46th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit, July 25-28, 2010, Nashville, TN
Fiber Optic Mass Gauging System for Measuring Liquid Levels in a
Reduced Gravity Environment
Ryan M. Sullenberger1, Wesley M. Munoz2, M.P. Lyon3, K. Vogel4, A.P. Yalin5
Colorado State University, Fort Collins, CO, 80523, USA
Valentin Korman6
K Sciences GP, LLC Huntsville, AL 35814
Kurt A. Polzin7
NASA-Marshall Space Flight Center, Huntsville, AL 35812
The performance of a compact and rugged fiber optic based liquid volume
sensor is presented. The sensor consists of a Mach-Zehnder interferometer capable
of measuring the amount of liquid contained in a tank at microgravity conditions
(or any gravitational environment) by detecting small changes in the index of
refraction of a gas contained within a sensing region. By monitoring changes in the
fringe pattern as the system undergoes a small compression provided by a piston,
the ullage volume of a tank can be directly measured, allowing the liquid volume to
be determined. In our demonstration, two tanks containing different volumes of
liquid are independently exposed to the measurement apparatus such that the
amount of liquid in each can be found; the two tanks representing tank levels at
different periods during a mission. A flight test will help mature the technology
beyond the laboratory environment.
I. Introduction
he lack of gravity in the space environment makes it challenging to measure the amount of liquid
propellant remaining in storage tanks. Various mass gauging systems have previously been
attempted, but no current working solution exists with the capability of determining liquid
volumes in zero-gravity, without adding significant mass to the propellant tank. An optical mass gauging
system equipped with a modified Michelson Interferometer was recently developed at NASA - Marshall
Space Flight Center (MSFC) and has been shown as a promising option to measure liquid volumes at high
precision using a small, lightweight system1. The optical mass gauge is capable of monitoring critical onorbit storables, such as cryogenic fluids (fuels or oxidizers), water and other liquids.
The present contribution describes recent efforts to modify the aforementioned optical mass
gauging system to be compact, lightweight, and to utilize a fiber optic based Mach-Zehnder
interferometer. The use of fiber-optics in place of traditional optics (mirrors and beam splitters) allows
T
1
Undergraduate Student, Mechanical Engineering, Student member AIAA.
Undergraduate Student, Mechanical Engineering.
3
Undergraduate Student, Mechanical Engineering, Student member AIAA.
4
Undergraduate Student, Mechanical Engineering.
5
Associate Professor, Mechanical Engineering, Member AIAA.
6
Director of Research.
7
Propulsion Research Engineer, Propulsion Research and Technology Applications Branch, Propulsion Systems
Department. Senior Member AIAA.
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American Institute of Aeronautics and Astronautics
2
the sensor to be ruggedized to withstand the extreme vibrations and accelerations experienced during
launch and flight. The new sensor has been fabricated for laboratory testing as well as to be part of a
payload to be placed on a sounding rocket (Terrier-Orion) for flight testing. The goal of the latter is to
show successful operation under flight conditions thereby maturing the technology.
a)
b)
Figure 1. a) A tank with liquid in normal gravity. b) A tank with liquid in microgravity.
II. Interferometer for Fluid Volume Measurement
In our approach, the amount of liquid contained within a tank is determined by measuring a change in
the index of refraction of the gas occupying the tank’s ullage volume. The change in refractive index
occurs as the gas experiences a pressure change provided by a piston compression. The pressure is
monitored using an interferometer that looks at the interference pattern generated by two optical signals.
We use a Mach-Zehnder interferometer in which the source beam is split into two (approximately equal)
legs; this first is a reference beam, while the second is referred to as the sensing beam as it passes through
a sensing region comprised of a gas cell connected to a propellant tank. The interferometer measures the
change in phase () of the beam passing through the gas cell, as is caused by varying index of refraction
due to varying gas density. (The contribution from the gas cell is isolated by measuring against the
reference leg.)
A photo-detector measures the phase change between the two beams based on the passage of fringes
(sequences of bright and dark light), as the two beams interfere during density fluctuation in the gas. A
change in the interference order will cause a fringe shift, m:
∆𝑚 =
∆𝜙 𝑙∆𝑛
=
2𝜋
𝜆
(1)
where 𝑙 is the difference between reference and sensing path lengths, ∆𝑛 is the change in refractive index
of the gas in the sensing region, and  is the wavelength of the light. Measuring the ullage volume of a
tank requires the ability to measure a pressure change by examining the fringe shift. A photo-detector
monitors fringe movement, and the number of fringes recorded can be related to a change in pressure (P)
by2:
𝜕𝑃 2𝑅𝑇𝜆
≈
𝜕𝑚
3𝐴𝑙
(2)
where R is the universal gas constant, T is the temperature of the gas, and A is the molar refractivity of the
gas. Assuming constant temperature, the pressure and density can be easily related to one another.
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III. Experimental
A. Sensor Design
Testing of the optical mass gauge was first performed in a laboratory environment and will be
followed by a flight-test onboard a Terrier-Orion sounding rocket. In order to maintain interferometer
alignment in the face of extreme accelerations and vibrations the sensor will experience during launch, we
elect to use a fiber optically based system in which all beams, with the exception of the beam passing
through the gas cell, are transmitted through fiber optics. Further details of the fiber hardware
configuration are given in Section X below. A compact Helium-Neon (HeNe) laser of 632.8 nm
wavelength was selected as the light source owing to the availability of laser and fiber components at this
wavelength. A HeNe laser provides a coherence length sufficient for simple interferometry with minimal
compensation. (The length of the reference leg was only crudely matched to the signal length, with
precision of ~1 mm.). Shown in Figure 2 is a generalized schematic of the fiber-optic Mach-Zehnder
interferometer with piston and tanks attached to the sensing region.
Figure 2: Schematic of fiber-optic mass gauging system.
In principle, the system can work with a single tank but for demonstration purposes we use two liquid
tanks containing different volumes of water (representing tank levels at different periods during a
mission) that can be independently exposed to the measurement apparatus such that the liquid volume in
each can be determined. At first, both tanks are closed off from the gas cell. Measurements are performed
by pumping the piston and looking at the resulting fringe change. With the piston open (i.e. retracted),
the measured gas occupies the cell and associated gas lines, a volume that we term 𝑉𝐶𝑒𝑙𝑙 . With the piston
closed (i.e. contracted), the same mass of gas additionally occupies the volume of this piston stroke, i.e. a
total area of 𝑉𝑃 +𝑉𝐶𝑒𝑙𝑙 . The action of pumping the piston causes fringes to be detected. We term this
number of fringes as the reference fringe count ∆𝑚𝑅𝑒𝑓 . The reference fringe count ∆𝑚𝑅𝑒𝑓 can be related
to the corresponding pressure change of the gas through equation (2). In the references state, the pressures
in the open and closed configurations (Pref,Open and Pref,Closed respectively) are given as:
𝑃 𝑅𝑒𝑓,𝑂𝑝𝑒𝑛 =
𝑛1 ∙ 𝑅 ∙ 𝑇
𝑉𝐶𝑒𝑙𝑙 + 𝑉𝑃
(3)
𝑛1 ∙ 𝑅 ∙ 𝑇
.
𝑉𝐶𝑒𝑙𝑙
(4)
𝑃𝑅𝑒𝑓,𝐶𝑙𝑜𝑠𝑒𝑑 =
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where n1 is the number of moles residing in the piston and cell combination. Assuming constant
temperature, the system experiences the following pressure change during the reference cycle:
𝑉𝑃
∆𝑃𝑅𝑒𝑓 = 𝑃 𝑅𝑒𝑓,𝐶𝑙𝑜𝑠𝑒𝑑 ∙ (
).
𝑉𝐶𝑒𝑙𝑙 + 𝑉𝑃
(5)
For tank volume measurement, the targeted tank (with unknown ullage volume 𝑉𝑇𝑎𝑛𝑘 ) is then opened
to the gas cell and piston volume and a second piston cycle is performed, producing a second fringe shift
∆𝑚 𝑇𝑎𝑛𝑘 . Fringe shift ∆𝑚 𝑇𝑎𝑛𝑘 corresponds to a pressure change in the system in the same manner
explained above, and:
𝑉𝑃
∆𝑃𝑇𝑎𝑛𝑘 = 𝑃𝑇𝑎𝑛𝑘,𝐶𝑙𝑜𝑠𝑒𝑑 ∙ (
)
𝑉𝑇𝑎𝑛𝑘 + 𝑉𝐶𝑒𝑙𝑙 + 𝑉𝑃
(6)
where
𝑃 𝑇𝑎𝑛𝑘,𝐶𝑙𝑜𝑠𝑒𝑑 =
𝑛2 ∙ 𝑅 ∙ 𝑇
.
𝑉𝑇𝑎𝑛𝑘 + 𝑉𝐶𝑒𝑙𝑙
(7)
And n2 is the number of moles of gas in the full volume.
The constants 𝑃2,𝐶𝑙𝑜𝑠𝑒𝑑 and 𝑃1,𝐶𝑙𝑜𝑠𝑒𝑑 are equal because:
𝑛1
𝑛2
=
.
𝑉1 𝑉1 + 𝑉2
(8)
Finally, the pressure changes can be related to the fringe shifts (by Eqn. 2) and:
∆𝑃𝑅𝑒𝑓
∆𝑚𝑅𝑒𝑓
(𝑉𝐶𝑒𝑙𝑙 + 𝑉𝑇𝑎𝑛𝑘 + 𝑉𝑝 )
𝑉𝑇𝑎𝑛𝑘
=
=
=1+
.
∆𝑃𝑇𝑎𝑛𝑘 ∆𝑚 𝑇𝑎𝑛𝑘
(𝑉𝐶𝑒𝑙𝑙 + 𝑉𝑝 )
𝑉𝐶𝑒𝑙𝑙 + 𝑉𝑃
(9)
By knowing the volume of the gas cell, the reference fringe shift, and the new fringe shift, the ullage
volume of the tank may be calculated. From there, by knowing the theoretical volume of the entire tank,
the volume of the liquid may be extracted. Volumes of the gas cell and tanks are initially estimated and
later determined experimentally.
B. Sensor Design and Hardware
The optical mass gauge sensor was designed to fit inside a Terrier-Orion sounding rocket payload subject
to constraints including size and weight requirements: the entire device must fit within a 23.62 cm
diameter by 12.1 cm tall space, and must have a mass less than 2.97 kg. Our final sensor design fits
within the required dimensions and weighs in at 2.90 kg.
Due to size constraints, the diameter of the piston was designed to be large (4.25 cm) so that the
actuation distance could be minimal; therefore a reasonably powerful actuation device was required.
Piston actuation was controlled using a 3.4 N-m servo capable of delivering ~760 N of linear force
through a rack and pinion design. The piston was designed to have approximately 1.25 cm of actuation, or
a total of ~18 mL of compression. Total piston actuation time was designed to take approximately 525
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ms. Through repeated tests, the piston held pressure up to 267
kPa; however, during operation, pressures above 67 kPa were
rarely observed.
The gas cell (including all connection tubing) has an
internal volume of approximately 60 mL. Each tank was
designed to be the same volume in size (~50 mL). For
simplicity, during each test, one of the tanks was kept
completely empty and the other was partially filled (~10 - 30
mL). For initial tests air was used as the sample gas (though
its molar refractivity is relatively low hurting sensitivity).
Single mode fiber-optic components were selected to
construct the interferometer. The use of fiber optics greatly
increases the likelihood of optical alignment after launch
(during free-fall) when the sensor begins to operate. Single
mode fiber eliminates internal fiber interference between
wave propagation modes, making it clear that the interference
is created by the measurement of interest2.
An inexpensive HeNe laser (previously used as a barcode
scanner) was used as the light source; the laser consisted of a
JDS Model 1007 HeNe tube. The HeNe laser accepted a 12
VDC power supply, allowing it to be powered from the
payload’s primary battery. The inexpensive HeNe laser was
found to possess a coherence length suitable for
interferometry. There. Owing primarily to insufficient
angular adjustments on the fiber coupling hardware, the fiber
launch stages were limited in their efficiency (power
coupling); of the 800 μW laser output, approximately 100
μW was successfully coupled (12.5%). The second fiber
launch (after passing through the gas cell) had an even lower
efficiency, ~5%. Considering power lost due to efficiency of Figure 3. Solid model (A) bottom view and (B)
the fiber splitters and launching, a total of 4 – 6 μW was top view of the optical mass gauge payload.
recorded as the final output of the interferometer. Despite
the low coupling efficiencies, the photodiode detector
(ThorLabs, PDA36A) used in the experiment (with a builtin gain adjustment) was more than adequate to detect a
strong signal and display a very good fringe visibility. A
photograph and solid model of the optical mass gauge
payload can be found in Figures 3 and 4, respectively.
Figure 4. Fully assembled optical mass gauge
payload.
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IV. Results and Discussion
A. Free Space Testing
Preliminary results were collected using a research class
HeNe laser (ThorLabs, HGR005) inappropriate for the
payload, and free space optics. A set of electronics was
assembled in order to control the piston compression as
well as sample data from the photo-detector and
communicate this data to a computer so that it could be
analyzed.
Two distinct fringe shifts are required to calculate the
volume of liquid contained within a single tank: a reference
fringe shift, and the fringe shift that occurs when a tank is
open to the system. The preliminary test setup consists of
Figure 5. Preliminary test setup using free-space
two different tanks containing different volumes of liquid,
optics.
therefore the sensor needs to measure three fringe shifts: the
reference fringe shift, the fringe shift that occurs when Tank
1 is exposed to the system, and the fringe shift that occurs when Tank 2 is exposed to the system. All
three fringe shifts (recorded using the preliminary setup) are shown.
Table 1. Testing Parameters
Table 2. Fringe shifts.
λ
632 nm
Item
Fringe Count
A
4.606E-6 (m3/mol) [3]
Ref. (Gas cell)
~15
R
8.34 (J/K*mol)
Tank 1 (empty)
~5.75
l
0.05 m
Tank 2 (water)
~7
T
297 K
P
1 ATM
Table 3. Calculated volumes.
Table 4. Change in pressure.
Item
Calculated Volume (mL)
Item
Delta P (PSI)
Ref. (Gas cell)
34.07
Ref. (Gas cell)
10.0
Tank 1 (empty)
54.40
Tank 1 (empty)
4.20
Tank 2 (water)
38.58
Tank 2 (water)
5.20
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Signal [Volts]
Time [Arbitrary]
Signal [Volts]
Fringe shift as a consequence of compressing the gas cell with a piston.
Time [Arbitrary]
Signal [Volts]
Fringe shift as a consequence of compressing the gas cell as well as an empty tank with a piston.
Time [Arbitrary]
Fringe shift as a consequence of compressing the gas cell as well as a semi-full tank with a piston.
The fringe shifts correlate exactly as they should, that is, the larger the ullage volume being measured, the
smaller the fringe count (a consequence of a smaller pressure change). The gas cell (smallest compressed
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volume in the system) has the highest fringe count, the empty tank (largest compressed volume) has the
smallest fringe count and the water tank lies somewhere in between. The fringe shifts above indicate that
the amount of liquid contained within Tank 2 was 13.5 mL. The actual amount of liquid contained within
the tank was 11.5 mL, therefore the system has an error of about 20%.
B. Results Recorded with Payload (Laboratory)
In this section we describe test results from the final fiber optic system (i.e. that shown in Fig. 3 and
Fig. 4). The final version of the optical mass gauge sensor was constructed in the spring of 2010. Initial
testing of the sensor indicated that there was a major source of noise in the interferometer signal during
piston actuation. It was discovered that the source of noise in the signal was due to the servo’s
mechanical vibrations during actuation. Once the vibrations were greatly reduced by mechanically
isolating the piston sub-assembly, the sensor became capable of recording much cleaner fringes. Shown
below is one of the data sets recorded using the optical mass gauge sensor:
Table 5. Fringe shifts
# of Fringes
Reference Tank 1
Tank 2
5.33
3.59
3.30
Table 6. Calculated Volumes
Measured Liquid Volume (mL)
=
10.33
Actual Liquid Volume (mL)
=
10.53
Percent Error
=
1.90%
Table 7. Testing Parameters (payload)
Figure 6. (A) Reference fringe shift, (B) Tank 1 fringe shift
and (C) Tank 2 fringe shift.
λ
632.8 nm
A
4.606E-6 (m3/mol) [3]
R
8.34 (J/K*mol)
l
0.0432 m
T
297 K
P
100,900 Pa
The data presented above is one of the most
accurate sets recorded using the sensor to date.
The liquid volume measured by the sensor was
10.33 mL, compared to the actual volume of
10.53 mL, less than 2% error. The entire time of
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testing, there were some difficulties in accurately counting the number of fringes mainly due to slight
variations in the fringe counts (± 0.25 fringes). With the tanks in the system being so small, there is not a
large difference in fringe count between an empty tank and a tank that is half full. For example, a
difference in fringe count (between the empty tank and the tank containing liquid) of approximately 0.3
fringes corresponds to a liquid volume difference of 10 mL. At the present test conditions, the relatively
weak dependence of fringe count on the tank volume, limits the achievable sensitivity. In actual
conditions, with larger tanks, larger fringe counts would alleviate some of these challenges. TRUE??
C. Vibration test
The optical mass gauge is tentatively scheduled for suborbital spaceflight onboard a Terrier-Orion
rocket (model 41.088 Koehler) on June 24th, 2010. During the early stages of flight, payloads are exposed
to harsh vibration environments and Z-axis loading up to 25G. NASA Wallops Flight Facility (NASA
WFF) provided vibration table testing specifications particular to 41.088 Koehler, so that early vibration
testing could be performed prior to arrival at NASA WFF in June.
A full scale vibration test of the optical mass gauge payload was performed on a vibration table (Sierra
Nevada Corporation, Louisville, CO). The payload was exposed to several different tests: two tests in the
Z-Axis (sine sweep and random), and a random test in the X and Y axes. An example test profile is shown
in Figure X. Zero components showed sign of damage upon visual inspection, and the optical components
remained in alignment (although there was slight laser coupling efficiency loss). In each test, the vibration
table was ramped up to a maximum of 7G’s, for no more than 30 seconds.
Acceleration (G-Force) versus Frequency (Hz)
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V. Future Work
There is much room for improvement in future iterations of the optical mass gauge, for example
acquisition of higher quality optical components such as replacing the gas cell windows (currently low
quality commercial window glass) with NBK-7 windows (or similar). Also, owing to the different
components in the two legs, improved choice of the optical splitter (currently 50:50) would serve to
provide a stronger signal beam, and consequently superior fringe visibility. Finally, replacing the current
fiber coupling hardware used on the gas cell with permanent pigtailed fiber couplers would aid optical
stability and especially as the payload experiences shock during launch.
Methods to isolate the mechanical vibrations from the optical components are being investigated. The
current iteration of the project suffers from mechanical vibration induced by the stepper motor inside of
the servo used to actuate the piston. Even during "free-fall", sounding rockets spin at a constant rate of 45 Hz for gyroscopic stabilization. An analysis or test should be performed to ensure that this constant
acceleration will not affect the interferometer output.
Design changes dealing with tank sizes and fluid management is something that is being investigated.
With the current iteration of the project, it was difficult to obtain accurate measurements because there
was not a significant difference in fringe counts for the liquid volumes being measured. As was
mentioned earlier, in order to obtain more accurate data, a larger fringe count is needed. The carious
volumes should be selected accordingly. Finally, the optical mass gauge needs a solution to keep liquid
from entering the gas cell. The use of a metal "mesh" that allows gas to pass through but prevents liquid
from entering was an idea that was never tested.
VI. Conclusion
In contrast to the optical mass gauging method, most current methods of propellant gauging require
some form of liquid stratification in order to operate. The simplest way to do this is to provide a small
acceleration to the spacecraft; this takes time and energy, and does not exactly provide a real-time
measurement system [CITATION_NEEDED]. For human spaceflight it would be advantageous for astronauts to
have a fuel gauge that can tell them instantaneously what their fuel level is without spacecraft movement
or stirring of tanks. In a human spaceflight situation, astronauts would never run down to the minute
residual fuel levels – with people onboard it is always necessary to keep far extra fuel for a safety margin.
A fiber-optic based liquid volume sensor has many advantages over the current used solutions: the sensor
is compact, rugged, and requires minimal modifications to existing tanks. Previous research at MSFC has
shown that such a sensor would only require a piston compression of roughly 100 cubic centimeters,
providing the device with enough sensitivity to measure 1,000 liter tanks1. The sensor is also swift when
taking measurements; the device can relay measurements in close to real-time.
Optical mass gauging technology for measuring liquid volumes in zero-gravity has only been tested in
a laboratory environment; never before has it been flight tested [CITATION_NEEDED]. At the time of this article,
the CSU optical mass gauge has been fully tested in a laboratory environment and is ready to be launched
on a Terrier-Orion sounding rocket on June 24, 2010 from Wallops Flight Facility in Virginia. A
successful flight-test will result in the data collected during flight matching the data collected in the
laboratory.
Acknowledgements
The authors of this paper would like to thank Omnis, Inc. for their generous financial donation that
seeded our research efforts. Financial support was also received from the Colorado State University
Senior Design Program, the Colorado Space Grant Consortium, and Masten Space Systems, Inc. We
would like to thank Jason Priebe, Dan Goodrich, and Lad Kurtis of Sierra Nevada Corp. for letting us use
their vibration table. We acknowledge technical guidance from Dr. John D. Williams (Colorado State
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University Electric Propulsion and Plasma Engineering Laboratory), Dr. Sachin Joschi and graduate
students Lei Tao, Brian Lee and Frank Loccisano from the Colorado State Laser Plasma Diagnostics
Laboratory.
References
Witherow, W., Pedersen, K., “Mass Gauging Demonstrator for Any Gravitational Conditions,” IRAD
2006. March 30, 2007
2
Sinko, J. E., Korman, V., Hendrickson, A., Polzin, K. A., , “A Fiber-Optic Sensor for Leak Detection
in a Space Environment,” 45th AIAA Joint Propulsion Conference, 3rd-5th Aug. 2009. AIAA-2009-5394
3
M. Born and E. Wolf, Principles of Optics, 7th expanded edition, Cambridge University Press,
Cambridge, 1999.
1
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