Design of safety mechanism for an industrial manipulator based on passive compliance

In recent years, collision safety between humans and robots has drawn much attention since human-robot cooperation is increasingly needed in various fields. Since positioning accuracy and collision safety are both important, an industrial manipulator should maintain very high stiffness for positioning accuracy in a normal situation, but exhibit very low stiffness when subjected to a collision force greater than the tolerance for human injury. To satisfy these requirements, we proposed in our previous research a safety mechanism composed of a linear spring and a double-slider mechanism for a service robot with a small payload. We modified this device to meet more stringent requirements for an industrial manipulator which usually has a payload higher than a service robot. Several experiments on static and dynamic collisions showed high stiffness of the safety mechanism in response to an external torque that was less than a predetermined threshold torque, but low stiffness that enabled absorption of the collision force when the external torque exceeded the threshold. Thus, positioning accuracy and collision safety were improved using the proposed design. Furthermore, a new safety criterion is suggested to verify the collision safety of a manipulator that uses the proposed safety mechanism.