Understanding 'Transverse Sensitivity'
The Transverse Sensitivity of a sensor refers to any output caused by motion,
which is not in the same axis that the sensor is designed to measure.
This phenomenon will be discussed with regard to both strain gauges and
accelerometers, but there are other types of sensors to which it could also
apply. In the case of a strain gauge this effect would be measured
perpendicular to the principal gauge axis, but for an accelerometer, a
cross axis effect can occur at any point around the sensor.
STRAIN GAUGE
The level of transverse sensitivity of a strain gauge is usually related to the
grid geometry and in particular its size, especially the length/width ratio.
A larger, longer gauge will generally have a much lower transverse sensitivity,
than a short narrow one. This is usually due to the ends of the grids where
the gauge material has to turn around the apex of the grid. Many foil gauges
have larger blocked ends to reduce this effect. The gauges are tested on a
uniaxial test piece and the output of a reference gauge fitted in the direction
of the principal strain is compared to the output of a gauge fitted at 90° to this.
The transverse sensitivity is expressed as a percentage such that:-
Transverse Sensitivity = (Test gauge strain / Reference gauge strain) x 100 (%)
The value of this should accompany any pack of strain gauges so that
measurements can be adjusted accordingly for greatest accuracy. Typical
values vary from 0.0% to 1.0% or more.
ACCELEROMETER
Vibration sensors are designed to measure vibration in one principal axis,
usually perpendicular to the fixing base. In the case of a triaxial accelerometer,
two of the sensing elements could be measuring in the same plane as the
mounting base so any high cross axis output sensitivity can effect results quite
significantly if the principal motion is at right angles to the base. Most
manufacturers will specify a nominal percentage figure of about 5% for
their sensors, but usually this figure will be measured lower than this and
should be quoted with any calibration. Every sensor should be individually
checked since a transverse effect is usually caused by misalignment of
parts during manufacture or by the nature of the optical axis of the crystals.
One way to test a sensor is to mount it at right angles to the axis of vibration
on top of a reference sensor.
There are other methods of testing but the transverse effect can vary
dependent on the direction of side vibration, consequently it is important
that the sensor is rotated during testing and a record made of the maximum
output. Sometimes the average of the maximum and minimum values
are recorded and an average taken for the calculation of transverse
sensitivity. Just like the strain gauge, this is expressed as a percentage
such that:-
Transverse Sensitivity = (Max. or Average output/Transducer sensitivity) x 100 (%)
For best accuracy it should be possible to obtain a sensor which is calibrated
and marked on the sensor at the point of maximum transverse sensitivity, or
a polar plot of the output could be supplied referenced to that point.
Next time you purchase a strain gauge or an accelerometer check that it has
been calibrated for transverse sensitivity.Information kindly provided by...
Ian Ramage - Techni Measure
Email: sales@techni-measure.co.uk
Website: www.techni-measure.co.uk
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