Control of a planar cable-actuated parallel manipulator with realistic cables

Düzgören, Onur
The cable-actuated parallel manipulators comprise a new class of robotic systems which utilize length-controlled unilateral force elements like cables or wires to move and orient an object. They provide several benefits over conventional parallel robots, such as larger workspace, simpler structure, and higher payload/manipulator weight ratio. However, the cables can only be pulled but not pushed. Besides, they may sag due to their own weight. Therefore, the cable-actuated manipulators pose challenges in modeling and control. In this thesis, a planar cable-actuated manipulator is studied. It consists of a payload and two cables with nonnegligible masses. Each cable is divided into a finite number point masses connected by massless rigid segments. The appropriate number of segments required for a realistic modeling is one of the issues studied in this thesis. After the modeling stage, an inverse-dynamics controller is developed in order to make the payload track a desired trajectory. This controller needs the angles of the cable segments, which are not feasible to measure. Therefore, two methods are proposed to estimate them. The first method is based on a lower order model of the cables with a smaller number of segments. The second method is based on the assumption that the cables remain in pseudo-static equilibrium. Moreover, the tension in each cable segment is monitored during the motion and the input forces are readjusted online to prevent slackness in any of the segments. For this purpose, an optimization algorithm is developed in order to determine the control forces with a compromise between the segment tensions and the tracking error. The performance of the controller is demonstrated and assessed through several simulations carried out by choosing the reference motions as square wave, deployment, and circular motions.


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Citation Formats
O. Düzgören, “Control of a planar cable-actuated parallel manipulator with realistic cables,” M.S. - Master of Science, Middle East Technical University, 2018.