## 2 Test Conditions  
Before starting the research on the usefulness of vibration signals for the recognition of printed objects, preliminary analyses of the examined 3D printer were performed. They concerned the identification of the main sources of vibrations in order to select the sensor mounting locations. The first attempts to install the sensor concerned the head printer and stepper motors. However, the obtained data did not allow to determine the length of the printed walls and the senses of the print head movement. Other sensor locations were looked for. Printer walls turned out to be the best choice. Sensor positioning was an important element of the success of the analyses. A particularly important issue was the proper levelling of the sensor and the alignment of the sensor axis with the X and Y axes--the directions of movement of the print head. Taking these elements into account made it possible to obtain results that allowed for the analysis of the possibility of recovering data on printed objects from the recorded vibration signals. The tests were conducted by using the Zortrax M200 Plus 3D printer. This is an FDM technology-based printer that uses the so-called thermoplastics in the production process, where plastics are used to form object geometries at temperatures around 200 \({}^{\circ}\)C. The printed object can be designed by using generally available CAD applications and implemented in the printing process after converting into a file supported by the printer (Figure 2). Objects printed using this technology are created by applying the individual layers of semi-liquid material (filament) extruded from the heated nozzle. The filament is a line with a diameter of 1.75 mm or 2.85 mm and is wound on a spool (a filament with a diameter 1.75 mm was used for conducted tests--Figure 3). On such a printer the head moves along the X and Y axes, while the printing platform moves along the Z axis (Figure 1). To isolate them from other vibration signals, the tests were conducted in a screening cabin. The printer was placed directly on a laboratory bench, while the accelerometer was glued to the printer housing (Figure 3). The results presented in this paper were preceded by numerous preliminary tests and recording of vibration signals, the source of which was the Zortrax M200 Plus printer. These tests were performed taking into account the different locations of the accelerometer on the upper part of the printer housing (on the rear, right, front and left sides, respectively--Figure 4). The usability of three types of accelerometers (ADXL345, MMA8451 and LIS2 DH) was tested. The analysis of spectrograms showed that the signals important for presenting the threat of non-invasive acquisition of data about the printed object are contained in a narrow band of frequencies (from several Hz to approx. 400 Hz). The quality of the spectrograms depended primarily on the type of accelerometer, but did not depend on its location. Finally, the most accurate LIS2 DH accelerometer was selected and placed in position 1, as shown in Figure 5. This sensor is used to measure an acceleration in three axes (X, Y and Z) in the ranges \(\pm\)2 g, \(\pm\)4 g, \(\pm\)8 g or \(\pm\)16 g, and it is capable of measuring accelerations with output data rates from 1 Hz to 5.3 kHz [22]. The module is powered with a voltage from 3.3 V to 5 V and communicates through an I2C/SPI bus. Measurements were taken at a speed of 1000 12-bit samples per second for each axis (X, Y, Z), at the maximum accelerometer sensitivity (\(\pm\)2 g). In practice, this is the highest sampling rate that can be obtained for this accelerometer without distorting data. Attempts to use built-in filters further lowered this sampling rate. The source of interesting vibrations was primarily the stepper motors forcing the printhead to move in the X and Y directions. These motors are located in the back of the printer (Figure 1). These are the original motors manufactured by Wantai and used in the M200 Plus printer and have the designation 42 BYGH408L21 (Figure 6).