A magnetic field response sensor comprises an inductor placed at a fixed separation distance from a conductive surface to address the low RF transmissivity of conductive surfaces. The minimum distance for separation is determined by the sensor response. The inductor should be separated from the conductive surface so that the response amplitude exceeds noise level by a recommended 10 dB. An embodiment for closed cavity measurements comprises a capacitor internal to said cavity and an inductor mounted external to the cavity and at a fixed distance from the cavity s wall. An additional embodiment includes a closed cavity configuration wherein multiple sensors and corresponding antenna are positioned inside the cavity, with the antenna and inductors maintained at a fixed distance from the cavity s wall

Taylor, Bryant D.

Woodard, Stanley E.

NASA Technical Reports Server

Source of Acquisition 
NASA Washington, D. c. 
Il1 111l1111111111ll US007075295B2kh III IIIl1111 /t-Il1 , 1111 ~ 7, c 1 
(12) United States Patent (io) Patent No.: US 7,875,295 B2 
Woodard et al. (45) Date of Patent: Jul. 11,2006 
(54) MAGNETIC FIELD RESPONSE SENSOR 
FOR CONDUCTIW MF,DIA 
(75) Inventors: Stanley E. Woodard, Hampton, VA 
(US); Bryant D. Taylor, Smithfield. VA 
(US) 
(73) Assignee: The United States of America as 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, DC (US) 
Subject to any disclaimer, the term of this 
patent is extended or adjusted uuder 35 
U.S.C. 154@) by 0 d q s .  
( * ) Notice: 
(21) Appl. No.: 10/839,448 
(22) Filed: Apr. 30, 2004 
(65) Prior Publication Data 
US 200510017711 A1 Jan. 27, 2005 
Related U.S. .4pplication Data 
(60) Provisional application No. 601467,841, filed on May 
1, 2003. 
(5i) int. Ci. 
GOlR 33/02 (2006.01) 
GOIN 2 7/00 (2006.01) 
(52) US. C1. ....................... 3241239; 3241244; 3241260 
(58) Field of Classification Search .._............. 3241239; 
2351439,449,451 
See application file for complete search history. 
(56) References Cited 
U.S. PL4TENT DOCUMENTS 
4,482,859 A * 11i1984 Foumier ..................... 3241661 
4,490,773 A 1211954 Moffatt 
4,594,640 A 6/1986 TatsiUni 
5,075.600 A 1211991 El-Hamamsy et al. 
5,420,757 A W995 Eberhardt et al. 
5,423,334 A 611995 Jordan 
6,025,735 A Z2000 Gardner et al. 
6.111,520 A SEOOO Men et al. 
6,278,379 B1 * 8/2001 Allen et al. ..._._._.... 340B70.16 
6,335,690 B1 112002 Konchin et al. 
6,463,798 B l  1012002 Niekerk et al. 
6,532.834 B1 3/2003 Pinto et al. 
6,515,958 B1 11/2004 K e d  et al. 
2001i0001311 A1 5i2001 Park et al. 
2004/0057589 A1 * 312004 Pedersen et al. .._......... 381/152 
2005!0028601 -41 * 22005 Pedersen et al. .............. 731718 
OTHER PUBLICATIONS 
John C. Butler,hthony J. Vigliotti, Fred W. Verdi, & Shawn 
M. Walsh, ‘Wireless, passive, resonant-circuit, inducevely 
coupled, inductive strain sensor,” Elsevier. Elsevier Science 
B.V., p. 63-66. 
(Continued) 
Primary Exuminer-Bot LeDynh 
(74) dttorney, Agent, or Firm-Robin W. Edwards 
(57) iiBSTRACT 
-4 magnetic field response sensor comprises an inductor 
placed at a fixed separation distance ftum a conductive 
surface to address the low RF transmissivity of conductive 
surfaces. The minimum distance for separation is deter- 
mined by the sensor response. The inductor should be 
separated tiom the conductive surface so that the response 
amplitude exceeds noise level by a recommended 10 dB. An 
embodiment for closed cavity measurements comprises a 
capacitor internal to said cavity and an inductor mounted 
external to the cavity and at a fixed distance tiom the 
cavity’s wall. An additional embodiment includes a closed 
cavity configuration wherein multiple sensors and corre- 
sponding antenua are positioned inside the cavity, with the 
antenna and inductors maintained at a fixed distance ftum 
the cavity’s wall. 
4 Claims, 5 Drawing Sheets 
51 f 50 
https://ntrs.nasa.gov/search.jsp?R=20060050049 2019-08-29T23:53:18+00:00Z
US 7,075,295 B2 
Page 2 
OTHER PUBLICATIONS 
K.G. On& C.A. ~ , . j ~ ~ ~ ,  C.L. Robbhs, R.S. siu$. 
“Jlesi@ and application of a wireless, passive, resouant- 
circuit euvirunmental mouitoring sensor,” Elsevier, Elsevier 
Science BY., p. 33-33. 
Keat Ghee Ong & Craig A. Grimes, “A resonant printed- 
circuit sensor for remote query monitoring of euviroumeutal 
parmeters,” Smart Materials Stnlt. 9 (2000), p. 421-425. 
* cited by examiner 
U.S. Patent Jul. 11,2006 Sheet 1 of 5 US 7,075,295 B2 
11 10 
FIG. I 
10 
FIG. 2 
U.S. Patent Jul. 11,2006 Sheet 2 of 5 US 7,075,295 B2 
10 
FIG. 3A 
10 
FIG. 4A 
FIG. 4B 
U.S. Patent Jul. 11,2006 Sheet 3 of 5 US 7,075,295 B2 
11 
l 3  7 
f- 50 51 
FIG. 5 
U.S. Patent Jul. 11,2006 Sheet 4 of 5 
61 
US 7,075,295 I32 
FIG. 6 
FIG. 7 
63 
61 
U.S. Patent Jul. 11,2006 Sheet 5 of 5 US 7,075,295 B2 
65 
82 
62 
FIG. 8 
US 7,075,295 B2 
1 
MAGNETIC FIELD RESPONSE SENSOR 
FOR CONDUCTIVE MEDL4 
CLAIM OF BENEFIT OF PROVISIONAL 
APPLICATION 
Pursuant to 35 U.S.C. Q 119, the benefit of priority h m  
provisional application hving US. Ser. No. 60/467,841. 
filed on May 1, 2003 is claimed for this nonprovisional 
application. 
This application is related to co-pending, commonly 
owned patent application US. Ser. No. lOl839.445, filed 
Apr. 30.2004. entitled “Magnetic Field Response Measure- 
ment -4cquisition System.” 
STATEMENT REGARDING FEDERALLY 
SPONSORED RESE.4RcH OR DEVELOPMENT 
The invention described herein was made in part by 
employees of he United States Government and may be 
manufactured and used by and for the Government of the 
United States for governmental purposes without the pay- 
ment of any royalties thereon or therefore. 
BACKGROUND OF THE NTNTION 
1. Field of the Iiivention 
The present invention relates generally to magnetic field 
response sensors for conductive media, more specifically 
conductive suhces and conductive closed cavities. It relates 
in particular to a magnetic field response sensor comprising 
an inductor separated by a fixed distance from the conduc- 
tive surface. The inductor radiates essentially in open space 
and transmits information gathered by the capacitive ele- 
ment. 
2. Description of the Related Art 
-4 magnetic field response sensor is a passive inductor- 
capacitive circuit designed to change correspondingly with 
a change in the physical state that the sensor measures. Use 
of inductors and capacitors to form resonant circuits is 
established in the literature. See. for example, D. Halliday 
and R. Resnick Fundanrental of Physics, 2nd Edition, 
Wiley, New York, pp. 624-634 or similar basic physics or 
electronics texts. 
US. Pat. No. 4,490,773 (Moffatt) teiches a capacitive 
pressure transducer that uses a separate inductance and 
capacitance, with the capacitor and inductor attached to the 
same framework. Moffatt teaches a method for overcoming 
the temperature problems associated with such design. US. 
Pat. No. 6.532,834 (Pinto et al.) teaches a capacitive pres- 
sure sensor having encapsulated resonating components, 
wherein the capxitor is located within a housing and the 
inductor is located outside of the housing. The resonance 
circuit is electrically coupled to an oscillator. 
Metal enclosures have low transmissivity for Radio Fre- 
quency (FR) energy and thus present problems for magnetic 
field response sensors. Proximity to conductive surfaces 
alters the inductance of magnetic fi-equency response sen- 
sors. -4s a sensor gets closer to a conductive d a c e .  the 
magnetic field energy of the sensor is reduced due to eddy 
currents being induced in the conductive surface. Existing 
measumment devices do not address the issue of RF trans- 
missivity. It is offen desirable to obtain measurements per- 
taining to conductive surfaces. Examples of closed cavities 
for which measurements are desired within the cavity 
include metal fuel tanks and landing gear struts. 
2 
SuMhlARY OF THE INVENTION 
Accordingly, it is an object of the present invention to 
provide a magnetic field response sensor for acquiring 
It is a fiirther object to provide a magnetic field response 
sensor that addresses the low Radio Frequency (RF) trans- 
missivity of conductive surfaces. 
In accordance with the present invention, a magnetic field 
response sensor comprises an inductor placed at a fixed 
separation distance from a conductive surface. 
5 measurements for conductive surfaces. 
BRIEF DESCRIPTION OF THE DRAWINGS 
FIG. 1 illustrates an embodiment of a nonconductive 
l5 spacer used to maintain a fixed separation of an inductor 
fiom a conductive surface. 
FIG. 2 illustrates an embodiment of a sensor in which an 
kductor is positiosed at a ked angle away from a conduc- 
tive surface. 
FIG. 3 illustrates an embodiment of a rotational deploy- 
able inductor. 
FIG. 4 illustrates an embodiment of a telescopic deploy- 
able inductor. 
FIG. 5 illustrates an embodiment having a capacitor 
25 embedded in a composite material having electrically con- 
FIG. 6 shows an embodiment of a magnetic field response 
FIG. 7 shows an embodiment of the inductive and capaci- 
FIG. 8 shows an embodment of a closed cavity measure- 
ments system having multiple sensors and antema located 
internal to the cavity. 
20 
ductive layers. 
sensor for closed cavity measurements. 
30 tive elements for closed cavity measurements. 
DETAILED DESCRIPTION OF THE 
INVENTION 35 
It is necessary in some applications to have a magnetic 
field response sensor’s capacitor aftixed to or embedded in 
a conductive surface. Mapetic field response sensors are 
4” passive inductor-capacitor L-C circuits. The inductor L is 
placed in parallel with the capacitor C, forming an L-C(p) 
circuit. An antenna can be used to produce either a broad- 
band time-varying magnetic field or a single hannonic 
mapetic field, with the magnetic field creating an electrical 
45 current in the sensor as a result of Faraday induction. The 
sensor will electrically oscillate at resonant electrical fie- 
quencies that are dependent upon the capacitance and induc- 
tance of the sensor. The oscillation occurs as the energy is 
harmonically transferred between the inductor (as magnetic 
50 energy) and capacitor (as electrical energy). When the 
energy is in the inductor, the magnetic fields produced are 
single harmonic radio frequencies whose hquencies are the 
sensor’s resonant fiequencies, and are dependent on how the 
physical measured properly changes the capacitance of the 
j5 circuit. The. antenna can also be used to receive the hannonic 
magnetic responses produced by the inductors. The receiv- 
ing antenna can be the same antenna used to produce the 
initial broadcast of energy received by the L-C circuit or 
another antenna can be used. When the same antenna is 
used, it must be switched from a transmitting antenna to a 
6o receiving antenna. h example of a magnetic field response 
system, described in related patent application US. Ser. No. 
101839,445, is herein incorporated by reference. The sensor 
described and claimed herein is suitable for use in the system 
described in such related application, as well as other 
Proximity to conductive surfaces aiters the inductance of 
the sensors. As the sensor gets closer to a conductive surface, 
65 magnetic field response system. 
US 7,075,295 B2 
3 
the magnetic field energy of the sensor is reduced due to 
eddy currents being induced in the conductive si&ce. 
Therefore, inductors cannot be &xed to or embedded in a 
conductive surface. It is necessary to have a means of fixed 
separation. A separation distance of at least 0.375 in was 
found to be suitable in experimental measurements. The 
minimum distance for separation is determiued by the sensor 
response. The inductor should be separated ffom the con- 
ductive surface so that the response amplitude exceeds noise 
level by a recommended 10 dB. 
Referring now to the drawings, FIGS. 1 and 2 illustrate 
embodiments for maintaining constant inductance levels. In 
FIG. 1, a dielectric nonconductive spacer 11 is used to 
maintain a fixed separation of the inductor 14 from the 
conductive surface 10. Nonconductive film 12 and capacitor 
13 are also shown. Although the inductance is less than what 
it would be if it were not in proximity to the conductive 
surface 10, the inductance is fixed. As long as the inductance 
is fixed, dl vaiations of the magnetic fie!d response are due 
to capacitance changes. FIG. 2 illustrates a sensor in which 
the inductor 14 is positioned at a fixed angle sway h m  the 
conductive surface 10. A lightweight stigener 20 is used to 
maintain the angle. Other suitable means for maintaining the 
angle may also be used. 
Numerous variations of inductor 14 mounting can be 
utilized. such as providing a housing that provides separa- 
tion from the conductive surface 10 as well as protection 
from impact darnage. Systems that have limited space but 
undergo deployment can have inductors that deploy during 
deployment of the system and maintain a fixed position after 
deployment is complete, including both rotational and tele- 
scopic deployable inductors. If capacitance is maintained 
fixed in value, changes in inductance resulting ffom varia- 
tion of the separation between inductor and conductive 
surface can be used to measure proximity to that surikce. 
This variation depends on the surface skin depth. FIGS. 3 
and 4 illusbate rotational and telescopic deployable induc- 
tors, respectively. FIGS. 3a and 4a show the sensors in their 
stowed positions. FIGS. 3h and 4b show the sensors in their 
deployed positions. Latches 16 maintain closure while in the 
stowed position. Rotational spring 15 deploys the retractable 
sensor. 
FIG. 5 illustrates an embodiment having a capacitor 13 
embedded in a composite material 50 having electrically 
conductive layers 51. 
Referring now to FIGS. 6 and 7, an embodiment of a 
magnetic field response sensor for closed cavity measure- 
ments in accordance with the present invention is shown and 
referenced generally by numeral 60. A capacitive element 61 
of the sensing element 70 is situated in a closed cavity 62 
and the inductive element 63 of the sensing element 70 is 
placed outside of the closed cavity 62. This allows the 
inductive portion 63 of the sensing element 70 to radiate in 
essentially open space and transmit the information gathered 
by the enclosed capacitive element 61. A broadband antenna 
can broadcast electromagnetic energy within the frequency 
range of the sensing element 70 and receive the emissions of 
the sensing element 70. which signals are processed to 
i d e n w  phenomena associated with the sensor. 
A narrow throat portion 71 of the sensing element 70 
connects the inductor 63 to the capacitor 61. The throat 71 
is of sdficient length to allow the capacitor 61 to be 
appropriately placed within the cavity 62. The inductor 63 is 
placed outside the cavity 62. The throat 71 is fed through the 
orifice 64 in the cavity wall 65 that is used to fill the cavity 
62 (e.g.. fiiel tank opening) andconnects the inductor 63 and 
capacitor 61 via elechical leads 66 to form a parallel circuit. 
Another embodiment i s  to have the inductor 63 and capaci- 
inductor 63 is mountd external to the cavity 62 and the 
... LO& 51 fzbricated as sqxmte units. h +&s cabcx%mat, the 
4 
capacitor 61 is mounted internal to the cavity 62. Electrical 
leads 66 are fed rhroiigh the orifice 64 that is used to fill the 
cavity 62 and connect the inductor 63 and capacitor 61 to 
form a parallel circuit. 
As discussed. the magnetic field produced from an elec- 
trically active inductor is eliminated when placed in very 
close proximity to an electrically conductive material. This 
means that antennae or inductors cannot be placed on the 
surface of an electrically conductive material or embedded 
1o in an electrically conductive composite materials (e.g., 
graphite fibers). Additionally, to use a conductive material to 
support an antenna made ffom metal foil or metal deposited 
on a thin film, the antenna must be separated, such as by use 
of a spacer. The thickness of the spacer is dependent on the 
amount of field strength that the antenna can lose without 
15 losing its ability to acquire its measurement. The same is true 
for the inductor used in the sensor. If the sensor is placed on 
a conductive surfkcel the capacitor can be placed in contact 
with the surface (a dielectric layer must separate the capaci- 
tor and the surface), but the inductor must be separated from 
20 the surface via a spacer. Similarly, the capacitor can be 
embedded within conductive composite layers but the induc- 
tor must be placed on the outside and separated. 
When the cavity contailling the sensor is inade of a 
conductive material and the antenna is external to the cavity, 
15 the inductor must also be external to the cavity to allow the 
sensor to be exposed to the antenna's varying magnetic field. 
As mentioned above, the inductor must be maintained in a 
fixed position relative to and separated &om the conductive 
surface. 
When a cavity containing multiple sensors is made of a 
30 conductive material, an a n t e m  80 can be placed internal to 
~e cavity 62, as illustrated h FIG. 8.An internal antenua 80 
allows all components of the sensors 82 to reside inside the 
cavity 62. The antenna 80 must be separated from the cavity 
wall's 65 conductive surface. Additionally, the inductors 
35 must be maintained in a fixed position relative to and 
separated from the cavity wall 65. Antenna leads 81 are fed 
through an orifice 64 in the cavity wall 65. 
Although the invention has been described relative to 
specific embodiments thereof, there are numerous variations 
40 and modifications that will be readily apparent to those 
skilled in the art in light of the above teachings. It is 
therefore to be understood that, within the scope of the 
appended claims, the invention may be practiced other than 
as specifically described. 
What is claimed as new and desired to be secured by 
Letters Patent of the United States is: 
1. A magnetic field response sensor for acquiring physical 
state measurements of a conductive material, comprising: 
an inductive element positioned at a fixed separation 
distance eom said conductive material; and 
a capacitive element in operative communication with 
said inductive element and embedded within said con- 
ductive material. 
2. The sensor of claim 1, wherein said fixed separation 
distance is maintained by a nonconductive layer disposed 
between said inductive element and said conductive mate- 
rial. 
3. The sensor of claim 1. wherein said inductive element 
is separated &om said conductive material such that said 
6o inductive element's response amplihide exceeds noise level 
by about 10 dB. 
4. The sensor of claim 1, wherein said conductive material 
is a composite material having electrically conductive lay- 
45 
55 
65 ers' 
* * * * *  


Magnetic Field Response Sensor For Conductive Media

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server