FR2614412A1 - Laser rate gyro, improvement to the device for eliminating parasitic rotations of the piezoelectric mirrors - Google Patents

Abstract

The subject of the invention is an improvement in the device for eliminating the parasitic rotations of the moving mirrors 15 of laser rate gyros, which are moved by piezoelectric ceramics 11. The parasitic rotations are due to the deformations of the outer parts 12 constituting the body of the moving mirror and fixed to the optical unit 1, under the effect of the accelerations of the actuation movement. These parasitic rotations are corrected, in the invention, by springs 24, fixed to the external structure of the gyro laser by screws 26, and which bear on a piece 30 fitted into a hole 38 pierced in the central part 13 of the piezoelectric mirror assembly, thus righting the outer parts 12. The invention applies to all lasers and in particular to gyro lasers.

Description

 The present invention relates to an improvement to devices intended to suppress parasitic rotations of the mobile mirror (s) of laser gyrometers, and which occur under the effect of accelerations due to mechanical activation or to certain vibrations.

 This invention applies to all triangular or square, single-axis or multi-axis laser gyros. It can also be applied to any laser provided with a cavity length servo by movable mirror.

 The operating principle of laser gyros is now well known and those skilled in the art will have no trouble following the explanations which follow.

Laser gyros, called in what follows gyrolasers, generally consist of:
- a triangular or square optical path, dug in
a block of insulating material with a low coefficient of
dilation (figure 1). Said optical path being
delimited by three or four mirrors and fixed on the
block.

- an amplifying medium, making it possible to generate in the
optical cavity two light waves rotating one
one way and the other the other. interference
between these two waves will allow the measurement of the
rotation of the gyroscope around an axis perpendicular to the
plan of the optical cavity.

- a device for mixing light waves, to
create interference fringes on a set of
photoelectric cells. The scrolling of said fringes
representing the angular rotation of the laser gyro, will
transformed by said photoelectric cells into
usable electrical signals.

 To avoid the effects of the well-known blocking phenomenon between the two light waves circulating in the optical path (blind zone), the optical block of the gyrolaser is generally driven by an alternating angular movement of small amplitude, around an axis generally perpendicular in the plane of the optical cavity and which can be the axis of symmetry of said optical unit. The frequency of the reciprocating movement is chosen such that the angular speed of the optical unit is most of the time outside the blind zone.

 This movement is created by an oscillating device consisting of an elastic assembly generally placed inside and in the center of the optical unit. The inertia of the block determines, with the stiffness of the elastic assembly, an oscillation frequency which will be maintained by a piezoelectric or electromagnetic motor system, controlled by an adequate electronic circuit.

 This oscillation movement, also called activation, causes accelerations on the different parts of the optical unit. These accelerations are all the more important as the point considered is distant from the axis of the device and the frequency chosen for activation is high.

 The optical unit itself is relatively insensitive to the deformations that these accelerations could cause. It is not the same for certain peripheral elements such as movable mirrors which are used to adjust the length of the cavity. These mirrors are also called piezoelectric mirrors because they are generally moved by a set of piezoelectric ceramics.

 Under the effect of alternative activation accelerations which are of course tangent to the movements of each point, these piezoelectric mirrors can deform and rotate around an axis parallel to the activation axis. This rotation, although very low, affects the performance of the gyrolasers.

 By a well-adapted design of the piezoelectric mirror, it may be possible to reduce these parasitic rotations, but it is very difficult to eliminate them completely.

The invention which is the subject of the French patent application registered under No. 87.02090 proposes a solution which completely eliminates the parasitic rotations of the piezoelectric mobile mirrors by correcting them by equal forces and opposite to the inertia forces which create these parasitic rotations, and this using elastic means or suitably tared springs, fixed on the part of the piezoelectric mirror preferably furthest from the center of rotation of the activation, bearing on the fixed structure of the gyrolaser or on an intermediate part linked to said structure and of which a component of the forces which they exert acts in the direction of the movements which the mirrors make under the effect of the activation.

 The object of the present invention is to improve the aforementioned device for eliminating parasitic rotations of piezoelectric movable mirrors by applying the correction forces supplied by the springs directly to the central part of the piezoelectric mirror, part at one end of which is located the mirror part proprt.'ent said.

 The invention will be better understood and other characteristics and advantages of the invention will appear from the following description, with reference to the appended figures.

Figure 1 recalls the principle of making a typical laser gyro.

Figure 2 shows, in top view, the direction of the different forces acting on the piezoelectric mirror (s) and due to the activation movements.

FIG. 3 shows the distortion and the parasitic rotation of the piezoelectric mirror under the effect of the acceleration of the activation.

FIG. 4 shows the principle of the device for eliminating parasitic rotations which is the subject of the aforementioned patent application No. 87.02090 and where springs suitably placed act on the piezoelectric mirror to counter deformations.

FIG. 5 shows the principle of embodiment of the device for eliminating parasitic rotations of the present invention.

Figures 6, 7, 8 and 9 show two of the preferred embodiments of the invention where leaf-shaped springs act on an extension towards the outside of the central part of the piezoelectric mirror assembly.

Figures 9 and 10 show another embodiment of the invention using wire-shaped springs.

FIG. 12 recalls another drawback of the laser gyro corrected by the device of the invention which is the subject of the above-mentioned patent application 87.02090 and to which the present invention also applies.

Figures 13 and 14 show a preferred embodiment of the invention which corrects the two faults by using a particular form of springs also acting on the central part of the piezoelectric mirror assembly.

Figures 15 and 16 show two other embodiments of the invention applied to a piezoelectric mirror assembly whose motor is integral with the outer membrane.

 As shown in FIG. 1, the laser gyrometer consists of an optical unit 1, mounted oscillating around an axis 16, thanks to a system of elastic blades 8 excited by piezoelectric ceramics 9.

 A triangular or square optical path is hollowed out in the block and filled with a mixture of helium and neon 3 excited by an electric current passing between one or more cathodes 4 and one or more anodes 5.

 The path is closed by mirrors 2 and one or more movable mirrors 10, driven by one or more piezoelectric motors 11. A movable mirror 10 and a piezoelectric motor 11 constitute the piezoelectric mirror assembly of which many variants may exist.

 An information output system is placed on one of the mirrors 2 and generally consists of at least one mixing prism 6 and of a set of photo-electric cells 7.

FIG. 2 represents a part of the optical unit 1 of the laser gyro and the piezoelectric mirror. This generally consists
- an external fixed part 12, generally
cylindrical, but which could take any other form
remaining within the scope of the invention.

- an inner movable part 13 held in the
fixed part by one or more thin membranes 14 which
allow it to move along its axis of
a few microns. This mobile part brings to its
end, located on the side of the optical unit, a part
reflective 15 constituting the mirror itself.

- a piezoelectric motor for which there are very
many realization solution. One of the solutions
commonly adopted is to use a bimetallic strip
piezoelectric 11 fixed by a set of parts
metal on the outer part 12 already described.

 All of these parts have an inertia which opposes the alternative acceleration of activation. The axis of the activation being at 16, the alternative acceleration is represented by the arrows 17. Because the activation acceleration 17 is in phase opposition with the angular displacement 18 of said activation, the force d inertia 19 opposite the acceleration will be in phase with said displacement. The external part 12 not being infinitely rigid will tend to deform alternately and in the same direction 18 as the displacement of the optical unit l.

 Figure 3 shows, by exaggerating it, the deformation which occurs and the parasitic rotation of the mirror.

 The peak amplitude of the activation movement is a. When the optical unit is moved by an angle a by the activation movement, the acceleration undergone by the unit and therefore by the piezoelectric mirror is maximum and directed along arrow 17. your force of inertia 19 drives the parts of the piezoelectric mirror in the direction of arrow 19.

 The outer part 12 is deformed as shown in Figure 3. The membranes 14 are also deformed as shown in the figure and the assembly rotates the central part 13 which will form an angle ss with its initial position.

 This angle ss, which varies in synchronism with the activation, can be very troublesome for the performance of the laser gyro.

 Figure 4 illustrates the principle of the invention which was the subject of the aforementioned patent application 87.02090 and which tends to suppress parasitic rotation ss.

 For this purpose, one or more springs 20 and 21 are placed between the outermost part of the piezoelectric mirror assembly (generally the motor) and the fixed structure 22 so that the springs are alternately compressed 20 and stretched 21 by the activation movements.

Therefore, the springs will exert, on the piezoelectric mirror, a force in phase opposition with the displacement and therefore in opposition with the inertial forces which tend to deform the piezoelectric assembly and to rotate the mirror. If the springs are properly calculated and adjusted, the force they exert can perfectly compensate for the inertial force and cancel the initial deformation as well as the parasitic rotation.
As an indication, if we reduce, by a suitable calculation, the mass of the piezoelectric mirror to an equivalent mass m placed at point A, at the right and in the axis of the springs, and if the frequency of activation is f , the total stiffness of the spring (s) must other:
r = m. (2.Xf) 2
FIG. 5 shows the principle of the present invention where the springs 20 and 21, instead of acting on the external part 23 act on the central part 13 of the piezoelectric mirror assembly, while always taking support on the fixed structure 22. To do this, this central part 13 is extended outwards, for example by a part 30 preferably in the form of a rod fitted into a hole 38 drilled in said central part 13 and passing through the piezoelectric ceramic bimetallic strip 11 by a hole 40 which will have been drilled there.

 One of the advantages of the invention lies in the fact that, if a relatively thin thickness of the outer membrane 39 has been chosen, its elasticity, under the effect of the springs 20 and 21, will allow relative displacement of the part 13 in part 12, so that the force of the springs can be reduced while obtaining perfect compensation. In this case, in fact, the part 12 will in reality be partially straightened, whereas FIG. 5 shows it completely straightened, but the elasticity of the membrane 39 will allow a complete straightening of the part 13.

 Note that the force of the springs must also be reduced due to the lever arm effect produced by the offset of the fulcrum towards the outside of the piezoelectric mirror assembly.

 Figures 6 and 7 explain a first embodiment of the invention where two blades 24 of spring material are fixed to the base 25 of the laser gyro by two screws 26 and press on the part 30 fitted into the hole 38 drilled in the part central 13 of the piezoelectric mirror assembly. The initial camber of the blades is such that they will remain in contact with the part 30 during all of the activation movements.

 For clarity in the drawings, the activation mechanism which links the optical unit 1 to the base 25 and which causes the oscillation of said optical unit relative to said base, is not shown in FIGS. 6, 8, 10 and 13.

 To ensure a better definition of the spring support point, and as in the aforementioned invention, it is advantageously possible to produce a shoulder, spherical or otherwise, 44 around the end of the part 30, as shown in FIGS. 8 and 9.

 To ensure a better definition of the fulcrum of the springs, and as in the aforementioned invention, these can also advantageously be produced in the form of wires 27 as shown in FIGS. 10 and 11. Said wires, taking support in two grooves 26 hollowed out circularly or not around the periphery of the end of the part 30, can be simply inserted and glued in two holes 29 previously drilled in the base 25 of the laser gyro.

The distance between the two holes and the initial camber.

of the two wires 27 being chosen such that they give, once the assembly is completed, an initial stress on the two wires sufficient to maintain them in pressure when the piezoelectric assembly moves under the effect of activation. The two wires can of course be fixed by any other means without departing from the scope of the invention.

 Figures 13 and 14 show an improvement of the previous device which overcomes another defect of the laser gyro as in the aforementioned invention.

 Indeed, the whole optical unit, activation mechanism and base is not infinitely stiff. It can present a resonance under the effect of vibrations applied to the base of the laser gyro and parallel to it.

 The arrow 31 in FIG. 12 presents this resonance movement, around a center of rotation 35 whose position depends on the geometry of the parts, under the effect of the excitation in vibration 32.

 This resonance produces, at the level of the piezoelectric assembly, an alternative acceleration 33 of which one of the components34 tends to deform the piezoelectric mirror and to misalign the optical cavity as the person skilled in the art will readily understand.

 The embodiment of Figures 13 and 14 allows to introduce a force parallel to the acceleration 34, equal and opposite to the inertial forces so that, here too, the deformations can be canceled.

 In this embodiment, the blades, which have horizontal 37 and vertical 47 parts, instead of being simply pressed on the end of the part 30 which extends the central part 13 of the piezoelectric mirror assembly, are fixed and preferably glued at the fulcrum 36 and have, by the effect of the horizontal parts 37, a vertical stiffness R calculated to compensate for the effect of the oscillations described above.

Recall that in the aforementioned invention, if F is the transverse resonant frequency, the total vertical stiffness of the two springs must be
R = m. (2.XF) 2
In the present invention, the stiffness R will have to be reduced to take account of the elasticity of the outer membrane 39 and of the lever arm effect of the part 30, as has been said above.

 Note that the acceleration 34 of FIG. 12, which occurs under the effect of the transverse resonance F, also has a horizontal component 45 which tends to move the central part 13 axially and therefore to disturb the length control cavity. This drawback can also be eliminated, and this is another advantage of the invention, if one makes sure that the stiffness R 'of the springs measured along the axis of movement of the movable mirror is adapted to this problem.

If M is the mass equivalent to the movable part of the piezoelectric mirror, the total stiffness of the two springs in the axis of the part 30 must be
R '= M. (2.zF) 2
Note also that, for horizontal vibrations and parallel to the plane of the piezoelectric mirror, the optical unit will have the same transverse resonance. In this case, the movements of the piezoelectric mirror will be very small, and the device of the invention will not cause disturbance.

 Note also that the coefficient of expansion of the material constituting the springs will preferably be chosen equal to that of the material constituting the activation support.

 FIG. 15 shows an embodiment of the invention applied to a piezoelectric mirror assembly the motor of which consists of two circular piezoelectric ceramics 41, bonded on either side of a thin outer membrane 42, itself fixed on the central 13 and outer 12 parts of the movable mirror, by gluing or by any other means. The rod 30, on which the springs will press, crosses the membrane and is also fitted into a hole 38 drilled in the central part 13.

 As shown in FIG. 16, the rod 30 can advantageously be replaced by a part 43 fixed and preferably glued to the outside 46 of the membrane and in the axis of the central part 13. The shape of the support points springs on this part 43, preferably spherical, and the type of springs used can be any one of those already described without departing from the scope of the invention.

 It is of course possible to use all the possible combinations of the solutions described above without departing from the scope of the invention.

 In general, any kind or arrangement of means, and in particular spider or cross-shaped springs, can be used to apply forces to the central part of the piezoelectric assembly so as to straighten it without leaving the frame. of the invention.

Claims (8)

R-rVENDTCATIONS. 1. Improvement to the device for eliminating parasitic 'rotations' of the movable mirror or mirrors of a gyrolaser of the type comprising - an optical unit (1) rotatably mounted on a support (25) and provided with at least one optical perimeter in closed loop, inside which two inverse laser waves are generated, - means (6 and 7) making it possible to mix these two waves in order to obtain a system of interference fringes on photoelectric cells, - mechanical activation means (8 and 9) allowing the optical unit (1) to oscillate relative to its support (25), - at least one movable mirror (10) provided with reflective layers and moved for example by a piezoelectric motor (II), and whose parasitic rotations produced by the accelerations are corrected by elastic means preferably constituted by suitably calibrated springs (20 and 21), bearing on the fixed structure (22) of the laser gyro, acting on the piezoelectric mirror assembly and of which a component of the forces which they exert acts in the direction of the movements which the mirrors make under the effect of the activation, characterized in that said device for eliminating the parasitic rotations of the piezoelectric movable mirrors applies the forces of correction provided by the springs (21) and (22) directly on the central part (13) of the piezoelectric mirror, preferably on an extended part of this central part, part (13) at one end of which is placed the mirror part proper (15). 2. Device according to the preceding claim, characterized in that the springs consist of blades (24) made of spring material, folded and fixed on the base (25) and pressing on a part (30) preferably in the form of rod, fitted into a hole (38) drilled in the central part (13) of the piezoelectric mirror assembly and passing through a hole 40 drilled in the piezoelectric ceramic bimetallic strip (11), the initial camber of said blades being such that after assembly, they will remain in contact with the part (30), whatever the activation movements. 3. Device according to claim 2, characterized in that the definition of the fulcrum of the blades forming the spring (24) consists of a shoulder (31), preferably spherical, produced around the end of the part (30 ) fitted into the central part (13) of the piezoelectric mirror assembly, shoulder on which said blades come to bear. 4. Device according to one of claims 1, 2 or 3, characterized in that the springs are produced in the form of wires (27) bearing in grooves (26) hollowed out around the periphery of the end of the part (30) fitted into the central part (13) of the piezoelectric mirror assembly and are fixed to the base (25) of the laser gyro by sinking into holes (29) pre-drilled in said base. The spacing between the two holes and the initial camber of the wires (27) being chosen so that they give an initial stress to said wires such that the latter remain in permanent contact with the part (30), whatever the movements of 'activation. 5. Device according to one of claims 1, 2 or 3, characterized in that it corrects several faults of the laser gyro using a form of blades forming a spring as they present, thanks to their vertical branches (47) and horizontal (37), stiffness in at least two directions, stiffness adapted to the defects to be corrected, the ends of the blades being fixed to the end of the part (30) fitted into the central part (13) of the piezoelectric mirror assembly , and preferably bonded to this part (30), the coefficient of expansion of the material from which said leaf springs are made being preferably chosen to be that of the material constituting the activation support. 6. Device according to one of the preceding claims, applied to a piezoelectric mirror whose motor consists of an outer membrane (42) on which are bonded two circular piezoelectric ceramics (41), the membrane itself being preferably bonded to the external (12) and central (13) parts of the movable mirror, characterized in that the springs (24) also press on a rod (30) which passes through the membrane (42) and is fitted into a hole (38) drilled in the central part (13). 7. Device according to one of the preceding claims, applied to a piezoelectric mirror whose motor consists of an outer membrane (42) on which are bonded two circular piezoelectric ceramics (41), the membrane itself being preferably bonded to the outer (12) and central (13) parts of the movable mirror, characterized in that the springs (24) exert their forces on a part (43) preferably glued to the center of the outer membrane (42), the part of said part (43) on which the springs come to be preferably of spherical shape. 8. Device according to any one of the preceding claims, characterized in that it applies to all lasers provided with movable mirrors and subjected to vibrations of resonances and in particular to all gyrolasers.