| Taking agricultural machinery as a carrier and taking agricultural science and technology innovation as a breakthrough, developing automation, informationization and intelligentification of agricultural machinery is an effective way to realize the raise of production quality and efficiency; it can meet the urgent demand of agricultural production and achieve the rapid development of agricultural mechanization. In order to reach the goal of autonomous following of vehicles, an autonomous following vehicles test platform was designed on the basis of two small electric modified vehicles. The design and implementation method on speed control system, heading angle following control system, following interval control system and collaborative control between guiding vehicle and following vehicle were conducted in depth. The main research work was summarized as below:1. The autonomous following vehicles test platform was set up. The platform was composed of core controller, actuators, detecting devices and wireless communication system of the control system for guiding and following vehicle. The digital signal processor TMS320F28335 was selected as the core controller of guiding and following vehicle’s control system. The actuators of two vehicles included speed regulating mechanism, steering and braking mechanisms. Speed control was realized by controlling differential motor’s rotation rate. Steering was realized by controlling the stepping motor. Braking was realized by controlling the linear actuator. In the detecting devices of two vehicles, the speed, front wheels’ deflection angle and linear actuator’s travel of the vehicle were measured by incremental type rotary encoder, absolute type rotary encoder and absolute type wire displacement sensor respectively; relative heading angle and following interval were measured respectively by reflection-type infrared sensor and ultrasonic ranging sensor. The wireless communication system of two vehicles was set up by wireless transport module UTC-1212, which is used to achieve communication of status information between two vehicles. RTK-DGPS was used to record two vehicles’ traveling track and verify the operational effect of the system.2. The design and realization of speed control system of two vehicles was accomplished. The mathematical model of differential motor drive system was determined according to the experimental data. A PI speed closed-loop controller was designed according to the speed closed-loop control structure; the controller was simulated and adjusted its parameters. Combining braking mechanism, hardware realization and actual performance test of speed control system were accomplished.3. The design and realization of heading control system of the following vehicle was accomplished. Detecting method and structural design of relative heading angle based on reflection-type infrared sensors was proposed, and the detection principle was analyzed in detail. By analyzing kinematical of following vehicle, mathematical relationship between front wheels’deflection angle and heading angle was derived. Steering system control model drove by stepping motor was designed, and the simulation of it was investigated under the Matlab-Simulink. A PD heading following controller was designed, and the parameters of controller were adjusted. The performance test of actual operation was accomplished.4. The design and realization of following interval control system was accomplished. A control method of vehicles’ interval based on ultrasonic ranging was proposed, a PD following interval controller was designed. The control system adopted a double-loop structure, in which the speed control loop was the inner loop and the following interval control loop was the outer loop. The controller was simulated and adjusted its parameters, and the performance test of actual operation was accomplished.5. Cooperative control algorithm of the guiding vehicle and the following vehicle was designed. According to the wireless communication system of two vehicles, cooperative control algorithm of two vehicles combined with the self-defined format and content of cooperative control command statement was designed. Matlab software was used to develop PC software of vehicles’ status information monitoring, realizing real-time display and retention of two vehicles’ status information in autonomous following.On the basis of the design of hardware and software, a series of performance testing experiments were carried out, test results were summarized as below:1. Performance test of speed control. The results of the speed control test were as follows:the target of travel speed was set to be 1.11m/s(4Km/h), driving on the flat cement road, the settling time of speed control system was 0.87s, the average of steady-state speed was 1.07m/s, the relative error was 3.6%, the mean square deviation of steady-state speed was 0.04m/s; driving under the condition of changing road, the settling time of speed control system was 0.85s, the average of steady-state speed was 1.06m/s, the relative error was 4.5%, the mean square deviation of steady-state speed was 0.05m/s; driving on the grass mud road, the settling time of speed control system was 0.97s, the average of steady-state speed was 1.04m/s, the relative error was 6.3%, the mean square deviation of steady-state speed was 0.09m/s. The above test results showed that speed control system has rapid response speed, high accuracy and good stability when running under actual road conditions.2. Performance test of heading following control. The results of the heading following control test were as follows:the maximum of settling time was 3.3s when targets of heading angle were respectively set to be ±5°, ±10°, ±15° and ±20°, there was no overshoot when targets of heading angle were respectively set to be ±5° and ±10°, the overshoot was 3.3% when target of heading angle was set to be ±15°, the overshoot was 5% when target of heading angle was set to be ±20°. The actual control effect of heading following was consistent with the simulation results, and has high accuracy and good stability.3. Overall performance test of autonomous following control. Under the cement road condition, the following interval was set to be 1.5m, the following speeds were respectively set to be 2Km/h and 5Km/h, the results of autonomous following control test were as follows:the biggest lateral deviation of the tracks between the guiding vehicle and the following vehicle was 8.1cm, the maximum of the average deviation was 2.8cm, the maximum of the variance was 4.5cm2; the biggest deviation of the following interval was 7cm, the maximum of the average was 152.6cm, the maximum of the variance was 7.4cm2. The results of autonomous following control test on the orchard path were as follows:the biggest lateral deviation of the tracks between the guiding vehicle and the following vehicle was 9.2cm, the average deviation was 3.3cm and the variance was 5.5cm2; the biggest deviation of the following interval was 9cm, the average was 153.7cm, the variance was 9.2cm2.The overall experimental results showed that the speed control system, heading following control system and following interval control system designed in this paper could realize autonomous following of vehicle, with high stability and reliability, which reflects the application prospects in complex agricultural environment. |
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