Designing Passive Safe Joint of a manipulator by the Planar Four bar Mechanism

In recent years, because of expanding human-robot interaction, especially in collaborative robots, safety in collision between a human and a robot has drawn much attention. A safe manipulator can be equipped with either active or passive system. A safe manipulator with a passive system usually consists of a mechanical elements, such as a spring which can absorb the collision force and reduce it. These joints are composed of a spring-loaded mechanism which is capable of acting as soon as a torque is dealt with, and by absorbing force from the spring causing the torque to fall to a safety limit. But for a torque lower than a limit, the rigid behavior of the manipulator which is necessary for the accurately is guaranteed. In this paper, design of revolute joint with nonlinear spring system is considered by a four-bar mechanism with a linear torsion spring so that it can be stiff for high values of an input torque for normal operation, but immediately after collision the torque will drop to its safety limit. We also show that the singularities of the four-bar mechanism can be used to design a secure joint so that the different behavior of the joint can be expected in different situations.