## 3 Materials and Methods  
### System Architecture  
The system overview of the integrated monitoring platform proposed in this research is shown in Figure 3.  
In this system, sensor devices attached to industrial machines are used to collect data on the industrial machines being manufactured. The collected data are sent to the gateway using LoRa communication, a type of Low Power Wide Area (LPWA), after edge processing at the sensor device. The gateway sends the packets received from the sensor device to the web server by Hypertext Transfer Protocol (HTTP) POST. The web server visualises the POSTed data in the form of a graph, linking it to the sensor device information. The operator can check the status of the equipment from the graph, and if any abnormality is found, the operator can provide feedback by maintaining the machine or changing the production conditions. Figure 1: Power spectrum of current waveforms [11]. Figure 2: Log likelihood by experimental condition [11]. Here, the data sent by the sensor device to the gateway are not the data as measured, but data that have been processed by edge processing. This edge processing calculates features for estimating the state of industrial machines from the data obtained from the sensors. To determine the content of edge processing, the measured data themselves must be analysed once. It is not suitable for the sensor device to send all the measured data to the gateway due to the communication capacity limitation of LoRa communication. Therefore, it is necessary to reduce the amount of data by adding some data processing to the measured data as edge processing in the sensor device and then transmitting the data. Based on the above, the operational phase of the integrated platform is defined in this system by dividing it into three phases: sensor data collection phase, sensor data analysis phase and industrial machine monitoring phase. In the sensor data collection phase, external sensor devices are used to collect data. This is because SMEs in the manufacturing industry tend to have legacy equipment, and legacy industrial machines without network functionality are still in use. The amount of data acquired per unit of time depends on the sensor used and the sampling rate. As these different data cannot be handled in one format, three data formats are defined for this system: Comma Separated Values (CSV) files, sensor data files and radio packets. A CSV file is a format in which data are recorded in comma-separated lines, where one line is the data for one measurement. Because a single line can have multiple columns separated by commas, multiple sensor data can be stored together in a single file. A sensor data file is a file in which the data from a single measurement, such as sound data, image data or video data, are written in one file according to the respective file protocol. It is not possible to include data from several sensors in one file, such as a CSV file. Wireless packets are packets sent by the sensor device to the gateway via LoRa communication. In the LoRa communication used in this system, the payload of a packet transmitted at a time should be limited to about 50 [B] to ensure stable communication. For the transmission frequency, the transmission interval should be at least one second, because high-frequency transmission from sensor devices will affect the communication quality in  
Figure 3: Schematic diagram of proposed system. an environment with multiple sensor devices. Due to the above requirements, it is difficult for wireless packets to continue transmitting the data measured by the sensors as they are. Therefore, wireless packets are used to transmit the results of processing measured data. However, if the data acquisition intervals using sensors are spaced, e.g., once every minute or once every ten minutes, it is possible to transmit the measurement data as they are. Of these data formats, CSV files and sensor data files are stored on the Secure Digital (SD) card installed in the sensor device.