Design and experiment of automatic leveling control system for orchards lifting platform

Abstract: Lifting platform for orchards plays a significant role in the fruit industry, because it successfully solves many difficult problems of picking fruit and trimming branches. However, there are some uncontrollable problems such as leveling instability, poor operational capacity in current machines. So it is necessary to design better leveling system to improve their working efficiency and stability. Domestic and foreign scholars have done a lot of researches on the leveling system for high-altitude lifting platforms, which provide important academic reference for the design and improvement of the leveling system for orchards lifting platform. There are higher leveling requirements because of poor terrain in orchards. United States, Australia and Japan are the earlier countries to produce lifting platform for orchards and their devices have achieved the automatic leveling. They are yet too expensive for Chinese farmers to afford. In China, it was not until 1980s that the lifting platform for orchards was made. Some types of the lifting platforms made in China realized mechanical leveling, but their leveling accuracy and working efficiency were lower. To this end an auto leveling control system was designed on the basis of the related information at home and abroad. Such a leveling control system consisted of lifting platform, leveling mechanism, hydraulic system and control system. By analyzing forces and movement of the leveling mechanism of the leveling control system, the dynamic mathematical model for the control system was set up. As we all know, the fuzzy control can decrease the overshoot, and PID (proportion, integral, derivative) control can increase the response speed, and Kalman filter can reduce signal interference. So the fuzzy PID control system with Kalman filter was designed. According to the information collected by inclinometer, the control system makes a decision, and then controls electromagnetic valve to drive the leveling cylinder to extend or retract, which can change the motion of the work platform and keep it level. The results of simulation showed that the performance of fuzzy PID control system was better than that of traditional PID. Firstly its rise time was 0.09 s, it's peak time was 0.12 s, its regulation time was 0.41 s, and they were respectively shortened by 43.75%, 47.82%, and 48.10%, respectively, which indicated the initial and overall speed of fuzzy PID control system were better. Secondly the maximum overshoot was 17%, which was reduced by 52.78% of traditional PID's, so fuzzy PID control system realized a good stabilization. Moreover in combination with Kalman filter, the leveling precision of fuzzy PID control system was improved greatly. Its steady state error was about 0.051° and was reduced by 44.57% of PID's. The experiments were done in different slopes (5.1°, 10.5°, 14.5° and 18.8°) and different loads (0, 50, 100, and 150 kg). The results showed that the performance of automatic leveling system was better compared with the original leveling control system. The response time of automatic leveling control system was shorter and was about 0.078 s. When the platform wasn't lifted, under the maximum load and the slope of 18.8°, the maximum automatic leveling error was 1.08°, the automatic leveling precision was increased by 1.69°. In the process of the platform lifted, the maximum automatic leveling error was 1.74o, and the automatic leveling precision was increased by 1.91°. In addition, there were some occasional phenomena such as leveling inaccurately and running slowly in the process of experiments. There were following main 3 reasons: 1) Irregular terrain disturbed the signals collected by angle sensors; 2) The workbench deviated from the set position because of hinge clearances; 3) With the action of motion inertia, the workbench swung back and forth when rising or falling too fast. So the following works will be carried out, including modifying the mechanical structure of lifting platform, and researching control algorithm and mathematical model of the control system to improve control accuracy and leveling efficiency. In conclusion, the automatic leveling control system can meet the design requirements and effectively improve leveling degree and work efficiency, and is well suitable for orchards.