Research On The Adaptive Hydraulic Active Suspension Of A Wheel-legged Off-road Vehicle Based On The Pressure Control

For the working characteristic, the engineering vehicle travels on various terrainsregularly. In view of that, the engineering vehicle needs to have not only good drivingperformance on flat road but also excellent passing performance on rough road. However,the traditional engineering vehicle has not high off-road performance in non-structuralterrains for the limitation of mechanical structure, transmission type and driving mode.Under the financial support of “211Project” and “863Plan”, a66wheel-legged off-roadvehicle is designed and researched in this paper. With adaptive hydraulic active suspension,the vehicle is able to adapt to terrain actively. Therefore, it could provide outstandingpassability and stability even in complicated three-dimensional terrains.For position and velocity control, electro-hydraulic servo control has been increasinglychallenged by electric servo control. Comparatively, for pressure control, the advantage ofelectro-hydraulic servo control is more and more remarkable. The hydraulic system of thattaking force as the controlled variable is called hydraulic force control system. Thecharacteristics of electro-hydraulic force servo control system are of fast response, highaccuracy, greater power and compact construction. Therefore, it is widely used inengineering field, such as material testing machine, depressing equipment of a rolling mill,tension control system, fatigue testing machine, load simulation equipment and so on.Taking force as controlled variable and displacement as random value, the electro-hydraulicpressure tracking control system is established in this paper. The structure and workingprinciple of system is analyzed. In view of asymmetric electro-hydraulic proportionaldirectional valve matching asymmetric hydraulic cylinder system characteristics, themathematical model of power mechanism is built. The rodless chamber pressure signal is fedback to ECU by pressure sensor in real time. After analysis and processing, output signalmakes wheel to actively adapt to external load. For position tracking control system, wheelflying and vehicle body subsidence will occur while vehicle wading and traveling in softterrain because of visual deviation between radar collected road signal and real road. Forpressure tracking control system, the above-mentioned will be avoided before instabilityoccurs. So the reliability is superior to that of displacement tracking control system. Thecharacteristics, such as high nonlinearity, parameter uncertainty, oil and elements inducedparameter variation, mean that electro-hydraulic force servo system is a nonlinear time-varying system. Therefore, reasonable control strategy plays a vital role forelectro-hydraulic pressure control system.With large inertia and small damping, load change will lead to dynamic qualitydeterioration, even instability. With large value, rapid change and sometimes even reachingor exceeding oil source pressure, external disturbances normally exerts a force upon the loadso as to have a strong impact on tracking performance of servo system. In order to eliminatethe effect of load change, external disturbance and cross coupling, structure invarianceprinciple based design project is proposed in this paper. Take the piston velocity as theobservable quantity with a first-order derivative element for compensator. The rodlesschamber pressure signal is fedback to ECU and converted to compensation quantity so as toeliminate the effect of external disturbance. Accordingly, the system is in top condition allthe while. Simulation analysis confirmed that feedforward compensation was effective. Thesuspension frequency characteristics in low frequency stage have been greatly improved, andbasically meet the requirement of off-road vehicle at low speed. Both redundant forcedisturbance and coupling interference are decreased to a certain extent. And the pressurejump will thus be eased while hydraulic cylinder piston rod reversing. Since fixed parametercompensation cannot offset the effect of internal disturbances such as system itselfnonlinearity and parameters time variability, it is crucial to design a superior controller forthe adaptive hydraulic active suspension, so as to ensure that the suspension performance isoptimal under off-road condition.Superior suspension system is an important guarantee for fast maneuverability andhandling stability of off-road vehicle in the wild unknown non-structural terrain conditions.For hydraulic active suspension, control unit and hydraulic cylinder actuator construct theclosed loop control system. According to vehicle motion state and external input, thehydraulic cylinder takes initiative to adjust and generate the desired force in order to controlsuspension. The control algorithm is crucial to ensure that the suspension control system isof effectiveness, low energy consumption and reasonable cost. The adaptive hydraulic activesuspension is proposed in this paper. The characteristics of asymmetric electro-hydraulicproportional direction valve matching asymmetric hydraulic cylinder are considered, such ashigh nonlinearity, parameter time variability, load disturbance, coupling interference,complex mathematical model, and so on. In view of above mentioned, the auto disturbancesrejection control (ADRC) is introduced. Being independent of exact mathematical model, thealgorithm could compensate for total disturbances effectively, to which controlled object uncertainty, unmodeled dynamics and external disturbance come down. Based on ADRCtechnical principle, the dynamic model of suspension system is built. With the separationcontrol principle, the double loop ADRC is designed. The block diagram of control system isgiven and the mathematical description of every part is established. The stability of controlsystem is analyzed using self stability region theory. Simulation analysis confirmed that thedouble loop ADRC had better performance than PID on rapidity, robustness and antidisturbance capability. For suspension system of complete vehicle, the pressure trajectorytracking experiment in low frequency stage and displacement servo experiment areperformed. Experimental results show that, under the control of double loop ADRC, pressuretracking accuracy is higher than that of PID, and displacement tracking error is lower thanthat of PID. The grounding pressure collected by data acquisition instrument changes in asmall range ensuring traction balanced drive of off-road vehicle in rough terrain. Thepressure data of real road show that the pressure control could track unknown terrainchanges under non-structural terrain condition with high speed and reliability. Therefore, thecontroller gives an excellent performance for the adaptive hydraulic active suspension.In consideration of the special structural feature of non-structural terrain, the first factoris stability and then driving behavior should be taken into account while off-road vehiclemoving. Reasonable design of mechanical structure is the basis for vehicle keeping not onlystatic stability but also dynamic stability. The structure symmetrical chassis is designed inthis paper with six independent wheel-legged movement units. With four-link articulatedarrangement, the vehicle has all-terrain adaptability. And stability and adaptability aregreatly improved. Besides mechanical structure, reasonable stability evaluation should beestablished. Based on stability pyramid model, the minimum stable angle is analyzed in bothstatic and dynamic state. Together standardized dynamic energy stability margin withstability pyramid model, the standardized dynamic energy stability cone is proposed in thispaper. The dynamic minimum tipping energy and tipping stability along the sideline areanalyzed. With square wave on behalf of the discontinuous terrain and sine wave on behalfof the non-structural terrain, the signal response experiments are performed. Experimentalresults show that the vehicle has excellent adaptability and stability. By comparing activesuspension system with passive suspension system, articulated angle and stability angle areanalyzed. Experimental results confirmed that the adaptive active suspension had strongadaptability to uneven terrain. And stability had been greatly improved.The66wheel-legged off-road vehicle is of multi-active drive system. In order to avoid parasitic loss, it is necessary to coordinate wheel speed so as to control vehicle motion.According to the path and bodywork attitude, control the motor speed and hydraulic cylinderaction. Using liquid resistance control technology, wheel edge motor flow is automaticallyassigned with the orifice regulation. Therefore, the speed of each wheel is automaticallyadjusted to realize traction balanced distribution of unilateral wheels. For motor speed, thereis no need for additional control system. The composition and working principle of the drivesystem are introduced. For the variable pump controlled constant displacement motor system,both the piston-swashplate model of pump and the motor speed model are established. Thedynamic characteristics of the system are analyzed. The basic motion of off-road vehicle isintroduced. The kinematical coordinate and constraint equations are established. Thekinematical mathematic models are established under different road conditions. Thequasi-static force balance equation is built. Together kinematical mathematic model withquasi-static force balance equation, the coordinated control rule is given. Taking Gaussianwhite noise as the rough road input and using1mm orifice for control, the pressure and flowof unilateral motor are simulated. Simulation analysis confirmed that orifice regulation iseffective and the multi-wheel coordinated control model was valid and effective. The steadyspeed and balanced traction are guaranteed. And traction efficiency and passing performancein complex terrains are improved.The research in this paper has given a new direction to further improve the off-roadperformance of the engineering vehicle.