Nuclear magnetic resonance gyroscope

A nuclear magnetic resonance gyroscope which derives angular rotation thef from the phases of precessing nuclear moments utilizes a single-resonance cell situated in the center of a uniform DC magnetic field. The field is generated by current flow through a circular array of coils between parallel plates. It also utilizes a pump and read-out beam and associated electronics for signal processing and control. Encapsulated in the cell for sensing rotation are odd isotopes of Mercury Hg.sup.199 and Hg.sup.201. Unpolarized intensity modulated light from a pump lamp is directed by lenses to a linear polarizer, quarter wave plate combination producing circularly polarized light. The circularly polarized light is reflected by a mirror to the cell transverse to the field for optical pumping of the isotopes. Unpolarized light from a readout lamp is directed by lenses to another linear polarizer. The linearly polarized light is reflected by another mirror to the cell transverse to the field and orthogonal to the pump lamp light. The linear light after transversing the cell strikes an analyzer where it is converted to an intensity-modulated light. The modulated light is detected by a photodiode processed and utilized as feedback to control the field and pump lamp excitation and readout of angular displacement.

BACKGROUND OF THE INVENTION 
This invention relates to a nuclear magnetic resonance gyroscope and in 
particular to a nuclear magnetic resonance gyroscope which utilizes 
parallel plate magnetic field coils to provide a uniform DC magnetic field 
about a resonance cell and transverse pumping of the resonance cell. 
Current nuclear magnetic resonance gyroscopes incorporate resonance cells, 
pump and readout lamps, optics, and associated electronics for control and 
signal processing. Isotopes are encapsulated in the resonance cell 
centrally positioned in a DC magnetic field generated by a hemmholtz or 
cylindrical field coil and a current source. The precession of the nuclear 
magnetic moment is sustained by an AC magnetic feedback field. The pump 
lamp is comprised of a single isotope which is excited to produce the 
light required for the optical pumping of the resonance cell. The readout 
lamp is identical in construction to the pump lamp and is used for 
determing angular changes. The techniques used in the readout process are 
the Faraday and Dehmelt. For both techniques intensity modulated light is 
detected by a photodetector. The signal is amplified, conditioned, and 
demodulated to produce the correct signal for control and information 
processing. Degradation of performance of existing nuclear magnetic 
resonance gyros occurs because limited magnetic field uniformity is 
provided in both the transverse and longitudinal direction; external 
magnetic fields couple with the DC magnetic field of the gyro altering the 
direction of the sensitive axis from that defined on the gyro case; phase 
shifts are introduced by a change in angle between the feedback field, 
light beam direction and DC magnetic field; and the feedback field 
interacts with the atomic sublevels of the isotopes reducing their 
relaxation time and affecting performance. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of this invention to provide a nuclear 
magnetic resonance gyro having a uniform magnetic field. It is another 
object of the invention to provide improved shielding so that external 
magnetic fields do not couple with the DC magnetic field and alter the 
direction of the sensitive axis. It is a further object of the invention 
to eliminate changes in angle between the feedback field, the direction of 
the light beam and the DC magnetic field. It is still a further object of 
the invention to provide a nuclear magnetic resonance with gyro with no 
feedback field, thereby preventing a reduction in the relaxation times of 
the isotopes. 
Briefly, these and other objects of the invention are achieved as follows: 
A resonance cell, utilizing Mercury isotopes Hg.sup.199 and Hg.sup.201 
encapsulated therein for sensing rotation, is centered in a parallel plate 
magnetic field coil. The coil structure produces an extremely uniform 
field in the region of the sample cell. It also serves as an additional 
shunt path for external fields. A pump lamp is excited by an RF oscillator 
and maintained at threshold by the use of a power amplifier. A feedback 
loop controls the gain of the power amplifier thereby modulating the light 
intensity at the precessional frequency of the nuclei (Lamour frequencies) 
for optical pumping. The modulated unpolarized light beam is transformed 
into modulated circular polarized light by directing it through a linear 
polarizer, quarter-wave plate combination. The circularly polarized light 
is then redirected by a mirror to the resonance cell for optical pumping 
along an axis transverse to the magnetic field. A readout lamp is excited 
in a similar manner as the pump lamp. The unpolarized light beam is 
directed through a set of lenses and a linear polarizer to produce 
linearly polarized light. The linear polarized light is redirected by a 
mirror to the resonance cell along another axis transverse to the magnetic 
field and orthogonal to the pump beam. The readout beam's plane of 
polarization is continuously varied by the motion the nuclei in the 
resonance cell. This oscillation is transformed into intensity-modulated 
light by a suitably oriented analyzer. The intensity-modulated light is 
then detected by a photodetector and utilized in the control and signal 
processing electronics for field and pump lamp control and angle 
processing. 
Other objects, advantages and novel features of the invention will become 
apparent from the following detailed description of the invention when 
considered in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawing, wherein like reference characters designate 
like or corresponding parts throughout the sevral views, there is shown in 
FIG. 1 a nuclear magnetic resonance gyro having three modular sections; a 
resonance cell container 10, a lens stack container 40 and a base 
container 60. 
Cell container 10 shown in FIGS. 3, 4 and 5 has mirror mounts 14a and b 
inserted through the walls thereof positioned orthogonal to each other 
between cover 12 and lens stack container 40. Mount 14a is centered on a 
readout beam R, axis X, passing through the center of a resonance cell 34 
and mount 14b is centered on a pump beam P axis Y orthogonal to the X axis 
and also passing through the center of resonance cell 34. A Z axis is 
defined as passing through the center of cell 34 and perpendicular to both 
the X and Y axis. Positioned in the center of cell container 10 is a cell 
cage 24 having inserted in the center thereof on the X axis a cylindrical 
spool 26. Spool 26 is used to secure the position of a resonance cell 34 
in the center thereof. Rotation and alignment of the symmetric axis of 
cell 34 is achieved by positioning spool 26 after withdrawing brake rod 
37. Resonance cell 34 comprises a transparent sealed quartz-pyrex envelope 
containing mercury vapor of odd isotopes Hg.sup.199 and Hg.sup.201. Cell 
34 is centrally positioned in a DC magnetic field H.sub.o generated by an 
electronically computer regulated stable current source which maintains 
field H.sub.o constant. Spool 26 having a hole 27 longitudinally 
therethrough for allowing the readout beam R to pass through to cell 34 
and having at one end therein a threaded portion 27a for insertion of an 
alignment tool (not shown) in place of photodetector 38. A U shaped notch 
28 is located in the center of spool 26 in line with the Y axis for 
permitting pump beam P to strike cell 34. A cavity 29 is located below U 
shape notch 28 and the center of spool 26 for holding cell 34 envelope 
sealing point. Spool cage 24 has a hole therethrough aligned on the X axis 
in which spool 26 is positioned. Brake rod 37 is positioned and held 
against spool 26 for maintaining the position of cell 34. Cage 24 with 
spool 26 and cell 34 therein is interposed between parallel plates 16a and 
b. Plates 16a and b are fabricated from a material having a high 
permeability such as silicon core c iron having a permeability equal to 
6,000. Proper separation between the upper and lower plates 16a and b 
respectively are maintained by cage 24. Cage 24 is cylindrical and has a 
lip extending above and around the perimeter of the top and bottom sides. 
FIGS. 7 and 8 are expanded views of the parallel plate magnetic field 
coils. FIG. 8 in particular shows a cross-sectional view illustrating the 
magnetic flux path .phi. through the parallel plates 16b across the air 
gap to parallel plate 16a through 16a to the top of coil 22. Plates 16a 
and 16b are disk shaped but have an area 19a and 19b about the center 
which is thicker than the outer periphery area of the disk shaped plates. 
Said thicker portions of plates 16a and 16b fit snuggly within the 
circumferential lip on the top and bottom of cage 24 respectively. The 
increased thickness of the center area of plate 16a and b provide an 
addition flux path to concentrate the magnetic field in the central area 
than would a flat disk. Parallel magnetic plates 16a and 16b are directly 
connected together by eight coils 22 each coiled about an adjustable core 
18 and symmetrically located about the perimeter of plates 16a and b. 
Coils 22 are connected in a series string and the current therethrough 
automatically regulates the DC magnetic field. Coil 22 wire is capable of 
withstanding 300 degrees C. Core 18 is comprised of high permeability 
material such as silicon core C iron and is internally threaded 
therethrough having an adjustable magnetic center rod 20 which can be 
turned into or out of core 18 to change the permeability thereof and hence 
the magnetic field H.sub.o. The adjustment provides the ability to fine 
tune the field H.sub.o in the central area of plate 16a and b which has 
magnetic irregularities due to nonuniformity in material. Grooves have 
been cut into the well of container 10 where the parallel plate 16b rests 
for a heater coil 17. Heater coils 17 are also placed over plate 16a to 
provide symmetrical heating of the resonant cell 34. Heater coil 17 
provides 600 watts at turn on. Coil structure 18 is positioned within the 
cell container 10 so that the axis of symmetry of hole 27 is aligned along 
axis X. Mounts 14a and 14b inserted through the walls of container 10, 
orthogonal to each other, extend partially into shafts 15a and 15b 
respectively. The center of 14a and 14b lie on the axis X and Y 
respectively. Mirrors 25a and 25b are attached to mountings 14a and 14b 
respectively and are positioned to reflect light received from lens well 
42a and 42b onto the X axis and Y axis respectively. Analyzer 36 is 
positioned at 57.40 degrees away from the perpendicular with respect to 
the direction of propagation of readout beam R provides the intensity 
modulated light I for detection by a photodetector 38 positioned on the X 
axis opposite of mirror 25a. Cell container 10, cage 24, spool 26, mirror 
mounts 14a and b and cover 12 are made of a ceramic material which 
requires no firing after machining and has physical characteristics 
conclusive to a gyro such as an extremely low magnetic susceptibility 
material. Mirror mounts 14a and b and respective mirrors 25a and b are 
used for directing the readout beam R and a pump beam P along the 
transverse axes into cell 34. 
A lens container 40 as shown in FIGS. 1, 3 and 4 has two threaded 
cylindrical wells 42a and 42b for containing in a stack configuration all 
the components necessary to produce circularly polarized light for optical 
pumping and linearly polarized light for readout respectively. The optical 
components threadingly stacked in well 42b are a quarter wave plate 44b, a 
polarizer 46b, a 2537 Angstrom filter 48b, a bi-concave lens 50b, an upper 
condenser 52b, and a lower condenser 54b. Well 42a contains threadingly 
stacked therein a polarizer 46a a bi-concave lens 50a, an upper condenser 
lens 52a, and a lower condenser lens 54a. Lens container 40 is fabricated 
from a non-magnetic reacting material such as a vulcanized fiber. 
Base block 60 comprises lamp housing 62a and 62b each partially inserted 
through the wall of container 60 orthogonal to each other. Housing 62a is 
centered on the X axis and 62b on the Y axis. A pump lamp 66a comprises a 
quartz envelope having the shape of a miniature dumb bell having two 
spherical sections connected therebetween by a capillary section 
encapsulating an isotope whose spectral characteristic match those of the 
isotopes in cell 34. Pump lamp 66a is maintained in position at one 
spherical end by a hole 64a and at the other spherical end by an 
excitation coil 68a coil thereabout. Excitation coil 68a receives current 
from a gain varied power amplifier 100. A readout lamp 66b having an 
encapsulated isotope has the same configuration as lamp 66a is positioned 
in housing 62b at one spherical end in hole 64b and at the other spherical 
end by an excitation coil 68b coiled thereabout. Light intensity of beams 
P and R in wells 42a and 42b are adjusted by diaphragms 69a and 69b 
respectively. The unpolarized intensity modulated pump light beam P from 
pump lamp 66b is directed through combination of lenses 54b, 52b, 50b, 
filter 48b, linear polarizer 46b and quarter wave plate 44b producing 
circularly polarized light and reflected from a mirror 25b onto axis Y 
into cell 34 transverse to field H.sub.o for optically pumping. 
Unpolarized readout light beam R from readout lamp 55a is directed through 
the lenses in well 42a through lenses 54a, 52a, and 50a and linear 
polarizer 46a and reflected from mirror 25a onto axis Y through cell 34 
transverse to field H.sub.o and orthogonal to pump beam P along axis Y. 
Lamps 66a and 66b have their center capillary portion, aligned with the 
optical axis of the lens stacks in well 42a and 42b respectively. 
Diaphragm 69a and 69b positioned above lamps 66a and 66b are used to 
restrict the amount of light into lens wells 42a and b. Base container 60 
and the lamp housings 62a and 62b are constructed from a non-magnetic 
fibrous material such as an acetyl resin. The isotope encapsulated within 
the readout lamp 66b has spectral characteristics different from the 
spectral characteristics of resonant cell 34 to ensure minimal absorption 
of the readout light beam R. The readout lamp 66b is excited from a 100 
mHz clock 102. As shown in FIG. 10c readout beam R interacts with the 
precessing nucleus of a mercury atom rotating its plane of polarization. 
An analyzer 36 is positioned at a 57.40 degrees angle away from the 
perpendicular with respect to the propagation of beam R to convert the 
translated linearly polarized light from cell 34 into intensity modulated 
light. A photodetector 38 is connected through the wall of container 10 on 
the X axis opposite mirror 25a for receiving intensity modulated light I 
and detecting the modulating signal Am and providing it as voltage 
fluctuations to electronic circuit 70. 
Electronic circuit 70 separates two precessional frequencies, uses them to 
control pump lamp excitation and gating of a high frequency oscillator 84 
into counting chains 78 and 90. The resultant counts represent the Lamour 
frequencies plus vehicle rotation rate to a resolution of microhertz. A 
computer 82 is connected to receive the resultant counts through a 
multiplexer 80 for processing and providing feedback to control the D.C. 
magnetic field H.sub.o. Electronic circuit 70 shown in FIG. 9 is connected 
as follows. A phase stable amplifier 72 comprising a voltage amplifier 
having no phase shift between input and output is connected to receive the 
detected modulation signal Am from photodetector 38 and to provide a 
composite signal C having two main frequency components consisting of 369 
Hz and 1000 Hz and deviations therefrom resulting from the precessing 
Mercury isotopes in cell 34. A lamour filer circuit 74 is connected to 
receive and separate composite signal C into the two main components. A 
sample rate multiplier 76 is connected to receive both components, convert 
them from a sinewave to a square wave of the same frequencies, provide two 
disable signals D.sub.1, each having a period one hundred times longer 
than the input signals, and provide an enabling signal E.sub.mi to a 
computer 82. An angle counter 78 is connected to receive signal D.sub.1 
and is used to count a 200 megahertz clock pulse from an oscillator 84 for 
the time period derived by multiplier 76. Angle counter 78 provides a 
count signal N.sub.1 equivalent to the number of pulses it received before 
receiving disable pulse D.sub.1 when enabled from computer 82 by signal 
E.sub.cl. A count multiplexer 80 is connected to receive count signal 
N.sub.1 and to transfer it to the computer 82. Computer 82 can be a 
general purpose computer programmed to receive count signal N, and perform 
arithmetic operations to calculate and provide the angle of rotational 
motion about a sensitive axis as defined by the direction of H.sub.o (Z 
axis). 
A frequency multiplier 86 is connected to receive the two frequencies and 
then by a multiplication technique well known in the art produce the sum 
frequency signal S all others being filtered. Signal S used to control 
H.sub.o field. An H.sub.o sample controller 88 is connected to receive 
signal S, provide a disable signal D.sub.2 that is 100 times the period of 
the incoming signal S and an enable signal E.sub.m2 to computer 82. The 
period multiplication increases the resolution of the final change in coil 
22 current. An H.sub.o control counter 90 is connected to receive 200 mHz 
oscillator signal from oscillator 84 and disable signal D.sub.2. When 
signal D.sub.2 occurs counter 90 stops incrementing and provides the count 
signal N.sub.2 when enabled from computer 82 by signal E.sub.c2. Computer 
82 is connected to receive count signal N.sub.2 and provide a feedback 
signal F.sub.d consisting of a 16 bit word indicative of the parallel 
plate magnetic field coil current control. A digital-to-analog converter 
92 is connected to receive signal F.sub.d to provide a feedback current 
F.sub.a which will increase/decrease the current I.sub.p from the power 
amplifier 94 to the coil 22 so as to maintain the sum frequency signal S 
constant. A pulse forming circuit 96 such as a Schmidt trigger is 
connected to receive the precessional frequencies components from Lamour 
filter circuit 74 and to provide a trigger pulse T, at the rate of the 
component frequencies. A phase control circuit 98 is connected to receive 
pulse T for providing a gating pulse G.sub.p having a predetermined pulse 
width and duty cycle to excite pump lamp 66b at the proper time and for 
the proper duration to be in phase with the precessing moments of cell 34 
nuclei. Lower amplifier 100 is connected to receive a 100 mHz excitation 
signal O and to provide a threshold level excitation and periodically 
pulsed by gating pulse G.sub.p increase excitation power. Pump lamp 
excitation coil 68b is connected to receive the gain varied excitation 
signal O from pump amplifier 100. Readout lamp excitation coil 68a is 
connected to receive excitation signal O continuously from 100 mHz 
oscillator 102. 
A triple nested magnetic shield not shown surrounds the gyroscope. The 
shield cylindrical axis of symmetry is aligned along the symmetric axis (Z 
axis) of the gyroscope. The shield provides attenuation of all external 
fields to a nominal value which shall not affect the performance of the 
instrument. 
Operation of the nuclear magnetic resonance gyroscope utilizes the 
intrinsic property of certain nuclei to determine angular displacement 
about a defined input axis. The nuclei have angular momentum, hence, a 
magnetic moment. When the nuclei are placed in a weak magnetic field, they 
precess about the direction of the field as defined by equation 1. 
EQU .omega.=.gamma.H.sub.o Equation 1 
.omega..sub.L is the precessional frequency; 
.gamma. is the gyromagnetic ratio, and H.sub.o is the DC magnetic field 
whose direction defines the input axis of the gyroscope. As illustrated in 
FIG. 10, field H.sub.o lies on the Z axis. An ensemble of aligned nuclei 
must be established in order to obtain a detectable signal. This is 
performed by a technique known as optical pumping. A beam of circularly 
polarized light is produced and directed at the resonant cell containing 
the atoms in a gaseous state. The light of proper wavelength is absorbed 
by the atoms. The atoms then align themselves along the direction of the 
pumping beam. According to the invention, the pump beam is directed 
perpendicular to the direction of the magnetic field H.sub.o as 
illustrated in FIGS. 9 and 10. The reoriented atoms now in a position 
perpendicular to H.sub.o experience a torque which causes them to precess 
about field H.sub.o. This is a free precession gyroscope. A feedback 
signal is established through the readout circuit and is utilized to 
modulate the pump lamp intensity. In this manner, the pump lamp is in 
phase with the precessing ensemble replacing those nuclei which have lost 
their coherence due to decay processes going on in the resonance cell. 
This establishes an equilibrium ensemble or net magnetc moment capable of 
producing a detectable signal. Readout of the net magnetic moment is 
achieved by utilizing the Faraday technique. A beam of linearly polarized 
light in the same plane as the pump beam being orthogonal is directed at 
the resonant cell 34. As the linear light traverses the cell, the plane of 
polarization varies due to its position with respect to the precessing net 
magnetic moment. The light is then transformed into intensely modulated 
light by an analyzer 36 and is sensed by a photodetector 38. The observed 
signal of .omega..sub.1 is given by equation 2. 
EQU .omega..sub.1 =.omega..sub.L +.omega..sub.R equation 2 
where .omega..sub.R is vehicle rotation sensed by the gyroscope along the 
direction of field H.sub.o. If a single nuclei were to be used, it would 
be extremely difficult to produce accurate angular measurements because of 
the stability requirement that would have to be imposed on H.sub.o field. 
To overcome this difficulty, two nuclei are placed in a single cell. The 
detected signal is then the sum of the individual signal from the cell 
given by equation 3. 
EQU .omega.=.omega..sub.1 +.omega..sub.2 equation 3 
where .omega..sub.2 is the precessional frequency of the second nuclei and 
is identical to equation 2 except for a change in sign for .omega..sub.R. 
Utilizing equation 3 and substituting in 1 and 2, the field control is 
expressed by equation 4. 
EQU H.sub.o =(.omega..sub.1 +.omega..sub.2)/(.gamma..sub.1 +.gamma..sub.2) 
equation 4 
Field control is a matter of measuring .omega..sub.1 +.omega..sub.2 and 
processing this information with the predetermined value of .alpha..sub.1 
and .alpha..sub.2. Electronically, the gyro output signal is separated 
then beat together to produce the sum and difference. The difference 
frequency is filtered leaving the sum which is independent of 
.omega..sub.R. The sum signal is then utilized to control an accounting 
chain from an extremely stable high frequency crystal oscillator. After 
the counting cycle is completed, the information is multiplexed into a 
computer for use in controlling a digital to analog converter for 
controlling the current field H.sub.o. This eliminates the requirement for 
an ultra stable current source required in a single nuclei approach. 
Vehicle angular information is based upon a difference between 
.omega..sub.1 +.omega..sub.2 and is given by equation 5. 
##EQU1## 
Equation 5 was based upon equation 2 and identical quation for 
.omega..sub.2. The difference frequency is utilized in the same manner as 
the sum frequency in the field H.sub.o and sum frequency in the H.sub.o 
control circuitry. The information after the gating is completed is 
multiplexed by multiplexer 82 into computer 82 and a calculation is 
performed to determine .omega..sub.R. 
Pump beam lamp 66b is excited by current supplied from pump amplifier 100 
through winding 68b. Upon excitation, unpolarized light is directed 
through diaphragm 69b through the lens polarizer and quarter wave plates 
stacked in lens well 46b. Now circularly polarized light beam P is 
reflected from mirror 25b onto the Y axis into resonant cell 34. 
Readout lamp 66a obtains excitation from 100 mHz clock 102 and emits 
unpolarized light through the lens stack in lens well 46a. Linearly 
polarized light beam R exits well 46a and is reflected from mirror 25a 
onto the X axis through the resonant cell 34 to analyzer 36 and then as 
amplitude modulated light to photodetector 38. The voltage fluctuation 
from detector 38 are separated into the two precessional frequencies and 
utilized to control pump lamp 66b excitation, the current flow through 
coils 22 and provide vehicle rotation rate. 
Some of the many advantages of the present invention should now be readily 
apparent. The invention provides parallel plate magnetic field coil which 
shunts external magnetic fields and produces an extremely uniform field in 
the region of the resonant cell. The invention uses a transverse AC 
pumping technique, thereby eliminating the effect of phase shifts which 
occur between the AC feedback magnetic field and the precessing nuclei, 
eliminating the angle change between the feedback magnetic field, light 
beam direction and the DC magnetic field, and reducing the relaxation time 
due to the interaction of the AC feedback magnetic field with the atomic 
sublevels. 
Obviously, many modifications and variations of the present invention are 
possible in view of the above teaching. It is therefore to be understood 
that within the scope of the appended claims the invention may be 
practiced otherwise than as specifically described.