Russian Journal of Nondestructive Testing, Vol. 40, No. 10, 2004, pp. 708–712. Translated from Defektoskopiya, Vol. 40, No. 10, 2004, pp. 89–95. Original Russian Text Copyright © 2004 by V. Shanin, O. Shanin.
OPTIC METHODS
The Use of Holographic Interferometry in Adjustment of Gyroscope Units V. I. Shanin and O. V. Shanin Tsiolkovskii Russian State Technological University Received February 17, 2004
Abstract—The possibility of employing holographic interferometry to regulate and adjust precision instruments is examined using certain gyroscope components as an example. The results of experimental investigation are presented.
INTRODUCTION Regulation or adjustment of an instrument is one of the final operations in the technology of its production, during which its parameters are brought up to predetermined values [1]. Successful completion of this operation depends on whether or not it inadequately takes into account the structural features and operating conditions of the instrument. As applied to a new generation of instruments (e.g., pressure-to-frequency transducers, accelerometers and gyroscopes based on untraditional physical principles), it was these distributions of microstrain fields and residual stresses that were inadequately controlled. Until recently, this operation was a serious technical problem. The most versatile method for investigating distributions of the strain fields and residual stresses is holographic interference [2]. It has already found wide utility for nondestructive testing in medicine (in stomatology in particular) and experimental mechanics. In this paper, we present the results of experimental investigations aimed at determining the efficiency of the methods of holographic interferometry in regulation and adjustment of units of aircraft instruments, namely, a gyromotor and an elastic suspension, which are the basic units of gyroscopic instruments. 1. ADJUSTMENT OF HYDRAULIC MOTORS WITH A SCREWED JOINT OF ELEMENTS A misalignment of the support races in ball-bearing supports is one of the commonest defects in the assembling of micromotors with a screwed joint of elements, e.g., gyromotors [3]. This defect leads to an increase in self-excited vibration, an accelerated wear of the support’s components, and, thereby, to premature failure of the gyromotor as a whole. A misalignment of the support races is caused by unequal tightening forces applied to the mounting screws during assembling of micromotors. The difference in tightening force results in nonuniform strains on the flanges and, finally, in the misalignment of the races. The characteristic features of assembling and adjustment of micromotors with a screwed joint of their elements dictate the use of the holographic interferometry method or, more precisely, real-time holographic interferometry. In essence, this method is known to consist of the following phases: obtaining a hologram of the initial micromotor state, illuminating it simultaneously with the micromotor, and observing the interferogram thereby produced [2]. This interferogram carries information on all changes in the state of the micromotor in the course of its assembling and adjustment. A block diagram of the experimental facility is shown in Fig. 1. It is based on the traditional scheme of diffuse reflection holography. A television system that acts as a display device is included in the facility as an obligatory component. This is due to the fact that provision of normal working conditions for an operator requires suitable spatial arrangement of a holder that clamps the controlled unit and a display device (in our case, this is a monitor of the television system). A general view of the micromotor used in the experiments is shown in Fig. 2a. It was inserted into a special holder based on a three-jaw chuck (conventionally used in the machine-tool industry) with a calibrated clamping force. The holder was rigidly held in place on the working surface of the holographic stage. The procedure for regulating the uniformity of the screw tightening consisted of the following operations. At first, the screws were tightened with a force 10% weaker than the required value. The micromotor 1061-8309/04/4010-0708 © 2004 MAIK “Nauka /Interperiodica”
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was illuminated from the flange side, and its hologram was recorded. After that, one of the screws was tightened with the required force. The other screws were tightened until the interferogram around each screw was identical to the interferogram obtained around the first one.
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As a result of experiments with a batch of 50 micromotors, we recorded characteristic interfero9 grams of thermal strains on micromotors tuned 7 either in the traditional way or following the holo10 graphic technique. Examples of the recorded holo11 grams are presented in Figs. 2b and 2c. Comparison shows that, when the micromotor is tuned using the traditional method, the microstrain fields around Fig. 1. Block diagram of the experimental facility: (1) each mounting screw in the micromotor flange differ laser; (2) mirror; (3) beam splitter; (4) light attenuator; (5, 6) collimator; (7) holder of the test object; (8) test from one another significantly. In the case where the object; (9) hologram; (10) pickup camera; and (11) holographic interferometry technique is used to tune monitor. the micromotor, the microstrain fields around each mounting screw are almost identical. Testing a batch of 50 micromotors, in which the screw tightening was regulated according to our technique, demonstrated a 10–15% improvement of their performance characteristics [4]. 2
2. ADJUSTING THE ELASTIC SUSPENSION OF A GYROSCOPE When searching for means of improving the accuracy of dynamically adjustable gyroscopes, we had to direct our attention to investigating the thermal strains of its elastic suspension using holographic interferometry methods. As a result, we ascertained that a holographic image of the thermal strains on the elastic suspension components has a totally definite form. In this case, the image is in unique correspondence with the presence or absence of quadrature disturbing moments. These moments result from the imperfect workmanship of the gyroscope suspension, e.g., the nonintersecting axes, the misalignment (linear or angular) of its axes and the principal axes of strain on its flexible elements working in bending, the existence of the torsional prestrain on its elements because of the production errors, and the difference in elastic stiffness of each element and the bearing as a whole [5]. It is exactly this fact that is responsible for the application of holographic interferometry to the elimination of quadrature disturbing moments. In contrast to the conventional electronic technique, this operation can be performed in a significantly shorter time and provides reliable results. As the object of investigation, we used an elastic suspension, the photograph and interior arrangement of which are presented in Fig. 3. For this particular design of the elastic suspension, the following factors are considered to give rise to the quadrature disturbing moments: the displacement and the tilt of the mutually perpendicular torsion bars, the lateral and axial displacements of the coplanar torsion bars, and the twisting of the torsion bars [5]. (‡)
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Fig. 2. Image of the micromotor and interferograms of the thermal strains: (a) image of the micromotor; (b) interferograms of the thermal strains after regulating the screw tightening in the traditional fashion; and (c) interferograms of the thermal strains after the regulation using the holographic interferometry method. RUSSIAN JOURNAL OF NONDESTRUCTIVE TESTING
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Fig. 3. Image of the elastic suspension and arrangement of its components: (a) image of the elastic suspension; (b) arrangement of elements inside the suspension; (1) wheel; (2) first frame; (3) second frame; (no. 1, no. 2, no. 3, no. 4) lock screws; (4, 5) torsion bars of the first frame; and (6, 7) torsion bars of the second frame.
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Fig. 4. Device for holding an elastic suspension in place and applying thermal loading during adjustment: (1) post; (2) collet mandrel; (3) coupling nut; (4) elastic suspension; (5) strip heater; (6) ceramic bushing; and (PS) power source.
Our experiments were conducted on the facility described in Section 1. Taking into account the structural features of the elastic suspension and the peculiarities of the control over the effect exerted by the quadrature moments on the intrinsic drift of the gyroscope, we developed the holder shown in Fig. 4. Post 1, which is bolted to the work surface of the holographic stage, is the main component of the holder. Inside the post, there is collet mandrel 2, with a mounting seat bored to the dimensions of the ceramic bushing that is slipped over the wheel of the elastic suspension. The depth of the recess is around 1.7 mm. This value was selected to provide free access to the end faces of the torsion bars. The collet mandrel is clamped using coupling nut 3. The thermal loading of the suspension is applied from a strip heater–resistor. In this case, the width of the heater must be such that the end faces of the torsion bars are freely movable through a certain angle with the help of a screwdriver. The screws rigidly join the collet mandrel to the post. Taking into account that the angular displacement of the torsion bars during their adjustment lies within the limits of a few angular minutes, the screwdriver used for this purpose is nothing else than a transmission–duplication mechanism equipped with a handle and a tip. Adjustment of an elastic suspension consists in turning the mutually perpendicular torsion bars through some angle in order to obtain a certain type of interferogram of thermal strains on the suspension components. The adjustment is performed according to the following procedure. After an elastic suspension is inserted into the holder, a hologram of its initial state at room temperature is recorded. The power source that supplies power for the heater is then switched on. After ten minutes, within which the temperature of the heater (its resistance is R = 90 Ω) reaches a value of 65°C, one starts the adjustment process. The suspension and the hologram are simultaneously exposed to light. To accomplish this, a lock screw on one of RUSSIAN JOURNAL OF NONDESTRUCTIVE TESTING
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Fig. 5. Interferograms of the thermal strains on the components of the elastic suspension during its adjustment: (a) interferogram of the thermal strains on the reference elastic suspension; (b) interferogram of the thermal strains on the elastic suspension after its processing; (c) interferogram of the thermal strains on the elastic suspension after the torsion bar of the first frame was turned through a certain angle; (d) interferogram of the thermal strains on the elastic suspension after the torsion bar of the second frame was turned through a certain angle.
the torsion bars in the first frame is released, and the angular position of the torsion bar is varied until the observed interferogram approximates the reference picture as close as possible. The angular position of the torsion bar obtained thereby is then fixed by the lock screw. To continue the adjustment, a lock screw on one of the torsion bars in the second frame is released and, by varying the angular position of the torsion bar, one strives for the closest fit of the observed interferogram to the reference picture. The angular position of the torsion bar obtained thereby is fixed by the lock screw. If it is impossible to achieve the identity of these interferograms, the suspension is adjusted by rotating the other two torsion bars. The potential of this technique was investigated in an experiment with a batch of elastic suspensions. Typical interferograms recorded during the experiment are shown in Fig. 5, along with the interferogram of thermal strains on the reference elastic suspension adjusted according to the conventional technique (Fig. 5a). In the interferogram, one can observe a distinct closed interference fringe on the wheel of the suspension, as well as parallel fringes on the frames (these are mutually perpendicular). In addition, parallel fringes, which make an angle of ~45° with the horizontal line, are observed on the inner race. The interferograms shown in Figs. 5b–5d display the phases in the adjustment of a suspension. The picture in Fig. 5b corresponds to the initial state of the suspension after its assembling. One can see that this interferogram bears no resemblance to the reference image. The interferogram in Fig. 5c presents the intermediate phase of the adjustment, in which the angular position of the first-frame torsion is changed. One can easily notice the similarity of separate fragments of this interferogram to the reference one. The interferogram in Fig. 5d shows the final phase in the adjustment of the elastic suspension, which is achieved by turning the torsion bar of the second frame. In this case, one can see that the interferogram of the adjustable suspension is undoubtedly similar to the reference image. RUSSIAN JOURNAL OF NONDESTRUCTIVE TESTING
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Tests of the dynamically adjustable gyroscope showed that the intrinsic drift of the gyroscope with an elastic suspension balanced according to this technique is equal in value to that of the gyroscope with a reference suspension. In addition, the adjustment took about 30 minutes to complete; moreover, most of this time was spent for wet photochemical processing of the photographic plate. Should fast-response recording media (e.g., photothermoplastic) be used, the adjustment time will be reduced to a few minutes. In conclusion, it should be noted that the holographic methods for recording and displaying information show great promise when applied to the adjustment and alignment of precision instruments that are still waiting for their practical implementation. CONCLUSIONS (1) Our experiments confirmed the high efficiency of holographic interferometry methods when applied to the adjustment and alignment of units included in products of the modern instrument-making industry, in particular, gyromotors with a screwed joint of their elements and an elastic suspension of a dynamically adjustable gyroscope. (2) A check-out and alignment complex has been developed on the basis of an UIG-12 holographic facility (Russia). It also includes a digital computer vision system, a set of auxiliaries for holding objects in place and carrying out their adjustment, an optical scheme, and procedures for adjustment and alignment. (3) The tests of a batch of gyromotors, in which the screws were tightened using the holographic interferometry method, showed a 10–15% increase in their service life. (4) By controlling the thermal strain field of an elastic suspension used in a dynamically adjustable gyroscope, it is possible to eliminate the effect of quadrature disturbing moments on the intrinsic drift of the gyroscope and to achieve thereby the predetermined values of its output parameters. In this case, the adjustment of one suspension takes only a few minutes, while the regulation of a gyroscope using the traditional procedure takes several hours. (5) The equipment and the procedures developed for adjusting the above-mentioned units of gyroscopic systems offer a means for extending the scope of tasks performed using coherent optics and holographic methods. REFERENCES 1. Idel’son, M.I., Filippov, K.P., Boitsov, I.A., et al., Tekhnologiya optomekhanicheskogo priborostroeniya (Technology of Optomechanical Instrument-Making Industry), Leningrad: Mashinostroenie, 1981. 2. Jones, R. and Wyeks, G., Hologram and Speckle Interferometry. Translated under the title Golograficheskaya i spekl-interferometriya, Moscow: Mir, 1986. 3. Akilin, V.I., Arkhangel’skaya, E.B., and Shanin, V.I., Holography Application for Diagnostics of Assembling Defects in Compact Engines, in Dokl. Vsesoyuznogo nauch.-tekhn. seminara “Metody i sredstva tekhnicheskoi diagnostiki v priborostroenii” (Proc. of All-Union Scientific and Technical Symposium “Methods and Means for Technical Diagnostics in Instrument-making Industry”), Dilizhan, 1985, p. 7. 4. Shanin, V.I., Coherent-Optical Methods for Presentation and Processing of Information in High-Technology Instrument-Making Industry, Extended Abstract of Cand. Sci. Dissertation, Moscow Institute of Radio Engineering, Electronics and Automation, Moscow, 1999. 5. Pel’por, D.S., Matveev, V.F., and Arsent’ev, V.L., Dinamicheski nastraivaemye giroskopy (Tuned Rotor Gyroscopes), Moscow: Mashinostroenie, 1988.
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2004