PRODUCTS & SERVICES
PRECISION ANTI-ALIASING
FILTERS FOR CDAQ AND CRIO
AEROSPACETESTINGINTERNATIONAL.COM // DECEMBER 79
Exposure to a space environment induces material
outgassing in ordinary accelerometers and cables.
Contaminants from outgassing can condense onto nearby
surfaces such as photo-optic devices and obscure them,
rendering them useless during their intended application.
During random vibration, swept sine or shock testing,
spacecraft payloads are often fitted with accelerometers in
inaccessible mounting locations. As the space structure is
built up around them, it can become impossible to remove
the accelerometers.
Sensors installed for ground vibration testing may
therefore remain on the structure even if they are no longer
needed for testing purposes. Also, before the vibration
testing occurs, it is typical to perform a thermal vacuum
operational checkout of the satellite. The accelerometers
are typically exposed to this extreme environment.
Model TLD356M155 is a hermetic, low outgassing,
triaxial, ceramic shear, ICP accelerometer with a sensitivity
of 2.5 mV/g, a ±2000 g measuring range, and an operating
temperature from -54 to +121C. Designed for shock and
vibration on satellites where mass loading is a concern,
this sensor weighs in at only 4.5 grams.
The sensor features a high measuring range (2000 g,
sensitivity of 2.5 mV/g) and high resolution (0.008 g rms).
This allows it to be used for sine, random and shaker shock
without the need to swap out accelerometers for different
measuring ranges.
Model TLD356M131 is a hermetic, low outgassing,
triaxial, quartz shear, ICP accelerometer with a sensitivity
of 10 mV/g, a ±500 g measuring range and an operating
temperature from -60 to +60C. The quartz sensing crystal
allows for a storage temperature from -190 to +121C and a
more stable thermal performance. This model is useful
because it does not need to be removed from a satellite
under test during cryogenic vacuum testing, which usually
P1451.4 TEDS that
self-identifies the
extremely easy for the
operator to trace. \\
When analog signals are sampled
by A/D converters, the sampled
data record could have come from
other signals outside of the
bandwidth of interest – aliases. The
Sampling Theorem states that an
analog signal must be sampled at
least twice the highest frequency
contained in the signal to prevent
aliasing. In other words, no
information higher than half the
sampling frequency can exist
in the signal, otherwise aliasing
will occur.
Signals above Fs/2 do exist in the
signal and require a filter to
attenuate them to an acceptable
level. If you do not properly filter
the input signal, the digitized
output will contain aliasing errors
that look like real data and since
the aliasing signals are unknown,
the errors are unknown and
cannot be corrected by
post-processing.
The CompactDAQ (cDAQ) and
CompactRIO (cRIO) platforms,
introduced by National
Instruments, provide a convenient
form factor for data acquisition
systems. However many A/D
modules do not have front end
filters requiring the user to provide
an external filter to provide
protection against aliasing.
Precision Filters offers a pair of
low-pass filter modules for the
cDAQ and cRIO platforms that
enable users to easily add high
performance anti-aliasing filters to
their system. The 9602 provides
6-pole filters with cutoffs
programmable from 10 Hz to
127kHz and includes an integrated
amplifier to gain up low-level
signals. The PFI-9608 has 4-pole
programmable filters with cutoffs
available from 10 Hz to 30 kHz. Both
modules feature Precision’s FLAT/
PULSE filter characteristics. \\
ACCELEROMETERS FOR
VIBRATION TESTING
SATELLITES
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occurs before vibration testing.
Both new sensors feature IEEE
accelerometer, making it
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