Page 82 - Instrumentation Reference Book 3E

Practice of length measurement for industrial use 67

the incoming radiation is not critical. Usually one photographic image, in a plate known as a hoio-
corner cube is held fixed to provide a reference gram, that captures the three-dimensional geo-
arm. The other corner cube must be translated metric detail of an object. The object of interest
from a datum position to give length readings, for is flooded with coherent radiation, of which some
the method is inherently incremental. Allowing reflects from the surfaces to be optically com-
both corner cubes to move enables the system to bined with reference radiation taken directly from
be configured to also measure small angles to the source. As the two are coherent radiations
extreme precision. their wavefronts combine to form a flat two-
The laser interferometer is; without doubt, the dimensional interference pattern. The hologram
superior length-measuring instrument for general bears little pictorial resemblance to the original
purpose industrial work but its cost, line-of-sight object and has a most unexpected property.
beam movement restriction, incremental nature, When the hologram is illuminated by coherent
and need for path condition control (for the best light the object can be seen, by looking through
work) does eliminate it from universal use. Its the illuminated hologram, as an apparent three-
dynamic measuring range covers from micro- dimensional object.
meters to tens of meters, a range not provided This basic procedure can be used in several
by any o'ther electronic length sensor. Recent forms to provide highly discriminating measure-
improvements in laser system design have ments of the shape of objects.
resulted ii.1 practical measurement systems even A first method places a similar object to that
for industrial processes like level in tanks and for which a hologram has been made in the image
open channels. space of that of the hologram. This: in effect,
Interferometry requires a reflector that gives superimposes the standard object over the real
adequate energy return without wavefront distor- object. Differences between the two can then be
tion. At optical wavelengths the reflecting surface decided by eye. This is not a very precise method
mst be of optical quality. Where very fine, but does suit some inspection needs.
micrometer discrimination is not required the In another method for using holography a sec-
use of microwave radiation allows interferometry ond hologram is formed on the same plate as the
systems that can operate directly on to the first was exposed on. The combined pair is devel-
normal machined surface of components being oped as a single plate. When viewed, as explained
machined. Acoustic methods can yield satisfact- above, this will reproduce an apparent object on
ory results when the accuracy needed is only of which are superimposed fringes that represent
the order of 1 part in 1000. shape differences between the two units. Each
Radiation methods can also make use of the fringe width, as a guide, represents detailed dif-
time of flight of the radiation. For light this is ferences of the order of the wavelength of the
around 300 mm in a nanosecond and for acoustic radiation. This form of holography is, therefore,
vibration from 300mm to 6m in a millisecond, a very powerful method for detecting small dif-
depending upon the medium. In "time-of-flight" ferences. It has been used, for example, to detect
methods the radiation, which can here be inco- imperfections in car tires (by slightly altering the
herent, is modulated at a convenient frequency or internal pressure) and to investigate shape
simply pulsed. The radiation returning from the changes in gas cylinders. It is very suitable for
surface of interest is detected and the lapsed time non-destructive testing but is expensive and
to go andl return is used to calculate the distance somewhat slow in its use.
between the source and the target. These methods Fast-moving objects can also be gauged using
do not have the same discrimination potential optical holography in the so-called time-lapse
that is ofrered by interferometry but can provide pulse holography method. Two holograms are
in certain applications (for EDM systems used exposed on top of each other on an undeveloped
over kilometer ranges) uncertainty of the order plate as mentioned above but in this situation
of a few parts in loG. By the use of more than one they are formed by the same object which
modulation frequency it is possible to provide presents itself periodically at known times, for
absolute ranging by this method, a feature that example a turbine blade rotating inside an
is clearly required for long distance measure- aircraft engine. The laser source is pulsed as the
ments. The need for a controlled movement path object passes using synchronized electronic circuitry.
over which the reflector must traverse the whole Holography is suitable for use with any form of
distance, as is required in incremental interfero- coherent radiation; optical, microwave, and
meters, is unworkabie in surveying operations. acoustic systems have been reported. It is also
The interference concept shown in Figure 3.18 possible to mix the radiations used at various
for one-dimensional movement can be extended stages in order to produce, and view, the holo-
to three-dimensional measurement application in gram with different absolute size scales. For
the holograph method. Holography provides a example, a seafloor sand-profile mapping system,