METHOD AND APPARATUS FOR MECHANICAL DITHER STABILIZATION OF A LASER ANGULAR SENSOR

Abstract: Three sets of laser angular sensors (14, 15, 16) corresponding to three angularly disposed sensing planes (11, 12, 13) are mounted on a common instrument block (10) and each provided with mechanical dithering devices (14a, 15a, 16a) for inducing an oscillatory mechanical motion to substantially cancel lock-in. A compensating mass (21) is mounted on the same instrument block (10) and indicates a selective controllable mechanical dithering device (21a). Dithering the compensating mass at an appropriately selected amplitude, frequency, and phase automatically compensates for dither interaction (coning) between the three sensors (14, 15, 16).

Description: METHOD AND APPARATUS FOR MECHANICAL DITHER STABILIZATION OF A LASER ANGULAR SENSOR The present invention relates generally to a laser angular sensor and, more particularly, to an apparatus and method of stabilizing certain adverse effects of mechanical dither used in such a sensor to prevent laser beam lock-in. This is a Continuation of United States Patent Application Serial No. 634,324, filed July 25, 1984 which was a Continuation-in-Part of Patent Application Serial No. 574,845, filed January 30, 1984. BACKGROUND A laser angular sensor consists primarily of first and second laser beams which move in opposite directions about a closed loop path enclosing the rotational axis with respect to which the sensor is to detect and determine angular change. Rotation of the apparatus about the axis changes the effective path length that each beam must travel, which, in turn, can be detected as a frequency difference between the two laser beams. The direction and amount of frequency change detected are indicative of the direction of physical rotation being sensed and the rate of this rotation, respectively. At low rates of rotation, the difference in beam frequency detected loses its proportionality to the rotational rate due to a phenomenon frequently referred to as "lock-in" in which the ' two laser beams exhibit a single frequency. One approach to solving the problem of lock-in that is commonly employed is to effect mechanical oscillation ("dither") of the laser sensor apparatus which, when added to the actual rate of change in rotation experienced by the laser beams, raise the frequency change detected above the critical lock-in limit. In practical application for navigational purposes of an aircraft, for example, three laser angular sensors are arranged with their sensitive axes orthogonally to each other and mounted on a common platform or instrument block. By sensing the rotational, change and rate of change in each of the three orthogonally related planes, angular movement of the vehicle or change in orientation of the vehicle from an initial direction can be continuously determined. To avoid lock-in each set of laser angular sensors has its own dithering apparatus. The three sensors are particularly effective when mounted on a common instrument block, however, when this is done, mechanical coupling of dithering motions between the laser sensors is experienced. This coupling of dither motion is frequently referred to as "coning" and can produce serious readout errors. Attempts have been made to solve the coning problem in the past by circuit processing techniques in which pick-off signals from the different laser angular sensors are modified in an attempt to eliminate coning error contributions; however, these attempts have not been completely satisfactory. One such attempt is disclosed in U. S. Patent No. 4,277,173 which relates to a technique for elimination frequency differences and phases differences in the three dither motions. The technique disclosed therein does not address the broader problems, which are: Cross dithering problems; the effects of coning; uneven dithering motions, and the possibility that a single ring laser gyroscope will cause dithering due to cross dithering effects. SUMMARY OF THE DISCLOSURE In the preferred embodiment of the present invention, four dithering bodies are flexibly mounted to a single instrument block to permit oscillatory mechanical motion in each of the bodies. Three of the dithering bodies are ring laser angular rotation sensors which are flexibly mounted with mutually orthogonally sensitive axes and each having a dither motion about its sensitive axis. The fourth dithering body may be either a ring laser angular rotation sensor or a compensating mass. In either case the fourth dithering body has an axis of dither rotation which is oriented, generally yet not necessarily, at equal angles with respect to the sensitive axes of the three angular rotation sensors to satisfy conditions of dynamic equilibrium with the other three angular rotation sensors thus relieving the instrument block from moment actions and the resulting dynamic rotational motion. Preferably the masses of- the four dithering bodies and the dynamic equilibrium is achieved by designing the four dithering bodies to have substantially the same mass and by mounting these bodies on a dither mechanism so that they may be driven in an oscillatory dither motion which has substantially equal frequency, phases and selected amplitudes. A substantially equal frequency of oscillatory motion is achieved by designing the four dither mechanisms to have substantially the same natural resonant frequency and by driving these dither mechanisms at this frequency. Substantially the same phase in the motion of the dithering bodies is- achieved by using a single power source such as a sine wave oscillator and by driving the dither mechanisms-in substantially the same phase. Finally, dynamic equilibrium is achieved by selecting the amplitudes of the dither drive signals individually to produce amplitudes of oscillatory motion by their respective dithering bodies so that the vector sum of the angular moments of the four dithering bodies is substantially zero. This dynamic equilibrium is maintained over a period of time by continuing to drive the dither mechanisms with signals having substantially equal frequency, substantially equal phase and the selected amplitudes, Dithered ring laser angular rotation sensors are disclosed in U.S. Patents Numbers 3,373,650 and 3,467,472. One type of known dithering mechanism comprises a flexure and piezo electric member mounted on the flexure. Each sensor or each sensor and compensating mass is mounted on such a dithering mechanism to subject it to a dither motion. The preferred embodiment is but one special case of solutions to the problem of coning of trials of ring laser angular rotation sensors that are within the scope of the invention. A generic teaching is also set forth below. DESCRIPTION OF THE DRAWING Figure 1 is a side elevational view of an instrument block showing three laser angular sensors being stabilized in accordance with the described method and apparatus. Figure 2 is a top plan view of the instrument block of Figure 1. Figure 3 is a schematic representation depicting pertinent motions and moments acting on the apparatus of Figure 1. Figure 4 is a functional block diagram of the compensating mass dithering control. DESCRIPTION OF A PREFERRED EMBODIMENT With reference now to the drawings, and particularly Figures 1 and 2, a one-piece instrument block 10 is seen to include three planar surfaces 11 - 13 which are flat surfaces precisely arranged to be mutually orthogonal to one another and serve as mounting surfaces for three individual laser angular sensors 14 - 16, respectively, shown in schematic form only. The instrument block 10 has by way of example, three feet, isolators or pedestals 17 - 19 for securing the instrument block, and ring laser angular sensors mounted thereon, to the aircraft or other vehicle whose movement is to be monitored. That is, the three angular sensors are all mounted on a common base or instrument block 10 with each sensing angular movements about an axis which is orthogonal to the plane of the surface to which it is mounted. It is known that by such a combination of three angular sensors, a vehicle carrying the instrument block and sensors mounted thereon will be capable of operating in three dimensions with any movement of the vehicle made in any of the three dimensions being detected by one or more of the sensors. In conventional manner, each of the ring laser sensors 14 - 16 includes a pair of laser beams moving in respectively opposite directions along a closed path about the sensing axis and means for determing change in frequency of the laser beams in response to angular motions experienced by the sensor about its sensitive axis. Additionally, as a preliminary requirement for application of the described method and apparatus each of the laser sensors incorporates dither means 14a, 15a, 16a, respec¬ tively, for producing an oscillatory mechanical movement ("dither") of the laser beam generating means and associated sensing apparatus mounted on the instrument block in a direction along the path of movement of the lasing beams to prevent or minimize lock-in of the light beams at low rates of rotation. Each sensor is dithered to prevent one problem, i.e., lock-in but dithering of three sensors creates another problem, coning by mechanical coupling of each dither motion to the other two sensors. Any such coupled dither which is sensed by another sensors result in an error in the output signal of that sensor. .The angular sensors with mechanical dithering means discussed to this point are of conventional construction and operation, and, therefore, will not be described in detail. A further dither axis Xc is shown on the instrument block which for simplicity is depicted in solid lines. The coning compensation means 21 having an axis of rotation X includes a compensating mass 21 and means 21a for dithering mass 21 about an axis orthogonal to surface 20 and equiangular with respect to all three sensitive axes of the three sensors. As will be set forth in the . description that follows, the angular momentum of the compensating means when resolved into components along each of the sensors axes will balance the dithering angular momenta so that the vector sum of all four angular momenta and the vector sum of the moments are both substantially zero. For the ensuing analysis reference is made to the schematic representation of Figure 3. The three dithered laser angular sensors are depicted as cylindrical masses having respective dithering moments M, , M_ and M_ of nearly equal phase and amplitude; the corresponding dithering rotation angles being α. , α and ~ _. Similarly, the compensating mass moment is M and the corresponding dithering angle of rotation is ~ . Assuming only small angles of rotation are involved, the dynamic equations of motion in Cartesian tensorial notation for the instrument block rotation, angular sensors and compensating mass in a dithering mode, are as follows: (I - Iδ i: ID J " I c n i n j ) »j + K ( θi - a,) +K(θ j n j - « c )n i K ι. . ή B : . = M. M n. c 1 (1) Iα 1. + K(σ 1. - Θ l.)' a -M 1. (2) ! c β c + K c ( «c - θ j Ω j) - M c (3) Where, θ. = rotation angles of instrument blocks about x. where i equals 1, 2, 3. a. = rotation angles of three orthogonal angular sensors. a = rotation angles of compensating mass. M. = dither active moment along ith axis M = compensating active moment of compensating dither mass. I.. = moment of inertia of the entire assembly including the sensors, the compensating mass, and the instrument block with respect to C. G. I = angular sensor moment of inertia about dither axis, I = moment of inertia of compensating mass. n. = compensating mass axis cosine angles with respect to the orthogonol dithering axes. K.. = angular stiffness coefficients of the support systems about the instrument block center of gravity. K = stiffness of a single dither mechanism. K = stiffness of the compensating dither mechanism. where all angles are measured with respect to mount fixed coordinates. Inertia is with respect to the center of gravity of each respective item. With the further assumption that the elastic center of the supports coincide with the center of gravity, and in that way securing separation of rotation from translation, on setting equation (1) equal to zero, yields: (Iπ - iδπ - I n . n . ) θ • + K ( θi - a 1. ) + c ( θj n. - a c ) n i + K. jθj =0 (4) Multiplying equation ( 3 ) by n . and adding to ( 2) , results in. . • * l a l. + I c ac ni. ' + K (aι. - Θ σ ι- )' + K c ( ac -θτ. nj. ) nι. = 0 ( 5 ) K K c In case of ——- = -^=— , equations (4) and (5) are c a set of six homogeneous equations for the six unknown θ- and (Iα. + I a n. ) . The steady state solution of these equations is, θ . = 0 , I ce . + = 0 ( 6 ) 1 I a n . C C 1 or θ . = ø I β. = 1 I c a c n 1. (7) This last relation also yields l al . + ιcecn1. = 0 Namely, the vectorial sum of all the angular momentums of the dithers is substantially zero. To summarize the analysis, the following correspondence is set forth: K c K When -■=— = —=— , upon setting M. + M n . j = ~ r c then θ« = 0 (which means there is no motion of the blocks) and l ά. + 1 ό n. = 0, and, vice versa, upon setting ' 1 c c 1 l ά 1- + IC άCn1. = 0, it follows that θ1. = 0 and M1. + MCN1. = 0. (The vector sum of the moments is substantially equal to zero.) This reverse relationship can be demonstrated following the same deviation shown above. The condition determines that the four dither mechanisms have substantially the same natural frequency relative to a stationary instrument block. This result demonstrates the elimination of coning and errors resulting therefrom. The moment of the fourth dither is given by M c = V (M χ ) 2 +(M 2 ) 2 + (M 3 ) 2 . Where the orientation of the fourth dither axis relative to the three other dither axes is defined to y the angle cosines Based on the foregoing analysis, it is clear that the selectively adjustable means 22 for dithering the compensating mass can be adjusted to a prescribed amplitude, frequency and phase of dither that will compensate for mechanical coupling of the dithering of laser sensors 14 - 16 and, in that way, substantially reduce, if not eliminate, coning errors. Although as an initial simplifying condition in derivation of the dynamic equations, it was assumed that the mounting plane 20 for the compensating mass intersects the sensor mounting planes 11 - 13 at the same angle, it is emphasized that this is not necessary and the angles of intersection with the different planes can differ. In other words the angles between the axis of rotation of the compensating mass and each of the sensitive axes need not be equal. This feataure is presented in Figure 1 by showing the compensating plane in dashed line which is skewed to the original plane 20 and, therefore, clearly intersects the respective planes 11 - 13 in angles that are substantially different. It is possible to vary the compensating mass, the compensating mass dithering amplitude, frequency or phase, or combinations thereof. However, the simplest and most direct construction would be to make the compensating mass identical to that of a sensor, and to adjust the compensating mass dither amplitude: its frequency and phase to be substantially identical to those of the three ring laser gyros. Figure 4 illustrates in function block form a generalized circuit schematic for separately adjusting the frequency, amplitude and phase of the compensating mass dithering means. In the first described embodiment the instrument block included three mutually perpendicular planar surfaces 11 - 13 on which the laser angular sensors are mounted with the mounting surface 20 for the compensating mass intercepting each of the surfaces 11 - 13 at the same angle. In a second described version the mounting surface 20 intercepted the surfaces 11 - 13 at different angles. It is further contemplated that the surfaces 11 - 13 may be angularly arranged other than mutually perpendicular which is depicted in Figure 1 by the dashed line surfaces 11' and 12'. The analytical basis for this further embodiment will now be presented. The case where the three laser gyros are in planes 11', 12' and 13 which are not mutually perpendicular will.now be considered. This case is best described in a affine system of coordinates with axes of rotation of the compensating mass and of the sensors which are each normal to a .respective dithering plane. In an affine system of coordinates, subscripts and superscripts will identify covariant and contravariant compon¬ ents, correspondingly. Using the same notation as in the previosuly described embodiments, we can write, (Ij -δl j. - Icn 1 n ~ .) Θ D + K(θ 1 - a ~ ) + K c(θ-'n ~ . - a c )' n x + K " ^ θ 1 = M 1 + M n 1 J ~ .' 1 a + K(σ - θ 1 ) = M J Icά'c + KC(αrC - θ J n_.)) = -M c Where the inertia and stiffness mixed tensors are defined as follows: l δ* - y . y dm ; K = ∑ n (r 2 δ - y is measured along the offine axes y. is derived from the relation y. = g ..y-^ - i IT where g.. are the covariant metric tensor components. The equations of motion in the oblique case (i.e., 11 - 13 not mutually perpendicular) , when using affine coordinates and written in covariant components are algebraically identical to the orthogonal case [equations (1) , (2) and (3)] . The dynamic coning compensation which was shown to apply in the case of mutually perpendicular ring laser gyros can then be inferred in the oblique case as well. More specifically, the vector sum of the previously defined dither moments is substantially equal to zero, the vector sum of the previously deferred angular momentums is zero, and all four dither mechanisms have substantially the same natural frequency relative to a stationary instrument block. The coning compensation means 21 comprises in its most general aspect a mass of indiscriminate character and means for dithering the mass in the manner described earlier herein. An especially advantageous embodiment of the compensation means 21 is achieved by mounting a redundant fourth laser sensor and dithering means on the flat surface 20. Not only would such a construction operate satisfactorily for purposes of the described invention, but also the redundant laser sensor can be readily and advantageously utilized as an active laser sensor in the event of, say, failure of one of the other three laser sensors 14 - 16. In Figure 4 there is shown a functional block diagram of one circuit for generating electrical signals for driving dither mechanisms 14a, 15a, 16a, and 21a into oscillatory motion. AC power source 22 comprises an oscillator circuit which generates a signal having a substantially sine waveform. Preferably the frequency of the signal generated by source 22 is approximately equal to each of the natural resonant frequencies of dither mechanisms 14a, 15a,- 16a and 21a. Amplifiers 24a, b, c and d each amplify this signal and apply it to dither mechanisms 14a, 15a, 16a, and 21a. Preferably amplifiers 24a, b, c and d are identical so that the frequency and phase of the sine wave signal from source 22 is not altered. Of course the gain of each amplifier can be separately set to prescribe a particular amplitude in the output signal from each and hence prescribe the amplitude of the dither motion of each dither mechanism and respective mass M, , 2 , M, and M to achieve cancellation of coning effects as taught above.

Claims: WHAT IS CLAIMED IS: 1. 1. A method for minimizing dither interaction of 2. two or more ring laser angle sensors (14, 15, 16) being 3. mechanically dithered, respectively, about their sensing 4. axes (X, , X-,, X- , said sensors being mechanically mounted 5. together for sensing angular motions of said sensors, which 6. comprises the steps of: 7. attaching a compensating mass onto said sensors 8. with angular freedom about an axis fixed to said sensors and 9. non-colinear with said sensing axes; and 10. dithering said compensating mass at a prescribed 11. frequency, amplitude and phase about its axis of angular 12. freedom to minimize the net rotational angular momentum of 13. said sensors produced by said mechanical dithering of the 14. individual said ring laser angle sensors. 1. 2. A method as in claim 1 wherein at least one of 2. the ring laser angle sensors each further includes a base 3. with respect to which it is dithered and wherein said bases 4. are mounted together to form an instrument block (10). 1. 3. A method as in claim 1 wherein said ring laser 2. angle sensors and said compensating mass are mounted on and 3. dithered with respect to a common mounting block. 1. 4. A method as in claim 1, in which said axes of 2. said sensors are orthogonal, and the angular momentum of said 3. compensating mass is equal to and directed opposite to the 4. vector sum of the angular momentums of dithering of said 5. sensors. 1. 5. In the combination of multiple ring laser angle 2. sensors (14, 15, 16) mounted together to form an instrument 3. block (10) for sensing angular motions of said block, 4. including means (14a, 15a, 16a, 24b, 24c, 24d, 22) for 5. mechanically dithering each sensor about its sensing axis 6. (X- j , X 2 , X,) at a prescribed frequency, amplitude and phase, 7. the improvement comprising: 8. a compensating mass mounted on said instrument 9. block for limited angular dithering about an axis (X ) which 10. has nonzero direction cosines relative to said sensing 11. axes; and 12. means for mechanically dithering said compensating 13. mass about its axis of rotation at a prescribed frequency, 14. amplitude, and phase to minimize rotational motions of said 15. instrument block resulting from dithering said ring laser 16. angle sensors about their said sensing axes. 1. 6. The device as claimed in claim 5 wherein said 2. more ring laser angle sensors each further includes a base 3. with respect to which it is dithered and wherein said bases 4. are mounted together to form said instrument block. 1. 7. The' evice as claimed in claim 5 wherein said 2. ring laser angle sensors are mounted on and dithered with 3. respect to a common mounting block. 1. 8. The device as claimed in claim 5 wherein the 2. amplitudes of the dither angle of each said sensor and of 3. said compensating mass are each individually prescribed to 4. minimize rotational motions of the instrument block. 1. 9. The device as claimed in claim 8, having three 2. sets of ring laser angle sensors which are substantially 3. identical and which are mounted with mutually orthogonal 4. sensing axes for sensing rotations about said sensing axes, 5. and said compensating mass is mounted with its axis of 6. rotation at a prescribed angle to each of said sensing axes. 1. 10. The device as claimed in claim 9 wherein said 2. compensating mass has a mass substantially equal to each of 3. said sensors and is mounted on said block with its axis of 4. rotation equiangularly located from said sensing axes. 1. 11. The device as claimed in claim 5, in which each 2. of the angular sensors dithering means oscillate at 3. substantially the same frequency, amplitude, and phase; 4. in which the dithering of the compensating mass is 5. accomplished at substantially the same frequency and phase as 6. that of each angular sensor dithering means; and 7. in which the dithering amplitude of the 8. compensating mass is adjusted to produce a vector sum of the 9. dithering moments of said sensors and said mass which is 10. substantially equal to zero and to produce a vector sum of 11. the angular momenta of the dithered sensors and the 12. compensating mass is substantially equal to zero. 1. 12. Ring laser angle sensing apparatus, comprising: 2. a unitary instrument block; 3. three ring laser angle sensors mounted on 4. respective surfaces of the instrument block; 5. separate dithering means associated with each ring 6. laser angle sensor for mechanically dithering each sensor 7. about its sensitive axis; 8. a compensating mass rotatably mounted on said 9. instrument block; and 10. selectively adjustable means for dithering the 11. compensating mass at a prescribed amplitude, frequency, and 12. frequency phase about its axis of rotation. 1. 13. The device as claimed in claim 12 having three 2. sets of ring laser angle sensors which are substantially 3t identical and which are mounted with mutually orthogonal 4. sensitive axes for sensing rotation about said axes, and said 5. compensating mass is mounted with its axis of rotation at a 6. prescribed angle with respect to each of said sensitive axes. 1. 14. The device as claimed in claim 13 wherein said 2. compensating mass has a mass substantially equal to each of 3. said sensors and is mounted on said block with its axis of 4. rotation equiangularly located with respect to each sensitive 5. axis- of each said sensor. 1. 15. A device as claimed in Claim 14 wherein said 2. compensating mass comprises a fourth ring laser angle sensor.

Inventors: FERSHT RENA S, FERSHT SAMUEL N, FERSHT, RENA, S., FERSHT, SAMUEL, N.
Applicants: LITTON SYSTEMS INC, LITTON SYSTEMS, INC.
Application year: 1985
Main classification: G01C19
IPC codes: G01C19, G01P9, H01S3