Systematic Design Of Optimal Disturbance Observer With Robust Performances And Its Application

High-performance motion control is one of the factors which promoting actioncapability of robotics. The methods in robot servo motion control area already hasbeen developed from the stage using simple feedback control into that of employingthe advanced control methods such as non-linear, robust and intelligent controls etc.,along with growth of the demand on the robotic control performances. For robotmanipulators that have serious interferences, changing parameters and unmodeleddynamics, it needs to adopt more effective control strategies in order to attain theperformance requirements of motion servo control. Disturbance observer is known asa powerful mean which can remove affects of the interferences and disturbances, andhas been widely used in robot joint motion control in order to realize high-speed highaccuracy positioning and tracking control.From viewpoint of optimal suppression of disturbances and sensor noises, thispaper investigates robust optimization and its application to control practice ofdisturbance observer. Detailed investigation works are followings:(1) As for design theory of disturbance observer, current disturbance observerdesign methods lack the systematicness and global optimality. To develop systematicdesign algorithm, the optimization cost functions including disturbance suppression,sensor noise removing and robustness are defined using H∞norm. By theoreticalanalysis and mathematical transformations, the new systematic design strategy foroptimizing disturbance observer is established: pseudo loop shaping concept isproposed to transform the problem designing disturbance observer into an equivalentpseudo loop shaping problem; on that basis, the demands on orders and structure suchas whole order, relative order and internal model order are investigated andtransformed to the structural restriction of pseudo plant and the order and structuralrestrictions on the frequency weighting functions in the cost functions. By thesetransformations, non-canonical H∞disturbance observer design problem withrestrictions can be turned into standard H∞disturbance observer design problemwhich satisfies all the preconditions and has not structural restrictions, so that candirectly use the systematic and straightforward solving algorithm to design theoptimal disturbance observer. (2) As for robust stability against model perturbation, current design methodonly guarantees the robust stability of local loop, but not that of whole closed-loopsystem. A new robust stability sufficient condition of the closed-loop system withdisturbance observer is proposed, on the basis of which the weighting functions areselected so that the standard H∞disturbance observer design method can guarantee therobust stability of closed-loop system to the plant's parameter variations. In thedesigning, the optimal trade-off between peak magnitude around cut-off frequencyand disturbance suppressing performance is optimally realized using the freedom ofselecting the weighting functions so that the disturbance observer can have optimaldisturbance removing performance in the condition of guaranteeing the robuststability.(3) The motion control system based on Q–filter scheduled disturbance observeris proposed, where multi Q–filters with different control purposes are switchedaccording to the system's state and control demands. In general, the disturbanceobserver with a fixed low-pass filter can not realize smooth dynamic transientresponse without vibration and overshoot and optimized performance of suppressingdisturbance at low frequency simultaneously. Q-filter scheduled disturbance observeris presented, which includes multi Q-filters in different functions and the filters arealternatively switched to system according to the results of operation of switchinglogic so as to realize the dynamic transient response without vibration and overshootand disturbance suppressing performance. All the Q–filters in the scheduleddisturbance observer are designed by robust standard disturbance observer designmethod and the switching logic guarrantees the stability of switching. simultaneously.(4) All the proposed design methods of disturbance observer are validated bydesign processes and experiments on the motion control of some practical plants andthe results show that these design methods are effective for servo control of robots.