A rating of control loops can be done by the characteristic equation of the control loop. This is formed by 1 + G0(s), whereby G0(s) is the transfer function of the open control loop. The zeros of the characteristic equation are also the poles of the closed loop. Modify the gain of the proportional-control and the I-part and watch the pollage change.

Control loop is stable: When complex conjugated poles occur, the control loop will start to oscillate. This can be seen when the step response overshoots. When the poles stay in the left half-plane, the control loop stays stable. On the lower left, the frequency curve and its elements of the loop are displayed. As long as the control loop is stable, the phase angle is:

|G0(s)| = 0 dB = 1 greater than -180°

Control loop is unstable: When the poles leave the left half-plane, the closed loop becomes unstable. This equals:

|G0(s)| = 0 dB = 1 smaller than -180°