GB2141868A

GB2141868A - Downhole ring laser gyro

Info

Publication number: GB2141868A
Application number: GB08415751A
Authority: GB
Inventors: Daryl C Stjern; Thomas M Wirt; Thomas Council
Original assignee: Sundstrand Optical Technologies Inc
Current assignee: Sundstrand Optical Technologies Inc
Priority date: 1983-06-20
Filing date: 1984-06-20
Publication date: 1985-01-03
Also published as: NL8401964A; BE899974A; JPS6018981A; FR2548357A1; SE8403264D0; NO842455L; GB8415751D0; IT8448410A0; AU2947684A; SE8403264L; ZA844479B; DE3422889A1

Abstract

A ring laser gyro assembly in which the length of closed loop path cavity 12 is maximised within a compact long narrow structure for use in a borehole has a body 10 of polygonal shape with two sides 30, 32 being substantially longer than the remaining sides 34, 36, 38, 40, each of the mirrors 20, 22, 24, 26 being disposed on a different one of the shorter sides. Lock-in is prevented by a mirror dither 45, 47, wherein two of the mirrors are vibrated 180 DEG out of phase, and a bias apparatus which includes two narrow flexure assemblies (50, 52) disposed adjacent the sides 30, 32 of the gyro body and piezoelectrically riven 180 DEG out of phase. Each flexure assembly comprises a pair of blades having an outer end secured to the gyro support means and an inner end coupled via mounting blocks to the gyro body. <IMAGE>

Description

SPECIFICATION Downhole ring laser gyro Technical field The present invention relates to ring laser gyros and particularly but not exclusively to ring laser gyros for use in a borehole such as an oilwell. Such gyros are referred to herein as "downhole ring laser gyros".

Background of the art Due to the small diameter of oil well boreholes, the size of the instruments which are used therein is critical. Known ring laser gyro assemblies are typically not suited for use downhole in a bore hole, alone or in an instrument cluster, because either the gyro assemblies are too bulky or the gyro is not sufficiently accurate.

Typical ring laser gyros include a cavity forming a closed loop path through which two counter-rotating laser beams travel. In known attempts to reduce the overall size of the gyro, the path length of the cavity has been decreased to as small as 6 cm. It has been found, however, that the greater the area enclosed by the cavity and hence, the longer the path length, the greater the accuracy of the gyro. Minimizing the path length in order to minimize the overall size of the gyro results in a gyro, such as the 6 cm gyro, which is not sufficiently accurate for most applications, including downhole applications.

One of the problems affecting the accuracy of a ring laser gyro is the phenomenon of lock-in of the two counter-rotating laser beams. In order to prevent lock-in, various dither techniques have been employed using external bias apparatus. Although known bias apparatus increases the accuracy of the gyro, such apparatus typically adds a considerable amount to the overall dimensions of the gyro assembly. Gyro assemblies employing such apparatus are therefore typically too bulky to be used in downhole applications.

Disclosure ofthe invention In accordance with the present invention the disadvantages of prior ring laser gyros as discussed above have been overcome. The ring laser gyro assembly of the present invention is both extremely accurate and very compact in order to be used downhole, for example, in an oil well borehole.

The downhole ring laser gyro has a body with a polygonal shaped cavity therein forming a closed loop path through which a pair of counter-rotating beams travel, a mirror being disposed at each corner of the cavity for reflecting the beams about the path.

The gyro body has a polygonal shape with two opposite sides thereof being substantially longer than the remaining sides which are short to form a long, narrow body for use downhole. Each of the mirrors is disposed on a different one of the shorter sides of the gyro body, the number of shorter sides of the body being equal to the number of mirrors to minimize the width of the gyro body. The cavity is disposed within the body of the gyro, with respect to the mirrors, such that the length of the closed loop path formed by the cavity is maximized for the narrow gyro body.

One technique used to prevent lock-in of the two counter-rotating laser beams is a mirror dither in which two of the mirrors of the gyro are vibrated 180 out of phase with respect to each other. The two dithered mirrors may be disposed on adjacent shorter sides of the gyro body, the cavity being disposed within the body with respect to the mirrors such that the longer sides of the cavity are parallel to the longer sides of the body. The two dithered mirrors may also be disposed on opposite shorter sides of the gyro body, the cavity being offset with respect to the body such that the cavity path length is maximized and the width of the gyro body minimized.

The downhole ring laser gyro assembly further includes a bias apparatus for imparting a dither to the body of the gyro in order to prevent lock-in of the two counter-rotating laser beams. The bias apparatus includes first and second flexure assemblies disposed adjacent opposite sides of the gyro body, each assembly being coupled between the body and a support. Means are provided for driving each of the flexure assemblies 180 out of phase with respect to one another in order to dither the body of the gyro about an input axis thereof. Each of the flexure assemblies is extremely narrow so as to add only a minimal amount to the overall width of the ring laser gyro assembly.

These and other objects and advantages of the invention as well as the details of an illustrative embodiment will be more fully understood from the following description and the drawings.

Brief description ofthe drawings Figure lisa perspective view of the downhole ring laser gyro assembly of the present invention; Figure 2 is a cross-section of the body of the ring laser gyro of Figure 1; Figure 3 is a cross-section of a second embodi mentofthe ring laser gyro body; Figure 4 is a top view of the ring laser gyro assembly of Figure 1 illustrating the bias apparatus for imparting a dither to the body of the gyro; Figure 5 is a perspective view of a flexure blade of the bias apparatus of Figure 4; Figure 6 is a transverse section through the ring laser gyro assembly in a housing; Figure 7 is a partial end view of the ring laser gyro assembly in the housing.

Best mode for carrying out the invention The downhole ring laser gyro, as shown in Figures 1-2, includes a body 10, which may be made of quartz, having a polygonal shaped cavity 12 therein forming a closed loop path. The cavity contains a gas or gases suitable for laser operation such as 90% helium and 10% neon art a pressure of 3 torr. A gas discharge is established between a cathode 14 and a pair of anodes 16 and 18, each of which is in communication with the cavity 12, to produce two counter-rotating laser beams. The beams are reflected around the closed loop path by mirrors 20, 22, 24 and 26 positioned at the corners of the cavity.

As the gyro is rotated about any input axis parallel to the Z-axis, the effective path length for one beam is increased while the effective path length for the other beam is decreased due to Doppler shifting. A beat frequency which is proportional to the rate of rotation, is produced in response to heterodyning of the two beams such as by means of a prism associated with the mirror 24. The beat frequency produces a fringe pattern which is detected by a dual photodiode 28 providing the output of the gyro.

The body 10 of the ring laser gyro is made very narrow and compact in order to be used in an oil.well bore hole, either alone or in an instrument cluster.

The width of the body taken along the X-axis is approximately 1 inch. The body, however, is substantially longer than it is wide. The length taken along the Y-axis is approximately 5 inches so as to accommodate a cavity therein having a path length of 10 inches or about 25 centimeters. Although cavities having smaller path lengths have been known, it has been found that the greater the area enclosed by the cavity and hence, the longer the path length, the greater the accuracy of the gyro.

In order to minimize the width of the gyro body but maximize the length of the closed loop path formed by the cavity, the body 10 of the gyro is formed having a polygonal shape with two non-adjacent or opposite sides 30 and 32 being substantially longer than the remaining sides 34-40 on which each of the mirrors is mounted, the remaining sides being short to form a long, narrow body. Further, the short sides 34-40 of the body 10 are equal in number to the number of mirrors required so as to minimize the overall width ofthe gyro.

The cavity 12 is formed having a long, narrow polygonal shape with two channels or gain tubes 42 and 44 being longer than the remaining gain tubes 46 and 48. The longer gain tubes are disposed adjacent the longer sides 30 and 32 of the body, the shorter gain tubes 46 and 48 intersecting the longer tubes 42,44 at the mirrors. The cavity 12 is positioned within the body of the gyro with respect to the mirrors such that the length of the closed loop path formed by the cavity is maximized. Although the cavity, as shown, is rectangular within an irregular hexogonal body, the cavity could have various other polygonal shapes. For example, a cavity forming a long, narrow triangle could be disposed within an irregular pentagonal body in accordance with the present invention.

In order to prevent lock-in of the two counterrotating laser beams, the downhole ring laser gyro employs both a body dither, as described in detail below, and a mirror dither in which each of the mirrors 20 and 22 is vibrated periodically in a direction perpendicular to its face. The mirrors 20 and 22 are diaphragm mirrors driven 180 out of phase so as to maintain the path length of the cavity constant. Each of the mirrors 20 and 22 has a cavity length control driver 45 and 47 which is responsive to the output of a single photodiode 49 associated with the mirror 26. The photodiode 49 monitors the intensity of the laser beams and provides a D.C.

output to the dithered mirrors so that the beams are maintained at mode center. Details of the two dithered mirrors and a control circuit for maintaining the 180 phase relationship are disclosed in a copending patent application Serial No. 462,548 assigned to the assignee of this application.

In one embodiment of the downhole ring laser gyro, as shown in Figure 2, the diaphragm mirrors 20 and 22 are positioned on adjacent shorter sides 34 and 36 of the gyro body 10. The cavity 12 is positioned within the gyro body such that the longer gain tubes 42 and 44 are parallel to the longer sides 30,32 of the body. This configuration results in a gyro assembly which is sufficently narrow for most applications.

However, to further reduce the width of the gyro while accommodating the diaphragm mirrors 20 and 22, which are largerthan the remaining mirrors, the ring laser gyro may be modified as shown in Figure 3. In this configuration,the diaphragm mirrors 20' and 22' are positioned on opposite shorter sides 34' and 38' of the body 10' and the cavity 12' is offset within the body such that the beams intersect the central portion of the mirrors. The sides 34' and 38' are made long enough to accommodate the large diaphragm mirrors 20' and 22', the sides 36' and 40' being shorter than the sides 34' and 38' in order to minimize the width of the gyro. The smaller mirrors 24' and 26' are positioned on the sides 36' and 40', closer to the respective adjacent longer sides 32' and 30' of the body in order to maximize the path length of the cavity 12'.The relative positioning of the mirrors and the offset cavity is a further feature enabling the overall width of the ring laser gyro assembly to be minimized while maximizing the length of the closed loop path formed by the cavity.

In addition to the mirror dither, the body 10 of the ring laser gyro is dithered by a bias apparatus. The bias apparatus vibrates the body of the gyro in a rotational mode about an input axis of the gyro to prevent lock-in of the laser beams, the bias apparatus being shown in detail with respect to Figures 1 and 4-7. The bias apparatus is extremely narrow, adding very little to the overall width of the gyro assembly, the bias apparatus including a pair of flexure assemblies 50 and 52 disposed adjacent the longer sides of the gyro body 30 and 32 respectively.

The flexure assemblies 50 and 52 are coupled to the body of the gyro through a pair of mounting blocks 62 and 64. The mounting blocks 62 and 64 are made of quartz and epoxied to the body 10 on opposite sides of a center input axis 65 of the gyro. The arrangement of the cathode 14, anodes 16 and 18, getter 19 and mounting blocks 62 and 64 on the same surface 67 of the gyro enables the overall height of the ring laser gyro assemblyto be minimized as well as the width thereof, the resulting structure being extremely compact.

The flexure assembly 50 includes a pair of flexure blades 54 and 56, which may be made of INVAR or beryllium-copper; materials which are of high strength and sufficiently stiff so that the only motion imparted to the gyro is the desired dither motion.

The flexure blades 54 and 56 are secured at their respective outer ends 58 and 60 to a support 61, shown in Figures 6 and 7, for the gyro and are coupled at their inner ends to the body of the gyro through the respective mounting blocks 62 and 64.

Similarly, the flexure assembly 52 includes a pair of flexure blades 66 and 68 made of INVAR or the like.

The blades 66 and 68 are secured at their respective outer ends 70 and 72 to the support 61 and coupled at their inner ends to the respective mounting blocks 62 and 64.

As described in detail below, the blades 54 and 56, are driven 180 out of phase with respect to each other as are the blades 66 and 68. Further, the blades 54 and 66 are driven 180 out of phase with respect to each other as are the blades 56 and 68. This drive results in a push-pull force applied to the mounting block 62 which is 180 out of phase with the resulting push-pull force applied to the mounting block 64.

The 180 phase shifted push-pull forces on the mounting blocks 62 and 64 cause the body of the gyro to vibrate in a rotational mode about the centre input axis 65.

The flexure blades 54, 56, 66 and 68 have the same construction so that only the flexure blade 56 will be described in detail with reference to Figure 5. The flexure blade 56 includes an outwardly extending flange 74 at its outer end 60. The flange has two holes 75 through which screws, such as screw 76 shown in Figure 7, extend for securing the flexure blade to the support 61. A portion 78 of the flexure blade at its outer end, extends inwardly towards the gyro body and limits the distance the gyro body can travel when dithered. A pad of rubber 80 is secured to the portion 78 to absorb any impact or relieve strain between the gyro body and the flexure blade at its outer end, which may result when the body 10 is dithered.

The flexure blade 56 at its inner end 84 includes a flange 90 which extends inwardly towards the body of the gyro and engages the mounting block 64. The inner end of the flexure blade 56 is secured to the mounting block 64 by three screws 96 extending through respective holes 98 in the flange 90 and into respective threaded holes 102 of the mounting block 64. The flexure blade 68 also has a flange 104 at its inner end, the flange having the same construction as the flange 90 of the flexure blade 56. The flange 104 is secured to the mounting block 64 by three screws 106 extending through respective holes 108 in the flange 104 and into the threaded holes 102 in the mounting block 64, the screws 106 entering the holes 102 from the end opposite from which the screws 96 enter.It is noted that three screws, extending through respective holes in one flange and the mounting block 64 and into respective threaded holes in the other flange, may be used to replace the six screws 96 and 106. Such single screws would allow the mounting blocks to be put into compression so as to maintain input axis stability.

The mounting blocks 62 and 64 are made of quartz each having a hexagonal cross-section. Each of the flange members of the flexure blades, such as flange 90, has a V-shaped indent so as to mate with the respective outwardly extending V-shaped side of the mounting blocks. Due to the V-shaped configuration of the mounting block sides which engage the flange members of the flexure assemblies, the blocks 62,64 minimize any forces, caused by mounting the flexure assemblies, which may be transmitted to the body 10 of the gyro. The quartz blocks 62 and 64 thereby allow the flexure essemblies to be mounted to the body of the gyro without warping the gyro or causing stress thereto.

The flexure blades at their inner ends 84 are notched with a generally U-shaped portion 114 as shown for the blade 56 in Figure 5. The U-shaped portions 114 of the flexure blades 54 and 56 extend between a pair of rubber washers 116 and 118 into slots formed in opposite sides of a damper 120. The damper 120 is centrally located adjacent side 30 of the gyro body 10. Similarly, the U-shaped portions 114 of the flexure blades 66 and 68 extend between a pair of rubber washers 122 and 124 into respective slots on opposite sides of a damper 126. The damper 126 is centrally located adjacent side 32 of the gyro body, directly opposite the damper 120.

A pad of rubber 128 is disposed between the damper 120 and the quartz body 10 of the gyro to relieve any strains therebetween. A pad of rubber 130 is similarly disposed between the damper 126 and the body 10 of the gyro. A preload is applied to the rubber 128,130 and the body 10 of the gyro by screws 134 and 135 extending through centrally located threaded holes disposed in the dampers 120 and 126 respectively. The dampers 120 and 126 essentially eliminate translation in the X-direction, caused by shock which may be on the order of 1,000 g's.

The dampers 120, 126 are secured to the support 61 for the ring laser gyro as shown in Figure 6. The dampers 120 and 126 include respective outwardly extending flanges 140 and 142 having a pair of threaded holes 146 and 148 therein through which respective pairs of screws 147, 149 extend. The screws further extend into threaded holes disposed in the support 61. Although the dampers are fixedly mounted to the support 61, when the flexure blades are driven, the inner ends 84 of the blades are caused to move in the damper slots towards and away from the body of the gyro, thereby imparting the dither motion thereto.

In order to drive the flexure blades, a piezoelectric transducer element is mounted on opposite sides of each of the blades. The elements are responsive to a drive voltage which may take the form of a sine wave or the like to impart the desired dither motion to the gyro body. Where the piezoelectric transducer elements are all to be driven with the same polarity voltage, the elements are mounted on the blades with different crystallographic orientations. For example, the transducer element 150 mounted on the outer surface of the flexure blade 54 may have a first orientation whereby, with the application of a positive voltage, the element expands. The transducer element 152 mounted on the inner surface of the flexure blade 54 has a second orientation such that with the application of a positive voltage, the transducer element 152 is also caused to expand.

Such an orientation will result in the flexure blade 54 pushing towards the mounting block 62 with the application of a positive polarity voltage, the blade pulling away from the block with the application of a negative polarity voltage.

Given the above orientation of the piezoelectric transducer elements associated with the flexure blade 54, the remaining transducer elements have the following orientations. The outer transducer elements 154 and 156 associated with the respective flexure blades 56 and 66 have the second orientations whereas the transducer elements 158 and 160 mounted on the inner surface of the respective flexure blades 56 and 66 have the first orientation.

The outer transducer element 162 of the flexure blade 68 has the first orientation whereas the transducer element 164 mounted on the inner surface of the flexure blade 68 has the second orientation. In general, the inner and outer piezoelectric transducer elements associated with one flexure blade have the same orientation as the respective inner and outer transducer elements associated with the flexure blade which is on a diagonal from the one flexure blade and have the opposite orientation of the inner and outer elements associated with the flexure blade which is directly across the gyro from the one flexure blade. It is noted, that instead of alternating the orientation of the transducer elements on each of the flexure blades, the transducer elements may have the same orientation but may be driven by voltages of opposite polarity to impart the desired dither motion.

The ring laser gyro assembly is enclosed in a housing, including a cover 170 and the support 61 which forms the base thereof. The housing cover 170 is secured to the support by means of bolts 172 or screws. The support 61 has a flat bottom 174 which facilitates mounting of the gyro assembly in an instrument cluster or the like.

Claims

1. A ring laser gyro assembly for measuring rotation about an axis, wherein the gyro body is polygonal in shape with a cavity therein receiving an array of mirrors to define a closed loop path around which counter-rotating light beams travel, and two opposite side of the polygon are substantially longer than the remaining sides with the mirrors being mounted on the shorter sides.

2. A ring laser gyro according to claim 1, wherein one of the mirrors is mounted on each of the shorter sides.

3. A ring laser gyro according to claim 2, wherein the gyro body is hexagonal in shape with two opposite sides of the hexagon being substantially longer than the remaining sides, so as to define a rectangular closed loop path for the light beams.

4. A ring laser gyro assembly according to any preceding claim, further including means forvibrating two of the mirrors 1800 out of phase with respect to each other.

5. A ring laser gyro assembly according to any preceding claim, further including means disposed adjacent each of the long sides of the gyro body for imparting a dither motion to the gyro body about an input axis of the gyro.

6. A ring laser gyro assembly according to claim 5, wherein the means for imparting the dither motion comprises a first flexure assembly coupled between the gyro body and means supporting the gyro body, being disposed adjacent one of the longer sides of the gyro body; a second flexure assembly coupled between the gyro body and the means supporting the gyro body, being disposed adjacentthe other of the longer sides of the gyro body; and means for driving each of the flexure assemblies 1800 out of phase to dither the body of the gyro about the input axis.

7. A ring laser gyro assembly according to claim 6, further including a pair of mounting blocks secured to a surface of the gyro body on opposite sides of the input axis for coupling each of said flexure assemblies to the gyro body.

8. A ring laser gyro assembly according to claim 7, wherein each of the mounting blocks has Vshaped sides extending outwardly toward each of theflexure assemblies and each oftheflexure assemblies include a member having a V-shaped indent for mating with the outwardly extending V-shaped sides of the mounting blocks.

9. A ring laser gyro according to claim 7 or claim 8, wherein the body of the gyro and each of the mounting blocks are of quartz, and the mounting blocks are bonded to the body of the gyro by an epoxy adhesive.

10. A ring laser gyro assembly according to any of claims 7 to 9 wherein the gyro further includes a cathode and a pair of anodes each of which is in communication with the cavity, the cathode, anodes and mounting blocks being mounted on the same surface of the gyro body.

11. A ring laser gyro assembly according to any preceding claim, wherein the cavity is disposed within the body such that two sides of the cavity are parallel to the two longer sides of the body.

12. A ring laser gyro assembly according to any preceding claim, wherein the cavity is offset within the body of the gyro.

13. A ring laser gyro assembly for use in a borehole, substantially as described herein with reference to the drawings.

14. In a ring laser gyro assembly for measuring a rate of rotation about an input axis, the gyro having a polygonal shaped body with a cavity therein forming a closed loop path through which a pair of counterrotating beams travel, the assembly including means for supporting the body of the gyro, an improved bias apparatus to prevent lock-in of the beams, comprising: first and second flexure assemblies disposed adjacent opposite sides of the gyro body, each flexure assembly including a pair of flexure blades, each blade having an outer end secured to the gyro supporting means and an inner end coupled to the body of the gyro; and means for driving the flexure blades of the first and second assemblies at the same frequency, the flexure blades of each pair being driven 1800 out of phase with respect to each other and the pair of flexure blades of said first assembly being driven 1 80" out of phase with respect to the pair offlexure blades of said second assembly to impart a dither to the body of the gyro.

15. A ring laser gyro assembly according to claim 14further including first and second mounting blocks secured to a surface of the gyro body on opposite sides of an input axis of the gyro, for coupling the inner ends of the flexure blades to the body of the gyro.

16. The ring laser gyro assembly of claim 15 wherein the flexure blades of the first and second flexure assemblies apply a push-pull force to each of the mounting blocks, the force applied to the first block being 180 out of phase with respect to the force applied to the second block so as to vibrate the body of the gyro in a rotational mode about said input axis of the gyro.

17. The ring laser gyro assembly of claim 15 wherein a flexure blade of each of said first and second flexure assemblies engage opposite sides of said first mounting block the and otherflexure blades of said first and second flexure assemblies engage opposite sides of said second mounting block.

18. The ring laser gyro assembly of claim 17 wherein the mounting blocks are configured to be put into compression when engaged by the flexure blades so as to maintain input axis stability.

19. The ring laser gyro assembly of claim 17 wherein the sides of the mounting blocks which engage the flexure blades are formed in the shape of an outwardly extending V, the flexure blades at their inner ends including a member having a V-shaped indent for mating with the V-shaped sides of the mounting blocks.

20. The ring laser gyro assembly of claim 15, wherein the body of the gyro and each of the mounting blocks are of quartz.

21. The ring laser gyro assembly of claim 15 wherein the gyro further includes a cathode and a pair of anodes each of which is in communication with the cavity, the cathode, anodes and mounting blocks being mounted on the same gyro surface so as to minimize the height and width of the gyro assembly.

22. The ring laser gyro assembly of claim 14 wherein each of said flexure assemblies includes a damper for engaging the inner ends of the flexure blades, the dampers being disposed directly opposite one another along an axis perpendicular to the axis about which the gyro is dithered, the damper preloading the body of the gyro to eliminate translation of the body in the direction of the axis along which the dampers are disposed.

23. The ring laser gyro assembly of claim 22 wherein each of said dampers is secured to the supporting means of the gyro assembly, the dampers engaging the flexure blades so as to allow movement of the blades towards and away from the body of the gyro.

24. The ring laser gyro assembly of claim 14 wherein the driving means includes a piezoelectric transducer element disposed on opposite sides of each oftheflexure blades.