A contactless detection system can be done as follows. Illuminate the area around the thread with a broad spectrum light. Then put a background behind the thread that is a contrasting color to that of the the thread. (IE. If the thread is white use a black background. If the thread is black use a white background). Now affix a lens or set of lens which can magnify the thread near the 5mm hole. Next you get a low cost video camera to look at the thread through the lens. You should be able to do this with any of the current variety of low cost "web cams". The secret will be in the design of the magnification sustem so that the image of the thread looks large with respect to the resolution of the image sensor in the camera. The thread image can be fuzzy but it should be visable across say as many as 10 or more pixels. It can be a good idea to orient the camera image sensor so that the thread image is "vertical" or "horizontal" to the thread direction.

The next step is to get a DSP with image capture memory (or a PC frame grabber with an SBC type PC). The DSP (the lower cost solution these days) or the SBC PC (Single Board Computer PC's have more than enough horsepower now) can do a very simple image analysis on the captured frames to determine if the thread is within the proper place. When doing the image analysis it is not necessary to evaluate the complete frame of video memory data either. You can peer into the memory and look for short rows of pixels that are at right angles to the direction of the thread. IE, see the picture below where the image scan areas are defined in the red color.



The system is relatively simple to program and one of the bit more difficult things I have encountered when evaluatining images such as this is defining the edge of the thread in the image capture zone. If you use a monochrome camera this job gets a lot easier but is still possible in color is you consider the luminosity of the pixels instead of the absolute RGB values of the pixels.

Michael Karas