A general mathematical function for hydraulic actuators is reviewed in this paper. This model is used for the application of Computed Torque and Variable Structure controllers to a hydraulic industrial robot, the Workmaster, formerly manufactured by Thorn EMI Robotics. The model based controllers are examined to demonstrate the effectiveness of feedback and feedforward compensation in hydraulic systems. It is shown that a large performance improvement is obtained through feedforward and feedback compensation. A sliding mode component is added to model based compensation in order to improve the system’s performance by providing additional control action when there are modeling inaccuracies resulting from nonlinear friction effects.

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