Ring Laser Gyroscope With Tilting Mirrors

1. Field of the Invention

The present invention relates to minimizing the effects of lock-in for an RLG and more particularly to dithering the optical path of the RLG by tilting the mirrors.

2. Description of the Prior Art

A ring laser gyro must overcome the problem of "lock-in" to be a practical device for measuring low rotation rates. One commaon method to minimize the effects of lock-in for an RLG is to mechanically dither the RLG. This is accomplished by using a flexure to sinusoidally rotate (dither) the RLG laser frame assembly with respect to its mounting plate. The sinusoidal motion has a large peak angular velocity but low angular displacement. The time that this input rate is below the lock-in threshold is thereby greatly reduced. The mechanical energy involved with dithering an RLG laser frame assembly presents problems when designing a system with three or more ring laser gyroscopes. Precautions must be made to avoid coupling of mechanical energy to other gyros or to the system.

One possible alternate approach is to dither the mirrors. Several patents (U.S. Pat. Nos. 4,686,683, 4,410,276, 4,410,274, 4,653,919 and 4,592,656) describe the use of mirror actuators that translate the mirror in a linear motion however none of these patents describe the improved performance technique of dithering the mirrors in an angular motion.

The present invention does not employ the dithering of the RLG mirror(s) in a linear motion as described in the prior art but rather the dithering of the mirrors in an angular motion. The optical path for a resonant RLG is defined by the location and angles of the mirrors. By sinusoidally tilting the mirrors, the optical path can be sinusoidally rotated (dithered), thereby imparting the desired sinusoidal angular rate.

This shows an improvement over the prior art of mechanical dithering the entire RLG by drastically reducing the dithering inertia, increasing the dither frequency, decreasing the dither amplitude and increasing peak angular velocity.

The present invention therefore includes the use of state of the art RLG design with the addition of tilting mirrors. The tilting mirrors are capable of tilting the surface of the mirror. The mirrors are driven by the same drive signal to all mirrors therefore eliminating the need to precisely control the amplitude and phase.

FIG. 1 illustrates an RLG with four tiltable mirrors.

FIG. 2 illustrates one possible configuration that will tilt the surface of a mirror.

FIG. 3 illustrates all four mirrors tilted by angle .theta. and the optical path rotated by .theta.'.

FIG. 1 shows a ring laser gyroscope 10 with frame 12, cathode 14, anodes 16 and four tiltable mirror assemblies 18, 20, 22, 24. Mirrors 18 and 20 have path length control devices 26. Mirror 22 has prism 28 and heterodyne detector 30 with output signals 32. Mirror 24 has detector support structure 34 for intensity detectors 36. The signals 38 and path length control electronics 40 are used to control the path length control devices 26 to maintain a constant length of optical path 42. The four tiltable mirror assemblies are controlled with a sinusoidal drive signal 44.

FIG. 2 shows the tiltable mirror assembly 50 made up of a moated mirror 52, support structure 54 and drive PZTs 56 and 58. PZTs 56 and 58 are electrically wired such that a voltage on sinusoidal drive signal 70 causes the length of PZT 56 to increase while the length of PZT 58 decreases. This applies a force on center post 60, resulting in a deformation of the thin moated webs 62 and a tilting of the mirror surface and optical coating 64. When this mirror is mounted on frame 66, the optical beam 68 is partially transmitted through the mirror coating 64 and passes through openings in the PZTs 56 and 58.

FIG. 3 shows a geometric ray trace of the optical path 102 defined by the four mirrors 100. When the four mirrors are tilted by angle .theta. to angular position 100', the optical path 102 rotates to optical path 102'. The angle between optical path 102 and 102' is .theta.' defined by

where r is the radial distance 104 from mirror 100 to the center of the gyro and R is the radius of curvature of mirror 100.

It is not intended that this invention be limited to the hardware or operational procedures shown disclosed. This invention includes all of the alterations and variations thereto as encompassed within the scope of the claims as follows.