WO1990004779A1

WO1990004779A1 - Metal corrosion detection

Info

Publication number: WO1990004779A1
Application number: PCT/GB1988/000876
Authority: WO
Inventors: Julian Marsden Carroll
Original assignee: Julian Marsden Carroll
Priority date: 1987-04-15
Filing date: 1988-10-18
Publication date: 1990-05-03
Also published as: GB2203549A; GB8709033D0; GB2203549B

Abstract

Apparatus is described for corrosion detection, especially detection of corroded wall ties. An array of Hall effect sensors (10) is linked by the magnetic field of a permanent magnet and the output from the sensors are multiplied by devices (30, 32, 34, 36, 44) and amplified by amplifier (42) to drive a meter supply (46). The patterns of voltages generated within the array of sensors differ between the situation where the field of the permanent magnet is substantially undisturbed, where it is disturbed by the presence of a sound wall tie, and where it is disturbed to a lesser extent by the presence of a corroded tie, due to the reduced ferrous content of a corroded tie. In a method of detecting corroded wall ties, a unit having the array of Hall effect sensors and the permanent magnet is positioned adjacent a wall with the magnet more remote from the wall than the sensors and the unit is moved to a wall position at which the sensors exhibit a voltage pattern, and a reference signal is generated by processing the sensor voltage. The unit is then moved to a position at which the sensors exhibit a different voltage pattern and the sensor voltages are again processed to establish a measurement signal, and the reference signal and measurement signal are compared.

Description

g Title: Metal Corrosion Detection

Field of the invention

This invention relates to apparatus for and a non- intrusive method of detecting metal corrosion, more especially corrosion which occurs in cavity wall ties.

Background to the invention

Cavity walls comprise two skins held in fixed relationship by metal ties, either of wire, mild steel strip or cast iron. Such ties commonly corrode with passage of time, eventually to give rise to cracking of the masonry and possible collapse of the wall. It is clearly desirable to replace corroded wall ties before damage can result, but currently the only reliable method of inspecting ties for corrosion is a destructive one, which entails the removal of bricks or other wall blocks to expose the ties to view.

The invention

According to one aspect of the present invention there is provided apparatus for corrision detection, especially detection of corroded wall ties, comprising: i an array of Hall effect sensors;

a permanent magnet generating a field through the array of sensors;

SUBSTITUTE SHEET processing circuitry responsive to the voltages generated at the sensors in the array; and

means for displaying and/or recording one or more output signals dependent on the voltages generated in the sensors.

The principle of the invention is that the patterns of voltages generated within the array of sensors differ between the situation where the field of the permanent magnet is substantially undisturbed, the situation where it is disturbed by the presence in its vicinity of a sound wall tie, and the situation where it is disturbed to a lesser extent by the presence in its vicinity of a corroded tie, due to the reduce ferrous content of the corroded ties.

It is possible simply to multiplex the sensor voltages into a volt meter and display device, so that a simple comparison of the amplitude of the output signals in differing situations is indicative of wall ties which are sound and wall ties which are badly corroded. A further aspect of the invention therefore concerns a preferred method of detecting metal corrosion.

According to this further aspect of the invention, a method of detecting metal corrosion, especially detection of corroded wall ties, comprises the steps of:

locating an array of Hall effect sensors in the field of a permanent magnet;

positioning the sensor array adjacent a wall with the

SUBSTITUTE SHEET magnet more remote from the wall;

moving the magnet and array to a wall position at which the sensors exhibit a voltage pattern and processing the voltages to establish a reference signal;

moving the magnet and array to a position at which the sensors exhibit a different voltage pattern and processing the voltages to establish a measurement signal; and

comparing the reference signal and measurement signal.

Preferably, in this method, the individual sensor values taken during determination of the reference signal and determination of the measurement signal are stored, the difference values are determined for each sensor, and the difference values are summed. In some instances, only the voltages of sensors in the central region of the array may be utilised in this method.

In a preferred method, the individual sensor voltages are stored for a tie-absent wall position, thereby to establish a background reference pattern, the individual sensor voltages are stored for one or more sound wall tie positions, thereby to establish a mean sound tie reference pattern, the sensor voltage differences for at least some of the sensors are determined to establish a sound tie difference signal pattern, the individual sensor voltages are stored for a suspect wall tie position, thereby to establish a measurement voltage pattern, the sensor voltage differences (for at least some of the sensors) are determined relative to the background reference pattern in order to establish a suspect tie difference signal pattern, and the sound tie difference signal pattern is

SUBSTITUTESHEET compared with the suspect tie difference signal pattern.

In connection with this method, it will be understood that a background reference pattern is indicated by a minimum sum of the sensor voltages and a sound tie by a maximum value thereof. Centering on a wall tie is indicated by maximum sensor voltages in the central region of the array. A typical sensor array would be a two-dimensional array, having a plurality of sensors extending in two mutually perpendicular directions.

In the preferred method, the sum of the difference signal voltages from the sound tie may be compared with the sum of the different signal voltages for a suspect tie. A substantial difference in the two sums is indicative of a severely corroded tie. In practice, it is badly corroded wall ties which are most important to detect, as distinct from ties suffering from only limited corrosion, and it is an advantage of this invention that the apparatus and method is most sensitive when there is in excess of 50% corrosion.

Summation values obtained for corroded wall ties will vary to some extend with position (orientation and depth) of the tie within the wall, but badly corroded ties will always be detectable. Moreover, an analysis of the pattern of sensor voltages through the array, as by means of a suitable algorithm, can enable information as to the positioning of the corroded tie to be derived, thus assisting in the work necessary to effect a replacement.

In a practical apparatus, the magnet and sensor array may constitute a portable unit, and the processing, measuring and display circuitry may be incorporated in a second unit

SUBSTITUTESHEET attached by a cable to the first unit.

A power supply, preferably of a rechargable kind, is required for the array of Hall effect sensors, and this may also form part of the second unit.

If desired, however, at least part of the display equipment may be incorporated in the first unit to facilitate the task of positioning the magnet and array at different sites on the wall. One suitable form of basic display is an LCD meter coupled to the sensors via two rotary switches which in combination select a particular sensor for reading on the meter.

Description of embodiment

The corrosion detection apparatus and method in accordance with the invention is exemplified in the following description, making reference to the accompanying drawings, in which:

Figure 1 is a block circuit diagram of the apparatus, and

Figures 2 to 4 are tables exemplifying results achieved with the exemplary apparatus.

Referring to Figure 1, the reference 10 denotes a 9 x 7 array of Hall effect sensors, conveniently sprague UGN 3503U linear Hall effect sensors. This array 10 forms part of a sensor head 12 which also includes a ferrite permanent magnet. Figure la shows the circuit element associated with one sensor.

The remaining parts of the apparatus are incorporated in a supervisory unit 14 connected to the sensor head 12 by 40- way connectors 16, 40-way cables 18 and 40-way plugs/sockets 20.

In a supervisory unit, a controlled power supply (Figure lb) includes a sealed lead acid battery 22, a battery indicator 24, recharging leads 26, and a d.c/d.c convertor 28. The power supply provides outputs of 0 volts and 5 volts to the sensor head 10.

The remainder of the supervisory unit comprises circuitry for processing, measuring and displaying the voltages,^, developed at the Hall effect sensors in use. This circuitry includes four stores 32 to 36, two rotary switches 38, 40, the latter forming part of a drive circuit, generally referenced 42, and which includes an arithmetic unit 44, for a digital display module 46.

Figures 2 to 4, comprising tables 1, 2 and 3, respectively show the pattern of different signal sensor outputs obtained for a wire tie, a sound cast iron tie, and a corroded cast iron tie. The different signals are obtained by substracting the sensor voltages developed in the presence of a tie from the voltages developed in the absence of a tie, ie, when the field of the permanent magnet is substantially undisturbed. The voltage pattern in the last mentioned case is substantially uniform over the entire array.

In all these tables it can be seen that highest values are in general but not universally obtained from the central region of the array, indicating that a sensor head has been approximately centred on a non-visible tie in a wall under examination. In the case of the corroded tie, it

SUBSTITUTE SHEET can be seen that the highest values are again generally in the centre of the array, the values of the different signals are much reduced, because the disturbance of the magnetic field is substantially less.

It will be understood that the above described and illustrated embodiment may be modified in various ways within the scope of the invention hereinbefore defined.

SUBSTITUTESHEET

Claims

1. Apparatus for corrision detection, especially detection of corroded wall ties, comprising:

an array of Hall effect sensors;

a permanent magnet generating a field through the array of sensors;

processing circuitry responsive to the voltages generated at the sensors in the array; and

2. Apparatus as claimed in claim 1 wherein the sensor voltages are multiplexed into a volt meter and display device, so that a simple comparison of the amplitude of the output signals in differing situations is indicative of wall ties which are sound and wall ties which are badly corroded.

3. Apparatus as claimed in claims 1 or 2 in which the sensor array, is a two-dimensional array, having a plurality of sensors extending in two mutually perpendicular directions.

4. Apparatus as claimed in any of claims 1 to 3, in which the magnet and sensor array constitute a portable unit, and the processing, measuring and display circuitry are incorporated in a second unit attached by a cable to the first unit.

5. Apparatus as claimed in claim 4 wherein a power supply for the array of Hall effect sensors, forms part of the second unit.

6. Apparatus as claimed in claim 4 wherein part of the display equipment is incorporated in the first unit to facilitate the task of positioning the magnet and array at different sites on the wall.

7. Apparatus as claimed in claim 6 wherein the basic display is an LCD meter coupled to the sensors via two rotary switches which in combination enable a particular sensor to be selected for reading on the meter.

8. A method of detecting metal corrosion, especially detection of corroded wall ties, comprising the steps of:

locating an array of Hall effect sensors in the field of a permanent magnet;

positioning the sensor array adjacent a wall with the magnet more remote from the wall;

moving the magnet and array to a second position at which the sensors exhibit a different voltage pattern and processing the voltages to establish a measurement signal; and

comparing the reference signal and measurement signal.

9. The method as claimed in claim 8 in which the individual sensor values taken during determination of the reference signal and determination of the measurement signal are stored, the difference values are determined for each sensor, and the difference values are summed.

10. The method as claimed in claim 9 in which only the voltages of sensors in the central region of the array are utilised.

11. The method of claims 8 or 9 or 10 wherein individual sensor voltages are stored for a tie-absent wall position, thereby to establish a background reference pattern, the individual sensor voltages are stored for one or more sound wall tie positions, thereby to establish a mean sound tie reference pattern, the sensor voltage differences for at least some of the sensors are determined to establish a sound tie difference signal pattern, the individual sensor voltages are stored for a suspect wall tie position, thereby to establish a measurement voltage pattern, the sensor voltage differences (for at least some of the sensors) are determined relative to the background reference pattern in order to establish a suspect tie difference signal pattern, and the sound tie difference signal pattern is compared with the suspect tie difference signal pattern.

12. The method of any of claims 8 to 11 in which the sum of the difference signal voltages from the sound tie are compared with the sum of the difference signal voltages for a suspect tie.

13. Apparatus for corrosion detection constructed and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.

14. Method of corrosion detection substantially as herein described.