Adaptive Backstepping-Flatness Control Based on an Adaptive State Observer for a Torque Tracking Electrohydraulic System

In practical applications, electrohydraulic servo systems (EHSSs) are known to be highly nonlinear. On account of high cost, space limitation, technical matters, etc., some of the state variables of the EHSS cannot be measured. Moreover, it is common that some of the physical parameters are uncertain. To the authors' knowledge, it is difficult to design a controller to achieve high trajectory-tracking precision in a wide frequency range for the nonlinear EHSS with unmeasurable state variables and uncertain parameters based on a single control method, and few articles are available for this problem, thus this paper proposes a compound controller: a nonlinear adaptive backstepping-flatness controller based on a nonlinear adaptive state observer. The observer is designed for highly precise state estimations in presence of nonlinear dynamics and uncertain parameters. The flatness method is originally applied to deal with the system with estimated state variables and parameters. Based on the observer, with integration of the backstepping method and the flatness method, the proposed controller is designed with lower design complexity and fewer feedback variable derivatives that ensure the proposed controller is easy to follow for control engineers and can help to avoid serious input voltage saturation. Simulation results show effectiveness of the proposed controller; furthermore, this paper shows how to implement the proposed controller on a torque tracking EHSS prototype and experimental results demonstrate its effectiveness as the results are compared with that of three previously reported controllers.