Neil D. Sims

Vibration control using smart fluids : A state-of-the-art review

A smart fluid is defined as one in which the resistance to flow can be controlled through the application of an electric or magnetic field. Such fluids can be used as the basis for constructing controllable damping devices that can generally outperform traditional passive dampers without involving the cost, weight, and complexity problems associated with fully active schemes. In this paper, the authors present a state-of-the-art review of smart fluids in vibration control. A comprehensive survey article appeared as recently as 1996, but progress since then has been so rapid and dramatic as to warrant an update. After summarizing the operating mechanisms of the two key smart fluids-electro-rheological (ER) and magneto-rheological (MR)-it is shown how they can be harnessed for vibration control. Progress over the past three years is categorized under four headings: the rise of MR fluids, the development of effective mathematical models of ER and MR fluids, the emergence of techniques for dynamic control, and the exploitation of promising new areas of application. The paper concludes with a discussion of possible avenues for future development. Some problems that await resolution are also mentioned.

Controllable viscous damping: an experimental study of an electrorheological long-stroke damper under proportional feedback control

It is now well known that smart fluids (electrorheological (ER) and magnetorheological) can form the basis of controllable vibration damping devices. With both types of fluid, however, the force/velocity characteristic of the resulting damper is significantly nonlinear, possessing the general form associated with a Bingham plastic. In a previous paper the authors suggested that by using a linear feedback control strategy it should be possible to produce the equivalent of a viscous damper with a continuously variable damping coefficient. In the present paper the authors describe a comprehensive investigation into the implementation of this linearization strategy on an industrial scale ER long-stroke vibration damper. Using mechanical excitation frequencies up to 5 Hz it is shown that linear behaviour can be obtained between well defined limits and that the slope of the linearized force/velocity characteristic can be specified through the choice of a controller gain term.

Energy harvesting from human motion and bridge vibrations: An evaluation of current nonlinear energy harvesting solutions:

A large quantity of recent research into the harvesting of electrical energy from ambient vibration sources has been focused on the improvement of device performance via the deliberate introduction of dynamic nonlinearities. In addition to this, the realisation that most of these kinetic energy sources are stochastic in nature has led to many studies focusing on the response of energy harvesters to random vibrations (often Gaussian white noise). This differs from early studies in which it was assumed that ambient vibration sources were sinusoidal. The aim of the present study is to take current nonlinear energy harvesting solutions and to numerically analyse their effectiveness when two real ambient vibration sources are used: human walking motion and the oscillation of the midspan of a suspension bridge. This study shows that the potential improvements that can be realised through the introduction of nonlinearities into energy harvesters are sensitive to the type of ambient excitation to which they are subjected. Additionally, the need for more research into the development of low-frequency energy harvesters is emphasised.

Milling workpiece chatter avoidance using piezoelectric active damping: a feasibility study

During the milling of thin-walled and flexible structures such as those manufactured for the aerospace industries, workpiece chatter can become a key factor that limits productivity. Chatter is a form of self-excited vibration that can be stabilized by increasing the damping of the vibrating structure. In the present study, an experimental investigation is described which assesses the feasibility of using piezoelectric active vibration control to mitigate workpiece chatter in milling. A positive position feedback control strategy is used, and a series of milling tests is performed, which demonstrate a sevenfold improvement in the limiting stable depth of cut. The practical issues concerning the application of this technique are then discussed.

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.

A unified modelling and model updating procedure for electrorheological and magnetorheological vibration dampers

Magnetorheological (MR) and electrorheological (ER) dampers are known to exhibit nonlinear behaviour which can make it difficult to predict their performance, particularly when they are integrated into engineering structures. As a result it can be impossible to properly assess the feasibility of using such semi-active devices to solve practical engineering problems. In this paper, a new model format is proposed which represents an extension of earlier work by the authors. The proposed model is more general and yet maintains the physical significance of key parameters. A novel model updating (or system identification) technique is developed so that the model can account for the behaviour of various configurations of device without the need for prior knowledge of the fluid properties. The technique relies upon the iterative adjustment of the models stiffness parameter so that the quasi-steady behaviour of the device can be estimated. Correlation between a bi-viscous model and the estimated quasi-steady behaviour is used as the criterion for choosing the most suitable value of stiffness. The modelling technique is completed by establishing empirical shape relationships between the pre-yield parameters, post-yield parameters, yield force and the applied excitation conditions. The modelling and identification procedures are applied to an MR damping device and the results are validated by comparing predicted and experimental responses under both non-sinusoidal and broadband excitation conditions.

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.

Active constrained-layer damping: A state-of-the-art review

Abstract In this paper the authors discuss the progress that has been made over the past decade in active constrained-layer damping (ACLD). ACLD treatments combine the best features of passive and active control of structural vibrations. By way of introduction the paper describes well-established techniques for passive control of structural vibrations and noise. A concise discussion of the development of so-called ‘smart’ (or ‘intelligent’) actuators and sensors and the emergence of suitable control algorithms show how passive techniques were extended to produce ACLD. A comprehensive literature review follows. It is shown how the passive and active components of ACLD complement each other to enable control of both high and low frequency modes of vibration. The active elements allow structures to adapt to suit a changing environment while the passive elements provide a fail-safe mechanism. Because of the available technology, these benefits are available without significant penalties in terms of cost, weight and complexity.

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.

The self-excitation damping ratio : A chatter criterion for time-domain milling simulations

Regenerative chatter is known to be a key factor that limits the productivity of high speed machining. Consequently, a great deal of research has focused on developing predictive models of milling dynamics, to aid engineers involved in both research and manufacturing practice. Time-domain models suffer from being computationally intensive, particularly when they are used to predict the boundary of chatter stability, when a large number of simulation runs are required under different milling conditions. Furthermore, to identify the boundary of stability each simulation must run for sufficient time for the chatter effect to manifest itself in the numerical data, and this is a major contributor to the inefficiency of the chatter prediction process. In the present article, a new chatter criterion is proposed for time-domain milling simulations, that aims to overcome this drawback by considering the transient response of the modeled behavior, rather than the steady-state response. Using a series of numerical investigations, it is shown that in many cases the new criterion can enable the numerical prediction to be computed more than five times faster than was previously possible. In addition, the analysis yields greater detail concerning the nature of the chatter vibrations, and the degree of stability that is observed.