In a hybrid electric vehicle (HEV), the braking system is composed of friction braking and regenerative braking. When a driver presses the brake pedal, each braking system collaborates and applies braking torque. The friction brake is a hydraulic system which has a slow response time and the regenerative brake is an electric system which responds quickly. Such characteristics bring a control problem, especially transient characteristic of shifts between regenerative brake and friction brake, because the hydraulic system cannot follow the response time of the electric system. The friction braking torque is also governed by the friction coefficient which changes with temperature. This causes the braking torque to be generated differently with the demanded braking torque, without considering the temperature. Due to these problems, the driver would feel uncomfortable and the vehicle would be unstable resulting from the difference in response time and variance of the friction coefficient when pressing the brake pedal. Hence, it is essential to coincide the settling time of friction and regenerative braking system regarding the temperature. To solve these problems, the hydraulic system was mathematically modelled using the flow and continuity equations and the electric system was modelled using the d-q transformation and voltage equation. The temperature estimation model of the brake components was developed using the heat transfer methods which are conduction, convection and semi-infinite solid. The brake temperature was calculated by the finite difference method (FDM). With the mathematical model of hydraulic and electric systems, the coincidence control for the settling time of both systems was established. It was also possible to find the friction coefficient and calculate the braking torque by using the temperature estimator. In this paper, the numerical simulation was carried out to verify these control algorithms. The difference in response time between friction and regenerative braking system was reduced and the transient characteristic was improved. Also, the braking torque was compensated with the temperature, and the difference between demanded and actual braking torque lessen using the algorithms.
The sideslip angle is used to control a vehicle for safety, however, it is hard to estimate the sideslip angle. In many researches, the estimator needs many unattainable variables, such as yaw inertia or cornering stiffness, to estimate the sideslip angle. This paper proposes the observer uses geometrically derived equations. The estimator needs a compensation value to estimate the sideslip angle instead of unattainable variables. The compensation value compensates the error from the imperfections of the formulas and nonlinearity motion at high speed. Estimator tunes its compensation value automatically, and uses the least squares method for estimating the compensation value and to apply to a real vehicle. Estimator is developed by LabVIEW, and gets the simulation result through CarSim. The algorithm of finding a proper compensation value is simply using the sensor data that are already mounted on vehicles, such as yaw rate, longitudinal speed, and lateral acceleration. The real vehicle test was conducted to verify the proposed algorithm. From the results, the algorithm can estimate the sideslip angle without unattainable variables.
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