# Review: A Survey on Configurations and Performance of Flow-Mode MR Valves
## 2 Method--Selection of Criteria  
In this section, we carry out a detailed analysis of the structure of a flow-mode MR actuator, then propose criteria for the actuator and valve classification based on the outcome of this analysis. Briefly, consider a typical MR actuator featuring a single cylinder tube housing and a control valve assembly--see Figure 1. The valve incorporates one dual-coil electromagnet with one (homogenous) annular channel. The assembly is attached to a rod. The electrical connection to the coils in series is through the thru-hole in the rod. The pressurized gas chamber (not shown) is for volume compensation due to rod motion and fluid expansion due to temperature. Let us briefly recall how the actuator works. In the non-energized condition, the current is not supplied to the coil(s), and no magnetic flux is generated in the annular channel. The flow losses in this condition are minimal and depend on the geometry of the flow channel and material properties of the fluid (viscosity, density) as well as the piston speed. As soon as the serial coils are energized, the magnetic fluxes that are induced in the electromagnet structure travel through the core into the annular gap, enter the outer ring, and then back into the core via the flow channel. These elements form the primary path for the magnetic flux. As the housing is usually ferromagnetic, portions of the fluxes enter the housing, too, and travel back to the rod via the MR fluid, thus, bypassing the annulus. As a result the fluid is energized in the regions above the active sections--the fluid's magnetic field dependent yield stress increases and the outcome is the resistance-to-flow build-up. The flow through the annulus can be initiated only when the pressure drop across the piston exceeds the resulting breakaway pressure due to the yield stress increase. Note that the fluid in this condition behaves in a rather complex fashion. Simply speaking, as soon as it enters the active zones its rheology changes from the near-Newtonian one to that of a pseudo-solid (Bingham) type only to return to the original condition in the non-active regions over the coil. That is the opposite of the situation in their electrical (ER) counterparts where the entire surface of the flow channel is energized, and thus contributes to the field-dependent losses as a whole. The exemplary actuator incorporates several components which can be the basis for the classification of particular actuator configurations regardless of the operating mode, namely, the housing and the valve. Analyzing the valve's structure in detail, it is apparent that a set of classification criteria can be provided based on the design of the following elements: solenoid, solenoid's position relative to the housing, coil assembly, electrical connections (electrical interface to the power supply), and flow channel(s). The criteria can be extended to incorporate the optional bypass and the sensory (displacement, velocity, magnetic flux) or fail-safe add-ons. In detail, a control valve incorporates at least one electromagnet assembly. Its sole purpose in the structure is to induce the magnetic field of sufficient strength for activating the fluid in the area over the magnetic poles and should meet particular performance requirements in terms of flux dynamic range, flux linearity vs the inducing current (ideal scenario) and response time. Next, considering the position of the electromagnet relative to the housing, the electromagnets can be placed inside the housing or on the outside of the tube. The external assemblies are usually stationary (integrated into the housing), whereas the internal electromagnets are usually attached to a moving rod although stationary configurations can be observed in the literature, too. Particular electrical connections (for supplying the voltage to the coil terminals) follow the method of attachment of the electromagnet to the rod and its relative position to the housing. The electromagnets incorporate at least one coil assembly wound transversely or radially.