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How laser profilers works

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How laser profilers works

1. 1. How Laser Profilers Work C-Tec Oliver Conow Sept.08.2010
2. 2. Three Main Components of Laser Profilers Camera: To record laser image using CCD chip CCD chip (Charged Coupled Device) is a sensor used in digital cameras to Lens record images. Laser: To illuminate pipe surface Computers and Analysis Software: To calculate and process data 1
3. 3. CAMERA: Laser Profilers Share Some Basic PrinciplesThere are two methods that laser profilers use to make measurements, alaser circle and a laser range finder, both of which use the grid or pixelsof a video image to base their measurements off of.To understand how laser profilers work we will use common graph paperthat has hundreds of squares on it and compare it to a CCD chip whichcontains thousands of squares, (pixels).(CCD "Charged Coupled Device." A CCD is a sensor used in digital cameras to recordstill images). Sheet of graph paper with hundreds of squares. 2
4. 4. LASER : Laser Circle Method  If you know the dimensions of each square on a graph paper, you can mathematically figure out dimensions of a drawn circle. Graph paper with drawing of circle.  Instead of using graph paper, laser profiler technology uses the pixels in an image.  Instead of drawing with a pencil, the outline of a pipeline is traced with a laser plan.  Software finds the laser circle in the image and makes complex measurements to determine the dimensions of the image.Camera image of pipe wall illuminatedwith a laser circle  A laser profiler moves along a pipeline and multiple measured circle dimensions are used by sophisticated software to recreate a pipeline profile.Image of measured points used torecreate pipe profile. 3
5. 5. LASER: Geometric Range Finder Method  Two lines drawn on graph paper from fixed positions, and which eventually intersect, would create a triangle.  From the perspective of the edge of your paper, you would see two dots. Depending on how far away the lines are from the edge, the dots would be farther or closer together.  If you knew the dimensions of each square on the graph paper, you can mathematically figure out the distance to any point along the laser lines.Graph paper with drawing of lasersfrom a range finder.  Instead of using graph paper, laser profiler technology uses the pixels in an image.  Two lasers are projected at the pipeline wall from a known reference point, which results in two dots on the image.  Software finds the two dots on the image and calculates radial measurements.  By rotating the rangefinder and making measurements in a circular motion, software is able to determineCamera image of pipe wall with twolasers pointed at it. pipeline dimensions.  A laser profiler moves along a pipeline and radial measurements are used by sophisticated software to recreate a pipeline profile.Image of measured points used torecreate pipe profile. 4
6. 6. COMPUTERS AND ANALYSIS SOFTWARE: Image Correction & CalibrationImage Correction: Using the pixels of a CCD as a tool to measure pipeline dimensionrequires image correction due to lens flaws and the “fish eye effect”, also called barreldistortion.Laser Image Image Image recorded with Image oncircle focused captured distortion due to lens Corrected image screen ason through On fish eye effect / barrel after Tsai matrix ofpipe lens CCD chip distortion calibration pixelswallCalibration: The pixel value is acquired through calibration of the laser profiler. Similar toknowing the dimensions of each square on the graph paper, the analysis software needsto know what value each pixel on the CCD chip represents in relation to a physicalmeasurement.Physical measurements are translated in to Software is taught what physical measurementMeasurement values by pixels. equals what pixel measurement and this relation is used by the software to calculate the pipe dimensions. 5
7. 7. Some Factors Affecting Laser Profiler AccuracyFactors which affect a laser profiler’s accuracy include:1. POOR IMAGE: Anything that distorts the laser image such as fog, water, reflection, outside light sources or dirt.2. SPEED: The speed at which the profiler travels will have an effect on the scan resolution. The faster it travels, the lower the resolution will be.2. BEING CENTERED: To make radial measurements and not have image distortion due to perspective within a pipeline, laser profilers need to be centered in the pipeline.3. SIZE OF LASER CIRCLE: A laser circle filling a minimum of 75 % of the screen will ensure a pixel ratio that allows for accurate measurements.4. SOFTWARE: Correct parameters, calibrations and filters must be set to process the data. Non proprietary software will also affect accuracy.5. MEANDERING: Meander between the camera recording the laser circle will result in false measurements and reduced accuracy.6. LASER CIRCLE THICKNESS: A thick laser circle will allow a wide range of possible measurements reducing accuracy. It is like marking precise measurements with a paint brush.7. CALIBRATION: Units out of calibration or poorly calibrated may produce inaccurate results.When used properly, laser profiler can provide invaluable accuratemeasurements that aid in quality control and pipeline assessments. 6