WO1999004221A1

WO1999004221A1 - Ring laser gyro readout

Info

Publication number: WO1999004221A1
Application number: PCT/US1998/014884
Authority: WO
Inventors: Rodney H. Thorland
Original assignee: Honeywell Inc.
Priority date: 1997-07-17
Filing date: 1998-07-17
Publication date: 1999-01-28
Also published as: CA2286197A1; EP0995077A1; JP2002509610A

Abstract

Ring laser gyros have mirrors that allow signals to leave the ring laser gyro and provide information about the status of the ring laser gyro. The wedge mirror (2), which is one of these mirrors, is aligned such that the signals leaving the ring laser gyro from this mirror will provide accurate ring laser gyro information.

Description

RING LASER GYRO READOUT

BACKGROUND OF THE INVENTION

Fig. 1 shows the structure of a ring laser gyro (RLGs) 1 with a wedge mirror 2. The wedge mirror 2 receives two beams 4 and 6 which contain information regarding the ring laser gyro 1. The two beams 4 and 6 are both moving in lasing paths in a laser cavity of the ring laser gyro 1. One beam 4 is depicted by a dotted line in Fig. 1 and the other beam 6 is depicted by a solid line. However, since both beams are moving through the same lasing paths but in opposite directions, Fig. 1 will have the appearance that only beam 6 exists in the ring laser gyro 1 but actually both beams 4 and 6 are in the ring laser gyro 1.

The first beam is called a direct beam 4 and goes through the wedge mirror 2 reflecting within the wedge mirror 2 and then out of the wedge mirror 2. Looking at Fig. 1 , beam 4 is moving in a clockwise direction and so beam 4 will be coming from the left direction. The second beam is referred to as the wedge beam 6 and comes from the opposite direction of the direct beam 4 or the counter-clockwise direction in the ring laser gyro 1. The wedge beam 6 reflects off the slant 7 of the wedge mirror 2 before reflecting through the wedge mirror 2 and then out of the wedge mirror 2 like the direct beam 4. The slant of the wedge mirror 6 is at an angle, called the wedge angle, so that a specific divergence angle is obtained between beams 4 and 6. The wedge angle in current use is 19 degrees and 31 minutes.

Fig. 2 shows an enlarged side view of the wedge mirror 2 and the beams 4 and 6 coming from the wedge mirror 2. The two light beams 4 and 6 come from the ring laser gyro 1 through the wedge mirror 2. As can be seen in Fig. 2, some of the beams 4 and 6 exit the wedge mirror 2 immediately while some of the beams 4 and 6 reflect in the mirror 2 before finally exiting the wedge mirror 2. After leaving the wedge mirror 2, the beams 4 and 6 reach sensors 8 which pass on the light beams 4 and 6 to be processed so that ring laser gyro information is determined. 5 is a reference point which will be used to help visualize the wedge mirror 2 from a top view later in the discussion. Fig. 3 shows a top view of the two beams 4 and 6 on the sensors 8 which are two silicon sensors. The two sensors 8 will provide AC signal outputs which are phase shifted. The sign of the phase shift is detected and this is used to determine the direction of the gyro rotation. Also, as can be seen in Fig. 3, there is an overlap 9 in the two beams 4 and 6. The overlap 9 of the two beams 4 and 6 also produces the information for gyro readout regarding modulation and light intensity.

Also, a mask can be placed before the sensors 8. The mask has slits so that the fringes, which represent the intensity of the beams 4 and 6, come through the slits in the mask and an interference pattern is created. The slits of the mask are spaced so that adequate readout of the fringes can be obtained. However, if the ring laser gyro 1 rotates at all, the fringes may be blocked by the mask in which there would be no intensity. As a result, the mask is tilted at an angle from the fringe direction of the pattern. The direction of the fringe motion will determine the direction of the rotation of the ring laser gyro 1. Masks and interference fringe patterns are well known in this area of technology and will not be discussed in any further detail here.

However, a problem develops when variations with temperature and vibration result in laser cavity alignment changes. Fig. 4 shows the two beams 4 and 6 on the sensors 8 when these variations occur. As can be seen, the beams 4 and 6 have moved upward from Fig. 3. Also, the area covered by the beams 4 and 6 is unequal in each of the respective sensors 8. Looking at Fig. 4 for example, more area of the beams 4 and 6 is in the first sensor 8a than in the second sensor 8b. As variations occur, the beam area in one sensor will increase, while the beam area in the other sensor will decrease. This is very undesirable.

Fig. 5 shows why it is undesirable to have uneven beam area coverage in the sensors. The problem is in the sensor output signal amplitude changes. The sensor output signals from the ring laser gyro 1 can be plotted to form lissajous patterns. As was mentioned, when variations occur, the sensors 8 would receive unequal portions of the beams 4 and 6. This would result in a lissajous pattern such as Fig. 5. In the example of Figs. 4 and 5, the area covered in sensor 8a is greater than 8b and this is shown in Fig. 5 with a larger 8a sensor output signal than an 8b sensor output signal. This is problematic since certain minimum signal amplitudes must be maintained in order for gyro information to be faithfully determined. Further, the amplitude would vary when variations occurred forming different shapes which would result in inaccurate gyro output information. For example, one shape could be a line in which no amplitude would exist and erroneous gyro information would be obtained. The desired lissajous pattern would be a circle that is consistently maintained despite variations that may occur in the ring laser gyro environment. Therefore, it would be desirable to have a more accurate readout set-up so that ring laser gyros may operate more reliably.

SUMMARY OF THE INVENTION A ring laser gyro has mirrors that allow beams to pass through the ring laser gyro. These beams contain information that is used in determining the status of the ring laser gyro. The mirrors must be specifically situated on the ring laser gyro block so that the beams will provide accurate information regarding the ring laser gyro status.

BRIEF DESCRIPTION OF THE PRESENT INVENTION Fig. 1 shows a ring laser gyro with beams moving within lasing paths of the ring laser gyro block. Fig. 2 shows a side view of a pair of beams passing through the wedge mirror and hitting sensors.

Fig. 3 shows a top view of the pair of beams hitting the sensors. Fig. 4 shows a top view of the pair of beams hitting the sensors after a variation in the environment has occurred. Fig. 5 shows a lissajous pattern of the beams after a variation in the environment has occurred.

Fig. 6 shows a top view of the wedge mirror rotated at an angle on the ring laser gyro block.

Fig. 7 shows a side view of the present invention and a pair of beams passing through the wedge mirror and hitting the sensors.

Fig. 8 shows a top view of the pair of beams hitting sensors after passing through the wedge mirror of the present invention.

Fig. 9 shows a top view of the pair of beams hitting the sensors after passing through the wedge mirror of the present invention after variations in the environment have occurred.

Fig. 10 shows the lissajous pattern of the beams, when the same effects on the ring laser gyro in Fig. 5 have occurred, with the present invention. DETAILED DESCRIPTION OF THE PRESENT INVENTION

The present invention presents a way to eliminate the problems with amplitude as a result of variations in the environment. Fig. 6 shows the structure of the present invention looking at the ring laser gyro 1 from a top view. Again, 5 is just a point of reference in helping to visualize the present invention. The wedge mirror 2 is rotated by a specific angle on the ring laser gyro block 10. It is well known how a wedge mirror 2 is attached to a RLG block 10 and no further detail describing the attachment of the wedge mirror 2 will be discussed here. The wedge mirror 2 is rotated on the ring laser gyro block 10. The angle of the preferred embodiment is .75 degrees between the central axis 12 of the wedge mirror 2 and the central axis 14 of the ring laser gyro block 10. The wedge mirror 2 can be turned in either direction as long as it is .75 degrees from the central axis 14 of the ring laser gyro block 10. .75 degrees is chosen to be the appropriate_angle in use with the readout mask spacings. However, if the mask spacings were to change, the angle would be adjusted to a new appropriate value.

Fig. 7 shows that the slant 7 of the wedge 2 is at a wedge angle of 19 degrees, 16 minutes and 16 seconds give or take 30 minutes from the top 16 of the wedge mirror 2. All the measurements presented so far is only for gyros made of BK-7. However, other materials would require other measurements which could be arrived at through analysis.

Fig. 7 also shows the beams as they impact the sensors 8 similar to Fig. 2. One beam 6 is still in the plane of the paper, but the other beam 4 is coming out of the paper. The desired end result of changing the wedge angle together with the rotation of the wedge is to achieve the same divergence angle between beams 4 and 6 with the divergence out of the plane rather than in the same plane as was shown in Fig. 2.

Fig. 8 shows a top view of the beams 4 and 6 impacting the sensors 8. The beams 4 and 6 are now perpendicular from before and so the positioning of the sensors 8 are now changed by 90 degrees. When variations occur as a result of the environment as shown in an example in Fig. 9, the amplitude measurement is unaffected because the area of the beams 4 and 6 now remains equal in both of the sensors 8 a and 8b.

Fig. 10 shows the lissajous pattern of Fig. 9 using the present invention. As can be seen by Fig. 10, a near perfect circle exists. A near perfect circle exists because the beams are nearly equal in sensor 8a and sensor 8b of Fig. 9. The circle shape provides ample amplitude for accurate processing to be made in determining ring laser gyro information. Further looking at Fig. 9, the circle shape is retained no matter what the variations are since as the beams 4 and 6 move up and down as before, the areas are maintained nearly equal in sensor 8a and sensor 8b. As a result, amplitude measurements remain reliable despite environment changes. This also provides stability in determining the status of the gyro.

The invention has been described herein in order to comply with the Patent Statutes and to provide those skilled in the art with the information needed to apply the novel principles and to construct and use such specialized components as are required. However, it is to be understood that the invention can be carried out by specifically different components and materials, and that various modifications, both as to the component details and materials, can be accomplished without departing from the scope of the invention itself.

Claims

CLAIMSThe embodiments of the invention in which an exclusive property or right is claimed are defined as follows:

1. A ring laser gyro, comprising: a ring laser gyro block having a plurality of sides; paths in the ring laser gyro block wherein a plurality of beams travel through the paths; plurality of mirrors to maintain the beams traveling through the paths; and at least one of the plurality of mirrors, having a top surface, rotated on the ring laser gyro block to accurately provide beams containing information concerning the ring laser gyro.

2. The ring laser gyro of claim 1 wherein the sides of the ring laser gyro block have a symmetry axis along the length of the sides and the one mirror has a symmetry axis along the length of the top surface with the symmetry axis of the mirror at an angle with the symmetry axis of the ring laser gyro block.

3. The ring laser gyro of claim 2 wherein the angle is .75 degrees.

4. The ring laser gyro of claim 1 wherein the one of the plurality of mirrors is a readout mirror.

5. The ring laser gyro of claim 4 wherein the readout mirror has a plurality of sides with one side inclined.

6. The ring laser gyro of claim 5 wherein the angle of the inclined side is approximately 19 degrees from the top surface of the one mirror.

7. The ring laser gyro of claim 1 wherein the ring laser gyro block is made of BK-7 material.

8. A ring laser gyro, comprising: a ring laser gyro block having a plurality of sides; paths in the ring laser gyro block wherein a plurality of beams travel through the paths; plurality of mirrors to maintain the beams traveling through the paths; at least one of the plurality of mirrors, having a top surface, rotated on the ring laser gyro block wherein the one mirror allows beams to travel out of the ring laser gyro block; and a plurality of sensors situated to receive the beams from the one mirror so that ring laser gyro status can be accurately determined from information in the beams.

9. The ring laser gyro of claim 8 wherein the sides of the ring laser gyro block have a symmetry axis along the length of the sides and the one mirror has a symmetry axis along the length of the top surface with the symmetry axis of the mirror at an angle with the symmetry axis of the ring laser gyro block.

10. The ring laser gyro of claim 9 wherein the angle is .75 degrees.

11. The ring laser gyro of claim 8 wherein the one of the plurality of mirrors is a readout mirror.

12. The ring laser gyro of claim 11 wherein the readout mirror has a plurality of sides with one of the sides at an incline.

13. The ring laser gyro of claim 12 wherein the angle of the incline is approximately 19 degrees from the top surface of the one mirror.

14. The ring laser gyro of claim 8 wherein the ring laser gyro block is made of BK-7 material.

15. The ring laser gyro of claim 1 further comprising a plurality of sensors situated to receive the beams provided by the one mirror to accurately determine information concerning the ring laser gyro.