Imperfect Actuators

10.4 Conclusion

Hysteresis is sometimes observed in pneumatic and piezoelectric actuators. Under relay feedback, the effects of actuator hysteresis on estimation errors are explored. Comparisons are made between the ideal and saturation relays. As expected, the ramp behavior of the saturation relay can alleviate the overestimate of K_u. If the width of hysteresis DB is available (e.g. estimated, from bump test, etc), the shapes of the relays can be modified to accommodate the imperfection and to provide better accuracy. Moreover, a procedure for simultaneous identification of the width of the hysteresis and ultimate properties is also proposed. Simulation results show that good estimates of ultimate properties can be obtained. It provides better reliability for relay feedback identification and opportunities for improved control under imperfect actuators. Finally, a word of caution is required: regardless of how smart the autotuner is, the best approach to handling an imperfect actuator is to have it fixed.