What does MRF stand for
Actuators based on magnetorheological fluids (MRF)
Magnetorheological fluids (MRF) are suspensions of fine, magnetically polarizable particles (carbonyl iron powder with a particle diameter of a few µm) in an oil-based carrier fluid. Under the influence of a magnetic field, the particles form chains in the direction of the field lines, which means that the flow resistance of the MRF can be changed in a highly dynamic and largely linear manner. This effect can be used technologically, especially in:
- controllable dampers or in
- Clutches and brakes.
As part of the BMBF-funded project "MRF Brake", fundamental aspects of the development of MRF actuators in the field of drive technology are being researched. The aim of the research project is the holistic modeling and conception of the novel actuators for a functionally reliable design. Another BMBF-funded project, "HLD-Fluidbrake", is researching into solutions for brakes with high performance. The advantages of using magnetorheological fluids are especially the longer service life and better thermal resistance. One goal of this research project is the investigation of magnetorheological fluids under the influence of high speeds in order to use the knowledge gained to develop actuator geometries that are optimized for the application and that enable the fluid to be controlled specifically for different operating states.
The actuators developed as part of the two funding projects represent a considerable potential for improvement, especially for applications in automation and automotive technology. The main advantages of MRF actuators over conventional, friction-lining-based brakes and clutches are
- a reproducible, easily adjustable braking torque,
- a low-noise mode of operation as well
- extensive wear and tear and low maintenance.
In this regard, in addition to the funded projects, research projects are also being worked on that deal with the development of MRF actuators for automation and automotive applications. One potential application of MRF actuators is the motion control of motor vehicle doors. In addition to compliance with high requirements with regard to installation space, weight and torque, the representation of a currentless holding torque is absolutely essential, for which new types of actuator topologies have been developed.
In addition, research is being carried out on function-integrated actuator systems which combine MRF-based brakes and clutches with electric motors in one integrative component and which, despite an internal magnetic excitation system, do not require any disadvantageous sliding contacts for energy transmission. A special design of the electronic power supply in connection with the excitation systems makes it possible, for example, to generate a motorized torque in addition to the clutch function. Therefore, these hybrid actuators are suitable for applications that require a torque that can be activated in addition to the actual clutch function, e.g. for additional consumers that can be activated or in the drive train of hybrid vehicles.
For the energy-efficient operation of the MRF actuators in the drive train of vehicles as well as in fast-rotating industrial applications, measures to avoid drag torques due to the viscous fluid properties are necessary. For this purpose, a fluid movement control was developed which, when idling, displaces the MRF into a so-called inactive area by a spatially transient magnetic field and allows it to remain stationary. As a result, one of the shearing surfaces is no longer in engagement with the MRF fluid, as a result of which the viscous losses can be completely eliminated.
The research advances described above with regard to MRF actuators form the basis for the development of energy-efficient multimode transmissions for use in electric hybrid vehicles in the BMWi joint project PHEVplus. For this purpose, MRF coupling elements are to be researched in order to increase the efficiency of the hybrid drive train, which among other things have no permanent drag torque in the open state due to the fluid movement control and thus contribute to increasing the range of the plug-in hybrids in electrical operating mode.
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