From magnetic signals to wavelets and wirelets

From magnetic signal to wavelet and wirelet

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Historical facts


From the beginnings to the first inspection machines

The principle of magneto-inductive testing was discovered by the English in the years 1830 to 1850 with an application dedicated to the inspection of gun barrels. A variant of magnetic rope inspection was developed by MacCann & Colson in the mines in South Africa around 1906. In 1931, Prof. Woernle of the IFT Institute, Stuttgart developed the first magneto-inductive coil technology for intercepting and measuring the voltage induced by the leakage magnetic flux emitted by a wire break in a steel wire rope. Starting in 1931, MRT technology entered the golden age of the development with for instance the Swiss (Signum) and Polish (Krakow Institute) inspection machines.

The era of machines

The source of magnetization of electromagnetic origin (Integra) was replaced in 1970 by permanent magnets (Kündig). In 1981, Polish Stachurski introduced Hall sensors to measure the loss of metallic cross section of a rope. The ferrite magnets are in turn supplanted by the rare earth magnets of the Neodymium type in the early 1990s. Until then, research focused on machine technology. A shift took place from 1985 onwards to the era of software, which took a closer look at signal analysis with an acceleration from 1999 onwards, born of the pioneering work of Mrs Weischedel, Tytko, Chaplin, Dohm and Nussbaum. Dohm is one of the first to point out the weaknesses of MRT analysis by organizing a robin test round demonstrating erroneous predictions in the context of mine cables. A theoretical vacuum related to the prediction of wire breaks is partially filled by the work of Nussbaum which is the cornerstone of future research.

The beginnings of predictive analysis

The first softwares dedicated to the analysis of rope magneto-inductive signals are designed by S. Winter, followed by S. Messmer, G. Hinterndorfer and others. Although intensive efforts have been made to improve the design of the auscultation machines, it should be noted that the results have been much worse in the analysis and interpretation of magnetic signals. It is important to remember that steel wire rope technology has evolved considerably with the use of increasingly high-grade wire and a more hostile environment: increasing rope operating speed and a constant trend more cycles of fatigue in a shortened time.

The era of wavelet predictive analysis?

Pr. Pistòra who invented the defectoscope in 1999 is the first to project the idea of ​​using wavelets as a tool for processing magneto-inductive signals. Faced with the theoretical and algorithmic difficulties of wavelet analyzes but also facing a deaf silence of the scientific community, he had to resign himself to giving up his idea which was later taken up by other contemporaries from 2014. In parallel, Mr Weischedel from NDtech was a pioneer who began early in 1985 to use signal processing techniques to isolate the signatures of defects hidden in rope ground signal. In 2013, he developed an ingenious technique called "wire rope roughness" to create indicators of the evolution of defects built on raw signals. Unknowingly, he had constructed a cousin filtering technique rediscovering the precepts of wavelet analysis applied to magnetic signals.
MRT rope testing procedure

Challenges and limits of magneto-inductive analysis

Magnetic rope testing is like a chain comprising four links: the machine, the in-situ test, the analysis and the interpretation of the magnetic signals and finally the reporting step. A fragile or broken link has important implications for the quality of the evaluation. The magneto-inductive method has limitations related to the physics of the process but also to a "faulty" analysis of the signals
  • the method is not injective: wire breaks are associated with peak signatures. Behind a signature of peaks, however, can be tethered one or several broken wires
  • The energy of the signatures is not uniform, which creates mask effects due to lack of contrast: one tuck tail end in a splice easily masks the presence of an attached broken wire
  • The signatures of ruptures emerge from a background noise which is not a noise but an information on the damage of the rope
  • Conventional analysis does not allow to monitor the development of wear, corrosion or fatigue
  • The reliability of the conventional method is less than 50%

Can we do better ?

Improve reliability?
- Can the sharpness of the analysis be improved? Yes
The magnetic signals comprise a host of information which has never been explored, in particular the frequency information
- Filter or not filter? No
Attempts to pre-filter signals to eliminate rope ground noise were failures. Signatures of magnetic anomalies have a spectrum that intersects the bandwidth  of the ground signal. The strategy to be adopted is thus: Separate the information locally but without ever filtering it
Identify mechanisms of damage?
magnetic signature of wire break

MRT, wavelet and wirelet ?

Nussbaum was the first to establish the theoretical basis for the induced signals of wire breaks: a broken wire signature is constructed as the superposition of two signals - a signal U1 of loss of cross section of the wire and a signal U2 returning from the section of the wire and has an evanescent waveform which has two negative amplitude nipples separated from a central peak of positive amplitude.
It is striking to note the similarity of the magnetic signal of a wire break with for example the Mexican hat wavelet or the Cauchy wavelet. The first idea is therefore to use a wavelet transform to analyze the content of magnetic signals to identify the presence of these broken wire signatures. After the resemblance traits, it is easy to see nuances between the candidate wavelet patterns. The second idea that follows naturally is the following: the signature of a wire break is itself a wavelet. This is the starting point of wirelets, a family of wavelets whose construction is dictated by the laws of physics.

Mexican hat wavelet

mexican hat wavelet

mexican hat wavelet

mexican hat wavelet

Cauchy wavelet

Cauchy wavelet

Cauchy wavelet

Cauchy wavelet


11-17 Teknova seminar

Teknova seminar 2017

Letscan was invited to give a lecture at the Teknova Steel and Fiber ropes seminar in Grimstadt, Norway entitled "Explore the hidden world behing MRT signals" to professionnals of the offshore industry

04-17 OIPEEC award

OIPEEC 2017 award

Letscan was awarded the 2017 OIPEEC price for its Wirelet innovation, a numerical tool enabling to explore the time-frequency content of magneto-inductive signals which greatly improves the assessment of rope condition

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