GB2414083A

GB2414083A - Proximity sensor system

Info

Publication number: GB2414083A
Application number: GB0509497A
Authority: GB
Inventors: William Woodruff
Original assignee: RISCO GROUP UK Ltd
Current assignee: RISCO GROUP UK Ltd
Priority date: 2004-05-10
Filing date: 2005-05-10
Publication date: 2005-11-16
Also published as: GB0410385D0; GB0509497D0

Abstract

A sensor unit (1) for use in a proximity sensor system having control means with connector means for operably connecting the control means to the sensor unit, comprises an antenna element (3) and resonance adjusting means for adjusting the resonance of the antenna element. The resonance adjusting means is provided by means for adjusting the reactance of the antenna element, by adjusting either the inductance or capacitance of the antenna element. The inductance adjusting means comprises coupling means (7, 9, 15, 17) for coupling to the antenna at different points to change the length of the antenna element whilst the capacitance adjusting means comprises coupling different capacitors to the antenna element, both of which change the resonant frequency of the antenna element. The sensor unit may be used in intruder alarm systems.

Description

DETECTION SYSTEM

This invention relates to detection systems and in particular to proximity sensors for detecting the presence of an electronic tag. The invention is especially concerned with the use of such proximity sensors as part of intruder alarm systems.

Conventional intruder alarm systems require an authorised person or homeowner to remeans a code number which must be inputted via a keypad into a control unit to activate and deactivate the alarm system.

Such systems therefore require an accessible keypad that must be located close to an entrance so that the system can be deactivated within a predetermined period to prevent the intruder alarm sounding. Where there are a large number of authorised persons who may wish to gain access to a premises, for example an office building or factory, the security of the system could be compromised if separate codes were to be issued to each individual because there would be a greater chance of an intruder guessing a correct code. One option would be to increase the number of digits in the code but it is difficult to reliably remeans code numbers having more than 4 or 5 digits.

In a development of intruder alarm technology, proximity sensors have been proposed that rely on the detection of an electronic tag carried by an authorised person to activate and deactivate the alarm system.

Proximity sensors are well known in other fields and typically comprise a detection unit having a sensor which includes an antenna, and a control for driving the sensor and processing the detected signals. These units have a limited detection range and must be placed near the area in which detection is required.

Various kinds of proximity sensors are known, all of which rely on the interaction of an electronic tag with a sensor inside a detection unit. For example they may rely on the electronic tag generating an RF signal which can be detected by the sensor, a change in capacitance of a sensor unit caused by the proximity of the tag, or a change in the current drawn by the sensor unit as a result of inductive coupling between the tag and sensor unit.

The idea behind the RF system is that the antenna creates an electromagnetic field over a certain region. Transponders containing microchips are energised when their circuits lie across (i.e. at an angle to) field lines of the electromagnetic field. Energising the transponder causes it to emit a response signal. The same or another antenna can detect the response signal and identify the transponder which has entered the antenna zone.

Alternatively the tag and antenna can act as two halves of an air-coupled transformer such that when the tag is brought into proximity to the antenna the current flow through the antenna is altered and this can be detected as a signal.

In the case of capacitive proximity sensors, the sensor contains a capacitor which generates an electrostatic field. When the tag is placed close to the sensor it interacts with the electrostatic field and alters the capacitance of the sensor, which change can be detected.

A corresponding range of tags are known. A popular kind for use with RF systems avoids the need for any built-in power source because it includes a 'transponder' containing an inductive microcircuit which can be energized by the electromagnetic field of a suitable antenna so as to emit a characteristic electromagnetic identification signal for detection, e.g. by the same or another antenna.

In the case of inductively coupled proximity sensors, the tag acts as the second half of a transformer, the primary half being the antenna in the sensor.

Whilst the electronic tag can be kept small, a drawback of such detection units is that they are bulky and so cannot necessarily be located in the most convenient positions.

The present inventors have noted that the sensor part of the detection device can be separated from the control/processing part. In other words, the sensor, e.g. a coil or loop, can be placed in a separate housing from the control. The sensor unit and the control unit can then be sited separately. This has the advantage that the sensor unit can be much smaller and have a lower profile than the combined conventional sensor/processing unit. It can therefore be placed in more convenient locations, both in terms of being less obtrusive and also in terms of improving the detection efficiency of the sensor. So, for example, a small low profile sensor unit could be located close to or within the frame of a door.

Although separate sensor and control units have advantages, the present inventors have noted a drawback which limits their usefulness in intruder alarm systems. The wire used to connect the sensor unit and the control unit will form part of the tuned detection circuit. This is because the wire forms part of the antenna and so influences the frequency at which it will oscillate, which in turn affects the detection performance of the sensor unit. In short, an antenna of fixed inductance will only operate with a wire of a specific length when the driving signal applied to the antenna is kept the same.

If the wire is too long or too short, the capacitance of the wire will cause the frequency of oscillation of the coil to be detuned, with a negative affect on detection performance.

The length of the wire is therefore important because the circuitry in the control unit which drives the antenna is conveniently set up for a specific predetermined antenna having a known resonance. The control unit and antenna are "tuned" to a particular frequency. If the antenna is changed or modified, by either replacing the antenna or by adding a different length of wire to it, the control circuitry would also need to be changed.

However, changing the driving circuitry is impractical during installation of the system because the circuitry is pre-programmed during manufacture. Producing a control unit in which the driving circuitry could be reprogrammed by an installer would be expensive. Similarly, producing multiple versions of the control unit would be costly and inconvenient for the installer.

The present inventors have found that a modified sensor unit can address the drawbacks discussed above.

At its most general we propose that the resonance of an antenna element in a sensor unit can be adjusted (possibly reversibly) so that different lengths of wire for connecting the sensor unit to a control unit can be used. We also propose that existing sensor units can be modified to incorporate means of adjusting the resonance of the antenna element therein.

A first aspect of our proposals is a sensor unit for use in a proximity sensor system, the system having a control means with connector means for operably connecting the control means to the sensor unit, the sensor unit including an antenna l O element and resonance adjusting means for adjusting the resonance of the antenna element.

As used herein "resonance" means the frequency at which a circuit or element will resonate (i.e. resonant frequency).

The resonance adjusting means is preferably either: an inductance adjusting means; or a capacitance adjusting means; or both. These approaches are discussed below.

The control means, when connected to the sensor unit with the connector means is operable to drive the antenna element to generate an oscillating field, and to monitor that field. For example, the control means (e.g. processor means) may be operable to process proximity detection signals from (e.g. generated in) the antenna element by the proximity of an electronic tag.

Suitably, the proximity sensor is part of an intruder alarm system. The intruder alamm system can be for domestic or commercial premises.

The connector means may include conductive cables arranged to connect operative ends of the antenna element to the control means. The sensor unit is preferably connectable to the control means, for example by connector means of length l - 10 m.

The sensor unit can act as the primary half of an air-coupled transformer, the second half being the tag. The sensor unit preferably includes only a single antenna element.

The sensor unit allows an antenna element in a remote sensor unit to be connected to a control means using any length of cable. There is no requirement to use pre-cut lengths. This permits a wider choice of locations for the sensor unit and simplifies installation.

A further advantage is that more than one sensor unit can be connected to a control means without modification of the control unit because the resonance of the antenna element can be adjusted to accommodate additional antenna elements, for example in separate sensor units.

The resonance of the whole circuit can remain unchanged, it is preferably only the resonance of the antenna element that is adjusted.

In the case where the resonance adjusting means is provided by an inductance adjusting means, the inductance adjusting means preferably includes a first coupling means coupled to the antenna element and second coupling means (e.g. an antenna tap) for providing a second coupling at a variable spacing from the first coupling means.

The first and second coupling means can provide the "input" and "output" for the drive signals applied to the antenna element by the control means, via connector means.

When coupled to the antenna element, the second coupling means can form an "antenna tap". When the antenna element is connected to the control means via the first coupling means and the second coupling means, the inductance of the antenna element will depend on the spacing between the first and second coupling means and/or the position of the first and second coupling means with respect to the antenna element.

The second coupling means, which provides a second coupling at a variable spacing from the first coupling means, is preferably either: a plurality of second coupling means coupled to the antenna element and spaced from the first coupling means; or a movable second coupling means that can be moved so as to be coupled with the antenna element at variable points along the antenna element; or both.

In other words, the variable spacing of the second coupling means from the first coupling means is achieved either by providing two or more second coupling means (so that the spacing can be selected by choosing one of the second coupling means) or by a (e.g. one) movable second coupling means whose spacing from the first coupling means can be varied by moving the second coupling means.

In a simple example, there may be two second coupling means (e.g. fixed with respect to the antenna element), making three coupling means in total (including the first coupling means). The antenna element can be connected to the control means via any two of the three coupling means. The spacing between any two of the coupling means is related to the inductance of the antenna element when connected to the control means. Thus, the installer has a simple means of selecting a variable inductance.

The plurality of second coupling means can for example be two, three, four, five, six or seven coupling means.

Typically the antenna element will have a coupling means at each end of the antenna element. Suitably, the first coupling means is located at one end of the antenna element. Where the antenna element is a coil, preferably there are coupling elements at respective ends of the coil. In accordance with the preferred features discussed above, one of the coupling means may be movable (to provide a variable spacing from the other coupling means) or the sensor unit may include a further coupling means.

In the case where the sensor element includes a first coupling means and two second coupling means, thereby forming two terminal coupling means and one intermediate coupling means the antenna element can be connected to the control means for example by either: connecting both terminal coupling means to the control means; connecting a first terminal coupling means and the intermediate coupling means to the control means; or by connecting the second terminal coupling means and the intermediate coupling means to the control means.

In these different arrangements the inductance of the antenna element will be different because the length of the antenna element connected to the control unit will have changed as a result of connecting the antenna element to the control means at different points along the length of the antenna element. Of course, where the intermediate coupling means is spaced equally from the terminal coupling means, there are only two combinations of antenna element length that can be selected.

It is preferred that the antenna element is a coil. In which case the location of an antenna tap along the length of the antenna element (e.g. as measured from the first coupling means) can correspond to a given number of turns of the coil. The coil is not particularly restricted as to the geometry of the coil, for example the core can have a circular or square cross-section. The nature and thickness of the coiled wire, the radius of the coil and the number of turns can be selected in accordance with the skilled worker's knowledge in the field. The sensor unit preferably includes only a single coil.

The following optional and preferred features are described with reference to a coil antenna element but can apply equally to other antenna element geometries, e.g. loops.

In general, for larger distances between the sensor unit and control means, and hence a longer connecting wire therebetween, the number of turns can be reduced by connecting the coil to the control means via a second coupling means closer to the first coupling means to accommodate the change in capacitance of the wire. For example, a wire of length 0 - 5 m could be connected to the antenna element via the first coupling means and a second coupling means spaced 100 turns from the first coupling means, whereas a wire of length 5 - 10 could be connected to the antenna element via the first coupling means and a second coupling means spaced e.g. about 70 turns from the first coupling means.

Where the antenna element is a loop or other geometry, the spacing of the second coupling means from the first coupling means may also change the geometry of the detection field created when the antenna element is connected to the control unit.

The second coupling means may include a simple electrical connector extending from the antenna element but it can also include an antenna branch whose size and shape can further modify the inductance and geometry of the antenna element and hence its performance.

For example, the antenna branch may include a curved length of conductive wire so that, in the case where the antenna element is a loop, connection of the control means to the antenna branch can form a smaller antenna element loop. In this particular example, a change in field geometry could thereby be avoided because it is only the size of the working antenna that would change, not the shape.

In other examples a change in field geometry may be desirable and an appropriately shaped antenna branch can be used.

The sensor unit may include at least two and preferably at least three second coupling means spaced from the first coupling means. For example, the sensor unit may include two, three, four, five, six or seven second coupling means. This permits a greater choice of inductances. In turn this means that a wider range of wire lengths between the sensor unit and control means can be accommodated, and hence a wider range of distances between the two units.

Suitably the second coupling means are in electrical contact with the antenna element. As an example second coupling means are spaced from the first coupling means at positions corresponding to about one quarter, one half and three quarters of the coil length. Suitably this corresponds to one quarter, one half and three quarters of the turns of the coil. This provides a convenient way of selecting a wide range of useful antenna element inductances so that a similarly wide range of wire lengths can be accommodated.

Similarly, second coupling means may additionally or alternatively be located at positions corresponding to about one third and two-thirds of the turns of the coil.

Preferably the sensor unit includes a housing and suitably the second coupling means extend from the antenna element to the housing where they are joined to sensor unit terminals so that each second coupling means is accessible from outside the housing. This makes it easier for an installer to wire the sensor unit to the control means. Suitably, the sensor unit terminals are wiring posts, sockets or plugs.

To further simplify the installation of the sensor unit the sensor unit terminals may each be colour coded, either directly or on an appropriate part of the housing.

Similarly, the sensor unit terminals or housing may be labelled.

The sensor unit may include switching means, e.g. one or more switches to simplify the selection of coupling means so that a single pair of sensor unit terminals can be connected to a plurality of different combinations of the first coupling means and second coupling means. Alternatively or additionally, the sensor unit may include jumpers to select combinations of first coupling means and second coupling means.

In preferred embodiments, the second coupling means are fixed to the antenna element by, e.g. solder. In the case of a coil antenna element, attachment of the second coupling means to the coil can be simplified by providing the coil with one or more loops (i.e. turns) that extend radially beyond the periphery of the coil so as to provide attachment sites for the second coupling means.

In other embodiments the second coupling means may form a unitary body with the antenna element. In still further embodiments the second coupling means may be selectively detachable from the antenna element.

In the case where the means for providing a second coupling means coupled to the antenna element at a variable spacing from the first coupling means is a movable second coupling means, the movable second coupling means is preferably selectively detachable so that it can be brought into electrical contact with the antenna at different positions along the length of the antenna element (e.g. at different spacings from the first coupling means). Preferably, the movable second coupling means is slidably movable with respect to the antenna element. This arrangement may advantageously permit a large number of antenna element inductances to be selected.

The sensor units can be located inside or outside a building. In the case where a unit is for location outside, it is preferred that the sensor unit includes a housing that is waterproof.

A sensor unit having a first coupling means and second coupling means for providing a second coupling that can be coupled to the antenna element at a variable spacing from the first coupling means is particularly suitable for operation outside a building.

An advantage of a separate sensor unit is that it is smaller than conventional combined sensor/control units and has a lower profile. This means that it can be hidden from view or installed within an existing structure, e.g. a door frame which inherently protects it from tampering.

The sensor unit may also include a status indicator. Typically this is a light or lights whose illumination, brightness or rate of flashing can provide the observer with status information. For example, the sensor unit may include a light visible through the housing, or attached to the housing, that is illuminated when the system is on, indicating that, for example an intruder alarm will sound if a door is opened by someone who isn't carrying an appropriate electronic tag. The light preferably turns off when the sensor unit detects the presence of an electronic tag and so indicates to the observer that it is safe to enter the building.

Another method of varying the inductance, in addition to or as an alternative to the methods described herein, is to provide the antenna element with an adjustable core. For example, where the antenna element is a coil, it can be provided with an adjustable core to provide a range of continuously variable inductance values. Preferably the adjustable core is an adjustable ferrite core.

Suitably, the position of the core with respect to the coil can be altered, for example by providing core adjusting means to e.g. extend the core into or retract it from the coil.

Turning now to the case where the resonance adjusting means is provided by capacitance adjusting means, preferably said means include a capacitor arranged in series or in parallel with the antenna element.

The sensor unit may also include said capacitor as a means of modifying the resonance of the antenna element, in addition to or instead of the second coupling means.

Where capacitors are used the sensor unit preferably includes a capacitor reversibly connectable in series or in parallel across the antenna element. Preferably the capacitor is connectable in series with the antenna element. The capacitor can preferably be switched between an "on" position in which it is connected in series or parallel with the antenna element and an "off" position in which it is not part of the circuit. Suitably the sensor unit may include switching means, e.g. a switch or jumper for effecting the switching.

When the capacitor is not connected with or across the antenna element the antenna element performs as if the capacitor were not there at all. When the capacitor is switched to an "on" position (e.g. by changing a jumper position or actuating a switch) and the antenna element is connected to the control means, the resonance of the antenna element can be changed. This permits control over the resonance of the antenna element in a similar way to the second coupling means.

Typically, the antenna element is connected to the control unit via coupling means attached to respective ends of the antenna element.

The capacitor can have a fixed capacitance or be a variable capacitor.

Where the capacitor is a variable capacitor, preferably the sensor unit includes capacitance selection means for selecting a desired capacitance.

An advantage of the use of capacitors is that the sensor unit need only have one pair of sensor unit terminals because the change of resonance is achieved by operating a switch or moving a jumper.

The capacitance and number of the capacitors may be selected so that the modified antenna element accommodates a shorter or longer connecting wire between the sensor unit and the control means. Examples of typical capacitor values include 470 pF to 33000pF, e.g. 470pF to 3300pF.

The capacitor may be a single capacitor or multiple capacitors arranged in series.

Two or more capacitors may be reversibly connectable across the antenna element in this way to accommodate a range of wire lengths.

In preferred embodiments a first capacitor is connectable in series with the antenna element and a second capacitor having a different capacitance to the first capacitor is also connectable in series with the antenna element such that either the first or second capacitor can be connected in series with the antenna element.

Preferably the sensor unit includes a direct connector path (e.g. a wire) which is connectable to the antenna element so as to join the sensor unit to the control means without the resonance adjusting means being part of the circuit.

A further advantage of providing means for adjusting the resonance of the antenna element by either or both of second coupling means and capacitor is that more than one antenna element can be connected to a control means without having to retune or adapt the control means. This is possible because the total resonance of the load attached to the control means can be kept the same by reducing the reactance (e.g. inductance) of the individual components (e.g. the antenna elements) that make up the load.

In practice this means that where two antenna elements are desired to be connected to a control unit the reactance (e.g. inductance) of each antenna element can be halved so that the total load experienced by the control unit is the same as a single antenna element. For example, the inductance can be reduced, e.g. halved, by providing a second coupling means at a point corresponding to e.g. about one quarter of the turns of the coil, or by connecting a capacitor in series or in parallel with the coil, as discussed above.

By adjusting the inductance of the antenna element in this way it is possible to connect two or more, e.g. two, three, four, five or six, sensor units in series to the control means without adjusting the control means.

The present inventors have found that the range and detection performance of the sensor units connected in series to the control means is not significantly diminished despite the reduction in inductance. It is believed that this is because the flux of the coil changes by a relatively small amount for a given change in inductance.

Thus, two sensor units can be connected in series to the control means and still detect the presence of an electronic tag. A particularly useful application of this advantage is where an interior and an exterior sensor unit are required.

This aspect of our proposals also includes a proximity sensor system having a sensor unit as described herein and a control means. The control means can be of the type well known in the art. Preferably the control means can be connected to at least two sensor units, for example two, three, four, five, six or seven sensor units.

The proximity sensor preferably interfaces with an electronic tag and can include such a tag. Suitably the tag contains an inductive circuit that can be powered by coupling with the antenna element in the sensor unit. For example when a circuit within the tag passes through an electromagnetic field generated by the sensor unit.

Preferably, the tag does not include a power source. Alternatively, the tag may include a power source, for example a battery.

The tag preferably includes an ID circuit for storing an ID code. The ID circuit may be re-programmable.

The control means can be a known control means. It may contain an antenna element so that it can also act as a sensor unit. Preferably, the control means itself contains an antenna element and resonance adjusting means as described above.

This permits a sensor unit (i.e. an additional antenna element) to be connected to the control means because the resonance of the antenna element in the control unit and the antenna element in the sensor unit can be adjusted.

The proximity sensor is preferably part of an intruder detection system.

Suitably it includes an audible alarm.

The intruder alarm apparatus described herein suitably conforms to the strict requirements of the UK Association of Chief Police Officers policy on intruder alarms, as set out in draft guidelines DD243.

A second aspect of our proposals is the use of a sensor unit having an antenna element and resonance adjusting means for adjusting the reactance of the antenna element in a proximity sensor having a control means with connector means for operably connecting the control means to the sensor unit. The optional and preferred features of the first aspect also apply to this aspect.

A third aspect of our proposals includes a method of installing a proximity sensor having a sensor unit including an antenna element, a control means with connector means for operably connecting the sensor unit to the control means, the method including the step of adjusting or selecting the reactance of the antenna element. The optional and preferred features of the previous aspects also apply here.

A further aspect of our proposals is a method of modifying a sensor unit having an antenna element for use in a proximity sensor having a control means with connector means for operably connecting the control means tothe sensor unit, the method including the step of providing resonance adjusting means for adjusting the reactance of the antenna element. The optional and preferred features of the previous aspects also apply to this aspect.

Embodiments of our proposals will now be described with reference to the accompanying drawings in which: Figure 1 is a schematic circuit diagram of a sensor unit, being a first embodiment of the invention; Figure 2 is another schematic circuit diagram of a sensor unit, being a second embodiment of the invention; and Figure 3 is a further schematic circuit diagram of a sensor unit, being a third embodiment of the invention.

Fig.1 shows a first embodiment of our proposals. A sensor unit 1 includes an antenna element 3, preferably a coil, but alternatively a loop or other geometry, and housing 5. The coil has a first coupling means 7. The first coupling means 7 extends from one end of the coil to the housing 5 of the sensor unit where it is joined to sensor unit terminal 11.

Three second coupling means (antenna taps) 9, 15, 17 are spaced from the first coupling means 7 along the antenna element 3.

The coil has approximately 100 turns. Approximately 30 turns from the first coupling means 7 is a second coupling means 15 that is electrically connected to the coil. Another second coupling means 17 is electrically connected to the coil about 30 turns from the other end and a further second coupling means 9 is connected to the coil at the end opposite first coupling means 7, i.e. 100 turns from the first coupling means.

The plurality of second coupling means 15, 17, 9 each extend from the coil to the housing 5 where they are joined to sensor unit terminals 19, 21, 13.

Thus, the housing includes four sensor unit terminals 1 1, 13, 19, 21. The terminals are colour-coded so that an installer can see which terminals he should use depending on the length of wire he uses to join the sensor unit to the control unit.

For example, where the wire length is to be less than 5 m, the installer would choose terminals 7 and 9, corresponding to the full length of the coil. Where the wire length was to be greater than 5 m, for example 8 m, the installer would choose terminals 15 and 9, corresponding to about 70% of the turns of the coil. Preferably, the second coupling means are unequally spaced to give a wider range of inductance options for a given number of second coupling means.

Where two sensor units are to be connected to the same control means, the installer would select terminals 7 and 17 for a wire length of 0 - 5 m, and terminals and 17 for a wire length of 5 - 10 m.

A coil antenna element as shown in Fig.1 can act as one half of an air coupled transformer. The other half of the transformer is electronic tag 23 that will be carried by an authorised person. The primary coil antenna element 3 generates a field through which the tag 23 moves when it is brought close to the sensor unit. The tag 23 couples with the primary coil 3 and generates a signal, which modulates the current flow through the primary coil 3. This current modulation is detected by control means 25, which interprets the signal and recognises a valid ID code.

Fig. 2 shows a second sensor unit 31. The sensor unit has a housing 33 with sensor unit terminals 35, 37. This sensor unit also includes a coil 39, which could alternatively be a loop or other geometry. The coil has a first coupling means 41 and a second coupling means 43 extending from respective ends of the coil to the terminals 35, 37.

Arranged in parallel across the coil are two capacitors 45, 47. In series with the capacitors are jumpers 49, 51. These jumpers permit one or both capacitors 45, 47 to be switched "on" and "off" independently.

For example, the first capacitor 45 may be switched on by closing jumper 49 so that the coil 39 generates a different field when it is energised. This can be used to accommodate a shorter or longer wire between the sensor unit 31 and control means 53.

Similarly, the second capacitor 47 can be switched "on" by jumper 51, independently of the first, to adjust the coil properties so that a second sensor unit can be connected to the control means 53.

Fig 3. shows a third sensor unit 59. The sensor unit has a housing 61 with sensor unit terminals 63, 65. The sensor unit includes a coil 67, which could alternatively be a loop or other geometry. In this example the coil has 48 turns, but a different number of turns is also possible, e.g. between 20 and 200 turns. t

The sensor unit includes resonance adjusting means 69, which includes first capacitor 71, second capacitor 73 and direct connector path 75. Any one of these three components may be connected in series with the coil by connecting the appropriate jumpers 77, 79, 81. In this way, the first and second capacitors can be switched "on" or"off".

For example, the first capacitor 71 may be switched on by closing jumper 77 so that the coil 67 generates a different field when it is energised. This can be used to accommodate a shorter or longer wire between the sensor unit 59 and the control means.

The first capacitor has a capacitor value of 2.7nF, although different values can be used. The second capacitor has a capacitor value of 33nF, although different values can be used.

These preferred embodiments have been described by way of example and it will be apparent to those skilled in the art that many alterations can be made that are still within the scope of the invention. t

Claims

1. A sensor unit for use in a proximity sensor system, the system having a control means with connector means for operably connecting the control means to the sensor unit, the sensor unit including an antenna element and resonance adjusting means for adjusting the resonance of the antenna element.

2. A sensor unit according to claim 1, wherein the means for adjusting the resonance of the antenna element includes capacitance adjusting means.

3. A sensor unit according to claim 2, wherein the capacitance adjusting means include a capacitor connectable in series or in parallel with the antenna element.

4. A sensor unit according to claim 3, wherein the sensor unit includes a switch or jumper for connecting the capacitor to the antenna element.

5. A sensor unit according to claim 4, wherein the capacitor has a fixed capacitance.

6. A sensor unit according to claim 4, wherein the capacitor is a variable capacitor.

7. A sensor unit according to claim 6, wherein the sensor unit includes capacitance selection means for selecting a desired capacitance.

8. A sensor unit according to any one of claims 3 to 7, wherein two or more capacitors are connectable to the antenna element.

9. A sensor unit according to any one of claims 3 to 8, wherein the capacitor or capacitors are reversibly connectable to the antenna element.

10. A sensor unit according to any one of claims 3 to 9, wherein the capacitor values of the capacitor lies in the range 470pF to 33000pF.

11. A sensor unit according to any one of claims 3 to 10, wherein the capacitance adjusting means includes a first capacitor connectable in series with the antenna element and a second capacitor having a different capacitance to the first capacitor connectable in series with the antenna element such that either the first or second capacitor can be connected in series with the antenna element.

12. A sensor unit according to any one of the previous claims, wherein the sensor unit includes a direct connector path which is connectable to the antenna element so as to join the sensor unit to the control means without the resonance adjusting means being part of the circuit.

13. A sensor unit according to any one of the previous claims, wherein the resonance adjusting means includes an inductance adjusting means.

14. A sensor unit according to claim 13, wherein the inductance adjusting means includes first coupling means coupled to the antenna element and second coupling means for providing a second coupling at a variable spacing from the first coupling means.

15. A sensor unit according to claim 14, wherein the second coupling means includes a plurality of second coupling means coupled to the antenna element and spaced from the first coupling means.

16. A sensor unit according to claim 15, wherein the plurality of second coupling means are antenna taps.

17. A sensor unit according to claim 16, wherein the plurality of second coupling means are spaced substantially equally along the length of the antenna element.

18. A sensor unit according to claim 17, wherein there are two second coupling means fixed with respect to the antenna element and the antenna element can be connected to the control means via any two of the first and second coupling means.

19. A sensor unit according to claim 16, wherein the plurality of second coupling means comprises second coupling means spaced from the first coupling means at positions corresponding to about one quarter, one half and three quarters of the coil length. i

20. A sensor unit according to any one of claims 14 to 19, wherein the second coupling means includes an electrical connector extending from the antenna element.

21. A sensor unit according to any one of claims 14 to 20, wherein the sensor unit includes a housing and the second coupling means extend from the antenna element to the housing where they are joined to sensor unit terminals so that each second coupling means is accessible from outside the housing.

22. A sensor unit according to claim 21, wherein the sensor unit terminals are wiring posts, sockets or plugs.

23. A sensor unit according to claim 24, wherein the sensor unit terminals are colour coded.

24. A sensor unit according to any one of claims 21 to 23, wherein the sensor unit includes one or more switches or jumpers for selection of coupling means so that a single pair of sensor unit terminals can be connected to a plurality of different combinations of the first coupling means and second coupling means.

25. A sensor unit according to any one of claims 14 to 24, wherein the antenna element is a coil having one or more loops that extend radially beyond the periphery of the coil so as to provide attachment sites for the second coupling means.

26. A sensor unit according to claim 14, wherein the second coupling means includes a movable second coupling means that can be moved so as to be coupled with the antenna element at variable points along the antenna element.

27. A sensor unit according to claim 26, wherein the moveable second coupling means is selectively detachable from the antenna element so that it can be brought into electrical contact with the antenna element at different positions along the length of the antenna element.

28. A sensor unit according to claim 26 or claim 27, wherein the movable second coupling means is slidably movable with respect to the antenna element.

29. A sensor unit according to any one of claims 1 to 28, wherein the resonance adjusting means includes an adjustable core for the antenna element.

30. A sensor unit according to claim 31, wherein the antenna element is a coil and is provided an adjustable core to provide a range of continuously variable inductance values.

31. A sensor unit according to claim 30, wherein the adjustable core is an adjustable ferrite core.

32. A sensor unit according to any one of claims 29 to 31, including core adjusting means to extend the core into or retract it from the antenna element.

33. A sensor unit according to any one of the previous claims, wherein the sensor unit includes conductive cables arranged to connect operative ends of the antenna element to the control means.

34. A sensor unit according to any one of the previous claims, wherein the sensor unit includes a status indicator

35. A sensor unit according to claim 34, wherein the status indicator is a light or lights whose illumination, brightness or rate of flashing can provide the observer with status information.

36. A sensor unit according to any one of the previous claims, wherein the proximity sensor is part of an intruder detection system.

37. A sensor unit according to any one of the previous claims, wherein the proximity sensor includes an audible alarm.

38. A sensor unit according to any one of the previous claims, wherein the sensor unit is for location outside a building and the sensor unit includes a waterproof housing.

39. A sensor unit according to any one of the previous claims, wherein the antenna element is a coil.

40. A proximity sensor system having a sensor unit according to any one of the previous claims, and a control means.

41. A proximity sensor system according to claim 40, wherein the control means can be connected to at least two sensor units.

42. A proximity sensor system according to claim 40 or claim 41, including an electronic tag, which can be detected by the sensor unit.

43. Use of a sensor unit having an antenna element and resonance adjusting means for adjusting the reactance of the antenna element in a proximity sensor having a control means with connector means for operably connecting the control means to the sensor unit.

44. A method of installing a proximity sensor having a sensor unit including an antenna element, a control means with connector means for operably connecting the sensor unit to the control means, the method including the step of adjusting or selecting the reactance of the antenna element.

45. A method of modifying a sensor unit having an antenna element for use in a proximity sensor having a control means with connector means for operably connecting the control means to the sensor unit, the method including the step of providing resonance adjusting means for adjusting the reactance of the antenna element.

46. A sensor unit substantially as hereinbefore described in any one embodiment, with reference to and as shown in the accompanying drawings.