Using magnetic method a rope expert have a possibility to estimate the
rope condition. In conjunction with visual examination this method may
be applied to determine the moment when the rope should be discarded.
Various equipment for different application ranges is available.
Although magnetic NDT of wire ropes has been in regular use in a number of countries for 30 or more years, it is still not commonly known NDT method. This method is well known and recognised in application areas such as inspection of hoisting ropes in deep mines and inspection of ropeways.
Equipment recently used for non-destructive testing of steel wire ropes generally uses the same method, "permanent magnet method". The method is based on magnetisation of the rope with permanent magnets and detection of rope anomalies indirectly by magnetic sensors. This method is somewhere called "DC" magnetic method because of previously used Direct Current excitation coils, opposite to previously used Alternating Current coils (out-dated AC method).
Since very first introduction in Poland (at AGH university), for latest over 20 years almost all manufacturers supply sensing heads where permanent magnets longitudinally magnetise a length of rope as it passes trough the head. A constant magnetic flux that magnetises the rope must be strong enough to create condition near magnetic saturation of the rope length.
Various types of sensors have been applied by some manufacturers of instruments across the world. Sensors provide different signals depending on the design of the magnetic concentrators and type, number and location of sensing devices. Inductive coils and/or Hall generators are popularly used as sensing devices. However generally, due to its application concept, sensors can be divided into two types:
|
i. | LF sensors, i.e. Local Fault or Local Flow sensors;
|
|
ii. | LMA sensors, i.e. Loss of Metallic cross-sectional Area. |
LF type discontinuity in the rope, such as broken wire or corrosion pit creates radial magnetic flux leakage and LF sensor detects it as the rope passes trough the sensor. LF sensor is placed coaxially around the rope, centrally between magnetic poles of the magnetising circuit. Its signal is rather qualitative then quantitative. However this signal provides information about presence of local fault and also more or less information about its magnitude.
LMA sensor measures total axial magnetic flux in the rope as an absolute magnitude or variations in a steady magnitude of the magnetic field. This signal is proportional to the volume of steel or the change in steel cross-sectional area. It provides information about loss of steel due to missing wire, continuous corrosion or abrasion.
LMA sensors are located in various places, almost within magnetising circuit or nearby it.
When absolute value is displayed it is somewhere called TCMA, i.e. "total change of metallic area".
If an NDT instrument is designed to detect primarily either LF or LMA, but not both, it is called "single function" instrument. "Dual function" instrument detects both, separately.
Two categories of equipment to test ropes have been supplied:
|
i. | simplified auxiliary testers for detecting and indicating localised flaws or loss of metallic cross-sectional area with a light flash or an acoustic signal;
|  |
|
ii. | high-end instrumentation with strip chart and/or computer recording which is capable of estimating loss of metallic cross-sectional area and localised losses, and features the real aid to determine true deterioration of the rope. |  |
The first of the above categories is less expensive than the second category and the instruments are almost simple tools, mostly hand-held. This kind of instruments makes visual examination of the rope more convenient and reliable. Sometimes, like Meraster MD-20 Tester, they are equipped with a recording signal output, which allows their application as sensing head for detailed inspection of the rope.
The second category of instrumentation is intended to perform detailed tests. In conjunction with visual examination they may be applied to determine the moment when the rope should be discarded. Generally this instrumentation consists of two units: a sensing head; and a signal processing/recording instrument. Sometimes the signal processing part and a standard chart recorder are supplied as separate units. Now some suppliers offer portable computers and software for use instead of chart recording. Detectability of rope defects depends mainly on the sensing head employed but readability of its signals and ease of operation depend mainly on recording/processing instrument.
The sensing head brings the running sector of wire rope to the condition close to magnetic saturation and senses magnetic fields. All reputable manufacturers employ at least double-channel sensing system: one to detect localised losses (LF), and the other one to detect the distributed loss of metallic cross-sectional area (LMA or TCMA). Only some types of Polish-made and German heads are equipped with additional channels to estimate the depth inside the rope of a localised loss position.
Detecting capabilities of sensing heads vary between manufacturers and rope constructions. They depend on strong magnetisation capability, shape of magnetic concentrators in the sensor and operating principle of the sensor. In order to measure running rope length (and speed of relative movement), some manufacturers supply heads equipped with special transducer for indicating rope/head movement as an electric signal. Some manufacturers use it to synchronise the strip chart feed with the rope/head travel. This signal is also useful to compensate the speed influence on the inductive coil signal.
Processing electronics depends on the sensor types and equipment features. For example the Hall generator sensor requires supply control and compensation of DC component of its signal, and the inductive sensor signal needs rope speed compensation to achieve good performance of the instrumentation. Some instruments have additional circuits that make them more convenient in use, e.g. rope length/speed measuring circuits.
A strip chart recorder seems to be indispensable in each fully functional wire rope NDT instrument, as a third part of an instrumentation set, or integrated with the electronic processing part of the equipment. Mostly, manufacturers of these NDT instruments use standard, stand-alone or OEM unit recorders. Almost, it is a two-channel digital thermal array printer or sometime analogue pen recorder. A recorder appropriate for this sort of application must be equipped with drive control to achieve good correlation between the recording and the wire rope at any non-controlled rope speed, within test speed range. The recording should be performed at real time mode, instantly. Meraster MD120 Defectograph is an example of extremely task dedicated recording instrument.
In addition, specialised computer software is supplied as an extension of the equipment capabilities. However some manufacturers supply software and notebooks instead of chart recording instruments. This way seems to be easier today but mainly for suppliers. Actually, most of NDT users prefer instant ease readable strip chart recording then signal runs displayed on notebook screen.
Meraster MD120 Wire Rope Defectograph
Based on many years of previous experience, the first model of this instrument was introduced in 1994. Since this date MD120 series has been supplied to rope experts in Poland and around the world and it has been recognised as a valuable state-of-art instrument
Apart from the standard features of reliable instrumentation, mentioned above, the unique features of the MD120 Defectograph are: capability of determining the rope defect depth location inside the rope; running integral method for easy read out of high density of defects; zoom replay of recording; solid state memory (computer compatibility); automatic printing of annotations; automatic set up after entering the specific rope code ("settings + rope code" memory).
The Defectograph equipped with a suitable sensing head with a three-channel sensor, records test signals in four measurement channels. Two channels of inductive sensors (inner and outer coils) are intended for detecting "localised losses". Relation between recorded values in both these channels indicates depth of the defect position inside the rope. Channel of Hall-effect sensor signal is provided for detecting of "distributed loss of metallic cross-sectional area"; Fourth channel, integral of the main inductive sensor (inner coil) signal is intended for indicating the totalled "localised losses" along a rope sector.
Fig 4: Example of a
Wire rope test chart
|
This last channel needs some explanation to understand its unique role. There are two advantages of this recording, particularly for mining hoist ropes, where broken wires are concentrated. First, more readable indication of a real damage resulting from broken wires, located close to each other, than in "localised losses" channel. Second, set-up of integration range in instrument according to rope discard criteria "number of broken wires in any x diameter length" allows indicating total losses in appropriate rope sectors lengths.
The instrument operates continuously, in the "running integration" mode, where integration is being performed on a length in the next rope sector. The instrument is recording current values of the integral (total of losses) of previous rope sector, last "x" metres length. If the length of integration range is set appropriate to discard criteria, it gives direct readable indications of rope sectors in which the number of broken wires probably exceeds value of the discard criteria.
During the rope NDT procedure performed in-situ, audio-alarm and "Zoom Replay" capabilities are useful. The Defectograph generates the audio-signal when the pulse value in the "localised losses" channel has exceeded pre-set alarm level. When a significant rope defect has been observed during recording, the user can stop the rope (or head) movement and recording of signalss, and then may replay a previous recording in the zoom mode. Defect position may be read out precisely and found in the rope. Visual examination of the rope sector in question should then be made, additionally.
Solid state memory is an option. This is a credit card size SRAM IC Memory Card conforming to the PCMCIA (PC Card) standard. PCMCIA cards are compatible with almost notebook computers. Also PCMCIA slots can be added to most of personal computer systems. In certain rope NDT conditions, for instance subject to magnetism, this method of data transfer has many advantages. With this option, the Defectograph may store additionally an all-rope test record in the memory card. Capacity of the recording depends on the card version, e.g. 1 MB card can storage test of a rope of 600 m in length and 4 MB - 2400 m. Then data may be sent easily and quickly to a computer via the PCMCIA slot. This way, the user can archive many test records for further comparative analysis and can employ software to help him in his work on rope test results. Also data from Memory Card may be replayed on a strip chart with an MD120 Defectograph, including old test records from computer storage memory.
The recorder prints automatically the number of annotations on strip chart, e.g. rope length in metres, a rope code set by the operator; recorder settings, direction of movement, date and time. Before a rope test, the user can enter into the instrument a specific identification code, which will be printed on the chart, and test settings like channel sensitivities will be stored with this identification code in non-volatile memory in the instrument. If the same codes are entered in future, the same settings may be applied automatically.
The recorder may operate in one of two main modes: chart feed synchronous to rope movement; or chart feed at constant selectable speed. Recording is done by means of a thermal array line printing on thermal paper. All of the instrument settings and measured values are displayed on a liquid crystal display. Any instrument setting may be changed with one only knob-push-button.
The instrument is designed for field service. Built in aluminium covered case with handle, the MD120 Defectograph is easy to carry. MD120 operates from a built-in rechargeable battery or various external power sources, AC or DC. Automatically microprocessor controlled recharging while external power is connected is provided.
Field service and user-friendly oriented functionality of the MD120 in conjunction with its capability of computer aided post-testing analysis make this instrument useful as well as every-day tool for rope expert and as a source of data for researchers and developers of methods. Easy access to the test records with computer software tools seems to be a real aid to make faster progress in the development of rope evaluation methods.
