Dither apparatus

Abstract: A dither motor (1) for dithering a plurality of inertial sensors (5) includes an outer support ring (40), a plurality of reeds (24) and a plurality of mounting supports (26). The outer support ring may be generally circular or polygonal in shape. Each reed of the plurality of reeds has a hub end and an outer end wherein the hub ends are connected to a common hub (22) and the ring ends are connected to the outer support ring (40). The plurality of reeds are arranged to be disposed at equal angles from each other and extend radially outwardly from the hub. The plurality of mounting supports (26) extend outwardly from the hub (22) and each mounting support is located between two of the plurality of reeds (24). The mounting supports are also equally spaced apart. Further, each reed has a means (20A - 20F)for oscillating connected thereto such as piezoelectric elements (Fig. 1).

Description: Field Of The Invention This invention relates generally to a dither apparatus according to the preamble of claim 1 and particularly to ring laser gyroscope rotation sensors. Still more particularly, this invention relates to an apparatus for dithering a plurality of ring laser gyroscopes using a common dither motor to reduce lock-in effects. BACKGROUND OF THE INVENTION Ring laser angular rate sensors are well known. One example of a ring laser angular rate sensor is US-A-4,751,718. Present day ring laser angular rate sensors, commonly known as ring laser gyros, include a thermally and mechanically stable laser block having a plurality of formed cavities for enclosing a gap. Mirrors are placed at the extremities of the cavities for reflecting laser beams and providing an optical close- looped 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 a sensor. Conventionally, a piezoelectric actuator is connected to a suspension system which causes the block of the sensor to oscillate angularly at the natural mechanical resident frequency of the suspension system. This 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 effects. Dither suspension mechanisms in the prior art are typically directed toward dithering a single ring laser gyro with a single dedicated dither motor imbedded in the hub of each ring laser gyro. Examples of such systems are US-A-4,653,918, and US-A-4,349,183 as well as the above-referenced US-A-4 751 718. Dither motors for dithering multiple ring laser gyro beams within an integral ring laser gyro block are also known. For example, US-A-3,503,688, entitled "Multiple Axis Laser Angular Rate Sensor", discloses a solid block support apparatus which supports three sets of counter rotating laser beams. A shaft and block are vibrated or dithered about the axis of the shaft by a single mechanical dithering means. Components of the dither are presented to each triangular path and all three of the triangles are simultaneously dithered. US-A-4,856,901, entitled "Velocity Control System Using Piezoelectric Transducers", discloses a design strikingly similar to the dither concept used in the '688 patent. Three ring laser gyro paths integrated into a solid block are mechanically dithered by a single dither mechanism through a cube diagonal. Other art provides various dither schemes. US-A-4,801,206, entitled "Simplified Ring Laser Gyroscope Dither Control and Method", is also directed to a dither mechanism. A dither flexor is mounted between a ring laser gyroscope body and a support such that the ring laser gyroscope may be dithered to have angular oscillations about a sensing axis. US-A-4,711,575, entitled "Non-pendulous Counter-Balance Dither Mechanism For Laser Gyro", discloses a laser gyro dither mechanism utilizing a three spring suspension system. US-A-4,710,027, entitled "Method and Apparatus for Mechanical Dither Stabilization Of A Laser Angular Sensor", discloses an apparatus whereby one, two, three or four dither mechanisms selectively may be simultaneously energized. The '027 patent is directed to individual dithering means connected to each of at least three ring laser gyros which are selectively energized. US-A-4,653,918, entitled "Low Q Body- Dithered Laser Gyro Assembly", discloses a low Q ring laser gyro dither motor assembly. US-A-4,597,667, entitled "Dither Controller For Ring Laser Angular Rotation Sensor", discloses a dithering apparatus which is inserted into the hub of a ring laser gyro. The mechanism includes a pickoff sensor to measure parameters relating the sensor body position to a support means. US-A-4,349,183, entitled "Spring For A Ring Laser Gyro Dither Mechanism", discloses a flexure spring assembly for a ring laser gyro dither mechanism using flexure springs between the hub and the rim. Each spring is driven by four piezoelectric crystal wafers. US-A-4,309,107, entitled "Laser Gyro Dither Mechanism", discloses a laser gyro dither mechanism utilizing a three-spring suspension system. US-A-4 534 648 discloses a spring suspension mount for a gyro sensor block comprising a set of common gyros. US-A-4 425 040 discloses a ring laser gyroscope cluster comprised of an outer support and an inner element with reeds extending between the outer support and the inner member. This prior art device uses a stable inner element and an oscillating outer support to which the gyroscopes are mounted. Electromechanical force transducers act together with arms extending from the support through openings within the stable element. Below the whole structure and within a bottom case a vibration damper is arranged. Departing from those prior art devices, it is the object of the present invention to devise a dither motor for a plurality of discrete ring laser gyros in a small package volume. This object is achieved by the characterizing features of claim 1. Further advantageous embodiments of the inventive dither motor and its use may be taken from the dependent claims. The cluster dither mounting apparatus provided by the instant invention offers significant advantages over the prior art schemes. One advantage is that the invention allows the fabrication of inertial measurement units in a package volume which is smaller than the packaging volume generally available with prior art schemes. Further, there is only a single dither motor in the system hardware and software elements are simplified. As a result, the cost of such units built in accordance with the present invention is low. For example, the packaging volume in one application of the invention fits a 9,4 cm (3.7 inch) diameter x 5,1 cm (2 inch) length. SUMMARY OF THE INVENTION The present invention provides a dither motor for dithering a plurality of inertial sensors. The dither motor includes an outer support ring, a plurality of reeds and a plurality of mounting supports. The outer support ring may be circular or polygonal in shape. Each reed of the plurality of reeds has a hub end and an outer end wherein the hub ends are connected to a common hub and the ring ends are connected to the outer support ring. The plurality of reeds are advantageously arranged to be disposed at equal angles from each other and extend radially outwardly from the hub. The plurality of mounting supports extend outwardly from the hub and each mounting support is located between two of the plurality of reeds. The mounting supports are also advantageously equally spaced apart. Further, each reed has a means for oscillating connected thereto. In one aspect of the invention, the outer support ring comprises a substantially triangular shape. In yet another aspect of the invention the plurality of reeds are tapered such that the hub end has a first height and the outer end has a second lesser height. In one particular embodiment of the invention, the first height is greater than the second height. In yet another aspect of the invention, the dither motor as provided by the invention has at least three radial reeds. In yet another aspect of the invention, the means for oscillating are piezoelectric elements affixed to each of the plurality of reeds. Other objects, features and advantages of the present invention will become apparent to those skilled in the art through the Description of the Preferred Embodiment, Claims, and drawings herein wherein like numerals refer to like elements. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 schematically shows an exploded view of an inertial reference apparatus including the cluster dither motor as provided by one aspect of the invention. Figure 2 shows, in detail, one alternate embodiment of the cluster dither motor of the invention including a plurality of ring laser gyros attached thereto. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Figure 1 is an exploded view of an inertial measurement apparatus including the cluster dither motor of the invention. The inertial measurement apparatus 10 includes a cover 50, printed circuit boards 52, cluster dither motor apparatus 1, and housing 54. The housing and cover may comprise a substantially polygonal shape as, for example, a substantially triangular shape or a substantially square shape. Alternatively, the housing and cover, together with the other elements may be fabricated in a circular or similarly rounded shape, such as an ellipsoid. The housing 54 and cover 50 may typically be comprised of metal. The printed circuit boards 52 may advantageously include circuit elements well known in the art for operating an inertial measurement unit. The cluster dither motor 1 comprises an outer support ring 40, a plurality of springs or reeds 24, a hub 22 and a plurality of mounting supports 26. Fastened to each of the plurality of reeds 24 by well known means, such as epoxy or glue or equivalent fastening mechanisms, are means for oscillating 20. The means for oscillating 20 may be, for example, piezoelectric elements 20A - 20F. The piezoelectric element 20A, for example, is poled such that application of a driving signal across it causes the piezoelectric element to selectively expand or contract. The piezoelectric element 20B is also poled and has a pair of opposing electrodes (not shown) connected thereto. The piezoelectric elements 20A and 20B may have opposite polarities so that application of the same driving signal thereto causes one element, for example element 20A, to expand while the other element 20B contracts. If the elements 20A and 20B have the same polarities, then the driving voltages must have opposite polarities in order to achieve the desired effect of alternately expanding and contracting. The piezoelectric elements 20C, 20D, 20E and 20F have polarities and driving voltages substantially identical to the piezoelectric elements 20A and 20B, respectively. Therefore, if the piezoelectric elements 20A, 20C and 20E contract while the piezoelectric elements 20B, 20D and 20F expand, reeds 24 will deform so that the dither motor 1 rotates about the hub 22. A change in sign of the driving signal causes an opposite direction rotation of the dither motor. Shown attached to each of the plurality of the mounting means 26 are a plurality of ring laser gyros 5. The ring laser gyros 5 may be of the type as described in US-A-4,751,718. The ring laser gyros are advantageously polygonal in shape and may typically have square or triangular blocks. Referring now to Figure 2, an alternate embodiment of the invention is shown having a generally circular shape. It will be understood by those skilled in the art that the cluster dither motor need not be a circular shape in order to function. The cluster dither motor 101 of the invention shown in Figure 2 comprises an outer support ring 240, a hub section 222, supports 226, oscillating means 20 and a plurality of radially extending reeds 224. In one aspect of the invention employing at last three ring laser gyros 5, the plurality of support reeds 224 may advantageously be three equally spaced reeds. Each of the reeds have a hub end and an outer support end. The reeds are attached at their hub ends at the hub 222 and are equally spaced and project radially outward to the outer support ring 240 where they are attached at their outer ends at junctions which advantageously include mounting holes 230. The reeds are tapered radially so as to have a first height at the hub end and a second, lesser height at the junction with the support ring. The ratio of the first height to the second height depends upon the amount of thrust to be experienced along a thrust axis 300 which runs perpendicularly through the center of the hub section 222. In one example of the invention designed for a thrust of 100,000 g's along the thrust axis, the first height was advantageously made approximately 5 times greater than the second height. Those skilled in the art with the benefit of the instant disclosure will appreciate that a typical ratio of the first to second end is advantageously at least 2:1. In operation, electrical leads (not shown) are attached from the ring laser gyro electronics 60 and printed circuit boards 52 to the piezoelectric elements 20 to cause the plurality of reeds to oscillate or dither. The dithering motion is then passed through the supports 226 onto the ring laser gyros 5, thereby preventing the ring laser gyro "lock-in" condition.

Claims: A dither motor (1) for dithering a plurality of inertial sensors (5), comprising (a) an outer support ring (40); (b) a plurality of reeds (24), each reed having a hub end and an outer end wherein the hub ends are connected to a common hub (22) and the outer ends are connected to the outer support ring (40) wherein the plurality of reeds are arranged to be disposed at equal angles from each other extending radially outwardly from the hub and wherein each reed has a means (20A - 20F) for oscillating connected thereto; and (c) a plurality of mounting supports (26) extending outwardly from the hub (22) with each mounting support being located between two reeds, whereat each one of said plurality of inertial sensors being mounted to each one of said plurality of mounting supports (26). Dither motor according to claim 1, characterized in that the outer support ring (40) has a substantially polygonal shape. Dither motor according to claim 2, characterized in that the hub end has a first height and the ring end has a second height, the plurality of reeds (24) are tapered such that the first height is greater than the second height. Dither motor according to claim 3, characterized in that the plurality of reeds (24) comprises at least three reeds. Dither motor according to claim 1, characterized in that the means for oscillating comprises a piezoelectric element (20A - 20F). Dither motor according to claim 1, characterized in that the outer support ring (40) has a substantially circular shape. Dither motor according to one of claims 1 to 6, characterized by its use in an inertial measurement apparatus, including a plurality of discrete ring laser gyros (5) as said inertial sensors, with said dither motor comprising means for simultaneously dithering each of the plurality of discrete ring laser gyros.

Inventors: HANSE JOEL G, HANSE, JOEL G.
Applicants: HONEYWELL INC, HONEYWELL INC.
Application year: 1992
Main classification: G01C19
IPC codes: G01C19, H01S3