Editorial

Compiled by J. F. Dunne
Email   j.f.dunne@sussex.ac.uk

Magnetorheological dampers are playing an increasingly important role in many areas of vibration control. This increased role stems from better understanding of the properties of rheological fluids, more sophisticated methods of damper manufacture, improved magnetorheological (MR) damper characterisation, and the use of advanced control strategies. An important feature of MR dampers is their potentially-broad suppression frequency range, from low-frequency seismic response in building structures to vibro-acoustic frequencies in aircraft panels.

Vibration control and MR dampers

Control of vibration is possible using passive, semi-active, or fully active methods. Passive control is the least expensive in the sense that good design and use of inherent system damping (or the addition of a simple damping device) can often achieve the required response characteristic. But passive vibration control is invariably a compromise. To move to a totally desirable suppression capability, fully active control is needed. This option is expensive since it uses actuators to apply control forces, and it may be impossible to implement, or may have unacceptable power consumption. Semi-active vibration control, by contrast, relaxes the necessary compromise of passive systems but avoids the expense and power demands of fully active systems. It does this by actively controlling damping, namely by modifying the way energy is dissipated or released from a system. Semi-active systems have received considerable attention over the past two decades, particularly in structural dynamics. Although initial attention focused on electro-rheological (ER) dampers, within the past decade it has become clear that magnetorheological dampers (invented more than 60 years ago) have the greater future. And despite MR dampers being more expensive, they are more practical because they operate at lower voltage and power, have wider load range, higher thermal capacity, and greater tolerance to damper fluid contamination.   

Practical applications of MR dampers vary enormously both in scale and frequency. Examples include vibration control in seismically-induced building structures, body-motion control in vehicle suspension systems, suppression of cable vibrations in bridges and guyed-towers, control of panel vibrations in aircraft wings, and elimination of whirling motion in rotor systems. Various models have been proposed to characterise the visco-elastic and hysteretic behaviour of MR dampers, including the Bingham, Bouc Wen, Dahl, fuzzy logic, neural network, and polynomial models. In this Virtual Edition, some exciting developments in MR-based vibration suppression are highlighted, focusing on improved characterization, and on better control strategies.

Rheological fluid properties

MR dampers exploit the properties of rheological fluids which behave as Bingham plastics. The yield shear stress τ_y and the post-yield marginal viscosity μ are modified by the presence of a magnetic field. This property is achieved by adding ferromagnetic spheres (around 5 μm diameter) to a low-viscosity mineral or silicone carrier oil. MR damper designs can be much closer (than ER dampers) to a conventional hydraulic damper, and in principle, can operate with substantially smaller active fluid volumes (around 1 cm³).  However energy dissipation damage to the MR fluid is a limiting factor which must be compensated for. Consequently to give an adequate operating life, a working volume around 100 times the minimum is needed. In practical operation, control of yield stress τ_y is most important, although mixed-mode operation is possible. Application of a magnetic field (typically involving a current up to 1 A being passed through a field coil) allows the damper characteristics to be modified in milliseconds. When the field is constant, a highly nonlinear characteristic is created (including the possibility of lock-up, where the damper effectively becomes a stiff spring). When no field is applied, the damper may still function at reduced level in a fail-safe mode.  A suitably controlled field offers virtually an infinite range of possible characteristics that can form part of a vibration control strategy. The use of permanent magnets to create the field offers an (uncontrolled) MR damper with absolutely zero power demand (which is not possible with ER dampers).

MR damper characterisation

Improved MR damper characterisation has received recent attention by Hong et al. (2008), building on earlier work in Hong et al. (2005). Using non-dimensional representation, a design methodology for an MR damper in mixed-mode operation has been developed and tested experimentally. Four dimensionless parameters are defined, namely: (i) the Bingham number  (the ratio of dynamic yield stress to viscous stress), (ii) the damping force, (iii) the dynamic range, and (iv) the geometric ratio (hydraulic amplification).  The physical design parameters can be determined from these dimensionless groups. In the 2005 work, a sky-hook controller was adopted to study vibration isolation using an MR-based  mount. In the 2008 work, two different Bingham numbers are proposed, in terms of piston velocity or average valve-gap duct velocity. The Bingham number and hydraulic amplification parameter are found to be the two most important factors that determine MR damper characteristics.  At much higher frequencies, experimental characterisation of an MR fluid in the frequency range 50 to 100 kHz has been undertaken by Kim and Park (2005). The measured speed of sound and attenuation is cleverly used to determine the storage and loss modulus in wave transmission, when the direction of the magnetic field is either parallel or orthogonal.

Low frequency suppression

Turning specifically to vibration control applications at low frequency, Du et al. (2005) adopted an H ∞ control strategy for a vehicle suspension system using a polynomial model of an MR damper. Simulation results point to the benefits of MR damping for fully random forcing.   Tsampardoukas et al. (2008), by contrast, adopted a hybrid balance control strategy to demonstrate the benefits of MR damping on the response of a commercial truck, the aim being to reduce road damage without compromising driver comfort. The Bouc Wen model was adopted, coupled to a conventional viscous damper.

Building responses to seismic disturbances are also of low frequency, but the forces involved (and the damper size) are massive. Ying et al. (2005) proposed an optimal control strategy for suppression of MDOF seismic response of buildings fitted with MR dampers. The following year, Yan and Zhou (2006) proposed an integrated fuzzy logic strategy for reduction of seismic response in the presence of uncertainty. Control constraints involved the use of multi-objective functions to minimize peak displacement and peak acceleration, and a genetic algorithm was used to implement the fuzzy logic controller. More recently, Ying et al. (2007) described an optimal stochastic control strategy (for use with both MR and ER dampers). This was successfully applied to the control of an inverted pendulum.

Vibration suppression of inclined sagging cables, excited through wind loading and support disturbances, is a difficult problem in low-frequency, three-dimensional stochastic control.  Zhou et al. (2006) undertook a numerical study of cable motion suppression using an MR damper, by exploiting the Dahl model. Suppression of both in-plane and out-of-plane vibration was found to be significant using MR dampers compared with viscous dampers. The following year, Zhou et al. (2007) further reported the benefits of MR damper suppression, as applied to both sub- and super-harmonic resonances of shallow cables under harmonic axial excitation of a support. One adverse finding to emerge from that study was that attempts to suppress cable vibration, using MR dampers with certain control algorithms, may actually result in chaotic motion rather than suppression.

Suppression at higher frequencies

At higher frequencies, recent reports of MR-based vibration suppression are also encouraging. Zhu et al. (2005) examined the use of a disk-type MR damper in suppressing synchronous shaft whirl. The benefits were also explored of putting the damper into lock-up mode to change system stiffness. Initially the magnetic field of the damper was analysed using the Finite Element method, and the predictions were experimentally verified. This FE model was then used to refine the damper design. A bang-bang control strategy was adopted. Suppression of cantilever plate vibrations using permanent-magnet shear-type MR dampers has also been explored by Pranoto et al. (2004). The motivation for this stems from aircraft wing panel vibrations, where MR dampers successfully meet a control specification that cannot be met by conventional piezoelectric sandwich dampers.   

It is evident from this brief survey that MR dampers are playing a critically important role in the suppression of vibration. Their role is clearly set to continue and grow in significance.

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