The Optimal Design and Simulation of End-effector of Working Robot Yan CHEN 1,a, Zhi-Lin JIANG 1,b, Jia-Wei LI 1, Xiang-Jun ZOU 1, Jia-Sheng WANG 1

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1 2016 International Conference on Mechanics Design, Manufacturing and Automation (MDM 2016) ISBN: The Optimal Design and Simulation of End-effector of Working Robot Yan CHEN 1,a, Zhi-Lin JIANG 1,b, Jia-Wei LI 1, Xiang-Jun ZOU 1, Jia-Sheng WANG 1 1 College of Engineering, South China Agricultural University, Key Laboratory of Key Technology on Agricultural Machine and Equipment, Ministry of Education, Guangzhou , China a cy123@scau.edu.cn, b @qq.com Keywords: End-effector, Clamp module, Cutting module, Optimal design, Kinematical simulation. Abstract. One designation of end effector was raised in order to make robot grasp spherical target effective and cut the conjunction off the matrix. The end effector consists of clamp module and cutting module. The clamp module clamps a spherical target with leadscrew motivated linkage system driven by motor. And cutting module cuts the target with extended blade under the control of steering gear. A kinematic model and static balance model was established and a double targets optimal function and constraint condition is obtained according to the geometry and load-bearing relations. The best composition of key parameters of the end effector was obtained with an optimal design to the size of linkages and power output of the link mechanism of the end effector with MATLAB. A kinematical simulation of clamp module and cutting module of the end effector were done with ADAMS. By means of analyzation, cutting characteristic curve of cutting module and kinematic relation curve of key structures in clamp module are acquired. With these methods, the reasonability in structural design of this scheme was turned out. Also, a robot end effector satisfied various performance requirements were achieved eventually. Introduction The use of robots in agricultural production can improve labor productivity, improve the agricultural production environment, prevent pesticides, chemical fertilizers and other damage to the human body, so that it can improve the quality of work [1]. The current agricultural work often used on the robot in a field is fruit and vegetable picking, sorting, grafting and transplanting, when the introduction of agricultural robot in fruit and vegetable harvesting, processing procedures, end effector is often used in grasping operation [2]. To developed a robot in application to vegetable production, the study of the end actuator is the key, which has a great impact on the quality of its work efficiency and operation. For many years, many researchers at home and abroad has been studied a lot about the end actuator for the development and application in fruit and vegetable harvesting robot, such as literature [2-8] of the apple, watermelon, tomato, cucumber, kiwi and strawberry harvesting robot end effector. The operations are clipping to fruit and vegetable body or fruit stalk and cutting or wringing the fruit stalk, according to the respective fruit and vegetable harvesting properties.

2 The planting area and yield of Citrus in Guangdong are third and second in the country. Though the team has studied the citrus harvesting robot, there are still a variety of problems which hinder the actual production. This paper proposed a design scheme of an end effector of an operation robot which is used to harvesting, according to its harvesting properties, and the optimized design and simulation were conducted. These process for provided the foundation to the development of citrus robot end effector, but also provided a reference of optimization design for spherical fruit and vegetable robot end effector. The Designation and Optimization of the End Effector The Structure and Working Principle of the End Effector According to the growth pattern of citrus and the requirement of harvesting, the end effector must have the function of clamping fruit and cutting off the fruit stalk. On the basis of the above results, this paper presents a design scheme of the citrus picking end effector, which is mainly composed of clamping module and cutting module, as shown in Figure 1. The clamping module mainly comprises a driving mechanism, a linkage mechanism and arc finger, cutting module composed of a power output cutting device and extended blade. Note: 1.Cutting servo 2. Stalk 3. Extended blade 4. Citrus 5. Arc finger 6.Screw rod 7.Machine frame 8.Linkage mechanism 9.Slider 10.Clip motor 11.Servo bearing Figure 1. The general structure of citrus picking end-effector for working robot. When the end effector is working, the screw rod transmission mechanism, driven by the clip motor, drive the slider in the bottom plate of the machine frame to make linear movement; linkage 8 do planar motion with the linear movement of the slider. The connecting rod mechanism drive finger to clamp citrus. When reached the preset stable clamp to force, cutting servo drive the overhanging blade to rotate and cut the citrus fruit stalk, After that, servo work reversed and extended blade reset. Finally, clamping motor work reversed, fingers loosen, citrus shed and picking motion is finished. Optimal Design of Connecting Rod Mechanism Kinematic Model. In order to analyze the process of the execution of the fruit of Citrus picking, optimize the structure parameters and obtain the optimal parameters combination, it is necessary to establish the kinematic equation of the end clamping module. Clamping module and connecting rod mechanism motion diagram, as shown in Figure 2, the dotted line in the figure means the movement trajectory of slider, each connecting rod and clamping finger, the coordinates of each point as follows shows A(LOA, 0), B (LOA +LABcos1, LABsin1), C(LFK, LKO), G (LCK +LCGcosε, LKO +LCGsinε).

3 In order to avoid the excessive size of the end effector, the C point coordinates are (25, 60). According to the kinematic relationship of Figure 2, the relationship between the parameters of the clamp and the sliding block is obtained by using the Matlab software: L (sin sin ) s L (cos cos ) AB 2 1 2atan( ) AB 2 1 (1) In this equation, (25- L )( L (25- L ) - L ) (25- L ) ( L (25- L ) - L ) = -acos( ) OA AB OA BC OA AB OA BC LAB (3600+(25- LOA ) ) 3600+(25- LOA ) 4 LAB (3600+(25- LOA ) ) -(25- L )( L +900+(25- L ) - L ) (25- L ) ( L +900+(25- L ) - L ) = -acos( + 1- (1- )) OA AB OA BC OA AB OA BC (900+(25- LOA ) ) LAB 900+(25- LOA ) 4 LAB (900+(25- LOA ) ) Where: LOA is the distance between the center of slider and joint A,mm; LAB is the length of the rod AB, mm; LCG is the length of rod CG, mm; 1 is the angle between rod AB and x axis on initial condition,º; 2 is the angle between rod AB and x axis after the movement of slider AO,º; ε is the angle between the rod CG and x axis,º; Φ is the angle of rod BC,º; η1 is the angle between AC and x axis oninitial condition,º; η2 is the angle between AC and x axis after the movement of slider AO,º; λ1 is the angle between AC and AB on initial condition,º; λ2 is the angle between AC and AB after the movement of slider AO,º. Static Model. Clamping force diagram as shown in figure 3. CFDG is a parallel quadrilateral structure. Fo is the reaction force (equal to the clamping force and the opposite direction) of the citrus, and P is the force screw rod perform to the slider. The angle between the line of P and X axis is, the angle between side BC and CG in the connecting rod GCB P shown asυ. The stress of the connecting rod AB is when the clamping finger is in the 2sin static balance. According to the principle of moment balance, the mathematical relationship between the force acting on the sliding block and the reaction force between the clamping fingers is as follows: 2FL o CG cos(180 ) tan P L BC (2) Where, LCG the length of linkage CG, mm;

4 Figure 2. The diagram of clamping mechanism motion force. Figure 3. The diagram of clamping mechanism. Parameter Optimization. The structure parameters of the connecting rod mechanism in the end effector not only affect its overall performance, but also affect its volume and weight. LZ is the sum of the length of the connecting rod, the LZ value smaller, the smaller the total weight of said connecting rod, the end effector lighter; P for wire rod on the slide force, the smaller this value, the lower the energy that the motor drives the screw rod to rotate and drives the sliding block to move meaning that the required motor power can be reduced. Therefore, the objective optimization function is given: min( L ) 2( L L L 2 L ) (3) Z OA AB BC CG 2FL O CGcos(180 ) tan min( P) L BC (4) If the connecting rod AB is too short or the connecting rod BC is too long, the clamping opening angle would be too small easily. If the connecting rod CG is too short, the clamping fingers hold the citrus would hit the bottom plate. Also, the size of the slider affects the motion stability of the linkage mechanism and the weight of the clamping module. Therefore, the length of rod AB, BC, CG and the slider OA should fulfill the condition: 10 LAB 45, 5 LBC 20, 20 LCG 45, 8 LOA 15. The movement of sliders should be controlled in a certain range, in order to avoid that the end of the base size is too long. The maximum value of the travel of the slider is 40mm according to the average diameter of citrus. In addition, in order to ensure that the end effector can clamp different sizes of citrus successfully, the closed angle of the finger-fixed rod BC should be satisfied: 45 The length of AC should follow the geometry relationship of plane triangle: LAC LAB LBC. This problem is a multi-objective constrained optimization problem, concluded from the optimization variables, target function and constraint conditions above. The optimization problem is solved with Matlab, according to the kinematic and static mathematics model of clamp model. In the end the best length combination among all rods in linkage mechanism is obtained, as shown in table 1.

5 Tabel 1. Optimal combination of length of the linkage of the linkage mechanism. LOA LAB LBC LCG 8mm 35 mm 38 mm 20 mm Simulation of Picking End Effector With ADAMS In order to analyze the performance of grasp module and cutting module of the citrus picking end effector and verify the rationality of its structural design, a kinematical simulation of grasp module and cutting module of the end effector were done with ADAMS. The following analysis is based on the simulation result. The Law of the Variation from the Clamping Force to Holding Time According to Fig.4, during the grasping process, when the gripping fingers touch the citrus, the clamping force rush up quickly, but during the rising process it would reduce and restore immediately. The smaller the diameter of citrus is, the more obvious this phenomenon becomes. The clamping force is getting stabilized when it is close to 7N, which shows that the gripping fingers can grasp citrus stably when the clamping force reach 7N. Besides, the larger the citrus diameter is, the less holding time it needs. Figure 4. Curves of the relationship between clamping force and holding time. Motion Analysis of Rod AB Driven by the leadscrew, rod AB is in plane motion, which can be decomposed into two different direction, respectively x and z. The x-direction is the translation direction of the leadscrew and the z-direction is the normal direction of translation of the slider. According to Fig.5, The speed curve of rod AB is smooth, which shows that rod AB moves steadily. With the continuation of the grasping process, the speed in x-direction rush to the top immediately and then reduce evenly. However, the speed in z-direction increase slowly and then reduce slowly. Within the simulation range, under the same grasping-motor speed, the large the diameter of citrus is, the faster the rod speeds in x-direction and z-direction are.

6 a. x-direction b. z-direction Figure 5. The speed curves of rod AB. Motion Analysis of Rod BC One end of rod BC is attached to rod AB by a pin, and the other end is attached to gripping fingers. Under the action of rod AB, rod BC rotates around a fixed spindle. According to Fig.6 and Fig.7, during the grasping process, the rotation angle curve and the angular velocity curve of rod BC is smooth. With the continuation of the grasping process, the rotation angle increases steadily, and the angular velocity reduces slowly, which reflects that rod BC moves steadily. Figure 6. Curve of the relationship between the rotation angle and time. Figure 7. The speed curve of rod BC. Cutting Performance Analysis According to Fig.8, with the rotation of the cutter, the sliding cutting angle declines. When the cutter begins to cut the stalk, the cutting force rush up quickly and the sliding cutting angle is at this point. When the cutter completely cut off the stalk, the cutting force drop instantaneously and the sliding cutting angle is at this point. According to the result of the cutting simulation experiment, the sliding cutting angle of the cutter reduces continuously from the cutter touching the stalk to completely cutting it off, but it remains at a large level. According to the sliding cutting experiment of fruit stalk experiment [10], the larger the sliding cutting angle is, the less the cutting force and cutting power are. The simulation result shows that the design of the cutting module ensure a fine cutting result.

7 Figure 8. Curves of the relationship between cutting force, sliding cutting angle and time. Conclusion 1) Experiments on grasping citrus by the end effector of the robot were carried out. The result shows that the arc gripping fingers have a better grasping performance because the minimum clamping force of the arc griping fingers is less than that of the surface gripping fingers. 2) According to the way of picking citrus, a design end effector of robot was carried out, in which a kinematic model and static balance model was established. According to the picking requirements and the mechanical properties of the picking machine, the optimal size combinations of all the linkage rods were acquired with Matlab. 3) A picking simulation analysis of the end effector was carried out. The result shows that the linkage rods in the grasp module moves steadily, and the little displacement of these rods bring a large displacement to the gripping fingers. The gripping fingers keep a low speed when it is grasping citrus, which can protects the citrus from impact damage easily. In the cutting module, the cutter keeps a large sliding cutting angle to cut off the stalk, which has a good cutting effect. Acknowledgement This work was supported by Science and Technology Project of Guangdong Province (2015A , 2014A , 2015A ), and Industry University Research Science and Technology Project of Guangdong Province (2014B ). References [1] L.B. Zhang, S.M. Ji, F. Xu, X. Zhang, Y.H. Wan and X.R. Zheng: Main Application Domains and Key Techniques of Agricultural Robots, Vol. 30 (2002) No.1, pp (In Chinese) [2] S. Hayashi, K. Ganno, Y. Ishii ans I. Tanaka: Development of a Harvesting End-Effector for Eggplants, Vol. 13 (2001) No. 2, pp [3] Y. Edan, D. Rogozin, T. Flash and G.E. Miles: Robotic Melon Harvesting, Vol. 16 (2001) No.6, pp

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