Bonded
Strain Gauges
Discrete metal or silicon strain gauges
are usually bonded (glued) to the surface
where strain is to be measured, and provide
an output proportional to the average strain
in their active area. The typical gauge
factor is around 2; a strain of 1 µin./in.
would produce a resistance change of 2 µ
/ . Unstrained resistance ranges from 120
to several hundred ohms. Because a significant
length of wire or foil is necessary to provide
high unstrained resistance, metal strain
gauges cannot be made extremely small.
Unbonded Strain Gauges
Unbonded strain gauge transducers use relatively
long strands of strain gauge wire stretched
around posts attached to a linkage mechanism.
The linkage is designed such that when pressure
increases, half of the wire is farther stretched
and the other half is less so. The primary
advantage of unbonded over bonded is a higher
gauge factor, on the order of 3. Because
no adhesives are required, they can also
be designed and fabricated for use at higher
temperatures. Unbonded strain gauge transducers
tend to be large.
Sputtered Strain Gauges
Strain gauge material may be sputtered onto
a nonconductive diaphragm to create the
strain gauges . Location and orientation
are controlled by masking, and the molecular
bond created by the sputtering process eliminates
any problems with adhesive bonding. Gauge
factors are similar to those of unbonded
gauges. Surface preparation and other process
controls are quite critical. The fabrication
process offers some of the advantages of
a silicon diaphragm, such as good linearity
and high natural frequency, as well as the
good temperature characteristics of metal
gauges.
Semiconductor Strain Gauges
These devices are made of semiconducting
silicon. Their gauge factor is dependent
on the doping level—more lightly doped,
higher resistivity material has a higher
gauge factor. However, it also has greater
thermal sensitivity, causing both resistance
and gauge factor to change significantly
with temperature. Discrete silicon strain
gauges are used just as are metal gauges,
glued to the strained surface in the desired
orientation to provide maximum sensitivity
for pressure measurement. In addition to
their higher gauge factor (which provides
higher sensitivity), they are also smaller,
allowing more miniaturization.
Bonded Discrete Silicon Strain Gauges
Early silicon strain gauge transducers used
discrete silicon strain gauges bonded with
adhesives to the surface of a strained element.
These devices were similar to bonded metal
strain gauges, except that the silicon types
provided much higher output and had greater
temperature errors. Furthermore, the silicon
gauges were smaller than metal gauges, so
the sensors could be made smaller.
Diffused Diaphragm Sensors
Discrete strain gauges, metal or or silicon,
require tedious microassembly for installation,
but diffused diaphragm sensors can be fabricated
using semiconductor masking and processing
techniques. This approach provides precision
location and orientation of the gauges for
optimum linearity and sensitivity, allows
extreme miniaturization, and reduces assembly
costs. It also removes the variability of
the adhesive and its application.
Sculptured-Diaphragm Sensors
Early diffused silicon diaphragm pressure
transducers used a simple, flat silicon
diaphragm of uniform thickness. Silicon
microfabrication techniques (MEMS) allow
great flexibility in the mechanical design
of the diaphragm.
Anisotropic etching provides precise control
of etching directions in the silicon crystal.
Extremely small yet complex shapes can be
fabricated, permitting the diaphragms to
be shaped for optimum combinations of linearity,
sensitivity, and frequency response characteristics.
Capacitive pressure sensor
In this capacitive pressure sensor two
metal plates are separated by a dielectric
material. This forms a capacitor. This component
is connected in parallel with inductor.
This LC circuit determines the oscillator
frequency. If object strikes the sensor,
the plate spacing momentarily decreases.
This causes increase in capacitance but
drop in oscillator frequency. When object
move away from the transducer the foam spring
back and the plate back to the original
spacing. The oscillator frequency returns
to normal. If the dielectric material is
maintained constant, this mechanism provides
a very repeatable transducer. The primary
advantages are low hysteresis; good linearity,
stability, and repeatability; static pressure
measurement capability; and a quasi-digital
output.
Piezoelectric
Transducers
Piezoelectric (PE) pressure transducers
use stacks of piezoelectric crystal or ceramic
elements to convert the motion of the force-summing
device to an electrical output. Quartz,
tourmaline, and several other naturally
occurring crystals generate an electrical
charge when strained. Specially formulated
ceramics can be artificially polarized to
be piezoelectric, and they have higher sensitivities
than natural crystals. Unlike strain gauge
transducers, PE devices require no external
excitation. Because their output is very
high impedance and their signal levels low,
they require special signal conditioning
such as charge amplifiers and noise-treated
coaxial cable.
Because the PE transducers are self-generating,
dependent on changes of strain to generate
electrical charge, they are not usable with
DC or steady-state conditions. They have
an inherent low-frequency rolloff that is
dependent on the signal conditioning's low-frequency
time constant.
Their primary advantage is their ruggedness,
and, without integral electronics, their
usefulness at high temperatures. If not
properly compensated, though, they are sensitive
to shock and vibration and may exhibit large
changes of sensitivity with temperature
variations.
Elastomer pressure sensor
Elastomer conducts electricity very well.
It has foam like consistency so that it
can be compressed. Conductive plates are
attached to the plates. When pressure appears
at some point in elastomer pad than material
compressed and this lowers its electrical
resistance.
This is detected as in increase in current
between plates. The greater the pressure
becomes, the more the elastomer compressed
and current increases more.
|