learn more

Area sensors with pattern projection

Simply measure large areas

Area sensors with pattern projection basically work according to the triangulation principle. Depending on the design variant, a distinction can be made between pattern projection sensors and photogrammetry sensors. In both methods, the angular relationships between the beam paths for imaging and pattern projection or between several imaging beam paths or combinations of these are used to determine the workpiece topography. With the pattern projection sensor (also fringe projection sensor, Fig. 19c), a projector projects a fringe pattern with an exactly known geometry onto the material surface.

<p>Fig. 19: Multidimensional distance sensors: a) Focus variation: Werth 3D Patch or confocal area sensor (NFP), b) Laser line sensor, c) Pattern projection sensor, d) Photogrammetry sensor</p>

Pattern projection sensor

With a pattern projection sensor (also known as a fringe projection sensor, see Fig. 19c), a projector projects a fringe pattern with an exactly known geometry onto the material surface. Similar to the light section method, the patterns generated in this way are captured by a camera and subsequently analysed by software. The properties of the imaging beam path (magnification, imaging error) must be precisely taken into account.

Measuring with projector and camera

If the three-dimensional surface of the object to be measured is completely within the measuring range of the sensor, no movement of the coordinate axes is required, in contrast to a simple line sensor. In order to achieve a high-resolution with clear assignment of the points to their spatial coordinates, different line patterns are usually projected and analysed one after the other. With the "phase shift" method, it is possible to realise a type of sub-pixeling[1]. In principle, the patterns are shifted step by step. The surface can thus be captured in a denser grid of points.

Photogrammetric sensor

Photogrammetry sensors (see Fig. 19d) are based on capturing the object surface from two different orientations with one camera each. According to the triangulation principle, the spatial coordinates of each recognised object feature are calculated using angular relationships.

Measuring with multiple cameras

As the measuring object is usually not sufficiently structured, a two-dimensional grid is projected onto the surface. The resulting pattern is captured by the two cameras and then analysed. Unlike with a fringe projection sensor, the accuracy of the projection has no influence on the measurement result. The projection is only used to generate structures on the workpiece surface. The actual triangulation process is carried out with the two cameras.

Measuring without the influence of projection

As a result, this method is also less sensitive to influences from the workpiece surface such as differences in brightness and surface interference. These only cause changes to the line geometry which, as described above, have no influence on the measurement result. The applications of these sensors are in car body metrology and the measurement of plastic parts.