# Review: A Survey on Configurations and Performance of Flow-Mode MR Valves
## 4 Analysis  
In this section, we discuss the typical configurations of MR valves in relation to their performance envelopes. Specifically, the configurations are illustrated in Figures 7 and 8, and the performance envelopes they are capable of are highlighted simply in Figure 9. The range of analyzed structures covers the benchmark structure and its functional variations--bypass mechanisms, force asymmetry, etc. Structural means for maximizing the active area ratio and the dynamic range by varying the number of coils, flow channels, and coil's orientation relative to the core and the flow path are discussed here, too. The material is complemented by a brief analysis using the proposed shear stress metric.  
Figure 6: Shear gap, \(D_{m}\)—circumferential diameter, \(L_{g}\)—gap length.  
Figure 7: MR valves: configurations, 1—core, 2—coil(s), 3—primary flow path(s), 4—ring, 5—bypass, 6—(non-magnetic) spacer ring, 7—permanent magnet. (**a**) Benchmark valve [24,25]; (**b**) Variable height [69]; (**c**) Annulus location [77]; (**d**) Bypass valve [59,60,78,79]; (**e**) High active area ratio [80]; (**f**) Fail-safe valve [81]; (**g**) Dual annulus (concentric) [66]; (**h**) Dual annulus, adapted from [66,77,82].  
Figure 8: MR valves: configurations (continued), 1—core, 2—coil(s), 3—primary flow path(s), 4—ring, 6—(non-magnetic) spacer. (**a**) Multiple coils [51; 52]; (**b**) Equivalent ‘two-coil’ valve [83]; (**c**) Pinch valve [9]; (**d**) Serpentine flux path [32; 84; 85]; (**e**) Multi-coils (radial) [50; 55; 56; 86]; (**f**) Dual-coil multipoles (radial) [87]; (**g**) Multi-pole valve (toroidal) [88; 89]; (**h**) Multi-pole valve (toroidal)—adapted from [89].  
There may be numerous reasons why a particular configuration may be preferred over another one within the context of a given application. Let us consider it briefly in the context of performance over the entire velocity (flow range). To begin with the benchmark valve, its single-slope envelope and its relatively high dynamic range may meet most semi-active vibration control application needs, however, the sharp transition at zero flow rate (see Figure 9a) eliminates such configuration from any practical use in vibration control.  
Figure 9: Steady-statepressure-flow rate \(\Delta P-Q\) characteristics (envelopes) of MR valves: (**a**) benchmark, no bypass, (**b**) benchmark, hydraulic bypass, (**c**) benchmark, flux bypass, (**d**) benchmark & hybrid hydraulic and flux bypass, (**e**) parallel flow channels, (**f**) variable slope bypass—1, (**g**) variable slope bypass—2, (**h**) asymmetric valve.  
This issue can be avoided with dual-slope damping force envelopes characterized by a high damping coefficient at low velocities (flow rates) and a small one at medium and high velocities. The solution is far more practical than the simple benchmark valve, however, the bypass or a combination of bypasses restricts the low-speed performance of the valve to a single value of the damping coefficient. This, in turn, may raise the need for a valve with the ability to vary the low-speed damping coefficient independently or in line with the flux in the main control circuit. Adding additional (energized) parallel flow paths (as well as magnetic circuits) allows shaping the upper and lower limits of the performance envelope according to the needs, however, at the expense of the simplicity and cost. Although the particular below-mentioned structures seem to have the ability to tune the damping force envelope almost to any needs, the valve's complexity increases tremendously. It seems that an optimized benchmark valve with the bypass path for the force roll-off at low-speed (flow rate) meets the requirements of most semi-active applications in the vibration control area. The observation is supported by our evaluation results using the shear stress metric below in the section.