GB2385182A

GB2385182A - Safety switching device for a machine guard

Info

Publication number: GB2385182A
Application number: GB0202650A
Authority: GB
Inventors: Mabruk Farrah; Matthew Bagley
Original assignee: MECHAN CONTROLS Ltd
Current assignee: MECHAN CONTROLS Ltd
Priority date: 2002-02-06
Filing date: 2002-02-06
Publication date: 2003-08-13
Anticipated expiration: 2022-02-06
Also published as: GB2385182B; GB0202650D0

Abstract

A safety switching device which can be used to monitor the presence of a machine guard, has two parts 1, 2 which move away from each other when the guard is moved and thereby trigger an alarm. Each part contains a transmitter 9, 12 and a receiver 7, 14, and one part 2 also contains a switch 17 for actuating the alarm. The transmitter 12 of the switch-containing part transmits power to the receiver 7 of the other part 1 which is rectified and smoothed to provide D.C. operating power for this other part 1 so that a battery is not required in this other part. The transmitter 9 of the other part 1, when powered, transmits operating signals to the receiver 14 of the switch-containing part 2 which when the parts are in close proximity, are used to operate the alarm switch 17. The parts 1, 2 also contain respectively a magnet 4 and a Hall-effect magnetic switch 18 which operate to allow passage of signals to the alarm switch 17 only when the parts 1, 2 are in close proximity.

Description

SAFETY SWITCHING DEVICES This invention relates to safety switching devices of the kind which may be used to indicate the operational state of a machine or to cut off power for safety when a machine guard is removed or for other purposes where the proximity of two parts is to be monitored.

It is known to use magnetically operated reed switches in safety switching devices. Movement of a magnet relative to the switch causes the switch to change between open and closed states. Switching devices of this nature can provide effective control but reliance solely on interaction between a magnet and a reed switch lacks security; the switching device is not at all tamper proof and can be readily overridden for example by use of an external hand held magnet.

It is also known to use a slave circuit, with a coil in parallel with a capacitor and a resistor, which is movable relative to a switching circuit incorporating a powered coil. The slave circuit is tuned to apply a damping effect to the powered coil when in close proximity and this can operate the switching circuit. Whilst this has a higher degree of security than reliance solely on interaction between a magnet and a reed switch, it is not wholly tamper proof and may lack reliability insofar as it may be susceptible to external spurious signals.

An object of the present invention is to provide a reliable safety switching device which is highly resistant to tampering.

According to the invention therefore there is provided a safety switching device comprising first and second parts positionable in safety to each other, said parts each having respective transmitting and receiving circuits, and the second part having a switching circuit, wherein, for the first part the receiving circuit is adapted to provide a power output for powering the transmitting circuit, and, for the second part the transmitting circuit is adapted to transmit power for reception by the receiving circuit of the first part and the receiving circuit is adapted to be actuated by signals received from the transmitting circuit of the first part for operation of the switching circuit.

With this arrangement, the first part provides a powered signal transmission, rather than a passive or slave effect, whereby security can be more readily assured. Moreover, the power for the signal transmission is derived from the second part whereby the first part can produce the powered signal transmission without requiring an on-board battery or other power source thereby permitting simple and convenient construction and reliable operation.

For additional security, preferably at least one of the parts has a magnet and the other has a magnetic switch whereby the switching circuit can only operate when the magnet and magnetic switch are in predetermined juxtaposition.

Any suitable kind of magnetic switch may be used but, in a particular

preferred embodiment the switch is a Hall effect device.

Preferably the magnet is provided in the first part and the magnetic switch is in the second part.

With regard to the power transmitting circuit of the second part, this preferably comprises a coil and an oscillator which may operate at 10 kHz.

The receiving circuit of the first part preferably also comprises a coil which may be tuned e. g. by a capacitor to resonate at the frequency of the transmitting circuit of the second part (e. g. 10 kHz), and which may be connected to appropriate components, such as a rectifying diode and a capacitor, to provide a smoothed DC output.

With regard to the transmitting circuit of the first part, this preferably comprises a coil and an oscillator which when powered produces an AC signal at say 70 kHz. The receiving circuit of the second part preferably also comprises a coil which may be connected to an amplified detection circuit arranged to produce a DC output for triggering the switching circuit.

The aforementioned Hall effect device, or other magnetic switch may be interposed before the switching circuit such that the operation of the switching circuit is determined by the state of the magnetic switch as well as the output of the detector circuit.

The switching circuit may take any suitable form. For example it may comprise one or more relays operated by a trigger circuit or the like. The relays may have contacts for interrupting a power circuit or for any other

purpose.

The switching circuit may also operate an indicator such as an LED to indicate the switching condition of the circuit.

The device of the invention may be used in any suitable context for any suitable purpose. Thus, the device may be used for monitoring a machine guard for safety purposes whereby movement of the guard causes the parts of the switching device to move away from each other thereby to sound an alarm or switch off the machine or take other action.

The first part is preferably movable whereas the second part is preferably fixed.

The sensitivity may be such that the operational state of the switching circuit changes when the parts are moved apart by more than say 5-10 mm.

The invention will now be described further by way of example only and with reference to the single figure of the accompanying drawing which shows a block circuit diagram of one form of a switching device according to the invention.

Referring to the drawing, a switching device for a machine guard has two parts 1,2.

The first part 1 is an actuator which comprises electronic components on a printed circuit board within a rectangular plastics housing 3, and is mounted on the machine guard so as to be movable therewith. The housing

3 also contains a rod-shaped permanent magnet 4 which may be located within a hole in the printed circuit board.

The second part 2 which also comprises electronic components on a printed circuit board within a rectangular plastics housing 5 is mounted on a fixed machine construction near to the machine guard. This part has a lead 6 connected to external AC electrical supply.

The components of the first part 1 comprise a coil 7 connected to an AC to DC power conversion circuit 8, and an AC signal generating circuit 9 connected to the power conversion circuit 8 and to a second coil 10.

The components of the second part 2 comprise: a power supply and rectification circuit 11 connected to the lead 6, an AC signal generator 12 connected to a coil 13, a receiving circuit 14 connected to a second coil 15, an AC to DC converter circuit 16 which produces a DC output which is fed to a final switching circuit 17 via an intermediate switching circuit 18. This circuit 18 incorporates a Hall effect device 19.

The final switching circuit 17 operates relays 19 which have contacts connected to an LED and to an external switching connection for the machine or for a machine safety alarm (not shown).

When the machine guard is in the closed safety position the parts 1 and 2 are in close proximity (say less than 5-10 mm away from each other). In this condition the switching device operates as follows : The power supply 11 produces a low DC voltage (say 24v) using e. g.

a rectifier diode and a smoothing capacitor. This powers the AC signal generator 12 which comprises an oscillator which causes the coil 13 to transmit a 10 kHz signal.

This signal is received by the coil 7 of the first part 1 and is rectified and smoothed by the circuit 8 to produce a low DC voltage power output.

This power output operates the AC signal generator 9 which comprises an oscillator which causes the coil 10 to transmit a 70 kHz signal.

This signal is picked up by the coil 15 of the receiving circuit 14 of the second part 2 and is rectified by the detection circuit 16 to give a DC switching voltage. Preferably the circuit 14 is tuned and/or filtered so that it is responsive only to a narrow frequency range. Amplification may also be provided.

This switching voltage is applied to the switching circuit 17 via the Hall effect device 19 of the circuit 18, which is maintained in a switched-on condition by the action of the magnet 4.

The switching circuit 17 is triggered so as to cause power to flow through the relays 19 and thereby switch on the LED and hold closed the contacts of the relays 19 which are connected to the machine.

In the event that the first part 1 is moved more than 5-10 mm away from the second part 2, the power circuit 8 no longer produces a power output so the signal generator 9 no longer produces a transmitted signal.

Accordingly, the receiving circuit 14 no longer produces a switching voltage whereby the relay contacts open. At the same time the Hall effect device 18 reverts to an open condition so any residual or stray switching voltage cannot operate the switching circuit 17.

The Hall effect device 18 may be connected such that when not switched by the magnet south pole it allows operation of a switch (e. g. a transistor) which grounds the signal line between the detection circuit 16 and the switching circuit 17.

With this arrangement reliable secure operation can be attained. In particular, to effect switching of the switching circuit it is necessary to employ both a correctly positioned magnet and a signal transmission of the correct frequency. Tampering cannot therefore readily be achieved. In this respect it is to be understood that the output of the signal generator 9 is controlled so that the frequency is held within a narrow range centred on the desired frequency, and the receiving circuit 14 is tuned and/or filtered so as to be responsive only to this narrow range.

Moreover, the movable part 1 achieves controlled signal transmission without requiring any on-board battery or other similar power source thereby facilitating reliability and simplicity of construction.

It is of course to be understood that the invention is not intended to be restricted to the details of the above embodiments which are described by way of example only.

Claims

CLAIMS 1. A safety switching device comprising first and second parts positionable in proximity to each other, said parts each having respective transmitting and receiving circuits, and the second part having a switching circuit, wherein, for the first part the receiving circuit is adapted to provide a power output for powering the transmitting circuit, and, for the second part the transmitting circuit is adapted to transmit power for reception by the receiving circuit of the first part and the receiving circuit is adapted to be actuated by signals received from the transmitting circuit of the first part for operation of the switching circuit.
2. A device according to claim 1 wherein the first part derives power for the signal transmission solely from the second part.
3. A device according to claim 1 or 2 wherein at least one of the parts has a magnet and the other has a magnetic switch whereby the switching circuit can only operate when the magnet and magnetic switch are in predetermined juxtaposition.
4. A device according to claim 3 wherein the magnetic switch is a Hall-effect device.
5. A device according to claim 3 or 4 wherein the magnet is provided in the first part and the magnetic switch is in the second part.
6. A device according to any one of claims 1 to 5 wherein the power

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transmitting circuit of the second part comprises a coil and an oscillator.
7. A device according to claim 6 wherein the oscillator operates at 10kHz.
8. A device according to any one of claims 1-7 wherein the receiving circuit of the first part comprises a coil turned to resonate at the frequency of the transmitting circuit of the second part.
9. A device according to claim 8 wherein the coil of the receiving circuit of the first part is connected to components to produce a smoothed D. C. output.
10. A device according to any one of claims 1-9 wherein the transmitting circuit of the first part comprises a coil and an oscillator which when powered produces an A. C. signal.
11. A device according to claim 10 wherein the A. C. signal is at 70 kHz.
12. A device according to any one of claims 1-11 wherein the receiving circuit of the second part comprises a coil connected to an amplified detection circuit arranged to produce a D. C. output for triggering the switching circuit.
13. A device according to any one of claims 1 to 12 wherein the switching circuit comprises one or more relays.
14. A device according to any one of claims 1 to 13 wherein the switching circuit operates an indicator to indicate the switching

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condition of the circuit.
15. A safety switching device substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.
16. A safety switching device according to any one of claims 1 to 15 when used for monitoring a machine guard whereby movement of the guard causes the first and second parts to move away from each other.