RU2795841C1 - Fiber optic temperature sensor - Google Patents

Abstract

FIELD: measuring technology.

SUBSTANCE: invention relates to measuring technology and can be used in fiber-optic sensors designed to measure the temperature of large-volume cement, or under the influence of external destabilizing factors on products of rocket-space and aviation technology. A fiber-optic temperature sensor is claimed, containing a source and receivers of radiation, input and output optical fibers fixed in a sleeve opposite the reflective surface of the shank inside the housing. Moreover, a cylindrical tube made of a material with a high coefficient of thermal expansion is additionally introduced into the device, coaxially located inside the body, and the inner diameter of the cylindrical tube is at most equal to the outer diameter of the narrow part of the shank and the outer diameter of the narrow part of the sleeve. The coefficients of linear expansion of the body materials α_B, shank α_S, bushing α_BS and cylinder α_C are determined by the relation X ₀=(0,3…0,7)d _C, where d _C is the diameter of the optical fiber core, the lengths of the body l _B and the cylinder l _C are interconnected by the expression l _B=(0,95…1)l _Cα_C/α_B.

EFFECT: reducing internal stresses and, accordingly, increasing the mechanical reliability of the device in large temperature ranges (for example, in the range of -50°C…+100°C); exclusion of sensor housing deformations caused by deformations of a hard environment (for example, concrete) that affect the accuracy of temperature measurement; high accuracy of temperature measurement due to the possible implementation of the compensation conversion of optical signals directly in the zone of measurement information perception; ensuring the tightness of the structure.

10 cl, 3 dwg

Description

The invention relates to measuring technology and can be used in fiber-optic sensors designed to measure the temperature of objects in various sectors of the national economy and, first of all, to measure the temperature of large-volume cement during its hardening, and then during the operation of the structure. In addition, the sensor can be used to measure temperature under the influence of external destabilizing factors on products of rocket, space and aviation technology.

Known fiber-optic temperature sensor containing lighting and receiving light guides, the first ends of which are connected respectively to the light source and the receiver, and the second ends are connected to the capsule. The common input-output of the splitter is fixed in the capsule. An intracapsular mirror is located in the cavity of the capsule, with the help of which the input light flux coming through the splitter from the lighting fiber is reflected, and the reflected output light flux is transmitted in the opposite direction through the same splitter to the receiving fiber [patent RU 2256890]. The intracapsular mirror is made at the end of the rod, fixed with its opposite end at the bottom of the capsule.

Known fiber-optic temperature sensor, consisting of a capillary fixed in it with optical fibers, flat and smooth ends of which are at a distance from each other (patent RU 2334965, figure 5). One of the fibers is the input and output of the sensing element. The fibers are attached to the capillary with glue or fusible glass. When the temperature changes, the length of the capillary changes, as a result of which the base of the sensor (the distance between the ends of the fibers) changes.

The disadvantages of the above sensors are:

- their vulnerability from external mechanical influences, primarily from the impact of cement;

- the dependence of the measurement results on the bending of optical fibers during assembly and the influence of external influencing factors during operation.

A fiber-optic temperature sensor is known, chosen by the applicant as a prototype, containing a capillary with an optical fiber fixed in it and a reflective element installed at a distance from the end of the optical fiber, a housing with a through hole and two blind holes coaxial to it, one of which has a thread for a screw , the shank of which is installed in a through hole and has a reflective end [patent RU 2393431].

The disadvantages of the prototype are:

- the coefficients of thermal expansion of the materials from which the sensor elements are made, primarily the capillary and housing, are not taken into account, which will lead to a significant measurement error, as well as at large temperature changes (for example, in the range of minus 50 ° С ... + 100 ° С) to large internal stresses and, accordingly, to a decrease in the mechanical reliability of the device;

- when such a sensor is installed in hardening cement, the body will experience deformations, the levels of which are comparable in value with the measurement range of the sensor, which is determined by the change in the distance between the reflector and the end of the optical fibers, respectively, all metrological characteristics obtained in the process of grading and calibrating the sensor are significantly will change;

- the design of the sensor is leaky, therefore, under conditions of possible humidity on the reflective surface and on the ends of optical fibers, water droplets (lenses) may appear at elevated temperatures, and frost at low temperatures, which will introduce a significant additional error;

- the measurement results depend on the bending of optical fibers during assembly and the influence of external influencing factors during operation, which also leads to large additional errors.

As a result of a search through the sources of patent and technical information, no devices were found with a set of essential features that coincide with the proposed invention and provide the claimed technical result.

The technical result of the invention are:

- reducing internal stresses and, accordingly, increasing the mechanical reliability of the device in large temperature ranges (for example, in the range of minus 50°С ... +100°С);

- exclusion of sensor housing deformations caused by deformations of a hard environment (for example, concrete), which affect the accuracy of temperature measurement;

- high accuracy of temperature measurement due to the possible implementation of the compensation conversion of optical signals directly in the zone of measurement information perception;

- Ensuring the tightness of the structure.

The specified technical result is achieved by the fact that:

- a fiber-optic temperature sensor containing a source and receivers of radiation, input and output optical fibers fixed in a sleeve opposite the reflective surface of the shank inside the housing, characterized in that

- again introduced a cylindrical tube made of a material with a high coefficient of thermal expansion, coaxially located inside the body, and the inner diameter of the cylindrical tube is at most equal to the outer diameter of the narrow part of the shank and the outer diameter of the narrow part of the sleeve, the ends of the cylindrical tube are hermetically connected to the inner ends of the wide part of the shank and the wide part of the bushing, and the coefficients of linear expansion of the materials of the body α _K , shank α _XB , bushing α _VT and cylinder α _C are determined by the relation:

the initial distance X ₀ between the reflective surface and the end of the optical fibers is determined by the expression:

where d _C is the diameter of the core of the optical fiber,

body length l _K and cylinder l _C are interconnected by the expression:

- a through hole is made in the wall of the cylinder to fill the free space between the shank, the sleeve and the cylindrical tube with an inert gas, in which a sealing part (for example, a pin / screw) is hermetically installed

- the through hole can be sealed by soldering with solder;

- with a gap relative to the wide part of the shank in the body, a cover made of the same material as the body is hermetically fastened;

- the free space between the inner surfaces of the body and cover and the outer surfaces of the cylindrical tube, bushing and wide part of the shank is filled with a heat-conducting compound;

- in the bushing there is a side blind hole in which a fixed mirror is fixed at a distance X ₀ relative to the end face of the additional bushing, with additional incoming and outgoing optical fibers fixed;

- all optical fibers are stretched through the shank, hermetically connected to the body from the side of the sleeve;

- the gap between the fixed mirror and the end face of the additional sleeve is filled with an inert gas.

The essence of the proposed invention is illustrated by drawings.

Figure 1 shows a general view of a fiber optic temperature sensor, figure 2 is a simplified block diagram of signal conversion; figure 3 - graphical dependency K ( X )=F( X )/F ₀ .

The sensor contains a housing 1 made of a material with good thermal conductivity (for example, a copper alloy), inside which is placed a sensitive element, which is a cylinder 2 made of a material with a high coefficient of thermal expansion α _C , for example, aluminum. On both sides of the cylinder 2 there is a shank 3 and a sleeve 4 made of materials with small coefficients of thermal expansion α _ХВ , α _VT , respectively, for example, steel 36НХТЮ, and the coefficients of linear expansion of the body materials α _K , shank α _ХВ , sleeve α _VT and cylinder α _C are determined by relation (1).

The reflecting surface 5 of the shank 3 is opposite the ends 6 of the working inlet 7 (POF) and the working outgoing 8 (OOV) optical fibers fixed in the sleeve 4, and at normal temperature (calibration temperature) the initial distance between the surface 5 and the end 6 is determined by the expression (2 ).

The outer diameter of the narrow part of the shank 3 is equal to the inner diameter of the cylinder 2 (with bore and shaft tolerances providing a sliding fit, eg H7/g6). Similarly, the outer diameter of the narrow part of the sleeve 4 is equal to the inner diameter of the cylinder 2, and is also set to a sliding fit. The wide part of the sleeve 4 is threaded onto the inner wall of the cylindrical tube and rigidly fixed on it, for example, using an adhesive.

The inner ends 9 and 10 of the wide part of the shank 3 and sleeve 4, respectively, are tightly connected to the ends of the cylinder 2, for example by welding or soldering 11 and 12, respectively.

A through hole 13 is made in the wall of the cylinder 2 to fill the free space between the shank 3 and the sleeve 4 with an inert gas, in which the sealing part 14 is hermetically installed (for example, a pin installed by rolling and then soldering, or a screw is screwed in and sealed with adhesive).

Using the thread on the narrow part of the shank 3, the sensitive element is fixed on the internal thread of the cylindrical tube 2.

From the side of the wide part of the shank 3 with a gap relative to the shank 3 in the body 1, a cover 15 of the same material as the body 1 is fixedly fixed.

The free space between the inner surfaces of the body 1 and the cover 15 and the outer surfaces of the cylindrical tube 2, the sleeve 4 and the wide part of the shank 3 is filled with a heat-conducting compound 16.

In the sleeve 4, a side blind hole 17 is made, in which a fixed mirror 18 is fixed at a distance X ₀ relative to the end of the additional sleeve 19, with additional input 20 and output 21 optical fibers fixed (see Fig. 2). Hole 17 before installing the sleeve 19 is filled with an inert gas (for example, argon).

All optical fibers are pulled through the shank 22, hermetically connected to the sleeve 4, filled with a sealant and fixed in the housing 1.

The sensor works as follows.

When the ambient temperature changes by ΔT, it is transferred through the heat transfer process through the body 1, the heat-conducting paste 16, to the cylindrical tube 2, which changes its length due to the large coefficient of thermal expansion. Since the shank 3 and sleeve 4 with low expansion coefficients are rigidly fixed at the ends of the cylindrical tube 2, the reflective surface 5 of the shank 3 moves relative to the ends 6 of the optical fibers.

The light flux from the radiation source 23 (for example, an IR LED) is fed through the working POV 7 into the measurement zone, falls on the mirror surface 5 of the shank 3, is reflected from it and enters the working POV 8 (Fig. 2). Through OOV 8, the luminous flux is directed to a working radiation receiver 24 (for example, a photodiode), where it is converted into an electrical signal, the value of which is proportional to the distance between surfaces 5 and 6.

The luminous flux from the radiation source 23 through additional POV 20 is fed into the zone of the fixed mirror 18, falls on its mirror, is reflected from it and enters the input of the additional OOV 21. Through the OOV 21, the luminous flux is directed to an additional radiation receiver 25 (for example, a photodiode), where is converted into an electrical signal, the value of which is proportional to the distance X ₀ . Since the additional mirror is installed in the short part of the sleeve 4 with a low coefficient of thermal expansion (for example, 36NKhTYu), the distance X ₀ remains unchanged during the measurement process.

Optical radiation Ф ₁ ( X ) ~ Ф ₁ ( T ) and Ф ₂ ( X ₀ ), carrying measurement information about the measured temperature, arrive at the radiation receivers 24 and 25 of the respective channels (see Fig. 2). Radiation receivers convert optical signals Ф ₁ ( T ) and Ф ₂ ( X ₀ ) into electrical I ₁ ( T ) and I ₂ ( X ₀ ) respectively. These signals are fed to the input of the electronic information conversion unit 26, where they are further converted, for example, an operation corresponding to the ratiometric conversion is performed:

where k is the coefficient of proportionality.

Establishing a causal relationship between the claimed features and the achieved technical effect will be carried out as follows.

Housing 1 is made of a material with good thermal conductivity to reduce the inertia in the transfer of temperature from the environment to the sensing element 2 and, accordingly, reduce the dynamic error. The thickness of the case 1 is selected from the same considerations, as well as to ensure the strength of the sensor design if it is located in a hard deformable medium, for example, in concrete.

Cylindrical tube 2, which acts as an element that converts the measured temperature into a change in the distance X between the ends 5 and 6, is made of a material with a high thermal expansion coefficient α _C , for example, aluminum, to provide a large modulation depth and high sensitivity of optical signal conversion.

Shank 3 and bushing 4 are made of materials with low coefficients of thermal expansion _in order to exclude a change in their length with temperature changes, thereby providing the calculated values of the distance between the ends 5 and 6, and α _ХВ ≈α _ВТ .

To exclude deformation of the body 1 with temperature changes due to the difference in the coefficients of thermal expansion of the body 1 and the cylindrical tube 2, it is necessary that the change in the length Δ L _K of the body 1 be equal to the change in the length of the cylinder Δ l _C , that is, the condition is fulfilled:

If the ambient temperature changes, then the length l _C of the cylindrical tube 2 and the length L _K of the body 1 will change in accordance with the expressions:

where l _C0, L _C0 are the lengths of the cylindrical tube 2 and body 1, respectively, under normal conditions (calibration temperature);

Δ T - range of temperature change.

Substituting expressions (6) and (7) into (5) we get:

But in view of the fact that in the drawings the linear dimensions must correspond to the standard values [GOST 6639-69 Interstate standard. Basic norms of interchangeability. Normal linear dimensions], then a correction factor equal to (0.95 ... 1) is introduced. As a result, we obtain condition (3).

Since the length of the body 1 is significantly longer than the length of the cylindrical tube 2 (see figure 1), then to avoid deformation of the body 1 of the sensor when the temperature changes, it is necessary that α _C > α _K .

The wide part of the shank 3 and the cover 15 are located relative to each other with a certain gap so that the sensitive element (the combination of cylindrical tube 2, shank 3 and bushing 4) can move freely along the body 1 with temperature changes without causing deformation of the body 1. And, on the contrary, when the body 1 was deformed, for one reason or another, the sensitive element was not deformed.

To transfer the ambient temperature, carried out due to the process of heat transfer (thermal conductivity), the body 1 and the cylindrical tube 2 are made of a material with good thermal conductivity, a thermally conductive composition 16 is placed between them, which has good thermal conductivity (for example, thermal paste KPT-8 or 52022) and reduces temperature resistance on the path of heat flow propagation.

The initial length lц0 is chosen in such a way that its change in the measurement range provides a large depth of modulation of the optical signal (up to 30 _% ) and a linear conversion function Ф( Х ) of the luminous flux from a change in the distance Х between surfaces 5 and 6 .

In figure 3a, as an example, the results of calculating the dependence K ( X )=F( X )/F ₀ are shown, where F ₀ is the initial luminous flux introduced into the measurement zone according to POV 7.

=12° (см. фиг. 3б).Analysis of the graphic dependence K ( X ) shows that it is necessary to work either on the ascending or descending branch of the dependence. For example, the dependence K ( X ) in the range (0.7 ... 0.8) mm is plotted for an optical fiber with a core diameter d _С = 200 μm, with an aperture angle

=12° (see Fig. 3 b ).

Significant linearization dependenceTO(X) can be achieved if the reflective surface 5 moves in the range of 0.25d _c …0.75d _c along the axisX relative to the ends of 6 optical fibers (OV). For example, whend _C=200 µm movement along the axisX will be 100 µm. Changing the starting distanceX ₀ between ends 5 and 6 leads to a change in the conversion sensitivity up to 30%, dependenceK=f(X) linear, the depth of modulation of the optical signal is more than 50%.

As an example, figure 3 shows the graphical relationship K=f ( X ) for different values of X ₀ .

In view of the fact that the temperature can both decrease and _increase , the initial distance X ₀ should be in the middle of the range 0.25 d _c ... 0.75 d _c 50 ... 150 µm), that is, 0.5 d _c (for example, for OF with d _C = 200 µm - X ₀ = 100 µm).

At the same time, if the sensor is designed to measure either negative or positive temperatures, then the initial distance X ₀ can be shifted either towards smaller values of microdisplacements, or towards larger values. In addition, various dimensional deviations are possible during the assembly of the sensor. Therefore, the final value of X ₀ is determined by expression (2).

The introduction of the hole 17, in which the mirror 18 and additional input 20 and output 21 optical fibers are fixed, is necessary to implement the compensation conversion of the sensor signals. The introduction of a compensation channel makes it possible to implement, for example, a ratiometric transformation described by expression (4), which makes it possible to reduce additional errors due to:

- bending of optical fibers during assembly and operation;

- influence of external influencing factors;

- changing the parameters of the power source, radiation sources and receivers, etc.

The present invention is a technical solution to the problem, which is new, industrially applicable and has an inventive step, i.e. the proposed invention meets the criteria for patentability.

Claims (15)

1. A fiber-optic temperature sensor containing a source and receivers of radiation, input and output optical fibers fixed in a sleeve opposite the reflective surface of the shank inside the housing, characterized in that a cylindrical tube made of a material with a high thermal expansion coefficient is inserted into the sensor, coaxially located inside body, and the inner diameter of the cylindrical tube is at most equal to the outer diameter of the narrow part of the shank and the outer diameter of the narrow part of the sleeve, the ends of the cylindrical tube are threaded hermetically connected to the inner ends of the wide part of the shank and the wide part of the sleeve, and the coefficients of linear expansion of the body materials α _K , shank α _XV , bushings α _VT and cylinder α _C are determined by the relation α _C > α _K > (α _XV ≈ α _WT ), the initial distance X ₀ between the reflective surface and the end of the optical fibers is determined by the expression X ₀ \u003d (0.3 ... 0.7) d _C , where d _C is the diameter of the core of the optical fiber, and the lengths of the body l _K and the cylinder l _C are interconnected by the expression l _K \u003d (0.95 ... 1) l _C α _C / α _K . 2. Fiber-optic temperature sensor according to claim 1, characterized in that a through hole is made in the cylinder wall. 3. Fiber optic temperature sensor according to claim 2, characterized in that the free space between the shank, bushing and cylindrical tube is filled with an inert gas. 4. Fiber-optic temperature sensor according to claim 3, characterized in that a sealing part is hermetically installed in the through hole. 5. Fiber optic temperature sensor according to claim 3, characterized in that the through hole is sealed by soldering with solder. 6. Fiber-optic temperature sensor according to claim 1, characterized in that with a gap relative to the wide part of the shank in the body, a cover made of the same material as the body is hermetically fastened. 7. Fiber-optic temperature sensor according to claim 6, characterized in that the free space between the inner surfaces of the body and cover and the outer surfaces of the cylindrical tube, bushing and wide part of the shank is filled with a heat-conducting compound. 8. Fiber-optic temperature sensor according to claim 1, characterized in that the sleeve has a side blind hole in which a fixed mirror is fixed at a distance X ₀ relative to the end of the additional sleeve, with additional input and output optical fibers fixed. 9. Fiber optic temperature sensor according to claim 8, characterized in that all optical fibers are pulled through a shank, hermetically connected to the body from the side of the sleeve. 10. Fiber-optic temperature sensor according to claim 8, characterized in that the gap between the fixed mirror and the end of the additional sleeve is filled with an inert gas.

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