Product category:
Vision and scanning systems
News Release from: Cognex UK | Subject: Machine vision systems. software
Edited by the Manufacturingtalk Editorial
Team on 17 October 2005
Vision system checks watch faces
automatically
Checking watch faces for various characteristics is a standard task when carried out visually, but automatic inspection placed great demands on the hardware and software.
In the luxury watch making industry the production volumes are high and the quality requirements are particularly stringent The assembly work is often carried out manually and the possible sources of error are correspondingly many and varied
This article was originally published on Manufacturingtalk on 28 May 2001 at 8.00am (UK)
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In the drive to optimize the manufacturing process and increase the quality of the end product, quality control obviously also needs to be optimized and the potential human error factor to be compensated for by means of appropriate control processes.
This is an application that is responsible for 100% inspection of hand-mounted watch dials.
This involves inspecting the correct position of the figures, their size, their orientation and their colour.
Further reading
Complex vision technology in an affordable unit
Cognex have introduced the In-Sight 2000, an entirely new type of industrial machine vision sensor at a cost usually associated with "smart cameras"
Vision systems retrofitted like CNC
Special purpose machine tools and assembly systems, with Cognex machine vision systems as an integral part, are being supplied to the automotive industry by Douglas Curtis Machine Tools
Vision system dedicated to electronic production
Cognex has introduced Display Inspect - a machine vision software and hardware package for inspecting displays, keypads, and enclosures on LCD and LED display-based products.
The challenge of this task arises from the immense diversity and combination possibilities of the types of mounted parts and dials.
The inspection cell must be generally configured so that yet unknown combinations of dials and figures can be processed in the future.
* Inspection constraints - functionality: In inspection using image processing, four criteria must be checked, each placing different demands on the image processing system.
A first stage would consist in determining the correct position of the figures on the watch dial (ensuring, for example, that an eight is correctly positioned where an eight should be).
In a second stage, the figure itself is checked to ensure that its size, its orientation (angular) and its colour correspond to the standard model.
These four inspections place quite different demands on the image processing system as a whole.
* Universality - the hardware and software must be so configured that, within geometrical boundaries, the combination possibilities for different watch dials and figures are unlimited and open.
It must be possible to identify reliably a multitude of forms (symbols, Arabic and Roman numerals) in many sizes.
Colour recognition may be limited to distinguishing between three types of gold, partly in combination with precious stones.
* Reliability - the quality requirements of this industry demand a miniscule number of measurement faults when this inspection is carried out (to attain a reject rate target of <100ppm).
As well as the optical controls, a complete inspection station with a high degree of autonomy had to be developed.
The watch dials are brought to the machine stacked on pallets, and their separation, measurement and restacking on the pallets after measurement take place automatically.
The entire system is connected to a data management program, which handles the preparation and evaluation of the statistical data regarding the error sources.
* Solution strategies - in preliminary testing, hardware and software component tests were carried out on a representative batch of dials in order to determine feasibility with regard to the essential features.
It turned out that, as well as the functionality of the software, the lighting was the key to the success of the operation.
The variation possibilities of forms, surfaces and materials in the figures and dials (e g, mother-of-pearl, CRFP, etc.) require universal lighting that generates optimal contrast between the outside edges of the figures and the dial while at the same time allowing colour measurements to be made.
This lighting must also remain homogeneous locally and over time.
Initial tests were carried out with prototyping software from Cognex (QuickStart in VisionPro).
With this tool it was possible to develop and test the measurement concept rapidly.
Within this environment, the optimal configurations were evaluated and the user-relevant functionalities were determined.
The following factors were then evaluated: * Position and orientation of the figures - the coordinates system is defined by taking as its reference an obvious object on the dial (one that is always present).
Then, using Geometric Pattern Matching (GPM), each individual object on the dial is localized in relation to this reference object.
From these measurements are derived the quality values of the localized objects and their coordinate data.
The model data for the GPM are either fed in by means of CAD data (DXF files) or by a teach-in process.
These model data are then administered in a database.
* Object size - the size of the active object is examined using the GPM scale factor and an edge-finder tool.
* Colour - the colour coordinates (RGB) are assessed as grey values using a histogram and compared, using statistical algorithms, with the nominal value for the individual colours (chromaticity coordinate in a three-dimensional space).
The colour evaluation is one of the critical operations in this problem, as the basic material of all the objects to be controlled is identical (yellow gold) and hence demonstrates similar characteristics in terms of spectral emissitivity.
Furthermore, in 'jewellery-related' applications, there is a wide variety of surface structures of objects (dull, shiny, etc) and this adds yet another degree of complexity to the colour measurement.
* The solution - process - the figures are identified with a bar code and positioned on pallets beneath the image processing system.
Using a vertical movement, the figure is brought precisely to the ideal lighting position.
Images are now taken sequentially with a variety of lighting (background, direct lighting and dark ground illumination), and these are evaluated according to the aforementioned criteria.
During the evaluation, the next dial is brought into position.
Parts showing errors are marked for subsequent sorting.
The image data of the parts showing errors are kept in a database and made available to the data management program.
* Hardware - the focus versus pixel definition requirements entail the use of a high-definition camera (1.45 Megapixel, Firewire) that can also take colour pictures of high sensitivity in the requisite spectral band (the maximum emissitivity of the basic material of the figures on the dial is at a wavelength of approximately 600nm).
The colour picture is taken using a Bayer filter.
Lighting is implemented in various stages and consists of background lighting (to determine the external shape of the dial), a dome with integrated coaxial lighting, and dark ground illumination.
The different forms of lighting can be switched on individually and are infinitely variable.
The lighting comes from Volpi AG (Schlieren) and was specially developed for this project.
* Software - there are high functional requirements for the user software.
The operator must be able to configure new references rapidly in line with the process parameters.
On account of the wide diversity of parts and combination possibilities, an expert system is required that offers the operator powerful, menu-driven support for the settings/configuration.
A vision library from Cognex is used (VisionPro with an add-on colour module).
Access within the library to the individual functions is provided by means of ActiveX controls and COM elements.
Much of the project's success can be attributed to the powerful GPM algorithms (PatQuick, PatFlex, PatMax) from Cognex.
* Installation - the entire installation was constructed by INSYS Industriesysteme (Muensingen) and completed using hardware components from iTECH (image processing and axis systems).
The system control (industry PC) and software were also supplied and produced respectively by iTECH.
The image processing is an integral component of the system control.
Operations as a whole are also controlled using the same software application.
The computing power enables a cycle time of around 3s per dial.
The installation itself offers high availability when working multiple shifts.
* Summary/outlook - the industrial image processing used in this context has contributed decisively to maintaining the high quality level of the end product, or even increasing it, while offering a quantum leap in the efficiency of the process.
The difficulty for the system integrator of this application is the multitude of combination possibilities of the dials and symbols/figures and their generalization in the functionality of the individual inspections.
Further types of inspection may be are in view for the dial, such as checking the quality of the printing on the dial, precise control of the watch movement, and so on. Request a free brochure from Cognex UK ...
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