Adjusting the active joint stiffness of a collaborative robot arm for force control

Collaborative robots are being used in machining task more and more. However, they have the disadvantage of elasticity in their joints, causing position errors, vibration, and poor performance in applications where contact forces are required. The main goal of this article is to study the effect of the joint stiffness in force control tasks. For this purpose, the active stiffness of the joints has been modified according to theoretical deductions. The experiments have been achieved on a UR3 robot through an inner/ outer force control loop. A theoretical and experimental analysis shows the advantage of this type of control, which allows adjusting the active stiffness when interacting with a different environment rigidity.