Application Note
On the New Frontiers of Precision
AN-2
Signal Conditioning for the NASA Vibro-Acoustic
Test Facility Plum Brook Station
Introduction
The Space Launch System, or SLS, is a United States heavy-lift vehicle
being designed by NASA as a follow-up to the Constellation Program.
It will replace the retired Space Shuttle and provide the USA with
new advanced capability for human exploration of space. In support
of the Constellation and the SLS programs, NASA desired to fully
simulate the spacecraft’s mechanical and acoustic vibrations along
with thermal vacuum and electromagnetic conditions during launch,
on planetary surfaces and in deep space. Unfortunately, a facility
to conduct such large scale testing was nonexistent and had to be
constructed. NASA chose the Plum Brook Station test facility in Sandusky, Ohio for the site, as it was already home to the world’s largest
space environment thermal vacuum chamber. As part of the project,
NASA constructed the world’s most powerful spacecraft acoustic and
thermal-vacuum test chamber, the world’s most powerful low-frequency mechanical vibration test stand, and developed the capability
to perform electromagnetic interference/compatibility (EMI/EMC)
The Space Power Facility at NASA’s Plum Brook Station Courtesy NASA
testing on full-scale spacecraft. To support the test and measurement
aspects of the world-class test facility, a data measurement system
was needed to interface to spacecraft and facility sensors and then
condition, digitize, display, analyze, store and archive the crucially
important test data.
Solution
NASA chose the Precision Filters 28000 Signal Conditioning system for
the data system “front-end” to interface to the spacecraft and facility
sensors and provide amplified filtered sensor signals to the downstream data acquisition system.
Support for a wide mix of sensors was required throughout the vibration, acoustic and thermal-vacuum test chambers. These sensors
include Integrated Electronics Piezo-Electric (IEPE) accelerometers,
IEPE/charge microphones, 4-arm strain gauges, load cells and thermocouples as well as general purpose voltage monitoring inputs. A total
of 800 channels of sensor conditioning were required for the initial
phase of the installation, with future expansion to 1,536 channels. Due
to the wide range of test protocols, and the high cost and importance
of the tests, the data system had to be flexible, reliable, traceable (to
NIST standards) and above all, test data had to be highly accurate and
fully defendable. Together the signal conditioning front-end and the
data acquisition system comprise the Plum Brook Station High Speed
Data Acquisition System (HSDAS).
Precision Filters’ 28000 Signal Conditioning system is a “mix-andmatch” transducer conditioning system and has more than 25 plug-in
signal conditioning cards to meet almost any sensor conditioning
requirements. To meet the vibration measurement requirements of
the Plum Brook HSDAS, the Precision 28316C 16-Channel Isolated IEPE
Conditioner card was selected. The high density of the 28316C card
provides 544 channels using three 6U chassis with future upgrade
capacity to 768 channels within the same cabinet space.
The 28316C card supports programmable filtering, gain and IEPE
supply current from 1 to 8 mA. For transducer health monitoring purposes, sensor open, short and bias level are monitored and reported.
The conditioners have 4-pole programmable low-pass Butterworth
filters with cut-offs programmable from 300 Hz to 30 kHz and a
wideband (filters bypassed) band width of 190 kHz. Programmable
high pass filters provide cutoffs of 0.25 or 10 Hz. The 28316C card has
an additional buffered output per channel available on sixteen front
panel coaxial SMB-type connectors to allow diagnostic monitoring, or connection to auxiliary recording devices without corrupting
the primary data path. To facilitate the logistics of system setup and
gain scaling to the calibrated sensitivity of 544 sensors, the 28316C is
compatible with Transducer Electronic Data Sheet (TEDS) equipped
accelerometers. Calibrated sensitivity and other calibration data is
read directly by the attached 28316C, and made available to the host
computer. While most other TEDS readers on the market are limited to
a maximum cable length of 400 feet, the Precision Filters proprietary
Long Distance TEDS (LDTEDS) can operate with cable lengths as long
as 1,500 feet.
NASA vibration testing methodology utilized triaxial accelerometers
with 4-pin microdot connectors at the accelerometer (X,Y,Z, and
common ground). This posed a technical problem as traditional
single-ended IEPE conditioners would have created multiple ground
loops in the input wiring caused by the shared ground connection
at the sensor end and the common ground at the signal conditioner
end. The proprietary balanced push-pull IEPE supply and isolated
input of the 28316C breaks the ground loops while allowing full electrostatic shielding of the X, Y and Z signal lines. The circuitry has been
demonstrated to have approximately 30 dB noise improvement over
single-ended IEPE conditioners.
For other test measurement solutions visit our web site at www.pfinc.com or send e-mail to pfinfo@pfinc.com
NIST Traceable Calibration
Precision Filters 28000 Signal Conditioning System
All test and measurement systems require periodic calibration. Typically, this means that test systems are dismantled,
cards uninstalled and shipped either to an in-house cal
lab or back to the manufacturer, requiring an average of
30 days of downtime. NASA required automated in-situ
calibration as the expected test schedule does not tolerate
one-month per year downtime for equipment calibrations.
Precision Filters’ built-in test hardware and software allows
the user to perform NIST traceable calibration tests on-site
without removing the system from the equipment rack.
Future Expansion
28000-TEST
Test Subsystem
28000
Mainframe
For future expansion to charge mode capability, the 28304
plug-in card provides charge mode conditioning with fullscale charge range from 2.5 pC to 100,000 pC and input
noise as low as 0.005 pC. For system, sensor and cable
health checks, the 28304 supports voltage substitution,
shunt calibration and T-insertion.
28000
Mainframe
When the test regimen requires Wheatstone bridge conditioning, the 28104A plug-in card is compatible with ¼, ½
or full bridges. The card is capable of bridge auto-balance
and shunt calibration through voltage insertion or precision resistor applied as a shunt cal. The voltage excitation
supply utilizes balanced voltage excitation to improve
rejection of high frequency common-mode signals. The
28104A includes a 4-pole programmable filter with selectable flat amplitude response Butterworth characteristics
for spectral analysis, or pulse Bessel type characteristics
for time domain uses including transient (shock) and
waveform analysis.
28000
Mainframe
Whether the sensor type is voltage, IEPE, bridge, or charge,
each channel of the 28000 is fully programmable to provide sensor scaled, amplified and filtered data acquisitionready signals to the HSDAS downstream A/D converters.
(11) 28316C IEPE
Conditioner Cards
(5) Expansion
Card Slots
(11) 28316C IEPE
Conditioner Cards
(5) Expansion
Card Slots
(12) 28316C IEPE
Conditioner Cards
(4) Expansion
Card Slots
• The HSDAS IEPE sensor conditioning subsystem consists of three
6U high 28016 mainframes each equipped with a low noise power
supply, integrated cooling and a 28000-BIF1-FT Controller card.
• (34) 28316C 16-Channel Isolated IEPE Conditioner cards
providing 544 IEPE sensor channels.
• One 28000-TEST Test Subsystem used for performing validation
and in-situ NIST traceable calibrations.
• For future expansion, the three mainframes provide fourteen
additional card slots providing up to 224 additional sensor
channels.
28000 Expansion Plug-in Cards
28316C Isolated IEPE Accelerometer
Conditioner with LDTEDS™
28304 Quad Charge/IEPE Conditioner
with Long Distance TEDS™
• 16 channels/card,
• 4 channels per card,
256 channels/system
• IEPE conditioner or AC Filter/
Amplifier with balanced
differential input
• Isolated channel input allows
conditioning of grounded IEPE
accelerometers without
introducing ground loops
• Precision Filters LDTEDS (Long-
Distance Transducer Electronic Datasheet)
compliant
• Gain x0.5 to x512 with 0.05% resolution
• Auto-gain setup for best Analog-to-Digital
128 channels/system
28104A QuadChannel Wideband Bridge
Conditioner
• 4 channels/card, 64 channels/system
• Balanced programmable constant voltage
source with remote sense
• Up to 200 kHz “filtered’ or 700 kHz “wideband” bandwidth
• Up to 20 V excitation delivered to the
bridge
• 2 to 10-wire plus shield input interface
Converter (ADC) dynamic range
• Dual mode: piezoelectric or IEPE
• Ground Sense Input Mode
conditions grounded sensors
• Up to 100 kHz “filtered” bandwidth
or 370 kHz “wide-band” bandwidth
• T-insertion for health test of
inaccessible accelerometers
• Programmable IEPE current to 0, 4, 8, 12 mA
• Two charge conversion ranges for 10,000
or 100,000 pC FS inputs
• Programmable amplifier: x0.25 to x8192
with 0.05/vernier
For more information, please contact Alan Szary, Precision Filters, Inc. at 607-277-3550, ext: 7237 or ars@pfinc.com.
ISO 9001:2008 CERTIFIED QUALIT Y
P8475 AN-2 Rev. -
Copyright © 2013 by Precision Filters, Inc.
Precision Filters, Inc.
Telephone: 607-277-3550
240 Cherry Street
E-mail: pfinfo@pfinc.com
Ithaca, New York 14850
Web Site:www.pfinc.com
™LDTEDS is a Trademark of Precision Filters, Inc.