Research On Hydraulic Control System For 4WD Front-axle Articulated Vehicle

China has a large number of paddy fields,which are mainly used for the rice cultivation.Due to the site-flatness requirements of rice cultivation,paddy fields usually exist in the form of small plots.Meanwhile,southern China is affected by the hilly terrain,the southern paddy fields are mainly in the form of irregular small plots.Therefore,the requirements for maneuverability of paddy machinery are higher than other agricultural machinery.Consequently,China still uses a large number of walking tractors as paddy fields' tillage machinery.However,there are still problems such as low efficiency and high labor intensity when using walking tractors for paddy field operation.Therefore,research and development a four-wheel paddy field vehicle with better working characteristics is meaningful to increase productivity and reduce labor intensity.Imitated from walking tractors steering method,a front axle articulated steering 4WD vehicle is developed by South China Agricultural University College of Engineering.This vehicle driven by a hydraulic wheel-motors,has the advantages of simple structure,small turning radius,and can directly make a U-turn to the next line when working in the paddy field.It is expected to be an important supplement to our country's small-scale walking machine.In order to realize the control of the front axle articulated steering 4WD vehicle,the vehicle needs to be equipped with a corresponding hydraulic control system.By studying vehicle's manipulative mechanics,this paper construct a mathematical model of front axle articulated steering 4WD vehicle.It can be concluded that by controlling the flow of the front axle wheel motor,the motion control of the vehicle can be realized,including the front and rear axle speed control,front and rear axle yaw rate control,and chassis attitude control.In the vehicle direction control,propose a back-pressure-streeing scheme.By separately control the back pressure of parallel hydraulic motors fixed on the front axle.The motor will run in different speeds that cause articulated-front-axle steering.By setting kingpin ahead of the front axle can obtained the aligning torque,which can make the vehicle walk straight.According to the steering simulation test,have the following conclusions: By setting kingpin ahead of the front axle,the vehicle will have the aligning torque.;The lower speed,the greater the steady-state steering angle that the vehicle can obtain;The cracking pressure of the relief valve is positively related to the steering angle of the vehicle;The back-pressurestreeing scheme can reduce or avoid hydraulic shock.In order to achieve vehicle driving brake,a back pressure brake scheme is proposed.By increases the outlet port pressure of each motor,makes the back pressure greater than the inlet pressure,the motor will generating braking torque.According to the steering simulation test,have the following conclusions: When the vehicle brakes,the greater back pressure,the greater the reverse acceleration obtained by the vehicle;As the back pressure increases linearly,the reverse acceleration obtained by the vehicle also shows a linear increase;In the ramp braking simulation,for a certain slope,the vehicle can be stationary on the slope when the back pressure is above a certain value.In order to achieve negative load control of the hydraulic system,a back-pressure-balance scheme is proposed.According to the vehicle simulation test result,when the vehicle work in a positive load,the motor is in a safe zone regardless of whether there is any hydraulic resistance in the outlet path.In this case,the greater the resistance in the outlet path,the more energy will waste.When the vehicle work in negative load and the hydraulic resistance of outlet path is insufficient,the pump oil supply will less than the motor oil demand.In this case,the motor appear pump-effect,the hydraulic components may not work properly due to lack of oil.After consider the comfort and ease of handling of the vehicle,three-stage back pressure control strategy has been make.By detecting the motor inlet pressure,when the vehicle work in positive load,reduce relief valve cracking pressure linearly,increase pressure energy usage.When the vehicle motor can only supply resistance,maintain relief valve cracking pressure,provide stable braking force.When the motor work in negative load,increases the relief valve cracking pressure,keeps the hydraulic motors from overrunning.According to the back-pressure-brake,back-pressure-streeing and back-pressure-balance scheme,design and manufacture the vehicle hydraulic system prototype.And conducted the experimental research.The results showed that:1,By controlling each outlet pressure of the hydraulic motors fixed in the vehicle front axle,can make the motor get different speed,realize the directional control of the vehicle.The larger the control signal of the proportional relief valve,the smaller the steady-state steering radius obtained by the chassis,which meets the design expectations;According to signal mutation test,the system inlet pressure did not show hydraulic shock,which ensures the safety of the hydraulic system.According to Straight walking test,set kingpin 0.021 m ahead of the front vehicle front axle,the yaw rate of the chassis does not exceed 5%.It is considered that the vehicle has a straight walking ability.2,By controlling system outlet pressure,can let the vehicle obtain brake function.The greater braking signal,the greater the reverse acceleration of the vehicle can obtain.Compare with 2WD mode,4WD mode can provide greater braking force with the same braking signal.3,The back pressure balance system can reduce the outlet hydraulic resistance when the vehicle walk in positive load condition.And provide the corresponding brake force to prevent the motor from overunning during the vehicle walk in negative load.which means that this back pressure balance system is feasible for the front axle articulated steering 4WD vehicle4,According to the off-road test,the vehicle off-road capability will be much better in 4WD mode.Paddy field walking test show that after the front axle articulated steering 4WD vehicle can travel smoothly and steadily.