Meersburg, Germany--Although engineers in quality and test departments generally use mechanical tactile sensor systems to map the 3-D coordinates of complex shapes, such techniques suffer from well-known drawbacks. Requiring long measurement times, these methods are unsuitable for soft or sensitive surfaces.
Breuckmann GmbH supplies non-laser optical measurement equipment based on white light that is designed for a production environment. Depending upon the application, users can employ two systems: The optoCAM system for applications that require absolute measurements--as in rapid prototyping; and optoSIS for relative measurements such as those required to detect 3-D surface defects.
These systems operate on a triangulation principle incorporating what Breuckmann refers to as the Miniaturised Projection Technique (MPT). In this method, a halogen lamp or flash-lamp projects an illuminated pattern onto the measurement object through a wafer carrying a grid structure. A CCD camera records the projected pattern, which consists of an MPT grid of varying line width. Maximum projected line density ranges from 64 to 256 lines per millimeter.
Next, a computer processes the received grid image to work out the position of the illuminated points in 3-D coordinates. To combine high accuracy with absolute position information, the projection sequence includes gray code and phase shift. One grid structure suffices in some applications for relative measurements or time-critical processes.
Ulrich Krackhardt of Breuckmann's Technical Sales points out an advantage of the MPT method: "Compared to using LCD displays to project the image, the MPT method yields significantly smaller and lighter sensors without forfeiting accuracy and resolution."
A calibration routine partially automates the determination of geometrical parameters. This calibration process also takes into account aberrations in the optical imaging system. It automatically masks out features such as holes, where no sensible coordinate data is found.
If necessary, a number of shots or "patches" are taken of the object from different viewing angles. Doing so produces 3-D coordinates of a few thousand to a few million measurement points, depending on the application. The resulting patches can be merged in a simple manner. The system achieves a resolution of 20 to 30 ćm over an of area of 12 x 15 cm.
"Its low weight and small size" says Krackhardt, "make the measurement system very suitable for robotic applications." As one example he notes quality control of pressed parts, where the system detects small dents and bulges in seconds.
Currently in operation at the Mercedes-Benz plant, Sindelfingen, the system measures the surface quality of parts output by a press line. The complete procedure takes just a few seconds and, in a three-way decision, states whether the part is good, scrap, or needs rework.
Additional details...Contact Ulrich Krackhardt, Breuckmann GmbH, Torenstrasse 14, D-88709 Meersburg, Germany, Tel.: +49 7532 1563, FAX: +49 7532 9377.