CA2238566A1 - Method and apparatus for monitoring of tensioned cables - Google Patents
Method and apparatus for monitoring of tensioned cables Download PDFInfo
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- CA2238566A1 CA2238566A1 CA 2238566 CA2238566A CA2238566A1 CA 2238566 A1 CA2238566 A1 CA 2238566A1 CA 2238566 CA2238566 CA 2238566 CA 2238566 A CA2238566 A CA 2238566A CA 2238566 A1 CA2238566 A1 CA 2238566A1
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N29/00—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object
- G01N29/14—Investigating or analysing materials by the use of ultrasonic, sonic or infrasonic waves; Visualisation of the interior of objects by transmitting ultrasonic or sonic waves through the object using acoustic emission techniques
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N2291/00—Indexing codes associated with group G01N29/00
- G01N2291/01—Indexing codes associated with the measuring variable
- G01N2291/011—Velocity or travel time
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N2291/00—Indexing codes associated with group G01N29/00
- G01N2291/10—Number of transducers
- G01N2291/102—Number of transducers one emitter, one receiver
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N2291/00—Indexing codes associated with group G01N29/00
- G01N2291/26—Scanned objects
- G01N2291/262—Linear objects
- G01N2291/2626—Wires, bars, rods
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Abstract
Structural cables, for example those formed of a large number of wires and used in suspension bridges or cable-stayed bridges, are monitored by acoustic sensors.
Breakage of a wire forming part of the cable gives rise to an acoustic signal.
The signal is recorded and its location is found from the relative times at which it arrives at two sensors. Where sensors are located at the cable anchorage or on bands along the length of a suspension bridge cable, they can also monitor slippage of wires relative to the band or cable anchorage. The system uses high impedance sensors and shielded cable.
The outputs of different sensors are distinguished by a multiplexing technique or by identifying signals.
In another embodiment, sensors placed about the circumference of a cable record signals from wire breaks and the signals are plotted to determine the location of the broken wire within the cross-section of the cable.
Breakage of a wire forming part of the cable gives rise to an acoustic signal.
The signal is recorded and its location is found from the relative times at which it arrives at two sensors. Where sensors are located at the cable anchorage or on bands along the length of a suspension bridge cable, they can also monitor slippage of wires relative to the band or cable anchorage. The system uses high impedance sensors and shielded cable.
The outputs of different sensors are distinguished by a multiplexing technique or by identifying signals.
In another embodiment, sensors placed about the circumference of a cable record signals from wire breaks and the signals are plotted to determine the location of the broken wire within the cross-section of the cable.
Description
s METHOD AND APPARATUS FOR MONITORING OF TENSIONED CABLES
to Field of the Invention The invention relates to the detection of breakage or slippage of wires forming part of cables under tension, which wires form part of structural cables.
is Background of the Invention Suspension cables are widely used as structural elements in bridges and the like. For example, a suspension bridge or support for carrying a pipeline across an 20 obstacle such as a river typically will have at least one suspension cable.
This is anchored at both sides of the gap to be traversed, and crosses the gap in a catenary curve, usually between two pylons or the like. Vertical cables of smaller diameter (called suspender ropes) are looped over the main catenary suspension cable, and these suspender ropes extend downwards to support the roadway or pipeline to be carried 25 across the gap. A typical suspension bridge has two main suspension cables, but a pipeline may have only one.
Tensioned cables are also used in other forms of construction. For example, buildings are sometimes built with a central pylon, and tensioned cables run down to the ground to make a tent-like structure or else hang suspended concrete slab floors from the pylon. Buildings which need large floor areas without columns or walls, such as arenas or hangers, often have roofs supported by tensioned cables.
Bridges known as cable-stay bridges have one or more pylons along their length, with inclined tensioned cables running from the pylons to support the roadbed.
In many cases tensioned cables are exposed to the elements. This is particularly the case in bridges and the like, where the cables are totally unprotected (except for whatever paint or anti-rust coating is placed on them). Bridges are also are frequently in high corrosion environments, such as near salt water.
Cables of the type used in a large suspension bridge are bundles of smaller wires. A typical wire has a diameter of approximately 0.1 - 0.2 inch (4-5 mm.
approx.) A cable is frequently formed of 10,000 or more of such wires clustered together.
Typically, the cable is formed with a circular cross section, although other cross sections are also used. Typically, also, the wires are not twisted together, but are laid parallel to each other. Main suspension cables in suspension bridges are surrounded at intervals by clamps known as "bands" which clamp the wires together to ensure that the wires do not separate, and also serve as the point from which smaller diameter cables known as 2o suspender ropes are hung from the main cable. Cables may have the spaces between wires filled with grease or the like and may have their surfaces covered or protected in some way to protect the wires from corrosion.
At each end of each catenary suspension cable of a typical suspension bridge, it is anchored to the ground in a cable anchorage, where groups of wires are spayed apart and anchored separately. The groups of wires are clamped in anchor structures filled with potting metal, to prevent moisture and the like from attacking the ends of the wires.
Similarly, other tensioned cables often end in sockets filled with poured zinc, and may have wedged anchors. The generic term "anchorage" will be used in this disclosure to indicate the place where the end of a tensioned cable is secured. The term "clamp" will be used to indicate anything which holds a cable, such as an anchorage or a band.
Individual wires in a cable may break from corrosion at any point along the cable. There is also breakage due to fretting of wires together, which usually occurs within the anchorages, especially if the wires have not been immobilized with respect to one another by the use of potting metal. Even when potting metal is present, some fretting may occur.
1o There is also another problem which occurs around the anchorages or bands, which is slippage of some wires within the band or anchorage.
Various methods have been used to attempt to inspect cables for breakage. Visual inspection is of course possible, as by moving a television camera along the cable, but this shows only the state of the external cladding of the cable, not the individual wires. Wires inside the cable bundle cannot be seen, and therefore their state cannot be ascertained. It has been known to spread the cable for a short period of time in one or more selected locations using a sledge hammer and a wedge, so as to be able to inspect visually the wires inside the cable. However, this requires the inspector to 2o approach the cable in the place where inspection is to be done. This can be very difficult, as many portions of the cable are suspended in mid air.
Some indication of the state of the interior wires of the cable bundle can be obtained by moving a magnet along the cable to determine whether there are magnetic anomalies, but this does not provide accurate or detailed information as to broken wires.
It has also been proposed (as in U.S. patent 5 456 113 of Kwan et al) to use passive or active monitoring of a magnetostrictive effect to determine wire breakage, but this proposal has not found practical application, so far as is known to applicant.
to Field of the Invention The invention relates to the detection of breakage or slippage of wires forming part of cables under tension, which wires form part of structural cables.
is Background of the Invention Suspension cables are widely used as structural elements in bridges and the like. For example, a suspension bridge or support for carrying a pipeline across an 20 obstacle such as a river typically will have at least one suspension cable.
This is anchored at both sides of the gap to be traversed, and crosses the gap in a catenary curve, usually between two pylons or the like. Vertical cables of smaller diameter (called suspender ropes) are looped over the main catenary suspension cable, and these suspender ropes extend downwards to support the roadway or pipeline to be carried 25 across the gap. A typical suspension bridge has two main suspension cables, but a pipeline may have only one.
Tensioned cables are also used in other forms of construction. For example, buildings are sometimes built with a central pylon, and tensioned cables run down to the ground to make a tent-like structure or else hang suspended concrete slab floors from the pylon. Buildings which need large floor areas without columns or walls, such as arenas or hangers, often have roofs supported by tensioned cables.
Bridges known as cable-stay bridges have one or more pylons along their length, with inclined tensioned cables running from the pylons to support the roadbed.
In many cases tensioned cables are exposed to the elements. This is particularly the case in bridges and the like, where the cables are totally unprotected (except for whatever paint or anti-rust coating is placed on them). Bridges are also are frequently in high corrosion environments, such as near salt water.
Cables of the type used in a large suspension bridge are bundles of smaller wires. A typical wire has a diameter of approximately 0.1 - 0.2 inch (4-5 mm.
approx.) A cable is frequently formed of 10,000 or more of such wires clustered together.
Typically, the cable is formed with a circular cross section, although other cross sections are also used. Typically, also, the wires are not twisted together, but are laid parallel to each other. Main suspension cables in suspension bridges are surrounded at intervals by clamps known as "bands" which clamp the wires together to ensure that the wires do not separate, and also serve as the point from which smaller diameter cables known as 2o suspender ropes are hung from the main cable. Cables may have the spaces between wires filled with grease or the like and may have their surfaces covered or protected in some way to protect the wires from corrosion.
At each end of each catenary suspension cable of a typical suspension bridge, it is anchored to the ground in a cable anchorage, where groups of wires are spayed apart and anchored separately. The groups of wires are clamped in anchor structures filled with potting metal, to prevent moisture and the like from attacking the ends of the wires.
Similarly, other tensioned cables often end in sockets filled with poured zinc, and may have wedged anchors. The generic term "anchorage" will be used in this disclosure to indicate the place where the end of a tensioned cable is secured. The term "clamp" will be used to indicate anything which holds a cable, such as an anchorage or a band.
Individual wires in a cable may break from corrosion at any point along the cable. There is also breakage due to fretting of wires together, which usually occurs within the anchorages, especially if the wires have not been immobilized with respect to one another by the use of potting metal. Even when potting metal is present, some fretting may occur.
1o There is also another problem which occurs around the anchorages or bands, which is slippage of some wires within the band or anchorage.
Various methods have been used to attempt to inspect cables for breakage. Visual inspection is of course possible, as by moving a television camera along the cable, but this shows only the state of the external cladding of the cable, not the individual wires. Wires inside the cable bundle cannot be seen, and therefore their state cannot be ascertained. It has been known to spread the cable for a short period of time in one or more selected locations using a sledge hammer and a wedge, so as to be able to inspect visually the wires inside the cable. However, this requires the inspector to 2o approach the cable in the place where inspection is to be done. This can be very difficult, as many portions of the cable are suspended in mid air.
Some indication of the state of the interior wires of the cable bundle can be obtained by moving a magnet along the cable to determine whether there are magnetic anomalies, but this does not provide accurate or detailed information as to broken wires.
It has also been proposed (as in U.S. patent 5 456 113 of Kwan et al) to use passive or active monitoring of a magnetostrictive effect to determine wire breakage, but this proposal has not found practical application, so far as is known to applicant.
Monitoring of the acoustic waves from cable breakage has also been known.
Indeed several bridges in France have been monitored for years to determine wire breakage.
However, the system used requires smart sensors with a power supply, which greatly limits the flexibility and usefulness of the technique.
Summary of the Invention The present invention provides a method and apparatus for monitoring of a cable which registers breakage of wires as it occurs, and gives the location of such breakage. In many cases, it registers slippage of the cable through a clamp, for example through a cable anchorage. The method and apparatus of the invention can also register breakage of a wire in a cable which is directly or indirectly in physical contact to the cable being monitored, such that an acoustic wave can propagate from the breakage to the cable being monitored. The method is non-destructive, and does not require inspectors to go into normally inaccessible locations.
According to the method, acoustic sensors are located at longitudinally spaced points along the cable. The acoustic sensors are monitored, and sounds made by breaking wires are identified. The locations of the breaking wires along the cable are 2o determined by noting the arrival times of such sounds at different longitudinally spaced sensors, and calculating the location of origin of such sounds from the difference in arrival times.
In a further aspect, cable slippage in a clamp is determined by positioning at least one acoustic sensor on or adjacent to adjacent to a clamp, and noting the sounds made by slippage.
In a further aspect, the acoustic sensors according to the invention are high impedance accelerometers which are joined to a central processing station by shielded lines, for example coaxial cable with electrically shielded junction boxes. In a preferred embodiment, a charge amplifier is used at the central controller to boost the signal received from the sensors.
In a further aspect, multiplexing means are provided to reduce the number of cables required to monitor a large number of sensors, and a multiplexing method is disclosed to identify from which sensor the sounds come. In an alternate embodiment, sensors provide identifying information when they transmit a sound, so that the sensor which transmits the sound can be identified.
In a further aspect, signals are analyzed to determine the distance over which wire breakage weakens the cable.
In a further aspect, a method and apparatus are provided to determine the radial direction from the center line of the cable to the breaking wire. Means axe provided including at least three sensors positioned around the circumference of the cable, and a method is provided for determining the radial direction of the breakage from the center line of the cable.
Brief Description of the Drawings Fig 1 is a plan view of a suspension bridge, showing one of the main suspension cables, suspender ropes and the apparatus of the invention in a typical arrangement.
Figure 2 is a plan view on a larger scale than Figure 1, of an anchorage for the end of a suspension cable of a suspension bridge, and illustrating a sensor according to the invention and a data acquisition device Figure 3 is a plan view, on a larger scale than Figure 1, of a portion of a suspension cable showing sensors according to the invention positioned on bands where suspender ropes cross the suspension cable, Figure 4 is a sectional view through Figure 3 on the lines 4--4.
Figure 5 is a diagrammatic view showing one method of connecting sensors electrically to the data acquisition device.
l0 Description of the Invention (i) Longitudinal location of cable breakage along the Cable According to the invention, acoustic sensors are placed at selected locations along the cable, and their output is continuously monitored. The output is recorded, at least temporarily, so that it can be retrieved if an event of apparent interest is determined to have happened. The acoustic sensors can be accelerometers or microphones having an impedance over 1 megaohm. which measure the deflection of the surface of the cable as an acoustic or other wave propagates through the cable..
The sensors are preferably high impedance accelerometers having an impedance of over 1 megaohm. A particularly preferred detector is a high impedance piezoceramic disc having an impedance of about 100 megaohms.
It is known that accelerometers often provide a signal which is distorted by having a maximum at a particular frequency, and a lesser response at other frequencies. Therefore, all of the sensors in any single installation are preferred to be of the same type, so that all will provide signals which are distorted similarly.
If desired, the central data collection location can be equipped with means to correct the expected _ $ _ distortion of the type of accelerometer used. However, if all sensors distort similar frequencies in similar predictable ways, it may be suitable not to correct the distortion, but merely to use and compare the distorted signals.
The sensors are connected to a central data collection location by a shielded cable network. The shielding is to reduce or prevent noise which would otherwise prevent the signal from being distinguished when received at the data collection location. Shielding can be accomplished by using coaxial cable, with each connection between a cable and a sensor, amplifier or other cable being made in a junction box protected by a Faraday cage.
The use of high impedance devices provides some protection in case of, for example, a lightning strike on the cable, and also permits a large number of sensors to be attached to the same data collection line. The sensors are placed at known locations along the cable.
At the data collection location, a charge amplifier compares the voltage of the input with the voltage of a comparator, and provides an amplified output. Thus, the signal is transmitted to the data collection location in analogue form, without an amplifier 2o located at each sensor. This reduces the cost of the apparatus required from that of the prior art, and makes it unnecessary to have a power supply or batteries at each sensor.
Surprisingly, it is found that an acceptable signal can be transmitted a distance long enough to permit instrumentation using this arrangement for even long bridges.
In one test, unamplified signals from a piezoelectric ceramic disc were successfully transmitted along a coaxial cable 8000 ft. (approx. 2440 m.) in length and successfully amplified.
In a preferred embodiment of the invention, sensors are located at or near cable nodes. A cable node is defined as a place where two cables intersect and are directly or indirectly connected in a way that transmits acoustic energy from one to the _g_ other without significant damping. (as for example where a band joins a suspension cable to a suspender cable) or a place where a cable end is clamped to another structure (as for example where a catenary cable is anchored at a end in a cable anchorage or where a suspender rope is secured in a hanger where a cable stay is anchored in a poured zinc socket.)..
It is not necessary to place a sensor at every node, provided that the sensors are sufficiently close to one another so that a wire break will be sensed by at least two sensors. The effective distance over which this can be done depends on the to attenuation of acoustic signals within the cable and the information which is sought. For example, in a suspension bridge, where only monitoring of a catenary suspension cable is desired, a sensor can be placed for example on every second one of the nodes where a suspender cable is hung from the suspension cable. 'The signals will propagate through the intervening nodes, and will be received at the sensors, provided that the distance between them is not far enough for attenuation of the signal to a level where it cannot be recognized. However, if there is a wire break in the suspender rope attached to the intervening node, such an arrangement may not provide sufficient information about such break for complete characterization of it.
2o The speed of sound in the cable is usually a known quantity, but if desired it can be measured by propagating an acoustic wave through the cable (as for example by hitting the cable with a pneumatic hammer) and determining the time taken for the acoustic wave to pass along the cable between two sensors a known distance apart.
When a wire breaks, it causes an acoustic event, and sends acoustic and pressure waves in both directions along the cable. These will hereinafter be called acoustic waves and the sensors for them will be called acoustic sensors, it being understood that the waves pass through the cable and that the sensors can either pick up sound (as by a microphone in close proximity to the cable) or vibrations in the cable itself (as by a piezoelectric transducer located on a cable band or the outer surface of the cable.). Thus, the location of the break can be determined by measuring the difference in the length of time that is taken for the signal from the breakage to reach the first sensor on either side of the breakage, and by knowing the distance between the two sensors. It is preferred that there be a sensor at or near any node which is the termination of the cable being monitored, so that the location of breakages near the cable end can be determined accurately. Where the cable passes through a node (for example if the cable is a catenary suspension cable and the node is the point where a vertical suspender rope passes over it) the signal will propagate through the node. Thus, where monitoring a to cable which continues through a node, it is not necessary to have a sensor at that node.
However, it is preferred to have sensors at each node, as such sensors will acquire useful information as discussed hereinafter about wire slippage and the nature of the event causing the signal.
It is found that acoustic waves attenuate fairly rapidly in cables.. The degree of attenuation appears to depend on the size and nature of the cable.
For example, in a catenary suspension cable of 2'(approx. 0.6m.) diameter having 10000 parallel wires of 0.2" (approx. 0.78 cm.) diameter and bands spaced every 40' (approximately 12.2 m.), attenuation was approximately 12 db between sensors on adjacent bands. Sensors must 2o be placed close enough to one another, having regard to cable size, composition and spacing of bands, so that useable signals can be obtained from at least two sensors if a wire break is to be located. Wire slippage gives generally a signal of much lower amplitude than wire breakage, so generally, it is desirable to have at least one sensor on each node where slippage is suspected.
Tests can be conducted with the particular cable to show the nature of the acoustic event produced by a wire breakage, so that such event can be recognized and distinguished from other acoustic events which may cause acoustic waves within the cable. Also, once an event is recognized as a likely wire break, the record of other acoustic events immediately before it can be reviewed to see if a likely cause can be determined. For example, the acoustic waves caused by sawing are recognizable, and a sawing sound immediately preceding a wire break indicates vandalism. On a bridge, such a sequence could be used to trigger an alarm to send personnel to catch the vandal.
Where a cable is has sufficient sensors (such as for example at its nodes) to enable the location of wire breaks to be determined with some accuracy, it is possible to use this information to determine the replacement schedule for that cable.
On a suspension bridge, it is not necessary to put a sensor or a group of 1o sensors at every node where a suspender rope intersects a catenary cable.
Sensors on the catenary suspension cable will identify wire breakage in vertical cables which pass over bands on that catenary cable, as the waves is transmitted to the catenary cable through the band. Such information will tell where along the catenary cable the waves were transmitted to the catenary cable.. For a particular cable, the transmission of a wave t s caused by wire breakage in a suspender rope to that cable through the bands will have a particular signature waveform which can be distinguished from that of a wire breakage in the cable itself. However, the location at which the wave was transmitted to the cable can be located in like manner as if there had been a wire break at that place.
This permits determination of which of the vertical cables has had a wire break. It is useful to 2o know how many wire breaks have occurred in a particular vertical cable, whether or not the precise locations of these wire breaks can be determined. After a certain number of wire breaks are known to have occurred in a particular vertical cable, it may be decided to replace that vertical cable, even though the precise locations of the wire breaks are not known, or it may be desired to put sensors on that vertical cable.
(ii) Placing Sensors Circumferentially Around the Cable Although it is preferred to use the high impedance sensors disclosed in part (i) above in this aspect of the invention, other sensors can be used for the practice of this aspect (ii) of the invention, provided they are of sufficient sensitivity to record slippage as hereinafter discussed. Thus, low impedance acoustic or pressure sensors with associated microprocessors, or even magnetostrictive sensors can be used if they have the requisite sensitivity. The use of such other sensors is not preferred, however, as they require a power supply or batteries, which increase the cost.
In this embodiment of the invention, there are at least two and preferably 1 o four or more sensors spaced around the circumference of the cable at a single longitudinal location, preferably a node. Where there is slippage of a wire in the cable at the longitudinal location, it is found that this causes signals to be received at the sensors around the circumference, and that the commencement of the signals occurs at slightly different times, if the most direct paths to the sensors are of different length. From the time difference, the location of the slipping wire within the cross-section of the cable can be calculated. Where there are only two sensors, an ambiguous result is sometimes obtained. However, if there is another set of two sensors spaced longitudinally from the first set (for example if the two sets of two sensors are on adjacent nodes of the cable), then it is frequently possible to get an unambiguous result. This is done by locating each 2o of the two sensors in each set at 180 degrees from each other around the circumference of the cable, and by placing the sets at adjacent nodes so that they are offset 90 degrees from each other. If the same wire slippage is recorded at both nodes, the results from the two sets of sensors can be combined to eliminate the ambiguity.
The use of two or more sensors circumferentially spaced about the cable can also provide useful information on development length. The development length of a cable is defined as being the distance over which a cut in the wire has an effect. It will be recalled that each of the wires is tightly in contact with the other wires of the cable.
Thus, when the wire is cut, it may not lose tension over its entire length, depending on the length. Instead, after a certain distance (which varies with the nature of the wire and the cable), its contact with other wires of the cable causes it to have as much tension as if it had not been cut. It is found for example, that in a particular cable of 1000 wires, and a length of 1000 feet (305 m. approximately), the deliberate cutting of one wire has no effect on the tension of that same wire 100 feet (30.5 m. approximately )away.
It appears that the tension imparted to the wire by its contact with other surrounding wires is enough to prevent tension being lost. The development length for the particular cable is therefore defined as being less than 100 feet (30.5 m.) Thus, if there are two breaks in the same wire 600 feet (approx. 183 m.) away from one another, the portion between the 1 o two breaks retains tension, even though it is severed from its anchor points at both ends.
When a wire break occurs longitudinally away from a set of circumferentially spaced sensors which are all at the same longitudinal location along a cable, the sensors sense the acoustic or pressure wave passing longitudinally along the wire. This wave, because it has propagated longitudinally along the cable, is found to reach all of the circumferential sensors in any one longitudinal location (hereinafter called a "sensor group" substantially simultaneously. However, if the location of the sensor group is longitudinally within the development length of the cable from the break, there will be a separate wave generated by the slippage of the broken wire relative to the other wires. The signal from this wave is usually of lesser amplitude than the signal caused by the breakage, but is distinguished from it by the fact that it arrives at the different circumferential sensors of the sensor groups at slightly different times.
Therefore, if all signals at a sensor group resulting from a wire break are identical, it can be concluded that the sensor group is beyond the development length away from the break. If they exhibit differences, it can be concluded that the sensor group is within one development length of the break.
When the locations of the breaks in wires in a cable are known, a determination can be made as to whether cable replacement or repair is necessary by considering the total number of breaks and the number of breaks which are within the development length of each other. If breaks are scattered from one another by more than the development length, it may be acceptable not to replace the cable after a certain total number of breaks has occurred. However, if there are numerous breaks within one development length of each other, it may be prudent to replace the cable when the same total number of breaks has occurred.
(iii) Collecting the Signals The use of at least one sensor at each node requires the use of a large number of sensors on a large structure, such as a long suspension bridge.
There must be means for the sensors to transmit signals which they have sensed to a recording device.
A first embodiment of signal collection has a common data line for a group of sensors, with each sensor on the line having means for generating a unique identifier signal. The means, which may be operated by a small battery located at the sensor, generates a signal unique to that sensor a predetermined amount of time after the sensor has sent a signal of greater than a desired preset magnitude. This permits the data 2o recording device to distinguish between signals from different sensors.
In another embodiment of signal collection, there is a plurality of data channels. Each longitudinally spaced location where sensing is required has two electrically independent sensors attached to different channels. The connections are such that each sensing location has its two sensors attached to a unique pair of channels. As an event at the sensing location triggers both sensors, a signal received on any two channels defines the sensing location (e.g. a node) from which the signal has come.
In a third embodiment of data collection, the is a data channel, and a locator circuit. Each sensor is connected to the data channel, and forms an open circuit to the locator circuit. A constant voltage (AC or DC) is applied across the open circuit.
When a sensor sends a signal of greater than a preset magnitude, it also closes the circuit to the locator circuit. The voltage causes current flow through the locator circuit and the resulting current is measured. Each sensor has a resistance associated with it in the locator circuit, so that the total resistance of the locator circuit has a different value depending on which sensor has sent data. The amount of current therefore shows which sensor sent the data. The sensor which sent the data then opens its circuit after a 1o predetermined interval so that the process can be repeated as has been described. This embodiment is only preferred, however, in situations where the sensors connected to the common data channel are spaced sufficiently far apart that they will not receive signals from the same wire break. Thus, for example, when this arrangement is used with longitudinally spaced sensors, there may be for example four dat channels, with each fourth sensor being connected to the same channel, provided that the degree of attenuation in this cable is such that the waves are unlikely to affect more than one sensor on a dat channel Specific embodiments of the invention will now be illustrated with 2o reference to the drawings.
Turning to Figure 1 of the drawings, 10 is a suspension bridge which runs from land mass 1 to land mass 2 over water indicated as 3. The bridge has two vertical pylons 22, and a roadway 20.
The roadway 20 is suspended from two suspension cables, which are slung between the two pylons 22, 23 and attached to the ground either side of the water 3. Only one of these suspension cables is shown. The other one (which would lie directly behind it in the illustration) is identical.
The suspension cable 12 has a first portion 26, which runs between a cable anchorage 24 and a clamp 28 on the top of pylon 22. The second portion 30, is suspended between clamp 28 on pylon 22 and clamp 33 on the top of pylon 22. A
fourth portion 34 descends from clamp 23 to cable anchorage 25.
At intervals along cable 12 are bands 14, from which suspender ropes 16 (which are wire ropes) are suspended. The suspender ropes terminate in hangers 18, which support transverse cables (not shown) on which the roadway 20 is built.
The anchorages 24, 25, the hangers 18 and the bands 14 constitute nodes according to the to teaching herein.
According to the invention sensors 40, 42, 44, 48 are placed at selected nodes on the bridge. For example, sensors 40 are placed on every second band of the segment 30 of cable 12, except that one node which would otherwise have a sensor 40 has instead sensors 46, which will be described later. There are also sensors 40 on some but not all of the wire ropes 16 which intersect with segments 26 and 34 of cable 12.
Other sensors, indicated as 42, are placed at the cable anchorages at both ends of cable 12. Still other sensors 44 are placed on bands 28 and 32, which are on pylons 22 and 23 respectively.
A few sensors 48 are also placed on selected ones of the hangers 18 which join the suspender ropes to road-supporting cables.
Figure 2 shows, in schematic form, detail of the anchorage 24. The anchor age 24 has an anchor 25. Cable 26 is spayed into groups of wires 26a, 26b, 26c and 26d, each of which passes into anchor 25 where it is anchored by conventional anchor means (not shown). Although only four such groups are shown a typical cable is stayed into many more than four.
Many types of anchor means are known, and it is intended that any of the known types of anchor means could be used. The spayed end of the cable terminates in the anchor means and is usually protected against corrosion by being potted into potting metal or otherwise mobilized and protected from moisture.
Placed at a convenient location on the splayed groups of wires are sensors 60a, 60b, 60c and 60d,one for each group of wires. The sensors are preferably high impedance accelerometers, or piezoelectric ceramic discs as described earlier.
to Sensors 60a, 60b, 60c and 60d may be attached to the associated groups of wires in any suitable way such as by gluing with a suitable adhesive or attaching it with a small clamp. Where the groups of wires have been painted or wrapped with a protective material" it is usually found not to be necessary to remove the paint or wrapping, as acoustic signals will travel through the paint or wrapping to the sensor without substantial distortion.
Sensors 60a ...60d convert acoustic signals which they receive to electrical signals, which are transmitted by cables 53a...53d to a trunk cable 52 and thence to a recording and processing location 54. Location 54 may be (for example) a 2o recording means to record the signals for later transmission to a central office for analysis, or a computer at which the signals are analyzed. Although recording/analyzing means 54 is shown as located close to anchor 25, it need not be in this location. Instead, it can be in any convenient place where it can receive signals by means of cables such as cable 52. Alternately, it can receive signals propagated to it by radio or infrared linkages from all or some of the sensors, if this is desired.
Figures 3 and 4 show a cable band 14 on segment 30 of the suspension cable 12. Many such bands have a single sensor on them. However, this particular band 9indicated as 46) has a plurality of sensors.. In the embodiment shown there are four such sensors, spaced at intervals of 90 degrees around the cable band. The sensors are 74, 76, 78 and 80.
Sensor 74 is connected to trunk cable 52 by means of cable 82. Sensor 76 is connected to trunk cable 52 by means of cable 84.. Cable 86 connects sensor 80 to trunk cable 52, and cable 88 connects sensor 78 to trunk cable 52. It is noted that the sensors, and the cables connecting them to the trunk cable, are oriented so that they do not lie along the top of the band. This is because there is sometimes an inspection platform or walkway, generally indicated as 89, on the top of the bands (and sometimes l0 along the suspension cable joining the bands) and it is desirable to have these sensors and cables where they will not be inadvertently damaged by someone walking on them.
Band 46 may be of any convenient type and has a channelgenerally indicated as 70 for suspender rope 16.. The band also has clamping means 71 indicated by dashed arrows 71 to hold it in place on cable segment 30..
Figure 5 shows a plurality of sensors 100a....100~ Each has a connection 101 a...101 f to a data channel 103 which leads to data collection device 103. Each also has a connection 104a...104f to a locator circuit 105. The locator circuit is an open circuit 2o connected to a source of AC or DC power 106. It has a series of identical known resistors 107a...107f, each of which is associated with a sensor as shown. Each sensor also ahas an open switch (108a...108f)which will complete the open circuit if closed.
When a sensor (eg.l00e) is triggered by an acoustic event, it causes its switch (108e) to close, completing the circuit. This causes the known voltage 106 to flow through circuit 105 to ground 110, causing a current to register in ammeter 110. The current will depend on how many of the resistors 107a...107f have been traversed by the current.
In this case resistors 107a to 107e are traversed, but not resistor 107f. The unique value of the current indicates which sensor has been triggered.
While the above description shows preferred embodiments of the present invention, it is to be understood that the invention is not limited thereby and that in light of the present disclosure of the invention, various other alternative embodiments will be apparent to persons skilled in the art. Accordingly, it is to be understood that changes can be made without departing from the scope of the invention as particularly pointed out and distinctly claimed in the claims set forth below.
Indeed several bridges in France have been monitored for years to determine wire breakage.
However, the system used requires smart sensors with a power supply, which greatly limits the flexibility and usefulness of the technique.
Summary of the Invention The present invention provides a method and apparatus for monitoring of a cable which registers breakage of wires as it occurs, and gives the location of such breakage. In many cases, it registers slippage of the cable through a clamp, for example through a cable anchorage. The method and apparatus of the invention can also register breakage of a wire in a cable which is directly or indirectly in physical contact to the cable being monitored, such that an acoustic wave can propagate from the breakage to the cable being monitored. The method is non-destructive, and does not require inspectors to go into normally inaccessible locations.
According to the method, acoustic sensors are located at longitudinally spaced points along the cable. The acoustic sensors are monitored, and sounds made by breaking wires are identified. The locations of the breaking wires along the cable are 2o determined by noting the arrival times of such sounds at different longitudinally spaced sensors, and calculating the location of origin of such sounds from the difference in arrival times.
In a further aspect, cable slippage in a clamp is determined by positioning at least one acoustic sensor on or adjacent to adjacent to a clamp, and noting the sounds made by slippage.
In a further aspect, the acoustic sensors according to the invention are high impedance accelerometers which are joined to a central processing station by shielded lines, for example coaxial cable with electrically shielded junction boxes. In a preferred embodiment, a charge amplifier is used at the central controller to boost the signal received from the sensors.
In a further aspect, multiplexing means are provided to reduce the number of cables required to monitor a large number of sensors, and a multiplexing method is disclosed to identify from which sensor the sounds come. In an alternate embodiment, sensors provide identifying information when they transmit a sound, so that the sensor which transmits the sound can be identified.
In a further aspect, signals are analyzed to determine the distance over which wire breakage weakens the cable.
In a further aspect, a method and apparatus are provided to determine the radial direction from the center line of the cable to the breaking wire. Means axe provided including at least three sensors positioned around the circumference of the cable, and a method is provided for determining the radial direction of the breakage from the center line of the cable.
Brief Description of the Drawings Fig 1 is a plan view of a suspension bridge, showing one of the main suspension cables, suspender ropes and the apparatus of the invention in a typical arrangement.
Figure 2 is a plan view on a larger scale than Figure 1, of an anchorage for the end of a suspension cable of a suspension bridge, and illustrating a sensor according to the invention and a data acquisition device Figure 3 is a plan view, on a larger scale than Figure 1, of a portion of a suspension cable showing sensors according to the invention positioned on bands where suspender ropes cross the suspension cable, Figure 4 is a sectional view through Figure 3 on the lines 4--4.
Figure 5 is a diagrammatic view showing one method of connecting sensors electrically to the data acquisition device.
l0 Description of the Invention (i) Longitudinal location of cable breakage along the Cable According to the invention, acoustic sensors are placed at selected locations along the cable, and their output is continuously monitored. The output is recorded, at least temporarily, so that it can be retrieved if an event of apparent interest is determined to have happened. The acoustic sensors can be accelerometers or microphones having an impedance over 1 megaohm. which measure the deflection of the surface of the cable as an acoustic or other wave propagates through the cable..
The sensors are preferably high impedance accelerometers having an impedance of over 1 megaohm. A particularly preferred detector is a high impedance piezoceramic disc having an impedance of about 100 megaohms.
It is known that accelerometers often provide a signal which is distorted by having a maximum at a particular frequency, and a lesser response at other frequencies. Therefore, all of the sensors in any single installation are preferred to be of the same type, so that all will provide signals which are distorted similarly.
If desired, the central data collection location can be equipped with means to correct the expected _ $ _ distortion of the type of accelerometer used. However, if all sensors distort similar frequencies in similar predictable ways, it may be suitable not to correct the distortion, but merely to use and compare the distorted signals.
The sensors are connected to a central data collection location by a shielded cable network. The shielding is to reduce or prevent noise which would otherwise prevent the signal from being distinguished when received at the data collection location. Shielding can be accomplished by using coaxial cable, with each connection between a cable and a sensor, amplifier or other cable being made in a junction box protected by a Faraday cage.
The use of high impedance devices provides some protection in case of, for example, a lightning strike on the cable, and also permits a large number of sensors to be attached to the same data collection line. The sensors are placed at known locations along the cable.
At the data collection location, a charge amplifier compares the voltage of the input with the voltage of a comparator, and provides an amplified output. Thus, the signal is transmitted to the data collection location in analogue form, without an amplifier 2o located at each sensor. This reduces the cost of the apparatus required from that of the prior art, and makes it unnecessary to have a power supply or batteries at each sensor.
Surprisingly, it is found that an acceptable signal can be transmitted a distance long enough to permit instrumentation using this arrangement for even long bridges.
In one test, unamplified signals from a piezoelectric ceramic disc were successfully transmitted along a coaxial cable 8000 ft. (approx. 2440 m.) in length and successfully amplified.
In a preferred embodiment of the invention, sensors are located at or near cable nodes. A cable node is defined as a place where two cables intersect and are directly or indirectly connected in a way that transmits acoustic energy from one to the _g_ other without significant damping. (as for example where a band joins a suspension cable to a suspender cable) or a place where a cable end is clamped to another structure (as for example where a catenary cable is anchored at a end in a cable anchorage or where a suspender rope is secured in a hanger where a cable stay is anchored in a poured zinc socket.)..
It is not necessary to place a sensor at every node, provided that the sensors are sufficiently close to one another so that a wire break will be sensed by at least two sensors. The effective distance over which this can be done depends on the to attenuation of acoustic signals within the cable and the information which is sought. For example, in a suspension bridge, where only monitoring of a catenary suspension cable is desired, a sensor can be placed for example on every second one of the nodes where a suspender cable is hung from the suspension cable. 'The signals will propagate through the intervening nodes, and will be received at the sensors, provided that the distance between them is not far enough for attenuation of the signal to a level where it cannot be recognized. However, if there is a wire break in the suspender rope attached to the intervening node, such an arrangement may not provide sufficient information about such break for complete characterization of it.
2o The speed of sound in the cable is usually a known quantity, but if desired it can be measured by propagating an acoustic wave through the cable (as for example by hitting the cable with a pneumatic hammer) and determining the time taken for the acoustic wave to pass along the cable between two sensors a known distance apart.
When a wire breaks, it causes an acoustic event, and sends acoustic and pressure waves in both directions along the cable. These will hereinafter be called acoustic waves and the sensors for them will be called acoustic sensors, it being understood that the waves pass through the cable and that the sensors can either pick up sound (as by a microphone in close proximity to the cable) or vibrations in the cable itself (as by a piezoelectric transducer located on a cable band or the outer surface of the cable.). Thus, the location of the break can be determined by measuring the difference in the length of time that is taken for the signal from the breakage to reach the first sensor on either side of the breakage, and by knowing the distance between the two sensors. It is preferred that there be a sensor at or near any node which is the termination of the cable being monitored, so that the location of breakages near the cable end can be determined accurately. Where the cable passes through a node (for example if the cable is a catenary suspension cable and the node is the point where a vertical suspender rope passes over it) the signal will propagate through the node. Thus, where monitoring a to cable which continues through a node, it is not necessary to have a sensor at that node.
However, it is preferred to have sensors at each node, as such sensors will acquire useful information as discussed hereinafter about wire slippage and the nature of the event causing the signal.
It is found that acoustic waves attenuate fairly rapidly in cables.. The degree of attenuation appears to depend on the size and nature of the cable.
For example, in a catenary suspension cable of 2'(approx. 0.6m.) diameter having 10000 parallel wires of 0.2" (approx. 0.78 cm.) diameter and bands spaced every 40' (approximately 12.2 m.), attenuation was approximately 12 db between sensors on adjacent bands. Sensors must 2o be placed close enough to one another, having regard to cable size, composition and spacing of bands, so that useable signals can be obtained from at least two sensors if a wire break is to be located. Wire slippage gives generally a signal of much lower amplitude than wire breakage, so generally, it is desirable to have at least one sensor on each node where slippage is suspected.
Tests can be conducted with the particular cable to show the nature of the acoustic event produced by a wire breakage, so that such event can be recognized and distinguished from other acoustic events which may cause acoustic waves within the cable. Also, once an event is recognized as a likely wire break, the record of other acoustic events immediately before it can be reviewed to see if a likely cause can be determined. For example, the acoustic waves caused by sawing are recognizable, and a sawing sound immediately preceding a wire break indicates vandalism. On a bridge, such a sequence could be used to trigger an alarm to send personnel to catch the vandal.
Where a cable is has sufficient sensors (such as for example at its nodes) to enable the location of wire breaks to be determined with some accuracy, it is possible to use this information to determine the replacement schedule for that cable.
On a suspension bridge, it is not necessary to put a sensor or a group of 1o sensors at every node where a suspender rope intersects a catenary cable.
Sensors on the catenary suspension cable will identify wire breakage in vertical cables which pass over bands on that catenary cable, as the waves is transmitted to the catenary cable through the band. Such information will tell where along the catenary cable the waves were transmitted to the catenary cable.. For a particular cable, the transmission of a wave t s caused by wire breakage in a suspender rope to that cable through the bands will have a particular signature waveform which can be distinguished from that of a wire breakage in the cable itself. However, the location at which the wave was transmitted to the cable can be located in like manner as if there had been a wire break at that place.
This permits determination of which of the vertical cables has had a wire break. It is useful to 2o know how many wire breaks have occurred in a particular vertical cable, whether or not the precise locations of these wire breaks can be determined. After a certain number of wire breaks are known to have occurred in a particular vertical cable, it may be decided to replace that vertical cable, even though the precise locations of the wire breaks are not known, or it may be desired to put sensors on that vertical cable.
(ii) Placing Sensors Circumferentially Around the Cable Although it is preferred to use the high impedance sensors disclosed in part (i) above in this aspect of the invention, other sensors can be used for the practice of this aspect (ii) of the invention, provided they are of sufficient sensitivity to record slippage as hereinafter discussed. Thus, low impedance acoustic or pressure sensors with associated microprocessors, or even magnetostrictive sensors can be used if they have the requisite sensitivity. The use of such other sensors is not preferred, however, as they require a power supply or batteries, which increase the cost.
In this embodiment of the invention, there are at least two and preferably 1 o four or more sensors spaced around the circumference of the cable at a single longitudinal location, preferably a node. Where there is slippage of a wire in the cable at the longitudinal location, it is found that this causes signals to be received at the sensors around the circumference, and that the commencement of the signals occurs at slightly different times, if the most direct paths to the sensors are of different length. From the time difference, the location of the slipping wire within the cross-section of the cable can be calculated. Where there are only two sensors, an ambiguous result is sometimes obtained. However, if there is another set of two sensors spaced longitudinally from the first set (for example if the two sets of two sensors are on adjacent nodes of the cable), then it is frequently possible to get an unambiguous result. This is done by locating each 2o of the two sensors in each set at 180 degrees from each other around the circumference of the cable, and by placing the sets at adjacent nodes so that they are offset 90 degrees from each other. If the same wire slippage is recorded at both nodes, the results from the two sets of sensors can be combined to eliminate the ambiguity.
The use of two or more sensors circumferentially spaced about the cable can also provide useful information on development length. The development length of a cable is defined as being the distance over which a cut in the wire has an effect. It will be recalled that each of the wires is tightly in contact with the other wires of the cable.
Thus, when the wire is cut, it may not lose tension over its entire length, depending on the length. Instead, after a certain distance (which varies with the nature of the wire and the cable), its contact with other wires of the cable causes it to have as much tension as if it had not been cut. It is found for example, that in a particular cable of 1000 wires, and a length of 1000 feet (305 m. approximately), the deliberate cutting of one wire has no effect on the tension of that same wire 100 feet (30.5 m. approximately )away.
It appears that the tension imparted to the wire by its contact with other surrounding wires is enough to prevent tension being lost. The development length for the particular cable is therefore defined as being less than 100 feet (30.5 m.) Thus, if there are two breaks in the same wire 600 feet (approx. 183 m.) away from one another, the portion between the 1 o two breaks retains tension, even though it is severed from its anchor points at both ends.
When a wire break occurs longitudinally away from a set of circumferentially spaced sensors which are all at the same longitudinal location along a cable, the sensors sense the acoustic or pressure wave passing longitudinally along the wire. This wave, because it has propagated longitudinally along the cable, is found to reach all of the circumferential sensors in any one longitudinal location (hereinafter called a "sensor group" substantially simultaneously. However, if the location of the sensor group is longitudinally within the development length of the cable from the break, there will be a separate wave generated by the slippage of the broken wire relative to the other wires. The signal from this wave is usually of lesser amplitude than the signal caused by the breakage, but is distinguished from it by the fact that it arrives at the different circumferential sensors of the sensor groups at slightly different times.
Therefore, if all signals at a sensor group resulting from a wire break are identical, it can be concluded that the sensor group is beyond the development length away from the break. If they exhibit differences, it can be concluded that the sensor group is within one development length of the break.
When the locations of the breaks in wires in a cable are known, a determination can be made as to whether cable replacement or repair is necessary by considering the total number of breaks and the number of breaks which are within the development length of each other. If breaks are scattered from one another by more than the development length, it may be acceptable not to replace the cable after a certain total number of breaks has occurred. However, if there are numerous breaks within one development length of each other, it may be prudent to replace the cable when the same total number of breaks has occurred.
(iii) Collecting the Signals The use of at least one sensor at each node requires the use of a large number of sensors on a large structure, such as a long suspension bridge.
There must be means for the sensors to transmit signals which they have sensed to a recording device.
A first embodiment of signal collection has a common data line for a group of sensors, with each sensor on the line having means for generating a unique identifier signal. The means, which may be operated by a small battery located at the sensor, generates a signal unique to that sensor a predetermined amount of time after the sensor has sent a signal of greater than a desired preset magnitude. This permits the data 2o recording device to distinguish between signals from different sensors.
In another embodiment of signal collection, there is a plurality of data channels. Each longitudinally spaced location where sensing is required has two electrically independent sensors attached to different channels. The connections are such that each sensing location has its two sensors attached to a unique pair of channels. As an event at the sensing location triggers both sensors, a signal received on any two channels defines the sensing location (e.g. a node) from which the signal has come.
In a third embodiment of data collection, the is a data channel, and a locator circuit. Each sensor is connected to the data channel, and forms an open circuit to the locator circuit. A constant voltage (AC or DC) is applied across the open circuit.
When a sensor sends a signal of greater than a preset magnitude, it also closes the circuit to the locator circuit. The voltage causes current flow through the locator circuit and the resulting current is measured. Each sensor has a resistance associated with it in the locator circuit, so that the total resistance of the locator circuit has a different value depending on which sensor has sent data. The amount of current therefore shows which sensor sent the data. The sensor which sent the data then opens its circuit after a 1o predetermined interval so that the process can be repeated as has been described. This embodiment is only preferred, however, in situations where the sensors connected to the common data channel are spaced sufficiently far apart that they will not receive signals from the same wire break. Thus, for example, when this arrangement is used with longitudinally spaced sensors, there may be for example four dat channels, with each fourth sensor being connected to the same channel, provided that the degree of attenuation in this cable is such that the waves are unlikely to affect more than one sensor on a dat channel Specific embodiments of the invention will now be illustrated with 2o reference to the drawings.
Turning to Figure 1 of the drawings, 10 is a suspension bridge which runs from land mass 1 to land mass 2 over water indicated as 3. The bridge has two vertical pylons 22, and a roadway 20.
The roadway 20 is suspended from two suspension cables, which are slung between the two pylons 22, 23 and attached to the ground either side of the water 3. Only one of these suspension cables is shown. The other one (which would lie directly behind it in the illustration) is identical.
The suspension cable 12 has a first portion 26, which runs between a cable anchorage 24 and a clamp 28 on the top of pylon 22. The second portion 30, is suspended between clamp 28 on pylon 22 and clamp 33 on the top of pylon 22. A
fourth portion 34 descends from clamp 23 to cable anchorage 25.
At intervals along cable 12 are bands 14, from which suspender ropes 16 (which are wire ropes) are suspended. The suspender ropes terminate in hangers 18, which support transverse cables (not shown) on which the roadway 20 is built.
The anchorages 24, 25, the hangers 18 and the bands 14 constitute nodes according to the to teaching herein.
According to the invention sensors 40, 42, 44, 48 are placed at selected nodes on the bridge. For example, sensors 40 are placed on every second band of the segment 30 of cable 12, except that one node which would otherwise have a sensor 40 has instead sensors 46, which will be described later. There are also sensors 40 on some but not all of the wire ropes 16 which intersect with segments 26 and 34 of cable 12.
Other sensors, indicated as 42, are placed at the cable anchorages at both ends of cable 12. Still other sensors 44 are placed on bands 28 and 32, which are on pylons 22 and 23 respectively.
A few sensors 48 are also placed on selected ones of the hangers 18 which join the suspender ropes to road-supporting cables.
Figure 2 shows, in schematic form, detail of the anchorage 24. The anchor age 24 has an anchor 25. Cable 26 is spayed into groups of wires 26a, 26b, 26c and 26d, each of which passes into anchor 25 where it is anchored by conventional anchor means (not shown). Although only four such groups are shown a typical cable is stayed into many more than four.
Many types of anchor means are known, and it is intended that any of the known types of anchor means could be used. The spayed end of the cable terminates in the anchor means and is usually protected against corrosion by being potted into potting metal or otherwise mobilized and protected from moisture.
Placed at a convenient location on the splayed groups of wires are sensors 60a, 60b, 60c and 60d,one for each group of wires. The sensors are preferably high impedance accelerometers, or piezoelectric ceramic discs as described earlier.
to Sensors 60a, 60b, 60c and 60d may be attached to the associated groups of wires in any suitable way such as by gluing with a suitable adhesive or attaching it with a small clamp. Where the groups of wires have been painted or wrapped with a protective material" it is usually found not to be necessary to remove the paint or wrapping, as acoustic signals will travel through the paint or wrapping to the sensor without substantial distortion.
Sensors 60a ...60d convert acoustic signals which they receive to electrical signals, which are transmitted by cables 53a...53d to a trunk cable 52 and thence to a recording and processing location 54. Location 54 may be (for example) a 2o recording means to record the signals for later transmission to a central office for analysis, or a computer at which the signals are analyzed. Although recording/analyzing means 54 is shown as located close to anchor 25, it need not be in this location. Instead, it can be in any convenient place where it can receive signals by means of cables such as cable 52. Alternately, it can receive signals propagated to it by radio or infrared linkages from all or some of the sensors, if this is desired.
Figures 3 and 4 show a cable band 14 on segment 30 of the suspension cable 12. Many such bands have a single sensor on them. However, this particular band 9indicated as 46) has a plurality of sensors.. In the embodiment shown there are four such sensors, spaced at intervals of 90 degrees around the cable band. The sensors are 74, 76, 78 and 80.
Sensor 74 is connected to trunk cable 52 by means of cable 82. Sensor 76 is connected to trunk cable 52 by means of cable 84.. Cable 86 connects sensor 80 to trunk cable 52, and cable 88 connects sensor 78 to trunk cable 52. It is noted that the sensors, and the cables connecting them to the trunk cable, are oriented so that they do not lie along the top of the band. This is because there is sometimes an inspection platform or walkway, generally indicated as 89, on the top of the bands (and sometimes l0 along the suspension cable joining the bands) and it is desirable to have these sensors and cables where they will not be inadvertently damaged by someone walking on them.
Band 46 may be of any convenient type and has a channelgenerally indicated as 70 for suspender rope 16.. The band also has clamping means 71 indicated by dashed arrows 71 to hold it in place on cable segment 30..
Figure 5 shows a plurality of sensors 100a....100~ Each has a connection 101 a...101 f to a data channel 103 which leads to data collection device 103. Each also has a connection 104a...104f to a locator circuit 105. The locator circuit is an open circuit 2o connected to a source of AC or DC power 106. It has a series of identical known resistors 107a...107f, each of which is associated with a sensor as shown. Each sensor also ahas an open switch (108a...108f)which will complete the open circuit if closed.
When a sensor (eg.l00e) is triggered by an acoustic event, it causes its switch (108e) to close, completing the circuit. This causes the known voltage 106 to flow through circuit 105 to ground 110, causing a current to register in ammeter 110. The current will depend on how many of the resistors 107a...107f have been traversed by the current.
In this case resistors 107a to 107e are traversed, but not resistor 107f. The unique value of the current indicates which sensor has been triggered.
While the above description shows preferred embodiments of the present invention, it is to be understood that the invention is not limited thereby and that in light of the present disclosure of the invention, various other alternative embodiments will be apparent to persons skilled in the art. Accordingly, it is to be understood that changes can be made without departing from the scope of the invention as particularly pointed out and distinctly claimed in the claims set forth below.
Claims (9)
1. A method of monitoring a tensioned cable which comprises:
positioning a plurality of first acoustic sensors along the cable at known locations monitoring such sensors for signals noting the relative time of arrival of an acoustically-sensed signal at a at least two such sensors determining from the relative arrival times of such signal the location along the cable of the origin of the signal.
positioning a plurality of first acoustic sensors along the cable at known locations monitoring such sensors for signals noting the relative time of arrival of an acoustically-sensed signal at a at least two such sensors determining from the relative arrival times of such signal the location along the cable of the origin of the signal.
2. A method as claimed in claim 1, in which each said first acoustic sensor has in association with it a second acoustic sensor which is displaced from it a desired distance around the circumference of the cable.
3. A method as claimed in claim 2, in which the signals from the said first sensor and the said second sensor associated with it are compared, and the comparison is used to locate the origin of the signal within the circumference of the cable.
4. A method as claimed in any of claims 1-3, in which at least one of said known locations is adjacent a location where such cable is attached to another cable.
5. A method as claimed in either of claims 3 or 4 in which said second acoustic sensors are accelerometers haveing an impedance greater than One megaohm.
6. A method as claimed in any of claims 1-5, in which said first acoustic sensors are accelerometers having an impedance greater than one megaohm.
7. Apparatus for monitoring a tensioned cable, comprising a plurality of acoustic sensor means positioned at known locations along the cable recording means for recording signals sensed by such sensor means timing means recording when such signals are recorded by the recording means, so that the relative arrival times of the signal of at least two such sensors can be determined.
8. Apparatus as claimed in claim 1, additionally comprising means for calculating the location of the event causing such signal along the cable from said relative arrival times.
9. Apparatus as claimed in either of claims 7 or 8, additionally comprising a common data channel for transmitting signals from at least two of said acoustic sensor means to said recording means, and means for distinguishing which of said two sensor means has sensed such transmitted signals.
Priority Applications (10)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2238566 CA2238566A1 (en) | 1998-05-25 | 1998-05-25 | Method and apparatus for monitoring of tensioned cables |
| DE69811494T DE69811494D1 (en) | 1997-06-11 | 1998-06-11 | METHOD AND DEVICE FOR MONITORING TENSIONED CABLES |
| JP54638598A JP2001508547A (en) | 1997-06-11 | 1998-06-11 | Method and apparatus for monitoring a cable in tension |
| KR1019997011696A KR20010013684A (en) | 1997-06-11 | 1998-06-11 | Method and apparatus for monitoring of tensioned cables |
| AU80059/98A AU8005998A (en) | 1997-06-11 | 1998-06-11 | Method and apparatus for monitoring of tensioned cables |
| PCT/CA1998/000575 WO1998057166A1 (en) | 1997-06-11 | 1998-06-11 | Method and apparatus for monitoring of tensioned cables |
| CN98806065A CN1260044A (en) | 1997-06-11 | 1998-06-11 | Method and apparatus for monitoring of tensioned cables |
| EP98928035A EP0988539B1 (en) | 1997-06-11 | 1998-06-11 | Method and apparatus for monitoring of tensioned cables |
| AT98928035T ATE232979T1 (en) | 1997-06-11 | 1998-06-11 | METHOD AND DEVICE FOR MONITORING TENSIONED CABLES |
| CA002290390A CA2290390A1 (en) | 1997-06-11 | 1998-06-11 | Method and apparatus for monitoring of tensioned cables |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2238566 CA2238566A1 (en) | 1998-05-25 | 1998-05-25 | Method and apparatus for monitoring of tensioned cables |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2238566A1 true CA2238566A1 (en) | 1999-11-25 |
Family
ID=29275781
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2238566 Abandoned CA2238566A1 (en) | 1997-06-11 | 1998-05-25 | Method and apparatus for monitoring of tensioned cables |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2238566A1 (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN110823404A (en) * | 2019-11-29 | 2020-02-21 | 武汉智汇高桥科技股份有限公司 | Intelligent monitoring system for suspension bridge construction |
-
1998
- 1998-05-25 CA CA 2238566 patent/CA2238566A1/en not_active Abandoned
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN110823404A (en) * | 2019-11-29 | 2020-02-21 | 武汉智汇高桥科技股份有限公司 | Intelligent monitoring system for suspension bridge construction |
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