Note: Descriptions are shown in the official language in which they were submitted.
29 .~
GCD 80-1 ~
I'
NUCLEAR ~GNETIC XESONANCE GYRO f'
Cross-Re~erence to a Related Paten _~pplication
....
APPLICATION
'~',' '.
This is an improvement o~ the invention in U. S. Patent
. . i.
4,157,495 which issued June 5, 1~79 to Bruce C. Grover, Edward
Kanegsberg, John G. Mark and Roger L. Meyer for a Nuclear
Magnetic Resonance Gyro, which patent is assigned to Litton
Systems, Inc., the assignee of this application.
:-- ; . . . ::
~ - BACKGROUND OF THE INVENTION
~ ' ' - ' ,,
' ' ' ' - .:
~O This invention relates to the creation and detectior. o --
nuclear magnetic resonance. More particularly this invention
relates to the a~plication of nuclear magnetic resonance in an
- angular rate sens~r.
:, . , . . ;:
. .
- A number of approaches have been suggested in the prior
1~ art ~or implementing the basic concept o~ a n~clear magnetic
resonance NMR gyroscope. In general, they utiliæe a nuclear
magnetic resonance controlled oscillator and derive rotational
information from the phases of the nuclear moment Larmor
prccession signal~ by means of suitable phase comparison and
zo magnetic ~ield control circuitry.
' ,.
.
Page l
. , ~
3L6aJ5~9
GCD 80-1
These devices contain significant deficiencies which
limit the development o a useful instrument. For instance, such
devices have been lirnited by relatively short relaxation times of
the gases which have been employed. Also, the strong direct
5 coupling ~etween these gases and the light which is employed as
the means of magnetic moment ali~nmert or magnetic momen~
detection can limit both the relaxation times and the
signal-to-noise ratio, and thererore can also limit the potentlal
usefulness of such instruments.
In U.S. Patent 4,157,495 a nuclear magnetic resonance
(hereinafter referred to as "NMR") angular rate sensor or
gyroscope is disclosed that operates on the principle of sensing
inertial angular rotation rste os~ sr7~ nt about a
sensitive axis of the device as a shl~t in the Larmor precession
15 frequency or phase, respectively, of one or more isotopes that
possess nuclear magnetic moments. The gyroscope is composed of
an angular rotation sensor and associated electronics. The - -
principal elements of the sensor are a light source, an NMR cell,
a photodetector, a set of magnetic shields and a set of magnetic
20 field coils. The principal elements of the electronic.s are
signal proc~ssing circuits for extracting the Larmor precession
frequency and phase information as well as circuits for
gen~rating and controlling various magnetic fields, both steady
and varying sinusoidally with time, that are necessary for the
2~proper operation o~ the device.
Page 2
1 164529 GCD ~0-I
lhe NMR cell is mounted within a set of magnetic shields
in order to attenuate external magnetic ~ields to acceptably low
levels. Magnetic field coils are used to apply very uni~orm
magnetic fields t~ the NMR cell. Both a steady field and an AC
5 carrier field are applied along the sensitive axis oF the device L
and AC feedback fields are applied along one of the transverse
axes. The DC magnetic fields along both transverse axes are
controlled to be substantially zero. The N~IR cell con~ains a
single alkali metal vapor, such as rubidium, together with two
10 isotopes of one or ~ore noble gases, such as krypton-83, and
xenon-129, or xenon l31. A buffer gas such as helium may also be
contained in the cell.
The NMR cell is illuminated ~y a beam~o~ circularly
polarized light that originates from a source such as a rubidium
15 lamp or a rubidium solid state laser and which passes througn the
cell at an angle with respect to the steady magnetic fieldc
Absorption o~ some o~ this light causes the atomic magnetic
~oments of the rubidium atoms to be partly aligned in the
direction of the steady magnetic field. This alignment is partly
20 trans~erred to the nuclear magnetic moments of the noble gases,
and these moments are caused to precess about the direction of
the steady magnetic field, which in turn creates magnetic fields
tha~ rotate at the respective Larmor precession frequencies of
the two noble gases. These rotating fields modulate the
25 precessional motions o~ the rubidium magnetic moments, which in
Page 3
1164529 GCD BO~
turn prodllces corresponding modulations of the transmitted light,
therehy making is possible optically to detect the Larrnor
precession frequencies of the two noble gases.
. .
The modulations of the light intensity are converted into
electrical signals by a photodetector, and these signals are then
electronically demodulated and filtered to provide signals-at the
Larmor precession frequencies of the two noble gases. The
dif~erence between the t~o precession frequencies is used
accurately to control the steady magnetic field so that it is
constant. One of the noble gas precession frequencies is
compared to a precision reference frequency, and the resul~ing
difference ~requency is a measure of the angular rotation rate of ~ -
the g~roscope~
The two detected noble gas precession signals are also
15 ~sed to generate two AC feedback magnetic ~ields at the Larmor
~ . - . . ...... . . .... . . . .
precession frequencies of the noble gases, and these are
responsible for sustaining the precession of the nuclear magnetic
moments of the noble gases. The use of an AC carrier magnetic
field facilitates the optical detection of the precessing noble
20 gas moments,and it provides means for controlling the DC magnetic
fields along the two ~ransverse axes of the gyroscope.
According to ~he patent, the NMR gyroscope includes means
for the simul~aneous alignment of ~he nuclear magnetic moments of
at least two nuci~ar moment gases, thereby constituting a nuclear
Page 4
116~529 ~ GCD ~0-l
,
ma~netic momen~ alignment device; the means for achieving
sustained precession of these moments, thereby constituting a
nuclear magnetic resonsnce oscillator capable of sustained
oscillations, the means for ~he optical detection of these
precessing nuclear moments thereby constituting a nuclear
magnetic resonance detection device; the ~eans for accurately
controlling the internal magnetic field of the device; and the ~ '-
means for the accurate measurement of ~he frequency or phase of.
the detected nuclear moment precession signal of at least one of
the nuclear moment gases to provide a measurement of the angular
rotation rate or angular displacement, respectively, o~ the
device with respect to inertial space, thereby constituting an
NMR gyroscope~ . .
- ` . ;.
More particularly, a steady magnetic field is applied to
an NMR cell which is substantially shielded from other steady
magnetic fields The NMR cell contains a gas or vapor of a
substance that possesses a magnetic moment that can be aligned by
optical p~mping, together with one or more additional gases, each
of which possesses a nuclear magnetic moment~ The NMR cell is 1.
illuminated by optical pumping liqnt whicA has a directional
component that is parallel to the direction of the steady
magnetic field and which has the proper wavelen~th to be absorbed
by the optically pumpable substance and partially align the , I .
magnetic momen~s o~ that substance~ The nuclear moments of the
2~ nuclear moMent gases are ca~sed to become aligned and are caused
to precess at their respective Larmor precession ~requencies
D~ 5
~ 4 5 2 9 GCD 80-1
about the direction o~ the steady magnetic field. An AC magnetic
field at a suitable carrier ~requency is also applied to the NMR
cell, and the cell .is illuminated by detection light ~hich has a
directional component that is orthogonal to the c~irection of the
AC carrier magnetic field and which has a wavelength that is
essentially the s~me as that of the op~icai p~ping light. The
intensity of the part of the detection light that is trans~itted ~.
by the cell is modulated in accordance with the totality of the
magnetic fields present in the cell, including the magnetic
fields that are generated by the precessing nuclear maynetic
moments. These modulations of the trans.nitted light intensity
are detected by a photodetector, after which they are
; electronically demodulated to obtain signals at the Larmor
~ precession frequencies of the nuclear momen~ gasPsS.-:
.' ,
1~ In one embodiment of the patented invention, the alignment of the nuclear magnetic moments of each nuclear moment gas is
accomplished by collisional interactions between the atoms of the
optically pompable substance and the atoms of the nuclear moment
gas or gases. Sustained precession of the nuclear magnetic
moments o~ each nuclear moment gas is accomplished by the
application of an AC feedback magnetic field at the Larmor
precession frequency of the nuclear moment gas in a direction
that is ortho~onal to the direction of the steady magnetic field.
The AC carrier magnetic ~ield is applied at substantially the
Larmor precession frequency o~ the optically p~npable substance
and in a direction that is substantially parallel to the
Page 6 .
~ 4 ~ 2 9 ~CD 80-1 ,
direction of the steady magnetic field, thereby permittiny the
device to be operated at higher values of the steady rbagnetic
field strength and with correspondingly hi~her Larmor precession
frequencies for the nuclear moment gases.
. . ,
In the preferred embodiment, an optically pumpable
substance such as a single alkali metal vapor is placed in an N~lR
cell together wi~h two noble gases, and the nuclear magnetic
moments of both noble gases are aligned simultaneously by
collisional interactions between the atoms of the sing:le alkaIi
~etal atoms and the atoms of the two noble gases. In this
pre~erred embodiment of the invention, the alkali me~al is
rubidium and the noble gases are xenon-129 ! and xenon 131.
. Another feature of the patent invo].ves the use of at least
one buffer gas in substantial quantities ;.n the NMR cell~.
' ' ' ' - '
In accordance with still another feature of the paten~,
the magnitude of the steady magnetic field is ca~sed to remai.n
constant by feedback control of the field in such a way that the
difference between the Larmor precession frequencies of the two
noble yases in the N~lR cell is caused to be equal to a
predetermined constant value.
In accordance with yeL another ~eature of the patent, one .
of the Larmor ~recession frequencies is compared to a precision
reerence ~requency and the resulting difference ~requency is
I 164529 GCD 80-1
utilized to provide a measuremen~ of angu:Lar displacement or
an~ular rate of the device about the direction of the steady ,,
magnetic field. !:
SU~ RY OF T~JE INVENTION
'; - ' ~,
It is contemplated b,y this invention to include ~wo rather
than one alkali metal vapors within the MMR en~elope. Oné of the
vapors, for example rubidium, is used because it is easily
excited or pumped by light from a rubidium lamp or a laser at the
rubidium wavelength~ The other alkali metal vapor, for example
cesium, is easily pumped by a cesium lamp or laser at the cesium
; 15 wavelength. The cesium within the NMR enclosure,is modulated at
the Larmor precession frequencies of the two nuclear magnetic
moment gases such as xenon 129 and xenon 131. The cesium vapor
;' is illuminated, for example, by a cesium lamp or a laser, and the
transmitted cesium radiation is modulated at the Larmor
20 precession frequencies of the two nuclear moment gases. The ',
transmitted light is detected, and the detected signals are used
in a manner identical to that déscribed in patent 4,157,495.
' ' ,
It is therefore an object of this invention to provide an
N~R gyroscope usin~ one pumpable vapor and a different sensing
vapor. The words "different vapor" are defined herein to include
di~ferent isotopes o~ the same vapor, particularly where the
vapor is an alkali metal vapor.
.
. Page 8
~CD ~0-l ~
~ ~452~ I~
It is a more specific object of this invention to use two
- alkaline vapors in an NilR gyro.
It is still a more speciic object of this invention to
use rubidium vapor as a pumping vapor and cesium vapor as a
5 detection vapor in an ~MR gyro which uses two aligned nuclear ,;
moment gases precessing at their Larmor precession frequencies.
. .
- DESC~IPTION OF THE DRAWINGS
The only figure i5 a conceptual diagram illustrating the
p~ocesses of optical pumping and of modulation of ~he itensity of
the light that is transmitted by the NMR cell.
' " .'.
DETAILED DESC~IPTIG~ E
PREFER~ED E~ODI~
The figure is a conceptual diagram illustrating for each
of the noble gases the process of optical p~nping and of
modulation of the intensity of the light that is transmitted
5 through the NIIR cell 28. Because these processes are so similar
for the two noble gases, they are illustrated and descrihed ~or
only one of the two noble gases. The circularly polarized
plunping light, ~or example from a laser at the rubidium
wavelength, which enters the NMR cell 28 has a component 64 along
the z-axis. Through the interactions of the opticai pumping
light 64 and the steady magnetic field 68, the rubidium atoms ~0
have their magnetic moments aligned preferentially in the
z-direction; ~y interatomic collisions, this magnetic moment
Page 9
` ~
, - GCD 80-1
~ 164~29
alignment is transferred rom the rubidium atoms 60 to the noble
gas nuclei 62 and to the cesium atoms 61.
'
A sinusoidal AC feedback magnetic field 70 that is matched
in frequency and phase to the Larmor precession frequency of the
coliective magnetic moment of the noble gas nuclei 62 is applied
in the x-direction and serves to torque the magnetic moment of
these nuclei to the X-y plane. This component of noble gas . ',
nuclear magnetic mom~nt then precesses in the x-y plane at the
noble gas Larmor precession frequency abou~ the s~eady magnetic
field 68. This preces~ing ,nuclear magnetic moment component
creates a nuclear precession magnetic field that rotates'in the
x-y plane.
,
The detection light 66 at cesium wavelength, for example~
from a cesîum l~mp or a laser, interacts ~7ith the cesium atoms
1~ which are under the influence of the steady magnetic field 68, a
superimposed AC carrier magnetic field 69, and the y-component of
the nucle~r precession field. This interaction causes the
intensity of the x-component of the transmitted cesium light 72
- to be modulated at the carrier frequency, with a modulation
20 envelope 7~ at the nuclear precession frequency, These llght
modùlations are then converted into electrical signals by the
photodetector 40. The electrical signals may be used by an
electronic circuit to crea~e signals which are measures o~
angular velocity o~ khe gyro as in patent 9,157,495.
Page 10
;
~ ~4~19 GCD ~0-1 ;
.
RELATED PATENTS
PATENT NO. INVENTOR TITLE ISSUED
. . . _ _ ,~ --_ _ _ __ _
4,157,495 B.C.Grover, et al. Nuclear Magnetic 6/5/79
Resonance Gyro
3,103,623 I.A. Greenwood, Jr. Nuclear Gyroscope 9/10/63
3,103,624 I A. Greenwood, Jr. Nuclear ~yroscope 9/10~63
e a .
3,396,329 A. Salvi Magnetic Resonance 8/6/68
Magnetometers for
Measuring ~eak Mag-
netic Fields From
Aboard a Moving Vehicle
; as a Plane
3,404,332 A.Abragam, et al. Magnetic Resonance
~evices for Accurately
~easuring Magnetic
Fields in Particular
Low Magnetic Fields, on
Board of a Movable Body
3,500,176 A. Xastler, et al. Method and Apparatus 3/10/70
for Controlling a l~ag- ~-
netic Fiel~ E~lploying
Optically P~mped
Nuclear Resonance
3,S13,381 ~. Ha2per, Jr. Off-Resonant Light as /19/70
Probe of Optically
Pumped Alkali Vapors
3,729,674 J.R.Lowdenslager Digital Nuclear Gyro- /24/73
scopic Instrumentation
and Di~ital Phase Locked
Loop Therefore
~_ ~__ ....................... _ ...... _. . _ .. .
Pago 11
~ 18~52~
- GCD 80-1
',
In conclusion, the present invention has been described in
terms of particular elements and particular physical arrange-
ments, but it is clear that reasonable alternatives, such as the
use of different optical paths accomplishing the sarne results, o~
the use of different combinations of the noble yases or the use
of a different pumpable substance than rubidium and cesium, or
the use of other va].ues for the freq~encies or magnetic fields
mentioned in the foregoing specification, may a].l be within the
. scope of the present invention.
. .
. .
'.
.
.
: , "' ' ' ' ' ~
. . .
