# Review: A Survey on Configurations and Performance of Flow-Mode MR Valves
## 2 Method--Selection of Criteria
The air gap in the electromagnet acts as a flow channel of constant height with stationary surfaces constituting the flow path (as in flow-mode or pinch mode). A valve may feature several parallel flow channels, thus splitting the flow into several paths of equal or different resistance. A hydraulic bypass is located in parallel to the main flow path(s); its main contribution is in shaping the valve's performance at near-zero flow rates. With ideal bypasses their function is to degrade the pressure drop down to zero at the zero flow rate. This is  
Figure 1: Example: MR actuator assembly. usually achieved by diverting a portion of the fluid flow to a non-energized flow path. A conventional digressive valve is a rare specimen in MR valves [21], and its efficiency can be questioned. Sensory mechanisms are often integrated into the housing or the control valve, or implemented in the control logic (flux control) [22]. Based on the analysis of the prior art, it is evident the simplest benchmark is a single-tube MR actuator with one control valve (having a single transversely wound coil assembly, one annular-type primary flow channel, and one bypass path--optional) attached to the moving rod and no sensory features. Although necessary, the pressurized gas container's location is of secondary importance. The function of the electromagnet is to ensure a uniform (or near-uniform) distribution of magnetic flux in the air gap (radially) and maximum (longitudinally). The flux distribution in the annulus is intrinsically nonhomogenous due to radial effects, however, for simplicity's sake, we assume the uniform flux distribution in the channel--the flat-plate approximation results in little error [23]. With the exception of the bypass, all these features can be observed in, e.g., [8; 24]; see [25] for the description of an MR valve with a bypass path. Therefore, all the other configurations will be analyzed concerning the benchmark structure. The output of the actuator is usually represented in the form of the static force-velocity plot at various levels of the input current (magnetic flux, flux density)--see Figure 2, or pressure vs flow rate characteristics (more common for a valve). With the actuator such data are obtained by exciting the actuator with constant velocity or sine displacement inputs or a test rig and at fixed levels of the magnetic field in the annulus, then recording the peak forces corresponding to particular velocity amplitudes. For comparison, MR valve characteristics are acquired on a flow bench where the response of the valve (pressure drop) is measured as a function of flow rate and the magnetic field. In the example shown, the bypass presence translates into the high slope of the force-velocity curves below approximately 0.25 m/s. Above the threshold velocity, the flow through the valve is dominated by the annular gap resistance. Fluid properties aside, the forces in the first operating regime (below the knee point) can be altered mainly by modifying the bypass geometry, whereas the actuator's performance in the second regime depends on the primary flow channel geometry (annulus) as well as the electromagnet's capability to energize the fluid. The actuator decomposition elements, the basis for comparison and classification of various configurations of MR actuators and the valves (which deviate from the benchmark design) are then highlighted in Figures 3 and 4, as well as Table 1. To clarify, the information concerns only flow-mode valves with (homogenous) annular flow paths. The structures with composite flow channels (usually in the form of an annular gap in series with a radial one) are not accounted for here. In this review, we limit our considerations to homogenous annular flow channels for the ease of the valve-to-valve comparison; the valves with performance-enhancing composite flow paths merit a separate review.