GB2185147A

GB2185147A - Laser angular rate sensor

Info

Publication number: GB2185147A
Application number: GB08630989A
Authority: GB
Inventors: Graham J Martin
Original assignee: Litton Systems Inc
Current assignee: Northrop Grumman Guidance and Electronics Co Inc
Priority date: 1984-01-09
Filing date: 1986-12-30
Publication date: 1987-07-08
Also published as: DE3500044A1; GB2152739A; FR2557970A1; IT8547526A0; SE8500057L; NO844141L; SE8500057D0; IT8547526A1; IL73151A; GB8630989D0; GB2152739B; CA1252551A; JPS60160677A; GB8430489D0; GB2185147B; IT1181841B; FR2557970B1; SE458722B

Abstract

A laser angular rate sensor has four mirrors (18 to 21) appropriately arranged at the corners of a square to reflect two oppositely directed laser beams around a closed path 17. Vibrators (22,23) interconnected with two adjacent mirrors (18,19) for effecting oscillation perpendicular to the mirror major surface are arranged to operate at 180 degree phasing. A vibrator drive circuit is controlled by actual mirror displacement to tune and maintain dither amplitude at a predetermined value which preferably is 0.271 of the laser radiation wavelength corresponding to the modulation index of the zeroth order Bessel function, Jo. The vibrator drive circuit incorporates the capability for dithering when the angular rate of input to the gyro is close to zero, but switches off for higher values. <IMAGE>

Description

GB 2 185 147 A 1

SPECIFICATION

Laser angular rate sensor The present invention relates generally to a laser angu I ar rate sensor, and, more particu larly, to such a sensors having a mirror mechanical oscillation for overcoming lock-in errors that occur during low angular rate sensing.

A laser angular rate sensor, or ring laser gyro, has two counterrotating monochromatic laser beams moving around a closed path by successive reflections from three orfour mirrors. On rotation of the sensor about its sensing axis, the effective path 15 length for the two beams is changed resulting in a frequency differential between the beams which is proportional to the angular rotation rate. At low rotation rates, where the frequency differential between the two laser beams would be expected to be small, 20 it is found that the beams tend to "lock-in" or oscillate atthe same frequency so that a frequency differential is not detected.

One general approach in the prior art for ell minating lock-in has been to mechanically vibrate (dither) the laser angular rate sensor in orderto raise low sensor rotation rates out of the lock-in range. Although useful in reducing lock-in, this so- called body dither does not completely remove lock-in and it is generally undesirable to subject the entire 30 sensorto sustained vibrations.

An alternative dither scheme described in United States 3,533,014 consists in each mirror of a threemirror sensor being sinusoidally vibrated in a direction parallel to the reflective surface. This is difficult to achieve in practice since relatively large shearing forces are needed to move the mirrors in such a manner.

A still further approach setforth in United States Patent4,281,930 involves vibrating all three mirrors 40 of a laser gyro orthogonally to the reflective surface. Although vibrating the mirrors in this manner is relatively easyto achieve, unless a correct phase relationship is precisely maintained the cavity length forthe beams will change during the dither cycle which is undesirable.

According to one aspect of the invention there is provided a laser angular rate sensor comprising four mirrors, means for generating f irst and second oppositely directed laser beams to be reflected from the four mirrors so as to traverse a closed path about an input rate sensing axis of the sensor, first vibrating means for moving a first one of said mirrors perpendicularly to the reflective surface of said first mirror, second vibrating means for moving a second 55 of the mirrors immediately adjacent said first mirror perpendicularly to the reflective surface of said second mirror, and drive circuit means for energiz ing said f irst and second vib - rating means in a 180 degre phase relationship and to the same pred- 60 etermined amplitude.

One embodiment is a laserangular rate sensor having four mirrors appropriately arranged atthe corners of a squareto reflecttwo oppositely directed laser beams around a closed path which lies along the optical axis of the cavity, dithering means being interconnected with two adjacent mirrors for effecting longitudinal oscillation of these mirrors (i.e., perpendicularto the mirror major surface), the relative phase forthe two mirrors being maintained at 180 70 degrees sothatthe cavity length will be unchanged. A dithering means drive circuit is controlled by monitoring actual mirror displacement in ordertotune and maintain the dither amplitude at a predetermined value which preferably is substantially 75 0.271 of the laser radiation wavelength corresponding to the modulation index of the zeroth order Bessel function, J0. The dithering means drive circuit incorporatesthe capabilityfor dithering when the angular rate of inputto the gyro is close to zero (i.e. in 80 the lock-in band), but switches off forvalues outside the dither band.

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the 85 accompanying drawings, in which:

Figure 1 is a schematic representation of a laser angular rate sensor; Figure2 is a sectional, elevational view of a mirror and mirror dithering means.

Figure3 is a function blockcircuit diagram for dither drive and control; and Figure 4 is an alternate circuit for dither drive and control.

Figure 1 shows a laser angular ate sensor 10. in- 95 cluding a one-piece instrument block 11 with a cavity 12 extending along a square path.. Typically, a mixture of neon and helium is contained in the cavity at a very low pressure (e.g. 3torr). Calhodes 13 and 15 are provided togetherwith respective anodes 14 and 100 16. On establishing an elevated potential difference between cathode 13 and anode 14 as well as between cathode 15 and anode 16, two monochromatic laser beams are generated which pass in opposite directions along the cavity axis indicated generally 105 bythe line 17.

Mirrors 18to 21 are located at respective comersof the square cavityand servefirstof all to directthe two laser beams along thecavity path indicated generally bythe line 17. Thetwo adjacent mirrors 18 and 110 19 include piezoelectric mechanical oscillators 22 and 23, respectively, which when energized vibrate or dither the mirrors in a direction perpenclicularto the flat reflecting surface of the mirrors. When the mirrors have a curved reflecting su rface, then mirror 115 dither is along the optical axis, so that reference herein to the perpendicular are intended to be interpreted to coverthe direction along the optical axis of a curved reflecting surface.

Turning nowto Figure 2, piezoelectric vibrators or 120 oscillators 22, 23 satisfactory for present purposes include a material that expands and contracts as an electric potential differential is applied to and removed from opposite faces. Accordingly, application of a cyclically changing voltage potential to the 125 piezoelectric material cases an alternating lengthening and shortening of the material which, in turn, acts through the vibrator housing 25to ditherthe mirror. An excellent means for producing this dither isthe piezoelectric ceramic operating as a bimorph 130 described in United States Patent No.4,383,763, 2 GB 2 185 147 A CONTROLLABLE MIRRORS by T.J. Hutchings, et al.

For the ensuing description of a first form of mirror control and drive circuit reference is now made to Figure 3. Typically, in a laser gyro a mirror (20) is par- tially transmissive to the laser beams which allows them to impinge upon conventional optics and photo detector means 26 to produce signals responsiveto the laser beat frequency wave representative of the input rateto the gyro. Signals from 26 arefed 10 into conventional count processing circuitryfor producing output information on both the magnitude and the sign of the input rotation. The input rate information is also fed into a microprocessor 28,the purpose of which is to decide if the gyro is well out- side the lock band.

A sine wave generator 29 provides an A.C. signal to a variable gain amplifier 30 which, in turn, is interconnected with a driver 31 for the piezoelectric vibrator 22 of mirror 18. Similarly, the sine wave signal from generator 29 is applied to a second amplifier32 the output of which is fed to a phase shifter 33 where the phase is changed by 180 degrees before application to driver 34for mirror 19 piezoelectric vibrator 23. As to operation of the dither drive circuitdescri- 25 bed to this point,the two mirrorvibrators 22 and 23 are cyclically driven atthe samefrequency and amplitude, but 180 degrees out of phase. This phase difference insuresthatthe cavity length forthetwo laser beams is maintained constantwhich is desir- 30 able.

The absolute value of the countfrom the optics and photo detector 26for an integral number of mirror oscillations is accumulated in a counter35A synchronizing pulse is sentfrom the sine wave gene- 35 ratorto the counter in orderto accomplish this. The pulse count of 35 is converted to an analog equivalent in digital-to-analog convertor 36which is applied to both gain controls of amplifiers 30 and 32 to effect a corresponding change in the output magni- tude of drivers 31, 34.

The microprocessor 28 provides an enabling signal to the sine wave generator 29 on line 37 as long asthe gyro input iswithin the predetermined lock-in range. When the gyro input is well in excess 45 of the undithered lock-in limit, but before first-order lock band created bythe ditheratthe first harmonic of the ditherfrequency is reached,the sine wave generator is disabled so that mirror dither is completely terminated atthattime.

Figure 4 depicts a furtherversion of ditherdrive and control circuit. As in the first described embodiment, a sine wave generator38 is interconnected through an amplifier39 and driver40to cyclically dither mirror 18. Also, as before,the sine wave gene- 55 ratorapplies an A.C. voltagethrough the amplifier 41, phase shifter42 and driver43 to dither mirror 19 atthe samefrequency and amplitude as mirror 18, but 180 degrees out of phase.

Partially transparent mirror 21 passes a single 60 laser beam to photodiode 44 which forms a signal corresponding to the single beam intensity. This signal will contain an A.C. component as the normal gyro heterodyneputput, because of backscatter within the cavity primarilyfrom the mirrors. Output of the photodiode44 is formed in peak-to-peak detector45 and an error signal created in 46. This signal is used to control the gain of amplifiers 39 and 41 such thatthe mirrors are servoed to be driven atan amplitude that minimizesthe A.C. component (atthe 70 gyro ouput beatfrequency) on the signal from photo diode44.

In the practice of the described embodimentstwo immediately adjacent (i.e. consecutive) mirrors in a laser gyro are dithered atthe same frequency and 75 amplitude, but 180 degree phase relationship, along a path perpenclicularto the mirror plane. Both versions automatically discontinue mirror dithering when the gyro input rate exceedsthe lock-in range.

Claims

80 CLAIMS

1. A laser angular rate sensor comprising four mirrors, means for generating first and second oppositely directed laser beamsto be reflected from the 85 four mirrors so as to traverse a closed path about an input rate sensing axis of the sensor, first vibrating means for moving a first one of said mirrors perpendicularlyto the reflective surface of said first mirror, second vibrating means for moving a second 90 of the mirrors immediately adjacent said first mirror perpendicularlyto the reflective surface of said second mirror. and drive circuit means for energizing said first and second vibrating means in a 180 degree phase relationship and to the same pred- 95 etermined amplitude.

2. A laser angular rate sensor as in Claim 1, in which said drive circuit means inpludes means for interrupting operation of said first and second vibrating means when the sensor is reqeiving an angular 100 input rate in excess of a predetermined lock-in rate.

3. A laser angular rate sensor as in Claim 1 or2, in which said drive circuit means are such that. in operation, said first and second vibrating means are energized to provide a vibration amplitude substanti- 105 ally equal to 0.271 of the laser beam wavelength.

4. A laser angular rate sensor as claimed in claim 40 1, substantially as hereinbefore described with refereneeto Figures 1 to 3 or Figures 1,2 and 4ofthe accompanying drawings.

Printed for Her Majesty's Stationery Office by Croydon Printing Company W K) Ltd, 5i87, D8991685. Published by The Patent Office, 25 Southampton Buildings, London, WC2A l AY, from which copies maybe obtained.

4. A ring laser gyro, comprising:

means for creating two counterrotating laser beams and including more than two mirrors; means fortranslating only two of said mirrors in 110 an oscillating mode atthe same frequency substantially only perpendicularly to the reflective surface of said mirrors; and and meansfor phasing the movementof both of saidvibrating mirrorsto maintain constant primary 115 laser beam path length as said vibrating mirrors are displaced.

5. A ring laser gyro comprising:

means forming a closed loop optical cavity containing an active lasing medium for generating coun- 120 terrotating laser beams therein, the frequency difference between the light beams being a measure of the rate of rotation experienced bythe ring laser gyro, said cavity forming means including more than two mirrors for reflecting said light beams; and means for vibrating only two of said mirrors in translation at the same frequency in a direction only perpendicularto the surface of the mirror, with said vibrating mirrors having non-zero amplitudes of vibration and having phases of vibration to causethe 130 total distance around said closed loop to remain sub- Z, X_ 3 GB 2 185 147 A 3 stanti a I ly constant.

6. A ring laser gyro as in Claim 4 and 5,wherein there are only four mirrors.

7. A ring laser gyro as in Claim 4,5 or 6,wherein 5 said two of said mirrors are successive mirrors in the set of m i rro rs.

8. A laser angular rate sensor substantially as hereinbefore described with reference to Figures 1 to 3 or Figures 1, 2 and 4of the accompanying 10 drawings.

New claims or amendments to claims f i led in 6.2.87 Superseded claims 1-8 New or amended claims:- 1-4 CLAIMS 1. A laser angular rate sensor comprising more than two mirrors; means for generating first and 20 second oppositely directed laser beams to be reflected from the mirrors so as to traverse a closed path about an input rate sensing axis of the sensor, means forvibrating two of said mirrors in respective directions along their respective optical axes or, in the 25 case wherein a mirror has a planar reflective surface, in a direction substantially perpendicularto said reflective surface, and drive circuit meansfor energizing said vibrating means so asto maintain a constant primary laser beam path length as said mirrors are vibrated,the drive circuit means being such that, in operation said vibrating means are energized to provide a vibration amplitude substantially equal to 0.271 of the laser beam wavelength.

2. A ring laser gyro as in claim 1, wherein there 35 are only four mirrors.

3. A ring lasergyro as in Claim 1 or2,wherein said two of said mirrors are successive mirrors in the set of m i rro rs.