Dither generation for ring laser gyroscopes

Details Dither generation for ring laser gyroscopes Dither generation for ring laser gyroscopes

1994-06-30

1996-05-14

McGraw, Vincent P.

Optics: measuring and testing

By dispersed light spectroscopy

Utilizing a spectrometer

G01C 1968

Patent

active

055173047

DESCRIPTION:

BRIEF SUMMARY
This invention relates to the application of dither to ring laser gyroscopes (RLGs) in order to overcome shortcomings in methods hitherto proposed for the purpose.
Mechanical dither is an established technique for the removal of the errors present in conventional RLGs due to the lock-in effect at low rotation rates. The technique consists of the application of an oscillatory rotation of the RLG assembly about an axis perpendicular to the lasing plane (which is the plane in which the RLG is sensitive to rotation) with an amplitude sufficient to provide angular rates which exceed the maximum rates which in the particular application may be applied to the RLG.
Thus, with a typical oscillatory frequency in the region of 400 Hz, the nett rate seen by the RLG moves through the lock-in range of, typically, 1.degree. to 2.degree. per second fairly rapidly so that only very small non-linearity errors remain. The technique has the very significant advantage that no angular position bias can directly result from the applied dither.
There are, however, a number of disadvantages. Firstly, the dither motion applied to the RLG results in a sinusoidal error signal superimposed on the RLG output. This necessitates continuous compensation by optical or electronic means or navigation limitations with rate updates only over integral dither cycles.
Secondly, instantaneous operation of the RLG from switch-on is no longer possible since the mechanically resonant dither takes some time to build up.
Thirdly, the dither motion is transmitted to other navigation sensors and to other instrumentation, particularly if the host vehicle is light and freely suspended, as in satellite applications. This motion produces errors and an audible sound which may be unacceptable in covert submarine navigation.
Since RLGs operate by the monitoring of the phase variation in the output of a ring laser, then in principle the equivalent of a mechanical dither superimposing a mechanical sinusoidal oscillation on the RLG can be produced by the application of a sinusoidal phase variation to the laser output.
One method of achieving such an optical dither is to subject one of the non-transmitting mirrors of the RLG to a transverse Kerr effect; by this means, the application of a sinusoidal electric current through an electromagnet induces a corresponding phase variation in the reflected light. The phase variation is not sinusoidal; because of a hysteresis effect, the magnetisation of the electromagnet remains close to a saturation value for a longer part of the cycle than would be in the case of a sine wave and the magnetisation and hence the phase variation tends towards a square wave function. In practice this is advantageous since the time period in which the superimposed dither value is close to zero (and does not therefore assist in the avoidance of lock-in) is a smaller proportion of the dither cycle.
The potential effect of this is limited by the saturation values achievable in the material of the field-directing pole pieces of the magnet, and in the present state of development of magnetic mirrors a direct replacement of mechanical dither by optical dither is not possible. Thus, if a generous .+-.10.degree. per sec is allowed for the lock-in range and the system is specified to 120.degree. per sec, the simulated dither amplitude needs to exceed .+-.130.degree. per sec.
This invention consists of a navigation system comprising a RLG, characterised by including: means for inducing an oscillatory phase change in the output of the laser which simulates a oscillatory dither whose amplitude is in excess of the sum of a defined lock-in range and a defined, narrower control zone; and means for activating said oscillatory dither when the rotation rate applied to said RLG is within said control zone but to apply to the laser output a dither fixed at its full amplitude in the sense of the rotation rate applied to the RLG whenever said rotation rate is outside said defined control zone.
It is a relatively straightforward matter to compensate for the simulated

REFERENCES:
patent: 4968136 (1990-11-01), Lim et al.

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