A method for evaluation and comparison of parallel robots for safe human interaction, applied to robotic TMS

Transcranial magnetic stimulation (TMS) is a noninvasive method to modify behaviour of neurons in the brain. TMS is applied by running large currents through a coil close to the scalp. For consistent results it is required to maintain the coil position within millimetres of the targeted location, but natural head sway and practitioner fatigue may hinder this. Serial robots are currently used to assist the application of TMS. However, their low stiffness limits the performance and their high moving mass limits the operating speeds for safety reasons. Since 6-DOF parallel robots combine high stiffness with low moving mass, they have potential for fast, safe and accurate positioning required for TMS during activities. For choosing the safest parallel manipulator, we developed an evaluation method using robotic safety criteria, including motor speed, motor acceleration, force transmission, and workspace accuracy. The method is applied for evaluation and comparison of the Delta robot with rotation head, the Hexaglide robot, and the Hexa robot. The Hexa robot shows to have the best safety characteristics. This paper also presents the design and evaluation of four controller strategies for the Hexa robot. These strategies include the application of a straightforward PID control (PID), a PID control with Jacobian Feed-Forward (PID J), a PID control with a Spring Force Control (PID O⁺) and a combination of these (PID J O⁺).