| At present,apple picking is mainly realized manually by auxiliary equipment,which has high labor intensity,low efficiency and high cost.Therefore,many scholars have developed a variety of apple picking end-executors such as adsorption type,twist type and cutting type for mechanized apple harvesting.However,because mechanically harvested apples are not easy to retain the fruit stem,and the rigid contact between the fruit and the end-effector during picking is easy to cause fruit damage,which will affect the subsequent transportation and storage.Therefore,the study of end-effector handleretaining picking and non-destructive picking will be the key technology to realize mechanized and automated apple picking.Hand-picked apples are usually pulled up and down or pulled and twisted.In this paper,according to the idea that apples can be picked stably by hand and then broken up or down around the joint point of the fruit stem and the branch,that is,the apple moves around a certain point in a circle,a picking executive mechanism with approximate remote movement center is proposed and studied.The main research contents are as follows:1)Aiming at the problem that it is difficult to keep the stalks of apples picked by mechanization,taking red Fuji apples from an apple orchard in Weihai,Shandong Province as the research object,a picking scheme based on remote motion center mechanism was proposed to move around the joint of the stalks and branches in a 120°circular motion.According to the definition correlation between virtual motion center mechanism and remote motion center mechanism,8 kinds of remote motion center mechanism were synthesized by the method of configuration synthesis,and one of them was selected as a compact single degree of freedom remote motion center mechanism,and the payload force and self-locking phenomenon of the end point of the remote motion center mechanism were analyzed,so as to provide alternative configuration for approximate remote motion center picking.2)Design the offset rotary end gripper based on the physical characteristics of Apple.Determine the geometric characteristics and mechanical properties of the apple to determine the finger configuration of the manipulator and the transmission mode of tendon rope-pulley drive,analyze the apple stable grasping force and the optimal grasping position.The updown rotation bias distance of the end gripper and the motion range of the telecentric point of the remote motion center mechanism are determined according to the length of the fruit handle.The force feedback design of the manipulator is carried out by the thin film pressure sensor to realize stable and non-destructive grasping.The apple stable grasping test was carried out,and the minimum gripping force required by 60-100 mm apple stable grasping was 10N(single finger)by changing the threshold of pressure sensor.The apple static compression test was conducted to measure the minimum damage pressure of 40 N,and the upper and lower limits of grasping force were obtained,so the threshold was set as 20 N.The device was designed to measure the torsion force in different rotating picking methods,and the average torsion force of biased rotating picking was 25 N and that of axial rotating picking was 40 N.The static tensile test of fruit stem was completed,and the average binding force between apple stem and parent branch was determined to be 30 N,and the breaking force required for picking fruit stem by telecentric motion was determined.3)Considering the realization of the motion picking function of the end effector similar to the remote motion center and the design of the overall structure,the structure and size of the telecentric mechanism were designed according to the length and working space of the end gripper.By combining the remote motion center mechanism and the biased rotating end gripper,a kind of apple picking end effector based on the remote motion center mechanism was designed.The end-effector is modeled in 3D.4)In order to verify the accuracy of the theoretical calculation and the reliability of the end-effector,kinematics simulation software was used to simulate and analyze the parameters such as workspace,displacement and velocity when the end-effector remote motion center was picked.The static structure of the key parts of the remote motion center mechanism was analyzed and the strength was checked by finite element software.5)According to the research on the effect of picking and the factors influencing the success rate of picking in the actual experiment,the processing and assembly of the endeffector were completed and the test platform was built.The bias rotating picking experiment of end-effector was carried out in apple orchard.The results showed that when the rotation Angle of end-effector exceeded 180°,the rotation speed was higher than30r/min and the bias distance was constant,the success rate was above 95%.In the laboratory,apple growth scene was simulated to carry out the motion picking experiment of apple approximate remote motion center.Under the high-speed camera,the working space was about 120° arc.Response surface analysis was used to design orthogonal experiments to explore the effects of swing Angle,swing speed(synchronous pulley rotation speed)and offset distance of the end hand on the success rate of picking the remote moving center.It was concluded that when other two factors were the same,When the swing Angle is over 45°,the swing speed is between 30r/min-60r/min,and the bias distance of the end gripper is over 30 mm,the approximate remote motion center picking can be achieved,and the handle retention rate of successful picking is about 85%.Finally,it is found that the swing Angle has the most significant effect on the success rate of picking apple remote motion center. |
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