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Scanning
Steel Cord Conveyor Belts With the "BELT C.A.T.™" MDR System
By Dieter W. Blum
[April 1996]
[Summary]
[Introduction]
[Belt Construction]
[Belt
Scanning Requirements] [Prior
NDT Methods] [Real
World Scanning Difficulties]
[Belt
C.A.T.™ Introduction] [Belt
C.A.T.™ Scanning System] [Scanner
Head] [Data
Acquisition System] [Data Processing System]
[Belt C.A.T.™
Scan Report] [When
to Scan] [Conclusion]
[References]
Summary
This paper
describes the "BELT C.A.T.™" scanning system for the
magnetic flux based inspection of the steel cord carcass in conveyor
belts. The "BELT C.A.T.™"system is based on a newly patented
Magnetostatic Differential Reluctance measurement technique, and
generates data that is unaffected by belt speed fluctuations, belt
flutter, residual magnetic patterns within the belt, high strength
ambient EM fields, steel cord size or spacing, and can scan loaded
conveyor belts running at full operational speed.
The "BELT C.A.T.™"
system is quickly installed for scanning, requiring the positioning of a
single sensor assembly on only one side of the belt, thereby creating
minimal disruption to conveyor operation. In addition to being the only
available system inherently able to produce three-dimensional graphical
displays of data that are intuitively comprehensible, the "BELT
C.A.T.™" system also provides for the precision
analysis and classification of splices and all known belt fault types.
as well as their exact transverse and longitudinal location in the
conveyor belt. [Back]
1. Introduction
The economic
viability of most mining operations is reliant on the dependable
operation of large conveyor systems. Conveyors have been shown to be the
most economical means of transportation for a wide range of bulk
materials. In recent years, as throughput tonnages have been increasing,
we have seen wider, longer and steeper incline steel cord conveyor
systems exhibiting higher tensions. During this same period we have also
seen belt technology improve, resulting in the current trend towards
conveyor systems having lower safety factors. To ensure consistent
conveyance of the product, the reliability of the constituent components
of the conveyor system becomes the highest priority.
The availability required of the conveyor system dictates that both continuous and preventative
maintenance programs be in place and be adhered to. At regular
intervals, testing is performed on critical components such as drive
motors, gear-boxes, pulleys, shafts, bearings, couplings etc. Test
measurements are taken via vibration analysis, ultrasound and laser
technology, and microscopic inspections of contaminants in oils, greases
etc. are also performed. As such, it can be seen that a significant
amount of time and money is spent on component life and failure analysis
and prediction. And yet, the most critical and costly component of the
conveyor system - the conveyor belt itself - is left largely untested.
To date, it has been very difficult to find a reliable and informative
NDT inspection tool (capable of providing precise and accurate data
analysis) for steel cord conveyor belts. [Back]
2.
Belt Construction
Fig.1: Section of steel
cord belt
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The conveyor
belt consists of two basic parts, the carcass, which is
responsible for the load tension and the top and bottom covers,
which serve to carry the product and protect the carcass from
product and pulley/idler damage. In typical high tension systems,
the carcass consists of steel cords placed side by side and
embedded in the rubber core between the top and bottom covers as
illustrated in Fig. 1. Long conveyor systems are comprised
of multiple rolls (@350 m each) of belt, and large systems can be
comprised of more than 50 such rolls of conveyor belt.
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The individual
belt rolls are connected together by splices as shown in Fig. 2.
In the belt section between splices, the steel cords handle 100%
of the load tension, and in the splice between belt sections, the
rubber handles 100% of the load tension. If either of these fails,
the conveyor halts, material flow stops and if the failure is
catastrophic the mine can shut down. [Back]
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Fig. 2: Section of 2-step splice
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3. Belt Scanning Requirements
What we require is
a means to "see" inside the belt to the underlying steel cord
carcass in order to analyze the condition of the steel cords and predict
failure potentials. The steel cord characteristics of interest are
external cord corrosion as illustrated in Fig. 3a, internal cord
corrosion and fretting damage as shown in Fig. 3b, touching cord breaks
as shown in Fig. 4, gapped cord breaks as shown in Fig. 5 and the cord
ends within splices as illustrated in Fig. 6. [Back]
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Fig.
3: External and internal cord corrosion
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Fig.
4: Touching cord break
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Fig.
5: Gapped cord break
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Fig.
6: Splice cord ends
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4. Prior Art NDT Methods for Conveyor Belts
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Fig.
7: Typical (HARRISON) "CBM" NDT system
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To date, we
have had to rely on various eddy current, inductive coupling, RF
tank loading or leakage flux based methods that have their roots
in the magnetic cable testing techniques pioneered in the late
1800's through early 1900's (see
for example: BURROWS patent, [1]).
Many of these techniques have
been advanced and refined, although most of the work to date has
been related to the NDT flaw inspection of weld joints, pipes and
elevator hoist cables (example
references: HIRAMA et al. patent, [2); ANDO et al. patent, [3];
VAN DER WALT patents, [4] and [5]).
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At the end of the 1970's, the
Commonwealth Scientific and Industrial Research Organization (CSIRO)
in Australia, conducted research and applied some of these methods
towards the testing of steel cord conveyors (this
work was performed by Dr. A. HARRISON; HARRISON patent, [6]).
This resulted in the development of a NDT system that became known
as the "Conveyor Belt Monitor" or "CBM", and
this was commercially deployed and has been widely used. One
version of the "CBM" system is shown in Fig. 7.
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However, this system suffers from a
number of limitations: As
indicated in the above patent, this technique is susceptible to
variations in the air gap between the steel cords and the transmit and
receive coil pole faces. To overcome this problem requires either belt
stabilization (to physically stop flutter) or the placement of
transmitter and receiver coil assemblies on both the top and bottom
sides of the belt in order to provide for the differential cancellation
of belt flutter.
This technique exhibits low sensitivity in that small faults
contribute only minutely to the output signal of each receive coil. This
means that small faults can go undetected.
The method provides poor transversal fault position resolution, due to
the transverse magnetic summation of many steel cords.
The output signal of each receive coil is simply plotted on an analog
chart recorder. This form of data acquisition and hard copy output
results in a loss of longitudinal (time axis) fault position resolution,
which causes great difficulty when attempting to interpret the hard copy
data.
The above described technique has been found to be prone to generating
errors in response to residual areas of magnetization that are commonly
found in the steel cords of conveyor belts (see
HARRISON patent, [7] and HARRISON [8]).
Although a
breakthrough when introduced, the "CBM" NDT system lacks the
accuracy and detail required of a useful steel cord conveyor NDT system.
Even so, this NDT system has been the tool of choice for almost two
decades.
Recent European advents are very
similar to the above described "CBM" approach and as a result
suffer from the same lack of accuracy, detail and difficulty in
interpreting scan data. [Back]
5. Real-World Scanning Difficulties
Scanning a steel
cord conveyor belt is not particularly easy. For practical purposes, one
should be able to perform a scan with the belt moving at normal
operating speed (as with a fixed-speed drive system), with the belt
possibly empty, but probably loaded. This eliminates the use of current
non-real time X-ray techniques, which even at best, when used to inspect
splices, suffer from imaging distortion and the need to manually align
many photographic images in order to create a complete view of a splice.
The moving conveyor belt also exhibits
varying types of physical motion or vibration, consisting of severe
vertical displacements (flutter) and lateral shifting from side-to-side.
The conveyor belt can also have an uneven cover surface, the conveyed
product can stick to the belt surface, and cables can be broken and can
protrude from the belt surface, thereby hindering physical attempts to
stabilize the belt (i.e., against flutter).
The steel cord faults of interest
consist of internal or external cord corrosion, small and large gap cord
breaks, touching breaks and cord ends. Belt speed fluctuations are also
common and must be taken into account. Electromagnetic noise and
disturbances from sources such as drive motors, drive electronics,
lights and even the magnetic fields due to the earth and adjacent steel
structures all contribute to the magnetic ambient when scanning.
Moisture, dust and extreme temperatures at the scan site must also be
accommodated. [Back]
[Summary]
[Introduction]
[Belt Construction]
[Belt
Scanning Requirements] [Prior
NDT Methods] [Real
World Scanning Difficulties]
[Belt
C.A.T.™ Introduction] [Belt
C.A.T.™ Scanning System] [Scanner
Head] [Data
Acquisition System] [Data Processing System]
[Belt C.A.T.™
Scan Report] [When
to Scan] [Conclusion]
[References]
6.
Belt C.A.T.™ Introduction
Rapid progress in
electronic and signal processing technology during the early to mid
1990s have allowed for the development of the next generation steel cord
conveyor belt scanning system - the "BELT C.A.T.™" (Cable
Anomaly Tomography).
The development of the "BELT
C.A.T.™" has produced a considerable amount of novel and
proprietary technology pertaining to the precision testing of steel cord
conveyors and has resulted in the issuance of significant patents (NINNIS
patent, [9]; BLUM patent, [10]). [Back]
7. The Belt C.A.T.™ Scanning System
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The "BELT
C.A.T.™" scanning system consists of four fundamental
components as shown in Fig. 8. The first is the scanner
head, which is placed above one side of the belt to be scanned.
The second component is a rugged, field-deployable, high-speed
data acquisition and recorder system, the third is a combination
of advanced data processing hardware and software, and the fourth
is the hard copy scan report and steel cord event analysis
produced by this system. [Back]
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Fig. 8:
BELT C.A.T.™ system
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8. Scanner Head
The scanner head
is comprised of one to four 66 cm modules that interlock to form a
rectangular unit that spans the width of the belt. It takes
approximately 15 minutes to place and align the tube either under or
over the belt. Scanning need only be done on one side of the belt and
steady rolls or idlers are not required to dampen belt flutter.
The scanner head operates on a
patented Magnetostatic Differential Reluctance measurement
principle (BLUM patent. [10]) which employs the generation of opposing high-density magnetic fields
from within each side of the scanner head. The steel cords within the
conveyor belt pass by at a distance of 3 to 8 cm from the head and
effectively collapse these opposing fields as shown in Fig. 9.
The collapsed flux then flows through static magnetic flux sensors
placed on 10 mm centers across the entire width of the head. These
sensors essentially "follow" individual cords as the belt
passes over the scanner head.
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Fig
9. BELT C.A.T.™ scanner head
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Any physical vibration and vertical
displacement (flutter) of the belt is canceled by the patented principle
of operation of the scanner head and have no effect on the sensed data.
As the principle of operation is based on measuring direct changes in
the reluctance of the magnetic path through the steel cords, as opposed
to measuring the flow of eddy-currents or utilizing magneto-inductive
coupling, we can precisely detect corrosion, touching breaks, and even
cords with individual strand breakages.
Besides the magnetic flux sensors,
each individual scanner module also contains electronic circuitry for
the conditioning and normalization of the sensor signals, as well as for
the sampling and quantization thereof. The resulting digital data is
then sent to the data acquisition and recorder system at high speed via
a noise immune fiberoptic link. [Back]
9. Data Acquisition System
The data
acquisition and recorder system shown in Fig. 10, consists of a rugged,
field-deployable, solid-state computer system with integrated LCD
touchscreen, packaged into an enclosure similar to a fat laptop. It also
houses the power supply system for the scanner modules. The belt data is
obtained by freezing (holding) all the output signals from the magnetic
flux sensors in a single slice across the belt width and then quantizing
each sensor's output signal.
The data acquisition system obtains up
to 256 transverse samples of the belt cords across a 2,640 mm wide belt
for each slice taken.
The data acquisition system utilizes
an automatic variable rate slice control system that continuously
adjusts itself to actual belt speed without requiring the use of a
tachometer. Successive belt slices are taken at 5mm intervals along the
entire length of the belt, with each slice consisting of digitized data
collected across the entire width of the belt. As a result, the
longitudinal fault position resolution provided by the "BELT
C.A.T.™" scanning system is stated as being ± 5 mm, but
since the scanner head functions in a track and hold mode, it is
actually capable of capturing steel cord events that are much smaller in
longitudinal length (of almost zero length, as in the case of touching
breaks). For a belt traveling at 7 m/sec and having a width of 2,640 mm,
the data acquisition and recorder system collects 750,000 samples per
second.
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Fig. 10: BELT C.A.T.™ data
recorder
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As an integral part of the data
acquisition system, a unique data recorder solution was developed. This
uses solid-state electronics (no hard disk drives) to stream the
acquired sensor data to dual tape drives simultaneously for redundant
storage, thereby ensuring scan data integrity.
As it is normal practice to collect
and store data for two complete revolutions of the conveyor belt in
order to demonstrate the precise correlation of both the magnitude of
detected steel cord events, as well as their location, a typical scan of
a wide belt on a long conveyor system will generate as many as 4 billion
data samples that will have been stored on each of the two tape drives.
At the end of a scan, the tapes are removed, one for safekeeping and the
other to be sent for processing. [Back]
10.
Data Processing System
The tape
containing acquired scan data is subsequently downloaded into a signal
processing computer for analysis.
Each and every data sample that was
collected during the scan is analyzed, and special filtering methods and
algorithms process the scan data. The system only reports steel cord
events that are considered to be anomalies or faults. Any interference
from drive motors, belt flutter, inherent magnetism etc., is eliminated.
The processed data is printed as a color hard copy scan report
automatically and may also be recorded onto videotape for visual archive
purposes. [Back]
11.
Belt C.A.T. ™ Scan Report
The scan report
consists of two parts, a color graphical summary of two complete
revolutions of the belt, and a graphic detail of each significant steel
cord fault and each splice. Also, each graphic detail is accompanied by
specific assessments and suggestions. In both of these graphical
presentations, the belt rubber is considered to be transparent and is
therefore not shown.
The scan report is meant to be viewed
as if one were walking down the belt on the carry side and in the
direction of travel. If no steel cord events were encountered during
scanning, one will see a white (blank) belt.
When a steel cord event is
encountered, one will see the fault or anomaly indicated by a colored
image. Red indicates a detected loss of mass in the steel cord
carcass; Green indicates a detected increase of mass in the steel
cord carcass.
If corrosion is detected, as shown in Fig.
11-A, one will see a series of green dots in the longitudinal
direction and if the corrosion is severe enough, these will be followed
by a series of red dots.
In the case of touching cord breaks,
one will see an area of either red or green dots oriented transversely.
The color is dependent on the degree of butting of the individual cord
strands. The larger the image in the transverse direction, the more
steel cords have been damaged in this manner. An example touching break
is shown in Fig. 11-B.
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Fig. 11: BELT C.A.T.™ color
graphical summary
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If there is a small air gap or a
larger open span between broken cord ends, one will see the presence of
red indicating the span of the gap as shown in Fig. 11-C (four
broken cords with gap) and Fig.
11-D (four cables missing).
These two opposing colors thereby
outline steel cord faults and anomalies and allow us to pinpoint the
location and magnitude of the fault. They also graphically show the
difference between mass lost (i.e., touching break, cord missing or
corrosion) and mass added, as for example, in a three cord patch or
repair illustrated in Fig. 11-E.
The "BELT C.A.T. ™"
scanning system also provides for accurate analysis of splices, as
to the type of splice, the angle and length of steps of overlapping
cables, location of cord butts and overall workmanship, in order to
verify that splices have been completed to specifications. Fig. 11-F
shows a single step splice detail containing a center butt.
Only the "BELT C.A.T. ™"
scanning system shows a complete image of the integrity of the steel
cord carcass embedded within the rubber of a conveyor belt. [Back]
12.
When to Scan
After the initial
installation of a steel cord conveyor belt system, a scan should be
performed to provide an audit in order to verify the manufacturer's
supply of the belt and to show any damage which may have occurred during
installation. Each splice will be accurately displayed in order to
verify splice lay-up according to specifications. This first belt scan
will then serve as a base-line reference for future scans of the belt.
Another scan should be performed as
the belt ages, if there are changes in belt operation, or as belt damage
occurs or is suspected. BELT C.A.T. ™ scans become a valuable
maintenance tool as steel cord damage can be easily classified as to
severity, and can be repaired or respliced.
Changes in splice cord end positions
or their breakage due to dynamic fatigue can be spotted and repaired or
replaced. This can prevent catastrophic failure and dramatically extend
the life of the belt. [Back]
13. Conclusion
For the first
time, we are able to "visually" examine the condition of the
steel cords inside the complete conveyor belt. We can now diagnose the
condition of the steel cords, predict running damage and belt life,
analyze splices and comment on catastrophic failure potentials.
"BELT C.A.T. ™"
scanning systems are currently being placed into production and will be
available worldwide through authorized licensees. [Back]
[Summary]
[Introduction]
[Belt Construction]
[Belt
Scanning Requirements] [Prior
NDT Methods] [Real
World Scanning Difficulties]
[Belt
C.A.T.™ Introduction] [Belt
C.A.T.™ Scanning System] [Scanner
Head] [Data
Acquisition System] [Data Processing System]
[Belt C.A.T.™
Scan Report] [When
to Scan] [Conclusion]
[References]
References
[1] Method and
apparatus for testing magnetizable objects by magnetic leakage; U.S.
Patent # 1,322,405 to C.W. BURROWS, Nov. 18,1919.
[2] Electromagnetic inspecting
apparatus for magnetizable wire rope; U.S. Patent # 4,427,940 to HIRAMA
et al., Jan. 24,1984.
[3] Apparatus for detecting surface
flaws of a pipeline by electromagnetic induction; U.S. Patent #
4,742,298 to ANDO et al,, May 3, 1988
[4] Method of and apparatus for
magnetic saturation testing a wire rope for defects; U.S. Patent #
4,827,215 to VAN DER WALT, May 2,1989.
[5] Method of and apparatus for
detecting cross sectional area variations in an elongate object by the
non-inductive measurement of radial flux variations; U.S. Patent #
5,036,277 to VAN DER WALT, July 30,1991.
[6] Monitoring of elongate
magnetically permeable members; U.S. Patent # 4,439,731 to A. HARRISON,
March 27,1984.
[7] Magnetic detection of air gaps
formed at breaks in conveyor belt cords; U.S. Patent # 4,864,233 to A.
HARRISON, Sept. 5,1989.
[8] HARRISON, A.: Use of reference
signatures to monitor belt splices; SME Bulk Material Handling by
Conveyor Belt, Technical Symposium Compendium, pp. 49-53.
[9] Damage detection apparatus and
method for a conveyor belt having magnetically permeable members; U.S.
Patent # 5,426,362 to R. NINNIS, June 20,1995, Assignee: Conveyor Belt
Technology Corp.
[10] Apparatus and method of damage
detection for magnetically permeable members; 1992 U.S. Patent
Application S/N # 07/954,485, to D.W. BLUM, Notice of Allowance given
April 1996, Assignee: Conveyor Belt Technology Corp. (Issued as U.S.
Patent #'s 5,570,017 and 5,847,563.)
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