finalreport csu

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. Introductionhe 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 on-orbit storables, such as cryogenic fluids (fuels or oxidizers), water and other liquids.

T 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

1 Undergraduate Student, Mechanical Engineering, Student member AIAA.2 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.

American Institute of Aeronautics and Astronautics1

http://www.aiaa.org/content.cfm?pageid=230&lumeetingid=2347#zz2347

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 MeasurementIn 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=∆ ϕ2π

=l ∆ nλ (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∂ m

≈ 2 RTλ3 Al (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.


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 V Cell . 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 V P+V Cell. 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

V Cell+V P(3)


PRef ,Closed=n1∙ R∙T

V Cell. (4)

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:

∆ PRef =P❑Ref , Closed ∙( V ❑P

V Cell+V P) . (5)

For tank volume measurement, the targeted tank (with unknown ullage volume V Tank ¿ is then opened to the gas cell and piston volume and a second piston cycle is performed, producing a second fringe shift ∆ mTank. Fringe shift ∆ mTank corresponds to a pressure change in the system in the same manner explained above, and:

∆ PTank=PTank ,Closed ∙( V P

V Tank+V Cell+V P) (6)

where

P❑Tank ,Closed=n2 ∙ R ∙T

V Tank+V Cell. (7)

And n2 is the number of moles of gas in the full volume.The constants P❑2 ,Closed and P❑1 ,Closed are equal because:

n1

V 1=

n2

V 1+V 2. (8)

Finally, the pressure changes can be related to the fringe shifts (by Eqn. 2) and:

∆ PRef

∆ PTank=

∆ mRef

∆ mTank=

(V Cell+V Tank+V p)(V Cell+V p)

=1+V Tank

V Cell+V P. (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 fit s 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 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 the fiber splitters and launching, a total of 4 – 6 μW was recorded as the final output of the interferometer. Despite the low coupling efficiencies, the photodiode detector (ThorLabs, PDA36A) used in the experiment (with a built-in 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.

American Institute of Aeronautics and Astronautics

Figure 4. Fully assembled optical mass gauge payload.

Figure 3. Solid model (A) bottom view and (B) top view of the optical mass gauge payload.

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IV. Results and Discussion

A. Free Space TestingPreliminary 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 two different tanks containing different volumes of liquid, 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

λ 632 nm

A 4.606E-6 (m3/mol) [3]

R 8.34 (J/K*mol)

l 0.05 m

T 297 K

P 1 ATM

Table 2. Fringe shifts.

Item Fringe Count

Ref. (Gas cell) ~15

Tank 1 (empty) ~5.75

Tank 2 (water) ~7

Table 3. Calculated volumes.

Item Calculated Volume (mL)

Ref. (Gas cell) 34.07

Tank 1 (empty) 54.40

Tank 2 (water) 38.58

Table 4. Change in pressure.

Item Delta P (PSI)

Ref. (Gas cell) 10.0

Tank 1 (empty) 4.20

Tank 2 (water) 5.20


Figure 5. Preliminary test setup using free-space optics.

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Sign

al [V

olts

]

Time [Arbitrary]Fringe shift as a consequence of compressing the gas cell with a piston.

Sign

al [V

olts

]

Time [Arbitrary]Fringe shift as a consequence of compressing the gas cell as well as an empty tank with a piston.

Sign

al [V

olts

]

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


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

Reference Tank 1 Tank 2

# of Fringes 5.33 3.30 3.59

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)

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 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 testThe 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


Figure 6. (A) Reference fringe shift, (B) Tank 1 fringe shift and (C) Tank 2 fringe shift.

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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)

V. Future WorkThere 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 4-5 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 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.

References1Witherow, W., Pedersen, K., “Mass Gauging Demonstrator for Any Gravitational Conditions,” IRAD

2006. March 30, 20072Sinko, 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-2009-53943M. Born and E. Wolf, Principles of Optics, 7th expanded edition, Cambridge University Press,

Cambridge, 1999.


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