JPH0552574A - Path-length control assembly for ring laser gyroscope - Google Patents

Abstract

PURPOSE: To simplify structure by equipping an assembly providing a mirror or the like having a film bent in the axial direction and a backing plate for driving the mirror, the film and the like and supporting and driving a transducer. CONSTITUTION: The basic elements of a path length control assembly 70 support a mirror housing 74 and a transducer to function as a driver, and have a backing plate 76 sandwiched between piezoelectric elements 78, 80. The element 78 has an inside annulus 77 for receiving a mirror column 75 of the housing 74. These elements 78, 80 are connected to the front face and the rear face of the plate 76. The housing 74 supports a mirror surface 84 and functions as a film surface 82, which is a mirror base plate for the mirror surface 84. The all elements (for instance, housing 74 or the like) for constituting the assembly 70 except for the elements 78, 80 uses the same material having a relatively low thermal expansion coefficient so that influence of different thermal expansion is reduced. The assembly 70 can operate over a wide temperature range.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION This invention relates to path length controllers for ring laser gyroscopes, and more particularly to improved and simplified path length control assemblies.

[0002]

BACKGROUND OF THE INVENTION The Ring Laser Gyroscope was developed as a logical alternative to mechanical inertial gyroscopes. Based on the Sagnac Effect principle, ideally a ring laser gyroscope has the least moving parts and allows very precise rotation detection. In the original model, the ring laser gyroscope has a design such that it has at least two counter-propagating electromagnetic waves (eg, light) that oscillate in an optical ring cavity. When this ideal ring laser gyroscope is stationary, no rotation is indicated by the sensor. When the ring cavity of a laser gyroscope rotates about its central axis, the waves propagating in the opposite direction produce beat frequencies. Well above the characteristic lock-in zone, a linear relationship is established between the beat frequency and the rotational speed of the gyroscope with respect to the reference inertial frame.

Practical ring laser gyroscopes require various adjustment mechanisms to achieve an ideal linear relationship between beat frequency and gyroscope rotational speed. This ideal ring gyroscope features beat notes that are proportional to rotational speed,
This two-mode planar ring laser gyroscope requires velocity biasing or mechanical dithering to prevent counter-propagating waves from locking at low rotational speeds. Mode locking is a major problem to overcome at low rotational speeds, where the ring laser gyroscope produces a false indication that the device is not rotating.
If the rotational speed of the ring laser gyroscope starts at a value above the speed at which lock-in occurs and is then decelerated, the frequency difference between the beams disappears at some input speed. Lock-in results from the coupling of light between the beams. An effective means to overcome the lock-in effect of the counter-propagating modes of light in a two-mode gyroscope is to mechanically dither the mirror or gyroscope body.

In addition to this, it is necessary to control and monitor the optical path length of the gyroscope to ensure that the resonant cavity operates in the center of the atomic spectral gain curve. Rings for diverse applications
The laser gyroscope is required to operate in a wide temperature range, for example, a range of -55 ° C to + 75 ° C. The laser beam emitted by the active gain region of the gyroscope propagates around the ring laser by reflection from the surfaces of at least three mirrors, so that thermal expansion of the frame causes a large change in the cavity resonant wavelength. Therefore, it is possible to provide a path length control mechanism for changing the optical path length of the gyroscope ring resonator to preserve the fundamental resonance of the cavity to which all the sensing instrument elements of the gyroscope are tuned. is necessary.

Even when low expansion glass materials are used to fabricate the monolithic frame that supports the optical cavity path between the mirrors, the path length of a ring laser gyroscope changes significantly when the temperature changes. Receive. This change corresponds to a magnitude of 5 wavelengths or more at the resonant frequency of the light produced by the gaseous active medium, eg the helium-neon mixture. In an active path length control system, changes in path length due to thermal expansion and contraction are
It is monitored by the detection electronics and provides feedback information for driving one or more piezoelectric transducers.

Another important function of the path length control assembly is to keep the resonant frequency of the ring laser cavity at the peak or center of the nonuniform line of the gain medium. Due to the dispersion effect caused by the deviation of the resonant frequency from the peak of the gain curve, the two counter-propagating rays exhibit different refractive indices, and the resulting optical path length, which causes a false gyro output, or bias. Providing is well known to those skilled in the art. Furthermore, in addition to being temperature dependent, this bias is highly elastic because its magnitude varies depending on the uncontrolled offset between the cavity resonant frequency and the gain line center.

As shown in the prior art FIGS. 1A and 1B, preserving the optical path length means gain center of the atomic spectral resonance gain profile 10.
This means that line 12 should come to the center of this curve at the point of maximum intensity. In order to hold this setting through various temperatures, the piezoelectric transducer driven mirror assemblies 16 and 18 are axially oriented to compensate for changes in path length of the ring laser optical path 20 as shown in FIG. It is electrically activated to move to. Light intensity detectors 22 and 24
Are positioned on either side of fixed mirrors 26 and 28 to detect rotation. Individual transducer driven mirror
Assemblies 16 and 18 include piezoelectric actuators mounted on a flexible annulus surface or driver to achieve axial bowing of these reflective surfaces.

To date, path length control assemblies, such as assemblies 16 and 18 of FIG. 1A, have been used.
Various schemes for are proposed. Some of these designs for path length controllers include US Pat. No. 4,824,253 issued to Butler on April 25, 1989 (owned by the same assignee as the present invention); 1989. Rejugation (Lju
U.S. Pat. No. 4,861,161 issued to
U.S. Pat. No. 4,691,323 issued to Rejang on Sep. 1, 1987, and Jul. 9, 1980.
There is a design disclosed in UK patent GB 2,037,455 issued to ReJang and Williams on the day. United States Patent No. 4,861, issued to Rejang
No. 161 discloses a particular technique for minimizing mirror tilt in the design of PLC assemblies.

FIG. 2 shows an example of the complex structure of a path length control assembly according to the prior art. Path length controller mirror assembly 30 includes a membrane-type mirror housing 38 that supports a mirror 44, as shown, which mirror gyroscope frame 40 to reflect light in the optical path from its surface.
It is located so as to face. The housing 38 has an outer cylinder 34 and a central mirror post 36. Mirror pillar 3
6 annulus surface 48 and housing 38 outer annulus surface 5
2 is level with the circular backing plate 32.
The circular backing plate 32 is sandwiched between the mirror housing 38 and the driver body 50. The driver body includes a driver post 54 which, upon activation of the cavity length control assembly, provides a driver post surface 56.
Is moved axially along the direction indicated by 42.
This axial movement of the driver post 54 causes the mirror post 36 to move.
Causes an axial movement relative to the flexible mirror membrane 46 to allow the mirror 44 to move axially between a rest position and a warped position 44 '(shown in dashed lines). To be done. The driver body 50 has an outer surface 58, which is level with the backside of the backing plate 32.

Two piezoelectric elements 60 and 62 are located on opposite sides of the rear end of the driver body 50. These elements are typically bimetallic or bimodal materials, and when they are alternately polarized by applying a voltage to them through electrical terminals 64 and 66, driver body 50 and driver post 54 are required in axial direction 42. Accordingly, the driver body 50 is moved back and forth along the central axis of the driver body 50. This movement results in moving the backing plate to the new position 68 and moving the vibrating membrane 46 of the mirror housing 38 to the new position 72, all of which result in the mirror surface 44 being directed toward 44 '. And move it axially as required. As shown in prior art FIG. 2, a complex structure including a separate driver body 50 is required to achieve a properly operating balanced path length control assembly. The complexity of such construction requires the drive of the mirror 44 in the axial direction 42,
And the driver 50 is required for two purposes to avoid destabilizing the dynamic symmetry of the assembly 30. However, the more complex the path length controller, the more costly it is to design and manufacture the gyroscope.

What is needed is a simple approach for manufacturing a path length controller assembly that consists of fewer components than the prior art path length controllers and does not sacrifice performance. Disclosed herein is a path length controller assembly for a ring laser gyroscope that includes a mirror mounted on (or forming a part of) a mirror housing that includes an axially deflectable membrane. The mirror housing also includes a mirror post bonded to the deflectable membrane. The mirror housing is located horizontally with the backing plate. The backing plate has at least one transducer mounted on and supported by the backing plate for axially deflecting the backing plate in response to an electronic input signal. In this way, the backing plate has the function of supporting the piezoelectric transducer element and the mirror housing, as well as driving the film and the reflecting mirror through the mirror post. Thus, this backing plate has the dual function of supporting the transducer and axially driving the mirror column due to the bimorphic effect of the piezoelectric element.

This transducer element usually consists of a pair of transducers, the first transducer being a flat bimorphic disk supported on one side of a backing plate and the second transducer being It is a flat bimorphic ring that defines the central opening. The central opening of the second transducer allows the mirror post of the mirror housing to be inserted on one side of the backing plate, while the other side of the backing plate supports the first transducer. The tilt of the path length control assembly is minimized due to the concentricity of the first and second transducers to the mirror post.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT As shown in FIG. 3, a path length control assembly is located at one of the three or four corners of a ring laser gyroscope. Mirror surface 84
Reflects light along the optical path 87. This reflective surface is
Mounted on the mirror membrane 82, the membrane 82 forms the forward surface of the mirror housing 74. A dual function backing plate / driver 76 is located at the back end of the mirror housing 74.

A more detailed view of the elements that make up the path length control assembly of the present invention is shown in FIGS. As shown, the basic elements of the path length control assembly 70 include a mirror housing 74 and a backing plate 76 that acts as a driver in its dual capabilities and is sandwiched between piezoelectric elements 78 and 80. .. This piezoelectric element 78 is used for the mirror column 7 of the mirror housing 74.
It has an inner ring 77 for receiving 5. The piezoelectric elements 78 and 80 are fixedly bonded to the front and back of the backing plate / driver 76, respectively, using epoxy cement.

The mirror housing 74 includes a forward surface, which supports the mirror surface 84 and also the membrane surface 8.
Functions as 2. This film surface 82 is also the mirror surface 8.
4 functions as a mirror substrate. The mirror housing 74 has an outer cylinder 86, which provides symmetry and balance to the path length control assembly 70.

Preferably, all of the elements that make up the assembly 70 (eg, piezoelectric elements 78 and 80) (eg,
The mirror housing 74 and the backing plate / driver 76) are made of the same material so that the effects of different thermal expansions are reduced. Materials with relatively low coefficient of thermal expansion, eg Cerit, Zerodur, ULE (ultra low expansion)
Glass or the like is most desirable for manufacturing the path length control assembly 70. By doing so, the assembly 70 is
It is possible to operate over a wide temperature range from -55 ° C to at least + 75 ° C.

In operation, an electrical annulus is shown along electrical lines 88A and 88B as shown. Piezoelectric elements 78 and 80 have the two inner surfaces closest to the backing plate get the same (eg, negative) charge, while the outer surfaces of elements 78 and 80 have the same opposite sign (eg, positive). Note that it is charged to get the charge. Thus, when they are voltage activated,
Piezoelectric elements 78 and 80 include a path length control assembly 79.
Has a tendency to bend in the axial direction along the central axis (4-4 in FIG. 3). Then, the piezoelectric element bends the backing plate and moves the mirror column 75 in the axial direction. Exhaust holes 90 allow pressure equalization within path length control assembly 70, and piezoelectric elements 78 and 80 of wires 88A and 88B.
To allow passage from the inside to the outside surface of the housing 74 and to provide electrical connection to the outside of the assembly 70. Although only a single vent hole is needed through the mirror housing 74 to achieve the required pressure equalization (as the mirror membrane 82 moves in and out along the axial direction of the assembly 70), on the other hand,
The prior art design requires at least one pair of pressure equalization holes on the side of the driver body 50, as shown in FIG.

Moving the mirror post 75 of the mirror housing 74 moves the reflective mirror surface 84 back and forth along the central axis of the mirror assembly 70, thereby achieving control of the active cavity, or optical path length. .. Previously, an additional driver body 50 (of prior art FIG. 2) was required to achieve the required axial movement, but in the present invention this is a dual function backing plate 76, eg, , Using a driver / piezoelectric element support plate. This design allows for considerable cost savings because the elements of driver body 50 can be eliminated.

A particularly fatal source of error in the performance of all path length control assemblies is the tilt of the mirror, or
Movement of the mirror in a direction other than the vertical axis. As shown in the prior art (i.e., U.S. Pat. No. 4,861,161 issued to Ljung),
The tilt of such a mirror is
Ring laser gyroscope output bias
Cause a shift.

An effective technique for testing the mirror tilt of a path length control mirror assembly is to perform a "mode scan". Mode scanning is
While applying the cell voltage to the path length control piezoelectric elements 78 and 80, a light intensity detector, such as the detector 2 of FIG.
This is a method of simultaneously monitoring the outputs of 2 and 24. Under such a test, the path length control mirror moves through these maximum number of design modes, and the trajectory of the output signal from the photodetector shows a curve (gain profile) as shown in FIG. ) Is shown. The change in the maximum value of a series of gain profiles under a certain mode scanning is an index of the tilt of the mirror. The path length control assembly and mirrors of the present invention were tested to determine the tilt of the mirror according to conventional modal scanning methods. Prior Art (US Patent Nos. 4,8
61, 161) in a completely different way
The path length control assembly of the present invention has an inherent simplicity compared to previous designs. Because of this simplicity of design, including the absence of point contacts between elements, the tilt testing of the mirrors of the present invention worked very well. Multiple path length control assemblies have been manufactured and tested in accordance with the teachings of the present invention. Two these assemblies are rings
Various gyroscope performance tests were performed, including a mode scan test integrated into the laser gyroscope to check the tilt of the path length control mirror. The results of these tests showed that there was no measurable change in the maximum gain profile during the seven mode scans (which were within the design goals) during this mode scan.

While the preferred embodiment has been shown, it is possible to envisage other equivalent embodiments of the invention which use the basic teachings and principles described herein, and which are also preferred. It can perform similar functions. For example, if the forces could be balanced across the surface of the backing plate 76,
In order to achieve control of the cavity length, the mirror housing 7
Any symmetrical shape for 4 can be used. Therefore, these other embodiments having substantially equivalent functions or structures are also included in the scope of the claims of the present invention.

[Brief description of drawings]

FIG. 1A is a drawing of a ring laser gyroscope for illustrating the operation of a cavity length control system according to the prior art, and FIG. 1B is a gyro operating at the resonance frequency of the ring laser gyroscope according to the prior art. 3 is a graph illustrating the atomic gain medium curve of a scope.

FIG. 2 is a cross-sectional view of a path length control assembly according to the prior art.

FIG. 3 is a perspective view of one preferred embodiment of a path length control assembly for a ring laser gyroscope of the present invention.

4 is line 4 of FIG. 3 of the inventive path length control assembly.
FIG. 4 is a sectional view taken along line -4.

5 is an enlarged isometric view of the device of FIG.

[Explanation of symbols]

 70 Path Length Control Assembly 74 Mirror Housing 75 Mirror Post 76 Backing Plate 78,80 Piezoelectric Element

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Mohamed M. Terrani United States, 91361 California, Westlake Village, Rusted Oak Drive 31587

Claims (10)

[Claims] 1. A path length control assembly for use in a ring laser gyroscope, the assembly comprising: a mirror including an axially deflectable membrane, a mirror mounted on a housing; A mirror column bonded to the membrane; a backing plate; and at least one transducer axially deflecting the backing plate in response to an electronic input signal bonded onto the backing plate; Driving a mirror column, the membrane and the mirror; whereby the backing plate has the dual function of supporting the at least one transducer and axially driving the mirror column. Ring Laser Gyroscope Path Length Control Assembly. 2. The at least one transducer comprises two transducers: a first said transducer is a flat bimorphic disc joined to one side of said backing plate; a second said transducer. A ring laser gyroscope as claimed in claim 1, characterized in that it is a flat bimorphic ring defining a central opening into which a mirror post is inserted, the second transducer being joined to the other side of the backing plate. Path length control assembly. 3. The at least one transducer comprises two transducers: a first said transducer being a flat bimorphic ring defining a central opening joined to one side of said backing plate; and a second. Is a flat bimorphic ring defining a central opening of the same size as the opening of the first transducer into which the mirror column is inserted, and the second transducer is the other of the backing plate. The ring laser gyroscope path length control assembly of claim 1, wherein the ring laser gyroscope path end control assembly. 4. The path length control assembly for a ring laser gyroscope of claim 1, wherein the mirror housing has only one ventilation hole through which pressure equalization of the assembly is achieved. 5. The mirror housing and backing
The plate is made from a low coefficient of thermal expansion material selected from the group consisting of Cervit, Zerodour, and ULE glass;
The ring laser of claim 1, wherein the path length control assembly is operable over a wide temperature range.
Path length control assembly for gyroscope. 6. In an improved ring laser gyroscope, said ring laser gyroscope includes: a resonant optical cavity formed in a ring configuration defining a closed optical path; said resonant optical cavity comprising said path length. A path length control assembly for adjusting the optical path length in response to changes in the optical path length; the path length control assembly further comprising: a mirror mounted on a mirror housing including an axially deflectable membrane; A mirror column bonded to the deflectable membrane; a backing plate; and at least one piezoelectric element axially deflecting the backing plate in response to an electronic input signal bonded on the backing plate. The backing plate drives the mirror column, the membrane, and the mirror; The backing
A ring laser gyroscope, wherein a plate has a dual function of supporting the at least one piezoelectric element and axially driving the mirror column. 7. The path length control assembly further comprises: the at least one piezoelectric element consisting of two elements, the first said element being a flat bimorphic disk bonded to one side of the backing plate, and The second element is a flat bimorphic ring defining a central opening into which the mirror post is inserted, the second element being joined to the other side of the backing plate. Ring laser gyroscope. 8. A path length control assembly for use in a ring laser gyroscope, the assembly comprising: a mirror; a means for mounting the mirror on an axially deflectable membrane; A membrane-bondable mirror column; at least one transducer means; and means for supporting the transducer means and for axially driving the mirror column; bonding on the support and drive means At least one transducer means axially deflects the support and drive means in response to an electronic input signal; the support and drive means axially displaces the mirror post, the membrane, and the mirror; This allows the support and drive means to support the at least one transducer and is undesirable Path length control assembly and providing a dual function of driving the mirror post and the mirror in the axial direction without causing inclination of the error. 9. Means for supporting said transducer means and for axially driving said mirror column comprises a single backing plate; and on said membrane capable of deflecting said mirror in said axial direction. 9. The path length control assembly of claim 8 wherein the means for mounting on the mirror comprises a single mirror housing. 10. The at least one transducer means comprises two transducers, the first transducer being a flat morphic disc joined to one side of the support and drive means, and the second. The transducer is a flat bimorphic ring defining a central opening into which the mirror post is inserted, the second transducer being joined to the other side of the support and drive means. The path length control assembly of claim 8.