## 3 Materials and Methods
### External Attachable Sensor Device  
The integrated monitoring platform proposed in this research uses sensor devices that can be attached externally to industrial machines during the sensor data collection phase and the industrial machine monitoring phase. This section describes the configuration and operation of these devices. First, the configuration and operation of the SD card-compatible accelerometer device used in the sensor data collection phase of the proposed system are described. The appearance of the SD card-compatible accelerometer device is shown in Figure 5.  
The main element of this device is a microcontroller board called Grand Central M4 Express featuring the SAMD51 (hereafter referred to as 'Grand Central') [16], manufactured by Adafruit Industries (New York, NY, USA). The reason for selecting this microcontroller board was the richness of its interfaces. Grand Central has 62 General Purpose Input Output (GPIO) ports. Among them, 16 ports can be used for analogue input and support many communication standards such as hardware Serial Peripheral Interface (SPI), Inter-Integrated Circuit (I2C) and Universal Asynchronous Receiver Transmitter (UART). This makes it possible to control a large number of sensors and acquire data from a single microcontroller board. As an acceleration sensor, an ICM-20948 [17] from TDK Corporation (Tokyo, Japan) is used. This sensor is a nine-axis inertial measurement unit with not only a three-axis accelerometer but also a three-axis gyro sensor and a three-axis magnetometer. As this  
Figure 5: Accelerometer devices compatible with Secure Digital (SD) cards. device only measures acceleration, only the three-axis accelerometer is enabled. The measurable range of acceleration can be selected from four levels, \(\pm\)2 g, \(\pm\)4 g, \(\pm\)8 g and \(\pm\)16 g, and can be set according to the machine to be measured. The device is also equipped with a First-in First-out (FIFO) for temporarily storing sensor data, making it suitable for applications where sensor data are read out at regular intervals. In the implementation of this device, only the accelerometer is installed in the industrial machine to be measured, while the microcontroller board is installed in a remote location. Therefore, the acceleration sensor is connected to the microcontroller board by extending it with shielded conductors. I2C is used for communication with the microcontroller. A module with a PCF8523, manufactured by Adafruit Industries (New York, NY, USA) is used as the real-time clock (RTC). The purpose of installing a RTC is to record the measurement time of the data. When a CSV file containing measurement data is saved on an SD card, the measurement time can be recorded in the file so that the data can be checked against the data of which product was being manufactured using the production record data. This makes it possible to compare data from the same product production. As with the acceleration sensor, I2C is used for communication with the microcontroller. The power supply to the device is via an Alternating Current (AC) adapter from a 100 V AC power supply. Grand Central is equipped with a 5.5 mm/2.1 mm centre plus Direct Current (DC) connector and can accept input voltages from 6 V to 12 V. Simply connecting the power supply to the device is not enough to check whether the sensor device itself has started working properly. A reset button with a Light Emitting Diode (LED) is, therefore, used. The LED inside the button is illuminated when the operation starts using the GPIO pins of Grand Central to inform the operator that the device is working properly. Conversely, if the connection to the sensor fails or the SD card recognition fails, the LED blinks to inform the operator that there is a fault with the device. This button is connected to the reset pin of Grand Central and can be reset by pressing the button. Here, the firmware of the accelerometer device that supports SD cards is described. The device starts up and begins operating when connected to a power source via an AC adapter.