MANAGING TECHNOLOGY
Made to measure
New and refined technologies are helping customers carry out more
accurate measurements of often difficult surfaces, says Chris Jones
THE USE of non-contact displacement
technologies in the field of precision
measurement is growing rapidly. The
main reasons for this are that
customers need to measure much
more accurately— to sub-micron or
even nanometer resolutions — and to
measure against difficult surfaces or
surfaces that cannot be touched
during the measurement process.
These include silicon, glass, plastics,
miniature electronic components,
medical components and even
food-based surfaces.
This rapid growth has pushed the
development of new technologies
and the adaptation of existing
technologies to meet the new
measurement requirements and
improve measurement accuracies and
resolutions.
So it is more important than ever
to understand the strengths and
limitations of each non-contact
measurement principle when selecting the
correct sensor technology for the task.
In practice, besides eddy current and laser
triangulation sensors, capacitive and confocal
sensors are proving popular with customers.
But non-contact displacement sensors come in
a variety of shapes, sizes and measurement
principles.
The key is selecting the most appropriate
sensing technology for the customer’s
application.
The eddy current measurement principle is
an inductive measuring method based on the
extraction of energy from an oscillating
circuit. This energy is required for the
induction of eddy currents in electrically
conductive materials.
A coil is supplied with an alternating
current, which causes a magnetic field
to form around it. If an electrically conducting
object is placed in this magnetic field, eddy
currents are induced, which form an
electromagnetic field. This acts against the
field of the coil, which also causes a
change in the impedance of the coil. The
controller calculates the impedance by
considering the change in amplitude and
phase position of the sensor coil.
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The advantage of the eddy current is that it
can be used on all electrically conductive,
ferromagnetic and non-ferromagnetic
metals. The size of the sensor is relatively
small compared with other technologies
and the temperature range is high due
to the resistance measurement of the sensor
and cable.
The highly accurate technology is
immune to dirt, dust, humidity, oil, high
pressures and dielectric materials in the
measuring gap.
Yet there are restrictions. The output/input
signal ratio depends on the electric and
magnetic features of the target material.
Therefore, individual calibration is necessary.
The maximum length of the cable is 15m and
the diameter of the sensor increases as the
measuring range increases.
With the capacitive principle, the sensor
and target operate like a parallel plate
capacitor. The two plate electrodes are formed
by the sensor and the opposing target.
If an AC current with constant
frequency flows through the sensor
capacitor, the amplitude of the AC voltage
on the sensor will equal the distance
between the capacitor electrodes. An
adjustable compensating voltage is
simultaneously generated by the
sensor system’s amplifier. After
demodulation of both AC voltages,
the difference is amplified and
output as an analogue signal.
Capacitive sensors are able to
achieve equal input/output signal
ratios and they have ideal sensitivity
to metals. They also offer high
temperature stability, as changes in
the conductivity of the target have no
effect on the measurement. And they
can measure insulators.
However, they are only ideal in
clean, dry applications and, like eddy
current technology, they only have a
relatively short cable length.
In the laser triangulation
principle, laser diode projects a
visible point of light onto the surface
of the object being measured. The
back scattered light reflected from
this point is then projected on to a
CCD array by a high-quality optical lens
system. If the target changes position with
respect to the sensor, the movement of the
reflected light is projected on the CCD array
and analysed to output the exact position of
the target.
This unique measuring principle enables
displacements and distances to be measured
very precisely. It can even measure on diffuse
and spectral surfaces.
Confocal technology offers nanometre
resolution and operates almost independently
of the target material. It can provide one-sided
thickness measurement of transparent
materials. This sort of sensing system can be
offered in miniature radial and axial versions
for measuring drilled or bored holes. Instead
of a laser, it uses white light.
Yet the technology is limited because the
sensor must be close to the target and the
beam requires a clean environment.
The right sensor manufacturer will help
customers choose the correct sensor for their
application. In some cases hybrid technologies
may be appropriate.
Chris Jones is managing director of precision
sensor manufacturer Micro-Epsilon (UK)
the EnGIneeR 29 SEPTEMBER–12 OCTOBER 2008
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