AU654230B2

AU654230B2 - Optical fiber type temperature distribution measuring apparatus

Info

Publication number: AU654230B2
Application number: AU33804/93A
Authority: AU
Inventors: Osamu Iida; Yuuichi Ushizima
Original assignee: Kawasaki Steel Corp
Current assignee: JFE Steel Corp
Priority date: 1992-02-28
Filing date: 1993-02-25
Publication date: 1994-10-27
Anticipated expiration: 2013-02-25
Also published as: GB2264559B; AU3380493A; GB9303852D0; GB2264559A; JPH05240717A

Description

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AUSTRALIA

Patents Act 1 990

ORIGINAL

COMPLETE SPECIFICATIOP~ STANDARD PATENT 654230 Cr,'

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A A A S CA At t t t Invention Title: OPTICAL FIBER TYPE TEMPERATURE DISTRIBUTION MEASURING APPARATUS p. S

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The following statement is a full description of this invention, including the best method of performing it known to us: GH&CO REF: P19210-BB:TJS:RK p OPTICAL FIBER TYPE TEMPERATURE DISTRIBUTION MEASURING

APPARATUS

BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates to an optical fiber type temperature distribution measuring apparatus for measuring a temperature distribution in an electric power facility, plants of various types, or the like by utilizing Raman scattering light, and in particular, to such an apparatus in which the spatial resolution (or distance resolution) is t improved.

DESCRIPTION OF THE PRIOR ART Recently, as described in an article "Raman scattering light utilized distribution type temperature sensor" f (magazine "SENSOR TECHNOLOGY", Vol. 9, No. 7, May 1989, pp.

1 30 to 34), an optical fiber type temperature distribution measuring apparatus for simultaneously measuring temperatures at multiplicity of positions by using a single optical fiber has been proposed. This apparatus utilizes a phenomenon in which the intensity ratio between a Stokes' line and an anti-Stokes' line which are two components of Raman scattering light changes sensitively depending on a temperature of an optical fiber. In the measurement, a light pulse is transmitted into the optical fiber, and a time (hereinafter referred to as a delay time) until Raman 1I- 11 ft 1 1

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back scattering light returns to a transmitting end of the optical fiber is measured to determine a position at which the scattering light is generated. On the other hand, a temperature of the optical fiber at the position, that is, the position at which the optical fiber is installed is determined from the intensity ratio. Furthermore, by detecting the Raman back IAealter light from respective positiors along the optical fiber on the time division basis, the temperatures at respective positions along the optical fiber, that is, a temperature distribution along the optical fiber can be obtained. The principle of the S t e measurement in this apparatus is illustrated in Fig. 5, and a waveform of the Raman scattering light is shown in Fig. 6, and a relationship between intensity ratio and temperature *6 9 is shown in Fig. 7.

Specifically, as shown in a block diagram in Fig. 4, an optical fiber 2 is installed along an object 1 to be measured in an electric power facility, a plant or the like, and a light pulse 18 is transmitted into the optical fiber *o4oo° within n measuring section 3, from a pulse semiconductor laser 5 which is driven by a pulse driving circuit 4.

o. o Subsequently, Raman back scattering light 19 from each position along the optical fiber 2 is received in the measuring section 3, and a Stokes' line and an anti-Stokes' line which are two components of the Raman back scattering light are splitted or separated by two types of interference filters 7 and 8 in an optical branching filter 6, and the 17 2 2- 4 161 THE CLAIMS DEFINING THE INVENTION AR AS FOLLOWS: THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS: rrr r6rr r*tr r F

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rt CI LI. I t rF tO tt L C e crr rr or tr r rra c r~ .e r rr r cr intensities of the splitted Stokes' line and anti-Stokes' line are respectively photoelectric converted by first and second avalanche photodiodes (APDs). Then, the intensities of these two components are A/D converted in a high speed averaging processing unit 11, and the A/D converted intensities are respectively stored in a memory at locations respectively corresponding to delay times. After all the Raman back scattering light 19 returned from the optical fiber 2, a light pulse 18 is again transmitted into the optical fiber 2, and the detection of Raman back scattering light 19 is carried out, and the obtained intensities are stored by adding to the respective previously stored intensities in the locations of the memory. After repeating these operations a predetermined times (for example, several thousands of times), the intensity values stored in each of the locations of the memory are divided by the number of times of the repetition to obtain an average value. The purpose of this precessing for averaging is to prevent a measurement error from being introduced because of the very weak Raman back scattering light. Thereafter, in the high speed processing unit 11, the intensity ratio is obtained for each of the positions on the basis of the average intensity values of the Stokes' line and the anti-Stokes' line, and the obtained intensity ratios are delivered to a data processing unit 12. In the data processing unit 12 temperature distribution information is obtained on the basis of the intensity ratio at each of the positions along -3 II II I CI I the optical fiber 2. The temperature distribution information is displayed on a screen of a display 13. In this respect, in obtaining the temperature from the intensity of ratio between the Stokes' line and the anti- Stokes' line, a map prepared beforehand by experiments and calculations is used.

However, in such a prior art optical fiber type temperature measuring apparatus, the following problems are involved.

In the prior art apparatus mentioned above, the temperature of the object 1 to be measured is measured as an average value in each segment corresponding to a light pulse *width. For this reason, in order to measure the temperature distribution accurately, it is necessary to enhance the spatial resolution by narrowing the light pulse'width and by shortening a time width for enabling time division Ssampling time interval). However, to narrow the light pulse width means a reduction of data which is averaged, and this naturally results in a deterioration of the accuracy of temperature measurement. Considering these situations, the practical light pulse width, that is, the spatial resolution S1 has been selected to be about I meters. Accordingly, the prior art apparatus cannot be applied to measure an object which requires a spatial resolution which is less than 1 m.

In this case, a conventional apparatus employing a thermocouple had to be used.

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4r ir 4 1 SUMMARY OF THE INVENTION The present invention was made in view of the problems mentioned above, and it is an advantage of at least a preferred embodiment of the present invention to provide an optical fibre type temperature distribution measuring apparatus capable of improving the spatial resolution in the measurement of the temperature distribution without deteriorating the accuracy of temperature measurement.

The present invention provides an optical fibre type temperature distribution measuring apparatus, wherein a light pulse is transmitted into an optical fibre installed in an object to be measured, and a temperature distribution value representing a series of segment average temperatures of segments of the optical fibre is measured on the basis of the intensity of Raman back scattering light and a time period from the transmission of the light pulse until the Raman back t scattering light returns, said optical fibre type temperature distribution measuring apparatus comprising: light pulse oscillating means for oscillating the light pulse having a predetermined pulse width; selective switching means provided at a light pulse transmitting side of the optical fibre and including a plurality of fibre length adjusting optical fibres respectively having different lengths so that a mutual phase difference is saller than a distance resolution in temperature measurement, said selective switching means selectively connecting the plurality of fibre length adjusting optical fibres to th.. optical fibre to form a plurality of detection routes having different lengths; and calculating means for calculating a plurality of temperature distribution measurement values respectively corresponding to the plurality of detection routes, and for calculating a temperature distribution value representing a series of temperature values of a series Sof contiguous subsegments of the optical fibre, wherein a i-f: f: 4$ Li I(t 4(44 1144Q It It

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i: II t ;I 9210-BB L, r segment is divided into a predetermined number of subsegments and the segment length corresponds to the predetermined pulse width of the light pulse.

Preferably a constant temperature tank is further provided at the light pulse transmitting side of the optical fibre, and the constant temperature tank maintains the optical fibre at a predetermined temperature for a range of the fibre longer than the predetermined pulse width of the light pulse.

In an apparatus in accordance with one embodiment of the present invention, when two systems of the detection routes having lengths diffE:rent from each other by a half of the length corresponding to the distance resolution in the temperature measurement, or a half of the light pulse width, the calculating means first calculates the temperature distribution measurement values of the segments of the two systems at respective C c t ransmitting end portions, and thereafter, the temperature distribution measurement value of a 20 subsegment which is a half of the segment and which does cc r.

i i not overlap the segment of the other system is calculated. Then, based on this temperature distribution measurement- value and the temperature distribution C 1.

Smeasurement value of the next tc

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-1I -1 segment, the temperature distribution measurement value of the next half of the segment (subsegment) is calculated. In this manner, by repeating a similar precessing, two times of the spatial resolution can be obtained while measuring by using the same light pulse width as that in the prior art.

Furthermore, when the constant temperature tank is provided, the temperature of the optical fiber within the constant temperature tank can be used as a reference value in calculating the temperature values of the succeeding subsegments.

BRIEF DESCRIPTION OF THE DRAWINGS at* Fig. 1 is a block diagram of an optical fiber type a a temperature distribution measuring apparatus in an embodiment of the present invention.

Fig. 2 is a graph of a temperature distribution measurement value in the embodiment.

a •Fig. 3 is a graph of a temperature distribution measurement value in the embodiment.

ao Fig. 4 is a block diagram of an ontical fiber type temperature distribution measuring apparatus in the prior a art.

SFig. 5 is a schematic view illustrating the principle of the operation of the optical fiber type temperature distribution measuring apparatus.

Fig. 6 is a diagram of a waveform of Rarian scattering light.

7 '.11 Fig. 7 is a graph of a relationship between intensity ratio and temperature.

DESCRIPTION OF THE PREFERRED EMBODIMENT The embodiment of the present invention will be described with reference to the drawings. Like reference numerals designate like or corresponding parts throughout the drawings.

With reference to Fig. 1, similar to the prior art apparatus, a pulse driving circait 4 for oscillating a light pulse, and a pulse semiconductor laser (hereinafter referred ri, to as LD) 5 are provided in a measuring section 3, and a 1t t light pulse 18 emitted by the LD 5 is transmitted into an optical fiber 2 through an optical branching filter 6.

However, in the present embodiment, within the measuring section 3 and at a base portion of the optical fiber 2, there is 'srovided with an optical switch 14 as a selective switching means, and this optical switch 14 is driven in synchronism with the LD 5 by the pulse driving circuit 4.

The optical switch 14 includes a pair of fiber length adjusting optical fibers 15 and 16 having lengths different cc from each other, for example, by a half (0.5 m) of a distance resolution which is equal to a pulse width (1 m in the present embodiment) of the light pulse 18, and when the optical switch 14 is driven, either one of the fiber length adjusting optical fibers 15 or 16 is instantly connected to the optical fiber 2. Accordingly, in the apparatus of the -8- J i r c" -r present embodiment, twc types of optical fibers 2 (i.e.

detection routes) having lengths different from each other by 0.5 m are provided. In the present embodiment, a shorter detection route is referred to as a first route, and a longer route is referred to as a second route.

On the other hand, in the vicinity of the measuring section 3, there is disposed with a constant temperature tank 17 for accommodating a part of the optical fiber 2, and the temperature of the optical fiber 2 is maintained constant in a predetermined segment (1 m in the present embodiment). The optical fiber 2 exiting from the constant temperature tank 17 is installed along an object 1 to be measured similar to the prior art apparatus.

Furthermore, two types of interference filters 7 and 8 built in the optical branching filter 6, first and second avalanche photodiodes (hereinafter referred to as APD) 9 and and a high speed averaging processing unit 11 are also provided in the measuring section 3, and these parts are similar to that in the prior art apparatus. A data processing unit 12a as a calculating means, and a display 13 are provided at the outside of the measuring section 3. As compared with the data processing unit 12 in the prior art apparatus, a data processing unit 12a in the embodiment calculates two temperature distribution measurement values which are displaced in phase from each other, on the basis of the intensity ratio between a Stokes' line and an anti- Stokes' line at each position cilong the first and second 9-

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i t*R\ ill -ji i l'bi:- i: eti C Lt $7 1 routes delivered from the measuring section 3, and further, based on the two temperature distribution measurement values respectively for the first and second routes, calculates a temperature distribution value consisting of a series of temperature values of respective subsegments, each of the subsegments equals a half of the segment corresponding the pulse width.

The operation in the embodiment will be described.

In the measurement of the temperature distribution along the optical fiber 2, first, the LD 5 and the optical switch 14 are driven by the light pulse driving circuit 4 to transmit light pulses successively and at the same time, the detection routes are switched between the first route and the second route.

For example, first, when a light pulse 18 is transmitted to the first route, a scattering is caused at each position along the optical fiber 2, and back scatterning light returns to the transmitting end of the optical fiber 2. A Stokes' line and an anti-Stokes' line which are two components of the back scatterning light are splitted or separated by the two types of interference filters 7 and 8, and photoelectric converted respectively by the first and second APDs 9 and 10. Subsequently, in the high speed averaging processing unit 11, the intensities of the two components are A/D converted and stored in locations in a memory respectively corresponding to delay times. The procedure described above is similar to that in the prior 10 art apparatus. However, in the present embodiment, after all the back scattering light from the first route is returned, a light pulse is transmitted to the second route, and in a similar procedure, the intensities of the two components are stored by adding to the previous respective intensities in the memory in the high speed averaging processing unit 11. After repeating the above-mentioned operation for a multiplicity of times, the intensities are divided by the number of times of the repeated operations to perform an averaging processing for each of the first and second routes. Subsequently, the intensity ratio between the intensity of the Stokes' line and the intensity of the anti-Stokes' line is obtained for each position and for each of the first and second routes.

I I Thereafter, in the data processing unit 12, two temperature distribution measurment values respectively for the first and second routes as shown in Figs. 2 and 3 are "c't produced based on the intensity ratios delivered from the measuring section 3. These two temperature distribution 1 mmeasurement values respectively for the first and second routes are phase displaced by 0.5 m due to the difference in the route lengths. The measurement starting point in Figs.

2 and 3, that is, a reference point in the distance is selected at an entrance of the constant temperature tank 17, a point P in Fig. I. In this case, the determination of the measurement starting point is made by calculating a delay time of the back scattering light based on a velocity L of -11 4 p i i resolution in temperature measurement, said selective switching means selectively connecting the plurality of fibre length adjusting optical fibres to the optical 1_1 d i 1 i; 1 r i i

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the light pulse in the optical fiber 2 and a distance between the LD 5 and the constant temperature tank 17.

Furthermore, the characters TI T2 T in Fig. 2 represent segment average temperatures of a series of segments in the first route, each of the segments corresponding to 1 m, and the characters T 2 1

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22 T Fig. 3 represent segment average temperatures of a series of segments in the second route, each of the segments corresponding to 1 m. The characters tl, t2, Figs. 2 and 3 represent subsegment average temperatures of a series of subsegmerts in the first and second routes, each of the subsegments corresponding to m or a half of the segment, however, at this stage, these subsegment average temperatures have not yet known.

Subsequently, in the data processing unit 12, the subsegment average temperature of each subsegment of 0.5 m is calculated on the basis of the two temperature distribution measurement values each consisting of the series of segment average temperatures of 1 m segments.

Firstly, a first and a second subsegment average temperature tl and t2 are calculated. In the first route, since the segment of 0 to 1 m is within the constant temperature tank 17 as described in the foregoing, the temperature is maintained at the constant value. Accordingly, both the subsegment average temperatures tI and t2 are equal to the segment averace temperature T 11 and thus, tl t2 T 11 Next, in the data processing unit 12, the subsegment

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i ii i i r i t average temperature t3 is calculated. In the second route, since the segment average temperature T21 of the segment of to 1.5 m is an average value of the subsegment average temperatures t2 and t3, T21 T2 t3 2 and thus, t3 2 T t2. Here, since t2 T it is obtained that t3 2 T21 Tt Furthermore, in a similar procedure, the subsegment average temperatures t4 can be obtained from the segment average temperature T12 Of the segment of 1 to 2 m in the first route and subsegment average temperature t3.

In other words, T12 t3 t4 2 and thus, t4 2 T12 t3 2 T12 2 T21 T i Likewise, after obtaining the subsegment average temperatures t5, and so on, sequentially, the data processing unit 12 forms temperature distribution information from these subsegment average temperatures tl, t2, t3, and this temperature distribution information is displayed on a screen of the display 13.

As described above, in the apparatus in the present invention, by providing the optical switch 14 incorporating therein the two fiber length adjusting optical fibers 15 and 16, and the constant temperature tank 17, it is possible to obtain 'the spatial resolution as large as two times the spatial resolution in the prior art apparatus. However, the present invention is not limited to this embodiment. For example, when the number of routes is increased by incorporating three or more fiber length adjusting opcical fibers in the optical switch 14, it is possible to ,further l f 13 ii I c;, 4t enhance the spatial resolution. Furthermore, in the present invention, although the constant temperature tank 17 is used as a reference in dividing the optical fiber into the segments, it is also possible as a reference for temperature correction. Moreover, so long as the temperature is stable, a test room or the like may be used as the constant temperature tank. Furthermore, in the present embodiment, although it is described as to the case wherein the distance resolution is equal to the pulse width of the light pulse which is emitted from the semiconductor laser, the present invention is not limited to this, and an arbitrary distance resolution which is determined from a light pulse width and a time interval which is feasible for time division (sampling) may be used.

By virtue of the arrangement as described above, the present invention provides the following advantages In the ptical fiber type temperature distribution measuring apparatus in the present invention, a plurality of temperature distribution measurement values with their phases displaced from each other are obtained by switching a plurality of routes by the optical switch, and based on these values, a temperature distribution measurement value including a series of subsegment average temperatures is obtained, in which each segment corresponding to the distance resolution in the temperature measurement is divided into a plurality of subsegments. As a result, the spatial resolution in the temperature distribution 14 rCL u o 1I i measurement is improved without deteriorating the temperature measurement accuracy and without modifying the prior art apparatus to a great extent, and the application field of the apparatus can be extended significantly.

Furthermore, when the constant temperature tank is provided, the temperature of the optical fiber within the constant temperature tank is used as a reference value in dividing into the segments, and the calculation of the temperature distribution measurement value can be achieved with a very high accuracy.

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Claims

1. An optical fibre type temperature distribution measuring apparatus, wherein a light pulse is transmitted into an optical fibre installed in an object to be measured, and a temperature distribution value representing a series of segment average temperatures of segments of the optical fibre is measured on the basis of the intensity of Raman back scatter'ing light and a time period from the transmission of the light pulse until the Raman back scattering light returns, said optical fibre type temperature distribution measuring apparatus comprising: light pulse oscillating means for oscillating the light pulse having a predetermined pulse width; selective switching means provided at a light pulse transmitting side of the optical fibre and including a plurality of fibre length adjusting optical mutual phase difference is smaller than a distance resolution in temperature measurement, said selective switching means selectively connecting the plurality of fibre length adjusting optical fibres to the optical C CI fibre to form a plurality of detection routes ha-ving different lengths; and C calculating means for calculating a plurality of c c c ttemperature distribution measurement values respectively corresponding to the plurality of detection routes, and for calculating a temperature distribution value representing a series of temperature values of a series of contiguous subsegments of the optical fibre, wherein a segment is divided into a predetermined number of subsegments and the segment length corresponds to the predetermined pulse width of the light pulse.

2. An optical fibre type temperature distribution measuring apparatus according to claim 1 further comprising: a constant tCemperature tank located at a downstream side of said selective switching means, for Vi -411/19210-66 V ja u- I= A 1LIe1 cy ratio at eacn or the positions along 3 K:: 17 maintaining a temperature of a range of the optical fibre at a predetermined temperature, said range being at least longer than the predetermined pulse width of the light pulse.

3. An optical fibre type temperature distribution measuring apparatus, substantially as herein described with reference to the accompanying drawings. Dated this 22nd day of August 1994 KAWASAKI STEEL CORPORATION By their Patent Attorney GRIFFITH HACK CO. fCcE C Ci C Ct ~c C C C C C C L C C LC C C. CC C: C CC, I C :C CC /12 RAB /1 9210-BBI i" ABSTRACT OF THE DISCLOSURE off- oco (A

64.' An optical switch (14) is disposed with in a measuring section and connected to a base portion of an optical fiber The optical switch (14) includes a pair of adjusting optical fibers (15, 16) having lengths different from each other by a half of a distance resolution, and the optical switch (14) is driven by a pulse driving circuit in synchronism with a semiconductor laser which emits a light pulse. When the optical switch (14) is driven, either one of the adjusting optica_ oers (15, 16) is selectively connected to the optical fiber so that two detection routes having different lengths are formed. The temperature distribution measurement values respectively for the two detection routes and having a phase displaced from each other corresponding to the half of the distance resolution are calculated in a high speed averaging processing unit and then a temperature distribution value along the optical fiber is calculated in a data processing unit Furthermore, a constant temperature tank (17) is disposed near the measuring section to accommodate a predetermined segment of the optical fiber, and the temperature of the optical fiber is maintained constant for the predetermined segment.