For efficient fast control of suspension systems with magnetorheological dampers controlled by semi-active algorithms, the time response of the magnetorheological damper is one of the most important parameters which influences the performance of the suspension system. The time response of commercial magnetorheological dampers with common controllers is in the range of tens of milliseconds, which is too long for efficient real-time control of car suspension. This article describes the ways how to design magnetorheological damper with short response time. First part describes the elimination of long time response caused by high inductance of electrical circuit of magnetorheological damper using current controller. The second part describes elimination of long time response caused by eddy currents which are induced in the magnetic circuit of magnetorheological damper. The new piston of the magnetorheological damper is designed from material with high relative permeability but low electrical conductivity (ferrite). The time responses of magnetic induction and damper forces were measured and compared to original commercial Delphi damper. Results showed rapid improvement in time response when the current controller is used and when the piston is constructed from ferrite materials.

The paper deals with design, simulation and experimental testing of a magnetorheological (MR) valve with short response time. The short response time is achieved by a suitable design of an active zone in combination with use of a ferrite material for magnetic circuit. The magneto-static model and the simplified hydraulic model of the MR valve are examined and experimentally verified. The development the MR valve achieves an average response time 4.1 ms and the maximum dynamic force range of eight.

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Eddy currents are the main reason causing for the long response time of a magnetorheological (MR) damper. Eddy currents are often unwanted parasitic phenomenon for many electromagnetic machines working with an alternating magnetic field. Their reduction can be secured by the use of material with high electrical resistivity such as ferrites or soft magnetic composites. These materials, however, exhibit bad mechanical properties and cannot be used in mechanically loaded parts. Eddy currents can also be reduced by the appropriate structure which must secure high conductivity for the magnetic flux but low electrical conductivity for the electric current flowing perpendicularly to the magnetic flux. This leads to complex structures which, in most cases, cannot be manufactured by conventional methods. This paper describes the design, manufacturing and verification of simulations of the magnetic circuit for a MR damper. Structured magnetic cores printed by selective laser melting technology connects the benefits of low-carbon steel (good mechanical properties, high magnetic saturation and high relative permeability) with benefits of sintered materials (high electric resistivity). The results proved that using the potential of additive manufacturing can not only reduce the eddy currents (and thus shorten the response time and reduce losses), but significantly reduce the weight as well. This technology enables the combination of performance parameters of electromagnetic machines, which cannot be reached by any other existing method.

Magnetorheological dampers seem to be suitable for the adaptive car suspension systems. For proper operation of the semi-active algorithms (skyhook, groundhook), the force response of the damper must be fast enough. In this paper, the response time of the Delphi vehicle MR damper is examined and the sources of the overall force time response on the control voltage are discussed. One of the main sources of the response time seems to be electro-magnetic circuit of the MR damper. A principle of an optimal controller for reducing response of the coil's current on the control voltage is designed. However, the measured overall force time response with fast current controller was not reduced as expected. Therefore, a FEM simulation of the magnetic circuit was made. It shows that after an optimization of the current controller, eddy currents in the coil's core cause long time response and therefore they are the limiting factor of the response time of the MR damper. These simulations were verified by the measurements and some recommendations about improving the pistons construction are given at the end.

Sedimentation stability is one of the most important features of magnetorheological (MR) fluids. Clay-based additives are known for improving the stability of the MR fluids. This article describes the dependency of the clay-based additive concentration on the sedimentation stability and the rheological properties of MR fluids in non activated state (without magnetic field). The sedimentation was measured for two different base oil viscosities, two different carbonyl iron particle sizes, and additive concentration between 2 and 6 wt%. The measurements showed that the sedimentation rate exponentially decreases with the additive concentration, while the yield stress is rising. The measurements of rheological properties also showed the dependency of rheological properties of MR fluid with a clay-based additive on loading history. The influence of carrier fluid viscosity or particle size has a minor effect on the sedimentation in comparison with the clay-based additive. The addition of 6 wt% slows down the sedimentation by more than 3000 times compared to MR fluid without additives. The MR fluid with 4.85% of clay-based additive achieves slightly better sedimentation stability than commercial MR fluid LORD MRF122.

This paper deals with the design and tests of a magnetorheological fluid seal (MRFs) using the innovative concept of the magnetic circuit, which allows the achievement of a promising trade-off between burst pressure and friction of the seal. Low friction torque and low burst pressure are typical for a ferrofluid seal (FFs). Replacement of the ferrofluid by magnetorheological fluid increases the burst pressure of the seal but the friction torque of the seal increases too. The optimum for sealing application is low friction torque and high burst pressure. The presented design is based on the pinch mode of magnetorheological fluid. The geometry of the seal was determined by a magnetostatic model. Subsequently, the chosen concept of the seal was manufactured and tested. Pinch MRFs achieved lower friction torque than common (standard) MRFs and a higher burst pressure than any FFs.

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