Page 68 - Instrumentation Reference Book 3E

easurement of length

P. H. SYDENHAM

3.1 introduction an excellent analysis of transducers, including
those used for length measurement.
Length is probably the most measured physical
parameter. This parameter is known under many
alternative names-displacement, movement, 3.2 The nature of length
motion.
Length is often the intermediate stage of sys- Efficient and faithful measurement requires an
tems used to measure other parameters. For understanding of the nature of the parameter
example, a common method of measuring fluid and of the pitfalls that can arise for that particu-
pressure is io use the force of the pressure to lar physical system domain.
elongate a metal element, a length sensor then Length, as a measured parameter. is generally
being used to give an electrical output related to so self-evident that very little is ever written about
pressure. it at the philosophical level. Measurement of
Older methods were largely mechanical, giving length is apparently simple to conceptualize and
readout suited to an observer’s eyes. The possibil- it appears easy to devise methods for converting
ity of using electrical and radiation techniques to the measured value into an appropriate signal.
give electronic outputs is now much wider. Pneu- Space can be described in terms of three length
matic techniques are also quite widely used, and parameters. Three coordinate numbers describe
these are discussed in Part 4. the position of a point in space regardless of the
Length can now be measured through over kind of coordinate framework used to define that
thirty decadic orders. Figure 3.1 is a chart of point’s coordinates. The number of coordinates
some common methods and their ranges of use. can be reduced if the measurement required is in
In most cases only two to three decades can be two dimensions. Measuring position along a
covered with a specific geometrical scaling of a defined straight line only requires one length-sen-
sensor’s configuration. sing system channel; to plot position in a defined
This chapter introduces the reader to the com- plane requires two sensors.
monly used methods that are used in the micro- Length measurements fall into two kinds. those
meter to subkilometer range. requiring determination of the absolute value in
For further reading, it may be noted that most terms of the defined international standard and
instrumentation books contain one chapter, or those that determine a change in length of a
more, on length measurement of the modern gauge length interval (relative length). For rela-
forms, examples being Mansfield (1973), Norton tive length there is no need to determine the gauge
(1969), Oliver (1971), and Sydenham (1983, 1984). interval length to high accuracy. Measuring the
Mechanical methodology is more generally length of a structure in absolute terms is a differ-
reported in the earlier literature on the subjects of ent kind of problem from measuring strains
mechanical measurements, tool-room gauging, and induced in the structure.
optical tooling. Some such books are Batson and Descriptive terminology is needed to simplify
Hyde (193L), Hume (1970), Kissam (1962), Rolt general description of the measuring range of a
(P929), and Sharp (1970). In this aspect of length length sensor. Classification into microdisplace-
measurement the value of the older books should ment, industrial. surveying, navigation. and ceies-
not be overlooked for they provide basic under- tial is included in Figure 3. I.
standing of a technique that is still relevant today in The actual range of a length sensor is not
proper application of modern electronic methods. necessarily that of the size of the task. For
For the microdisplacement range see Garratt example, to measure strain over a long test
(1979), Sydenham (1969; 1972), and Woltring interval may make use of a long-range, fixed-length,
(1975); for larger ranges see Sydenham (1968) standard structure which is compared with the
and Sydenham (1971). Neubert (1975) has written object of interest using a short-range sensor to