The science of phenomenon that a material changes its viscoelasticity when subjected to a magnetic field is called Magneto-Rheology (MR), and the function is called the MR effect. The typical material is an MR fluid containing micronsized ferromagnetic particles dispersed in an oil. In addition to the MR fluid, this review article also covers a high-fluidity dry MR fluid consisting of oil-free ferromagnetic particles powder with higher MR effect and higher environmental resistance, and an MR elastomer consisting of ferromagnetic fine particles dispersed in a silicone rubber and cured. The advanced MR technologies ranging from the fabrication and MR effect evaluation of various functional fluids and soft materials showing the MR effects to the application technologies to various devices and systems have been discussed by mainly introducing the author's research and development cases.

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Japan is frequently suffered from giant earthquake. Representative giant earthquakes are Great Hanshin-Awaji and Tohoku Earthquake and Tsunami. Several research institutions estimate the scale of damage occurred in Nankai Trough Earthquake in the future. Some methods which protect buildings from these earthquakes are earthquake resistant, seismic isolation system, and vibration control. We have developed oil dampers as vibration control devices. The developed oil damper using valves and silicone oil have fixed characteristics. Variable damping force damper which adjusts damping force to vibrational strength is expected that the damper efficiently controls vibration includes random waves such as earthquakes. Magneto Rheological Fluid (MR Fluid) significantly changes the characteristics when magnet field is applied. MR damper which is used the typical property has been developed as variable damping force damper. And, taking advantage of MR Fluid’s characteristics of being heavier than ordinary oil, MR damper using MR Fluid as the operating fluid has been developed. This report introduces these developed products.

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This explanatory paper investigates the fundamental characteristics and aircraft applications of magnetorheological fluid brakes (MRBs). MR fluids exhibit field-dependent yield stress, enabling rapid, controllable braking without mechanical friction. Experiments on single-disc, multi-disc, cylindrical MRBs revealed current-proportional torque, particle-size effects, and significant temperature dependence. Scaling analysis showed that aircraft-level torque requires multi-disc structures and optimized magnetic circuits. A rejected takeoff (RTO) simulation using a long short-term memory (LSTM)-based MRB model demonstrated stable slip control and sufficiently deceleration.

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Magnetorheological (MR) fluids, a type of functional fluid, are capable of rapidly and reversibly altering their rheological properties in response to an external magnetic field. This unique characteristic enables their application in various mechatronic devices that require semi-active control of resistance and transmission forces. To integrate MR fluids into such force-control devices, two essential components are required: magnetic poles for applying a magnetic field to the fluid layer, and a mechanical structure capable of converting changes in the fluid’s dynamic properties into device output. This paper provides an overview of rotary-type MR fluid devices in which the magnetic poles and the moving components are integrated and perform rotational motion. As application examples, we introduce two devices recently developed by the authors: a miniature MR fluid device using a cylindrical rotor, and a fail-safe MR fluid device using both electromagnets and permanent magnets.

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Haptic feedback is required for medical palpation; however, conventional force display devices face limitations in miniaturization and localized stiffness presentation. This paper introduces magnetorheological (MR) fluid as an effective dynamic actuator, utilizing changes in its viscosity to present stiffness. Two developed devices are described, both employing an MR fluid cell sealed in fingertip-sized silicone resin. These devices generate a reaction force when the finger presses against them. The first device, a fingertip-mounted type, demonstrated the feasibility of presenting a stiffness distribution through fingertip contact with a magnetic stage featuring a prearranged permanent magnetic field. The second device employs an electro-permanent magnet (EPM) to instantly switch the magnetic flux density applied to an MR fluid cell. Sensory evaluation using this device confirmed the perception of stiffness differences through magnetic switching, verifying the potential for biological stiffness presentation using MR fluid. Future work will focus on EPM miniaturization, magnetic flux density enhancement, and establishing control methods to realize a real-time stiffness display system applicable to telemedicine, surgical training, and virtual-reality technology.

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The LLC resonant converters are used as server power supplies in data centers, and further efficiency improvements are required as the information society develops. The LLC transformer used in LLC resonant converters requires a coupling coefficient of approximately 0.8 to utilize the resonance phenomenon between the leakage flux and the resonant capacitor. Typically, the coupling coefficient is adjusted by introducing a gap in the inner leg of the core. However, this causes fringe flux to act on the windings, leading to AC copper losses due to proximity effects. This paper confirms that using AC copper loss reduction windings in LLC transformers reduces the coupling coefficient from 0.93 to 0.88 while maintaining excitation inductance, simultaneously decreasing AC resistance by 37.9%. This improves the efficiency of LLC resonant converters by 0.54 percentage points and reduces losses by up to 4.1W

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