BACKGROUND OF THE INVENTION The present invention relates to ring laser gyroscopes. More particularly, it relates to a path length control element for the cavity of a ring laser gyroscope.

A ring laser gyroscope is basically a laser apparatus having a ring type resonant cavity, typically triangular in configuration. The laser beam is directed around the triangular path by suitable mirrors positioned at each of the corners of the triangular structure. In most cases there are two laser beams traveling in opposite directions relative to each other around the ring. The positioning of the mirrors in the corners of the ring, or triangle, not only direct the laser beams down the channels of ^" the resonant cavity but also determine the path length of the resonant cavity. As is understood in the art relating to gas lasers, the path length of the resonant cavity must be maintained at an integral number of wave lengths of the laser beam. Since such laser gyroscopes are subjected to wide extremes of temperature, ranging from between -65 to +185 degrees

Fahrenheit, it is apparent that the system would tend to change its dimensions in its transitions between such extremes of temperature. In order to maintain the laser gyroscope at its proper operating condition, efforts have been made in the past to minimize the affect of such changes in dimension. Among other things, the block defining the resonant cavity for the ring laser is made of a substance having a low thermal coefficient of expansion such as Cervit. While the

> substances exhibit a very low thermal coefficient expansion, they are not sufficient to prevent undesired shifts in the path length of the resonant cavity. To further compensate for such dimensional changes, there have been efforts to reposition mirrors at each of the corners of the ring cavity to restore the path length to its desired dimension. Such an arrangement is shown in U.S. Patent 3,481,227, issued in the name of Theodore J. Podgorski. In that patent, a stack of piezoelectric ceramic discs may be energized to drive a control system carrying the corner mirror in response to signals derived from a closed-loop servo type system. The stack of ceramic discs provide a rather cumbersome structure and are

themselves subject to dimensional changes with temperature, thereby introducing a variation in the scale factor of the correcting circuitry.

In Patent 4,113,378, issued in the name of Sidney Shutt, a somewhat similar structure is provided but in which the piezo ceramic discs are provided with split electrodes whereby to produce not only a transitional movement of the mirror, but also a rotational movement. In Patent 4,383,763, issued to Hutchings et al, there is shown and claimed a structure specifically arranged to eliminate the undesired rotational movement of the mirror and which shows a driving number comprised of a pair of piezoelectric discs sandwiched about a metallic membrane. An adjustable screw provides an adjustable thrust bearing.

While each of the foregoing systems was designated to meet a specific need, they each fall short of the high order of stability required to meet the requirements of the present wide temperature range conditions. The arrangements having stacked discs of ceramic discs have the disadvantage that they, too, are temperature sensitive in addition to being bulky. The structure providing the rotational or bending mode provides a structure which would not be suitably

structurally stable under a high stress condition. The structures of the type shown in Patent 4,383,763 which provides the structure necessary to the maintain a transitional motion only of a mirror still involves the use of a metal adjusting screw and the metal diaphragm sandwiched between the two piezoelectric ceramic discs. The metallic structure, even under carefully controlled selection, still provides a difference in thermal expansion coefficients. The adjacent positioning of the ceramics and the metal diaphragm produce thermal strains in the interface thereof. All of this contributes to variations in scale factor in connection with the extreme range of temperature to which the structure is ^' exposed. SUMMARY OF THE INVENTION

It is, accordingly, an object of the present invention to provide an improved piezoelectric driver element which exhibits improved characteristics over an extremely wide range of temperature variations. It is another object of the present invention to provide an improved piezoelectric driver element as set forth which is suitable for use in a path length controller for ring laser gyro assembly.

In accomplishing these and other objects, there has been provided, in accordance with the present invention, a piezoelectric driver assembly

which includes piezoelectric discs bonded on opposite sides of a driver diaphragm. The driver diaphragm, or plate, is made of the same ceramic material ^' such as Cervit as the main transducer block resulting in no thermal stresses between the driving diaphragm and the transducer. Similarly, the bonding of the piezoelectric ceramic discs to the ceramic driver diaphragm introduces little to no thermal stress between the piezoelectric discs and the driver diaphragm, or plate, bonded to the transducer block, there is no interfacing metallic structure such as a adjusting screw to interpose a thermal response difference producing a scale factor shift.

BRIEF DESCRIPTION OF THE DRAWINGS A better understanding of the present invention may be had from the following detailed description when read in the light of the accompanying drawings in which:

Figure 1 is a schematic block diagram of a ring laser gyro system embodying the present invention;

Figure 2 is a cross-sectional view of a prior art structure over which the present invention is an improvement;

Figure 3 is a cross-sectional view of a piezoelectric driver assembly embodying the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings in more detail, there is shown in Figure 1 a schematic diagram of a ring laser gyro embodying the present invention. The apparatus is represented as a main block 2 which is shown as being triangular in shape. In a preferred embodiment, the main block 2 is formed of a glass-like substance having a relatively low thermal coefficient of expansion, such as Cervit or Zerodur. The block has formed therein a cavity 4 defining a triangular path and which is preferably filled with a lasing gas such as the traditional helium-neon mixture. By conventional means (not shown) , a pair of counter-rotating laser beams 6 are introduced into the cavity 4 and travel in opposite directions about the triangular path defined by the cavity 4. At each of the corners of the block 2 and facing into the cavity 4, there is positioned reflecting means or mirror assemblies 8, 10, and 12, respectively.

The mirror assembly 12 is illustrated as being associated with detector 14. In that respect, the mirror assembly 12 is, in effect, a beam splitter. That is, a portion of the laser beams 6 are

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reflected back into the resonant cavity while another portion of the beams are transmitted through the reflector assembly into suitable photoelectric detector means 14. For purposes of simplification of the present explanation the detector means 14 is shown merely as a block. The details thereof are not essential to the present invention. It might be pointed out, however, that there are two aspects of detector means 14. First, the detector 14 detects differences in the frequency of the two counter-rotating beams as a function of the physical rotation of the ring laser gyro assembly. The detector 14 also responds to changes in the magnitude, of the laser signals which are indicative of changes in the path length of the resonant cavity, effectively detuning the cavity. It is this latter signal that is applied along a feedback path 16 to a regulated voltage source 18 which is, in turn, connected to a piezoelectric driver element 20. The driver 20 is secured to and drives the transducer structure forming a part of the mirror assembly 10. As the path length of the resonant cavity tends to change, as a function of shift in the temperature, the detector 14 will produce an output signal indicative of the amount of that shift. That signal is applied by means of the feedback path 16 to the voltage source 18 causing the

driver 20 to be moved, in response thereto, in a direction and in an amount to tend to restore the resonant cavity path length to its original position. The mirror or reflector 8 may be either a simple reflecting mirror or may be a duplicate of the adjustable mirror 10.

In Figure 2 there is shown, on an enlarged scale, a transducer assembly of a type heretofore used in a ring laser gyro for the purpose of controlling the path length of the resonant cavity. In that structure, a transducer body 22 is formed of a material such as the hereinabove referenced Cervit or Zerodur. The body.22 -is configured with a central post 24 of substantial dimension and an outer wall member 26 is in the form of an annulus. The central post 24 and the outer wall 26 are connected by a first and second relatively thin diaphragm or membrane 28 and 30, respectively, with the central post 24 and the outer wall 26 extending somewhat beyond of the surface of the membrane 30. The body 22 includes a transverse hole 32 which extends through the side wall structure 26 and the center post 24 at a position between the two membranes 28 and 20. A metallic pin 34 is inserted into the transverse hole 32. The pin 34 has a threaded hole therethrough which is positioned in

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alignment with an axial hole 36 in the center of the center post 24. As will be seen hereinafter, the pin 34 comprises a nut captive for a screw 38.

A driver element 40 is secured to the transducer body 22 by means of the screw 38. The driver element 40 includes a first and second piezoelectric ceramic disc 42 and 44, respectively, which are bonded together with a conductive coating 46 there between. The piezoelectric ceramic disc 44 is, in turn, bonded conductively to one surface of a metallic disc armature of driver plate 48. This disc 48 is preferably made of a metallic alloy having a low coefficient of thermal expansion such as Kovar. The disc 48 has a peripheral rim which is held in firm engagement with the upper surface of the transducer body 22 by means of the screw 38. A metallic electrode 50 is deposited on the upper surface of the piezoelectric disc 42. The screw 38 is tightened in the nut 34 to securely hold the driver element 40 on the transducer body 22. The tightening of the screw 38 also established a predetermined preload condition on the transducer assembly.

The leads 52 and 54 from the regulated voltage sources 18 (Figure 1) are connected to the electrodes of the piezoelectric ceramic discs 42 and 44. The lead 52 is* connected to the two outermost

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electrodes 50 and 48 while the lead 54 is connected to the electrode 46 sandwiched between the two piezoelectric ceramic discs 42 and 44. Thus the two ceramic discs 42 and 44 respond to electric signals applied to the leads 52 and 54 in a bi orph manner to cause a relative motion of the metallic disc driver plate 48 which, in turn, results in a piston-like motion of the central post 24, opposite from the driver 40, carries a mirror 56. The mirror 56 comprises the positionally adjustable reflecting means represented by the mirror assembly 10 of Figure 1.

The structure shown in Figure 2 has a disadvantage, comparable to that of the structures shown in Patent 4,383,763, in that the scale factor of the transducer assembly is initially set by the adjustment of the screw 38 relative to the pin-nut 34. In the extremes of temperature to which such devices must be exposed, the thermal expansion of the adjusting screw 38 would tend to cause a shift in the scale factor of the adjusting circuitry.

Additionally, the juxtaposition of the metallic disc 48 between the ceramic discs 44 and the Cervit block 22 would tend to produce thermal strains in the assembly. Again, with the two bimorhic discs 40 and 42 positioned together and both on the same side of the metallic disc 48, would also tend to create

thermally activated stress in the nature of a bimetallic deformation which would similarly tend to contribute to the scale factor instability at the wide temperature ranges noted. In overcoming the aforementioned disadvantages of the structure shown in Figure 2, as well as constituting a significant improvement over the art heretofore cited, there has been provided, in accordance with the present invention illustrated in Figure 3, a transducer assembly which includes a transducer body 58. The transducer body 58 is identical or substantially identical to the transducer body 22 as shown in Figure 2. Although the transducer body 58 may be identical to the transducer body 22, it may differ in that there is no necessity for the transverse hole 32 or the axial hole 36. As before, the transducer body 58 includes a central post 60 and an outer wall 62 of relatively substantial thickness dimensions. The outer wall 62 and the central post 60 are connected together by a first and a second relatively thin membrane member 64 and 66, respectively. The first membrane 64 defines a planar surface with the associated end of the post 60 and the wall 62. A mirror 68 is affixed to the remote end of the center post 60 opposite from the planar surface.

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A significantly different driver assembly 70 is provided for driving the transducer assembly to compensate for the thermal changes in cavity length. The driver assembly 70 includes an armature or driver plate 72 which is preferably formed of the same material, i.e. Cervit, as the transducer body 58. The armature or driver plate 72 is in the form of a disc having a relatively thick peripheral rim 74, a central hub 76 and a relatively thin membrane 78 interconnecting the rim 74 and the hub 76. One surface of the membrane may be coplanar with the upper surface of the hub 76. The other surface of the hub 76 is coplanar with the corresponding surface of the rim 74. On that surface of the driver plate 72 wherein the rim 74 and the hub 76 are coplanar, the surface of the membrane 78 defines an annular recess. A first annular ceramic piezoelectric element is positioned in the recess defined by the configuration of the plate 72. A second annular ceramic piezoelectric element 82 is positioned on the opposite side of the membrane 78 from the element 80. In an exemplary embodiment, the elements 80 and 82 were made of piezoelectric material. The opposite faces of each of the piezoelectric elements 80 and 82 are provided with an electrically conductive coating which may be in the form of a very thin metallic film deposited

thereon. These electrically conductive surfaces will, as will be seen hereinafter, constitute electrodes for the exitation of the piezoelectric elements. The two piezoelectric elements 80 and 82 are securely bonded, respectively, to the opposite faces of the membrane 78 of the armature or driver plate 72. One surface of the peripheral rim 74 of the driver plate 72 is securely bonded, as by optical bonding or by epoxy cement, to the planar surface of the transducer body 58 at the surface of the end wall 62. Similarly, the coplanar surface of the hub 76 is bonded to the planar surface of the block 58 in the position of the proximate end the center post 60.

A first electrical lead 84 from a controlled, regulated voltage source, such as the voltage source

18 of Figure 1, is connected to the upper (as shown in Figure 3) electrode of both of the piezoelectric ceramic discs 80 and 82. A second electrical lead 86 from the controlled, regulated voltage source is connected to the lower (as shown in Figure 3) electrodes of both of the piezoelectric ceramic discs 80 and 82.

In a response to a detected change in the path length of the resonant cavity, the detector 14, as previously mentioned, applies a control signal to the regulated voltage source 18. This unit, in turn.

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applies a controlled signal to the leads 84 and 86, respectively, and thence to the corresponding electrodes of the piezoelectric ceramic discs 80 and 82. Those signals cause the ceramic discs to deform axially. The deformation of the piezoelectric ceramic discs cause the driver plate 72 to be similarly deformed. Since the driver plate is bonded to the transducer body 58, the deformation of the driver plate 72 causes an axial displacement of the central post 60 relative to the outer wall 62 of the transducer body 58. The displacement of the center post 60, in turn, causes a displacement of the mirror 68, thereby changing the path length within the resonant cavity of the ring laser block 2. The movement of the mirror 68 will be in a direction to restore the path length of the resonant cavity substantially to the predetermined path length commensurate with the resonance of the laser beam.

Whereas, in previous transducer structures, the transducer body 58 was constructed of material such as Cervit and the armature or driver plate was formed of a metal such as Kovar; in the present invention, the driver plate 72 is formed of the same materials, such as Cervit, as the transducer body 58. With both the driver plate 72 and the transducer body 58 being made of the same material, they will have

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identical responses to the wide variations in temperature to which the apparatus would be exposed. Therefore, the driver plate 72 may be bonded securely to the surface of the transducer body 58 without introducing thermal stresses at the interfaces thereof.

Whereas in the structure shown in Figure 2, the driver plate was secured to the transducer body by means of a metal screw and nut arrangement, in the apparatus in accordance with the present invention there is no metallic interconnection between the driver plate and the transducer body, the transducer plate being securely bonded to .the transducer body at . the periphery and at the central post/hub interface. As previously noted, the metallic interconnection introduces a difference in thermal response characteristics between the metal and the glass-like substance of the transducer body. This difference in temperature response characteristics introduces instabilities in the scale factor of the correcting circuitry at the extremely wide temperature range to which the apparatus is subjected.

With the piezoelectric discs 80 and 82 being made of a ceramic material and the driver plate 72 being made of the glass-like substance such as Cervit, there is very little difference in their thermal

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coeffiσients of expansion, thereby reducing to a negligible amount thermal stresses at the interface between the piezoelectric discs and the driver plate 72. Further, with the two piezoelectric discs being bonded to opposite faces of the driver plate 72 and effective "bi-metallic" action which would result from the differences in thermal coefficients of expansion are neutralized. This is in contradistinction to the bi-metallic effect experienced in the structure such as that shown in Figure 2 wherein the resulting bi-metallic effect would produce a further instability in the scale factor of the correcting circuitry.

There has been provided, in accordance with the present application, an improved piezoelectric transducer apparatus especially useful in the correcting of the path lengths of a ring laser gyro. The improved structure is characterized in the greater stability of the scale factor of the correcting circuitry as well as a reduction in thermal stresses when apparatus is subjected to a variation in temperatures ranging from -65 to +180 degrees Fahrenheit.