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
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3. CAMERA:
Laser Profilers Share Some Basic Principles
There are two methods that laser profilers use to make measurements, a
laser circle and a laser range finder, both of which use the grid or pixels
of a video image to base their measurements off of.
To understand how laser profilers work we will use common graph paper
that has hundreds of squares on it and compare it to a CCD chip which
contains thousands of squares, (pixels).
(CCD "Charged Coupled Device." A CCD is a sensor used in digital cameras to record
still images).
Sheet of graph paper with
hundreds of squares.
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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 illuminated
with 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 to
recreate pipe profile.
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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 lasers
from 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 determine
Camera image of pipe wall with two
lasers 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 to
recreate pipe profile. 4
6. COMPUTERS AND ANALYSIS SOFTWARE:
Image Correction & Calibration
Image Correction: Using the pixels of a CCD as a tool to measure pipeline dimension
requires image correction due to lens flaws and the “fish eye effect”, also called barrel
distortion.
Laser Image Image Image recorded with Image on
circle focused captured distortion due to lens Corrected image screen as
on through On fish eye effect / barrel after Tsai matrix of
pipe lens CCD chip distortion calibration pixels
wall
Calibration: The pixel value is acquired through calibration of the laser profiler. Similar to
knowing the dimensions of each square on the graph paper, the analysis software needs
to know what value each pixel on the CCD chip represents in relation to a physical
measurement.
Physical measurements are translated in to Software is taught what physical measurement
Measurement values by pixels. equals what pixel measurement and this relation
is used by the software to calculate the pipe
dimensions.
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7. Some Factors Affecting Laser Profiler Accuracy
Factors 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 accurate
measurements that aid in quality control and pipeline assessments.
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