Signal Conditioning for Rocket Engine Fuel Flow Measurements On

Signal Conditioning for Rocket Engine Fuel Flow Measurements On
Application Note
On the New Frontiers of Precision
AN-5
Signal Conditioning for Rocket
Engine Fuel Flow Measurements
Introduction
NASA’s John C. Stennis Space Center has tested rocket engines for more
than 40 years. The testing performed at Stennis has been vital to the U.S.
space program. The A and B test complexes house vertical stands that
were used in the ‘60s to test Saturn-V rocket engines. During the shuttle
era the stands were used to test the Space Shuttle Main Engine (SSME).
Today, the center supports a variety of programs including testing of
engines for the cutting-edge Space Launch System (SLS). This application
note discusses the signal conditioning solution for the measurement of
fuel flow to the engine under test.
To create thrust, chemical rocket engines require a combustible fuel and
an oxidizer (like liquid oxygen). Accurately measuring the rate volume
of an engine’s fuel/oxidizer consumption is critical for measuring performance parameters of the rocket, such as specific impulse: the ratio of fuel
mass consumed to thrust produced per second. Specific impulse measurements determine payload size and mission fuel requirements.
Pulse Signal to
Instrumentation
NASA Stennis
B1/B2 Test Stand
Solution Highlights
• Scalable, universal conditioning
solution supports a variety of
transducers in one high-density
mainframe.
Magnetic
Pickup Coil
• Programmable band-pass filtering of
flow-meter signals reduces out-band
noise.
Internal Turbine Blade
with Embedded Magnets
Fuel/Oxidizer
Flow
Fuel Line
Simplified Diagram of a Turbine Flow Meter
A turbine flow meter in the fuel lines
monitors fuel/oxidizer consumption. An
internal turbine turns as fluid flows across
the blades, and magnetic pickups generate
electrical pulses. The pulse frequency of the
flow meter output is proportional to the
volumetric flow of fuel and oxidizer to the
rocket engine.
Flow meter signals often have overshoot,
ringing, and noise on the pulse signal, which
makes reliable triggering and frequency measurement extremely difficult. The mechanism
inside the flow meter produces noise with
frequency content above and below the
pulse signal. Variability in noise content from
sensor to sensor creates new and evolving
signal conditioning challenges.
After 30 years of service, the signal
conditioning system for Stennis’s test
stands needed an upgrade. The legacy
system failure rate was increasing, the repair
costs were high, and many of the parts
were no longer available. It was desired to
have a replacement solution with computer
programmable setup and one where
automated on-site calibration could
be performed.
To address changing flow measurement
challenges and reduce system setup,
verification, and maintenance costs, NASA
chose Precision Filters’ 28000 system with
the 28524 Frequency-to-Voltage Converter
and 28608B Filter/Amplifier for the frontend signal conditioning of its fuel flow
monitoring system.
• Frequency-to-DC-voltage converter
reliably measures flow meter pulse
outputs to determine fuel/oxidizer
volumetric flow.
• 28000 FAT provides fully automated,
in situ NIST-traceable calibration
tests.
• 28000 Go/No-Go test provides
automated validation of signal
conditioner settings.
Rocket Engine Test at NASA Stennis
For other test measurement solutions visit our web site at www.pfinc.com or send e-mail to pfinfo@pfinc.com
Solution
Precision’s 28000 system supports a mix of
signal conditioning cards for strain, shock,
vibration, frequency-to-voltage, and antialiasing filters. As shown in the channel block
diagram below, the fuel monitoring system
front-end signal conditioning uses the
28608B Filter/Amplifier card configured with
band-pass filters to buffer, amplify, and filter
the flow meter pulse signals. The band-pass
filter capability is effective at cleaning up the
raw flow meter signal harmonics and noise
prior to frequency conversion. Since each
filter’s cutoff frequency is independently controlled, the center frequency and bandwidth
of the band-pass response can be changed to
reject interfering signals and harmonics that
vary from sensor to sensor.
System Self-Test and
Calibration
10 V
5V
Programmable Trigger
Threshold Set at 1.0 V
Precision Filters’ built-in test capability lets
users perform NIST-traceable calibration
tests without removing the system from the
equipment rack—saving NASA hours of pretest manual validation. Every card function
is exercised and all data-critical performance
characteristics are measured and compared
to published specifications.
2.5 V
0V
–2.5 V
The Precision 28524 Frequency-to-Voltage
Converter card accurately measures the frequency of the flow meter signal and outputs
a precise DC level proportional to frequency.
The DC output range can be independently
programmed for each channel, allowing
users to scale the DC output to the flow
meter frequency range of interest. The 28524
has four averaging time constants for the DC
output, so users can select smoothed or rapid
output response.
To deal with the most difficult flow meter
signals, the 28524 card features a configurable trigger. Programmable trigger polarity
(positive or negative), trigger hold-off (in
seconds), and trigger level settings allow for
reliable measurement of frequency on signals
with ringing, overshoot, crossover distortion,
or glitches.
Glitch
–5 V
–10 V
Figure 1: Programmable Trigger Threshold
Set Above the Glitch
Figure 1 shows a signal from an actual flow
meter. The expected periodic waveform is
present, with a glitch in the middle of each
signal cycle that has a slight variation from
cycle to cycle. These glitches can cause false
triggers by creating additional rising and
falling edges near the zero crossing point at
random zero crossings.
One method that provides reliable triggering is to set the 28524 trigger point above or
below the glitch point in the signal cycle, as
shown by the red line in Figure 1.
While the FAT verifies channel parameters,
the Go/No-Go test quickly verifies each channel’s run-time settings. Channel settings—
including gain, filter setting, DC offset, and
noise levels—are quickly measured, verified,
and reported, proving that the equipment is
functioning properly.
PFI Equipment Used
for Rocket Engine Fuel
Flow Tests
Another approach is the trigger hold-off
feature as shown in Figure 2. The trigger
circuit is disabled for the hold-off time to
ignore false edges.
Trigger
Glitch Threshold
Trigger
Glitch Threshold
28016 Signal Conditioning System with
28000 Sub-Test System
Hold Off Hold Off Hold Off
Period Period Period
Converted Signal
Converted Signal
No Trigger Hold-Off
Creates False Triggering
Trigger Hold-Off Enabled
Prevents False Trigger
Figure 2: Programmable Hold-Off
Pre-Filter
Gain
Flow
Meter
Input
Post-Filter
Gain
4-Pole HP
& 4-Pole LP
Prog. Filter
28608B-HP4F/LP4FP Programmable
Band-Pass Filter/Amp
Gain/Atten.
Input
Filtering
Edge Selection
Hold-Off Prescaling
Conversion
Trigger
Offset
Output
Span
Offset
Pulse
Out
Output
Filter
28524 Frequency to Voltage Converter
Channel Block Diagram of NASA Stennis Flow Meter Signal Conditioner
28608B Filter/Amplifier
Card
28524 Frequency-toVoltage Converters
VDC
Output
28000 Graphical User Interface (GUI)
For more information, please contact Doug Firth, Precision Filters, Inc.
at 607-277-3550 or doug@pfinc.com.
ISO 9001:2008 C ERTIFIED QUA LIT Y
P8489 AN-5 Rev. -
Copyright © 2016 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
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