Full suspension mountain bicycles exhibit unwanted suspension movement during pedalling. Damper manufacturers
frequently adopt what is known as platform damping to overcome this problem. Such dampers resist low frequency
pedalling inputs due to the presence of a threshold or 'platform' damping level. However, this platform compromises
shock absorption ability over rougher terrain.
In this paper, the authors describe a prototype rear shock absorber that utilises magnetorheological (MR) fluids to
implement semi-active platform damping. Results from recent field trials will be presented, and the current status of
commercialising the system will be discussed.
Smart fluid dampers can undergo large temperature changes due to the heating associated with energy dissipation. Such
heating will alter the fluid's properties and could degrade control system performance. For example, previous work by
the authors has shown that the stability of an MR damper under feedback control is dependent on the fluid's
compressibility and viscosity. In the present study, a temperature dependent model of a magnetorheological damper is
developed from experimental data, and it is shown that the fluid's yield stress, viscosity and compressibility parameters
vary significantly. An experimental and numerical control study is then performed to investigate the resulting effects of
temperature on the stability of two feedback controllers - a PID controller, and a proportional controller. Experimental
results indicate that both controllers can exhibit a reduction in stability with increasing temperature, particularly if the
controller gains are not suitably chosen. The temperature dependent MR damper model predicts this behaviour well, and
it is shown that the change in viscosity has the most significant effect on stability. Future work could focus on the
resulting effect on a complete vibration system, devices with different modes of operation, and alternative controllers.
Magnetorheological (MR) fluids provide a novel solution to adapt damping levels in aircraft landing gear, so that optimal performance can be achieved over a wide range of conditions. The present study helps to demonstrate the feasibility of this solution by sizing an MR valve within the constraints of an existing commercial (passive) oleopneumatic shock strut. Previous work on MR landing gear has tended to focus on potential control strategies rather than design and sizing issues. However these latter aspects are of great importance in aircraft systems, where space and weight are vital design constraints.
To aid the sizing analysis performed in this study, accurate quasi-steady and dynamic impact models of passive and MR oleopneumatic landing gears are developed. The model is validated against experimental data incorporating the passive device, which is then used as a benchmark for the MR designs and to assess fail safety. The dynamic model is particularly important as it incorporates fluid compressibility, which may be a significant contributor to the overall response of the device in an impact scenario. The present study also aims to give further insight into high velocity MR valve flow, which will be inevitable during impulsive loading. This area remains largely unexplored and particular importance is given to valve Reynolds number since turbulent values are known to reduce device performance. The feasibility of an MR landing gear will be largely dependant on these factors.
In recent years, much research has focused on the development of effective control strategies for smart fluid dampers. In particular, skyhook control principles are frequently shown to demonstrate significant performance improvements over conventional passive systems. However these investigations are often either model-based and assume that the controlled damper can accurately track a prescribed force, or they are based on on/off type control strategies where such accurate tracking is not required.
In this paper, the authors present an investigation of a magnetorheological (MR) skyhook controlled SDOF mass isolator subject to broadband input excitations. The semi-active element is an MR smart fluid damper. The study utilises feedback linearisation, which is demonstrated experimentally, to convert the non-linear damper into a linear controllable device. This approach can be effectively harnessed to implement skyhook control since it permits the accurate tracking of a desired force within the controllable limits of the MR damper.
Using a validated model of an MR damper, it is demonstrated that feedback linearisation can yield significant performance improvements over more simplistic on/off control strategies. The same strategy could be integrated within larger scale vibrating structures (such as vehicle suspensions or aircraft landing gear) to implement more complex control strategies, e.g. optimal control.