GB2407126A - Magnetic safety interlock and monitoring circuit - Google Patents

Abstract

The electromagnetic locking system controls access to potentially dangerous sites (2). An energisable solenoid (7, 37) can interact with an armature (8, 38) to hold an access door (3) or the like closed. The relative disposition of the solenoid (7, 37) and the armature (8, 38) is monitored by a sensor winding (20) encircling a core (19) of the solenoid (7, 37). Two microprocessors (14) produce alternate pulses of a pulse train that is superimposed on an energising direct current supplied to a primary winding of the solenoid (7). The sensor winding (20) is linked to the microprocessors (14) which detect and analyse currents induced therein by the pulses. The two microprocessors (14) thus monitor each other's operation as well as the status of the locking system. The system is controlled by one or more key switch units (9, 23, 29) and may control more than one access door (3). Interlocks may be provided between the system and equipment within the hazardous site (2), such that its operation is halted when the access door (3) is unlocked or forced open, preferably after a delay to allow the hazard to dissipate.

Description

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MAGNETIC SAFETY INTERLOCK AND MONITORING CIRCUIT

The present invention relates to a magnetic locking system for use on a door, gate, hatch or the like. More particularly, but not exclusively, it relates to such a magnetic locking system comprising a safety interlock for hazardous machinery, or the like, accessible through the door, etc. It also relates to a monitoring circuit applicable to such systems, and to a safety enclosure provided with such a system.

It is a requirement of most health and safety regimes that access to hazardous areas should be strictly controlled. For example, moving parts of machinery and electrically live components should be fenced off or otherwise enclosed to prevent a worker or passer-by coming into contact therewith, and so being injured or even killed. Since there will inevitably be a need to maintain or adjust such equipment, means of access is required, such as doors or gates in fences, doors to machinery rooms, access hatches in machinery casings and the like. The term "access means" should be understood as referring to any such closable aperture, whether sufficiently large for a person to pass therethrough, or only for a hand or even a finger. These : ë.:e en- :e.: . . access means will need to be locked when not in use, to keep out the curious, the careless or even the malicious.

It is common practice to use electromagnetically fastened locking devices in such applications, as they are capable of rapid release when required, they have no moving parts which might jam or wear out, and they are compatible with a wide range of activating means, such as conventional keys, coded keypads, coded transponders and so forth, as desired.

However, when such locking mechanisms are used in conjunction with hazardous areas, rather than merely to control access for security purposes, there are additional requirements.

To be thoroughly safe, it is necessary that the hazard is inactivated when the access means is open - e.g. the locking system should be adapted to cut power to machinery within the area as well as releasing the access means.

The requirements for such interlock systems are becoming increasingly strict, and it is generally desirable to have some form of fail-safe arrangement (particularly since the more safety systems are installed, the more there is a tendency for operators, etc. to rely on the safety systems rather than their own awareness). Thus, it is preferable that the hazard is inactivated when the access means is open for any reason, not just as a result of deliberate operation of the locking system.

Simple mechanical contacts can be used to indicate that an access means is open, but they can easily become jammed, blocked by debris or even deliberately defeated, for example by an operator trying to cut corners and save time. They also do not indicate a situation in which the electromagnetic lock on the access means has failed, but the access means is still e: - .. .- .-e a..

closed. This could make it possible for the access means to be opened and the area entered before the interlock has fully operated and machinery within has slowed down safely, for

example.

Attempts have been made to monitor the continued operation of electromagnetic locking devices with reed switches (see, for example, US Patents Nos. 4,703,962 and 5,261,713) or Hall-effect sensors (see, for example, US Patent No. 4,287,512 and European Patent Application No. 0821123). Each of these can detect the presence of a magnetic field. A majority of electromagnetic locks comprise an electromagnet and an armature, one mounted to a frame of the access means and one to a moving door, gate, hatch, etc. thereof. The magnetic field around an energised electromagnet changes, depending on whether it is in contact with an armature or not. Thus, a correctly placed reed switch or Hall-effect device can both sense whether the locking device is energised or not and sense when the locking device is energised but the access means is ajar or open.

Unfortunately, problems have been encountered with such arrangements. The reed switch or Hall-effect device can be defeated by extraneous magnetic fields, and it also only operates effectively when very accurately positioned. Either the failsafe system must rely on the reed switch, etc. never becoming even slightly displaced (despite machinery vibrations, slammed doors, etc), or the reed switch, etc. must be so securely fixed against movement that replacement becomes impossible if it fails. Furthermore, both reed switches and Hall-effect devices can fail while continuing to indicate a safe condition.

The system disclosed in IJS Patent No. 6271751 is intended as an improvement on the above systems, but it is optimised for security applications and does not consider safety interlocks. 1#.

. :: . ::e. ale: . . It is believed that the system disclosed therein would not be a completely adequate answer to the problems of safety systems. For example, a truly reliable fail-safe arrangement would automatically cut power if it itself malfunctions as well as if the locking device fails or is defeated.

With the advent of modern equipment safety standards, such as BS EN954-1: 1997 and the EC Machinery Directive 98/37/EC, the need for an effective fail-safe arrangement has become even more important, particularly for the machines, equipment, etc defined therein as "Category 4", requiring extremely high levels of protection and safety.

Clearly, a locking system with the degree of reliability required for Category 4 equipment safety would also be highly effective in a conventional security application.

A monitoring system that could be added to an existing locking system, to bring it up to the standards referred to above, would also be of significant benefit.

It is hence an object of the present invention to provide an electromagnetic locking system for access means, comprising a safety interlock, that obviates the above problems and provides the above benefits. It is also an object of the present invention to provide an electromagnetic locking system for general use that detects, more reliably than hitherto, whether it is open, closed or has failed. It is another object of the present invention to provide a monitoring circuit for such locking systems, and also to provide a safety enclosure provided with such a locking system. e:

..:e.e:. . be: . s According to a first aspect of the present invention, there is provided an electromagnetic locking system for access means as defined herein, comprising selectively energisable solenoid means, armature means cooperable therewith to hold the access means closed, and monitoring means, adapted to detect a relative disposition of the solenoid means and the armature means, comprising sensor winding means encircling a core of the solenoid means and means to detect a current induced in the sensor winding means.

Preferably, the monitoring means comprises means to superimpose a train of pulses on an energising current supplied to the solenoid means, thereby inducing corresponding currents in the sensor winding means.

Advantageously, the monitoring means comprises means to analyse a waveform of the current induced in the sensor winding means.

The monitoring means may then be adapted to distinguish a waveform of said induced current when the solenoid means and armature means are in contact, one with the other, from a waveform of said induced current when the solenoid means and armature means are displaced, one from the other.

The analysis means may be adapted to calculate a decay constant of the waveform of the induced current and to distinguish said "in-contact" and "displaced" waveforms by means of their respective decay constants.

The monitoring means may comprise microprocessor means comprising pulse generating means and analysis means. :

I.. ë.e The monitoring means may comprise two or more said microprocessor means.

Each of said two or more microprocessor means may then in turn provide pulses for said train.

The two or more microprocessor means may analyse the waveforms of the induced current, each independently of the others.

Failure of one of said two or more microprocessors is thus indicated by the absence of a waveform induced by a pulse due to be provided by that microprocessor, as detected by the remaining microprocessor or microprocessors.

The locking system is preferably provided with means selectably to activate or inactivate the solenoid means.

Said activation means may comprise key switch means, key pad means, swipecard reader means or transponder detection means.

Said key switch means may comprise mechanical key means or magnetic key means.

The locking system may comprise a plurality of said activation means.

In a first embodiment, the monitoring means is adapted to indicate, for example by activating alarm means, that the solenoid means and armature means arc disposed one away from the other while the locking system is activated.

:..* I. Pe& i 7 The monitoring means thus acts to alert a user that the access means has been left open or that the access means is being opened inappropriately.

In a second, preferred embodiment, the monitoring means is operatively linked to control means for potentially hazardous equipment accessible through the access means, so as to prevent operation of the equipment when the solenoid means and the armature means are displaced one from the other.

The monitoring means thus acts as a safety interlock to prevent access to the potentially hazardous equipment while It is in operation.

The activation means of the locking system may be also be operatively linked to the control means, so as to prevent operation of the equipment while the solenoid means is inactivated.

Advantageously, the activation means is adapted first to prevent operation of the equipment and then to inactivate the solenoid means, following a delay period.

Thus, the operation of the equipment is allowed to cease before access thereto through the access means is permitted.

Preferably, the locking system is provided with indicator means to display an operational status of the locking means and of the equipment.

The indicator means advantageously comprises a plurality of lightemitting means, colour- coded according to a status to be indicated. d

, * The locking system may be provided with means to select a desired attractive locking force between the solenoid means and the armature means.

The locking system may be so adapted that while the solenoid means is inactivated, a residual attraction exists between the solenoid means and the armature means sufficient to oppose their accidental separation.

The locking system may be provided with means to select a desired residual attraction.

The locking system may be provided with a plurality of cooperable pairs of solenoid means and armature means.

Each of said plurality of solenoid means may then be provided with sensor winding means.

According to a second aspect of the present invention, there is provided a monitoring circuit for an electromagnetic locking system, comprising sensor winding means encircling a core of solenoid means of the electromagnetic locking system, and means to detect a current induced in the sensor winding means.

Preferably, the monitoring circuit comprises means to superimpose a train of pulses in an energising current supplied to the solenoid means, thereby inducing corresponding currents in the sensor winding means.

According to a third aspect of the present invention, there is provided an electromagnetic locking system for access means as defined herein, comprising selectively energisable ]: tIt q'. ee. . t solenoid means, armature means cooperable therewith to hold the access means closed, and monitoring means adapted both to monitor a relative disposition of the solenoid means and the armature means and to monitor operation of the monitoring means itself.

Preferably, the monitoring means is adapted to signal that the solenoid means and the armature means are disposed one away from the other, and to signal a malfunction in its own operation.

Advantageously, the monitoring means is so operatively linked to an item of equipment that said signals cause power to the equipment to be cut off or the equipment otherwise to be inactivated.

The locking system may comprise a locking system as described in the first aspect above.

According to a fourth aspect of the present invention, there is provided a safety enclosure for potentially hazardous, in operation, equipment, the safety enclosure being provided with access means to allow entry to an interior thereof and an electromagnetic locking system as described in the first or third aspects above, operatively connected to the access means.

The safety enclosure may be provided with a plurality of lockable access means.

Preferably, the locking system is so operatively linked to the equipment that operation of the equipment is prevented when the or any one of the access means is unlocked. ce

:.. e: .. ë. . .e Advantageously, the locking system is so operatively linked to the equipment that operation of the equipment is prevented when the or any one of the access means is open.

Optionally, the locking means is so operatively linked to the equipment that operation of the equipment is prevented should the locking means or components thereof malfunction.

An embodiment of the present invention will now be more particularly described, by way of example and with reference to the accompanying drawings, in which.

Figure I is a schematic plan view of a first locking mechanism of the present invention, mounted to an access point in a protective barrier; Figure 2A is an exploded plan view of components of the locking mechanism shown in Figure 1; Figure 2B is an exploded elevation of the components shown in Figure 2A; Figure 3 is an elevation of a programmable logic controller of the present invention; Figure 4 shows, schematically, a monitoring circuit of the present invention; Figure 5 is a graph against time of currents flowing through the circuit of Figure 4.

Figure 6A is an enlargement of a portion of the graph of Figure 5, when the locking mechanism is open; Figure 6B is an enlargement of the same portion of the graph of Figure 5, when the locking mechanism is closed; Figure 7 is a perspective view of a first key switch unit for a locking system embodying the present invention; Figure 8 is a perspective view of a second, magnetic key switch unit for a locking system embodying the present invention; ee.

e. ee. . Figure 9 is a frontal elevation ofthe magnetic key switch unit shown in Figure 8; Figure lOA is a plan view of a second locking mechanism of the present invention; Figure IOB is a side elevation of the locking mechanism shown in Figure IOA; Figure 10C is a perspective view of the locking mechanism shown in Figure IOA; Figure 11 is a frontal elevation of a control board for a locking system embodying the present invention; and Figure 12 is a schematic representation of a control circuit associated with the control board shown in Figure l 1.

Turning now to the Figures and to Figure I in particular, a protective barrier, such as a fence 1, has been set up to enclose a hazardous area 2, within which an item of machinery may be operating. A door 3 is provided in the fence I as an access point for when the machinery requires adjustment, maintenance, repair or replacement. The door 3 is mounted by means of a hinge 4 to a frame 5 which comprises part of the fence 1. The hinge 4 and frame 5 are arranged such that the door 3 can only be opened away from the hazardous area 2, for example by manually pulling on a handle 6 mounted thereto. It is hence difficult to barge through the door 3 straight into the hazardous area 2, deliberately or accidentally.

The door 3 is held closed by a solenoid 7 mounted to the frame 5 and a corresponding armature 8 mounted to the door 3. When the solenoid 7 is energised, it is electromagnetically attracted into contact with the armature 8, fastening the door 3. The locking mechanism is engaged and released by means of a keyswitch unit 9 (or a keypad unit, or a swipecard unit, or a range of other conventional equivalents), which passes or interrupts a power supply for the solenoid 7. This power supply is provided from a central power source along cabling 10, ::e :. ::. ... . . . . . which may be as much as one kilometre long if a large number of doors 3 are being controlled.

In a typical arrangement, when the solenoid 7 and armature 8 are in direct contact, a force of approximately 100kgf is required to separate them. Thus, it is possible for one or two persons manually to pull the door 3 open, for example to provide for emergency situations in which urgent access is required, but a key for the keyswitch unit 9 is not immediately available. The attractive force dies away rapidly if the solenoid 7 and armature 8 are spaced apart, however, so it is essential to detect if this is the case. Otherwise, the locking mechanism might appear closed, but could be defeated by a casual or accidental tug on the handle 6.

It is hence necessary for the exact status of the locking mechanism to be monitored.

Furthermore, to make guarding of the hazardous area fail-safe, an interlock mechanism should be included, which both cuts power to the machinery within the hazardous area when the locking mechanism is deliberately released using the keyswitch unit 9, and when the locking mechanism (and hence the door 3) is open for any other reason, justifiable or not.

The monitoring circuit for this purpose is described in detail below (see Figures 4 to 6B).

The armature 8 is conveniently mounted to the door 3 by means of a planar mounting plate 11, which is apertured to receive screws, bolts or the like to secure it both to the armature 8 and to the door 3. The solenoid 7 is also provided with an apertured mounting plate, here a cranked mounting plate 12, by which it may be mounted to a lintel of the frame 5, above the door 3. (A rebate in the door 3, necessary to accommodate the solenoid 7 in contact with the armature 8, is omitted for clarity).

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The system as a whole is controlled by a programmable logic controller PLC 13 of known form, which may conveniently be positioned adjacent the central power source and be linked to the solenoid 7 by means of the cabling 10.

A monitoring circuit for the locking mechanism, as shown in Figure 4, may be provided in the form of individual electronic components, or may substantially be simulated by the PLC 13, suitably programmed.

Two independent microprocessors 14, here labelled 'A' and 'B', are programmed so as alternately to send signals via an OR gate 15 to a pulsing unit 16, which is connected to the DC power supply 17 for the solenoid 7. In response to each signal from either microprocessor 14, the pulsing unit 16 superimposes a small pulse on the constant DC current supplied by the DC power supply 17. The superimposed pulse has an amplitude of about five percent of that of the underlying DC current. The waveform of the current I' flowing through the cabling l0 to the solenoid 7 is thus as shown in Figure 5.

The solenoid 7 comprises at least one primary winding 18 around a core 19. The current 1' passing through the primary winding 18 creates a magnetic field which urges the solenoid 7 and the armature 8 together. Since the strength of the magnetic field depends on the magnitude of the current I the field will rise and fall with each pulse in the current I' These rises and falls are insufficient to have a significant effect on the attraction between the solenoid 7 and the armature 8.

However, a small sensing winding 20 also encircles the core 19 of the solenoid 7. Each change of the magnetic field will thus induce a voltage across the sensing winding 20, :. :.. ë ec. . ec.

generating a current 12 therein. This induced current I2 is split and passed to each of two amplifiers 21, one of which is connected to microprocessor 14 A and the other to microprocessor 14 B. Each microprocessor 14 thus receives an indication of any changes in the magnetic field of the solenoid 7. The waveform of the induced current I2 is generally as shown in Figure 5.

In detail, however, the waveform of the current I2 induced by each pulse in the current I' depends on the exact relative disposition of the solenoid 7 and armature 8. Figure 6A shows the waveform when the door 3 is open and the armature 8 is too far from the solenoid 7 to have any effect. When the current It rises or falls, the magnetic field responds rapidly, and so does the induced current 12, as shown. The resulting waveform can be characterized by a time constant To, which is here relatively small.

When the solenoid 7 and armature 8 are in contact, however, the inductance of the solenoid 7 changes significantly. The magnetic field of the solenoid 7 is much slower to settle down after each change in the current It, and so the induced current 12 is much slower to die away (see Figure 6B). Thus, a waveform is produced characterized by a time constant T2 where T2 is significantly greater than To The inductance of the solenoid 7 (and hence the value of the time constant) is found to be very sensitive to any air gap established between the solenoid 7 and the armature 8. The time constant will therefore drop very rapidly from T2 towards To as soon as the door 3 is opened for any reason. Each of the microprocessors 14 will respond to such a drop by cutting off power to the machinery within the hazardous area 2.

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If the current It to the solenoid 7 is cut off, either because the keyswitch unit 9 has been operated to release the locking mechanism, or because of a power failure, a large spike will be observed in the current I2 as the magnetic field of the solenoid 7 collapses, and then nothing further. The microprocessors 14 are programmed to cut olf power to the machinery in response to this event also.

If one of the microprocessors 14 were to fail, only alternate pulses would be superimposed on the current 17 and the remaining microprocessor 14 would sense the absence of a response in the induced current 12 where one would be expected. Again, this would result in the remaining microprocessor 14 cutting off power to the machinery.

Thus, each microprocessor 14 also checks on the continued operation of the other, rendering the system fail-safe, whereas a single microprocessor 14 might fail, leaving the machinery permanently on. Although microprocessor failures are rare, this extra level of safety may be vital in "Category 4" situations.

Gross changes in the magnetic field, for example due to attempts to defeat the locking mechanism with another magnet, will produce induced currents I2 having very different waveforms to those shown. The microprocessors 14 would respond to these, too, by cutting offpower to the machinery.

Each of these different failure modes would also be notified accordingly to the PLC 13, which would display and optionally record them for later reference.

:. :.. I..e '. . The system shown may have additional features, which arc associated with the routine control of the locking system by means of the keyswitch unit 9. Here, the "off" disposition of the keyswitch unit 9 does not completely cut off power to the solenoid 7, but instead lowers it, so that there is a small residual attraction between the solenoid 7 and the armature 8. This is sufficient to prevent the door 3 swinging open, but allows easy manual opening when required.

As an additional level of safety, when the locking mechanism is released using the keyswitch unit 9, power to the solenoid 7 is cut off only after a delay period following cutting-off of power to the machinery within the hazardous area 2. Thus, there is more time for fast- moving parts to idle down, capacitors to discharge, and so forth, before access can be gained.

The power to the solenoid 7 may also or alternatively be reduced gradually. For example, it could be reduced over a period of twenty or thirty seconds, either to a level where the door 3 can easily be opened against the magnetic attraction, or to zero. The PLC 13 allows a range of such delays and reductions to be selectively imposed, depending on how rapidly the particular hazard within the area 2 dissipates when the power thereto is cut off.

A particularly useful feature in practice is the provision of an indicator light unit 22, ideally adjacent or integral with the keyswitch unit 9. This has three lights, controlled by the PLC 13 to indicate the status of the locking mechanism and the machinery within the area 2, using a "traffic light" system. A red light indicates that the machinery is in operation, and the door 3 is locked. An amber light indicates that the machinery is stopping, but the door 3 is still locked (either at full strength or partially). A green light indicates that the machinery is safe, and the door 3 may be opened. Further visual or audible signals may optionally be provided 1.

c e to indicate locally that the door 3 has been opened without the keyswitch unit 9 having been operated first.

A particular advantage of the keyswitch unit 9 is that an operator can remove the key therefrom, when the machinery is off and the door 3 is unlocked ("green"), and take it with him inside the hazardous area 2. This prevents anyone resetting the system and starting up the machinery while he is in the hazardous area 2.

Another advantage of the system described is that anyone who, despite all the levels of safety provided, still finds himself within the hazardous area 2 with the machinery running, merely has to push the door 3 very slightly open for the machinery to be cut off.

The locking and interlock system described would be suitable for a wide range of hazardous areas, including enclosures or rooms containing machinery with accessible moving parts, rooms containing high voltage electrical equipment, rooms with dangerously noisy machinery, X-ray facilities, enclosures containing laser equipment, and so forth. The only situations in which it would probably not be appropriate would be where a hazard remains even after power has been cut off (e.g. lift shafts) or is slow to dissipate (e.g. very high temperatures). It could easily be applied to access panels or hatches in machine casings, to ensure that the internal mechanism of the machine can only be accessed when the machine is not working.

The monitoring circuit described would also be useful for security locks, even where no safety interlocks are required, as it provides a very sensitive and reliable indication of a secure door being opened, or being left apparently closed but actually slightly ajar. Instead :e:e ceee i derl of being connected to an interlock to cut off power to machinery, the microprocessors 14 could be connected to any desired alarm system. The failsafe performance of the circuit could also be an advantage in particularly high security areas.

Figure 7 shows a first combined key switch unit 23, corresponding to both the key switch unit 9 and the indicator light unit 22 shown in Figure 1. A conventional mechanical key 24 is turnable in a lock 25 having "on" and "off" positions. The key switch unit 23 is provided with a red 26, an amber 27 and a green high-intensity LED warning light 28, disposed in a "traffic-light" array.

The red light 26, when illuminated, indicates that the machinery within the hazardous area 2 is running, and that the door 3, etc. is locked. The amber light 27, when illuminated, indicates that the machinery is in the process of stopping, but some hazard remains; the delay period described above is still in progress, so the door 3 is still locked. The green light 28, when illuminated, indicates that the machinery has stopped, the door 3 is unlocked, and the hazardous area 2 may be entered. The key 24 may be removed from the lock 25 when the green light 28 is on,

so that an operator may carry it with him when entering the hazardous area 2. The machinery may only be restarted by returning the key 24 to the lock 25, and turning it to the "on" position, with the door 3 to the hazardous area 2 already closed.

Should the locking mechanism be forced open, or should a system failure be detected by a monitoring circuit, as described above, all three warning lights 26, 27, 28 are illuminated. In such a "lockout condition", the machinery cannot be restarted with the key 24 alone. Instead, ::. :e:. :: .

. . . . e.

a specific reset procedure must be followed at the PLC 13 or other control device. This discourages unnecessary use of the lockout condition as a shortcut for turning off the machinery and entering the hazardous area 2.

Figures 8 and 9 show a second combined key switch unit 29. This employs a magnetic key containing an array of permanent magnets, arranged to form a unique code. The magnetic key 30 may be attached to either a Run panel 31 or a Safe panel 32 on the key switch unit 29, being held in place by the magnets. A reader behind each panel 31, 32 analyses the coded array of magnets to confirm that an attached key 30 is the correct one for that key switch unit 29.

Placing the key 30 on the Run panel 31 corresponds to turning the mechanical key 24 of the first unit 23 to "on", while moving it to the Safe panel 32 (or, optionally, merely removing it from the Run panel 3 l) corresponds to turning the mechanical key 24 to "off'.

In place of the red 26, amber 27 and green 28 warning lights of the first combined key switch unit 23, the second combined key switch unit 29 is provided with illuminable RUN 33, WAIT 34 and SAFE 35 indicia, which are lit in red, amber and green, respectively, according to the same principles as described above for the first combined key switch unit 23.

A magnetic key 30 can rapidly be made up to a desired code by authorised personnel, who can also program the readers in the key switch unit 29 to the same code by means of the PLC 13, or equivalent. Once made up, a magnetic key 30 is a sealed block, clearly visible in use and very hard to damage or lose. The key switch unit 29 is also completely enclosed and sealed. Hence, unlike a conventional key 24 and lock 25 mechanism, there is nowhere for :.. :.. . eee. ....e contaminants to collect, and no moving parts that might become clogged or corroded. This is a significant benefit in industrial applications such as cement or pigment production. Such units 29 are also advantageous in applications such as food or pharmaceutical processing, where high standards of cleanliness and hygiene are required.

In a possible variation on the first key switch unit 23, a plurality of locks 25 could be provided, each having its own corresponding key 24 assigned to a particular operator. The machinery could only be run with every key 24 turned to "on". Similarly, the second combination key switch unit 29 could be provided with a plurality of corresponding pairs of Run panels 31 and Safe panels 32, each pair having a corresponding magnetic key 30. In either case, an operator need only turn off the system with his own key 24, 30 to render the machinery sale and unlock the door 3 to allow entry, preferably taking his key 24, 30 with him so that the machinery cannot be restarted.

A plurality of separate key switch units 9, 23, 29 may be used, perhaps adjacent a number of separate doors 3 or other access points but each linked to a common PLC 13 or other control unit. In this case, the machinery within the hazardous area 2 could only operate if each door 3 is closed and each key switch unit 9, 23, 29 is turned to "on"/"Run", as appropriate. If any access point or door 3 is forced, the lockout condition may optionally be indicated on the adjacent key switch unit 23, 29 alone.

In less safety-critical applications, the locking system may be operated with conventional push-buttons, instead of the key arrangements described above. For example, a first push- button is mounted externally of the hazardous area 2, adjacent the door 3, and a second push button is mounted internally of the hazardous area 2. In normal use, the machinery would be .e.

e turned off for access by pressing the external push-button, but to switch it on again, first the internal push-button is pressed, then the door 3 is closed and the external push-button is pressed, all within a first predetermined time limit. As above, when the machinery is turned off, there is a delay period to allow it to become safe before the door 3 is unlocked. "Traffic light" warning LEDs or the like, as described above, may be used in conjunction with the push-button system. As an added safety feature, the system may be programmed so that instructions from the external push-button are only accepted after a second predetermined time limit, shorter that the first, so as to ensure that the operator pressing the internal push- button has time to leave, should some-one else press the external push- button. In the push button arrangement, either forcing the door 3 or pressing the internal push-button while the machinery is running would lead to the lockout condition.

Figures IOA and lOC show a second locking mechanism 36 as an alternative to that shown in Figures 2A and 2B. This comprises a solenoid module 377 mountable to a fixed part of an access points such as a door frame 57 and an armature module 387 mountable to a moveable part of the access points such as a door 3 leaf. The solenoid module 37 comprises a solenoid coils selectable energisable with electrical current provided through a conventional "Ml277 connector 39. The monitoring circuitry described above and below is also connected through the Ml2 connector 39. When energised7 the solenoid module 37 is magnetically attracted to a steel plate of the armature module 38. The particular example shown requires only 0.25 Amps DC to provide a holding force of twenty-five kilograms.

The respective modules 37, 38 may be mounted either so that, as the door 3 closest the armature module 38 approaches the solenoid module 37 generally perpendicularly to their contact surfaces 427 or so that the armature module 38 approached generally parallelly to the ::. A::. :-:..

. . . . contact surfaces 42. In the first case, the modules 37, 38 are held directly together by the magnetic attraction therebetween (shown by arrow 40), while in the second case, they are held together by resultant frictional forces (shown by arrows 41).

Each module 37, 38 has a casing of ABS or a similar high-impact plastics material, with a contact surface 42 of stainless steel. They are thus suitable for both "wet" and "dry" applications and for both "dirty" and "clean" applications. It has been found that as long as the modules 37, 38 are so mounted that they approach within five millimetres of complete contact when the door 3 is fully closed, they will perform effectively; precisely-aligned contact is not essential.

Figure l l shows a face of a control board for a locking system embodying the present invention as seen by a user, and Figure 12 shows, schematically, the operative connections within the control board of Figure 11 and leading to a remainder of the system.

The group of connections marked ITB represent a user terminal block. 2TB comprises user connections to the magnet (ie the solenoid 7 or 37). IPL comprises connections to one or more key switch units 9, 23, 29. 2PL comprises connections to the magnet. Each of D14, D15, Dl6, Dl7, Dl8, Dl9, D20 and D25 is a selectably illuminable LED.

KEY A and KEY B are connections to two separate key switch units or to a single key switch unit provided with two keys.

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COM is a common +24V connection for the RUN, WAIT and ENTER (or SAFE) lamps (26, 27, 28 or 33, 34, 35 respectively), operated by respective relays RL3, RL4, RL5 and confimned on the control board by LEDs Die, Dl 9, D20 respectively.

Connections marked OSSD or OSD are linked to a monitoring circuit as shown in Figure 4, providing two cross-monitored safety relay outputs (OSD1 & OSD2 or OSSDI or OSSD2) to an emergency stop, operated by relays RLI & RL2 respectively, in case of the door 3 being forced. Operation of either of these relays is confirmed on the board by LED D17.

A bank of user-operable switches SWI allows adjustment of various parameters of the locking system. SW1-1&2 allow the locking force between the solenoid 7, 37 and the armature 8, 38 to be adjusted, here in four steps ranging from 25% to 100% of the maximum possible force. SWI-3&4 allow the residual attraction between the solenoid 7, 37 and the armature 8, 38, when the door 3 is unlocked, to be adjusted, here in four steps ranging from 2% to 20% of the maximum possible force. SWI -6 to 8 allow adjustment of the length of the delay period between cutting-off power to the machinery and unlocking the door 3 to allow access. Here, eight steps are possible, ranging between one and 128 seconds. Diode D25 varies in brightness depending on the voltage being supplied to the magnet solenoid 7, 37 along 2TBI and 2. D14 is a simple power-on indicator, for a main power supply linked to I TB I, 2 and 3.

The control board shown comprises one further feature not referred to above. The monitoring circuit shown in Figure 4 is in this case only operated when the door 3 is locked, and the solenoid 7, 37 is powered to give a locking force. However, as mentioned above, the solenoid 7, 37 is kept partially powered when the door 3 is unlocked, to prevent the door 3 : : . -I: e:. .- :e:..

swinging open. It is necessary to check whether the door 3 is actually closed, before the machinery can be activated using the keyswitch units 9, 23, 29 or the pushbutton arrangement described above. Using the monitoring circuit might result in a door 3 that was merely left ajar being registered as a forced door, leading to a full lockout condition, whereas all that would be needed would be for the activation of the machinery to be delayed until the door 3 is safely closed. Therefore, a small AC signal is superimposed on the main DC supply, provided to the magnet solenoid 7, 37 along 2TB1 and 2TB2 and through the M12 connector 39 to the primary solenoid winding 18. When the door 3 is closed and the armature 8, 38 is in contact with the solenoid 7, 37, this AC signal passing through the primary solenoid winding] 8 induces an AC signal in the secondary sensing winding 20. This signal is passed back to the control board through the M12 connector 39 and 2TB3 and 2TB4. Thus, the system can detect whether the door 3 is open or closed when the monitoring circuit of Figure 4 is not in operation.

As well as a physical control board as shown, it is also possible to use a computer display screen, controlled by a conventional keyboard and mouse, allowing the same inputs and displaying the same information.

While the systems described above comprise a monitoring arrangement fully integrated with a magnetic locking system, it is also envisaged that monitoring arrangements of the type shown in Figure 4 could be used to monitor existing magnetic lock systems, as an add-on.

The monitoring arrangements could also be used purely to indicate lock status or problems, rather than having a full interlock to machinery, etc.

Claims (1)

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   1. An electromagnetic locking system for access means as defined herein, comprising energisable solenoid means, armature means cooperable therewith to hold the access means closed, and monitoring means adapted to detect a relative disposition of the solenoid means and the armature means, sensor winding means encircling a core of the solenoid means and means to detect a current induced in the sensor winding means.

   2. A system as claimed in claim 1, wherein the monitoring means comprises means to superimpose a train of pulses on an energising current supplied to the solenoid means, thereby inducing corresponding currents in the sensor winding means.

   3. A system as claimed in either claim 1 or claim 2, wherein the monitoring means comprises means to analyse a waveform of the current induced in the sensor winding means.

   4. A system as claimed in claim 3, wherein the monitoring means is adapted to distinguish a waveform of said induced current when the solenoid means and armature means are in contact, one with the other, from a waveform of said induced current when the solenoid means and armature means are displaced, one from the other.

   :. :.e ë e.. ë.e 5. A system as claimed in any one of the preceding claims, wherein the analysis means is adapted to calculate a decay constant of the waveform of the induced current thereby to distinguish different waveforms.

   6. A system as claimed in any one of the preceding claims, wherein the monitoring means comprises at least one microprocessor means, the or each of which includes pulse generating means and analysis means.

   7. A system as claimed in claim 6, comprising two or more microprocessor means, each of which provides pulses for said train.

   8. A system as claimed in claim 7, wherein each of the two or more microprocessor means analyses the waveforms of the induced current independently of the others.

   9. A system as claimed in any one of the preceding claims, further comprising activation means including key switch means, key pad means, swipe-card reader means or transponder detection means.

   10. A system as claimed in claim 9, wherein said key switch means comprises mechanical key means or magnetic key means.

   A system as claimed in any one ol the preceding claims, further comprising alarm means to indicate that the solenoid means and armature means are disposed one away from the other while the locking system is activated.

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   12. A system as claimed in any one of claims I to 10, further comprising control means for potentially hazardous equipment accessible through the access means, operatively linked to the monitoring means so as to prevent operation of the equipment when the solenoid means and the armature means are displaced one from the other.

   13. A system as claimed in claim 12, wherein activation means of the locking system are also operatively linked to the control means, so as to prevent operation of the equipment while the solenoid means is inactivated.

   14. A system as claimed in claim 13, wherein the activation means is adapted first to prevent operation of the equipment and then to inactivate the solenoid means, following a delay period.

   15. A system as claimed in any one of the preceding claims, further comprising indicator means to display an operational status of the locking means and of the equipment.

   16. A system as claimed in any one of the preceding claims, wherein a residual attraction exists between the solenoid means and the armature means when the solenoid means is inactivated sufficient to oppose their accidental separation.

   17. An electromagnetic locking system for access means, substantially as described herein with reference to the Figures of the accompanying drawings.

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   18. A monitoring circuit for an electromagnetic locking system, comprising sensor winding means encircling a core of solenoid means of the electromagnetic locking system, and means to detect a current induced in the sensor winding means.

   19. A monitoring circuit as claimed in claim 18, further comprising means to superimpose a train of pulses in an energising current supplied to the solenoid means thereby inducing corresponding currents in the sensor winding means.

   20. A monitoring circuit for an electromagnetic locking system substantially as described herein with reference to the Figures of the accompanying drawings.

   21. An electromagnetic locking system for access means, as defined herein, comprising selectively energisable solenoid means, armature means cooperable therewith to hold the access means closed, and monitoring means adapted both to monitor a relative disposition of the solenoid means and the armature means and to monitor operation of the monitoring means itself.

   22. A safety enclosure for potentially hazardous, in operation, equipment, the safety enclosure being provided with access means to allow entry to an interior thereof and an electromagnetic locking system as claimed in any one of the preceding claims, operatively connected to the access means.

   23. A safety enclosure as claimed in claim 22, wherein the locking system is so operatively linked to the equipment that operation of the equipment is prevented when the or any one of the access means is unlocked.

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   24. A safety enclosure as claimed in claim 22, wherein the locking system is so operatively linked to the equipment that operation of the equipment is prevented when the or any one of the access means is open.

   25. A safety enclosure as claimed in any one of claims 22 to 24, wherein the locking means is so operatively linked to the equipment that operation of the equipment is prevented should the locking means or components thereof malfunction.

   26. A safety enclosure substantially as disclosed herein with reference to the Figures of the accompanying drawings.