Magnetorheological fluid
A magnetorheological fluid (MRF) is a suspension of magnetically polarizable particles ( carbonyl iron powder ) that are finely distributed in a carrier fluid.
The particles of magnetorheological fluids are around one to three powers of ten larger than those of the ferrofluid , so that an MRF - unlike a ferrofluid - solidifies when a magnetic field is applied . Mechanism: the particles are polarized and form chains in the direction of the field lines . The alignment of the particles makes the suspension more viscous with increasing field strength . An MRF can be changed drastically, quickly and reversibly in a magnetic field .
Magnetorheological fluids have been known since the 1940s. Only in recent years has it been possible to avoid negative properties such as abrasion , sedimentation and aging by using special additives and polymer surface coating of the particles, so that use in series products is now possible.
composition
In magnetorheological fluids, spherical particles with a diameter of 1 to 10 micrometers and a high degree of purity (produced via synthesis ,> 99% pure iron) are used. The particles themselves are therefore non-toxic and, due to their high purity, are also used, for example, as food supplements in iron tablets. When used in magnetorheological fluids, the excellent electromagnetic and mechanical properties are decisive. Mineral oil and synthetic oils , ethylene glycol and water are used as carrier liquids . Additives such as stabilizers and viscosity improvers are added as auxiliaries . Among other things, these prevent sedimentation and agglomeration . The proportion of iron powder is 70-90% by weight and 20-40% by volume. The liquid has a dark gray color, its density is between 2.2 and 4 g · cm −3 .
Mechanical and magnetic properties
The viscosity of the liquid is 0.07 Pa · s to 14 Pa · s at a shear rate of 10 s −1 .
The temperature influences the viscosity of the MRF depending on the base fluid; the operating temperature is between −40 ° C and 150 ° C.
When the magnetic field is applied, the magnetorheological effect predominates; Changes in viscosity then take place in less than a millisecond , since only the particles have to be aligned. Thus, changes in force can be implemented within a few milliseconds depending on the design, see measurement (3000 N / ms). The shear stress behaves in a magnetic field-free space like a Newtonian fluid and in a magnetic field like a Bingham fluid .
BH curve of a typical magnetorheological fluid
Shear stress over B-field of a typical magnetorheological fluid
Measurement of the response behavior of the MRF damper
All values depend on the mixing ratio and the carrier liquid.
Operating modes
The strength of the force transmission in the fluid depends, among other parameters, on the stress on the fluid. Three relevant operating modes are known:
- In valve mode, the fluid moves through a channel through which a magnetic field flows perpendicular to the direction of movement of the fluid. MRF attenuators use this method.
- In the case of direct shear stress, two plates move, between which the fluid is located. The magnetic field flows through the fluid perpendicular to the direction of movement of the plates. This use is found in clutches and brakes.
- In squeeze mode, the highest forces are achieved. The force acts on the fluid in the direction of the magnetic flux.
Possibilities of generating magnetic fields
Depending on the field of application and application, it is advisable to generate the magnetic field differently:
| Magnetic field generation | comment | Characteristic (example MRF valve) |
|---|---|---|
| Permanent magnet ( permanent magnet ) | The system works independently of electrical energy, the magnetic field strength z. B. depends on the distance between the magnet and the magnetorheological fluid and can be continuously adjusted via this. This is not possible with electrorheological fluids . | |
| Electric coil | The magnetic field is proportional to the coil current and can be set as required and quickly. Thus, an automatic setting z. B. possible depending on sensor values. With large adjustment ranges required, the energy consumption is also greater. | |
| Combination of permanent magnet and electric coil | In this system, the magnetic field of a permanent magnet is weakened or strengthened by a magnetic field from an electric coil. This is e.g. B. useful if a fail-safe function is required or a switching state is prevailing and thus energy can be saved. | |
| Remanence | In simplified terms, an adjustable permanent magnet is used in this system to generate the magnetic field. A hard magnetic material is magnetized to the desired value by an electromagnetic pulse. As a result, such a system only requires electrical energy when there is a change in state and is also conditionally fail-safe. |
If a system with magnetorheological fluid is correctly, efficiently and EMC- compliantly designed, it will not cause any interference in other systems.
application areas
Variable damping to adapt to changed loads / loads, d. H. dynamic control of viscosity through the use of sensors and electromagnets
- Vehicle Applications: suspension - shocks (such as the Audi TT , R8 , Acura MDX , Chevrolet Corvette , Ferrari 599 GTB ), dampers in seats, engine mounts (for example, Porsche ) and all-wheel drive - couplings
- Construction: dampers in bridges and high-rise buildings
- Medical technology: prostheses
- Household appliances: washing machine with MRF damper (for example Zaboon from Toshiba ) to reduce washing time, for more washing power, less vibrations, less noise and a compact, simple design.
- Technical optics: Processing of surfaces by means of magnetorheological polishing
See also
Web links
- Dissertation Florian Zschunke: Actuators based on the magnetorheological effect (PDF 6 MB)
- Patent Specification: General Motors Corp. Oxidation Resistant Magnetorheological Fluid at PatentDe
Individual evidence
- ^ Artur Grunwald: Design and Optimization of Magnetostrictive Actuator (dissertation) . Ed .: Dublin City University, Ireland. August 2007.
- ↑ MRF suspension damper in the new Acura MDX 2010 ( Memento of the original from September 9, 2010 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ Engine mounts: Dynamic engine mounts from Porsche for the 911 Turbo and 911 GT3 models ( memento of the original from January 2, 2010 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ First large-scale product on the market outside of the automotive industry: Toshiba washing machine with MRF damper (Zaboon) - available in Japan since October 2010 ( Memento of the original from August 15, 2010 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked . Please check the original and archive link according to the instructions and then remove this notice.
