Evaluation Engineering - 22

AUTOMATED TEST

Alliance Sensor
Group's LZ-19 SS
linear position
sensor

THE 3 R'S OF
ANALOG POSITION
SENSOR-BASED
MECHANICAL MEASUREMENTS
by Edward Herceg
Those of us old enough to remember
the "good old days" recall that grade
school focused on learning the 3 R's: reading, 'riting, and 'rithametic. In the world of
sensors, there are also 3 R's: repeatability,
resolution, and response. As important as
these sensor parameters are, there is often
confusion in the mind of users about exactly what they mean and in what ways
they tend to interact with each other. This
article attempts to explain these 3 R's for
position sensors and to dispel any confusion that exists.

Definitions
Repeatability is a measure of the variation between outputs of a sensor-based
measuring system for repeated trials of an
identical mechanical input in a constant
environment. Common practice is to use
at least three repeated inputs, but five or
more identical inputs are considered to
be an even better sample for determining this parameter. Repeatability is usually evaluated by applying an averaging
process to the variations in output values observed for the multiple trials. It is
typically specified as a percentage of fullscale output or full span output (FSO),

EVALUATION ENGINEERING NOVEMBER 2019

but sometimes it is specified in absolute
terms such as parts per million or fractions of the mechanical units applicable
to the actual sensor-based measurement.
A constraint on repeatability measurement is that the trial inputs have to be
applied in the same way, usually from a
lower value to higher value, to eliminate
any effects from hysteresis. Hysteresis error is a measure of the difference in system output when the mechanical input is
rising up to the desired input value from
a lower value, compared to an identical
input coming down from a higher input
value, to the desired value. For most contactless position sensors, hysteresis error
is smaller than repeatability error.
An example demonstrating repeatability can be seen in Figure 1, which
shows a spring-loaded position sensor in
a typical gaging stand being calibrated
with a precision gage block of 0.5000-inch
dimension. The sensor delivers an output
of 0 to 10 V DC full scale for 0 to 1 inch
of probe movement. The tip of the sensor
is moved inward to allow the gage block
to be inserted between the tip and a flat
base, and then released to contact the
block. In five trials, system outputs are:

5.0012, 5.0016, 5.0013, 5.0010, and 5.0015
V DC. The average value is slightly over
5.0013 V and the maximum variance is
±0.0003 V, which is equal to ±30 ppm of
FSO, or 0.003% of FSO.
Resolution is a measure of the smallest change in the input to a sensor-based
measuring system that will produce a
measurable change in the electrical output from that system. While this may
seem like a fairly simple concept, it is
impacted by factors external to the sensor itself, the most significant of which is
the signal-to-noise ratio of the system's
analog output. Electrical noise present on
the system's output can reduce the effective resolution of the system by masking
any small changes in the sensor's output.
For example, if the sensor's resolution
specification is 0.25 mV, but the system
output noise and ripple is 2 mVp-p, clearly
sensor output changes smaller than 2 mV
will not be discernable within that noise.
Thus, the actual system resolution is only
about 12% of what the sensor resolution
specification offers.
Like repeatability, resolution is often
specified as a percentage of FSO, but may
also be specified in absolute terms, like

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