The world of electro-active fluids
This page is dedicated to magneto and electro rheological fluids. Many articles and websites discuss about the principles and many links are available from this page.
Summary
- Principle of magneto-rheological fluids (MR)
- Principle of electro-rheological fluids
- What about the ferro-fluids?
- Applications
- Modellings
- Links
Principle of magneto-rheological fluids (MR)
Magnetorheological (MR) fluids are materials that respond to an applied field with a dramatic change in their rheological behavior.
The essential characteristic of these fluids is their ability to reversibly change from a free-flowing, linear, viscous liquid to a semi-solid with a controllable yield strength in milliseconds when exposed to a magnetic field. In the absence of an applied field, MR fluids are reasonably well approximated as Newtonian liquids. For most engineering applications a simple Bingham plastic model is effective at describing the essential, field-dependent fluid characteristics. (A Bingham plastic is a non-Newtonian fluid whose yield stress must be exceeded before flow can begin; thereafter, the rate-of-shear vs. shear stress curve is linear.) In this model, the total yield stress is given by: A typical MR fluid consists of 20%–40% by volume of relatively pure, soft iron particles, typically 3–5 microns, suspended in a carrier liquid such as mineral oil, synthetic oil, water, or glycol. A variety of proprietary additives similar to those found in commercial lubricants are commonly added to discourage gravitational settling and promote particle suspension, enhance lubricity, modify viscosity, and inhibit wear. The ultimate strength of the MR fluid depends on the square of the saturation magnetization of the suspended particles. MR fluids made from iron particles exhibit maximum yield strengths of 30–90 kPa for applied magnetic fields of 150–250 kA/m (1 Oe . 80 A/m). MR fluids are not highly sensitive to moisture or other contaminants that might be encountered during manufacture and use. Further, because the magnetic polarization mechanism is not affected by the surface chemistry of surfactants and additives, it is a relatively straightforward matter to stabilize MR fluids against particle-liquid separation in spite of the large density mismatch. Most devices use MR fluids in a valve mode, direct-shear mode, or combination of these two modes. Examples of valve mode devices include servovalves, dampers, and shock absorbers. Examples of direct-shear mode devices include clutches, brakes, and variable friction dampers.
Principle of electro-rheological fluidsER fluid was invented in the 1940s. Electro-Rheological (ER) fluids change their physical properties in the presence of an electric field. The fluids change from a free-flowing liquid to one with a finite static yield stress, giving them properties consistent with a solid or gel when the field is turned on. The speed of the transition between two states is typically less than 1 millisecond (more than 1 kHz). This gives rise to new possibilities of fast mechanical control techniques. ER valves are fast enough, for example, to play music through a hydraulically operated ER loudspeaker. Smart holds a number of patents associated with the manufacture and application of ER fluids. We are happy to discuss potential requirements with our customers, as often it is possible to modify the fluid composition and hence customise its characteristics for a specific application. Smart produce several standard fluids including a general purpose fluid LID 3354S designed for high volume applications that require good ER performance at an affordable price. As mentioned on the previous page, ER fluids change to a gel like solid when an electric field is applied. Electric fields in the order of 1kV to 4kV per millimetre are applied to the fluid to produce a high finite static yield stress. In the video on the left a microscopic view of an ER fluid shows the effect of applying an electric field. First, when no voltage is applied, the ER fluid is in its natural fluidic state, then an electric field is applied to the fluid and all the ER particles form a one dimensional chain from electrode to electrode.
What about the ferro-fluids?Applications
joints ferrofluides
prothèses
amortisseurs automobiles
ceintures de sécurité
berceau moteur
embrayage
systèmes antisismiques
interfaces haptiques
miroirs déformables