# Multipole Multi-Layered Magnetorheological Brake with Intermediate Slots
###### Abstract
The disc-type brake can be used for applications where the accessible space is thin. In contrast, the drum-type brake is suitable for applications where the space is long and cylindrical. [PERSON] et al. [13] developed a mathematical model for drum-type MR brakes. LORD Corporation designed and manufactured a simple single-disc brake [14] with a maximum resistance of 4 Nm. [PERSON] investigated the performance of this brake theoretically and experimentally. [PERSON] and [PERSON] introduced a disc-shaped MR brake design with a simple disc construction [15]. [PERSON] designed a new configuration, i.e., a T-shaped drum-type brake, to combine drum and disc-type models with individual advantages [16; 17]. From these papers, it is understood that the brake torque increases with the increase in the magnetic field. However, once the saturation limit is touched, torque no longer increases. Therefore, multi-layered brakes were introduced to increase the maximum torque of the brake. [PERSON] [18] used multi-drum architecture to achieve higher torque at a lower volume. [PERSON] [19] developed a multi-layered magnetorheological brake for knee orthosis applications. In all these types of brakes, the magnetic flux flow area is limited; therefore, these brakes are low-braking-torque generators, and their torque-volume ratios (IVR) are lower. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **MRF-140 CG** & **MRF-132G** & **MRF-122 EG** \\ \hline Viscosity at 400c, Pa-s & 0.280 & 0.112 & 0.042 \\ \hline Density, g/cm\({}^{3}\) & 3.64 & 3.09 & 2.28 \\ \hline Flashpoint, 0c & \textgreater{}150 & \textgreater{}150 & \textgreater{}150 \\ \hline Thermal conductivity at 250c, W/m0c & 0.28–1.28 & 0.25–1.06 & 0.21–0.81 \\ \hline Operating Temperature, K & 233 to 400 & 233 to 400 & 233 to 400 \\ \hline Solids Content by Weight, \% & 85.44 & 80.98 & 72 \\ \hline Yield Strength at 200 kA/m, kPa & 57 & 42 & 32 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Properties of commercially available MR fluids. On the other hand, using multipole magnetic coil construction, the torque of the brake can be enhanced without increasing its size; therefore, the TVR significantly increases. In this multiple-pole brake, even though one of the coils fails, the brake does not fail. Therefore, it is safer than conventional MR brakes. [PERSON] [20] introduced a higher TVR brake with a multipole drum brake. In that design, the generated magnetic flux will orthogonally travel all over the MR fluid, which gives the maximum possible performance. Figure 1 shows the six-pole arrangement, flux travel direction, and the MR fluid with a magnetic field. Later, [PERSON] [21] attempted to combine multi-layer and multipole concepts with his multipole bilayer MR brake. However, that design is limited to two layers only. The biggest challenge to developing multi-layers in a multipole concept brake is the magnetic flux flow to the outer layers. Flux always tries to flow to the neighbor pole via the nearest inner rotor plates (flux bypassing) in the multipole concept. Therefore, the outside MR layers receive very little magnetic flux. Eventually, a multi-layer concept cannot be applied to multipole brakes. Recently, [PERSON] et al. [22] proposed a radial multipole multi-layered (MPML) magnetorheological (MR) brake, achieving high torque density. Their design employed inner and outer poles to realize the multiple poles concept. However, this arrangement differs from [PERSON]'s multipole concept, as it follows a multi-directional pattern rather than a true multipole configuration. Moreover, the placement of inner and outer poles occupies additional volume, ultimately reducing the overall torque-volume ratio. Furthermore, the outer poles limit the mechanical advantage, as the perpendicular distance from the MR layers to the brake's center is decreased. To address these limitations, this study aims to design an MPML drum-type MR brake with an enhanced torque-volume ratio and mechanical advantage.