Braking pressure control in electro-hydraulic brake system based on pressure estimation with nonlinearities and uncertainties

Abstract Superior to the conventional brake systems, brake-by-wire (BBW) systems can produce faster response and better performance. This paper presents an adaptive sliding mode hydraulic pressure controller based on a hydraulic pressure estimator to track desired hydraulic pressure for ‘sensorless’ electro-hydraulic brake system (EHB) in the presence of both nonlinearities and uncertainties. There are no add-in sensors (i.e. pressure sensor, position sensor) equipped in the sensorless EHB. The rotation angle of the motor is available by the braking control unit, and it can be transformed to the position of the master cylinder piston via the kinematic relationship. The position is utilized as the input of the whole control system. The friction of the EHB is analyzed and the position-dependent Coulomb + viscous friction model is presented to represent the friction. The system is also influenced by the pressure-position relationship, which is described as perturbations that are nonlinearly parameterized by unknown time-varying parameters. The pressure-position relationship is modeled using a quadratic polynomial, and the nonlinear interconnected observation approach is applied to the system for estimating the unknown parameter and pressure. To eliminate the influence of parametric uncertainties on the control performance, an adaptive module is employed. The uncertainties are also caused by the brake pads wear and the temperature effect. The sliding mode control (SMC) algorithm is used in the pressure-tracking loop to improve the robustness for disturbance. The interconnected stability of the whole closed-loop system is analyzed based on the Lyapunov Theorem. The performance of estimator and controller is verified via simulations and experiments. The results show that the proposed control system provides satisfactory tracking performance and robustness in the presence of uncertainties, nonlinearities and disturbances in the diverse working region.