EP0399026A1 - Instrument for measuring reinforcing bars - Google Patents

Abstract

The described instrument, which is used to determine the diameter of a buried reinforcing bar, comprises, in a first embodiment, a search coil (20) excited by an electric pulse intended to induce eddy currents in a bar of stainless steel, a member (45) for alternately coupling two integrators (25, 25a) to the coil, in response to two main sampling pulses which are temporarily separated, in order to obtain a measurement of the currents of Eddy induced at each sampling pulse, as well as a member (46) used to calculate the ratio of the integrated signals thus sampled and to compare this ratio with a table of known ratios for stainless steel bars of standard dimensions. In another embodiment, the instrument comprises at least one search coil (20) excited in order to induce eddy currents in a high resistance steel bar, a member serving to couple at least one integrator ( 25) to a search coil in response to a sampling pulse, a member for calculating the ratio between an integrated signal from a coil placed in a first position and an integrated signal from a coil placed in a second position, as well as a member for comparing this ratio with a known ratio table for high-strength steel bars of standard dimensions.

Description

INSTRUMENT FOR MEASURING REINFORCING BARS

The present invention concerns the location and measurement of hidden or buried bars, particularly reinforcing bars using a pulse-induction metal detector.

It is known to provide a detector which comprises a search coil, usually in the form of a figure of eight coil or other coil having a net zero area-turns product, means for energising the search coil with a first electrical pulse a d means for coupling an integrator to the search coil in response to a sampling pulse so as to obtain a measure of eddy currents induced in the bar.

It is also known to use a detector head comprising a search coil to detect hidden bars under a cover by taking a first reading with the detector head on the surface of the cover, and a second reading with the head spaced away from the cover. The readings are converted into pairs of values of cover depth and compared for all possible bar diameters to determine the correct bar diameter and cover depth.

The present invention is particularly concerned with the determination of diameter of hidden stainless steel and high-tensile steel bars.

SUMMARY OF THE INVENTION

According to one aspect of the invention, the delay time between the induction pulse and the first sampling pulse is reduced to ten microseconds in order that the eddy currents flowing in a stainless steel bar can be sampled.

According to a further aspect of the invention, at least two temporally displaced sampling pulses are provided and means are provided to compute the ratio of the integrated signals sampled in response to the respective pulses in order to derive a determination of the diameter of the stainless steel bar.

According to a third aspect of the invention means are provided to derive a measure of the diameter of a high-tensile steel bar from measurements of signal strength with the search coil disposed in relatively orthogonal directions.

According to a further aspect of the invention means are provided to derive a measure of the diameter of a high-tensile steel bar from a comparison of signals received from two search coils.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates certain waveforms appearing at various parts of a basic detector;

Figure 2 is a schematic drawing of a metal detector and associated processing device;

Figure 3 is a schematic drawing of a search coil of the kind preferably employed in the present invention;

Figure 4 is a schematic diagram of one embodiment of the system according to the invention; and

Figure 5 is a diagram showing waveforms occurring at various points in the embodiment shown in Figure 4.

Figure 6 is a schematic diagram of a further embodiment of the system according to the invention.

Figure 7 is a diagram showing waveforms occurring at various points in the embodiment shown in Figure 6.

DETAILED DESCRIPTION OF THE INVENTION

In order to detect hidden ferrous bars, which may be bars embedded in concrete as reinforcing bars, it is known to use a pulse induction method by which a unidirectional pulse of current is sent repeatedly through a search coil. Such pulses are shown at 1 in Figure 1A. The leading edge of the current pulse through the search coil is not critical but it is preferred to cut off the current pulse as rapidly as possible. As a result, a large transient voltage is generated across the coil. Figure IB shows the variation of voltage across the search coil, the voltage pulses being shown at 2 and the reverse transient at 3. The current through the coil produces a magnetic field which will collapse as rapidly as the current through the coil. If there is a conductive target in the vicinity, the collapsing field will induce eddy currents to flow in the target, in opposition to the change in magnetic field. Such eddy currents decay with time. If the size and conductivity of the target are sufficient, and the cessation of the primary current in the coil is sufficiently rapid, the decaying eddy currents persist at a significant strength for some time after the cessation of the transient voltage associated with the cessation of the primary current in the coil. These eddy currents generate a decaying secondary magnetic field which is picked up by the search coil in which is induced a small, decaying voltage across the coil in addition to, and for a time subsequent to, the voltages which would have been present due to the self inductance of the coil in the absence of such a target.

Figure IB shows at 4 a noise signal in the absence of a conductive target and the signal 5 induced in the search coil by the eddy currents flowing in the conductive target.

It is necessary also to generate at least one sampling pulse 6, which starts at a controlled time after the end of the cessation of the main current pulse. The delay is chosen to be longer than the time during which primary transients are present across the coil but short enough so that a signal from a metal target, if present, is still of adequate magnitude. Figure IC shows the sampling pulses and Figure ID the samples determined in response to the sampling pulses.

Figure 2 illustrates a system including a search coil 20, a transmitter 21, a pulse generator 22, a receiving amplifier

23, a switch 24, made operative by a sampling pulse, and an integrator amplifier 25. A transmitted pulse 1 is fed from the pulse generator through the transmitter amplifier to the search coil 20. At the specified time after the cessation of current pulse 1 a sampling pulse 6 is generated by the pulse generator 22 to close the electronic switch 24 and allow the integrator amplifier 25 to receive a signal from the search coil 20, by way of the preamplifier 23. The integrator amplifier 25 provides a direct voltage proportional to the area of the sampled signal. The integration is effective to remove high frequency noise present on the signal.

Quite often, a second sampling pulse is generated at a time sufficiently after the first that the signal from any metal if present has decayed away. The integrator amplifier will integrate the difference between the first and second signal samples to cancel out any low frequency noise picked up by the coil due to powerline radiation or variation in the earth's magnetic field.

As described so far, the system constitutes a basic metal detector. The signal at the output of the integrator increases with the proximity of the target and also with its size and depends moreover on the conductivity of the target. The output can be displayed in any convenient manner. Figure 3 illustrates a preferred form of search coil. The coil 31 is of a figure of eight configuration and is shown as a single turn coil, though it is actually constituted by a multiplicity of turns. It behaves as two adjacent, approximately square coils, carrying current in opposite directions. The field produced beneath the search coil is parallel to the plane of the coil. If the search coil is aligned with a bar 32 as shown in Figure 3, the principal field direction is along the length of the bar and the only currents generated will flow in well defined paths around the circumference of the bar. In this configuration eddy- current signal strength will vary with diameter of the bar in a consistent way. Also, if the bar is of ferrous metal, the field at the bar will be magnified by the effective relative permeability of the steel and so too will the eddy-current signal strength; no such magnification occurs if the field is transverse to the bar nor if the bar is substantially non-magnetic; for example, stainless steel.

The . remaining part of Figure 2 shows a microprocessor-based signal-to-depth converter. The output of the integrator amplifier goes to an analog-to-digital converter 26, of which the output can be read by microprocessor 27 which is controllable by means of keypad 28 and drives a display 29.

The signal strength V (e.g. at the output of amplifier 25) is related to the distance of the bar from the coil by an empirically derived equation. This is expressible in the form: where A is a constant determined by the number of turns in the coil, the bar size and other factors, Z is the distance of the bar and C is related to the coil size. From a large number of experimental measurements of the signal strength for a range of values of the distance of the bar and repeated for different sizes of bar, a look-up table can be constructed and stored. In operation the keypad connected to the microprocessor may be used to select the diameter of the bar being located and the quantity relating the distance of the bar in terms of the coefficients can be displayed.

As mentioned previously, stainless steel is essentially non-magnetic and would therefore provide less signal than a magnetic metal of the same electrical conductivity. Moreover, stainless steel has a very low conductivity compared to most other metals and alloys and so the eddy currents generated therein decay very quickly.

This can be accommodated by reducing the delay time between an energising pulse 1 and the next sampling pulse 6 to the order of ten microseconds. There is comparatively little loss of signal from the larger bar sizes during the delay time and even the smaller sizes in which there is a significant loss of signal still provide sufficient signal by the time of the sample pulse. The ability of the instrument to detect stainless steel is assisted by the configuration of the search coil. The eddy currents are flowing in an optimum path and the twisted winding has effectively a zero area-turns product.

Since the signal from a stainless steel bar at a given distance depends on its diameter, the display of distance or depth relies on knowing the size of the bar. Since the bar is invisible (in concrete, for example) the size may not be known and so a means of determining size is highly desirable.

This aspect of the present invention relies on an appreciation that the decay time of the eddy current is dependent upon the diameter of the bar. The variation of decay time with bar size could be observed on an oscilloscope connected to the output of the receiver but the information would be lost by the process of integration.

Accordingly it is now proposed that two principal sampling pulses be used (in addition to any sampling pulse used for low frequency noise cancellation) . An early sampling pulse would occur a short time after the cessation of the main current pulse and a late sampling pulse would occur a corresponding time later, overlapping the early sampling pulse if appropriate.

The corresponding early and late signal samples together with an even later sample for low frequency noise cancellation, as mentioned earlier, may be fed to separate integrators.

Figure 5 illustrates a modified form of Figure 1 employing two main sampling pulses 6, 6a occurring at approximately 10 microsecond intervals after the cessation of-=the current pulse 1 applied to the search coil. As may be seen, two samples 7 and 7a are obtained, having different signal strengths V.and ₂-

A system based on the use of two main sampling pulses is shown in Figure 4. The pulse generator 22, the transmitter 21 and the receiver 23, and the search coil 20 are similar to the foregoing. However, the system includes two switches 41 and 42, one of which is operated by an 'early' sampling pulse 6 from the pulse generator, while the other is operated by a late sampling pulse 6a from the pulse generator. Each switch is operative to couple the output of the receiver amplifier 23 to a respective integrating amplifier 25, 25a. The outputs of these amplifiers, V₁ and V₂, may be coupled to an analog-to-digital converter 43 according to the state of a bistable device 44.

The analog-to-digital converter produces an end of conversion (EOC) output which can be used to toggle the bistable device which operates switch 45 to switch the input of the analog-to-digital converter between the two integrators for alternate conversion cycles. The output of the bistable device may be passed to a microprocessor 46 as a status bit so that the microprocessor knows which of the two signals it is receiving.

In an alternative embodiment separate analog-to-digital converters may be used to digitize the outputs of the integrators.

The microprocessor 46 is arranged to calculate the ratio V₂/V, (with a decimal shift if required) . These signals are related exponentially and in particular by the equation V₂ = v.. exp (-t/T), where t is the time differential between the early and late sample pulses and T is the time constant of decay of the eddy current, and could be calculated but in fact is not required explicitly. Measurement of a variety of sizes of bar over a range of distances show that the aforementioned ratio, is substantially independent of distance. It also shows that the ratio is quite different for different bar sizes. The average values of ₂/V_. for any two adjacent sizes differ by more than the spread of V₂/V. in either. The microprocessor programme therefore may contain tables of the maximum and minimum values of the ratio for each standard bar size so that the measured ratio can be compared with the tables and the correct diameter deduced.

In principle, a different approach could be to find empirically an equation relating bar size to signal ratio and to realise a programming technique which could produce a diameter from a continuum of values. In practice this is not necessary, since bars always exist in discrete standard size .

The method according to this invention turns out to have a number of advantages. The ratio is independent of the vertical distance from coil to bar and independent of horizontal displacement, so that the search coil need not be exactly over the bar. Secondly, the ratio varies very little with the angular alignment between the axis of the search head and the line of the bar. Thirdly, if a structure contains several parallel bars of the same size in close proximity, the net decay time remains the same.

Moreover, owing to the readily distinguished ratios for different bar sizes, it may be possible to distinguish between the different grades of stainless steel currently in use. If the stainless steel bar sizing system is presented with a ferrous bar (such as high-tensile steel) the early/late sampling method will still furnish a repeatable ratio, even though the decay of eddy currents is not strictly exponential. It happens that the equivalent time constant is not merely dependent on the diameter of the bar but also on the effective permeability of the- steel. The effective permeability of a specimen of magnetic material is less than its true permeability, and is determined predominantly by the length-to-diameter ratio of the object. This effectively cancels out the diameter dependency and, although the system can recognise high-tensile steel, no further useful information can be obtained. If it be required to determine the depth or distance of bars which are suspected to be of high-tensile steel, the basic system described with reference to Figures 1 and 2 is adequate. However, this will give no indication of bar size.

In one method which may be used to obtain an indication of the size of a high-tensile steel bar, two phenomena already mentioned may be exploited. When the axis of the search coil and, accordingly, its magnetic field vector are aligned along the length of the bar, the signal from the eddy currents is magnified by the effective relative permeability of the bar, but no such magnification occurs if the field and bar are orthogonal. Second, the effective permeability of a bar decreases with an increase in the diameter of the bar.

It is found that, for any given bar size, the variation with depth of the ratio of the signal strengths obtained with the coil across and along the bar is small; but the average ratio does vary with bar size, the difference in the ratios for adjacent bar sizes being at least comparable to, and usually greater than, the spread of the ratio with distance.

Accordingly, as for the stainless steel bar sizing method, diametric determination can be implemented by fitting an equation to a ratio-versus-diameter plot, so that diameter may be derived from a ratio by evaluating a formula, or by the compilation and storage of tables of maximum and minimum ratios for each standard bar size using a look-up technique.

The instrument need not involve any more hardware than the basic cover meter shown in Figure 2. It is necessary only for the keypad to contain keys for the operator to indicate when the search head is respectively parallel and orthogonal to the bar respectively. The microprocessor then stores the values of signal strength at the instants of each of the two key closures, divides one by the other using scaled integer division, and consults a look-up table stored in the program.

This method provides -single, discrete evaluations of bar size at points along the bar.

As previously mentioned, it is known to use a detector head comprising a search coil to detect the diameter of hidden bars under cover by taking a first reading with the detector head placed directly on the surface of the cover, and a second reading with the detector head spaced a certain distance away from the cover by means of a spacer. The two readings are used to derive pairs of cover depths for all possible bar diameters, until a pair of values are found which differ by an amount equal to the known spacer distance. The bar diameter size corresponding to this pair of values, and the cover depth derived from the direct, first reading at that size, should then be the actual bar diameter and cover depth.

In practice, however, the derived cover depth varies only slightly with changes in assumed bar diameter, and the difference between the values of cover depth derived from a pair of readings changes even more slowly with postulated diameter. This fact, coupled with the compounding of errors in this difference value, results in the answer being rarely unambiguous.

According to a further aspect of the present invention there is provided an improved bar-sizing method wherein, as before, a direct and a spaced reading are taken, but these are retained as readings of signal strength, V, and V respectively, and not converted into values of cover depth. It has been found that the ratio of spaced signal to direct signal, ^V _s/^V _d' -^{s a} function of the distance from the surface of the cover (which may, for example, be concrete) to the centre of the bar, but independent of the diameter of the bar.

A graph of V /V. versus distance-to-centre is first prepared from measurements at known cover depths, and thereafter the distance to the centre of an unknown bar can easily be determined. In order to determine the diameter of the bar the absolute value of the direct signal V, is converted into a value of cover depth for all possible bar sizes on a trial and error basis, until a value is found for the cover depth which is compatible with the distance to the centre of the bar already derived.

Since the cover depth derived from signal strength increases with increasing bar size, but the cover depth derived by subtracting the bar radius from the distance from the distance-to-centre decreases with increasing bar size, the "point of intersection" between these two relationships is far less ambiguous than in the known method described above.

A detector head may have a planar search coil positioned inside and nearer to one face of the head than the other. The first, direct reading is taken with the coil nearer the lower face of the head. To take the second, spaced reading, the head is turned over so that the coil is nearer its upper face, and so is effectively spaced away from the cover surface by a known amount. Thus the need for a separate spacer is avoided. This method could be followed using the circuitry and hardware described in Figure 4, merely by incorporating in the software appropriate algorithms and a look-up table. However, even this improved method requires two discrete measurements, and the head must be physically turned over between them.

Figure 6 is a schematic diagram of a system whereby this aspect of the invention may be implemented in an improved manner, without the disadvantage of requiring two discrete measurements.

This is achieved by placing two separate windings in the head; one search coil 20 near the lower surface and the second search coil 20a some distance above, and thus nearer the upper surface of the head. The selection of "direct" or "spaced" readings may then be accomplished by switching the pulse induction circuitry to the lower or upper coil respectively. This changeover switching could be effected as required, but it would be more efficient to switch between coils on alternate transmitter-pulse cycles generated from pulse generator 22 according to the state of bistable device 44. As in the embodiment shown in Figure 4, the two integrators 25 and 25a following the receiver amplifier 23 may be used to accumulate the lower and upper signals essentially simultaneously.

To switch between coils with a semiconductor switch in fact causes some technical difficulties: the peak current to be passed is of the order of 0.25 amperes, the peak voltage present is about 100 volts, and the received signal (and hence the maximum permitted breakthrough of spurious pulses) is only of the order of a few microvolts; and this combination is not easy to achieve.

However, the transmitter required for driving the coils is, in essence, a simple transistor switch. Thus, one way of switching the transmit pulse between two coils is to use two such switches 21 and 21a, one for each coil, activated alternately by logic-level gating 60 and 60a of the existing transmit control pulse.

The problem of switching the received signal between the coils could be obviated by duplicating the receiver amplifier; but such a duplication is considered undesirable. It is therefore proposed that a "compromise" method be used whereby the two transmitter switches 21 and 21a drive the lower and upper coils 20 and 20a respectively with alternate transmit pulses, and a single receiver pre-amplifier 23 remains connected to the lower coil 20 at all times. The sample pulses are gated via logic-level gating 61 and 61a to operate the sampling analogue gates 62 and 62a of the two integrators 25 and 25a also on alternate transmit cycles such that, when gate 62 or 62a is closed, the integrator 25 or 25a respectively receives a signal from the search coil 20 by way of the preamplifier 23. Thus a "direct" reading is made using a "close" coil for both transmit and receive alternates, while a "spaced" reading using a "spaced" transmit coil and a "close" receive coil. The difference between the spaced and direct readings will now be less (in fact, the ratio will move nearer unity), and so the precision of the final results will not be quite so good as if two receiver pre-amplifiers were used; but the main advantage of this proposal is that the estimated cover depth and diameter of unknown bars will be processed and presented immediately and continuously, rather than as discrete spot measurements.

Figures 7A to 7K illustrate waveforms occurring at various points in the embodiment shown in Figure 6.

Figure 7A shows the transmit pulse generated by the pulse generator 22.

Figure 7B shows the sample pulse.

Figures 7C and 7D show the waveforms at the outputs of the bistable device 44 when the upper and lower coil are respectively selected. Figures 7E and 7F show the upper and lower coil current respectively which result from the transmit pulse.

Figure 7G shows the voltage induced in the lower search coil by the eddy currents flowing in the bar, where signal 71 is induced when the transmit pulse flows in the upper search coil, and signal 72 is induced when the transmit pulse flows in the lower search coil.

Figures 7H and 71 show the upper and lower sampling pulses which are output from logic-level gating 61 and 61a respectively.

Finally, Figures 7J and 7K show the upper and lower, "spaced" and "direct", samples V and V, of the lower coil voltage determined in response to the sampling pulses.

As in the embodiment shown in Figure 4, the outputs, V and

V., of the integrator amplifiers 25a and 25 may be coupled to a two-channel, analog-to-digital convertor.

Claims

1. An instrument for determination of diameter of a hidden stainless steel bar comprising a search coil (20) energised by an electrical pulse to induce eddy currents in the bar, means (45) for alternately coupling at least two integrators (25, 25a) to the search coil in response to at least two main sampling pulses, said sampling pulses being temporally separated, to obtain a measure of the induced eddy currents at each sample pulse, and means (46) for computing the ratio of integrated signals thus sampled.

2. An instrument for determination of diameter of a hidden stainless steel bar according^' to claim 1 wherein the two main sampling pulses occur at intervals of between 5 and 15 microseconds after cessation of the electrical pulse.

3. An instrument for determination of diameter of a hidden stainless steel bar according to claim 1 or claim 2 wherein the integrated signals are input to an analog-to-digital converter (43) according to the state of a bistable device.

4. An instrument for determination of diameter of a hidden stainless steel bar according to any preceding claim wherein the computed ratio of integrated signals is compared with a table of known ratios for standard stainless steel bar sizes.

5. An instrument for the determination of diameter of a hidden high tensile steel bar comprising at least one search coil (20) energised by an electrical pulse to induce eddy currents in the bar, means for coupling at least one integrator (25) to a search coil in response to a sampling pulse, and means for computing the ratio of an integrated signal resulting from eddy currents induced by a coil placed in a first position with an integrated signal resulting from eddy currents induced by a coil placed in a second position.

6. An instrument for determination of diameter of a hidden high tensile steel bar according to claim 5, comprising one search coil wherein the coil, in the first position, lies in a first plane and the coil, in the second position, lies in the said first plane and orthogonal to the first position.

7. An instrument for determination of diameter of a hidden high tensile steel bar according to claim 5 wherein a coil, in the first position, lies in a first plane, and a coil in the second position lies in a plane parallel to the first plane but displaced a set distance perpendicular to the first plane.

8. An instrument for determination of diameter of a hidden high tensile steel bar according to claim 7 comprising two coils; a first coil (20) being fixed the set distance apart from a second coil (20a) .

9. An instrument for determination of diameter of a hidden high tensile steel bar according to claim 8 comprising two integrators (25, 25a) which may be coupled to the first coil.

10. An instrument for determination of diameter of a hidden high tensile steel bar according to any of claims 5 to 9 wherein the computed ratio of integrated signals is compared with a table of known ratios for standard high tensile steel bar sizes.

11. An instrument for determination of diameter of a hidden high tensile steel bar according to any preceding claim wherein an additional sampling pulse is used for low frequency noise cancellation.

12. An instrument according to any foregoing claim and adapted to provide selectively a measure of the depth of cover of the bar.