FEATURE Machine Vision
Three simple steps to improve your machine vision system
Earl Yardley from Industrial Vision Systems looks at some of the critical parts of the vision inspection chain and what needs to be addressed
W
hat’s the secret to a perfect vision system installation? That’s what all industrial automation
and process control engineers want to know. How do you guarantee that the machine vision system has been set up to the best possible engineering standard? And how can you apply this knowledge to refine the current vision systems on the line?
Step 1: Lighting and filters What wavelength of light are you using and how does it compare to the sensitivity of the camera, coupled with the colours on the part requiring inspection? An intelligent approach to segmenting the scene is based on the correct lighting setup, the angle of the machine vision lighting and the way contrast is created based on the part’s colour against the light. Coloured illumination may intensify or suppress defined colours in monochrome imaging. This way, the contrast helps recognise relevant features, which is decisive for an application-specific and optimally-matched vision solution. For example, blue light cast onto a multi- colour surface will only be reflected by the blue content. In contrast, red content illuminated in blue appears exceptionally dark.
resolution. Selecting an appropriate filter may improve the current system’s quality if ambient light is a problem. Bandpass filters are frequently one of the simplest methods to significantly improve image quality.
Step 2: Camera resolution and field of view A vision system’s spatial resolution refers to the sensor’s active available pixels. It’s important to consider how many pixels are available for image processing, relative to the defect’s size or computation to achieve. To improve your machine vision system, check what size defect you need to see, and then work out the field of view relative to the available pixels. However, bear in mind, having a single-pixel resolution for measurement is not good enough, as the system will fail the Measurement Systems Analysis (MSA) and Gauge R&R studies – tests used to determine measurement accuracy.
an area from the background from another object. But, because it uses the fundamental pixel grey level to determine the segmentation, it can easily be affected by light changes, which will ultimately change the pixel value and lead to changes required to the threshold level. This is one of the most common faults in a system that has not been set up with a controlled lighting level. So, if you are reviewing a threshold level change, you should also check the lighting and filters used on the system. 2. Calibration – perhaps not calibration in the true sense of the word, but if the camera on the vision system has been knocked out of place, it may have detrimental effects on the inspection outcome, algorithm execution and the yields of the vision system. Hence, re- calibration may need to be done. Check the camera is in-line and that the field of view is still the same. Focus and light should also be consistent. Some measurement-based machine-vision systems will come with a calibration piece or a Dot-and-Square Calibration Target. These generally come with a NIST Traceability Certificate, so you know that the artefact is traceable to national standards. This calibration target should be placed, and the system checked against it.
Combining a filter mounted on one end of the lens allows blocking all unwanted ambient light and passing only the necessary wavelength of light required, increasing contrast and
10 July/August 2022 | Automation
Step 3: Vision algorithm optimisation Review the software and the underlying sequence of inspection for the image- processing system. There will be some fundamental areas which should be checked, including: 1. Thresholding – a simple look-up table that places pixel levels into a binary 1 or 0, depending whether they sit above, below or in-between levels. The resulting image is a pure black- and-white image, used to segment
CONTACT:
Industrial Vision Systems
www.industrialvision.co.uk
automationmagazine.co.uk
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