Cameras and processors inspect wooden-ruler parts

When making linear measurements, carpenters, masons, and electricians often use stiff wooden folding rulers that can be expanded as required to get accurate information.

Dec 1st, 2001
Th 81047

When making linear measurements, carpenters, masons, and electricians often use stiff wooden folding rulers that can be expanded as required to get accurate information. For high accuracy, vendors construct these rulers with straight, unwarped wood that does not contain defects such as knots or cracks.

In the manufacture of these rulers, individual pieces of wood are joined with brass fittings. Before this assembly process can occur, the integrity of each wooden part must be checked. Rather than perform this task manually, a German-based manufacturer enlisted the help of hema Electronik GmbH (Aalen, Germany) to produce a vision-based inspection system for automatically inspecting each wooden part before assembly.


To inspect parts for use in wooden folding rulers, hema Electronik has developed a CompactPCI-based vision-inspection system that uses multiple C40 DSPs to process images from two linescan cameras and frame grabbers.
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During operation, wooden pieces measuring 24 x 1.6 cm are presented to the imaging system on a conveyor belt. To rapidly inspect one side of the wooden pieces, designers used two 20-MHz CL-CB 2048 linescan cameras from Dalsa Imaging (Waterloo, Ont., Canada) that have since been superseded by the Spyder camera series. Image data are captured by two separate VSP-1 frame-grabber boards from hema Electronik.

Under control of a Motorola CompactPCI CPU board, the CompactPCI-based vision system uses a hema Electronik cPCI-DSP4 board that operates as a bridge between the CompactPCI bus and a hema Electronik Link-Bus backplane. With the backplane, an extension of Texas Instruments (TI; Dallas, TX) C40 digital-signal-processing (DSP) communication ports, five DSP3 boards are connected to one DSP4 board.

To perform the image processing required to inspect 17 wooden parts per second, the vision system transfers captured images from the two frame-grabber boards to the five DSP3 boards. Each DSP3 board contains two C40 DSPs. In operation, image-analysis functions operate on each of the five DSP3 boards, and each board's two processors operate on individual images simultaneously.

According to Norbert Zuppa, sales engineer at hema Electronik, all the software was developed for the system using TI's ANSI C compiler, hema Electronik's libraries and hipe image-processing software, and Sinectonalysis (Reno, NV) DSP libraries for signal and image processing.Under control of the DSP4 board, also connected via the Link-Bus, the vision system sends pass-fail decisions through digital input/output interfaces to control an automated product rejection mechanism that sorts good and bad wooden parts. After the parts are sorted, good parts are shipped to the next production stage, where they are manufactured into folding rulers. According to Zuppa, the DM320,000 ($150,000) system, which took more than one-and-a-half years to develop, is being offered to other manufacturers to perform similar operations.

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