Electrorheological Devices Research Articles

The research progress of electrorheological fluids

Electrorheological (ER) fluids are such smart materials whose rheological properties (yield stress, viscosity, etc.) can be reversibly and continuously controlled using an external electric field. They are colloids composing of dielectric particles and insulating liquids. They switch from a liquid-like state to a solid-like state within a millisecond with the aid of an electric field, which is called the ER effect. ER fluids can therefore be used as electrical and mechanical interfaces in various industries, including the fast acting valves, clutches, brakes, shock absorbers, accurate polishing, robotics and tactile displays. Since the ER effect was first described by Winslow in 1949, ER fluids show a promising prospect in the application in various industries and a great deal of research interest in ER fluids and ER devices has been stimulated. A large body of literature on ER fluids, the mechanism of the ER effect, and the design of industrial applicable ER devices has been published. In the meantime, after the invention of ERF, a number of theories are put forward, such as Fibrillation Theory, “Water Bridge” Theory, Double layer Theory, Particle Polarization Theory and so on. Ma et al. calculated that the theoretical upper bound on conventional ER static yield stress is 10 kPa based on first-principles calculations. The highest yield strength of the dielectric electrorheological fluid obtained by Lu et al. in the experiment is 5 kPa. However, electrorheological fluids are not industrialized practically because of the low yield strength. In recent years, Wen developed giant electrorheological fluid that can reach a yield strength of 130 kPa, breaking the theoretical upper bound of traditional ER static yield stress. Later, Lu invented polar-molecule-dominated electrorheological fluids whose solid state can reach yield strength of 200 kPa. With the inventions of the giant electrorheological fluid and polar-molecule-dominated electrorheological fluids with high yield stress under low electric field, there is a new opportunity for ER fluids to apply to the industrial application because their yield strength of the new kind of ER fluids is more than 40 kPa, the lowest practical limit. However, the problems about sedimentation and redispersibility of electrorheological fluids still restrict its wide application. With the research of surfactant and the development of the hollow multilayer porous nano particles, the stability of electrorheological fluids has greatly improved. This paper focuses on the study of the giant electrorheological fluid and its application in smart microfluidics. Specifically, it gives a detailed introduction of micropumps and micro-valves and smart electroresponsive droplets in microfluidics based on giant electrorheological fluids. This paper also offers a review of the composition, macroscopic properties, microscopic mechanism and applications of electrorheological fluids, as well as a summary of its current research status and its future development. Research of the stability and service life of electrorheological fluids will definitely be the most popular topic in the future, thus accelerating the progress of industrialization with the settlement of the issues discussed above.

Electrokinetic instabilities of non-dilute colloidal suspensions

An experimental investigation of electrokinetic instabilities (EKIs) of non-dilute colloidal suspensions in microchannels is presented. The addition of charged colloidal particles to a solution can alter the solution's electrical conductivity and permittivity as well as the average particle electrophoretic mobility. In this work, a colloidal (500 nm polystyrene) volume fraction gradient is achieved at the intersection of a Y-shaped polydimethylsiloxane (PDMS) microchannel. The flow becomes unstable when the electroviscous stretching and folding of the conductivity and permittivity interfaces exceed the dissipative effects of viscous forces and particle diffusion. The suspension conductivity as a function of the particle volume fraction is presented. The critical conditions required for flow instability are measured along with a scaling analysis which shows that the flow becomes unstable due to a coupling of applied electric fields and the electrical conductivity and permittivity gradients in the flow. The flow becomes unstable at a critical electric Rayleigh number ofRae= 1.8 × 105for a wide range of applied electric fields spanning three orders of magnitude and colloid volume fractions varying two orders of magnitude. EKIs of non-dilute colloidal suspensions may be important for applications such as the electrophoretic deposition of micropatterned colloidal assemblies, electrorheological devices and on-chip electrokinetic (EK) manipulation of colloids.

Braking responses of inertia/load by using an electro-rheological (ER) brake

Electro-rheological (ER) actuators are potential devices in modern industrial applications. This is due to the excellent performances of the ER devices such as large output dynamics and fast speeds of responses. By using its fast dynamic responses, the ER device or ER brake is studied as a possible actuator to halt the robot arm rapidly. Although many research studies of the ER brake were conducted in the past, but the details of the ER braking dynamics is not clearly understood. Therefore in this paper, a mathematical model is developed and the modelled results are compared with the experimental output velocity responses of the ER brake. After the model validation is completed, the ER brake is subjected to various loads, electric fields, fluid temperatures and volume fractions. By observing the effect of each input variable, the trend behaviour of the ER braking velocity response is investigated in a bid to identify suitable parameters that can halt the robot arm at the fastest time durations. In addition, the braking displacement responses of the attached loads are examined in order to understand the variations of the load position errors at different input conditions. This study is aimed to establish the fundamental knowledge of using the ER brake in future ER-robotic applications.

Primal-dual Newton Methods in Structural Optimization

We consider the numerical solution of optimization problems for systems of partial differential equations with constraints on the state and design variables as they arise in the optimal design of the shape and the topology of continuum mechanical structures. After discretization the resulting nonlinear programming problems are solved by an “all-at-once” approach featuring the numerical solution of the state equations as an integral part of the optimization routine. In particular, we focus on primal-dual Newton methods combined with interior-point techniques for an appropriate handling of the inequality constraints. Special emphasis is given on the efficient solution of the primal-dual system that results from the application of Newton’s method to the Karush–Kuhn–Tucker conditions where we take advantage of the special block structure of the primal-dual Hessian. Applications include structural optimization of microcellular biomorphic ceramics by homogenization modeling, the shape optimization of electrorheological devices, and the topology optimization of high power electromotors.

Electrorheological Effects at High Shear Rate

Much attention has been given to electrorheological (ER) fluids because of the ER effect, which has been described by a large number of researchers as a notable increase in the apparent viscosity of a fluid upon the application of an electric field. The description of ER effects is, however, not accurate at high shear rates. To clarify the discrepancy, we analyze and compute the apparent viscosity as a function of shear rate for ER fluid flow between rotating coaxial cylinders in the presence of an electric field. The theoretical predictions show that the increase of electric intensity contributes little to the apparent viscosity enhancement at high shear rates, while ER effects for ER fluids with a higher polarization rate still exist and ER devices possess controllability in this regime. Description of the ER effect by the apparent viscosity leads to an unrealistic conclusion that ER effects disappear at high shear rates, because the apparent viscosity of ER fluids approaches the value for Newtonian fluids. Therefore, it is concluded that the proper description of ER effects, i.e., one that holds uniformly for any strain rate when ER effects exist, is manifested by a remarkable increase in the extra stress rather than in the apparent viscosity of ER fluids.

YIELD STRESS MODELING OF ELECTRORHEOLOGICAL FLUIDS USING NEURAL NETWORK

Electrorheological (ER) fluids are a kind of smart materials whose rheological properties can be rapidly changed by applied electric fields. Many potential industrial applications of ER technology have been proposed. In order to formulate better ER fluids and design ER devices, it is important to predict the yield stress of ER fluids based on the ER fluids components and the operating conditions. This paper proposes a new method for predicting the yield stress of ER fluids with neural network (NN). A multilayer perceptron with a single hidden layer of neurons is used to model the ER effect. The data for training and test were produced from the simulation of previous proposed mathematical models. The Levernberg–Marquardt back propagation algorithm was selected for fast learning. The results show the neural network model can well approximate the previous theoretical model, and the predicted outputs of NN agree nearly with the theoretical model values under the same input, all of which demonstrate that it is possible to generate a robust NN model for rapidly predicting the yield stress of ER fluids under different input parameters.

Shear mode ER transfer function for robotic applications

Electro-rheological (ER) fluids are becoming popular in modern industrial applications. The advantage of employing ER devices is due to the ease of energizing the ER fluids at fast speeds of response. One innovation in ER applications could be in the positioning control of the robotic arm using an ER clutch. In order to actuate the manipulator, the ER output torque response is required. However, the behaviour of this ER torque response at different input conditions is not clearly understood. Therefore, in this paper, a sample study of the ER output torque is conducted. The ER output torque responses at different input parameters are studied carefully for the establishment of an appropriate ER transfer function in shear mode. This transfer function will serve as an important feature in future ER-actuated robot arm's control process.

PERFORMANCE OF PIN ELECTRODE ER VALVES

This work concentrates on the development of an uncommon type of electrodes to be used in electrorheological devices. Rather than using electrodes with constant gap, pin electrodes are used, in order to achieve variation in the gap. The device described has pins running across the direction of the flow. The ER fluid moves through rows and columns of pin electrodes of different polarities. One of the main problems would be the development of the head pressure drop as the number of rows of pins increases. The work presents different configurations that could be constructed using pin electrodes. One of the configurations is developed and tested as to the pressure gradients that could be achieved when different magnitudes of electric potentials are applied. The results are compared to the ones obtained when two of the constructed modules are aligned one after the other for the tests.

DESIGN PROGRAM IN GRAPHIC USER INTERFACE ENVIRONMENT FOR AUTOMOBILE ER DEVICES

This work presents a design and analysis program for vehicle devices utilizing an electrorheological (ER) fluid. The program is operated in graphic user interface (GUI) environment and the initial window is consisted of four subprogram modules which are related to ER shock absorber, ER seat damper, ER engine mount, and ER anti-lock brake system (ABS), respectively. In order to execute each module, both material properties and design parameters are to be chosen by the user. Then, the output display window shows the field-dependent performance characteristics to be considered as design criteria. In addition, control performances of the vehicle system equipped with ER devices are displayed in time and frequency domain. In order to demonstrate the effectiveness of the proposed program, ER shock absorber and ER ABS are designed and manufactured and their performance characteristics are evaluated.

Design and analysis programme of electrorheological devices for vehicle systems

In this work, a dynamic program for design and analysis of electrorheological (ER) devices for vehicle systems is developed in a graphic user-interface (GUI) environment. The initial window of the program shows four subprogram modules which are related to the design of ER shock absorber, ER seat damper, ER engine mount and ER anti-lock brake system, respectively. Each module consists of three structures: input design parameter, dynamic analysis, and output display. In order to demonstrate the effectiveness of the proposed program, the ER shock absorber and ER engine mount are designed and manufactured on the basis of the program. A comparison of damping force characteristics between analysed and measured results is undertaken.

CHARACTERISTICS OF A YIELD STRESS SCALING FUNCTION FOR ELECTRORHEOLOGICAL FLUIDS

The electrorheological (ER) fluids exhibit a drastic change in rheological and electrical properties. Among these properties, yield stress is one of the critical evaluation parameters of the performance of ER devices. The published experimental data of yield dependence on the electric field strength and particle volume fraction are inconsistent due to the time dependence of material properties and measuring conditions. In this paper, we present a universal function, descriptive of the normalized yield stress, via scaling of the applied electric field strength. This scaling equation hybridizes both the polarization and conductivity models. Yield stress data for various ER fluids are collapsed onto a single curve for a broad range of electric field strengths, suggesting that the proposed scaling equation is adequate for predicting the ER property. Furthermore, the yield stresses, obtained from two different measuring techniques (static and dynamics methods), were also examined.

Multi-Duct ER Dampers

This paper describes the design, construction, testing and modeling of controllable damping devices utilizing electro-rheological (ER) materials. The rheological properties of ER materials (yield stress and viscoelasticity) are extremely sensitive to electric fields. Modulations of the electric field in an electrorheological damper results in a corresponding change in device forces. The key feature of the ER devices described in this paper is a set of multiple concentric annular ducts through which the ER material flows. The annular ducts are formed by a set of concentric metallic tubes, which may be electrically charged with a high voltage potential, or electrically grounded. Three designs targeting different force levels (2-6 kN), are designed, tested and modeled. The device analyses used in the design incorporate a simplified closed form relation for the flow behavior of ER materials. Evolutionary and algebraic device models are fit to the measured force response of the devices over a range of harmonic frequencies and at zero and high electric field magnitudes. The evolutionary model is motivated by observations and simulations of ER materials in simple shear, and the algebraic model is convenient for control and feedback linearization applications. The algebraic model is evaluated with data collected with random excitation and with switching electric fields. The three devices display different levels of pre-yield viscoelasticity. Friction, due to seals in the device, was not accounted for in the initial design, and affected the behavior of the device as tested.

Predictive active control of MDOF structures

This paper presents a theoretical study of a predictive active control system used to improve the response of multi-degree-of-freedom (MDOF) structures to earthquakes. As an example a building frame equipped with electrorheological (ER) dampers is considered. The aim of the design is to find a combination of forces that are produced by the ER dampers in order to obtain an optimal structural response. The mechanical response of ER fluid dampers is regulated by an electric field. Linear auto-regressive model with exogenous input (ARX) is used to predict the displacements and the velocities of the frame in order to overcome the time-delay problem in the control system. The control forces in the ER devices are calculated at every time step by the optimal control theory (OCT) according to the values of the displacements and of the velocities that are predicted at the next time step at each storey of the structure. A numerical analysis of a seven-storey ER damped structure is presented as an example. It shows a significant improvement of the structural response when the predictive active control system is applied compared to that of an uncontrolled structure or that of a structure with controlled damping forces with time delay. The structure's displacements and velocities that were used to obtain the optimal control forces were predicted according to an ‘occurring’ earthquake by the ARX model (predictive control). The response was similar to that of the structure with control forces that were calculated from a ‘known’ complete history of the earthquake's displacement and velocity values, and were applied without delay (instantaneous control). Copyright © 2000 John Wiley & Sons, Ltd.

Control Characteristics of ER Devices

The successful development of a new generation of technology incorporating an electro-rheological (ER) fluid requires three principal ingredients; advanced ER fluid, reliable mechanical mechanism and efficacious control scheme. This paper discusses the third one. After describing general, but stringent requirements for semiactive or active control systems featuring the ER fluid, feedback control characteristics of some specific ER devices are presented; a frequency-dependent feedback control of mode shape in a smart flexible structure containing the ER fluid, an optimal control of vibration in a semi-active suspension installed with ER shock absorbers, and a neural network control of position in a hydraulic ER valve-cylinder system.

Active control of MDOF structures with supplemental electrorheological fluid dampers

A method for design of an active control system for multistorey structures using Electrorheological (ER) dampers is presented. Incorporated at various levels of a structural frame, ER dampers are used to improve the response of the structure during earthquakes. Optimal control theory was used to design the ER devices. The aim of the design is to find the most suitable combination of the minimum required forces produced by the ER dampers to obtain the optimal structural response. The mechanical response of ER fluid dampers is regulated by an electric field, depending on the displacements and velocities of the frame. Numerical analysis of an ER damped seven-storey structure is represented as an example. Significant improvement of the structural response was obtained using optimal active controlled ER dampers compared to passive controlled and uncontrolled structures.

Electrorheological Dampers, Part II: Testing and Modeling

Electrorheological (ER) materials develop yield stresses on the order of 5–10 kPa in the presence of strong electric fields. Viscoelastic and yielding material properties can be modulated within milli-seconds. An analysis of flowing ER materials in the limiting case of fully developed steady flow results in simple approximations for use in design. Small-scale experiments show that these design equations can be applied to designing devices in which the flow is unsteady. More exact models of ER device behavior can be determined using curve-fitting techniques in multiple dimensions. A previously known curve-fitting technique is extended to deal with variable electric fields. Experiments are described which illustrate the potential for ER devices in large-scale damping applications and the accuracy of the modeling technique.

Applications of electro-rheological fluids in vibration control: a survey

Electro-rheological (ER) fluids are now regarded as one of the most versatile of the materials available for building smart structures and machines. In principle, ER fluids promise an elegant means of providing continuously variable forces for the control of mechanical vibrations. In practice, the development of industrial devices has been hampered by the unavailability of suitable ER fluids. Prompted by recent advances in ER fluid development this paper provides a comprehensive survey of ER devices for vibration control. The key modes of operation are identified and progress towards a unified approach to visualizing the macroscopic behaviour is summarized before presenting a comprehensive survey which includes contributions to the identification of ER fluid dynamics and the application of feedback control. The discussion of results includes some thoughts on future trends.

The effect of electrical transients on the shear stresses in electrorheological fluids

The effect of time‐varying electric fields on the stresses in model and commercial electrorheological (ER) fluids in steady shear were surveyed. The dependence of the time‐averaged shear stress with the frequency of an applied alternating (ac) square‐wave electric field reveals two classes of fluids: those that exhibit substantial ER shear stresses with static (dc) and low frequency ac fields, and those that perform poorly in dc fields but show substantial ER activity in ac fields. These contrasting behaviors are consistent with the existence of two different means by which ER activity is generally produced: mismatches of the conductivities and dielectric constants, respectively, between the suspended particles and suspending fluids. Sudden changes in the applied electric field were found to induce a variety of transient stress phenomena; while the detailed response of the disparate materials studied did differ, many similarities were observed. The characteristic times for the growth and decay of the stress were found to depend inversely on the shear rate; they varied only weakly with the amplitude of the applied field. Rise and decay times as short as a few ms were observed at the highest shear rates utilized. The inverse relationship between the characteristic rheological response times and the shear rate is consistent with the necessary roles of both field‐induced structure formation and shear‐induced strain growth in producing an ER shear stress. Implications of this work include that high shear rates must be designed into ER devices in order to achieve short response times.

Influence of orientation of electric field on shear flow of electrorheological fluids

It is possible, in experimental flows of electrorheological (ER) fluids or in actual ER devices based on shear flows, that the electric field may have a component normal to and/or parallel to the fluid velocity. An analytical study of this possibility is presented here. The analysis is carried out using a constitutive equation for the three‐dimensional response of ER fluids [Rajagopal and Wineman, Acta Mechanica 91 (1992)] in which the electric field influences the response in two ways: (a) by affecting the material parameters, and (b) by contributing to stress components because of the interaction of the electric field vector and the shearing. It is shown that the viscosity is altered by a component of the electric field in the direction of the fluid velocity. Moreover, the viscosity is changed if the shear direction is reversed. The model is used in a study of the flow between parallel plates. It is found that the velocity field need not be symmetric about the midplane and the tractions on the plates may differ.

DESIGN AND DEVELOPMENT OF FLOW BASED ELECTRO-RHEOLOGICAL DEVICES

The foundations for the design of electro-rheological (ER) actuating devices that use Poiseuille flow are laid. Systems of measurement to assess ER fluids are described including a high pressure flow apparatus. This was used to examine alternative valve geometries and fluids. A method is shown for the reduction of basic flow data to excess shear stress and shear rate. Standard actuator arrangements are reviewed before the design of a flying control surface servomechanism is discussed. The evaluation of this validates the methods and indicates that the forces developed are within striking distance of those required for operational systems.