Study On Grasping Model And Compliance Control Of Gripper Of Apple Picking Robot

China is the country with highest apple production in the world.At present,China government is calling for promoting the development of agricultural machinery equipment vigorously to improve the efficiency of apple picking and reduce the labor costs.Thus,researches on relevant technologies of picking robot are imminent.End-effector of harvesting robot is the key equipment which contacts with target crops directly.The rate of fruit damage is very high due to the softness of apple tissue.It is urgent to reduce the grasping damage of apple and improve the stability and flexibility of the grasping system.The stress and its distribution of apple during the process of grasping as well as the control of the end-effector on the grasping force are the main factors that affect the apple grasping effect.This paper arms at revealing the damage mechanism of apple and realizing the flexible grasping of apple by the way of the research on biomechanical properties of apple,the grasping complex model of fruit,the finite element analysis of apple stress and its distribution as well as impedance control.Main research contents are shown as follows:(1)A complex model of fruit grasping by end-effector was established.Based on the viscoelastic behavior of apple,the Burgers model of apple was determined by compression experiment.And then,the process of uniform grasping of fruit was divided into three stages: uniform loading,grasping deceleration and stress relaxation.The grasping complex model of fruit was established according to the dynamic characteristics of each stage as well as the apple Burgers model,and solved by MATLAB,then the change regulation of contact force,fruit deformation and grasping environment equivalent stiffness were achieved during the grasping process.(2)The finite element grasping complex model was established and solved by finite element method.According to the biological characteristic parameters of apple and the physical characteristic parameters of the end-effector finger,the contact geometry model consists of end-effector,apple skin,flesh and core was established in ANSYS software.Then,the finite element grasping complex model was obtained by meshing and establishing the contact pair.Finally,the contact force of the fruit under different velocities was obtained according to the grasping complex model of fruit and used to define the loading force applied to the finite element model of end-effector,so the uniform grasping process was realized indirectly in ANSYS.The variation of stress and deformation of apple were achieved by solving the finite element grasping complex model,and the damage mechanism of apple was analyzed based on the results of the solution.(3)A force-based impedance controller was designed to ensure the compliance of the end-effector during the grasping process.According to the grasping system of picking robot,the mathematical model of end-effector was established,and the principle of impedance control was introduced.Then,according to the particularity of the grasping process and the uncertainty of the grasping environment as well as the damage analysis of apple,the expected position input of the impedance controller and the grasping environment model were obtained by solving the grasping complex model of fruit.Then,the force-based impedance control algorithm was designed,simulated and debugged by MATLAB.Finally,the influence of impedance controller parameters on contact force was found,and the impedance control system with time-varying parameters was designed,which improved the control performance of the system.(4)The grasping experiment under open-loop and closed-loop modes were designed by using grasping platform of picking robot.The open-loop experiment measured contact force at different grasping velocities and apple deformation through the sensor,and the damage of apple contact area was judged by observation.The experiment results verified the accuracy of the grasping complex model of fruit and the feasibility of the finite element method applied to apple damage analysis during the grasping process.Several closed-loop grasping experiments were carried out by using improved and unimproved force-based impedance control.It was found that the improved impedance control was superior to the unimproved one in reducing overshoot of contact force measured by the sensor and improving grasping stability.