In recent years the automotive industry has been working towards intelligent suspension systems that adapt to various road conditions to provide a superior ride and improved road handling. So called semi-active devices, in particular smart fluid dampers, are a viable method of implementing such a system. Despite the fact that magnetorheological (MR) dampers have been used in a number of commercially produced vehicles to date, there is little published information on the control of such devices. Building upon a successful modelling approach developed initially for electrorheological (ER) dampers at the University of Sheffield, a computational model was developed and implemented to simulate the behavior of an MR damper. A proportional force feedback control methodology was adopted and applied to the model with the intention of linearizing the output response. The smart fluid damper is therefore forced to behave in a manner equivalent to a linear damper, with the advantage of having a controllable viscous damping coefficient. Whereas previous research has almost exclusively concentrated upon the controller gain and its influence on the range of linearization which is possible to achieve, this investigation focuses on the time response of the MR fluid and its profound impact on the ability of the control method to linearize the output. Results will be presented which show that the fluid time response introduces a high frequency oscillation into the force/velocity output responses. Simultaneously, at higher excitation frequencies non-linear output responses will be demonstrated. As the fluid time response increases, the oscillations seen at low frequencies reduce but conversely the non-linear output of even moderate excitation frequencies becomes apparent. This result shows the need for a compromise between a larger range of controllability with the introduction of noise at low frequencies, or a smaller, yet noise-free range of controllability. This result may have significance when considered in the wider context of smart fluid applications. The instability and long-term degradation of smart fluids alongside other smart fluid phenomena such as 'in-use fluid thickening' indicate that the fluid time response is apt to change as the fluid is used. With a control system which has been demonstrated to be sensitive to fluid time response this change would of course be detrimental. The authors hope to highlight fluid time response as an important consideration in the design of smart fluid control systems.

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