Variable Stiffness Suspension Systems

Olugbenga Moses Anubi

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Abstract

Improvements over passive suspension designs is an active area of research. Past approaches utilize one of three techniques; adaptive, semi-active, or fully active suspension. An adaptive suspension utilizes a passive spring and an adjustable damper with slow response to improve the control of ride comfort and road holding. A semi-active suspension is similar, except that the adjustable damper has a faster response and the damping force is controlled in real-time. A fully active suspension replaces the damper with a hydraulic actuator, or other types of actuators like electromagnetic actuators, which can achieve optimum vehicle control, but at the cost of design complexity. The fully active suspension is also not fail-safe in the sense that performance degradation results whenever the control fails, which may be due to either mechanical, electrical, or software failures. Recently, research in semi-active suspensions has continued to advance with respect to capabilities, narrowing the gap between semi-active and fully active suspension systems. Today, semi-active suspensions (e.g using Magneto-Rheological (MR), Electro-Rheological (ER) etc) are widely used in the automobile industry due to their small weight and volume, as well as low energy consumption compared to purely active suspension systems. However, most semi-active design concepts are focused on only varying the damping coefficient of the shock absorber while keeping the stiffness constant. Meanwhile, in suspension optimization, both the damping coefficient and the spring rate of the suspension elements are usually used as optimization arguments. Therefore, a semi-active suspension system that varies both the stiffness and damping of the suspension element could provide more flexibility in balancing competing design objectives. This work considers the design, analyses, and experimentation of a new variable stiffness suspension system. The design is based on the concept of a variable stiffness mechanism. The mechanism, which is a simple arrangement of two springs, a lever arm, and a pivot bar, has an effective stiffness that is a rational function of the horizontal position of the pivot. The effective stiffness is varied by changing the position of the pivot while keeping the point of application of the external force constant. The overall suspension system consists of a horizontal control strut and a vertical strut. The main idea is to vary the load transfer ratio by moving the location of the point of attachment of the vertical strut to the car body. This movement is controlled passively, semi-actively, and actively using the horizontal strut. The system is analyzed using an \(\mathcal{L}_2\)-gain analysis based on the concept of energy dissipation. The analyses, simulation, experimental results, show that the variable stiffness suspension achieves better performance than the constant stiffness counterpart. The performance criteria used are; ride comfort, characterized by the car body acceleration, suspension deflection, and road holding, characterized by tire deflection.