GODDARD, Tristan (Bruntingthorpe Industrial EstateLutterworth, Leicestershire LE17 5QZ, GB)
CHAMPION, Barry (Bruntingthorpe Industrial EstateLutterworth, Leicestershire LE17 5QZ, GB)
SABIN, Barrie (Bruntingthorpe Industrial EstateLutterworth, Leicestershire LE17 5QZ, GB)
PULHAM, Richard (Bruntingthorpe Industrial EstateLutterworth, Leicestershire LE17 5QZ, GB)
GODDARD, Tristan (Bruntingthorpe Industrial EstateLutterworth, Leicestershire LE17 5QZ, GB)
CHAMPION, Barry (Bruntingthorpe Industrial EstateLutterworth, Leicestershire LE17 5QZ, GB)
SABIN, Barrie (Bruntingthorpe Industrial EstateLutterworth, Leicestershire LE17 5QZ, GB)
AGRATECH LTD (Desford RoadEnderby, Leicester LE19 4AT, GB)
| CLAIMS: 1. Apparatus for detecting perturbations in the local magnetic environment, the apparatus comprising: a capacitance device including a first and second capacitive plate, the second capacitive plate being separated from a magnet section of the first capacitive plate by a distance; and a means for receiving a magnetic element on the magnet section of the first capacitive plate, wherein movement between the magnet section of the first capacitive plate and the second capacitive plate indicates a change in capacitance. 2. The apparatus according to claim 1, wherein the first capacitive plate further comprises an outer section and the magnet section of the first capacitive plate is connected to the outer section. 3. The apparatus according to claim 2, wherein the magnet section of the first capacitive plate is movable with respect to the second capacitive plate. 4. The apparatus according to claim 2 or 3, wherein the first capacitive element comprises a groove between the magnet section and the outer section. 5. The apparatus according to claim 2 or 3, wherein the first capacitive element comprises a plurality of separation members between the magnet section and the outer section. 6. The apparatus according to claim 5, wherein the plurality of separation members meander between the magnet section and the outer section of the first capacitive plate. 7. The apparatus according to any of claims 2 to 5, wherein the second capacitive plate comprises an outer section that is fixed with the outer section of the first capacitive plate, and second capacitive plate further comprises a central section that is fixed with respect to the magnet section. 8. The apparatus according to any preceding claim further comprising a third capacitive plate arranged on another side of the first capacitive plate to the second capacitive plate so as to form two capacitors. 9. The apparatus according to any of claim 8, wherein a fourth capacitive plate is arranged between the first and third capacitive plate. 10. The apparatus according to claim 9 further comprising a magnetic element attached to the first and fourth capacitive plate. 11. The apparatus according to claims 8, 9 or 10 further comprising a housing wherein the capacitive device is contained within the housing. 12. The apparatus according to claim 1, wherein the first capacitive plate comprises a cut-out section and the first capacitive plate is arranged to rotate with respect to the second capacitive plate. 13. The apparatus according to any preceding claim further comprising means for calculating the capacitance of the capacitance device comprising means for determining a change in capacitance and means for judging whether a permanent magnet is near the capacitance device on the basis of the change in capacitance. 14. The apparatus according to claim 13 wherein the means for determining a change in the capacitance comprises a capacitance bridge circuit comprising an inductive voltage divider and a null detector. 15. The apparatus according to claim 14, wherein the inductive voltage divider includes a plurality of rotary switches for making a decade selection. 16. The apparatus according to claim 14, wherein the inductive voltage divider includes a plurality of precision relays for making a decade selection and a microcontroller means for controlling the precision relays. . 17. The apparatus according to any of claims 14 to 15 wherein the null detector comprises a lock-in amplifier comprising a plurality of electronic switches. 18. A method for detecting perturbations in a local magnetic environment near a capacitance device, the device including a first and second capacitive plate, and the second capacitive plate being separated from a magnet section of the first capacitive plate by a distance, the method comprising calculating the capacitance of the capacitance device, and determining a change in capacitance and on the basis of the change in capacitance, judging whether a permanent magnet is near the capacitance device. 19. An electronic article surveillance (EAS) system comprising an apparatus according to claim 13, wherein the apparatus is adapted to detect the presence of the magnetic element that is attached to an article. |
The present invention relates to an apparatus and method for detecting the presence of a magnetic element that creates perturbations in a local magnetic environment. In particular, the present invention relates to a detection apparatus and method for detecting the presence of one or more magnets, the apparatus and method being particularly suitable for use in an electronic article surveillance (EAS) system where articles are provided with markers or tags and the presence of the markers or tags is detected to prevent unauthorised removal of the articles from a controlled area.
EAS systems are designed to prevent unauthorized removal of an item from a controlled area. In an electromagnetic (EM) EAS system, tags designed to interact with an electromagnetic field located at the exits of the controlled area are attached to articles to be protected. If a tag is brought into the electromagnetic field, the presence of the tag is detected and appropriate action is taken. For a controlled area such as retail store, the appropriate action taken for detection of an EAS tag may be the generation of an alarm. The tag material has a specific permeability (the ability to conduct magnetic flux) and in current electromagnetic systems, the detection is at the retail store entrance and the shopper walks between pedestal units. One part of the pedestal unit emits a low frequency magnetic field which varies sinusoidally. At the peaks and troughs of the emitted signal, the tag becomes saturated. From the perspective of the receiving sensor, the saturated tag becomes invisible. Sophisticated processing of the received signal can synchronise the saturation of the tag with the transmitted signal.
In radio frequency (RF) EAS systems, a radio frequency signal is sent to a tag from a transmitter pedestal. The tag has a resonant circuit (inductor and capacitor) which re-radiates a radio frequency signal. The signal is detected and amplified by the sensing apparatus located at a receiver pedestal.
The above EAS systems all require a transmitter to stimulate the EAS tag.
From a first aspect, the present invention provides an apparatus for detecting perturbations in a local magnetic environment, the apparatus comprising a capacitance device including a first and second capacitive plate, the second capacitive plate being separated from the first capacitive plate by a distance, and a means for causing relative movement between the first and second capacitive plate, wherein movement between the first and second capacitive plate indicates a change in capacitance. The first capacitive plate comprises means for receiving a magnetic element.
The first capacitive plate is preferably provided with a permanent magnet in the receiving means and the presence of perturbations in the local magnetic environment that may be created by a permanent magnetic element near the first capacitive plate including the permanent magnet causes the first capacitive plate to move relative to the second capacitive plate. When the magnetic is mounted on the first capacitor plate, it is in direct mechanical connection with the plate and this allows precise control of the deformation of the capacitor plates without needing to take into account the properties of other substrates which are not required for the magnet to be mounted on the first substrate.
The apparatus may further comprise means for calculating the capacitance of the capacitance device, and means for determining a change in capacitance and on the basis of the change in capacitance to judge whether a permanent magnet is near the capacitance device.
The differential movement of the two capacitive plates caused by the presence of the permanent magnet near the first capacitive plate causes a change in the capacitance due to the capacitance value being calculated on the basis of the distance between the capacitive plates and the area of the overlap of the capacitive plates. The distance between the plates or the area of overlap can be varied by producing differential movement between the plates which is achieved by providing one of the capacitive plates with a magnet.
From a second aspect the present invention provides a method for detecting perturbations in the local magnetic field near a capacitance device, the device including a first and second capacitive plate, the second capacitive plate being separated from the first capacitive plate by a distance, the method comprising calculating the capacitance of the capacitance device, and determining a change in capacitance and on the basis of the change in capacitance, judging whether there are perturbations in the local magnetic environment near the capacitance device. These perturbations may be caused by a permanent magnet, electromagnet, and/or a material with high magnetic permeability (eg. iron or steel).
In one embodiment, the first capacitive plate comprises a magnet section to which the magnet is mounted and the magnet is preferably located in the centre of the magnet section, and second capacitive plate is fixed with respect to the first capacitive plate in a layered structure. Adjacent each outer surface of the first and second capacitive plate, an end cap is provided such that the first and second capacitive plate is sandwiched between the end caps.
Preferably, a groove is provided between the magnet section of the first capacitive plate and an outer section of the first capacitive plate. The groove may be annular. This improves deformation of the first capacitive plate when a magnetic element such as a permanent magnet is brought into close proximity to the magnet mounted in the first capacitive plate.
Advantageously, the groove is replaced with a one or more of separation members that create one or more gaps between the magnet section and the outer section of the first capacitive plate and provide a resilient connection between the magnet section and the outer section. The separation members provide improved deformation in the magnet section of the first capacitive plate when a permanent magnet is brought into close proximity of the first capacitive plate.
In one embodiment, the separation members meander between the magnet section and the outer section of the first capacitive plate. This type of shape is advantageous in providing repeatable deformation when a permanent magnet is brought into close proximity of the first capacitive plate.
In a preferred embodiment, a third capacitive plate is further provided such that the second and third capacitive plate is positioned on either side of the first capacitive plate. That is, the third capacitive plate is arranged on another side of the first capacitive plate to the second capacitive plate. End caps are provided adjacent each outer surface of the second and third capacitive plate to form a layered capacitive plate structure. With the first capacitive plate being common to both the second and third capacitive plates, two capacitors are effectively formed with this arrangement. An advantage of this arrangement is that if the first capacitive plate moves away from the second fixed capacitive plate when a permanent magnet is brought into close proximity of the first capacitive plate, it will move closer to the third capacitive plate and the differential capacitance signal that results is increased thereby improving sensitivity in the arrangement. In another embodiment, a fourth capacitive plate is provided and this is the same as the first capacitive plate. The magnetic element is attached to a side of the first and fourth capacitive element and is positioned between the first and fourth capacitive element. On a side of the first capacitive element opposite to the side to which the magnet is attached, the second capacitive plate is positioned and on a side of the fourth capacitive element opposite its magnet side, the third capacitive plate is positioned.
Spacers may be positioned between the first and second conductive plates and between the third and fourth capacitive plates.As well as the magnetic element separating the first and fourth conductive plates, at least one spacer element may be provided.
In another embodiment, the first capacitive plate is adapted such that an area of overlap between the first capacitive plate and the second capacitive plate can be changed. This is preferably achieved by the second capacitive plate being separated from the first capacitive plate by a distance, and the first capacitive plate being adapted to rotate with respect to the second capacitive element. The second capacitive plate is fixed and formed of sections that align with the first capacitive plate and overlap with corresponding sections of the first capacitive plate. The first capacitive plate preferably comprises one or more cut-out sections and is rotatable to allow the cut-out sections to rotate into and out of alignment with the second capacitive plate.
Outputs of the first, second and /or third capacitive plates in the above embodiments are provided to a signal conditioning circuit where further processing is carried out. The signal conditioning circuit includes a capacitance bridge.
The apparatus is particularly advantageous in an EAS system and provides a low cost and compact system not requiring a transmitter to excite a tag that is attached to an article.
Embodiments of the invention are described below in more detail, by way of example, with reference to the accompanying drawings in which:
Fig 1 shows a basic circuit diagram of capacitor circuit useful for the understanding of the present invention;
Fig 2 shows a schematic diagram of the operating principle of the invention based on the capacitor shown in Fig. 1 ;
Fig 3 shows basic detection circuitry provided to Fig. 2 to detect a nearby magnet; Fig 4 shows a schematic diagram of a second embodiment of the present invention; Fig 5 shows a side view of a plate of a capacitive sensor including a magnet used in the second embodiment of Fig. 4;
Fig 6 shows a cross sectional view of the complete capacitive sensor of the second embodiment;
Fig 7 shows a schematic diagram of a third embodiment of the present invention; Fig 8 shows a perspective view of the capacitive sensor used in the third embodiment;
Fig. 9 shows a perspective view of a capacitor plate of the capacitive sensor of the third embodiment;
Fig. 10 shows a schematic diagram of a fourth embodiment of the present invention; Fig. 11 shows a perspective view of the capacitive sensor used in the fourth embodiment;
Fig. 12 shows a perspective view of the capacitive sensor of Fig. 11 with an end cap removed to show a capacitor plate of the capacitive sensor;
Fig. 13 shows a perspective view of a sensor used in the fifth embodiment with a cover removed;
Fig. 14 shows a perspective view of a capacitive reference plate used in Fig. 13; Fig. 15 shows a perspective view of the sensor of Fig. 13 with the cover connected to the base of the housing;
Fig. 16 shows a simplified side view of a sensor used in a sixth embodiment;
Fig. 17 shows a perspective view of a capacitive sensor plate used in Fig. 16;
Fig. 18a shows a simplified side view of a sensor used in a seventh embodiment with magnification used along the X axis and Fig. 18b shows the sensor without magnification;
Fig. 19 shows a basic circuit diagram of the signal conditioning circuit of a first embodiment of the present invention;
Fig. 20 shows a schematic diagram of an inductive voltage divider that can be used in the signal conditioning circuit of Fig. 19;
Fig. 21 shows another type of inductive voltage divider circuit that can be used in the signal conditioning circuit of Fig. 19; Fig. 22 shows a schematic diagram of a null detector that can be used in the signal conditioning circuit of Fig. 19;
Fig. 23 shows a circuit diagram of the null detector of Fig. 22;
Fig. 24 shows a basic circuit diagram of the signal conditioning circuit of a second embodiment of the present invention;
Fig. 25 shows a schematic diagram of a first embodiment of the rotational approach for sensing a nearby magnet;
Fig. 26 shows a schematic diagram of a second embodiment of the rotational approach for sensing a nearby magnet;
Fig. 27 shows a schematic representation of a third embodiment of the rotational approach for sensing a nearby magnet;
Fig. 28 shows the rotatable capacitor plate used in Fig. 27 with the cut outs from the disc shown shaded in Fig. 28a;
In one embodiment of the present invention, a magnetometer sensor comprises one or more capacitors with parallel capacitive plates and at least one of the plates is capable of moving with respect to the other plates(s) when a magnetic element is brought into close proximity of the plate. By magnetic element it is meant an element that creates perturbations in a local magnetic environment. This could be a permanent magnet, electromagnet and/or a material with high magnetic permeability (eg. iron or steel).
Movement of the a capacitive plate is achieved in the preferred embodiment by attaching a magnet to the movable capacitive plate such that the plate is moved with respect to the other plates when the magnetic element is brought into close proximity of the movable plate. The differential movement between the plates causes a change in capacitance due to the relationship between capacitance and various parameters of the capacitor such as distance between the plates of the capacitor.
The basic principles behind the first embodiment are described with reference to Fig. 1. A capacitor circuit is shown comprising a battery 1, a resistor Rl, a capacitor CI and a switch SW1. The capacitor CI is an electrical device that stores charge. In its simplest form, it consists of two metal plates 2,3 separated by an insulating medium (e.g. air). If a static voltage is applied across the capacitor circuit, there will initially be no charge on the capacitor CI before switch SW1 is closed. Once the switch SW1 is closed, charge will flow onto the capacitor plates 2,3 until the voltage across the capacitor CI is equal to that across the terminals of battery 1. If the voltage V is not static but varies then the charge on the capacitor CI will vary as current flows into and out of the capacitor plates 2,3. Because of this, capacitors are used as sensors with alternating signals. There are a number of parameters which control how much charge the capacitor CI can store - these are:
1) The respective size of the capacitor plates 2,3, in particular the shared area that they overlap;
2) The distance d between the plates 2,3; and
3) The medium between the plates 2,3
The relationship can be defined by the equation:
_ c.Area
Where C is the measured capacitance
ε is the dielectric constant of the medium between the plates 2,3
d is the distance between the plates 2,3
Area is the area of overlap of the two plates 2,3
The inventors have found that a magnetic field from a magnetic element can influence the above parameters, thereby causing a change in capacitance C that can be used to detect the presence of the magnetic element.
Referring to Fig. 2 which shows the operating principle of the first embodiment, capacitor CI is adapted to be influenced by the presence of a magnet 4 due to changes of the distance d between the parallel capacitor plates 2,3. A small magnet 5 is fixed to a first plate 2 of the capacitor CI . As the magnet 4 is moved relative to the capacitor plates 2,3 it affects the small magnet 5. This in turn affects the distance d between the plates 2,3. As distance d is changed, it changes the capacitance value of the capacitor CI. As the value for distance d will typically be sub-millimetre, it will be sensitive to small changes in the distance induced by the changes in the localised magnetic environment. Fig. 3 shows circuitry provided to the arrangement shown in the first embodiment for indicating the presence of the nearby magnet 4. The capacitor CI and the magnet 5 in Fig. 2 form a capacitive sensor 10 for sensing the magnet 4. The parallel capacitor plates 2,3 are formed of aluminium sheets fixed apart with plastic shims (not shown) at the edges and the magnet 5 is bonded to the rear of the first plate 2. To detect the changes in capacitance induced by a nearby magnet 4, the capacitive sensor 10 is included in an oscillator circuit 6. This circuit 6 uses the capacitive sensor 10 as the key frequency determining element in an oscillator (not shown) that forms part of the oscillator circuit and oscillates in the audio frequency range. The oscillating circuit 6 is used to drive a power amplifier 7 which in turn was connected to a loudspeaker 8. As the permanent magnet 4 is bought closer to the capacitive sensor 10, the frequency of the oscillator varies and can be detected by the change in frequency from the speaker 8.
This arrangement demonstrates the principal of operation and gives a detection range of about 2m between magnet 4 and the capacitive sensor 10.
Referring to Fig. 4, 5, and 6, a second embodiment of a capacitive sensor 20 is shown. The capacitive sensor 20 is similar to that in the first embodiment but the capacitance plates 12,13 are smaller and mechanical resistance members in the form of tension springs 14 are attached continuously around the circumference of each plate 12,13. The plates 12,13 include etching rings 15, which are preferably annular. Referring to plate 12 which is shown in Fig. 5, a centre section 12a and outer section 12b is produced. The centre section 12a can move relative to the outer section 12b which is firmly anchored to the other capacitor plate 13 and both are electrically isolated from each other. The centre section 12a has a magnet 5 attached to each surface of the centre section 12a at the centre of the section 12a. The capacitor plate 13 has an opening to receive the magnet 5 of the capacitor plate 12. The opening is large enough to allow movement of the magnet through the opening but not to cause movement of the capacitor plate 13. To vary the strength of the tension springs 14, different thickness and widths of annular ring 15 can be produced. The plates 12,13 are similarly constrained and therefore can move together due to local disturbances. The advantage of this type of sensor 20 is that distortions or bending under the influence of local vibrations is reduced. Disturbances due to a permanent magnet 4 in the immediate vicinity will cause differential movement of the two capacitor plates 12,13 and hence, a change in the capacitance of the sensor 20.
The two capacitor plates 12,13 are separated by a non-conductive element which is preferably a plastic shim 16 that enables predictable and repeatable separations. The gap between the plates 12,13 can be increased by stacking more shims. The two capacitor plates 12,13 and the plastic shim 16 are held together by a pair of housing members 17 and on the outside of each housing member is an end plate 18. The magnet 5 is bonded to the capacitor plate 12. Screws 19b are used to secure the respective end plate 18 to its respective housing member 17. Pins 19c are used to locate and maintain the two halves of the sensor 20 together and pass from the housing members 17 through the capacitor plates 12,13 and the plastic shim 16.
Instead of two magnets 5, a single magnet can be mounted through the centre of the centre section 12a of the capacitor plate 12 as long as the magnet is fixed to the centre section such that movement of the magnet also causes movement of the centre section 12a.
The overall structure of the capacitive sensor is compact as it provides a layered stack of discs sandwiched between end cap discs.
In a third embodiment as shown in Fig. 7, 8 and 9, another type of capacitive sensor 30 is provided which is based on that in the second embodiment. The capacitive sensor 30 comprises end cap discs 24 with a stack of discs sandwiched between them. The end cap discs are formed of non-magnetic electrically conducted material such as aluminium, copper or brass. The discs are capacitor plate 22 including a magnet 35, capacitor plate 23, a non- conductive (electrically insulating) element such as a plastic shim 26 to separate the two capacitor plates 22,23, and a housing members 29. The capacitor plates 22,23 are each provided with outlet extensions 27,28 which allow them to be connected to a signal conditioning circuit (explained later).
The capacitor plate 22 of this embodiment is described in more detail with reference to Fig. 9. In this particular figure, the magnet 35 is not shown. The capacitor plate 22 is circular due to simplicity of construction but is not limited to such a shape. The plate 22 is formed from any easy to manufacture electrical conducting non-magnetic material such as aluminium, copper, or brass. The same shape and material is used for the capacitor plate 23. A recess is provided in the centre of the capacitor plate 22 for receiving the magnet 35 and for fixing to the capacitor plate 22. A corresponding hole is provided in the capacitor plate 23 for receiving the magnet 35 when the capacitive sensor 30 is assembled. As with the second embodiment, the capacitor plate 22 comprises a centre section 22a and an outer section 22b. However, to improve deformation of the centre section 22a, the etched annular ring is replaced with support members 25 for the centre section 22a with gaps 25a between each support member 25. Each support member 25 has a meandered or curved pattern to weaken the support of the central section 22a such that the length of the support members 25 is as large as possible without causing permanent deformation of the centre section. A longer support member 25 provides for more displacement of the moveable section 22a. A meander pattern provides improved deformation compared to the annular ring or linear support members (not shown).
With the capacitor plate 22 including the meander support members 25, when in use with the magnet 35 in position, the capacitive sensor 30 is capable of providing improved repeatability of readings. The closed nature of the sensor 30 assists in improving sensor stability. That is, it is difficult for ambient environment changes to influence the sensor 30.
A fourth embodiment is now described with reference to Figure 10, 1 1 and 12. This embodiment is based on the third embodiment, however, a dual capacitor arrangement is provided instead of the single capacitor of the third embodiment. That is, the sensor in this embodiment is based on a combination of two closed sensors 30 of the third embodiment.
Sensor 40 is formed of a number of disc shaped plates stacked on top of one another to form a compact sensor assembly. The capacitive sensor 40 comprises end cap plates 44 formed of non-magnetic electrically conductive material such as aluminium, copper or brass. The plates are electrically conductive capacitor plate 42 including a magnet 55 in the centre of the plate, two further electrically conductive capacitor plates 43 fixed on either side of the capacitor plate 42, non-conductive (electrically insulating) elements such as plastic shims 46 to separate the capacitor plate 42 and the two capacitor plates 43, and a plurality of housing members 49. The capacitor plate 42 is provided with connectors 47a and fixed capacitor plates 43 are each provided with connectors 47b which allow them to be connected to a signal conditioning circuit (explained later) and that can be provided separately to the sensor. The connectors 47a,47b include screening connected to the end plates 44. The capacitor plate 42 can be considered a common plate to the two capacitor plates 43 on either side of it and this configuration creates two capacitors and a dual capacitor arrangement. In use, the capacitor plate 42 containing the magnet 55 moves away from one fixed plate 43, and moves closer to the other fixed plate 43 and advantageously, this increases the differential signal and the sensitivity and stability of the sensor 40.
The capacitor plate 42 is similar to the corresponding plate in the third embodiment in that it comprises a centre section 42a and an outer section 42b. A plurality of support members 45 are provided between the centre section 42a and the outer section 42b with a number of gaps 45 a between the support members 45. The centre section 42a comprises a seat for receiving the magnet 55 such that the magnet is attached to the capacitor plate 42 and movement of the magnet 55 also causes movement of the centre section 42a. A corresponding opening (shown in Figure 12) is provided in each fixed capacitor plate 43 to receive the magnet such that the magnet can move relative to the opening, however, the opening does not cause movement of the fixed capacitor plate 43. Instead of the seat, another holding method can be provided to secure the magnet 55 to the centre section 42.
The support members 45 provide elastic support of the centre section 42a to the outer section 42b and allow deflection of the centre section 42a. Compared to the third embodiment, the number of support members is increased and space between each support member is minimised. Furthermore, the diametric width of the outer section 42b is decreased compared to the third embodiment such that the diametric width W of the support member section is relatively large compared to the third embodiment. The support members 45 are curved to a greater extent than the third embodiment and the length of each support member 45 is greater. The proportion of the centre section 42a to the overall capacitor plate 42 and therefore the proportion of the capacitor plate that is displaced by the presence of a magnetic material 4 is larger than that in the third embodiment.
The capacitor plates 43 have an identical mechanical structure to the capacitor plate
42 and the capacitor plates 43 are fixed in their outer sections to the outer section of the capacitor plate 42 via the plastic shims 46. The plates 43 have a hole that is aligned with the position of the magnet 55 on the plate 42 so that the magnet is movable within the hole. This ensures that the capacitor plates 42,43 are affected equally by local disturbances so that this type of disturbance does not affect the differential signal representing the relative displacement between capacitor plates. The material used for the capacitor plates when the plates are at a microscopic level (micro-electronic mechanical structure, MEMS) can be silicon with aluminium deposition.
Another embodiment of the sensor is now described with reference to Figs. 13 to 15. The sensor 50 is arranged in a similar differential configuration to the sensor 40 and operates on the basis of the same principles where there are two capacitors as in sensor 40. However, the various capacitive plates are completely enclosed within a sensor housing 1. This can improve the tolerance of the sensor to local electro-magnetic interference and general environmental noise. Since the sensor is a completely screened assembly, it only responds to a local perturbation in the magnetic environment. This improved stability increases the resolution of the signal conditioning electronics and reduces fluctuations due to ambient noise.
Considering the structure of this embodiment in detail, the sensor 50 has a reference capacitor plate 53 which is similar to the capacitor plate 43 of the sensor 40 in that it is formed of a large moveable centre section 53a and an annuler outer section 53b with a plurality of support members 54 arranged in a meandered pattern between the centre section 53a and outer section 53b. A hole 56 is provided in centre of the centre of the centre section 53a such that a permanent magnet 57 coupled to a capacitive plate (not shown) similar to the plate 42 of the sensor 40 is movable within the hole 56 if a magnetic material is detected near the sensor 50.
The housing 51 is formed of a base 58, and a cover 59. As shown in Fig, 13, a hollow cavity 60 and annular ledge 61 is provided in the base 58. The outer section 52b of the capacitive plate can be attached to ledge 61 by any appropriate means which in this embodiment is a plurality of screws (not shown). The diameter of the reference plate 53 is smaller that the diameter of the housing 51 and therefore the plate 53 can be contained entirely within the housing.
A set of two reference capacitor plates of a similar diameter to a sensor capacitive plate (not shown) are positioned on either side of sensor capacitive plate to provide the differential configuration as in the sensor 40. Plastic shims (not shown) can isolate the plates. Although the sensor capacitive plate is not shown, it will be appreciated that it will have an identical mechanical structure to the reference capacitor plate 53 except that instead of a hole to receive a magnet, the magnet that is to be received through the hole is coupled to the centre of the plate. Each side of the plate can be provided with a magnet.
The sensor 50 of this embodiment can be considerably smaller is size to that in the previous embodiment without having an adverse affect on the sensitivity of the sensor (eg. sensor 50 has a diameter of around 6 cm whereas sensor 40 has a diameter of around 12 cm). The capacitive plates can be manufactured using conventional processes such as a photochemical machining (PCM) process or photo electro forming (PEF) process.
Referring to Figs 16 and 17, Fig. 16 shows a simplified side view of an embodiment of an alternative sensor 70 which operates on the same principles as the sensors 40, 50. The housing which is similar to that in Figs 13 and 15 is not shown. The sensor 70 is shown with the horizontal or X axis magnified in order to clearly show the respective features. The size of the sensor is much smaller than the view provided in Fig 16 (a factor of 10 smaller). Plates 72,73 have only three separation members 74 compared to the larger number of separation members provided in the capacitive plates of sensor 50. The members 74 are thinner. The size and number of the separation members 63 can be varied with the number of members having an effect of the deformation of the capacitive plate and the sensitivity of the overall sensor.
The central capacitive sensor plate 72 has bonded thereto two magnets that are preferably rod shaped Neodymium Iron Boron (NIB) magnets 75. As with previous embodiments, it is interaction of these magnets 75 with remote magnets (not shown) that cause displacement of the sensor plate 72. Adjacent to either side of the sensor plate 72 are the reference plates 73. These are the sensor plate for the two capacitors. Displacement of the sensor plate causes a change in the ratio of these capacitors. Insulative members 76 are provided to separate each plate from an adjacent plate and these can be in the form of plastic shims as in previously described embodiments.
In this embodiment, as well as the plates 72, 73 being formed differently compared to previous embodiments such that there mass is much smaller compared to other embodiments, the magnets 75 are much larger than the sensor plate 72 and they dominate the mass of the sensor 70.
Referring to Fig 18a, a simplified side view of an embodiment of an alternative sensor 80 is shown. The sensor 80 is shown with the horizontal or X axis magnified in order to clearly show the respective features. The size of the sensor is much smaller than the view provided in Fig 18a (a factor of 10 smaller) and Fig.18b is a representation of the sensor 80 without the magnification applied.
The sensor 80 is similar to sensor 70 in that it includes mechanically similar reference plates 83 and a sensor plate 82 which have a central section 82a and an outer section 82b with separation members 84 between the central section 82a and outer section 82b. The structure of the sensor plate 82 is similar to that in Fig. 17 and both sensor plates 82 are identical in this embodiment.
The sensor 80 differs to previous embodiments in that two sensor plates 82 are provided and a single magnet 85 is attached between the two sensor plates 82. The magnet 85 can be a rod shaped Neodymium Iron Boron (NIB) magnet. The attachment is between the two central sections 82a of the sensor plates 82. The outer sections of sensor plates are attached to each other by a non-conductive spacer member 86 to provide stability to the assembly and reduces drift in the measurements. On the other side of the sensor plates 82 to the magnet 85 are reference plates 83. The reference plates 83 have a solid central section 83a without any holes since the magnet 85 is not required to be received therein as is required in previous embodiments. Insulative members 86 are provided on the outer portion of the reference plates to separate each reference plate 83 from an adjacent sensor plate 82 and these can be in the form of plastic shims as in previously described embodiments.
With this assembly, the drift of the sensor is reduced as the mass of the assembly is not dominated by the magnet 85 and distortion and permanent deformation of the sensor plates can be alleviated.
The plates and magnets of the sensor 80 can be fully screened by being securing in a housing such as that in Fig. 13 and the housing can be fixed in an enclosure (not shown) at a relevant position. The sensor 80 can be connected to the signal conditioning circuitry that is located outside the housing.
It is possible to sense perturbations in the local magnetic fields out to a range of at least 6m and the sensitivity is typically around luT using a sensor 80.
It will be appreciated by the skilled person that in the embodiments described herein, the plates can be adapted as long as the required differential characteristics are achieved. For example, in some embodiments, the number of separation members could be varied and as already explained a groove could be provided instead of the separations members although advantageous results are achieved with the separation members.
Signal conditioning and processing circuitry is required to process the signals from the sensors described hereinbefore. The method of signal conditioning adopted is a capacitance bridge. Bridge methods have the advantage that they allow a small change in a variable to be detected and isolated from a large base signal.
Fig. 19 shows a basic capacitance bridge circuit 90 as a first embodiment of the signal conditioning circuitry of the present invention. The circuit 90 comprises a sine wave generator and power amplifier 91 generating a frequency f, a matching transformer 92, a reference capacitor 93 providing a capacitance Cr, a sensor capacitor 94 providing a capacitance Cs in parallel to the reference capacitor 93, a null detector 95 and an inductive voltage divider 96 with a divide ratio n. For the bridge circuit 90 to be balanced, the null detector 95 must detect no net current flow from combination of the reference capacitor 93 and sensor capacitor 94.
The currents are given by:
Is = l.n.f.Cs.E Ir = -2.n.f .Cr.n.E Is = -Ir
2.n.f.Cs.E = 2.n.f.Cr.n.E
Cs = Cr.n
Cs
n =—
Cr
Similarly it can be shown: ACs
An =
Cr
Hence, if the bridge is balanced then a perturbation introduced by the introduction of a magnet 4 in the first to fourth embodiment, can be detected by the change in ratio of the inductive voltage divider 96. Therefore the accuracy and resolution of the inductive voltage divider 96 and the null detector 95 improve the performance of the detection of the magnet 4.
If a voltage is applied across the voltage divider 96, it should produce an accurate and scaled representation of this voltage signal at its output. For example, if the input signal has magnitude of 10V (AC signal only) it will produce a scaled representation of the input voltage with only the magnitude of the signal changing. In formulaic terms:
Output Voltage = n x Input voltage Where n can be any ratio from 1 to 1 /(number of decades of voltage division). For simple low resolution measurements, resistive voltage dividers can be used. However, as the number of decades increases, the errors introduced by successive decades become prohibitive. Inductive voltage dividers 96 are used instead. Inductive voltage dividers can be used in combination with resistive dividers if desired. Here the inductive voltage divider 96 will produce the first few decades of voltage division and then the resistive voltage divider can fill in the last few decades.
Another advantage of the inductive voltage divider 96 is it can typically handle much higher input voltages than commercial resistive dividers. To maximise signals from the capacitance bridge and, minimise the influence of ambient noise, the voltage across the bridge is set at 40V.
Fig. 20 shows a first embodiment of an inductive voltage divider 96 where the decade selection is by a plurality of rotary switches 97.
Fig. 21 shows a second embodiment of voltage divider 106 providing three more decades of resolution in the voltage divider (1 part in 100,000) 106. With this higher resolution divider 106, the rotary switches of the first embodiment of the inductive voltage divider 96 are replaced with a plurality of precision relays 107 and controlled by a microcontroller 108.
Fig. 22 shows a schematic diagram of a type of null detector 95 that is used to balance the bridge in Fig 19 i.e. cancel out the currents from either arm of the bridge. If the bridge is not correctly balanced the additional signals due to the presence of a magnet 4 could be missed. A lock-in amplifier (also known as a phase sensitive detector) is shown in Fig. 22 and provides the functionality that is required by the null detector. The lock-in amplifier uses a sample of the input signal to lock-in and detects traces of this signal amongst an abundance of noise.
The lock-in amplifier has a plurality of electronic switches 109 as shown in Fig. 22.
These are opened and closed synchronously with the signal from the capacitance bridge oscillator (reference signal). On consecutive half cycles of the reference signal, the input signal is added to the output either directly or inverted. This rectifies any portion of the input signal at the same frequency and phase as the reference signal. The signal is then averaged by the output filter. All signals at frequencies different to the reference signal will cancel over several wavelengths with only a small residual. The signal in-phase and at the same frequency will produce an average over all cycles. This method can improve the signal to noise of the detector and exclude vast amount of out of phase noise. The advantages of the lock-in amplifier are:
1) It detects readily the signals from the bridge and provides sufficient amplification such that small errors in the balancing of the bridge can be detected.
2) It rejects ambient noise in particular mains borne noise and disturbances from other electrical equipment.
3) It reacts quickly, a detector that takes minutes to provide a reading would not be suitable in a retail environment where people are streaming past the detector.
4) It is compact and low cost - multiple detectors are likely to be required at a retail store entrance and the final equipment must be unobtrusive.
A circuit diagram of the lock- in amplifier of Fig. 22 is shown in Fig. 23.
Fig. 24 shows a circuit diagram of a second embodiment of the signal conditioning circuitry of the present invention. In this embodiment, changes in the resistive nature of the sensor introduced a "quadrature" signal that was ninety degrees out of phase with the main signal from the magnetic capacitor plate of the sensor. The effects of unwanted impedances for the capacitor (as no capacitor is ideal) are compensated for as the resolution of the sensor can lead to the impedances becoming significant (whereas for normal capacitor arrangements they are much smaller) and affecting the capacitance measurement. To increase the sensitivity, the ability to "null" out the quadrature signal is provided.
Furthermore, the modified circuit enables greater flexibility by providing more control.
The preferred sensor of the third embodiment and the signal conditioning circuit of the second embodiment has been found to be capable of stable detection of a small permanent magnet at a range up to 2 metres.
Another capacitive sensor approach is based upon the compass effect rather than approach relating to the relative distance of the plates. In the compass effect approach, metal parallel plates are rotated with respect to each other. This affects the area of overlap of the plates and hence, directly the capacitance (Area in equation (1) above). There are two embodiments to this approach, the first embodiment is shown in Fig 25 and is a variable capacitor geometry.
In this embodiment the capacitive sensor 1 10 comprises two capacitor plates 1 1 1,1 12. The capacitor plates 1 1 1 ,1 12 are semicircular and in a first position (Fig. 25a) they have no overlap and the capacitance is minimised. In the middle position, (Fig. 25b) a quadrant of each semicircle overlaps to give half of the maximum capacitance value. In the final position, (Fig. 25c), the two plates 11 1,1 12 are fully overlapped and the capacitance reaches its maximum value.
A second embodiment shown in Fig. 26 comprises a capacitive sensor 120 uses three capacitor plates 121 ,122,123 - one of which can move to change the area of overlap and hence the capacitance (see equation (1 )).
In a first position (Fig. 26a), there is no overlap between plate 122 and plates 121 and 123 and hence, the capacitance is minimised. Plate 122 is rotatable with respect to fixed plates 121 ,123. In the middle position (Fig. 26b), the plate 122 only has rotated and the overlap is approaching 50% with a corresponding percentage of the maximum capacitance. In the final position (Fig. 26c), the plate 122 overlaps both plates 121 ,123 and the capacitance reaches it maximum value. The plate 122 can continue to rotate and the area of overlap between plate 122 and plates 121,123 will reduce down to the first position. To make the configuration sensitive to localised magnetic variations, two miniature magnets (not shown) are bonded onto the plate 122 aligned along the radial axis. Then, if a magnet is bought close to the configuration it will either attract or repel plate 122 and the rotation of this plate 122 will be detected by the corresponding capacitance change.
Fig. 27 shows a capacitive sensor 130 based on the configuration in Fig. 26. That is, the three capacitor plates version as this provides improved flexibility.
This embodiment comprises a housing in the form of a fixed etched upper plate 133a arranged opposite a lower etched plate 133b. At each corner of the upper and lower plate 133a,133b is a support member 137 extending transverse to the respective plate. Located between the upper and lower plates 133a, 133b is a printed circuit board 134 and a rotatable capacitor plate 132 that is mounted on a rotatable pivot in the form of a spindle 135. A reduction in friction associated with the rotating capacitor plate 132 is achieved through bearings. In this embodiment, "jewel" bearings 136 are used to minimise frictional losses. The rotatable capacitor plate 132 is fitted to the spindle 135 and the jewel bearing 136 are fitted to the upper and lower plate 133a, 133b of the housing.
With reference to Fig. 28, in order to ensure rigidity, the rotatable capacitor plate 132 is made from a disc with quadrants 132a etched out to give the basic geometry of Fig. 26. In addition, two slots 132b are provided on a radial axis preferably diametrically opposite each other. These are used to accurately locate permanent magnets (not shown) that will rotate the plate 132 in the presence of a permanent magnet near the sensor 130.
The distance between the rotating plate 132 and the static plates (not shown), can be adjusted by the thumb wheel adjusters (not shown). Through rotation of the rotatable capacitor plate with respect to the static plates, the overlap of the capacitor plates will vary. The static plates do not necessarily have to be discs having an identical shape to the rotatable plate 132 but the shape is such that the extent of overlap can be changed in order for the capacitance to change.
Signal conditioning circuitry similar as described with reference to Figs. 19 to 24 and similar to that required in respect of the linear displacement capacitive sensor can be used to detect a change of capacitance. Both types of magnetometer sensor (linear and rotary) along with the signal conditioning circuitry can be used in an electronic article surveillance (EAS) system to determine perturbations in the local magnetic field and therefore determine whether a permanent magnet that is attached to an article in a controlled area such as a retail environment is near the sensor. A number of sensors can be provided in a sensor array near an entrance /exit of the retail environment in a frame structure to "magnetically image" the entrance / exit of a store. The apparatus (sensor and signal conditioning circuitry) is immune to changes in the ambient conditions (temperature, electric fields etc) due to the differential nature and the fully screened sensor geometry.
The sensors, having the ability to detect perturbations in the local magnetic field i.e. in the presence of the earth's magnetic field, can detect permanent magnets, electromagnets and ferromagnetic materials that distort locally the earth's magnetic field. These types of magnetic materials can be applied to articles to form the magnetic tags of the EAS system.
It will be appreciated that although certain features of the invention are described in the context of separate embodiments but may also be provided in combination in a single embodiment. Conversely, various features of the invention which are described in the context of a single embodiment, may also be provided separately or in any suitable subcombination.