GB2142756A

GB2142756A - Safety switching system

Info

Publication number: GB2142756A
Application number: GB08310370A
Authority: GB
Inventors: Roland Myers
Original assignee: BWR MECHANICAL SERVICES LIMITE
Current assignee: BWR MECHANICAL SERVICES LIMITE
Priority date: 1983-04-16
Filing date: 1983-04-16
Publication date: 1985-01-23
Also published as: GB8310370D0

Abstract

A safety switching system is provided for controlling supply of electric power to a potentially dangerous machine having 38 protective gates. Each gate has a plug (3) which has to be disengaged from a socket (2) before the gate can be opened. Each plug and socket assembly (1) is connected to two logic devices (11, 12) such as relays which in turn are connected to a main switching device (4a) capable of disconnecting power from the machine. A parity check is performed on the two logic devices (11, 12) by appropriate circuitry (23, 24) and the switching device is disabled in the event that there is a parity failure due for example to a fault in one of the logic devices. <IMAGE>

Description

SPECIFICATION Safety switching system This invention relates to safety switching systems particularly although not exclusively for controlling supply of electrical operating power to potentially dangerous machines.

With electrically operated machines having movable parts, such as large flywheels, gear wheels with exposed teeth, transfer equipment, robotic implements and the like, which are capable of causing serious injury to any person accidentally contacting same, it is known to provide a protective gate or guard in front of the movable part in conjunction with a switching system which acts automatically to disconnect the supply of electrical operating power from the machine when the gate or guard is opened.

Moreover, in order to provide a fail-safe facility in the switching system it is known to utilise a pair of switching relays arranged such that operating power can only be supplied when both relays are simultaneously in the normal energised switching modes. However, this known arrangement is not wholly satisfactory since failures can occur in the switching system which may not be immediately apparent and which do not fail safe. In particular it can happen that in course of time contacts of first one then the other of the relays fuse in the closed positions corresponding to the normal energised switching modes.

An object of the present invention is to provide a safety switching system with which the likelihood of a dangerous failure occurring can be essentially avoided.

According to the invention therefore there is provided a safety switching system for control ling supply of electrical operating power to a potentially dangerous machine or the like, comprising: an adjustable electrical device operable to produce different electrical outputs respectively in different states of adjustment thereof, a main switching device adapted to be interposed between said electrical power supply and said machine or the like, and at least two logic devices adapted to be connected to said adjustable electrical device and to be switchable between different states in correspondence with said different adjustment states of said electrical device, said logic devices being connected to said main switching device so as to permit actuation thereof only when both (or all) said logic devices are in a predetermined said state thereof, characterised in that a parity checking circuit is provided which is operable to take protective action in the event that both (or all) said logic devices are found not to be in the same said state thereof.

With this arrangement, it will be appreciated that a particularly effective fail-safe facility can be provided.

The switching system may incorporate a single said adjustable electrical device with its respective plurality of logic devices. Alternatively, there may be a plurality of said electrical devices each with its respective plurality of logic devices and in this case the arrangement is preferably such that all logic devices for all electrical devices have to be in the said predetermined states thereof before the said main switching device can be actuated.

Preferably the (or each) adjustable electrical device has only two logic devices associated therewith.

With regard to the (or each) adjustable electrical device this may take any suitable form and thus for example may comprise a plug and socket assembly, or alternatively an arrangement of cam-operated limit switches or any other suitable form of switch or transducer or the like. In a particularly preferred embodiment, a plug and socket is used in conjunction with a concealed diode bridge whereby the requisite electrical output obtained by interengaging the plug and socket cannot readily be simulated by tampering with exposed terminals when the plug and socket are disengaged.

With regard to the said logic device these may be electromagnetic relays or solid state switching devices or gates or any other suitable devices.

The said parity checking circuit may incorporate auxiliary switching elements associated with said logic devices (e.g. auxiliary relay contacts) which are normally changeable between two states, the checking circuit acting to disable the said main switching device in the event that all such auxiliary elements do not change states simultaneously. Where relay contacts are used, a positive gradient thermistor may be connected across the contacts to avoid "chatter" problems.

It is visualised that the invention will find particular application in the context of potentially dangerous machines protected by multiple gates or guards each with a respective associated said electrical device. However, the invention is not intended to be restricted to this and the safety system thereof may be used in any suitable context for any suitable purpose.

The invention will now be described further by way of example only and with reference to the accompanying drawings in which: Fig. 1 is a diagrammatic representation of a logic unit and a contactor constituting parts of one form of a switching system according to the invention; and Fig. 2 is a diagram illustrating the mode of connection of the logic unit to a power supply and to a signal input device.

The illustrated switching system is used to control supply of electrical operating power to a potentially dangerous machine provided with protective gates. Each protective gate has associated therewith a respective six-pin plug and socket assembly 1 (Fig. 2) which is so arranged that, in practice, the socket 2 has to be disengaged from the plug 3 before the gate can be opened. Thus the plug 3 may be fixed to the gate and the socket 2 may be provided at the end of a length of cable anchored to a fixed position close to the gate.

As shown in Fig. 1, the system comprises a logic unit 4 and a contactor unit 4a which may be provided in a common housing or in two separate housings as required.

As shown in Fig. 2, the logic unit 4 contains a step-down transformer 5 adapted to provide 24V a.c. from a 1 1 OV a.c. power supply (not shown) live and neutral lines 6, 7.

The 24V a.c. transformer output is fed to two pins of each of the different sockets 2 via connecting cables 8. Two further pins of each socket 2 are connected to a respective input connection 9 of the logic unit. Each plug 3 contains a diode bridge 10 connected to four of its pins, such bridge being concealed within the structure of the plug. When any socket 2 is engaged with its associated plug 3, a d.c. voltage derived from the 24V a.c.

source rectified by the diode bridge contained in the plug is applied to the respective input connection 9.

The logic unit further contains successive pairs of electromagnetic relays 11, 12, there being one such pair 11, 1 2 for each plug and socket assembly 1. Each relay has first contacts 1 3 or 14 which close when energising power is supplied to the relay coil, and two further contacts 1 5, 1 6 or 1 7, 18 which alternate between respective open and closed states in correspondence with energisation and de-energisation of the coil. That is contact 1 5 (or 17) is open when 16 (or 18) is closed and vice versa.

The first contacts 13, 14 are joined respectively in two series circuits 19, 20 each such circuit being connected at one end to the aforesaid 1 10V a.c. live line 6 in a manner yet to be described and at its other end to the neutral line 7 via a respective winding 21, 22 of a contactor in the contactor unit 4a.

The second contacts 1 5, 1 6, 17, 18 of each relay 11, 12 are connected in parallel connected series circuits 23, 24 which in turn are connected as a series circuit 25 between the 11 OV a.c. live line 6 and the neutral line 7 via a further contactor winding 26. A respective neon lamp 27 is connected across each pair of series circuits 23,24 and a further neon 28 is connected across the entire series circuit 25.

The contactor unit 4a contains the said contactor having the aforesaid three windings 21, 22, 26 which, in dependence on the energisation thereof, controls actuation of various contactor switches 29-32. Three such switches 29 control connection of operating power from a supply (not shown) to the machine (not shown), the arrangement being such that two of these switches associated with the windings 21, 22 have to be closed whilst the other of these switches associated with the winding 26 is open to permit supply of power to the machine.Other contactor switches 30, 31, 32 are connected respectively between the series circuits 19, 20 and the windings 21, 22, between the series circuit 25 and the windings 26, and between the series circuit 25 and the supply end 33 of the series circuits 19, 20.

A pair of series connected push buttons 34 are connected between the supply end 33 of the circuit 19, 20 and the neutral end of the circuit 25. A positive gradient thermistor 35 is connected between supply line 6 and the supply end 33.

As indicated in Fig. 2, each input 9 is connected across the windings of the respective associated pair of relays 11, 1 2 via a light-emitting diode 36.

Fuses 37, 38, 39 are provided respective on the primary and secondary sides of the transformer 5 and in the live line 6. Neon indicators 40, 41 are connected to the supply line 6 in the logic and contactor units 4, 4a.

In use, the operation of the system is as follows: each socket 2 is engaged with the respective plug 3 and a.c. supply is connected to the logic and contactor units via the live and neutral lines 6, 7 whereby the winding 26 is energised via the series circuit 25. The buttons 34 are then pressed simultaneously to cause power to be connected to the series circuits 19, 20 via the series circuit 25 which effects energisation of the windings 21, 22 and subsequently, as a consequence of changes in the various switches 29-32, deenergisation of the windings 26 and latching of a.c. live to the series circuits 19, 20.

In the event that any socket 2 is disengaged from the respective plug 3, in the normal course of events the respective contacts 1 3 and 14 open disrupting the series circuits 19, 20 and thereby de-activating the windings 21, 22 so as to prevent supply of operating power to the machine. Re-insertion of the plug will then reverse this procedure and permit operation of the machine.

As the contacts 1 3, 14 switch from closed to open states and vice versa in correspondence with engagement and disengagement of any plug and socket, the auxiliary contacts 1 5-1 8 of the pertaining relay pair 11, 12 in the normal course of events switch between a state at which contacts 1 5 and 1 7 are open and 1 6 and 18 are closed and a state at which contacts 1 5 and 1 7 are closed and 1 6 and 18 are open. Normally therefore the integrity of the series circuit 25 is maintained.

The thermistor acts to complete this circuit 25 during the short period in which the contacts 1 5-1 8 are moving thereby to avoid "chatter" problems. However, in the event that any one relay fails because for example contacts have fused together and it is capable of switching, there will be a parity failure. That is, the state of contacts 1 5 and 1 6 will not be identical respectively with the state of the contacts 1 7 and 1 8. This will disrupt the circuit 25 thereby removing power from end 33 and also causing the neon 28 and also the respective neon 27 to be illuminated which effects identification of the nature of the error and also the location of the faulty relay.

With the above-described embodiment an effective fail-safe facility is provided and the possibility of a machine being operable when a gate is open can therefore be avoided.

The use of diode bridges 10 in the plug and socket assemblies prevents ready tampering with the system. That is, if the socket terminals are bridged with loose wires when the socket 2 is disengaged from the plug 3 this will return 24V a.c. to the respective input 9 rather than the requisite d.c. voltage and this will not actuate the switching system.

It is of course to be understood that the invention is not intended to be restricted to the details of the above embodiment which are described by way of example only. Thus for example the system may be adapted to operate on 220V a.c. instead of 1 10V a.c.

Claims

1. A safety switching system for controlling supply of electrical operating power to a potentially dangerous machine or the like, comprising: an adjustable electrical device operable to produce different electrical outputs respectively in different states of adjustment thereof, a main switching device adapted to be interposed between said electrical power supply and said machine or the like, and at least two logic devices adapted to be connected to said adjustable electrical device and to be switchable between different states in correspondence with said different adjustment states of said electrical device, said logic devices being connected to said main switching device so as to permit actuation thereof only when both (or all) said logic devices are in a predetermined said state thereof, characterised in that a parity checking circuit is provided which is operable to take protective action in the event that both (or all) said logic devices are found not to be in the same said state thereof.

2. A system according to claim 1 wherein there is a plurality of adjustable electrical devices each with its respective plurality of logic devices.

3. A system according to claim 2 wherein all logic devices for all electrical devices have to be in the said predetermined states thereof before the said main switching device can be actuated.

4. A system according to any one of claims 1 to 3 wherein the or each adjustable electrical device has only two logic devices associated therewith.

5. A system according to any one of claims 1 to 4 wherein the or each adjustable electrical device comprises a plug and socket assembly.

6. A system according to any one of claims 1 to 4 wherein the or each adjustable electrical device comprises cam-operated limit switches.

7. A system according to claim 5 wherein the plug and socket assembly incorporates a concealed diode bridge.

8. A system according to any one of claims 1 to 7 wherein the said logic devices comprise relays.

9. A system according to any one of claims 1 to 8 wherein the said parity checking circuit incorporates auxiliary switching elements associated with said logic devices which are normally changeable between two states, the checking circuit acting to disable the said main switching device in the event that all such auxiliary elements do not change states simultaneously.

10. A system according to claims 8 and 9 wherein said auxiliary switching elements comprise auxiliary relay contacts.

11. A system according to claim 10 wherein a positive gradient thermistor is connected across the relay contacts.

1 2. A system according to claim 1 substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

1 3. A potentially dangerous machine protected by multiple gates or guards and provided with a safety switching system according to any one of claims 1 to 12, said gates or guards each having a respective associated said electrical device of said system.