Research On Tire-Road Contact Pressure Distribution Measurement Based On Optical Principle

Tire is the only part of a vehicle contacting with the road surface,the contact force between tire and road determines the driving,braking and steering performance of the vehicle.The grounding performance of tire is also closely related to the contact force,so the accurate measurement of tire-road contact pressure is imperative for explorative studies into tire development and vehicle performance.At present,the accurate measurement of the tire-road contact pressure distribution of actual tires is mainly performed by sensors,but densely arranging sensors is expensive and its size limits the resolution.The frustration of total internal reflection(FTIR)method can not only reduce the cost,but also improve the resolution of the measurement results.In order to measure tire-road contact pressure with the optical principle,a test bench is developed.The strength and stiffness of the bench is verified by the finite element analysis method.A toughened glass plate is used to simulate the road surface,a specific mechanism and stepper motor are used to control the camber angle,corner angle,load and rolling speed.When measuring the tire-road contact pressure with the test bench,the pressure in the contact region between tire and road(toughened glass plate)is closely related to the gray value of the image taken by the camera,so the grey value needs to be calibrated to obtain the real contact pressure.There are many factors that affect the relation between the pressure and gray value.Firstly,the camera is controlled to perform calibration experiments at different focal lengths and apertures,and the reasonable camera parameters are determined based on the calibration curve;secondly,taking the impact of ambient light into account,a black box was established to solve the problem of calibration over time;finally,considering the influence of color,saturation,roughness and creep of the material on the calibration,a reasonable calibration material is selected.In order to accurately separate the contact region and non-contact region in the image,a new image processing algorithm is proposed.First,the camera is calibrated based on the Zhang's calibration method to correct the image distortion caused by the camera lens curvature,glass and so on.Then,the image is pre-processed by maximum grayscale method and median filter.Finally,the image is segmented based on the Sauvola algorithm and Gaussian mixture model to obtain the contact region,which is converted into contact pressure distribution according to the correlation coefficient obtained by calibration.Based on the above algorithm,C# and OpenCV are used to write a software,which can efficiently process the pictures obtained by the test,convert them into contact pressure distribution and output corresponding footprint characteristic evaluation indexes.Three PCR(205/55R16)tires with different patterns were taken as the research object.The contact pressure distribution of them under static loading conditions are measured by the optical test bench and Tekscan.Fourteen kinds of footprint characteristic evaluation indexes were selected to compare the results obtained by two methods.The relative error is within the reasonable range,which proves the reliability of the FTIR method.The dynamic tire-road contact pressure distribution of the tire under different parameters were studied,and the trend of contact pressure and the footprint characteristic evaluation indexes of the tire under different loads,inflation pressures,camber angles and corner angle were obtained.