Optics: measuring and testing – By light interference – Rotation rate
Reexamination Certificate
1998-12-31
2001-11-20
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
Rotation rate
C356S465000
Reexamination Certificate
active
06320664
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention concerns fiber optic gyroscopes having vibration-error reduction schemes, and more particularly alleviating false indications of rotation rate due to rectification of vibration at vibration frequencies in the gyroscope operating environment.
Fiber optic gyroscopes are an attractive means with which to sense rotation of an object supporting such a gyroscope. Such gyroscopes can be made quite small and can be constructed to withstand considerable mechanical shock, temperature change, and other environmental extremes. Due to the absence of moving parts, they can be nearly maintenance free, and they have the potential of becoming economical in cost. They can also be sensitive to low rotation rates that can be a problem in other kinds of optical gyroscopes.
A fiber optic gyroscope, as shown in
FIG. 1
, has a coiled optical fiber wound on a core and about the axis thereof around which rotation is to be sensed. The optical fiber is typical of a length of 50 to 2,000 meters, or so, and is part of a closed optical path in which an electromagnetic wave, or light wave, is introduced and split into a pair of such waves to propagate in clockwise (cw) and counterclockwise (ccw) directions through the coil to both ultimately impinge on a photo detector. Rotation &OHgr; about the sensing axis of the core, or the coiled optical fiber, provides an effective optical path length increase in one rotational direction and an optical path length decrease in the other rotational direction for one of these waves. The opposite result occurs for rotation in the other direction. Such path length differences between the waves introduce a phase shift between these waves for either rotation direction, i.e., the well-known Sagnac effect. This gyroscope is known as the interferometric fiber optic gyroscope (IFOG). The use of a coiled optical fiber is desirable because the amount of phase difference shift due to rotation, and so the output signal, depends on the length of the entire optical path through the coil traversed by the two electromagnetic waves traveling in opposed direction, and so a large phase difference can be obtained in the long optical fiber but in the relatively small volume taken by it as a result of being coiled.
The output light intensity impinging on the photo detector and hence the current emanating from the photo detector system photodiode (PD), in response to the opposite direction traveling electromagnetic waves impinging thereon after passing through the coiled optical fiber, follows a raised cosine function. That is, the output current
32
depends on the cosine of the phase difference &phgr;(&OHgr;) between these two waves as shown in FIG.
2
. Since a cosine function is an even function, such an output function gives no indication as to the relative directions of the phase difference shift, and so no indication as to the direction of the rotation about the coil axis. In addition, the rate of change of a cosine function near zero phase is very small, and so such an output function provides very low sensitivity for low rotation rates.
Because of these unsatisfactory characteristics, the phase difference between the two opposite-direction traveling electromagnetic waves is usually modulated by placing an optical phase modulator, or what is sometimes referred to as a bias modulator, in the optical path on one side of or adjacent to one side of the coiled optical fiber. In order to achieve sensitive detection of rotation, the Sagnac interferometer is typically biased at a frequency f
b
by a sinusoidal or square wave modulation of the differential phase between the counter-propagating beams within the interferometric loop. As a result, one of these oppositely directed propagating waves passes through the modulator on the way into the coil while the other wave, traversing the coil in the opposite direction, passes through the modulator upon exiting the coil.
In addition, a phase-sensitive detector PSD serving as part of a demodulator system or a digital demodulator is provided to receive a signal representing the photo detector output current. Both the phase modulator and the phase-sensitive detector can be operated by the modulation signal generator or a synchronized derivative thereof at the so-called “proper” frequency to reduce or eliminate modulator-induced amplitude modulation.
FIGS. 3
a,
3
b,
4
a
and
4
b
show the effect of modulation and demodulation over the raised cosine function. In
FIGS. 3
a
and
3
b,
the phase difference &Dgr;&phgr; of the gyroscope optical waves are modulated with a sine wave bias modulation
33
for the cases of &OHgr;=0 and &OHgr;≠0 respectively. The resulting modulated intensity output
34
of the photo detector vs. time is shown to the right of the raised cosine function. As
FIGS. 3
a
and
3
b
show, for &OHgr;=0 the phase modulation is applied symmetrically about the center of the raised cosine function and for &OHgr;≠0 the phase modulation is applied asymmetrically. In the first case, the output is the same when the sensor is biased at point A as when it is biased at point B, giving only even harmonics of f
b
on the photo detector output. In the second case, the outputs at A and B are unequal, giving significant photo detector signal content at f
b
, which is indicative of rotation rate. This signal content at f
b
, recovered by the phase sensitive demodulator (PSD), is proportional to the rotation rate &OHgr;. The signal also changes sign for an oppositely directed rotation rate.
FIGS. 4
a
and
4
b
show the case of square wave modulation
36
for &OHgr;=0 and &OHgr;≠0, respectively. Here, in practice, square wave modulation produces modulation transients
38
by the value of switching &Dgr;&phgr; from point A to point B on the raised cosine function. These are shown by the vertical lines in the resultant modulated photo detector current vs. time, which is proportional to the optical intensity impinging on the photo detector for an ideal photo detector. Again, in the absence of rotation, the output
37
at points A and B are equal, while the presence of rotation makes the output unequal for the “A” half periods and “B” half periods.
In the square wave demodulation process depicted in
FIGS. 5
a,
5
b
and
5
c,
the signal component synchronous with the bias modulation frequency f
b
is recovered from the photo detector signal by multiplying by a square wave demodulator reference waveform
39
of zero mean, synchronized to the bias modulation. The average or DC component
41
of the resultant demodulated output
40
is proportional to rotation rate.
One other method of recovering the rotation rate, shown in
FIG. 6
, is that of a digital demodulation scheme where the output
37
of the photo detector in a square wave modulated system is sampled at points A
i
during the first half cycle and points B
i
during the second half cycle. The sample event is represented by an arrow. Each sample
42
is converted from an analog signal to a digital one and the difference between the digital sum of the A
i
's and the digital sum of the B
i
's is proportional to &OHgr;.
In all of these cases, the PSD/digital demodulator output is an odd function having a large rate of change at zero phase shift, and thus changes algebraic sign on either side of zero phase shift. Hence, the phase-sensitive detector PSD/digital demodulator signal can provide an indication of which direction a rotation is occurring about the axis of the coil, and can provide the large rate of change of signal value as a function of the rotation rate near a zero rotation rate, i.e., the detector has a high sensitivity for phase shifts near zero so that its output signal is quite sensitive to low rotation rates. This is possible, of course, only if phase shifts due to other sources, that is, errors, are sufficiently small. In addition, this output signal in these circumstances is close to being linear at relatively low rotation rates. Such characteristics for the output signal of the demodulator/PS
Kaliszek Andrew W.
Laskoskie Clarence E.
Sanders Glen A.
Fredrick Kris T.
Honeywell Inc.
Turner Samuel A.
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