Abstract:

With the development of computer and automation control technology, robots have gradually entered the field of agricultural production. The application of agricultural robots can improve labor productivity, product quality and working conditions, solve the problem of labor shortage, and promote the intellectualization of agricultural production process. Fruit harvesting is the most time-consuming and laborious part of agricultural production. Since the skin of fruit is relatively fragile, it is easy to cause damage in the process of grasping. Therefore, some flexibility is necessary for the grasping device. As the end of the picking, robot directly acts on the part of the grasping object, the manipulator has attracted more and more attention of scientific researchers because of its light weight, small size, low energy consumption, high flexibility and low cost. Manipulator is the core component of robot, which is installed on the end of picking robot and acting on the object directly. In order to improve universality and flexibility, reduce the damage to the fruits, and shorten the design cycle, the flexible manipulator with simple structure and self-adaptive function was designed to achieve favorable grasp of fruits. The manipulator developed based on 3D printing has the advantages of rapid prototyping, low experimental cost and easy to assemble, etc. Flexible manipulator consists of flexible finger, wrist, base and pneumatic components. Its general action process is opening, grasping, moveing and putting down. However, flexible manipulator combines the two processes of moving and putting down into swallowing, which reduces the execution of the motion and improves the grasping performance and efficiency of the manipulator. Pneumatic components and wrist were printed from flexible materials and the material is thermoplastic urethane and polylactic acid respectively. The wrist is an integral part with flexibility. The use of pneumatic components can achieve the wrist bending, driving flexible fingers self-adaptive deformation to grasp the fruit. The manipulator is placed on the vertical sliding platform of the four-wheel platform, which can move up and down, and the four-wheel platform can move freely in all directions. The single wrist has two rotational degrees of freedom. The kinematics model of single wrist was established by combining constant curvature deformation and D-H coordinate method. On this basis, the functional validation test and safety test of flexible manipulator were carried out. In the safety test, the thin pressure sensor was used as the detection element of the contact force signal between the finger and the grasping object. The experiment results show that the pneumatic components of the flexible manipulator meet the design requirements and the driving wrist is flexible. The manipulator has certain flexibility, and can adapt to the shape of the fruit for self-adaptive grasping. The self-adaptive grasping effect of the manipulator is remarkable, and the fruit skin is intact. Moreover, the flexible manipulator has a favorable self-adaptive function based on the structural design and the complexity of the control system is deduced. In addition, it will provide reference for the design of the flexible grasping mechanism.

Key words: flexible manipulator, 3D printing, kinematics, self-adaptive function, contact force