EP0874998A1

EP0874998A1 - Optical measuring method and device for measuring a magnetic alternating field with an expanded measuring range and good linearity

Info

Publication number: EP0874998A1
Application number: EP97900955A
Authority: EP
Inventors: Thomas Bosselmann; Peter Menke
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1996-01-18
Filing date: 1997-01-03
Publication date: 1998-11-04
Also published as: JP2000503388A; CA2243211A1; US6114846A; DE19601727C1; WO1997026547A1

Abstract

After passing through a Faraday sensor arrangement (3), light (L) to be measured, which is polarized in a linear manner, is divided into two partial light signals (L1, L2) with polarization planes inclined at 45°. A measuring signal (M) which is proportional to the tangent of the Faraday angle of rotation is derived from the two partial light signals.

Description

description

Optical measuring method and optical measuring arrangement for measuring an alternating magnetic field with an extended measuring range and good linearity

The invention relates to a method and an arrangement for measuring a magnetic alternating field. A magnetic alternating field is understood to mean a magnetic field which only has frequency components which differ from zero in its spectrum.

Optical measuring arrangements for measuring an electrical current in a current conductor are known which are based on the magneto-optical Faraday effect and are therefore also referred to as magneto-optical current transformers. In the case of a magneto-optical current transformer, linearly polarized measuring light is transmitted through a Faraday sensor device which is arranged in the vicinity of the current conductor and consists of an optically transparent material which shows the Faraday effect. Because of the Faraday effect, the magnet: field generated by the current causes the plane of polarization of the measuring light to rotate by an angle of rotation p which is proportional to the path integral over the magnetic field along the path traveled by the measuring light ir the sensor device. The proportionality constant is called the Verdet constant V. The Verdet constant V generally depends on the material and the temperature of the sensor device and on the wavelength of the measuring light used. in general, the sensor device surrounds the current conductor, so that the measuring light rotates the current conductor at least once in a practically closed path. The angle of rotation p is in this case essentially directly proportional to the amplitude I of the current to be measured according to the relationship p = N • V ■ I (1), where N is the number of revolutions of the measurement light around the current conductor. The Faraday angle of rotation p is pclarimetrically determined by a folarization analysis of the NEN measurement light determined to obtain a measurement signal for the electrical current.

For polarization analysis, it is known to break down the measuring light after passing through the sensor device from an analyzer into two linearly polarized light components L 1 and L 2 with polarization planes oriented perpendicular to one another. Polarizing beam splitters such as, for example, a Wollaston prism or a simple beam splitter with two downstream polarizers, whose polarization axes are rotated by π / 2 or 90 ° relative to one another, are known as analyzers. Both light components L1 and L2 are each converted into an electrical intensity signal T1 or T2 by an assigned photoelectric converter, which is proportional to the light intensity of the respective light components L1 and L2. A measurement signal is made from these two electrical signals

T = (Tl - T2) / (Tl + T2) (3), which corresponds to the quotient of a difference and the Suinme of the two intensity signals Tl and T2

(WO 95/10046). This measurement signal T is the same when neglecting interference

T = sin (2p + ζ) = sin (2 * N * V * I * ζ) * 4 ⁾ , where ζ is one of the angle between the plane of polarization of the measuring light when coupled into the Faraday element and an excellent optical axis of the analyzer dependent offset angle for I = 0 A.

Although the Faraday measuring angle p itself gernäß equation (1) is a linear and therefore unique function of the current I is the test signal T according to equation (4) extends only over a maxi ^¬ π times / 2 (or 90 °) wide range of angles a unique radio ^¬ tion of Meßwinkeis p. Thus, with these polarimetric magneto-optical current transformers, only electrical currents that are within a current measuring range can be clearly measured

(Current measuring intervals, measuring ranks)) MR of the interval length | MR | = π / (2-NV) ⁽ 5 ⁾ lie. It can be seen from equation (5) that the quantity | MR | of the current measuring range MR of a magneto-optical current converter by the choice of materials with different Verdet constants V for the Faraday element and / or by the number N of the cycles of the measuring light around the current conductor. A larger current measuring range is obtained if the product N * V is set smaller in the denominator. However, such a choice of a larger current measuring range MR inevitably entails a reduced measuring resolution (measuring resolution) MA of the current transformer for a given display resolution. The measurement resolution MA is the amount | MS | the measuring sensitivity MS of the current transformer is defined. The measuring sensitivity MS corresponds to the slope of the characteristic of the magneto-optical current transformer at an operating point and is equal to MS = dT / dl = 2 ■ N • V • cos (2-NV * I + ζ) in a two-channel evaluation according to equation (4) (6). It can be seen immediately from equation (6) that a reduction in the product N * V in both evaluation methods leads to a reduction in the measurement resolution MA = | MS | leads.

A magneto-optical current transformer is known from EP-B-0 088 419, in which two Faraday glass rings are arranged parallel to one another around a common current conductor, which consist of Faragay materials with different Verdet constants and thus each for different current measuring ranges have. Each Faraday glass ring is assigned a transmitter unit for transmitting linearly polarized measurement light into the glass ring and a two-channel evaluation unit for calculating a respective measurement signal for the respective Faraday angle of rotation. The two measurement signals of the two evaluation units are fed to an OR gate, which determines a maximum signal from the two measurement signals. This maximum signal is used to switch between the measuring ranges of the two glass rings. Different measuring ranges of the two glass rings can also be achieved with the same glass material for both glass rings by measuring light different Wavelength is used. The wavelength dependence of the Faraday rotation is exploited.

From "SENSOR 93 Kongreßband IV Bll. 1, Be ten 137 to 144" is known in magneto-optical current transformers for protective purposes for measuring, alternating currents, in which linearly polarized light is split into two partial light signals after passing through a Faraday optical fiber and each of these light partial signals is fed to an analyzer. The natural axes (polarization axes) of the two analyzers are oriented at an angle of 45 ° or 58 ° to each other. The light incidences let through by the analyzers are only normalized by division by their direct components, which are obtained by peak value rectification. A product of the standardized signals is then formed and this

Product then differentiated. The Faraday rotation angle is obtained directly through integration. This gives a signal which is proportional to the current and is therefore not subject to any measuring range restrictions. However, this method is comparatively complex.

From EP-B-0208593 a magneto-optical current transducer is known in which linearly polarized measuring light by pres ^¬ a fen a conductor surrounding the Faraday optical fiber by a beam splitter into two Lichcteilsignale is divided and an analyzer to ^¬ is performed each of these partial light signals. The natural axes of the two analyzers are directed at an angle of 0 ° or 45 ° to the coupling polarization of the measuring light. This provides an analyzer, a first sinusoidal signal at the output and at the output of the other _A nalysators a second cosine signal. These two signals are in each case ambiguous, oscillating functions of the current in the current conductor, which are phase-shifted from one another by an angle of 90 °. For these examples the ambiguous signals is now an unambiguous measurement signal composed by comparison of the sign and the Be ^¬ slow the measured values of the first sinusoidal signal and the second, cosine-shaped signal. As soon as the sine and cosine magnitudes are equal, i.e. with an integer multiple of 45 °, depending on the signs of the sine and cosine, a switch is made from a unique branch of the first, sinusoidal signal to a unique branch of the second, cosine-shaped signal or vice versa . The measuring range of this known magneto-optical current transformer is therefore in principle not restricted. However, the method is an incremental method with the result that the operating point for zero current only has to be reset if the electronics of the current transformer fail.

The invention is based on the object of specifying a method and an arrangement for measuring a magnetic alternating field with an expanded measuring range and good linearity.

This object is achieved according to the invention with the features of claims 1 and 4, respectively. Linearly polarized measurement light is converted into a Faraday effect

Coupled sensor device, which is arranged at least during the measurement process in the alternating magnetic field. The measuring light passes through the sensor device at least once and is then divided into two linearly polarized partial light signals, the polarization directions of which are directed at an angle of essentially an odd multiple of 45 ° or π / 4 to one another. The two partial light signals are each converted into an electrical intensity signal, which is a measure of the light intensity of the associated partial light signal. An AC signal component and a DC signal component are determined from a first of the two electrical intensity signals and a DC signal component from a second of the two intensity signals. The alternating signal component essentially contains all frequency components of the alternating magnetic field. In contrast, the two DC signal components contain essentially no frequency components of the magnetic alternating field. There is now a measurement signal for the derived alternating magnetic field, which is proportional to a quotient of two intensity-standardized signals, a first of the two intensity-standardized signals corresponding to the quotient of the alternating signal component and the DC signal component of the first intensity signal and a second of the two intensity-standardized signals to the quotient of the second Intensity signal and its DC signal component corresponds. On the one hand, this measurement signal is practically independent of undesired fluctuations in the intensity of the measurement light and, on the other hand, it is a clear function over an angular range of approximately π for the Faraday angle of rotation by which the plane of polarization of the measurement light in the sensor device is rotated due to the magnetic field, for example via the open angular range] -π / 2, -r-π / 2 [. Furthermore, the measurement signal has excellent linearity in a large area around an operating point in the middle of the measurement area.

Advantageous refinements and developments of the method and the arrangement result from the claims dependent in each case on claim 1 or on claim 4.

In an advantageous embodiment, an effective value is formed from the measurement signal for precision measurement as a measure of the effective value of the alternating magnetic field.

To measure an electrical alternating current, the sensor device is arranged in the magnetic alternating field inductively generated by the alternating current.

For further explanation of the invention, reference is made to the drawing

Referred to in their

1 shows an exemplary embodiment of an arrangement for measuring a magnetic alternating field and in particular for measuring an electrical alternating current and

2 shows in a diagram the ef fect ivwert of the measured current as a function of Faraσay-Meßwir.kεi are each illustrated schematically.

1 shows an embodiment of an arrangement for measuring an alternating magnetic field H, which can be generated in particular by an electrical current I in a current conductor 2. A sensor device 3 which shows the magneto-optical Faraday effect is arranged in the alternating magnetic field H. In the embodiment shown, the sensor device 3 is formed with a single-mode optical fiber, which preferably surrounds the current conductor 2 in the form of a measuring winding with at least one turn. A tempered optical fiber (annealed fiber) is preferably provided, which is characterized by low linear birefringence and practically negligible circular birefringence. However, the sensor device 3 can also be formed with one or more solid bodies, preferably made of glass, which show the Faraday effect and in particular surround the current conductor 2 as a polygonal ring body. Means are provided for coupling linearly polarized measuring light L into the sensor device 3. The polarization direction of the electric field strength vector of the measuring light L when coupling into the sensor device 3 is hereinafter referred to as the coupling polarization direction of the measuring light L. The means for coupling the measuring light L into the sensor device 3 can, as shown, contain a light source 9 and a polarizer 10 for linearly polarizing the light from the light source 9, or also a linearly polarized light source such as a laser diode. In the embodiment shown, the polarization axis (transmission axis) of the polarizer 10 specifies the coupling polarization direction of the measuring light L. The linearly polarized measuring light L coupled into the sensor device 3 passes through the sensor device 3 and is passed to a beam splitter 4 after passing through the sensor device 3. The beam splitter 4 divides the measuring light L into two light components L1 'and L2' with the same polarization. For example, the beam splitter 4 can be positioned at an angle of as 45 ° to the direction of propagation of the measuring light L inclined partially transparent mirror can be formed. A first polarizer 5 is now arranged in the optical path (beam path) of the first light component L1 ', which forms a first light component signal L1 projected onto its polarization axis P1. A second polarizer 6 is arranged in the light path of the second light component L2 'and forms a second light component signal L2 projected onto its associated polarization axis P2. The polarization axis Pl of the first polarizer 5 and the polarization axis P2 of the second polarizer 6 close at least approximately an angle

α = (2n + 1) - 45 ° or α = (2n + 1) - (π / 4) ⁽ 7 ⁾

one with the integer n. Preferably, the polarization axis Pl of the first polarizer 5 is directed at an angle of at least approximately + 45 ° or + π / 4 or - 45 ° or - π / 4 to the coupling polarization direction of the measurement light L and the polarization axis P2 of polarization of the second ^¬ gate 6 at an angle of 0 ° or 0 to Einkoppelpolari¬ sationsrichtung of the measuring light L.

The two partial light signals L1 and L2 are now fed to an assigned photoelectric converter 7 and 8, respectively. Each photoelectric converter 7 and 8 associated with ^¬ partial light signals Ll and L2 in each case one electrical ^¬ ULTRASONIC intensity signal Sl or S2 converts which respectively is a measure of the intensity of the respective partial light signal Ll L2. In general, the electrical intensity signal S1 or S2 is proportional to the light intensity of the associated light part ^signal L1 or L2. The output of the first photoelectric transducer 7 is now electrically connected 12 comparable with the input of a high pass filter 11 and ^¬ with the input of a low pass filter. The high-pass filter 11 forms an AC signal component AI of the first intensity signal S1 and the low-pass filter 12 a DC signal component Dl of this first intensity signal Sl. The separation frequencies of high-pass filter 11 and low-pass filter 12 are set so that the alternating signal component AI contains all frequency components of the alternating magnetic field H and the direct signal component D1 is independent of the alternating magnetic field H. The alternating signal component AI of the first intensity signal S1 is fed from an output of the high-pass filter 11 to a first input of a divider 14. The DC signal component D1 of the first intensity signal S1 is fed from an output of the low-pass filter 12 to a second input of the divider 14. The divider 14 now forms the quotient signal AI / Dl from the alternating signal component AI to the direct signal component Dl of the first intensity signal S1. This quotient signal AI / Dl is an intensity-normalized ≤ signal, ie independent of changes in intensity of the measuring light L, for example due to fluctuations in intensity of the light source 9 or attenuation losses in the light path of the measuring light L or the first partial light signal L1. The output of the second photoelectric converter 8 is electrically connected to the input of a low-pass filter 13 and a first input of a divider 15. The low-pass filter 13 forms a DC signal component D2 of the second intensity signal S2. The separation frequency of the low-pass filter 13 is set such that the DC signal component D2 contains no frequency components of the magnetic alternating field H. At an output of the divider 15 there is now a quotient signal S2 / D2, which corresponds to the quotient from the second intensity signal S2 and its DC signal component D2. This quotient signal S2 / D2 is also an intensity-normalized signal and is therefore independent of changes in intensity in the measuring light L and in the second partial light signal L2. Since intensity changes in the light paths of the two light line signals L1 and L2 are also compensated for by the intensity normalization, multimode fibers can also be used to transmit the two light part signals L1 and L2. The two standardized signals AI / Di and S2 / D2 are each fed to an input of a further divider 16. The divider 16 forms the Quotients of the two standardized signals Al / Dl and S2 / D2 as a measurement signal

M = (Al / Dl) / (S2 / D2) (8),

that can be tapped at an output 30 of the arrangement.

This measurement signal M is similar to the function tan (p) of the Faraday angle of rotation p around which the direction of polarization

(Polarization plane) of the measuring light (L) is rotated in the sensor device 3 due to the magnetic alternating field H. However, the tangent function tan (p) is now an unambiguous function of the rotation angle p over an angular interval with an interval length of approximately π, namely for -π / 2 + 2mπ <p <+ π / 2 + 2mπ with the integer m. A measuring range which is practically twice as large as that obtained with the measuring signals proportional to sin (2p) obtained according to the prior art is thus obtained.

In a preferred embodiment, means 17 are provided for forming the effective value M_ _f - of the measurement signal M, which is used as a measure for the amplitude (amount) of the alternating magnetic field H or as a measure for the effective value I « _J; of an electrical current I in the current conductor 2. FIG. 2 shows the effective value K _{IÜ of} the measurement signal M for a sinusoidal electrical current I = 2 ^{ύ, i} l _#t . sin (ωt) plotted over an angular range of 0 ° to approximately 60 ° for the effective value p «__ of the Faraday rotation angle p = 2 ° ' ⁵ p _{9t i} sin ⁽ ωt ⁾ . The effective value I _{β £ f of} the electrical current I then results from the relationship p _β; ; = 2 NV l _{ii t} with the number of turns N of the fiber coil (measuring winding) and the Verdet constant V of the sensor device 3. To form the effective value M _* -. Any known analog or digital circuit can be used. The effective value M, _{£ f of} the measurement signal M can also be subjected to a subsequent linearization, preferably with the aid of a digital signal processor. The linearized effective value M _.f: - ⁱⁿ , which then depends linearly on the angle of rotation p, is applied to an output 20. Of course, the RMS value M, - :; be given to an output, not shown.

Instead of the high-pass filter 11, a subtractor can also be provided to form the alternating signal component AI of the first intensity signal Sl, which subtractor forms the difference Sl - Dl from the first intensity signal Sl and its direct signal component Dl formed by the low-pass filter 12, which corresponds to the alternating signal component AI. Conversely, instead of the low-pass filter 12 to form the DC signal component Dl of the first intensity signal S1, a subtractor can also be provided, which subtracts the difference S1-AI from the first intensity signal S1 and its AC signal component AI formed by the high-pass filter 11, which is just the DC signal component Corresponds to Dl. Furthermore, the low-pass filter 13 can also be replaced by a high-pass filter for forming an AC signal component A2 of the second intensity signal S2 and a subtractor for forming the DC signal component D2 of the second intensity signal S2 by subtracting the AC signal component A2 from the second intensity signal S2. Finally, the analog filters shown can also be replaced by digital filters and upstream analog / digital converters.

Arithmetic means for deriving the measurement signal M according to the relationship (8) from the alternating signal component AI and the direct signal component Dl of the first intensity signal S1 and from the second intensity signal S2 and its direct signal component D2 can of course also be used instead of the analog divider 14, 15 and 16 digital computing means can be provided, in particular a microprocessor or a digital signal processor with an upstream analc / digital converter. In front- Both digital filters and digital arithmetic means are preferably provided. The analog / digital conversion then takes place before the digital filters.

The optical coupling of the various optical components of the measuring arrangement is preferably supported by collimator lenses (not shown) for focusing the light.

Instead of the transmission type shown in FIG. 1, in which the measuring light L passes through the sensor device 3 only once, an arrangement of the reflection type can also be provided, in which the measuring light L reflects back into the sensor device 3 after a first passing through the sensor device 3 with the aid of a mirror and passes through the sensor device 3 a second time in the opposite direction before it is fed to the beam splitter 4.

Claims

claims

1. A method for measuring an alternating magnetic field (H), in which a) linearly polarized measuring light (L) is coupled into a sensor device (3) arranged in the alternating magnetic field (H) and showing the Faraday effect, b) the measuring light (LR) after passing through the sensor device (3) at least once is divided into two linearly polarized partial light signals (L1, L2), the polarization directions of which are at an angle (oc) of essentially an odd multiple of 45 ° or π / 4 are directed towards each other, c) the two partial light signals (L1, L2) are each converted into an electrical intensity signal (S1, S2), which is a measure of the light intensity of the associated partial light signal (L1, L2), d) from one An alternating signal component (AI) and a direct signal component (Dl) are determined in the first of the two electrical intensity signals (S1) and a direct signal component (D2) is determined by a second of the two intensity signals (S2) is, the AC signal component (AI) contains essentially all frequency components of the magnetic alternating field (H) and the DC signal components (D1, D2) essentially contain no frequency components of the magnetic alternating field (H), e) a measurement signal (M) for the alternating magnetic field (H) is derived, which is proportional to a quotient of two intensity-standardized signals (Al / Dl, S2 / D2), a first of the two intensity-standardized signals (Al / Dl) being the quotient of the alternating signal component (AI) and corresponds to the DC signal component (Dl) of the first intensity signal (S1) and a second of the two intensity-standardized signals (S2 / D2) corresponds to the quotient of the second intensity signal (S2) and its DC signal component (D2).

2. The method according to claim 1, in which an effective value (M, _{f £} ) is formed from the measurement signal (M) as a measure of the effective value of the alternating magnetic field (H).

3. The method according to any one of the preceding claims for measuring an electrical alternating current (I), in which the sensor device (3) is arranged in the alternating magnetic field (H) generated by the alternating current (I).

4. Arrangement for measuring an alternating magnetic field (H) with a) a sensor device (3) showing the Faraday effect, b) means (9, 10) for coupling linearly polarized measuring light (L) into the sensor device (3), c ) Means (4,5,6) for splitting the measuring light (L) after passing through the sensor device (3) at least once into two linearly polarized partial light signals (L1, L2), the polarization directions of which are at an angle (α) of essentially an odd multiple of 45 ° or π / 4 are directed to each other, d) means (7,8) for converting the two partial light signals (L1.L2) in each case into an electrical intensity signal (S1, S2), which is a measure of the light intensity of the low-level light signal (L1, L2), e) means (11, 12, 13) for forming an alternating signal component (AI) and a DC signal component (Dl) from a first of the two electrical intensity signals (S1) and for forming a DC signal component (D2) from a second de r two intensity signals (S2), the alternating signal component (AI) containing essentially all frequency components of the alternating magnetic field (H) and the direct signal components (D1, D2) containing essentially no frequency components of the alternating magnetic field (H), f) averages ( 14, 15, 16) for deriving a measurement signal (M) for the alternating magnetic field (H), which is proportional to a quotient of two intensity-standardized signals (Al / Dl, S2 / D2), a first of the two intensity-standardized signals (Al / Dl) corresponding to the quotient of the alternating signal component (AI) and the direct signal component (Dl) of the first intensity signal (S1) and a second one the two intensity-standardized signals (S2 / D2) corresponds to the quotient of the second intensity signal (S2) and its DC signal component (D2).

5. Arrangement according to claim 4, which comprises means (17) for forming an effective value (M, «) of the measurement signal (M) as a measure of the effective value of the alternating magnetic field (H).

6. Arrangement according to claim 4 or claim 5 for measuring an electrical alternating current (I), in which the sensor device (3) can be arranged in the alternating magnetic field (H) generated by the alternating current (I).