| Electric-wheel technology that integrates power,transmission and brake system into the hub,cancels the traditional complex automobile transmission,so the mechanical part of the vehicle has been greatly simplified.Electric-wheel drive vehicles are ideal platform for future general purpose drive automotive,and have good prospects and technology development potential.Therefore,we proposed a new kind of non-trapezoid drive-by-wire driving steering system based on electric-wheel.It can precisely control the steering angle of each wheel to ensure that all wheels roll around the same center while steering.Based on this,the mechanism,implementation,control strategies and the influence of electric-wheel driving steering system on handling stability were researched.The feasibility and superiority of electric-wheel driving steering system was verified through virtual test and a small prototype electric vehicle test.Firstly,the paper described the vehicle steering history,current situation and development trend,as well as major automotive power steering technology research in China and abroad.Based on this the study meaning and content is proposed.After the mechanical model of electric-wheel driving steering system was established,the feasibility of electric-wheel driving steering system has been analyzed.Based on this,the whole vehicle mechanics model was established.After the whole vehicle force was analyzed,it is found that the steering-wheel-drive torque inside and outside is different in order to realize electric-wheel driving steering.With this,the active steering yaw moment can be achieved and reduce the vehicle's understeer characteristics.So the vehicle can have a faster steering response.Moreover because the force of electric-wheel driving steering system is constrained by ground adhesion,the optimal adhesion control strategy based on wheel attachment coefficient change rate is proposed to make full use of the ground adhesion.Through the analysis of multi-mode steer,it is found that only non-trapezoid steering system can meet the requirements of vehicle turning flexibility.Based on this,the non-trapezoid steering system has been proposed,and the coordination control mathematical model of wheel driving torque and turning constraints mechanism has been built.Electric-wheel model and permanent magnet brushless DC motor model used for calculating longitudinal force between ground and wheel were established for later virtual test analysis.Based on the designed steering control mode and mechanism parameters,Pacejka'89 tire model based on magic formulas was selected,and three steering systems were designed.The truck multibody kinetic model with 23 degrees of freedom of electric-wheel driving steering system was established with simulation software ADAMS.The control strategy model was established with software Simulink,including ADAMS_S subsystem passing parameters,air resistance subsystem,front wheel steering control subsystem,the subsystem setting the initial conditions,and the driving force control subsystem.Based on this,co-simulation virtual test platform was set up for electric-wheel driving steering test analysis.To further verify the feasibility of electric-wheel driving steer,a scaled electric-wheel driving steering small prototype vehicle was designed and produced after appropriate simplified the actual vehicle.With the installation of control systems and test sensors,and programming SCM,RTX51 Tiny multitasking real-time operating system was established.It can measure wheel speed,electronic throttle position,and steering angle.It can realize lower computer steering control with the cooperating of electric-wheel driving steer and steering constraint mechanism.With Lab VIEW software programming,the upper computer can realize serial communications,calculation of the inner and outer wheel steering angle,sending of steering control command,as well as real-time display of vehicle running parameters.Communication between the upper and lower computer is achieved by Bluetooth wireless.With the communication,the measurement and control of small prototype vehicle is carried out.The small prototype vehicle system can provide a test platform for the later analysis of drive-by-wire low-speed steering test based on electric-wheel driving steer.In order to verify the performance of electric-wheel driving steer,three steering modes were established.These modes are ordinary electric power steering system,electric-wheel driving steering system,and electric-wheel driving & constraint mechanism driving steering system.With the ADAMS-Simulink co-simulation virtual test platform built in front,a series of comparative tests were performed.These tests are low adhesion coefficient road angle step input response test,high adhesion coefficient road angle step input response test,and sinusoidal input response test,etc.A series of measurement results were analyzed.These results were vehicle steering angle,yaw rate,lateral acceleration,side slip angle,centroid trajectory,driving speed,wheel slippage rates and slip rate,steering driving torque,and wheel driving torque.It is found that electric-wheel driving & constraint mechanism driving steering system is superior to the other two steering systems,and makes vehicle have faster steering response.However,steering response too fast could prolong steering stability convergence time,and make peak value larger.For example,while angle step input testing on low adhesion coefficient road,stabilization time of mode 3 is less 12.77 percent than the mode 1,mode 2 is longer 8.51 percent than mode 1;the yaw rate peak value of mode 3 is larger 24.46 percent than mode 1,mode 2 is larger 18.23 percent than mode 1.While angle step input testing on high adhesion coefficient road,stabilization time of mode 3 and 2 are both longer 30.95 percent than the mode 1;the yaw rate peak value of mode 3 is larger 19.21 percent than mode 1,mode 2 is larger 26.56 percent than mode 1.While sinusoidal response test,mode 3 yaw rate peak is respectively larger 22.00 percent and 20.34 percent than mode 1,mode 2 is respectively larger 42.64 percent and 60.02 percent than mode 1.These have a negative impact on handling stability,so it should be controlled.With the small prototype vehicle,a series of tests were performed.These tests are stationary steering angle step,fixed circumference steering,steering angle step with 1 km/h running,double lane with 1 km/h running,and straight running.These tests verified the feasibility of electric-wheel driving steering.From curves of wheel steering angle,it can be found that the actual angle of the two front wheels is basic consistent with the theoretical,but there is a small margin of glitches mutation on the local scale,indicating that the designed electric-wheel steering control system can achieve the requirements of steering control,but control accuracy and stability should to be improved,especially the stability control of steering constraint mechanism needs to be further strengthened.Finally,for electric-wheel driving steering system,each wheel driving torque can be controlled separately.Based on this feature,integrated coordination control strategy between electric-wheel driving steering and whole vehicle mechanics was presented.This control strategy was also combined with the whole vehicle yaw rate control and ground adhesion force optimal control.With virtual co-simulation platform,high-speed steering tests were carried out based on integrated coordination control strategy.It is found that on high adhesion coefficient road,based on the electric-wheel driving steer,with Direct Yaw Control,it can make the vehicle have faster steering response,and moreover the convergence time shorten 43.29 percent showing that the steering stability convergence speed is also faster;on low adhesion coefficient road,only with Torque Control System,Direct Yaw Control can work well.Especially the Torque Control System based on wheel attachment coefficient change rate can ensure the full use of ground adhesion,make vehicle travel by predetermined trajectory,and make vehicle have better handling and stability.The vehicle without Torque Control System can't steer,and the travelling trajectory is nearly straight. |
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