GB2353361A

GB2353361A - Proximity switch comprising relatively movable tuned circuits

Info

Publication number: GB2353361A
Application number: GB9919401A
Authority: GB
Inventors: Andrew John Scott Walton-Green; Ian Howard Moth
Original assignee: WALTON GREEN ANDREW JOHN SCOTT
Current assignee: WALTON GREEN ANDREW JOHN SCOTT
Priority date: 1999-08-18
Filing date: 1999-08-18
Publication date: 2001-02-21
Also published as: GB9919401D0

Abstract

A switching circuit comprises first and second tuned circuits 34, 28 which are movable relative to each other, for example, one circuit may be mounted on a door 14 and the other attached to the surround 16. The first tuned circuit 34 is driven to form an oscillator having an amplitude of oscillation dependent upon the relative position of the passive tuned circuit 28. A logical signal is produced in response to the level of oscillation in the first circuit 34 and this may control a power switch. The circuit may be used as a safety switch or to activate an alarm system. A plurality of circuits of different oscillation frequencies may be provided for use in a security system. The first tuned circuit may be mounted on electrical apparatus and the second tuned circuit may be worn by an operator to prevent unauthorised use thereof.

Description

TITLE Switching Circuits

DESCRIPTION

This invention relates to switching circuits, to safety circuits employing such switching circuits, to electrical apparatus having such safety circuits, and to security systems employing such switching circuits.

The invention was originally conceived in connection with a safety switch for mains- operated vending machines, but it does have other applications. In the case of a mains-operated vending machine, it is usually necessary to open a door of the machine in order to restock it, and care needs to be taken that the person restocking the machine is not electrocuted. All of the electrical circuitry in the machine could be sufficiently insulated to avoid any risk of electrocution, but this would be expensive. Alternatively, as is already known, a safety switch may be fitted to the door and/or door frame which automatically switches the machine off when the door is opened. One known form of safety switch is a purely mechanical switch or microswitch which is fitted to the door or frame and which is closed when the door is closed. Problems with such switches are that they have moving parts which eventually wear out, they need to be properly aligned, they are susceptible to dirt, dust, moisture, etc., and their effect can be easily overridden, whether intentionally or accidentally. Another known form of switch is provided by a magnetically-operable reed switch which is mounted on, say, the door frame, and a complementary permanent magnet which is mounted on the door. Again, such reed switches have moving parts which can eventually wear out or fail and have the potential to be overridden, whether intentionally or accidentally.

One aspect of the present invention is concerned with providing a switching circuit for use in the above application and other applications, which does not need to have any moving parts, which has a good tolerance to relative aligni-nent, which can easily be made immune to dirt, dust and moisture, and which cannot easily be overridden accidentally.

In accordance with this aspect of the invention, there is provided a switching circuit, comprising: first and second complementary tuned circuits which are movable relative to each other; driving means for electrically driving the first tuned circuit to form an oscillator having a level of oscillation dependent upon the relative position of the tuned circuits; and signalling means for producing a logical signal in dependence upon the level of oscillation in the first tuned circuit.

When the switching circuit is used in a safety circuit for an electrical apparatus, the tuned circuits are preferably arranged so that they are normally in at least one relative position when the apparatus is intended to be operable and are normally in at least one other relative position when the apparatus is required to be isolated, and switching means are provided for switching a supply of electricity to the apparatus in dependence upon the logical signal.

Preferably, the tuned circuits are normally adjacent each other when the apparatus is intended to be operable and are normally remote from each other when the apparatus is required to be isolated; and the switching means is arranged to connect the supply of electricity to the apparatus only when the logical signal indicates that the level of oscillation in the first tuned circuit is below a predetermined threshold.

As an additional failsafe, the safety circuit may further include means for detecting the electrical current supplied to the apparatus, and a logic circuit for generating a cut-out signal for the apparatus when the detected current is incompatible with the logical signal.

When the safety circuit is used with an electrical apparatus having two relatively movable members (such as an access door and door surround), the tuned circuits may be mounted on those two members.

The safety circuit may also be used with an operator-controlled apparatus (such as a power tool). One of the tuned circuits may be disposed in a position adjacent which a part of an operator's body should be placed for proper operation of the apparatus (e.g. their hand on a particular handle). The other tuned circuit may be adapted to be worn on or adjacent, or held by or adjacent, that part of their body.

Such switching circuits may also be used as sensors in security systems such as burglar alarm systems and other alarm systems. For example, the tuned circuits may be arranged to be mounted on relatively movable parts of a building (for example as door or window sensors). In the case where a plurality of such switching circuits are provided, they preferably have different oscillation frequencies so that it is even move difficult for an intruder to defeat them. Alternatively, one of the tuned circuits may be arranged to be statically mounted, with its complementary tuned circuit being arranged to be worn or held by a person, and thus a covert 4cpanic" alarm may be provided which might be used in domestic situations or by bank tellers.

A specific embodiment of the present invention will now be described, purely by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a perspective view of an electrically-operated vending machine; and Figure 2 is a block diagram of a safety circuit provided in the machine of Figure 1.

Referring to the drawings, an electrically-operated vending machine 10 has a cabinet 12 and a door 14 hinged in a door surround 16. Various conventional electrical circuits 18 are included in the cabinet 12 for purposes including coin checking, product dispensing, control and lighting, these receiving power from live and neutral mains wires 20, 22.

A safety circuit of the machine 10 will now be described particularly with reference to Figure 2. A small power supply unit 24 is connected to the live and neutral wires 20,22 to supply low-voltage electricity to the remainder of the safety circuit, and a power switch 26 such as a triac is placed in the live wire 20 between the power supply unit 24 and the conventional circuits 18. The power switch 26, mains side of the power supply unit 24 and the portion of the live wire 20 leading to the power switch are fully insulated or placed out of reach of people accessing the machine 10 through the door 14.

A passive tuned circuit 28 comprising a loop-connected inductor 30 and capacitor 32 is mounted on the edge of the door 14. A complementary active tuned circuit 34 comprising a loop-connected inductor 36 and capacitor 38 is mounted on the edge of the door surround 16 so that it is adjacent the passive tuned circuit 28 when the door 14 is closed. The inductance of the inductor 36 and the capacitance of the capacitor 38 are similar to those of the inductor 30 and capacitor 32, respectively, so that the passive and active tuned circuits 28,34 have matched natural frequencies.

The active tuned circuit 34 is driven at its natural frequency by a driver circuit 40. The driver circuit 40 also provides a rectified and smoothed output 42 of the level of oscillation which is supplied to an inverting input of a comparator 44, the non- inverting input being provided with a DC reference signal 46. The output of the comparator 44 is supplied as a control input to the power switch 26, so that the power switch 26 is turned on when the output of the comparator 44 is high.

In a prototype of the safety circuit, the inductance L of the inductors 30,36 was about 95 nH, and the capacitance C of the capacitors 32,38 was about 27 pF, so that the natural frequency, 1/(21r(LC)'), of the active and passive tuned circuits 28,34 was about 100 MHz.

The spacing between the tuned circuits 28,34 when the door 14 was closed was about 10 mm.

The active tuned circuit 34 was driven so that, with the door 14 open and therefore with the active tuned circuit 34 unaffected by the passive tuned circuit 28, the level of oscillation at the output 42 was about 2.0 V. When the door 14 was closed so that the passive tuned circuit 28 was brought into close proximity to the active tuned circuit 34, some of the energy from the active tuned circuit 34 was drawn off by the passive tuned circuit 28, and the level of oscillation at the output 42 decreased to about 1.0 V. Accordingly, with the DC reference signal 46 set at 1.5 V, the power switch 26 was turned off when the door 14 was opened and was turned back on when the door 14 was closed.

It will be appreciated that many modifications and developments may be made to the 10 embodiment of the invention described above.

For example, as a failsafe, the output from the comparator 44 and a signal from a current detector 48 provided on the portion of the live wire 20 between the power switch 26 and the conventional machine circuits 18 may be fed to a failsafe logic circuit 50. If the output from the comparator 44 is low, but current is detected in the live wire 20, this circuit 50 15 produces an output signal which is fed to a cut-out (not shown) to trip the whole machine.

Also, a switch (not shown), such as a key switch, operable by a service engineer may be provided so that the engineer may work live on the machine 10 with the door 14 open.

It should be noted that the embodiment of the invention has been described above purely by way of example and that many other modifications and developments may be made 20 thereto within the scope of the present invention.

Claims

1 A switching circuit, comprising: first and second complementary tuned circuits which are movable relative to each other; driving means for electrically driving the first tuned circuit to form an oscillator having a level of oscillation dependent upon the relative position of the tuned circuits; and signalling means for producing a logical signal in dependence upon the level 5 of oscillation in the first tuned circuit.

2. A switching circuit, substantially as described with reference to the drawings.

3. A safety circuit for an electrical apparatus, comprising: a switching circuit as claimed in claim 1 or 2, the tuned circuits being arranged so that they are normally in at least one relative position when the apparatus is intended to be operable and are normally in at least one other relative position when the apparatus is required to be isolated; and switching means for switching a supply of electricity to the apparatus in dependence upon the logical signal.

4. A safety circuit as claimed in claim 3, wherein: the tuned circuits are normally adjacent each other when the apparatus is intended to be operable and are normally remote from each other when the apparatus is required to be isolated; and the switching means is arranged to connect the supply of electricity to the apparatus only when the logical signal indicates that the level of oscillation in the first tuned circuit is below a predetermined threshold.

5. A safety circuit as claimed in claim 3 or 4, further including means for detecting the electrical current supplied to the apparatus, and a logic circuit for generating a cut-out signal for the apparatus when the detected current is incompatible with the logical signal.

6. A safety circuit for an electrical apparatus, substantially as described with reference to the drawings.

7. An electrical apparatus having a safety circuit as claimed in any of claims 3 to 6.

8. An electrical apparatus as claimed in claim 7, and having two relatively movable members on which the tuned circuits are respectively mounted.

g. An electrical apparatus as claimed in claim 8, wherein one of the members is an access door, and the other member is a surround for the access door.

10. An electrical apparatus as claimed in claim 7, wherein one of the tuned circuits is disposed in a position adjacent which a part of an operator's body should be placed for proper operation of the apparatus, and the other tuned circuit is adapted to be worn on or adjacent, or held by or adjacent, that part of their body.

11. An electrical apparatus, substantially as described with reference to the drawings.

12. A security system having at least one sensor provided by a switching circuit as claimed 5 in claim 1 or 2.

13. A security system as claimed in claim 12, wherein the tuned circuits of the switching circuit, or of at least one of the switching circuits, are arranged to be mounted on relatively movable parts of a building.

14. A security system as claimed in claim 12 or 13, wherein one of the tuned circuits of the switching circuit, or of at least one of the switching circuits, is arranged to be statically mounted, and the complementary tuned circuit is arranged to be worn or held by a person.

15. A security system as claimed in any of claims 12 to 14, wherein a plurality of such switching circuits are provided having different oscillation frequencies.

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