Cascade linear control strategies with output feedback have been studied at the Maha Fluid Power Research Center to demonstrate robust control for displacement-controlled rotary actuation. These strategies have been mainly investigated for closed-loop actuator control where the operator specifies the actuator position to close the loop. This paper presents an extension of the work developed for this kind of actuation by introducing a non-linear control strategy for open-loop applications (i.e. the operator closes the loop via a joystick). The test bench, a 1.5 ton hydraulically-operated end-effector with a range of motion of 270° is utilized to validate the obtained control law. The proposed control scheme, an adaptive robust control (ARC) law, ensures system stability and robustness for a wide range of motion while eliminating the linear controller approach limitations. Furthermore, changes in the plant behavior are taken into account through online parameter adaptation. To emphasize on the advantages of ARC, a deterministic robust control (DRC) law has been derived from the ARC. Results show that the advantages of online parameter adaptation lead to a dramatic increase on the actuator position accuracy. In addition, the ARC results are compared to the cascade controller developed by Grabbel in 2004.

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