Abstract
Variable admittance control is commonly used for a collaborative robot to achieve the compliant or accurate cooperation according to human's intention. However, existing research seldom investigates such a human-robot collaboration coupled with an extra environment with unknown stiffness. If the end-effector that is guided by a human with various intended motion contacts the unknown environment, the interaction might become unstable. Additionally, current research for this physical human-robot-environment interaction use two force sensors to address the issue, and hence the cost of the robot is likely to increase and it reduces the flexibility to many applications. Therefore, in this paper, we address the issue of physical human-robot interaction coupled with an extra environment whose stiffness is unknown. To achieve this, the condition of robot admittance is rigorously proved in accordance with different human intended motion and environmental stiffness. Moreover, a variable admittance control scheme is proposed based on human intention, environmental force and environment stiffness using the combination of a force sensor and a force observer. Simulation and experiments are conducted to demonstrate the effectiveness of the proposed control scheme.