Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
  • G01B7/00—Measuring arrangements characterised by the use of electric or magnetic techniques
  • G01B7/02—Measuring arrangements characterised by the use of electric or magnetic techniques for measuring length, width or thickness
  • G01B7/06—Measuring arrangements characterised by the use of electric or magnetic techniques for measuring length, width or thickness for measuring thickness
  • G01B7/10—Measuring arrangements characterised by the use of electric or magnetic techniques for measuring length, width or thickness for measuring thickness using magnetic means, e.g. by measuring change of reluctance
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
  • G01B7/00—Measuring arrangements characterised by the use of electric or magnetic techniques
  • G01B7/12—Measuring arrangements characterised by the use of electric or magnetic techniques for measuring diameters
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
  • G01V3/00—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation
  • G01V3/08—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation operating with magnetic or electric fields produced or modified by objects or geological structures or by detecting devices

Definitions

• the present invention concerns the location and measurement of hidden or buried bars, particularly reinforcing bars using a pulse-induction metal detector.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• the collapsing field will induce eddy currents to flow in the target, in opposition to the change in magnetic field.
• 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.
• 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.
• 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
• a transmitted pulse 1 is fed from the pulse generator through the transmitter amplifier to the search coil 20.
• 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.
• 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.
• 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.
• 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.
• FIG. 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.
• 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.
• 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.
• 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.
• FIG. 4 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.
• 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, V 1 and V 2 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.
• EOC end of conversion
• 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.
• separate analog-to-digital converters may be used to digitize the outputs of the integrators.
• 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.
• 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.
• the ratio varies very little with the angular alignment between the axis of the search head and the line of the bar.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• the peak current to be passed is of the order of 0.25 amperes
• the peak voltage present is about 100 volts
• 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.
• the transmitter required for driving the coils is, in essence, a simple transistor switch.
• 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.
• FIGS 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.
• 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.
• V., of the integrator amplifiers 25a and 25 may be coupled to a two-channel, analog-to-digital convertor.

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• 1989
  • 1989-12-04 EP EP19900900261 patent/EP0399026A1/de not_active Withdrawn
  • 1989-12-04 WO PCT/GB1989/001447 patent/WO1990006488A1/en not_active Application Discontinuation

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