2 DOF cartesian force limiting device for safe physical human-robot interaction

This paper presents a device that significantly increases the safety level of suspended robots whose end-effector orientation remains constant with respect to the vertical direction (e.g. Scara-type suspended robots). The device is a two-degree-of-freedom (DOF) parallel mechanism with a parallepipedic architecture on which two revolute joints have been replaced with commercially available torque limiters. The device is implemented as a mechanical connection between the robot and the effector. It is rigid unless excessive horizontal forces are applied on the end-effector, for example during a collision. The level of force that activates the mechanism is set by properly adjusting the threshold of the torque limiters. Furthermore, a collision can be rapidly detected with a limit switch placed on one of the links of the mechanism and a signal can be sent directly to brakes that will stop the robot, without passing through a controller and thus improving the reliability and reaction-time of the safety system. By mechanically disconnecting the robot from its end-effector, the device ensures that the person involved in the collision is only subjected to the inertia of the end-effector and thus potential injuries are greatly reduced. A prototype of the proposed device has been built to validate the concept and to study its behaviour for collisions with different velocities and orientations.