Jakub Roupec

Design of magnetorheological damper with short time response

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.

Limiting factors of the response time of the magnetorheological damper

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 coils 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 coils 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.

The behavior of the MR fluid during durability test

The article describes results of durability test of a magnetorheological fluid (MRF), which was carried out in rheometer of own design. The rheometer design enables to measure the rheological properties of MR fluid and to expose it to a long-term loading simultaneously, without any manipulation of the measured sample. During the durability test a change of the two most important parameters of Bingham model describing the behavior of MR fluids can be followed – dynamic viscosity and yield stress. In this paper the yield stress and viscosity were evaluated depending on temperature in OFF-state. The results show a significant change of yield strength during durability test depending on temperature of loading. Dependence of yield stress on temperature was proved. The viscosity decreased by 36% from its initial value after the dissipation of 9÷20kJcm−3 from total 1.2 MJcm−3 and then has remained the same until the end of durability test. Viscosity was evaluated depending on temperature.

Stability of Magnetorheological Effect during Long Term Operation

The article describes a determination of magnetorheological (MR) effect stability during long term operation. The experiments were carried out in a custom design rheometer, which is built in a mechanical pulsator. The rheometer design allows measurement of the rheological properties of MR fluid and its exposure to a long-term loading simultaneously, without any manipulation of the measured sample. The experiments were performed under conditions which correspond to real conditions in an automotive MR damper. The results of long term operation have shown that during the durability test the yield stress increases up to five times in the fluid off-state due to in-use-thickening (IUT). The same increase was identified in the on-state. These results disprove the hypothesis that a measurable decrease of MR effect arises during the long term operation due to the degradation of magnetic properties.

Problems of FEM Analysis of Magnetic Circuit

This paper compares four different approaches to modeling of magnetic circuit in the ANSYS software. The approaches differ in various inputs of magnetic field source and varying levels of axial symmetry. The accuracy and the solution time were the main evaluation criteria. The analysis was performed on a simple magnetic circuit. The analysis results were compared with measurements on experimental device that was made for this purpose. The results show that planar solution in ANSYS Classic is the fastest and the highly accurate approach. The problem solution such as 3D axisymmetric task with the definition of magnetic field source with the stranded type is the most accurate, but more time-consuming.

Identification of MR Fluids Properties in Mechatronic Damping Elements

Mechatronic systems often use a magnetorheological fluids for the damping of movement. A significant dependence of their rheological properties on a magnetic field is the main reason. Therefore these fluids are relatively well controllable in mechatronic systems. For correct control of these elements it is important to assign to MR fluids a suitable rheological model of behavior. This model describes flow and viscosity dependences by various magnetizing currents and various shear rates. This paper is concerned with measurement problems of flow and viscosity curves on standard and original rheometers. Based on measured data, a behavior of MR fluid can be mathematically described, a relevant model can be compiled, a behavior of different MR fluids can be compared or changes of mechanical properties during the measurement can be observed.