Hybrid impedance/time-delay control from free space to constrained motion

In this paper, hybrid impedance/time-delay control is used for a robot to successfully achieve contact tasks without changing a control algorithm or controller gains throughout all three modes: free space, contact transition and constrained motion. In order to absorb impact forces and stabilize the system upon collision with a stiff environment, a nonlinear bang-bang impact controller is developed. The proposed controller uses hybrid impedance/time-delay control in free space and this control input alternates with zero when no environment force is sensed due to loss of contact. The discontinuous on-off control action depending on the state of contact quickly dissipates the impact energy during impact transient. After impact transient, the hybrid impedance/time-delay control algorithm is employed, which achieves optimal responsiveness and has good disturbance rejection properties. Therefore, this bang-bang control method provides stable interaction between the robot with severe nonlinear joint friction and a stiff environment and achieves rapid response. The proposed controller requires specific knowledge of only one system parameter, the inertia, for its implementation. It is shown via experiments that a robot can successfully work with only one control algorithm from free space to constrained motion under the nonlinear bang-bang impact controller. The proposed controller can be best applied for robots working in unstructured environments.