Design and test of end-effector for navel orange picking robot

Abstract: Navel orange is rich in nutritive value and has high edible and medicinal value, and it is loved by people. With the increase of planting area and yield of navel orange, Gannan navel orange industry has developed into an industry cluster which integrates production, logistics and deep processing. Also, mechanical and intelligent picking is an inevitable trend in modern agriculture. The picking robot is one of the important equipment of intelligent agriculture, which plays an important role in promoting the agricultural transformation and upgrading. As one of the most important parts of the picking robot, the end effector, has a direct influence on the success rate, damage rate and the work efficiency of the picking operation. In order to improve mechanical and automatic picking level, meet the requirements of non-destructive picking for navel orange and solve the shortage of labor force, a new picking end-effector was designed. It mainly consisted of 3 parts, i.e. adsorption mechanism, clamping mechanism and rotary cutting mechanism. The fruit could be quickly separated from the fruit cluster by the adsorption mechanism, and stably and non-destructively clamped by the clamping mechanism, and the rotary cutting mechanism mainly realized the separation of the fruit and the fruit stalk. At the same time, the mathematical model of navel orange was established by fruit index test, which was a standard ellipsoid. The mechanism model was established on the basis of the navel orange equatorial plane and with the horizontal diameter as the index. Each joint was designed to V type and the transverse diameter range of navel orange harvested was determined as 50-100 mm. In order to ensure the finger surface and recoverable diameter range of arbitrary navel orange have 4 tangent points in holding and the 4 sensors can feed back pressure value in real time, the parameters of the finger were simulated and optimized by CAD, finally obtaining the ideal parameters. The minimum positive pressure was determined by the static stress analysis of navel orange to 0.77 N. In the compression experiments of the equatorial surface of navel orange with different diameters at the different speeds of loading, which were 100 and 200 mm/min, the maximum deformation was determined as 5 mm and the maximum positive pressure was 25.57 N. In the friction coefficient experiment with 3 common materials which were rubber, foam and silica gel, the silica gel was chosen as buffer material and the coefficient of friction was 1.275 by comparing the results of repeated tests of each material. Through the analysis of a series of experiments above, a finger feedback system with a resistive thin-film pressure sensor was designed and a rationally designed drive motor was used to achieve stable and non-destructive picking requirements. What was more, hardware and software of the control system of end effector were designed. Hardware system mainly contained electric relay, stepper motor, motor drivers, force sensor, I/O (input/output) board, solenoid valve and embedded motion controller. Main-computer control interface was designed by MFC (microsoft foundation classes) frame. MFC frame is always used in VC++ software. Program of submitted computer was designed in Language C. Through the above design, the highly integrated hardware that could be controlled by visual software was realized. Lastly, the reliability of end effector for picking navel orange was examined though experiments after the whole platform was established. In the experiments, different stepper motor speeds were took as the influencing factors, and the picking time of single fruit, the picking success rate and the damage rate as the indices, and the experiments were carried out 105 times. The test results show that the average picking success rate is 94.28% when the speed of the stepper motor is 250 r/min, the picking time of single fruit is 1.76 s, and the fruit damage rate is 0. The end effector is proved to be useful and reliable, and provides reference for research on the development of key components of the late navel orange robotic picking.