Thesis - Leigh Moody.pdf - Bad Request - Cranfield University

Chapter 3 / Sensors / Gyroscopes
_ _
As mentioned earlier the RLG suffers from low angular rate “lock-in”
caused by imperfect lasing path mirror reflectivity. Back scattered light
returns from whence it came causing the two beams to synchronise and the
standing wave to become locked to the moving cavity, instead of the inertial
reference, causing a dead-band. The mechanical solution is to superimpose
a zero mean oscillation (dither) onto the input angular rate some 2-3 orders
higher than the lock-in threshold. Less time is then spent in the lock-in
region although there remains a residual random drift in the output after the
dither is removed. This is the dominant RLG error noise characteristic and
careful design is required to prevent rectified noise from appearing within
their operational bandwidth.
SCALE FACTOR STABILITY (ppm)
1000
100
10
1
0.1
0.000015
MECHANICAL
Aircraft
grade
RLG
0.00015 0.0015 0.015 0.15 1.5 15 150
3.4-2
FOG
BIAS STABILITY (deg/hr)
Earth
Rate
QUARTZ
SILICON
MICROS
Figure 3-23 : Gyroscope Relative Performance
Figure 3-23 compares the accuracy of the different types of gyroscope - data
supplied by Dr. P. Jones from original material published by QinetiQ.
3.4.2 Description
The triad of rate-integrating, single-axis gyroscopes shown in Figure 3-24
provides inertial angular increments (M_W_INC), and average inertial
angular rates (M_W_BCB), with respect to the IMU reference frame (U).
The model is flexible enough to accommodate the following types of
gyroscopes:
• Mechanical, torque re-balanced floated rotor gyroscope
• Ring laser gyroscope
• Fibre optical gyroscope
• Resonant gyroscopes
There is a wealth of literature on gyroscopes. The work of Lawrence [L.10]
and The Institute of Electrical and Electronics Engineers (IEEE) standard [I.1]
are useful references.