Current sensor

Abstract

A current sensor (1) is disclosed. The current sensor comprises a belt (3) running between first and second ends (4, 5). The belt comprises a line of magnetic field sensors (6), each having at least one magnetic field sensing axis (7), evenly spaced along the belt and orientated such that each of the magnetic field sensors has a magnetic field sensing axis aligned along the line. The belt is sufficiently bendable to allow the ends of the band to be brought together to form a loop (9; Fig. 3).

Description

[0001] Current sensor

[0002] Field

[0003] The present invention relates to a current sensor capable of measuring a de current in flowing through a conductor.

[0004] Background

[0005] A Rogowski coil can be used to measure an ac current flowing through a conductor by detecting the magnetic fields generated by the current using Faraday's law. US 7 164 263 B2 describes a current sensor which is hinged to allow clamping to a conductor, and which can be used to measure de and ac currents in the conductor.

[0006] Summary

[0007] According to a first aspect of the present invention there is provided a current sensor. The current sensor comprises a belt running along a line between first and second ends. The belt comprises a line of magnetic field sensors, each having at least one magnetic field sensing axis, evenly spaced along the belt and orientated such that each of the magnetic field sensors has a magnetic field sensing axis aligned along the line. The belt is sufficiently bendable to allow the ends of the band to be brought together to form a loop.

[0008] The current sensor can be used to measure a de current passing through a conductor, such as a busbar.

[0009] Each magnetic field sensor may have more than one magnetic field sensing axes. Thus, for each magnetic field sensor, one of the magnetic field sensing axes is aligned with the line.

[0010] The belt is preferably an elongate belt. The belt can be laid flat.

[0011] The belt may comprise alternating first and second sections between the first and second ends. The first sections may support the magnetic field sensors, and the second sections may interconnect the first sections. The first sections may be relatively long, and the second sections may be relatively short. The first sections may be relatively stiff, and the second sections may be relatively flexible. This can help to obtain a sum of potential vectors measured by the magnetic field sensors which is zero or substantially zero. The second sections may comprise hinges, such mechanical hinges or living hinges. The belt may comprise a continuous flexible strip between the first and second ends. The first sections may include stiffeners, such as plates.

[0012] The current may comprise wiring connecting the magnetic field sensors. The current sensor may comprise a flexible printed circuit board providing the wiring or at least part of the wiring. The current sensor may comprise discrete wires providing the wiring or at least part of the wiring.

[0013] The magnetic field sensors may comprise tunnelling magneto resistance bridges. The magnetic field sensors may be excited in parallel. The magnetic field sensors are read in parallel. The magnetic field sensors may be three-axis sensors. The magnetic field sensors may be configured to have a sufficiently wide bandwidth for measuring a powerline frequency and at least first harmonic of the powerline frequency. The powerline frequency may be between 50 and 60 Hz. The sufficiently wide bandwidth may be between 10kHz and 100 kHz.

[0014] The current sensor may further comprise a coil wound along a centre line running along the belt. This can allow an ac measurement or a further ac measurement to be performed.

[0015] The current sensor may further comprise a fastening arrangement for releasably securing together the first and second ends of the belt.

[0016] The magnetic field sensors may be spaced by a given pitch and the belt, a first magnetic field sensor closest to the first end and a last magnetic sensor closest the second end are configured such that when the loop is formed the first and last magnetic field sensors are adjacent and spaced by the given pitch.

[0017] The current sensor may further comprise an electrically insulating sheath, wherein the belt is disposed inside the electrically insulating sheath. The electrically insulating sheath may take the form of an over mould or a sleeve. The electrically insulating sheath may be configured (for instance, consist of a suitable material and be sufficiently thick or otherwise provide a gap) to provide protection to an external voltage of between 0 VDC to 1500 VDC or more.

[0018] The electrically insulating sheath may be configured (for instance, be sufficiently thick) to limit or avoid a permanent magnet affecting the magnetic field sensors and / or damaging the magnetic field sensors (for example, by affecting a reference layer or pinned layer in a TMR). The permanent magnet may have a remanence of up to IT or 1.5T.

[0019] The current sensor may further comprise magnetic shield, wherein the belt is disposed inside the magnetic shield. The magnetic shield may comprise a sheath of magnetic soft material, for example, in the form of a braided sheath or flexible tube. The magnetic shield is configured (for instance, is sufficiently thick) to limit or avoid an external permanent magnet affecting the magnetic field sensors and / or damaging the magnetic field sensors (for example, by affecting a reference layer or pinned layer in a TMR). According to a second aspect of the present invention there is provided a current sensor system comprising the current sensor of the first aspect and instrumentation for reading the current sensor.

[0020] The current sensor system may be configured to measure a de current, an ac current and / or an ac current having a de component.

[0021] According to a third aspect of the present invention there is provide a method comprising installing the current sensor of the first aspect by bending the band around a conductor and carrying out a measurement of de current through the conductor.

[0022] According to a fourth aspect of the present invention there is provided a DC sensitive current sensor made of a flexible linear array of magnetic field sensing devices.

[0023] Regions around sensors may be stiffened so that field vectors sum to zero under wider range of bending. The sensors may be tunnelling magneto resistance bridges.

[0024] The sensors output averaging may be obtained by sharing excitation and output connections across the sensors.

[0025] 3-axis sensors may be used, the two orthogonal axes being used to trim the sensor axis in software to align in the direction of the strip, independent of sensor chip manufacturing or attachment processing.

[0026] The senor array bandwidth may be sufficient to cover line frequencies and harmonics of interest as well as the DC component.

[0027] Coil based di / dt pickup may be provided in addition to the DC sensitive elements, and a composite frequency dependant output is provided.

[0028] The spacing of the sensors may be maintained at the joint when the assembly is connected around a conductor. Brief Description of the Drawings

[0029] Certain embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

[0030] Figure 1 is a plan view of a current sensor in the form of a belt comprising a plurality of magnetic field sensors arranged along the belt;

[0031] Figure 2A is a cross-sectional view of the sensor and a first arrangement of electrical insulation and magnetic shield;

[0032] Figure 2B is a cross-sectional view of the sensor and a second arrangement of electrical insulation and magnetic shield;

[0033] Figure 2C is a cross-sectional view of the sensor and a third arrangement of electrical insulation and magnetic shield;

[0034] Figure 3 is perspective view of the current sensor shown in Figure 1 bent into a loop around a conductor;

[0035] Figure 4 is a schematic circuit diagram of a magnetic field sensors, a signal source and an amplifier;

[0036] Figure 5 is a plot of meter factor as a function current for a working model of the sensor;

[0037] Figure 6 illustrates a first current sensor comprising magnetic field sensors supported on flexible sections of a belt which can kink and which can result in the sum of potential vectors measured by the magnetic field sensors being non-zero; and Figure 7 illustrates a second current sensor comprising magnetic field sensors supported on stiffened sections of a belt which are joined by hinges which can help to resist formation of kink and which can result in the sum of potential vectors measured by the magnetic field sensors being non-zero

[0038] Detailed Description

[0039] Referring to Figure 1, a current sensor 1 which is capable of measuring de current in a conductor 2 (Figure 2) is shown.

[0040] The current sensor 1 comprises an elongate belt 3 (or "band") running between first and second ends 4, 5 comprising a line of magnetic field sensors 6, 6i, 62,..., 6n-i, 6n evenly spaced at a pitch, p, along the belt. The magnetic field sensors 61, 62,...,6n-i, 6n may take the form of tunnelling magnetoresistive (TMR) devices. The magnetic field sensors 61, 6z,...,6n-i, 6n are orientated such that their magnetic field sensing axes 7 are aligned along the belt 3 parallel to a centre line 8 of the belt. The belt 3 is sufficiently bendable to allow the ends 4, 5 of the belt 3 to be brought together to form a loop 9. The belt 3 is flexible and can be laid out flat. A fastening arrangement 10, for example, in the form a clip 11 and corresponding lugs 12 can be used to secure the ends 4, 5 together.

[0041] The elongate belt 3 may comprise a strip 13 of flexible material. This may be provided by a flexible printed circuit board 14 (or "flex circuit") which comprises a plastic substrate 13, for instance a strip of polyamide having a thickness t, supporting wiring 15 in the form of conductive tracks. For example, a substrate may have a thickness of 0.25 mm. A flexible printed circuit board 14 need not, however be used. For example, discrete wires may be used.

[0042] The strip 13 may be continuous, in other words, be a single strip. The belt 3, however may be formed from discrete strips which are joined together.

[0043] The elongate belt 3 may comprise alternating first and second sections 21, 21i, 212,...,21n-i, 21n, 22, 221, 222,..., 22n-i, 22n. The first sections 21, 21i, 212,..., 21n-i, 21nrunning between the first and second ends 4, 5 support the magnetic field sensors 6, 6i, 62,...,6nand the second sections 22, 22i, 222,... ,22ninterconnect the first sections 21, 21i, 212,...,21n. In this case, n = 11, but n may be larger or smaller.

[0044] The first sections 21, 21i, 212,...,21n-i, 21nare relatively long having a first length, li, and the second sections 22, 22i, 222,..., 22n-i, 22nare relatively short, having a length, I2.

[0045] The first sections 21, 21i, 212,...,21n-i, 21nare relatively stiff and the second sections 22, 22i, 222,..., 22n-i, 22nare relatively flexible. The first sections 21, 21i, 212,...,21n- 1, 21nmay be stiffened, for example, with a stiffener, such as a plate, or treated chemically or thermally to change the properties of the material.

[0046] The second sections 22, 22i, 222,..., 22n-i, 22 may take the form of hinges, such as living hinges or mechanical hinges.

[0047] The belt 3 may include outer flexible insulation 23 (Figure 2) encapsulating the sensors 61, 62,...,6n-i, 6n and other parts such as sensor support structure (such as strip 13, strips and hinges or other arrangement) and wiring 15.

[0048] A first sensor 61 which is closest to the first end 4 is supported by a first end section 21i. A last sensor 6nwhich is closest to the second end 6 is supported by a second end section 21n. The second end section 21nis split into two parts 21n,a, 21n,t>. The first part has 21n,ais connected to the first end section 21i and has a length, a. The second part has 21n,b is connected to the second end section 212 and has a length, li- a. Thus, when the first and second ends 4, 5 are brought together, the first and last sensors 61, 6n are separated by the same distance as the other sensors, namely, by a pitch, p, where p = h + I2.

[0049] Referring still to Figure 1, the belt 3 is connected, via a connector 31 to a plug 32 via a screened connection cable 33 The plug 32 is used to connect the sensor 1 to instrumentation 34 (or "measuring device") such as a hand-held meter, logging unit, oscilloscope, power quality analyser, or the like.

[0050] The clip 11 engages with lugs 12 joining the two parts 21n,a, 21n,b of the second end section 21nand leaving the separation of first and last sensors 61, 6nthe same as between the other sensors 6.

[0051] Referring to Figures 2A, 2B and 2C, the sensor 1 may have an electrically insulating sheath 35 (or sheaths) along the length of the belt 3. The belt 3, including the sensors 6, are disposed inside the sheath 35. The sheath 35 may comprise, for example, a thermoplastic elastomer.

[0052] Referring in particular to Figures 2A, the sheath 35 may take the form of a plastic over mould.

[0053] Referring in particular to Figure 2B, the sheath 35 may take the form of plastic continuous sleeve.

[0054] Referring in particular to Figure 2C, there may be more than one layer of electrically insulating material.

[0055] Additionally, or alternatively, the sensor 1 may have a magnetic shield 36 along the length of the belt 3. The belt 3, including the sensors 6, and, if present, the electrically insulating sheath may be disposed inside the magnetic shield 36. The magnetic shield 36 may comprise a soft magnetic material which may be braided to form a braided sheath or embedded in a polymer to provide a flexible tube

[0056] The electrically insulating sheath 35 and, if present, the magnetic shield 36 is (are) sufficiently thick to provide electrical protection by providing a separation, s, between an external voltage and the sensors 6. For example, the separation may be sufficient to provide insulation from a voltage of up to 1500 VDC or more.

[0057] The magnetic shield and, if present, the electrically insulating sheath 35 is (are) sufficiently thick to provide magnetic protection from a permanent magnet 37 which may have a remanence of up to IT, 1.2T or more. A magnetic field can affect the measurement and may even damage the sensor, for example, by reorientating a reference (or "pinned") layer in the sensor.

[0058] In some cases, a separate sensor (not shown) or an additional magnetic sensing axis of the sensors 6 may be used to detect the presence of an external magnetic field. This can be used to detect interference or tampering and the sensor 1 can, thus, signal to the user that its reading can no longer be trusted and / or that the sensor can no longer be used.

[0059] Referring also to Figure 3, the sensor 1 can be bent around a conductor 2, such as a busbar, to form a loop 9.

[0060] Referring to Figure 4, an example of a direct interconnection circuit 40 is shown which is connected to a signal source 41 and an amplifier 42 having an output 43.

[0061] Each sensor 6 has a Wheatstone bridge-like configuration of four elements 38i, 382, 383, 384 and four junctions 39i, 392, 39s, 394. The sensors 6 are arranged in parallel, excited by a common lines VCC, GND and measured using common lines +, -.

[0062] First, second, third and fourth connections 44, 45, 46, 47 are made to a composite bridge providing a volt per volt of excitation output proportional to the current in the conductor 2. This output signal can be given to the user directly, with a calibration factor, for example gain (mV / V / amp) and / or as a de offset (A or mA). These values may be trimmed to predefined numbers by additional trimming resistors (not shown) for instance gain O. lmV / V / amp, offset =0, or provided by built in electronic memory (not shown) in the connector or QR code etc. Buffer resistors (not shown) may be placed between the bridge outputs and the summing lines to reduce dependency on part to part bridge impedance variation at the expense of increased output impedance if needed.

[0063] The sensor 1 has a length, L, of 240 mm and an aperture diameter, D, of 70 mm. For a 240 mm-long aperture, the linear range is estimated to be 4kA. The sensor 1 can have a radial thickness, r, of about 3 mm depending on the thickness of insulation, i.e., used for encapsulation, needed. The sensor 1 can exhibit a bandwidth of greater than 1 > lkHz and good immunity to external currents. Immunity can be defined as, for a current flowing in a conductor outside the sensor (in other words, flowing outside the loop), the measured current being less than 1% of the measured current for a current flowing in a conductor inside the loop.

[0064] Referring also to Figure 5, a plot of meter factor as a function of current is shown.

[0065] The plot shows that the sensor 1 exhibits less than 1.5% linearity in the range from 0.1 A to 4A and less than 0.5% linearity in the range from 4 A to 1 kA.

[0066] As hereinbefore described, the sensor 1 can provide a flexible dc-capable Rogowski which can be formed, for example, using a flexi circuit. The sensor 1 has evenly- spaced magnetic field sensors 6 along its length, with their magnetic sensitivity axes 7 all aligned along the length of the strip 3. The output signal can be the sum or average of each of the field sensor outputs.

[0067] Referring to Figures 6 and 7, the belt 3 can be stiffened around each sensor 6 which can help ensure zero sensitivity to uniform external magnetic fields, as the sum of the signal outputs add to zero. Also, stiffening can provide mechanical strength which can help protect the electrical connections to the sensor 1.

[0068] The following is provided by a way of a non-limiting explanation. A magnetic field sensor with a sensitivity axis along a particular vector measures the projection of the magnetic field onto that vector. If an arrangement of magnetic field sensors have sensitivity axes along a set of vectors, and that set of vectors sum to zero, then, in the presence of a uniform magnetic field, the sum of the outputs of that set of sensors will be zero. If the set of vectors do not sum to zero, then the sum of the outputs of the sensors will be sensitive to a uniform magnetic field. Thus, for a set of sensors with sensitivity axes locally fixed to the tangent of a closed path, the sum of the vector directions of those axes do not necessarily sum to zero due to curvature at higher spatial frequencies than the curvature is sampled by the sensor spacing.

[0069] As explained earlier, the magnetic axes 7 of the sensors 6 are aligned with the strip 3, and the sensitivities of the sensors 6 are matched and the spacing of the sensors are matched. This allows application of Gauss's law and the sum of the sensor signals to be equated with the magnetic field loop integral around the conductor, and hence the net enclosed current. The periodic nature of the sensing elements allows for the joint made when wrapping the assembly 3 around a conductor 2 to preserve the periodicity. Both the nominal pitch, p, and the mechanical response of the flexure can be made similar at the joint to the other gaps between sensors.

[0070] Modifications

[0071] It will be appreciated that various modifications may be made to the embodiments hereinbefore described. Such modifications may involve equivalent and other features which are already known in the design, manufacture and use of current sensors and component parts thereof and which may be used instead of or in addition to features already described herein. Features of one embodiment may be replaced or supplemented by features of another embodiment.

[0072] Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel features or any novel combination of features disclosed herein either explicitly or implicitly or any generalization thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. The applicants hereby give notice that new claims may be formulated to such features and / or combinations of such features during the prosecution of the present application or of any further application derived therefrom.

Claims

Claims1. A current sensor which comprises:■ a belt running along a line between first and second ends and comprising :- a line of magnetic field sensors, each having at least one magnetic field sensing axis, evenly spaced along the belt and orientated such that each of the magnetic field sensors has a magnetic field sensing axis aligned along the belt, wherein the belt is sufficiently bendable to allow the ends of the band to be brought together to form a loop.

2. The current sensor of claim 1, wherein the belt comprises alternating first and second sections between the first and second ends, wherein:■ the first sections support the magnetic field sensors; and■ the second sections interconnect the first sections.

3. The current sensor of claim 2, wherein:■ the first sections are relatively long; and■ the second sections are relatively short.

4. The current sensor of claim 2 or 3, wherein:■ the first sections are relatively stiff, and■ the second sections are relatively flexible.

5. The current sensor of claim 4, wherein the second sections comprise hinges.

6. The current sensor of claim 4 or 5, wherein the second sections comprise living hinges.

7. The current sensor of claim 1 or any one of claims 2 to 4, wherein the belt comprises:■ a continuous flexible strip between the first and second ends.

8. The current sensor of claim 7, wherein the first sections include stiffeners.

9. The current sensor of claim 1 or any one of claims 2 to 8, comprising :■ wiring connecting the magnetic field sensors.

10. The current sensor of any one of claim 1 or any one of any one of claims 2 to claim 9, comprising:■ a flexible printed circuit board providing the wiring or at least some of the wiring.

11. The current sensor of claim 9 or 10, comprising:■ a discrete wires providing the wiring or at least some of the wiring.

12. The current sensor of claim 1 or any one of claims 2 to 11, wherein the magnetic field sensors comprise tunnelling magneto resistance bridge.

13. The current sensor of claim 1 or any one of claims 2 to 12, wherein the magnetic field sensors are excited in parallel.

14. The current sensor of claim 1 or any one of claims 2 to 13, wherein the magnetic field sensors are read in parallel.

15. The current sensor of claim 1 or any one of claims 2 to 14, wherein the magnetic field sensors are three-axis sensors.

16. The current sensor of claim 1 or any one of claims 2 to 15, wherein the magnetic field sensors are configured to have a sufficiently wide bandwidth for measuring a powerline frequency and at least first harmonic of the powerline frequency.

17. The current sensor of claim 1 or any one of claims 2 to 16, further comprising:■ a coil wound along a centre line running along the belt.

18. The current sensor of claim 1 or any one of claims 2 to 17, further comprising:■ a fastening arrangement for releasably securing together the first and second ends of the belt.

19. The current sensor of claim 1 or any one of claims 2 to 18, wherein:- the magnetic field sensors are spaced by a given pitch,- the belt, a first magnetic field sensor closest to the first end and a last magnetic sensor closest the second end are configured such that when the loop is formed the first and last magnetic field sensors are adjacent and spaced by the given pitch.

20. The current sensor of claim 1 or any one of claims 2 to 19, further comprising:■ an electrically insulating sheath, wherein the belt is disposed inside the electrically insulating sheath.

21. The current sensor of claim 20, wherein the electrically insulating sheath is configured to provide protection to an external voltage of between 0 VDC to 1500 VDC or more.

22. The current sensor of sensor of claim 20, wherein the electrical shield is configured to help limit or to avoid a permanent magnet affecting or damaging the magnetic field sensors.

23. The current sensor of claim 1 or any one of claims 2 to 22, wherein further comprising:■ a magnetic shield, wherein the magnetic shield is configured to help limit or to avoid a permanent magnet affecting or damaging the magnetic field sensors.

24. The current sensor of sensor of claim 23, wherein the magnetic shield comprises a sheath formed of magnetic soft material.

25. A current sensor system comprising:■ the current sensor of claim 1 or any one of claims 1 to 19; and■ instrumentation for reading the current sensor.

26. The current sensor of claim 25, configured to measure a de current, an ac current and / or an ac current having a de component.

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