Note: Descriptions are shown in the official language in which they were submitted.
Z~32
BACKGROUND OF Tl IE I NVENTION
1. Field of the Invention:
The invention pertains broadly to laser gyroscope systems.
More particularly, the invention relates to laser gyroscope systems
in which high precision and high resolution output signals are re-
quired.
2. Description of the Prior Art:
One of the primary problems which must be overcome to
provide a useful laser gyroscope system is that of mode locking.
In an uncompensated system in which no provision is made for
overcoming this problem, for low angular rates of rotation,
the frequency difference produced between wave pairs circulating
in their laser gyroscope cavity are less than would be predicted
were it not for the mode locking phenomena. In fact, the actual -
frequency difference output only asymptotically approaches
the desired linear relationship between output frequency
difference and rate of rotation as the actual rate of rotation
is increased.
Numerous laser gyroscope structures have been proposed
for eliminating or substantially reducing this mode locking -
problem. Among the most successful of these systems is that
shown and described in the United States Patents Nos. 3,741,657
and 3,854,819, both to l~eimpe Andringa and assigned to the
present assignee. In the patented systems, beams of four
seperate frequencies propagate around a closed laser gyroscope
path defined by four mirrors. Two beams circulate in the
clockwise direction and two in the counterclockwise direction.
Of the two clockwise circulating beams, one is of left-hand
circular polarization and the other of right-hand circular
q~
~l~Z69Z
polarization as is also the case for the two counterclock-
wise circulating beams. In the preferred embodiment, the
two beams of right-hand circular polarization are of higher
frequency than those of left-hand circular polarization. A
Faraday rotator structure provides the frequency difference or
splitting between the beams of clockwise and counterclockwise
rotation while the crystal rotator structure provides the
frequency splitting between the beams of right-hand and left-
hand polarization.
The relative frequency positions of the beams of four
different frequencies are shown in the diagram of FIG. 2.
To avoid the mode locking problem, the Faraday rotator
provides sufficient frequency splitting between the beams of
frequency fl and f2 as well as between the beams of frequencies
f3 and f4 at a zero rate of rotation and for all anticipated
rates of rotation such that no mode locking can occur and the
system is biased substantially outside the nonlinear region
where mode locking occurs.
At rest, the frequency difference between the beams of
frequency fl and f2 is the same as that between the beams of
frequencies f3 and f4. As the laser gyroscope system is rotated
in a first direction, the beams of frequencies fl and f2 will
move together in frequency while those of f3 and f4 will
move apart in frequency by the same amount. For rotation in
the opposite direction, the beams of frequency fl and f2 will
move apart in frequency while tho~e of f3 and f4 will move
together by the same amount.
To produce an output signal having a frequency in
proportion to the ra~e of rotation, a first two output frequency
difference signals having frequencies ~fl = f2 - fl and ~f2 =
~ ~2 ~ ~ ~
f4 - f3 are formed. A final output signal ~f = af2 ~ ~fl is
then formed. To provide an indication of the total amount of
rotation, two counters are provided, one of which is incremented
by the ~fl signal and the other by the ~f2 signal. The output
of one counter is digitally subtracted from that of the other
thus providing a digital signal indicative of the total amount
of rotation of the system.
Although this system described in the Andringa patents
has been found to function quite satisfactorily for numerous
applications, in still further applications it has been
found desirable to provide an output signal indicating either
the amount of rotation or rate of rotation having a higher
degree of precision than quantizing the ~fl and Qf2 signals at
one pulse per cycle of the signals can provide.
:L~32~
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention
to provide a laser gyroscope system having a highly precise
output signal.
Moreover, it is an object of the present invention to
provide such a system in which quantization of the output
signal is achieved at a much higher rate than straight-
forward quantization of the normal output signals would provide.
These, as well as other objects of the invention, may be
met by providing the combina~ion of means for-providing a closed-
loop path for sustaining propagation of electromagnetic waves
of a plurality of frequencies and a phase-locked loop coupled
to an output from the path, the output having a signal related
to the frequency of at least one of the waves propagating
around the closed-loop path. Preferably, the output signal
has a frequency substantially equal to the difference in
frequency between at least two of the electromagnetic waves.
The closed-loop path providing means may include at least
three reflecting means and a laser gain medium. First and
second frequency dispersive elements are also disposed in the
path. The phase-locked loop preferably includes means for
varying the difference in frequency between at least two of the
electromagnetic waves. One of the frequency dispersive elements
is preferably a Faraday rotator body. The frequency varying
means may then be a coil for producing a magnetlc field in
the Faraday rotator body in response to an output signal
from the phase-locked loop. As used herein, the term signal
refers to information conveyed upon a shgle or plural lines.
The invention may further be practiced by providing ~he
combination of means for providing a closed-loop path
,
65~Z
for sustaining propagation of electromagnetic waves of two
frequencies, means for producing a first signal having a
frequency equal to the difference in frequency between the
electromagnetic waves of two frequencies, means for producing
a second signal of a predetermined fixed frequency, means for
producing a third signal having an amplitude in proportion to
the difference in phase between the first and second signals,
and means for varying the frequency differ~nce between the
two electromagnetic waves in accordance with a parameter of
the third signal. In the preferred embodiment, the frequenc~
varying means maintains the frequency difference between the
two output beams at a constant value over a predetermined range
of rates of rotation. The means for producing the third signal
preferably includes phase detecting means with the first and
second signals being coupled to inputs of the phase detecting
means and a low-pass filter means. Further, there may be provided
means for amplifying the third signal with the frequency varying
means being coupled-to the output of the amplifying means.
Prior to or following the amplifying means there may be provided
means for summing the third signal, amplified or not, with
a fixed signal or voltage. But still further there may be
provided means for repetitively changing, t~at is reversing,
the direction of current flow through the frequency varying
means.
Objects of the invention may also be met by providing
the combination of means for providing a closed-loop path for
sustaining propagation of electromagnetic waves of two
frequencies, detecting means for producing a first signal having
a frequency equal to the difference in frequency between the
previously mentioned two frequencies, means for amplifying the
9~
first signal, phase detecting means with the first signal
being coupled to a first input of the phase detecting means,
means for producing a second signal of fixed frequency with
that signal being coupled to a second input of the phase
detecting means, low-pass filtering means coupled to the output
of the phase detecting means, means for amplifying the
output of the low-pass filtering means which may be incorp-
orated as a single unit with the low-pass filtering means, and
a coil coupled to the output of the means for amplifying the
output of the low-pass filtering means with the coil being
positioned to vary the frequency difference in accordance
with the amplitude and direction o-f the field produced by the
coil. There may further be provided means for cyclically
changing or reversing the direction of current flow through
the coil. The means for cyclically changing the direction of
current flow through the coil includes switching means coupled
to the output of the means for amplifying the output of the
low-pass filtering means and means for cyclically operating
the switching means. There may further be included for
producing a digital output signal means for converting an
analog signal to a digital slgnal with the input of the
converter means being coupled to the output of the low-pass
filtering means amplified or not. The output of the low-pass
filtering means may be summed with a fixed voltage either
prior to or following ampllfication. The cavity includes a
Faraday rotator element with the coil being positioned so
that the field produced by the coil extends within the body
of the Faraday rotator element. Alternately, the coil may
be positioned around the laser gain medium of the cavity.
-6-
.
~L326~2
In accordance with the present invention, there is
provided in combination: means for providing a closed-loop
path for sustaining propagation of at least two circularly
polarized counter-rotating electromagnetic waves of different
frequencies; means for providing a predetermined frequency
difference between said two counter-rotating waves when said
path is at rest; means for producing an electrical signal
having a frequency equal to the difference in frequency between
said two counter-rotating waves; and means for producing an
output signal which varies as a function of the rotation rate
of said path comprising a phase-locked loop, said phase-locked
loop comprising a phase detector coupled to said electrical
signal.
In accordance with the present invention, there is
also provided in combination: means for providing a closed-
loop path for sustaining propagation of two counter-rotating
electromagnetic waves of different frequencies; means for pro-
viding a predetermined frequency difference between said counter-
rotating waves when said path is at rest; means for producing - .
a first signal having a frequency equal to the difference in
frequency between said two counter-rotating electromagnetic
waves; means for producing a second signal of a predetermined
fixed frequency; a phase-locked loop comprising a phase detector
for producing a third signal haYing an amplitude in proportion -
to the difference in phase between said first and second
signals; and means for varyi.ng the frequency difference between
said two counter-rotating electromagnetic waves as a function
of said third signal.
In accordance with.the present invention, there is
also provided in combination: means for providing a closed-
loop path for sustaining propagation of two counter-rotating
electromagnetic waves of different frequencies; a detector for
- 6a -
,
.3z~2
producing a first signal having a frequency equal to the differ-
ence in frequency between said two counter-rotating waves; phase
locking means comprising a phase detector having said first
signal coupled to a first input thereof; means for producing a
second signal of fixed frequency; said second signal being
coupled to a second input of said phase detector; low-pass
filtering means coupled to the output of said phase detector;
means for amplifying the output of said low-pass filtering
means; and a coil coupled to the output of said means for ampli-
fying the output of said low-pass filtering means, said coil
being positioned to vary said frequency difference as a function
of the field produced by said coil.
In connection with.th.e present invention, there is
also provided in combination: means for providing a closed-
loop propagation path for sustaining propagation of electro-
magnetic waves of different frequencies; means for producing a
difference signal having a frequency equal to the difference in
frequency between tT~o counter-rotating ones of said waves;
means for providing a phase-locked loop comprising a phase
detector coupled to said difference signal; and means for pro-
ducing an output signal which varies as a function of the error
signal produced by said phase-locked loop.
In accordance with the present invention, th.ere is
also provided in combination: means for providing a closed- ;
loop propagation path for sustaining propagation of electro-
magnetic waves of different frequencies; means for pxoducing a
difference signal having a fre~uency equal to the difference
in frequency between two counter-rotating ones of said waves;
means for producing a reference signal; means for comprising
phase detecting means for producing a phase locked control
signal having a parameter determined by the difference in phase
between said difference signal and said reference signal; means
- 6b -
, ~
2~;~2
for varying the frequency difference between said two of said
waves as a function of said control signal; and means for
producing an output signal as a function of said control
signal.
- 6c -
., .
- -
- ~
" ~ ~: ' ' ' : -
1~L3~
BRIEF DESCRIPTION OF THE DRAWINGS.
FIG. 1 is a diagrammatic view of a laser gyroscope system
in accordance with the invention;
FIG. 2 is a diagram illustrating the frequency line
distribution and gain medium characteristics of the laser
gyroscope system of the invention;
FIGS. 3 - 6 and 9 are block dlagrams of various embodiments
of the invention: and
FIGS. 7 and 8 taken together are a detailed schematic
diagram of the embodiment of the invention shown in FIG. 4.
~3~ 2
DESCRIPTION OF THE PREFERRED E~BODIMENTS.
Referring first to the view of FIG. 1, there is shown
therein in diagrammatic form a laser gyroscope system in
which the present invention is used to advantage. A
generally rectangularly shaped laser gyro cavity 110 is defined
by mirrors 12-15. Along one leg of cavity 110 in the path of
the electromagnetic waves which propagate around the path
is disposed laser gain medium l0. L~se~-gain medium 10 may
be formed as a sealed chamber containing gases such as a mixture
of isotopes of helium and neon. The amount of gain afforded
the various electromagnetic waves is, as is well known, a
function of frequency of the waves. As shown in the view of
FIG. 2, the laser gain curve 11 for the chosen gain medium is
substantially bell shaped. Along the leg of laser gyro cavity
110 opposite laser gain medium 10 is disposed polarization
dispersive structure 16. Two seperate polarization dispersive
elements are provided within polarization dispersive structure 16.
Crystal rotator 17 provides a delay or, equivalently,
a phase shift, for circularly polarized waves that is different
for one sense of circular polarization than for ~he other.
That is, the delay or phase shifts are different for waves
of right-hand circular polarization than or thcse o~ left-
hand circular polarization. Moreover, the delay is reciprocal
in that the delay imparted to a-particular wave depends only
upon its sense of polarization and not upon its direction
of propagation through the crystal.
Adjacent crystal rotator 17 within polarization dispersive
structure 16 is positioned Faraday rotator 18. Faraday rotator
18 is constructed using either a crystalline or noncrystalline
center core through which is applied a constant magnetic
g
~3~
field from an external permanent ~agnet, not shown. Fused
quartz is the preferred material. Faraday rotator 18 provides
a delay or phase shift which is different for waves propagating
through it in one direction than the other. The delay imparted
the waves is independent of the sense of polarization. A second
magnetic field is produced in the body of Faraday rotator 18
by Faraday rotator bias coil 114. The magnetic field produced
by Faraday rotator bias coil 114 may be in either direction
depending upon the direction of current flow in the coil.
Faraday rotator bias coil 114 forms part of a phased-locked
loop circuit 5 as will be described below.
Referring again to the view of FIG. 2, it shows that
waves of four distinct frequencies fl, f2, f3, and f~ are
produced by the apparatus shown in FIG. 1. The waves of
frequencies fl and f4 are waves propagating in the clockwise
direction while the waves of frequencies f2 and f3 are waves
propagating in the counterclockwise direction. The waves of
frequencies fl and f2 are left-hand circularly polarized while
those of frequencies f3 and f~ are right-hand circularly
ZO polarized. As may be appreciated from the description above,
the splitting between the left-hand and right-hand circularly
polarized beams is caused by crystal rotator 17 while the
splitting between the clockwise and counterclockwise beams is
produced by Faraday rotator 18.
As the system of FIG. 1 is rotated about its sensitive
axis, for a first direction of rotation, the waves of
frequencies ~3 and f4 move closer together in frequency while
those of fl and f2 move apart in ~requency by the sa~.e~amount
as f3 and f4 move together. For the opposite direction of
rotation, the waves of ~requencies fl and f move closer together
~L~3Z~;9~
in frequency-while those of f3 and f4 move apart in frequency
again by the sane.amount.
To produce an output signal indicative of the rate of
rotation of the system or, alternatively, of the total
amount of rotation Since ~ predetermined time, two different
g ls ~fl f2 ~ fl and ~f2 = f4 ~ f3 are formed. At rest
~fl - ~f2 To form an output signal indicative of the rate of
rotation at any particular instant, a second difference signal
f = ~f2 ~ ~fl is formed. To determine the total amount of
rotation since a predetermined time, an integral of the f signal
is performed. The integral may be formed with an analog circuit
but is preferably done digitally for increased accuracy.
The frequency difference signals afl and ~f2 are produ~ed
by output structure 112. Mirror 14 is constructed to be partially
transmitting so that a small portion of each of the four
waves circulating in laser gyro cavity 110 are passed through
the mirror to output structure 112. Clockwise propagating
C~te~
waves pass through mirror 14 along path 30 while the~clockwise
circulating beam waves are coupled out along path 31. The
extracted beams pass through quarter-wave plates 32, the thick-
ness of which is chosen in accordance with well-known principles
such that the circularly polarized waves are converted to
linearly polarized waves with the linearly polarized waves,
corresponding to the waves of right-hand circular polarization,
being substantially orthogonal to those corresponding to the
waves of left-hand circular polarization.
The linearly polarized waves are split into beams of
substantially equal amplitude by half-silvered mirrors 33 and 34.
The four beams are then passed through polarization analyzers
35 to produce the four beams at 41, 42, 43, and 44 each of
~L~3;~69~:
which contains a wave of only one of frequencies fl, f2, f3,
and f4 as the polarization analyzers pass only one angle of
linearly polarized waves. The waves having frequencies fl
and f2 are shone upon half-silvered mirror 47 and reflected
towards detector diode 48 while those of frequencies f3 and
f4 are reflected by half-silvered mirror 45 to detector diode
46. Detector diodes 46 and 48 are reversed biased by voltage
sources 49 and 50 to produce the desired operating characteristics
as is well known in the detector diode art. Detector diodes
46 and 48 produce an output signal which has a frequency
equal to the difference in frequency between the two input
waves incident upon each diode. The output signals appear
across resistors 51 and 52. Higher frequency output signals
such as those having a frequency equal to the sum of the
frequencies of the incident waves are filtered out by the
stray capacitances appearing across each diode and do not form
a part of the output signal.
In systems operation, it is desirable that the waves of
the four frequencies be cen~ered symmetrically about the peak
of the gain curve. To this end, a piezoelectric transducer
68 is provided to mechanically position mirror 12 to adjust the
total path length within laser gyro cavity 110 to properly
center the four frequencies. To derive a signal for operating
piezoelectric transducer 68, signals are formed having an
amplitude in proportion to the total amplitudes of the cor-
responding afl and af2 signals and a difrerence formed between
the two amplitude related signals. The output difference signal
of course has a zero amplitude when the waves of the four
rsquencies are properly centered upon the gain curve. The
output difference signal is of one polarity when the four waves
- 11 ~
~3Z~9%
,~o /~ ~, t,"
are off center in one direction and the opposite p~r~
when the waves are off center in the other direction. The
average amplitude signals are formed by the network including
diodes 61 and 62J resistors 63, 64, and 65, and capacitor 66.
The output difference signal is formed by differential amplifier
67, the output of which is coupled to the input leads of
piezoelectric trandsucer 68.
In accordance with the principles of the invention, a
phase-locked loop circuit 5 receives the input frequency
difference signals and from them produces a highly precise
output signal representing the rate of rotation of the system
or the total amount of rotation or both of these. Also, in the
embodiment shown in FIG. 1, phase-locked loop circuit 5
produces a signal coupled to Faraday rotator bias coil 114
for controlling the frequency difference between at least one
of the wave pairs. In some embodiments, Faraday rotator coil
18 may be omitted.
Referring next to the block diagram of FIG. 3, the
operation of phase-locked loop circuit 5 will be described in
more detail. The ~fl frequency difference signal from output
structure 112 is coupled to one input of phase detector 116.
To the other input of phase detector 116 is coupled the
output oS ~e~e~ence clock 118. The output signal from ph~se
detector 116 representing the difference in phase between
the reference clock signal and ~fl signal is coupled to amplifier
120 through low-pass filter 119. Low-pass filter 119 may
alternately follow amplifier 120 or may be incorporated ~here-
with such as in a feedback arrangement. The output of amplifier
120 is coupled back to Faraday bias coil 114 amplified by coil
driver amplifier 115.
- 1~2 -
1~3;~;9~
The signal applied to Faraday bias coil 114 from the
output of amplifier 115 causes a,-,lagnetic field to be produced
in the body of Faraday rotator 18 of FIG. 1 such that the change
in frequency of two of the four waves propagating within laser
gyro cavity 110 caused by rotation of the system are opposed
and nulled out over wide range of rates of rotation. Thus,
as the system is rotated, one of the frequency dîfference
signals will not change. However, the rate of rotation will
be precisely indicated by the amplitude of the output signal from
amplifier 120.
The system shown in FIG. 3 has a number of advantages
over the prior art. First, there is no quantization error in
the rate output signal as the rate of rotation is indicated by
a highly precise analog voltage and not by the frequency of a
' ' s-bj~Gti
signal which is of course s~e~ to the quantization error.
If a digital output is desired, the output signal from amplifier
120 may be digitized using an analog-to-digital converter at
any desired level of precision. Secondly, with tne apparatus
shown in FIG. 3, there is no residual error caused by the lock-
in effect because no change of frequency takes place within
laser gyro cavity 110 for the signals from which the output
is derived.
Referring next to the block diagram of FIG. 4, there is
shown a further embodiment of the invention. The system shown
in the embodiment of FIG. 4 operates in the same manner as that
shown in FIG. 3 but with the addition of a second phase-locked
the
loop to the system. T~ e ~f2 output signal from output
structure 112 is coupled to one input of second phase detector
133. A low-pass filter 134 and an amplifier 137 are coupled to
33 the output of phase detector 133 in the same manner as the system
~3;~69Z
discussed in reference to FIG. 3. To the output of amplifier
137 is coupled high precision voltage-controlled oscillator
138. The frequency of the output signal produced by voltage-
controlled oscillator 138 is determined by the amplitude of the
output signal from amplifier 137. Frequency divider 135 divides
down the output signal from voltage-controlled oscillator 138
producing an output signal which has the same frequency as
~f2 when the system is at rest.
With the embodiment of the invention shown in FIG. 4, the
afl frequency difference is maintained at a constant value
independent of the rate of rotation of the system. Because
the afl signal is a constant, the af2 signal will vary by
twice the amount for a given rate of rotation than it would if
the Qfl signal were also permitted to vary. Thus, the analog
output signal VOUt from amplifier 137 has an amplitude twice
that as would be produced in a nonphase-locked system and
with no addition of noise.
Referring next to the block diagram of FIG. S, there is
shown still another embodiment of the invention. In the
embodiment shown in FIG. 5, the phase-locked loop circuitry is
entirely outside laser gyro cavity 110. The afl and af~
signals are amplified by buffer ampli-fiers 141 and 142 and
coupled as the two inputs to double balanced mi~er 140. The
output signal af = ~f2 ~ afl from double balanced mixer 140
is of a frequency which is directly indicative of the rate
of rotation of the system. However, as the af signal has a
typical frequency range of 100 to 500 ~. for a typical
laser gyro cavity construction, a simple digitization of the
af signal quantizing the signal as one counter pulse per cycle
of the signal contains a large amount of quantizing error. This
~L~3~
error is largely eliminated with this embodiment of the
invention.
The DC level of the ~f signal is set by DC restorer
circuit 143 such that a zero frequency of the Qf signal,
corresponding to a zero rate of rotation of the system, produces
a zero output voltage. The DC restored signal is coupled to
one input of phase detector 144 which is the input point of the
phase-locked loop circuitry. The output of phase detector 1~4
is coupled through low-pass filter 145 and amplifier 146. As
in the previous embodiments, the output of amplifier 146 is
coupled through voltage controlled oscillator 148 and frequency
divider 147 to the second input of phase detector 144.
Two output signals are produced from the phase-locked
loop. The VOUt signal from amplifier 146 is an analog
signal the amplitude of which is in direct proportion to the
rate of rotation of the system. This analog signal is highly
precise and has no quantiz&tion error. The second output
signal V' is produced at the output of voltage-controlled
out
oscillator 148. The frequency of the V~out signal is N times
that of the ~f signal. Thus, the V~out signal may be digitized
with a precision of N times that of a digitization of the ~f
signal. In the circuitry implementation described below, N
is typically of the order of 233. Hence, it is readily
appreciated that a large reduction in quantization error has
been achieved with the invention.
Referring now to the view of FIG. 6, there is shown a
block diagram of still another embodiment of the invention.
This embodiment employs two phase-locked loop circuits producing
an output signal VOUt which again has a frequency N times that of
~f. With the embodiment shown in FIG. 6, however, it is not
. .
~3~9Z
necessary to actually produce the Qf signal. A highly
precise analog output signal VOUt is nonetheless produced
in direct proportion to the rate of rotation of the system.
The ~fl signal from output structure 112 is coupled to
the input o phase detector 151 while the ~f2 signal is coupled
to the corresponding input of phase detector 157. The outputs
of phase detectors 151 and 157 are coupled through low-pass
filters 152 and 15~ to amplifiers 153 and 158 as in previously
described embodiments. The outputs of amplifiers 153 and 158,
again as in previously described embodiments, are coupled
back to the inputs of phase detectors 151 and 157 through
voltage-controlled oscillators 155 and 159 and frequency
dividers 154 and 160.
The highly precise analog output signal VOUt is produced
by forming the difference between the output signals from
amplifiers 153 and 158 through difference amplifier 162. Besides
providing a highly precise analog indication of the rate of
rotation the polarity of the VOUt signal indicates the direction
of rotation of the system.
The outputs of voltage controlled oscillators 155 and
159 are coupled to the two inputs of double balanced mixer 161.
The output signal V'out from double balanced mixer 161 has
a frequency which, as in the previously described embodiments,
is in direct proportion to the rate of rotation of the
system with the quantization error reduced by a factor of N.
In FIG. 7 is shown a schematic diagram of the phase-locked
loop circuitry coupled to the ~fl signal. The sinusoilally
shaped ~fl signal is coupled through capacitor 264 to pulse
forming network 261 which converts the sinusoidal signal to
pulse form with one pulse being generated for each cycle of the
~L3~65~
Qfl signal. Pulse forming network 261 is not shown in the block
diagram view of FIG. 4 as it may not always be needed depending
upon the form of the ~fl signal produced by the output s~ructure
of the gyro system. Pulse forming network 261 includes thres-
hold detector 257 connected in a Schmidt trigger configuration
so that noise present upon the ~fl signal ~oes not cause false
trigger~ng.
The output of pulse forming network 261 is coupled through
inverter 231 to one input of phase detector 130. Phase detector
130 is functionally implemented by integrated circuit digital
phase detector 232. Integrated circuit phase detector 232 has
two output lines Ul and Dl. If, for example, the R input leads
in phase the V input, the Ul output will remain at a fixed
positive DC voltage while the Dl output will be pulsed with low
going pulses of a width depending upon the phase difference.
If the V input leads the R~input, the Dl output will remain
at the fixed positive voltage while the Ul output is pulsed.
Reference clock 131 produces a pulsed signal Of controllable
frequency which is coupled to the R input of phase detector ~30
through inverter 230. As it is generally easier to obtain vary
stable frequency sources at frequencies of, for example, ;0 MHz
and above, a 70 MHz oscillator 205 provides the initial clock
pulse source for reference clock 131. The output of oscillator
205 is coupled to the clock inputs of emitter coupled logic
flip-flops 210 and 212. A divide by four function is provided by
these two flip-flops. The inverted and noninverted outputs of
flip-flop 212 are coupled to the base inputs of dual transistor
215. Transistor 215 is coupled in a differential amplifier
configuration and biased so as to convert the emitter coupled
logic output levels from flip-flop 212 to levels acceptable fOI'
17 -
' ~ :
~3Z6g~
transistor-transistor logic circuitry. The output is taken
across collector resistor 218 and coupled to the input of inverter
221. Inverter 221 provides clock buffering to the clock inputs
of four-bit binary counters 222 and 223. Counters 222 and 223
are connected serially in a count-down configuration. Switches
227 and 228 are coupled to the preset inputs of the counters
with biasing provided by resistors 226. A reset pulse for both
counters 222 and 223 is produced at the output NAND gate 224 each
time a count of zero is reached. The output signal to phase
detector 130 is taken as the highest order bit output from
counter 223. In this configuration, counters 222 and 223
provide a variable pulse frequency dividing circuit with the
division factor dçtermined by the settings of switches 227 and
228. In systems operation, switches 227 and 228 are set to
provide a zero phase difference output from phase detector 130
when the system is inertially at rest.
The Ul and Dl outputs from integrated circuit phase
detector 232 are coupled through resistors 233 and 234 to the
inverting and noninverting inputs respectively of integrated
differencial amplifier circuit 241 within amplifier 136.
Frequency compensation is provided to amplifier circuit 241
with capacitors 240, 243, ~nd 244. The function of low-pass
filter 132 is provided by two seperate RC circuits, one formed
of resistor 242 and capacitor 235, is coupled between the
noninverting input of amplifier circuit 241 and ground. The
other, formed of resistor 238 and capacitor 236 is coupled in
a feed-back arrangement between the output and ~4~inverting
input of amplifier circuit 241. The output of amplifier circuit
241 is coupled to the input of coil driver amplifier 139 at the
noninverting inpu~ of amplifier circuit 249. Faraday bias coil
- 18 -
s
~3~
114 is coupled between the output of amplifier circuit 249
and its inverting input. Frequency compensation is provided
amplifier circuit 249 by capacitors 248, 250, and 251.
The operation of the ~f2 phase-locked loop will be explained
with reference to FIG. 8. The Af2 signal is coupled to pulse
forming network 305 through capacitor 304. Pulse forming
network 305 functions as in the circuit of FIG. 7. The circuitry
and operation of phase detector 133 is also the same as described
in conjunction with FIG. 7 as is that of low-pass filter 134
and ampliier 137.
The output of amplifier circuit 322 within amplifier ~
is coupled through resistor 338 to the control voltage input of
VCO integrated circuit 340 within voltage-controlled oscillator
138. VCO circuit 340 has a 70 MHz output for a zero value of
input signal. As is well known in the voltage-controlled
oscillator art, the RF output of V~O circuit 340 varies in
proportion to the changes in the input signal.
The output from voltage-controlled oscillator 138 is
coupled to the input of frequency divider 135. Frequency divider
135 operates in the same manner as the similar circuit in FIG. 7.
The value of N is set by switches 360 and 361. N is predetermined
by the relationship fvco/Qf2 where ~f2 is taken for the system
at rest. For the chosen value of fvco = 70 MHz for the output
frequency from voltage-controlled oscillator 138 for a zero
input and for a trpical value of ~f2 of 300 KHz, N = 233.
Although the circuits of FIGS. 7 and 8 have been described
in conjunction with the block diagram of FIG. 4, each of the
circuits of FIGS. 7 and 8 may function in the circui~s indicated
by the other block diagrams shown herein for the equivalent
circuits indicated.
- 19
~ ~ 3~69;~
Referring next to the diagram of FIG. 9, there will be
described an embodiment of the invention which operates using
a two-frequency laser gyro cavity. Laser gyro cavity 402
as shown in FIG. 9 is similar to laser gyro cavity 110 of
FIG. 1 but with crystal rotator 17 omitted. Of course, other
, - configurations of two-frequency laser gyroscope cavities may
be used as well. The two output beams from laser gyro cavity
402 are optically coupled to output structure 403 which beats
the beams together forming an output signal having a frequency
equal to the difference'in frequency between the two beams.
This difference frequency signal is amplified by amplifIer
404. The amplified signal is coupled to,one input of phase
detector 406.
The other input of phase detector 406 is coupled to
the output of reference clock source 405. Reference clock
source 405 produces an output signal of constant frequency and
phase. Phase detector 406 thereby produces an output signal
which has a parameter related to the difference in phase
between the frequency difference signal and signal produced
by reference clock source 405. Th~s may be represented, or
example, as an~mplitude or pulse width on one or more output
lines. The output of phase detector 406 is passed through
low-pass filter 407 which produces an output control signal
having an amplitude in proportlon to the phase difference
between the two input signals to phase detector 406. This
control signal is summed by signal summer 408 with an ofset
bias voltage 409 of fixed amplitude. The amplitude of the
bias signal is determined so as to produce the desired amount
of fre~uency splitting between the two beams at inertial rest
and for the expected range of rates of rotation.
-20-
~32~
The summed signal is amplified and buffered by coil
driver amplifier 410. The output of coil driver amplifier
410 is coupled back to Faraday bias coil 415 through dither
switch 414. Dither switch 41~ is cyclically moved back and
forth between the two positions shown to successively pass the
current through Faraday bias coil ~15 first in one direction
then the other. This is the well-known dither technique
used in previous two-frequency laser gyroscope s~stems. However,
in such systems, there is no feedback component present in
the current coupled to the Faraday bias coil. Dither switch
4 which is preferably implemented as an electronic switch
such as by using field effect transistors, is operated by
dither switch driver amplifier 413 which amplifies the symmetric
square wave output from dither oscillator 412.
In this embodiment the output of low-pass filter 407 may
be used directly as an analog output signal indicative of the
rate of rotation. Also, there is provided an analog-to-digital
con~erter 411 for converting the output signal VOUt to digital
form.
In still another embodiment, the Zeeman effect is used
to maintain a constant frequency difference. In that embodimen~,
the feedback coil is positioned around the laser gain medium.
The magnetic field produced in the gain medium by the current
flowing in the coil determines the amount of frequency difference
between the various beams. This technique may be used in either
dithered or ~on-dithered systems and may be used in combination
~Jith separate Faraday bias splitting.
~ ' '
:
1~3~i92
This concludes the description of the preferred embodiments
of the invention. Although preferred embodiments have been
described, it is believed that numerous modifications and
alterations thereto would be apparent to one having ordinary
skill in the art without departing from the spirit and scope
of the invention.
1,
, . ,~
ll~Z~Z
APPENDIX
Parts List for Circuits of FIGS. 7 ~ 8
Resistors
211, 214, 216, 346, - 560
349, 353
213, 351 - 100
218, 352 - -330
226, 337, 359 - lK
233, 234, 325, 326 - 5.6K
238, 242, 323, 329 - 150K
253, 262, 263, 307, 309 - lOK
256, 313 - 1.8K
259, 314 - 220K
260, 308 - l.SM
338 - 4.7K
341 - 200
342 - 3.9K
Capacitors
219, 220, 333, 335, - 470 p~. -
347, 356
240, 248, 320 - 1500 p.
235, 236, 324, 328 - 0.047 Mf.
239, 2433 244, 246, - 0.1 M.
247, 250, 251, 252,
254, 258, 304, 310,
312, 316, 317~ 318,
319, 331
264, 343 - 1000 pf.
Coils
114 - - 35.4 turns #34 magnet
wire, 1/2 " diame~er
334 - 1 MH.
-23-
,
~32~;~2
Transistors
215, 350 - 2N3810
Integrated Circuits
210~ 21Z, 357, 358 - Motorola MECL 10131
221, 225, 230, 2319 - Texas Instruments
332, 354, 362 SN 74H04
222, 223, 357, 358 - Fairchild 93516DC
224, 363 - Texas Instruments
SN 74H10
232, 330 - Motorola MC 4344
257 3 305 - National LMll9
340 - Motorola K1085A-375-
73-70 ~z.
-24-
, .
