Note: Descriptions are shown in the official language in which they were submitted.
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ELECTROMAGNETIC ANALYSIS OF
CONCRETE TENSIONING WIRES
Field of the Invention
This invention relates to a method of non-destructive inspection of
concrete conduits and cylinders, such as for example water pipes and
water reservoir vessels, which are reinforced with metal wires. The
invention also relates to apparatus for carrying out such inspections.
Background of the Invention
There are many wire-reinforced concrete structures in use to contain or
to conduct pressurized fluids, for example forming conduits in piping
systems for water or forming water reservoir vessels. Typical concrete
conduits are formed of concrete pressure pipe. Concrete pressure pipe
consists of a thin steel cylinder, over which a layer of concrete is cast.
Metal reinforcing wires are wound helically, either directly onto the metal
cylinder or onto a layer of concrete cast on the cylinder. Often, a second
layer of concrete is cast over the metal reinforcing wires. The exterior of
the pipe is then finished with a layer of mortar.
Concrete vessels used for water storage (as for example to contain
water for distribution) are usually cylindrical in cross-section, although
they are occasionally oval in cross-section. The vessel is wrapped
around its circumference with wire to provide compressive force to the
concrete of the reservoir to support the water contained within the vessel
forming the reservoir. Typically, the wire is of circular cross section,
although other cross-sections (e.g. rectangular) are known as well.
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The purpose of the reinforcing wires is to keep the concrete that they
overlie in compression. Over time, the wires may corrode and eventually
break. When this happens, it is possible that a rupture of the concrete
conduit or reservoir vessel will occur, leading to escape of the
pressurized fluid which it contains.
It is very expensive to replace an entire conduit or reservoir vessel.
Therefore, it is preferred to carry on some sort of inspection procedure,
to determine where wires have broken. This permits remedial work to be
carried out only in locations that need it.
Prior techniques of inspection have not been completely successful.
Some work has been done with remote eddy field current devices, and
U.S. patent 6,127,823 of Atherton has' proposed simultaneously using
remote eddy field effects and transformer coupling effects for inspection..
However, as admitted in that patent, the interpretation of the test results
is complicated. Further, because the device of the Atherton patent
preferably has a spacing of two to three pipe diameters between its
exciter coil and its detector coil, it is not suited to detecting wire breaks
near the ends of the pipeline, i.e. within two to three pipeline diameters
of the end.
Brief Description of the Invention
According to the invention, an inspection device is provided for concrete
pipes or vessels having a cylindrical wall reinforced with wires wound
around the wall, or concrete vessels having an oval wall with wires
wound around the wall. The device has one or more detectors proximal
to the wall to be inspected. The detector can be inside or outside the
wall. When the inspection device is used for inspecting pipelines, the
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detector is preferably inside the wall, attached to a vehicle which can be
pulled through the pipeline.
In one embodiment, the detector is a coil having an axis parallel to the
axis of the pipeline, and with an edge proximal to the wall of the pipeline.
In a preferred embodiment, there are two detectors, axially spaced from
each other. Preferably, where the detectors are coils, the detector coils
have a diameter considerably less than the diameter of the pipeline
being examined, and more preferably, not more than one-third of the
diameter of the pipe being examined.
In another embodiment, the detector is a non-coil detector of
electromagnetic fields, preferably a giant magneto resistive (GMR)
sensor. Preferably, the detector comprises three GMR sensors, with
their axes of sensitivity to magnetic flux orthogonal to one another. The
magnetic flux in the direction desired to be measured (for example,
along an axis parallel to the axis of a cylindrical pipeline or vessel) is
measured by measuring the flux in the three orthogonal directions
represented by the three detectors, and resolving the vectors to
determine the flux in the desired direction.
In one manner of operation, the invention provides a driver coil to create
an electromagnetic field, which creates a current flow through the wires
forming part of the wire-wound concrete pipe or vessel. The voltage and
other effects induced by this current in a detector are then measured.
Preferably, the driver coil has its axis orthogonal to the detectors, which
may be radial to the pipe or vessel in one manner of orientation of the
driver coil in relation to a cylindrical pipe or vessel being inspected. The
axis of the driver coil will be discussed with relation to a cylindrical pipe
or vessel, which is the normal case. If a pipe or vessel has an oval cross
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section, the two axes are parallel to one another. In that case, the term
"the axis" used herein means either of the parallel axes.
It is preferred that the axis of the driver coil lies in a plane extending
across the pipe or vessel, that is transverse to an axis of the pipe, and
intersecting the detector. Where there are two detectors, the axis of the
driver coil is preferably in a plane at right angles to an axis of the pipe
and intermediate the two detectors. This has the advantage that there is
no separation along the axis of the pipeline between the detector and
the driver. This permits measurements to be taken up to only a few
centimetres of the end of the pipe, which is not possible with apparatus
where an axial separation must be maintained. Although not preferred,
it is possible to use the invention with an axial separation along the pipe
between the detector and the driver coif. Distances of up to 3.05 m. (10
feet) separation in a 6.1 m. (20 foot) diameter pipe have been found to
work. However, such axial separation has no benefit, requires a longer
mount for the equipment, and prevents taking readings near the ends of
pipes.
In one embodiment, the detector is offset from the driver coil along an
inner surface circumference of the pipe. The detector may be
diametrically opposite the pipe from the driver coil. Where the detector is
a coil, the axis of the detector coil is preferably parallel to the axis of
the
pipe. It is possible to have a driver coil that is not completely
diametrically opposed to the detector, but it is preferred that the radius
along which an axis of driver coil is, should at least be on a side of the
central axis of the pipe that is remote from the detector. For large
diameter pipes, such as 6.1 m. (20 foot) diameter pipes, it is preferable
not to have the driver coil diametrically opposed from the detector, but
circumferentially offset from it, to reduce the length of the equipment
mounting boom on which the detector and driver coil are mounted.
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In one method of operation, the invention provides a driver coil to create
an electromagnetic field, which creates a current flow through the wires
that wrap a concrete cylinder, such as a water reservoir or a very large
diameter pipe. The voltage and other effecfis induced by this current in a
detector located proximal to an exterior surface of the concrete cylinder
remote from the driver coil are then measured.
The detector is remote from the driver coil along an outer surface
circumference of the concrete cylinder. The detector may be
diametrically opposite the cylinder under test from the driver coil. For
large diameter cylinders, for example 6.1.m. (20 foot) diameter pipes or
water reservoirs of even larger diameter, if the driver coil is not
diametrically opposed from the detector, it is circumferentially offset from
it. Where the detector is a coil, the axis of the detector coil is parallel to
the axis of the cylinder under test.
There can be appreciable interference to the signal produced by the
detector through direct magnetic flux coupling between the driver and
the detector, for example, the magnetic flux formed within the pipe,
between the detector and the driver. To counter this, it is preferable to
orient the detector axis to be orthogonal to the driver axis. Further
reduction of the direct magnetic flux coupling between the driver and the
detector can be obtained by placing a substance of high permeability,
which shields magnetic flux, in a position to block or substantially
attenuate such magnetic flux. Mu-metal is a metal alloy that is expressly
built to prevent passage of magnetic force, so a shield of mu-metal is
preferred.
In a particularly preferred embodiment of the invention, a detector device
according to the invention is mounted on a vehicle movable through the
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pipe. The vehicle is provided with a means for determining its location
or distance of travel precisely. The vehicle proceeds down the pipe,
while logging information from the magnetic pipe inspection and location
or distance information. The location determination means provides a
precise location, so that the .information that is received about the state
of the wires can be correlated to a particular location along the pipe.
Optionally, the vehicle is also fitted with means to propel it through the
pipe, and hydrophone means, which can carry out an acoustic
examination of the walls as the vehicle is passing it through.
The vehicle is preferably sized so that, in a large pipeline, it can be
placed in the pipeline through inspection ports, which are spaced along
the pipeline. The vehicle can also be stopped at such inspection ports
for the recharging of batteries and removal of recorded data. If desired,
a whole or partial analysis can be done at the vehicle progresses
through or traverses the pipe. The results can be displayed graphically
to the operator. Alternatively, the data can be removed and analyzed at
a remote location.
In one of its aspects, the invention provides an inspection apparatus for
detecting discontinuities in spirally wound metallic wires embedded the
wall of a concrete pipe, comprising a detector for producing an output
responsive to a magnetic flux and a driver means to create magnetic flux
and located not more than one pipe diameter along the axis of the pipe
from the detector. Preferably, the detector is a coil oriented with an axis
parallel to the concrete pipe, and the driver means is a coil with an axis
orthogonal to the to the axis of the concrete pipe for inducing a current in
the wires. The detector is oriented proximal to a surface of the pipe,
preferably an inside surface.
In a preferred embodiment, the inspection apparatus includes
displacement sensor means to produce an output representative of at
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least one distance of the detector a from a known location and means
for causing the detector to move along a wall of such pipe as well as
means for storing outputs corresponding to the flux detected by the
detector and the displacement of the detector from such known location.
In another of its aspects, the invention provides a method of detecting
discontinuities in spirally wound metallic wires reinforcing a concrete
pipe, comprising providing a driving signal to a driver means having an
axis oriented orthogonal to the axis of a concrete pipe and disposed
proximal to an inside surface thereof to generate an induced current in
said wires and providing a detector for producing an output responsive
to a magnetic flux in a direction axial to said pipe, the detector being
located in close proximity to an interior wall of a pipe and within one pipe
diameter of the driver along the axis of the pipe. In accordance with the
method, the detector is moved along the wall of the pipe, and the output
and the location of the detector is recorded as it moves.
In another aspect, the invention provides~a method of testing the spirally
wound metallic wires reinforcing the wall of a concrete pipe, by
generating a driving signal with a signal generator, providing a detector
located in a fixed position relative to the signal generator and not more
than one pipe diameter axially along the pipe therefrom, moving the
detector and signal generator along the pipe, detecting a signal with said
detector in response to said driving signal and determining the location
along the pipe where the detector is located at the time each detected
signal is detected. Preferably, the periodic driving signal is generated at
more than one frequency, and the detector signal is recorded over a
range of locations traversed along a length of pipe.
In another of its aspects, the invention provides a method for testing the
spirally wound reinforcing wires embedded in the wall of a concrete pipe
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along a length thereof using apparatus including magnetic flux
production means and magnetic flux detector means disposed proximal
to a surface of the pipe, the magnetic flux production means and the
magnetic flux detector means in a spaced relationship to the other and
axially disposed within one pipe diameter to the other. The magnetic
flux production means produces a magnetic field in response to a driving
signal and the magnetic flux detector means produces a detector signal
in response to magnetic flux, location indication means and control
means operatively connected to said location indication means, to said
magnetic flux means and to said detector means. The method
comprises providing a driving signal of at least one frequency; receiving
a detector signal; producing an output representative of the detector
signal corresponding to the in-phase and quadrature components of the
detector signal in relation to the driving signal; and recording the output
representative of the detector signal and the location; whereby at least
one output is recorded over a range of locations traversed along a length
of pipe.
The invention will be further described with respect to the drawings, in
which:
Brief Description of the Drawings
Figure 1 is s cross-section through a pre-stressed concrete pipe,
showing schematically a first form of detector according to the invention
within such pipe.
Figure 2 is a cross-section of a similar pipe, showing a second
embodiment of the inventive detector system.
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Figure 3 is a cross-section through a similar pipe, showing a third
embodiment of the inventive detector system.
Figure 4 is a cross-section through a similar pipe, showing a fourth
embodiment of the detector system according to the invention.
Figure 5a is a schematic cross-section through a length of pipeline. In
order to demonstrate a wire breakage, wires (which would in reality be
concealed behind the metal lining in the view shown) are shown.
Further, the drawing is not to scale, and dimensions have been distorted
so that detail of wire placement and wire breaks can be shown.
Figure 5b is a plot of voltage against distance using the detector of the
invention, on the pipe of Figure 5a.
Figure 5c is a schematic cross-section through a length of pipeline,
which is not to scale and has distorted dimensioris similar to Figure 5a,
with two wire breakages shown.
Figure 5d is a plot representative of an output representative of the
detector signal phase against distance produced in accordance with a
method of the invention, from an inspection of the pipe of Figure 5c.
Figure 6 is a vehicle designed to pass through a pipe according to the
invention, and having a detector system according to the invention
placed on it.
Figure 7 is functional schematic diagram of the electronic signal
elements of an embodiment of the invention.
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Figure 7a is a functional schematic diagram of two sensors connected in
a common polarity configuration.
Figure 7b is a functional schematic diagram of two sensors connected in
a reverse polarity configuration.
Figure 8 is functional schematic diagram of the electronic signal
elements of a preferred embodiment of the invention.
Figure 8a is a graph of a vector output representing the in-phase and
quadrature components of a received signal output from a lock-in
amplifier.
Figure 9 is a graph showing detector trace plots produced in accordance
with the invention for two exemplary driving frequencies.
Figure 10 is a graph showing detector trace plots produced in
accordance with the invention for a plurality of driving frequencies
, Figure 11 is a graph showing a detector trace plot of a component of a
detector output produced in accordance with the invention for a single
driving frequency.
Figure 12 is a graph showing a plot of a component of a detector output
and a corresponding plot of a phase shifted component of a detector
output produced in accordance with the invention for a single driving
frequency.
Figure 13 is a graph of a vector output representing the in-phase and
quadrature components of a received signal output from a lock-in
amplifier transposed by an angle alpha.
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Figure 14a is a cross section through a pre-stressed concrete pipe,
showing schematically a preferred arrangement of the driver and
detector around the pipe under test.
Figure 14b is an elevation view of the pipe and arrangement of Figure
14a, with the protective mortar of the pipe is not shown so that the
underlying structure can be viewed.
Figure 15a is a cross section through a pre-stressed concrete pipe,
showing schematically an alternate arrangement of the driver and
detector around the pipe under test from the arrangement of Figure 14a.
Figure 15b is an elevation view of the pipe and arrangement of Figure
15a, where the protective mortar of the pipe is not shown so that the
underlying structure can be viewed.
Figure 16 is a top view of a pre-stressed water reservoir vessel, showing
schematically an arrangement of the driver and detector around the
vessel under test.
Figure 17 is an elevation of the vessel and arrangement of Figure 16.
Figures 18a, 18b, 18c and 18d are cross sections through a pre-
stressed concrete pipe, showing schematically alternate preferred
arrangements of the driver and detector disposed about a pipe under
test.
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Detailed description of the invention
The preferred embodiments of the invention will now be described with
reference to the Figures. Figure 1 shows a cross-section through a pre-
stressed concrete pipe generally indicated as 10. A pre-stressed
concrete pipe of this sort has an inner metal cylinder 11. Depending
upon the type and grade of pipe, either pre-stressing wires are wound
directly onto the cylinder, or a layer of concrete is cast onto the cylinder,
and the pre-stressing wires are wound on the layer of concrete. Some
pipes also have a layer of concrete cast inside the pipe, separating the
metal cylinder from the interior. Other pipes have two layers of pre-
stressing wires, with layers of concrete between them, outside the metal
cylinder. Another layer of concrete or protective mortar is cast around
the wires to complete the pipe. Pipes are sold under the designation
ECP, LCP, SP5 and SP12, and are usually designed to meet AWWA
standards C301 and 6304. All of these types of concrete pressure pipe
can be examined using the detector of the present invention.
In Figure 1, pipe is shown as having a metal cylinder 11, wrapped with
wires 12 embedded in concrete 13.
The pipe inspection apparatus is shown schematically at 15. The
apparatus comprises a detector 16. This detector is preferably a coil
detector capable of detecting magnetically induced fields or currents in
the pipe being examined.
In the embodiment shown, the detector 16 is a coil, which is adapted to
receive magnetic flux and convert it into a measurable electrical current
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and voltage. However, instead of a coil, any other detector of magnetic
flux could be used. Another particularly preferred sensor is a giant
magneto resistive sensor, or GMR sensor. Such sensors will be
described henceforth in this application as "GMR sensors".
When detector 16 is a coil, it is located so that the axis of the coil is
parallel to the centre axis 14 of the pipe. The detector 16 is placed so
that it almost touches the wall of the pipe. It is preferred that the
a
detector does not touch the wall, as this would impede movement of the
detector along the interior surface of the wall. However, the gap
between the detector 16 and wall of the pipe 10 should be kept as small
as is conveniently possible, having regard for the fact that the detector is
to be moved along the length of the pipe.
Reference numeral 19 represents a diameter of the pipe, which passes
through detector 16. At the opposite end of the diameter 19 from
detector 16, there is a driver coil 17, Preferably the driver coil is driven
with low frequency alternating current, for example from 20 hertz to 300
hertz but may be driven with a pulse. The coil is located so that its axis
is orthogonal to the axis of the pipe being inspected. The driver is
placed by a wall of the pipe, and, in the preferred arrangement, it is
preferable that the driver be disposed as close as possible to a wall of
the pipe. Having regard to the fact that the apparatus will be moved
along the pipe, it is not desirable to have the driver 17 drag against the
wall of the pipe in operation of the apparatus.
Optionally, a shield 18 of a high permeability material, that is material
that impedes the passage of magnetic flux therethrough, is placed
between the detector 16 and the coil 17. A suitable. material is mu-
metal. The purpose of the mu-metal shield is to prevent magnetic flux
passing through the contents of the pipeline directly to the detector 16
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from the driver coil 17. It is intended that the primary signal picked up by
the detector 16 should be the signal that is made by an induced current
in the wires 12, because of the driver coil 17. A signal caused by
magnetic flux in the contents of the pipeline would add noise to this
signal. In a very large pipeline, particularly when the pipeline has been
drained for inspection, the signal passing directly from the driver 17 to
the detector 16 is often insignificant (large pipelines, for water
transmission, are often several meters in diameter). However, where
the pipeline is smaller, and particularly when the pipeline is filled with
water, direct signals through the contents of the pipeline may be a
problem, so the shield 18 is desirable in such circumstances. The
desirability of the shield 18 can be determined by doing sample
measurements with and without the shield, to see whether the shield
makes an appreciable difference in the measurements.
Figure 2 is a view similar to that of Figure 1, showing a second
embodiment of the inspection device. Reference numerals, where they
are the same as used in Figure 1, designate subject matter the same as
in Figure 1 in this and all subsequent figures.
In the embodiment of Figure 2, the inspection device is generally shown
as 20. It has detector means provided by two detectors 21 and 22, for
example coils, which are spaced from one another along a common axis
23 by a distance less than the diameter 19 of the pipeline. Preferably,
the spacing of the detectors is small, such as from about 7.5 centimetres
to preferably not more than half the diameter of the pipeline.
Alternately, the detector means is a giant magnetoresistive (GMR)
sensor which has an axis responsive to magnetic flux analogous to an
axis of a coil detector. A GMR sensor provides an output which is a
change in resistance that is induced in the GMR sensor by magnetic
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flux. To provide an analogous structure to the two detectors 21 and 21
of Figures 3 or 3, the detector means is a pair of GMR and the output is
a change in resistance induced in the GMR detects by magnetic flux.
Where it is desired to resolve a magnetic field in three dimensional
space, three detectors oriented along each orthogonal axis can be used.
For example, where the detector means has three detectors, each
oriented to be responsive to a magnetic flux along a corresponding
orthogonal axis.
In Figure 3, the inspection device 30 has again two detectors 21 and 22
on a common axis 23. In this case however, there is a driver coil 27,
which is not diametrically offset from the two detectors. The driver coil
27 can be located anywhere on the internal circumference of the
pipeline, so long as it is far enough away so that the magnetic flux
produced by current in the wires of the pipe are distinguishable from any
stray magnetic fields from the driver coil. In this case, the driver coil is
located approximately 45 degrees offset from the vertical diameter 19
across the pipe as shown by the angle x, and the coil is disposed radially
of the pipe. Particularly in large pipes (for example the 20 foot (6.1 m.)
diameter pipe mentioned above, it is preferable that the driver coil be
offset from the vertical diameter, even if the driver coil is not axially
offset
along the pipe from the detector. The driver coil is offset
circumferentially from the detector means by an angle x which preferably
is at least 10 degrees. Where the driver coil is offset circumferentially
from the detector means by an angle x, the circumferential offset is
preferably a distance of at least one meter. This permits having a
smaller set of booms on which to mount the detector and drive coil, and
sometimes in large pipes results in a cleaner signal.
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Figure 4 shows a detector with the same arrangement as Figure 3.
However, in Figure 4 the detector has a driver coil, which is wrapped
around an axis radial to the pipeline and spaced at a distance "I" from
the diameter 19 between the detectors 21 and 22. The distance "I" is
less than one pipeline diameter. Although not shown, magnetically
impermeable barriers, such as barriers 18 in Figures 1 and 2, can be
placed in the line of sight between driver coils 27 or 28 and detectors 21
and 22.
Generally, it is preferred not to offset, along the axis of the pipe, the
driver coil from a line 19 extending orthogonal to the axis of the pipe to
the detector coil or (in the case of two detector coils, a line located
midway between the two detectors). If there is no offset of the driver
relative to the detector, measurements can be made right to the end of a
pipe section. Further, the signal detected is often clearer, with less
"noise" as induced currents pass through fewer wires in the pipe before
generating the major part of the magnetic flux proximal to the detector
apparatus.
Figure 5a shows, in schematic form, a pipeline 60, having a series of
pipes 61, 70 and 80. These are laid end to end to form the pipeline, and
are connected by the well-known bell and spigot system. The pipeline is
not shown to scale. Typically, the pipe sections would be of the order of
three meters in length, and pipe diameters would be of the order of one
and one half - two meters.
Pipe 61 is shown as joined to pipe 70 by a bell 63 which is part of pipe
61. A spigot 71 from pipe 70 is inserted into the bell, and suitably
sealed. Similarly, pipe 70 has a bell 73, into which spigot 81 of pipe 80
is inserted and sealed. Each pipe is a concrete pressure pipe, having an
internal metallic cylinder (numbered as 64 for pipe 61, 74 for pipe 70 and
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84 for pipe 80). This is wrapped (either with or without an intervening
layer of concrete as described) with helical reinforcing wire. For pipe 61,
this wire is 62. For pipe 71, the wire is shown as 72 and for pipe 80, the
wire is shown as 82. Only a very few wires schematically are shown for
each pipe. In actual practice, the pipe would be closely wound with such
wires, and there could be several layers of wire, separated by layers of
concrete. The wires are overlaid with concrete or protective mortar to
make the pipe.
In the drawing, a few selected wires are visible. In actual fact, these
would not be seen in a cross-sectional view of the pipe, as the metal
cylinders 64, 74 and 84 would hide them. However, they are shown for
the purpose of illustrating what happens when there is a break. A break
is shown in one wire at 85.
Figure 5b is a plot of voltage against distance along the pipe, for one
detector such as shown at 16 in Figure 1. Driver coil 17 in Figure 1 is
generating a periodic signal at a selected frequency in the range
frequency of 20 - 300 hertz, and detector 16 is receiving a voltage.
Figure 5b shows a plot of this voltage against the distance travelled by
detector 16 along the pipe. Detector 16 and driver coil 17 are rigidly
linked, so that they each travel at the same speed.
Figure 5b is a plot of voltage (on axis 90 of the plot) against distance
travelled (on axis 91 of the plot). Only positive voltages are plotted on
this plot. As will be seen, the pattern of the plot of voltage against
distance is that there is a peak, as shown 93, as the detector traverses
each of the bell and spigot connections. In between, there is a relatively
flat portion. Thus, while the detector is traversing pipe 61, there is
initially a peak 98 as the detector passes over the bell and spigot
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connection just before pipe 61, then a flat portion 92 as it traverses that
pipe length. When it approaches bell and spigot connection 63, 71, the
voltage rises again to a peak 93. After it has reversed that connection
and is traversing pipe 70, the voltage again drops to a relatively flat
portion 94.
After a series of pipes have been traversed, it becomes possible to
determine an average voltage for the peak when a bell and spigot
connection is passed. This average voltage is shown as 95. The voltage
95 is the average of peaks 98 and 93. Similarly, it is possible to predict
an average voltage when the detector is passing over a section of the
pipe that does not have a bell and spigot connection. This average
voltage is shown as 96. In the example given, it is the average of the flat
portions 92 and 94.
In the example given, as the detector 16 traverses the bell and spigot
connection 73, 81, a further peak 99 is obtained in the voltage. This
peak is approximately the same as peaks 98 and 93, as is expected
from the previous peaks for bell and spigot connections. However, after
dropping from peak 99, the voltage first drops approximately to average
96 as shown at 97, then rises again as shown at 200, then drops again
as shown at 201 to approximately average 96, before rising again to
another peak for a bell and spigot connection, as shown 203. The result
is a "bulge" 200, which indicates that there is an anomaly in the pipe
section being examined. This anomaly is indicative of a broken wire in
pipe 80, as is shown in Figure 5a at 85. (The drawings are not to scale).
When there isea single detector, it is possible to find the broken wire with
a fair degree of accuracy, as being approximately at the midpoint of
peak 200 in the curve of Figure 5b. However, the accuracy can be
greatly increased by using two sensors, as shown at 21 and 22 in Figure
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2. The two sensors (for example receiver coils) can be very close
together (for example, about 0.6 cm apart) or can be spaced from each
other by a longer distance, such as for example 60 cm. Generally, the
total effect of the wire windings is symmetrical upstream and
downstream of the detector coils. However, when there is a
discontinuity in the helically wrapped wires, such as a wire break, this
unbalances the effect, and a very large difFerence in the signal received
at the two receiver coils is found. By analyzing the plots of the voltage
against distance of the two coils, a very precise position can be given for
the break in the wire.
In Figure 5c, a pipe section similar to that of figure 5a is shown. The
same reference numerals are used to identify the same things. In this
pipe, there are two broken wires at 75 and 85.
Figure 5d is a plot similar to that of figure 5c. However, the plot is
produced from the phase relationship of the received signal to the driver
signal which is represented as a phase angle, a voltage representative
of the in-phase component of the received signal, a voltage
representative of the quadrature component of the received signal, or a
voltage representative of either the in-phase or quadrature component of
the received signal translated by a selected angle alpha, all of which are
referred to herein as "the detector phase." A plot of the detector phase
gives rise to peak and trough patterns, including positive and negative
values, instead of the peaks shown in Figure 5b.
As is shown in the plot, the pattern of the detector phase against
distance is that there is an excursion resulting in peak-trough
combinations at each of 98, 93 and 99 as the detector traverses each of
the bell and spigot joint connections. In between, there is a relatively flat
portion, which approximates the average 96 for the pipe portions
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between bell and spigot. Thus, while the detector completes traversing
pipe 61, there is initially a diverging excursion resulting in peak and
trough pair 98 as the detector passes over the bell and spigot
connection 58, 59 on entering pipe 61, then a substantially flat portion 92
(approximating average 96) as it traverses the pipe length. When the
detector approaches bell and spigot connection 63, 71, the plot makes a
diverging excursion again to form peak and trough pair 93. After it has
traversed that connection and is traversing pipe 70, the plot again drops
to a flat region. However, in this case, the substantially flat region is
interrupted by a small peak 200 and a small trough 201, which is a
diverging excursion corresponding to wire break 75. When the detector
approaches bell and spigot connection 73, 81, the plot makes a
diverging excursion again to form peak and trough pair 99. After the
detector has traversed that connection and is traversing pipe 80, the plot
again drops. However, instead of reaching a substantially flat portion
corresponding to line 96, it instead ramps down to a trough 97, and then
rises to a small peak 203. The ramp and peak excursion corresponds to
wire break 85.
After a series of pipes has been traversed, it becomes possible to
determine an average, of the upper and lower excursion of the peaks
when a bell and spigot connection is passed. These averages are
shown as upper and lower lines 95. Each of the upper and lower lines 95
is the average of the respective upper or lower peak excursions of the
peak pairs, for example, 98, 93, and 99. Similarly, it is possible to
predict an average signal when the detector is passing over a section of
the pipe, which does not have a bell and spigot connection. This
average is shown as 96.
Figure 6 shows a vehicle equipped with one embodiment of the
apparatus of the invention (in this case the embodiment of Figure 2).
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The vehicle is designed to pass through the pipelines fio detect broken
wires and other discontinuities. In Figure 6, a concrete pipe is generally
indicated as 100. The pipe 100 is formed of concrete 103 having helical
wires 102 as reinforcement. Frequently, the pipe 100 has a steel
cylinder 101 disposed interior to concrete 103 providing a steel-lined
concrete pipe. Additional concrete lining may be provided interior to the
steel cylinder 101. Although the pipe as shown in the figure does not
have a concrete lining within the steel cylinder, the operation is the same
when the pipe does have a concrete lining.
The pipe is provided at intervals with inspection hatches. Hatch 105 is
one of such hatches. It has a flange 107, on which a hatch cover 106
rests removably. The hatch has an opening of a width shown by the
arrow "a"
The inspection vehicle 150 has a body 170, with wheels 171. fn the
present embodiment, there are eight wheels extending outwardly along
radii of the pipe spaced 90 degrees from each other, with two wheels
171 on each radius. Six of these wheels are shown in the drawing. Two
others are behind the body 170. Each wheel 171 is on an axle 172,
which is mounted to body 170 by means of a suitable axle support 173.
Preferably the axle support 173 includes springs 174, so that the wheels
will deflect from any discontinuity on the surface of the pipeline.
Extending from the front of the body 170 of the vehicle, in the direction .
that the vehicle will travel within a pipeline, is a support bar 165, which,
as shown, is disposed axially of the pipeline. Connected to this at right
angles is a frame 164, constructed from a non-ferromagnetic material
such as aluminium or fibreglass to avoid a driver influence on the
detector apparatus via the frame. Frame 164 has the detector
apparatus mounted on an end remote from the driver 163. In the
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embodiment depicted, the detector apparatus is a .pair of detectors 161
and 162 mounted on opposed ends of a crossbar 166. Preferably, the
detectors are co-axial detector coils with their axis parallel to the central
axis of the pipe. Alternately, the detectors are GMR sensors when GMR
sensors are used, it is particularly preferred to have each detector made
up of three GMR sensors with their axis of sensitivity orthogonal to one
another, so as to detect the magnetic field from all directions. The two
detectors 161 and 162 are spacedly disposed from each other
approximately 10 cm apart. As discussed, the preferred spacing of the
detectors can be from approximately 0.6 cm to 60 cm, depending on the
size of the pipeline and precision desired. Each of the detectors 161
and 162 is responsive to a magnetic flux in its vicinity. For example,
each detector is a coil which has a current induced in it by magnetic flux
in its vicinity. Control means, generally referenced by block 169, are
provided in the body of the vehicle to measure the current in each coil,
and to record the current measured.
At the other end of frame 164 is a driver coil 163. This driver coil is
driven by an alternating current in the 1 - 300 hertz range, or a pulse, of
sufficient magnitude to induce a current flow in the wires of the pipe. A
current source 191 is connected to driver coil 163 to provide the driving
current. Magnetic barrier 168, preferably composed of mu-metal, is
provided to block stray magnetic fields from extending through the
volume enclosed by the pipeline between driver coil 163 and the
defector apparatus, namely detectors 161 and 162.
A displacement sensor 176 to provide location information of the
apparatus as it extends along the length of the pipe is provided, for
example, an odometer. The displacement sensor 176 is connected to
one of the wheels 171, to provide a displacement measurement based
on the distance travelled by that wheel. Displacement sensor 176
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produces a signal indicative of the distance travelled along the pipe,
which is recorded with the recording of the outputs of detectors 161 and
162, to provide a record of distance travelled. Alternately, instead of
being a displacement sensor, 176 can be a GPS locator or similar
. device that can determine and record its location.
The vehicle can be manually moved through the pipeline by being
"walked" by an attendant, or it can be pulled through the pipeline by a
wire line. However, it is especially preferred to have the vehicle
autonomous. In this case, the vehicle can be placed in the pipeline at a
point where there is a suitable opening. The vehicle is made so that it
can be disassembled into parts, which can be handed into the pipeline
through the inspection port 105 or similar ports. Thus, no single
component of the vehicle has all of its dimensions greater than the
distance represented by arrow "a". The result is that the vehicle can be
passed into the pipeline in sections when the pipeline is depressurized,
and can be assembled. Then, the operators can leave the pipeline and
close off the inspection port, letting the vehicle remain in the pipeline.
If desired, the autonomous vehicle in the pipeline could have a motor
means 195 sufficiently powerful to power it, either against or with the
current of the pipeline (obviously, powering it with the current of the
pipeline is preferable, as less power is required). If could also have
battery means 194 to power this motor. However, it is preferred that the
vehicle be carried along by the flow of the pipeline. In the present
embodiment, there is a deflector 190 mounted on the back (i.e.
upstream) end of the vehicle. When the flow~of the fluid in the pipeline
(usually water) hits the deflector, it pushes the device in a downstream
direction (to the right in Figure 6). An alternate way of propelling the
device would be by deploying a parachute downstream (i.e. to the right
of the device in Figure 6) to pull it through the pipeline.
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Suitably, the vehicle according to .the invention can also be used to do
other types of examination of the pipe as it passes through. For
example, Figure 6 shows hydrophone 180 mounted on body 170 of the
vehicle. The hydrophone 180 senses sounds in the vicinity of the
vehicle as it passes through the pipe. The data produced by hydrophone
may be stored in a data storage device 181 along wifih location data
produced by the displacement sensor 176. The hydrophone data can be
used to indicate sites of possible leakage and other information as
known in the hydrophone art. Other types of sensors can also be
mounted on the machine.
The vehicle can also be equipped with automatic data transfer
capabilities. Thus, as the vehicle approaches an inspection port 105, an
operator can trigger it to transmit the data that it has received and an
operator on the other side of inspection port 105 can operate a probe to
receive this data by wireless modem, acoustic modem, or inductive
coupling and/or recharge the batteries as needed. Alternately, as the
vehicle reaches an inspection port, a barrier can be placed in the
pipeline at the inspection port to stop the vehicle. The line can then be
depressurized and the inspection port can be opened. The vehicle can
then be examined for its physical condition, the data that it has collected
can be downloaded, and the batteries, which provide the output
generated by the AC current of the driver coil can be recharged.
It is also within the scope of the invention to provide motor means 195
and battery 194, for use if the vehicle becomes stuck between inspection
stations, or if the current in the pipeline becomes insufficient to move it.
If desired, this can be triggered by a sound signal of a predetermined
sort, which can be sent from an inspection port 105 and received by
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hydrophone 180. There are of course other known means to control
movement of the vehicle, as by having it drag a wire line.
The output signal from such a vehicle can be presented as a graph such
as that shown in Figure 5b. It is extremely easy to notice from such a
graph where a wire break has occurred. It is also possible, however, to
have initial processing done on the vehicle, so that the vehicle will
prepare a supplementary data stream, which generates an exception
when there is a voltage which is not at the voltage indicated as 95 or 96
in Figure 5b, and which is not part of the smooth transition between
them. Thus, the voltage registered at 200 would be noted as an
exception. Thus, a signal would be generated showing each exception,
together with the distance (according to the displacement sensor) at
which the exception occurred.
It is within the invention to use other distance measuring means other
than a displacement sensor 176 mounted on the vehicle. For example
means for measuring the velocity of the vehicle and the time that it has
moved at that velocity are suitable, particularly if there are calibrating
means at inspection ports to provide a calibration as the vehicle passes.
A location sensor such as a GPS sensor can also be used.
Suitably, the vehicle can also have a means to control its velocity when it
is passing through the pipe. For example, the system can, when it does
on site processing and indicates that there is an anomalous signal which
could represent a wire break, have means to change the angle of
deflector 190, so that it will move more slowly through the pipe for a
predetermined distance thereafter. Alternately, the deflector can be
adjusted so that the device will stop completely before a predetermined
time, after which the deflector then is moved so that the vehicle will
continue moving through the pipe. Other inspection means, for example
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hydrophone 180, can be used to provide acoustical signalling which can
be utilized to determine if there is continued wire breakage occurring at
or around a location where there is an anomalous signal.
In most pre-stressed water or sewage pipe, fluid flow within the pipe is
one to ten feet per second. This is a very convenient speed at which to
carry out inspection. Inspection with the autonomous vehicle of. the
invention permits the inspection to be done without interrupting flow or
emptying the pipe.
The invention also comprises instrumentation exterior to the pipe (for
example at access ports 105, which recognize signals emitted by the
vehicle). For example, the vehicle can have a transponder 211, which
responds to a sound emitted by a fixed location transponder 210, for
example on access port 105, and responds with a sound of its own,
thereby giving location information as it passes by inspection ports so
equipped. This equipment can also be used for calibration of the
distance measurement as discussed above.
It is preferred that attachments 164, 165 and 166 are provided with
mechanical damping means, so that mechanical vibration is kept to a
minimum, as such vibration can lead to electrical noise which could
effect the quality of the signal being received.
Figure 7 shows a functional schematic diagram of a circuit that can be
used to implement the electronic components of the present invention.
An output signal to drive the driver coil 163 of the invention is obtained
from output line 200. An amplifier 202 provides sufficient signal power
on output line 200 to produce a magnetic field from driver coil 163 that is
efficacious for the purposes of the electromagnetic inspection of the
present invention. An opamp is shown as the amplifier 202 in the
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functional schematic diagram. Power transistors or a more elaborate
amplifier circuitry can be used to provide the amplifier 202 that will
function to produce the signal power for output on output line 200. A
variable frequency signal generator 204 provides a periodic signal for
input to amplifier 202. Microprocessor 206 provides the parameters of
the periodic signal. The parameters define the periodic signal produced,
which is preferably, a sine wave, but can include, for example, a square
wave or a sawtooth wave periodic signal. A sine wave is preferable as it
provides a single frequency output signal. A pulse signal can also be
used to obtain a transient response signal at the detectors. The type of
the periodic signal and its repetition rate or frequency is set by
microprocessor 206 over data communications line 208. In this
arrangement, microprocessor 206 controls the parameters of the
frequency and type of wave of the periodic signal that is to be produced
by the variable frequency signal generator 204. The parameters may
define a periodic signal that is a range of frequencies, for example, 20 -
300 Hz, which are to be continuously produced by the signal traversed
over a given time period, such as a few seconds or milliseconds.
Parameters defining this type of driving signal produce a continuous
frequency ,sweep over the range of interest repeated for each
successive time period.
Output from the detector apparatus 16 is received on input tine 213
where it is supplied to an input of a multiplier 216. A reference signal
derived from the output signal appearing on output line 200 is also
supplied to an input of multiplier 216 via an attenuator 218. The output
signal of the multiplier is low pass filtered at 217 and supplied to input
amplifier 212. In this manner, the signal arriving at the input amplifier
212 is a signal that represents differences between the driver coil signal
and the detector signal. The filtered signal output from multiplier 216 is
amplified by input amplifier 212 and converted to a digital signal by a
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digital signal processor (DSP) 214. Thus the digital signal of DSP214 is
derived from the output of the sensor 16. While only one input line 213
and corresponding amplifier 212 and DSP 214 is shown in the diagram,
an additional input line can be provided if desired. If there are two
detectors, for example 161 and 162 as depicted in the other figures, and
each is to provide an independent input to the microprocessor 206 for
processing, then a second path, duplicating input line 213, multiplier
216, amplifier 212 and DSP 214 is provided. To use two detectors to
provide a single input signal on input fine 213, the output from detectors
161 and 162 is coupled together. The detectors can be coupled in a
common polarity or a reverse polarity configuration. Figure 7a shows
the coupling of detectors 161 and 162 together in a common polarity
configuration. In this configuration, the signal output of the detector
apparatus is coupled in an additive fashion such that the sum of the
outputs of each of the detectors 161 and 162 adds to the signal that is
provided on input line 213. Figure 7b shown the coupling of detectors
161 and 162 together in a reverse polarity configuration. In this
configuration, the signal output of the detector apparatus is coupled in
an subtractive fashion such that the difference of the outputs of each of
the detectors 161 and 162 is the signal that is provided on input line 213.
By using more than one detector, different locations and orientations of
detectors can be achieved.
A displacement sensor 176 provides input to microprocessor 206
representative of the location of the vehicle in the pipe. When the
apparatus of the present invention is placed in a pipe as depicted in
Figure 6, the magnetic coupling between driver coil 162 and the detector
apparatus is manifested by variations in the input signal provided to
amplifier 212. Microprocessor 206 receives digital signals
representative of the input signal over data line 220 and performs
computations based on the received digital signals to produce a visually
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perceptible output on display device 220. Preferably, the visually
perceptible output is graph on a display device 220. Display device 220
can be a computer monitor such as a CRT or LCD, or, display 220 can
be a printer that produces a printed output of the signal. The output
produced may include processing performed on the signal, for example
to provide numeric outputs as graph axis, produce averages traces or
change trace colours. The signal received by the microprocessor 206
can be stored in a data storage device 181, which can be a magnetic
disk or other suitable form of storage device such as, for example, a
floppy disk or CD.
Because the output of detector 16 is subject to noise, it is preferable to
use a phase sensitive detector, or a lock-in amplifier, on the detector
output in place of multiplier 216 and amplifier 212. A phase sensitive
detector multiplies the signal received from the detector with the
reference, or transmitter, .signal and integrates the resulting product
signal to produce a DC signal representative of the amplitude of the
received signal. A description of manner of operation and use of a
phase sensitive detector and lock-in amplifier may be found, for
example, in the publication DSP Lock-In Amplifier model SR830,
Revision 1.5, November 1999 published by Stanford Research Systems
of Sunnyvale California.
Figure 8 shows a functional schematic diagram of a preferred
embodiment of a circuit to implement the electronic components of the
present invention. The driver configuration is a shown in Figure 7,
however, the input received from the detector apparatus 16 on line 213
is supplied to a lock-in amplifier, for example, a model SR830
manufactured by Stanford Research Systems of Sunnyvale, California.
The type of periodic signal is preferably a sine wave, but can include, for
example, a square wave or a sawtooth wave periodic signal. If other
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signals than a sine wave are used, the lock-in amplifier will provide an
output reference only to the fundamental frequency of the periodic
signal, the higher harmonic components will be discarded.
Microprocessor 206 controls the parameters of the driving signal. For
example, the sweep frequency range and time frame, or a frequency,
the type of wave as parameters of the periodic signal that is to be
.produced by the variable frequency signal generator 204.
Figure 8a shows an output from the detector apparatus 16 is received on
input line 213 where it is supplied to an input of lock-in amplifier (LIA)
219. A reference signal input to LIA 219 is derived from the output
signal appearing on output line 200, which may be reduced in magnitude
if needed by an attenuator 218. The LIA 219 provides two output signals
on 221 and 223, which represent the magnitude and phase of the AC
signal produced by detector 16 at the frequency corresponding to the
reference frequency. The output on 221 and 223 can take either the
form of an (X, Y) value pair as Cartesian co-ordinates, which define the
in-phase (shown as X on the I axis) and quadrature (shown as Y on the
Q axis) components of the received signal. The received signal may
also be represented in polar co-ordinates as an (R, Theta) value pair.
Using either co-ordinate method, the value pair defines a vector
representative of the received signal in relation to the fundamental
frequency of the driving signal.
The value pair produced by LIA 219 is converted to a digital form by
DSP's 214 and supplied to microprocessor 206 where it is stored in
storage 181. While only one input signal path comprising detector 16
and corresponding input line 213, LIA 219 and DSP's 214 are shown in
the diagram, an additional input path can be provided for each additional
detector. For example, if there are two detectors 161 and 162 as
depicted in the other figures, and each is to provide an independent
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input , signal to the microprocessor 206 for processing, then each
detector would have a signal path comprising input line 213, LIA 219 and
DSP's 214.
Figure 9 is a graph of an output provided on display 220. The detector
apparatus produces an output signal while traversing a length of pipe,
which is stored in storage 181. This data is used to produce the graph
of Figure 9. The graph provides the distance of travel of the detector
apparatus along the length of the pipe is shown as the horizontal axis of
the graph. The vertical axis of the graph represents a voltage level
output of the LIA 219 output as received signal at DSP 214. The plotted
voltage level, can be either the X component, that is the in-phase
component of the received signal, output of LIA 219, or the Y
component, that is the quadrature component of the received signal,
output of LIA 219. The plot may be produced at the time the pipe is
tested or may be produced at a subsequent time from the data stored in
storage 181.
The plot of Figure 9 is a plot produced from a multiple frequencies and
provides a multi-frequency analysis of the pipe. In the plot of Figure 9,
the multi-frequency analysis is performed using two selected separate
frequencies. Thus, there are two traces shown in the graph, each
relating to a difFerent frequency of a periodic signal output to the driver
coil. Trace 222 is a trace produced at a first selected frequency and
trace 224 is a second trace produced at a second selected frequency,
which differs from the first frequency. The frequencies are chosen such
that the slope of the traces produced by a broken wire shows a reversal
when the same pipe section is surveyed by the two different frequencies
but the traces produced by other features of the pipe do not result in a
slope reversal. In the example of the graph of Figure 9, the response
represented by line 222 is a response corresponding to a sine wave
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driver signal at frequency of 85 Hz. The trace produced at 224 is the
response corresponding to a sine wave driver signal at a frequency of 35
Hz.
The response of the detector is different at the two frequencies and the
differences in the response produced by the detector apparatus at the
two frequencies provides information to determine whether anomalies
such as wire breaks are present in the helically extending tensioned
wires 102 of the pipe. The frequencies depicted in the graph plots are
selected from the range of frequencies at which the magnetic inspection
test of the pipe was conducted. Naturally, the range of frequencies that
a pipe was tested at may be many more than those that are
subsequently used to produce a particular graph. The response to wire
breaks shown in the region labelled 226 has a positive sloping excursion
for the 222 trace and a negative slope extending excursion for the 224
trace. The reverse in sign of the slope at the differing frequencies
provides an indication that a wire break is present in the region of 226.
The response from wire breaks to frequencies, chosen in this way,
results in a trace pair that has diverging excursion slopes where wire
breaks exist, but non-diverging slopes where other features exist, such
as pipe joints. The traces may form a mirror image excursion in the
region of the anomaly as depicted in region 226 of the trace. The
response in the region at 228 shows similar mirror image excursion and
reversed sign slopes that indicate a wire break anomaly. The driver coil
and detector are in a plane substantially orthogonal to the axis of the
pipe under inspection. Region 230 of the graph of Figure 8 shows a
response when the detector passes through the region of a bell and
spigot pipe joint, which provides a consistent excursion of the traces for
each frequency. That is, the excursion slope of each trace has the same
sign. Each has a positive sloping excursion, or each has a negative
sloping excursion.
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The response trace of the detector apparatus to a wire break provides a
diverging signal response trace at selected different frequencies in the
region when a wire break is located. However, when the detector
passes over a pipe joint, the selected different frequencies produce
excursions with each excursion having the same sign slope. In this
manner, the driving signal frequencies can be used to produce traces
that distinguish between wire breaks and pipe joints.
The manner of supplying differing frequency driving signals to perform a
magnetic inspection test of a pipe can be achieved by several methods
of operation of the inspection apparatus. In one method of operation,
the detector apparatus is passed through the pipe to be inspected
several times. Each pass has a different frequency that is tested. In a
first pass a first driving frequency produces one trace, for example one
of the traces appearing in the graph of Figure 8. The detector apparatus
is repositioned to the same start position and a second pass occurs at a
frequency different than the first. In the example shown in Figure 7 of
the drawings, a first pass was made at 35 Hz, the vehicle was
repositioned to the start position and a second pass was performed at
the 85 Hz driver frequency.
Another manner of operating the apparatus is to perform a pass in each
direction at a different frequency. In this manner of operation, a first
pass along the pipe length occurs at a first frequency. When the end of
the course of traverse of the pipeline that the inspection is to be
performed for has been reached, the driving frequency is changed to the
second desired frequency. From the end position, the detector
apparatus moves in a reverse direction back toward the start point and
detection is perFormed at a second driver frequency. In this manner,
each traversal of the detector apparatus through the pipeline in either a
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forward direction or a reverse direction will produce a trace. Thus, in this
manner of operation one half of the number of traversals is required than
would be required by operating the apparatus in a forward direction only.
A third manner of operation of the detector apparatus is to advance the
detector apparatus to discrete locations within the pipe. At each discrete
location, the driver signal is swept over a range of frequencies or
stepped through the various frequencies that are to be used in the pipe
inspection. In this manner of operation, the detector apparatus is
advanced incrementally and at each test location, the test frequency is
to the desired settings. To produce the trace of Figure 8 in this manner
of operation, the detector apparatus is positioned at a first position. At
that position, the variable frequency signal generator 204 is operated to
produce a periodic signal at 35 Hz and the detector signal is captured.
The periodic driving signal is set to the next frequency, 85 Hz, and the
detector signal is then captured. When all of the periodic driving signal
frequencies of interest have been produced at the position, the detector
apparatus is then advanced to the next position. At the next location,
the periodic driver signal frequency cycle is repeated. Operating the
detector apparatus in this manner requires only a single traversal of the
pipe section to be inspected. At the conclusion of the traversal, data
points are collected to produce all of the traces representative of all the
frequencies that the detection occurred at during the course of the
traversal of the pipe.
A fourth method is to move the inspection apparatus through the pipe
continuously while continuously changing the frequency of the driver
apparatus. When the frequency of the driver apparatus is changed
sufficiently rapidly relative to the velocity of the inspection apparatus, all
of the frequencies of interest are applied every few inches of
displacement along the pipe. Traces representing each individual
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component frequency of interest can be produced from data recorded
during the inspection process. Thus, for example, the frequency can be
varied continuously from 100 Hz to 200 Hz over a period of 1 second
and data can be recovered at a sampling rate of 15,000 points per
second. The when it is desired to view the results at a particular
frequency, the sampling points that were captured at that frequency or
the closest available frequency can be viewed.
Figure 10 is a graph showing traces resulting from traversal of a pipe
section selected from a plurality of different periodic driving frequencies
extending over the range of 20 to 300 Hz, namely 24 different periodic
driving frequencies. Where a graph showing a plurality of periodic
driving frequencies is produced, it is preferable to select a frequency
separation of each periodic driving frequency from another such that the
range of frequencies extends at least over one octave and the individual
frequencies are separated from each other by at least one eighth of an
octave. For example, use of a range of at least 2 octaves will enable
selection of about 17 frequencies, where each separated by one eighth
of an octave. Wider separation of individual frequencies than one eighth
of an octave will produce useful results but will reduce the number of
frequency traces from 17 over the range. Conversely, narrower
separation of individual frequencies than one eighth of an octave for
each trace will increase the number of,frequencies plotted from 17 over
the range. However, the individual traces produced by the narrower
separated individual frequencies may not provide significant additional
difFerences to warrant use of such narrower selected frequency
separation.
In the traces of the graph of figure 9, a wire break is manifested at 240,
which show a plurality of signal trace or plots that have positive sloping
and negative sloping excursions in the range of 240. Thus, there is a
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sign reversal in the slopes of the excursions of the various traces in the
region of 204, which is indicative of the presence of a wire break at that
location. When performing the test to produce data for production of the
graphs, it is desirable to obtain test data points for a particular frequency
at a particular location as quickly as possible. The less time taken to
gather data for each data point, will increase the number data points
available from a pipe inspection test session over a given time period.
Figure 11 is graph showing a plot of a component of a detector output
produced by the apparatus of the invention for a single periodic driving
frequency. The plot of Figure 11 represents a trace produced at a single
driving frequency over the traversal of the pipe under inspection. The
plot shows the output of either an X or Y component of the output of LIA
219 as the vertical axis for locations along the pipe as the horizontal axis
of the plot. Large excursions 300 and 302 occur where the detector
apparatus of the invention crosses over a pipe joint. Relatively smaller
excursions 304 are representative of an anomaly which may warrant
further consideration, as will be described in more detail with reference
to Figure 12.
Figure 12 is graph showing a plot of a component of a detector output
and a corresponding plot of a phase shifted component of a detector
output produced by the apparatus of the invention for a single periodic
driving frequencies. One plot of Figure 11 is a plot of a component of a
detector output 306 corresponding to the plot depicted in Figure 11. The
plot may be produced from either the X, or in-phase, component or the
Y, or quadrature, component of the output of LIA 219 provided on lines
221 or 223 of Figure 8. The corresponding outputs X or Y for the vector
of the received signal are as described and shown with reference to
Figure 8a. The vector of the received signal may be transposed by an
angle alpha, the process of which is described in more detail with
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reference to Figure 13. Transposition of the component by an angle
alpha results in a second plot 308 of the transposed corresponding
component (which is either the in-phase or quadrature component). The
angle of transposition, alpha, is selected to provide a slope reversal of
an anomaly of interest as shown in the region of the plot at the area
referenced by numeral 310. Production of the plot 308 based on
transposition of the detector vector by the transposition angle alpha
results in a mirror image form of plot only at the region of interest,
namely region 310 which corresponds to an anomaly. The larger
excursions occurring at known bell and spigot pipe joints at areas 312 of
the plot do not provide a mirror image plot in the transposition plot as
illustrated when referring the two plots 306 and 308 in the regions 312.
Thus selection of the transposition angle alpha is made such that the
known anomalies occurring at a bell and spigot pipe joint do not result in
mirror image excursions between the recorded plot 306 and the
transposed plot 308, calculated based on the transposition angle alpha.
Figure 13 is a graph of a vector output representing the in-phase and
quadrature components of a received signal output from a lock-in
amplifier. The in-phase axis I has a component value X corresponding
to the received vector and the quadrature axis Q has a component value
Y corresponding to the received vector. The vector may be described in
polar co-ordinates as having a length R and a phase theta relative to the
driving frequency. The vector may be transposed by an angle alpha,
which will cause the in-phase and quadrature components of the
received vector to assume new values. Figure 13 illustrates the
transposition transformation of one vector or data point pair by an angle
alpha. This transposition is performed against all logged data point pairs
of a data set to produce the corresponding trace 308, which is
transposed by an angle alpha as depicted in Figure 12.
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Figure 14a is a cross section through a pre-stressed concrete pipe,
generally indicated by 10, showing schematically a preferred
arrangement of the driver and detector positioned exterior to, or around,
the pipe under test. The pre-stressed concrete pipe typically has an
inner metal cylinder 11. Depending upon the type and grade of pipe,
either pre-stressing wires are wound directly onto the cylinder, or a layer
of concrete 13 is cast onto the cylinder, and the pre-stressing wires 12
are wound on the layer of concrete. As noted previously, some pipes
also have a layer of concrete cast inside the pipe, separating the metal
cylinder from the interior volume of the pipe. Generally, another layer of
concrete or protective mortar is cast around the wires to complete the
pipe.
The pipe inspection apparatus is shown disposed on the exterior of the
pipe and comprises a driver coil 17 and a detector 16. The detector is
preferably a coil detector capable of detecting magnetically induced
currents in the pipe under inspection. The detector is adapted to receive
magnetic flux and convert it into a measurable electrical current and
voltage. The detector 16 is placed proximal to the exterior surface of the
pipe. It is preferred that the detector does not touch the pipe surface, as
this would impede movement of the detector along the exterior surface
of the wall. However, the gap between the detector 16 and wall of the
pipe 10 should be kept as small as is conveniently possible, having
regard for the fact that the detector is to be moved along the length of
the pipe.
Reference numeral 19 represents a diameter of the pipe, which passes
through detector 16 in the arrangement of Figures 14a and 14b. In this
arrangement, driver coil 17 is disposed at the opposite end of the
diameter 19 from detector 16. The driver coil is driven with the same low
frequency alternating current that has been previously described. As
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previously described, it is preferable that the driver be placed as close
as conveniently possible to the wall of the pipe. Having regard to the
fact that the apparatus will be moved along the pipe, it is not desirable to
have the driver 17 dragging against the exterior wall of the pipe in
operation of the apparatus.
Figure 14b is an elevation view of the pipe and arrangement of Figure
14a, where the protective mortar of the pipe is not shown so that it is
possible to view the pre-stressing wires 12. For clarity, only a few
representative wires are shown. The driver coil 17 is visible, but the
detector coil 16 is obscured behind pipe 10.
Figure 15a is a cross section through a pre-stressed concrete pipe,
showing schematically a alternate arrangement of the driver and
detector around the pipe under test from the arrangement of Figure 14a.
The pipe inspection apparatus is shown disposed on the exterior of the
pipe and comprises a driver coil 17 and a detector 16. Each of the driver
17 and the detector 16 is placed proximal to but not touching the exterior
surface of the pipe as is driver coil 17. The driver and detector coils are
positioned so that the axis of the pipe (see 14 of Figure 15b) is normal to
a line extending between the driver 17 and detector 16. As previously
described, the driver coil is driven with a low frequency alternating
cu rrent.
Figure 15b is an elevation view of the pipe and arrangement of Figure
15a, in which the protective mortar of the pipe is not shown to enable
viewing of the pre-stressing wires 12. For clarity, only a few
representative wires are shown.
Figure 16 is a top view of a pre-stressed water reservoir vessel 250,
showing schematically an arrangement of the driver and detector around
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the vessel under test. The inspection apparatus is shown disposed on
the exterior of the reservoir vessel 250 and comprises a driver coil 17
and a detector 16. Each of the driver 17 and the detector 16 is placed
proximal to but preferably not touching the exterior surface of the
reservoir vessel 250 as is driver coil 17. The driver and detector coils
are positioned so that the axis of the reservoir vessel (see 14 of Figure
15b) is normal to a line, shown in Figure 16, extending between the
driver 17 and detector 16. As previously described, the driver coil is
driven with a low frequency alternating current. In operation of the
arrangement of Figure 16, the driver and detector are maintained in a
spaced relationship and the apparatus is moved along the axis of the
water reservoir vessel 250. Multiple passes of the vessel can be
performed using a number of inspection processes.. The apparatus can
traverse the water reservoir vessel under test. With each test, the
distance between the driver 17 and the detector 16 can be changed, or
the radial location of driver and detector apparatus around the perimeter
of the water reservoir can be changed. Another variation is to provide
the driver at a fixed location and extend the receiver along a line of the
perimeter of the water reservoir oriented parallel to the axis of the water
reservoir.
Figure 17 is an elevation of the vessel and arrangement of Figure 16
with the pre-stressing wires 12 exposed for clarity.
Figures 18a, 18b, 18c and 18d are cross sections through a pre-
stressed concrete cylinder showing schematically alternate preferred
arrangements of the driver and detector disposed about the cylinder
under test. In these embodiments, the driver and detector are disposed
on opposite sides of a cylinder under test. The arrangement of
apparatus of Figures 18a through 18d are less preferred as maintaining
the orientation and relative position of the detector to driver as the
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apparatus extends along the cylinder under test is required. In the
arrangement of each of Figures 18a through 18d, the inspection
apparatus, comprising a driver coil 17 and a detector 16, is shown
disposed on proximal to but not touching a surface of the cylinder 10 to
allow movement of the apparatus along the surface of the cylinder.
In Figure 18a, reference numeral 19 represents a diameter of the
cylinder, which is parallel to the axis of driver 17 and passes through
detector 16. In this arrangement, driver coil ~ 17 is disposed on the
exterior of cylinder 10 along the diameter 19 from detector 16, which is
disposed on an interior side of cylinder 10.
In Figure 18b, driver 17 is disposed on the exterior side of cylinder 10
under test. The axis of driver 17 is oriented toward detector 16 such that
the line extending between the driver 17 and detector 16 is normal or
orthogonal to the axis of the cylinder under inspection. In this
arrangement, driver coil 17 is disposed on the exterior of cylinder 10 and
detector 16 is disposed on an interior side of cylinder 10:
In Figure 18c, reference numeral 19 represents a diameter of the
cylinder, which is parallel to the axis of driver 17 and passes through
detector 16. In this arrangement, driver coil 17 is disposed on the
interior of cylinder 10 along the diameter 19 from detector 16, which is
disposed on the exterior side of cylinder 10.
In Figure 18d, driver 17 is disposed on an interior side of cylinder 10
under test. The axis of driver 17 is oriented toward detector 16 such that
the line extending between the driver 17 and detector 16 is normal or
orthogonal to the axis of the cylinder under inspection. In this
arrangement, driver coil 17 is disposed on an interior side of cylinder 10
and detector 16 is disposed on an exterior side of cylinder 10. The line
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extending between driver 17 and detector 16 is normal or orthogonal to
the axis of the cylinder 10 under test.
While the invention has been shown with respect to certain
embodiments, it will be understood that many variations to such
embodiments will be evident to a person skilled in the art, and it is
intended that all such evident variations should be protected.
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