Magnetorheological Device Development and Testing: Student Projects Augmenting Research

Journal of Engineering Technology, Spring 2004 by Vavreck, Andrew N

Abstract

Magnetorheological fluids are adaptive-passive materials with many interesting applications, stemming from their ability to convert rapidly from a liquid to a solid under the application of a magnetic field. This paper describes an extensive research program at Penn State Altoona, with efforts in modeling, design, and application of these materials, that is supported by a variety of student projects, particularly capstone design projects. Students gain exposure to a cutting-edge technology, learn about design processes and tools, and acquire technical skills in instrumentation, controls, materials, and sensors. Faculty gain effective technical support that can help farther their research work as well as enhance their teaching experiences through the exploration of this new field.

Introduction

Magnetorhcological (MR) fluids typically consist of suspensions of ferromagnetic particles (on the order of a few microns in diameter) in an oil or water base, with a dispersant added to prevent settling. When a magnetic field is applied to the liquid, the particles acquire magnetic poles in alignment with the impressed magnetic field. Each particle thus attracts adjacent particles, forming a lattice or framework within the fluid, as seen in figure 1.

This structure and the dipole forces resulting from the impressed magnetic field cause the fluid to change a few milliseconds after the application of the field from the consistency of house paint to that of peanut butter. In addition to the obvious change in the fluid's rheology, its thermal conductivity also varies with the impressed magnetic field. MR fluid is a prime example of a class of materials called adaptive-passive materials, those whose properties can be altered in real time to respond to a dynamic situation. In vibration suppression, adaptive-passive materials are an excellent compromise between simple passive materials approaches, which cannot adjust to changing situations, and active approaches, which require more energy and can contribute to system instabilities.

Typical yield stresses for cleclrorheological (ER) fluids (MR fluids precursors that are energized by electric fields rather than magnetic fields) range from 0.3 to 0.7 psi, while MR fluids exhibit higher yield stresses, on the order of 7 to 15 psi.1 Viscosity with no impressed field ranges from 0.004 to 0.006 lbf sec/ft^sup 2^ for both fluids. The rapid response of these fluids, a broad dependence of the fluid rheology on field magnitude, and the practical application of these fluids as part of damping treatments have made this area ripe for research in both the microscopic (fluid constituents, particle and fibril motion, and polarization effects) and macroscopic (damping models, viscoelastic/viscoplastic responses, and device design) regimes. ER and MR fluids have been used in dampers, clutches, brakes, valves, vibrators, dynamic vibration absorbers, and as a substrate for lens polishing.2,3

The most successful commercial applications of these fluids to date have been in variable resistance elements in rotary brakes for exercise equipment and in linear dampers for ride enhancement in commercial truck seats.4 Figure 2 shows a Lord Corporation model RD-1005-3 monotube MR damper (next to a quarter for size estimation), and figure 3 shows the force produced by this damper as a function of piston speed and current. This device relies on the passage of MR fluid through a hole in the piston that is part of a magnetic circuit. Energizing the circuit solidifies the fluid in the hole, increasing the resistance to piston motion.

In the Lord Corporation model MRB-2 107-3 rotary brake shown in figure 4, a rotor spins through a chamber that contains MR fluid. A coil converts the fluid inside the chamber into a semisolid, thus increasing the shear stress and torque required to turn the rotor. The brake has a diameter of approximately 3.6 inches.

Because MR dampers behave in a nonlinear fashion, unlike conventional viscous dampers, and because MR brakes are more difficult to model than conventional friction brakes, testing of these devices helps develop parametric models that can be used to predict device performance and develop control strategies.5 Typically, servomotors, in the case of rotary devices, and electrodynamic shakers, in the case of linear devices, are used to impart controlled displacement while the resultant torque or force is recorded. While somewhat expensive, these test beds are well suited when a highly controlled displacement is required. As part of an ongoing research and educational program in MR fluid application and modeling at Penn State Altoona, engineering technology (ET) students have developed a low-cost, versatile test bed for linear dampers. System principles have also been incorporated in other student projects, enabling many future engineering technologists to explore various facets of instrumentation, mechanical design, data acquisition, and adaptive-passive devices while providing critical technical support to faculty research.