An improved dither-stripping scheme for strapdown ring laser gyroscopes

Of the various non-linearity sources in a ring laser gyroscope (RLG), the most limiting one is the "mode-locking" or "lock-in" phenomenon. To avoid the mode-locking, the laser cavity is subjected to a fixed frequency to-and-fro motion, called mechanical dithering. This forces the RLG to stay outside the lock-in zone. The RLG output, however, reflects the dither rate as an additive noise, which must be removed to obtain the true body rate. The dither removal is by far the most important task in RLG signal processing, as the dither residue limits the accuracy with which the RLG measures angular rate and hence angular position. Two dither removal techniques commonly used in RLGs are: (1) the dither-stripping scheme using dither pick-off sensor; and (2) the dither removal by digital filtering scheme. Both techniques have their own strengths and weaknesses and hence are not applicable in all RLG-based positioning and tracking problems. This paper proposes a "third alternative scheme" for dither removal that essentially combines the strengths of the two existing schemes, provides comparable measurement accuracy and meets the time constraint in typical RLG-based applications. A comparative study of the three schemes with online single axis RLG data is also presented.