A Study on Generation of Passive Softness at Load-end by the Soft-Actuation with Stepping Motor

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Softness at load ends has been expected to introduce soft and safe motion on robots and/or manipulators. In this study, a driving scheme of stepping motor, so called "soft actuation", was proposed and examined experimentally.(1) Soft actuationStepping motors have inherent passive softness against external force, since they have no mechanical contact except for shaft bearings. When the current through the stepping motor is appropriately lowered, the threshold for miss-stepping can be decreased to generate passive softness. For example, it was demonstrated experimentally that a robot arm driven under the soft actuation regime can smoothly catch a free falling object.(2) Torque reference control of stepping motorUnder the soft actuation regime, the torque reference control of stepping motor is available, if the torque angle is regulated appropriately. The torque characteristics which show acceptable linearity were presented experimentally. In this scheme, the action of stepping motor can expressed as the torque in the equation of motion, and it means that lots of well known motion control methods can be applied on the robotic systems driven with stepping motors.(3) Experimental results obtained from serial link manipulators driven with soft actuation1. Reasonable position/velocity controllability under torque reference regime was achieved.2. An object picking and piling task without height information was performed successfully.3. An evading motion of obstacle was demonstrated with groping strategy.4. A peg-in-hole experiment without direct force and/or compliance control was presented.It is generally not easy to achieve controllability and softness in motion simultaneously, but the proposed scheme can successfully provide both acceptable position/velocity controllability and passive softness against collision or restriction of load ends of robotic manipulators.