二指手的欠驱动手指，能够模仿人的拇指和中指，实现对不同物体的抓持并完成抓放的任务。针对一般物体，利用丝传动的欠驱动手能更好地模仿人的手指，从而实现两个手指对于物体的抓持，并完成抓放任务。利用抓持矩阵的方法，建立二指多关节手对物体有摩擦接触的抓持模型，并将抓持矩阵逐步向外扩展到手指的各个关节处，进而建立扩展抓持矩阵。利用接触点处抓持矩阵确定了一类平面物体抓持规划中接触点的位置应该满足的条件，并在最小力的原则下计算物体在不同的位姿下抓持力的大小。利用上述规划的接触点进行运动学反解，进而从联动关节处的抓持矩阵提取出欠驱动手指两个耦合关节力矩信息，得出丝传动的半径设计和反向弹簧设计的原则。利用扩展抓持矩阵求得力矩分量并用之验证此法与常规求法求得的力矩是等效的，从而验证此方法的正确性并对丝传动进行位置与力的计算。

A two fingered hand with two under-actuated fingers can complete the task of grasping and pick-and-place of different objects by imitating people’s fingers. For general objects, the fingers with tendon-driven can be more like people’s to grasp and to pick and place by using just two fingers. The two-fingered hand’s grasping objects with frictional contracts using grasp matrix is modeled and the grasp matrix is expanded to the joints of fingers to build the expanded grasp matrix of every rank. By using grasp matrix of the contracts, the position condition is derived that the contracts must satisfy for a kind of planar objects and the magnitude of the contract forces is computed with different positions and orientations under the principle of minimum force. Using the planned contracts above, the inverse kinematics is done and the moment of two coupling joints of the under-actuated fingers is obtained from grasp matrix of the two coupling joints, to design the radius and inverse springs for tendon-driven. The moment obtained from the expanded grasp matrix proves that this method and normal method are equal which prove it is right and effective, and then the position and force of the tendon driven are computed.