GB2579019A

GB2579019A - Test apparatus

Info

Publication number: GB2579019A
Application number: GB1818500.9A
Authority: GB
Inventors: Zurek Stanislaw
Original assignee: Megger Instruments Ltd
Current assignee: Megger Instruments Ltd
Priority date: 2018-11-13
Filing date: 2018-11-13
Publication date: 2020-06-10
Anticipated expiration: 2038-11-13
Also published as: GB2579019B; GB201818500D0

Abstract

A flexible electric current sensor 1 with a solenoidal 2 core divided into a plurality of flexible magnetic sections 4 extending along its length. Between some of the plurality of magnetic sections are gaps 5 which retain their size and shape during flexing. Preferably, the flexible sections are composed of a flexible magnetic material and are substantially tubular and may have different diameters (fig 4). The spaces between the magnetic sections may be partially filled by a non-magnetic material and may not contain an active electronic device. The windings of the coil can be non-uniform around the core. Preferably, there are a plurality of coils where each coil is disposed, at least partially, around one of the gaps or where each coil is disposed around one of the flexible magnetic sections. The magnetic sections can be embedded in a non-magnetic flexible material and may also be threaded onto a flexible wire.

Description

Intellectual Property Office Application No. GII1818500.9 RTM Date:14 March 2019 The following terms are registered trade marks and should be read as such wherever they occur in this document: Mu-metal Intellectual Property Office is an operating name of the Patent Office www.gov.uk/ipo

FLEXIBLE CURRENT SENSOR

Technical Field

The present invention relates to a flexible electrical current sensor, in particular a flexible electrical current sensor with a winding and an at least partially magnetic core having an effective magnetic permeability that is maintained during flexing.

Background

Flexible current sensors, in the form of a Rogowski coil (herein "RC"), have been known since the end of the 19th century and are commonly used today for measuring electrical currents. Modern versions typically have a non-magnetic core, for example made from silicon rubber or plastic.

F. Alves, "High-frequency behaviour of magnetic composites based on FeSiBCuNb particles for power electronics", IEEE, Trans. Magnetics, Vol. 38 (5), 2002, p. 3135 describes a structure with magnetic flakes dispersed in a non-magnetic matrix.

Ren, Shiyan, Jiang Cao, and Huayun Yang. "Research of a novel Rogowski coil with special magnetic core." Precision Electromagnetic Measurements Digest, 2008. CPEM 2008. Conference on. IEEE, 2008 describes a Rogowski coil with a core having embedded particles.

Summary

According to a first aspect of the present invention, there is provided a flexible electrical current sensor comprising a winding disposed about an at least partially magnetic core, the core being elongate and comprising a plurality of flexible magnetic members distributed along the length of the core, wherein the plurality of flexible magnetic members are arranged within the flexible electrical current sensor so that a respective gap is provided between at least some adjacent flexible magnetic members such that the size and shape of each respective gap is maintained during flexing of the flexible electrical current sensor.

This allows the core, and the flexible current sensor as a whole, to have an effective magnetic permeability that is controllable to predictable tolerances at the design stage, by the design of the flexible magnetic members and the gaps between adjacent flexible magnetic members, and that is maintained during flexing of the flexible current sensor. The effective magnetic permeability of the present flexible current sensor may therefore have greater controllability and predictability than known current sensors, while flexibility of the current sensor allows it to be useful in a wide range of applications involving measurement of electrical current.

In an embodiment of the invention, the plurality of flexible magnetic members comprises magnetic members having at least two rigid parts connected by a flexible joint. This allows a gap of fixed dimensions to be provided between adjacent magnetic members, to provide precise control of the permeability of the core. The flexible magnetic member, being composed of rigid parts and which have a flexible joint engineered to controlled tolerances, may also provide precise control of the permeability of the core which is maintained during flexing.

In an embodiment of the invention, the plurality of flexible magnetic members comprises magnetic members shaped to allow flexing at defined parts of each flexible magnetic member. This allows the permeability of the magnetic member to be maintained during flexing.

In an embodiment of the invention, the plurality of flexible magnetic members are substantially tubular. This provides a format that may be reliably manufactured to controlled tolerances.

In an embodiment of the invention, the flexible magnetic elements have slots which allow the flexible magnetic member to flex. This allows precise control of the flexibility of the flexible magnetic members, which may be tubular, by the use of precise cutting techniques, for example laser cutting.

In an embodiment of the invention, the plurality of tubular flexible magnetic members comprises tubular flexible magnetic members of different diameters, configured to provide radial gaps between adjacent smaller diameter tubular members and larger diameter tubular members. This allows for efficient manufacturing, providing gaps with dimensions that may be precisely set.

In an embodiment of the invention, the plurality of flexible magnetic members comprises magnetic members composed of a flexible magnetic material. Each magnetic member composed of a flexible magnetic material may comprise rigid ends configured to define the gap between the respective flexible magnetic material and the adjacent flexible magnetic members. This allows a convenient implementation of a flexible magnetic member, while allowing a precisely controlled gap to be provided, since the dimensions of the gap may be maintained by suitable supports while the magnetic member flexes. The gaps may increase the tolerance of the design to variability of the permeability of the flexible magnetic material.

In an embodiment of the invention, the plurality of gaps comprises gaps at least partially filled by a non-magnetic material. The plurality of gaps may comprise gaps formed by connecting adjacent flexible magnetic members using a rigid non-magnetic material. This provides a convenient way of allowing the dimensions of the gaps to be maintained during flexing of the flexible current sensor.

In an embodiment of the invention, the plurality of gaps comprises gaps which do not contain an active electronic device. This provides a simpler implementation which is easier to manufacture.

In an embodiment of the invention, the plurality of magnetic members is arranged in a single layer and the gaps between adjacent magnetic members are configured such that adjacent magnetic members in the single layer do not have an overlap in the plane of the single layer. This may provide increased flexibility.

In an embodiment of the invention, the winding is disposed about the at least partially magnetic core.

In an embodiment of the invention, the distribution of the windings of the coil is non-uniform longitudinally along the core. The winding may comprise a plurality of coils, each coil being disposed at least partially around a respective gap of the plurality of gaps. In another embodiment, the winding may comprise a plurality of coils, each coil being disposed about a respective flexible magnetic member of the plurality of flexible magnetic members. This provides alternative methods of current sensing.

According to a second aspect of the invention, there is provided a flexible electrical current sensor comprising a winding disposed about an at least partially magnetic core, the core being elongate and comprising a plurality of magnetic members arranged along the length of the core, wherein the plurality of magnetic members is arranged to provide a plurality of gaps between adjacent magnetic members of the plurality, each magnetic member having opposing surfaces of which at least one is at least partially spherical, such that at least parts of at least one of the at least partially spherical surfaces defines the gap between the adjacent magnetic members, such that the size and shape of the gap are maintained during flexing of the flexible electrical current sensor. This allows the core, and the flexible current sensor as a whole, to have an effective magnetic permeability that is controllable to predictable tolerances at the design stage, by the design of the flexible magnetic members and the gaps between adjacent flexible magnetic members and that is maintained during flexing of the flexible current sensor.

In an embodiment of the invention the plurality of magnetic members are embedded in a non-magnetic flexible material. This provides an effective way of maintaining the dimensions of the gaps In an embodiment of the invention, the plurality of magnetic members are threaded onto a flexible wire. This provides a robust implementation of a flexible sensor while maintaining permeability during flexing.

Further features and advantages of the invention will become apparent from the following description of preferred embodiments of the invention, given by way of example only, which is made with reference to the accompanying drawings.

Brief Description of the Drawings

The invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a flexible current sensor according to embodiments of the invention, showing use to measure a current in a conductor; Figure 2 is a schematic diagram of a flexible current sensor according to embodiments of the invention with a core having fixed gaps and flexible magnetic members formed as jointed rigid parts; Figure 3 is a schematic diagram of a flexible current sensor according to embodiments of the invention, having flexible magnetic members composed of flexible or slotted material; Figure 4 is a schematic diagram of a side-on view of a flexible current sensor comprising flexible magnetic members which are formed as concentric tubes according to embodiments of the invention; Figure 5 is a schematic diagram of a perspective view of the concentric flexible tubes of Figure 4, shown with laser-cut slots; Figure 6 is a schematic diagram of a perspective view of a flexible current sensor according to embodiments of the invention, shown with a non-magnetic material covering the tubes of smaller diameter to precisely define the gaps between adjacent tubes; Figure 7 is a schematic diagram of a flexible current sensor according to embodiments of the invention, having flexible magnetic members; Figure 8 is a schematic diagram of a flexible current sensor according to embodiments of the invention, having flexible magnetic members made from a flexible magnetic material and having rigid ends defining a gap of fixed dimensions; Figure 9a and 9b are schematic diagrams of part of the core of a flexible current sensor according to embodiments of the invention, illustrating principles of operation with vertical and longitudinal gaps; Figure 10 is a schematic diagram of a perspective view of a flexible current sensor according to embodiments of the invention, composed of rigid hemispherical magnetic members embedded in a flexible non-magnetic substrate Figure 11 is a schematic diagram of an exploded view of the core of the arrangement of Figure 10; Figure 12 is a schematic diagram of a perspective view of a flexible current sensor according to embodiments of the invention, comprising a single layer of rigid magnetic spheres embedded in a flexible non-magnetic material, Figure 13 is a schematic diagram of an exploded view of the core of arrangement of Figure 12, Figure 14 is a schematic diagram of a perspective view of a flexible current sensor according to embodiments of the invention, comprising magnetic elements comprising pairs of rigid spheres, interconnected by connection pieces composed of flexible or rigid non-magnetic material configured so that the dimensions of the gaps between the spherical surfaces of adjacent magnetic elements are maintained when the sensor is flexed.

Figure 15 is a schematic diagram of an exploded view of the core of the arrangement of Figure 14; Figure 16 is a schematic diagram of a perspective view of a flexible current sensor according to embodiments of the invention, comprising magnetic elements in the form of rigid spheres threaded on a wire.

Figure 17 is a schematic diagram of an exploded view of the core of the arrangement of Figure 16; Figure 18 is a schematic diagram of a flexible current sensor according to embodiments of the invention with a core having fixed gaps and flexible magnetic members formed as jointed rigid parts, in which the winding comprises a series of interconnected coils disposed at least partially around the gaps between the flexible magnetic elements; and Figure 19 is a schematic diagram of a flexible current sensor according to embodiments of the invention with a core having fixed gaps and flexible magnetic members formed as jointed rigid parts, in which the winding comprises a series of interconnected coils each disposed around a respective flexible magnetic element.

Detailed Description

Embodiments of the invention are herein described with reference to the accompanying drawings. In the following description, for the purpose of explanation, numerous specific details of certain examples are set forth. Reference in the specification to "an example" or similar language means that a particular feature, structure, or characteristic described in connection with the example is included in at least that one example, but not necessarily in other examples. It should further be noted that certain examples are described schematically with certain features omitted and/or necessarily simplified for ease of explanation and understanding of the concepts underlying the examples.

Rogowski coils (RCs) are used mainly for measuring alternating currents (AC). They may provide reliable performance and linearity at currents of kilo-amperes (kA), in a wide frequency range up to megahertz (MHz). However, the non-magnetic core means that their sensitivity is relatively low and they typically cannot measure accurately below 1 ampere (A). For normal RC construction, 10 milli-amperes (mA) may be the lowest current measurable. Generally, the term "Rogowski coil" implies a sensor with a non-magnetic core. However, sensors with magnetic (or partially magnetic) cores may also be referred to as Rogowski coils.

The operation of Rogowski coils and the present flexible current sensor is based on a relationship between an input current passing through, and being measured by, the coil and a generated output voltage. With the relationship known, the output voltage can be measured and the input current inferred. This relationship may be given by: A* 12011 (14,,H) VOIR dt)i where Vo", is the output voltage, dh,(t)/dt is the time differential of the input current as a function of time Im(I), pr is the relative magnetic permeability of the medium, po is the magnetic constant or magnetic permeability of vacuum, A is the cross-sectional area of the core about which turns of wire are wound, / is the magnetic path length, and the sum is over each turn i up to the total number of turns N. The sum of partial voltages, each partial voltage corresponding to the voltage through each turn i of the coil, may also be referred to as a space integral.

For better proportionality between the output voltage Void and input current Ln(I), the other terms in the equation should be kept as constant as possible throughout the core of the current sensor, an is a natural constant, while A and 1 are relatively straightforward to manufacture such that they are constant throughout the coil. Thus, a current sensor where it, is constant and the spacing between the turns of the coil is constant (uniform winding) would improve the measurement accuracy of a current.

The relative magnetic permeability of a material is related to a magnetic field strength H (measured in amps per metre) and a magnetic flux density B (measured in tesla) by B = fib fir H. Thus, a constant itt, gives a linear relationship between B and H. Introducing an air gap into a magnetic core may reduce the magnetic permeability pr of the core while also increasing the linearity of the relationship between the magnetic field strength H and magnetic flux density B -sometimes referred to as a B-H curve or loop. The effective magnetic permeability /Jeff of such a magnetic core with a gap may be considered to be: Ptore "lett =jgap Acore ± 1 where score is the magnetic permeability of the core material, Icore is the length of the core, and (gap is the length of the air gap. This equation is based on several assumptions, such as: the cross section area of the magnetic circuit is constant at every point of the circuit, and is the same for the core and for the gap; the length of the air gap is much shorter than the total path length of the magnetic core; the magnetisation is uniform and fringing effect is neglected; and the permeability of the core material is much greater than the permeability of air gap.

It is an object of the present invention to provide a current sensor that is flexible and has increased uniformity of effective magnetic permeability, even during flexing, such that measurements of low current (for example, current below I A) may be made with a higher value of certainty than known current sensors. For example, in current sensors where magnetic powder, particles or flakes is/are dispersed in a non-magnetic matrix or substrate, it may be difficult to determine with sufficient accuracy the number of such particles present in a given volume of material. The uniformity of such a structure must therefore be estimated by statistical averaging. In the present flexible current sensor, however, the uniformity of the structure is controlled by mechanical properties, and so may be determined and controlled to greater accuracy. The effective magnetic permeability of the present flexible current sensor may therefore have greater controllability and predictability.

Figure 1 is a schematic diagram of a perspective view of a flexible current sensor 1 according to embodiments of the invention, showing use to measure a current in a conductor 12. The flexible current sensor is arranged in a loop around the conductor 12, which may be any conductor carrying a current, such as a wire or cable, or for example the earthed leg of a current carrying installation such as an electricity pylon. If the flexible current sensor is sufficiently long, it may be wrapped several times around the conductor carrying the current to be measured. The flexible current sensor is typically elongate as shown, and has two ends 6, 7 which may be brought together and held in place to form a closed loop around the conductor 12 to make a measurement of current, and released to form an aperture 13 to allow the current sensor to be placed around the conductor and then removed. It is also possible to have a sensor without the opening, so that the cable with current would have to be threaded through the loop. This might be advantageous in practice in certain applications, because for installed sensors it will limit theft opportunities. The current sensor has a winding, which is typically disposed about an at least partially magnetic core. The winding is formed as a coiled conductor which may be wound with a uniform pitch in some embodiments, and with a non-uniform pitch in other embodiments, and may comprise interconnected coils connected in series or in parallel. One end of the winding is connected via conductor 8 to test equipment 11, typically a current meter, and the other end of the winding is also connected, via a return wire running along the current sensor, by a conductor 9 to the test equipment 11.

Figure 2 is a schematic diagram of a flexible current sensor 1 according to embodiments of the invention with a core having fixed gaps 5 and flexible magnetic members 4. As shown in Figure 2, the flexible electrical current sensor 1 comprises a winding 2, shown here with windings of uniform pitch, and an at least partially magnetic core, the at least partially magnetic core being elongate and comprising a plurality of flexible magnetic members 4 disposed successively along the elongate partially magnetic core. The at least partially magnetic core may also comprise a flexible non-magnetic material (not shown in Figure 2), such as a polymer, to give the core support and protection and to support the winding. As shown in Figure 2, the plurality of flexible magnetic members 4 is configured to provide a plurality of gaps 5 between adjacent flexible magnetic members 4 of the plurality, each gap 5 being configured such that the size and shape of the gap are maintained or generally maintained during flexing of the flexible electrical current sensor 1. The gap 5 may be essentially of fixed dimensions, so that the flexibility of the current sensor does not derive from any movement of the fixed gap, but instead from flexing of the magnetic members. The provision of gap has the benefit of reducing the effect of variability of the permeability of the magnetic members on the overall permeability of the core.

In some embodiments the flexible magnetic member is made from a flexible material, which may be shaped or cut to increase its flexibility in defined parts of the member, and in other embodiments the flexible magnetic member may comprise rigid parts connected by one or more flexible joints, such a pivots or hinges or other kinds of flexible mechanical joint providing well controlled registration between the parts. There may be small gaps within a magnetic member, for example due to the tolerances of the flexible mechanical joint, but these are typically much smaller than the gaps provided between adjacent magnetic members so that the effect of these small gaps on the permeability of the at least partially magnetic core is negligible, typically not affecting the resultant permeability of the composite core in any significant way. This is in contrast to the main gaps, that is to say the gaps provided between adjacent magnetic members, whose presence significantly lowers the resultant permeability of the core.

In the example shown in Figure 2, the plurality of flexible magnetic members 4 comprises magnetic members having at least two rigid parts connected by a flexible joint. This allows a gap 5 of fixed dimensions to be provided between adjacent magnetic members, to provide precise control of the permeability of the core. The flexible magnetic member 4, being composed of rigid parts and which have a flexible joint engineered to controlled tolerances, may also provide precise control of the permeability of the core which is maintained during flexing. This allows the core, and the flexible current sensor as a whole, to have an effective magnetic permeability that is controllable to predictable tolerances at the design stage, by the design of the flexible magnetic members and the gaps between adjacent flexible magnetic members, and that is maintained during flexing of the flexible current sensor.

In the alternative embodiment of Figure 3, the flexible current sensor has flexible magnetic members 4 comprising a single piece, which is made to be sufficiently flexible for the intended use as illustrated by Figure 1. The flexible magnetic member may be composed of flexible material, such as flakes of magnetic material in a non-magnetic flexible matrix or substrate. Alternatively, the flexible magnetic member may be made of a material such as a metal that is made more flexible by shaping of cutting at appropriate parts of the member. Similarly to the embodiments shown in Figure 2, it is the magnetic members that are flexible and the gap is fixed, i.e. the gap has fixed dimensions. This allows the permeability of the core to be maintained during flexing of the current sensor. The flexible magnetic members shown in Figure 3 may be shaped to allow flexing at defined parts of each flexible magnetic member, for example by the provision of slots of an appropriate shape, to allow the permeability of the magnetic member to be maintained during flexing, as shown for example in Figures 4, 5 and 6.

Figure 4 shows a flexible current sensor comprising flexible magnetic members which are formed as concentric tubes 4a, 4b. As shown in Figure 5, the tubes 4a, 4b may have an arrangement of slots, which may be laser-cut, or cut may any other means such as chemical etching or mechanical milling to allow the tubes to flex. Alternatively the tubes may be made of a flexible material that does not need slots. Adjacent tubes may be of different diameters, so that a smaller tube may fit inside a larger tube in a region of overlap, defining a gap 5 between the overlapping parts of the smaller and larger tubes. As shown in Figure 6, a non-magnetic material 10 may cover the tubes of smaller diameter to precisely define the gaps between adjacent tubes. The substantially tubular flexible magnetic members may be reliably manufactured to controlled tolerances, arid the dimension of the gap may be maintained during flexing of the flexible current sensor because the gap is formed between rigid, typically un-slotted, parts of the tubes defining the gap. The slots allowing the flexible magnetic members to flex allow precise control of the flexibility of the flexible magnetic members in manufacture, by the use of precise cutting techniques, for example laser cutting Figure 7 shows a flexible current sensor according to embodiments of the invention, having flexible magnetic members 4 with gaps 5 defined between the members, the gaps being between faces of the magnetic members which are perpendicular to the longitudinal axis of the sensor, in which the plurality of flexible magnetic members may comprises magnetic members composed of a flexible magnetic material. The flexible magnetic material may for example be a magnetic powder, particles, grains or flakes embedded in a flexible non-magnetic substrate such as a rubber. The presence of a plurality of gaps of controlled dimensions reduces the effects of the variability of the distribution and concentration of the magnetic particles within the substrate, and allows a predictable and maintained magnetic permeability of the core to be reliably provided, and to be maintained during flexing.

As shown in Figure 8, each magnetic member 4 comprising a flexible magnetic material may comprise rigid ends 14 configured to define the gap 5 between the respective flexible magnetic material and the adj acent flexible magnetic members. This

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facilitates the flexibility of the magnetic member, while allowing a precisely controlled gap 5 to be provided. The dimensions of the gap may be maintained by suitable supports bearing against the rigid ends while the magnetic member flexes. The presence of gaps may increase the tolerance of the design to variability of the permeability of the flexible magnetic material. As shown in Figure 8, the gaps may be at least partially filled by a non-magnetic material. Alternatively or in addition, the gaps may be formed by connecting adjacent flexible magnetic members using a rigid non-magnetic material. This provides a convenient way of allowing the dimensions of the gaps to be maintained during flexing of the flexible current sensor.

In each of the embodiments shown, the gaps do not contain an active electronic device, so that a simpler implementation is provided which is easier to manufacture. Figure 9a and 9b are schematic diagrams of part of the core of a flexible current sensor according to embodiments of the invention, illustrating principles of operation with longitudinal and vertical gaps respectively. For example, Figure 9a corresponds to the embodiments with longitudinal gaps as in, for example, Figures 4, 5 and 6, in which the concentric tubes overlap to define the gap. Figure 9b corresponds to embodiments with vertical gaps, for example the embodiments of Figures 2, 3, 7 and 8, 15, 16, 18 and 19, where the magnetic members are arranged in a single layer and the gaps between adjacent magnetic members are configured such that adjacent magnetic members in the single layer do not have an overlap in the plane of the single layer. This may provide increased flexibility compared with the longitudinal gaps.

Referring to Figure 9a, an effective magnetic permeability it/comp of the core may be given by: Pmat itecm1P = 2g * t s Pmat ± 1 where: pmat is the magnetic permeability of the magnetic elements; g is the distance between magnetic elements 4; t is the thickness of the magnetic elements; o is the length of the overlap region, along a longitudinal axis of the core, between magnetic elements 4; s is the length of the magnetic elements along the longitudinal axis of the core 3. In some examples, the quantities g, t, o, and s may be averaged over the entire core. In other examples, the magnetic elements 4 may be uniform and arranged homogeneously along the length of the at least partially magnetic core, such that these quantities will have substantially the same value for each magnetic element 4 and each relationship between magnetic elements 4.

The quantity 2g * t/o * s may be considered to be an effective gap length of the at least partially magnetic core comprising two layers of magnetic elements 4 (analogous effective gap length expressions may be specified for partially magnetic cores comprising a different number of layers). For example, when comparing the above equation for the effective magnetic permeability itcomp of the core shown in Figure 9a to the general equation for effective magnetic permeability pen of a core with a gap, it can be seen that the ratios Igap//,,," and 2g * t/o * s are related. In an at least partially magnetic core 3 made up of multiple uniform elements, each of length e, the length sofa magnetic element 4 is related to the total effective length of the magnetic elements 4 in the core 3 by a factor of e. This factor is cancelled out when multiplying the effective gap length for an element () by e.

In some embodiments, the ratio Kt() of the average distance g between the first and second layers of magnetic elements 4; and the average overlap o between magnetic elements 4 in the first and second layers is substantially constant, due to the provision of a gap whose size and shape are maintained during flexing. In alternative embodiments, the size and shape of the gap may be allowed to change during flexing, in a controlled way that is designed to maintain the ratio g o during flexing of the flexible electrical current sensor I. For example the rati og o may be maintained during flexing to within 10% of a value at rest, in other words when the sensor is unflexed. Thus, by using magnetic elements 4 with uniform thickness t, the effective gap length of the at least partially magnetic core, and therefore its effective magnetic permeability //comp, may be kept constant. This allows for the linearity of the relationship between B and H, and between VOW and An to be maintained throughout the at least partially magnetic core, and during flexing of the electrical current sensor 1. The magnetic elements 4 may also have a high relative magnetic permeability of at least 500, 1000 or 10,000. This allows for high sensitivity of the electrical current sensor I, with measurements of below 1 mA possible, while being flexible so that it can be wrapped

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around a current-carrying structure, and thus useful in a wide range of applications. Figure 9b shows the equivalent g and o values in the case of the vertical gap between magnetic members. In both cases, in various embodiments, either the gap is maintained of fixed dimensions so that both g and o are fixed, or else the deformation of the gap shape is controlled, so that the g/o ratio is maintained within limits. So, a fixed gap may be provided and the magnetic members may be flexible, as already described. Alternatively, the magnetic members may be rigid, and the gap may be maintained by design of the interface, for example by using a spherical or partially spherical interface to allow rotation of one magnetic member relative to an adjacent member without changing the gap shape or size. Alternatively, in some embodiments the gap may be allowed to distort in a controlled way so that the g/o ratio is maintained within tolerable limits. This may for example be achieved with the arrangement of 9a, in which the magnetic members are rigid, and the gaps are filled with a flexible non-magnetic material, which is arranged to compress at one end of the gap and stretch at the other end of the gap during flexing, so that the g/o ratio is approximately maintained. In some embodiments the gap in Figure 9a is rigid and the structure is flexible because the magnetic members, in the form of the metal strips, can be bent, typically in the middle, during flexing.

In some embodiments the winding may comprise coils which are arranged around or in the gaps of a structure similar to that shown in Figure 9a. In such an embodiment, and also in embodiments similar to that shown in 9b, the coils are disposed about a longitudinal axis of the magnetic flux. This may be true for a single winding of uniform pitch, because the main direction of the global flux is around the length of the sensor, as well as for the winding comprising a series or parallel arrangement of coils. In the latter case, this is because the coil will have to encircle the local vectors of the flux jumping through air gap, as in Fig. 9a or Fig. 9b, as well as the main flux flowing in the magnetic elements, as in Fig. 19.

Figure 10 -17 show embodiments in which rigid magnetic members are disposed so that the gaps between them may have a size and shape that is in effect maintained during flexing of the flexible electrical current sensor. This maintains the permeability of the core by in effect maintaining the dimensions of the gap. This may be achieved by having magnetic members having opposing faces, or at least one opposing face, that is substantially a section of a sphere, so that as it moves relative to the adjacent member, the gap stays the same size and shape. In this case, the gap is in effect fixed. In each case, the magnetic member may for example be composed of mu-metal, and the non-magnetic material may be a flexible polymer. A magnetic member may be referred to as a magnetic element.

Figure 10 shows a flexible current sensor composed of rigid hemispherical magnetic members 15, 17, which may be composed of mu-metal for example, embedded in a flexible non-magnetic substrate 16, as shown in an exploded view in Figure 11. The spherical shapes of the adjacent faces of the magnetic members, held in place by the flexible non-magnetic substrate maintain the size and shape of the gaps between magnetic members when the current sensor is flexed. In some embodiments, the size and shape of the gaps may change with greater degrees of flexing, but in effect the Wo ratio is maintained due to the design of the interface. As shown in Figure 10, the flexible electrical current sensor comprises a winding 2 disposed about an at least partially magnetic core, the at least partially magnetic core being elongate and comprising a plurality of magnetic members 15, 17 disposed successively along the elongate partially magnetic core. The magnetic members 15, 17 are configured to provide a plurality of gaps between the adjacent magnetic members, each magnetic element having opposing surfaces of which at least one is at least partially spherical, such that at least parts of at least one of the at least partially spherical surfaces defines the gap between the adjacent magnetic members, such that the size and shape of the gap are maintained during flexing of the flexible electrical current sensor. This allows the core, and the flexible current sensor as a whole, to have an effective magnetic permeability that is controllable to predictable tolerances at the design stage, by the design of the flexible magnetic members and the gaps between adjacent flexible magnetic members and that is maintained during flexing of the flexible current sensor. As shown in Figure 10, the magnetic members are embedded in a non-magnetic flexible material. This provides an effective way of maintaining the dimensions of the gaps.

Figures 12 and 13 show a flexible current sensor comprising a single layer of rigid magnetic spheres 20 embedded in a flexible non-magnetic material 18, 19. In this example, the flexible non-magnetic material is shown as two pieces. Alternatively, the spheres can be embedded into the flexible non-magnetic material.

Figures 14 and 15 show a flexible current sensor comprising magnetic elements 22 comprising pairs of rigid spheres, interconnected by connection pieces 21 made of flexible or rigid non-magnetic material configured so that the dimensions of the gaps between the spherical surfaces of adjacent magnetic elements are maintained when the sensor is flexed. Alternatively, the magnetic elements could be embedded in a flexible non-magnetic substrate.

Figures 16 and 17 show a flexible current sensor comprising magnetic elements in the form of rigid spheres 23 threaded on a wire 25. The spheres are enclosed in a flexible non-magnetic cover 24, to provide additional support and protection for the core. The non-magnetic cover 24 can be used to define precisely the gap between the spheres. Non-magnetic cover 24 may be a non-magnetic coating on each sphere, of precise thickness. In this way, the magnetic insides of such coated beads, that is to say the magnetic members, will be separated by a precise non-magnetic spacing as defined by the thickness of the coating.

This provides a robust implementation of a flexible sensor while maintaining permeability during flexing.

As shown in the examples of Figures 2, 3, 4, 6. 7, 8, 10, 12, 14 and 16, the 20 winding is disposed about the at least partially magnetic core, and the distribution of the windings is uniform along the winding.

As shown in Figures 18 and 19, the distribution of the windings of the coil is non-uniform longitudinally along the core. The winding may comprise a plurality of coils, each coil being disposed around or within a respective gap of the plurality of gaps.

In another embodiment, the winding may comprise a plurality of coils, each coil being disposed about a respective flexible magnetic member of the plurality of flexible magnetic members. This arrangement of the winding may be applied to any embodiment of flexible current sensor, with rigid or flexible magnetic members, which may also be referred to as magnetic elements, and including the examples of Figures 2, 3, 4, 6. 7, 8, 10, 12, 14 and 16, in which the winding may be may comprise a plurality of coils, each coil being disposed about a respective or rigid magnetic member or may be placed around gaps or in gaps between magnetic members. F'

Figure 18 shows an example of a flexible current sensor with a core haying fixed gaps and flexible magnetic members 4 formed as jointed rigid parts, in which the winding comprises a series of interconnected coils 26 disposed around or in the gaps 5 between the flexible magnetic elements 4. A return wire 27 may be provided, or a second layer of winding may be used so that a connection to each end of the winding is accessible at one end of the current sensor.

As an alternative, Figure 19 shows a flexible current sensor with a core having fixed gaps 5 and flexible magnetic members 4 formed as jointed rigid parts, in which the winding comprises a series of interconnected coils 28 each disposed around a respective flexible magnetic element 4. A return wire 29 is provided. As described in connection with Figure 18, a second layer if winding may be used as a return path, and the second layer of winding may be used instead of a return wire for any of the embodiments of the invention The embodiments of Figures 18 and 19 provide alternative methods of current sensing.

The conductors 8 and 9 are for connection to a test unit 11, as shown in Figure 1. The test unit 11 may display a measured current value, for example, and may have controls for controlling the test apparatus. The flexible current sensor 1, and where applicable the non-magnetic substrate, is sufficiently flexible to allow the flexible current sensor 1 to be opened sufficiently wide and arranged about a current-carrying structure 12 in some embodiments. For example, in the embodiment shown in Figure 1, the flexible current sensor 1 is flexed to create an opening 13 for the current-carrying structure 12 to be passed through. The flexible current sensor 1 may be flexed to close the opening 13 and form a closed loop, for measuring the current passing through the current-carrying structure 12. In other embodiments, the flexible current sensor 1, and where applicable the non-magnetic substrate, may be sufficiently flexible to allow the flexible current sensor 1 to be wrapped around the current-carrying structure 12 with multiple turns. In examples, the current-carrying structure 12 may be any conductor for example a wire, a cable, or a metallic structural member such as a support leg of an electricity pylon.

Figure 1 shows schematically a test apparatus comprising the flexible current sensor 1 according to any of the described embodiments connected to the test unit 1 L The flexible current sensor comprises a return wire connected to, or as part of, the winding 2 which is not shown in the figure. The return wire or second layer of winding travels from an end of the winding at an end of the flexible current sensor Ito the other end of the flexible current sensor 1. In this way, the opening 13 may be created by flexing the flexible current sensor 1, with the winding 2 and the return wire not obstructing the opening 13, so that the flexible current sensor 1 may be arranged about a current-carrying structure. The return wire connects to the test unit 11 to complete a circuit with the other end of the winding 2 being connected to the test unit also.

In some examples, the flexible current sensor I may have a releasable joint to open and secure the ends of the flexible current sensor 1 at the opening 13.

Figures 1-4, 6-8, 10, 12, 14, 16, 18 and 19 show a flexible electrical current sensor 1 comprising a winding 2 disposed about an at least partially magnetic core. The at least partially magnetic core comprises at least one magnetic element 4, 15, 17, 20, 22, 23. The at least partially magnetic core may, in some examples, comprise magnetic and non-magnetic parts and hence is referred to as at least partially magnetic. The at least partially magnetic core may significantly increase sensitivity of the present current sensor compared to Rogowski Coils with non-magnetic cores, for example by a factor of around 30 to 50 times, or even exceeding 250 times for small currents. Thus it is generally possible, in each of the embodiments of the invention, to achieve greater sensitivity with such gapped structure, while maintaining excellent mechanical flexibility, than it is the case for powder/flakes filled magnetic rubber. For the latter the concentration of particles would have to be very high (e.g. exceeding 70% of volume ratio) to achieve the same sensitivity, thus making it less mechanically flexible or prone to cracking.

Compared with current sensors having a core comprising magnetic particles or flakes dispersed in a non-magnetic matrix, the positioning of the discrete magnetic element(s) 4 in the present flexible current sensor 1 may be controlled to greater precision. Thus, instead of an inhomogeneous distribution of magnetic particles, giving varying magnetic couplings between each pair of particles due to their varying non-uniform separations, the magnetic elements 4, or particular parts of a singular magnetic element 4, in the present flexible current sensor 1 may be positioned to a greater accuracy and precision relative to one another. This allows for a greater controllability and predictability of the effective magnetic permeability of the core.

Each respective gap is configured such that the effective magnetic permeability of the at least partially magnetic core is maintained during flexing. In some examples, the effective magnetic permeability of the at least partially magnetic core is much greater than 1, for example at least 500, which may improve the sensitivity of the flexible electrical current sensor 1 allowing smaller currents, for example 1 mA and below, to be measured. This provides a significant improvement in sensitivity over comparable RCs, with high-sensitivity RCs measuring currents in the order of 10 mA.

In an example, the effective magnetic permeability of the at least partially magnetic core is at least 2. In other examples, the effective magnetic permeability of the at least partially magnetic core 3 is approximately 30, or 100.

In certain cases, the effective magnetic permeability of the at least partially magnetic 3 core is maintained during flexing to within 1%, 2% 3%, 4%, 5%, or less than 10(11) of an effective magnetic permeability value when the core 3 is at rest or unflexed.

In some embodiments, each respective gap is configured to maintain a substantially constant volume of the respective region of overlap 5 provided by the at least one magnetic element 4.

The at least partially magnetic core may comprise a plurality of magnetic elements 4, with the magnetic elements 4 arranged in at least one layer disposed on the flexible non-magnetic substrate. The magnetic elements 4 may be affixed to the flexible non-magnetic substrate by an adhesive, for example the flexible non-magnetic substrate may have an adhesive layer. The magnetic elements 4 may be metallic strips in certain cases.

The at least partially magnetic core may comprise a first layer of magnetic elements 4 disposed on a first side of the flexible non-magnetic substrate, and a second layer of magnetic elements 4 disposed on an opposite side to the first side of the flexible non-magnetic substrate In other embodiments, the at least partially magnetic core may comprise multiple layers of magnetic elements 4 disposed on opposite sides of the flexible non-magnetic substrate.

The relative magnetic permeability p of the magnetic elements 4 may be greater than 500 or 1000 in some examples, and preferably at least 10,000. The magnetic field is guided along a magnetic element 4, because it is energetically much easier for the field to flow inside of a high-p magnetic element than through the non-magnetic substrate 6. However, the magnetic elements 4 have finite length and so at the end of the element 4, the field is forced to jump across to the next high-p magnetic element via the shortest possible path, which will start and end perpendicularly to the surfaces of adjacent magnetic elements 4. In some embodiments the lines of field may enter material at another angle, so that the filed will not start and end perpendicularly to the surfaces of adjacent magnetic elements.

In another embodiment, the at least partially magnetic core comprises a plurality of magnetic elements 4 arranged in at least one layer and disposed at least partially within the flexible non-magnetic substrate. For example, instead of the magnetic elements 4 being disposed on a surface of the flexible non-magnetic substrate, the magnetic elements 4 may be embedded, partially or fully, in the flexible non-magnetic substrate. The embedding may be achieved, for example, by over-moulding in rubber or via 3D printing, or by a lamination processes similar to that used in making printed circuit boards In certain cases, the magnetic elements 4 may be skewed at an angle relative to the longitudinal axis of the at least partially magnet core, for example the magnetic elements may have a longitudinal axis that is not parallel nor orthogonal to the longitudinal axis of the flexible non-magnetic substrate. In these cases, adjacent magnetic elements 4 may overlap one another in a common plane.

In another embodiment, the plurality of magnetic elements 4 are arranged in at least two layers, wherein each layer of magnetic elements is disposed at least partially within the flexible non-magnetic substrate, such that each magnetic element 4 in a layer overlaps with a magnetic element 4 in an adjacent layer.

In examples, a magnetic element 4 may be a magnetic member, such as a solid piece or strip of material that, as a whole, comprises magnetic properties. In other examples, a magnetic element 4 may be formed by a depression or cavity in the non-magnetic substrate 6 that is at least partially filled with a magnetic powder or liquid. For example, the concentration of magnetic powder or liquid having a defined boundary with the non-magnetic substrate 6 may be considered a magnetic element 4. A plurality of such cavities in the non-magnetic substrate, each at least partially filled with magnetic powder or liquid, may therefore be considered a plurality of magnetic elements 4.

In certain cases, the magnetic element(s) 4 described herein may have a relative magnetic permeability of at least 500, or at least 1,000, or at least 10,000. The magnetic element(s) may comprise magnetically soft material, for example iron (Fe), nickel (Ni), or cobalt (Co), or a mixture comprising at least two of these three metals such as a nickel-iron alloy. In an example, the magnetic elements(s) 4 are made from an alloy comprising 80% nickel and 20% iron, which is known to have high initial permeability (for low magnetic fields). In other cases, the magnetic element(s) 4 may be formed from nanocrystalline or amorphous ribbon, or electrical steel, for example cut into strips. In other examples, the magnetic element(s) may comprise a ferrite, oxide, or powder-based material. In certain cases, the magnetic element(s) may be flexible. In these cases, the magnetic element(s) may flex when the at least partially magnetic core is flexed, with the effective magnetic permeability of the at least partially magnetic core maintained during flexing.

In any of the embodiments and examples described herein, the winding 2 disposed about the at least partially magnetic core 3 may have winding of a uniform pitch. As the value for the output voltage depends on a space integral over all turns i of the winding 2 up to the total number of turns AT, a uniform winding of the winding 2 allows the same value for output voltage to be determined regardless of the position of the current sensor with respect to the electrical conductor being measured. Thus, in cases where the winding 2 has a winding of a uniform pitch, the present flexible current sensor I may be used more reliably, as its position relative to the electrical conductor being measured does not affect the measurement reading.

According to a further aspect of the present invention, there is provided a flexible electrical current sensor comprising a winding disposed about an at least partially magnetic core, the at least partially magnetic core comprising at least one magnetic element, wherein the at least one magnetic element is configured to provide one or more regions of overlap such that a respective gap is provided in each region of overlap, each respective gap being configured such that the effective magnetic permeability of the at least partially magnetic core is maintained during flexing This allows the core, and the flexible current sensor as a whole, to have an effective magnetic permeability that is controllable at the design stage, by the design of the positioning and overlap of the at least one magnetic element, and that is maintained during flexing of the flexible current sensor. The effective magnetic permeability of the present flexible current sensor may therefore have greater controllability and predictability than known current sensors, while flexibility of the current sensor allows it to be useful in a wide range of applications involving measurement of electrical current.

In some embodiments of the invention, each respective gap is configured to maintain a substantially constant volume of the respective region of overlap provided by the at least one magnetic element.

In some embodiments of the invention, the at least partially magnetic core comprises a plurality of magnetic elements, and the magnetic elements are arranged in at least one layer disposed on a flexible non-magnetic substrate. This allows the core, and the current sensor as a whole, to be flexible while having an effective magnetic permeability that is controllable by the positioning of the magnetic elements on the nonmagnetic substrate In some embodiments of the invention, the at least partially magnetic core comprises a first layer of magnetic elements disposed on a first side of the flexible nonmagnetic substrate, and further comprises a second layer of magnetic elements disposed on an opposite side to the first side of the flexible non-magnetic substrate. This allows the spacing, and thus magnetic coupling, between magnetic elements in the same and/or adjacent layer to be controllable. The effective magnetic permeability may therefore be set by the positioning of the magnetic elements, and maintained during flexing of the at least partially magnetic core.

In some embodiments of the invention, a ratio g/6 of the average distance between the first and second layers of magnetic elements g, and the average overlap between magnetic elements in the first and second layers o, is maintained during flexing.

In some embodiments of the invention, the at least partially magnetic core comprises a plurality of magnetic elements and the magnetic elements are arranged in at least two layers such that each magnetic element in a layer overlaps with a magnetic element in an adjacent layer. Each magnetic element is arranged to pivot about a point in a region of overlap with another magnetic element. Pivoting of the magnetic elements relative to one another, in some embodiments in more than one plane, allows the current sensor to be flexible, while the spacing of the layers, and of the magnetic elements within the layers, allows the effective magnetic permeability of the at least partially magnetic core to be controllable.

In some embodiments of the invention, the flexible electrical current sensor comprises one magnetic el ement arranged substantially helically. In other embodiments of the invention, the flexible electrical current sensor comprises two magnetic elements arranged as intertwining strips.

In some embodiments of the invention, the winding comprises winding of a uniform pitch This allows the current sensor to be more accurate, i.e. improves measurement uncertainty, due to the relationship between the current being measured and the output voltage.

In some embodiments of the invention, the magnetic element(s) have a relative magnetic permeability of at least 500. In some embodiments, the magnetic element(s) comprise magnetically soft material.

In an embodiment of the invention there is provided a flexible electrical current sensor comprising a winding disposed about a longitudinal axis of an at least partially magnetic core, the core being elongate and comprising a plurality of flexible magnetic members disposed successively along the length of the core, wherein the plurality of flexible magnetic members are arranged within the flexible electrical current sensor so that a size and shape of gaps, provided between at least some ad] acent flexible magnetic members, are generally maintained during flexing of the flexible electrical current sensor.

In an embodiment of the invention there is provided a flexible electrical current sensor comprising a winding disposed about an at least partially magnetic core, the core being elongate and comprising a plurality of flexible magnetic members disposed successively along the length of the core, wherein the plurality of flexible magnetic members are arranged within the flexible electrical current sensor so that a size and shape of gaps, provided between at least some adjacent flexible magnetic members, are generally maintained during flexing of the flexible electrical current sensor Embodiments have been described with fixed gaps and flexible magnetic members, or flexible gaps designed to maintain the permeability of the sensor during flexing, by maintaining their size and shape by a rolling action between partially spherical faces, or by distortion in a controlled way the maintains the g/o ratio within acceptable limits. This allows the sensor to be designed with predictable magnetic permeability within reliable tolerances, allowing accurate measurements of current with a flexible sensor. The above embodiments are to be understood as illustrative examples of the invention. It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the IS embodiments, or any combination of any other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.

Claims

CLAIMS1. A flexible electrical current sensor comprising a winding disposed about an at least partially magnetic core, the core being elongate and comprising a plurality of flexible magnetic members distributed along the length of the core, wherein the plurality of flexible magnetic members are arranged within the flexible electrical current sensor so that a respective gap is provided between at least some adjacent flexible magnetic members such that the size and shape of each respective gap is maintained during flexing of the flexible electrical current sensor.
2 A flexible electrical current sensor according to claim 1, wherein the plurality of flexible magnetic members comprises magnetic members having at least two rigid parts connected by a flexible joint.
3 A flexible electrical current sensor according to claim 1, wherein the plurality of flexible magnetic members comprises magnetic members shaped to allow flexing at defined parts of each flexible magnetic member.
4. A flexible electrical current sensor according to claim 3, wherein the plurality of flexible magnetic members are substantially tubular.
5. A flexible electrical current sensor according to claim 3 or claim 4, wherein the flexible magnetic members have slots which allow the flexible magnetic member to flex.
6. A flexible electrical current sensor according to claim 4 or claim 5, wherein the plurality of flexible magnetic members comprises tubular flexible magnetic members of different diameters, configured to provide radial gaps between adjacent smaller diameter tubular members and larger diameter tubular members.
7. A flexible electrical current sensor according to claim 1, wherein the plurality of flexible magnetic members comprises magnetic members composed of a flexible magnetic material.
8. A flexible electrical current sensor according to claim 7, wherein each magnetic member composed of a flexible magnetic material is arranged so that each gap is delimited by a rigid end of adjacent magnetic members.
9. A flexible electrical current sensor according to any preceding claim, wherein the plurality of gaps comprises gaps at least partially filled by a non-magnetic material.
10. A flexible electrical current sensor according to any preceding claim, wherein the plurality of gaps comprises gaps formed by a rigid non-magnetic material, which connects respective, adjacent flexible magnetic members.
11. A flexible electrical current sensor according to any preceding claim, wherein the plurality of gaps comprises gaps which do not contain an active electronic device.
12. A flexible electrical current sensor according to any preceding claim, wherein the plurality of magnetic members is arranged in a single layer and the gaps between adjacent magnetic members are configured such that adjacent magnetic members in the single layer do not have an overlap in the plane of the single layer. .
13. A flexible electrical current sensor according to any preceding claim, wherein the distribution of the windings of the coil is non-uniform longitudinally along the core.
14. A flexible electrical current sensor according to claim 13, wherein the winding comprises a plurality of coils, each coil being disposed at least partially around a respective gap of the plurality of gaps.
A flexible electrical current sensor according to claim 13, wherein the winding comprises a plurality of coils, each coil being disposed about a respective flexible magnetic member of the plurality of flexible magnetic members.
16. A flexible electrical current sensor comprising a winding disposed about an at least partially magnetic core, the core being elongate and comprising a plurality of magnetic members arranged along the length of the core, wherein the plurality of magnetic members is arranged to provide a plurality of gaps between adjacent magnetic members of the plurality, each magnetic member having opposing surfaces of which at least one is at least partially spherical, such that at least parts of at least one of the at least partially spherical surfaces defines the gap between the adjacent magnetic members, such that the size and shape of the gap are maintained during flexing of the flexible electrical current sensor.
17. A flexible electrical current sensor according to claim 16, wherein the plurality of magnetic members are embedded in a non-magnetic flexible material.
18. A flexible electrical current sensor according to claim 16 or claim 17, wherein the plurality of magnetic members are threaded onto a flexible wire.