Depending on the requirements in terms of the quality of the inner wall of a bore, quality controls can be a necessity. If, for example, a thread has been cut in the bore, then the inspection will make sure that the thread area is free of e.g. drilling chips. A visual inspection by correspondingly trained employees is the simplest and cheapest way to carry out a quality control. However, it is difficult for testing personnel to maintain consistently high levels of concentration over longer periods of time – particularly if the throughput rate is high, the results of the inspection process will be subject to significant fluctuations. In comparison to automated solutions, productivity is limited even under optimum conditions.
One alternative is to carry out checks with tactile gauges. Although this method offers consistent quality, it offers a different disadvantage, which in some cases may rule it out altogether: since the inspection process is performed with mechanical means, contactless inspections are not possible – which means that the gauge scrapes along the surface of the object under testing. This carries the risk that, by touching it, the measuring equipment itself changes the surface of the object it is supposed to be checking. In addition, the method is relatively time-intensive, which restricts throughput.
A third option is to use endoscopes for visual inspection of the affected surfaces. However, the endoscopes used typically for industrial applications tend to have been developed for individual measurements under laboratory conditions rather than for use in production environments. This means that endoscopes are usually quite effective – but they are not very robust in the face of the mechanical effects that can never be fully ruled out in industrial series production set-ups that are designed for high throughput.
Contactless inspections at high throughput rates
As part of a customer solution, image processing specialist Vision & Control developed a visual testing device for automated quality control together with system integrator SSR Technik. The setup uses an industrial camera with a 360° wide angle lens with distortion correction, as well as corresponding lighting and dedicated image processing software. In the field of workpiece handling, standardised components are used across the board here for infeed, separation, positioning, transport and rejection. The system is capable of contactless testing of up to 100 workpieces per minute.
In this type of internal contour inspection as an application of image processing, the lighting represents a special challenge because standard products do not meet the requirements of such an application. This is why the system developers Vicolux decided to choose lighting devices from V&C, which could be adapted to this task thanks to their flexible concept to enable reliable lighting of the bore in particular in incident light mode. They can also be used to deliver transmitted light setups. The light colour can be varied depending on the material of the workpiece and the purpose of the inspection; in addition, system integrators can also choose between light sources with directional light or scattered light. A careful and professional layout of the overall system prevents any shading by the camera. The described system can be used within wide physical limits, including bores with a depth equal to three times their diameter, and is suitable for use with bore diameters from just 0.5 mm.
Camera with integrated processing unit
The heart of the inspection system is the camera together with the image processing software. In our example, a camera of type pictor® from Vision & Control was used, the images of which were evaluated by the software vcwin. This family of cameras features a high-performance integrated computing unit, which means that the image processing can take place directly in the machine environment. The combination of pictor® and vcwin delivers a capable synthesis of image acquisition and analysis.
The software vcwin, which is tailored to this camera, not only offers an integrated user interface for a wide range of different image acquisition and image processing tasks, but is also remarkably adaptable to the relevant tasks in hand. The integrated tools include virtual probing functions. These can be used to realise processes that are comparable to mechanical probing – but, in contrast to these, they are entirely contactless and thus free of the risks or disadvantages of a mechanical quality inspection. With its integrated user guidance, the software provides users with a tool that will enable them to create and modify case-specific evaluation algorithms themselves after brief training – without the need for major prior programming skills.
This arrangement also detects the correctness of the thread intake and run-out, as well as the completeness of the thread. Mechanical residue throughout the entire drilling path and damage to the bore hole base on blind hole bores are detected. Here, it is up to the user whether he or she receives the outcome of the inspection process as a good/bad decision or as a quantitative indication in numerical form.
One of these described systems is in use at the company Vosseler Umformtechnik, a SME in Hildburghausen/Thuringia.
published in: Elektronik Praxis 12/2019