Simulation and experiment of dual channel PID control for hydraulic steer by wire system of tractor

The hydraulic steering by wire system abandons the mechanical connection between hand wheel and frontsteering wheels, and thus the driver′s intent is delivered via electrical signal.The actuator of hydraulic steering by wiresystem is the hydraulic cylinder.ECU (electronic control unit) generates the corresponding electric current to control theopening of electro hydraulic proportional servo valve after receiving the steering signal, which correspondingly leads to thedisplacement of steering hydraulic cylinder.The displacement direction determines the steering direction and themagnitude determines the steering angle.At present, asymmetric hydraulic cylinders are mostly adopted.However, due tothe piston areas are varied as the piston rod moves in different directions, even if the opening of electro hydraulicproportional servo valve and the pressure are fixed, the displacements of the piston rod vary under stretch and retractingcondition.Combining hydraulic steering by wire system with asymmetric hydraulic cylinder, we know that accuracyadjustment of steering is essentially the adjustment of the hydraulic cylinder piston rod′s displacement on the left or rightside.Therefore, the traditional PID(proportional integral derivative) control of hydraulic steering by wire system is notefficient for precise control of steering angle.Considering the error is either positive or negative in steering angel, toimprove control accuracy, the hydraulic cylinder can be regarded as different hydraulic cylinders with different pistonareas working alternately.This paper presented the idea of dual channel PID control, in which way the 2 hydrauliccylinders (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, wereadjusted 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 aswell as the time consuming defect of other software, we used SimHydraulics to build the physical model of hydraulicsteering by wire system conforming to the real platform, whose preferences consulted the reality; the simulation offollowing 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 controlprogram to be a controller of the hydraulic steering by wire system platform.The steering wheel angle was the input signalof controller, and the product of the actual angle and the steering ratio was a feedback signal.The output signal of thecontroller was the current controlling the electromagnetic proportional servo valve.The experiments of the steering wheelwere completed under the control of single channel and dual channels on platform; and the effect of dual channel PIDcontrol 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 indifferent directions were almost the same, and they were both superior to the performance of the traditional PID control, forwhich the following error was 1.315° and the step response time was 0.334 s.Moreover, compared with single channel PIDcontrol, the error of dual channel 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 andnegative direction were almost the same.Thus, it is concluded that the system under the control of dual channel PID hassuperior following performance and control precision as well as more desirable stability and reliability.