Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
1. Field of the Invention:
The invention pertains broadly to laser gyroscope systems
More particularly, the invention relates to four-frequency laser
gyroscope systems in which high precision output signals are re-
quired.
2. Description of the Prior Art:
One of the primary problems which must be overcome to
providé a useful laser gyroscope system is that of mode locking.
In an uncompensated system in which no provision is made for
overcoming this problemJ 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 Keimpe Andringa and assigned to the ~-~
present assignee. In the patented systems, beams of four
separate frequencies propagate around a closed laser gyroscope
path defined by four mirrors. Two beams circulate in the
clockwise direction and two in the counterclockwise direckion. ~
Of the two clockwise circulating beams, one is of left-hand ~;
circular polarization and the other of right-hand circular
. - ~ , , .
polarization as is also the case for the two counterclock-
wise circulating beams. In ~he preerred embodiment, the
two beams of right-hand circular polariza~ion are of higher
frequency ~han those of left-hand circular polarization. A
Faraday rotator structure provides the frequency difference or
splitting between the beams o~ 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 ro~ated
in a first direction, the beams of frequencies fl and f2 will
move together in requency while those of f3 and f4 will
move apart in frequency by the same amount. For rotation in
the opposite direction, the beams o frequency fl and f2 will
move apart ln frequency while those of f3 and f4 will move
together by the same amount.
To produce an output signal having a frequency in
proportion to the rate of rotation, a irst two output frequency
difference signals having frequencies ~fl = f2 - fl and ~f2 =
.
- r ~
f4 - f3 are formed. A final output signal hf = ~2 ~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 Qfl 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 o the system.
Although ~his 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 ~f2 signals at
~one pulse per cycle of the signals can provide.
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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 quanti~ation of the output signal is achieved at
a much higher rate than straightforward quantiza~ion of the nor~al output -
signals would provide.
According to a broad aspect of the present invention, there is
provided in combination, means for providing a close-loop propagation
path for sustaining propagation of electromagnetic waves of a plurality
of frequencies, means for producing a first signal having a frequency
which is a function of the difference frequency between two of said waves,
means for producing a reference signal, and means for reducing the quan-
tization error of said first signal comprising a phase detector coupled -to said first signal producing means and said reference signal producing
means to produce a second signal as a function of the phase difference
between said first signal and said reference signal. ~- -
According to a more specific aspect of the invention, there is
provided the combination of means for providing a closed-loop path for
sustaining propagation of electromagnetic waves of a plurality af frequen-
cies and means for producing a first signal having a frequency equal to a
.
predetermined multiple of the diference in frequency between two of said ~
;
waves. The multiplicative factor is preferably an integer factor much - ~
greater than unity. A second signal is produced which has a frequency ~; -
equal to the difference in frequency be~ween the two waves. The first ~
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signal is phase locked to the second. Phase detecting means is provided
with the second signal coupled to the input of the phase detecting means
and the output of the phase detecting means being coupled to the means
for producing the first signal. Prequency dividing means is also provided
which divides the frequency of the ~irst signal to substantially the same
frequency as the second signal. The output of the dividing means is
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coupled as the second input of the phase detecting means to thus orm a
phase-locked loop.
The invention may further be practiced by providing the cornbin~
ation of mQans for providing a closed-loop propagation
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path for sus~aining electromagnetic waves of at least four
frequencies, means for producing a difference signal having a
frequency equal to the difference in frequency between ~wo of
the waves, means for producing a signal having a predetermined
or fixed frequency, means for producing a control signal
having parameters by the difference in phase between the
difference signal and the signal having the predetermined or
fixed frequency, and means for varying the frequency difference
between the two waves as determined by the control signal. In
preferred embodiments, the frequency difference between ~he two
waves is maintained at a cGnstant value over at least a prede-
termined range of rates of rotation of the path providing
means. The frequency difference varying means includes means
for producing a magnetic field in response to the control signal.
The control signal producing means may in turn include phase
detecting means the inputs of which are coupled to the means
for producing a signal having the predetermined fixed frequency
and to the dif~erence signal producing means, low-pass filte~
means, and amplifying means coupled either to the input or
output of the low-pass filter means or the amplifying means
and low-pass filtering means being constructed as a single unit.
The~closed-loop~propagation path providing means preferably
includes at least four reflecting means, first and second
frequency dispersive means disposed in the path, and a laser
gain medium.
Objects of the invention may also be met by providing
the combination o means for providing a closed-loop propagation
path for sustaining propagation o electromagnetic waves of at
least four frequencies, means or producing a irst difference
signal having a frequency equal to the dierence in frequency
between a chosen first two of the waves, means or producing a
second difference signal having a requency equal to the difference
in frequency between a chosen second two of the waves, means
~or producing a irst control signal having a parameter determined
by the difference in phase between the first difference signal and
the signal having the predetermined fixed frequency, means for
varying the frequency difference between the first two of the
waves in accordance with ~he first control signal, means for
producing a second control signal having a parameter determined
by the difference in phase between the second dif~erence signal
and a feedback control signal, and means or producing the feedback
control signal in response to the second control signal wherein
the feedback control signal has a frequency determlned by the
second control signal. In the preferred embodiments, the frequency
difference varying means maintains the frequency difference
between the first two of the waves at a constant value over at
least a predetermined range of rates of rotation of the path
provlding means. The path providing means may include first and
second frequency dispersive elemènts particularly one of which is
Faraday rotator body so that the frequency difference varying
means may comprise a coil for producing a magnetic field in the
Faraday rota~tor body. The means for producing the first control
signal and the means for producing the second control signal may
each separately comprise phase detecting means having an input
coupled to the corresponding difference signal, low-pass filter
means and amplifying means coupled to ~ the low-
pass filter means. In the preerred embodiments, the frequencles
of the ~irst two o the waves are either both abave or below the
frequencies of the second two of the waves. ~`
Still further, objects of the invention may be met by providin~
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the combination of means for providing a closed-loop proyagation path for
sus~aining propagation of electromagentic waves of at least four frequen-
cies, means for producing a first difference signal having a frequency equal
to the difference in frequency between a first two of the waves, means for
producing a second difference signal having a frequency equal to the differ-
ence in frequency between a second two of the waves, means for producing a
third difference signal having a frequency equal to the difference in fre-
quency between the first and second difference signals, phase detecting
means with the third difference signal being coupled to a first input of a
phase detecting means, low-pass filter means, amplifying means coupled to
the low-pass filter means, amplifying means coupled to the low-pass filter ,
means with the input of the low-pass filtér means or amplifying means cou-
pled to the output of the phase detecting means, and means for producing a
control signal having a frequency determined by the value of an input par- ~-
ameter with the input of the control signal producing means being coupled
to the output of the amplifying means and the output of the control signal
producing means being coupled to a second input of the phase detecting
means. The means for producing the third difference signal preferably in- -~
cludes means for amplifying the first difference signal, means for amplify-
ing the second difference signal, and a double balanced mixer the inputs of
which are coupled to the means for amplifying the first and second differ- ~
ence signals. The control signal producing means comprises a voltage-con- `
trolled oscillator and frequency dividing means coupled ~o the output of
the voltage-controlled oscillator.
Still further, the invention may be practiced by providing the -
combination of means for providing a closéd-loop propagation p~th for sus-
taining propagation of electromagnetic waves of at least four frequencies,
means for producing a first diference signal having a frequency equal to
the difference in frequency ~
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between a first ~wo of the waves, means for producing a second
difference signal having a frequency equal to the difference in
frequenc~ between a second two of the waves, first and second
phase-locked loops with the irst phase-locl~ed laop coupled to the
means or producing the first difference signal and the second
phase-locked loop coupled to the means for producing the second
difference signal. Each of the phase-locked loops includes phase
detecting means with the corresponding difference signal being
coupled to one input of the phase detecting meansj7 low-pass
filter means, and means for producing a con~rol signal having a
frequency determined by the value of an input parameter with the
control signal producing means being coupled to the output of the
amplifying means and with the output of the control signal producing
means being coupled to a second input of the phase detecting
means. Further, in the combination is included means for produclng
an output signal having a parameter determined by the dif~erence
in frequency~bet~een the control signals of the first and second
phase-locked loops. The control signal producing means of each
of the first~and second phase-locked loops has a voltage-controlled
oscillator with the input of each being coupled to the output of
the~amplifying means and the output of each being coupled to an
input of the output signal producing means and ~requency dividing
means the input of which is coupled to the output of the voltage-
controlled oscillator and the output of each being coupled to
the second input of the phase detecting means. The propagation
path in the preerred embodiments includes four or more reflecting
means and first and second requency dispersive elements.
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D~SCRIPTION 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 o the laser
gyroscope system of the invention;
FIGS. 3-6 are block diagrams o various embodiments of
the invention; and
FIGS. 7 ~ 8 taken together are a detailed schematic
diagram of the embodiment of the invention shown in FIG. 4.
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DESC~IPTION OF THE PREFE~RED EMBODIMENTS.
, . .
Referring first to the view o:~ 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 deined
by mirrors 12-15. Along one leg o cavity 110 in the path of
the electromagnetic waves which propagate around the path
is disposed laser ga~.. medium 10. Laser gain medium 10 may
be formed as a sealed chamber containing gases such as a mixture
of isotopes of helium and neon. The amount o gain aforded
the various electromagnetic waves is, as is well known, a
function o~ frequency of the waves. As shown in the view of
FIG. 2, the laser gain curve 11 ~or the chosen gain medium is
substantially bell shaped. Along the leg o 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, e~uivalently,
a phase shift, for circularly polarized waves that is different
for one sense of circular polarization than for the other.
..... _ _ _ _ . _
That is, the~delay or phase shifts are diferent or waves
of right-hand circular polarization than for thcse of left-
hand circular polarization. Moreover, the delay is reciprocal
ln that the delay imparted to a particular wave depends only
upon its sense o 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
- 10 -
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field from an external permanent nagnet, 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 blas coil 114 forms part of a phased-locked
loop circuit S as will be described below.
Referring again to the view o FIG. 2, it shows that
waves of four dlstinct frequencies fl. f2, f3, and 4 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 f4 are right-hand circularly
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 PIG. l is rotated about its sensitive
axis, for a first direction o~ rotation, the waves of
frequencies 3 and f4 move closer togéther in ~requency while
those of fl and f2 move apart in frequency by the same amount
as f3 and f4 move together. For the opposite direction of
rotation, the waves of frequencies fl and f2 move closer together
in frequency while those of f3 and 4 move apart in frequency
again by the same amount.
To produce an output signal indicative of the rate of
rotation of the system or, alternatively, of the total
amount of rotation since a predetermined time, ~wo different
g s ~fl f2 ~ fl and ~f2 = f4 ~ f3 are ormed. At res~
~fl = ~f2. To form an output signal indicative of the rate of
rotation at any particular instant, a second difference signal
f = Qf2 ~ ~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 ~fl and ~f2 are produced
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
~o~n~e ~
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
- 12 -
. .
4~
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 1
and f2 are shone upon half-silvered mirror 47 and reflected
towards detector diode 48 while ~hose of frequencies f3 and
f4 are reflected by half-silvered mirror 45 to detector diode
~6. Detec~or diodes 46 and 4g are reversed biased b~ voltage
sources 49 and S0 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 lncident 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 centered symmetrically abou~ the peak
of the gain curve. To this end, a piezoelectric transducer
68 is provided to mechanically position mirror 12 to adjust the
total ~ath length within laser gyro cavity 110 to properly
center the our 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 al and Qf2 signals and a difference formed between
the two amplitude related signals. The output difference signal
of course has a zero ampli~ude when the waves of the four
frequencies are properly centered upon the gain curve. The
output difference signal is of one polarity when the four waves
-13 -
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are off center in one direction and the opposite ~ r~
when the waves are off center in the other direction. The
average amplitude signals are formed by the network including
diodes 61 and 62~ resistors 63, 64, and 65, and capacitor 66.
The output diference signal is formed by diferential ampli~ier
67, the output of which is coupled to the input leads of
piezoelectric trandsucer 58.
In accordance with the principles of the invention, a
phase-locked loop circuit S receives the input frequency
difference signals and from them produces a highly precise
output signal representing the rate o-rotation of the system
or the total amount o 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 requency diference 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 o~ re~erence clock 118. The output signal from phase
detector 116 representing the diference in phase between
the reerence clock signal and ~1 signal is coupled to amplifier
120 through low-pass filter 119. Low-pass ilter 119 may
alternately ollow amplifier 120 or may be incorporated there-
with such as in a eedback arrangement. The output o~ ampliier
120 is coupled back to Faraday bias coil 114 amplified by coil
driver ampliier llS.
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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 ~IG. 1 such ~ha~ 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 o rotation Thus,
as the system is rotated, one of the frequency difference
signals will not change. However, the rate of rotation will
be precisely indicated by the amplitude of the output signal from
lO ~ ampliier I20.
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
signal which is Oe course sugject to the quantization error.
If a digital output is desired, the output signal from amplifier
120 may be dlgi~tlzed using an analog-to-digital converter at
any desired level of precision. Secondly, with the 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 derlved.
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
loop to t-he system. ~ af2 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
the output of phase detector 133 in the same manner as the system
- 15 -
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discussed in reference to FIG. 3. To the output o~ amplifier
137 is coupled high precision voltage-controlled oscillator
138. The frequency of the ou~put 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
af2 when the system is at rest.
With the embodimen~ of the i~vention 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 glven rate of rotation than it would if
the ~fl 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 nolse.
Referring next to the block diagram of FIG. 5, there is
~ shown still another embodiment of the invention. In the
~embodiment shown ln FIG. 5, the phase-locked loop clrcuitry is~
entirely outslde laser gyro cavity 110. The afl and ~f2
signals are amplified by buffer amplifiers 141 and 142 and
coupled as the two inputs to double balanced mixer 140. The
output signal af = af2 - afl from double balanced mixer 140
is of a frequency which is directly indicative o~ the rate
of rotation of the system. However, as the af signal has a
B typical frequency range of 100 to 500 ~. for a typical
laser gyro cavity construction, a simple digitization of the
~f signal quantizing the signal as one counter pulse per cycle
of the signal contains a large amount of quantizing error. This
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error is largely eliminated wi~h this embodiment of theinvention.
The DC level of the af signal is set by DC restorer
circuit 143 such that a zero frequency of the ~f signal~
corresponding to a zero rate o 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 144
is coupled through low-pass filter 145 and amplifier 146. As
10 , in the previous embodlments, 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 slgnal 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
preclse and has no quantization 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 digiti~ed
with a precision of N times that o~ a digitization of the ~f
slgnal. 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 PIG. 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 Çrequency N times that of
~f. With the embodiment shown in FIG. 6, however, it is not
- 1.7 -
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necessary to actually produce the Q 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 of phase detector 151 while the ~f2 signal is coupled
to the corresponding input o phase detector 157. The outputs
of phase detectors lSl and 157 are coupled through low-pass
filters 152 and 156 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
` ~ o rotation of the system.
~ The outputs o voltage controlled oscillators 155 and
159 are coupled to the two inputs of double balanced mixer 161.
The~ou~put signal V'OUt from double balanced mixer 161 has ~
a requency which, as in the previously described embodiments,
is in direct proportion to the rate o rotation of the
system with the quantization error reduced by a factor o N.
In FIG. 7 is shown a schematic diagram of the phase-locked
loop circuitry coupled to the Ql signal. The sinusoidally
shaped Ql 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
- 18
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~fl signal. Pulse ~orming 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 structure
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 Qfl signal does not cause false
triggering.
The output of pulse forming network 261 is coupled through
inverter 231 to one inpu~ of phase detector 130. Phase detector
10 ~ 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 o~tput 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..nputj 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 130
through inverter 230. As it is generally easier to obtaln vary
stable frequency sources at frequencies of, for example, 50 MHz
and above, a 70 MHz oscillator 205 provides the initial clock
pulse source for reference clock 131. The output o~ oscilla~or
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 21S is coupled in a diferential amplifier
coniguration and biased so as to convert the emitter coupled
logic output levels from flip-flop 212 to levels acceptable for
19
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transistor-transistor logic circuitry The output is taken
across collector resistor 218 and coupled to the input o inverter
221. Inverter 221 provides clock bufering to the clock inputs
of four-bit binary counters 222 and 223. Counters 222 and 223
are connected serially in a count-down coniguration. 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 ~he 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 2Z3. In this configuration, counters 222 and 223
provide a variable pulse frequency dividing circuit with the
division factor determined by the settings of switches 227 and
228. In systems operation, switches 227 and 228 are set to
pro~lde 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
invertillg 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 Z35, i5 coupled between the
noninverting input of amplifier circuit 241 and ground. .The
other, formed of resistor 238 and capacitor 236 is coupled in
B a feed-back arrangement between the output and ~4ninverting
input of amplifier circuit 241. The output of amplifier circuit
241 is coupled to the input of coil driver amplifier 139 at the
noninverting input of amplifier circuit 249. Faraday bias coil
- ZO
" ~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 reerence to FIG. 8. The ~f2 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 amplifier 137.
The output of amplifier circuit 322 within amplifier 322
is coupled through resistor 338 to the control voltage input of
VC0 integrated circuit 340 wi~hin voltage-controlled oscillator
~138. VC0 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 ou~put of VC0 circuit 340 varies in
proportio~ 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/~f2 where ~fz is taken for the system ~ -
at rest. For the chosen value of fvco = 70 MH7 for the output
frequency from voltage-controlled oscillator 138 for a zero
input and for a typical value of ~f2 of 300 KHz, N i~ 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 circuits indicated
by the other block diagrams shown herein for the equivalent
circuits indicated.
~ 21 -
., , , ": . . . . . . .
: `~
This concludes thc description of the preerred embodiments
of the invention. Although preferred embodiments have been
described, it is believed that numerous modi~ications ~d
alterations thereto would be apparent to one having ordinary
skill in the art without departing rom the spirit and scope of
the invention,
:
-22
.. .. . .
,; , . . ..
~3f~
APPENDIX
Parts List _or Circuits of FIGS. 7 ~ 8
Resistors
2ll, 214, 216, 346, - 560
213, 351 - 100
218, 352 - 330
226, 337, 359 - lK
233, 234, 325, 326 - 5.6K
238, 242, 323, 329 - l50K
253, 262, 263~ 307, 309 - lOK
2~56, 313 - 1.8K
Z59, 314 - 220K
260, 308 : - 1.5M
,
:~ 338 - ~ 4.7K
341 : - 200
34Z ~ ~ - 3-9K
Capacitors : ~ : :
9 356~ 333~ 335~ ~ 470 pf. ~ ~
`:~ 240,:~Z48, 32~0 - l500 pf. ;.
235, 236, 324, 328 - 0.047 Mf. ~ :
:
: 239, 243, 244, 246, - 0.1 Mf
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 " diameter
334 - 1 MH.
Z3 ~
, , ,. , ,, , , , . ,, , " , ,
~3~
Transistors
215, 350 - 2N3810
Integrated Circuits
210, 212, 357, 358 - Motorola MECL 10131
221, 225, 230, 231, - Texas Instruments
, 4, 362 SN 74H04
222, 223, 357, 358 - Fairchild 93516DC
224, 363 - Texas Instruments
SN 74H10
232, 330 . - Motorola MC 4344
257, 305 - National LMll9
340 - Motorola K1085A-375-
73-70 MHz.
:
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24
