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This limitation exists because all digital calipers have capacitance
position sensors - conductors arrayed on the scale and on
the slider form a row of capacitors bridging the gap between
slider and scale. The measured capacities yield the displacement.
If correctly designed, capacitive sensors are immune to external
electrical and magnetic fields, and are easy to protect against
dust and metal chips - a set of wipers will usually do. But
if a drop of water, oil or coolant seeps in and bridges the
gap between the scale and the slider, it will distort the
sensor’s output by locally magnifying capacitance - up
to 80 times its value in the case of water or water-based
coolants.
Because of this,
a digital caliper should be used carefully in a machine shop
environment: a splash of coolant or even sweaty palms may
intermittently alter or interrupt its function. Another type
of sensor was needed to make calipers compatible with wet
environments - an inductive sensor. It consists of flat windings
on the slider and an array of copper areas on the scale. Very
short pulses (20 nanoseconds) applied to any one of the windings
induce eddy currents in the copper areas facing them, which
in turn inducts voltages in the other windings. Sampling these
voltages and repeating the process generates a waveform ultimately
yielding the position. Even though the pulse current rises
to about 200 milliamps, the caliper’s average current
consumption is kept below 50 microamps by the low pulse repetition
rate.
Coupling is now
done by magnetic fields - liquids do not distort magnetic
fields, their presence between scale and slider goes unnoticed.
Because of their low impedance, inductive sensors are not
sensitive to electrical fields. And they do not react to magnetic
fields either, but only to their sudden changes. ‘Sudden’
means less than a tenth of a microsecond, so there is no need
to worry if the caliper gets accidentally clamped on a magnetic
chuck. The inductive sensor not only matches its capacitive
predecessor in dry environments, but also keeps working in
wet ones. The whole digital caliper was designed to be water-resistant
to IP54 as defined in the IEC529 standard. The IP54 code’s
first digit ‘5’ means dust-protected. The second
digit ‘4’ means that the caliper is protected against
splashing water - it has to withstand a shower from every
direction for 10 minutes. This is more than what’s likely
to happen to the caliper in a machine shop.
© Fred V. Fowler Co., Newton, MA.
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