| Anthropomorphic prosthetic hand is a kind of bionic device mainly for the requirements of the amputees. Which consists many subjects, such as biomedicine, robotics, computer science and control theory. Its future development tends to be anthropomorphic, intelligent and intuitional control. However, the machanim, control system, sensor system and control methods are far away from natural human hand, which is postponed to practical usage. Therefore, this paper focuses on the technical which is relative to the design of multi-DOF anthropomorphic prosthetic hand, aiming at improving the tactile sensing, the stable grasping, for the purpose of finding dynamic grasp control way based on the tactile sensor signal feedback to improve the controllability. The main research subjects are: prosthetic hand embedded control system, fingertip three-dimension force tactile sensor and its static characteristics, prosthetic hand adaptive force-tracking impedance control, and dynamic grasping based on the tactile sensor signal feedback.This paper made detailed comparison about the status in quo of prosthetic hand from overseas and domestic, the comparison is dedicated to the key problems of prosthetic hand design, such as machanism, control system configuration, sensor configuration, and control methods. With this condition, considering the daily requirements of the handicapped, an embedded control system of prosthetic hand was designed using modularization and multilevel control system approach. A DSP&FPGA-based multilevel control system is developed, which consists of finger control sub-system and palm control sub-system, in which the drive, sensor and control systems are integrated in the finger. The RS485serial communications interface is adoped in communication between finger control sub-system and palm control sub-system. With the aim of real-time and high-speed communication between the prosthetic hand control system and PC, the USB3.0communication sub-system is designed. In order to real-time measure EMG for controlling prosthetic hand motion, the multi-channals EMG measure system is integrated into the embedded control system.For achieving abundant tactile sense and local self-determination control for prosthetic hand, fingertip flexble three-dimension force tactile sensor is designed. This tactile sensor has been realized in a13array of unit sensors, where each unit sensor responds to normal and shear stresses in all three axes respectively. The three-axes force measure model is established based on mechanism analysis and finite element analysis. The surface of the fingertip is discriminated into several parts based on the grasping modalities analysis. Both fingertip machanism and tactile sensor are designed specifically. For the crosstalk effect due to sensor array, the scanning circuit based on the zero potential method is designed, making both the high speed measure of sensor array signals and avoid crosstalk effect can be acquired. To test the sensor, a tactile sensor performance test system is designed. The sensor is calibrated using support vector machine regression analysis, and its static characteristics are experimentally tested. This fingertip three-dimension force tactile sensor system can provide necessary locality and force vector information of contact for the anthropomorphic prosthetic hand in the process of grasping.The grasp functionality of the prosthetic hand depends on the position and force control of a single finger to a certain extent, thus the adaptive force-tracking impedance control method was applied by considering the contact flexibility and the force-tracking precision. The kinematic and mechanical models of the finger are modified using contact position information from the tactile sensor. The accuracy force information of the contact point is acquired using tactile sensor and torque sensor. In order to improve the force-tracking precision, the force error is added into the impedance control, and the force-tracking impedance control is realized using adaptive discrete-time sliding mode impedance control algorithm. The indirect adaptive algorithm is applied to estimate the parameters of the environment, and then real-time adjusts the parameters of impedance control and reference track for grasping unknown object. According to the experiments of contacting environment with different stiffness, the force-tracking impedance algorithm can track the desired contact force with a relatively high accuracy by considering the contact flexibility, which establishes the foundation for the dynamic grasp.When prosthetic hand grasps unknown objects, outside disturb and object mass changing will lead unstable grasp, which may induce grasp failing. To solve this problem, the dynamic grasping strategy based on three-dimension tactile signal feedback is proposed. The grasping force of the finger is dynamically adjusted by variable universe fuzzy control algorithm so that the ratio of normal force to tangential force stays inside the safety region. Thus the fingertips without slip or detaching from the object while grasping it. This dynamic grasping strategy allows real-time control of the hand since it only requires a few numerical calculations. If the dynamic responses of all fingers are not strictly equal, it may happen that the grasping force applies an unexpected resultant force or moment on the object at a moment in the process of grasp, which will break stable grasp. This question has been avoided by proposed dynamic grasp strategy with a little compatation expense. In order to detect and control slip, a cumulative summation algorithm is proposed to detect the sudden switch from the static friction coefficient to the dynamic friction coefficient. When slip event is detected, the grasp force is increased to control slip. |
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