Magnetorheological shock absorbers : modelling, design and control.

Magnetorheological (MR) fluids enable the rapid and continuous alteration of flow resistance via the application of a magnetic field. This unique characteristic can be utilised to build semi-active dampers for a wide variety of vibration control systems, including structural, automotive, and aeronautical applications. As an example, MR fluids could enhance the performance of aircraft landing gear, which are subject to widely varied and unpredictable impact conditions with conflicting damping requirements. In this thesis, a numerical sizing methodology is developed that enables the impact performance of MR landing gears to be optimised. Using real data provided by landing gear manufacturers, the sizing methodology is applied to both lightweight aircraft, and large-scale commercial jets in order to demonstrate scalability. For both aircraft types, results indicate that the peak force and the severity of fatigue loading can be enhanced over a wide range of impact conditions. However, it is shown that MR landing gears can be heavier than passive systems. To validate the numerical approach, a prototype MR landing gear shock strut is designed, fabricated, and tested. Good correlation between the model and experiment is demonstrated, particularly for low velocity excitations. MR dampers exhibit highly non-linear force-velocity behaviour. For landing gear impacts, it transpires that this behaviour can be used to an advantage, where it is shown that an acceptable performance can be obtained using open-loop control i.e. with a constant magnetic field. However, this non-linear behaviour is highly undesirable for other scenarios (e.g. an aircraft taxiing), and as a consequence, the choice of an effecti\'e control strategy remains an unresolved problem. A further aim of this thesis is therefore to develop effective control techniques for broadband excited MR vibration systems. Through an extensive series of numerical and experimental investigations, case studics representative of the general single-degree-of-freedom and two-degree-of-freedom vibration isolation problem are presented. In the experiments, the hardware-in-the-Ioopsimulation method is adopted, which provides an excellent means to bridge the gap between theory and practice when the behaviour of a specific component is complex. Here, the MR damper is physically tested, whilst the remainder of the structure is simulated in real-time. The results demonstrate that the chosen control strategy can provide significant performance benefits when compared to more commonly used strategies and equivalent passive systems. Furthermore, the control strategy is shown to be insensitive to factors such as the type of input excitation.