Note: Descriptions are shown in the official language in which they were submitted.
CA 02356239 2001-08-27
LOW FREQUENCY ELECTROMAGNETIC ANALYSIS OF
PRESTRESSED CONCRETE PIPE TENSIONING STRANDS
Field of the Invention
This invention relates to a method of non-destructive inspection of concrete
conduits
reinforced with metal wires, and to apparatus for carrying out such
inspections.
Background of the Invention
There are many concrete conduits in use to conduct pressurized fluids, for
example in
piping systems for water. 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.
The purpose of the reinforcing wires is to keep the concrete in compression.
Over time,
the wires may corrode and eventually break. When this happens, it is possible
that a
rupture of the concrete conduit will occur, leading to escape of the
pressurized fluid
which it contains.
It is very expensive to replace an entire conduit. 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.
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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 for wire wound concrete pipes
is
provided. The device has one or more detectors proximal to the inner wall of
the pipe.
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, fhe 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 magnetoresistive (GMR) sensor.
The invention provides a driver coil to create an electromagnetic field, which
creates a
current flow through the wire strands forming part of the pre-stressed
concrete pipe. The
voltage and other effects induced by this current in a detector are then
measured.
Preferably, the driver coil has its axis radial to the pipe. It is preferred
that the axis of
the driver coil lies in a plane extending diametrically across the pipe, that
is transverse
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to the 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 the axis
of the pipe and
intermediate of 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 coil. Distances of up to 10 feet
separation in a 20
foot diameter pipe have been found to work. However, such arrangement has no
benefit, requires a longer equipment mounting and prevents taking readings
near the
ends of pipes.
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. For large
diameter pipes, such as 20 foot diameter pipes, it is preferable not to have
the driver
coil not diametrically opposed from the detector, but even though not
diametrically
opposed from the driver coil, the detector is circumferentially offset from
it. Where the
detector is a coil, the axis of the detector coil is parallel to the axis of
the pipe. It is
possible; however, 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.
There can be appreciable interference to the signal through magnetic flux
formed within
the pipe, between the detector and the driver. To counter this, it is
preferable to place 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.
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In a particularly preferred embodiment of the invention, a detector device
according to
the invention is mounted on a vehicle movable through the 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 sized so that, in a large pipeline, it can be placed in the
pipeline through
inspection ports, which periodically occur in the pipeline. It 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 ca 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 analysed at a remote location.
In one of its aspects, the invention provides an inspection apparatus for
detecting
discontinuities in a spirally wound metallic pre-stressing reinforcements
embedded in
concrete pipe, comprising a driver coil oriented with an axis radial to a
concrete pipe for
inducing a current in a metallic pre-stressing reinforcement of the concrete
pipe and
detector apparatus oriented proximal to an inside surface of the pipe and
disposed
axially along the pipe from said driver means not more than one pipe diameter
from a
plane orthogonal to the axis of the pipe and common to said driver means, the
detector
apparatus for producing an output responsive to a magnetic flux. The apparatus
includes displacement sensor means to produce an output representative of at
least
one distance of the detector apparatus a from a known location and means for
causing
the detector apparatus to move along an inside surface of such pipe as well as
means
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for storing outputs corresponding to the detector apparatus and the
displacement
sensor.
In another of its aspects, the invention provides a method of detecting
discontinuities in
spirally wound metallic pre-stressing elements of a concrete pipe comprising
providing a
driving signal to a driver having an axis oriented radially to a concrete pipe
and proximal
to an inside surface thereof to generate a periodic induced current in pre-
stressing
elements extending substantially circumferentially of the pipe and providing a
detector
for producing an output responsive to a magnetic flux in a direction axial to
a concrete
pipe, the detector located in close proximity to an interior wall of a pipe
and within one
pipe diameter of a plane orthogonal to the axis of the pipe and common to the
driver. 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 yet another of its aspects, the invention provides a method of testing the
tension
strands of a pre-stressed concrete pipe along a length thereof using apparatus
including
magnetic flux production means and magnetic flux detector means each proximal
to an
interior surface of a pipe and in a spaced relationship to each other, the
magnetic flux
production means for producing a magnetic field in response to a periodic
driving signal
and the detector means for producing a detector signal in response to magnetic
flux and
locating means to indicate a location along said pipe and control means
operatively
connected to said locating means, to said magnetic flux means and to said
detector
means. The method comprising performing the steps of providing a periodic
driving
signal of at least one frequency, receiving a detector signal; and recording
the detector
signal in relation to the frequency of the periodic driving signal and the
location
indication over a range of locations traversed along a length of pipe.
And in yet another of its aspects, the invention provides a method of testing
the tension
strands of a pre-stressed concrete pipe along a length thereof using apparatus
including
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magnetic flux production means and magnetic flux detector means each disposed
proximal to an interior surface of the pipe in a spaced relationship to the
other and
axially disposed within one pipe diameter to the other, the magnetic flux
production
means for producing a magnetic field in response to a periodic driving signal
and the
detector means for producing 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
comprising providing a periodic 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
fundamental frequency of the driving signal; and recording the output
representative of
the detector signal in relation to the fundamental frequency and the location;
whereby at
least one fundamental frequency 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.
Figure 3 is a cross-section through a similar pipe, showing a third embodiment
of the
inventive detector system.
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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 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.
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.
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Figure 9 is graph showing detector traces output produced by the apparatus of
the
invention for two periodic driving frequencies.
Figure 10 is graph showing detector traces output produced by the apparatus of
the
invention for a plurality of periodic driving frequencies.
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.
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.
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.
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
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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 C304. 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 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 and voltage.
However,
instead of a coil, any other detector of magnetic flux could be used. Another
particularly
preferred sensor is a giant magnetoresistive 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 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.
The driver coil is driven with low frequency alternating current, for example
from 20
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hertz to 300 hertz. The coil is located so that its axis is radial to the pipe
being
inspected. Again, 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 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
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
two detectors 21 and 22, for example coils, which are spaced from one another
by a
distance 23 less than the diameter 19 of the pipeline. Preferably, the
distance of
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spacing 23 is small, such as a few inches and preferably not more than half
the
diameter of the pipeline. The two detectors are set on an axis, which is
parallel to the
central axis 14 of the pipeline.
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 does not need to be precisely
diametrically
across the pipeline, but may instead 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 strands of the pipe exceed 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, and the coil is disposed radially of the pipe.
Particularly in
large pipes (for example the 20 foot 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. 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.
Figure 4 shows a detector with the same arrangement as Figure 3. However, in
Figure
4 the detector (numbered 25) 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
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of a pipeline. 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 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 cylinder 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.
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Figure 5b is a plot of voltage against distance along the pipe, for one of
detector such
as shown at 16 in Figure 1. The voltage is represented on axis 90 of the graph
and the
plot or trace is made against distance travelled, which is shown on axis 91 of
the graph.
Exciter coil 17 in Figure 1 is generating a frequency of 20 - 300 hertz, and
detector 16 is
producing a voltage output. Figure 5b shows a plot a voltage representative of
a
detector output plotted against the distance travelled by the detector
apparatus.
Detectors 16 and 17 are rigidly linked, so that they each travel at the same
speed.
As is shown in the plot, 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 trough. Thus, while the detector is
traversing pipe
61, there is initially a peak 98 as the detector passes over the bell and
spigot connection
just before pipe 61, then a trough 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 trough 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, which 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 troughs
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
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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 202.
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.
When there is a 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 2. The two detectors (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 on the
detectors
is symmetrical upstream and downstream of the detector apparatus. 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
detectors is found. By analysing the plots of the voltage against distance of
the two
detectors, a very precise position can be given for the break in the wire.
Figure 6 shows a vehicle equipped with one embodiment of the apparatus of the
invention (in this case the embodiment of Figure 2). The vehicle is designed
to pass
through the pipelines to 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.
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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 E-ate.
The inspection vehicle 150 has a body 170, with wheels 171. In 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 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 crentral
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
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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 20 - 300 hertz range of sufficient magnitude to
induce a current
flow in the wire strands of the pipe. An AC 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 detector 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 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 which 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 E-A-~. The result is that the
vehicle can
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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.
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 with 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
receive this data. 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
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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 generate the AC current for 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 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.
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Suitably, the apparatus 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 Figure 5b have means to change the
angle
of deflector 190, so that it will move more slowly through the pipe or
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 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.
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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
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. 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
produces 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 line 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
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detector signal. The filtered signal output from multiplier 216 is amplified
by input
amplifier 212 and converted to a digital signal by a digital signal processor
(DSP) 214.
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 it 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. If an
independent signal from each of two detectors is to be processed by
microprocessor
206, 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 line
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 ana performs computations based on the received
digital
signals to produce a visually perceptible output on display device 220.
Preferably, the
visually perceptible output is graph on a display device 220. Display device
220 can be
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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 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.
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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, A) 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 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, or selected portions of 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 or "X - Position along Pipe" axis of
the graph.
The vertical axis of the graph represents a voltage level output of the LIA
219 output as
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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 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 tension strands 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
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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.
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
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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 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
40153529.3
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CA 02356239 2001-08-27
traversal, data points would be 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 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. But the individual traces produced
by the
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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 at 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 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
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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 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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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.
40153529.3
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