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A hybrid position/force controller is presented for a 6 DOF hydraulic manipulator. The controller has been implemented on a Kraft telerobotic slave which has been modified to accommodate a 6 DOF force/torque sensor at the wrist. The controller is implemented within the task frame, and both the position and force are controlled with a non-conventional hybrid control method. Positional accuracy control is maintained at the joint level by using a joint error prediction method based on measured joint torques which have been low-pass filtered. This prediction method eliminates the need for integral gains, which introduce unwanted limit cycling. Dynamic stability is maintained and Cartesian positional error is held to less than 0.2 inches. Conventional hybrid control is based on the ability to control joint torques, but hydraulic actuator torque can not generally be directly controlled. We instead employ a feedback loop which adjusts positional commands along force controlled DOFs until desired end effector force/,moments have been realized. This feedback loop has been implemented in both joint and Cartesian space. The joint space feedback method is based on the observed joint error verses joint torque characteristics used in the positional accuracy control portion. The joint space method has better force tracking capabilities than the Cartesian method, but is not stable for all robot configurations. Cartesian space feedback method has sufficient force tracking for a useful range of tasks, and is stable for all configurations.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only. |
