NASA Tech Briefs, July 2010 7

Although there are several methodsfor determining liquid level in a tank,there are no proven methods to quicklygauge the amount of propellant in atank while it is in low gravity or underlow-settling thrust conditions where pro-pellant sloshing is an issue. Having theability to quickly and accurately gaugepropellant tanks in low-gravity is an en-abling technology that would allow aspacecraft crew or mission control to al-ways know the amount of propellant on-board, thus increasing the chances for asuccessful mission.

The Radio Frequency Mass Gauge(RFMG) technique measures the elec-tromagnetic eigenmodes, or naturalresonant frequencies, of a tank con-taining a dielectric fluid. The essentialhardware components consist of an RFnetwork analyzer that measures the re-

flected power from an antenna probemounted internal to the tank. At a res-onant frequency, there is a drop in thereflected power, and these invertedpeaks in the reflected power spectrumare identified as the tank eigenmodefrequencies using a peak-detectionsoftware algorithm. This informationis passed to a pattern-matching algo-rithm, which compares the measuredeigenmode frequencies with a data-base of simulated eigenmode frequen-cies at various fill levels. A best matchbetween the simulated and measuredfrequency values occurs at some filllevel, which is then reported as thegauged fill level.

The database of simulated eigenmodefrequencies is created by using RF simu-lation software to calculate the tankeigenmodes at various fill levels. The

input to the simulations consists of afairly high-fidelity tank model withproper dimensions and including inter-nal tank hardware, the dielectric proper-ties of the fluid, and a definedliquid/vapor interface. Because of smalldiscrepancies between the model andactual hardware, the measured emptytank spectra and simulations are used tocreate a set of correction factors for eachmode (typically in the range of0.999–1.001), which effectively accountsfor the small discrepancies. These cor-rection factors are multiplied to themodes at all fill levels. By comparing sev-eral measured modes with the simula-tions, it is possible to accurately gaugethe amount of propellant in the tank.

An advantage of the RFMG approachof applying computer simulations and apattern-matching algorithm is that the

Radio-Frequency Tank Eigenmode Sensor for Propellant Quan-tity GaugingThis sensor has applications in cryogenic liquid storage tanks.John H. Glenn Research Center, Cleveland, Ohio

the return will be separated (sheared)from the outgoing beam by twice thedistance between the impact pointand the center of the retroreflector.Provided that shear amount is smallenough, the return will hit the aper-ture of the steering mirror and goback through the beam splitter and be

imaged on the back end of the scan-ner with the shear sensor. A telescopeplaced in front of the shear sensorserves to compress the image of the re-turn beam to the size of the detector.

To acquire the retroreflector withinthe field of view of the shear sensor, thesystem operates by first performing an

open loop search for the retroreflectortarget. Once a return from the retrore-flector optic is detected, a servo loop isclosed with the fast steering mirror andshear sensor to center the laser beam onthe vertex of the retroreflector. Oncelocked, any motion of the retroreflectorwill be tracked by keeping the servoerror small. Once in track mode, the IRrangefinder can be used to give rangemeasurements. Bearing measurementsare available from a local sensor used bythe steering mirror.

In comparison to flash LIDAR sys-tems, this work represents a system withmuch less complexity and a lower cost.The rangefinder used by the sensor sys-tem is a low-cost COTS (commercialoff-the-shelf) unit. The camera in aflash LIDAR system is replaced with amuch lower cost, two-dimensionalshear sensor that reports only the cen-ter of light of the image. This sensorserves as both a detector for determin-ing whether or not the retroreflector ishit by the pointing laser and as a feed-back sensor for the tracking systemwhen the retroreflector is moving.

This work was done by Joel F. Shields andBrandon C. Metz of Caltech for NASA’s JetPropulsion Laboratory. For more information,contact iaoffice@jpl.nasa.gov. NPO-47001

The Formation Control Testbed Optical Pointing Loop hardware is shown. The sensor system is com-posed of a laser rangefinder, fast steering mirror, back-end shear sensor, and a large-aperture, open-face retro target.

Fast Steering Mirror(not visible)

LaserRangefinder

Sensor Boresight

Retroreflector(on calibration block)

Laser for BearingMeasurement

(fiber optic feed)

PositionSensingDevice

8 NASA Tech Briefs, July 2010

High-Temperature Optical SensorThe technology significantly extends applicability of optical sensors to high-temperature envi-ronments.John H. Glenn Research Center, Cleveland, Ohio

A high-temperature optical sensor(see Figure 1) has been developed thatcan operate at temperatures up to 1,000°C. The sensor development processconsists of two parts: packaging of afiber Bragg grating into a housing thatallows a more sturdy thermally stable de-vice, and a technological process towhich the device is subjected to in orderto meet environmental requirements ofseveral hundred °C.

This technology uses a newly discov-ered phenomenon of the formation ofthermally stable secondary Bragg grat-ings in communication-grade fibers athigh temperatures to construct robust,optical, high-temperature sensors. Test-ing and performance evaluation (seeFigure 2) of packaged sensors demon-strated operability of the devices at 1,000°C for several hundred hours, and dur-ing numerous thermal cycling from 400to 800 °C with different heating rates.

The technology significantly extendsapplicability of optical sensors to high-temperature environments includingground testing of engines, flightpropulsion control, thermal protec-tion monitoring of launch vehicles,etc. It may also find applications insuch non-aerospace arenas as monitor-ing of nuclear reactors, furnaces,chemical processes, and other high-temperature environments whereother measurement techniques are ei-ther unreliable, dangerous, undesir-able, or unavailable.

This work was done by Grigory Adamovsky,Jeffrey R. Juergens, and Donald J. Varga of

Glenn Research Center and Bertram M. Floydof Sierra Lobo, Inc. Further information iscontained in a TSP (see page 1).

Inquiries concerning rights for the com-mercial use of this invention should be ad-

dressed to NASA Glenn Research Center, In-novative Partnerships Office, Attn: SteveFedor, Mail Stop 4–8, 21000 BrookparkRoad, Cleveland, Ohio 44135. Refer toLEW-18381-1.

Figure 1. A Probe packaged and connectorized with a fiber Bragg grating (FBG) inside. The FBG islocated at the end of the probe inside a smaller-diameter ceramic tube.

Figure 2. Performance Characteristics: (a) Wavelength stability of a sensor exposed to 1,000 °C for500 hours and (b) wavelength readings as a function of temperature during thermal cycling from 400to 800 °C.

Wav

elen

gth

.nm

1304

1306

1308

1310

400 500 600 700 800

Furnace, Temperature °C

(b)

Peak

Wav

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1311.8

1311.6

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predictions can be verified through test-ing on Earth, and the results can be ex-trapolated to low-gravity liquid configu-rations using simulations of liquidconfigurations that would be likely tooccur in low gravity. Such liquid configu-rations can also be solved using othercomputer software tools such as the Sur-face Evolver code. RF computer simula-tions are routinely used in the RF andcommunications industry to design orpredict performance of RF devices. The

same software tools can be used to calcu-late the electromagnetic eigenmodes oflarge tanks with a two-phase fluid distri-bution. By having a pre-built library oftank eigenmode simulations, the meas-ured tank eigenmode spectra can becompared with the library of spectra todetermine the unknown amount of pro-pellant in the tank.

This work was led by Gregory A. Zimmerli,David A. Buchanan, Jeffrey C. Follo, Karl R.Vaden, and James D. Wagner of Glenn Re-

search Center; Marius Asipauskas of Na-tional Center for Space Exploration Research;and Michael D. Herlacher of Analex Corp.Further information is contained in a TSP(see page 1).

Inquiries concerning rights for the com-mercial use of this invention should be ad-dressed to NASA Glenn Research Center, In-novative Partnerships Office, Attn: SteveFedor, Mail Stop 4–8, 21000 BrookparkRoad, Cleveland, Ohio 44135. Refer toLEW-18373-1.