The effect of actuator saturation on the performance of PD-controlled servo systems

Perhaps the most often utilized means of closed-loop control of a servo sys tem is proportional-derivative (PD) control. Linear analysis methods sugges t the best tracking performance is achieved at maximum possible proportiona l and derivative gains. Maximum gains, however, drive the actuators into sa turation, which renders the system nonlinear and the linear analysis invali d. This paper investigates the effect of actuator saturation on servo syste m tracking performance by formulating a frequency-based tracking performanc e measure roughly equivalent to the linear system -3 dB bandwidth. The prop osed measure utilizes a series of band-limited pseudo-random tracking input s to characterize the 'bandwidth' of the (nonlinear) saturating system. Num erical simulations based on this measure show that, for a servo system that exhibits actuator saturation, the best tracking performance is not achieve d at maximum gain. Instead, performance improves up to a given gain, then b egins to recede as the gain is increased further. The simulations also show that avoiding actuator saturation to ensure linear behavior significantly sacrifices tracking performance. The measure of tracking performance is com pared with the -3 dB bandwidth utilized in linear analysis techniques, and the two are shown to be well correlated. (C) 1999 Elsevier Science Ltd. All rights reserved.