Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates generally to a
laser angular rate ~ensor, and, more p~rticularly, to
an improved mirror mechanical o~c~llation tec~nlque in
~uch an angular rate ~en~or for overcoming lock-in
errors that occur during low angular rate sensing.
BACKGROUND
A la~er angular rate ~en~or, or rinB laser gyro,
has two counterrotating monoobromatic la3er b~ams
moving around a clo~ed path by ~ucces~ive reflections
from three or four mirrors. On rotation of the ~ensor
about it~ sensing axi~, the effective path length for
the two beams is changed resulting in a fr~quency
differential bet~een the beams which is proportional to
the angular rotation rate. At low rotation rate~ where
the frequency differential between the ~wo 12~er beams
would be expected to be small, it i~ found that the
beam~ tend to "lock-in" or oscillate at the same
frequency so that a frequency dlfferential ie not
detected.
One general approach $n the prior art for
eliminatlng lock-in has been to mechanically vibrate
(dither) the la~er angular sensor ln order to rai~e low
~ensor rotation rate3 out of the lock~in range.
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Although useful in reduclng lock-ln, thi~ so-called
body dither does not completely remove lock in and it
is generally undesirable to sub~ect the entire ~ensor
to sustained vibrations.
s
An alternative dither ~cheme described ln United
States 3,533,014 consists in each mirror of a three-
mirror being sinusoidally vibrated in a direction
parallel to the reflective surface. This l~ difficult
to achieYe in practice since relatively large ~hearing
forces are needed to move the mirrors in such a manner.
.
A still further approach set forth in United
State~ Patent 4,281,933 involveq vibrating all three
lS mirrors of a laser gyro orthogonally to the reflective
surface. Although vibrating the mirrors ln this manner
i~ relatively easy to achieve, unless a correct pha e
relation~hip is precisely maintained the cavity length
for the beam~ will change ~uring the dither cycle which
20 is undesirable.
SUMMARY OF T~E DISCLOSURE
A la~er angular rate ~ensor has four ~nirror~
appropriately arranged at the corners of a 3quare to
reflect two oppo~itely directed laser beams around a
closed path which lies along the optical axis of the
cavity. Dithering means are interconnected with two
-3-
adjacent mirrors for effecting longitudirlal o~cillatlon
of these mirrors (i.e., perpendicular to the mirror
major surface), the relative phaae for the two mirrors
being maintained at 180 de8ree~ so that the cavity
length will be unchanged~ The dithering means drive
circuit is controlled by monitori~g actual mirror 5~~
displacement in order to tune and maintaln the dither
amplitude at a predetermined value which preferably i5
0.271 of the laser radiation wavelength corre~pon~ing
to the modulation index of the zeroth order Bessel
function, JO.
The dlthering means drive circult incorporate
the capability for dithering when the angular rata of
input to the gyro is close to zero (i.e., in the lock-
in band~, but switches off for values outside the
dither band.
2~ DESCRIPTION OF THE DRAWING
Figure 1 is a qchematic repreaentation of a la~er
angular rate sensor in accordance with the present
invention.
~5 Figure 2 is a sectional, elevational view of a
mirror and mirror dithering ~eans.
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Figure 3 is a function block circuit diagram for
dither drive and control.
Figure 4 is an alternate circuit for dither drive
and control.
DESCRIPTION OF PREFERRED EM~ODIMENTS
Turning now to the drawing and particularly
Flgure 1, a laser angular rate sensor 10 is seen to
include a one-piece instrument block 11 with a cavity
12 extending along a square path. Typically, a mixture
of neon and helium are contained in the cavity at
very low pre~sure (e.g., 3 torr). On e~tablishing an
elevated potential difference between cathode 13 and
anode 14 as well as ~etween cathode 15 and anode 16,
two monochromatic laser beams are generated whlch pa~s
in opposlte direction~ along the cavity axis indicated
generally by the line 17.
Mirrors 18-21 are loc~ted at each corner of the
square cavity and serve first of all to direct the two
laser beams along the cavity path indicated generally
as at 17. The two ad~acent mirrors 18 an~ 19 include
piezoelectric mechanlcal osoillators 22 and 23,
respectively, which when energlze~ vibrate or d~ther
the mirrors in a direction perpendicular to the flat
rerlecting surface of the mirror. When the mirror~
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have a curved reflecting 3uface, ~hen mlrror dither la
along the optical axi~.
Turning now to Figure 2I piezoelectric vlbrator~
or oscillator~ 22, 23 3ati~faotory for present purpo~e~
include a material that expand~ and contract~ a~ an
eleotr~c potential differential 1~ applled to and
removed from opposite face~ Accordlngly, application
of a cyclically changing voltage potential to the
piezoelectric materlal cau~e~ an alternatin~
lengthening and ~hortening of the mat~rial which, in
turn, acts through the vibrator housing 25 to dither
~he mirror. An excellent mean3 for producing thia
dither is the plezoelectric ceramlo operaking a~ a
bimorph described in United State~ patent No.
4,383,763, CONTROLLA~LE MIRRORS by T. J. Hutchings, et
al.
For the ensuing de~crlptlon of a fir~t form of
2~ mirrQr control and drive circuît reference is now made
~,~ to Fisure 3. Typically, in a la~er gyro a mirror t20)
iq partially transmis~ive to the laser beams which
allow~ them to $mpinge upon conventional optics and
photo detector meanq 26 to produce signals re~pon~ive
to the laser beat frequeney wave repre~entativ~ o~ the
input rate to th~ gyro. Signal3 ~rom 26 are fed lnto
conventional count proce~ing circultry for producing
output information on both the magnitude and th~ siBn
of the input rota~ion. The input rate i~ormati~n is
also fed into a microprocessor 28, the ~urpose of which
is to decide if the gyro is well outside the lock band.
A sine wave generator 29 provides an A.C. signal
to a variable gain ampllfier 30 which, in turn, is
lnterconnected with a driver 31 for the plezoelectric
vibrator 22 of mirror 18. Slmilarly, the 3ine wave
signal from generator 29 ia applied to a second
amplifier 32 the output of which i~ fed to a phase
shifter 33 where the phase i5 changed by 180 degrees
before application to driver 34 for mirror 19
piezoelectric vlbrator 23. As to operatlon of the
dither drive cicuit described to this point, the two
mirror vibrator 22 and 23 are cyclically driven at the
same frequency and amplitude, but 180 de8rees out of
phase. This phase difference insureY that the cavity
length for the two laser beam~ is maintained constant
which is desirable~
The absolute value of the count from the optic~
2~ and photo deteckor 26 for an lntegral number of ~irror
oscillation~ is accumulated in a counter 35 A
~ynchronizing pulse is sent from the ~ine wave
generator to the counter in order to accompli~h thi~.
The pul~e count of 35 i~ converted to an analog
equivalent in digital-to-analog ¢onvertor 36 which iq
applied to both gain controls of amplifiers 30 and 32
to effect a corresponding change in the output
magnitude of drivers 31, 34.
~ ~r ~æ~
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The microprocessor 28 provides an enabllng ~lgnal
to the sine wave generator 29 on line 37 aq long as the
BYro input i~ within the predetermined lock-in range.
When the gyro input is well in exce3g of the undithered
lock-in limit, but before first-order lock band created
by the dither at the first harmonic of the dither
frequency is reached, the sine wave generator is
disabled so that mirror dither iq completely terminated
at that time.
Figure 4 depicts a further version of dither
drlve and control circuit. As in the first described
embodiment, a ~ine wave Benerator 38 i~ interconnected
through an amplifier 39 and driver 4G to cyclically
dither mirror 18. A1YO~ a~ before, the sine wave
generator applies an A.C. voltage through the a~nplifier
41, pha3e shifter 42 and driver 43 to dither mirror 19
at the ~ame frequency and amplitude a~ mirror 18, but
180 degrees ouk of p'nase.
Partially transparent mirror 21 passes a single
la~er beam to photodiode 44 which form~ a qignal
corresponding to the single beam lnten~ityL This
signal will contain an A.C. component as the normal
~5 gyro heterodyne output 9 becau~e of back~catter within
the cavity primarily from the mirror~. Output o~ the
photodiode 44 i~ formed in peak-to~peak detector 45 and
an error signal created in 46. This ~ignal i3 used to
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control the gain of amplifie~a 39 and 41 such tha~ khe
mirrors are servoed to be driven at an amplitude that
minimizes the A.C. component (at the gyro output beat
frequency) on the ~ignal from photodiode 44O
In the practice of the described invention two
immediately adjacent (i.e., con~ecutive) mirror~ in a
laser gyro are dithered at the aame freq~ency and
amplitude, but 180 degree phase relationsh~p, alang a
path perpendicular to the mirror plane. Both veraion~
automatically di~continue mlrror ditherin~ when the
8Yro input rate exceed~ the lock-ln range.
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