This experiment clearly demonstrates the important role of feedback in reducing the transmission of disturbances to the system output. The student designs a simple PD controller for a rigid body plant. Integral action and a lead/lag filter are then designed to separately augment the controller. All three designs are implemented and tested for their transmission of low and high frequency disturbances (0.1 and 2.0 Hz) as shown in Figure 1. As seen in the figure, the PD controller provides only moderate attenuation of nearly equal magnitude for the low and high frequency disturbances.
With the addition of the integrator, the low frequency disturbance is highly attenuated, but the high frequency one is actually amplified. The lead/lag filter does little to change the low frequency performance but is effective in attenuating the high frequency disturbance.
These effects are studied in terms of the open loop gain of each controller. The closed loop transfer function between the disturbance and the output (see Figure 2) is then obtained and is seen to agree well with the experimental results. Thus the student learns through first hand experience that the effectiveness of control regulation depends on the open loop controller gain characteristic and the frequency band of the disturbance itself.
