Arm-Hand Systems As Hybrid Parallel-Serial Systems: A Novel Inverse Kinematics Solution

In this paper, we aim to solve inverse kinematics of the integrated robotic arm-hand systems to achieve precision grasping, provided the desired grasp configuration (contact points + contact normals). The key insights of our approach are three-fold. First, we propose a human-inspired thumb-first strategy and consider one finger of the robotic hand as the "thumb" to narrow down the search space and increase the success rate of the algorithm. Second, we formulate the arm-thumb serial chain as a closed chain while other fingers are still as serial chains such that the entire arm-hand system is controlled as a hybrid parallel-serial system. The closed-chain formulation truncates and simplifies the task hierarchy of the entire arm-hand system. Third, we attach a virtual revolute joint to the thumb’s tip with its rotation axis aligning with the thumb’s contact normal to allow this virtual joint to act as the embodiment of the thumb’s functional redundancy. By selecting the thumb’s joints including the virtual revolute joint as the active joints of the arm-thumb closed chain, the arm-thumb system’s self-motion (i.e., the palm pose) and the thumb’s functional redundancy can be directly controlled without using the null space projection. This provides a new possibility to control the self-motion of robot manipulators. Simulation results will demonstrate the advantages and superb performance of the proposed approach for solving the problem of inverse kinematics of achieving precision grasps compared to other classical approaches based on the Damped Least-Squares method [1] in terms of the average success rate (96% v.s. 12%).