Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/02—Measuring direction or magnitude of magnetic fields or magnetic flux
  • G01R33/04—Measuring direction or magnitude of magnetic fields or magnetic flux using the flux-gate principle
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/02—Measuring direction or magnitude of magnetic fields or magnetic flux
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03F—AMPLIFIERS
  • H03F2200/00—Indexing scheme relating to amplifiers
  • H03F2200/261—Amplifier which being suitable for instrumentation applications

Definitions

• this device comprising: - an element made of magnetic material whose permeability is likely to vary between at least two different values as a function of the amplitude of the magnetic field in which it is placed, and
• the invention also relates to a system for modulating a high frequency magnetic field as a function of a low frequency magnetic field used in this measurement system.
• measurement systems combine magnetic field sensors with devices to shift the spectrum of the low-frequency magnetic field to higher frequencies where excess noise does not exist.
• the device for shifting the spectrum is formed from a source of high frequency magnetic fields and a tube in hard ferromagnetic material placed in the high frequency magnetic field.
• the amplitude of the high frequency magnetic field is chosen so as to saturate the ferromagnetic material with a frequency twice that of the high frequency magnetic field.
• This alternative saturation of the ferromagnetic material corresponds to a variation in the permeability of this material between a value ⁇ i for which the material is saturated and therefore transparent to external magnetic fields, and a value ⁇ 2 for which the material is opaque to magnetic fields and forms so a screen.
• the senor is placed inside the tube and the tube is exposed to the low frequency magnetic field to be measured.
• the low frequency magnetic field can only reach the sensor when the tube is transparent to external magnetic fields.
• the "Chopper” device chops the low frequency magnetic field at a frequency twice the high frequency magnetic field and only this chopped magnetic field is measured by the sensor.
• the chopped magnetic field can be seen as an amplitude modulation of the high frequency magnetic field as a function of the low frequency magnetic field. Consequently, this system makes it possible to shift the spectrum of the low frequency magnetic field towards higher frequencies, here, twice that of the high frequency magnetic field and therefore to benefit from the better sensitivity of the sensor for these higher frequencies.
• This system requires the production of a controllable magnetic screen surrounding the sensor. This can turn out to be difficult to achieve and of a considerable bulk.
• the invention aims to remedy this drawback by proposing a measurement system having the same advantages, but without using a controlled magnetic screen.
• the subject of the invention is therefore a low frequency magnetic field measurement system, characterized:
• said source is able to create a high frequency magnetic field intended to be amplitude modulated, this high frequency magnetic field combining with the magnetic field to be measured to create a magnetic field resulting from both the magnetic field to be measured and the high frequency magnetic field,
• the magnetic field sensor is associated with the element made of magnetic material so as to measure a magnetic field which is a function of the magnetic field created inside this element by the resulting magnetic field.
• the amplitude of the resulting magnetic field B app corresponds to the vector sum of the low frequency magnetic field B ext and the high frequency magnetic field B a .
• the amplitude of the field B app therefore has a high frequency component and a low frequency component.
• the magnetic material is placed in this field B app .
• the apparent permeability of this magnetic material is likely to vary between two different values ⁇ i and ⁇ 2 depending on the amplitude of the magnetic field in which it is placed, that is to say, here, the field B app .
• the senor When the amplitude of the field B app is less than the threshold B t , the sensor measures a field proportional to ⁇ i x B app , while when the amplitude of the field B app is greater than the threshold Bt, the sensor measures a field proportional to ⁇ 2 x B app .
• the variations in the amplitude of the field B app are essentially caused by the variations in the field B a .
• the fact that the permeability of the magnetic material varies as a function of the magnetic field in which it is placed makes it possible to modify the amplitude of the high frequency component of the field measured by the sensor.
• the amplitude of the high frequency component of the measured field is less than that which would have been measured if the permeability of the magnetic material did not vary.
• the measured high frequency component is clipped as soon as the threshold B t is crossed.
• the time during which the measured field is proportional to ⁇ 2 x B app is a function of the value of the amplitude of the field B ex t. Indeed, the more the value of the amplitude of the field B ext is close to, or even greater than the threshold B t , the longer the amplitude of the resulting field B app remains above the threshold B t .
• the system described above makes it possible to modify the amplitude of the high frequency component measured as a function of the amplitude of the field B ex t.
• the system described above therefore carries out an amplitude modulation of the high frequency component measured by the sensor as a function of the field B ext without using a controlled magnetic screen.
• the curve representing the value of the permeability of the element made of magnetic material as a function of the amplitude of the magnetic field in which it is placed has at least two plates substantially constant permeability value connected together by a slope corresponding to a breaking amplitude of the magnetic field, and the source is also capable of creating an additional magnetic field which combines with the magnetic field to be measured so as to approximate the amplitude of the component continuous or low frequency of the magnetic field resulting from the breaking amplitude;
• It includes a device for controlling the amplitude of the additional magnetic field to maintain the continuous or low frequency component of the resulting magnetic field equal to the breaking amplitude to within one error, this error being proportional to the variations of the magnetic field at measure ;
• - the amplitude of the high frequency magnetic field is substantially equal to the breaking amplitude;
• Said element made of magnetic material has an antenna shape to increase the density of the magnetic flux of the magnetic field to be measured received by the magnetic field sensor;
• - Said element made of magnetic material is in the form of at least one cylindrical bar forming said antenna; - The source of magnetic fields comprises a coil extending around said at least one cylindrical bar;
• the magnetic field sensor has an overlapping frequency below which the sensitivity of the sensor decreases, and said frequency of the high frequency magnetic field is at least equal to twice this overlapping frequency;
• the element made of magnetic material is made of soft ferromagnetic material.
• the subject of the invention is also a system for amplitude modulation of a high frequency magnetic field by a continuous or low frequency magnetic field, characterized in that it comprises:
• a source capable of creating the high frequency magnetic field intended to be amplitude modulated, this high frequency magnetic field combining with the continuous or low frequency magnetic field to create a magnetic field resulting function of both the continuous or low frequency magnetic field and the high frequency magnetic field,
• the permeability of this magnetic material being likely to vary between at least two different values depending on the amplitude of the magnetic field in which it is placed,
• a magnetic field sensor associated with the element made of magnetic material so as to measure the magnetic field created inside this element by the resulting magnetic field, the measured magnetic field corresponding to the high frequency magnetic field modulated as a function of the field magnetic continuous or low frequency.
• FIG. 1 is a schematic view of a system according to the invention.
• FIG. 2 is a graph representing the asymptotic value of the apparent permeability of a magnetic material used in the system of Figure 1 for different magnetic fields;
• FIG. 3 is a graph representing the evolution of the value of the magnetic field measured in the system of FIG. 1, and
• FIG. 4 is a schematic view of a variant of the system of FIG. 1.
• FIG. 1 represents a system for measuring the amplitude variations of a low frequency magnetic field B ex t designated by the general reference 2.
• the system 2 comprises a conventional Hall effect magnetic field sensor 4 of micrometric size, also known by the term "Hall ⁇ -probe".
• This sensor 4 is for example formed on a moderately doped conductive layer.
• this sensor 4 has a flat active face 6 in the form of a Greek cross with four rectangular branches.
• the active face 6 is represented as being vertical in the perspective view of FIG. 1.
• Each branch typically measures between 160 and 320 ⁇ m in length and has a width of between 40 and 80 ⁇ m.
• the active face 6 of such a magnetic field sensor is only sensitive to the component of the magnetic field perpendicular thereto.
• branches intended to be crossed by a current I are here aligned on the vertical, while the branches between which a voltage is measured are aligned on the horizontal.
• the upper vertical branch is connected, via a connection terminal 10, to a current source 11.
• the lower vertical branch is connected by a connection terminal 12 to ground.
• the two horizontal branches are connected, via respective connection terminals 14 and 16, to the inputs of a synchronous voltage detector 18.
• This detector 18 is able to measure the amplitude of the fundamental frequency of the voltage signal received at its inputs. This amplitude is noted, here, V b (t).
• This sensor 4 has, like all conventional magnetic sensors, background noise whose amplitude is uniformly distributed and excess noise added to the background noise for low frequencies. This excess noise is also known by the English term "1 / f noise”. The frequency from which excess noise begins to appear is referred to here as the crossover frequency.
• the system 2 includes a device 26 for shifting the frequency spectrum of the low frequency magnetic field B ex t to be measured towards higher frequencies.
• the device 26 comprises an element such as an antenna
• the antenna 28 made of soft ferromagnetic material associated with a source of high frequency magnetic fields. Soft ferromagnetic materials have the advantage, compared to other possible magnetic materials, of not exhibiting hysteresis during variations in magnetic fields.
• the antenna 28 is, for example, a cylindrical bar of round or square section. This cylindrical bar is arranged perpendicular to the active face 6 and substantially in the center of the latter.
• the end of this bar facing the active face is either in contact with it or in the vicinity of the latter, it ie spaced a few micrometers from it.
• the antenna 28 is, here, spaced 7.5 ⁇ m ⁇ 2.5 ⁇ m from the active face 6.
• the amount of magnetic flux picked up by this antenna 28 and transmitted to the sensor 4 depends on the shape and the material of this antenna.
• the quantity of magnetic flux picked up depends mainly on the permeability of the magnetic material and the ratio of the length to the diameter of the antenna.
• the term permeability alone is used to designate the apparent permeability.
• the length of the antenna 28 is large relative to its diameter.
• the length of the antenna 28 is between 3 mm and 20 mm, while its diameter is between 15 ⁇ m and 45 ⁇ m for " ⁇ -deHall probe" sensors. This diameter is of the order of the size of the active surface of the sensor 4.
• the length of the antenna 4 is 5 mm, while its diameter is 30 ⁇ m, in the case where the branches each have a dimension of 160 by 40 ⁇ m.
• the permeability of the magnetic material of the antenna 28 varies as a function of the amplitude of the magnetic field B app in which it is placed.
• the curve ( Figure 2) representing the permeability as a function of the amplitude of the field B app successively presents three plates 36, 38 and 40 of substantially constant value.
• the plates 36 and 40 correspond to a value of the permeability ⁇ 2
• the plate 38 corresponds to a value of the permeability ⁇ -i. ⁇ i is very clearly greater than ⁇ 2 .
• Tray 36 covers all negative values up to a threshold
• the permeability increases suddenly to reach the plateau 38.
• the plateau 38 extends from the threshold - Bt to a threshold B t .
• the value of the permeability decreases suddenly to reach the plate 40.
• the plate 40 extends to infinity from the threshold B t .
• the ferromagnetic material chosen has a high permeability ⁇ i, that is to say greater than 1000.
• the ferromagnetic material chosen is a amorphous material rich in cobalt having a permeability ⁇ -i equal to 10 3 and thresholds ⁇ Bt equal respectively to ⁇ 130 ⁇ T. This material is sold by XT Inc., 1744 William Street, Suite 104, Montreal, Quebec, Canada H3J 1 4.
• the source of high frequency magnetic fields is here produced using a coil 42 wound uniformly along the length of the antenna 28 and connected to a current generator 44.
• the generator 44 is capable of generating an alternating current of frequencies between 10 kHz and 1 MHz so as to create, inside the winding 42, a high frequency alternating magnetic field B a .
• the frequency is chosen to be greater than or equal to twice the recovery frequency of the sensor 4.
• the generator 44 generates sinusoidal or triangular current signals.
• the generator 44 is also capable of superimposing on the alternating current a current controlled by the amplitude of the field B ex t.
• This controlled current creates, inside the winding 42 a magnetic field B c .
• the amplitude of the servo current is controlled so as to try to maintain at all times the cumulative amplitudes of the fields B c and B ex t equal to the value of a constant amplitude setpoint.
• the value of this constant amplitude setpoint is here chosen equal to the value of the threshold Bt. This choice makes it possible to maximize the linearity and the sensitivity of the system 2.
• the system 2 also includes a servo device 48.
• the input of the device 48 is connected by via a low-pass filter 50 at the output of the synchronous detector 18.
• the output of the device 48 is, in turn, connected to an input of the generator 44 for controlling the intensity of the controlled current.
• the servo device 48 is conventional and will therefore not be described here in more detail. It is simply recalled that because of the imperfections of such servo devices, the amplitudes of the fields B ex t and B c are actually connected by the following formula:
• the amplitude of the intensity of the alternating current is chosen so that the amplitude of the alternating field B a is equal to the value of the threshold B t .
• FIG. 3 represents the value of the magnetic field B direction measured by the sensor 4 as a function of the field B app applied to the antenna 28.
• the amplitude of the field B sen s is proportional to the apparent permeability ( ⁇ i or ⁇ 2 ) of the magnetic material of the antenna 28 multiplied by the amplitude of the field B ap .
• this graph presents three successive linear sections 52, 54, 58 corresponding respectively to the plates 36, 38 and 40 of permeability values.
• the antenna 28 is placed inside the coil 42, so that it is placed in the fields B a and B c created by this coil.
• the antenna 28 is exposed to the low frequency magnetic field at measure B ex t. Consequently, the field B app , in which this antenna 28 is placed, results from the vectorial sum of the fields B a , B c and B and .
• the amplitude of the field B ext cumulated with the amplitude of the field B c can be considered constant, since their amplitudes vary a lot . slower than that of field B a .
• the field B ap can therefore be represented as being formed by a continuous component represented by the vertical dotted line 60 in FIG. 3 and by an alternative component represented by the vertical sinusoid 62 of amplitude B É .
• the amplitude of the field B e ⁇ t varies.
• Line 60 is therefore spaced from a vertical line passing through the threshold Bt by the error ⁇ .
• Field B sen s. measured by the sensor 4 during this same period, is represented by the signal in the form of a horizontal sinusoid 64.
• the amplitude of the field B sense is proportional to ⁇ i x B app as long as the sinusoid 62 does not exceed the threshold B t . Beyond this threshold Bt, the amplitude of Bsens is proportional to ⁇ 2 x B app . Consequently, since ⁇ 2 is very much less than ⁇ -i, the exceeding of the threshold Bt by the sinusoid 62 results in a clipping of the alternating component 64 measured by the sensor 4.
• Such a clipped sinusoid 64 corresponds to a fundamental frequency or first harmonic whose amplitude is smaller than if this sinusoid 64 was not clipped (shown in phantom in Figure 3).
• the clipping of the sinusoid 64 is therefore a function of variations in the amplitude of the low frequency magnetic field.
• the high frequency component measured by the sensor 4 is amplitude modulated as a function of variations in the low frequency magnetic field.
• the signal measured by the sensor 4 comprising, in particular, the amplitude-modulated high frequency component, is transmitted, in the form of an electrical signal, to the synchronous detector 18.
• This synchronous detector 18 extracts only the amplitude V b (t) of the received signal.
• This amplitude V b (t) is proportional to the variations in the amplitude of the field B e t.
• the antenna 28 makes it possible to increase the flux density of the magnetic field B ext picked up and measured by the sensor 4.
• the system 2 described here also makes it possible to improve the signal / noise ratio of the sensor 4, in particular at low frequencies.
• the choice of the amplitude of the field B a equal to the threshold Bt makes it possible to guarantee that the slightest variation of the amplitude of the field B ex t will result in a variation of the amplitude of the first harmonic of the high frequency component measured by the sensor 4.
• the creation of the additional magnetic field B c makes it possible to place the sinusoid 60, in the absence of the magnetic field B ex t, on the breaking point between the two linear sections 54 and 58. Consequently, a displacement on the right or on the left line 60 results in a variation of the amplitude of the high frequency component proportional to the amplitude of the displacement.
• the operation of system 2 under these conditions is therefore linear.
• the servo device 44 makes it possible to limit the displacements of the line 60 in an area close to the breaking point, that is to say in an area where any displacement of the line 60 results in a corresponding modulation. of the high frequency component of the field B se ⁇ s.
• the output V b (t) is only proportional to the variations of the field B ex t and not to the amplitude of the field B ext .
• the system 2 has no offset or "Offset" of the signal Vt, (t) depending on the amplitude of the field B ⁇ ⁇ t .
• the system 2 is also able to measure magnetic fields whose frequency is greater than 100 kHz, that is to say magnetic fields which are generally not considered to be low frequency. However, it has been found that system 2 also makes it possible to improve the sensitivity of the measurement for these magnetic fields greater than 100 kHz. Thus, the system 2 can be used to measure any magnetic field whose frequency is lower than the cut-off frequency of the servo device 48.
• FIG. 4 represents a different arrangement of the antenna 28.
• the largest side of a cylindrical bar forming an antenna 78 is arranged parallel to the active surface of the sensor 4 in contact or spaced a few micrometers from this active surface.
• the operation of this variant is identical to that of the system 2 except that in this variant, the sensor 4 measures a radial component of the magnetic field created inside the element made of magnetic material by the field B.
• Terror ⁇ of the servo device 48 is a function of the times the amplitude and frequency variations of the field B ex t.
• the signal delivered by the sensor 4 is therefore a time signal, the spectrum of which is a function of these variations in amplitude and frequency.
• the antenna is associated with other types of magnetic field sensors, such as magneto-resistive sensors.
• the shape, dimensions and position of the antenna 28 can be modified with respect to the embodiment described here so as to adapt, for example, to a different orientation of the low frequency magnetic field to be measured.
• Different shapes and dimensions for the antenna also modify the field gain of the antenna, that is to say the amplification of the low frequency magnetic field by the antenna.
• the evolution of the permeability as a function of the amplitude of the magnetic field in which the magnetic material is placed has at least one non-linearity which makes it possible to vary the amplitude of the field created inside. of this magnetic material as a function of the amplitude of the field in which it is placed.
• the system is adapted to also measure the amplitude of the magnetic field B ext .
• a processing unit is connected to the output of the synchronous detector 18 and to the output of the servo device 48.
• This processing unit is able to calculate the amplitude in absolute value of the field B ex t from the value of the variations V (t) of the field Bext and the value of the amplitude of the field B c deduced from the servo current applied by the servo device 48.
• the processing unit can, for example, calculate l amplitude of field B ext from formula (1).

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• Physics & Mathematics (AREA)
• Condensed Matter Physics & Semiconductors (AREA)
• General Physics & Mathematics (AREA)
• Measuring Magnetic Variables (AREA)

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• 2003
  • 2003-03-13 FR FR0303133A patent/FR2852398B1/fr not_active Expired - Fee Related
• 2004
  • 2004-03-12 EP EP04720030A patent/EP1601986A2/de not_active Withdrawn
  • 2004-03-12 WO PCT/FR2004/000620 patent/WO2004083872A2/fr not_active Ceased

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