SENSORS
degradation due to thermal cycling. With a
bias current of 2 mA and down to about 77
Kelvin the sensitivity drops with about
20% and below this temperature
decreases faster down to about 65%.
The loss is less at 4.2 Kelvin with a
bias current of 4 mA, but this is at the
expense of more dissipation. Driven
by the high dissipation at 4.2 Kelvin
the researchers now want
to investigate if the accelerometer can
be read out intermittent, using the same
test setup, without influence on the
gain stability which is shown to be
temperature dependent. \\
Henk van Weers and Martin Eggens are senior
mechanical design engineers at SRON, Heino Smit is
an electrical design engineer at SRON and Marine
Dumont is business development manger of test and
measurement at Kistler Instrument AG
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AEROSPACETESTINGINTERNATIONAL.COM // SHOWCASE 2020 145
5
4 // Internal view of the
test cryostat with the
Kistler accelerometer
placed in the center of the
copper bottom in a cold
environment
5 // The test setup of the
test cryostat, shaker and
reference accelerometer
clamped at an optical table
6 // Graph of the sensitivity
of the Kistler accelerometer
related to the temperature
for 2 different bias currents
SPECIFIC ACCELERATION SENSOR DESIGNS
Typical vibration measurements call for usage
of IEPE accelerometers (Integrated Electronic
Piezoelectric). Such sensor designs are based on
a piezoelectric element. When subjected to the
load from a seismic mass, the element generates
a charge proportional to the acceleration which
in turn is converted to a voltage by the internal
electronics.
Standard sensors from the market are mostly
using a piezoceramic as the piezoelectric
element. These designs , provide an economical
solution while insuring very good performance
but unfortunately exhibit poor temperature
stability performance when subjected to
temperature fluctuations. In such cases, sensing
elements based on quartz are preferred when
the application environment is thermally active.
Kistler went even one step further over the past
15 years, by developing a family of synthetic
single crystals, called Piezostar, which have very
unique properties.
The PiezoStar family of crystals has improved
with a low temperature coefficient of sensitivity
and higher piezoelectric sensitivity compared
to quartz requiring less mass and less electrical
amplification and lower noise. Less element
mass equates, as well to higher mounted
resonance frequencies, better frequency
response and allows smaller overall housing
designs. Some accelerometers, such as the
8712B5D0CB discussed in this article are
available in a cryogenic version which is qualified
within Kistler factory to measure in temperatures
down to -196°C.
A sensor such as the 8712B5D0CB, being
based on the IEPE technology, will require a
specific IEPE electronics to be used that can
survive -196°C temperature levels and even lower.
To do so, Kistler has developed a proprietary
coating to protect the electronic components
and allow for them to perform properly in a
cryogenic environment.
In addition, at extreme cryogenic levels, such
as 4K, IEPE sensor output bias voltage increase
substantially thus requiring a higher constant
current supply open circuit voltage (compliance
voltage). Typically a compliance voltage of
34VDC or higher is required for the extreme
cryogenic temperatures requiring specifically
designed couplers.
TEST RESULTS
The cryogenic vacuum environment – the
cryostat used for testing the actual TES
detector arrays, provided the first testbed
for testing the accelerometers. From this
test, the researchers concluded that the
accelerometer still operates at a
temperature of 3 Kelvin.
Another cryostat was required to
monitor the other properties. In the second
test cryostat the accelerometer has been
thermally cycled 10 times, from room
temperature to temperatures of 77 Kelvin
and 4.2 Kelvin. After this test, the researchers
found no degradation of the accelerometer.
To get an indication of the sensitivity,
noise and dissipation of the accelerometer
with relation to the temperature,
researchers placed the test cryostat on an
optical table together with another
accelerometer and a small shaker. They
measured the response of the Kistler
accelerometer by operating the shaker at a
constant low level of 0.5g and a chosen
frequency of 40Hz, which has no influence
on the dynamics of the test cryostat.
The Kistler accelerometer was measured
during warmup from the lowest
temperature of 4.2 Kelvin until room
temperature, without any liquid helium or
liquid nitrogen which would otherwise
disturb the measurements.
The dissipation was determined by
measuring the bias voltage with bias
currents of 2 mA and 4 mA during the
warmup of the accelerometer. Finally, the
researchers measured the noise at 77
Kelvin and 4 Kelvin in the test cryostat
without any liquids.
From their tests, the researchers
observed that the accelerometer is still
operating at 4.2 Kelvin, without any
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