The step response of a 3 pole system is shown in the applet below.

The open and closed loop transfer function for the system is

G(s) = K/(s(T1s + 1)(T2s + 1)), C(s)/R(s) = K/(s(T1s + 1)(T2s + 1) + K))

Since this is a 3 pole system then instability is possible.

The parameter values (1.5, 1.0, 1.0, 2.0, 4.0) shows the step response when the system with poles at 0, -1/T1, -1/T3 is oscillatory. Unstable for K > 1.5.

Values (2.0, 1.0, 1.0, 2.0, 4.0) show oscillation with poles at 0, -1/T1, -1/T2. Unstable for K > 2.0.

Values (9.0, 1.0, 1.0, 2.0, 2.0) show oscillation of type 0 with poles at -1/T1, -1/T2, -1/T3. Unstable for K > 9.0.

Gif image below shows how applet should appear.

COPYRIGHT © 2006 Cuthbert Nyack.