Abstract
Abstract: Agricultural engineering operations are often conducted in complex ground conditions such as fields on a hill, forest land, and hilly land. Compared with 6 degrees of freedom (DOFs) parallel robot, lower-mobility parallel robot is especially suitable for picking, planting and transporting crops in complex ground conditions with the advantages of fewer actuators, fewer components, easy control, easy fabrication, low cost, and so on. One translational and two rotational DOFs parallel mechanism is a representative of lower-mobility parallel robot. In order to expand the application of lower-mobility parallel robot in agriculture, this paper presented a novel 2PRC-PRS parallel mechanism, which has one translational DOF and two rotational DOFs. The motion characteristics of the mechanism and its number of DOFs were analyzed based on the theory of reciprocal screw, and through establishing the moving coordinate system and the fixed coordinate system of the parallel platform, the kinematic screw system of each branch and the corresponding constraint wrenches were calculated, the reciprocal kinematic screws of the constraint wrenches of each branch were solved, and then the DOF nature of the moving platform was analyzed based on the kinematic screws of the moving platform. It concluded that the moving platform had 3 DOFs, which were respectively the movement along the vertical direction and the rotations along the two directions which are perpendicular in the horizontal plane. After analyzing the number of DOFs and the motion characteristics of the 2PRC-PRS parallel mechanism, whether the DOF of the moving platform was instantaneous or not was checked. The results showed that the DOF of the 2PRC-PRS parallel mechanism was not instantaneous, and it was continuous. By adopting Z-X-Y Euler angle to describe posture and position of the parallel platform, the inverse position of the parallel platform was calculated. The model machine of the 2PRC-PRS parallel platform was designed and manufactured; combined with the servo motion controller produced by the Bernecker&Rainer industrial automation company, add the appropriate peripherals, design and set up a network motion control system based on Powerlinks bus for the 2PRC-PRS parallel platform, and assemble the electric control cabinet, the control system included human-computer interaction module, master display module, servo-driven module, sensing module and security module. Adopt modularization approach, and based on the ST programming language, develop the 2PRC-PRS parallel platform control software on Automation Studio 3.0 software development platform. It included interactive interface, security alarm, calibration by returning to zero, uniaxial control, multi-axis control, I/O control, spindle control, and so on, which satisfied the basic demands of the experiment. Then the relevant experiments were carried out based on parallel platform, cabinet and control software. Through the regulation of the speed loop and the position loop of the servo motor, the servo motor parameters tuning was completed, and the electronic gear test was completed by writing electronic gear control program under the position control mode. When the electronic gear ratio was 4:1, and the driving shaft speed was set to 2000, the actual value of the driven shaft speed was 500, and the displacement change of the driving shaft and the driven shaft were also in line with expectations, so the electronic gear experiment was successful, and the control system of high precision met the design requirements. The trajectory planning of the moving platform was carried out and the inverse data were calculated, then the interpolation calculation was conducted. And the interpolation moving data point was carried out by inputting the parallel platform inverse data into the control software. The moving platform moved in accordance with the moving data point after it came back to the initial position and it achieved the expected effect. Finally, combined with the network communication technology, the wireless network remote control experiment was performed separately on IOS mobile terminal under the intranet and PC (personal computer) under extranet, which basically achieved the expected target, and also found the signal transmission delay phenomenon, especially under the condition of wide-area the internet image delay was more obvious. This issue will be further researched. This paper is aimed to provide the theoretical and experimental foundation for the agricultural lower-mobility parallel robot and provide the reference for the researching of this mechatronics equipment.
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