DITHER SUSPENSION MECHANISM FOR A RING LASER ANGULAR RATE SENSOR

Abstract: A pair of flexure plates (20, 30) having an outer toroidal rim (22) extended from a hub (24) by a plurality of radial springs (26) are utilized to ridgedly sandwich a laser block (12). A housing (40) having an aperture (42) for receiving the sandwiched assembly is welded to the outer toroidal rims (22) to provide a dither suspension mechanism for a ring laser angular rate sensor.

Description: The present invention relates to a dither suspension mechanism for a ring laser angular rate sensor according to the preamble of claim 1. Ring laser angular rate sensors are well known and exemplified by U.S. Patents 3,323,411 and 3,373,650. Present day ring laser angular rate sensors generally include a thermally and mechanically stable laser block having a plurality of formed cavities for enclosing a gas. Mirrors are placed at the extremeties of the cavities for reflecting laser beams and providing an optical close-loop path. Associated with such sensors is a phenomenon called lock-in which has been recognized for some time in the prior art and has been solved by rotationally oscillating the sensor (dither). The means for accomplishing the dither conventionally includes a piezoelectric actuator connected to a suspension system which causes the block of the sensor to oscillate angularly at the natural mechanical resonant frequency of the suspension system. The dither motion is superimposed upon the actual rotation of the sensor in inertial space. The prior art includes various approaches to recover inertial rotation data free from dither but this does not form part of the present invention. Dither suspension mechanisms in the prior art include those shown, by way of example, in U.S. Patent 4,321,557, U.S. Patent 4,411,527 and U.S. Patent 4,436,423. US-A 4 309 107 as well as US-A-4 321 557 and DE-Al-3 109 789 show the same dithering device where the laser block is mounted at one side of a dither suspension mechanism including a flexure plate connected by springs to an outer toroidal rim. With such a known device the dither axis is not stiff enough and may flex when lateral accelerations occur. US-A-4 477 188 already shows a device where a cube-shaped ringlaser gyro is mounted between two dither springs; however nothing is shown with respect to the mounting of said dither springs. Advances in the art of ring laser angular rate sensors have now made it possible to provide laser sensors having a laser block in the order of 1 inch square (6,5 cm²). Reduction of the size of the sensor has many advantages including lower cost and lower weight. Dither suspension systems of the prior art do not lend themselves well to small sensor blocks. This is so since the dither mechanism would have to be reduced in size, which generally causes the dither mechanism cost to increase. Further, many dither mechanisms require a large aperture in the laser block for receiving the dither mechanism, and securing the mechanism to the inner wall of the block created by the aperture. Therefore, it is an object of the present invention to provide a low cost dither suspension mechanism for ring laser angular rate sensors which is simple to build resulting in low cost. This object is achieved according to the characterizing features of claim 1. Further advantageous embodiments of the dither suspension mechanism may be taken from the dependent claims. The dither suspension mechanism of the present invention utilizes a pair of flexure plates including an outer toroidal rim and an inner hub coupled together by webs acting as springs. A housing forms, in part, the dither suspension mechanism. The housing has an annular aperture sufficiently sized to receive the combination of a laser block ridgedly secured together between the inner hubs of the flexure plates. The sandwich of the two flexure plates and laser block is inserted into the housing aperture. The outer toroidal rims of the flexure plates are ridgedly secured to the annular inner wall of the aperture in the housing. With respect to the figures of the attached drawing, the inventive dither suspension mechanism shall be further described, where Figure 1 is an exploded view showing a ring laser angular rate sensor and the dither suspension mechanism. Figure 2 is a cross section of a flexure plate. Figure 3 is a cross section of a dither suspension mechanism and laser sensor assembly. Figure 4 is a cross section of another dither suspension mechanism and laser block assembly. Figure 5 shows an alternate flexure plate construction. Referring now to the drawings, Figure 1 shows a ring laser angular rate sensor 10 including a laser block 12. An example of a laser sensor and laser block are shown in U.s. Patent 3,390,606 and U.S. Patent 3,467,472. Laser block 12 is illustrated as having a triangular optical closed-loop path including mirrors 13, 14, and 15, and electrodes 16, 17, and 18. Also shown in Figure 1 are flexure plates 20 and 30, a housing 40, a cover 50, header plate 60, threaded stud 65 and hex nut 66. Flexure plates 20 and 30 each include a toroidal rim 22, a central member 24, and three springs 26. Spring 26 are shown as webs. The central member acts as a hub. As illustrated in Figure 2, the webs are contoured so as to be somewhat recessed from the outer surfaces 27 and 28 of rim 22 and central member 24. Figures 1 and 2 show both the central member and the rim being of equal width, but may be contoured or shaped otherwise to reduce cost and weight, and perhaps the spring constant of the system (i.e. the resonant frequency). The flexure plates are preferably of unitary construction using a suitable material such as metal. Laser block 12 includes an aperture through the block in parallel with the sensor axis. Further, flexure plates each include an aperture concentric with its central axis. The apertures of flexure plates 20 and 30, and laser block 12 allow a threaded stud 65 to pass therethrough. Housing 40 is preferably constructed from a material suitable for being welded to flexure plates 20 and 30. Housing 40 includes an annular aperture 42 being bounded by an inner wall 43 of curvature radius slightly greater than the outer portion of the toroidal rim 22 of flexure plates 20 and 30. In the preferred embodiment of the invention, the height of the housing is substantially equal to the sum of the widths of the two toroidal rims of flexure plates 20 and 30 and the width of block 12 as diagramatically illustrated in the simplified cross-section drawing of Figure 3. Housing 40 also includes a toroidal shaped flange 41 for mounting in an inertial sensor package. In Figure 3 stud 65 in combination with hex nut 66 ridgedly secures together block 12 between the hubs or central members 24 of each of the flexure plates. The external surface of toroidal rims 22 of flexure plates 20 and 30 are ridgedly secured to the annular inner wall of housing 40 by welding, for example by laser welding, and the like. Secured to the top and bottom part of the housing are cover 50 and header plate 60. Cover 50 and header plate 60 may be secured to the housing 40 by a variety of techniques including welding the parts together, threading the parts together, and the like. Header plate 60 includes electrical pins for connections to the gas electrodes and the laser sensor readout assembly of laser 10 (not shown). As illustrated in Figures 1 and 3, flexure plate 30 includes piezoelectric devices 70 attached to springs 26 for electro mechanically flexing the springs 26 to cause flexing in a circumferential direction so as to be capable of oscillating the gyro block 12 relative to the housing 40. Application of piezoelectric devices attached to springs 26 for obtaining the intended function is well known in the art. An alternate arrangement within the scope of the present invention is illustrated in Figure 4. In Figure 4, the toroidal rims of flexure plates 20 and 30 are welded to the top and bottom surfaces of housing 40. As before, the arrangement illustrated in Figure 4 provides a dither suspension mechanism to obtain the intended function of oscillating the block relative to the housing. While the arrangement shown in Figures 1, 2 and 3 contemplates welding the outer toroidal rim to the housing, it should be understood that use of threaded members could accomplish the same intended function of ridgedly securing the toroidal rims to the housing. Specifically, aperture 42 of housing 40 could be threaded to receive a threaded toroidal rim internally. Alternatively, the toroidal rim could have a threaded extension for being threaded on the external surface of the housing. Shown in Figure 5 is an alternate arrangement for either of flexure plates 20 and 30. In Figure 5, flexure plate 200 is essentially that of flexure plate 20 in Figure 1 except for the elimination of the toroidal rim. Flexure plate 200 includes a central member 224 and springs 226 having first and second ends 230 and 231. The first ends of each of the springs 226 extend from central member 224 acting as spokes to a hub. Preferably, the second ends 231 radially extend away from central member 224 have an outside radius of curvature so as to accomodate the annular inner wall radius of housing 40 to allow a welding operation between the second ends of the springs and the housing. Flexure plate 200 shown in Figure 5 may likewise be used for flexure plate 30 in which piezoelectric devices 70 are mounted in a similar manner as shown in Figure 1. Advantages of flexure plate 200 include lower overall weight and additional mechanical mounting opportunities. For example, housing 40 could be slotted to match the radial extensions of springs 226 to aid in the assembly operation. Although the preferred embodiment illustrated in Figure 1 shows two flexure plates securing together laser block 12, only flexure plate 30 is required. That is, only a single flexure plate having the piezoelectric devices bending the springs to cause motion in a circumferential direction is necessary in performing the intended function. It should be understood that the invention is not limited to the exact details of construction shown and described herein for obvious modifications will occur to persons skilled in the art, and particularly those skilled in the art of mechanical assembly. Although the embodiments of the invention illustrated in the accompanying figures show a triangular laser block, the invention is not limited thereto. Laser block 12 merely serves as a driven member and the dither suspension mechanism can be used to transmit circumferential motion to the driven member. Specifically in the art of laser sensors, the driven member could also be a rectangular laser sensor as well as a cube laser sensor (three laser sensors in one block), and the like.

Claims: Dither suspension mechanism for a laser angular rate sensor comprising a laser block (10,12) driven about an axis of said dither suspension mechanism, a first flexure plate (20) having a central axis, said first flexure plate having, a central member (24), and a plurality of springs (26) having first and second ends, each of said first ends extending from said central member, each of said springs being capable of flexing in a circumferential direction to allow limited relative motion between said central member and said second ends; a second flexure plate (30) having a central axis, said second flexure plate having, a central member (24), and a plurality of springs (26) having first and second ends, each of said first ends extending from said second flexure plate central member, each of said springs of said second flexure plate being capable of flexing in a circumferential direction to allow limited relative motion between said central member and said second ends; characterized by : a housing (40) having an aperture (42) for receiving said laser block (10,12); means (65,66) for rigidly securing together said laser block (10,12) and said central member (24) of said first and second flexure plate (20,30); and means for rigidly securing said second ends of said springs (26) to said housing (40). Mechanism according to claim 1, characterized in that said housing aperture (42) is bounded by an annular inner wall (43) about a central axis therethrough, said aperture (42) capable of receiving said laser block (10,12) and said first and second flexure plate (20,30), and each of said second ends of said springs (26) are rigidly secured to said annular inner wall (43). Mechanism according to claim 1, characterized in that said laser block (10,12) includes an aperture in parallel with the input axis of said sensor, and an aperture in said first and second flexure plate (20,30) for receiving a stud (65) therethrough. Mechanism according to claim 1 with said first and second flexure plate (20,30) comprising an outer toroidal rim (22) concentric with its central axis; and said plurality of springs (26) extending from said outer toroidal rim (22) and said central member (24); characterized by means for rigidly securing said toroidal rim (22) of said first and second flexure plate (20,30) to said housing (40). Mechanism according to claim 4, characterized in that said housing aperture (42) defines an annular inner wall (43) about a central axis therethrough, said aperture capable of receiving said laser block (12) and said first flexure plate (20,30), and said toroidal rim (22) of said first and second flexure plate is rigidly secured to said annular inner wall (43). Mechanism according to one of claims 1 to 5, characterized in that to the top and bottom part of said housing (40) a cover (50) and a header plate (60) are secured.

Inventors: HANSE, JOEL G., KILLPATRICK, JOSEPH E.
Applicants: HONEYWELL INC.
Application year: 1986
Main classification: G01C19
IPC codes: H01S3, G01P9, G01C19