Abstract:
A system for monitoring a conveyor belt having magnetically permeable cords, has an AC magnetic field generator for generating an alternating magnetic field to magnetize the cords, in use; a magnetic field sensing unit for sensing the alternating magnetic field provided, in use, by the cords and for providing signals representative of the alternating magnetic field; arid a processor for processing the signals to monitor continuous parts of the cords. The system further has a DC magnetic field generator for erasing an AC field previously generated by the AC magnetic field generator, the AC magnetic field generator being positioned between the DC magnetic field generator and the magnetic field sensing unit. The processor also determines the speed of travel of the belt, and the position in space of a lateral edge of the belt.

Description:
TECHNICAL FIELD 
     This invention relates to monitoring of conveyor belts. More particularly it relates to a system for and a method of monitoring conveyor belts having magnetically permeable cords. It extends to a conveyor belt arrangement which has the system. 
     BACKGROUND 
     It is well known that the complete failure of steel cord-reinforced conveyor belts as used on conveyor belt structures in various mining and industrial applications can have catastrophic results. As such, condition monitoring of these conveyor belts has become common practice, the objective being to identify damage to conveyor belts and thus to effectively maintain conveyor belts. 
     A typical steel cord-reinforced conveyor belt as herein envisaged is made up of elongated conveyor belt sections, typically in the order of 300 m in length. 
     Each section comprises a central layer of multi-stranded steel cords sandwiched in a substantially equally-spaced, parallel configuration between two rubber layers, the sections being connected by means of splices. A splice between two sections is formed by overlapping the ends of the two sections by one to five meters and vulcanizing the sections together. When the sections are so connected, the cords of the sections in the overlapping region are arranged in a pattern in which alternating cords of the sections lie in a parallel adjacent relationship. 
     It is known to monitor conveyor belts for cord breaks by magnetizing the cords using a permanent magnet array and then detecting fringing magnetic fields resulting from breaks. The condition of splices may also be monitored in this manner. 
     However, this technique has a serious deficiency. There is only a detectable magnetic field just above the cord break or just above the cord end. Thus the magnetic image above intact cords is blank—i.e. they are magnetically invisible. 
     If for example for a new belt, here are many unbroken long cords, then after magnetizing, the magnetic image will be blank except for a single dip at one end (the south pole) of these cords and a peak at the other end (north pole). There is in fact a very small dipole magnetic field that exists between these very separated poles. However, since the cords of a new belt segments are typically 2-30 Om apart, the resulting magnetic field is very small and difficult to detect. 
     With a new belt it is desirable to know the number of cords thereof and their spacing, which is not possible to do with such existing technology. Also, in some applications cords are placed across the belt at 45° in order to detect rips, since if a longitudinal rip occurs, this will cut these diagonal cords and produce additional north/south pole pairs where none were previously present. However again, intact transverse cords are invisible with present magnetic field detection technology. It is again desirable to be able to detect the presence of the diagonal cords and hence confirm that the rip detection functionality was intact. 
     It is thus an object of this invention to provide a method of and an apparatus for the above purpose and in respect of which the above inadequacies are at least ameliorated. 
     SUMMARY 
     According to the invention there is provided a system for monitoring a conveyor belt having magnetically permeable cords, which includes an AC magnetic field generator for generating an alternating magnetic field to magnetize the cords, in use;
         a magnetic field sensing unit for sensing the alternating magnetic fields provided, in use, by the cords and for providing signals representative of the alternating magnetic fields; and   a processor for processing the signals to monitor continuous parts of the cords Further according to the invention there is provided a method of monitoring a conveyor belt having magnetically permeable cords, which includes   generating an alternating magnetic field to magnetize the cords in an alternating manner;   sensing the magnetic fields provided by the cords and providing signals representative of their magnetic fields; and   processing the signals to monitor continuous parts of the cord.       

     Still further according to the invention, there is provided a conveyor belt arrangement, which includes
         a belt having a plurality of magnetically permeable cords; and   a system for monitoring the belt as described above, the AC magnetic field generator and the magnetic field sensing unit thereof being positioned adjacent the belt and longitudinally spaced from one another.       

     The sensing unit may comprise an array of spaced magnetic field sensors, the signals from the sensors being processed. The sensors may have a sensing axis, such that the magnetic field strength in that direction is sensed. The magnetic field sensing unit may then have sensors suitably oriented to sense two, or all three, of the components of the magnetic field at spaced positions across the belt. The spacing thereof may be sufficiently small to provide the desired resolution. 
     The system may have a DC magnetic field generator for supplying a DC magnetic field to erase the alternating field previously supplied by the AC magnetic field generator. Thus, the AC magnetic field generator may be positioned between the DC magnetic field generator and the field sensing unit. The presence, spacing and position of intact cords may be determined by means of the invention. The degree of overlap in splices may also be determined. In addition the transverse position of an edge of the belt may be monitored and the speed of travel of the belt may be measured. To enable the system to monitor the position of the edge of the belt, the sensing unit may be wider than the belt and may extend beyond the ends thereof. 
     The processor may generate images representing the continuous parts of cords of the belt and the system may include a display for displaying the images. 
     The system may include data acquisition equipment for processing signals received from the sensors and for supplying data to the processor. The data acquisition equipment may have multiple channels or may be of the multiplexed type. If multiplexing is utilised then either analogue or digital multiplexing may be utilized. 
     It will be appreciated by those skilled in the art that the system may also include a belt speed determining means for determining the speed of travel of the belt in a longitudinal direction, and hence the longitudinal position of the belt at each sampling point. The belt speed determining means may include an encoder connected to a pulley of the conveyor belt arrangement. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described by way of non-limiting examples, with reference to the accompanying schematic drawings, in which 
         FIG. 1  shows schematically a conveyor belt arrangement in accordance with the invention; 
         FIG. 2  shows schematically an array of sensors which is part of the system of  FIG. 1 ; 
         FIG. 3  shows schematically an embodiment of data acquisition equipment which is part of the system of  FIG. 1 ; 
         FIG. 4  shows schematically how the vertical position of a cord in the matrix of the conveyor belt is determined graphically 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , a conveyor belt arrangement in accordance with the invention is designated generally by reference numeral  10 . The conveyor belt arrangement  10  has a conveyor belt  12 , a DC magnetic field generator  14 , an AC magnetic field generator  15 , a magnetic field sensing unit  16 , an encoder  18  for determining the speed of travel of the conveyor belt  12  and data acquisition equipment  20  for processing signals received from the magnetic field sensing unit  16  and for supplying data to a processor  60 . The processor  60  supplies images to a display unit  61  to be displayed thereby. The direction of travel of the conveyor belt  12  is indicated by arrow  22 . The DC magnetic field generator  14  and the magnetic field sensing unit  16  extend across the conveyor belt  12  and are mounted about 4 cm to 5 cm above the conveyor belt  12 . The encoder  18  is connected to a pulley of the conveyor belt arrangement  10 , to monitor the speed of travel of the conveyor belt  12 . The DC magnetic field generator  14 , the AC magnetic field generator  15  and the magnetic field sensing unit  16  all extend beyond the sides of the conveyor belt  12  about 50 cms which length exceeds the distance by which the belt tracks sideways (in the transverse direction) during a full belt revolution. The AC magnetic field generator  15  is excited at a frequency of between 4 Hz and 50 Hz. 
     It will be appreciated that the direction of travel  22  defines a longitudinal direction, with a transverse direction being defined across the conveyor belt  12  and a perpendicular direction being defined perpendicular to the conveyor belt  12 . 
     The conveyor belt  12  transports bulk material such as coal, iron ore and the like. It is constructed of a rubber matrix in which is imbedded a number of cords  24  that are comprised of braided strands of steel wire that run along the length of the conveyor belt  12 . 
     These cores are thus magnetically permeable. Typical belts have cord spacing of 10 mm to 25 mm. Clearly, the number of cords  24  in the conveyor belt  12  will depend on the spacing of the cords  24  and the width of the conveyor belt  12 . Although only four cords  24  are shown in  FIG. 1  it will be appreciated that in practice a larger number of cords  24  will typically be used. 
     The AC magnetic field generator  15  is an array of electromagnets or solenoids, that is placed across the belt as shown in  FIG. 1 . The axis of each electromagnet or solenoid is oriented perpendicular or parallel to the conveyor belt  12 . The DC magnetic field generator  14  is placed upstream of the AC magnetic field generator  15 . The DC magnetic field generator  14  is still required to erase the previous cycle&#39;s AC field. The magnetic field sensing unit  16  is placed downstream of the AC magnetic field generator  15 . 
     In use, as is known in the art, an alternating magnetic field is generated by the DC magnetic field generator  14  which magnetises the cords  24  with an alternating magnetic field, along their entire lengths. These magnetic fields are sensed by sensors of the magnetic field sensing unit  16 . Signals provided by the magnetic field sensing unit  16  are processed by the data acquisition equipment  20  which provides data to the processor  60 . The processor processes the data, as is explained further below. 
     Referring now to  FIG. 2 , an embodiment of the magnetic field sensing unit  16  is shown. This magnetic field sensing unit  16  has an elongated carrier  26  on which are mounted a number of groups  28  of sensors  30 . 1 ,  30 . 2  and  30 . 3 . The sensors  30 . 1 ,  30 . 2  and  30 . 3  each have a sensing axis. Each group  28  has a perpendicular sensor  30 . 1 , a longitudinal sensor  30 . 2  and a transverse sensor  30 . 3 . The perpendicular sensor  30 . 1  of each group has its associated longitudinal sensor  30 . 2  on its left side and its associated transverse sensor  30 . 3  on its right side. The groups  28  are spaced apart a distance indicated by arrows  32 . The perpendicular sensors  30 . 1  have a sensing axis  34 , the longitudinal sensors  30 . 2  have a sensing axis  36 , and the transverse sensors  30 . 3  have a sensing axis  38 . It will thus be appreciated that when the carrier is placed in position across and above the conveyor belt  12 , the perpendicular sensors  30 . 1  will point down, the longitudinal sensors  30 . 2  will point in the direction of travel  22  and the transverse sensors  30 . 2  will point across the conveyor belt  12 . Thus, the perpendicular sensors  30 . 1  will measure the vertical component of the magnetic field, the longitudinal sensors  30 . 2  will measure the longitudinal component of the magnetic field and the transverse sensors  30 . 3  will measure the transverse component of the magnetic field at each position across the conveyor belt  12  as the belt travels below it. As indicated above the signals from the longitudinal sensors  30 . 2  and the transverse sensors  30 . 3  are interpolated to provide representative signals at the centre of their associated perpendicular sensors  30 . 1 . The carrier  26  and the sensors  30 . 1 ,  30 . 2  and  30 . 3  provides a single array  40 . 
     The groups  28  are spaced about 10 mm apart. 
     The sensors  30 . 1 ,  30 . 2  and  30 . 3  are Hall effect sensors and are supplied by Allegro Microsystems, with part number A1302KLHLt-T. 
     It will be appreciated that in many applications it will be sufficient to determine only the perpendicular magnetic component and the conveyor belt arrangement  10  could use only perpendicular sensors  30 . 1 , in which event they may be spaced about 4 mm. 
     Referring now to  FIG. 3  an embodiment of data acquisition equipment  20  for processing the signals received from sensors  30  is shown. The equipment  20  has a buffer  46  for each sensor  30  which supplies an analog multiplexer  48  with buffered signals. The output of the multiplexer  48  is supplied to an A/D converter  50 , the digital output of which is supplied to a field programmable gate array (FPGA)  52  via a data bus  54 . The FPGA  52  is connected to the multiplexer  48  and the converter  50  by command links  56  and  58 . The FPGA  52 , in turn, supplies data signals to a processor  60 . 
     The analog voltages from each of the sensors  30 . 1 ,  30 . 2  and  30 . 3  are first amplified and filtered by the buffers  46 . The filtered analog values are fed into the n-channel multiplexer  48 . n is the number of sensors  30 . 1 ,  30 . 2  and  30 . 3 . The command outputs from the FPGA  52  determines which analog input value is switched through to the output of the multiplexer  48 . Typically devices with only a maximum of 16 channel multiplexers are available. However, the number of channel inputs can be increased by connecting a number of slave multiplexers to one master multiplexer. For example, the outputs of sixteen 16-channel slave multiplexers can be connected to the inputs of a single master multiplexer. This particular configuration will result in the equivalent of a single 16×16=256 channel multiplexer. The analog voltage outputs from, typically 256, channels are converted to their digital values by the single A/D converter  50 . The A/D converter  50  must be capable of sampling at a rate equal to n.f samp  where n is the number of analog channels and f samp  is the sampling frequency of each channel. The FPGA  52  directs the required convert signal to the A/D converter  50 , and controls the A/D converter thus determining the sampling rate. The digital outputs from the A/D converter  50  are received by the FPGA  52  and sent to the processor  60  via a suitable (e.g. ISA) bus. 
     With reference to  FIG. 4 , there is a graphical indication of the manner in which the position of a cord in the belt matrix may be determined. Using only the perpendicular components of the magnetic field from the sensors is adequate to identify the transverse and longitudinal position of a cord, but this does not give its vertical position in the belt matrix. In order to do this at least one other magnetic field component needs to be measured. For example if the transverse and perpendicular components are plotted in a plane perpendicular to the cords, then the vectors, when extrapolated in both directions in the region of the poles, will be directed to the cords, and in fact will intersect the center of the cords. An example of this result is shown in  FIG. 4  where the vector magnetic field, measured by each sensor at z=0, when extrapolated, meet at the center of the cords located at z=−5. This technique works since there is a one to one correspondence between the magnetized cords and the resulting fields. Care should be taken to only extrapolate those vectors that have a significant perpendicular component. For example, those lines which originate between the cords and marked  68  do not intersect at the cords. 
     The processor  60  determines from the data supplied to it, the transverse position of each cord  24 , at each longitudinal sample point, and plots these to provide an image of the cords  24  of the conveyor belt  12  showing where they are positioned relative to the sides of the conveyor belt  12 . This image is then displayed on the display unit  61 . Similarly, vertical position of each cord  24  in the belt matrix is determined along the length of the cord  24 , an image generated, and then displayed on the display unit  61 . 
     Since the steel cords  24  in the conveyor belt  12  are oriented parallel to the direction of movement, a common damage mode is when a stake or piece of metal pierces the cord and then rips the rubber along its length between two cords  24 . It is known to place one or more cords in a patch that is vulcanised to the top of the conveyor belt  12  with the additional (thinner) cord strands oriented at ˜45 degrees to the conveyor belt  12  to provide a rip detector. With the invention, the rip detector cords are magnetized and the AC modulation is able to confirm that the lines of diagonal rip cords are present and intact. 
     It will be appreciated that it is possible, with the invention, to identify the edge cords  24  of the conveyor belt  12  and their spatial positions. It is thus also possible, with the invention to determine the spatial positions of the edges of the conveyor belt  12 . An edge tracking plot of the conveyor belt  12  for one revolution is a useful technique in confirming the correct alignment of the splices and pulleys. If the pulleys and splices are not correctly aligned, then there will be excessive sideways (in the transverse direction) motion of the belt during a revolution. 
     Those skilled in the art will appreciate that it is possible to determine the speed of travel of the belt. This non contact belt speed measurement has advantages over the normal techniques that use proximity sensors attached to tachymeter wheels or directly to the belt pulleys. Thus, the processor  60  also determines the speed of travel of the conveyor belt  12 . 
     The processor  60  further determines the degree of overlap of splices of the conveyor belt  12 .