WO2008151369A1

WO2008151369A1 - Method and apparatus for the condition monitoring of structures

Info

Publication number: WO2008151369A1
Application number: PCT/AU2008/000840
Authority: WO
Inventors: Kenneth Davey
Original assignee: Structural Monitoring Systems Ltd
Current assignee: Structural Monitoring Systems Ltd
Priority date: 2007-06-12
Filing date: 2008-06-12
Publication date: 2008-12-18
Anticipated expiration: 2009-12-12

Abstract

An apparatus (10) for the condition monitoring of structures comprises a sensor cavity (12) formed on or in the structure being monitored, a pressure storage vessel (14) and a monitoring device (16). The sensor cavity (12), vessel (14) and monitoring device (16) are connected in series with each other to form a closed circuit. High fluid flow impedance (22) is connected in series between one end of the cavity (12) and the vessel (14) with an adjustable flow device (24) coupled in series between the impedance (22) and the vessel (14). The device (24) enables matching of volume to impedance ratios between a portion of the circuit comprising the cavity (12) and a portion comprising the vessel (14). A pressure adjustment device (28) is coupled between impedance (22) and the adjustable flow device (24) enabling a recharging or discharging of the cavity (12) and vessel (14) in a proportional or balanced manner so that the monitoring device (16) does not provide false condition monitoring signals.

Description

METHOD AND APPARATUS FOR THE CONDITION MONITORING OF

STRUCTURES

Field of the Invention

The present invention is based on a method and system for condition monitoring of structures and, in particular but not exclusively, to the condition monitoring of structures subjected to varying ambient pressures such as aircraft or submarines .

Background of the Invention

The present Applicant has been involved in the research and development of sensors and monitoring systems for condition monitoring of structures and in particular failure critical structures such as aircraft frames and other components, and load bearing metal and concrete structures. This research and development has led to various innovative systems and methods such as described in US Patent No . s : 5,770,794; US 6,539,776; US 6,591,661; US 6,715,365; and US 6,720,882.

Extensively, the systems and methods described in the above referenced US Patents are largely single -ended having a sensor cavity that has one end connected via a high fluid flow impedance to a pressure source which is different to (i.e. greater or lower than) ambient pressure, with an opposite end of the cavity terminating on or in the component or monitoring device or otherwise sealed to ambient pressure. A monitoring device is connected in parallel across the high fluid flow impedance. A miniscule flow of air through the impedance, which may arise from a crack penetrating the cavity, produces a pressure change across the impedance that can be monitored by the monitoring device to provide an indication of the condition of the component. The present invention arises from further research and development in the area of condition monitoring.

Summary of the Invention

According to the present invention, there is provided a condition monitoring system comprising:

a sensor cavity formed on or in a component or structure to be monitored; a pressure storage vessel containing fluid at a pressure different to ambient pressure; and, a monitoring device which provides an indication when a difference in fluid pressure between the cavity and the pressure vessel exceeds a threshold level;

wherein the sensor cavity, pressure vessel and monitoring device are connected together in series to form a closed fluidic circuit.

The system may further comprise a pressure adjustment port provided in the series circuit, the port located between the sensor cavity and the pressure vessel at a point where a ratio of volume to fluid flow impedance of a first portion of the circuit from the port to one side of the monitoring device, is substantially equal to a ratio of volume to fluid flow impedance of a second portion of the circuit from the port to an opposite side of the monitoring device.

The system may further comprise an adjustable flow device connected in series between the port and either the first portion of the circuit or the second portion of the circuit, the adjustable flow device configured to vary volume or impedance or both volume and impedance of the portion of the circuit in which the device is series connected to facilitate a matching of volume to impedance ratio with the other portion of the circuit.

In one form of the invention, the adjustable flow device may comprise an adjustable flow impedance such as a needle valve. However, in an alternate embodiment, the adjustment flow advice may comprise a conduit or container of adjustable volume.

In one embodiment the adjustable flow device is in series connection between the port and the pressure vessel .

The condition monitoring system may further comprise a high fluid flow impedance connected in series between the sensor cavity and the pressure vessel.

The system may further comprise a pressure adjustment source coupled to the port, the pressure adjustment source operable to vary fluid pressure within the circuit.

In one embodiment, the pressure adjustment source may comprise a vacuum pump or a vacuum vessel. However, in an alternative embodiment, the pressure adjustment source may comprise a positive pressure pump or a positive pressure vessel.

The pressure adjustment source may be operated continuously; or, periodically on the basis of a sensed change in pressure in the pressure vessel in comparison to ambient pressure.

According to a further aspect of the present invention, there is provided a method of condition monitoring comprising :

forming a sensor cavity on or in a component or structure to be condition monitored; connecting opposite ends of the sensor cavity to be in fluid pressure communication with a pressure vessel and a monitoring device to form a closed series circuit; and, operating the monitoring device to provide an indication when a difference in fluid pressure between the sensor cavity and the pressure vessel exceeds a threshold level.

The method may further comprise adjusting fluid pressure within the circuit by connection of a pressure adjustment device at a port in the circuit, the port being at a location where a ratio of volume to impedance of a first portion of the circuit from the port to one side of the monitoring device is substantially equal to a ratio of volume to impedance of a second portion of the circuit from the port to an opposite side of the monitoring device.

That is the method may comprise balancing a volume to impedance ratio of a first portion of the circuit containing the sensor cavity with a volume to impedance ratio of a second portion of the circuit containing the pressure vessel and coupling a pressure adjustment device to a port in the circuit located at a junction of the first and second portions of the circuit.

In one embodiment the method comprises operating the pressure adjustment device continuously. However, in an alternate embodiment, the pressure adjustment device is operable periodically to maintain a pressure differential between fluid pressure in the vessel and ambient pressure.

Brief Description of the Drawings

Embodiment of the present invention will now be described by way of an example only with reference to the covering drawings in which: Fig 1. is a schematic representation of a condition monitoring system in accordance with the present invention in a steady pressure state;

Fig 2. is a schematic representation of the system depicted in Figure 1 in a changing pressure state.

Fig 3. is a schematic representation of the system shown in Figures 1 and 2 depicting the fluid flow in the system in the event of a crack or flaw developing in a component or structure being monitored by the system;

Fig 4 is a schematic representation showing fluid flow in an embodiment of the system depicted in Figures 1 and 2 in the event of condensate being drawn into a sensor cavity of the system via a crack or flaw formed in a component or structure being monitored;

Fig 5. is a schematic representation of second embodiment of the system;

Fig 6 is a schematic representation of a third embodiment of the system; and

Fig 7 is a schematic representation of a fourth embodiment of the system comprising a combination of the second and third embodiments depicted in Figures 5 and 6 respectively .

Description of Preferred Embodiments

Referring to the accompanying drawings and in particular Figure 1, an embodiment of a condition monitoring system 10 comprises a sensor cavity 12, a pressure storage vessel 14, and a monitoring device 16. The sensor cavity 12, vessel 14 , and monitoring device 16 are connected in series with each other to form a closed circuit. This circuit is closed to the extent that opposite ends 18 and 20 of the sensor cavity 12 are in fluidic pressure communication with the vessel 14. That is end 18 is in fluid pressure communication with the vessel 14 via a high fluid flow impedance 22, while end 20 is in fluid pressure communication with the vessel 14 via the monitoring device 16.

The sensor cavity 12 is formed on or in a component or structure being the subject of condition monitoring. The monitoring device 16 is in fluid communication between end 20 of the sensor 12 and the vessel 14, and provides a signal when the difference in fluid pressure between the sensor cavity 12 and the storage vessel 14 exceeds the threshold level. As explained in greater detail, this may occur in the event of a crack or flaw being formed in the component or structure which penetrates the sensor cavity 12; or, a blockage in the sensor cavity 12.

The system 10 further comprises an adjustable flow device 24 connected in series between the high fluid flow impedance 22 and the vessel 14. A pressure adjustment port 26 is provided in the fluidic circuit between the impedance 22 and the pressure adjustment device 24. A pressure adjustment source 28 is coupled to the port 26 via a conduit 30.

To assist in the understanding of the operation of the system 10, assume for example that the vessel 14 contains air at say -20KPa relative to ambient pressure (i.e. a relative vacuum) , and the sensor cavity 12 is in the form of a manifold adhered to the surface a component or structure being monitored. Also assume at this time that there is no crack or flaw in the component or structure that intersects or otherwise provides a fluid flow path to the sensor cavity 12. Due to natural permeability of materials, and the provision of imperfect joints between adverse components of the system 10, there is likely to be a miniscυle flow of fluid into the cavity 12, through the impedance 22 and the pressure adjustment device 24 to the vessel 14. The monitoring device 16 which may be in the form of a differential pressure switch is set to close when a pressure difference between the sensor cavity 12 and the vessel 14 is greater than the pressure difference caused by the permeable air flow into the sensor cavity 12. In a steady pressure state depicted in Figure 1, where there is no flaw or crack providing the fluid flow path to the sensor cavity 12, the pressure differential caused by the permeable air flow, represented by the small black arrows, does not exceed a threshold level of the monitoring device 16 and thus is the monitoring device 16 does not provide any signal indicative of this threshold level being exceeded.

However, the reference to Figure 3, in the event of a crack 32 breaching the sensor cavity 12 there will be an increase in the flow of air into the sensor cavity 12 and thus an increase in the pressure differential on opposite sides of the monitoring device 16. In the event that this increased pressure differential exceeds the threshold level, the monitoring device 16 sends a signal indicative of the presence of the crack 32.

The monitoring device 16 being represented as a differential pressure switch comprises a body 34 divided into two cavities 36 and 38 by a diaphragm 40. The cavity 36 is a fluid communication with end 20 of the sensor 12 via a duct or tube 42. The cavity 38 of the monitoring device 16 is in fluid communication with the source 14 via a duct or tube 44.

An inflow of air into the sensor cavity 12 via the crack 32 provides an increase in fluid pressure in the duct 42 which is transmitted to the cavity 36 resulting in a bowing or deformation of the diaphragm 40 causing a subsequent increase in fluid pressure in the duct 44 which is transmitted to the vessel 14. The extent of defamation of the diaphragm 40 is dependent on the pressure differential between the cavity 12 and the vessel 14 caused by the inflow of air to the crack 22. This in turn is dependent on the size and period of opening of the crack 32. The monitoring device 16 may be adjusted to set the predetermined threshold pressure differential which, when exceeded, causes the monitoring device 16 to provide a signal indicative the presence of the crack 32.

It can be recognized from the above description that due to the natural permeability of materials and imperfect seals there is most likely to be a small steady flow of air into the cavity 12 which in time will deplete the vacuum in the vessel 14. In order to maintain the operational status of the system 10, it is thus required from time to time to re-establish appropriate vacuum levels within the vessel 14. This is achieved by operating the pressure adjustment device 28. However, simply coupling the vessel 14 to a vacuum pump to reestablish the vacuum level will induce additional flow of air through the impedance 22 causing a increased pressure differential across the monitoring device 16 which may exceed the threshold level causing the device 16 to provide a false indication of the existence of a crack or flaw.

Embodiments of the present invention avoid this by adjusting the pressure state of the vessel 14 and the sensor cavity 12 proportionately. This is achieved by balancing the volume to impedance ratio of two portions of the circuit on opposite sides of the port 26. Thus, there is a proportional flow of air to the device 28 from the vessel 14 and the sensor cavity 12 thereby maintaining any existing pressure differential between the sensor cavity 12 and vessel 14. Accordingly, the state of the monitoring device 16 is unaffected by the fluidic connection between the device 28 and the vessel 14 and sensor cavity 12.

This relationship is depicted in Figure 2 where the small black arrows depict low rate of air flow evacuated from the sensor cavity 12 into the high fluid flow impedance 22 and the duct 30 while the large black arrows depict a much higher rate of air flow from the vessel 14 through the adjustable flow device 24 and subsequently through the port 26 and duct 30 to the device 28. While there is a substantially different rate of flow of air through the sensor cavity 12 and the vessel 14, there is also a substantial difference in the impedance to these respective flows. By matching the ratio of volume to impedance of air flowing on opposite sides of the port 26, the pressure differential on opposite sides of the monitoring device 16 is maintained constant .

To achieve this balance in volume to impedance ratio the adjustor flow device 24 can be configured to enable either an adjustment in: impedance between the vessel 14 and the port 26; the volume between the vessel 14 and the port 26; or, both the impedance and the volume for the portion of the circuit between the vessel 14 and the port 26.

For example, the adjustable flow device 24 is illustrated in the Figures as comprising a duct 46 in which is provided an adjustable needle valve 48. By adjusting the position of the needle valve 48, the impedance in the duct 46 can be adjusted so that the ratio of volume to impedance from opposites sides of the port 26 can be matched. Alternatively, the duct 46 may initially be provided as a relatively long length of large bore tubing and the length subsequently being adjusted, for example by cutting, to provide the required balance in ratio of volume to impedance on both sides of the port 26. Thus the port 26 may be considered as the junction between a first portion of the circuit having the sensor and a second portion of the circuit containing the vessel 14, with the adjustable flow device 24 being considered as a volume to impedance ratio matching or tuning system which enables matching the volume to impedance radio for the first and second portions to be matched,

The balancing of the volume to impedance ratios is most conveniently performed on initial installation and setup of the system 10. This balancing may be performed in two stages. Firstly, assuming the high fluid flow impedance 22 has a fixed impedance, the flow impedance of the duct 46 is determined by the minimum volume of the sensor cavity 12 that can be expected to be monitored in service.

Secondly, for larger volumes of sensor cavity 12, than the predetermined minimum, the needle valve 48 is used to further restrict the rate of flow of fluid from the vessel 14 to favour the larger volume of air that needs to be extracted from the larger sensor cavity 12 to restore the relative rates of flow required for balance. For example, when the system 10 is initially installed, the system 10 may be evacuated at a rapid rate by providing the pressure adjustment source 28 as a vacuum storage device and allowing fluid communication with the port 26 through the duct 30. The needle valve 48 is then adjusted until a minimal differential pressure excursion of the monitoring device 16 is achieved. The needle valve 48 is now fixed at the setting.

If the restriction to flow provided in the duct 46 by the needle valve 48 is insufficient, on subsequent application of the source 28 during use of the system 10 there would be a sudden rise in pressure differential which would then decay until becoming stable at a level commensurate with the normal permeability of the system 10. This peak, provided it is above the threshold differential level, will cause a false reading.

If the impedance of the duct 46 is excessive, the differential pressure will fall suddenly to provide a negative peak and subsequently rise to stabilize at the level commensurate with the normal permeability of the system 10.

The significance of the balanced pressure adjustment of the system 10 becomes apparent when one considers the task of maintaining the vessel 14 at a constant vacuum level relative to ambient pressure conditions of an aircraft that is rapidly climbing or diving. During the climb, additional air must be removed from the vessel 14 and during descent, air must be admitted. High airframe loads can be experienced, especially during a dive (and turbulence) , and the first opportunity to detect an opening crack would present itself. Under these conditions, a stable relationship between the pressures within sensor cavity 12 and vessel 14 would be vital.

Testing has shown that a sub-millimetre crack can be detected during a climb at the rate of over 30,000ft/ minute and descent of 60,000ft/ minute using the vessel 14 as a constant vacuum source. This performance is well beyond requirements . The constant vacuum source for the test was derived from a small side channel blower type vacuum pump with an electronic controller.

To maintain the vessel 14 at a constant vacuum relative to ambient pressure (i.e. to act as a constant vacuum source) the device 28 is operated when necessary to either evacuate the vessel 14 (when altitude increases) or relieve vacuum from the vessel 14 (when altituide decreases) . This may be achieved by the provision of a controller which upon detecting a variation between the pressure level in the vessel 14 and ambient pressure being greater than a threshold difference operates the device 28 to readjust the pressure differential between the vessel 14 and the ambient pressure to the required range .

However, in an alternate embodiment, rather than operating the source 28 periodically as required to maintain the constant pressure differential between the vessel 14 and ambient pressure, the device 28 maybe run continuously. This provides an unregulated vacuum in the vessel 14 as the pressure differential between the vessel 14 ambient pressure will change with altitude of the aircraft. Nevertheless, the stable relationship between the sensor cavity 12 and the vessel 14 is maintained due to the balancing of the volume to impedance ratios on opposite sides of the port 26. In this regard, a test using a small peristaltic vacuum pump running continuously at its ultimate performance of about -98 KPa was found to detect sub millimetre cracks at the same rate of altitude change as described above .

With the unregulated system, a change in overall sensitivity to a crack would occur, however, sensitivity would be highest at lower altitude where most turbulence is experienced by the aircraft. This is beneficial because turbulence is most likely to create cyclic loads causing cracks to subsequently open and close. As the cracks open, there is an increased flow of fluid into the sensor cavity 12. In the event that the flow is sufficient to cause the pressure differential between the vessel 14 and cavity 12 greater than the pressure differential threshold setting of the monitoring device 16, the crack will be detected. Sensitivity is most significant when a crack closes than when, or as it opens. A closed crack creates the same structural weakness as a an open crack but due to restricted air flow requires greater instrument/system sensitivity to detect.

In the event that a crack becomes larger during the cyclic loading, the length of time at which the pressure differential is exceeded is likely to increase with each opening of the crack. The monitoring device 16 may comprise or be coupled to a signal integration means, for example an electronic counter which runs when the threshold level is exceeded, but stops when the pressure differential is below threshold level. This provides an effective automatic device for continuous unattended operation where a crack is detected by the integration of time rather than as single alarm signals on each occurrence of a crack opening to the extent that it causes a pressure differential greater than the threshold level. ^•

Referring to Figure 4, the affect of moisture or contaminant ingestion into the crack 32 breaking into sensor cavity 12 is shown, Moisture or contaminant 50 tends to block and mask the crack 32 when the crack 32 first appears but upon entering the sensor cavity 12 it quickly tends to block the cavity 12. A blockage in sensor cavity 12 deprives duct 42 of fluid or pressure communication with the vessel, and the influence of device 28. The effect of permeable flow though walls of duct 42, and a portion of sensor cavity 12 (notionally represented by the associated small black arrows) produces a rise in pressure that is fluidly transmitted to differential pressure transducer device 16 resulting in the device 16 providing a signal falsely indicating that the crack 32 is sufficiently large to cause the pressure differential threshold of the device 16 to be exceeded.

Further, the signal produced by the device 16 will have different characteristics (e.g. different frequency response) than when there is a crack 32 with no ingestion of moisture or contaminant 50. This allows a blockage induced signal to be distinguished from a crack induced signal. Inducing additional perturbation of the pressure in vessel 14 by means of operation of the pump 28 according to a predetermined regime will induce an imbalance in the device 16 with characteristics sufficient to confirm the identity of the cause as a blockage or crack .

In order to understand how such a small amount of moisture can cause blockage, a typical sensor cavity 12 may comprise an elongate gallery of typically 250 micron width or less and have a height of perhaps 100 micron. The blocking effect of the moisture 50 is enhanced by surface tension.

Validation tests have shown that the ingestion of a few millimetres of water into the sensor cavity 12 can sufficiently block the sensor cavity 12 so as to influence the differential pressure transducer device 16 and draw attention to the presence of the flaw.

A further test to observe the masking effect on a crack of a 2mm sealing coat of Petroleum Jelly resulted in detection of the crack after about 100 cycles of the crack opening and closing under cyclic load and ingesting a small globule of the jelly.

Many cyclic loads can be encountered by an airframe when flying through turbulence and the resultant flexing of a crack with attendant surface tension forces, and the attractive internal low pressure promotes fluid entry in the crack.

The device 16 can be a differential pressure transducer connected by electrical conductors to an amplifier in the same manner as the cited patents, however, a differential pressure switch, set at a predetermined trigger level above expected permeable flow levels to give warning that a crack has been detected, provides simplicity as bulk and electrical interference is a consideration especially with high performance aircraft.

Due to the extremes of temperature, the device 16 could be subjected to temperatures at the tropopause typically to - 60 degrees Celsius. Accordingly, if a device 16 such as miniature switch rated for low temperature service having, for example, a Mylar diaphragm 40 is not available, or cannot be placed in a suitably warmed location on the structure, then a temperature controlled electrical heating jacket 60 surrounding the device 16 may be provided as shown in Figure 5.

Figure 5 schematically shows a practical layout of the system 10 where the device 16 comprises a differential pressure switch. The impedance 22, port 26, duct 46 and monitoring device 16 are housed inside a vessel 14a. The vessel 14a acting also as the vacuum source as per the vessel 14 in Figures 1-4. The vessel 14a may for example have a volume of 0.075 litre. The cavity 12 is coupled by conduit 42 to the device 16 via a port 54 on the vessel 14a, and a conduit 56 connects end 18 of conduit 12 to the impedance 22 via a port 58 on the vessel 14a.

The differential pressure switch 16 is surrounded by a heater 60. A wire S provides signal communication from the device 16, while wires V+ and V- provide power to the device 16 and the heater 60 from a power supply 62 (shown in Figure 7) . A temperature controlling means such as a thermistor and simple amplifier 64 is provided to maintain the device 16 above its minimum operating temperature (for example say -4 degrees Celsius) . A removable plug or sealing means 64 is shown to allow access for adjustment of device 16 to set the desired trigger level above the maximum expected permeability effect.

The needle valve 48 is sealed in the vessel 14a to allow convenient adjustment and is configured to restrict one end the duct 48. The other end of the duct 48 is connected to the port 26 which further connects both high impedance to fluid flow duct 22 and the duct 30, with the duct 30 connecting to pump 28. Pump 28 could be situated in a distant location on the aircraft or be replaced by some other pressure/ vacuum source .

Temporarily opening the port 64 and applying vacuum to the port 64, will upset the pressure balance of the system 10 and cause the monitoring device 16 to produce an adverse signal and therefore is a valuable feature to assure the integrity of the system 10. The duct 30 may be isolated during this step. This facility could be provided by a miniature change-over type solenoid activated valve (not shown) and controlled automatically at intervals for total system integrity checking.

When the vessel 14a can be located close to the sensor cavity 12, simple ducts 42 and 56 allow the connection. An advantage of this short connection is that the total volumetric capacity of the ducting associated with the sensing cavity 12 is minimal. This improves the frequency response of the system 10 and therefore increases sensitivity to the transient leakage pulses that would be apparent when a crack 32 under cyclic load first intercepts a sensing cavity 12.

Figure 6 shows a configuration of the system 10 where the sensor cavity 12 is remote from a vessel 14b. This configuration may be appropriate for example to avoid the vessel 14b from exposure to low temperature or when there is insufficient space for the vessel 14b. In these examples the heater 60 may not be required. Also depending on the electrical connection system it may be possible to dispense with wire V- .

In this configuration, the high impedance 22 is not contained within the vessel 14b, but is used to form at least part of the connecting duct 56 which connects with a simple connection duct 66 located inside vessel 14b. This arrangement helps improve frequency response as previously mentioned. Duct 42 can also be of minimal size as it only needs to carry minuscule flow sufficient to deflect the diaphragm of the device 16.

Figure 7 shows a schematic drawing of a system 10 comprising a combination of the systems 10 shown in

Figures 5 and 6 in a notional installation on an aircraft. A common pump 28 is connected via: a duct 30a to a vessel 14a, corresponding to vessel 14a of Figure 5; and, duct 30b to vessel 14b corresponding to vessel 14b of Figure 6. The power supply 62 is connected by conductor V+ to both vessels 14a and 14b. Signal wires Sa and Sb connect to vessels 14a and 14b respectively and to corresponding warning indicators 45 and 47. A signal return wire SR provides connection of the warning indicators 45 and 47 to the power supply 62 via conductor V- which is also connected to capsule 14a to allow internal warming of its enclosed differential pressure switch (not shown) .

The sensor cavity 12a is defined within a manifold 70 and its attachment to component 72 being condition monitored is representative of the situation where the vessel 14a is located proximate to a sensor cavity 12a similarly to the arrangement of Figure 5.

The sensor cavity 12b is defined within a manifold 74 attached to a component 76 being condition monitored and is representative of the situation where the capsule 14b is located remotely from a sensor cavity 12b similarly to the arrangement of Figure 6. The dotted outline 78 defines a warm zone protected from the low ambient temperatures experienced within an aircraft flying at high altitude.

Figure 7 is representative of a system 10 where the vessels 14a/14b provide a vacuum and the pump 28 is a vacuum pump. However, a relative positive pressure may be used rather than a vacuum if the sensor cavities 12a and 12b are in the form of cavities for in or between components of structure rather than formed on components of structure. For example the cavities may be in the form of cavities or galleries formed internally in a stringer or frame.

A plurality of further cavities 12, 12a, 12b could be monitored and arranged in series for assurance of continuity .

A simple electronic counter (not shown) may be employed and used as a recording means in conjunction with the indicators 45 and 47. Signals produced by the devices 16 produced by a crack opening up under transient loads in turbulence, which activate the indicators 45 and 47 can then be recorded. This arrangement together with a miniature vacuum pump, activated for a predetermined time by an inertia type of switch (such as advantageously mounted mercury switches or the like) during periods of turbulence or high "G" manoeuvres, allows a simple automatic periodic monitoring method that records the presence of a crack when the crack is first expected to be advantageously detectible and then gives follow up confirmation that the crack exists.

Additionally, the device 16 could be used to activate a solenoid actuated miniature indexing device for the monitoring of the propagation rate of a crack. The indexing device may be of the form described in Australian Patent No 2001254524, "SYSTEM AND METHOD FOR THE DETECTION AND PROPAGATION MEASUREMENTS OF FLAWS IN A COMPONENT OR STRUCTURE" , and in particular shown in Figure 8 of that specification.

In operation, the first detected differential pressure pulse (above a predetermined level of differential pressure) from the momentary opening of a crack under a transient load, would activate a solenoid and index a device to isolate the effected sensor cavity typically corresponding to the "channels" referred to in Figures 8, 9, and 10 of the Australian Patent No 2001254524 above. The indexing device would then remain dormant until the next said sensor cavity or "channel" was influenced by the advancing crack. Various additional signalling means and or direct observation of the indexing device could be used of course to indicate that the said indexing device is responding to an intercepted crack.

The impedance 22 utilised in embodiments for the system 10 may comprise a micro bore duct, for example a PE tube having a bore diameter of about 0.28 mm and a length in the order of 3 metres. However, the impedance 22 may also take other forms such as a permeable membrane, sintered glass, or any other means sufficient to produce a detectable pressure drop in response to permeable flow of fluid through the material defining or forming the sensor cavity 12, including a surface of a component on which the sensor cavity 12 is formed when the cavity is formed on a surface rather than inside a component or structure. The duct 30 may have an internal diameter of about 0.8 mm. When the system 10 is a vacuum system, i.e. the vessel 14 providing the pressure source 28 as a peristaltic pump provided a convenient mechanism for discharging and recharging vacuum levels within the vessel 14 as a peristaltic pump maybe reversed to either increase vacuum level or decrease vacuum level.

Modifications and variations to embodiments of the present invention which would be obvious to a person of ordinary skill in the art to be deemed to be within the scope of the present invention the nature of which is to be determined following the above description and appended claims .

Claims

1. A condition monitoring system comprising: a sensor cavity formed on or in a component or structure to be monitored; a pressure storage vessel containing fluid at a pressure different to ambient pressure; and, a monitoring device which provides an indication when a difference in fluid pressure between the cavity and the pressure vessel exceeds a threshold level; wherein the sensor cavity, pressure vessel and monitoring device are connected together in series to form a closed circuit.

2. The system according to claim 1 further comprising a pressure adjustment port or junction provided in the series circuit, the port or junction located between the sensor cavity and the pressure vessel at a point where a ratio of volume to fluid flow impedance of a first portion of the circuit from the port to one side of the monitoring device, is substantially equal to a ratio of volume to fluid flow impedance of a second portion of the circuit from the port to an opposite side of the monitoring device.

3. The system according to claim 2 further comprising an adjustable flow device connected in series between the port or junction and either the first portion of the circuit or the second portion of the circuit, the adjustable flow device configured to vary volume, or impedance, or both volume and impedance of the portion of the circuit in which the device is series connected to facilitate a matching of volume to impedance ratio with the other portion of the circuit.

4. The system according to claim 3, wherein the adjustable flow device comprises an adjustable fluid flow impedance.

5. The system according to claim 4 wherein the adjustable fluid flow impedance comprises a valve.

6. The system according to claim 3 wherein the adjustment flow advice comprises a conduit or container of adjustable volume.

7. The system according to any one of claims 3 to 6 wherein the adjustable flow device is in series connection between the port or junction and the pressure vessel .

8. The system according to any one of claims 1 to 7 further comprising a high fluid flow impedance connected in series between the sensor cavity and the pressure vessel.

9. The system according to any one of claims 2 to 8 further comprising a pressure adjustment source coupled to the port or junction, the pressure adjustment source operable to vary fluid pressure within the circuit.

10. The system according to claim 9 wherein the pressure adjustment source comprises a vacuum pump or a vacuum vessel .

11. The system according to claim 9 wherein the pressure adjustment source comprises a positive pressure pump or a positive pressure vessel .

12. The system according to any one of claims 9 to 11 wherein the pressure adjustment source is operated continuously.

13. The system according to any one of claims 9 to 11 wherein the pressure adjustment source is operated periodically on the basis of a sensed change in pressure in the pressure vessel in comparison to ambient pressure .

14. A condition monitoring system comprising: a sensor cavity formed on or in a component or structure to be monitored; a pressure storage vessel containing fluid at a pressure different to ambient; a high fluid flow impedance coupled in series between one end of the sensor cavity and the pressure vessel; and, a monitoring device coupled in series between an opposite end of the sensor cavity and the pressure vessel, wherein the monitoring device provides an indication when a difference in fluid pressure between the cavity and the pressure vessel exceeds a threshold level.

15. The system according to claim 14 further comprising an adjustable flow device connected in series between the impedance and the vessel, the adjustable flow device configured to facilitate a matching of a volume to impedance ratio of a first portion of the circuit containing the impedance and the sensor cavity with a volume to impedance ratio of a second portion of the circuit containing the adjustable flow device and the vessel .

16. The system according to claim 15 further comprising a port or junction between the adjustable flow device and the impedance; and, the pressure adjustment source coupled to the port or junction, the pressure adjustment source operable to vary fluid pressure within the circuit.

17. A method of condition monitoring comprising: forming a sensor cavity on or in a component or structure to be condition monitored; connecting opposite ends of the sensor cavity to be in fluid pressure communication with a pressure vessel and a monitoring device to form a closed series circuit; and, operating the monitoring device to provide an indication when a difference in fluid pressure between the sensor cavity and the pressure vessel exceeds a threshold level.

18. The method according to claim 17 further comprising adjusting fluid pressure within the circuit by connection of a pressure adjustment device at a port or junction in the circuit, the port or junction being at a location where a ratio of volume to impedance of a first portion of the circuit from the port or junction to one side of the monitoring device is substantially equal to a ratio of volume to impedance of a second portion of the circuit from the port to an opposite side of the monitoring device.

19. The method according to claim 17 comprising balancing a volume to impedance ratio of a first portion of the circuit containing the sensor cavity with a volume to impedance ratio of a second portion of the circuit containing the pressure vessel and coupling a pressure adjustment device to the circuit at a junction of the first and second portions of the circuit.

20. The method according to claim 18 or 19 comprising operating the pressure adjustment device continuously.

21. The method according to claim 18 or 19 comprising operating the pressure adjustment device periodically to maintain a predetermined pressure differential between fluid pressure in the vessel and ambient pressure .