DE102018205722A1 - Distance measurement by reflectometry in optical waveguides - Google Patents

Abstract

The invention provides a sensor arrangement for distance measurement and a method for operating such a sensor arrangement. Two partial reflecting portions of an optical waveguide are fixed at two points whose distance is to be measured. By means of reflectometric measuring methods, the distance between the partially reflecting areas and thus between the two points can be measured precisely and accurately. For this purpose, the sensor arrangement is formed with: an optical waveguide (20) having a first partial light reflector device (21) and a second partial light reflector device (22) biased between the first position and the second position; light introducing means (12) for introducing intensity-modulated light into the optical waveguide (20); andan evaluation device (19) for determining a frequency spectrum of light reflected at the first and second partial light reflector means (21, 22); wherein the evaluation means (19) is further adapted to output an output signal based on the determined frequency spectrum.

Description

Technical field of the invention

The invention relates to a sensor arrangement for distance measurement by means of reflectometry in optical waveguides and to a method for operating a sensor arrangement for distance measurement by means of reflectometry in optical waveguides.

State of the art

In many industrial and energy applications, distances must be measured without contact and very precisely. Frequently, a relative shift of machine parts in operation relative to each other must be determined. In particular in hard-to-reach areas, at high temperatures and in machine parts at high-voltage potential, conventional electrical sensors based on e.g. Capacitive or inductive measuring principles to their limits.

An example of such measuring methods is a displacement of a stator winding of a large electric machine against a housing or against a laminated core. In such situations it may sometimes be necessary to measure displacements with resolutions of +/- 10 micrometres at absolute distances of 10 meters in measuring ranges of +/- 20 millimeters.

Various optical technologies for distance measurement are known. Optical triangulation sensors are comparatively accurate and available for various measuring ranges, but have disadvantages in harsh environments, since the optoelectronic transmitting and receiving electronics is sometimes sensitive to external influences. In addition, the sensor head such triangulation sensors is relatively large.

Known methods for interferometric distance measurement are often very expensive and can not perform absolute, but only relative measurements.

Furthermore, distance sensors exist, which determine the propagation time of a directed light pulse between a sensor and a measurement object (optical time domain reflectometry, OTDR). For large distances partially laser optical free-jet sensors are used, which are based on interferometric basis or on the measurement of the light transit time.

Also known is the optical frequency domain reflectometry (OFDR). OFDR does not work like the OTDR technology in the time domain but in the frequency domain. In the OFDR method, a statement about the local temperature profile is obtained if the backscatter signal detected during the entire measuring time is measured as a function of the frequency and thus complex (complex transfer function) and then Fourier-transformed.

An OFDR method is for example in US Pat. No. 7,515,276 B2 described.

Compared to the known methods, the present invention solves the problem of providing a particularly accurate and accurate measurement method for absolute distances even under difficult environmental conditions.

Summary of the invention

According to a first aspect of the invention, this object is achieved by a sensor arrangement for distance measurement, comprising: a first optical waveguide having a first partial light reflector device at a first position within the optical waveguide and a second partial optical reflector device at a second position within the optical waveguide;
wherein the optical fiber is biased at least between the first position and the second position;
a light-emitting device for introducing light into the optical waveguide;
an evaluation device for determining a frequency spectrum of light reflected at the first partial light reflector means and light reflected at the second partial light reflector means;
wherein the evaluation device is further configured to output an output signal based on the determined frequency spectrum.

By a partial light reflector device is meant any region, or any element, within an optical waveguide capable of partially transmitting light introduced into the optical waveguide, i. partially, to reflect and to partially transmit.

The light-emitting device may comprise a light source, such as a light source. a laser diode for generating light. The light provided by the light-emitting device can also be labeled as a light signal, as a test signal or as a probe signal.

In particular, the output signal can indicate a distance between the first and the second partial light reflector device, for example by the output signal carrying a message over a light transit time difference or directly over a distance. An evaluation of the output signal for Actual determination of the distance in distance measuring units can for example be carried out downstream by an external calculation unit.

In particular, the evaluation device may be a microcontroller or an application-specific integrated circuit (ASIC).

• - Einleiten von intensitätsmoduliertem Licht in den ersten Lichtwellenleiter;
• - Empfangen von an der ersten partiellen Lichtreflektoreinrichtung reflektiertem Licht;
• - Empfangen von an der zweiten partiellen Lichtreflektoreinrichtung reflektiertem Licht;
• - Bestimmen eines Frequenzspektrums des von der ersten und der zweiten partiellen Lichtreflektoreinrichtung reflektierten Lichts;
• - Bestimmen eines Abstands zwischen der ersten partiellen Lichtreflektoreinrichtung und der zweiten partiellen Lichtreflektoreinrichtung basierend auf dem bestimmten Frequenzspektrum.

Furthermore, according to a second aspect of the invention, a method for operating a sensor arrangement according to the first aspect of the invention is provided, with the steps:

• - introducing intensity-modulated light into the first optical waveguide;
• Receiving light reflected at the first partial light reflector means;
• Receiving light reflected at the second partial light reflector means;
• Determining a frequency spectrum of the light reflected by the first and second partial light reflector means;
• Determining a distance between the first partial light reflector means and the second partial light reflector means based on the determined frequency spectrum.

The device according to the invention as well as the method according to the invention make it possible to determine a distance by means of an OFDR method, wherein even with large absolute distances small changes in distance can be precisely and accurately measured.

Further advantages and variants will become apparent from the dependent claims and from the description with reference to the figures.

According to some advantageous embodiments, the sensor arrangement comprises at least one further partial light reflector device at at least one further position within the first optical waveguide. Also, the reflected light signals of these at least one other, e.g. third, partial light reflector means may be evaluated and used to determine the distance and / or to generate the output signal. For example, the reflected light signals of the at least one further partial light reflector device can be used for plausibility checking or for even more accurate determination of the distance.

According to some advantageous embodiments, at least one of the partial light reflector means is realized as an air gap. A small air gap leads to the so-called Fresnel reflection and provides a back reflection of a few percent of the incident light or light signal.

According to some advantageous embodiments, at least one of the partial light reflector devices is realized as a structure inscribed with a laser with a different refractive index, in particular as a fiber Bragg grating.

According to some advantageous embodiments, at least one of the partial light reflector means is formed as an optical waveguide piece of different refractive index spliced into the first optical waveguide.

According to some advantageous embodiments, the sensor arrangement comprises a protective tube, in which the first optical waveguide extends at least in regions. Through the protective tube, the optical waveguide is at least partially protected against external influences. Advantageously, the protective tube is mechanically decoupled from the optical waveguide when it is stretched open.

According to some advantageous embodiments, the optical waveguide comprises a polymeric optical fiber ("POF" or "plastic optical fiber") or consists of one or more polymeric optical fibers. Optical fibers made of such fibers allow a higher mechanical strain than optical fibers, for example of quartz glass.

According to some advantageous embodiments, the light introduction device and the evaluation device are connected to a plurality of optical waveguides each having at least two partial light reflector devices at different positions within the respective optical waveguide. The evaluation device can be designed to determine the frequency spectrum of the light reflected at the respective partial light reflector devices of each optical waveguide and to output at least one output signal based thereon. Thus, with little effort in hardware, a plurality of distances can be determined. In this case, all partial light reflector devices in different absolute optical distances within the optical waveguides are particularly advantageously arranged by the evaluation device, so that the light signals can be time-multiplexed.

list of figures

• 1 ein schematisches Blockdiagramm einer Sensoranordnung gemäß einer Ausführungsform des ersten Aspekts der vorliegenden Erfindung;
• 2 eine schematische Schrägansicht eines Teils einer Sensoranordnung gemäß einer weiteren Ausführungsform des ersten Aspekts der vorliegenden Erfindung; und
• 3 ein schematisches Flussdiagramm zum Erläutern eines Verfahrens gemäß einer Ausführungsform des zweiten Aspekts der vorliegenden Erfindung.

The present invention will be explained in more detail with reference to the exemplary embodiments given in the schematic figures. It shows:

• 1 a schematic block diagram of a sensor arrangement according to an embodiment of the first aspect of the present invention;
• 2 a schematic oblique view of a portion of a sensor arrangement according to another embodiment of the first aspect of the present invention; and
• 3 a schematic flowchart for explaining a method according to an embodiment of the second aspect of the present invention.

The accompanying figures are intended to convey a further understanding of the embodiments of the invention. They illustrate embodiments and, together with the description, serve to explain principles and concepts of the invention. Other embodiments and many of the stated advantages will become apparent with reference to the drawings. The elements of the drawings are not necessarily shown to scale to each other.

In the figures of the drawing are the same, functionally identical and same-acting elements, features and components - unless otherwise stated - each provided with the same reference numerals.

Detailed description of the embodiments

1 shows a schematic block diagram of a sensor arrangement 10 for distance measurement according to an embodiment of the first aspect of the present invention.

The sensor arrangement 10 includes an optical fiber 20 which preferably comprises a polymeric optical fiber ("POF") or consists of one or more polymeric optical fibers.

In the optical fiber 20 At a first position is a first partial light reflector means 21 formed, which in the optical waveguide 20 introduced light partially reflected. At a second position in the optical fiber 20 is a second partial light reflector device 22 formed, which in the optical waveguide 20 introduced light also partially reflected.

The two partial light reflector devices 21 . 22 may partially reflect light in equal proportions or in different proportions. There may also be more than two partial light reflector means within the optical fiber 20 be provided.

As already explained above, the partial light reflector devices 21 . 22 in particular as air gaps, as a fiber Bragg grating or as spliced optical waveguide pieces of refractive index other than the remaining optical waveguide 20 be educated. Several of the at least two partial light reflector means 21 . 22 may be formed in the same way; but it can also all partial light reflector devices 21 . 22 be formed in various ways.

The first and second partial light reflector means 21 . 22 are each fixed at the points between which a distance L is to be measured, for example on two machine parts. For example, one may be the first partial light reflector device 21 on a stator winding and the second partial light reflector means 22 be arranged on a housing of the stator winding.

The optical waveguide is preferred 20 in its rest position, eg at a zero position of the distance to be measured L , pretensioned. In this way, also negative changes in length of the optical waveguide 20 be detected because due to the bias of the optical fibers 20 then advantageously contract, rather than sag about without its length change.

The sensor arrangement 10 also has a light-emitting device 12 for introducing intensity-modulated light into the optical waveguide. For this purpose, the light introduction device 12 for example, a laser diode 13 and one with the laser diode 13 coupled light modulation device 14 , eg a Mach-Zehner modulator.

Through an optical element 16 through, which may be, for example, a beam splitter or an optical isolator, is light from the light-emitting device 12 in the optical fiber 20 initiated. In the optical element 16 it may also be, for example, a 1x2 coupler or a 1x4 coupler.

Via a photodiode 18 for converting the reflected light signals into electrical signals is an evaluation device 19 to the optical element 16 connected. The evaluation device 19 For example, it may be formed as a microprocessor or as another evaluation circuit structure, eg an ASIC, an FPGA or the like. That of the partial light reflector devices 21 . 22 reflected light is transmitted through the optical element 16 passed to the evaluation device 19. About the optical element 16 is also originally in the optical fiber 20 coupled-in light signal from the light-emitting device 12 the evaluation device 19 provided.

From the evaluation device 19 can, using the originally coupled light signal, a frequency spectrum of the partial light reflector means 21 . 22 reflected light, ie a frequency response. t, so that the frequency response can be correlated with the originally emitted light signal.

Subsequently, the frequency response can be Fourier transformed to obtain a result in the time domain or time domain information, which in turn can be deduced from the distance L of the partial light reflector devices 21 . 22 allows. Thus, already the output signal of the evaluation 19 the distance L between the partial light reflector means 21 . 22 indexed and understood as a distance measurement signal. The evaluation device 19 may also include a circuit, or a software component, which performs the Fourier transform of the particular frequency spectrum, so that the output signal of the evaluation device 19 already specify time range information. The evaluation device 19 can also be designed so that their output signal directly to be measured distance L between the partial light reflector devices 21 . 22 or indicates.

One or more of the above-described steps may optionally also be performed by a terminal 50 , In particular, a mobile terminal such as a laptop, a tablet, a smartphone or the like, are performed, which to the evaluation 19 is connectable. The evaluation device 19 can accordingly an interface for connecting a terminal 50 , in particular a mobile terminal.

It is understood that the sensor arrangement 10 also several optical fibers 20 may include, in which by the light introduction device 12 a light signal is introduced and a respective frequency response by the evaluation device 19 is determined. The optical element 16 For this purpose, for example, can be coupled to a 1x2 optical coupler or a 1x4 optical coupler, or even consist of such a coupler.

In this case it is advantageous if all partial light reflector devices have different absolute optical distances to the evaluation device 19 exhibit. This is done for example by appropriate supply fibers in the individual channels, ie in front of the individual optical waveguides 20 , realized. In this way, a multiplexed evaluation of the frequency responses of the various optical fibers 20 possible.

The multiple optical fibers 20 a single sensor arrangement 10 For example, they can all be arranged at the same distance L or to measure different correlated distances so that the individual distance measurement results can be used for mutual plausibility or for calculating a statistical average (eg, an arithmetic mean or a median). Thus, the accuracy of the distance measurement can be further improved by simple means.

2 shows a schematic oblique view of a part of a sensor arrangement 10 for distance measurement according to another embodiment of the first aspect of the present invention.

The sensor arrangement 10 according to 2 is a variant of the sensor arrangement 10 according to 2 and differs from this in that in addition a protective tube 40 is provided, in which the optical waveguide 20 at least partially.

In 2 are also fixation devices 31 . 32 shown which the partial light reflector means 21 . 22 biased in the rest position in the distance L hold according to a zero position. The fixing devices 31 . 32 can be realized in simple cases, for example by gluing or clamping. The optical fiber 20 However, it can also be fixed in a ferrule, which is mechanically held, so that the optical waveguide 20 can be biased by a mechanical device. Decreases the distance L , takes the bias in the optical fiber 20 ab, the distance between the partial light reflector means 21 . 22 also decreases, and the frequency response of the optical fiber 20 changes accordingly. The distance increases L , takes the bias in the optical fiber 20 , the distance between the partial light reflector means 21 . 22 increases, and the frequency response of the optical fiber 20 changes again.

3 shows a schematic flowchart for explaining a method according to an embodiment of the second aspect of the present invention for operating a sensor arrangement 10 according to an embodiment of the first aspect of the present invention.

The method according to 3 Thus, according to all with respect to the sensor arrangement according to the invention 10 adapted variants and modifications and vice versa.

In one step S10 becomes intensity modulated light, or in other words a light signal, in the first optical fiber 20 initiated, for example, as in the foregoing with respect to the light-emitting device 12 described. In particular, the step S10 two Sub-steps include: a step S11 producing light, such as through the laser diode 13 , and one step S12 intensity modulating the generated light, such as by the light modulation means 14 as described above.

In one step S20 is from the first partial light reflector means 21 receive reflected light. In one step S30 is from the second partial light reflector means 22 receive reflected light.

In one step S40 is, by the evaluation 19 and / or the terminal 50 , a frequency spectrum of that of the first and second partial light reflector means 21 . 22 received light as described above.

In one step S50 is, by the evaluation 19 and / or the terminal 50 , the distance L determined based on the determined frequency spectrum as described above.

In the foregoing detailed description, various features have been summarized to improve the stringency of the illustration in one or more examples. It should be understood, however, that the above description is merely illustrative and not restrictive in nature. It serves to cover all alternatives, modifications and equivalents of the various features and embodiments. Many other examples will be immediately and immediately apparent to one of ordinary skill in the art, given the skill of the art in light of the above description.

The exemplary embodiments have been selected and described in order to represent the principles underlying the invention and their possible applications in practice in the best possible way. As a result, those skilled in the art can optimally modify and utilize the invention and its various embodiments with respect to the intended use.

A simple idea of the invention can be summarized as follows: Two partial reflecting areas of an optical waveguide are fixed at two points whose distance is to be measured. Reflectometric measuring methods allow a precise and accurate measurement of the distance between the partially reflecting areas and thus between the two points.

LIST OF REFERENCE NUMBERS

10

sensor arrangement

12

Light introducing means

13

laser diode

14

Light modulator device

16

optical element

18

diode

19

evaluation

20

first optical fiber

21

first light reflector device

22

second light reflector means

31

first fixing device

32

second fixing device

40

thermowell

50

terminal

S10

Initiate a light signal

S11

Generating light

S12

Intensity modulating light

S20

Receiving reflected light

S30

Receiving reflected light

S40

Determining the frequency spectrum

S50

Determining a distance

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• US 7515276 B2 [0008]

Claims (10)

A sensor arrangement (10) for distance measurement, comprising: a first optical waveguide (20) having a first partial light reflector means (21) at a first position within the optical waveguide (20) and a second partial light reflector means (22) at a second position within the optical waveguide (20 ); wherein the first optical fiber (20) is biased at least between the first position and the second position; a light-emitting device (12) for introducing intensity-modulated light into the optical waveguide (20); and an evaluation device (19) for determining a frequency spectrum of light reflected at the first and second partial light reflector means (21, 22); wherein the evaluation device (19) is further adapted to output an output signal based on the determined frequency spectrum. Sensor arrangement (10) according to Claim 1 , with at least one further partial light reflector device at at least one further position within the first optical waveguide (20). Sensor arrangement (10) according to Claim 1 or 2 , wherein at least one of the partial light reflector means (21, 22) is realized as an air gap. Sensor arrangement (10) according to one of Claims 1 to 3 wherein at least one of the partial light reflector means (21, 22) is formed as a fiber Bragg grating. Sensor arrangement (10) according to one of Claims 1 to 4 wherein at least one of said partial light reflector means (21, 22) is formed as an optical waveguide piece of different refractive index spliced into said first optical waveguide (20). Sensor arrangement (10) according to one of Claims 1 to 5 , with a protective tube (40), in which the first optical waveguide (20) extends at least in regions. Sensor arrangement (10) according to one of Claims 1 to 6 wherein the optical waveguide (20) comprises a polymeric optical fiber. Sensor arrangement (10) according to one of Claims 1 to 7 wherein the light introducing means (12) and the evaluating means (19) are connected to a plurality of optical waveguides (20) each having at least two partial light reflector means (21, 22) at different positions within the respective optical waveguide (20); and wherein the evaluation device (19) is designed to determine the frequency spectrum of the light reflected at the respective partial light reflector means (21, 22) of each optical waveguide (20) and to output at least one output signal based thereon. Sensor arrangement (10) according to Claim 8 , wherein all the partial light reflector means (21, 22) are arranged at different absolute optical distances within the optical waveguide (20) of the evaluation device (19). Method for operating a sensor arrangement (10) according to one of the Claims 1 to 9 , comprising the steps of: - introducing (S10) intensity-modulated light into the first optical waveguide (20); Receiving (S20) light reflected at the first partial light reflector means (21); - receiving (S30) reflected light from the second partial light reflector means (22); - determining (S40) a frequency spectrum of the light reflected by the first and second partial light reflector means (21, 22); - determining (S50) a distance (L) between the first partial light reflector means (21) and the second partial light reflector means (22) based on the determined frequency spectrum.

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