H-infinity Control Design of an Active Vehicle Suspension System
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The ever-increasing requirements in drive dynamics, comfort and efficiency are challenges that the automotive industry faces when designing new vehicles. Development of advanced suspension systems and drive dynamics control systems are opportunities to meet these challenges. This thesis examines the optimal control problem of a new active suspension system that is being developed at Daimler AG. Several different control structures are tested with the goal of finding the one that has the best comfort and energy-efficiency qualities. The thesis is divided into two main topics. The first one discusses suspension systems and introduces a new active suspension actuator that is being developed at Daimler. The actuator is modeled, linearized and combined with a quarter-car model to create a complete system model. In the second main section optimal and robust control is discussed with a focus on the H-infinity-method. This method aims to find the controller that stabilizes the system and minimizes the worst-case gain from disturbance inputs to system outputs. Six different controllers are designed, discretized and tested by applying them to a detailed nonlinear model of the system. Three of the configurations control the system in a cascaded manner and the rest control the system directly based on the vehicle body acceleration and road level signals. An advanced nonlinear two-degree-of-freedom (2DOF) controller serves as a benchmark system that helps to assess the performance of the new controllers. The study indicates that all six controllers are able to stabilize the system and increase comfort. Comfort performance was assessed with power spectral density (PSD) graphs of vehicle body acceleration data that was measured during test track simulations. The best cascaded control structure has a near identical performance with the 2DOF controller while the best direct suspension controller is able to improve comfort on low frequencies even more than the other systems without increasing the power consumption. Additionally, utilizing preview measurements of the road level further increases the bandwidth of the controller. Implementing the controllers on a testbed would be the next step of the project but it is outside the scope of this work. Future work also includes studying and improving the robustness properties of the controller by using parameter variation during control design.