WO1996034279A1

WO1996034279A1 - Method and device for measuring ferro-magnetic objects

Info

Publication number: WO1996034279A1
Application number: PCT/SE1995/000444
Authority: WO
Inventors: Göran Larsson; Jan Sundström
Original assignee: Larsson Goeran; Sundstroem Jan
Priority date: 1995-04-24
Filing date: 1995-04-24
Publication date: 1996-10-31
Also published as: EP0823053A1; FI973993A; FI973993A0; AU2685595A

Abstract

Method and device for detecting anomalies in objects of ferro-magnetic materials, characterized in that a measuring means (9) comprising at least one coil (20), the coil being placed against an object (1) to be measured, that said coil (20) is provided with alternate current, inducing eddy currents in the object (1) giving rise to magnetic fields which vary depending on the anomalies, that the resulting magnetic fields are detected by said measuring means and that said detected magnetic fields are a measure of the degree of anomalies in the object (1).

Description

METHOD AND DEVICE FOR MEASURING FERRO-MAGNETIC OBJECTS

Technical field

The present invention relates to a method and device for measuring ferro-magnetic objects with eddy current technology.

Background of the Invention

Today a lot of different techniques are employed for measuring objects to obtain different results, such as measuring thickness, detection of cracks and impurities, control of weld quality, detection of corrosion etc. For these different areas, different technologies have been developed, such as ultra-sonic, penetration techniques, measuring of flux and eddy current.

For measurement and control of corrosion and other anomaUes in the material on the inside as well as outside where it is not possible to have access to the outside, like faults on, for instance, pipes placed in the ground, containers and reservoirs with access only to the inside, lamp posts, etc., specific techniques and devices have been developed for the specific application. For instance, the patent US-A-3,543,144 discloses an apparatus for inspection of well pipes. The apparatus utilises a technique iu which a magnetiser produces flux to saturate the part of the pipe to be inspected. In order to do this, a core surrounded by a winding is brought inside the pipe.

The flux thus generated in the core flows through a closed loop including pole pieces and a cylindrical section of the casing between the pole pieces. Between said pole pieces and in contact with the casing are a number of detectors placed around the circumference of the casing. Each detector has a set of coils. One of these detects flux perturbations caused by flaws, cracks or other anomaUes in the casing and the other detects eddy current which is an indication if the fault is on the inside or outside. This method works weU for oil weU pipes and the like where the pipes are ferromagnetic and can be magnetically saturated. However, the pipe has to be empty in order to work, i.e. no cables or other objects can be present inside the pipe.

Other methods such as eddy current techniques have successfuUy been employed for detection of cracks and the like anomaUes on non- ferro-magnetic objects, for instance in the aviation and nuclear industry. The eddy current techniques are used to find anomaUes that are maximum 1-2 mm below the surface. Another area of use for this method is to measure the thickness of coating layers.

Ultra-sonic techniques can penetrate deep into materials and are often used to check welds or measure of thickness. This technique requires that a contact media is placed on the ultra-sonic sender which limits the use to certain appUcations.

All the above techniques have found their use for certain appUcations where they have proved successful. But to this day, none of these methods have proven useful in measuring the other side of a ferro-magnetic object in a simple and easy way when there is no access to the other side and/or without magneticaUy saturating the object, like for instance in measuring external corrosion on lamp posts under ground level without freeing that area of the post.

Brief Description of the Invention

One object of the present invention is to provide a method and apparatus with a speciaUy designed transmitter/receiver which detects and indicates the defects and anomaUes of an object of ferro-magnetic material, especiaUy on surfaces with no direct access, such as the outside of pipes, the part of street lamp posts that are placed under ground, the outer side of reservoirs and tanks that are placed underground, without magnetic saturation and which does not require contact medias and can be used through layers of paint or other surface layers without affecting the test result.

NormaUy when a magnetic material is tested, the material has to be magneticaUy saturated in order that the magnetic field of the detector shaU not be disturbed by the magnetising ability of the material. This is done in order to increase the penetration abiUty of the eddy currents. With the novel transmitter/receiver, it is no longer necessary to saturate the material magneticaUy.

This is achieved with a method for detecting anomaUes in objects of ferro-magnetic materials, characterized in that a measuring means comprising at least one coil is placed against the object to be measured, that the coil is provided with alternate current, inducing eddy currents in the object, giving rise to magnetic fields which vary depending on the anomaUes, that the resulting magnetic fields are detected by the measuring means and that the detected magnetic fields are a measure of the degree of anomaUes in the object. Detailed Description of a Preferred Embodiment

A detailed description of the method and an embodiment wiU be described below with reference to the attached drawings, wherein

Fig. 1 shows a perspective side view of an embodiment of the measuring device according to the invention,

Fig. 1A is a cross section taken along the line I-I in fig. 1,

Fig. 2A shows an enlarged cross- sectional front view of one arrangement of the transmitting and receiving coils in the probe,

FFiigg.. 22BB shows an enlarged cross-sectional side view of one arrangement of the transmitting and receiving coils in the probe,

Fig . 3A shows an enlarged cross-sectional front view of a second arrangement of the transmitting and receiving coils in the probe,

Fig- 3B shows an enlarged cross- sectional side view of a second arrangement of the transmitting and receiving coils in the probe,

Fig. 4 shows a cross- section of the probe along the line IV-IV in figs. 2B and 3B, and

Fig. 5 is a cross-sectional side view and shows an example of how the method according to the invention is used with an apparatus to detect anomaUes on a lamp post.

An embodiment of an apparatus 10 according to the invention, for measuring inside cylindrical objects such as lamp posts, shown in the figures comprises an elongated tube shaped part 11, referred to as arm, with a longitudinal axis A. The arm 11 is beudable and remains in the desired bent shape. It is preferably made of a bendable metaUic hose 14 covered with a protective resiUent sheath 15, for example rubber or plastic, fig 1 A. In one end of the arm 11, a probe 9 is attached.

The probe 9 comprises a transmitting coil 20 and a receiving coU 22, figs. 2, 3 and 4, arranged under and adjacent to each other as seen along the axis A, with each coil being transverse to said axis A. Two different sets of coils may be used. One set is shown in fig. 2 and comprises two ferrite cores wherein the first is a transmitter 20 of the primary magnetic field into the object to be tested and the other core is the receiver 22 of the secundary magnetic field, a so caUed ''Driver Pick-up Probe". The set shown in fig. 3 comprises a transmitter 20, 22 with a number of ferrite cores connected to each other with bridges and is fed with low frequency. The coUs 20, 22 are protected and held in place by a housing or a cover 24. The cover 24 may have the same cylindrical shape as the arm 11 but is preferably shaped so that an area 26 in front of the coils 20, 22 have a somewhat convex shape as seen along the arm 11. This area 26 is distinct in that the rest of the circumference of the probe 9 is separated with corner parts 28, fig. 4.

Tne other end of the arm 11 is preferably provided with a handle 12.

The coils 20, 22 are connected to electrical wires 13 which run inside the arm 11 and up through the handle 12. The wires 13 are then connected to an eddy current detector and processor device 8 of known kind, such as for example Phasec 3.4 or 2.2 manufactured by Hocking NDT Ltd, but of course any device for detecting and displaying eddy current signals can be used. Preferably, the device is provided with a CRT display (Cathode Ray Tube) and memory faciUties in order to display and store measuring signals.

The eddy current device 8 feeds the transmitting coil 20 with alternate current via the electrical wires 13 inside the arm 11. The probe 9 with the coUs 20, 22 is put against the object whose outside is to be measured. The eddy currents induced by the transmitting coil 20 into the object penetrate the material and gives rise to magnetic fields which vary depending on the dimensions and anomaUes of the material. It has been found that the lower the frequency of the eddy currents the deeper it will penetrate, and far deeper without saturation of the object for ferro-magnetic materials than has been thought possible. Therefore, the transmitting cott 20 is speciaUy developed to use frequencies in the range of 1-100 kHz. Objects with a thickness of four millimetres have successfully been measured with this method.

However, the lower the frequency, the lower the sensitivity, thus it is a choice of frequencies for the specific object to be measured as regards to thickness and material so that the eddy currents penetrate the object with as high sensitivity as possible.

The resulting magnetic field is detected by the receiving coil 22 and is a measure of the degree of anomaUes present in the object. The receiving 22 coU is preferably of the same type as the transmitting coil 20. The signal from the receiving coU is sent to and

_* displayed on the display of the eddy current device. One coU could also be used as both sender and receiver. However, with only one coil, the sensitivity decreases and thus two coils are preferred. In order to be able to evaluate the signal from the receiving coil 22, the device is caUbrated against a reference object with the same material as for the object to be measured, and with predefined reference errors made on the reference object, before it is used for the first time in a specific appUcation.

»

The coils 20, 22 of the probe 9 are put against the reference object on the opposite side of the reference errors. The errors could for example be grooves that are ground to specific depths, e.g. one, two and three millimetres. The eddy current device feeds the transmitting probe 20 with alternate current which in turn induces eddy currents in the object which gives rise to magnetic fields. The detected resulting magnetic field, i.e. the output signal from the receiving probe, is then caUbrated so that it has a certain ampUtude on the display of the eddy current apparatus. As an example, if the object to be measured has a thickness of four millimetres and the eddy current apparatus is caUbrated so that the ampUtude from the reference error of one miUimetre corresponds to the distance between two lines in the grid of the display of the apparatus, two millimetres corresponds to two lines etc., it is then very easy to obtain a percentage of the errors by just looking at the ampUtude in that with these reference errors there is a twenty-five percent difference between the lines on the display. Of course, other reference errors may be chosen.

By the foregoing description it is to be understood that it is not the actual thickness of the object that is measured, but information is obtained of how affected^lthe measured item is. The apparatus is also caUbrated such that the material of the object to be measured provides peaks that are vertical on the display. If another material is encountered or a change has occurred in the material, the peak on the display has a different phase angle due to the difference in electrical conductivity for different materials. This is especiaUy useful in detecting, for example, if the galvanisation on a galvanised measured object has worn off in some areas on the object. This is immediately displayed by a different phase angle of the peak on the display.

It has also been found that clearer and more distinct peaks are obtained if the probe is moved with some speed over the area with anomaUes. If the probe passes over a hole, a peak is obtained at the edge between the surface and the hole. When the probe moves further over the hole, i.e., when the probe is out of contact with the surface, the signal resets. When the other edge of the hole is encountered, another peak is obtained but with a 180° phase difference.

If the probe loses its contact with the surface during measuring, this is also displayed by peaks with phase angles of about 45°. Unwanted indications are thus easy to separate in that they have a phase angle different from vertical.

The foUowing is an example of how the measuring technique is used for checking and evaluating lamp posts 31, fig. 5. These are often made of ferro-magnetic steel and are galvanised to withstand corrosion. They are usuaUy placed in a foundation 32 of concrete placed in the ground. A rubber ring 33 around the lamp post 31 in the area between the post and the foundation is thought to act as a seal. This is often not enough to prevent water from entering the space between the post 31 and the foundation 32 and is the area where corrosion is usuaUy formed.

Before the measuring procedure begins, and as described above, the eddy current device 38, which is connected to the probe 39 via electrical wires 43, and the probe 39 are caUbrated against a reference object with the same material and thickness as the posts to be measured and with reference errors in order to obtain the ampUtude and desired phase angle of the peak. During the measuring procedure, the aπn 41 with the probe 39 is inserted into the inspection opening 34 of the lamp post 31. The probe 39 is first held against the inside at an area where the outside is free of errors in order to reset the system-

As weU as being indicated on the display 48, the operator can easily feel if the probe

39 has come out of contact with the surface due to the distinct area on the probe 39 in which the coUs are placed. The probe 39 is then moved down and upwards along the inside of the post with the area with the coils in contact with the inside surface. The operator "scans" the lower area 37 and the measuring values are stored in the eddy current device 38 and the position of the probe 39 at each value is recorded. The arm

41 could be provided with some sort of indicating means 16, fig. 1, which indicates how deep the probe is inserted into the post.

From the values it can be estabUshed how affected the lamp post is and if it has to be replaced. If an area with corrosion is encountered, as is often the case around the outside base of the lamp post where it is inserted into the concrete foundation, the eddy current coils will detect this and a peak wiU be displayed on the apparatus, the ampUtude of the peak depending on how affected the area is, i.e., how deep the corrosion has entered into the material.

In contrast to the conventional methods for inspecting lamp posts and the method disclosed in US-A-3,543,144, a very quick and accurate estimation of the state of the lamp post is obtained without saturating the post and with the cables 6 stiU inside the post. Thus, instead of removing one lamp post and inspecting it, and if it is badly affected, changing aU lamp posts that are of the same age, as is very common, only individual posts that are found badly affected when they are checked with the method of the invention are replaced. This drasticaUy reduces the costs for replacing street Ughting. In this case the eddy current device is preferably portable so that the measuring device can be operated by one or maybe two persons.

It is to be understood that the present invention is not limited to the embodiment described and can be changed within the scope of the patent claims to be foUowed. Thus, the probe may have any desired shape depending on the appUcation as long as the transmitting and the receiving coUs are placed beside each other and with one of their ends facing the surface to be measured. The mounting of the probe can naturaUy be done in any suitable fashion depending on the appUcation, and may, for example, be directly handheld.

Thus, the measuring device can be designed in a variety of ways within _the scope of the present invention. Because any eddy current apparatus can be used, it is also to be understood that multi-channel devices can be used in which unwanted signals can be filtered. Signal thresholds may also be used with these apparatus.

Claims

PATENT CLAIMS

1. A method for detecting anomalies in objects of ferro-magnetic materials, characterized in that a measuring means (9) comprising at least one coil (20), the coil being placed against an object (1) to be measured, that said coil (20) is provided with alternate current, inducing eddy currents in the object (1) giving rise to magnetic fields which vary depending on the anomalies, that the resulting magnetic fields are detected by said measuring means and that said detected magnetic fields are a measure of the degree of anomalies in the object (1).

2. A method according to claim ^characterized in that the magnetic fields from said induced eddy currents penetrate said object (1) and that said detected resulting magnetic fields are a measure of the degree of anomalies on the other side of said object as seen from the measuring means side.

3. A method according to claim 1 or 2, characterized in that the measuring means (9) comprises one transmitting coil, (20) inducing eddy currents in the object to be measured and one receiving coil (22) detecting the resulting magnetic field.

4. A method according to claims 1 to 3, characterized in that the degree of anomalies is determined by comparison with known reference errors.

5. A method according to claim 4, characterized in that the measuring means (9), before the measuring procedure, is calibrated against a reference object with the same material as the object to be measured and provided with predefined reference errors.

6. A method according to claim 5, characterized in that said reference errors consist of grooves of different depths.

7. A method of detecting anomalies in lamp posts according to any of the claims 1 to

4, characterized in that said eddy current inducing coil (20) is put in contact with the inside of said post (31), that the coil (20) is moved along the inside of said post, inducing eddy currents which gives rise to magnetic fields in the material of said post, detecting the resulting magnetic fields and that said detected magnetic fields are a measure of the degree of anomalies in said lamp post

8. A method according to claim 7, characterizedin that said detected resulting magnetic fields are a measure of the degree of anomaUes on the outside of said lamp post.

9. An apparatus for detecting anomaUes in objects of ferro-magnetic materials, comprising an eddy current device and a measuring means connected electricaUy to said eddy current device, characterizedin that said measuring means (9) comprises at least one coU operable of operating at a frequency in the range of 1-100 kHz.

10. An apparatus according to claim 9, characterizedin that said measuring means comprises a transmitting coil (20) operating at a frequency in the range of 1- 100kHz, operable of inducing eddy currents which gives rise to magnetic fields and a receiving coil (22) operable of detecting resulting magnetic fields .

11. An apparatus for detecting anomaUes in lamp posts according to claim 9, characterized in that said measuring means (39) is placed at the end of an elongated part (41) operable to be inserted inside said lamp post.

12. An apparatus according to any preceeding claim, characterizedin that said probe comprises two coUs, one transmitting coil (20) and one receiving coil (22), each of said coils comprising at least one ferrite core.