# Review: A Survey on Configurations and Performance of Flow-Mode MR Valves
## 1 Introduction  
Smart materials are capable of sensing external stimuli, i.e., temperature, light, electric field, magnetic field, stress, and reacting to them in a coordinated fashion. In many application fields, e.g., vibration suppression, medical systems, noise control, the implementation of a specific smart material has led to spectacular results, e.g., smart material-based actuators, drive systems, motors, sensors, etc. For instance, magnetorheological (MR) fluids (suspensions of micron-sized particles in a non-conductive carrier liquid) undergo a dramatic transition from that of a Newtonian fluid to a pseudo-solid. As discovered originally by [PERSON] [1], the material develops a yield stress when exposed to magnetic field. The so-called MR effect is stable, reversible fast and of sufficient magnitude to be utilized in real-world applications [2]. The range of prototype MR fluid-baseddevices is vast incorporating automotive suspension dampers and powertrain mounts, washing machine dampers, rotary clutches and brakes, exoskeletons, optical finishing systems, haptics, military gun recoil systems, etc. [3]. In particular, the technology has been successfully commercialized in automotive suspension systems and high-quality optics [4; 5]. Regrettably, their electrical counterparts, electrorheological (ER) fluids, have failed to deliver commercial applications due to serious deficiencies, e.g., sensitivity to metallic impurities, lower yield stress and a very narrow operating temperature range [6]. Their yield stress is on the order of magnitude lower than that of MR fluids forcing the use of long flow channels and tight gaps, and it is limited by the maximum (breakdown) electric field strength. Their operating temperature range is narrow (usually 0-80 \({}^{\circ}\)C), and the response time varies with temperature [7]; the operating temperature range of MR fluids well exceeds that range. Moreover, their sensitivity to metallic contamination forces an extremely strict control process during the assembly, and the leakage current across the gap between the electrodes increases with temperature, resulting in high power consumption by these devices. Thus, despite the large interest from the industry in the early 1990s, a successful application of ER fluids has yet to materialize. The magnetorheology is based on a similar principle as their ER counterparts. Again, the actuators are unique devices in many aspects. Their operating principle is based upon the modification of specific material properties (yield stress) by external magnetic fields in order to drive changes in the force (or torque) output of these actuators. In essence, the technology is valveless. As a result, the devices are structurally simple. At the device-level, they consist of three fundamental components or modules: housing, fluid, and solenoid valve (electromagnet). An external low-voltage supply is also needed for powering the actuator. This is in contrast to valve-based semi-active solenoid actuators in which flow restriction variations are achieved by manipulating the flow geometry--an advantage over the conventional semi-active actuators, and the MR valves do not incorporate small moving and precise components. The technology is not, however, rid of drawbacks. Despite the extensive research over the last 30 years, fluid stability, fluid tribology, increased weight, etc., are engineering challenges that the technology needs to cope with. By a fashion, the actuators can be operated in at least one of the operating modes: flow (valve), shear (rotary), squeeze or gradient pinch [8; 9]. The flow-mode seems to have been best utilized in vibration control applications, the shear-mode in rotary dampers and clutches, whereas the squeeze-mode in small-stroke mounts. The gradient pinch mode is an interesting variation of flow-mode featuring highly non-uniform and deliberate distributions of magnetic flux in the MR valve's flow channels [10].