Optics: measuring and testing – By light interference – Rotation rate
Reexamination Certificate
1999-10-21
2001-07-03
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
Rotation rate
C356S464000
Reexamination Certificate
active
06256101
ABSTRACT:
BACKGROUND
A) Field of the Invention
The present invention relates to an open loop fiber optic gyroscope and more particularly to an open loop fiber optic gyroscope which can measure ultra-high rates of rotation.
B) Background of the Invention
FIG. 1
shows a conventional open loop fiber optic gyroscope
1
. The gyroscope
1
includes a light source
3
, a bidirectional coupler
5
, a splitter
7
, a modulator
9
, a fiber optic sense coil
11
, a detector
13
, a signal processor
15
and a phase modulation drive circuit
27
.
Operationally, light from the light source
3
passes through the bidirectional coupler
5
to the splitter
7
. At the splitter
7
, the light is split into two beams
8
and
10
. The beam
10
passes through the modulator
9
and travels in a counterclockwise direction through the fiber optic sense coil
11
. The light beam
8
travels in the clockwise direction through the fiber optic sense coil
11
and then passes through the modulator
9
.
After passing through the fiber optic coil
11
, as is known in the art, the two beams
8
and
10
are recombined by the splitter
7
and then travel back through the bidirectional coupler
5
and to the detector
13
. The output of the detector
13
enters signal processor
15
, first being amplified by preamplifier
17
and then converted to digital format by the A/D converter
19
.
The output of the A/D converter
19
is demodulated by demodulator
21
and then averaged by signal processor
23
. The output of the signal processor
23
is then forwarded output processor
25
. It is the signal processor
15
which, in accordance with the techniques described below, calculates the rotation rate of the gyroscope.
The output of the detector
13
represents the intensity of beams
8
and
10
after being recombined by the splitter
7
. The signal processor
15
compares the measured intensity of the recombined beams to an interference curve, which is illustrated in
FIG. 2
, to determine the phase difference between the beams.
As is known in the art, the phase difference between the beams
8
and
10
is proportional to the rotation rate of the gyroscope
1
. Thus, if the phase difference between beams
8
and
10
is known, the rotation rate of the gyroscope
1
can be computed.
Referring to
FIG. 2
, when the rotation rate of the gyroscope is zero, the output of the detector
13
rests at point X on the curve which means that no phase shift exists. When a rotation rate is applied to the gyroscope
1
, the output of the detector
13
moves in one direction or the other depending on the direction in which the gyroscope is rotating. For example, if the gyroscope
1
is rotating in the clockwise direction, the output of the detector shifts in a direction toward point A on the curve. Alternatively, if the gyroscope
1
is rotating in the counterclockwise direction, the output of the detector shifts in a direction toward point B on the curve. This phenomenon is referred to in the art as either a rate induced phase shift or Sagnac phase shift.
Since the slope of the interference curve is relatively flat at point X, the sensitivity of the curve at this region to changes in phase shift is low. As such, small directional movements of the rotational rate along this region of the interference curve can not be accurately measured.
To improve the sensitivity and provide a technique for detecting small changes in phase shift, the phases of the beams
8
and
10
of gyroscope
1
are shifted to allow measurements to occur at points A and B on the interference curve. This technique is referred to in the art as phase modulation.
Phase modulation is accomplished by modulating the phase of beams
8
and
10
to allow measurements of the recombined beam to occur at points A and B. The modulator
9
and phase modulation drive circuit
27
shown in
FIG. 1
are the devices which perform the phase modulation.
The phase modulation drive circuit
27
contains a square wave generator
31
and amplifier
29
. The generator
31
produces a square wave signal which directs the amplifier
29
to apply signals A and B to the modulator
9
. When signal A is applied to the modulator
9
, the phases of beams
8
and
10
which pass through the modulator, are adjusted such that the output of the detector
13
is measured at or near the region A of the interference curve. Similarly, when signal B is applied to the modulator
9
, the phases of beams
8
and
10
which pass through the modulator, are adjusted such that the output of the detector
13
is measured at or near region B of the interference curve.
As is known in the art, the measurement obtained at or near region B of the curve is then subtracted from the measurement obtained near region A of the curve. This calculated difference is then used to determine changes in rotational rate of the gyroscope
1
.
While the conventional gyroscope described above allows for measurements to occur at the more sensitive points on the interference curve, it still has significant drawbacks. In particular, the conventional gyroscope does not have the capability of accurately measuring high rates of rotation which induce Sagnac phase shifts in the range of 90°.
For example, if the conventional gyroscope
1
were to rotate in the clockwise direction at a very high speed and in turn induce a Sagnac shift in the range of 90°, the output of the detector
13
would shift in a direction toward point D on the interference curve given the enormity of the rate induced phase shift. When this occurs, the signal processor
15
cannot distinguish between rotations occurring at points B and D and, as such produces a false reading. Similar errors occur if the output of the detector occurs at point C on the interference curve.
In view of the foregoing, if the conventional gyroscope is placed on an object which at points during its flight has speeds of rotation which induce Sagnac phase shifts in the range of 90°, the gyroscope
100
will produce false readings. In view of this problem, there currently exists a need for a gyroscope that can measure ultra-high rates of rotation.
SUMMARY OF THE INVENTION
It is accordingly an object of the present invention to provide a gyroscope that can measure ultra-high rates of rotation.
It is another object of the invention to provide a gyroscope that can measure ultra-high rates of rotation, is small in size, durable and easily manufactured.
In accordance with the invention, the rotation rate of an object is measured by a fiber optic gyroscope which generates an alarm signal when the rotation rate of the object exceeds a predetermined threshold. The alarm signal is applied to a drive circuit which drives a phase modulator associated with the gyroscope. Different alarm signals are issued for different excessive rates of rotation which are measured by the gyroscope thereby resulting in a more severe modulation being performed by the phase modulator.
The drive signals from the drive circuit direct the phase modulator to offset Sagnac phase shifts which occur when the rotation speed of the object reach certain thresholds. The offsetting phase shifts allow for accurate measurements to occur on the interference curve and, as such, allow for the gyroscope to produce accurate measurements when the object is rotating at high rates.
In accordance with one embodiment of the invention, a fiber optic gyroscope for measuring a rotation rate of an object is disclosed where the gyroscope comprises: a light source means for generating a light beam; a bidirectional coupling means for receiving and forwarding the light beam generated by the light source means; a splitting means, which receives the light beam forwarded from the coupling means, for splitting the light beam into a first beam and a second beam; a fiber optic sense means for receiving the first and second beams; a phase modulator means for adjusting the phase of the first and second beams; a detector means for detecting the intensity of the first and second beams; a signal processor means for determining the rotation rate of the gyroscope bas
Goodwin James E.
Lo Pei-hwa
L-3 Communications Corporation
Turner Samuel A.
Winston & Strawn
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