Wheel Slip Control in ABS Brakes using Gain Scheduled Optimal Control with Constraints
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In a conventional antilock brake system (ABS), the wheel slip will oscillate around a "critical slip" within some given thresholds. This oscillation will have as side effects a noticeable vibration for the driver and limitations in ABS performance. Thus, the actual friction force between tyre and road will oscillate around a "maximum" point. The level of complexity present in current production ABS systems has serious limitations for further development and analysis. This thesis looks at the analysis and design of an ABS controller using a continuously adjustable electromechanical actuator where the ABS aims to control the slip of the wheel to arbitrary setpoints provided by a higher level control system such as the electronic stability program (ESP). Thus, maximum friction force can be obtained together with a vibration free braking. This thesis contributes to stability and robustness analysis of a nonlinear ABS controller with respect to uncertainty in the road/tyre friction using Lyapunov theory, frequency analysis and experiments with a test vehicle. A communication delay between the ABS controller and the electromechanical actuator together with the actuator dynamics introduce phase losses and the effect of these performance limitations are also analysed. This thesis contributes to model-based nonlinear wheel slip controller design, as an explicit gain scheduled LQR design method was used for controller design. Full-scale results are presented for a Mercedes car (E220) equipped with a brake-by-wire system and electromechanical actuators for various test scenarios, which show that high performance and robustness are achieved. The test scenarios consist of straight-line braking on different road surfaces (ice, snow, dry asphalt, wet asphalt and inhomogeneous asphalt/plastic coated surface) and a single experiment for braking in a turn on dry asphalt. The main results of this dissertation have been published in international journals, at international conferences and as a book chapter.