Note: Descriptions are shown in the official language in which they were submitted.
CA 02332473 2001-O1-29
MONITORING SYSTEM FOR PRESTRESSED CONCRETE PIPES
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 which 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. Further, the size of the Atherton device means that it is
difficult to deploy. It
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cannot usually be put into a pipeline except at the ends, as it is too large
to be placed in
through a standard access port.
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 in abutting relationship with 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 one edge abutting the wall of the pipeline. In a preferred
embodiment, there are two
such coils, axially spaced. Preferably, 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
force,
preferably a giant magnetostrictive sensor (GMS).
Where possible it is preferred not to have a specific exciter or driver coil,
but instead to
measure, using the detector, an existing electromotive force in the pipe. Such
electromotive
force can be for example, a ground loop. Such a ground loop can often be
created (if it is not
already present), by applying a direct current voltage to the pipeline at a
place where it is
accessible, for example at its origin or at an access port along its length.
Where an existing electromotive force within the pipe is not present, it is
within the scope of
the invention to provide a driver coil to create an electromagnetic force
which creates a
current through the wires forming part of the compressed concrete pipe. The
voltage induced
by this current in the detector is then measured. Preferably, the driver coil
is located
diametrically across the pipe from the detector. Its axis is radial to the
pipe. It is preferred that
the axis of the driver coil lies in a plane extending diametrically across the
pipe which
intersects the detector. Similarly, where there are two detectors, the axis of
the driver coil is
preferably in a plane 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.
This is not
possible with apparatus where an axial separation must be maintained.
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Although less preferred, it is possible to offset the detector axially along
the length of the
pipe. The detector is preferably, even though offset, diametrically opposed
from the detector.
The axis of the detector coil lies along the radius of the pipe. It is
possible; however, to have
the driver coil not completely diametrically opposed, but it is preferred that
the radius along
which the driver coil lies should at least be on the side of the central axis
of the pipe remote
from the detector.
Where there is a driver coil directly diametrically across the pipe from the
detector or
detectors, 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,
in a position which blocks such magnetic flux, a substance which shields
magnetic flux. Mu-
metal is a metal which 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 pipe. The vehicle is
provided with a
means for determining its location precisely. The vehicle proceeds down the
pipe, while
logging information on the stated walls, particularly on broken wires. The
vehicle is
provided with a precise location determinator, so that the information which
it receives about
the state of the wires can be correlated to a particular place on the wall.
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.
The invention
also comprises a simplified analysis system for the data received, which
locates broken wires
within the wire wrapping of the pipe, without detailed analysis of the results
received.
The invention will be further described with respect to the drawings, in
which:
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.
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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
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.
Detailed description of the invention
The invention will now be described with respect to the drawings.
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
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C101 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 12, 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 detector coil 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 magnetostrictive sensor, or GMS. Such sensors will be described
henceforth in this
1 S application as "GMS sensors".
When detector 16 is a coil, it is located so that the axis of the coil is
parallel to the centre line
14 of the pipe. The coil 16 is placed so that its edge almost touches the wall
of the pipe. It is
preferred that the coil not touch the wall, as this would impede movement of
the coil along
the interior surface of the wall. However, the air gap between the coil 16 and
wall of the pipe
10 should be kept as small as is conveniently possible, having regard for the
fact that the coil
is to moved along the length of the pipe.
-Reference Numeral 19 represents a diameter of the pipe, which passes through
coil 16. At
the opposite end of the diameter 19 from coil 16, there is a driver coil 17.
The driver coil is
driven with low frequency alternating current, for example from 20 hertz to
300 hertz. The
coil is located so that its axis is radial to the pipe being inspected. Again,
it is placed as close
as possible to the wall of the pipe, having regard to the fact that the
apparatus will be moved
along the pipe, and that it is not desired that the coil will drag against the
wall.
Optionally, a shield 18 of a material which does not permit the passage of
magnetic flux 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
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signal picked up by the detector 16 should be the signal which is made by an
induced current
in the wires 12, because of the magnetic flux 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 and appreciable difference in the measurements.
Figure 2 is a view similar to that of Figure l, 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
detector coils, 21 and 22, which are spaced from one another by a distance 23
less than the
diameter 19 of the pipeline. Preferably, the distance of spacing is not more
than half the
diameter of the pipeline. The two coils are set on a common axis which is
parallel to the
central axis 14 of the pipeline.
In Figure 3, the inspection device 30 has again two coils 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
detector coils. Instead, the a driver coil 27 is axially offset, not more than
one pipeline
diameter axially along the pipeline from the detectors 21 and 22.
Additionally, 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 from eddy currents through the wires exceeds the
magnetic flux from
eddy currents through the air separating the detectors 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, at a distance
approximately three quarters
of a pipe length axially offset from the two detectors 21 and 22.
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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 "1" from the diameter 19 between the
detector coils 21 and
22. The distance "1" 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 the driver coil from the plane 19 of
the detector coil or
(in the case of two detector coils, the plane midway between the two detector
coils). If there
is no offset, measurements can be made right to the end 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 detected by the
detector coils.
It is possible to space the driver at a distance greater than one pipe
diameter, but there appears
to be little advantage in doing so. When the driver is axially distant, the
effect which it gives
is similar to that of a ground loop in the pipe. It is sometimes simpler to
induce a ground
loop in the pipe from a distant point, such as an inspection port, by applying
a voltage to the
pipe and to a grounded distant point. This avoids the need for a driver coil
entirely. It also
permits a simpler vehicle on which to put the apparatus for moving through the
pipeline, as
no driver loop need be on the vehicle.
Where there are large natural ground loop currents, these currents can be
detected by the
detector. Preferably, a very sensitive detector, such as a GTD is used.
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.
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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 they would be hidden by the metal
cylinder 64, 74 and 84.
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 of
detector such as
shown at 16 in Figure 1. Exciter coil 17 in Figure 1 is generating a 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 and 17
along the pipe. Detector 16 and 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). As will be seen, the pattern of the plot of voltage against
distance is that there is a
peak, as shown 93, as each of the bell and spigot connections is traversed by
the detector. 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
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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 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 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 analysing the plots
of the voltage
against distance of the two coils, a very precise position can be given for
the break in the
wire.
Figure 6 shows a vehicle equipped with one embodiment of the detector 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.
Figure 6 shows a concrete pipe generally indicated as 100. The pipe is formed
of a steel
cylinder 101, helical wires 102 in the steel cylinder as reinforcement, and
concrete 103.
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Although the pipe as shown 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.
5 It has a flange 107, on which a hatch cover 106 rests removably. The hatch
has an opening
f-ate.
The inspection vehicle 1 SO 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
10 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 by a suitable axle support 173. Preferably the axle support is
provided with springs
174, so that the wheels will deflect from any discontinuity on the surface of
the pipeline.
Extending from the front of the vehicle 170, in the direction which the
vehicle will travel
within a pipeline, is a support bar 165, which, as shown, is axial to the
pipeline. Connected
to this at right angles is a bar 164 which does not conduct electric current,
so that it will not
carry current between the driver and detector coils. Bar 164 has mounted on it
at one end the
crossbar 166, with two detectors 161 and 162 spaced 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 a coil which has a current induced in it by magnetic flux in its
vicinity. Means (not
shown) are provided to measure the current in each coil, and to record the
current measured.
The recording apparatus is in the body 170 of the vehicle, and is
schematically shown at 169.
At the other end of bar 164 is a driver coil 163. This driver coil has an
alternating current in
the 20 - 300 hertz range (the precise frequency is not important, so long as a
suitable eddy
current is generated in the wires of the pipe). The power is provided to
driver coil 163 a
means of suitable AC generating current means 191 mounted in the vehicle. The
connection
to driver coil 163 is not shown. Barrier 168 of mu-metal is provided to block
stray magnetic
currents through the pipeline between driver coil 163 and detector coils 161
and 162.
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An odometer 174 is provided. This is connected to one of the wheels 171, to
measure the
distance travelled by that wheel. It records a signal indicative of the
distance travelled along
with the recording of the outputs of detectors 161 and 162, to provide a
record of distance
travelled.
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 one
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 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) 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 vehicle 170, with hydrophone data storage 181, also connected to
the odometer
174. The hydrophone records information as the vehicle passes through the pipe
in known
fashion, and this information 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.
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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
which it has received, and this data can be received by an operator on the
other side of
inspection port 105. 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 generate the AC current for the driver coil can be
recharged.
It is also within the scope of the invention to have a supplementary motor 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 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 odometer) at which the exception occurred.
It is within the invention to use other distance measuring means other than an
odometer.
This, 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.
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.
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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. During the period when the vehicle is stopped, other
inspection
means (such for example hydrometer 180) can be activated to see if there is
continued wire
breakage occurring at or around the location.
In most pre-stressed water or sewage pipe, flow 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
210, which recognize signals emitted by the vehicle). For example, the vehicle
can have a
transponder 211, which responds to a sound emitted by a transponder 210 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.
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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