Page 73 - Instrumentation Reference Book 3E

58 Measurement of length
to be faced is that good calibration requires con- the use of electronic sensing can be more product-
siderable time: the standard under calibration ive than the use of manually read micrometers and
must be observed for a time in order to ensure scales because of the reduced cost of the electronic
that it does have the long-term stability needed to part of the sensing system and the need for more
hold the calibration. automatic data processing. There can be little doubt
that solely mechanical instruments will gradually
become less attractive in many uses.
3.5 Practice of length
measurement for industrial use
3.5.2 Mechanical length-rneasuring equipment
3.5.1 General remarks Measurement of length from a micrometer to
A large proportion of industrial range meas- fractional meters can be performed inexpensively
urements can be performed quite adequately using an appropriate mechanical instrument.
using simple mechanical gauging and measuring These group into the familiar range of internal
instruments. If, however, the requirement is for and external micrometers, sliding-jaw calipers,
automatic measurement such as is needed in and dial gauges. Accuracy obtained with
automatic inspection, or in closed-loop control, these depends much upon the quality of manu-
then the manual methods must be replaced by facture.
transducer forms of length sensor. Modern improvements, which have been incor-
In many applications the speed of response porated into the more expensive units, include
needed is far greater than the traditional mechan- addition of electronic transduction to give direct
ical methods can yield. Numerically controlled digital readout of the value, making them easier
mills, for instance, could not function without to read and suitable for automatic recording.
the use of electronic sensors that transduce the Another improvement, for the larger throat
various axial dimensions into control signals. micrometers, has been the use of carbon fiber for
Initially, that is, in the 1950s, the cost of elec- throat construction. This has enabled the frame to
tronic sensors greatly exceeded that of the traditional be lighter for a given size allowing larger units and
mechanical measuring tools and their servicing increased precision.
required a new breed of technician. Most of these For the very best accuracy and precision
earlier shortcomings are now removed and today work, use is made of measuring machines incor-

Figure 3.7 Automatically-read length-measuring machine incorporating ruled scales. Courtesy, SIP Society Genevoise
d’ Instruments de Physique.