| Steering system determines the direction of the vehicle,which is one of the important factors that affect the vehicle's driving ability,stability and safety.At present,tractors use a full hydraulic steering system which can reduce the burden on the driver.However,there is no road-feeling,therefore the driver cannot feel the road through the resistance of the road information.Hydraulic oil is also constantly pumped into the redirector and then flows back to the tank while not turning,leading to a huge energy loss.The advanced steering technology was never applied on a tractor since tractor needs big power and it has to be low cost.However,with the change of the domestic agricultural situation,people's higher requirement on the tractor's comfort,stability,safety performance,and the cost reduction of electronic technology and the development of electro-hydraulic,hydraulic steering technology and wire steering technology have been merged together,therefore the hydraulic steering by wire technology has been developed with the electronic control technology as the core.This steering system consisting all of the advantages of wire-steering system and the advantages of hydraulic steering system of higher power,has been widely used in agricultural vehicles and vehicle engineering.The application of the hydraulic steering by wire system on a tractor was mainly studied.According to the advantages of the hydraulic steering by wire system and the electronic control technology,the ideal angle transmission ratio of the tractor on the road and the steering system control strategy are studied.All the tasks as follows:Firstly,to reduce the burden on the driver is one of the purposes of this study,so the person-tractor-terra closed-loop system was established,and the state equation was obtained.At the same time the accuracy of the closed-loop system had already been verified.Secondly,the method of fixed yaw rate gain to obtain the ideal angle transmission ratio was used in this paper.The yaw rate gain was usually obtained by experiment,so the method of intelligent was adopted in this paper to optimize the yaw rate gain.Handling stability of the tractor is necessary to be considered under road driving Since a tractor usually shifts work sites,drives on the road and the velocity is higher,.Therefore,based on the handling and stability evaluation index of the tractor,the yaw rate gain is optimized by using the genetic algorithm.And finally the relationship of the ideal variable-ratio steering with the steering angle and the speed is obtained.Then,a dual-channel fuzzy PID control strategy was proposed.For the asymmetric single rod hydraulic cylinder,a dual-channel fuzzy PID control strategy was presented.Based on Simulink/Simhydraulic,a physical model of the system was established.Then sine follow response,step response and random follow response simulation test were conducted.The steering effect of hydraulic steering by wire system with different control methods was analyzed.Finally,the dual-channel fuzzy PID control method was proved to be high precision and fast response and it can compensate the asymmetry of steering cylinder and remove influence.In addition,the sine follow,step response and random steering test are carried out on the existing hydraulic steering by wire system test platform,and the influence of different control methods on the steering accuracy was compared and analyzed to verify the superiority of the dual-channel fuzzy PID control method.At the same time,the steering system was modified based on the WORLD 854 tractor.A valve seat for the hydraulic circuit was designed after the modification.All the hydraulic components were installed in the valve seat to consist a hydraulic assembly in order to reduce the energy loss.The hydraulic steering by wire system was connected with the primary steering system in parallel.The three classic conditions of S-shaped,steady rotation,angular step are selected to complete the test in the field including two road surfaces before and after plowing.Based on the experimental results with different control strategies,it was verified that the dual-channel fuzzy PID control strategy not only can improve the control precision,but also compensate the asymmetry of single piston rod hydraulic cylinder.At last,the research work and the innovation point of this paper was summarized.Listing the future study points and development related to the area. |
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