RU2807020C1 - Fibre optic gyroscope sensing element - Google Patents

Abstract

FIELD: fibre optics.

SUBSTANCE: invention can be used in fibre-optic gyroscopes (FOG) of the interferometric type. The design of the FOG sensing element is based on a coil with an optical fibre, in the frame of which 3 hemispherical supports are installed, through which the sensing element (interferometer) contacts the spherical pins at the location where the FOG is installed. The FOG is fastened to the pins at the installation site by screws. The input and output of the light guide into the coil is carried out through grooves and holes made on the coil frame. The optical fibre spool is covered on the outside with a magnetic shield consisting of three layers of soft magnetic material and heat-insulating spacers between the layers. The main optical and electrical elements necessary for the operation of the FOG are located on the base. The light guide connecting the optical elements fits into grooves made in the disk. The input and output of the light guide into the FOG housing is carried out through a sealing sleeve installed in the magnetic shield. The transition of the light guide from the lower part of the structure to the upper is carried out by placing it in special radial grooves of the sleeve to limit its free movement. The internal volume of the FOG coil is divided into upper and lower parts by a plate, which is an additional magnetic shield.

EFFECT: decrease in the sensitivity of the FOG output signal to external disturbances.

1 cl, 6 dwg

Description

The invention relates to the field of fiber optics and can be used in fiber-optic gyroscopes (hereinafter referred to as FOG) of the interferometric type.

The closest analogue, adopted as a prototype, is a fiber-optic gyroscope (RF patent No. 2589450), the design of which contains a fiber circuit located in the internal volume of the protective screen, which is a sensitive coil consisting of a supporting frame with an optical fiber wound on it that maintains polarization, FOG base with a multifunctional integrated optical circuit (hereinafter referred to as MIOC) installed in the housing, implementing the “polarizer-splitter-phase modulator” function, optical fibers laid on the base, an input polarizer and a splitter.

The disadvantage of this design is that the coil frame is made of quartz ceramic, and the FOG is mounted not directly on the frame, due to its fragility, but through intermediate parts in the base of the FOG. This technical solution does not ensure the strength of the structure under mechanical stress and does not guarantee the stability of the fastening and high repeatability of the orientation of the FOG sensitivity axis in space during its subsequent reinstallation. Also, in this design, MIOS is not isolated from the influence of atmospheric influences.

The design of an interferometric type FOG is known (RF patent No. 2283475), which contains a fiber circuit located in the internal volume of the protective screen, which is a coil consisting of a supporting frame with an optical fiber wound on it that maintains polarization, a base with a MIOS installed in the housing, implementing the function "polarizer-splitter-phase modulator".

The disadvantage of this design is that the coil is mounted without the use of hemispherical supports, which provide thermal decoupling of the mounting nodes from the base and high repeatability of the orientation of the FOG sensitivity axis in space during its subsequent reinstallations. In addition, in this design the magnetic shield has only one layer, which may not be enough when operating in conditions of significant magnetic fields.

A technical solution is known (RF patent No. 2298819) with a sealed housing for MIOS, filled with inert gas, aimed at protecting MIOS from atmospheric influences.

The disadvantage of this design is that in the MIOS housing there are no technological holes that are closed with screw plugs with rubber rings to ensure that the housing is filled with dried inert gas, the presence of which allows for independent sealing without disassembling the MIOS unit. In addition, it is proposed to fasten the cover by laser welding, and not by gluing or seam-roller welding, and the fiber light guides are removed from the housing through holes in additional adapter bushings, which complicates the design.

The technical problem being solved is the development of a design for an interferometric type FOG sensing element that has reduced sensitivity to external influencing factors, namely mechanical (vibrations and shocks), climatic (ambient temperature and humidity) and electromagnetic influences.

The achieved technical result is a decrease in the sensitivity of the FOG output signal to external disturbing influences.

The technical result is achieved using the following design solutions:

- Selection of a specific material for the frame of the optical fiber reel, which has increased resistance to shock and vibration, and has a low coefficient of linear thermal expansion (hereinafter referred to as CTE), to reduce the influence of thermal influences.

- Installation of three hemispherical supports in the frame, ensuring precise orientation of the FOG sensitivity axis relative to the base mounting surfaces and increased stability of the spatial position under inertial loads. Thanks to the proposed spherical shape of the supports, the area of thermal contact between the interferometer and the base at the mounting points is reduced, which reduces the influence of ambient temperature on the interferometer.

- The use of a multilayer magnetic screen of a special design, which provides reduced sensitivity to an external magnetic field and at the same time creates thermal insulation of the interferometer body from environmental influences by alternating heat-insulating gaskets and heat-conducting layers of metal.

- Installation of a distributed system of temperature sensors in the area of the optical fiber coil to enable algorithmic compensation of the influence of temperature gradients on the drift of the output signal.

- Creation of a special design of a sealed MIOS housing with technological holes for filling with dried inert gas in order to reduce the sensitivity of the device to changes in environmental humidity.

The essence of the proposed invention lies in the fact that in the sensitive element of a fiber-optic gyroscope containing a fiber circuit located in the internal volume of the protective screen, which is a sensitive coil consisting of a supporting frame with an optical fiber laid on its surface that maintains polarization, a base installed in housing with an integrated optical circuit that implements the “polarizer-splitter-phase modulator” function, optical fibers laid on the base, an input polarizer and splitter, and a board for primary processing of thermal sensor data; unlike the prototype, the frame structure is load-bearing and is installed through specially oriented hemispherical supports on spherical fingers at the installation site without intermediate fastenings, which ensure a reduction in the thermal contact area at the interferometer installation site, as well as high-precision stable alignment of the interferometer; in this case, the frame is made of structural materials with a low coefficient of thermal linear expansion, and the protective magnetic screen, creating a magnetic shielding coefficient of at least 300 and providing thermal insulation of the interferometer, consists of three layers of cylindrical screens, each of which consists of two halves with an overlap of forming a cylinder and made of a soft magnetic alloy (permalloy), with heat-insulating gaskets between the layers; the sealed housing of the integrated optical circuit is filled with dried nitrogen and has technological plugs to create the possibility of re-sealing; A flexible printed circuit board with thermal sensors located on it is glued to the outer layer of the optical fiber, forming a distributed temperature measurement system.

The proposed invention is illustrated by drawings:

Fig. 1 – FOG sensitive element in section.

Fig. 2 – optical components of the design of the FOG sensitive element.

Fig. 3 – frame of a reel with optical fiber (optical fiber not shown) with installed hemispherical supports for basing at the installation site of the device:

a) View of the frame with supports from below,

b) Sectional view of the frame with supports

Figure 4 – magnetic screens of the FOG sensing element.

Fig. 5 – housing of a multifunctional integrated optical circuit (cover not shown).

Fig.6 – elements of the MIOS housing that ensure its sealing:

a) View of the MIOS building on the right.

b) Top view of the MIOS body.

Designations in Fig.1 - 6:

1- reel with optical fiber on a frame (hereinafter referred to as the reel),

2 - hemispherical coil supports,

3 - VOG fastening screws (hereinafter referred to as screws),

4 - multilayer magnetic screen,

5 – FOG base,

6 - sleeve with grooves for the light guide,

7 - sealing sleeve,

8 - additional magnetic screen,

9 - optical polarizer,

10 - optical splitter,

11 - sealed MIOS housing (hereinafter referred to as the housing),

12 - coil frame (hereinafter referred to as the frame),

13 – fixing pins (hereinafter referred to as pins),

14 - first half of layer 1 of the magnetic screen,

15 - second half of layer 1 of the magnetic screen,

16 - first half of layer 2 of the magnetic screen,

17 - second half of layer 2 of the magnetic screen,

18 - first half of layer 3 of the magnetic screen,

19 - second half of layer 3 of the magnetic screen,

20 - fluoroplastic tape,

21 - getinax gaskets in the frame area,

22 - getinax gaskets in the area where optical components are placed,

23 - MIOS,

24 - MIOS board,

25 - sealed leads,

26 - board with electrical elements,

27 - housing cover (hereinafter referred to as the cover),

28 - traffic jams.

Let us describe the proposed invention.

The key component of the FOG sensing element (interferometer) is coil 1 with an optical fiber on the frame. In order to ensure an unambiguous position of the optical fiber in the coil and, as a result, precise positioning of the FOG sensitivity axis, as well as to minimize changes in the length of optical paths and voltages in the coil that arise when the ambient temperature changes, the coil frame is made of structural materials with low KTLR. In the proposed FOG sensitive element, for the manufacture of the coil frame, materials can be used that have relatively low CTLR values and at the same time can be easily processed on precision metal-cutting equipment, for example, kovar, fiberglass or press material.

The coil frame is the basic element of the FOG as a whole and at the same time the main load-bearing element of the structure. The supporting surfaces of the frame, which determine the position of the fiber in the coil, are connected and processed relative to the connecting places for installing the FOG, in order to minimize the number of instrumental errors in the mating parts of the FOG structure, which as a result affects the accuracy of the orientation of the FOG sensitivity axis relative to the base installation surfaces. In the prototype, the coil was not a load-bearing element of the structure; the frame was made of a rather fragile material - quartz ceramics (kersil), which is less amenable to processing than kovar, fiberglass or press material, leading to a certain number of defects during manufacturing.

The base of the frame of the coil 1 and the FOG as a whole at the installation site is carried out using three hemispherical supports 2 (Figs. 3a and 3b) installed in the frame 12 of the coil 1. The hemispheres are in contact with the spherical pins (not shown in the figure) at the installation site of the FOG, which minimizes the contact area and, accordingly, thermal contact. The FOG fastening is secured using three screws 3, passing through the holes in the hemispheres and installed in the threaded holes in the spherical pins at the location where the FOG is installed. The material used to make the hemispheres must have high hardness, so the proposed design uses special steel with subsequent heat treatment cycles. The orientation of the direction of the grooves in the hemispheres of the FOG coil is of fundamental importance. The grooves in the two lower hemispheres are oriented along one line, while the groove in the third upper hemisphere forms a line direction perpendicular to the given one. The direction of the grooves of the hemispheres in the coil is fixed using pins 13.

The specified fastening provides thermal decoupling in the FOG fastening elements from its installation location in the devices due to point contact in the hemispherical supports, excluding the influence of heat flows of the external environment entering through the interferometer fastening unit. In addition, this method of fixing the FOG coil ensures its unambiguous fixation in six coordinates (three angular and three linear coordinates), guarantees high position retention under mechanical influences and repeatability of the orientation of the FOG sensitivity axis in space during its subsequent reinstallations at the level of 10 arc seconds. The frame is based with the exception of intermediate parts (body parts at the base of the FOG structure, as in the analogue - RF patent No. 2589450), which increases the stability of the fastening when the ambient temperature changes. In the prototype, the FOG mount is organized differently and does not provide thermal decoupling of the interferometer from the base and unambiguous fixation of the coil.

A magnetic shield in the FOG design is necessary to reduce the influence of magnetic fields on the output signal of the FOG, in particular, to reduce the influence of the Faraday effect. In the proposed design variant, a multilayer magnetic shield is implemented in order to achieve a magnetic shielding coefficient of at least 300 and ensure thermal insulation of the interferometer from the influence of external thermal fields. The magnetic screen assembly consists of three layers of cylindrical screens that repeat the geometry of the FOG coil and have additional depressions in the central part to ensure screen rigidity. Each of the screen layers consists of two halves 14-19 (Fig. 4), having an overlap along the generatrix of the cylinder.

For the manufacture of each of the three layers of the screen, sheets of soft magnetic material (permalloy) are used, with heat treatment to achieve certain values of magnetic permeability. Heat-insulating gaskets 21, 22 are installed in the radial and end gaps between adjacent layers, and the remaining cavities are filled with glue, for example K-400 or a similar epoxy-silicon glue, to prevent contact between the layers. By alternating heat-insulating and heat-conducting layers, effective heat distribution and reduction of thermal gradients are achieved. In this case, the required magnetic shielding coefficient is achieved in the FOG design with relatively small dimensions, which is of fundamental importance in the design of small-sized FOGs and devices based on them. If necessary, the number of magnetic shield layers in this design can be increased to achieve higher values of magnetic shielding coefficients with a corresponding improvement in thermal insulation properties. The prototype uses two single-layer screens (one covers the fiber spool, the second covers the entire FOG structure together with the MIOS), and there is no thermal insulation between the screens, which does not provide a sufficient shielding coefficient and does not create the possibility of increasing the number of layers to reduce sensitivity to temperature changes.

To implement phase modulation of a light wave in a FOG, MIOS is used. For the correct functioning of this element, there must be no condensation inside the volume where this element is located. Taking into account the wide operating temperature range of FOG, it is necessary to exclude the formation of condensation on the surface of the MIOS elements, which negatively affects the operation of the control electrode system. To seal the MIOS, it is proposed to use housing 11 (Fig. 5), which is a parallelepiped with an installed lid 27 (Fig. 6). To seal the internal volume of the housing 11, it is proposed to use adhesive joints using K-400 glue or a similar epoxy-silicon glue, which ensures the tightness of the internal volume of the housing in the range of extreme temperatures of FOG. The housing 11 has six sealed terminals 25 for soldering electrical conductors located on one of the sides of the housing. The sealed leads inside the housing are soldered onto the MIOS board 24 (Fig. 5) for its subsequent connection to the MIOS control electrode system.

From the outside of the case, the sealed terminals are soldered onto a board with electrical elements. Installation of MIOS 23 (Fig. 5) into housing 11, with pre-attached optical fibers, is carried out with cover 27 removed using a vacuum holder. The housing has two holes for filling with dry nitrogen gas; the seal is realized using technological plugs 28 with rubber rings (Fig. 6). The housing is checked for leaks using a helium leak detector. This design allows for refilling and sealing. This variant of the design of the sealed MIOS housing ensures the manufacturability of the assembly of this unit, taking into account the peculiarities of working with optical components, and also ensures a reliable hermetic connection in the mating parts. In the prototype, the tightness of the MIOS is not ensured.

To monitor the temperature throughout the entire volume of the FOG, a distributed system of temperature sensors is used, installed on a reel with optical fiber 1 and on MIOS 23. Several temperature sensors are distributed evenly along the reel with optical fiber, another similar sensor is installed near the MIOS inside the sealed housing of MIOS 11. All temperature sensors are connected to the electronic board for processing the FOG output signal, and their readings are used to algorithmically compensate for the drift of the FOG output signal caused by temperature changes. To improve the manufacturability of the FOG assembly and ensure the same distances between sensors from device to device, the coil temperature sensors are placed on a special flexible printed circuit board, which is glued to the coil. The prototype does not address the issue of thermal sensors.

Currently, FOG SEs of this design have been manufactured and tested. The test results of the prototype FOG confirmed the insensitivity to the effects of magnetic fields and humidity to the level of stability of the drift of this FOG and provided the possibility of using the FOG in ground-based equipment.

Thus, the declared technical result has been achieved.

Claims (1)

A sensitive element of a fiber-optic gyroscope containing a fiber circuit located in the internal volume of the protective screen, which is a sensitive coil consisting of a supporting frame with an optical fiber laid on its surface that maintains polarization, a base with a multifunctional integrated optical circuit installed in the housing that implements the function “polarizer-splitter-phase modulator”, with optical fibers laid on the base, an input polarizer and a splitter, and a board for primary processing of thermal sensor data, characterized in that the frame structure is load-bearing and, through three hemispherical supports, is mounted on spherical fingers at the installation site without intermediate fastenings, wherein the grooves of the two hemispherical supports are oriented along one line, and the groove in the third hemispherical support forms the direction of the line perpendicular to the given one, while the frame is made of structural materials with a low coefficient of thermal linear expansion, the protective magnetic screen consists of three layers of cylindrical screens, each of which consists of two halves with overlap along the generatrix of the cylinder, made of a soft magnetic alloy, with heat-insulating gaskets between the layers, the sealed housing of the integrated optical circuit, filled with dried nitrogen, has technological plugs to create the possibility of re-sealing, a flexible printed circuit is glued to the outer layer of the optical fiber a board with thermal sensors located on it, forming a distributed temperature measurement system.