GB2470775A

GB2470775A - Safety switching device with stainless steel housing

Info

Publication number: GB2470775A
Application number: GB0909683A
Authority: GB
Inventors: Mabruk Farrah
Original assignee: MECHAN CONTROLS Ltd
Current assignee: MECHAN CONTROLS Ltd
Priority date: 2009-06-05
Filing date: 2009-06-05
Publication date: 2010-12-08
Anticipated expiration: 2029-06-05
Also published as: GB2470775B; GB0909683D0

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. Part 1 contains a transmitter 9 and part 2 contains a receiver 7 and a switch 17 for actuating the alarm. The transmitter 9 of the part 1 transmits power to the receiver 7 of the part 2 so that a battery is not required the part 2. The transmitter 9 of the part 1, when powered, transmits operating signals to the receiver7 of the part 2 which when the parts are in close proximity, are used to operate the alarm switch 17. The parts 1, 2 comprise generally cuboidal stainless steel housing 3, 5 in which the transmitter 9, receiver 7 and switch 17 are housed.

Description

Safety Switching Device This invention relates to safety switches of the proximity kind. The safety switches are 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.

U.K. Patent No. 2385182 describes the use of a safety switch which comprises first and second parts positionable in proximity to each other, each having respective transmitting and receiving circuits. The second part also comprises a switching circuit which is connected to its receiving circuit. A powered coil in the transmitting circuit of the second part transmits an electromagnetic signal of a certain frequency which is received by the receiving circuit of the first part, when the first and second parts are within a certain distance from each other. This signal powers the transmitting circuit of the first part which transmits a return signal of another certain frequency to the receiving circuit of the second part. On receiving this return signal, the receiving circuit switches the switching circuit to a first state.

When the first part is moved above a certain distance from the second part, the signal transmitted by the transmitting circuit of the second part is no longer received by the receiving circuit of the first part. Accordingly, no power is provided to the transmitting circuit of the first part and no return signal is transmitted to the receiving circuit of the second part. Under this condition, the receiving circuit of the second part switches the switching circuit to a second state. In this way, the state of the switching circuit is dependent on the proximity of the first and second parts.

Since the first part provides a powered signal transmission, rather than a passive or slave effect, a high level of reliability is attained. Furthermore, since the power for the signal transmission is derived from the second part, 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.

However, since the first and second parts are normally contained in plastics housings, which cannot be readily cleaned, this safety switch is not suitable for use in ultra hygienic environments, for example in the food and pharmaceutical industries, and is therefore not suitable for use with applications and machines used in these industries.

Furthermore, housings of plastic material are susceptible to permeation.

It is an object of the present invention to provide a reliable safety switching device which is highly resistant to tampering, can be used in the food and pharmaceutical industries and can be cleaned easily.

According to the present invention there is provided a safety switching device comprising first and second parts positionable in proximity to each other, said first part having a transmitting circuit and said second part having a receiving circuit and a switching circuit, and its receiving circuit being adapted to be actuated by signals received from the transmitting circuit of the first part for operation of the switching circuit, characterised in that the first and/or second parts have at least a cover of stainless steel.

Preferably the first and/or second parts of the safety switch are at least substantially enclosed within respective stainless steel housings. More preferably, the first andlor second parts of the safety switch are entirely enclosed within respective stainless steel housings.

It is the current understanding in the art that, for a safety switch which relies upon transmission and reception of an electromagnetic signal for its operation, it would not be possible to cover or enclose parts of the safety switch which transmit and receive the signal within stainless steel housings without impeding the transmission or reception of the signal. This is on the basis that each housing would effectively act as a shield of the signal, blocking transmission and reception of the signal.

The applicant has discovered, contrary to the current understanding in the art, that covering or enclosing said first and second parts in stainless steel does not impede the transmission or reception of electromagnetic signals by said parts and accordingly does not impede the functioning of the safety switch.

The second part preferably further comprises a transmitting circuit and the first part preferably further comprises a receiving circuit. In this case, the receiving circuit of the first part is preferably adapted to provide a power output for powering the transmitting circuit of the first part and the transmitting circuit of the second part is preferably adapted to transmit power for reception by the receiving circuit of the first part.

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, since the power for the signal transmission of the first part is derived from the second part, 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.

Furthermore, since the first andlor second parts of the safety switch are covered or enclosed in stainless steel, the safety switch may be readily cleaned, is not susceptible to permeation or rust and accordingly is suitable for use in industries where hygiene is important, such as the food and pharmaceuticals industry.

A capacitor is preferably arranged within the receiving circuit of the second part to increase the sensitivity of the receiving circuit.

The present invention preferably has some or all features of the safety switch of U.K. Patent No. 2385182.

With regard to the power transmitting circuit of the second part, this preferably comprises a coil and an oscillator which may operate at 7-10 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. 7-10 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 60-80 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 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 away 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 I is an actuator which comprises electronic components on a printed circuit board, enclosed entirely within a generally cuboidal stainless steel housing 3, and is mounted on the machine guard so as to be movable therewith.

The second part 2 comprises electronic components on a printed circuit board, enclosed entirely within a generally cuboidal stainless steel housing 5, and is mounted on a fixed machine construction near to the machine guard. This part has a lead 6 connected to external 24 volt AC or DC 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 and intermediate switching circuit 16 which produces a DC output which is fed to a final switching circuit 17.

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 andlor filtered so that it is responsive only to a narrow frequency range. A capacitor 18 is arranged within the receiving circuit of the second part to increase the sensitivity of the receiving circuit.

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.

With this arrangement reliable secure operation can be attained. 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.

Furthermore, since the first and/or second parts of the safety switch are covered or enclosed in stainless steel, the safety switch may be readily cleaned, is not susceptible to permeation or rust and accordingly is suitable for use in industries where hygiene is important, such as the food and pharmaceutical industries.

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 first part having a transmitting circuit and said second part having a receiving circuit and a switching circuit, and its receiving circuit being adapted to be actuated by signals received from the transmitting circuit of the first part for operation of the switching circuit, characterised in that the first andlor second parts have at least a cover of stainless steel.
2. A safety switch according to claim 1, wherein the first and/or second parts of the safety switch are at least substantially enclosed within respective stainless steel housings.
3. A safety switch according to either of claims 1 or 2, wherein the first and/or second parts of the safety switch are entirely enclosed within respective stainless steel housings.
4. A safety switch according to any preceding claim wherein a capacitor is arranged within the receiving circuit of the second part to increase the sensitivity of the receiving circuit.
5. A safety switch according to any preceding claim wherein the second part further comprises a transmitting circuit.
6. A safety switch according to any preceding claim wherein the first part further comprises a receiving circuit.
7. A safety switch according to claim 6 when dependent on claim 5 wherein the receiving circuit of the first part is adapted to provide a power output for powering the transmitting circuit of the first part.
8. A safety switch according to either claim 7 or claim 6 when dependent on claim 5 wherein the transmitting circuit of the second part is adapted to transmit power for reception by the receiving circuit of the first part.
9. A safety switch substantially as described herein with reference to the accompanying drawings.