Dynamic analysis and experimental investigation of the free-to-contact motion for a flexible link manipulator

In the force control of an elastically flexible link manipulator, if there exists a gap initially between the end effector and the workpiece, the control task requires the manipulator to switch from the free motion control to the contact motion control. Thus, the contact force control will start with a non-zero initial state condition. These non-zero initial states in the system will generate an impact phenomenon which tends to allow the system to bounce back and become unstable. The purpose of this paper is to investigate the control of the flexible link during this process. In this paper, dynamic models of the flexible link manipulator in both free motion and contact motion are first derived. Due to the model changing between the two motion modes, state transform matrices between two modes are introduced. Finally, a joint angle-based force control strategy is investigated by using a constant angular speed at the first free space approaching stage. After making contact, a constant torque is applied to the link for a short period to suppress bouncing, and a step command followed by a cubic polynominal trajectory is used to achieve the desired force. Experimental results are demonstrated.