EP0186700A1

EP0186700A1 - Body dithered laser gyro assembly

Info

Publication number: EP0186700A1
Application number: EP85903473A
Authority: EP
Inventors: Gordon S. Kirkpatrick
Original assignee: Sundstrand Data Control Inc
Current assignee: Sundstrand Data Control Inc
Priority date: 1984-06-15
Filing date: 1985-06-07
Publication date: 1986-07-09
Also published as: NO860549L; DE3590265T1; GB8603598D0; JPS61502353A; IT1181685B; ZA854142B; GB2171246A; WO1986000130A1; IT8548214A0; AU4603685A

Abstract

Montage gyroscopique à laser à corps vibrant, conçu de manière à permettre la prédiction aisée et le réglage de la fréquence naturelle du montage. Dans un mode de réalisation, le montage gyroscopique à laser comporte une base (10), un mécanisme de support résilient (12) fixé à la base, un mécanisme de couplage (30, 40, 50) fixé au mécanisme de support, un gyroscope à laser (20) fixé au mécanisme de couplage et un mécanisme d'entraînement (70) connecté à la base et au mécanisme de couplage. Le mécanisme d'entraînement confère un mouvement vibrant au mécanisme de couplage, alors que le mécanisme de couplage couple ledit mouvement vibrant au gyroscope. Le mécanisme de couplage peut comporter un diaphragme (40) doté de parties intérieures (124) et extérieures (122), la partie intérieure du diaphragme étant connectée au mécanisme d'entraînement et la partie extérieure couplée au gyroscope à laser. Les parties de diaphragme intérieures et extérieures peuvent être connectées par une pluralité de rayons (126) permettant le couplage du mouvement de vibration.Gyroscopic laser assembly with vibrating body, designed to allow easy prediction and adjustment of the natural frequency of the assembly. In one embodiment, the laser gyro assembly includes a base (10), a resilient support mechanism (12) attached to the base, a coupling mechanism (30, 40, 50) attached to the support mechanism, a gyroscope laser (20) attached to the coupling mechanism and a drive mechanism (70) connected to the base and the coupling mechanism. The drive mechanism imparts a vibrating movement to the coupling mechanism, while the coupling mechanism couples said vibrating movement to the gyroscope. The coupling mechanism may include a diaphragm (40) having interior (124) and exterior (122) portions, the interior portion of the diaphragm being connected to the drive mechanism and the exterior portion coupled to the laser gyroscope. The inner and outer diaphragm parts can be connected by a plurality of spokes (126) allowing the coupling of the vibration movement.

Description

BODY DΓΓHERED LASER GYRO ASSEMBLY

Technical Field The present invention relates to ring laser gyros and more par¬ ticularly to an assembly for eliminating errors in the laser gyro output due to lock-in between the counterrotating laser beams.

Background of the Invention

In a laser gyro, two monochromatic beams of light are generated and caused to travel in opposite directions about a closed path perpendicular to the axis about which rotation is to be sensed. As the gyro is rotated about its sensitive axis, the effective path length for one beam is increased while the effective path length for the other beam is decreased. Because the frequency of oscillation of a laser is dependent upon the length of the lasing path, the gyro rotation causes a frequency difference between the two beams. The magnitude and sign of this frequency difference are indicative of the rate and direction of rotation respectively, and may be monitored to provide the desired gyro output.

As the rate of rotation of the laser gyro is decreased, the frequency difference between the two beams is similarly decreased. As very low rates of rotation, errors arise due to lock-in effects, whereby no frequency difference between the beams is observed. Lock-in arises where the frequency splitting between the two beams is small, causing coupling between the beams such that they oscillate at the same frequency. This results in a dead band or a lock-in region in which the gyro output does not track the input.

Various techniques have been employed in an attempt to eliminate lock-in at low rates of rotation. One such technique is to provide a dither motor to vibrate the body of the gyro about its sensitive axis^' with a sinusoidal dither motion at the natural frequency of the assembly. A variation of this technique is to add a random noise component to the sinusoidal drive signal. Such techniques produce a dither motion of the gyro body on the order of 0.01°, which has been found to be generally effective on reducing lock in. Prior body dithered laser gyro assemblies are generally made to have a relatively high , i.e., low damping, because very little power is required to dither a high Q assembly when it is driven at its natural frequency. However, a high Q assembly is very sensitive, and the amplitude of vibration imparted to the gyro in such an assembly is affected by unwanted external vibratory inputs.

Since three laser gyros are commonly mounted in a common instrument cluster to measure rotation rates about three orthogonal axes, such unwanted external inputs include vibrations caused by the dither motors of other gyros. For this reason, when laser gyros are mounted in a common instrument cluster, each gyro and motor assembly is preferably constructed to have a different natural frequency, and the corresponding dither motors are operated to drive each gyro at its respective natural frequency. In the past, constructing each gyro assembly to have a unique natural frequency, and predicting and adjusting such frequency, have been time-consuming and difficult tasks. Summary of the Invention

The present invention provides a body dithered laser gyro assembly that is adapted such that the natural frequency of the assembly can readily be predicted and adjusted.

In one preferred embodiment, the laser gyro assembly comprises a base, resilient support means mounted to the base, coupling means mounted to the support means, a laser gyro mounted to the coupling means, and drive means connected to the base and to the coupling means. The drive means imparts a dither motion to the coupling means, and the coupling means in turn couples such

- dither motion to the gyro. In one aspect of the invention, the coupling means comprises a resilient ring mounted in contact with the gyro, a diaphragm mounted in contact with the resilient ring, an alignment plate mounted in contact with the diaphragm, and fastening means for securing the gyro, resilient ring, diaphragm and alignment plate together. An inner portion of the diaphragm is connected to the drive means, and an outer portion is mounted in contact with the resilient ring and alignment plate. The inner and outer portions may be connected by a plurality of beams through which the dither motion is coupled. The support means comprises a plurality of resilient posts upon which the coupling means and gyro are suspended above the base. Such posts may extend through openings in the gyro.

In another aspect of the invention, the gyro has a central bore ex¬ tending through the gyro between exterior surfaces thereof. The coupling means is connected to the gyro only at such exterior surfaces and not at the central bore. The drive means may be positioned in the central bore such that the drive means does not contact the gyro.

These and other features that are advantages of the invention will be apparent in the detailed description and claims to follow, taken in conjunction with the accompanying drawings.

A Brief Description of the Drawings FIGURE 1 is an exploded, isometric view of the laser gyro assembly of the present invention.

FIGURE 2 is a top elevational view of a dither motor suitable for use in the present invention.

FIGURE 3 is a top elevational view of a diaphragm suitable for use in the present invention.

FIGURE 4 is a vibrational model of the laser gyro assembly of the present invention. Disclosure of the Invention

Referring initially to FIGURE 1, the assembly of the present invention comprises base 10 having four upstanding resilient posts 12 mounted thereon. Base 10 is typically mounted in an instrument package that includes two other laser gyro assemblies. Laser gyro 20 is mounted, in a manner described below, such that it is suspended a short distance above base 10. The laser gyro, shown schematically, has a generally octahedral shape with a large, cylindrical central opening 28. The laser gyro also includes small openings 22 and 24. Openings 22, 24 and 28 extend vertically completely through the body of - the gyro. Laser gyro 120 is part of subassembly 64 that also includes resilient ring 30, diaphragm 40, alignment plate 50 and cap 60. Resilient ring 30 is composed of a comparatively stiff, elastomeric material, such as silicone rubber, and rests directly on upper surface 26 of laser gyro 20. Diaphragm 40 comprises a thin, steel plate and is mounted directly on upper surface 36 of resilient ring 30. Portions of diaphragm 40 may be cut away to form slots, as described below. Alignment plate 50 is mounted directly on upper surface 46 of dia¬ phragm 40, and includes central opening 62 within which cap 60 is mounted. Alignment plate 50 also includes depending rods 52 having lower threaded portions 58. Rods 52 are used to fasten subassembly 64 together. In particular, rods 52 extend through aligned openings in diaphragm 40, resilient ring 30 and laser gyro 20, such openings in laser gyro 20 being designated by numeral 24. The length of rods 52 are such that when the rods are inserted through the diaphragm, resilient ring and gyro, threaded portions 58 extend beneath the -4- lower surface of laser gyro 20 and engage washers 54 and nuts 56 to fasten the entire subassembly 64 together.

Dither motor 70 is mounted to base 10 such that it is centrally located with respect to posts 12. Subassembly 64 is positioned over posts 12, such that the posts extend through aligned openings in laser gyro 20, resilient ring 30 and diaphragm 40, such openings in laser gyro 20 being designated by numeral 22. The upper ends 14 of posts 12 are fastened, such as by welding, to the lower surface of alignment plate 50. Openings 22, and aligned openings in resilient ring 30 and diaphragm 40, are sized such that posts 12 do not make contact with the laser gyro, the resilient ring or the diaphragm. As described below, the upper portions of dither motor 70 are fastened to diaphragm 40. When an appropriate drive signal is supplied to dither motor 70, it causes a dither motion in diaphragm 40 which is coupled to the laser gyro through sub¬ assembly 64.

Referring now * FIGURE 2, dither motor 70 includes central hub 72 and a plurality of arms 74 and 82 extending therefrom. Hub 72 includes central opening 73 that may be used for gaining access to base 10, if desired. Arms 74 include radial portions 76, circumferential portions 77 and cylindrical portions 78. Similarly, arms 82 include radial portions 84, circumferential por¬ tions 85 and cylindrical portions 86. Cylindrical portions 78 extend a short distance above the upper surface of dither motor 70, and each cylindrical portion 78 includes bore 80 that is internally threaded at its upper end for fastening its associated arm 74 to diaphragm 40, as described below. Similarly, cylindrical portions 86 extend a short distance below the lower surface of the dither motor, and each cylindrical portion 86 includes a bore 81 that is internally threaded at its lower end for fastening its associated arm 82 to base 10. The dither motor is positioned within central opening 28 of laser gyro 20. However, no portion of the dither motor makes contact with the laser gyro. Since the only portions of the gyro support and dither drive means that contact the gyro are resilient ring 30 and washers 54, it will be appreciated that thermal expansion of the various elements of the support and drive means do not affect gyro output or degrade gyro performance.

Piezoelectric crystals 90 and 92 are bonded to opposite sides of radial portions 76 and 84 of arms 74 and 82 respectively, preferably using a conductive cement. By using such a cement, arms 74 and 82 provide one electrical contact for each of the piezoelectric crystals. Each of the piezoelectric crystals is cut with respect to the crystallographic axes of the piezoelectric material such that an alternating electric signal applied between the outer surface of the crystal and the inner surface abutting arm 74 or 82 will cause the crystal to expand and contract in a radial direction, causing arm 74 or 82 to be deflected circumferentially. The static deflection of each crystal is dependent upon the amplitude and polarity of the applied field. The outer surfaces of crystals 90 are connected in parallel to a first conductor (not shown) to which a first dither drive signal is supplied, and the outer surfaces of crystals 92 are connected in parallel to a second conductor (not shown) to which a second dither drive signal, 180° out of phase with the first dither drive signal, is supplied. As a result, the two crystals attached to a given arm will always cooperate with one another in causing deflection of that arm, and the motion of arms 74 will be 180° out of phase with the motion of arms 82. Arms 74 and 82 will thereby cooperate to impart a dither motion to diaphragm 40 with respect to base 10.

A preferred construction for diaphragm 40 is illustrated in FIGURE 3. The diaphragm is constructed of stainless steel shim material having a thickness on the order of 0.01 inches. As indicated, the diaphragm includes eight cut-away sections 120 which effectively divide the diaphragm into outer portion 122 and inner portion 124 connected by beams 126. Outer portion 122 includes openings 128 and 130, openings 128 being adapted for the passage therethrough of posts 12, and openings 130 being adapted for the passage there¬ through of rods 52, as described previously in connection with FIGURE 1. Inner portion 124 includes openings 132 through which the inner portion 124 is secured to dither motor 70, and central opening 125 analogous to opening 73 in the dither motor. Bolts (not shown) are passed through openings 61 in cap 60 and openings 132 in diaphragm 40, and into internally threaded bores 80 in arms 74 of dither motor 70. Dither motor 70 thereby imparts a dither motion to inner portion 124 of diaphragm 40, which dither motion is coupled to outer portion 122 through beams 126. The dither motion of outer portion 122 is in turn coupled to subassembly 64 and thereby to laser gyro 20. Cap 60 prevents diaphragm 40, and in particular beams 126, from significant buckling out of the plane of the diaphragm in response to the forces generated by dither motor 70. Since the beams cannot buckle significantly, the natural frequency of the assembly is not strongly dependent on the thickness of diaphragm 40. The underside of cap 60 is slightly milled above the beams. In accordance with the present invention, it has been found that by controlling the length of beams 126 (by adjusting slots 120), the natural fre¬ quency of the laser gyro assembly can be adjusted and controlled. A vibrational model of the assembly of the present invention is set forth in FIGURE 4. The -6- model includes laser gyro 20, alignment plate 50, and base 10. The laser gyro and alignment plate are coupled by resilient ring 30, which is modeled by spring 110 having a spring constant K_R. The alignment plate is coupled to the base by posts 12, modeled by spring 112 having a spring constant K_p, and by diaphragm 40 and dither motor 70, modeled by series connected springs 114 and 116 having spring constants K~ and K„ respectively.

The effective coupling between the alignment plate and the base, K^, is given by:

^KAB ^■ 1§^ ^{+ K}P ^{' "} . ⁽¹⁾

If the polar moment of inertia of the alignment plate is much less than that of the laser gyro, then the effective spring constant between the gyro and the base, K_QB, is given by:

If resilient ring 30 is composed of a comparitively stiff material, then K„ will be very large compared to K . _B, and one can write:

^KM^KD (3) ^KGB ^{= K}AB ^{" K}M ^{+ K}D ^{• + K}P

The resonant frequency of the assembly is then:

where J_ is the polar moment of inertia of the laser gyro.

For the purpose of illustration, assume that it is desired to be able to adjust the resonant frequency of the laser gyro assembly between 500-600 Hz. The upper frequency, 600 Hz, would correspond to a solid diaphragm. In such a case, K_Q would be very large, and one can write:

^KGB " ^KM ^{+ K} _P ⁽⁵⁾

By combining equations (4) and (5) and setting f = 600, one obtains:

K_M + Kp = 1.42 10⁷ J_G (6) Thus for a given dither motor having a particular value of K_M, the thickness of posts 12 can be adjusted using equation (6) to produce an upper limit of resonant frequency of 600 Hz. Posts having diameters of 0.08 inches have been found suitable for a dither motor of the type illustrated in FIGURE 2.

If 500 Hz is substituted in equation (4), and if equations (3) and (4) are then combined, one obtains:

_{κ =} ^{[ 9}'^{87 lθ6 J}G - V ^KM (7) ^kD K M + K_p - 9.87 10^*

Equation (7) may be used to obtain the necessary spring constant for diaphragm 40. The lengths of slots 126 are then adjusted to obtain the required spring constant K_D. In one embodiment according to the present invention, diaphragm 40 has a diameter of 2.55 inches, a thickness of 0.01 inches, and the radially outer portions of slots 120 are located 0.7 inches from the center of the diaphragm. The slots are 0.03 inches wide, and the distance between adjacent slots, i.e., the width of beams 126, is 0.08 inches. In such an embodiment, if the laser gyro assembly has a natural frequency of 600 Hz when diaphragm 40 is solid, then the natural frequency of the assembly can be adjusted between about 550 and 250 Hz by varying the beam length between 0.1 and 0.4 inches, respectively. The beam width in this embodiment is varied by varying the distance that slots 120 extend radially inward.

It will be understood that the present invention may be embodied in other specific forms without departing from the central characteristics thereof. The present examples and embodiments are therefore to be considered in all respects as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope of the following claims.

Claims

-8-The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A laser gyro assembly, comprising: a base; resilient support means mounted to the base; coupling means mounted to the support means; 5 a laser gyro mounted to the coupling means; and drive means connected to the base and to the coupling means for imparting a dither motion to the coupling means, whereby the dither motion is coupled to the gyro by the coupling means.

2. The assembly of Claim 1, wherein the gyro is mounted to the coupling means such that the gyro is not in contact with the base.

3. The assembly of Claim 2, wherein the coupling means comprises a diaphragm having an inner portion connected to the drive means and an outer portion connected to the support means and to the gyro.

4. The assembly of Claim 1, wherein the gyro has a central bore extending through the gyro between exterior surfaces thereof, and wherein the coupling means is connected to the gyro only at said exterior surfaces and not at said central bore.

5. The 'assembly of Claim 4, wherein the drive means is positioned within said central bore but is not in contact with the gyro.

6. The assembly of Claim 1, wherein the coupling means comprises a resilient ring mounted in contact with the gyro, a diaphragm mounted in contact with the resilient ring, an alignment plate mounted in contact with the diaphragm, and fastening means for securing the gyro, resilient

> ring, diaphragm and alignment plate together.

7. The assembly of Claim 6, wherein the diaphragm comprises an inner portion connected to the drive means, an outer portion mounted in contact with the resilient ring and with the alignment plate, and a plurality of beams connecting the inner portion to the outer portion.

8. The assembly of Claim 7, wherein the support means com¬ prises a plurality of resilient posts, each post having a first end and a second end and being attached to the base at its_ first end and to the alignment plate at its second end.

9. The assembly of Claim 8, wherein the posts extend through openings in the gyro, the openings being sized such that the posts do not contact the gyro.

10. The assembly of Claim 6, wherein the* effective spring constant of the resilient ring is very large in comparison to the effective spring constant by which the alignment plate is coupled to the base.

11. The assembly of Claim 6, wherein the gyro has a central bore extending through the gyro between exterior surfaces thereof, and wherein the resilient ring is in contact with the gyro only at said exterior surfaces and not at said central bore.

12. The assembly of Claim 11, wherein the drive means is positioned within said central bore but is not in contact with the gyro.