David J. Peel

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.

Dynamic modelling of an ER vibration damper for vehicle suspension applications

In this paper, the authors describe the development of a mathematical model of a controllable vibration damper intended for eventual application to ground-vehicle suspension systems. The damper under investigation employs electro-rheological (ER) fluid as the working medium which enables a continuously variable damping force to be provided in response to an electrical control signal. There are some difficulties inherent in characterizing the ER dampers behaviour which the present study attempts to overcome. The paper begins by describing a novel form of non-dimensionalization which drastically reduces the number of variables required to characterize the quasi-steady behaviour of the ER fluid. The construction of the ER damper is described and, on the basis of physical reasoning, it is shown how a dynamic model can be derived by taking account of ER fluid inertia and compressibility. A recently developed iterative scheme is introduced in order to solve the resulting non-linear equations of motion. The paper concludes with a case study involving the application of the ER damper to controlling the lateral vibrations of a rail vehicle.

Decomposition of the Pressure Response in an ER Valve Control System

Experimental data from tests on an ER valve pertinent to the development of a con troller for high speed machine duty has been analysed to show that three common, underlying modes of response are present. This is demonstrated for a range of industrial scale flow velocities and electrode dimensions, in the time and frequency domains. The dependence of steady-state pressure on the electric field is also discussed.

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.

Electrorheological relaxation times derived from pressure response experiments in the flow mode

Abstract We examine the experimental pressure response for a set of electrorheological valves operating at several steady flow rates to step function voltage inputs and analyse the factors that may contribute to the complex forms observed. Using a numerical algorithm incorporating a viscoelastic model for the fluid the response of the valve flow profile to changes in rheology is found to be relatively fast compared to the experimental pressure response and thought not to feature as a significant contribution to the final shape. The dynamic interaction of the valve and its associated system of pump and connecting pipes is investigated using a multi-element lumped parameter model and reveals a frequency spectrum containing resonances comparable to the experimental results. A simple lumped parameter model is less successful. Using the multi-element model for the system an upper bound can be placed on the basic response time of the electro-stress itself under realistic engineering conditions.

Experimental study of an ER long-stroke vibration damper

The development of controllable suspension dampers for ground vehicles is the subject of much current research. In this paper the authors describe aspects of a design methodology for controllable dampers which use electro-rheological (ER) fluid as the working medium. This methodology is based upon a non- dimensional characterization of ER fluid data which allows measurements obtained from small-scale tests to be used to predict the behavior of industrial-scale vibration dampers. The ER damper is represented via a Bingham plastic constitutive relationship, augmented by terms to account for fluid inertia and compressibility. An industrial-scale test facility is described and the first available set of experimental results are presented. A comparison is made between model predictions and observed behavior.

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 an ER long-stroke vibration damper

Electrically-structured (ES) fluids offer a potentially elegant means of introducing greater flexibility into a range of industrial machines and structures, and are especially suited to controllable vibration damping. The authors have developed a technique for characterizing ES fluids, leading to a mathematical model and practical device design procedure, the principles of which are verified by testing with an industrial scale, long-stroke electro- rheological damper, suited to the needs of a rail vehicle lateral suspension. In this paper, the model is applied to predict the effect on the test damper steady state performance, and performance envelope, of the principle variables of operating temperature, mechanical displacement amplitude and frequency, and control field excitation, which influence the controllability and control requirements of the damper, and to show that the test damper should achieve the desired range of control.

Effect of Flow Rate, Excitation Level and Solids Content on the Time Response in an Electro-Rheological Valve

Diverse regimes of the fast time response of an electro-rheological fluid are iden tified in a presentation of experimental results which show the effects of the application of step and DC biased sine wave excitations to a series of set flows in one valve. The form of pressure response is complex and depends to some extent on the rate of flow and applied electric field magnitude. Some frequency domain behaviour is related to the step wave performance. Apart from their value as a foundation study for a new area of rheology, the results are important to the rapidly developing sub ject of flexibly operated smart machines and as an aid in target setting to developers of the hydraulic semi-conductors on which some of them are based.

Feedback control of an electrorheological long-stroke vibration damper

It is widely acknowledged that the inherent non-linearity of smart fluid dampers is inhibiting the development of effective control regimes, and mass-production devices. In an earlier publication, an innovative solution to this problem was presented -- using a simple feedback control strategy to linearize the response. The study used a quasi-steady model of a long-stroke Electrorheological damper, and showed how proportional feedback control could linearize the simulated response. However, this initial research did not consider the dynamics of the dampers behavior, and so the development of a more advanced model has been necessary. In this article, the authors present an extension to this earlier study, using a model of the dampers response that is capable of accurately predicting the dynamic response of the damper. To introduce the topic, the electrorheological long-stroke damper test rig is described, and an overview of the earlier study is given. The advanced model is then derived, and its predictions are compared to experimental data from the test rig. This model is then incorporated into the feedback control simulations, and it is shown how the control strategy is still able to linearize the response in simulations.