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Improving robot performance while simultaneously ensuring compliance and human safety has been appealing for a long time and remains a challenge. To this end, we propose a new approach for kinematic control of redundant manipulators to deal with multiple prioritized tasks and at the same time produce a novel compliance behavior in the null space of main task. Different from typical Voigt model based compliance control methods, the compliance control approach proposed in this paper is based on Maxwell model, which shows superior performance on impact absorption and contact reducing than its counterparts. In comparison with typical compliance control, the annoying and even harmful return force is removed and human comfortableness can be improved consequently. Besides, this novel compliance control method is implemented in the null space of higher priority task without disturbing the main end-effector task. The effectiveness of our approach has been practically evaluated and verified in experiments on a 7 DOF redundant collaborative robot manipulator. A novel cushion-like and plastic deformed whole body compliance has been realized while the continuity and quality of robot main task have been maintained. Promisingly, the approaches proposed in this paper are general and manipulator independent, which can also be applied to other emerging collaborative and even hyper redundant manipulators. Besides, research results of this work can also potentially inspire other robotic research related with human robot friendly interaction and collaboration. INDEX TERMS Multi-prioritized framework, null space compliance control, Maxwell model, plastic deformation.

Compliance has become one prerequisite of robots designed to work in complex operation environment where dynamic and uncertain physical contact or impact takes place frequently and even intentionally. Impedance control is a typical complaint control methodology. Standard impedance control is based on dynamics described by a spring and damper model connected in parallel way, which endues the robot an elastic behavior. In contrast, plastic deformation can be realized by Maxwell model in which spring and damper connect in series. In this study, a novel Cartesian impedance controller is constructed based on the Maxwell model. Implementation in a robot manipulator is executed to validate and analyze the proposed control law. A plastic deformation behavior of the robot manipulator is produced and certain extent compliance is achieved under the unpredictable impact or contact force exerted by human or other environment objects.