*4.1. Introduction to the Experimental Platform*

The experimental data sets for this study were derived from the UC Irvine Machine Learning Repository [51]. The data were collected from a hydraulic test bed that allowed a reversible change of the state or condition of various components, at the Mechatronics and Automation Technology Center of Saarbrucken University in Germany [52]. The hydraulic system consists of a primary working circuit (Figure 1a) and a secondary cooling-filtration circuit (Figure 1b), which are connected by the oil tank. In the working circuit with the main pump (electrical motor power of 3.3 kW), different load levels are cyclically repeated with the electro-hydraulic proportional valve (V11). It is possible that the typical cyclical operation and repeated load characteristic in an industrial application can be simulated by setting fixed working cycles with pre-defined load levels in the test. Meanwhile, the random load variations in mobile machines can be started by setting variable working cycles with pseudo-random load variations.

## *4.2. Data Acquisition System*

The condition monitoring system for the hydraulic system is equipped with several sensors for measuring process values. Among them, there are six pressure sensors (PS1–PS6), two flow rate sensors (FS1, FS2), five temperature sensors (TS1–TS5), one motor power sensor (EPS1), and one vibration sensor (VS1) with standard industrial 4–20 mA current loop interfaces connected to a data acquisition system. In addition, sensors integrating EIA-232 and EIA-485 buses for oil particle contamination (CS and MCS) and oil parameter monitoring (COPS) are installed in the condition monitoring system. In total, the above 17 sensor signals are stored while the hydraulic system repeats pre-defined constant working cycles with changing conditions of hydraulic components to identify typical signal patterns. The sampling rate of the above sensors is set within the range of 1 Hz to 100 Hz, respectively, according to the different types of collected state signals. The specific sampling rate of each sensor is shown in Table 1.

(**b**) Hydraulic system for the secondary cooling-filtration working circuit.



**Table 1.** Sampling rates of sensors.

The sensor data are connected and buffered on a Programmable Logic Controller (PLC) (Beckhoff CX5020) at run time and transferred to a computer by EtherCAT, where the data are stored for further analysis. It is possible to configure fault characterization measurements with a specifically developed tool such as LabVIEW and to, subsequently, perform them by using the PLC. Using this tool, measurements of the fault type, severity, and duration are taken, if necessary, to define different fault states, such as hydraulic pump internal leakage, hydraulic valve switching characteristic degradation, accumulator leakage, and cooler power degradation. Table 2 shows the components and respective parameters that are configurable to simulate fault scenarios. The experimental method has the advantages of not requiring damage to the mechanical structure of the hydraulic valve, repeatability, and reversibility of the fault state, and simple operation of the fault setting.

## *4.3. Hydraulic Valve Fault Setting and Data Acquisition*

According to the primary working circuit in Figure 1a, the working cycle (duration 60 s) consists of different segments with transient and static load characteristics performed by the electro-hydraulic proportional pressure valve (V11) and the directional valve (V10) to simulate a typical machine operation. Under pre-defined load conditions, the current set points of the directional valve are 100%, 90%, 80%, and 73% of the nominal value. Thus, a simulation is conducted for the hydraulic valve (V10) in the fault states of normal, slight, medium, and severe valve operation deterioration, and raw sensor data are collected during characterization.


**Table 2.** Hydraulic test bed: components and their simulated fault conditions.

The specific experimental process is under different fault conditions of the hydraulic valve (V10). The hydraulic system is cyclically operated for 2205 cycles with a pre-defined load on the electro-hydraulic servo valve (V11), and the running time of each cycle is 60 s. Among them, the number of cycles in which the hydraulic system is in an unstable state is 756 cycles, and, in a steady state, it is 1449 cycles. Therefore, in the steady state, the experimental data of 369 normal states, 360 slight fault states, 360 medium fault states, and 360 severe fault states are obtained, respectively, for a total of 1449 sets of data. Taking the partial pressure signals of the inlet (PS1) and outlet (PS2) measured by the hydraulic directional valve (V10) within the fixed working cycle with a duration of 60 s as an example, the changing rule of different fault states in the steady state is shown in Figure 2.

**Figure 2.** Fixed working cycle (measured by partial PS1 and PS2) with a pre-defined load.
