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UBC Theses and Dissertations

Study on discrete-force suspension system for a car on randomly profiled road Yang, Dajiang


An active suspension system possesses the ability to generate arbitrarily controllable forces in an automotive suspension. It has widely been accepted as the most promising car suspension scheme. Prototype active suspension systems have shown impressively smooth ride, firm attitude control ability and good handling quality. Present implementations are, however, not perfect and still leaves some problems which represent the limitations of an active suspension system. The most conspicuous problem of performance is ride harshness over small, sharp bumps. Active suspension is worse than the passive variety at isolating high frequency vibration because the force actuator bandwidth is not big enough to cover the frequency range for both sprung and unsprung masses. The energy consumption is also a problem. The power required by system's hydraulic pump is more than three horsepower. A new suspension scheme called discrete force suspension (DFS) is introduced to solve these problems. In order to improve the high frequency characteristics of the active suspension and significantly reduce the extra external power required by the hydraulic system , a new type of hydraulic actuator — digital force actuator (DFA) is developed to replace the conventional one. A DFA only outputs a limited number of force levels but does not restrict the flow in the system. Therefore, it transmits little high frequency disturbance through it and has a very little energy loss. It has been studied through theoretical analysis and numerical simulation. Results show satisfyingly small equivalent stiffness and damping coefficient, and a quite linear relation between output and input. Unfortunately, no sensible outcome was obtained for the physical experiment due to mistakes made during the manufacturing and design. Then the idea to control the low frequency body mode and the high frequency wheel mode separately by a DFA and a conventional damper (or a dynamic absorber) respectively is developed to avoid the necessity of wide bandwidth actuators and to achieve a better high frequency performance. Due to introducing DFA into the control system, a new control strategy — step-wise reference control, was also developed, analyzed and adopted to cope with the problem of discrete control force coming up with DFA. Numerical simulations have been performed for a quarter car model to demonstrate the feasibility and effectiveness of a DFS in terms of the independence of the DFA output to the suspension motion, the significant reduction of the extra power requirement of the DFS in comparison with an active suspension, and the vibration isolation characteristics.

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