Rip detector system
This is a continuation-in-part of application Ser. No. 331,708, filed Dec. 17, 1981, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     In long endless conveyor belts of the type having a body formed from an electrically insulating material, used to convey bulk material, there is a possibility of encountering a rip in the belt. Rips may occur, for example by sharp objects being dropped upon the belt at a loading station. It is desirable to promptly detect particularly longitudinal rips and, preferably, to shut down and repair the conveyor belt upon such detection, thereby reducing the possibility for damage to the belt. 
     In prior structures as described, for example, in U.S. Pat. No. 3,792,459 a conveyor belt rip detection system is disclosed of a type having a plurality of spaced antennae imbedded transversely at spaced-apart locations in the belt to periodically couple an electrical signal from a transmitter probe to a receiver probe while the belt, with its respective antennae, moves past the probes. The detected electrical signals function to monitor the integrity of the belt and to shut down the conveying system in response to a break in an antenna which would be typically indicative of a longitudinal rip in the belt. Detection of the antenna break permits belt repair or replacement before the break or rip elongates and destroys an extended portion or even the entirety of the belt or before the conveyor belt assembly ceases to function properly as a consequence of the rip. A rip, noted timely, can be corrected with relatively minimal cost and inconvenience. 
     In prior devices it has been the general practice to utilize antennae made of metallic screens or wires such as steel wires, extending transversely across the width of the belt, at spaced apart distances. Such screen or wire antennae function properly during normal operations for a period of time. 
     Frequently, however, due to movement of the belt and wires around conveyor rollers in the conveyor system, continued flexing stresses upon the screen or wires of the antennae may cause premature breakage of the wires, thus causing a malfunction in the rip detection system Metallic wires are also susceptible to breakage caused by ore dropped on the belt at a loading station. A broken antenna, caused by a failure of any nature, undesirably will give the appearance to a conveyor belt control system that a rip exists in the belt. The control system could then improperly shut down the conveying system when in fact no such rip exists in the belt. Alternate decisional programming of the control system may also be employed. 
     The electrical signals, in the form of charges, are readily conveyed across the belt through the antennae as the antennae move past the probe sites. Such charges must traverse non-conductive zones such as an air space between the probes and the adjacent surface of the belt and then through any belt segments interposed between the probes and the angennae where the antennae are imbedded within the belt. The movement of the charges through these non-conductive zones tend to decrease the signal to noise ratio of the signal being carried from probe to probe. These electrically non-conductive zones decrease the system reliability, thus permitting mistaken signals, or signals of inconsistent intensity, to be received by the detector probe in implementing the rip detection function. 
     Priorly, as described in U.S. Pat. No. 4,296,855 issued to Gary L. Blalock, antennae have been constructed of a knit or woven, carbon infused non-metallic fiber such as nylon. Such antennae have performed satisfactorily having mechanical properties superior to metallic screen type antennae. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improved electrical conductor particularly useful in conjunction with antennae for utilization in conveyor belt rip detecting systems. 
     In accordance with the invention, inserts of electrically conductive rubber are placed and vulcanized into the normally insulating conveyor belt adjacent the edges thereof configured to correspond in positioning to the location of an antenna imbedded within the belt. Each insert occupies a length from the edge of the belt toward the center thereof substantially equal to the corresponding dimension of the probes. In the direction of belt movement, the width of each insert is preferably slightly greater than a corresponding dimension of a probe to accommodate slippage of the antenna relative to the direction of belt motion during fabrication. Also, the conductive rubber inserts preferably are sufficiently thick to extend from the surface of the belt adjacent the probe inwardly to a point in contact or substantially in contact with the antenna. In this manner, the insulating electrical path through which charge signals must move in the rip detection function is minimized yielding an increased signal to noise ratio of the system, increased system reliability, and permitting wider variation in the magnitude of any gaps between the probe and the belt which can be tolerated. 
     In accordance with a preferred embodiment of the instant invention, each antenna is constructed principally of natural and synthetic rubber, blended with a significant portion of carbon black having a substantial electrical conductivity and with the remaining minority of components being wear resisting agents, antioxidants, vulcanization promoters and other additives generally extending the life of the inserts and improving their mechanical, chemical and electrical properties. When vulcanized into a conveyor belt at spaced apart points along the belt length, such rubber inserts can function with imbedded antennae for rip detection purposes. 
     A rip detecting system provided with a belt having conductive rubber inserts integrally vulcanized into the belt and with similarly formed antenna enjoys a reduction in premature breakage of the antennae since the conductive rubber antennae and inserts can undergo elongation more effectively than prior antennae without significant loss of substantial electrically conductive properties. Employing inserts of the instant invention assures great impact resistance for belts in which they are employed. Being vulcanized into the belt, the inserts of this invention exhibit provide improved mechanical and electrical integrity since the inserts and antennae are part of the belt composite, effectively being bonded as one with the belt. 
     In order to gain a better understanding of the invention as well as other advantages and further features thereof, reference is made to the following detailed description of the invention to be read in conjunction with the accompanying drawings together forming part of the specification. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic side elevational view of a conveyor assembly and control system adapted to utilize inserts constructed in accordance with the instant invention; 
     FIG. 2 is a schematic sectional view of the assembly of FIG. 1 taken through the belt antenna, inserts, sensor probes, and control circuitry; 
     FIG. 3 is a perspective view of the belt of FIGS. 1 and 2 with layers removed to show internal constructions, including the inserts; and 
     FIG. 4 is a sectional view of an alternate belt assembly employing inserts of the instant invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the drawings, FIG. 1 depicts an endless conveyor belt assembly constructed to utilize an insert made in accordance with the instant invention. The conveyor belt, generally designated as 10, is trained around a plurality of rollers 12 and 14 powered by a drive means such as a motor 16, to move the belt in a continuous cycle of operation. The conveyor belt assembly includes an upper zone or track 18 for conveying ore or other material between a loading station and a discharge station. Opposite the upper zone is a belt return zone 20. The edges of the belt are preferably turned up slightly at least along the upper zone 18 to minimize spillage of conveyed ore therefrom. 
     The belt 10 is schematically shown as traveling over a short distance between two rollers, 12, 14. It should be understood that belts 10 of this type are readily adapted for conveying ore over great distances including many hundreds or thousands of yards. While only two rollers, 12 and 14, are shown for illustrative purposes, many roller configurations may be suitably utilized to permit conveying of materials great distances, up extended heights and around corners. 
     Utilized in conjunction with the conveyor belt 10 and drive source 16 is a control mechanism or assembly 22 including electrical sensors or probes 24 and 26 fixedly positioned adjacent the lower surface of the upper track 18 of the belt, however, other probe locations to transmit and receive signals may be utilized. The probes are located adjacent opposite edges of the belt 10 defining a line generally perpendicular to the direction of belt travel and are adapted to monitor its movement and feed electrical signals to the control system 22. The control system 22 in turn is adapted to act in a servo-loop configuration, through appropriate circuitry to control electrical current to the motor 16. In this manner, the motive force to the belt may be terminated in response to detection of a rip as indicated by a breakage of any antenna 28 fixedly mounted within the belt for movement therewith. 
     In the preferred mode, a plurality of antennae are located at fixed, spaced intervals along the belt. These antennae generally are spaced at fifty to one hundred feet apart, more or less, spacing generally being a function of the belt size, speed, degree of rip detection protection desired and the control system 22 being utilized. The antennae are utilized to detect the presence of broken or unbroken belt segments moving past the probes 24, 26. A more complete understanding of a typical probe and control system can be had by reference to the aforementioned patent to Snyder. 
     Referring to the drawings, FIG. 3 depicts a preferred belt construction including an insert 29 made in accordance with the instant invention. The preferred insert 29 is shown in a conveyor belt wherein it may be beneficially utilized. The belt 10, as shown particularly in FIG. 3, may include steel cables 30 which assist in preventing the belt from stretching in a direction of belt movement and which provide greater strength to the belt. Reinforcement in both the length and width of the belt is provided by fabric sheets or breakers 32 and 34 on opposite sides of the centrally located cables 30. Layers 36 of vulcanized elastomeric material such as rubber, natural or synthetic, or combinations thereof blended for belting purposes, are located on opposite sides of the cables 30 and sheets 32 and 34. The cables 30, sheets 32, 34 and layers 36 all are present in continuous loop configuration for carrying out a conveying function. 
     Referring to the drawings, an alternate preferred belt construction employing the antennae of the instant invention is shown in FIG. 4. The belt 50 has rubber upper and lower cover layers 52 and 54 on opposite faces. Rubberized fabric or carcass layers 56 are located between the cover layers with fabric sheets or breakers 58 being located at interfaces between the rubberized fabric layers and cover layers. The breakers are constructed of a fabric material and function to aid in bonding the carcass layers 56 to the cover layers 52, 54 as well as adding impact resistance to the belt. The antennae 28 are imbedded in the belt, being located within the cover layer 54. The inserts 29 join the antennae 28 and an external surface of the cover 54 in electrical communication. The antennae 28 and inserts 29 are preferably vulcanized into the conveyor belt. 
     The breakers 58, covers 52, 54 and fabric carcass 56 extend continuously along the length of the endless belt. The antennae 28 are spaced along the length of the belt and are configured to carry electrical charges in the form of signals from a transmitter probe 24 to a receiver probe 26 for rip detection purposes. It should be understood, however, that the antennae 10, 50 of the instant invention can function equally well in a belt assembly with or without cables 30. Breaker plies 58 are preferably utilized in a location between the antennae and steel cables, if steel cables are employed. 
     Located between the layers of rubber are segments or antenna 28 schematically shown in the drawings as screens or girds. Each insert 29, and in much preferred embodiments each antenna 28 is fabricated of rubber compound mixed using conventional rubber mixing equipment. The rubber preferably contains a majority of natural or synthetic rubber in appropriate blends to constitute a majority of the composition. This majority may constitute from about 50% to 70% by weight of each composite antenna. Preferably, the rubber component should be between 58% and 63% by weight in the preferred embodiment. Particularly it is preferred that the rubber be one of natural rubber, styrene butadiene rubber (SBR), cis-polybutadiene, neoprene, Hydrin®, rubber chlorinated polyethylene, nitrile rubbers, silicon rubbers, chlorosulfonated polybutadiene, and mixtures thereof. 
     The second most prevalent component of the composite material of each insert and preferably antenna 28 should be electrically conductive carbon black. A preferred carbon black is presently marketed by Akzo Chemie Nederland B.V., a Dutch company, and sold under the trade name of &#34;Ketjen EC&#34;. The properties of this material are discussed in the September-October 1977 issue of Rubber Chemistry and Technology published by the Rubber Division, American Chemical Society, Inc. It is not sufficient in the practice of the instant invention that the carbon black be electrically conducting as that term is generally understood; the carbon black must be substantially electrically conductive such as Ketjen EC. 
     Ketjen EC has a lower bulk density, i.e. a lower weight per unit volume, than typical reinforcing carbon blacks commercially available today. Ketjen EC has a specific gravity of about 2.05 plus or minus about 2% as compared to typical carbon blacks which have a specific gravity of from about 1.82 to about 1.86 plus or minus about 2%. 
     The carbon black employed in the practice of the invention constitutes from about 10% to about 30% by weight of the composite antenna, and preferably from 20% to about 30%. Increased carbon loading is possible, but produces little in the way of enhanced conductivity and risks decreased insert durability due to lower rubber content. The remaining chemicals and additives of the antenna material include well known antioxidant agents, life extenders, wear resistant materials, vulcanization promoters, processing aids and the like and constitute between about 0% and 40% and preferably about 12% to about 26% by weight, of the insert material. Antennae 28 can be similarly constituted. 
     The constitutent components of the insert material are preferably mixed in well-known manner and calendared to an appropriate, even thickness. The calendared material can then be cut to desired shape and size to fit the belt for carrying electrical signals to the antennae 28. During fabrication, the constitutent interior portions of the belt are laid up or plied together prior to vulcanization. The electrically conductive antennae 28 are spaced along the length of the belt in a sandwich between the bottom cover layer and the composite interior belt structure portions, the inserts 29 are positioned bridging the cover 54 to provide an electrical pathway and then the inserts are vulcanized in the usual, well known manner with the remainder of the belt. Where the antennae 28 are vulcanizable, vulcanization of the belt curingly joins the inserts 29 and antennae 28. 
     During the process of vulcanization of the belt laminate, the elastomeric belt rubber surrounds each antenna, and where the antennae 28 are rubber coadheres with the rubber of each antennae to adheringly position each insert 29 antenna 28 in the belt at a proper spacing. The vulcanized rubber provides an excellent bond to forestall slippage of the inserts and any rubber antennae within the belt during conveyor operation. This non-slipping condition is desirable since slippage towards either belt edge or along the linear path of belt travel during conveyor operation could cause a malfunction of the detection function. 
     Where rubber, each antenna 28 is preferably about eight (8) to ten (10) inches in width and extends substantially fully across the transverse dimension of the belt or at least a distance extending from a position opposing one probe to a position opposing the other probe and sufficient to protect portions of the belt where ripping is likely to occur. Each antenna is normally between 0.030 and 0.060 inches in thickness and preferably about 0.032 inches in thickness and constitutes about 10% or less of the total belt thickness. The probes are generally about four (4) to six (6) inches in length measured in a direction perpendicular to belt travel and of a width of about eight (8) to ten (10) inches, substantially equal to that of the antennae, measured in the direction of belt travel. Variations in these dimensions are acceptable. The inserts 29 typically are of somewhat greater dimension than the length and width of the probes 24, 26, and are of sufficient thickness to bridge between the outside of the cover 54 and a point at least adjecent the antennae 28. The extra dimension, especially in the direction of belt movement, permits the emitted charges to be concurrently picked up and detected by the emitting and detecting probes even in those instances where an antenna becomes slightly skewed during fabrication or operation of the belt. The size and shape of the inserts is such that they will fit closely into precut openings of the outer or lower rubber layer of the conveyor belt and extend from the surface of the conveyor belt adjacent the probes preferably into contact with the antenna thereby decreasing the insulated pathway the charges would have to travel in a typical conveyor having a rip detection system. 
     During the vulcanization process of the resulting belt laminate, the elastomeric rubber on opposite sides of each antennae and adjacent the inserts cohere with the material of each antenna (where rubber) and the inserts to hold each antenna and inserts in the belt at the proper spacings. A rubber antenna and insert may curingly co-adhere. The rubber also creates an excellent bond in wire or screen antennae by flowing through interstices of each antenna to mechanically prevent slippage of the antennae in the belt as during operation. The rubber insert similarly can infiltrate the interstices. This non-slipping condition is desired since antenna slippage either sideways or along the linear path during operation could cause a reduction in the effectiveness of the detection function by separating the insert from the antennae. 
     When employed with lower covers of about 1/4 inch in thickness or greater, rubber antennas become fully incorporated into the belt during vulcanization and the belt assumes an even thickness along its length. If, however, thinner lower covers are employed, as for example, covers of about 1/16 inch or less, rubber antennas will be fixedly positioned in the belt but, the belt will have a greater thickness at the antenna locations than at other locations. The rip detecting capabilities of a system employing such a belt will not, however, be lost. Further, the greater thickness of the belt at the locations of the antennas will not normally detract from the mechanical operation of such belts. 
     Belts resulting from the instant invention are possessed of an insert having an electrical resistance across the insert of not more than the equivalent of 6000 ohms and preferably less than the equivalent of 3000 ohms when proportionated to the resistance across a 5.5&#34;×0.5&#34; plug of vulcanized antenna material 0.075&#34; in thickness, resistance of the plug being determined along the 5.5&#34; dimension. Rubber antennae would be possessed of similar electrical resistance properties. 
     EXAMPLE I 
     Four unvulcanized rubber blends were made in accordance with Table I using conventional rubber mixing techniques. 
     
                       TABLE I                                                     
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(parts by weight)                                                         
Blend #          1       2       3     4                                  
______________________________________                                    
Natural Rubber   50.00   45.00   40.00 30.00                              
Cis-polybutadien 34.37   34.37   34.37 34.37                              
ASTM 101 HA Oil                                                           
Styrenebutadiene 34.37   34.37   34.37 34.37                              
ASTM 101 HA Oil                                                           
400.sup.1TM.     --      5.00    10.00 20.00                              
Gum Rosin        2.00    2.00    2.00  2.00                               
Paratertiary-octyl                                                        
                 2.00    2.00    2.00  2.00                               
phenol-formaldehyde                                                       
Stearic Acid     1.50    1.50    1.50  1.50                               
ZnO              3.00    3.00    3.00  3.00                               
Dimethyl-butyl-  4.00    4.00    4.00  4.00                               
phenyl-Phenylenediamine                                                   
                 4.00    4.00    4.00  4.00                               
Paraffin         1.00    1.00    1.00  1.00                               
Trimethyl-dihydro-                                                        
                 5.00    5.00    5.00  5.00                               
quinoline        137.24  137.24  137.24                                   
                                       137.24                             
______________________________________                                    
 .sup.1 Trademark of the B. F. Goodrich Company                           
 
    
     Each of the rubber blends was then combined with selected carbon blacks, sulfur and accelerators, formed into a 5.5&#34;×0.5&#34; plug 0.75&#34; in thickness under vulcanization conditions and tested for electrical resistance across the plug, the results being set forth in Tables II-V. All entries are parts by weight unless otherwise noted. 
     
                                           TABLE II                                
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Square # 11  12  13  14  15  16  17  18    19                             
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Blend #1 137.24                                                           
             137.24                                                       
                 137.24                                                   
                     137.24                                               
                         137.24                                           
                             137.24                                       
                                 137.24                                   
                                     137.24                               
                                           137.24                         
Furnace carbon                                                            
         10.00                                                            
             20.00                                                        
                 30.00                                                    
black (HAF HS)                                                            
Vulcan C low         10.00                                                
                         20.00                                            
                             30.00                                        
conductive carbon                                                         
black                                                                     
Ketjen EC                        10.00                                    
                                     20.00 30.00                          
carbon black                                                              
Sulfur   0.60                                                             
             0.60                                                         
                 0.60                                                     
                     0.60                                                 
                         0.60                                             
                             0.60                                         
                                 0.60                                     
                                     0.60  0.60                           
Sulfenamide                                                               
         2.25                                                             
             2.25                                                         
                 2.25                                                     
                     2.25                                                 
                         2.25                                             
                             2.25                                         
                                 2.25                                     
                                     2.25  2.25                           
accelerators                                                              
Electrical                                                                
         *   *   *   *   *   *   *   25000 300                            
resistance, ohms                                                          
__________________________________________________________________________
 * = greater than 20 megohms                                              
 
    
     
                                           TABLE III                               
__________________________________________________________________________
Square # 21  22  23  24  25  26  27  28   29                              
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Blend #2 137.24                                                           
             137.24                                                       
                 137.24                                                   
                     137.24                                               
                         137.24                                           
                             137.24                                       
                                 137.24                                   
                                     137.24                               
                                          137.24                          
Furnace carbon                                                            
         10.00                                                            
             20.00                                                        
                 30.00                                                    
black (HAF HS)                                                            
Vulcan C low         10.00                                                
                         20.00                                            
                             30.00                                        
conductive carbon                                                         
black                                                                     
Ketjen EC                        10.00                                    
                                     20.00                                
                                          30.00                           
carbon black                                                              
Sulfur   0.60                                                             
             0.60                                                         
                 0.60                                                     
                     0.60                                                 
                         0.60                                             
                             0.60                                         
                                 0.60                                     
                                     0.60 0.60                            
Sulfenamide                                                               
         2.25                                                             
             2.25                                                         
                 2.25                                                     
                     2.25                                                 
                         2.25                                             
                             2.25                                         
                                 2.25                                     
                                     2.25 2.25                            
accelerators                                                              
Electrical                                                                
         *   *   *   *   *   *   *   6000 360                             
resistance, ohms                                                          
__________________________________________________________________________
 * = greater than 20 megohms                                              
 
    
     
                                           TABLE IV                                
__________________________________________________________________________
Square # 31  32  33  34  35  36  37  38  39                               
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Blend #3 137.24                                                           
             137.24                                                       
                 137.24                                                   
                     137.24                                               
                         137.24                                           
                             137.24                                       
                                 137.24                                   
                                     137.24                               
                                         137.24                           
Furnace carbon                                                            
         10.00                                                            
             20.00                                                        
                 30.00                                                    
black (HAF HS)                                                            
Vulcan C low         10.00                                                
                         20.00                                            
                             30.00                                        
conductive carbon                                                         
black                                                                     
Ketjen EC                        10.00                                    
                                     20.00                                
                                         30.00                            
carbon black                                                              
Sulfur   0.60                                                             
             0.60                                                         
                 0.60                                                     
                     0.60                                                 
                         0.60                                             
                             0.60                                         
                                 0.60                                     
                                     0.60                                 
                                         0.60                             
Sulfenamide                                                               
         2.25                                                             
             2.25                                                         
                 2.25                                                     
                     2.25                                                 
                         2.25                                             
                             2.25                                         
                                 2.25                                     
                                     2.25                                 
                                         2.25                             
accelerators                                                              
Electrical                                                                
         *   *   *   *   *   *   *   12000                                
                                         300                              
resistance, ohms                                                          
__________________________________________________________________________
 * = greater than 20 megohms                                              
 
    
     
                                           TABLE V                                 
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Square # 41  42  43  44  45  46  47  48   49                              
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Blend #4 137.24                                                           
             137.24                                                       
                 137.24                                                   
                     137.24                                               
                         137.24                                           
                             137.24                                       
                                 137.24                                   
                                     137.24                               
                                          137.24                          
Furnace carbon                                                            
         10.00                                                            
             20.00                                                        
                 30.00                                                    
black (HAF HS)                                                            
Vulcan C low         10.00                                                
                         20.00                                            
                             30.00                                        
conductive carbon                                                         
black                                                                     
Ketjen EC                        10.00                                    
                                     20.00                                
                                          30.00                           
carbon black                                                              
Sulfur   0.60                                                             
             0.60                                                         
                 0.60                                                     
                     0.60                                                 
                         0.60                                             
                             0.60                                         
                                 0.60                                     
                                     0.60 0.60                            
Sulfenamide                                                               
         2.25                                                             
             2.25                                                         
                 2.25                                                     
                     2.25                                                 
                         2.25                                             
                             2.25                                         
                                 2.25                                     
                                     2.25 2.25                            
accelerators                                                              
Electrical                                                                
         *   *   *   *   *   *   *   3000 270                             
resistance, ohms                                                          
__________________________________________________________________________
 * = greater than 20 megohms                                              
 
    
     While the instant invention is described above with regard to a preferred embodiment, it is intended to be covered broadly within the spirit and scope of the appended claims.