Note: Descriptions are shown in the official language in which they were submitted.
6~'1
This invention relates to a method and apparatus for
detecting a rip or flaw in a conveyor belt.
Conveyor belts comprising rubber or rubber-like
materials are in some uses susceptible to failure by tearing
or ripping. Early detection of such flaws is desirable to
prevent loss of conveyed materials and complete failure of,
or damage to, conveyor belts.
For that purpose various means such as trip cords and
tilt trays have been used mechanically to detect material
falling through a tear in the conveyor belt. Such monitors
are only sensitive to a gaping rip sufficient to permit
material to fall through, or to gross belt failure.
In another method the electrical continuity of
electrical conductors embedded in the belt composition is
monitored. However, that method requires the use of
expensive and non-standard belts and it is difficult to
maintain the conductors in a production environment.
of the methods employed to-date all are either
insensitive to some ways in which a belt can rip or are
otherwise impractical.
It would be particularly advantageous if a flaw or
incipient flaw likely to result in belt failure could be
detected so that preventative maintenance could be
conducted. None of the methods hitherto proposed for
monitoring conveyor belts has been useful for that purpose
The present invention uses ultrasonic vibrations,
referred to hereinafter as "ultrasound", for the purpose of
detecting discontinuties in the belting.
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2a
It has hitherto been believed that transmission of
ultrasound in rubber over distances of more than a few
centimeters was impracticable due to the high level of
attenuation of ultrasound in rubber. It was also thought
tihat attenuation would decrease continuously with decreasing
frequency.
The present invention stems from the discovery that
ultrasound may be propogated in belting comprising a rubber
or rubber like composition, and that particular frequencies
of ultrasound are attenuated to a much lesser degree than
expected and to a lesser degree than other higher or lower
frequencies.
The particular frequencies may be ascertained by
experiment for a particular belting composition and belting
thickness. There may be more than one particular frequency
at which the attenuation is low in comparison with
neighbourlng frequencies.
Surprising, it has been found possible to select an
ultrasound frequency at which attenuation in belting is
sufficiently low that a transmitter and receiver may be
spaced apart over a long path length, for example a meter or
more, enabling a transmitter and receiver to be situated
adjacent opposite side edges of an industrial scale conveyor
belt. Moreover the low attenuation enables a reLiable signal
to be propogated and detected at a distance from the
transmitter even when the intervening conveyor belting is of
a poor quality but intact.
,.
Preferred embodiments ox the method of-the invention
also permit small discontinuities throughout the length of a
belt to be discovered while the belt is in normal use and
enable identification of portions requiring preventative
maintenance to reduce the risk of a rip subsequently
occurring.
According to a first aspect the invention consists in a
method fox detecting a ri? in a conveyor belting
comprising rubber or rubber-liXe composition, the method
comprising the stews of:
(a) trans,nit'ing ultrasound of a particular frequency
to the belting at a first æone;
(D) detecting ultrasound of said particular frequency
propagated by the belting from the first zone to a
second zone spaced apart from the first; and
(c) issuing a signal responsive to the intensity of
ultrasound, if any, so detected whereby said signal
is indicative of discontinuities in the belting
intermediate said zones.
According to a second aspect the invention consists in
apparatus comprising:
a conveyor belt comprising a rubber or rubber liXe
material, the belt passing throuc3h a first zone and a
second zone spaced from the first,
go
tran~lucer means or producing uLtrasound at a
prede.ermined frequency,
cou?ling means for transmitting said ultrasound Jo the
Dell wnile it is in motion through the first zone, and
sensor jeans producing a signal responsive to said
ul~rasoun~ toe welt at toe second zone.
In preferred embodiments the invention is used to
monitor a conveyor belt while in motion. ultrasound
generated by a piezo-electric transducec is transmitted to
moving belting via a liquid or aqueous medium impinging on
the underside of the belting at a first zone. The ultrasound
is propogated along the belting to a second zone at which a
detector produces a signal responsive to ultrasound, also
coupled to it via a liquid or aqueous medium. Vaciatio;;s in
the received signal are indicative of discontinuities or
flaws of the belting intermediate the zones. or preference
the zones are s`paced apart on a line perpendicular to the
direction ox travel of the belt.
ay way of example only an embodiment of apparatus
according to the invention will now be described with
reference to the accompanying drawings wherein:-
Fig. 1 is a general arrangement according to the
invention.
Fig. 2 is a cross-section of a transducer assembly
useful for transmitting ultrasound to a conveyor belt in
motion.
Fig. 3 shows one part oE the means foc mounting the
apparatus of Fig. 2 in the arrangement of Fig. 1.
Fig. 4 shows a second part of the means for mounting the
apparatus of Fig. 2 in the arrangement of Fig. 1.
Fig. 5 shows a third part of the means for mountiny the
apparatus of Fig. 2 in the arrangement of Fig. 1.
Fig. 6 is a grapih showing the attenuation of ultrasound
in belting of 25 and 20 mm thickness at various frequencies.
With reference to Fig. 1 there is shown a cross-section
through an endless conveyor belt 1, transverse the direction
of conveyor travel.
The conveyor has a trough shaped cross-section and
belting 1 is supported along its length by means not shown in
Fig. 1.
The conveyor belting is typical of the wind used in the
mining industry comprising a steel cord reinforced rubber-like
composition having a width edge to edge of 1-2 meters and
having a thickness in the present example of 2~ or 25 mm.
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A first transducer assembly 2 is mounted adjacent to and
beneath conveyor belt 1 near one edge of the conveyor and a
second transducer assembly 2 is similarly mounted near the
opposite edge. One transducer assembly 2 together with
associated circuitry is a means for producing ultrasound and
the other with associated circuitry acts as a detector of
ultrasound.
In use a column of water extends between each transducer
assembly 2 and a zone 3 of belt 1 overlying that transducer
assembly.
Ultrasound may thus be transmitted from one transducer
assembly 2 via a water column extending from the transducer
to a zone 3 of belt 1, through belt 1 to a second zone 3
overlying the other transducer assembly, and via a water
column to ultrasound detection means associated with the
other transducer assembly 2.
A discontinuity in belt 1 between zones 3 may thus be
detected.
With reference to Fig. 2 there is shown in cross-section
a transducer assembly 2 useful either for transmitting
ultrasound to a belt in motion or for detecting ultrasound in
the belt. The transducer assembly comprises an upper tubular
case part 10 and a lower tubular case part 11 connected at
respective radially extending flanges 12 and 13 by fasteners
14.
Upper case part 10 has an upper radially extending
flange 15 to which is mounted a head 16 defining a chamber 17
provided with a water inlet tube 18 and a water outlet
orifice 19.
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Case parts 10 and 11 house a sub-assembly comprising two
discs 21,22 of pizeo-electric material (such as Philips
PXE42)separated at adjacent faces by metallic foil 23 and
clamped coaxially between metal cylinders 24 and 25 by sleeve
mounting on bolt 26 which extends through cylinder 24, disks
21 and 22, foil 23 and threadibly engages a threaded blind
bore 27 of cylinder 25.
A cylindrical plug 28 of a non-metallic solid material,
typically a plastics substance, is fixed at one end 28 to the
upper face of metal cylinder 25. The other end 30 of plug 28
is of reduced diameter over a length extending into cavity
17. O-rings 31 provide a seal between plug 28 and case 16.
Cylinder 25 is provided with a radially extending flange
32 which is clamped between O-rings 33 grooved in case
flanges 12 and 13 whereby the pizeo-electric crystal
sub-assembly is mounted.
Electrlcal connection 34 with metallic foil 3 serves as
a connection to one electric pole of pizeo-electric disks 1,2.
The other electrical pole is kept at earth potential by
connection 35 of the end face of each disc remote from foil 3
to earth potential. Cylinders 24 and 25 are of dissimilar
metal and the axial lengths are chosen having regard to the
velocity of ultrasound in the respective metals. The
combined sub-assembly connected by bolt 26 can vibrate
substantially in the axial direction at ultrasonic
frequencies with a fundamental resonance frequency occurring
when the wavelength of the vibration is equal to twice the
combined axial dimension.
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In use as a transmitter water fed under pressure via
inlet tube 18 to cavity 17 flows outwardly from orifice 19
and impinges on the underside of an overlying conveyor belt 1.
A continuous column of water thus extends between the
ultrasound transmitting face 30 of plug 28 and the overlying
zone ox belt 1 providing an ultrasound coupling medium,
notwithstanding motion of the belt. Flange 33 surroundiny
orifice 19 assists to maintain the water column while
conveyor belt 1 is in motion. ''
The ultrasonic receiver and transmitter are similar in
design and differ in respect of the electric circuit 36 shown
schematically. This circuit is of a type well known in the
art and comprises an impedance matching inductor in the
transmitter and in the receiver a similar inductor in
addition to a pre-amplifier. In the case oE the transmitter
electrical signals supplied through cable 37 are converted to
ultrasonic vibrations according to the piezo-electric effect
and in the receiver the vibrations are converted to electric
signals by the piezo-electric effect and amplified by the
pre-amplifier for subsequent analysis. A lug 38 is fixed to
case 11 to facilitate mounting.
Figure 3 shows a means of mounting the transducer
assembly 2 (referring collectively to the receiver and
transmitter) relative to a conveyor belt 1. Transducer
assembly 2 is fixed to a lever arm 40 by bolts through the
lug 38 in such a way that allows a small rotation about an
axis through bolt 41 so as to adjust and fix the angle of the
axis of the transducer to the surfaces of the belt. The
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lever arm 40 is fixed to a pipe clamp 42 by a lug ~3. Pipe
clamp 42 attaches over a roller 5 shown in Fig. 1 and in part
sectioned view in Fig. 4. Roller 5 is modiEied from a
typical idler roller and forms a pivot axis for transducer
assembly 2 such that the transducer assembly can follow the
random vertical movements of belt 1.
With reference to Fig. 4, roller 5 is provided with a
helical spring 51 mounted co-axially in the roller and
whereby a restoring torque acts on the roller with respect to
the fixed shaft 52 and in turn provides an upward force to
the transducer relative to the conveyor belts. Spring 5,1 is
secured at one end 54 to the roller by a collet 53 fastened
by a screw (not shown) to the inside wall of roller 5 and is
free of the shaft 52.
The other end of spring 51 is secured to the shaft 52 by
a similar collet 55 which is free of the inside wall of
roller 5. Each end of the spring is fastened to the
respective collet by a grub screw 56 (not sectioned).
Attached to the same roller 5 is a wheel arrangement 6 shown
in Fig. 5.
The separation of the face of the transducer at the
or.ifice 17 from the surface of belt 1 is controlled by wheel
arrangement 6. Wheel 61 is mounted on an axle 62 and fork 63
which is in turn mounted on a lever arm 64 at pivot axle 65.
This arrangement allows for the rotation of the wheel about
the axis of axle 65 so as to effectively adjust the
separation of the axle 65 from belt 1. Adjustment is made by
screw 66 which is screwed into lug 67 attached rigidly to the
~3~
lever arm 6~ and acts on t.he lug 6~ whicii is rigidly attached
to the York ~3. 'rhe assembly is clamped adjacent to the
transducec on the roller 5 by a clamp 69 similar to clamp
~2. Adjustment ox screw 66 results in rotation of the roller
5 and a change in the separation of the transducer from tne
belt. Contact ox the wheel and the belt is maintained by
roller spring 51.
With re~erènce to Fig. 1, rollers 5 are mounted in the
usual way on an idler frame 4. The transmitter transducer
assembly 2 and associated wheel 6 is mounted so as to
transmit ultrasound into one edge of the belt and the
receiver transducer assembly 2 with associated wheel 6 is
mounted on the opposite roller 5 to detect ultLasound at the
other edge of the belt.
Electrical energy is supplied to the transmitter at a
frequency for preference equal to the fundamental resonant
frequency of that transmitter in a series of short pulses.
Also for preference the frequency is such that the wavelength
of ultrasound in the conveyor belt is equal to one thickness
of belt.
When the ultrasound is of a particular frequency such
that the wavelength in beLting is equal or similar in
dimension to the belting thickness dimension, the ultrasound
attenuation is of the order of only 0.5-0.65 dB per cm of
path length, whereas at neighbouring higher and lower
frequencies the attenuation is significantly greater.
When reference to Fig. 6 there is shown a plot of
ultrasound attenuation in decibels per centimeter of path
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length against requency in belting of thicknesses. In a
steel cord reinforced belting of 25 mm thickness and at
frequency of 43.6 XHz the wavelength is about 25 mm and the
attenuation is at or near a minimum. By selecting a
frequency of 43.5 XHz ultrasound can be propogated and
detected over a path length exceeding one meter in the 25 mm
thick belt. At frequencies in excess of 100 kHz ultrasound
is Darely detectable over a path length in the belting of 20
cm and at lower frequencies the attenuation i5 also greater.
Likewise for a belt of similar construction but of thickness
20 mm, at a frequency of 47.6 kHz the ultrasound wavelength
is about 21 my and attenuation is at or near a minimum.
As is apparent from Fig. 6 there is another attenuation
minimum for~the 25 mm thicXness belting at around 39 ~Hz.
As will be apparent to those sifted in the art the
frequency is desirably selected having regard both to the
level of attenuation and the sensitivity of attenuation to
slight change in frequency.
Signals from the receiver are amplified, rectiied and
sampled at a certain time following each pulse of electrical
energy supplied to the transmitter. This delay takes into
account the time that the resultant pulse of ultrasound
travels from the transmitter to the receiver. An alarm
circuit monitors the level of ultrasound and issues an alarm
if this level falls below an adjustable threshold for an
adjustable period of time. This alarm is also inhibited if
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certain functions of the apparatus are not operational. Such
functions include power supplies and coupling water supply.
Thus the apparatus can be adjusted to be responsive to rips
and other faults that are of a predetermined severity and/or
oE a certain minimum length. The conveyor belt drive motor
is stopped if all alarm conditions apply.
The apparatus described is normally mounted at a
location on the conveyor belt so that it monitors the belt
immediately after passing through areas where rips are likely
to occur. These locations are usually immediately after the
belt is loaded. One problem occurs when the conveyor belt
contains relatively long sections that, because of their poor
physical condition, do not satisfactorily transmit
ultrasound. The belt rip detector can be made insensitive to
these sections by increasing the response time (and hence rip
length) of the alarm circuit. This, however, also increases
the minimum length of rip capable of being detected. To
overcome this problem a second rip detector is mounted to
monitor the belt immediately before passing through the area
where a rip is most likely to occur. The conveyor belt drive
motor is stopped only if the condition of the belt has
deteriorated between the two rip detectors.
Another application of the apparatus relates to its use
for detecting long term degradation of the belt and
identifying sections that require preventative maintenance.
This is done by compiling a log of the level of received
ultrasound throughout the entire length of the belt. In
addition, as an alternative condition for raising an alarm
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the 109 is compared in real time with the actual received
level of ultrasound and an alarm is raised when the actual
level differs from the log.
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