2-DOF Robot Arm with Variable Torque Limiters Realized by Electrostatic Film Motors

When a conventional robot arm, driven by magnetic motors and reduction gears, operates at high speed, it can produce an excessive force due to unexpected contact. Toward the development of a safe robot arm, the present study investigated a 2-degree-of-freedom robot arm driven by direct-drive electrostatic film motors. The motors operate synchronously, except when they are subjected to an excessive force. This loss of synchronicity is referred to as a "step-out" process, and in the present study it was used to produce a variable torque limiter. By setting appropriate limits, contact forces caused by unexpected collisions could be suppressed. The torque limiter was applied to a robot arm with a length of 50 cm. Experiments showed that the arm was impactresistant; when it was struck by a hammer, the torque limiter was activated and suppressed the collision force, allowing the robot arm to successfully resume its motion. When the robot arm unexpectedly hit an obstacle while moving at 1.2 m/s, the collision force was maintained at 21 N by the torque limiter, which is sufficiently small for a high-speed collision. The experiments verified the effectiveness of the electrostatic film motor as a variable torque limiter to realize safe contact.