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May, 2019

Optical analysis of Trabecular structures

Rapid prototyping, known as 3D printing or Additive Manufacturing, is a process that allows the creation of 3D objects by depositing material layer by layer. The materials used vary: plastic polymers, metals, ceramics or glass, depending on the principle used by the machine for prototyping, such as the deposit of the molten material or the welding of dust particles of the material itself by means of high-power lasers. This technique allows the creation of particular objects of extreme complexity including the so-called “trabecular structures“, structures that have very advantageous mechanical and physical properties (Fig. 1). They are in fact lightweight structures and at the same time very resistant and these characteristics have led them, in recent years, to be increasingly studied and used in application areas such as biomedical and automotive research fields.

Despite the high flexibility of prototyping machines, the complexity of these structures often generates differences between the designed structure and the final result of 3D printing. It is therefore necessary to design and build measuring benches that can detect such differences. The study of these differences is the subject of a Progetto di Ricerca di Interesse Nazionale (PRIN Prot. 2015BNWJZT), which provides a multi-competence and multidisciplinary approach, through the collaboration of various universities: the University of Brescia, the University of Perugia, the Polytechnic University of Marche and the University of Messina. 

The aim of this thesis was to study the possible measurement set-ups involving both 2D and 3D vision. The solutions identified for the superficial dimensioning of the prototyped object (shown in Fig. 2) are:

  1. a 3D measurement set-up with a light profile sensor;
  2. a 2D measurement set-up with cameras, telecentric optics and collimated backlight.

In addition, a dimensional survey of the internal structure of the object was carried out thanks to to a tomographic scan of the structure made by a selected company.

Fig. 1 - Example of a Trabecular Structure.
Fig. 2 - The prototyped object studied in this thesis.

The 3D measurment set-up

The experimental set-up created involved a light profile sensor WENGLOR MLWL132. The object has been mounted on a micrometric slide to better perform the acquisitions (Fig. 3).
The point cloud is acquired by the sensor using a custom made LabView software. The whole object is scanned and the point cloud is then analyzed by using PolyWorks. Fig. 4 shows an example of acquisition, while Fig. 5 shows the errors between the point cloud obtained and the CAD model of the object.
Fig. 3 - 3D experimental set-up.
Fig. 4 - Example of acquisition using the light profile sensor.
Fig. 5 - Errors between the measured point cloud and the CAD model.

The 2D measurment set-up

The experimental set-up involving telecentric lenses is shown in Fig. 6. Telecentric lenses are fundamental to avoid camera distorsion especially when high resolution for low dimension measurments are required. The camera used is a iDS UI-1460SE, the telecentric lenses are an OPTO-ENGINEERING TC23036 and finally the retro-illuminator is an OPTO-ENGINEERING LTCLHP036-R (red light). In this set-up a spot was also dedicated to the calibration master required for the calibration of the camera.
The acquisitions obtained have some differences according to the use of the the retro-illuminator. Fig. 7, 8 and 9 show some examples of the acquisitions conducted.
Finally, the measured object was then compared to the tomography obtained from a selected company, resulting in the error map in Fig. 10.

 

Fig. 6 - 2D experimental set-up.
Fig. 10 - Error map obtained comparing the measured object to the tomography.

If you are interested in the project and want to read more about the procedure carried out in this thesis work, as well as the resulting measurments, download the presentation below.

DOWNLOAD THE PRESENTATION