In the field of quality control, automatic inspection systems can greatly improve the production process, both in terms of precision and time consumption. In particular, for small superficial defect detection, high resolution techniques are required for surface profile measurement and 3D modeling with errors of units of microns. Among active systems for precise measurements, those based on laser triangulation scanners are very promising since they are contactless and not invasive, and at the same time it provides high precision and fast measurements. In the last decade, laser triangulation scanners have been widely used for quality inspection, surface profile measurement, 3D modeling and reverse engineering applications. These systems use a camera and a laser light to model 3D objects as the appearance of the laser spot changes according to the distance between the light source and the object surface. The design of a laser triangulation scanner presents many challenges as the project of the optical routing, the detection of the laser spot in the camera image plane, and the overall system calibration greatly affect the measure ability. Moreover, issues such as the laser reflective properties on the object surface, speckle noise, occlusions and sub-pixel precision in the detection of laser stripe peaks are fundamental to increase the precision of the final measurement. The objective of this project is the development of a 3D reconstruction system for the detection of surface defect with high precision. The system is based on a laser triangulation scanner that, by using an innovative acquisition procedure, overcomes the occlusion problems due to the presence of grooves on the object surface. In particular, the proposed system has a resolution of 15 µm which, to the best of our knowledge, has never been reached by using a standard laser triangulation scanner.
TYPE AND DATE
STARTING DATE – Jan 1, 2012
ENDING DATE – Dec 31, 2013
COST € 125.000,00
FUNDING € 125.000,00
Fig. 1. Drilling tool with defect and Reconstructed 3D surface.
The proposed system for 3D surface reconstruction consists of two fundamental components. On the one hand an ad-hoc experimental setup for the laser triangulation scanner has been designed in order to obtain high resolution measurements. On the other hand an innovative procedure for data acquisition has been defined in order to solve occlusion problems that usually occur in presence of interior cavities on the object. In particular, the aim of this work is the inspection of industrial objects such as drilling tools.
In Fig. 1 an example of helicoidally-shaped drilling tool with the correspondent main facets and angles, is shown. For this kind of objects, the main challenge is the reconstruction of the entire volume, including the internal faces (i.e. Flute, Flank, Cutting lip, etc.), in order to perform complete quality control measurements. The proposed system is based on an innovative acquisition procedure able to reconstruct the entire surface of these objects. The idea is to rotate the drilling tool about its main axis, namely the Drill axis. Due to the particular form of these objects some parts of the surface, such as the Flute or the Face, are not scanned by the system, since either light is not able to reach these regions or the tool itself hides these areas to the camera view. These occluded parts are then acquired giving a controlled spatial displacement of few millimeters, transversal to the laser optical axis. In this way the laser beam intercepts all the undercut regions which can be now observed in the camera field of view at different rotation angles of the tool. An analytical exact transformation function is then developed in order to compensate for the change of the reference system, in terms of both the translational shift of the system and the rotational alteration of the position of the tool.
Fig. 2. Schematic view of the system.
The system consists of three fundamental parts: a laser, a camera and a telecentric lens (see Fig.2). The object under test is positioned along the Z-axis of the (X,Y,Z) coordinate refence system, reported in the right-top corner of the figure. The origin of this reference system corresponds to the focuses position of both laser and camera.
The main components of the system are:
- Laser “LASIRIS T-MFL55” with wavelength λ = 660 nm;
- Camera “IDS-uEye USB-3 UI-3480CP-M”;
- Telecentric Lens “Lensation TC5M-10-110i”;
- Band-Pass filter tuned on the laser wavelength;
- Rigid arm, used to connect steadily the laser, the “camera” and the Telecentric lens;
- Micrometric Translation Stage on which the entire system is mounted to allow movements in two perpendicular directions: LEFT-RIGHT, FORWARD-BACKWARD;
- Precision Rotation Stage where the observed object is placed.
All the moving parts (“Micrometric Translation Stage” and “Precision Rotation Stage”), are provided with encoders which give the absolute position in the (X,Y,Z) reference system.
3D prototypal system (side view).
Section of a drilling tool to be reconstructed.
High resolution surface of the drilling tool section.