Evaluation and dynamic forecast of reliability on vehicle brake system based on time-dependent theory

Abstract: As one of the most important sub-systems ensuring vehicle's driving safety, the brake system, whose work reliability greatly determines the safety of car running in a large extent, is usually taken as an important object in studies of vehicle's braking reliability. While for brake system, the use reliability is related to the allowable braking distance, braking deceleration and other relevant parameters prescribed by travel safety regulations. The intention of this paper is to establish a model of evaluation and dynamic forecast on brake system's reliability based on time-dependent theory. Therefore the vehicle's braking performance was studied to analyze the use reliability of brake system and the uncertain factors (such as temperature, speed and wear) which can cause the fluctuation of braking performance and exist in brake system's whole lifetime were also analyzed. In addition, the impact extent of uncertain factors was quantified by fuzzy comprehensive evaluation method in this paper as well. Combined with the study of vehicle braking distances under different conditions, the model can not only assess the reliability of brake system in current time, but also dynamically predict that in future time.The evaluation and dynamic forecast model was developed as follow: 1) the fuzzy comprehensive evaluation matrix of each impact factor (Braking frequency T, braking force P and speed V) was obtained by using experts coring method and normalization; 2) the evaluation value for each impact factor was selected in their corresponding value ranges, according to actual values of T, P and V, such that the actual fuzzy comprehensive evaluation vector for each impact factor was determined;3) in order to reflect the impact on braking distance brought by time-varying uncertain factors, fluctuation coefficients and were computed, which respectively represent mean shift and variance volatility of the jth impact factor's evaluation values; 4) such that the comprehensive fluctuation coefficients and regarding to all impact factors influencing vehicle braking distance could be worked out, by calculating the average. Finally, taking the sliding caliper disc brake system of off-road vehicle as an example, the evaluation and dynamic forecast of the brake system's reliability by employing this model was conducted in this paper: 1) the data of braking distances changing over time was obtained by numerical simulation with Matlab; 2) the comprehensive fluctuation coefficients and were calculated by using the statistical formula of drift rate and fluctuation rate and the fuzzy comprehensive evaluation method mentioned above; 3) by comparing the theoretical value calculated by model with the practical value of braking distance, the practical application value of the calculation model was confirmed, with a deviation of 2.1% which considered very small; 4) to calculate predictive value of reliability and examine whether it was less than 0.5 or not, if so, we could know the braking distance would exceed the legal permission, and warning signals could be issued. The application example shows that the model is valid on assessing the reliability of the brake system of off-road vehicles, and the model can also dynamically predict the reliability of brake system in the future. In conclusion, the model established in this paper, whose simulation results agree with the actual experiences, has a good application prospect.