Sunday, November 14, 2010

Accuracy Testing with Precision Surface Plates

One of the most fundamental 3D metrology tests out there is the plane
test.  It is simple to conduct, revealing, practical, easy to analyze, and
correlates directly to the real world.  Here’s how it works:

1) Get a surface plate that covers as much area as you have room for. 
Granite plates are great.  Metal plates are magnificent.  Ceramic
plates are spectacular.  You get the picture.  There are 100’s of
suppliers out there.  Make sure you get one that is certified.  Grade
B, Grade A, and Grade AA are common terms you might encounter–they refer to how
flat a surface is using a series of standard tests.  You might also
encounter the term “surface finish,” but we’re really concerned with the
“flatness” here.  You’ll see why.

2) Measure the surface plate.  Collect as many points as you can. 
Keep every point.  Don’t throw away any points, or smooth the data. 
You’ll see why.

3) Using measurement software, fit a plane through the raw data.  There
are many software programs that can do this, but make sure you use every
point.  If you collected points on a corner or edge of the surface plate,
delete or ignore them because we’re only interested in flatness for the purpose
of this study.

4) Look for the following values on the plane fit: 1-sigma value, RMS value,
max deviation, min deviation.  The 1-sigma and RMS values are generally
similar for near-Gaussian error distributions.  The max and min deviations
represent a total bandwidth.  Most measurement systems are specified using
1-sigma, 2-sigma, or (max-min)/2 as their reference.  To get the 2-sigma
value, double the 1-sigma value.  Do these numbers 1) match what the
supplier is quoting for their accuracy? 2) meet the specifications for your
application?  Remember that the measurement tolerance has to be much
smaller than the manufacturing tolerance.  If your measurement software has
the ability to perform a “color map” relative to the plane, this “color map” is
useful for looking at how the errors are distributed throughout the plane. 
The numbers revealed by this test demonstrate how “globally accurate” the
measurement system is.  All metrology suppliers should quote a “global
accuracy” on their systems, and specify whether it is a 1-sigma, 2-sigma, or
(max-min)/2 accuracy.

5) Now, repeat steps 3 and 4, except this time fit a small plane through the
data.  1/10th of the total plane area would be a reasonble
size.  This time, instead of “global accuracy”, these numbers are
showing you the “background noise” of the measurement system.  The
“background noise” might be much smaller than the “global accuracy,” and is
useful for determining how small features such as circles, radii, and other
localized inspections will be influenced by random noise.

6) Consider the results.  Both “global accuracy” and “background noise”
provide useful information.  It is possible for a system to have a high
“global accuracy” number but a small “local accuracy” number.  It is
also possible for a system to have “global accuracy” and “local accuracy”
numbers that are similar.  Consider how each of these numbers affects your
inspection requirements.  Now consider that the reason we wanted to collect
a lot of points, and not smooth or filter the data is because both the local and
global accuracy numbers are important.

7) Consider the surface you just measured.  If it was granite, there
might be internal/local reflections due to the quartz in the granite that might
act as a source of local inaccuracy.  If it was metal, there might have
been specular or direct reflections that influenced the local accuracy.  If
it was ceramic, there might have been light penetration beneath the
surface.  Every material is different.  Do not dismiss this as
trivial!  Find out why these materials responded the way they did, because
they are going to influence your measurements!

8 ) Next, were the lights on or off during the measurement?  What kind
of lighting was present?  This is important for optical systems.  Was
the room free from vibration?  Do different operators conducting the same
test yield the same results?  All of these questions are significant
because they impact the outcome of the results.  Conduct this “plane test”
under varying conditions to determine how the measurement system is influenced
by its environment.

9) Finally, do not stop there!  Make a plane out of the actual material
you plan to measure.  If you are measuring titanium, machine a titanium
plane.  If you are working with carbon fiber, construct a carbon fiber
plane.  It is imperative that you test on your actual material, because
each material can respond differently to optical measurement in
particular.  Further yet, measure these materials in the real-world
environments in which they will be measured.  Look for global and local
accuracy in each case.

10) After this plane test has been conducted, try measuring the same plane,
but moving the measurement equipment.  Try measuring at the far extents of
the measurement system, close in to the measurement system, tilt the plane
relative to the measurement system, and even try it upside down or facing into a
wall, floor, or corner.  All of these results combine to indicate a “true”
system performance.

You will be amazed by what a plane measurement can reveal.  You’ll
quickly be able to determine whether your measurement system is within
specifications, and how much “random noise” there is relative to “global
inaccuracy”.  Testing different materials in different environments will
also start to reveal the limits of the equipment.  Remember, our goal is
not to perform the “perfect measurement.”  It is to better understand our
equipment, and how it can be used within our organization to improve our

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