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Laser Gyroscope
M S Prasad
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The Sagnac-effect.
The inertial characteristics of light can also be utilized,
by letting two beams of light travel in a loop in
opposite directions. If the loop rotates clockwise, the
clockwise beam must travel a longer distance before
finishing the loop. The opposite is true for
thecounter-clockwise beam.
Combining the two rays in a detector, an interference
pattern is formed, which will depend on the angular
velocity.
The Coriolis-effect. Assume a mass that is vibrating in the
radial direction of a rotating
system. Due to the Coriolis force working perpendicular to the
original vibrating
direction, a new vibration will take place in this direction.
The amplitude of this new
vibration is a function of the angular velocity. MEMS gyros
(MicroElectroMechanical
Systems), tuning fork and wineglass gyros are utilizing this
principle. Coriolis-basedgyros are typically cheaper and less
accurate than mechanical, ring laser or fiber optic
gyros.
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Laser Gyroscope
Path length CW = ( 2 R + R d/dt . T )
Path length for CCW = ( 2 R - R d/dt . T )
T = ( 2 R + R d/dt .T ) / C - (( 2 R - R d/dt . T)/C
= 4 R ^2/ C^2 d/dt ==== proportional to rate of
change of d/dt
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L = 4A/C^2 d/dt = 4 A/C d/dt
Where is A is area enclosed
A = pi R 2 N ( N no of turns of FO cable)
Phase shift = w . T
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Sagnac effect is very low for low rate of rotation. For better
accuracy we measure
the fringe phase shift.Also
L / L = f/ f or f = 4A f /CL d/dt
f = K d/dt where k = 4A/L Integrate it we .get integrating
Gyroscope
Lock in ProblemFrequency lock in at low angular rates caused due
to imperfection in cavities .
Dither the laser block about the input axis .typical dither
frequency is about 100Hz
Aim is to minimize the dwell time in lock on zone .
Example : Traingle of side length = 7.329 cms and height 6.2687
cms laser at
wave length = 0.6328 micrometersthen freq change = 0.32 hz (
angle change over a period ) taking = 1deg /hr
Laser Gyro
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High accuracy The RLG meets the dynamic range for a pure IN
system of being able to
measure angular rates from 0.01/hour to 400/s to the required
accuracy a dynamic
range of 108:1.
Insensitivity to acceleration The RLG has no acceleration
sensitive bias errors, as it is basedon optical effects rather than
inertial effects.
Very high rate range This is limited only by the noise/bandwidth
characteristics of the read
out electronics: 1,000/s is no problem.
Very high scale factor accuracyErrors are in the 5to10ppm
bracket.
Negligible warm up time Full gyro operation from the instant of
turn-on.
Excellent turn-on to turn-on performance Performance
capabilities can bemaintained over several years without
calibration.
Random noise uncertaintyThis is measured in degrees per hour,
andis one of the RLGs
most important error characteristics. The error is significantly
higherthan experienced with
angular momentum gyros. It affects the system heading
determination in the gyro
compassing phase during the initial alignment process. This is
because it extends the time
required to filter the Earths rate signal from the gyro noise in
order to determine the initialheading.
Very high reliability : 50 .000 Hrs to 100000 hrs MTBF. Small
volume ( 20 cms)
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