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Digimatic Calipers
A
refinement now popular is the replacement of the analog dial with
an electronic digital display on which the reading is displayed as a
single value. Some digital calipers can
be switched between centimeters or millimeters, and inches. All provide
for zeroing the display at any point along the slide, allowing the same
sort of differential measurements as with the dial caliper. Digital
calipers may contain some sort of "reading hold" feature, allowing the
reading of dimensions even in awkward locations where the display cannot
be seen. Ordinary 6-in/150-mm digital calipers are made of stainless
steel, have a rated accuracy of 0.001 in (0.02 mm) and resolution of
0.0005 in (0.01 mm). The same technology is used to make longer 8-in and
12-in calipers; the accuracy for bigger measurements declines to
0.001 in (0.03 mm) for 100–200 mm and 0.0015 in (0.04 mm) for
200–300 mm.
Many
Chinese-made digital calipers are inexpensive and perform reasonably
well. One point worth noting is battery current when they are turned
off. Many calipers do not stop drawing power when the switch is in the
off position; they shut down the display but continue drawing nearly as
much current. The current may be as much as 20 microamperes, which is
much higher than many established brands. Sometimes calipers may not
work properly when the battery voltage has dropped relatively little;
silver cells, preferably selected from a datasheet to have a constant
voltage for most of their life, may give a much longer usable life than
alkaline button cells (e.g., SR44 instead of LR44).
Increasingly,
digital calipers offer a serial data output to allow them to be
interfaced with a dedicated recorder or a personal computer. The digital
interface significantly decreases the time to make and record a series
of measurements, and it also improves the reliability of the records. A
suitable device to convert the serial data output to common computer
interfaces such as RS-232, Universal Serial Bus, or wireless can be
built or purchased. With such a converter, measurements can be directly
entered into a spreadsheet, a statistical process control program, or
similar software.
The serial digital output varies among manufacturers. Common options are
- Mitutoyo's Digimatic interface. This is the dominant name brand interface. Format is 52-bits arranged as 13 nibbles.
- Mahr (appears to offer Digimatic, RS232, and USB)
- Brown & Sharpe, Fowler, Starrett, Tesa, and SPI.
Like dial calipers, the slide of a digital caliper can usually be locked using a lever or thumb-screw.
Some
digital calipers contain a capacitive linear encoder. A pattern of bars
is etched directly on the printed circuit board in the slider. Under
the scale of the caliper another printed circuit board also contains an
etched pattern of lines. The combination of these printed circuit boards
forms two variable capacitors. The two capacitances are out of phase.
As the slider moves the capacitance changes in a linear fashion and in a
repeating pattern. The circuitry built into the slider counts the bars
as the slider moves and does a linear interpolation based on the
magnitudes of the capacitors to find the precise position of the slider.
Other digital calipers contain an inductive linear encoder, which
allows robust performance in the presence of contamination such as
coolants. Magnetic linear encoders are used in yet other digital
calipers.
Source: Wikipedia
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