2.1 PRODUCT SELECTION GUIDE
G
Gallium Orthophosphate GaPO4
The GaPO4 is the patented high
temperature resistant piezoelectric
material developed by AVL. It allows
high signal linearity and temperature
stability like no other material on
the market. The crystal material is
grown and manufactured only in the
headquarters of AVL. Even though
the hydrothermal growing process
of GaPO4 takes several months AVL
produces this piezoelectric material
in high quantities. In numbers this
means simultaneous growing up to
300 crystals which corresponds to
almost 200 kg crystal material per
year. Each crystal has a size larger
than a man’s palm giving thousands
of thin slices and cubes in the cutting
process which can be used with
high yield for hundreds of pressure
sensors.
The crystal structure of Gallium
Orthophosphate can be derived
from α-quartz by replacing silicon
alternatively with gallium and phosphorus,
see Figure 2.
α-Gallium Orthophosphate is stable
up to a temperature of 933 °C.
Above that it changes into the high
cristobalite type.
The excellent thermal behaviour
and the high sensitivity of Gallium
Orthophosphate have made great
performance advantages over quartz
and Langasite realized in AVL’s
uncooled pressure sensors.
Langasite crystals tend to have higher
longitudinal sensitivities than Gallium
Orthophosphate. However, if measurement
accuracy and precision are
of importantance terms like sensitivity
change and linearity of a sensor are
more relevant. These are the areas
where GaPO4 shows its superior
performance. The importance of the
sensitivity change becomes clear
in context with the sensor housing.
Designing a pressure sensor for
automotive applications requires the
piezoelectric crystal to be packed
into a rigid sensor housing. The
material of the sensor housing has
to be chosen in a way that the thermal
expansion of the sensor housing
and crystal cancel out to zero. With
Langasite crystals this process is
much more difficult and results in
higher design effort and costs.
Gallium Orthophosphate allows
a much better optimization of the
design which results in sensors with
higher measurement precision.
I
Insulation resistance
The insulation resistance is the
electrical (ohmic-) resistance measured
between the electrodes of
the sensor (electrical contacts of the
connector). Piezoelectric sensors
have to have a resistance in the
range of more than 1012 to ensure
proper operation.
A higher resistance value allows full
sensor performance in quasi-static
measurements. If liquids, moisture or
particles contaminate the connector
or start to enter the interior of the
sensor the electrical resistance can
drop. This indicates that the sensor
needs to be serviced immediately
by the manufacturer.
L
Linearity %
As the sensitivity defines how much
signal is generated per pressure unit
it is furthermore expected that this
sensitivity is the same for all applied
pressures. A variation in this context
is defined by the term called linearity.
The maximum deviation (+A, -A) is
expressed as a percentage of the
maximum pressure of the measuring
range which is called full scale output
(FSO). This value should be as close
to zero as possible.
Figure 2
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