GB2104283A

GB2104283A - Piezoelectric transducer

Info

Publication number: GB2104283A
Application number: GB08220717A
Authority: GB
Inventors: Alfred L Butler
Original assignee: Litton Systems Inc
Current assignee: Northrop Grumman Guidance and Electronics Co Inc
Priority date: 1981-07-24
Filing date: 1982-07-16
Publication date: 1983-03-02
Also published as: GB2104283B; DE3227451A1; FR2510336B1; IT8248865A0; CA1210842A; GB8431194D0; JPS5827387A; IL66379A0; FR2510336A1; GB2149570A; IT1148381B; IL66379A

Abstract

A transducer comprises a piezoelectric element (58) which may be mounted upon a thin, flexible diaphragm (54). The piezoelectric element (58) is coated with a metallic coating forming electrodes which permit a potential difference to be applied across the element for deforming it and displacing the diaphragm to produce useful motion. The metallic coating is interrupted to form first and second electrodes (62, 64). The piezoelectric element thus is driven by a voltage source connected to the first electrode (64) while a voltage detector connected to the second electrode (62) generates a sensor signal due to strain of the element, which is useful to determine the position of the transducer. <IMAGE>

Description

SPECIFICATION Piezoelectric transducer This invention relates to a piezoelectric transducer.

Piezoelectric transducers have been proposed for use within ring laser gyros for adjusting the cavity length of the gyro.

The essence of a ring laser gyro is that two light waves, circulating in opposite directions around the same path, undergo frequency shifts when the path is rotated. The frequency shifts are in opposite directions to produce optical frequency differences between the two waves. The two frequencies heterodyne at a common photo-dector giving rise to a beat frequency directly proportional to the angular rotational rate.

The laser cavity is often created within a quartz body having a number of apertures cut into it. The apertures form the inner passages which make up the cavity of the ring laser. Each corner of the cavity, formed by the intersection of the passages, is provided with a reflective mirror which reflects the two counter-propagating light waves from one passageway to the next.

For proper functioning of the ring laser, it is important that the amplitudes of the two oppositely circulating light waves which make up the gyro be substantially equal. The characteristic intermodulation of the two light waves is the classical form of amplitude modulation with maxima occuring when the sum of the instantaneous components are in phase, and the minima occurring when these components are 180 out of phase. If the amplitude of one of the two light wave signals is significantly greater than that of the other, the beat signal does not drop to zero.

Proper ring laser gyro action may be achieved by detecting the minimum points of the beat signal and then determining whether these points are actually zero. If the minimum points are not zero, the length of the laser cavity is adjusted to drive the minimum points toward zero. To adjust the length of the laser cavity a piezoelectric transducer is attached to one of the laser mirrors. A cavity control circuit drives the piezoelectric transducer and, in the prior art, receives its input signal from the output signal of the laser (U.S. Letters Patent No. 4,123,162 issued October31, 1978 to V.E. Sanders entitled MULTIOSCILLATOR RING LASER GYRO OUTPUT INFORMATION PRO CESSING SYSTEM assigned to the present applicants).

According to the present invention there is provided a piezoelectric transducer comprising a piezoelectric element having a first surface provided with a first electrode for establishing a potential difference between the first surface of the piezoelectric element and a second surface of the piezoelectric element to cause deformation of the piezoelectric element, and a second electrode for detecting a potential difference created between the first and second surfaces as a result of strain of the piezoelectric element.

A preferred embodiment in accordance with the present invention can be used to track the location of the ring laser corner mirrors by causing a piezoelectric transducer to generate its own feedback signal for indicating the position of the transducer.

The design of the transducer is preferably symmetrical since symmetry tends to eliminate thermal distortion inherent in a separately attached sensor.

Further, by using the same crystal structure for the driver and the sensor, direct monitoring of the driver by the sensor without intermediate attachments, such as mechanical or adhesive layers, eliminates many components which might cause error through manufacturing tolerance, assembly errors, mismatches in alignment, or thermal distortion.

For a better understanding of the present invention and to show how it may be carried into effect, reference will now be made by way of example, to the accompanying drawings, in which: Figure 1 is a top plan view, partially in section, showing a typical ring laser gyro; Figure 2 is a cross section taken along lines 2 - 2 of Figure 1 showing a piezoelectrictransducer; Figure 3 is a plan view of the piezoelectric transducer of Figure 2; Figure 4 is a side view of the transducer of Figure 3; Figure 5 is a plan view of an alternative form of piezoelectric transducer; Figure 6 is a side view of the transducer of Figure 5; Figure 7 is a plan view of another alternative form of piezoelectric transducer; Figure 8 is a side view of Figure 7; and Figure 9 illustrates the electrical connection of the transducer shown in Figures 1 - 4.

Referring now to the drawings, Figure 1 illustrates a typical ring laser gyro 10 which is formed in a body 12, e.g. quartz or an ultra-low expansion material, such as titanium silicate. The laser body 12 is constructed with four passages 14therein which are arranged to form a closed rectangular path. Alternative configurations such as a triangle or other polygon are known. Further the laser need not be planar. The passages 14 are sealed to retain a gas mixture consisting, typically, of approximately 90% helium and 10% neon in a vacuum of approximately 3 torr. It is understood that atmospheric pressure is approximately 760 torrs.

The body 12 is typically provided with two cathodes 16 and 18 and two anodes 20 and 22 which may be secured to the body in a manner that is known in the art. Other energizing means are known, for example, it is common to use a single cathode.

Gas discharges are established in passage 14 between cathode 16 and anode 20 and between cathode 18 and anode 22. A getter 24 may be provided to absorb impurities found within the gas in the passageway 14. Mirrors 28,30,32 and 34 are located at the four corners of the laser optical path formed within the passageway 14 of the ring laser gyro 10. The mirror 28 is mounted upon a photodetection device 36. The photo-detection device 36 produces the beat frequency signal generated by the counter-propagating laser beams as a measure of the angular rate of rotation of the ring laser gyro 10 about its sensing axis.

The mirror 34 is mounted upon a piezoelectric transducer 38 which controls the position of the mirror 34 to control the length of the laser cavity 14 of the ring laser gyro. The piezoelectric transducer 38 is shown in greater detail in Figure 2. Typically the transducer 38 has a housing 40 formed by a thin cup-shaped metallic member whose bottom surface forms a flexible diaphragm 42 which is mounted against the quartz body 12 and is sealed thereto by suitable bonding. The surface of the diaphragm 42 which mounts against the inner passages of the quartz body 12 supports the mirror 34. A post 44, used to drive the diaphragm 42, extends from the center of the diaphragm 42.The cup-shaped housing 40 is closed by a transducer housing 48 whose toroidally-shaped outer rim 49 includes, for example, eight spring-loaded fingers 50, only five of which are shown in Figure 2, that extend over the outer surface of cup 40. The spring fingers are provided with an arcuate surface 51 which is urged snugly against the outer surface of cup 40 and secured thereto, as by bonding. The outertoroidal rim 49 of the housing 48 is connected to a center hub 52 by a further thin, flexible diaphragm 54. Mounted on the inner and outer surfaces of the diaphragm 54 are disk-shaped piezoelectric crystals 56 and 58 respectively.

In the preferred embodiment, it is not necessary to use the inner piezoelectric disk 56. Alternatively, disk may be used solely as a stiffener. As best seen in Figures 3 and 4, the piezoelectric disk 58 is coated with a metal coating 59 to form two symmetrical concentric electrodes on the surface of the disk. The coating 59 is interrupted by a toroidally-shaped uncoated area or cut 60 which separates the concentric inner and outer electrodes 62 and 64. Interruption of the metal coating 59 may be accomplished by several methods including masking the areas of the metal coating 59 which are desired and abrading the unmasked areas, toroidal ring 60, that one wishes to remove.

A stud 66 is threadably mounted in the center of hub 52 for contacting the post 44 of diaphragm cup 40. The threaded stud 66 is adjusted until it firmly engages the inner surface of post 44. When an electrical potential is placed across the piezoelectric crystal 58, the crystal will go convexed, when viewed from the left-hand side of Figure 2, for removing a preset displacement of diaphragm 42 caused by the stud 66. This permits mirror 34 to move, thus adjusting the length of laser cavity passage 14.

As seen in Figure 9, the crystal 58 may be connected to an electrical potential by connecting the diaphragm 54to a common potential, such as the system earth, and connecting the outer electrode 64 between the common potential and a source of adjustable potential 68. The inner electrode 62 is then connected from the common potential at the diaphragm 54 to a voltage detector 70.

Should two crystals be used as shown in Figure 2, the common potential may be connected to a metallic coating over the full surface of the first crystal 56. In this configuration, the crystal 56 is driven to a concave position while the crystal 58 is driven to a convex position when viewed from the left-hand side of Figure 2.

As seen in Figures 3,4 and 9 the sensor represented by voltage detector 70 is preferably connected to the inner electrode 62 which is arranged over the portion of the crystal 58 that experiences maximum strain. It has been found experimentally that an annular piezoelectric crystal having a small internal diameter undergoes maximum stress and strain at or near its internal diameter.

However, as the internal diameter of an annular piezoelectric crystal increases in size, there are some situations in which the sensor undergoes maximum stress and strain near its outer diameter. The outer electrode 64 would then be used as a sensor. As shown in Figures 5 and 6, a diaphragm 72 may be provided with an aperture 74 having a large internal diameter and at least one surface upon which is mounted a piezoelectric crystal 76. The crystal is covered with a metallic coating 78 which is interrupted by a toroidal cut 80 for forming an inner electrode 82 and an outer electrode 84. The inner electrode 82 may then be connected to a source of adjustable voltage potential, such as the adjustable DC source 68, to drive the crystal 76. The outer electrode 84 then produces a voltage which may be measured by voltage detector 70 to produce a useful feedback signal.

In some uses of the invention not involving control of cavity length, it is desired to measure the flexure of a bar. For example, a thin elongated metallic bar is used in a spring system which mechanically dithers a ring laser gyro about its input axis. As shown in Figures 7 and 8, such a drive system may incorporate a bar-shaped flexure spring 86 against which is mounted a quadrilaterally-profiled piezoelectric crystal 88 which, in the embodiment shown, has a rectangular profile. The surface of crystal 88 is coated with a metallic coating 90 which is interrupted by a cut 92, generally perpendicular to the longitudinal axis of crystal 88, to form a major electrode 94 and a minor electrode 96. The minor electrode 96 is preferably located at one end of crystal 88 which is positioned adjacent a position of maximum strain of the spring 86.Electrode 96 acts as a sensor electrode to generate a sensor signal to be applied to a voltage detector 70. The major electrode 94 may then be connected to a source of variable potential 68 for contracting or expanding crystal 88 and driving spring 86.

Although Figure 9 shows a separate control voltage source 68 and sensor voltage measuring device 70, it is clear that a typical servo amplifier (not shown) could be used to drive the source 68, and the voltage measured at 70 could be the feedback voltage which is delivered to the servo amplifier.

While the piezoelectric transducer of the present invention has been described for use within a ring laser gyro, it will be understood that similar transducers may be used in any number of devices.

Further, the configuration shown as the preferred embodiment within the present invention may be modified to include many variations including a variation with a larger internal diameter, a quadrila teral profile or rectangular profile. Accordingly, the present invention should be limited only by the appended claims.

Claims

.1 A piezoelectric transducer comprising a piezoelectric element having a first surface provided with a first electrode for establishing a potential difference between the first surface of the piezoelectric element and a second surface of the piezoelectric element to cause deformation of the piezoelectric element, and a second electrode for detecting a potential difference created between the first and second surfaces as a result of strain of the piezoelectric element.
2. A piezoelectric transducer as claimed in claim 1, in which the first and second electrodes comprise metallic coatings on the first surface of the piezoelectric element.
3. A piezoelectric transducer as claimed in claim 2, in which the first and second electrodes are separated from each other by an uncoated portion of the first surface.
4. A piezoelectric transducer as claimed in claim 3, in which the piezoelectric element has a quadrilateral shape and in which the uncoated portion extends in a straight line.
5. A piezoelectric transducer as claimed in claim 3, in which the piezoelectric element has a circular shape and in which the uncoated portion extends in a circular line.
6. A piezoelectric transducer as claimed in claim 5, in which the radially inner coated portion comprises the second electrode.
7. A piezoelectric transducer as claimed in claim 5, in which the radially outer coated portion comprises the second electrode.
8. A piezoelectric transducer as claimed in any one of claims 2 to 7, in which the shapes of the electrodes are symmetrical.
9. A piezoelectric transducer as claimed in any one of the preceding claims, in which the second electrode is provided on the first surface at a region of the piezoelectric element which undergoes the greatest strain.
10. A piezoelectric transducer as claimed in any one of the preceding claims, in which the piezoelectric element is mounted on a surface of a flexible element whereby the application of a voltage to the first electrode causes displacement of the flexible element, and whereby displacement of the flexible element results in a voltage signal at the second electrode.
11. A piezoelectric transducer as claimed in any one of the preceding claims, in which the second surface of the piezoelectric element is provided with a third electrode.
12. A piezoelectric transducer as claimed in claim 11, in which a voltage source is connected between the first and third electrodes and a voltage detector is connected between the second and third electrodes.
13. A piezoelectric transducer as claimed in Claims 11 or 12, when appendantto claim 10, in which the flexible element constitutes the third electrode.
14. A piezoelectric transducer as claimed in claim 10, in which a stiffening element is mounted on a surface of the flexible element opposite the piezoelectric element.
15. A piezoelectric transducer as claimed in claim 14, in which a third electrode is provided on a surface of the stiffening element away from the flexible element, a voltage source being connected between the first and third electrodes and a voltage detector being connected between the second and third electrodes.
16. A piezoelectric transducer as claimed in claim 14 or 15, in which the stiffening element comprises a further piezoelectric element.
17. A piezoelectric transducer as claimed in any one of claims 1 to 10, in which the second surface of the piezoelectric element is provided with third and fourth electrodes, a voltage source being connected between the first and third electrodes and a voltage detector being connected between the second and fourth electrodes.
18. A piezoelectric transducer as claimed in claim 17 when appendant to claim 10 or in any one of claims 13 to 16, in which the flexible element comprises a diaphragm mounted in a housing.
19. A mirror assembly comprising a mirror and a piezoelectric transducer as claimed in claim 18, the mirror being mounted on the diaphragm.
20. A mirror assembly as claimed in claim 19, in which the mirror is mounted for displacement in a direction normal to its reflective surface, the diaphragm extending substantially parallel to the reflective surface of the mirror.
21. A piezoelectric transducer substantially as described herein with reference to, and as shown in, Figure 2, Figures 3, 4 and 9, Figures 5 and 6, or Figures 7 and 8 of the accompanying drawings.