MR Fluid Devices

Virtually all devices that use controllable MR fluids operate in a valvemode, direct-shear mode, or a combination of these two modes. Diagrams of the basic valve and direct-shear modes are shown in Fig. 6.76. Examples of valve-mode devices include dampers, and shock absorbers. Examples of direct shear-mode devices include clutches, brakes, chucking and locking devices, and some dampers. 

The pressure drop developed by a valve-mode device can be divided into two components, the pressure AP due to the fluid viscosity and APmr due to the magnetic field-induced yield. These pressures may be approximated by [131,157,158]  

An example of a simple valve-mode device is the RD-1005-3 linear damper by Lord Corporation shown in Fig. 6.77 [152]. As in the vast majority of all commercial MR fluid dampers, these dampers have an internal, axi-symmetric valve with an annular flow path. In this case the damper is a single-ended, mono-tube style having an internal rod volmne accumulator pressurized with nitrogen. As indicated in the Fig. 6.77, downward motion of the piston causes MR fluid to flow up through the annular flow channel. Application of current to the coil creates a magnetic field that interacts with the MR fluid in two regions where the magnetic flux crosses the flow channel. 

The range of force control that is possible with a valve-mode MR fluid damper is illustrated in Fig. 6.78. Here the force/velocity character that is typical of a passive hydraulic damper is compared to the range of forces possible with a MR damper. With appropriate control based on displacement, velocity or acceleration, any force profile between the upper and lower bounds can be realized. Unlike passive viscous dampers, with the MR damper it is easy to achieve large force at very low speed. 

MR with maximum current MR with constant current 

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The equations describing the on-state and off-state pressures or forces in MR fluid devices can be combined into a simple expression for the minimum active fluid volume, i. e. the volume of fluid acted upon by the magnetic field in a MR fluid valve [131], Such an expression is useful because it allows one to estimate the necessary size of a device and determine feasibility prior to developing a detailed engineering design. For many of the most widely used standard commercial MR fluids this expression takes the particularly simple form [159] ... 

Magnetorheological fluid actuators provide technology that enables effective semi-active control in a number of real-world applications. Automotive applications of MR fluid are significant and growing rapidly. Annual production of MR fluid is now on the order of hundreds of tons. It is estimated that more than one hundred thousand MR fluid devices are presently in use. This number is expected to rise into the millions as more automotive platforms adopt MR fluid based real-time controlled motion control systems. Due to their simplicity, low power, and inherent robustness, MR fluid devices have proven themselves in a wide variety of commercial applications. 

An example of a simple, direct-shear device is shown in Fig. 6.79. In this brake MR fluid is located between the faces of the disc-shaped rotor and the... 

Carlson, J.D. Catanzarite, D.M. and St. Clair, K.A. Commercial Magneto-Rheological Fluid Devices. Proc. 5th Int. Conf on ER Fluids, MR Fluids and Assoc. Tech., Sheffield (July 1995), W.A. Bullough, ed.. World Scientific, Singapore (1996), pp. 20-28... 

The discovery of both ER and MR fluids dates back to the late 1940s (Winslow 1947). ER fluid dampers have been developed, modeled, and tested for civil engineering applications (Erhrogott and Marsi 1992, 1993 Makris et al. 1995). Work on MR devices have been done by Spencer et al. (1997), Soong and Spencer (2002), Spencer and Nagarajaiah (2003), Carlson et al. (1995), and Dyke et al. (1996c-f). 

Carlson JD, Catanzarite DM, St Clair KA (1995) Commercial magneto-rheological fluid devices. In Proceedings of the 5th international conference on ER fluids, MR fluids and associated technology. University of Sheffield, Sheffield... 

To overcome limitations in displaying alternatively soft tissues and fluids by means of MR data and air, bone or metallic devices by means of CT... 

A much more detailed appraisal of the ER machine/device controller and typical treatments by CFD can be seen in [103,105]. All apphcations of CFD to MR and ER systems are likely to be reasonably comprehensive and precise once better performance data for the fluids becomes available. 

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