K-15KDetecting/measuring edges in the XY planeGray scale2551270(1)(2)(3)Tool positionTool244241220193977667525353243242220195947366545355244246220195947564565150Vision MeasurementVision measuring machines mainly provide the following processing capabilities.• Edge detection• Auto focusingFocusing and Z-axis measurement• Pattern recognitionAlignment, positioning, and inspecting a featureAn image is comprised of a regular array of pixels. This is just like a picture on ne plotting paper with each square solid-lled differently.Gray ScaleA PC stores an image after internally converting it to numeric values. A numeric value is assigned to each pixel of an image. Image quality varies depending on how many levels of gray scale are dened by the numeric values. The PC provides two types of gray scale: two-level and multi-level. The pixels in an image are usually displayed as 256-level gray scale.Pixels in an image brighter than a given level are displayed as white and all other pixels are displayed as black.Each pixel is displayed as one of 256 levels between black and white. This allows high-delity images to be displayed.Dimensional MeasurementAn image consists of pixels. If the number of pixels in a section to be measured is counted and is multiplied by the size of a pixel, then the section can be converted to a numeric value in length. For example, assume that the total number of pixels in the lateral size of a square workpiece is 300 pixels as shown in the gure below. If a pixel size is 10 μm under imaging magnication, the total length of the workpiece is given by 10 μm×300 pixels=3000 μm=3 mm.Edge DetectionHow to actually detect a workpiece edge in an image is described using the following monochrome picture as an example. Edge detection is performed within a given domain. A symbol which visually denes this domain is referred to as a tool. Multiple tools are provided to suit various workpiece geometries or measurement data.The edge detection system scans within the tool area as shown in the gure at left and detects the boundary between light and shade.Example of numeric values assigned to pixels on the tool(1) Scan start position(2) Edge detection position(3) Scan end position10 25512702-levelgray scale WhiteGrayBlackMulti-levelgray scale WhiteGrayBlack300 pixels10 µm480 pixelsCCD Camera lensVideo signalAmplierHigh-speed A/D converterFrame grabberDisplay screenPC640 pixelsImage StorageVision Measuring MachinesQuick Guide to Precision Measuring InstrumentsHigh-resolution MeasurementTo increase the accuracy in edge detection, sub-pixel image processing is used.An edge is detected by determining an interpolation curve from adjacent pixel data as shown below.As a result, it allows measurement with a resolution better than 1 pixel.Gray scaleTool positionGray scaleTool positionWhen enlarged...A position the system recognizes as an edge may be in error by up to one pixel width using normal image processing. This will prevent the execution of high-resolution measurement.Gray scaleGray scaleTool positionTool positionImage signal without sub-pixel processingImage signal with sub-pixel processingThe image signal prole approaches an analog waveform like this.
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