WO2015074119A1 - Retrofit stay cable assessment - Google Patents

Abstract

This disclosure concerns monitoring cables, such as stay cables supporting bridges. The cables comprise multiple strands which are electrically connected at one end and insulated from each other between the ends. A generator applies a signal to one strand and a sensor senses reflection signals of the stimulus signal at multiple sensing locations. The sensing locations are chosen such that a sensing distance between the sensing locations along the strands and via the first end is sufficiently large to allow sensing a time difference between the two or more reflection signals. A processor then determines a continuity of one or more of the multiple strands based on the reflection signals. This way, the reflection signals of the multiple strands can be separated and the continuity of one particular strand can be determined.

Description

Retrofit stay cable assessment Cross-Reference to Related Applications

The present application claims priority from Australian Provisional Patent Application No 2013904523 filed o 22 November 2013, Australian Provisional Patent Application No 2013904544 filed on 25 November 2013, the contents of which are incorporated herein by reference.

Technical Field

This disclosure concerns monitoring cables, for example, but not limited to, stay cables used to support bridges.

Background Ar

Cables are used widely to support vaiious loads, such as in cable stay bridges or cable cars. Many of these applications are intended to last for decades but the integrity of the cables cannot always be guaranteed for such a long time. As a result, it is important to assess the integrity of the cables to prevent premature failure.

Fig. la illustrates a cable stay bridge 100, such as the Anzac Bridge in Sydney, Australia. The bridge 100 comprises a bridge deck 102 o which traffic crosse the bridge 100. The bridge 100 further comprises two pylons 104 and 106 and multiple stay cables, such as exemplary stay cable 108. The Anzac Bridge, for example, comprises 128 stay cables, Fig. lb illustrates stay cable 108 in more detail. Stay cable 1.08 comprises 7 stands, such as strand 110, and each strand comprises five wires, such as wire 11.2. In the example of the Anzac Bridge, each cable comprises 25 to 75 Strands and each strand comprises 7 wires. While the wires 112 are in electrical contact to each other along the entire length of the cable 106, the strands 110 are insulated from each other along the length of the cable 106.

The strands 110 are mechanically secured at bot end of the cable 108 by a clampin mechanism (not shown) to provide a firm mechanical connection between the wires of the cable 108, the top of the pylon 104 and the bridge deck 102, As a result, mechanical loads from the bridge deck 102 are transferred via the stay cable 108 to the pylon 104. As a side effect, the clampin mechanism electrically connects all strands 110 of the cable and therefore forces the ends of the strands 1.10 t the same electrical potential or voltage. As a result, it is difficult to measure the strands 110 individuall to determine faulty strands although it would be possible to replace an individual faulty strand of cable 108.

Any discussion of documents, acts, materials, devices, articles o the like which has been included in the present specification is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present disclosure as it existed before the priority date of each claim of this application.

Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

Disclosure of Invention

A method for monitoring a cable comprising multiple strands which are electrically connected to each other at a first end of the cable and insulated from each other between the first end and a second end comprises:

(a) applying an electrical stimulus signal to a strand of the cable;

(b) sensing two or more reflection signals of the stimulus signal on two or more of the multiple strands at respective sensing locations, the sensing locations being such that a sensing distance between the sensing locations along the strands and via the first end is sufficiently large to allow sensing a time difference betwee the two or more reflection signals; and

(e) determining based on the reflection signals a continuity of one or more of the multiple strands.

Since the reflection signal is sensed at defined sensing locations, reflection signals from multiple strands arrive at different times and are not laid ove each other. Therefore, the reflection signals of the multiple strands can be separated and the continuity of one particular strand can be determined. Other methods can only determine the continuity of the entire cable or can only determine that one strand is faulty but not which strand i faulty. It is an advantage of the above method that a strand with a low continuity can be replaced without replacing the entire cable.

Applying the electrical stimulus signal may comprise applying the electrical stimulus signal to exactly one strand.

The method may further comprise repeating the steps of applying and sensing such that each repetition is performed on different combination of strands on which the reflection signals are sensed.

Deterrnining the continuity may comprise comparing the reflection signals to an expected reflection signal.

The electrical stimulus signal may be an electrical pulse.

Sensing the reflection signals may comprise determining one or more arrival times of one or more pulses of the reflection signals and determining the continuity may be based on the one or more arrival times. The method ma further comprise determining that the continuity is below a damage threshold if a difference is sensed between the arrival times on the multiple strands.

Sensing the two or more reflection signals may comprise sensing a first reflection signal that i sensed immediately after applyin the electrical stimulus signal and a second reflection signal that is sensed immediately after the first reflection signal and determining a continuity may be based on the second reflection signal.

The method ma further comprise identifying a faulty strand as the strand with the earliest second reflection signal.

Sensing the reflection signal may comprise using a signal sensor and the sensing distance may be based on a signal resolution of the signal sensor.

The signal resolution may be defined by a minimum length in time which is required between two signals for the signal sensor to correctly sense the two signals and the sensing distance may be greater than a travel distance that the stimulus signal travels during the minimum lengt in time.

The electrical stimulus signal may be a pulse over a spatial pulse length along the cable and the sensing distance may be greater than the pulse length.

The sensing distance may be greater than 30cm.

The method may furthe comprise transforming the sensed two or more reflection signals into a. frequency representation, wherein determining the continuity may be based on the frequency representation of the two or more reflection signals.

Determining the continuity may comprise comparing the frequency representation of the two or more reflection signals to an expected frequency representation.

Software when installed on a computer causes the computer to perform the method above.

A system for monitoring a cable comprising multiple strands which are electrically connected to each other at a first end of the cable and insulated from each other between the first end and a second end comprises:

a signal generator to apply an electrical stimulus signal to a strand of the cable; a sensor to sense two or more reflection signals of the stimulus signal o two or more of the multiple strands at respective sensin locations, the sensing locations bein such that a sensing distance between the sensing locations along the strands and vi the first, end is sufficiently large to allow sensing a time difference between the two or more reflection signals; and

a processor to determine based on the reflection signals a continuit of one or more of the multiple strands.

Optional features described of any aspect, where appropriate, similarly apply to the other aspects also described here.

Brief Description of Drawings

Fig. l illustrates a cable stay bridge.

Fig. lb illustrates a stay cable in more detail. An example will, be described with reference to

Fig. 2a schematically illustrates a system for monitoring a cable.

Fig. 2b illustrates a sensing distance between sensing locations of two sensors. Fig. 3a illustrates an example of a faulty cable.

Fig. 3b illustrates sensed reflection signals.

Fig. 4 illustrates a method for monitoring a cable.

Best Mode for Carrying Out the Invention

Fig, 2a schematically illustrates a system 200 for monitoring a cable 202. The monitoring system 200 performs a method for monitoring a cable as described with reference to Fig. 4. Similar to cable 108 in Figs, la and lb cable 202 comprises four strands 204, 206, 208 and 210. The four strands 204, 206, 208 and 210 are electrically connected to each other by a first clampin system 12, which connects the cable 202 to the bridge deck 102 at the lower end of the cable 202. Further, the four strands 204, 206, 208 and 210 are electrically connected by a second clamping system 214, which connects the cable 202 to the pylon 104 at the upper end of the cable.

The first clamping system 212 and the second clamping system 214 force the ends of the four strands 204, 206, 208 and 210 to a common electrical potential. As a result, applying a constant voltage to each sti'and individually and measuring the current to measure conductivity and therefore integrity of each strand is not possible. As long as there is one single intact strand in the cable, measuring the conductivity of the cable would indicate an intact, cable even if all other strands are broken.

The monitoring system 200 comprises multiple sensors 216, 218, 220 and 222 connected to a sensor interface circuit 224, The interface circuit 224 is connected to processor 226 which comprises a memory (not shown) to store dat and program code, such as software. Parts of the method 400 in Fig, 4 are implemented in software, such as compiled C++ code and executed by processor 226 to perform these parts.

In one example, the sensors 216, 218, 220 and 222 and the sensor interface circuit 224 are four identical commercially available time domain refieetometers, such a model HLl lOl by Hyperlabs Inc. The sensors 216, 218, 220 and 222 sense reflection signals of the stimulus signal on the strands 204, 206. 208 or 2:10, respecti vely. Each of the sensors 21 , 218, 220 and 222 has a sensing location generally indicated at 230. As explained in more detail further below, the position 230 of the sensors 216, 218, 220 and 222 defines a distance between the sensors 216, 218, 220 and 222, which causes a time difference in sensed reflection signals.

Fig. 2b illustrates a sensing distance 230 between sensing locations of sensors 218 and 220 along a first strand 206 and a second strand 208 via the first end 212. The sensing location of each of the sensors 216, 218, 220 and 222 is chosen such that the time difference can he measured by the sensing interface 224 and processor 226.

The first clamping system 212 is connected to a signal generator 228, such a a Time- domain reflectometer. The signal generator 228 applies an electrical stimuius signal to the cable 20 via clamping system 212, It is noted that the electrical stimulus signal may also be applied to one of the strands 204, 206, 208 or 210 by the sensors 216, 218, 220 and 222, respectively .

In one example, the electrical stimulus signal is an electrical pulse with a pulse duration of Ins and a peak voltage of 50V. Other parameters for the pulse are of course possible as long as the pulse is short compared to the propagation time of the signal to the fault location. Unfortunately, the fault could be located in the first few metres, which is why Ins is a good figure to choose, The peak voltage may be chosen so as to not present a shock hazard in case anyone come into contact with the impulse.

After the signal is applied to the cable 202, the signal propagates along the cable 202. The signal is reflected at the end of the cable 214 and returns along the strands 204, 206, 208 or 210 to the clamping system 212. Sensors 216, 218, 220 and 222 sense this reflection signal of the stimulus signal. In one example, the interface circuit 224 ha a timer for each sensor and a voltage threshold. Each time the voltage at the sensors 216, 218. 220 and 222 crosses the voltage threshold from low to high, the interface circuit 224 records the current time from the timer as the arrival time of a pulse. In some examples, the interface circuit 224 is integrated with the processor 226 into microcontroller with ail integrated A/D converter and the voltage threshold is represented by a binary value stored in a program memor of the microcont roller.

The processor 226 receives the arrival time and determines based on the arrival time. which is in turn based on the reflection signal, a continuity of the strands.

In case of a perfectl continuous cable, the pulse propagates equally along all of the strands, is reflected at the end 214 of the cable and returns to the sensors 216, 218, 220 and 222. However, over time the total cross sectional area of the wires decreases, such as caused by corrosion, and discontinuities appear, such as broken wires.

Determining a continuity means to determine to which degree the cable is continuou or whether there are discontinuities. For example, the result of determining a continuity may be "essentially continuous" in case of a non-faulty cable or "unsafe discontinuities" in case of a cable where the number of intact wires in the cable is below a specified threshold. A number value may also be assigned to the continuity, such that 1 indicates a continuous cable and 0 indicates a broken cable. Values between 1 and 0 may indicated various degrees of discontinuities, A damage threshold may be defined such that a strand is considered faulty if the continuity is below the damage threshold.

Fig. 3a illustrates an example where cable 202 has one faulty strand 208 and three intact strand 204, 206 and 210. Faulty means that the diameter of at least one wire of strand 208 is significantly reduced or broken uch that the pulse applied to the cable is at least partially reflected from that fault. Strand 208 ha a fault 302 at position B. which is 10 m away from end 212, for example. Sensors 216, 218, 220 and 222 are located at sensor locations 0.5 m from end 212, In this example, sensors 216, 218, 220 and 222 are capable of sensing a reflection signal as well as applying the stimulus signal to respective strands.

In this example, a pulse is applied to strand 208 by sensor 220 at a location of 0,5 m from end 212 of cable 202. As result, the sensing distance between the sensing locations along the strands and via the end 212 is 1 m. The pulse travels along strand 208 to fault 302, which covers a distance of 9,5 m. The pulse is reflected at fault 302 and travels back to sensor 220 by another 9.5 m. In total, the pulse travels 19 m, which results i a travel time of about 63ns. After being sensed by sensor 220, the pulse continues to end 212 where it is again reflected and continues to sensors 216, 218 and 222, Therefore, the pulse travels a further 1 m, that is, the sensing distance, between sensor 220 and the other sensors 216. 218 and 222. As a result, the pulse reflected from fault 302 arrives at sensors 216, 218 and 222 about 33 n after being sensed by sensor 220.

Hie interface circuit 224 measures the arrival time at sensors 216, 218, 220 and 222 and calculates a time difference between the arrival times. Most interface circuit have a lower limit of time that can be resolved. The distance between the sensors 216, 218, 220 and 222 defines the time difference and therefore, the location of the sensors 216, 218, 220 and 222 is chosen, such that the interface circuit 224 can determine a time difference between the signal from sensor 220 and the signal from sensor 218, for example.

In one example, the interface circuit. 224 comprises two signal sensors, such as A/D converters model maxl0 by Maxim Integrated, CA, United States, which are selectively connected to the strands by switches. The A/D converters have a sample rate of 2200 Msps, which results in a signal time resolution of 0.5 ns. Reflection signals that are closer together than 0.5 ns may consequentl be measured as a single signal.

The minimal sensing distance is based on this signal resolution and since the sensing distance results in a time difference of reflection signals of 3 its, this sensing distance is sufficientl large to allow sensing the time difference reliably. In othe words, the sensin distance is greater than a travel distance that the stimulus signal travels during the niiniffium length in time of 0.5 ns. For many available signal sensors, the sensing distance is greater than 30cm. In most examples, the sensing difference is greater than the pulse length, wliich again is 30cm for a 1 ns pulse.

In one example, the interface circuit 224 further comprises an FPGA, such as Virtex-7 by Xilinx, CA, United States, connected to the A/D converters. The FPGA may trigger the application of the stimulus pulse and allows high-speed read-out of the A/D converters and performs some basic pre-processing, such as defenmning the arrival time of the reflected pulses. The FPGA is interfaced to processor 226 to provide the arrival time to the processor 226 at relatively low speed after the measurement is completed. The processor 226 can then execute program code stored on program memory to determine the continuity of the strands, such as by comparing the arrival times of the reflection signals received from the FPGA.

The FPGA may also transform the sensed reflection signal into a frequency representation, such as by using an FPGA implementation of a Fast Fourier Transformation (FFT). In some examples, the window size of the FFT is equal to the pulse width and the FFT is triggered by the start of the pulse such that the entire reflected pulse is transformed by one FFT. The continuity of the cable 202 can then be determined based on the frequency representation of the reflection signal.

The processor 226 may then receive the transformed representation of the reflection signal from the FPGA and compare it to a previously stored expected frequency representation, such a a frequency representation of pulses measured while the cable 202 was still continuous, that is, not damaged.

Broadly available components can be used in order to use a sensing distance of 1 m. This is an advantage since most stay cables can be accessed within the first meter without tfte need for complicated machinery, such as lifts or ladders. Further, the sensors can be applied without dissembling the cable, which means the system 200 can be retro-fitted to existing bridges without major engineering works or bridge closures. Further, a large number of strands can be selectively coupled to the two A D converters such that over time, all strands can be tested automatically without the need for a large number of A/D converters . This reduces the overall cost of the system.

In one example, the system 200 operates remotely and transmits the determined continuity to a control centre via a wireless connection, such as a 3G/LTE cellular network.

Fig. 3b illustrates example signal 304, 306, 308 and 310 measured b sensors 216, 218, 220 and 222, respectively. In this example, the pulse is applied to the end 212 of the cable at time t-0 (indicated at 320). It takes the pulse 3.3 ns to travel from the end 212 to the sensors 216, 218, 220 and 222 and as a result, all four sensors 216, 218, 220 and 222 measure the stimulation pulse at t=3.3 ns (indicated at 322). Since strand 208 is damaged at 302, as can be seen, in Fig. 3a* the pulse is reflected from, that damage 30 and the reflected signal is sensed by sensor 220 at t— 63 ns (indicated at 324). The reflected signal continues and is reflected at end 212, Since the strands 204, 206, 208 and 210 are connected at the end 212, the reflected signal continues o the strands 204, 206 and 210 in reverse direction. As a result, sensors 216, 218 and 222 measure the reflection signal at t=66.3 ns.

There is a time difference 328 between the reflection signal 308 and the other reflection signals 304, 306 and 310. As explained above, the distance between the location of the sensors 220 and 218, for example, influences the time difference 328. Therefore, the sensing locations are chosen such that the time difference can be sensed.

As can be seen in Fig. 3 b each of the sensors 216, 218, 220 and 222 senses a first reflection signal at 322 that is sensed immediately after applying the electrical stimulus signal and a second reflection signal at 324 or 326 that is sensed immediately after the first reflection signal. Therefore, determining the continuit is based on the second reflection signal, This way, system 200 determines whether all second reflection signals arrive at the same time at 326, which means the cable is not damaged, or whether one or more second reflection signals arrive early at. 324, which means that strand is damaged (as in the example of Fig, 3b), The system 200 identifies a faulty strand as the strand with the earliest second reflection signal, that is the strand with reflection signal measured at 324.

I one example, the processor 226 detenirines the arrival times of the reflected pulses. The processor 226 then compares the arrival times of the pulses on the multiple strands. The processor 226 determines that a strand is faulty, that is, the continuity is below a damage threshold, if a difference is sensed between two arrival times. For example, there is a difference of 3.3 ns betwee ai ivai time of signal 308 and arrival time of signal 306 and therefore, processor 226 determines that there is a damage in the cable 202, The later reflection signals in Fig. 3 are die reflection of the stimulation pulse at the distal end 214. Fig. 4 illustrates a method 400 for monitorin the cable 202 of Figs. 2 and 3a. As described earlier, the cable 202 comprises multiple strands which are electrically connected to each other at the first end 212 of the cable and insulated from each other between the first end 212 and a second end 214.

The method 400 commences by applying 402 an electrical stimulus signal to a strand of the cable 202. In one example, the signal is applied at the first end 212 of the cable 202 such that the signal is effectively applied to all strands of the cable 202 at the same time.

Then, two or more reflection signals of the stimulus signal are sensed 404 on two or more of the multiple strands, such as strand 220 and strand 218, by sensors 220 and 21 at respective sensing locations 230. The sensing locations 230 are chosen such that a sensing distance between the sensing locations along the strands and via the first end is sufficientl large to allow sensing a time difference between the two or more reflection signals.

Based on the reflection signals a continuity of one or more of the multiple strands is then determined 406.

In one example, method 400 is repeated such that each repetition is performed on a different combination of strands on which the reflection signals are sensed. For example, in a first repetition reflection signals are sensed, on strands 204 and 206 by sensors 216 and 218 and in a second repetition reflection signals are sen seel on strands 208 and 210 by sensors 22 and 222,

At each repetition the stimulus signal may be applied at or near one sensor location, and therefore; at each repetition, the signal is applied at a different strand,

This method increases the abilit to determine a fault, without havin to increase the time discrimination. Repeating the measurement helps by giving some other measurements that can correlate with the hypothesis that a given strand is faulty, bearing in mind that reflectometry is sometimes complicated by the presence of false reflections. That may particularly be the case with electrical cables, where there are sometimes joins i the cable, or minor damage. In another example, the measurements for all pairings are compared to identify measurements which are unusual compared to the broader population of measurements. If a strand is damaged then it would be expected that, all measurements involving that strand show unusual characteristics. Various data analysis techniques can be used to identify unusual measurements. These techniques could include supervised or unsupervised machine learning techniques for example. Measurements are kept for future reference. If measurements are retaken at a late date and a strand has experienced damage in that time the resulting change in measurements will be apparent. That is, the processor 226 compares the reflected signal for each strand to a previously stored signal and determines that the continuity i unsatisfactory if the two signals differ significantly.

In yet a further example, the processor 226 trains a statistical classifier using the non- faulty strands and later uses the sensed reflection signals to classify the respective strand as either non-fault or faulty, which means the processor 226 determines the continuit of the strand.

It. will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the specific embodiments without departing from the scope as defined in the claims.

It should be understood that the techniques of the present disclosure might be implemented using a variety of technologies. For example, the methods described herein may be implemented by a series of computer executable instructions residin on a suitable computer readable medium. Suitable computer readable media may include volatile (e.g. RAM) and/or non-volatile (e.g. ROM, disk) memory, earlier waves and transmission media. Exemplary earner waves may take the form of electrical, electromagnetic or optical signals conveying digital data steams along a local network or a pu lically accessible network such as the internet.

It should also be understood that, unless specifically stated otherwise as apparent from the following discussion, i is appreciated that throughout the description, discussions utilizing terms such as "estimating" or "processing" or "computing" or "calculating" or "generating", "optimizing" or "determining" or "displaying" or "maximising" o the like, refer to the action and processes of a computer system, or similar electronic computing device, that processes and transforms data represented as physical (electronic) quantities within the computer system's registers and memorie into other data similarly represented as physical quantities within the computer system memories or registers or other such information storage, transmission or display devices.

Claims

CLAIMS:

1. A method for monitoring a cable comprising multiple: strands which are electrically connected to each other at a first end of the cable and insulated from each other between the first end and a second end, the method comprising:

(a) applying an electrical stimulus signal to a strand of the cable;

fb) sensing two or more reflection signals of the stimulus signal on two or more of the multiple strands at respective sensing locations, the sensing locations being such that a sensin distance between fire sensing locations along the strands and via the first end is sufficiently large to allow sensing a time difference between the two or more reflection signals^* and

(c) determining based on the reflection signals a continuity of one or more of the multiple strands.

2, The method of claim 1, wherein applying the electrical stimulus signal comprises applying the electrical stimulus signal to exactly one strand.

3. The method of claim 1 or 2, further comprising repeating the steps of applying and sensing such that each repetition is performed on a different combination of strands on which the reflection signals are sensed.

4. The method of any one of the precedin claims, wherein determining the continuity comprises comparing the reflection signals to an expected reflection signal.

5. The method of any one of the preceding claims, wherein the electrical stimulus signal is a electrical pulse.

6. The method of claim 5, wherein sensing the reflection signals comprise determining one or more arrival times of one or more pulses of the reflection signals and determining the continuity is based on the one or more arrival times.

7. The method of claim 6, further comprising determining that the continuity is below a damage threshold if a difference i sensed between the arrival times on the multiple strands.

8, The method of any one of claims 6 or 7. wherein sensing the two or more reflection signals comprises sensing a first reflectio signal that is sensed immediately after applying the electrical stimulus signal and a second reflection signal that is sensed immediately after the first reflection signal and determining a continuity is based on the second reflection signal.

9, The method of claim 8, further comprising identifying a faulty strand as the strand wit the earliest second reflection signal.

10. The method of any one of the preceding claims, wherein sensing the reflection signal comprises using a signal sensor and the sensing distance is based on a signal resolution of the signal sensor,

11. The method of claim 10, wherein the signal resolution is defined by a minimum length in time which is required between two signals for the signal sensor to correctly sense the two signals and the sensing distance is greater than a travel distance that the stimulus signal travels during the minimum length in time,

12. The method of claim 1.1, wherein the electrical stimulus signal is a pulse over a spatial pulse length along the cable and the sensing distance is greater than the pulse length,

13. The method of an one of the preceding claims, wherein the sensing distance is greater than 30cm.

14. The method of any one of the preceding claims, further comprising transformin the sensed two or more reflection signals into a frequency representation, wherein determining the continuity is based on the frequency representation of the two or more reflection signals.

15. The method of claim 14, wherein determinin the continuit comprises compaiing the frequency representation of the two or more reflection signals to an expected frequency representation.

16. Software that when installed on a computer causes the computer to perform, the method of any one or more of the claims 1 to 15.

17. A. system for monitoring a cable comprisin multiple strands which are electrically connected to each other at a first end of the cable and insulated from each other between the first end and a second end, the system comprising:

a signal generator to apply an electrical stimulus signal to a strand of the cable; a sensor to sense two or more reflection signals of the stimulus signal on two or more of the multiple strands at respective sensing locations, the sensing locations being such that a sensing distance between the sensing locations along the strands and via the first end is sufficiently large to allow sensing a time difference between the two or more reflection signals; and

a processor to determine based on the reflection signals a continuity of one or more of the multiple strands.