Research Of Steer-by-Wire Technology On Wheel Loder

The steering system in wheel loader determines the driving security, workingefficiency, fuel consumption and the labor intensity of driver. However, thetraditional steering system of wheel loader has no adjustable steering sensitivity andno road sense etc. In order to tackle between higher working efficiency and the morestability at high speed, and provide appropriate road sense to the driver, this paperapplies the steer-by-wire technology to wheel loader. It replaces the traditionalmechanical or hydraulic connect by connecting and controlling all the component ofsteering system through micro-electronics technology, using sensors to collect thedata of steering, then transfer them to the micro computer on the vehicle. Thecomputer processes these signals, makes decision and outputs actuating signalssuch as the steering degrees of vehicle. As there is no mechanical connect betweenthe hand wheel and the steering wheel, the limitation of tradition steering system iscompletely eliminated. Therefore the design and the assembly of the vehicles can besimplified greatly and the design of force and angle transfer characteristics becomesfreely. It is a significant innovation to the vehicles steering system.The wheel loader has the distinct characteristics under different operating modes.During the working mode, the vehicle's speed is low, but the steering angle variesgreatly and frequently. While in normal driving mode, the vehicle's speed is high. Asfar as these characteristics are considered, the steer-by-wire is state-of-the-arttechnology for the wheel loader: Under low speed working mode, the high steeringsensitivity mode is adopted to increase the work efficiency and decrease operator'slabor intensity because in this mode the steering wheel only rotates 180° for the fullsteering. While in the normal driving mode, in order to enhance operation securityand the stability, the low steering sensitivity is set. Therefore the steering wheel mustrotate 900° for the full steering. At the same time the wheel loader's maximum speedis only about 30km/h, while that of the automobile is beyond 100km/h, thesefeatures make the steer-by-wire technology applied in the wheel loader other than inthe automobile.This Ph.D. dissertation is funded by the project (No.20040336) named "Application ofSteer-By-Wire technology in wheel loader" from Bureau of Science and Technology ofJILIN province and the project named "Reaearch of Steer-By-Wire technology andApplication on Wheel Loader" funded by corporation. The original steering system ofwheel loader is replaced by a new steer-by-wire system which is suitable for thewheel loader's characteristic. It eliminates the traditional mechanical or hydraulicconnection by connecting and controlling all the component of steering systemthrough micro-electronics technology. This system integrates electro-hydraulicproportion control technology , computer technology and automatic controltechnology etc. The switch between the two steering sensitivity modes in the light ofthe wheel loader actual operation can be implemented. Also the proper road sensecan be provided to the operator. This technology applied to the wheel loader canbring our construction machinery's progress a great stride. Its entire machine workingperformance and the mobile flexibility can be enhanced greatly, the enhancementinternational competitive ability will increase, and the operator's labor intensity will bedecreased.This dissertation designs and develops the steering system in view of the articulatedwheel loader as following:1. The steering kinematics of articulated wheel loader is analyzed and the definitionof equivalent steering arm of force is put forward. The relationship between thesteering piston displacement and steering angle, and also that of arm of force (andequivalent steering arm of force) and the steering angle are derived. The insitusteering process of articulated wheel loader is analyzed. And the characteristics ofthe resistance moment under the insitu steering for both the single-bridge driving andthe double-bridge driving condition are studied. The simulation based on theoreticanalysis has been done in Matlab and validated by the experiment results fordouble-bridge driving wheel loader. And the analysis for the resistance moment of theinsitu steering is reasonable.2. The proportion directional valve, proportion directional valve control flow amplifiervalve, proportion pressure reduce valve control flow amplifier valve are designedrespectively for the steer-by-wire system and compared by experiments. Finally thethree-way proportion pressure reduce valve to control flow amplifier valve is adoptedto the steer-by-sire steering system. The hand wheel subsystem with force feedbackcharacteristic and adjustable steer sensitivity is developed.3. The steady characteristic of the flow amplifier valve is analyzed and itsmathematical model is established by the transfer function. The transfer functions forproportion pressure reduce valve control flow amplifier valve are deduced, and thedynamic characteristics of the system are analyzed.4. In MATLAB/SIMULINK environment, the simulation model of steer-by-wire systemis established, and the performance of the system with no calibration, conventionalPID controller and fuzzy self-tuning PID controller are compared, the simulationresults show that the fuzzy self-tuning PID controller has quick response time, goodanti-jamming ability, it can satisfied the requirement of actual steering system.5. In order to satisfy the needs of the electronic control system of the steer-by-wiresystem, the single-chip is developed with the PIC16F73B CMOS chip, containingthe electric circuits of reset, input, output, drive and display. And the steeringcontrol module can implement the multi-task and real-time features. The electroniccontrol unit software adopts the modular design method which guarantees systemhardware to complete the control task smoothly. Because of the complex operatingmode and the bad working conditions, the wheel loader will be influenced by all sortsof disturbances and the system failure will occur, therefore the system anti-jammingdesign is done to guarantee the system security and reliability under the complexworking conditions.6. Designed Hardware-in-the-Loop simulation system. Experiment was carried outaimed to the characteristic of the flow amplifier valve on this system, the test resultshave confirmed that the shortcoming of the flow amplifier valve. Also the experimentwas done for the steer-by-wire system. The responses of cylinder's displacement withno calibration, conventional PID controller and fuzzy self-tuning PID controller underinput signal of step and sine wave were tested. The simulation is validated by theexperiment. The results showed that the fuzzy self-tuning PID control algorithm canobtain good performance.7.The road test for the wheel loader installed the steer-by-wire system was carriedout, the test results indicated that the control performance of wheel loader used thesteer-by-wire system and control strategy proposed by this article is improved, at thesame time the problem between higher working efficiency and the stability at highspeed is solved, which increases the productivity and decreases the labor intensityof operator.The main innovations of this dissertation are as followed:1. The steer-by-wire technology is applied on wheel loader for the first timedomestically.The traditional steering system of wheel loader has no adjustable steeringsensitivity and no road sense etc. In order to tackle between higher workingefficiency and the more stability at high speed, and provide appropriate roadsense to the driver, compared with three kinds of schemes by lots of experiments,which are the proportional direction valve with big inner diameter, proportionaldirection valve to control the flow amplifier valve and three-way proportion pressurereduce valve to control the flow amplifier valve, the last one is adopted for thesteer-by-wire system. This control scheme implements the spool displacementcontrol by pressure directly other than that by pressure loss from the flow rateindirectly. The advantage is that the machining precision and oil temperature haveno effect on the control and the system control characteristics keep the same underevery condition. Meanwhile, the original open loop control is replaced by the closeloop control to increase the control precision.The hand wheel subsystem with force feedback characteristic adjustable steeringsensitivity is developed. This subsystem has two modes in which the hand wheelcan rotate within 180° and 900° according to working conditions of wheel loader.The hand wheel or steering lever can be selected. While the hand wheel is used,the road sense can be felt by the driver.2. The shortcomings of CAT flow amplifier valve are presented:1) the pilot oil in spring cavity of flow divider valve has not been coupled with thewheel loader operating mode, which causes the hydraulic steering systempressure oil overflow under high-pressure and the power loss increasing;2) The displacement of flow amplifier valve main valve spool is controlled by thepressure drop which the oil flow through the orifice located on the ends main valvespool, this will lead to the characteristic to be influenced by the processing precisionand the oil temperature. If the pressure directly controls the displacement of mainvalve spool, then the above factor can be avoided.3. The steady and dynamic characteristics of the flow amplifier valve analyzed.The steady characteristics of the flow amplifier valve are analyzed by itsmathematical model with the transfer function method. The dynamic characteristicsof the system with transfer function of proportion pressure reduce valve control flowamplifier valve are analyzed.4. The fuzzy self-tuning PID controller is developed in the steer-by-wire system.In view of the conventional PID control and the fuzzy control insufficiency, the fuzzyself-tuning PID control method is developed combining the PID control and the fuzzycontrol together, which good adaptability for the fuzzy and the high control precisionfor the PID controller. The simulation and the test results both showed that fuzzyself-tuning PID controller has quick response time, good anti-jamming ability. It cansatisfy the requirement of actual steering system.5. Development of software and hardware for electronic control unit.The PIC16F73B CMOS chip is adopted on the single-chip, which contains electriccircuits of reset, input, output, drive and display. And the steering control modulecan implement the multi-task and real-time features.