A Controlled Impedance Robot Gripper

The design of robot grippers has traditionally involved pneumatics or gear reduction electric drives that do not allow for the control of the stiffness of the mechanism. The manipulation of fragile objects such as eggs or light bulbs requires the gripper to be able to close on the object with minimal impact forces, and yet maintain a static grip force sufficient to firmly handle the object. This paper describes a two-fingered controlled impedance gripper where the impedance is imparted by electrical means. The analog control system allows independent control of the effective mechanical mass and damping of each finger, as well as additional control of common-mode versus differential-mode response. The gripper is modeled by a computer simulation that consists of a set of nonlinear differential equations with time-varying feedback parameters. In the final form the model has ten degrees of mechanical freedom and eight electrical poles, i.e., an 18th-order nonlinear differential equation. The dynamic equations are described and results given are compared with actual results obtained from laboratory experiments with the gripper. An Appendix describes the method of solution used for the nonlinear differential equations. The accuracy of the simulation has been verified by measurements on the actual gripper. This analysis has resulted in the design of a gripper control system capable of providing controlled compliance and reduced finger impact forces while maintaining a quick response and firm grasp.