Cognex machine vision achieves zero error rate

Sensor Review

ISSN: 0260-2288

Article publication date: 1 September 2004




(2004), "Cognex machine vision achieves zero error rate", Sensor Review, Vol. 24 No. 3.



Emerald Group Publishing Limited

Copyright © 2004, Emerald Group Publishing Limited

Cognex machine vision achieves zero error rate

Cognex machine vision achieves zero error rate

Keywords: Machine vision, Inspection, Cognex

Flawless product quality ensures maximum efficiency in subsequent assembly processes

One of the injection molding machines operated by Walter Söhner GmbH & Co, a manufacturer of precision plastic parts, produces about 5,000 coil forms an hour. In a standard three-shift cycle the output totals over 120,000 parts per day. The machine has already produced 62 million coil forms in two different versions. The most recent 50 per cent of these parts have benefited from a new quality inspection system. Jochen Neugart, Head of Production, proudly states, “Thanks to reliable monitoring by the image processing system, we have not delivered a single faulty part to the customer in a whole year. In other words, we have achieved an error rate of 0 ppm (ppm = parts per million) in real terms.”

Walter Söhner had already successfully deployed industrial image processing systems for quality assurance purposes in other areas of production, but the new application has broken entirely new ground. The aim was to reliably detect sporadically occurring defects in the plastic coil forms. When supplying the automotive industry with components like these, it is vital to eliminate defects that could disrupt subsequent production process. Walter Söhner required an absolutely dependable quality monitoring solution using industrial image processing that provided fully automatic sorting based on clear pass/fail criteria.


The coil forms are placed at intervals of 30 mm on a special fixture, in two rows of eight, and then inspected to check that they are fully formed. With a belt speed of 9 m/min, the maximum analysis time per fixture is 1.6 s. The high precision shape inspection of each coil form has to be completed within just 100 ms. The entire contour of each component is inspected and compared to a specified set of values by two cameras simultaneously capturing images from different positions and perspectives. The two cameras are angled at 40° to the horizontal, and are offset at 90° to each other. With two rows of parts in the fixture, two cameras and one stroboscopic lighting unit are deployed per row (Plates 1 and 2).

Plate 1 Two cameras simultaneously scan each of the rows of plastic parts

Plate 2 The entire contour of each of the coil forms is inspected in great detail

These demanding requirements could only be satisfied using a powerful PC-based vision system, supplied with advanced and reliable image processing tools. The key components of high- performance vision system supplied by Cognex is the MVS 8120 frame grabber and the PatInspect software package. PatInspect is a further development of Cognex's market-leading, high accuracy PatMax software technology. The PatInspect tools are invariant; that is to say, they are insensitive to changes in the object's position, orientation or scale, and unaffected by fluctuating light and contrast conditions. PatInspect adopts the principle of geometry-oriented object description to differentiate parts according to a series of individually and sensitively determined error criteria. The machine operators have been delighted by the very simple user interface. Minor adjustments can be made quickly during production by modifying just a few parameters. Inspection-levels can be defined in order to define the exact quality required.

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