MODELING AND CONTROL OF A FLEXIBLE SPHERICAL WRIST

In this paper, a very inexpensive, lightweight and simple wrist mechan ism is introduced. This wrist displays nonlinear torsional vibrations. This differs from conventional wrists in that structural flexibility in the mechanism is allowed to occur by design. In this paper, the dyn amic equations of this wrist are derived. System identification techni ques are then employed to obtain a linearized model. Various control s trategies are studied. It is shown that the input-output feedback line arization technique is not feasible for these nonlinear dynamic equati ons. It is also shown that the use of conventional rigid body PID cont rollers on this proto-type is inadequate. A tracking controller which compensates for the flexible dynamics of the wrist is implemented with encouraging results. This controller allows the end-effector to be pl aced at an arbitrary orientation with little vibration. The effect of the controller is to make the wrist appear to have a much higher struc tural stiffness. The compliant nature of this wrist allows simple forc e control strategies to be implemented. It is the combination of the w rist with the control algorithm which makes this design viable.