the class.

My doubt stems from Relative Stability Analysis in portion Stability and Algebraic Criteria of Control Systems

Answer 1

Right choice is (d) 3

The explanation: Solve using Routh array and for the system to be marginally stable, K = -2. Polynomial for sustained oscillation w = 3 rad/s.