拖拉机线控液压转向系统的双通道PID控制仿真与试验

Simulation and experiment of dual channel PID control for hydraulic steerby-wire system of tractor

拖拉机线控液压转向系统采用的单杆液压缸具有非对称性,为了提高转向系统的控制精度,提出了双通道PID(proportional integral derivative)控制方法,对液压缸活塞杆伸出和缩回的运动进行分通道控制。基于Sim Hydraulics模块建立线控液压转向系统的物理模型,对转向轮的跟随响应、阶跃响应进行仿真试验;同时搭建了线控液压转向系统试验台,进行台架试验,从而分析双通道PID控制对转向系统的影响。仿真试验得出双通道PID控制的跟随误差为0.473°、响应时间为0.273 s,且左、右转向跟随误差基本一致,均优于单通道PID控制,台架试验结果与仿真试验的效果一致。结果表明,线控液压转向系统在双通道PID控制下响应快,跟随误差更小,具有良好的跟随性和较高的控制精度。

The hydraulic steering-by-wire system abandons the mechanical connection between hand-wheel and front steering wheels, and thus the driver′ s intent is delivered via electrical signal. The actuator of hydraulic steering-by-wire system is the hydraulic cylinder. ECU（electronic control unit） generates the corresponding electric current to control the opening of electro-hydraulic proportional servo valve after receiving the steering signal, which correspondingly leads to the displacement of steering hydraulic cylinder. The displacement direction determines the steering direction and the magnitude determines the steering angle. At present, asymmetric hydraulic cylinders are mostly adopted. However, due to the piston areas are varied as the piston rod moves in different directions, even if the opening of electro-hydraulic proportional servo valve and the pressure are fixed, the displacements of the piston rod vary under stretch and retracting condition. Combining hydraulic steering-by-wire system with asymmetric hydraulic cylinder, we know that accuracy adjustment of steering is essentially the adjustment of the hydraulic cylinder piston rod′s displacement on the left or right side. Therefore, the traditional PID（proportional integral derivative） control of hydraulic steering-by-wire system is not efficient for precise control of steering angle. Considering the error is either positive or negative in steering angel, to improve control accuracy, the hydraulic cylinder can be regarded as different hydraulic cylinders with different piston areas working alternately. This paper presented the idea of dual-channel PID control, in which way the 2 hydraulic cylinders（positive or negative error） used different channels with different PID parameter values. The characteristic of dual-channel control was that when the errors, whose magnitudes were same and positivity and negativity were different, were adjusted with different PID values, the control current of electro-hydraulic proportional servo valves was not same. However, the difference of cavity work area finally guaranteed the same displacement of hydraulic cylinder piston rod, namely the accuracy of adjustment. In addition, in order to avoid the low accuracy of a simplified mathematical model as well as the time-consuming defect of other software, we used Sim Hydraulics to build the physical model of hydraulic steering-by-wire system conforming to the real platform, whose preferences consulted the reality; the simulation of following response and steering angle pulse test of the steering wheel was completed, the test rig of hydraulic steering-by-wire system was set up,and then IPA15F2K61S2 chip of STC was chosen, which was written to the dual-channel control program to be a controller of the hydraulic steering-by-wire system platform. The steering wheel angle was the input signal of controller, and the product of the actual angle and the steering ratio was a feedback signal. The output signal of the controller was the current controlling the electromagnetic proportional servo valve. The experiments of the steering wheel were completed under the control of single channel and dual channels on platform; and the effect of dual-channel PID control on the system was analyzed by combining simulation and experiment results. Under the dual-channel PID control, the simulation results showed that, the following error was 0.473° and the step response time was 0.273 s, the errors in different directions were almost the same, and they were both superior to the performance of the traditional PID control, for which the following error was 1.315° and the step response time was 0.334 s. Moreover, compared with single channel PID control, the error of dualchannel PID control decreased by 64%, and the response speed increased by 18%. The results of experiments on platform were consistent with the simulation, which indicated that under the control of dual-channel PID, the response of hydraulic steering-by-wire system was faster, the following error was smaller, and the errors in positive and negative direction were almost the same. Thus, it is concluded that the system under the control of dual-channel PID has superior following performance and control precision as well as more desirable stability and reliability.