measurement system, thus enablin g ultra-high
precision measurements (i.e. up to the 10
9
range
of the whole dimensions of the part to be mea-
sured). Due to the property of coherence in the
emitted radiation, the laser is the main technique
applied within high precision applications, not
only within the realm of dimensional control,
but also within a wide range of applications where
the physical characteristics of materials have to be
measured in a direct or indirect manner.
Laser applications on measurement syst ems go
from dimensional analys is to vibration measure-
ment. Some applications in material characteriza-
tion even use the laser as a power source in order
to produce mechanical waves. This is achieved by
impacting the laser beam onto targets and then
measuring the ultrasound reflections (LUS: laser
ultra sound inspe ction).
The laser is also used within optical applica-
tions in order to inspect transparent or semi-
opaque materials using a coherence light source
(OCT optical coherence tomography). How-
ever, even high resolution image analysis is
increasing its influence within the high precision
measurement domain, especially when in-line
inspection and testing of micro-parts are required.
Laser interferometers, LUS, OCT and image pro-
cessing can easily ensure resolutions of below
1 mm with very simple equipment and, in some
cases, at a very competitive cost.
Recalling these applications, one may also rec-
ognize the wide use of precise positioning systems
suitable to move the probe or the sensor, as shown
in Fig. 22-2. Precision movements are often
achieved with mec hatronic systems based on
piezo-actuators, sub-micrometric brushless
motors, galvo motors, moving coil motors, etc.
Note also that high precision mechanical devices
have to ensure optimal isolation from vibration
and other external disturbances.
Of particular importance, dimensional mea-
surement sensors are especially valuable because
by dimensional measurement it is possible to
retrieve a large amount of information about the
observed system, and not only from a purely
dimensional point of view. For example, laser dis-
placement sensors can report information about
dimensional, dynamical, thermal, and vibrational
behavior. Therefore, it may be useful to briefly
detail a number of such sensor s, especial ly of the
non-contact type, which are particularly suited
for micro-testing applications.
Mono- and Multi-dimensional
Measurement
Precise non-contact dimensional measurement
systems are largely based on the laser interferom-
eter technique (i.e. the Michelson interferometer).
Comprising just the interferometer, i.e. being a
system capable of detecting only differences on
beam paths, it is suitable for incremental, or dis-
placement, measurements. Typically, the time
response is very fast (typ. <100 ms) and the mea-
suring accuracy, by the use of interpolator
devices, easily reaches values below 10
8
m. The
laser interferometer system is very suitable for
coordinate measuring machine (CMM) applica-
tions. Very accurate measurements on micro-
parts are possible by placing the part on a posi-
tioning system (one stage: X or mul tistage: X-Y or
X-Y-Z table) provided with reflectors.
Other dimensional measurement techniques are
based on different principles, such as laser beam
triangulation, which use a laser source and a detec-
tor (CCD or PSD as represented by Fig. 22-4). In
this case the system performances depend on the
surface characteristics of the target material. Both
the precision and the distance range are affected by
material typology and surface texture.
Thickness Measurement
There are several techniques which are suitable
for the precise and ultra-precise measurement of
thickness. Inductive measurement, based on dif-
ferential measurement techniques, is a methodol-
ogy often adopted for low cost ap plications. This
method is affected by the type of material being
measured. Higher levels of stability and precision
are achieved by laser techniques which consist of a
laser beam shaped by the target. A sensor , in these
systems, consists of emitter and receiver elements,
with the field of measurement situated between
348 CHAPTER 22 Testing and Diagnosis for Micro-Manufacturing Systems