## 3 Definition of the Degradation Signal and Dataset Acquisition
### Data Acquisition and Analysis  
The hydraulic circuit shown in Figure 6 was established to collect the current signal of the HSV. The collected first current waveform signal was set as the template current waveform.  
The acquisition card was used to gather the electric current of the HSV. The current acquisition device used was an ACS712 Hall current sensor. This sensor converts the collected current signal into a voltage signal, which it outputs to the acquisition card.  
The data collection system contained a water hydraulic piston pump driven by an electric motor, which was used to provide a constant flow rate to the system, as well as a pressure relief valve, which regulated system pressure. The working pressure is set to 10 MPa and the switching frequency of the valve is 10 Hz. When the HSV fails, the valve spool is slightly worn, and the actuator temperature is very high and fails. Therefore, the main reason for the failure of this HSV is the damage to the actuator. Figure 7 is a partial current signal collected from the test. The figure suggests that the current waveform is not a complete rise and fall process, which is consistent with Figure 5. It shows the rationality of the signal acquisition process and results. At the same time, the performance degradation index of the HSV can also be defined according to the change of the current waveform.  
Figure 8 shows the current curve distance versus cycle number of the HSV. The original signal was filtered to obtain a clearer result, and the filtered curve was taken as the trajectory of the final degradation curve. As shown in Figure 8, the entire working state of the HSV could be divided into three stages: (1) a normal state; (2) a degradation state; and (3) a failure state. The state is defined according to the rising degree of the filtered curve and the corresponding g value. During the normal state, the curve is relatively flat, and during the degradation state, the curve has an obvious upward trend. When g continues to increase, the HSV can no longer work reliably, which is defined as the failure state. The number of the working cycles and g value corresponding to each state are shown in Table 1.  
Figure 6: Experimental system for current detection. (**a**) Schematic diagram. (**b**) HSV test bench.  
Figure 9 is the current signal diagram under three states. The figure suggests that with the performance degradation of the HSV, point B and point F gradually smooth and finally disappear. Figure 5 shows that point B and point F represent the opening response time and closing response time of the HSV, respectively, indicating that with the degradation of HSV performance, the switching response time gradually increases. When the HSV fails, there are no points B and F on the current curve, and the valve pool no longer acts. Moreover, with the performance degradation of the valve, the driving current gradually decreases, indicating that the resistance of the actuator gradually increases.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**No.** & **States** & **Cycles** & **g value** \\ \hline
1 & Normal state & 0–9 \(\times\) 10\({}^{3}\) & 0–2.7 \\
2 & Degradation state & 9 \(\times\) 10\({}^{3}\)–1.5 \(\times\) 10\({}^{4}\) & 2.7–7.6 \\
3 & Failure state & Above 1.5 \(\times\) 10\({}^{4}\) & Above 7.6 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Working cycles corresponding to degradation states.  
Figure 8: Parameter \(g\) versus cycle number.  
Figure 7: Current signal.  
Figure 9: Current diagram under three states.