# Research on Clustering-Based Fault Diagnosis during ROV Hovering Control
## 3 Configuration of ROV
### Back-EMF Measurement RPM Board  
The objective of this study was to achieve consistent thrust through RPM control of underwater thrusters that are identical but have different performances. The utilized sensorless BLDC underwater thrusters lack built-in encoders for RPM feedback. Therefore, a board was developed to calculate the back-EMF period for the RPM computation and feedback to the MCU. Immediate feedback during variable control is crucial for real-time RPM control. Back-EMF allows fast feedback because it appears almost immediately in the opposite direction of the voltage applied to the winding, according to [PERSON]'s law [46, 47].  
Figure 13 presents the back-EMF measurement circuit and board designed for RPM acquisition. An LM324 comparator detects the back-EMF and outputs it in pulse form, with the MCU reading the falling time of the pulse to calculate the RPM. Pins 4 and 11 of the LM324 are for power supply, connected to 5V and ground, respectively. Pins 2 and 3 are connected as shown in Figure 13a for detecting the back-EMF of the BLDC motor, and pin 1 outputs a pulse signal by comparing the two signals. The reliability of this method was confirmed through comparisons with a laser tachometer (manufactured by PeakTech, Germany) and an oscilloscope (manufactured by Tektronix, USA), as illustrated in Figure 14.  
Figure 12: GUI program for the ROV system.  
Figure 13: Back-EMF for RPM acquisition: (**a**) Measurement circuit; (**b**) measurement Board.  
To validate the back-EMF circuit, experiments were configured as shown in Figure 14, for comparing (1) the RPM calculated by the MCU from the back-EMF period, (2) the RPM derived from frequency measurements on an oscilloscope, and (3) the RPM measured by a laser tachometer. The specifications of the RPM tachometer and oscilloscope used for the verification are presented in Table 1.  
The ESC used in this study was the \"Basic ESC\" from BlueRobotics, which utilizes PWM as control inputs. This ESC was designed such that control inputs of 1100, 1500, and 1900 us result in the maximum reverse rotation speed, stop the motor, and result in the maximum forward rotation speed, respectively. The control inputs were varied discretely from \(-100\%\) to \(100\%\) in increments of 100. The experiment was repeated five times, and the average values were calculated. The results are presented in Table 2.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Control Input** & **Back-EMF Board** & **Tachometer** & **Oscilloscope** \\ \hline
1900 & 4253 & 4237 & 4230 \\
1800 & 3245 & 3239 & 3241 \\
1700 & 2131 & 2130 & 2132 \\
1600 & 1002 & 1000 & 998 \\
1400 & 801 & 803 & 799 \\
1300 & 1941 & 1933 & 1932 \\
1200 & 3058 & 3049 & 3049 \\
1100 & 4201 & 4179 & 4174 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Average RPM results for each method.  
Figure 14: Back-EMF circuit verification experiment.  
The average RPM values for each method with respect to the control input are presented in Table 2. The error rate between the back-EMF board and the tachometer reached a maximum of 0.53%, and the error rate between the oscilloscope readings reached 0.64%. According to these validations, RPM control was executed to achieve consistent thrust among underwater thrusters with different thrusts.