# Review: A Survey on Configurations and Performance of Flow-Mode MR Valves
## 1 Introduction
Hybrid MR actuators take advantage of at least the two operating modes for increased performance or specific performance characteristics [11; 12]. As a reminder, the basic set of application-specific performance requirements for a typical MR actuator should include [8]: (1) zero or minimum (safe) force (torque) in the off-state (deactivated) condition, (2) maximum damping force at the highest current (magnetic flux) condition, (3) power supply, (4) power consumption, (5) response time, (6) temperature operating range, heat dissipation. Other specifications may be related to friction, packaging, etc., not to mention manufacturability and cost. An analysis of prior art in the field reveals that the most common area of application of MR actuators is vibration control [6]. Moreover, the application field has been dominated by single-mode (flow-mode) devices, which is even more evident while analyzing, e.g., the commercial MR actuators. The automotive MR dampers are simple single-tube & flow mode devices with dual coil valves. Similar observations can be made while inspecting civil engineering applications [13]. Therefore, the emphasis of this review is on flow-mode MR actuators powered from external power supplies and their valves in particular. Energy-harvesting MR actuators are beyond the scope of this review [14]. Our motivation behind the topic review is simple. Although several reviews have already been published on the subject [15; 16; 17; 18; 19; 20], little attention has been paid to the generic performance of MR actuators. One important shortcoming of the previous review works is the lack of comprehensive and critical insight on the structures of MR actuators and the relationship with their performance. An even closer inspection of the prior art shows that very few MR configurations have a chance to stand up to the real-world requirements both in terms of the dynamic range, power draw, or the response time. Mostly, the previous studies summarize the research work performed with little attention to their design details and parameters and how they contribute to the damping force. Therefore, the purpose of this review study is to fill in the gap. To summarize, this study provides a systematic and organized review of existing configurations of flow-mode MR valves, highlights the contribution of specific features to the valve's output, and delivers a simple metric to evaluate their performance based on the ability to yield appropriate forces (pressures). To highlight the selection criteria for the review, the analyzed set of references spans 30 years (1992-2022). Due to the reasons above, the analyzed references observe only the flow-mode prototypes for use in vibration control systems. The study includes research papers, as well as patents and patent applications of commercial enterprises. We cover feasibility and design studies, valve geometry optimization examples, presentations of novel features, etc. Modeling papers fall into the review scope here provided that they serve the sole purpose of demonstrating the unique design features and their contribution to the performance envelope enhancement and shaping. The work is structured as follows: Section 2 provides the classification criteria based on the detailed analysis of various MR valve structures complemented by a fundamental analysis of the valve's operating principle and performance metrics in Section 3. In Section 4, we analyze the selected structures, their key features and performance characteris-tics. The analysis is complemented by a qualitative demonstration of the proposed performance metric. Finally, conclusions are drawn in Section 5. We proceed with the review by providing the classification criteria based on the detailed analysis of various MR valve structures, highlight the valve's operating principles along with fundamental equations and performance metrics. The considerations are then the basis for a detailed analysis of selected configurations of MR valves.