Lateral Acceleration Estimation Based On Wheel Speed Signal & Curving Braking Control Algorithm Research

During braking on a curve, a lot of vehicles may exhibit a turn-inward tendency due to weight transfers, namely occur oversteering. When the vehicular velocity is very high, the driver can not correct vehicular performance immediately; even cause wrong operation which aggravates the vehicles instability. Traditional ABS and EBD can not allocate brake pressure for avoiding oversteering tendency when the vehicle is braking on a curve. So curving braking control (CBC) system is proposed in this paper.The full text is divided into five chapters.Chapter one is introduction. CBC working principles and domestic and foreign development status quo are intruded, besides the content of the dissertation is put forward also.The calculations of CBC external control quantities, such as lateral acceleration, reference speed and slip ratio are introduced in chapter two. In this chapter, lateral acceleration is estimated by wheel speed sensor signal and points out that the estimation is disturbed by a lot of uncertain factors, for instance, manufacturing tolerance of vehicle system, tire wear, load change, tire pressure change, spare tire and so on. So, it is necessary to modify the estimated value and limit its maximum value to prevent CBC from controlling the vehicle by mistakes. The reference speed and slip ratio are also modified by the lateral acceleration value after finishing its estimation when the vehicle is driving on a curve.The maximum wheel speed law is applied to estimate vehicle reference speed in this paper. Four wheel speed values are not the same in most cases on a curve. In general situation, the outside wheel speeds are faster than inside wheel speed, the front wheel speeds are faster than rear wheel speeds. For calculating curving reference speed, the wheel speeds must translate into the center of gravity (CG) speeds, and then obtain reference speed by maximum wheel speed law. Before translating the wheel speeds into CG speeds, the speed deviations between the wheel speeds and CG speeds should be calculated. If the outside wheel speed is the largest one, it should subtract the deviation to obtain CG speed. On the contrary, if the inside wheel speed is the largest one, it should add the deviation. The method is also applied for the front wheels and rear wheels.Besides modifying curving reference speed, the curving slip ratio is needed to be modified too. The slip ratio stands for slip extent of tire. For driving on straight line, the slip ratio could be calculated by the slip ratio calculation formula directly. But, when the vehicle is turning, the CG speed is different from wheel speeds. In order to obtain curving slip ratios of individual wheels, it is necessary to translate the reference speed into the speeds at the points of wheels. Then the curving wheel slip ratios which are related to slip extent of tire are calculated from these speed values. Cornering identification, oversteering identification and CBC algorithm are introduced in chapter three. The CBC only activates during cornering, so cornering identification should be done in advance. In this paper, slip ratio or wheel speed deviation is selected according to different driving situation to identify cornering. Besides, the lateral acceleration and slip ratio are used for identifying oversteering. If the vehicle occurs oversteering, the CBC makes sure the brake pressure of inside wheels reduce immediately to prevent oversteering. Actually, the CBC mainly keeps outside wheels pressure constant and reduces inside wheels pressure. When the vehicle is braking on a curve, the CBC reduces inside wheel brake torque to create yaw torque which makes the vehicle turn outward.The simulation of CBC is introduced in chapter four. In chapter two and chapter three, the CBC external control quantities calculation and internal control algorithm are proposed. Based on these theories, the simulating models are built up in the Matlab/Simulink environment. The vehicle model is from CarSim software and the driver model was built up by the other team members. The simulation results show that the lateral acceleration estimation is reliable and with high precision, the corrective reference speed and slip ratios approach actual values closely and the CBC improves the cornering brake control performance and alleviates the driver's fatigue of driving.Chapter five is the full text conclusion and research prospect. In this chapter, the next key point of research is that CBC works harmoniously with ABS, EBD and other active safety systems, so that further improve cornering braking performances. In short, the CBC system which is described in the paper could improve cornering braking performances in certain extent, and based on which extends active safety system function.