RU2474798C2 - Fibre-optic pressure sensor - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: fibre-optic pressure sensor has a diaphragm unit which includes a diaphragm and its supporting component, a connecting piece, an optical fibre unit in which like ends of input and output optical fibres which are cut at a certain angle lie in a bushing on a triangle-shaped component and are pressed by a cover, and the other ends of the optical fibres are mated with a radiation source and a radiation receiver. The supporting component has at least two cavities linked by a hole with a smaller diameter than the diameter of the cavities, wherein the height H of the cavity in which the diaphragm is rigidly fixed is defined by the expression: H=h_d+λ, where h_d is the diaphragm thickness; λ is the wavelength of light flux. The connecting piece has three cavities. In the smaller cavity at a circular protrusion, there is a diaphragm unit on which an optical fibre unit, which is mounted in the new housing, is fitted in the second cavity. The connecting piece and the housing are rigidly connected to each other.

EFFECT: high accuracy of measuring pressure in conditions of varying ambient temperature.

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Description

The invention relates to instrumentation and can be used in the design and during the assembly of fiber-optic pressure sensors based on the optical tunnel effect in various sectors of the economy. The proposed sensor can be used to measure high pressures under conditions of changing ambient temperature in the range of ± 100 ° C on products of rocket and space technology (and presumably from minus 100 to + 500 ° C in other industries).

Known fiber optic pressure sensors (VODD), containing fiber optic bundles, the common ends of which are installed at a fixed distance from a reflective metal membrane, the pressure measurement process in which is carried out by recording changes in the intensity of the reflected light flux depending on the deflection of the membrane under pressure [1. Zhilin V.G. Fiber optic measuring transducers of speed and pressure. - M .: Energoatomizdat, 1987. - p.11-12; 2. Avdoshin E.S. Fiber optics in US military equipment // Foreign Electronics, 1989. - No. 11. - p. 98-99; 3. A.S. 1631329 G01L 11/00. Pressure meter; 4. Busurin V.I., Nosov Yu.R. Fiber Optic Sensors: Physical Basics, Calculation and Application Issues. - M .: Energoatomizdat, 1990. - p.40-41].

The disadvantage of such sensors is the high temperature error due to a change in the geometric parameters of the sensor. In particular, a change in the initial distance between the membrane and the end of the optical fibers due to a change in ambient temperature leads to a temperature error. A significant share in this error in pressure sensors is made by the membrane, which is explained by a change in its geometric dimensions and the elastic modulus of the membrane material. In addition, reflective-type EDFs have a low conversion sensitivity and a small modulation depth of the optical signal due to losses in the measurement zone due to the divergence of the light flux within the aperture angle of the optical fiber.

Known fiber-optic pressure sensor based on the optical tunnel effect, in which the listed disadvantages of the above sensors are practically eliminated [Busurin V.I., Nosov Yu.R. Fiber Optic Sensors: Physical Basics, Calculation and Application Issues. - M .: Energoatomizdat, 1990. - p.188]. The sensor contains optical fiber inlet and outlet, an optical prism, on which an annular gasket is applied, with a thickness approximately equal to the wavelength of the radiation source, a membrane that accepts the measured pressure and moves under pressure relative to the hypotenuse face of the prism.

Particularly attractive in this sensor is the possibility of using a small size quartz plate as a membrane (the radius of the membrane when measuring pressure in the range 0 ... 300 kgf / cm ² will be approximately 4 ... 5 mm). Such a constructive solution allows to reduce the error due to a change in the design parameters of the sensor when the ambient temperature changes. The use of quartz glass allows to reduce this error component to a minimum. So, for example, the temperature coefficient of linear expansion (TEC) of C5-1 grade silica glass is 5-10 ^-7 1 / ° С, and when the temperature changes in the range from minus 100 to + 500 ° С, the relative expansion of the material will be approximately 0.0025 %, a change in Young's modulus will lead to an additional error not exceeding 1% in this temperature range.

However, the well-known VODD based on the tunnel effect has a number of disadvantages: the sensor design is not adapted to work in harsh conditions of rocket and space technology and is not technological:

firstly, the design is leaky (it is technologically difficult to ensure the tightness of the prism-membrane assembly);

secondly, the interface between the OB and the prism is rather complicated: a rectangular prism is used as the main optical element (fiber), a rectangular triangle is at the base, therefore, to ensure the angle of incidence of light on the reflective surface, the role of which is the hypotenuse face of the prism, which is different from 45 degrees, it is necessary to modify the prism (in order to provide the necessary conversion sensitivity, it is necessary to accurately maintain the required angle to fractions of a degree - most often it is an angle of 47 degrees mustache). When finalizing, sophisticated expensive technological equipment and special measuring equipment should be used, which significantly increases the cost of the sensor manufacturing process;

thirdly, it is necessary to accurately establish the ends of the inlet and outlet optical fibers relative to the prism;

fourthly, if the sensor will be operated under the influence of vibrations, shock, etc., it is necessary to ensure reliable contact between the ends of the optical fibers and the faces of the prism. The known design does not provide this.

These disadvantages are eliminated in the closest in technical essence to the proposed invention VODD containing the inlet and outlet optical fibers, a membrane, an annular gasket, a part of a triangular shape [Pat. 2253850 RF, IPC ⁶ G01L 11/02, 19/04. VODD / E.A. Badeeva, A.V. Gorish, T.I. Murashkina, A.G. Pivkin; publ. 06/10/2005. Bull. No. 16].

However, the prototype sensor based on the tunnel effect has several disadvantages:

- in order to obtain a gap between the membrane and the end face of the supply and optical fibers equal to the wavelength of the optical radiation (to ensure the tunneling effect), a complex process of deposition of a metal strip on a quartz membrane, which is difficult to control, is necessary;

- when connecting the individual elements of the sensor to each other, for example, by welding, the process of squeezing this gasket is possible and, accordingly, the required clearance will not be ensured, respectively, the required level of the optical signal for the functioning of the sensor will not be achieved;

- during assembly, the non-parallelism of two surfaces may occur: the surface of the membrane and the end surface of the optical fibers facing the membrane, which sharply reduces the sensitivity of the conversion of the optical signal;

- requires complex technological and measuring operations in the manufacture.

Ultimately, the required metrological characteristics of the sensor will also not be achieved.

Thus, the prototype does not achieve a technical result, expressed in the manufacturability of the design and high accuracy of the sensor. A new design of the VODD on the tunnel effect, devoid of the above disadvantages, and the manufacturing process of the proposed sensor are proposed.

The specified technical result is achieved by the fact that in the VODD containing the membrane unit, which includes the membrane and the component supporting it, a fitting, an optical fiber unit, in which one ends of the input and output optical fibers cut at a certain angle are located in a sleeve on a triangular-shaped part and pressed against the lid, and the other ends of the optical fibers are docked to the radiation source and radiation receiver, new is that the supporting part has at least two cavities connected by an opening with a smaller diameter than the diameter etry cavities, wherein the height H of the cavity, wherein the diaphragm is rigidly attached, is given by:

where h _M is the thickness of the membrane;

λ is the wavelength of the light flux,

the nozzle has three cavities, and a membrane block is mounted on the annular protrusion in the smaller cavity, onto which the optical fiber block is mounted in the second cavity, fixed to the newly introduced housing, the inner size of which is equal to the external dimensions of the optical fiber block, the nozzle and the housing being rigidly between each other connected.

Thus, the alleged invention is a technical solution to the problem, which is a modernized (new), industrially applicable and inventive step, i.e. the present invention meets the criteria of patentability.

Figure 1 shows the design of the proposed sensor, in figures 2, 3, 4 - stages of manufacturing a measuring transducer of an optical fiber pressure sensor based on the tunnel effect, in figure 2a - the formation of the beveled ends of the optical fibers, in figure 2b - mounting of optical fibers in a metal sleeve , in figure 3a - the placement of optical fibers on the "cushion", in figure 3b - fastening of the optical fibers on the "cushion" using a lid and welding, in figure 3c - polishing of the optical fibers, in figure 4 - membrane unit.

The sensor contains a membrane unit 1, which includes a membrane 2 and its supporting part 3, an optical fiber block 4, where the inlet optical fibers (OWF) 5 and the outgoing optical fibers (OOW) 6 are located, a triangular shape part 7 (“pillow”), cover 8, sleeve 9, fitting 10 and housing 11 connected by welding 12 (Fig. 1).

Some ends of POV 5 and OOB 6 located in the measurement zone are cut off at an angle Θ defined by the expression

where n ₁ , n ₂ , n ₃ are the refractive indices of the core of the optical fiber, the medium between the membrane and the optical fibers, membranes, respectively,

passed through the sleeve, laid on the parts of a triangular shape 7 and pressed against the cover 8. The other ends of the POV 5 and OOB 6 are connected to the radiation source (AI) 13 and the radiation receiver (PI) 14, respectively. "Pillow" 7 is a part of a triangular shape with an apex angle of 2θ, with a recess that follows the shape of the optical fiber, respectively, the depth and width of the recess correspond to the external dimensions of the optical fiber. The length of the base of the triangle in section AA is equal to bd _OB . To eliminate optical fiber breakdowns, the angle at the apex is rounded, and the estimated radius of rounding of the surface of the recess on which the optical fiber is laid is determined by the expression (3)

The supporting part 3 has at least two cavities (in the example shown in FIG. 4, three cavities) connected by an opening with a smaller diameter than the diameters of the cavities, the height H of the cavity in which the membrane 2 is rigidly fixed is determined by the expression (1).

The fitting 10 has three cavities. Using the threaded part of the fitting, the sensor is fixed on the object, and through it a medium is supplied, the pressure of which must be measured. In the smaller cavity, a membrane unit 1 is mounted on the annular protrusion so that the membrane 2 faces the optical fiber unit 4. The membrane unit 1 is pressed by the optical fiber unit 4 fixed in the housing 11. The internal dimensions of the housing 11 are equal to the external dimensions of the optical fiber 4 To ensure rigidity and tightness of the structure, the nozzle 10 and the housing 11 are rigidly connected by welding 12, and the internal cavity of the housing 11 is filled with sealant 15.

To ensure the tunneling effect, the gap between the membrane 2 and the polished surface of the optical fibers 5 and 6 should be approximately equal to the length of the optical radiation AI 13, that is, λ. Accordingly, the dimensions of the membrane are selected so that its deflection under pressure is less than the wavelength. For example, if infrared radiation is used at a wavelength of 0.85 μm, the gap should be approximately equal to 1 μm, and the deflection of the membrane should not exceed this value.

The required clearance h is set by the height H of the cavity in which the membrane 2 is fixed, and the height h _{M of the} membrane 2 in the supporting part, the height of the recess being determined by expression (1) and, if necessary, brought to the desired value by polishing.

The sensor operates as follows (see figure 1).

From the radiation source AI 13, the light flux Ф ₀ along the POV 5 is directed toward the membrane 2 at an angle Θ, the value of which is determined by the shape and dimensions of the “cushion” 7, and is selected from the condition for ensuring the maximum conversion sensitivity and the modulation depth of the optical signal from expression (2) [Busurin V.I., Nosov Yu.R. Fiber optic sensors: Physical fundamentals, calculation and application issues. - M .: Energoatomizdat, 1990].

Under the influence of the controlled pressure P, the membrane 2 bends, and in the central part the gap between the membrane and the working ends of the optical fibers will be less than the initial value X ₀ . As a result of this, the reflectivity for electromagnetic waves in the region of the cut ends of the optical fibers changes; accordingly, the intensity of the light flux Ф (Р) reflected at an angle Θ from this region, which carries information about the measured pressure P and arrives via OOV 6 to PI 14, changes accordingly. PI 14 converts the optical signal into an electrical one.

Consider the main stages of the assembly process of this sensor (see figure 2, 3, 4):

1. Assembly of the block of optical fibers 4.

1.1. Optical fibers 5 and 6 are temporarily motionless fixed in metal lugs 16 (figure 2).

1.2. The optical fibers fixed in the tips 16 are cut off at an angle Θ and polished along section AA, after which the tips 16 are removed (Fig. 2a).

1.3. The ends of two optical fibers 5 and 6, in which the working ends are cut at an angle Θ, are glued into a metal sleeve 9 of 29NK steel at a distance b relative to each other so that the free ends of the fibers protrude above the plate surface to a height l, determined from the expression ( four)

1.4. Optical fibers 5 and 6 are stacked on a “pillow” 7 under a radius

To ensure accurate assembly, it is advisable that the tolerance on the dimension R is positive. This will allow, if necessary, bring it to the desired value (figa).

1.5. In order for the optical fibers to lie on the “cushion” 7, they are pressed against it from above by a metal cover 8, which, by welding 17, is fastened to the sleeve 9. The cover 8 in the center has a through hole with a width equal to the diameter of the optical fiber and a length of a = 2HtgΘ, where H≈d _OB . The free space under the lid is filled with an adhesive, for example, ceramic cement 18 (figb).

1.6. Part of the optical fibers, which turned out to be higher than the lid by the value of H, is cut along the surface BB and polished (figb).

2. Assembly of the membrane unit 1 (figure 4).

2.1. The membrane is rigidly fixed in the membrane block using sintered cement 19.

2.2. The required clearance h is set by the height H of the cavity in which the membrane 2 is fixed, and the height h _{M of the} membrane 2 in the supporting part, the height of the recess being determined by expression (1) and, if necessary, brought to the desired value by polishing.

3. Assembly of the fiber optic pressure sensor.

3.1. The membrane unit 1 is inserted into the nozzle 10, and the optical fiber unit 4 is installed in the housing 11.

3.2. The fitting 10 and the housing 11 are rigidly connected to each other by pulsed welding 12 so that their axis of symmetry coincides.

The technical result of the invention is as follows.

The proposed design excludes complex technological assembly operations.

The proposed new design of the fiber-optic pressure sensor based on the tunneling effect is operable under the harsh conditions of rocket and space technology and does not require complex technological and measuring operations during manufacturing.

This design of the sensor provides higher accuracy of pressure measurement in conditions of changing ambient temperature. At the same time, this technical solution does not require significant complication of the structural and circuit design of the sensor, respectively, and does not lead to unnecessary material costs.

Claims (1)

Fiber-optic pressure sensor containing a membrane unit, which includes a membrane and a component carrying it, a fitting, an optical fiber unit, in which one ends of the input and output optical fibers cut at a certain angle are located in the sleeve on a triangular shape and are pressed by a cover, and the other ends of the optical fibers are connected to the radiation source and the radiation receiver, characterized in that the supporting part has at least two cavities connected by an opening smaller in diameter than the diameters of the cavities, H Height of the cavity, wherein the diaphragm is rigidly attached, is given by:
H = h _M + λ,
where h _M is the thickness of the membrane;
λ is the wavelength of the light flux,
the nozzle has three cavities, and a membrane block is mounted on the annular protrusion in the smaller cavity, onto which the optical fiber block is mounted, mounted in the newly introduced housing, the inner size of which is equal to the external dimensions of the optical fiber block, the nozzle and the housing being rigidly between each other connected.

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