Aiming at the lack of all-purpose and effective planning study for apple picking robot's end-effector during the grasping process, which has caused great inconvenient in the accuracy of fruit picking process and design of end-effector. This paper studies the contact process of three-finger end-effector with apples. Taking contact of apples with fingers as point contact of friction between hard objects, and based on the fact that the contact force is decomposed into the orthogonal operating force component and internal force component, the regulation of internal force on contact stability is discussed. Stability would be attributed to the existence of the internal force of concurrent polygon and the position of the internal force concurrent node within the concurrent polygon. Regarding the circle center of concurrent polygon of the maximum inscribed circle as the intersection of three internal force action lines, we get the size of each internal force, and calculate the internal force meeting the friction cone constraints to avoid complex operation such as matrix operation. Eventually, a numerical example shows the feasibility of the method.
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