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Nonlinear control of hydraulic differential cylinders actuating a flexible robot

Using robots for heavy loads and/or huge operating ranges causes on the one hand that elastic deformations have to be taken into account. On the other hand, these robots demand actuators with a significant power density able to produce the desired high forces. This can profitably be reached by the use of hydraulic drives. A laboratory testbed of a flexible robot for testing various control concepts is considered. It is a robot with elastic links manufactured of spring steel. The three rotary joints are driven by hydraulic translation drives within closed kinematic loops to transform the translation of the drives into a rotation of the joints. To realize complex control concepts, e.g. for vibration damping, it is necessary to develop decentralised controllers for position and/or force control of the hydraulic actuators in a first step. One modern nonlinear control technique is the exact linearization via feedback which is used to develop a nonlinear controller to compensate the main nonlinear effects of a hydraulic differential cylinder.