## 2 Design and Transfer Function Acquisition of MRB
### MRB System  
In the MRB system, 40# steel was chosen for the magnetic circuit components, while 304 stainless steel was used for the non-magnetic parts. The MRF used in this work was MRF-350, the rheological performance parameters of which are shown in Table 1, and the B-H curve is shown in Figure 5.  
The MRB system consists of a control system, data acquisition system and mechanical transmission system, as shown in Figure 6. The control system is comprised of an upper computer, programmable power supply, 2013 and a frequency converter. The mechanical transmission system consists of a three-phase asynchronous motor, torque speed sensor and MRB. Finally, the data acquisition system is comprised of a digital sensor indicator and data acquisition card, which is mainly used to collect torque data. As can be seen from Figure 7, the designed MRB test-bed consists of a three-phase asynchronous motor, frequency converter, torque speed sensor, self-designed MRB, programmable power supply and computer. The parameters of the main devices are described in Tables 2-4. The function of the three-phase asynchronous motor is to provide the required speed and power of the system; the frequency converter can obtain the rotating speed of the system; the upper computer completes the calculation and analysis of the data uploaded by the data acquisition card, and sends signals to the programmable power to realize the system speed regulation. The digital display instrument converts the digital quantity output by the sensor into analog quantity, and the data acquisition card receives \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \begin{tabular}{c} **Magnetic** \\ **Particles** \\ \end{tabular} & \begin{tabular}{c} **Density** \\ **(g/mL)** \\ \end{tabular} & \begin{tabular}{c} **Temperature** \\ **(\({}^{\circ}\)C)** \\ \end{tabular} & \begin{tabular}{c} **Zero Field** \\ **Viscosity/mPas** \\ **(1000 S\({}^{-1}\), 0A)** \\ \end{tabular} & \begin{tabular}{c} **Shear** \\ **Stress/kPa** \\ **(10 S\({}^{-1}\), 4A)** \\ \end{tabular} & \
\begin{tabular}{c} **Yield Stress/** \\ **kPa (4A)** \\ \end{tabular} \\ \hline
\begin{tabular}{c} Carbonyl \\ iron powder \\ \end{tabular} & 2.9–3.1 & 40 & 350–450 & \(\geq\)70 & \(\geq\)45 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Rheological performance parameters of MRF-350. Figure 5: The B-H curve of the MRF. the voltage signal from the digital display instrument, which is solved and stored by the upper computer. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Radial Dimension** & **Axial Dimension** & \begin{tabular}{c} **Average** \\ **Maximum** \\ **Magnetic Field** \\ \end{tabular} & \begin{tabular}{c} **Current** \\ **Range** \\ **Range** \\ \end{tabular} & \begin{tabular}{c} **Brake** \\ **Torque** \\ \end{tabular} & \
\begin{tabular}{c} **Weight** \\ **of Soil** \\ \end{tabular} &
\begin{tabular}{c} **Inductance** \\ **Diameter** \\ \end{tabular} \\ \hline
300 mm & 110 mm & 0.6 T & 0–2 A & 4–30 N\(\cdot\)m & 24 kg & 1.5 H & 0.96 mm \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The main technical parameters of the cell-designed MRB. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Type** & **Input Channel** & **Accuracy** & **Input Range** & **Sampling Rate** & **Manufacturer** \\ \hline PC18735 & Double 16 & 0.1\% & \(\pm\)5 V & \(\pm\)500 KHz &
\begin{tabular}{c} Beijing Art Science \\ and Technology \\ Development Co., Ltd. \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The main parameters of the PCI data acquisition card. Figure 6: Basic block diagram of MRB. Figure 7: The MRB test-bed. The input signal current \(I_{1}\) was adjusted to 0.5 A, 1.5 A and 2.5 A, respectively. The speed of the motor was increased from 100 r/min to 800 r/min by changing the frequency of frequency converter. The variation of braking torque at different slip speeds was tested, as shown in Figure 8. Under the same excitation current, the braking torque increased slightly with the increase of the slip speed.