Patent Publication Number: US-2002000801-A1

Title: Method to obtain a temperature compensated output signal in an optical current measuring sensor

Description:
[0001] The invention relates to a method for obtaining a temperature-coefficient-compensated output signal in an optical sensor for measuring a periodically fluctuating electrical and/or magnetic field strength, according to the preamble of patent claim  1   
       [0002] Optical sensors of the cited type are known, in particular in the form of current measurement sensors for measuring the current strength of an alternating current conducted in a conductor, which produces in the immediate environment of the conductor an electromagnetic field with an electrical and magnetic field strength that fluctuates periodically corresponding to the current strength of the alternating current. The sensor measures, for example according to the Faraday effect, the periodically fluctuating magnetic field strength, according to which the current strength can be inferred.  
       [0003] Voltage measurement sensors operate in a similar manner for the measurement of an alternating voltage applied to a conductor, which voltage produces in the immediate environment of the conductor an electrical field with an electrical field strength that fluctuates periodically corresponding to the alternating voltage. This sensor measures, for example according to the Pockels effect, the periodically fluctuating electrical field strength, according to which the alternating voltage can be inferred.  
       [0004] The respective field strength is measured in that light that can be influenced by the field strength is sent through the periodically fluctuating field, and from this light that has been sent through and influenced two intensity signals are produced that are separate from one another and that comprise intensities containing intensity portions that fluctuate in chronologically periodic fashion in phase opposition to one another dependent on the field strength of the periodically fluctuating field.  
       [0005] The influencing of the light by the field can be based on various known physical effects, for example the cited Faraday or Pockels effect. As an example, the Faraday effect, frequently used in current measurement sensors, is explained in more detail, in which linearly polarized light from a polarizer is influenced by the magnetic field strength in such a way that the polarization plane of the polarized light is rotated, dependent on the field strength, in relation to the polarization plane determined by the polarizer, or in correspondingly periodically fluctuating fashion in the case of the periodically fluctuating field strength. The linearly polarized light influenced by the field strength can be supplied to an analyzer, which forms two polarized light portions perpendicular to one another from this light, whose intensities relative to one another depend, in phase opposition, on the polarization state of the field-strength-influenced light in relation to the polarization plane determined by the polarizer, and which supplement each other to form an overall intensity independent of the angular position of the polarization plane of the field-strength-influenced light. In this case, these light portions form the two intensity signals that comprise the periodically fluctuating containing intensities [sic].  
       [0006] Such intensity signals can be obtained in a similar manner in voltage measurement sensors which for example exploit the Pockels effect.  
       [0007] With the aid of the variable derived from the two intensity signals, which corresponds to the quotient of the difference of the intensities of the two intensity signals and the sum of these intensities and that [ . . . ] the direct portion and the periodically fluctuating alternating portion, to which a root-mean-square value can be allocated, the output signal of such a sensor, indicating the measured field strength and therewith the current strength or voltage, is obtained.  
       [0008] This output signal, in particular a periodically fluctuating portion contained therein, often exhibits a temperature drift whose cause is grounded in the physical effect on which the measurement is based, and/or in disturbances such as for example mechanical stresses and/or a linear double refraction.  
       [0009] In order to remove this problem, for example a temperature coefficient compensation method is proposed in relation to a magneto-optical sensor based on the Faraday effect, in which the two intensity signals are obtained in the manner described above with the aid of linearly polarized light from a polarizer, for example a laser diode, and an optical analyzer. In this known temperature compensation method, it is necessary to adjust the polarizer and analyzer very precisely to one another with respect to their angular setting, i.e., in such a way that the polarization plane determined by the polarizer and the polarization plane determined by the analyzer stand as precisely as possible at an angle of 45° to one another.  
       [0010] The invention indicated in claim  1  provides a new type of method for obtaining a temperature-coefficient-compensated output signal, having the advantage that in a sensor of the cited type, if the two intensity&#39;signals are obtained with the aid of a polarizer and analyzer, larger angular misadjustments are permissible between the polarizer and the analyzer in comparison to the known temperature compensation method.  
       [0011] The method according to the preamble of patent claim  1 , in which the quantity is derived from the two intensity signals, which quantity corresponds to a quotient of a difference of the intensities of the two intensity signals and the sum of these intensities, is known as −/+ intensity norming.  
       [0012] The basic idea of the invention underlying the inventive method permits, but is not limited to, this intensity norming, but rather can also be applied analogously to sensors in which what is known as AC/CD [sic] intensity norming is present, i.e., in sensors according to the preamble of patent claim  2 .  
       [0013] The invention indicated in claim  2  likewise provides a new type of method for obtaining a temperature-coefficient-compensated output signal, with the advantage that in a sensor of the named type, if the two intensity signals are obtained with the aid of a polarizer and analyzer, greater angular misadjustments are permissible between the polarizer and the analyzer in comparison to the known temperature compensation method.  
       [0014] In the inventive method according to claim  1  and/or  2 , angular misadjustments between the polarizer and the analyzer of more than 5° advantageously adversely affect the function of the inventive method only inessentially in the sensor itself. The reduction of sensitivity is low and is proportional to the cosine of the faulty angle between the polarization plane of the polarizer and that of the analyzer. This advantageously simplifies the design and manufacture of the sensor in which the inventive method is applied.  
       [0015] However, both in the construction according to claim  1  and also in the construction according to claim  2  the inventive method is not limited to sensors that operate with polarizer and analyzer, but rather can generally be applied also in other sensors according to the preamble of claim  1  and/or  2 , and in this way extends the technology.  
       [0016] The function to be used in the inventive method and determined by the calibration measurement can be approximated by a polynomial of a predeterminable degree (claim  2 ) and/or can be stored in approximate form in a lookup table (claim  3 ).  
       [0017] A preferred arrangement for the execution of a method according to claim  1  follows from claim  4 , and a preferred arrangement for the execution of a method according to claim  2  follows from claim  5 . 
     
    
    
     [0018] In the following specification, the invention is explained in more detail by way of example on the basis of the figures.  
     [0019]FIG. 1 shows, in a schematic representation, an output part, which produces the two intensity signals and the quantity derived therefrom, of an optical sensor with −/+ intensity norming, and an arrangement, coupled to this output part, for the execution of the method for obtaining a temperature-coefficient-compensated output signal in this sensor,  
     [0020]FIG. 2 shows, in a schematic representation, an output part, which produces the two intensity signals and the two normed quantities derived therefrom, of an optical sensor with AC/DC intensity norming, and an arrangement, coupled to this output part, for execution of a method for obtaining a temperature-coefficient-compensated output signal in this sensor,  
     [0021]FIG. 3 shows a diagram which [ . . . ] the percent deviation of the non-temperature-coefficient-compensated output signal of a magneto-optical sensor from its ideal value, and  
     [0022]FIG. 4 shows a diagram showing the percent deviation of the output signal, corrected with the inventive method, of the magneto-optical sensor given an angular misadjustment of 5° between the analyzer and the polarizer. 
    
    
     [0023] In the examples according to FIGS. 1 and 2, it is assumed that the optical sensor is a magneto-optical or electro-optical sensor, as described in more detail above, which measures the periodically fluctuating magnetic and/or electrical field strength of a magnetic and/or electrical field produced by a current or a voltage, e.g. exploiting the Faraday or Pockels effect.  
     [0024] Light L with a particular polarization p, produced by a polarizer (not shown), [ . . . ] sent through the periodically fluctuating field, which influences the polarization dependent on the magnetic or electrical field strength in such a way that after passing through the field the light L comprises a polarization p′ that is modified in relation to the particular polarization produced by the polarizer. Because the magnetic or electrical field or, respectively, the field strength thereof periodically fluctuates, the polarization p′ of the polarized light that has passed through this field also fluctuates periodically in relation to the polarization determined by the polarizer.  
     [0025] In the following, let it be assumed without limitation of generality that the sensor is a current measurement sensor exploiting the Faraday effect. In this case, the polarizer (not shown) produces light L with a determined linear polarization p, which is sent through an optical medium with a Verdet constant arranged in the periodically fluctuating magnetic field, for example a glass fiber coil surrounding a current-conducting conductor.  
     [0026] The periodically fluctuating magnetic field influences the linearly polarized light L in such a way that after passage through the field the linear polarization p′ periodically fluctuates in relation to the determined linear polarization. The light L [ . . . ] fluctuating polarization p′ is supplied to an analyzer  9  with a polarization plane that is set as precisely as possible at an angle of 45° to the polarization plane of the determined polarization p of the linearly polarized light L produced by the polarizer, for example a laser diode.  
     [0027] From the light L with the fluctuating linear polarization p′, the analyzer  9  produces  
     [0028] an intensity signal L 1  with a linear polarization p 1  with a fixed polarization plane and an intensity I 1 , containing an intensity portion that fluctuates periodically dependent on the periodically fluctuating field strength, and  
     [0029] an intensity signal L 2  of a linear polarization p 2  with a fixed polarization plane perpendicular to the fixed polarization plane of the intensity signal L 1  and with an intensity I 2  that contains an intensity portion that fluctuates periodically dependent on the periodically fluctuating field strength, whereby  
     [0030] the periodically fluctuating intensity portions of intensities I 1  or, respectively, I 2  of the two intensity signals L 1  and L 2  fluctuate periodically in phase opposition to one another, dependent on the periodically fluctuating field strength.  
     [0031] An example of such an analyzer  10  is for example a Wollaston prism, which is often used in such current measurement sensors.  
     [0032] In the example according to FIG. 1, the quantity P is derived from the two intensity signals L 1  and L 2 , corresponding to a quotient of, for example, the difference I 1 −I 2  of the intensities I 1  and I 2  of the two intensity signals L 1  and L 2  and the sum I 1 +I 2  of these two intensity signals I 1  and I 2 .  
     [0033] For the formation of this derived quantity P=(I 1 −I 2 )/(I 1 +I 2 ), the means  10  in FIG. 1 is provided, which preferably comprises a digital processor and digitally processes the supplied intensity signals L 1  and L 2 , although an analog processing is equally possible.  
     [0034] The derived quantity P characterizes the sensor as the sensor with −/+ intensity norming.  
     [0035] The derived quantity P naturally contains a direct portion P DC  and an alternating portion P AC  that fluctuates periodically corresponding to the magnetic field strength, to which alternating portion a root-mean-square value PACeff can be allocated.  
     [0036] For the inventive method in relation to the sensor with −/+ intensity norming, these quantities P DC , P AC  and P ACeff  are decisive.  
     [0037] Accordingly, from the derived quantity P in the means  11  in FIG. 1 the direct portion P DC  thereof can be obtained, while in the means  12  the alternating portion P AC  of the derived quantity P is obtained and the root-mean-square value P ACeff  of this portion P AC  is formed. P DC  can for example be obtained by means of low-pass filtering, and P AC  can for example be obtained by high-pass filtering or formation of P−P DC    
     [0038] The means  12  can for example be fashioned in such a way that a means  12   1  is provided that obtains the alternating portion P AC  from the derived quantity P, which alternating portion is supplied to a separate means  12   2 , in which the root-mean-square value P ACeff  is formed.  
     [0039] According to the invention, the temperature-coefficient-compensated output signal S is obtained by formation of the quotient  
       S=P   AC   /f ( P   DC   , P   ACeff )  
     [0040] from the alternating portion P AC  of the derived quantity P and the function f(P DC , P ACeff ), determined by the calibration measurement in the desired temperature range, of the direct portion P DC  and of the root-mean-square value P ACeff  of the alternating portion P AC  of the derived quantity P.  
     [0041] Given a digital signal processing, from the value at a particular sampling point of the direct portion P DC  and of the root-mean-square value P ACeff , the function value belonging to this sampling point of the predetermined function f(P DC , P ACeff ) is determined, and from this the value belonging to this sampling point of the output signal S is formed by formation of the quotient with the value belonging to this sampling point of the alternating portion P AC  and the functional value belonging to this sampling point. This method can be repeated from sampling point to sampling point.  
     [0042] For the formation of the functional value belonging to this sampling point, the means  13  is provided, to which the value belonging to this sampling point of the direct portion P DC  and the root-mean-square value P ACeff  belonging to this sampling point are supplied, and which outputs to the means  14  the functional value, belonging to this value of the direct portion P DC  and to this root-mean-square value P ACeff  of the predetermined function f(P DC , P ACeff ), as the functional value belonging to this sampling point of this function, to which means  14  on the other hand the value belonging to this sampling point of the alternating portion P AC  is supplied and which forms and outputs from this functional value and this value of the alternating portion P AC  the temperature-coefficient-compensated output signal S belonging to this sampling point.  
     [0043] The means  13  can contain a lookup table in which a functional value of the function f is respectively allocated to each pair of values consisting of a value of the direct portion P DC  and a root-mean-square value P ACeff  of the alternating portion P AC . The means  13  can also contain a processor that calculates the functional values of the function f allocated to the various value pairs from a value of the direct portion P DC  and a root-mean-square value P ACeff , e.g. in approximated fashion with the aid of a polynomial of a selectable order.  
     [0044] The value of the direct portion P DC  and of the root-mean-square value P ACeff  do not respectively change over a longer time period in comparison to the value of the alternating portion P AC , so that in this respective time period these values can be regarded as constant, and do not need to be determined as often as the value of the alternating portion P AC .  
     [0045] The example according to FIG. 2 is based on a sensor with AC/DC intensity norming. In this sensor, according to the invention a normed quantity S 1  or, respectively, S 2  is respectively derived from each intensity signal L 1  and L 2  from the analyzer  9 . The normed quantity S 1  derived from the intensity signal L 1  corresponds to the quotient I AC   1 /I DC   1  of the alternating portion I AC   1  and the direct portion I DC   1  of the intensity I 1  of the intensity signal L 1 , and the quantity S 2  corresponds to the quotient I AC   2 /I DC   2  of the alternating portion I AC   2  and the direct portion I DC   2  of the intensity I 2  of the intensity signal L 2 .  
     [0046] In the example according to FIG. 2, the normed quantity S 1  is formed by the means  15   1 , which comprises for example a means  15   11  for obtaining the alternating portion I AC   1 , a means  15   12  for obtaining the direct portion I DC   1  of the intensity signal L 1 , and a means  15   13  for forming the quotient I AC   1 /I D   1 . The normed quantity S 2  is formed by the means  15   2 , which comprises for example a means  15   21  for obtaining the alternating portion I AC   2  of the intensity signal L 2 , a means  15   22  for obtaining the direct portion  DC   2  [sic] of the intensity signal L 2  and a means  15   23  for forming the quotient I AC   2 /I DC   2 .  
     [0047] According to the invention, from the two derived quantities S 1  and S 2  the first quotient P′ DC  is derived from the sum S 1 +S 2  and the difference S 2 −S 1  of the two normed quantities S 1  and S 2 , i.e., P′ DC =(S 1 +S 2 )/(S 2 −S 1 ), and the periodically fluctuating second quotient P AC  is derived from the twofold product 2S 1 S 2  and the difference S 2 −S 1  of the two normed quantities S 1  and S 2 , i.e. P′ AC =2S 1 S 2 /(S 2 −S 1 ), and the temperature-coefficient-compensated output signal S is obtained by formation of the quotient P′ AC /f(P′ DC , P′ ACeff ) of the periodically fluctuating second quotient P′ AC  and the function f(P′ DC , P′ ACeff ), determined by the calibration measurement, of the first quotient P′ DC  and of the root-mean-square value P′ Aceff  of the periodically fluctuating second quotient P′ AC .  
     [0048] The first quotient P′ DC  corresponds to the direct portion P DC  of the derived quantity P according to FIG. 1, and the periodically fluctuating quotient P′ AC  corresponds to the alternating portion P AC  of this derived quantity P.  
     [0049] For the formation of the first quotient P′ DC , the means  16  according to FIG. 2 is provided, while the second quotient P′ AC  and the root-mean-square value P′ ACeff  of this second quotient P′ AC  are formed by the means  17 .  
     [0050] Similar to the means  12  in FIG. 1, the means  17  can for example consist of a means  17   1  for the formation of the second quotient P′ AC  and of a means  17   2  for the formation of the root-mean-square value P′ ACeff  of the second quotient P′ AC , whereby the second quotient P′ AC  is supplied to the means  17   2 .  
     [0051] The means  18  corresponds to the means  13  in FIG. 1, and the means  19  corresponds to the means  14  in FIG. 1.  
     [0052] This means that the means  18  is supplied with the value belonging to a sampling point of the first quotient P′ DC  and the root-mean-square value P′ ACeff  belonging to this sampling point of the second quotient P′ AC , and the means  18  outputs to the means  19  the functional value of the predetermined function f(P′ DC , P′ ACeff ) belonging to this value of the first quotient P′ DC  to this root-mean-square value P′ ACeff  as the functional value of this function belonging to this sampling point, to which means  19  on the other hand the value belonging to this sampling point of the second quotient P′ AC  is supplied, and which forms from this functional value and this value of the second quotient P′ AC  the temperature-coefficient-compensated output signal S′ belonging to this sampling point, and outputs this output signal.  
     [0053] The means  18  can contain a lookup table in which a function value of the function f is respectively allocated to each pair of values consisting of a value of the first quotient P′ DC  and a root-mean-square value P′ ACeff  of the second quotient P′ AC . The means  19  can also contain a processor that calculates the functional values of the function f allocated to the various value pairs of a value of the first quotient P′ DC  and a root-mean-square value P′ ACeff  of the second quotient P′ AC , for example approximated with the aid of a polynomial of a selectable order.  
     [0054] An essential feature of the inventive method is the inclusion also of the root-mean-square value P ACeff  of the alternating portion P AC  or, respectively, of the root-mean-square value P′ ACeff  of the second quotient P′ AC , and not only of the direct portion P DC  or, respectively, first quotient P′ DC .  
     [0055] As is known, given optical adjusting P DC  is a good measure for the sensor head temperature and thus for its momentary current sensitivity or, respectively, voltage sensitivity, a misadjustment between the analyzer and the polarizer causes however a dependence of the direct portion P DC  or, respectively, first quotient P′ DC  on the field strength, whereby the previous compensation of the temperature coefficient fails. In contrast, in the inventive compensation a field strength dependence of the direct portion P DC  or, respectively, first quotient P′ DC  is taken into account, and the temperature coefficient compensation works even given poor adjustment between analyzer and polarizer.  
     [0056]FIG. 3 shows, for example, the percent deviation of the non-temperature-coefficient-compensated derived quantity P according to FIG. 1 from its ideal value without linear double refraction δ, which likewise influences the light polarization and is caused by mechanical tensions, in particular given temperature fluctuations. The percent deviation is plotted against the circular double refraction 2ρ/°, which is directly proportional to the magnetic field strength, and against the linear double refraction δ/°, whose change with temperature essentially causes the temperature coefficient of the derived quantity P. If analyzer and polarizer are not adjusted precisely in relation to one another, the high measurement errors occurring here cannot be reduced by the application of the known temperature coefficient compensation methods. Given a de-adjustment of for example 5° between analyzer and polarizer, the compensated signal hardly differs from the uncompensated signal.  
     [0057]FIG. 4 shows, in the same type of representation as FIG. 3, the percent deviation of the signal S or, respectively, S′, corrected with the inventive method, from its ideal value without linear double refraction, given an angular misadjustment of 5° between analyzer and polarizer. A third-order polynomial was used for the function f, to be determined by means of calibration measurement. The percent error remains for the most part below 0.5%. The error actually to be expected during a measurement is much smaller, because it is not be expected that the linear double refraction  5  will change with temperature over such a large range. Fluctuations of the Verdet constant, which is decisive for the Faraday effect, of the optical medium of the sensor with temperature are likewise compensated given determination of the adaptation function f by means of calibration measurement. The precision can be further increased by selection of an adaptation function f in the form of a polynomial of a still higher order.