D C Batterbee

Magnetorheological landing gear: 1. A design methodology

Aircraft landing gears are subjected to a wide range of excitation conditions, which result in conflicting damping requirements. A novel solution to this problem is to implement semi-active damping using magnetorheological (MR) fluids. This paper presents a design methodology that enables an MR landing gear to be optimized, both in terms of its damping and magnetic circuit performance, whilst adhering to stringent packaging constraints. Such constraints are vital in landing gear, if MR technology is to be considered as feasible in commercial applications. The design approach focuses on the impact or landing phase of an aircrafts flight, where large variations in sink speed, angle of attack and aircraft mass makes an MR device potentially very attractive. In this study, an equivalent MR model of an existing aircraft landing gear is developed. This includes a dynamic model of an MR shock strut, which accounts for the effects of fluid compressibility. This is important in impulsive loading applications such as landing gear, as fluid compression will reduce device controllability. Using the model, numerical impact simulations are performed to illustrate the performance of the optimized MR shock strut, and hence the effectiveness of the proposed design methodology. Part 2 of this contribution focuses on experimental validation.

Magnetorheological landing gear: 2. Validation using experimental data

Aircraft landing gears are subjected to a wide range of excitation conditions with conflicting damping requirements. A novel solution to this problem is to implement semi-active damping using magnetorheological (MR) fluids. In part 1 of this contribution, a methodology was developed that enables the geometry of a flow mode MR valve to be optimized within the constraints of an existing passive landing gear. The device was designed to be optimal in terms of its impact performance, which was demonstrated using numerical simulations of the complete landing gear system. To perform the simulations, assumptions were made regarding some of the parameters used in the MR shock strut model. In particular, the MR fluids yield stress, viscosity, and bulk modulus properties were not known accurately. Therefore, the present contribution aims to validate these parameters experimentally, via the manufacture and testing of an MR shock strut. The gas exponent, which is used to model the shock struts nonlinear stiffness, is also investigated. In general, it is shown that MR fluid property data at high shear rates are required in order to accurately predict performance prior to device manufacture. Furthermore, the study illustrates how fluid compressibility can have a significant influence on the device time constant, and hence on potential control strategies.

Hardware-in-the-loop simulation of magnetorheological dampers for vehicle suspension systems

Abstract Magnetorheological (MR) fluids provide an elegant means to enhance vibration control in primary vehicle suspensions. Such fluids can rapidly modify their flow characteristics in response to a magnetic field, so they can be used to create semi-active dampers. However, the behaviour of MR dampers is inherently non-linear and as a consequence, the choice of an effective control strategy remains an unresolved problem. Previous research has developed a method to linearize the dampers force/velocity response, to allow implementation of classical control techniques. In the present study, this strategy is used to implement skyhook damping laws within primary automotive suspensions. To simulate the vehicle suspension, a two-degree-of-freedom quarter car model is used, which is excited by realistic road profiles. The controller performance is investigated experimentally using the hardware-in-the-loop-simulation (HILS) method. This experimental method is described in detail and its performance is validated against numerical simulations for a simplified problem. The present authors demonstrate that feedback linearization can provide significant performance enhancements in terms of passenger comfort, road holding, and suspension working space compared with other control strategies. Furthermore, feedback linearization is shown to desensitize the controller to uncertainties in the input excitation such as changes in severity of the road surface roughness.

Computational model of an infant brain subjected to periodic motion simplified modelling and bayesian sensitivity analysis

Non-accidental head injury in infants, or shaken baby syndrome, is a highly controversial and disputed topic. Biomechanical studies often suggest that shaking alone cannot cause the classical symptoms, yet many medical experts believe the contrary. Researchers have turned to finite element modelling for a more detailed understanding of the interactions between the brain, skull, cerebrospinal fluid (CSF), and surrounding tissues. However, the uncertainties in such models are significant; these can arise from theoretical approximations, lack of information, and inherent variability. Consequently, this study presents an uncertainty analysis of a finite element model of a human head subject to shaking. Although the model geometry was greatly simplified, fluid-structure-interaction techniques were used to model the brain, skull, and CSF using a Eulerian mesh formulation with penalty-based coupling. Uncertainty and sensitivity measurements were obtained using Bayesian sensitivity analysis, which is a technique that is relatively new to the engineering community. Uncertainty in nine different model parameters was investigated for two different shaking excitations: sinusoidal translation only, and sinusoidal translation plus rotation about the base of the head. The level and type of sensitivity in the results was found to be highly dependent on the excitation type.

Temperature sensitive stability of feedback controllers for MR dampers

Smart fluid dampers can undergo large temperature changes due to the heating associated with energy dissipation. Such heating will alter the fluids 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 fluids 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 fluids 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.

Design and performance optimization of magneto-rheological oleopneumatic landing gear

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.

Skyhook damping with linearized magnetorheological dampers

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.