## 5 Conclusions  
The paper presents the results of tests and analyses regarding the possibility of using vibration signals in non-invasion shape data acquisition for objects printed with a 3D printer. Signals that can be used in the infiltration process are generated both by stepper motors controlling the printhead and platform and by the head and platform during their operation. The tests used objects with horizontal cross-sections with a shape of a right-angled isosceles triangle, a circle and a square (trihedron, tube and tetrahedron, respectively). The recorded vibration signals were used to create spectrograms with features allowing to identify the printhead movements, which makes it possible to reproduce the shape of the printed object. The frequencies that can be used to identify the shape of the printed object range from approx. 20 Hz to 370 Hz. The frequency spectrum of the recorded signal makes it possible to determine the printhead movement trajectory. Oscillations characteristic for the printhead operation presented in the form of spectrograms allowed us to identify the printed object components (straight lines, arches). The knowledge of the H printhead movement speed allowed us to determine the length of printed components. For the trihedron object, the leg length values of 20.33 mm and 19.42 mm (the actual length of 20.00 mm) were determined, while the value of 28.73 mm was determined for the hypotenuse length (the actual length of 28.30 mm). The reproduction error for the printed \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{4}{c}{**The Error of Determining of the Wall Length**} \\ \cline{2-5}  & **Wall a** & **Wall b** & **Wall c** & **Wall d** \\ \hline Vibration signal & 4.60\% & 4.70\% & 3.95\% & 4.00\% \\ \hline \hline \end{tabular}
\end{table} 
Table 6: The errors of determining the tetrahedron (Figure 6(c)). \begin{table}
\begin{tabular}{c c c c} \hline \hline  & \multicolumn{4}{c}{**The Error of Determining of the Wall Length**} \\ \cline{2-4}  & **Side along \(X\) Axis** & **Side along \(Y\) Axis** & **Hypotenuse** \\ \hline Vibration signal & 1.65\% & 2.90\% & 1.52\% \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The errors of determining the wall length (Figure 6(a)). \begin{table}
\begin{tabular}{c c c c} \hline \hline  & \multicolumn{4}{c}{**The Error of Determining of the Quarters**} \\ \cline{2-4}  & **Quarter I** & **Quarter II** & **Quarter III** & **Quarter IV** \\ \hline Vibration signal & 3.24\% & 4.39\% & 0.06\% & 3.24\% \\ \hline \hline \end{tabular}
\end{table} 
Table 5: The errors of determining the quarters (Figure 6(b)). object walls was lower than 5%. It should be emphasized that it is easy to determine the direction of printhead movement based on spectrograms, while reproducing its sense is more difficult, if not impossible to specify. The obtained research results show the great usefulness of vibration signals in the process of non-invasive data acquisition. Not every element of the printer's construction, e.g., the platform or the print head, is a valuable source of vibration. An important issue of the correctness of the research was the proper positioning of the vibration sensor--levelling and compliance of the sensor axis with the directions of the print head movement. The lack of precision in the settings resulted in significant errors in recreating the shapes of printed objects. In the process of analyzing the recorded vibration signals, resulting in the acquisition of data on printed shapes, it is necessary to identify the starting and ending points of the printout of each level of a given shape. The possibility of reconstructing printed simple shapes is a very important conclusion in the conducted analyses. Obtained results show that the recognition of more complicated objects that are built from the simple analyzed shapes can be possible.