METHOD AND APPARATUS OF BELT TESTING

Abstract: A method of, and apparatus for, testing conveyor belts reinforced with dielectric material. At least one pair of plates (2, 3; 12, 13) are arranged to one side of the belt (6), substantially parallel to the belt and equally spaced from it. A time varying voltage is appplied to the capacitor (30) formed by the plates and the capacitor voltage is rectified and passed through a low pass filter (25). The filtered voltage represents the mass of the belt. If two sets of plates are located one to either side of the belt and the plates are electrically connected in parallel, the filtered voltage represents the belt thickness.

Description: METHOD AND APPARATUS OF BELT TESTING The present invention relates to the testing of conveyor belts and, in particular, to the testing of conveyor belts which are not reinforced with magnetically permeable members such as steel cords. Such belts are formed from elastomers such as natural and synthetic rubber and other materials including PVC and other suitable polymers. Some of these belts are reinforced with one or more layers of fabric woven from dielectric materials such as nylon, kevlar and the like. The fabric reinforcing is generally sandwiched between layers of the belt material. Alternatively, the reinforcing is woven from dielectric material and then impregnated with the belt cover material. This structure is generally used when the belt cover material is PVC or similar. As used herein the term "conveyor belts reinforced with dielectric material" is used to describe such belts having a reinforcing which is not magnetically permeable but is a dielectric material". BACKGROUND ART It is known from U.S. Patent No. 4,439,731 (to which Australian Patent No. 535,356) corresponds, to magnetically test conveyor belts reinforced with steel cords. Such conveyor belts comprise from 10 to 20% of all conveyor belts in use. Approximately 20 to 30% of all belts are formed from woven plastics materials and covered with a thin layer of PVC. The remaining belts, comprising approximately 50% of all belts, are formed from the above described ply fabric belts and are generally formed from rubber reinforced with layers of a woven fabric. Thus the present invention relates to the 80 to 90% of all conveyor belts which are reinforced with dielectric material. Such belts reinforced with dielectric material are used in many applications, particularly in coal mining and have a typical life of approximately five years. However, there is a need to monitor such belts during their life in order to detect any rotting of the reinforcement due to water impregnation, for e ample, arising out of a break in the covering material, the depth, of the covering material, the strength of any splices, and the like. There is also a need to detect and locate manufacturing defects such as folded ply layers. It is the object of the present invention to devise a method of, an apparatus for testing conveyor belts reinforced with dielectric material in a non-invasive manner. SUMMARY OF THE INVENTION According to one aspect of the present invention there is disclosed a method of testing a conveyor belt reinforced with dielectric material, said method comprising the steps of passing said belt between a first pair of substantially co-planar plates which extend on the same side of said belt transversely relative to the longitudinal axis of said belt, maintaining the spacing between said plates and said belt substantially uniform within predetermined limits, electrically interconnecting said plates to form a capacitor, applying an alternating voltage via said capacitor to a series connected half wave rectifier and low pass filter, and sensing the output of said filter. According to another aspect of the present invention there is disclosed apparatus for testing conveyor belts reinforced with dielectric material, said apparatus comprising a first pair of substantially co-planar plates located to one side of said belt with the spacing between said plates and said belt being substantially uniform within predetermined limits, said plates being electrically interconnected to form a capacitor, and a source of alternating voltage connected via said capacitor to a series connected half wave rectifier and low pass filter. If desired, in both the above described method and apparatus, the pair of plates can be duplicated on the opposite side of the belt with opposed plates aligned and electrically connected together. BRIEF DESCRIPTION OF THE DRAWINGS - An embodiment of the present invention will now be described with reference to the drawings: Fig. 1 is a schematic perspective view of the apparatus of the preferred embodiment of the present invention; Fig. 2 is a longitudinal cross-sectional view taken along the line II-II of Fig. 1; Fig. 3 is a schematic circuit diagram illustrating the circuit connections required of the apparatus of Figs. 1 and 2; Fig. 4 is a graph for one pair of the co-planar plates of Fig. 2 of the output voltage for the circuit of Fig. 3 as function of the mass of the conveyor belt for different conveyor belt to plate spacings; Fig. 5 is a graph for all the plates illustrated in Fig. 2 of the output voltage of the circuit of Fig. 3 as function of belt to plate distance as the belt is moved from an upper to a lower plate; Fig. 6 is a diagram illustrating two output waveforms for the circuit of Fig. 3 for a given length of fabric reinforced conveyor belt, and Fig. 7 is a diagram similar to that of Fig. 6 but for a solid woven fabric reinforced belt. DESCRIPTION OF THE PREFERRED EMBODIMENT Turning now to Figs. 1 and 2, the apparatus 1 of the preferred embodiment is illustrated. The apparatus 1 takes the form of two substantially co-planar, substantially parallel generally rectangular plates 2 and 3 each of which is provided with upturned edges 4. The edges 4 provide rigidity and act to deflect any proud material from the belt surface. The plates 2, 3 are supported above one (upper) surface 5 of a conveyor belt 6. The conveyor belt 6 has fabric or woven reinforcing made from dielectric material as schematically indicated at 7. Each of the plates 2, 3 is supported by a pair of electrically insulating spacers 8. The spacers 8 interconnect the plates 2, 3 with an inverted.dish shaped metal guard 9 (illustrated in phantom in Fig. 1) . As best seen in Fig. 2, the spacers 8 and guard 9 are arranged so that the plates 2, 3 are suspended above the belt upper surface by a substantially uniform distance h. As will be described hereafter in order to take some types of measurement, the apparatus 1 located above the upper surface 5 of the belt 6 is duplicated by a substantially identical apparatus 10 formed from a pair of plates 12, 13 having upturned edges 14. The plates 12, 13 are supported by spacers 18 mounted on a guard 19 again in substantially identical fashion to that of the apparatus 1. Again, the spacing between the plates 12, 13 and the lower surface 15 of the belt 6 is maintained substantially uniform and is preferably equal to the spacing h. As illustrated in Fig. 2, the plates 2 and 3 are connected to form a capacitor in which the plates are co-planar. In addition, the plate 12 can be connected to the plate 2 " and the plate 13 can be connected to the plate 3 via switch 17 so that the capacitor formed by plates 12 and 13 is effectively connected in parallel with the capacitor formed by plates 2 and 3. As also indicated in Fig. 2 the earth shields or guards 9 and 19 are earthed to provide immunity to external stray electric fields. Fig. 3 illustrates the electrical circuit 20 into which the capacitor 30 formed by plates 2,3, 12 and 13 is connected. One terminal of the capacitor 30 is connected to an oscillator 21 which is preferably provided with a square wave output having a peak to peak voltage of approximately 10V, a pulse repetititon rate of approximately 2MHz, and a 50% mark-space ratio. Frequencies in the range of from approximately 10kHz to approximately 10MHz can be used. Sine waves can be used but square waves are preferred. The other side of the capacitor 30 is connected to a diode 22 which forms a half wave rectifier and a resistor 23 and capacitor 24 which form a low pass filter 25. The output voltage V of the low pass filter 25 is recorded by means of a chart recorder 26, for example. For the apparatus of Figs. 1 and 2, with only plates 2 and 3 connected (and with plates 12 and 13 electrically disconnected by means of switches 17) the voltage V at the output of the low pass filter 25 as a function of the mass m of the conveyor belt 5 is illustrated in Fig. 4. This voltage V is directly proportional to the width of the belt 5, this dimension being directly proportional to the belt mass for a belt of substantially uniform thickness and density. However, the slope of the voltage to mass linear relationship is itself dependent upon the spacing h between the plates 2, 3 and the upper surface 5 of the belt 6. Turning now to Fig. 5 illustrated by means of a dot and dash line is the magnitude of the voltage V at the output of the low pass filter 25 for only the plates 2 and 3 as a function of the distance x of the plates 2 and 3 from the upper surface 5 of the belt 6. Similarly, illustrated by means of a two dot and dash line in Fig. 5 is the equivalent output for the plates 12 and 13 only as a function of the distance between the lower surface 15 of the belt 6 and the plates 12, 13. It will therefore be apparent, that if both sets of plates 2, 3 and 12, 13 are interconnected in parallel by switch 17 as illustrated in Fig. 2, a resultant curve for the voltage V at the output of the low pass filter 25 is obtained. This curve is illustrated by a solid line in Fig. 5 for maximum plate to belt separations of 2h. Since this curve passes through a shallow minimum, it will be apparent that at the nominal belt surface to plate spacing indicated at the minimum, the output of the low pass filter 25 is relatively insensitive to small variations in the belt-plate spacing when operating near x = 0 in Fig. 5. That is, the output of the low pass filter 25 is relatively insensitive to any flutter or vibration in the belt 6 as it moves between the plates 2, 12 and 3, 13 in the direction of the arrow as indicated in Fig. 1. This insensitivity for variations in belt to plate spacing is brought about by the nulling effect of the transducer geometry. Thus small variations in h can be tolerated in the range dh illustrated in Fig. 5. In practice dh is about one third of 2 h. Thus, in circumstances where the belt 6 cannot be prevented from transversely vibrating between the plates 2, 12 (3,13) it is possible to improve the nulling effect by increasing the spacing between the plates 2, 12 (3,13) and thereby increase the size of both h and dh. It will be apparent to those skilled in the conveyor belt art, that the apparatus 1, 10 of Figs. 1 and 2 can preferably be mounted in situ on the return run of a conveyor belt, and preferably between a pair of stabilising rollers 16 (illustrated in Fig. 2) . With all the plates 2,3 and 12, 13 connected as illustrated in Fig. 2 " with the switch 17 closed, and for the belt 6 as illustrated in Fig. 2, the upper of the two traces of output voltage V as a function time (and hence distance along the belt 6) illustrated in Figs. 6 and 7 is obtained. If only plates 2 and 3 are connected (plates 12 and 13 being disconnected via switch 17) then the output trace V as a function of time (and hence distance along the belt 6) is the lower trace illustrated in Figs. 6 and 7. The upper trace is termed a belt mass signature and the lower trace is termed a belt thickness signature. As seen in Fig. 6, a length of conveyor belt 6 extending between two splices 31 is illustrated in plan. In sequence the particular length of belt has a number of defects as follows:- 32 - folded ply layers 33 - edge cut 34 - holes or slits in or through the belts 35 - a section of rotted fabric 36 - edge loss of long duration 37 - edge loss of short duration, and 38 - change in belt thickness near the end of a ro11. For the upper of the two traces in Fig. 6 the mass of the belt 6 is effectively graphed and the nulling effect discussed in relation to Fig. 5 applies since the switch 17 is closed. Each of the above described defects 32-38 and the splices 31 produce a corresponding characteristic variation in the upper trace. The trace can be used as a signature at a first date and compared with a like signature trace of the same belt taken at a later date. Then these two signature traces can be used to located the position of, and to estimate the extent of, any degradation of the belt 6 and its splices 31. This degradation may be caused by water ingress and resultant rotting, for example. Similarly, two such traces can detect any exacerbation of any existing defect 32-38 or the occurrence of any new defect. The lower trace illustrated in Fig. 6 effectively illustrates the thickness of the rubber covering on the upper surface 5 of the belt 6 relative to the rollers 16. The lower trace of Fig. 6 includes the information of the upper trace but is not nulled and so shows thickness data as well. The irregular trace at the location adjacent the section of rotted fabric 35 indicates that there has been some wear of the rubber cover on the surface 5 at this location. Turning now to Fig. 7, the equivalent traces are shown for a length of solid woven fabric reinforced belt 46 which is again illustrated in plan. The belt 46 has a vulcanised splice 41, a region 42 of varying thickness, a slit 43, an edge loss section 44 and a metal clip joint 45. Each of these defects produces a corresponding variation in the belt traces similar to those found for ply reinforced belts. If one set of plates 2, 3 is positioned _at one location above the belt, and another set of plates 12, 13 is positioned at an adjacent but spaced apart location below the belt, the separate signals can be recorded simultaneously and used to determine whether a reduction in belt thickness at a given belt location is due to wear on the upper surface, the lower surface or both. INDUSTRIAL APPLICABILITY The foregoing describes only one embodiment of the present invention and modifications, obvious to those skilled in the art, can be made thereto without departing from the scope of the present invention. For example, the plates 2, 3, 12 and 13 and guards 9 and 19 are preferably made from galvanised steel so as to be inherently safe in a mining environment. Furthermore, where it is necessary to specify the mass or cover wear defects not only along the belt, but also across the belt, then it is possible to divide the plates 2, 3, 12 and 13 in the longitudinal direction of the belt as illustrated by broken lines in Fig. ' 1. Thus, .effectively, .three longitudinally aligned plates are used thereby forming three separate capacitors and providing three separate traces respectively indicative of one edge, the centre, and the other edge of the belt. Clearly numbers other than three are possible. It will be apparent that some of the above described defects such as folded ply layers 32, and changes in belt thickness 38, 42 are manufacturing defects. Thus the present invention can be used to check for manufacturing defects before a belt leaves the factory. In addition, some belts are formulated with carbon black and/or a filler in the covering material in order that the belt be static electricity conducting. For such belts it is difficult to obtain a belt mass signature. For these belts the thickness signature (and the fact that it includes edge effects data) is important. As the conductive belt surface rises and falls because of thickness variations, so the effective air gap of the capacitor changes giving rise to a thickness measurement. The arrangement of the capacitor plates 2, 3 , 12 and 13 can also be varied since a T-shaped configuration can be used for plates 2 and 3 with one plate forming the stem of the T and the other the cross piece. Plates with an aperture and a co-planar plate located within the aperture are also possible.

Claims: CLAIMS 1. A method of testing a conveyor belt reinforced with dielectric material, said method comprising the steps of passing said belt between a first pair of substantially co-planar plates which extend on the same side of said belt transversely relative to the longitudinal axis of said belt, maintaining the spacing between said plates and said belt substantially uniform within predetermined limits, electrically interconnecting said plates to form a capacitor, applying an alternating voltage v a said capacitor to a series connected half wave rectifier and! low pass filter, and sensing the output of said filter. 2-. A method as claimed in claim 1 including the step of electrostatically shielding the pair of plates. 3. A method as claimed in claim 1 including the step of locating a second pair of said co-planar plates on that side of said belt opposite to said first pair, and electrically connecting together via switch means each plate of said first pair of plates with the corresponding plate of the second pair of plates.- 4. A method as claimed in claim 3 including the step of electrostatically shielding each said pair of plates. 5. A method as claimed in claim 1 wherein said belt passes over two idler rollers, one located upstream of, and the other located downstream of, said plates with respect to the direction of relative belt movement. 6. A method as claimed in claim 1 wherein said plates are substantially rectangular, have upturned leading and trailing edges with respect to the direction of belt movement, and are substantially the same size. 7. A method as claimed in claim 1 wherein the frequency or pulse repetition rate of said alternating voltage is in the range of from 10 kHz to 10 MHz . 8. Apparatus for testing a conveyor belt reinforced with dielectric material, said apparatus comprising a first pair of substantially co-planar plates located to one side of said belt with the spacing between said plates and said belt being substantially uniform within predetermined limits, said plates being electrically interconnected to form a capacitor, and a source of alternating voltage connected via said capacitor to a series connected half wave rectifier and low pass filter. 9. Apparatus as claimed in claim 8 wherein the pair of plates is electrostatically shielded. 10. Apparatus as claimed in claim 8 including a second pair of said co-planar plates located on that side of said belt opposite to said first pair, each plate of each pair of plates being electrically connected to the corresponding plate of the other pair of plates via switch means . 11. Apparatus as claimed in claim 10 wherein each said pair of plates is electrostatically shielded. 12. Apparatus as claimed in claim 8 wherein said plates are located between a pair of spaced apart idler rollers over which said belt moves. 13. Apparatus as claimed in claim 8 wherein said plates are substantially rectangular, are substantially the same size and have upturned leading and trailing edges with respect to the direction of belt movement. 14. Apparatus as claimed in claim 8 wherein said source of alternating voltage has a frequency or pulse repetition rate in the range of from 2kHz to 10MHz.

Inventors: HARRISON ALEXANDER, HARRISON, ALEXANDER
Applicants: COMMW SCIENT IND RES ORG, COMMONWEALTH SCIENTIFIC AND INDUSTRIAL RESEARCH OR
Application year: 1987
Main classification: B65G43
IPC codes: B65G15, B65G43, G01N27