NASA Tech Briefs, July 2010 5

Technology Focus: Sensors

Sensors for monitoring surface wearand/or temperature without need forwire connections have been developed.Excitation and interrogation of thesesensors are accomplished by means of amagnetic-field-response recorder — anapparatus previously reported in “Mag -netic-Field-Response Measurement-Ac-quisition System” (LAR-16908), NASATech Briefs, Vol. 30, No. 6 (June 2006),page 28. To recapitulate: The magnetic-field-response recorder is placed near,

but not touching, the sensor of interest.This apparatus generates an alternatingmagnetic field that excites oscillationsin the resonant circuit, measures themagnetic response of the circuit, anddetermines the resonance frequencyfrom the response.

These sensors are related to the onesreported in “Wirelessly Interrogated Po-sition or Displacement Sensors” (LAR-16617-1), NASA Tech Briefs, Vol. 31, No.10 (October 2007), page 20. Like the

previously reported sensors, these sen-sors consist mainly of variable capacitorselectrically connected in series withfixed inductors. In a sensor of the pres-ent type as in the previously reportedones, the capacitance and, thus, the res-onance frequency, varies as a knownfunction of the quantity of interest thatone seeks to determine. Hence, the res-onance frequency is measured and usedto calculate the quantity of interest.

The upper part of the figure depictsone of the present sensors, wherein thecapacitor consists of multiple interdigi-tated plate electrodes oriented perpen-dicular to the wear surface of, and em-bedded within, a block of material, thewear of which one seeks to monitor. (Forexample, such a sensor could be embed-ded in a brake pad.) The embedment isperformed during the fabrication of thebrake pad or other block of wearing ma-terial. The electrodes are made of ametal that becomes worn away more eas-ily than does the material that one seeksto monitor. As the surface wears away,portions of the electrodes are also wornaway, reducing the capacitance. Thedepth of wear can be estimated straight-forwardly from the increase in the reso-nance frequency, using the known rela-tionship between the change inresonance frequency and the reductionin capacitance as a function of the depthof wear.

This wear sensor can be augmentedwith a temperature-measurement capa-bility by embedding, between two ormore of the electrodes, a dielectric ma-terial that is temperature-sensitive inthe sense that its permittivity exhibits aknown variation with temperature. Inthis case, the capacitance, and thus theresonance frequency, depends on boththe depth of wear and the tempera-ture. Hence, if the temperature isknown from a measurement by a differ-ent sensor, then the depth of wear canbe determined from the resonance fre-quency. Similarly, if the depth of wearhas been determined from a priormeasurement by a different sensor (orby this sensor at a known temperature)

Wirelessly Interrogated Wear or Temperature Sensors Such sensors could be embedded in brake pads. Langley Research Center, Hampton, Virginia

Capacitors Comprising Interdigitated Electrodes are connected to inductors to form resonant circuits. Inboth cases, the capacitance varies with the remaining thickness (t) and, thus, the depth of wear. Option-ally, a temperature-sensitive dielectric can be included to obtain a temperature-measurement capability.

Inductor

Interdigitated Electrodes

Wear Surface

Slat View

Temperature-SensitiveDielectric

Top View

SENSOR 1

t

Spiral Inductor

Interdigitated Electrodes

Wear Surface

Temperature-Sensitive Dielectric

SENSOR 2

https://ntrs.nasa.gov/search.jsp?R=20100024412 2018-06-20T22:12:45+00:00Z

6 NASA Tech Briefs, July 2010

and there has been no wear since theprior measurement, then the presenttemperature can be determined fromthe present resonance frequency.

The lower part of the figure depicts an-other sensor of the present type, contain-ing multiple sets of interdigitated elec-

trodes embedded parallel to the wearingsurface in a configuration such that thenumber of electrode pairs, and thus thecapacitance, decreases with the depth ofwear. Optionally, one or more of the setsof interdigitated electrodes can be em-bedded along with a temperature-sensi-

tive dielectric material to obtain a tem-perature-measurement capability.

This work was done by Stanley E. Woodardof Langley Research Center and Bryant D.Taylor of Swales Aerospace. Further informa-tion is contained in a TSP (see page 1). LAR-16591-1

The optical pointing sensor providesa means of directly measuring the rela-tive positions of JPL’s Formation Con-trol Testbed (FCT) vehicles withoutcommunication. This innovation is asteerable infrared (IR) rangefinderthat gives measurements in terms ofrange and bearing to a passive retrore-flector. Due to its reduced range of mo-tion, the range and bearing measure-ments are on the order of 10 times

better than those of the existing sensorsystem.

The retroreflector is placed on onerobot, and the range finder and steeringoptics are on another robot. The meas-urements are available on therangefinder-mounted robot, giving itrelative position knowledge to theretroreflector.

The system is composed of an HeNepointing laser, a SICK IR laser

rangefinder, a two-axis fast steeringmirror, a shear sensor, and a far-fieldretroreflector (see figure). The point-ing laser is injected into the opticalpath using a beam splitter andbounces off the steering mirror to-ward the retroreflector. If the retrore-flector is hit by the pointing laser, thebeam is returned with the exact oppo-site direction. When the beam impactwith the retroreflector is non-central,

Optical Pointing SensorThe sensor can be used as a digitizer of physical objects to extract shape data.NASA’s Jet Propulsion Laboratory, Pasadena, California

Standard microfabrication techniquescan be implemented and scaled to helpassemble nanoscale microsensors. Cur-rently nanostructures are often de-posited onto materials primarily byadding them to a solution, then apply-ing the solution in a thin film. This re-sults in random placement of the nanos-tructures with no controlled order, andno way to accurately reproduce theplacement. This method changes themeans by which microsensors withnanostructures are fabricated. The fun-damental advantage to this approach isthat it enables standard microfabrica-tion techniques to be applied in the re-peated manufacture of nanostructuredsensors on a microplatform.

The fundamental steps are first to de-fine a standard metal electrode patternof interdigitated fingers with parallel fin-gers that are saw-toothed. Nanostruc-tures are then added to a standard pho-toresist to form a dilute solution. Thephotoresist solution is then applied to

the microstructure. Before the solutionsolidifies, alternating electric fields areapplied across the electrodes in order toalign the nanostructures on the wafer.Once this photoresist later dries into afilm and is processsed, a second layer ofmetal is deposited on top of the firstlayer. The effect is to remove photoresistfrom the metal fingers, but leave thenanostructures that bridge the fingers tobe held in place by the top metal layer.Longer nanostructures, which are al-ready aligned across the fingers, will beheld in place by the top metal.

This buries the contacts of the nanos-tructures that are bridging the fingersbetween two layers of metal. The result isa microsensor fabricated using micro-fabrication techniques with alignednanostructures bridging the electrodesand buried electrical contacts.

Possible applications include emis-sions monitoring, leak detection, en-gine monitoring, security, fire detec-tion, extra-vehicular-activity (EVA)

applications, personal health monitor-ing, and environmental monitoring.Because this process is compatible withlow temperatures and thin-film sup-ports, it can be used in thin films forconductive coatings requiring electricalconnections.

A proof-of-concept of this approachwas demonstrated using alumina as thesubstrate, metals such as platinum as thebottom electrode and titanium as the topmetal layer, and both multiwalled carbonnanotubes and metal oxide nanowires asthe nanostructured material.

This work was done by Gary W. Hunter,Randall L. VanderWal, Laura J. Evans, andJennifer C. Xu of Glenn Research Center.Further information is contained in a TSP(see page 1).

Inquiries concerning rights for the commer-cial use of this invention should be addressedto NASA Glenn Research Center, InnovativePartnerships Office, Attn: Steve Fedor, MailStop 4–8, 21000 Brookpark Road, Cleve-land, Ohio 44135. Refer to LEW-18418-1.

Processing Nanostructured Sensors Using Microfabrication Techniques Nanostructured sensors have uses in safety, environmental monitoring, fire detection, and security. John H. Glenn Research Center, Cleveland, Ohio