# Research on Clustering-Based Fault Diagnosis during ROV Hovering Control
## 2 Faults and Methods
### Thruster Degradation  
The underwater thrusters used in this study were small BLDC thrusters. BLDC motors are widely used in underwater thrusters because of their high dynamic response, excellent speed versus torque characteristics, and high efficiency [35]. However, performance differences can occur even among identical underwater thrusters. In addition, the physical characteristics of underwater thrusters change over time, and corrosion and aging progress more rapidly in marine environments than on land. Thus, aged thrusters may not produce the same thrust for identical control inputs, owing to changes in motor characteristics [36]. To verify the changes in motor characteristics due to aging, four BLDC motors from BlueRobotics (Torrance, CA, USA), T200 thrusters commonly used on marine platforms [37], and five T60-30 thrusters from CiLab (Seoul, Republic of Korea) currently used on an ROV were employed.  
Figure 1: Imitated fault situations: (**a**) Normal (three propellers); (**b**) one-blade breakage; (**c**) two-blade breakage; (**d**) net; (**e**) rope.  
The experiment was conducted in an engineering tank with dimensions of \(4.5\times 2.5\times 2.8\) m\({}^{3}\) (L \(\times\) W \(\times\) H) and the system for data acquisition is shown in Figure 2. The T200s included a new thruster, one thruster from research conducted two years ago, and two thrusters from research conducted five years ago [38; 39]. For the T60-30s, a new thruster and four thrusters currently mounted on the ROV were used. The electronic speed controllers (ESC) used were BlueRobotics' \"Basic ESC\", which employs chopper amplifiers for pulse-width modulation (PWM) as control inputs. With control inputs of 1100, 1500, and 1900 us, this ESC operates in reverse at maximum speed, stops, and runs forward at maximum speed, respectively. The current sensor used was the WCS6800 manufactured by Winson (Hsinchu, Taiwan)--a Hall effect type sensor measuring up to 35 A with a 55 mV/A sensitivity. To measure the RPM of sensorless BLDC motors, a board was designed to read the period of the back-EMF and calculate the RPM, and its measurement principle and accuracy are discussed in Section 3.3. Control inputs ranging from -80% to 80% were continuously applied to the T200 and T60-30 thrusters, and consumption current and RPM data were acquired at a sampling rate of 10 Hz.  
The experimental results for the four T200 and five T60-30 thrusters are shown in Figure 3. Both types of thrusters have a \(\pm\)25 us dead zone around the stop point (PWM: 1500). Figure 3a,b shows the trends in the RPM and current consumption of the T200 thrusters with various control inputs. Although the T200 thruster consumption currents are consistent across the segments, as they approach closer to their maximum rotation speeds, differences in the RPM become noticeable. Figure 3c,d shows the trends in the RPM and current consumption for the T60-30 thrusters with various control inputs. For the T60-30 thrusters, while the RPM was consistent across segments, there were differences in the current consumption. The measured data from the underwater thrusters confirmed that even identical thrusters exhibited significantly different characteristics.  
Figure 2: System for measuring the thruster RPM and consumption current.