When a prosthetic hand grasps an object, a proper grasping force should be exerted according to the stiffness properties of the grasped object so that damage caused by excessive force or slide caused by insufficient force is prevented. To implement stiffness detection and simultaneously to prevent errors and defects caused by the use of force sensors and the difficulties in the direct measurement of the deformation of the grasped object, a force sensor-less method of contact stiffness detection is proposed for a single degree of freedom prosthetic hand with a self-locking mechanism. In this method, force sensor signals are replaced with motor current signals. An analytic solution of contact stiffness is obtained based on a linearized grasping model near the initial contact position between the prosthetic finger and the grasped object. Contact stiffness is thus calculated by using the current of the driving motor and the angular displacement of the prosthetic hand rather than directly through the ratio of the force to the deformation. Simulation and experiment results demonstrate the effectiveness and feasibility of the proposed method.
Abstract. A force estimation model using motor current signals was deduced in this paper for a tendon-driven prosthetic finger grasping objects with its distal phalanx. Models of the prosthetic finger were first established. As driving moment of each joint could be calculated form motor current, stable grasping force of the finger could be calculated by its statics mechanic model, that is, the grasping force is estimated using motor current signals. Then a PID controller based on the estimation method was designed with the estimated force as its force feedback signal. Based on the dynamics model of the prosthetic finger and a DC motor model, the estimation model and the PID controller were simulated in MATLAB, whose results indicated that the proposed grasping force estimation and control methods are effective.
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