Research On An Underactuated Hand For Fruit And Vegetable Picking

The21st century is an important period for the development of agricultural mechanization. When picking fruit or vegetable, damages resulted by excessive grasping force of the end-effectors is one of the key issues in designing the end-effectors. An end-actuator with a simpler and more versatile structure was designed in this paper by adopting the underactuated principle, to achieve non-destructive fruit and vegetable picking. The underactuated mechanism refers to the machine that has fewer drivers compared with the number of degree of freedom. The robotic hand designed based on underactuated principle is simpler and more reliable. Due to its simple structure and better adaptability to the shape of the object, it can fold the object completely with its fingers. Non-destructive harvesting can be achieved by a closed-loop force-feedback control algorithm that controls the maximum contact force.Through research of the underactuated principle, a three-finger gripper for fruit and vegetable picking which was driven only by a motor was designed and manufactured. In order to study the influencing factors of the finger contact forces, the contact force computing model of the finger joints was built by static analysis.Through structural optimization to achieve uniform contact force the final model size was determined. Dynamics simulation was done by using ADAMS for contact force analysis in the case of different torsion-spring stiffness and apples of different diameters.The circuit based on ARM chip was designed, the ARM control and the human-machine interface software was compiled to achieve control of the robot hand. The serial communication protocol was set to achieve transmission of the command and data between the ARM chip and the computer.Grasping experiments combined with force-feedback control was completed and the experimental results were analyzed. After completing parameter tuning, the open-loop control for single finger and closed-loop PID control for multi-finger grasping experiments were conducted, the experimental results are consistent with simulation results. A closed-loop control of the maximum gripping force was achieved and the frequency domain response characteristics of the closed-loop control system were analyzed.The experiments showed that the robotic hand can achieve the desired grasping function, maximum contact force control and has such features as simple structure, reliability, easily processed, small size and light weight.