Dirk Güth

Magnetic fluid control for viscous loss reduction of high-speed MRF brakes and clutches with well-defined fail-safe behavior

No-load losses within brakes and clutches based on magnetorheological fluids are unavoidable and represent a major barrier towards their wide-spread commercial adoption. Completely torque free rotation is not yet possible due to persistent fluid contact within the shear gap. In this paper, a novel concept is presented that facilitates the controlled movement of the magnetorheological fluid from an active, torque-transmitting region into an inactive region of the shear gap. This concept enables complete decoupling of the fluid engaging surfaces such that viscous drag torque can be eliminated. In order to achieve the desired effect, motion in the magnetorheological fluid is induced by magnetic forces acting on the fluid, which requires an appropriate magnetic circuit design. In this investigation, we propose a methodology to determine suitable magnetic circuit designs with well-defined fail-safe behavior. The magnetically induced motion of magnetorheological fluids is modeled by the use of the Kelvin body force, and a multi-physics domain simulation is performed to elucidate various transitions between an engaged and disengaged operating mode. The modeling approach is validated by captured high-speed video frames which show the induced motion of the magnetorheological fluid due to the magnetic field. Finally, measurements performed with a prototype actuator prove that the induced viscous drag torque can be reduced significantly by the proposed magnetic fluid control methodology.

Design of shear gaps for high-speed and high-load MRF brakes and clutches

Magnetorheological fluids (MRF) are smart fluids with the particular characteristics of changing their apparent viscosity significantly under the influence of a magnetic field. This property allows the design of mechanical devices for torque transmission, such as brakes and clutches, with a continuously adjustable and smooth torque generation. Especially the use of MRF in devices for applications with the demand on high rotational speeds and high loads can offer advantages, because more dissipation energy and thermal load peaks can be compensated by the torque generating fluid volume. However, high rotational speeds can cause a centrifugation of the particles contained in the MRF especially in idle mode when no magnetic field is applied. This can yield on the one hand to labile and unpredictable behavior of the torque response and on the other hand to a higher wear of the MRF or in the worst case to a destruction of the MRF with respect to a long-term view. For ensuring reliable braking or coupling conditions, in this contribution the development of a Taylor-Couette flow with axisymmetric toroidal vortices in axial shear gaps of MRF brakes and clutches is considered. The developing flow profiles in these shear gaps are modeled and analyzed in detail and finally design rules for axial shear gaps are introduced. Measurements at high rotational speeds up to 6000min−1 with a test actuator based on this design prove the positive influence of the vortex flow on the homogeneity of the MRF suspension and on the consistency and predictability of the braking or coupling torque even under long-term conditions.

MRF based clutch with integrated electrical drive

Actuators based on magnetorheological fluids represent an innovative technology to optimize brakes and clutches. Compared to conventional electromechanical systems, MRF actuators show improved and advanced characteristics featuring a smooth adjustable torque, a fast response time, a noiseless operation and a reduced design space. In the present paper, the authors propose an actuator design for a MRF based clutch combined with an integrated electric drive. The actuator design is a further development with enhanced performances considering the torque capacity. Both functionalities of this actuator can be controlled widely independent due to a suitable adjustment of the magnetic field. Therefore a model and a new control strategy are developed. The capability of this concept is proven by measurements of a new proof-of-concept prototype that provides an appropriated ratio between coupling and motor torque of 5∶1. This highly integrated mechatronical actuator system allows the design of innovative applications whereby novel features are feasible.

Characterization and modeling of the behavior of magnetorheological fluids at high shear rates in rotational systems

The behavior of magnetorheological fluids in rotational systems for torque transmission such as brakes and clutches offering a drum-shaped shear gap design is investigated for high-speed operation up to 6000 rpm or respectively high shear rates up to 34,000 s−1. The occurrence of a Taylor vortex flow is described and experimentally proven for the operation without and with an applied magnetic field. By measurements it is shown that a mixing effect due to the vortex flow avoid a disadvantageous particle centrifugation. In further measurements a weakening of the magnetorheological effect with increasing rotational speeds is observed. Utilizing a magnetic dipole model for calculating the yield stress with a common parameterization based on measurements, a prediction of the achievable torque as a function of the rotational speed is possible. This modeling approach provides an important tool for the design process of magnetorheological-fluid-based actuators for high rotational speed application.

Long-term stable magnetorheological fluid brake for application in wind turbines:

In this article, a novel brake for application in wind turbines developed with the focus on long-term stability is proposed. The brake, whose transmission of power is based on magnetorheological fluids, is designed for fail-safe operation under industrial standards. The long-term stability performance over a lifetime of up to 20 years can be ensured by the use of a Taylor-vortex flow in idle mode that causes a mixing effect for preventing particle separation. Beside a detailed description of the design, long-term measurements with requirements for use in wind turbines emulating a timely reduced aging of the magnetorheological fluid will be presented by applying Hardware-in-the-Loop simulations. The results show an outstanding torque performance over an emulated lifetime of 20 years of use in wind turbines that outpaces the capability of conventional brakes whose power transmission is based on dry friction.

Novel concepts for MRF based clutch systems with integrative functionalities

Actuators based on magnetorheological fluids have already been realized with a coupling function due to their special advantages, such as a fast response time and an excellent controllability of torque. In this paper novel concepts for integrated designs of MRF based clutches in combination with a shiftable electromotive actuator are presented. For an optimized use of the design space both functionalities, clutch and drive function, can be controlled widely independent by a suitable modulation of the magnetic field generated by one common magnetic excitation system. Measurement results of a realized proof-of-concept prototype demonstrate the possibilities of combining both functions in one actuator and show the potential of such a highly integrated mechatronic device.

Modeling Approach for a Fluid Movement Induced by Magnetic Forces for Viscous Loss Reduction of MRF Brakes and Clutches

A challenge that is opposed to a commercial use of actuators like brakes and clutches based on magnetorheological fluids (MRF), are durable no-load losses, because a complete torque-free separation due to the permanent liquid intervention is inherently not yet possible. In this paper, the necessity of reducing these durable no-load losses will be shown by measurements performed with a MRF brake for high rotational speeds of 6000min−1. The detrimental high viscous torque motivates the introduction of a novel concept that allows a controlled movement of the MRF from an active shear gap into an inactive shear gap, enabling a complete separation of the fluid engaging surfaces. This behavior is modeled by the use of the ferrohydrodynamics and simulations are performed for different transitions between braking and idle mode. Images of high speed video capturing, showing the motion of MRF induced by a magnetic field, are presented for the validation of the modeling approach. Measurements performed with a realized proof-of-concept actuator show that the viscous induced drag torque can be reduced significantly.Copyright

Design and characteristics of MRF-based actuators for torque transmission under influence of high shear rates up to 34,000s-1

High rotational speeds for brakes and clutches based on magnetorheological fluids represent a remaining challenge for the industrial or automotive application. Beside particle centrifugation effects and rotational speed-depending no-load losses, the torque characteristic is an important property that needs to considered in the design process of actuators. Due to missing experimental data for these operating conditions, in this paper the shear rate and flux depending yield stress behavior of magnetorheological uids is experimentally investigated for high rotational speeds or respectively high shear rates. Therefore a brake actuator with variable shear gap heights up to 4 mm is designed, realized and used for the experimental investigation, which are performed for a maximum shear rate of ƴ= 34; 000 s-1 under large magnetic elds. The measurement results point out a strong dependency between shear rate, magnetic ux density and resulting yield stress. For low shear gap heights, a significant reduction in the yield stress up to 10 % can be determined. Additionally the development of Taylor vortices is determined, which will not only occur in viscous case without an applied magnetic field. The measurement results are important for a reliable actuator design which should be used in application with high rotational speeds.

Energy-Efficient MRF-Clutch With Optimized Torque Density

The viscous losses at high rotational speeds in idle mode represent a major drawback for an energy-efficient operation of MRF brakes and clutches. In this paper, an innovative concept is presented that allows a complete torque-free idle mode of the mentioned actuators by the use of a MR-fluid control. Using magnetic forces, the MR-fluid control is capable to increase the efficiency and lifetime of MRF based actuators for torque transmission. An approach of designing actuators using the investigated MR-fluid control is applied which considers in detail the required magnetic excitation systems for enabling a MRF movement in the shear gaps. A clutch is presented and simulations of the transient switching behavior are performed. Measurements of a realized clutch underline the functionality and efficiency of those actuators. Additionally, further approaches for a shear gap design are introduced that increase the maximum torque capacity. By the use of a micro-grooved structure for the MR-fluid control, a torque-to-volume ratio can be achieved that is in the order of typical MRF actuators for torque-transmission with completely filled shear gaps.Copyright

MRF actuators with reduced no-load losses

Magnetorheological fluids (MRF) are smart fluids with the particular characteristics of changing their apparent viscosity significantly under the influence of a magnetic field. This property allows the design of mechanical devices for torque transmission, such as brakes and clutches, with a continuously adjustable and smooth torque generation. A challenge that is opposed to a commercial use, are durable no-load losses, because a complete torque-free separation due to the permanent liquid intervention is inherently not yet possible. In this paper, the necessity of reducing these durable no-load losses will be shown by measurements performed with a MRF brake for high rotational speeds of 6000min-1 in a first step. The detrimental high viscous torque motivates the introduction of a novel concept that allows a controlled movement of the MR fluid from an active shear gap into an inactive shear gap and thus an almost separation of the fluid engaging surfaces. Simulation and measurement results show that the viscous induced drag torque can be reduced significantly. Based on this new approach, it is possible to realize MRF actuators for an energy-efficient use in the drive technology or power train, which avoid this inherent disadvantage and extend additionally the durability of the entire component.