In the world of dimensional measurement, electronic gages make up a class of
instruments that are capable of detecting extremely small dimensional variations
on a surface element. The gage's electronic transducer can operate in a number
of different ways, typically LVDT (Linear Variable Differential Transducer) or
through a digital scale-based technology. The resulting signal can be amplified
or counted and is then available for display on an electronic readout or it can
be fed directly into a computer-based software system.Most of these
transducers are used in short-range comparative gages and can have sub-micron or
millionth of an inch resolutions. Besides having extremely fine resolutions,
fast response and the ability to collect data, another benefit of electronic
gaging is that transducer signals can be combined in mathematical formulas to
display dimensional or geometric conditions. The most common example is a
differential measurement where two transducers combine to produce a reading of
part size without regard to part position.
When parts are measured under ideal conditions—for example, when a part with
perfectly flat and parallel dimensions is placed on the reference table of a
gage that is extremely flat and perfectly perpendicular to the part—then results
should be pretty good without the need for differential measurement. However,
since neither we, our parts, nor our gages live in a perfect world, differential
gaging can be a decided benefit to the application.
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| Tracking error is a potential source of
error that must be accounted for when trying to make very precise
differential measurements using these fine resolution transducers. |
This type of signal combination is typically used on simple bench stands or
on complex fixture gages having multiple checks, where the part position cannot
be well controlled. In this case, any out-of-position is seen by both heads.
Since the heads are combined differentially, this shift is canceled out and only
the change of size is displayed. However, there is a catch.
There is a potential source of error that must be accounted for when trying
to make very precise measurements using these fine resolution transducers. Each
transducer has its own performance characteristics. These are repeatability,
linearity and calibration accuracy. Knowing the performance characteristics of
the transducers is important, but what you need to remember is that every
additional transducer adds another source of error to the measurement.
These errors may or may not be additive, but when they are, they can produce
significant errors in extremely demanding measurements. This “new” error is
called tracking error. Normally two heads are added differentially to produce a
diameter measurement. However, because of the manufacturing process, a two-point
measurement will not always suffice, and three, four or even six transducers are
combined to produce an average diameter. This can result in tracking error.
So how do you track tracking error? It requires a way to hold the
transducers, a display, and a means of displacing all the gage heads equally.
Let’s start with a basic two-headed differential check. After each transducer is
calibrated with the amplifier, place them in a gage calibrator or bench stand
where gage blocks can be used to precisely move each transducer by the same
amount. If you are using gage blocks, select a set that will allow you to record
at least three equally spaced points on each side of the transducer’s zero.
Mechanically position both transducers at electrical zero on the zero gage
block. Then switch the heads to the amplifier’s differential mode and note the
result in the display. Now replace the zero block with the series of gage blocks
and record the values (or if using the calibrator, move it to displace the
transducers at a number of points in their travel). The results should provide a
linear trace, but there will likely be more error than when each head is tracked
individually. This check should be performed with all the heads that would be
used in the mathematical calculation to determine the worst case error of the
system. In extreme cases it may be necessary to calibrate on the total system
response, but mostly you are apt to see non-linear results over different areas
of the signal’s range.
While it may not always be possible to correct for these errors over the
short range, knowing where they are may help you optimize the position of the
transducers, or to look for a different combination of transducers that provide
the best results.
Article courtesy of MMS Online.
