Andrew R. Johnson

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.

Smart Fluid Damping: Shaping the Force/Velocity Response through Feedback Control

It is now well known that smart fluids [electrorheological (ER) and magnetorheological (MR)] 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 non-linear, possessing the general form associated with a Bingham plastic. In a previous paper the authors showed that by using a linear feedback control strategy it is possible to produce the equivalent of a viscous damper with a continuously variable damping coefficient. In the present paper the authors illustrate an extension of the technique, by showing how the shape of the force/velocity characteristic can be controlled through feedback control. This is achieved by using a polynomial function to generate a set point based upon the damper velocity. The response is investigated for polynomial functions of zero, 1st and 2nd order. It is shown how the damper can accurately track higher order polynomial shaping functions, while the zero-order function is particularly useful in illustrating the dynamics of the closed-loop system.

The Electro-Rheological Clutch: Design, Performance Characteristics and Operation

Fluid power transmission based on the electro-rheological clutch is taken beyond the concept-proving stage. A typical electro-rheological fluid is characterized over a range of engineering conditions and is found to usefully approximate to a continuum of Bingham plastic form. The clutch is optimized from this standpoint, and the limits of its performance are estimated. The state of the art is discussed alongside an outline of the infrastructure required to achieve maximum potential.

Dynamic simulation and performance of an electro-rheological clutch based reciprocating mechanism

A reciprocating mechanism which utilizes two electro-rheological clutches is described. An industrial application of the mechanism is in winding filaments onto bobbins. The required traverse speed is 5 m s-1 with a turn-round period of 10-20 ms, the traverse length is 250 mm and the turn-round position must be electronically controllable and repeatable within the ±1 mm. These combined criteria of high-speed and controllability makes the use of electro-rheological fluids an attractive proposition. The operation of the reciprocating mechanism and the dynamic model used to simulate the performance are outlined. The simulation is verified by comparison with experimental results from a prototype mechanism. Simulations are made to illustrate the effect of various fundamental electro-rheological fluid characteristics, such as electro-shear stress, time delays and viscosity. These simulations are considered in relation to the requirements for the operation of the high-speed mechanism.

A Simple One Dimensional Robot Joint Based on the ER Linear Reversing Mechanism

The feasibility of using a well-known twin ER clutch linear reversing mechanism as a robotic actuator is demonstrated. High speed of response, displacement and positional accuracy can be obtained bi-directionally. A validated mathematical model of the apparatus provides a basis for the control strategy. Positional accuracy is enhanced by an ER brake which works in sequence with the clutch excitation switches. The robot arm, in rotational displacement is tested over a large number of cycles, speeds, displacements and inertial loads and the quality of control compared to that of competitive conventional servo motors.

Fluid Durability in a High Speed Electro-Rheological Clutch

The durability of an electro-rheological (ER) fluid was investigated by running a high speed ER clutch under different conditions and periods of operation. The tests involved running the clutch at 3000 rpm for a total period of twelve hours over a five day period. The tests subjected the fluid to a centripetal acceleration of 3000 m/s2, and were conducted with and without an excitation field of 2 kV/mm, and with and without shearing the fluid at shear rates up to 9500 s-1. The condition of the fluid was assessed periodically by measuring the torque response of the clutch to a step ap plication of voltage in respect of both magnitude and speed of response. Results at the two pole 50 Hz synchronous speed of 3000 rpm indicated that the particles in the fluid were centrifuged over the prolonged test periods. The application of a voltage across the fluid had a negligible effect on this particle migration. The effect of particle migration due to centrifugal and electro-static effects indicate future development requirements for these smart materials.

Design, testing, and model validation of an MR squeeze-flow vibration damper

Ongoing research at the University of Sheffield is currently concerned with the design and construction of magneto- rheological (MR) squeeze-flow vibration damper. Previous work has demonstrated the feasibility of employing such a device as the key component in a controllable vibration isolator. The work also demonstrated the inadequacies of existing mathematical models which do not account for the observed behavior of MR fluids in squeeze flow. In parallel with investigations into the behavior of MR dampers, a collaborative programme between the Universities of Liverpool and Sheffield is also in progress. Here attention is focussed on ER fluids in squeeze-flow and a new test facility has been constructed for use in the development and validation of mathematical models. It is anticipated that this collaborative programme will assist in the development of both ER and MR squeeze-flow models. In this paper, the authors present a summary of progress to date.

Smart fluid damping: shaping the force/velocity response through feedback control

It is now well known that smart fluids [electrorheological (ER) and magnetorheological (MR)] 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 non-linear, possessing the general form associated with a Bingham plastic. In a previous paper the authors showed that by using a linear feedback control strategy is it possible to produce the equivalent of a viscous damper with a continuously variable damping coefficient. In the present paper the authors illustrate an extension of the technique, by showing how the shape of the force/velocity characteristic can be controlled through feedback control. This is achieved by using a polynomial function to generate a set point based upon the damper velocity. The response is investigated for polynomial functions of zero, 1st and 2nd order. It is shown how the damper can accurately track higher order polynomial shaping functions, while the zero order function is particularly useful in illustrating the dynamics of the closed-loop system.

Modeling and control of a magneto-rheological vibration isolator

It is now well established that magnetorheological (MR) fluids can provide the basis for constructing controllable vibration damping devices. Moreover, the characteristics of MR fluids are generally compatible with industrial requirements and there is enormous scope for commercial exploitation. In this paper the authors describe the design and construction of a vibration isolator which incorporates an MR damper. The damper is unusual in that it operates in the squeeze-flow mode. A quasi-steady model of the MR damper is summarized and then extended to include the vibration isolator dynamics. Model predictions are compared with experimental results. It is shown that by employing the MR damper a wide range of control can be exercised over the transmissibility of the vibration isolator. Numerical experiments are used to show that a feedback control strategy can provide even more control over transmissibility.