HK40008356B - Inductive position detector - Google Patents

Description

Inductive position detectors

Technical Field

The present invention relates to an inductive displacement detector, which can be used to measure the displacement of a relatively movable body (entity, entity, main body).

Comments on the applicant's known technology

Various forms of inductive detectors have been used to measure position. The most well-known are resolvers and linear variable differential transformers or LVDTs. They have a long record of safe, reliable and accurate operation in harsh environments. Such inductive detectors typically use a wound transformer construction as their main component and are therefore large, bulky and costly. Recently, the authors have disclosed inductive detectors that use a printed circuit board (PCB) rather than a wound transformer construction to minimize size and cost while maximizing measurement performance. Such devices are sometimes referred to as inductive encoders and include the Incoder ^™ series of detectors manufactured by Zettlex Ltd. of Cambridge, England. Such detectors typically place their signal generation and processing electronics on a separate PCB away from the windings of the detector or away from the measurement path of the detector so that powering of the electronics does not interfere with the inductive sensing area. For some applications where space or weight is limited, this arrangement is not compact enough.

The present invention comprises the concept of a compact, efficient, accurate and robust inductive detector to detect the relative position of two or more bodies, and is applicable to various topologies. It is particularly suitable for accurate angular measurement in large through-shaft or hollow bore devices where axial or radial space is limited.

Summary of the Invention

In a first broad independent aspect, the present invention provides an inductive position detector comprising a first and a second body, at least one of the bodies being displaceable relative to the other along a measuring path, the first body comprising one or more antenna windings suitable for sending signals or for receiving signals or for sending and receiving signals; the first body further comprising an electronic circuit for powering the antenna windings for sending signals and for processing signals induced in the antenna windings; characterised in that the second body comprises a plurality of discrete target areas arranged along the measuring path; at least parts of the antenna windings and the electronic circuit overlap with different areas of the plurality of discrete areas; the discrete target areas are electrically conductive or magnetic; whereby the induced signals vary according to the relative positions of the first and second bodies.

This configuration is particularly advantageous because it allows the electronic circuitry, in certain embodiments, to be located in close proximity to the antenna windings, rather than being located in an embedded, remote location within the first body. Typically, such encoders must be of sufficient thickness to avoid errors in the position measurement caused by the electronic circuitry itself. This departure from conventional thinking allows for the construction of a significantly more compact detector because the construction is essentially free from any influence of the electronic circuitry, as the isolated target areas are advantageously segmented or spaced apart in some embodiments. By employing a significantly more compact detector, embodiments of the present invention allow for the benefits of inductive sensing to be applied over a wide range and in a variety of situations that would not be possible using conventional constructions.

In a subordinate aspect, the electronic circuit is positioned adjacent to the antenna winding. This approach defies conventional thinking, allowing for a highly compact configuration while also enabling precise measurement of the relative position of the subject. Preferably, the antenna winding and the electronic circuit are arranged contiguously. Preferably, the antenna winding and the electronic circuit are arranged substantially in the same layer of the first subject. Preferably, the antenna winding and the electronic circuit are arranged in substantially the same plane and spaced apart. Preferably, the antenna winding and the electronic circuit both face different or separate target areas.

In another subsidiary aspect, the first and second bodies are annular and configured to overlap one another, the one or more antenna windings being disposed in a first portion of the first body, and the electronic circuit being disposed in a second portion of the first body; the second portion being separate from the first portion. This configuration is particularly advantageous in forming a particularly compact annular configuration, in which the reduction in size is even more significant.

In another subsidiary aspect, said first body comprises a further portion separate from said first and second portions; said further portion comprising one or more further antenna windings.

In another subsidiary aspect, the first portion and the further portion are arranged diametrically opposite each other, and the second portion is arranged between the first portion and the further portion.

In another subsidiary aspect, the first body further comprises an insulating substrate and an electromagnetic shielding layer; the insulating substrate being a carrier for the antenna winding; one or more fasteners being provided to secure the substrate to the shielding layer; the fasteners being provided on the exterior of the first portion. This configuration is particularly advantageous as it allows for a robust and efficient construction while enabling precise position sensing and measurement.

In another subsidiary aspect, the first body comprises at least two separate adjacent antenna winding groups, which, in use, overlap at least two corresponding separate target area groups; each group of target areas forms a measurement path, whereby two different measurement paths are arranged adjacent to each other. This allows for accurate position determination in certain embodiments.

In another subsidiary aspect, the target area of the first measurement path differs in configuration relative to the target area of the second measurement path. This allows applying a coarser measurement to one of the groups and determining a finer measurement with the other group.

In another subsidiary aspect, said separated target regions form a periodic pattern of layered conductive regions.

In another subsidiary aspect, said separated target areas form a periodic pattern of magnetically permeable areas.

In another subsidiary aspect, the periodic pattern comprises a region perpendicular to the measurement path, the width of which varies continuously along the measurement path.

In another subsidiary aspect, the area is a circular area.

Preferably, the region is elliptical.

Preferably, the area is rectangular.

Preferably, the region is annular.

Preferably, the separated area is defined by two closed loops, one of the two closed loops being located within the other closed loop.

Preferably, the inner ring has a varying circumferential radius.

Preferably, the region is part of a periodically repeating shape.

Preferably, the region is defined by a plurality of separate windings, each of the separate windings being arranged in series with a capacitor to form a plurality of separate resonant circuits along the measurement path.

In a preferred embodiment, an inductive detector for measuring the relative position of multiple bodies along a measurement path is provided, comprising: an inductive target arranged along the measurement path; a layered antenna arranged facing a portion of the target; and electronic circuitry arranged along the measurement path; wherein the inductance of at least one winding in the antenna varies continuously in proportion to the relative position of the antenna and the target.

Preferably, at least a portion of the electronic circuit area is arranged in a plane parallel to the target.

Preferably, at least a portion of the electronic circuit area is arranged in the same plane as the antenna.

Preferably, at least a portion of the electronic circuit area faces at least a portion of the target.

Preferably, the sensing target has a periodic variation at a first pitch along the measurement path.

Preferably, the measurement path is taken from the list of linear, curved or circular.

Preferably, a metal surface faces the antenna.

Preferably, a metal surface faces the target.

Preferably, the distance between the target and the antenna perpendicular to the measurement path is smaller than the distance between the metal surface and the antenna.

Preferably, the distance between the target and the antenna perpendicular to the measurement path is smaller than the distance between the metal surface and the target.

Preferably, the target comprises at least one conductive area, wherein the conductive area forms a periodic variation of the sensing target at a first pitch along the measurement path.

Preferably, the target comprises at least one magnetic conductive region, and the magnetic conductive region forms a periodic variation of the inductive target at a first pitch along the measurement path.

Preferably, the antenna comprises a set of windings, the set of windings comprising: a transmitting winding; a first receiving winding comprising loops wound at a first pitch along the measuring path, wherein adjacent loops have opposite magnetic polarity; a second receiving winding comprising loops wound at the first pitch along the measuring path, wherein adjacent loops have opposite magnetic polarity, and the loops thereof are shifted a distance along the measuring path relative to the first receiving winding.

Preferably, the target comprises at least one conductive region, wherein the conductive region forms a second periodically varying sensing target at a first pitch along the measurement path.

Preferably, the target comprises at least one magnetically conductive region, and the magnetically conductive region forms a second periodically changing inductive target at a first pitch along the measurement path.

Preferably, said first and second variations form a unique spatial pattern along at least a portion of said measurement path.

Preferably, the antenna includes a second set of windings, the second set of windings including: a third receiving winding including loops wound at a second pitch along the measurement path, wherein adjacent loops have opposite magnetic polarities; a fourth receiving winding including loops wound at a second pitch along the measurement path, wherein adjacent loops have opposite magnetic polarities, and the loops thereof are shifted a distance along the measurement path relative to the third receiving winding.

Preferably, at least one winding from said first group overlaps with at least one winding from said second group.

BRIEF DESCRIPTION OF THE DRAWINGS

In the attached figure:

FIG1 shows a simplified schematic diagram in plan view of a detector in linear form.

Figure 2 shows the arrangement of the antenna windings.

Figure 3 shows a centreline section through a detector in the form of a rotating hollow bore.

4a, 4b, 4c and 4d show various sensing targets in plan view.

FIG5 shows a simplified schematic diagram in plan view of a detector in a linear format, wherein two sets of targets are arranged at different pitches for absolute position measurement.

FIG6 shows a schematic diagram of an electronic circuit suitable for operation with the detector when a winding pattern as shown in FIG2 is used.

FIG. 7 shows a surface of a detector arranged for absolute rotational position measurement with a stator and a rotor, wherein the stator has a plurality of antennas.

DETAILED DESCRIPTION

Figure 1 shows a simplified schematic diagram of the detector. A sensing target 1 and an antenna 2 are arranged for relative movement along a measurement path, i.e., the x-axis. A target carrier 9 carries the target 1, which is formed by a periodic pattern of conductive, layered target areas along the measurement path. This periodic pattern has a pitch L. The target carrier 9 is an insulating, layered substrate. The antenna 2 is a layered construction of conductive tracks on the layered, insulating substrate, preferably constructed using a multilayer PCB. The antenna 2 includes a transmitting winding 2c cooperating with two receiving windings 2a and 2b. The winding arrangement will be described in more detail later. The receiving windings 2a and 2b are spaced relative to each other along the measurement axis (x-axis). The transmitting winding 2c is powered by an electronic circuit 4 from a power source 3. The electronic circuit 4 advantageously occupies the same PCB area as the antenna 2, but is displaced along the measurement axis, facing another portion of the target. The electronic circuit 4 generates an AC signal, preferably in the range of 10 kHz to 10 MHz. Consequently, the transmitting winding 2c generates an AC electromagnetic field that surrounds at least a portion of the target 1. The mutual inductance between transmit winding 2c and receive windings 2a and 2b varies continuously depending on the position of target 1 along the measurement path. This arrangement allows energizing electronic circuit 3 to have substantially no effect on inductive sensing near antenna 2. Advantageously, any effect of energizing electronic circuit 4 on a nearby target is independent of the signal sensed by antenna 2. This is important for good sensor performance. Target area 1 can be made of either magnetically conductive or electrically conductive material and can be fabricated using a variety of techniques, including: copper areas etched into laminates, such as those used for printed circuit boards; one or more areas of conductive ink printed onto an insulating substrate; punched metal strips or mechanical features on machined or formed components. Etched copper PCB construction has been found to be advantageous due to the high level of consistency and accuracy inherent in photolithography. Windings 2a, 2b, and 2c are formed as tracks on a multilayer laminated printed circuit board, such as 0.8 mm thick FR4 with 1 ounce copper. Electrical isolation between winding sections can be achieved using plated through-holes at any intersections of the winding conductors. While this embodiment contemplates the use of one transmit winding and two receive windings, fewer sets of windings or significantly more sets of windings may be employed depending on the envisioned application.

Figure 2 shows a simplified version of antenna 2 and its windings 2a, 2b, and 2c. Each of receiving windings 2a and 2b is wound into two loops with opposite polarity and substantially the same area. Embodiments with a much larger number of loops, and with different shapes and configurations, are contemplated. The loops are configured to provide electrical balance between transmitting winding 2c and receiving windings 2a and 2b, such that in the absence of target 1, no or low signals are present at receiving windings 2a and 2b. Receiving windings 2a and 2b are displaced relative to each other along the measurement axis by ¼ of the pitch L. As antenna 2 is moved along this axis, the mutual inductance between transmitting winding 2c and receiving windings 2a and 2b varies continuously depending on the position of antenna 2 relative to target area 1. When maximum coupling occurs in first winding 2a, coupling in second winding 2b is at its zero value. The received voltages Vrx1 and Vrx2 in first and second receiving windings 2a and 2b vary sinusoidally and cosine-wise along the x-axis. The receiving windings 2a and 2b need not be sinusoidal as shown, but can be rectangular or other shapes, as will be understood by those skilled in the art. The position of the antenna 2 relative to the target 1 can be determined by a simple arctan calculation. Since the signal Vrx1 in the first receiving winding 2a is proportional to sin(x), and the signal Vrx2 in the second winding 2b is proportional to cos(x), the position "x" of the winding along the pitch can be obtained by the equation (L/2pi)*ARCTAN((Vrx1)/(Vrx2)).

Antenna 2 is connected to electronic circuitry 4, which is powered by power supply 3 and outputs a signal 5 based on the relative position of antenna 2 and target 1. Power supply 3 and output signal 5 are carried on the wires of a shielded multi-core cable. The electrical output can be in various forms, including serial peripheral interface (SPI), synchronous serial interface (SSI), RS-422A/B pulse, 0-5VDC, or 4-20mA. A 5VDC power supply 3 with a current of <100mA to the electronic circuitry is preferred. Preferably, a conformal coating is used to protect the detector printed circuit board for antenna 2 and electronic circuitry 3 from moisture and fluids.

While the bodies in Figures 1 and 2 are arranged to move primarily along a single linear axis (x), it is important to note that the present invention is not limited to linear motion. The present invention is particularly useful in providing a highly compact device for making precise angular measurements in large, through-shaft or hollow-bore arrangements where axial or radial space is limited. Figure 3 shows a centerline cross-section of a detector in the form of a rotating through-hole. The field emitted by antenna 2 substantially surrounds the isolated target area 1, but does not extend far beyond it due to good electromagnetic compatibility, but extends far enough to ensure that any small variations in the z and y axes do not produce measurement errors. Preferably, antenna 2 faces a metal surface 6, which partially shields antenna 2 from external far-field emissions, limits its field range, and minimizes the effects of other nearby metal components such as bearings, couplings, or fasteners. Preferably, the standoff distance (z ₁ ) should be kept as small as possible. For a target area 1 with a nominal width of 10 mm and a pattern pitch L of 10 mm, a standoff distance of 0.5 mm is suitable. The distance z2 of the metal surface 6 from the antenna 2 should be at least as great as the distance between the antenna 2 and the target 1, and preferably much greater than the distance between the antenna 2 and the target 1. For similar beneficial reasons, the target can also face the metal surface. The distance [z3] of the metal surface [7] from the target [1] should be at least as great as the distance between the antenna [2] and the target [1], and preferably much greater than the distance between the antenna [2] and the target [1]. Given that the preferred measurement algorithm is proportional, any small change in the standoff distance [ _z1 ] will not, within limits, have a significant effect on the measurement performance. This facilitates a mounting arrangement that allows for loose tolerances and is therefore inexpensive and easy to implement.

Figure 4 shows plan views of various isolated target regions 1 that can be used as a method for generating desired patterns along a measurement path. Figure 4a shows a target 1 produced by metal stamping. Because the sensing effect is primarily determined by the planar surface of the target 1, the thickness of the material is relatively unimportant and can range from tens of microns to tens of millimeters or more. The regions in 4a are triangular in cross-section and are spaced at regular intervals. The triangular forms can also be part of a raised configuration and do not necessarily have to be formed in layers.

Figure 4b shows a target produced using a printed circuit board approach [1], where the individual regions have been photolithographically etched onto an insulating substrate such as a 1.6mm FR4 grade laminate. The separate target regions can be either conductive or magnetic.

Figure 4c shows an alternative configuration in which the target area is generated using a square design, and the square has been stamped or machined from a metal strip. Other shapes of apertures may include circular, rectangular, pentagonal, hexagonal, etc. - i.e., any shape of aperture that, when spaced along the measurement path, induces a periodic variation in inductive coupling as the winding passes through it.

Variations in the target's pattern do not necessarily imply changes in the width of the conductive or magnetic material across the measurement path. Similar patterning effects can be achieved by increasing and decreasing the density of rows of drilled holes on a conductive substrate. Similarly, etching patterns of electrical conductors with increasing and decreasing density on the surface of an insulating substrate can also be effective.

Figure 4d shows another etching design, whereby two loops or shorted turns are used to create the conductive area. This technique is particularly advantageous because it helps shape the target 1, thereby producing a more linear output from the detector that would otherwise be generated with a simple, ordinary circle. Other higher-order harmonic components relative to the fundamental pitch of the target 1 can be added to the pattern of the target 1 to improve linearity.

The use of an inductively resonant target 1 as a detector is possible. This arrangement uses a winding placed electrically in series with a capacitor to form a resonant or tank circuit. While possible, it is generally not preferred because it is more difficult to minimize the effect of the energized electronic circuit on the target 1 facing the antenna 2.

Figure 6 shows a schematic diagram of the electronic circuit 4, which is largely self-explanatory. Furthermore, the power supply can be routed through an overvoltage protection circuit if voltage spikes are likely to occur. Reverse polarity protection (not shown for clarity) is also a common requirement. Preferably, the circuit 4 consists of an application-specific integrated circuit or surface-mount electronics on the same printed circuit board as the antenna 2. For maximum measurement speed, the analog multiplexer is eliminated and separate receive amplifier channels are used.

The embodiments described so far use a simple, periodic, regular variation of the target's range along the measurement axis, which provides incremental measurements. This can be particularly useful for velocity measurements; however, one disadvantage of the periodic arrangement is that the measured position is ambiguous over multiple intervals and is not absolute. Absolute position measurement can be achieved using a number of methods described in the following paragraphs, as well as permutations and combinations of these methods.

The first method is to convert the fuzzy or incremental output of the detector by electronic circuit 4, increasing or decreasing a count maintained in software each time a step is passed.

A second method of obtaining an absolute position measurement is to use a second, coarser pitch arrangement of a second target 1 and a second antenna 2 extending over the desired measurement scale. In this way, the approximate position can be determined using readings from the coarse scale and fine resolution readings obtained from the first repeating scale. The same concept can be extended to cover grayscale or binary scales.

Another way to obtain absolute measurements is to use the Vernier technique. A schematic diagram of this arrangement is shown in Figure 5. This technique uses two or more multi-pitch periodic patterns of separated target areas 1. A first series of target areas with a pitch of 8x is used together with a second series with a pitch of 9x. The fuzzy readings from each pattern can be combined to provide a unique indication of position. This unique indication will be maintained until the pitch of the lowest common multiple. In this example, the lowest common multiple is 72x. This length can be extended by selecting different pitches (e.g., 25x and 26x) or by adding a third pattern, etc.

FIG7 shows the face of a detector arranged for angle measurement. The stator and rotor are provided with multiple antennas and are arranged for absolute position measurement using two targets 1, an inner target and an outer target, whose pitches differ so as to form a unique (vernier) pattern within a single revolution. Facing the inner and outer targets 1 on the rotor are corresponding sets of inner and outer windings on the stator, labeled 2a1, 2b1, 2c1 and 2a2, 2b2, 2c2, respectively. As will be appreciated by those skilled in the art, using multiple winding pitches to maximize the extent of the windings around the ring will maximize measurement performance, as any local imperfections in the targets 1 or antennas 2 tend to be averaged out over multiple pitches. Advantageously, the third and fourth sets of windings, labeled 2a3, 2b3, 2c3 and 2a4, 2b4, 2c4, respectively, are arranged in other, preferably diametrically opposed, regions of the stator to minimize any effects due to non-concentricity between the rotor and stator. This type of arrangement is particularly useful because it provides space for countersunk mounting screws 8 or other suitable fasteners to be inserted into the gaps between the groups of target areas 1 and between the antenna areas 2. In applications with very tight space constraints, the use of plastic screws is advantageous in order to avoid the inductive effects that arise from metal screws. Furthermore, in applications with tight space constraints, the inner and outer antennas can be radially overlapped without any significant loss of signal fidelity.

Modifications and further embodiments

Detectors can be deployed in a variety of geometries including linear, rotational, curvilinear, and two-dimensional.

The present invention has no absolute size limits. These limits are imposed solely by the limitations of the manufacturing process, not the laws of physics. At one extreme, very long or large targets 1 can be produced using manufacturing methods such as, but not limited to, pressing or punching steel strips; laser cutting steel; electrodeposition onto ceramic or glass; printing conductive ink onto insulating substrates; bonding self-adhesive metallized discs onto insulating substrates, etc.

A variety of materials for target carrier 9 enable the detector to operate in a variety of environments. Glass is particularly advantageous due to its stability and low coefficient of thermal expansion. The target 1, antenna 2, and electronic circuitry 4 of the present invention can be partially or completely enclosed by a housing, shielding material, or sealant. The complete housing can be conductive, as long as its thickness between target 1 and antenna 2 is less than skin depth at the detector's excitation frequency.

In high-precision applications and in any of the aforementioned aspects, thermal expansion and contraction of detector components can lead to measurement errors and, therefore, loss of accuracy. These thermal errors can be counteracted by measuring the temperature and feeding the corresponding temperature coefficient into the position calculation performed in the electronic circuit 4. Advantageously, the temperature can be obtained by measuring the resistance of the conductive track on the antenna 2. In this way, for example, the average temperature adjacent to the target 1 can be measured, rather than localized hot or cold spots that may be experienced with conventional thermocouples.

The target 1 and target carrier 9 need not be rigid. If the conductive pattern is deposited on a flexible substrate (such as Mylar or polyester) then the detector can be deployed in more complex geometries, for example coiled and unrolled or adhered to complex surfaces or contours.

Within limits, variations in the position of target 1 relative to antenna 2 along axes other than the primary measurement axis do not affect the measured values. In particular, the distance _z1 between target 1 and antenna 2 along the z-axis can be varied without substantially altering the measured displacement. By adjusting the amplification factor used in electronic circuit 3 based on the amplitude of received signals Vrx1 and Vrx2, the range of acceptable variations can be extended. If the distance from target 1 to antenna 2 is large, the amplitude of received signals Vrx1 and Vrx2 will be small, and greater amplification should be applied. The opposite is true if the distance from target 1 to antenna 2 is small.

Thus far, the detector has primarily been described using a single transmitting winding 2c and two receiving windings 2a and 2b. As those skilled in the art will appreciate, various other arrangements and configurations of the windings in antenna 2 exist, including, but not limited to, windings arranged on either side of the target. Placing the antenna 2 windings on either side of target 1 is not preferred due to the mechanical limitations of this arrangement. For simplicity of construction and good measurement performance, arranging target 1 substantially in one plane and antenna 2 and electronic circuitry 3 substantially in a second plane, facing target 1, is preferred, as this provides a compact arrangement. Other possible excitation and position calculation techniques exist, such as using a high-frequency excitation frequency modulated by a lower-frequency signal to provide slower signal processing. However, this is not preferred due to its relatively slow operating speed and high complexity. Another embodiment swaps the transmit and receive functions, whereby the transmit winding described thus far becomes the receive winding, and vice versa. Yet another embodiment uses the phase of the received signal rather than its amplitude.

As an alternative to the winding pattern shown in Figure 2, other embodiments contemplate the use of a simple spiral winding whose inductance varies proportionally with the position of the target 1. In this approach, separate transmit and receive windings are avoided. This approach also enables the use of simple winding bobbins, thus reducing cost relative to more complex winding arrangements, which also tend to reduce measurement performance.

By simply moving the patterns away from each other (on the y-axis) and avoiding electrical connections, multiple targets 1 can be constructed on the same carrier 9. This configuration is particularly advantageous in detectors used in safety-related environments where electrical redundancy is required. In an electrically redundant system, multiple targets 1 can be formed on the same carrier 9 and can be detected simultaneously using multiple antennas 2.

Claims (19)

1. A sensing position detector comprising a first body and a second body, at least one of the bodies being displaceable relative to the other in a measurement direction along a measurement path, the first body comprising one or more antenna windings for transmitting and receiving signals; the first body further comprising electronic circuitry for powering the antenna windings for transmitting signals and for processing signals induced in the antenna windings; wherein the second body comprises a plurality of separate target regions disposed along the measurement path; the separate target regions being conductive or magnetically conductive; thereby, the induced signals vary according to the relative positions of the first body and the second body; wherein the antenna windings and the electronic circuitry for powering the antenna windings and processing signals from the antenna windings are positioned adjacent to each other; the antenna windings and the electronic circuitry are longitudinally arranged and spaced apart in the measurement direction; thereby, the antenna windings and the electronic circuitry overlap with different areas of the plurality of separate target regions. 2. The detector according to claim 1, wherein the electronic circuit is located in the same plane as the antenna winding. 3. The detector of claim 1, wherein the first body and the second body are annular and configured to overlap each other, the one or more antenna windings are disposed in a first portion of the first body, and the electronic circuitry is disposed in a second portion of the first body; the second portion is separate from the first portion. 4. The detector of claim 3, wherein the first body includes another portion separate from the first and second portions; the other portion includes one or more additional antenna windings. 5. The detector of claim 4, wherein the first portion and the other portion are disposed opposite each other in a diametrical direction, wherein the second portion is disposed between the first portion and the other portion. 6. The detector according to any one of claims 3 to 5, wherein the first body further comprises an insulating substrate and an electromagnetic shielding layer; the insulating substrate is a carrier for the antenna winding; one or more fasteners are provided to secure the substrate to the shielding layer; the fasteners are provided on the outside of the first portion. 7. The detector according to any one of claims 1 to 5, wherein the first body comprises at least two separate adjacent antenna winding groups, the antenna winding groups overlapping with at least two corresponding separate target region groups in use; each target region group forms a measurement path, whereby the two different measurement paths are arranged adjacent to each other. 8. The detector of claim 7, wherein the target region of the first measurement path is configured differently from the target region of the second measurement path. 9. The detector according to claim 1, wherein the separated target region forms a periodic pattern of layered conductive regions. 10. The detector according to claim 1, wherein the separated target region forms a periodic pattern of magnetically conductive regions. 11. The detector according to claim 9 or claim 10, wherein the periodic pattern includes a region perpendicular to the measurement path whose width varies continuously along the measurement path. 12. The detector according to any one of claims 1 to 5 and 8 to 10, wherein the region is a circular region. 13. The detector according to any one of claims 1 to 5 and 8 to 10, wherein the region is elliptical. 14. The detector according to any one of claims 1 to 5 and 8 to 10, wherein the region is rectangular. 15. The detector according to any one of claims 1 to 5 and 8 to 10, wherein the region is annular. 16. The detector according to any one of claims 1 to 5 and 8 to 10, wherein the separated target region is defined by two closed loops, one of which is located within the other. 17. The detector of claim 16, wherein the inner radius of its circumference varies. 18. The detector according to any one of claims 1 to 5 and 8 to 10, wherein the region is part of a periodically repeating shape. 19. The detector according to any one of claims 1 to 5 and 8 to 10, wherein the region is defined by a plurality of separate windings, each of the separate windings being arranged in series with a capacitor to form a plurality of separate resonant circuits along the measurement path.