## 3 The Dataset
### Data Collection Process
In hydraulic systems, pumps are most often used as a source of specific pressure to, for example, generate the force required using a hydraulic cylinder or torque using a hydraulic motor. For this reason, a forced pump load in the form of a given load torque was used when collecting training data. This causes the pump to generate the appropriate pressure. There are structural losses between the cooperating pump components, causing internal oil leaks, especially when operating under high pressure. This is a normal feature of these devices. Wear and tear or failure causes these leaks to become more severe. Hydraulic oil, which has a specific nominal kinematic viscosity according to ISO 3448 [24], is usually used as the working medium. To be able to observe the operational wear of the pump or its failure state, the viscosity of the working medium plays an important role. The built system uses ISO VG46 oil, which means that it has a nominal kinematic viscosity of 46 mm\({}^{2}\)/s at a temperature of 40 \({}^{\circ}\)C. Oil with this viscosity value is often used in hydraulic systems. The  
Figure 3: Test unit for data acquisition; (**a**) HPU model with tested pump; (**b**) Hydraulic motor and electric motor-adjustable load for the tested pump. Figure 2: Hydraulic diagram. viscosity of the oil strongly depends on its temperature; therefore, the measured parameters were recorded while the oil temperature was increased. The temperature in the oil tank, and in the suction line was increased from ambient temperature to approximately 50 \({}^{\circ}\)C. Because the data were collected in different months of the year, the starting ambient temperature ranges from around 16 \({}^{\circ}\)C up to 21 \({}^{\circ}\)C. The built system enables the registration of many parameters, but for the purposes of failure prediction research, seven of them were selected based on expert knowledge and the pump design. The data collected during the tests show the correct and damaged condition (worn valve plate) of the piston pump. In real hydraulic systems, the way the pumps are loaded is quite random, depending on many factors. These can be sudden pressure changes from a few bars when the pump is unloaded to 300 bar and more, but also smooth pressure changes when the system works with proportional parameter control. Taking these factors into account, both a stepped load and a sinusoidal waveform were used during data collection. Both types of load were applied randomly to better fit behavior to a real hydraulic system. Graphs showing the applied load torque are shown in Figure 4. The torque value was set in the range of 20-220 Nm, which corresponds to the obtained output pressure value in the range of around 25-235 bar. For the stepped type, the value of the applied torque was changed manually in periods ca. 4-5 min. For the sine wave, the period of sine was set to 300 s.  
The process parameters recorded during the tests were collected at three points in the system:  
* suction line: temperature of the oil
* the output line of the pump: pressure, temperature and flow
* leak line: pressure, temperature and flow  
The data acquisition system is quite extensive software running under Windows OS. Its limitation is that it is not possible to set a constant sampling frequency, and it depends on the operation of the operating system and the PLC controller. As a result, a sampling rate of approximately 40 samples (data records)/second was obtained. After collecting the data, the recorded values for each sensor were joined by the time stamp to form a sample record. Finally, each data sample consisted of seven values that describe the temperature, pressure, and flow on the suction line, the output line, and the leak line as described above. The data were collected for four different pump states, one with correct functioning and three representing the damaged state. In order to collect data on the damaged condition, a functional element in the pump was replaced with a damaged one. The damaged element was the valve plate.