Modeling And Control Algorithm Research Of Electric Steering System For Agricultural Vehicle

In order to develop the autonomous navigation technology of agricultural vehicle, track the desired front wheel angle accurately and rapidly, reduce labor and ensure the operation accuracy and production efficiency, using an experimental car as test platform, a speed control system model and a steering system model were built independently. Simple and efficient control algorithms were designed. The main research contents are as follows:1. A two-level distributed CAN-Bus network structure was adopted. In the system, the master node (an on-board computer) communicated with four slave nodes (front wheel angle measure node, speed measure node, steering motor control node, driving speed control mode) by CAN-Bus communication network. Good scalability and reliability of the data transmission have been gained.2. System hardware platform was built, which was divided into three parts according to different functions:the main control module, the information acquisition module and the execution module. The main control module was responsible for collecting the vehicle pose information, operating control algorithm, sending control commands and so on. Vehicle’s front wheel steering angle and speed information were collected in the second module. By using the absolute rotary encoder, direction and value of the front wheel angle were calculated by receiving the encoder’s voltage, which corresponded to the angle. By using the incremental rotary encoder, direction and value of the vehicle were determined by measuring the phase difference and number of the encoder’s output pulse. The execution module adopts mechanical structure. Accurate steering and speed control were realized by sending pulse signals of different frequencies to the step motor and sending analog voltage signals of different sizes to driving DC motor’s motor controller.3. By comparing the speed measured by Differential GPS and absolute rotary encoder with actual average speed, precision advantage of the encoder was confirmed. The physical relationship between actual vehicle speed and analog signal received by the driving DC motor’s controller was obtained according to experiments. A closed loop model of vehicle speed control system was built and a proportion control algorithm was designed. In order to achieve a balance between control time and driving stability, an optimal proportional was chose according experimental results. Experiments of speed control system on different roads showed that system could track the desired speed accurately and rapidly.4.According to the relationship between the input (stepper motor’s frequency) and output (steering angle) of the steering system, a second-order model was built. After stability analysis, two optimal PD controllers based on the IAE(integral absolute error)/ISE(integral square error) and ITAE (integral time absolute error)/ITSE(integral time square error) performance index were designed to enhance steering performance. In order to find a better steering controller, experiments were done when the vehicle was driving on the farmland and the cement road at the speed of 0.3 ms-1,0.5 ms-1,0.7 ms"1 respectively. Results showed that the steering system with the optimal PD controller based on ITAE/ITSE performance index, could track the desired front wheel angle smoothly under the desired input signals (±20° of the front wheel angle).The adjusting time was within 0.35s and the steady-state error was less than 0.5°.