Nonlinear feedback control of robot manipulator and compliant wrist

Many robotic applications require the direct contact of the end-effector with the environment. Passive compliance attached to the robot wrist, hand, or finger is desirable to produce smooth transitions between the free motion and contact, as well as to allow self-correction in order to accommodate geometric uncertainties in assembly and manufacturing. However, the use of passive compliance degenerates the positioning capability of the manipulator when the robot moves in free space. When the robot makes contact on workpiece, active adjustment of stiffness for various tasks in different directions is needed. We proposed to use passive compliance that is instrumented so that the system provides the necessary flexibility and also sensing to actively control the contact forces or to compensate the positioning error during motion and contact. In this article, the dynamic control of the manipulator with a compliant wrist is addressed. The measured deformation information of the instrumented compliant wrist is utilized in the feedback loop to increase the stiffness of the overall system in position control and to decrease the stiffness in force control. The dynamics model for both unconstrained and constrained cases is established. Applying nonlinear feedback control techniques, the dynamics of the manipulator-wrist system is linearized and decoupled, which allows the controller design to be carried out by using the linear system theory.