Basic MR Device Design Considerations

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]  

In this expression, forces are in N (or torques in Nm), speed v in m/s (or rad/s) and 14iin in m . The constant a equals 1 for direct-shear devices, while for valve-mode devices it has a value of approximately 2. This approximation is valid for any MR flmd having that is on the order 

For valve-mode devices the estimated miiurnmn active flmd volmne of 

Based on the above minimum active fluid volume, it is also possible to estimate the electric power required to power the electromagnet. For MR fluid that is operating near its maximum yield strength, the magnetic held energy density that needs to be established in the fluid is approximately 0.1 J/cm . Thus, in order to establish the required magnetic field H within a desired time interval At, the power source must be capable of supplying a minimum electric power Pei (in watts) that equals 0.1 J/cm times the fluid volume fmin (in cm ) divided by the time interval At (in seconds)  

for any application, the minimum information needed to estimate active fluid volume, minimum electric power and overall valve size is  

---