GB2585338A

GB2585338A - Release actuator

Info

Publication number: GB2585338A
Application number: GB1904583.0A
Authority: GB
Inventors: Robert Cairns Paul; Charles Cairns Oliver
Original assignee: Mjr Controls Ltd
Current assignee: Mjr Controls Ltd
Priority date: 2019-04-02
Filing date: 2019-04-02
Publication date: 2021-01-13
Anticipated expiration: 2039-04-02
Also published as: GB2585338B; GB201904583D0

Abstract

A release actuator for a safety shut down system is described comprising: a stator 2 including a coil 3 which can be energised to generate a magnetic field; an armature 1 including an armature formation disposed to be movable relative to the stator between a first position in proximity to the stator whereat it may be acted upon by the said magnetic field and a second position spaced apart from the stator; secondary urging means 4 disposed to act on the armature formation to urge the armature formation towards the second position; a latch mechanism selectively operable to lock the armature formation in the first position; an electrically operated latch release mechanism operable in an energised state to release the latch mechanism. A safety system incorporating a release actuator as described is also disclosed. Such a safety system may be applicable to Liquid Natural Gas (LNG) cargo transfer and bunkering for ship to ship, ship to shore and shore to ship transfer.

Description

RELEASE ACTUATOR

The invention relates to a release actuator for a safety system that actuates to put in a safe mode a potentially hazardous processes, machinery or system. The invention in particular relates to a release actuator that is configurable so as to operate in "energise to trip" mode. The invention in particular relates to a release actuator that is configurable so that it can be proof tested while in operation.

Introduction

Safety shut down systems play a critical role in ensuring the functional safety of potentially hazardous processes, machinery and systems. Such systems often include a final element actuator element, for example including or causing to operate a valve, brake, electrical circuit breaker or other operator that is responsible for taking the process, machine or system to a so called 'safe state' in order to prevent a catastrophic situation developing. Some examples of such situations include: injury to people; dangerous release of energy; uncontrolled release of hazardous or polluting substances; mechanical run away of loads; mechanical or structural failure and explosion or fire.

A typical simple application of such an actuator is in the provision of a pressure relief valve that opens to prevent overpressure or a safety or quick closing valve that closes to contain hazardous substances or to prevent an uncontrolled release. In the case of quick closing safety valves, closing is typically by an actuation mechanism that operates by release of mechanical energy stored in springs or by pneumatic or hydraulic energy stored in accumulators. Typically, applied power is used to retain the valve open against this stored energy, and when the power to the device is removed, the stored energy is released and the valve is closed. In other words, the device operates according to the 'de-energise to trip' failsafe philosophy.

As the opening action in these simple actuators is caused by release of stored mechanical energy once the power is removed, these actuators have a true 'failsafe' state in this system when power is off. There are also applications where the failsafe condition is to open the valve. However, it is more usually desirable to operate on a de-energise to trip' failsafe principle whereby when power to the device is removed, stored energy is released and the valve is opened.

In other safety applications, for example Liquid Natural Gas (LNG) Cargo Transfer and Bunkering for Ship to Ship, Ship to Shore and Shore to Ship transfer, an Emergency Release Coupling (ERC) is fitted in the transfer hose line to provide the critical safety function in a similar way to the quick closing safety valve described above. In a Ship to Shore LNG transfer application an unexpected increase in the excursion between the ship and the jetty can result in potential tightening of the transfer hoses and unexpected stress, parting or failure of the transfer lines. This is a highly hazardous situation and could potentially result in an unplanned and explosive release of LNG if measures were not taken to protect against it. The safety monitoring system and ERC protects against this and if such a developing situation is detected by the safety monitoring system, the ERC mechanically releases the transfer hoses and at the same time automatically closes off the flow of LNG to prevent spillage or release into the atmosphere. Conversely a spurious release of the ERC must not occur as this is not without hazards of its own as physically breaking the flow lines accidentally whilst transferring LNG is also inherently dangerous.

However, since the release system is also powered in such an example there is no true 'failsafe' state in this system and the usual 'de-energise to trip' failsafe philosophy of a quick closing safety valve cannot be used as if the power supply fails to the release system, and the safe condition is not necessarily to release the coupling and close its internal valve.

In order to realise a safety system that fulfils the required performance criteria and to guard against spurious coupling release it is necessary to use an 'energise to trip' approach rather than the more usual 'de-energise to trip' failsafe philosophy. This brings with it further problems as it is then necessary to regularly 'prove' that the safety system, including the final element (coupling release actuator) is fully operational and will release when ordered to do so.

For systems that require a high Safety Integrity Level (SIL) for example SIL2 rated systems, proving of the system is generally carried out by means of an automatic 'proof' test prior to a live transfer being carried out. Typical systems of this nature are either hydraulic or pneumatically operated, with the mechanical release of the coupling being actuated by a single hydraulic or pneumatic cylinder acting on the coupling release collar. Taking the hydraulic system as an example, the proof test generally comprises operation of the hydraulic system (valves, pumps, accumulators etc) in a predefined automatic test sequence whilst taking feedback readings from instrumentation (pressure transmitters, flow transmitters, temperature sensors etc). These readings are then compared with reference or 'fingerprint' data from a known good test to provide a high degree of confidence that the system is fully operational and that when a release is ordered it will happen.

This approach is known but has several limitations and disadvantages.

For example, it may not be possible to proof test in practice without actuating into the safe mode. Taking the Emergency Release Coupling (ERC) example as an illustration, it should be noted that when the transfer hoses are coupled and the ERC is connected the proof test cannot actually test the final element actuator, otherwise the coupling would release.

High integrity systems of the type described are inherently complex as they must be continuously available and therefore include high levels of redundancy. To achieve this level of safety performance in a hydraulic system for example, requires additional hardware such that failure of single elements i.e. hydraulic valves, does not affect the ability of the overall system to carry out a coupling release when required. Also the coupling must not release spuriously in the event of a component failure. As the complexity of the hardware increases to achieve this level of performance, so does the complexity of the proof testing required to ensure that, with as high confidence as possible, the system will perform as required when needed. This results in a highly complex hydraulic and control system.

A further complicating factor is that the final element in the system that physically actuates the release collar of the coupling is, in the example of a hydraulic system, generally a single hydraulic cylinder. This can be considered as a significant single point of failure in the system and greatly degrades the overall safety performance. Whilst investing heavily in terms of complexity and cost to ensure that the control and hydraulic system is continuously available, it is this single non redundant final element that contributes most to the Safety Integrity Level calculation and degrades the overall functional safety performance of the system.

Increasing the complexity of the control and hydraulic system inherently leads to increased costs. It also follows that the size of the system increases as the complexity increases. In applications where space is at a premium, such as on board ships where space is limited this is a distinct disadvantage.

In summary: the complexity, cost and size of the traditional electro-hydraulic solution is certainly not ideal for installation min all cases, especially where space is limited such as on board ships and it would be of great commercial and practical benefit to reduce the complexity of the system whilst maintaining or improving overall functional safety performance.

All electric safety valve actuators are known for other applications and generally comprise a mechanical release spring that is charged using an electrically driven spring charging mechanism with a small electric motor and ratchet system that compresses the spring. The principle is for example found on traditional air circuit breakers for protection of electrical distribution circuits. Once the spring is charged (compressed or extended) it is latched in its 'charged' state with a 'trigger' or 'trip' mechanism that is actuated via an electrical solenoid. When the actuator is required to operate, the release solenoid is operated (may be de-energise or energise to trip), releasing the stored energy in the spring to close or open the valve or circuit breaker.

A similar 'standard' spring release mechanism is not practicable for use in the large scale industrial systems envisaged for the invention, such as for example in the LNG release coupling application discussed above, for example because: * There is no simple way to carry out a proof test to verify the integrity of the actuator, other than to actually trip it.

* The single spring approach is similar to the existing single hydraulic cylinder approach and is a single point of failure * The requirement of the system to operate at very low temperatures could result in spring material brittle failure. A means of heating is also required to prevent 'icing' on the ERG release collar that would prevent the coupling actually parting -currently done by circulating heated oil through the hydraulic release cylinder * A mechanical trip mechanism could be prone to wear.

The invention seeks to overcome or mitigate one or more of the above disadvantages of conventional actuators, especially those based at least partly on hydraulic and/ or pneumatic systems.

The invention seeks to provide an electrically operated release actuator alternative to such systems. The invention in particular preferably seeks to provide an electrically operated release actuator that seeks to overcome or mitigate one or more of the above disadvantages of such actuators.

The invention in particular preferably seeks to provide a release actuator that is configurable so as to operate in "energise to trip" mode. The invention in particular preferably seeks to provide a release actuator that is configurable so that it can be proof tested while in operation.

Summary of Invention

In accordance with the invention in a first aspect there is provided: a release actuator for a safety shut down system comprising: a stator including a coil energisable to generate a magnetic field; an armature including an armature formation disposed to be movable relative to the stator between a first position in proximity to the stator whereat it may be acted upon by the said magnetic field and a second position spaced apart from the stator; secondary urging means disposed to act on the armature formation to urge the armature formation towards the second position; a latch mechanism selectively operable to lock the armature formation in the first position; an electrically operated latch release mechanism operable in an energised state to release the latch mechanism.

The stator and armature formation are thereby configured and relatively disposed such that when the coil is energised and de-energised with the latch mechanism unlocked, the actuator portion may be caused to move between the first position and the second position.

The armature formation may comprise or may be provided in association with and to act upon and move an actuation formation which is caused to be switchable between a first actuation state when the armature formation is in the first position and a second actuation state when the armature formation is in the second position.

One of the actuation states corresponds in use to an operational state and one of the actuation states corresponds in use to a safe mode. For example, the second actuation state corresponds in use to a safe mode.

Thus, the release actuator in accordance with the invention may be provided as an actuator for a safety system and in a more complete aspect of the invention a safety system comprises a release actuator in accordance with the first aspect of invention provided with a safety mechanism comprising a safe mode mechanism wherein the release actuator is operably coupled to the safe mode mechanism so as to cause the safe mode mechanism to be operated when the armature formation is in the second position.

The safe mode mechanism is any mechanism that operates to cause the system to be moved from an operational mode to a safe mode. Familiar examples include valves, which may be closure valves designed to close in a safe mode or release valves designed to open in a safe mode, braking systems, decoupling systems and the like, and may include the Emergency Release Coupling (ERC) example discussed above.

The stator and armature formation are configured and relatively disposed such as to function as a traditional 'spring-set' de-energise to apply actuator, such as may be known for instance in emergency braking systems. When the coil is electrically energised, the magnetic field set up ads on the armature potion to overcome the force applied by the secondary urging means and to hold the armature formation in the first position. When the coil is de-energised the secondary urging means acts on the armature formation to move the armature formation in such manner that the actuator portion is urged to move to a second position, and it will do so unless the latch mechanism is engaged to lock the armature formation in the first position.

Such an arrangement alone has limitations however, including: that the system is 'de-energise to trip' which does not fulfil the requirement in applications that require 'energise to trip' and could easily result in spurious release; that the device cannot be proof tested whilst it is in operation (similar to the disadvantage seen with the traditional existing hydraulic actuator).

In order to address these issues, the invention further includes a safety latch with a secondary electrically operated and for example solenoid operated 'energise to trip' latch release mechanism that prevents the armature releasing when the main coil is de-energised.

Release is a two-step process as follows: a. Remove the mechanical safety latch by energisation of the latch solenoid or other latch release mechanism; b. De-energise the main coil Overall release performance is therefore 'energise to trip'.

The addition of the 'energise to trip' latch release mechanism in addition to the holding coil enables the actuator to be proof tested whilst it is in operation. This offers significant advantages over the currently available systems.

In normal operation the latch release mechanism is released (energised) prior to de-energising the main coil therefore the latch mechanism is not subjected to any mechanical loading, that is it is operated unloaded so are not subject to wear.

However, on power supply failure to the main coil or failure of the main coil where the latch mechanism is mechanically restraining the armature, the latch mechanism may be configured to release with full load meaning that a release can still be produced if necessary.

Preferably alternatively, the power supplies to the main coil and latch release mechanism could be separate, with each power supply having a backup so that there is no single point of failure. Failure of the power supply to the main coil will not affect the power supply to the latch release mechanism and therefore a release can still be achieved by energising latch release mechanism.

In simple prior art devises the mechanical travel is typically a only few mm as a small air gap between the armature formation and coil body is required to enable the armature to be pulled into the coil body by the magnetic field produced by coil energisation. For some applications envisaged for the invention a greater travel for the armature formation.

In a preferred case therefore the release actuator may further comprise a secondary charging mechanism disposed to move the armature formation from the second position towards the first position whereat it may be held by the energised coil prior to use. This allows a much greater travel between the armature formation and coil body to be accommodated which would be too great for the coil alone to move the armature formation from the second position. The charging formation brings the armature formation into proximity of the coil, sufficiently close so that the magnetic field produced by the coil can hold the armature in place.

This charging mechanism can be a small electrical charging motor and/or a manual charging mechanism or air/oil pressure mechanism. The charging mechanism is necessary to bring the armature towards the coil body against the spring force.

The armature formation is disposed to be movable relative to the stator between a first position in proximity to the stator whereat it may be acted upon by the said magnetic field and a second position spaced apart from the stator.

In a convenient arrangement the armature formation has a planar face, the stator has a planar face, and the first position comprises a position with the armature formation in close face to face proximity to the stator.

Conveniently the armature formation is an armature plate.

Conveniently the stator is an annular stator, and the armature comprises an armature plate in a plane that sits parallel to the plane of the annular stator and a perpendicular projection that passes through the central aperture in the annular stator.

Preferably, at least the armature formation of the armature comprises a ferrous material. More preferably, the armature comprises a ferrous material.

Preferably, the stator comprises a ferrous stator body.

The secondary urging means is disposed to act on the armature formation to urge the armature formation towards the second position. Conveniently, the secondary urging means is disposed between the armature formation and the stator. In the case where the armature formation has a planar face, and the stator has a planar face, the secondary urging means is disposed between the respective planar faces.

Preferably the secondary urging means comprises at least one spring. Preferably the secondary urging means comprises a plurality of springs. Preferably, the spring(s) are positioned to be compressed as the armature formation moves towards the first position thereby to tend to urge the armature formation towards the second position.

Preferably, the spring(s) are helical springs.

Preferably, the latch mechanism comprises at least one mechanical latch. More preferably, the latch mechanism comprises at least a pair of mechanical latches.

Preferably, the electrically operated latch release mechanism operable in an energised state to release the latch mechanism is a selectively energisable solenoid.

Preferably, the electrically operated latch release mechanism has a separate electrical supply from that of the coil. That is to say, the coil is provided with a first supply of electrical power, and the electrically operated latch release is provided with a second supply of electrical power independent from the first supply of electrical power. This condition may be met if they have the same primary electrical supply, for example both having as their primary supply the said first supply of electrical power, but at least the electrically operated latch release mechanism has a separate backup or redundant supply independent from the primary supply of electrical power.

Brief Description of Drawings

The invention and its operation will now be described by way of example only with reference to the accompanying drawings, in which: Figure 1 is a simplified schematic diagram a release actuator in accordance with an embodiment of the invention; Figure 2 illustrates the method of charging the actuator of figure 1 without the power applied; Figure 3 illustrates the method of charging the actuator of figure 1 with the power applied; Figure 4 shows the actuator charged but unlatched; Figure 5 shows the actuator charged and latched; Figure 6 illustrates a method of proof testing.

Detailed Description

Figures 1 to 6 show a simplified schematic diagram a release actuator in accordance with an embodiment of the invention in various stages of configuration, to illustrate its operation in an example mode.

Figure 1 shows the basic elements of the release actuator of the example embodiment.

A ferrous armature (1) comprising a circular plate and actuator rod assembly moves inside a ferrous stator (2) that contains an electrical coil (3), which when a source of electrical direct current is applied, forms an electromagnet.

The circular plate of the armature (1) is held away from the circular face of the stator (2) by a plurality of mechanical springs (4) arranged in one or more circular arrays around the stator (2). The springs (4), once compressed provide the means of storing the mechanical energy required to operate the actuator and release the LNG coupling.

An example mode of preparation for operation is described with reference to figures 2 to 5.

Charging the Device Unlike a normal solenoid, with the large air gap (6) between the circular plate of the armature (1) and the circular face of the stator (2) fully developed (for example 40 mm) as shown in Figure 1, simply applying a source of electrical power to the electrical coil (3) is not sufficient to produce enough force to overcome the spring (4) force and attract the circular plate of the armature (1) towards the circular face of the stator (2).

Therefore, in order for the electromagnet to be able to hold the armature (1) at the circular face of the stator (2) the device must be 'charged' mechanically by means of application of an external mechanical force (7) to the circular plate of the armature (1) to move it towards the circular face of the stator (2). This can be provided by an electric motor and lead screw arrangement, an electro-hydraulic or manually pumped hydraulic cylinder or a mechanical ratchet type charging device, direct oil pressure or air pressure. Figure 2 shows this charging force represented by (7).

A source of electrical direct current (8) can be applied to the coil (3) either after the device has been charged (Figure 2) or before the device is charged (Figure 3). When charging the device without power applied, the armature disc (1) must be held in place by the mechanical charging force (7) until a source of electrical direct current (8) power applied to the coil (3) and the magnetic field is established.

When charging the device with power applied (Figure 3), once the armature plate (1) has reached the stator (2) it will be held by the magnetic field and the mechanical charging force (7) can be released immediately.

Figure 4 shows the device charged with the springs (4) fully compressed and the armature plate (1) held at the stator (2) by the electromagnet formed by the coil (3). Once charged the force (7) can be removed.

Latching & Releasing the Device The drawback of using only the coil (3) to hold the armature plate (1) in place is that it makes the device tle-energise to trip' and therefore vulnerable to accidental release of either the power supply is lost to the main coil or the coil or wiring to the coil fails. This is not acceptable.

The addition of mechanical latches (5) to the device that are spring set (latch) and energise to release (unlatch) addresses this issue. Unless the latch solenoids (not shown) are energised the actuator cannot release even if the power supply (8) to the main coil is lost or the coil (3) or wiring to the coil (3) fails. Release is a two stage process where firstly the latches (5) are released followed by de-energisation of the coil (3). Operation is similar to the 'safety' on a firearm trigger mechanism.

Figure 5 shows the device charged and held on the solenoid coil (3) and the latches (5) spring set i.e. de-energised. In normal operation with the device in this condition there is a small air gap between the armature disc (1) and the latches (5) and therefore the latches (5) make no mechanical contact with the armature disc (1). To release the actuator, the latches (5) are first withdrawn by energising their solenoids (not shown) so that they are clear of the armature disc (1) followed by de-energisation of the coil (3).

This method results in no mechanical wear or load on the latches during normal release.

If the coil (3) or its power supply (8) fails with the latches in place, the armature disc (1) comes to rest against the latches (5) which restrain it from further movement and prevent release. Should the device then require release the latches (5) can be retracted under full load to release the armature.

Such an arrangement fulfils the 'energise to release' philosophy and results in no mechanical wear during normal release. The coil (3) also provides a second 'heating' function that both prevents icing of the ERC release collar and also heats the mechanical parts of the device, including the springs, to ensure that they do not fail in operation due to the potentially low temperatures produced by cryogenic nature of the LNG transfer process.

Multiple (for example 3) latches provide good mechanical redundancy preventing unwanted release, but are disadvantageous as all latches are require to retract to allow a release to occur. Precautions are therefore necessary to ensure that all latches retract when required. Double solenoid coils (not shown) can be provided on each latch mechanism with the latch being able to release under full load using only one of the solenoids. Alternatively, the latch assembly can be located in a sliding block mount with a secondary solenoid operated release pin (not shown) that unlocks the slide if the latch does not release allowing the entire latch assembly to slide upwards and out of the way of the armature. Using this approach, a release can always be guaranteed.

On Line Proof Testing Figure 6 illustrates a method of proof testing the actuator on line. One of main benefits of the mechanical latches (5) in this configuration and unlike any system currently available is the ability to carry out a proof test of the device whilst it is operating but without instigating a release.

Maintaining mechanical, air or oil pressure used during the charging process in place of the mechanical latches could be used more simply to prevent accidental release if the power supply (8) is lost or the coil (3) or wiring to the coil (3) fails. However, on line proof testing is not possible with this simplified arrangement.

Continuous Monitoring the Latch Integrity (Proof Testing of Latches) If more than one latch (5) is used and the latches are designed such that n-1 latches (where n = total number of latches) can withstand the full load of the charged device should the coil (3) or its power supply (8) fail, then when the device is charged and in operation, each latch in turn can be energised, withdrawn, re-energised and reinserted by the control system (9) and its position monitored by means of a proximity switch or sensor (10). This can be used to prove latch operational integrity whilst the device is in operation.

Continuous Monitoring of Coil and Power Supply Integrity With the device charged and latched the current of the coil (3) can be continuously monitored by the control system (9) to verify the condition of the coil. Temperature can also be monitored (not shown).

Continuous Monitoring of Spring and Armature Integrity (Proof Testing of Spring/Armature) Monitoring the current profile of the coil (3) during coil energisation and pull in of the armature to the stator against a known good 'signature' can provide valuable information about the spring force and armature movement. This is already known in electromagnetic brake control where active current monitoring can be used to verify the spring condition and air gap.

With the device charged and a small air cap between the engaged latches (5) and the armature disc face, the coil (3) can be de-energised by the control system (9) with the device in operation and with the armature held by the latches (5). When the coil is de-energised the armature disc leaves the face of the stator and is restrained from further movement by the latches (5). The air gap that previously existed between the latches (5) and armature disc (1) is closed and reappears between the armature disc (1) and stator (2). Providing that the air gap is sufficiently small (< lmm) and there is a small amount of backlash provided between the actuator rod and the release collar this movement is permissible and does not put force onto the ERC release collar.

When the coil (3) is subsequently re-energised the very small air gap makes it possible for the armature disc (1) to be pulled towards the stator (2) to close the air gap. The current profile during pull in can then be monitored and compared by the control system (9) to a known good reference curve which provides valuable information about the pull in forces required, the armature position and the time required to pull the armature (1) back to the stator (2). This current is high during initial energisation with the air gap present between the armature disc (1) and stator (2) (depending on the force that needs to be overcome and the air gap) and decays as the air gap is closed and reduces to a low level once the air gap is fully closed. With a uniform airgap and assuming that there is no additional force on the actuator rod that is attached to the armature disc (1), the pull in current will be dependent entirely on the spring force. Therefore, the pull in current profile is a good indicator of the integrity of the springs.

A proximity switch or sensor is also embedded in the stator (2) that physically monitors the position of the armature disc (1) relative to the stator (2). This is used to physically verify that the armature moves during pull out/pull in and together with the reduction in current when the armature disc (1) reaches the stator (2) proves that the armature has once more moved back to the stator.

The ability to carry out this type of proof testing with the device in operation brings significant advantages over the existing solutions in the market where this is not possible.

Claims

CLAIMS1. A release actuator for a safety shut down system comprising: a stator including a coil energisable to generate a magnetic field; an armature including an armature formation disposed to be movable relative to the stator between a first position in proximity to the stator whereat it may be acted upon by the said magnetic field and a second position spaced apart from the stator; secondary urging means disposed to act on the armature formation to urge the armature formation towards the second position; a latch mechanism selectively operable to lock the armature formation in the first position; an electrically operated latch release mechanism operable in an energised state to release the latch mechanism.
2. A release actuator in accordance with claim 1 wherein the armature formation comprises or is provided in association with and to act upon and move an actuation formation which is caused to be switchable between a first actuation state when the armature formation is in the first position and a second actuation state when the armature formation is in the second position.
3. A release actuator in accordance with claim 2 wherein the first actuation state corresponds in use to an operational state the second actuation state corresponds in use to a safe mode.
4. A safety system comprising a release actuator in accordance with any preceding claim provided with a safety mechanism comprising a safe mode mechanism wherein the release actuator is operably coupled to the safe mode mechanism so as to cause the safe mode mechanism to be operated when the armature formation is in the second position.
5. A safety system in accordance with claim 4 wherein the safe mode mechanism is selected from one or more of: a closure valve configured to close in a safe mode; a release valve configured to open in a safe mode; a braking system; a decoupling system; and combinations thereof 6. 7. 8. 9. 10. 11. 12.
A safety system in accordance with claim 5 wherein the safe mode mechanism is an emergency release coupling.
A release actuator in accordance with one of claims 1 to 3 or a safety system in accordance with one of claims 4 to 6 wherein the release actuator further comprises a secondary charging mechanism disposed to move the armature formation from the second position towards the first position whereat it may be held by the energised coil prior.
A release actuator in accordance with one of claims 1 to 3 or 7 or a safety system in accordance with one of claims 4 to 7 wherein the secondary charging mechanism is selected from an electrical charging motor, a manual charging mechanism, an air/oil pressure mechanism or combinations thereof.
A release actuator in accordance with one of claims 1 to 3 or 7 to 8 or a safety system in accordance with one of claims 4 to 8 wherein the armature formation has a planar face, the stator has a planar face, and the first position comprises a position with the armature formation in close face to face proximity to the stator.
A release actuator in accordance with one of claims 1 to 3 or 7 to 9 or a safety system in accordance with one of claims 4 to # wherein the armature formation is an armature plate.
A release actuator in accordance with one of claims 1 to 3 or 7 to 10 or a safety system in accordance with one of claims 4 to 10 wherein the stator is an annular stator, and the armature comprises an armature plate in a plane that sits parallel to the plane of the annular stator and a perpendicular projection that passes through the central aperture in the annular stator.
A release actuator in accordance with one of claims 1 to 3 or 7 to 11 or a safety system in accordance with one of claims 4 to 11 wherein at least the armature formation of the armature comprises a ferrous material. More preferably, the armature comprises a ferrous material.
13. A release actuator in accordance with one of claims 1 to 3 or 7 to 12 or a safety system in accordance with one of claims 4 to 12 wherein the stator comprises a ferrous stator body.
14. A release actuator in accordance with one of claims 1 to 3 or 7 to 13 or a safety system in accordance with one of claims 4 to 13 wherein the secondary urging means is disposed between the armature formation and the stator.
15. A release actuator in accordance with one of claims 1 to 3 or 7 to 14 or a safety system in accordance with one of claims 4 to 14 wherein the secondary urging means comprises at least one spring.
16. A release actuator or a safety system in accordance with claim 15 wherein the secondary urging means comprise a plurality of springs.
17. A release actuator or a safety system in accordance with claim 15 or claim 16 wherein the spring(s) are positioned to be compressed as the armature formation moves towards the first position thereby to tend to urge the armature formation towards the second position.
18. A release actuator in accordance with one of claims 1 to 3 or 7 to 17 or a safety system in accordance with one of claims 4 to 17 wherein the latch mechanism comprises a mechanical latch.
19. A release actuator or a safety system in accordance with claim 18 wherein the latch mechanism comprises a pair of mechanical latches.
20. A release actuator in accordance with one of claims 1 to 3 or 7 to 19 or a safety system in accordance with one of claims 4 to 19 wherein the electrically operated latch release mechanism operable in an energised state to release the latch mechanism is a selectively energisable solenoid.
21. A release actuator in accordance with one of claims 1 to 3 or 7 to 20 or a safety system in accordance with one of claims 4 to 20 wherein the electrically operated latch release mechanism has a separate electrical supply from that of the coil.