**1. Introduction**

Magnetorheological fluid (MRF) is a non-colloidal suspension of magnetizable particles that are on the order of tens of microns (20-50 microns) in diameter. Generally, MRF is composed of oil, usually mineral or silicone based, and varying percentages of ferrous particles that have been coated with an anti-coagulant material. When inactivated, MRF displays Newtonian-like behavior. When exposed to a magnetic field, the ferrous particles that are dispersed throughout the fluid form magnetic dipoles. These magnetic dipoles align themselves along lines of magnetic flux. The fluid was developed by Jacob Rabinow at the US National Bureau of Standards in the late 1940's. For the first few years, there was a flurry of interest in MRF but this interest quickly waned. In the early 1990's there was resurgence in MRF research that was primarily due to Lord Corporation's research and development. Although similar in operation to electro-rheological fluids (ERF) and Ferro-fluids, MR devices are capable of much higher yield strengths when activated. For this advantage, many MRF-based mechanisms have been developed such as MR dampers, MR brake, MR clutch, MR valve... and some of them are now commercial. As well-known that performance of MRF based systems significantly depends on the activating magnetic circuit, therefore, by optimal design of the activating magnetic circuit, the performance of MRF-based systems can be optimized. Recently, there have been various researches on optimal design of MRFbased devices such as MR damper, MR valve, MR brake. The results from these studies showed that performance of MRF-based systems can be significantly improved via optimal design of the magnetic circuit of the systems.

Consequently, this chapter focuses on the methodology of optimal design of MRF-based devices. The chapter is organized as follows; in section 2, fundamentals and the theory behind MRF are overviewed. Section 3 deals with the modes used in MRF based devices and several different devices featuring MRF are discussed. In the fourth section, optimal

© 2012 Choi and Nguyen, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

design methodology of MRF-based mechanisms is considered. In this section, firstly the necessity of optimal design and the state of the art are discussed. Then the magnetic circuit analysis and the modeling of MRF devices are considered. In addition, the optimization problem of MRF devices is figured out and the methods to solve the problem are investigated. Section 5 is devoted to deal with a case study of MR valve optimal design. In this case study, several valve configurations such as single-coil, multiple-coil and annularradial MR valves are considered. The chapter is then closed by the conclusion
