Abstract:
A method for measuring the weight of material carried by a moving conveyor belt at an angle to the horizontal and passing over a belt scale, and a method for calibrating the belt scale under actual field conditions using previously determined calibration curves.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to conveyor belt weighing systems, and more particularly to a system, and a method for calibrating this system, for accurately weighing material transported by the system despite variations in the angle of inclination of the conveyor to the horizontal. 
     The weighing of material in many industrial situations, such as coal and iron mines, is accomplished by the continuous weighing method. This method uses weigh scales of the type with which the present invention is concerned in conjunction with belt conveyors that transport the bulk materials. The amount of conveyed material can be determined by continuously weighing the material passing over the weigh scale in a known period of time. In general, the continuous weighing method is extremely accurate as long as the belt scale is operated in a horizontal plane. 
     The difficulty has been that the weigh bridge is generally calibrated for a horizontal conveyor, and this calibration is not adjusted when the conveyor angle is changed either upwardly or downwardly. If the calibration is adjusted for a change in the conveyor angle, it is generally modified as a function of the cosine of the conveyor angle. Accordingly, for a positive (upward) conveyor angle, the modification factor used as 1/cos. Further, no modification factor is suggested for a negative (downward) conveyor angle. 
     Thus, there exists a need for a method of calibrating a belt scale wherein a calibration factor is defined to adjust the output signal of weigh bridge when the conveyor is inclined at either a negative or positive angle. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a method for weighing material conveyed by a moving belt and calibrating the weigh bridge which measures the weight of the material conveyed by said belt. 
     A belt conveyor weigh bridge assembly is installed in the frame work of a conveyor. The weigh bridge comprises low receiving weigh arms connected to the frame by mechanical hinges that allow the weigh arms to yield under a load imposed by the belt conveyor. The weigh bridge further comprises a weigh idler, and a load cell positioned in a receptacle below said idler. The output signal of the load cell together with the output signal of an electronic speed transducer, which monitors the belt speed, provides the necessary inputs to an integrator which multiplies the weight and speed signals to produce a totalized weight signal. The integrator, the speed transducer, and the weigh platform constitute a belt scale. 
     In accordance with the invention, the mechanical calibration factor is determined by first placing the weigh bridge in a level position with a known weight resting on the weigh arms whereby an output signal is produced from the load cell. Secondly, the known weight is placed above the load cell weigh idler position and again the output of the load cell is recorded. This procedure is repeated for the platform inclined at both positive and negative angles. In this manner, a set of non-linear curves relating the mechanical calibration factor to conveyor angles is generated for level, uphill, and downhill conveyor angles. 
     Accordingly, it is an object of this invention to provide a belt scale weighing system which provides an accurate reading despite variations in the conveyor belt angle. 
     A further object is to provide a method for calibrating the weigh bridge in the field when the conveyor angle changes due to field conditions. 
     The various features of novelty which characterize the invention are pointed with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which there is illustrated and described a preferred embodiment of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings, forming a part of this specification, and in which reference numerals shown in the drawings designate like or corresponding parts throughout the same. 
     FIG. 1 is a perspective view of the weigh bridge; 
     FIG. 2 is an elevtion view, partly broken away, of a weigh bridge; 
     FIG. 3 is a perspective view of the weigh bridge showing the position for holding the calibration weights on the weigh arms; 
     FIG. 4 is a perspective view showing the position for holding the calibration weights above the load cell; 
     FIG. 5 is a diagram illustrating a non-linear curve relating the calibration factor to a positive conveyor angle; 
     and 
     FIG. 6 is a diagram illustrating a non-linear curve relating the calibration factor to a negative conveyor angle. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 illustrates a conveyor belt 12 supported upon a plurality of support idlers 14. Beneath the conveyor belt 12 is a belt scale 10 for continuously weighing the material transported thereupon comprising a weigh bridge 17 having a frame 16, two spaced-apart weigh arms 18, and a yieldable mechanical hinge 20 connecting one end of each weigh arm 18 to said frame 16, a weigh idler 22 mounted on and transverse to said weigh arms 18 (bolts 19 merely hold the weigh idler in position) and a load cell 26. Parallel and spaced below the weigh idler 22 is a bar member 24 that impinges the load cell 26 which is positioned in a receptacle beneath said weigh idler 22. The weigh idler 22 is supported above the load cell 26, therefore the weigh idler 22 transmits the force generated by the weight of the material it supports to the load cell 26. The weigh idler 22 when placed on the weigh arms 18 is free to move up and down as material is processed over the belt scale. Consequently the load cell output is a function of the weight of the material resting upon a weigh span of the belt. A weigh span is defined as the length of belt 12 over which material is weighed. It is determined by idler spacing and is equal to one-half the distance from the weigh idler 22 to the downstream support idler 14 plus one-half the distance from the weigh idler 22 to the upstream idler 14. The weigh idler 22 therefor supports one-half of the total belt weight between the downstream and upstream idlers 14 plus one-half of the total material weight between the downstream and upstream idlers 14. 
     Generally to install the weigh bridge 17, one of the existing support idlers 14 is removed and replaced by the weigh bridge. The weigh bridge is generally placed in the conveyor with the hinge side toward the head pulley but may be placed with the hinge side toward the tail pulley. The weigh idler 22 is mounted on the weigh bridge 17 and takes the place of the support idler which was removed. 
     The total weight of material handled by the conveyor belt 12 during a period of time is a function of the linear velocity of the belt during that time. This velocity is measured by a speed pickup encoder (not shown). The speed pickup may be an optical encoding device, which outputs a fixed number of electrical pulses per revolution of its shaft, which is generally coupled to a pulley which has positive contact with the conveyor belt 12 at all times, such as the tail pulley. The head pulley is generally undesirable since the belt may slip in this location creating belt speed errors affecting scale accuracy. By counting the number of pulses which are outputted in a time period of known duration, by knowing the resolution (i.e. the pulses per revolution) of the encoder, and by knowing the diameter of the pulley on which the encoder is mounted, the belt speed is computed. The output signals of the load cell 26 and the speed pickup device provides the necessary inputs to an electronic integrator (not shown), which may be a microprocessor-based electronics package. Here the inputs are continuously integrated and recorded to provide readings of both the instantaneous rate of flow and cumulative weight of the material which has passed over the belt scale 10. The speed pickup device, integrator, and recorder are well known, commercially available items, the details of which form no part of the present invention. Thus, the load signal output of load cell 26 is appropriately processed and recorded in a manner well known to those skilled in the art. 
     As shown in FIG. 2, the yieldable mechanical hinge 20 generally comprised C-shaped members connected to said frame 16 and said weigh arms 18, for flexing motion in response to a load applied to said weigh idler 22. This mechanical hinge 20 supports a portion of the loading on the belt scale. A bending moment occurs in hinge 20 as weight is applied to the weigh bridge 10. The magnitude of this moment increases and thus causes a deflection onto the load cell 26 which is proportional to the magnitude of the load. Generally, the full range of load cell deflection is only 0.005 inches (0.127 mm), so the amount of bending movement at the mechanical hinge 20 must also be very small. Intermediate said mechanical hinge 20 and said frame 16 is a tare adjustment means 32 comprising a plate member 34 movably connecting said mechanical hinge 20 to said frame 16, and an adjustment screw 36 mounted on said frame 16 and cooperating with said plate member 34 to displace said hinge and said weigh arms in order to utilize the full range of the load cell 26, which in turn will make accuracy easier to achieve. The tare adjustment means 32 allows one to mechanically tare out the deadload of the weigh idler 22 and the empty conveyor belt 12. Therefore, a greater range of load cell deflection is made available purely for material loading. 
     In FIG. 3, calibration pins 38 are illustrated connected to said weigh arms 18 for holding precision calibration weights 40 in plce on said weigh arms 18 to obtain an output from said load cell 26. 
     As shown in FIG. 4, during static calibration of the belt scale 10, the weigh idler 22 is replaced by calibration member 42 in order that said calibration weights 40 may be placed above the load cell 26 to record an output from said load cell. The calibration member 42 must be the same weight and at the same elevation as the weigh idler 22. This elevation is referenced to the top of the center roller 44 of the weigh idler 22. If the load of the test weights 40 is not transmitted to the load cell 26 at a one-to-one ratio, it is necessary to accurately establish the true ratio. 
     In the field, the belt scale 10 must be adjusted to the customer&#39;s conveyor application, environment and service requirements. Accordingly, a mechanical calibration factor as illustrated in FIGS. 5 and 6 is determined from a previous static test utilizing test weights 40 to simulate a known load on the belt scale 10 and to establish a true ratio of the weights 40 above the load cell 26. These figures were obtained by, but not limited to, the following laboratory tests: 
     
                                           TEST 1__________________________________________________________________________POSITIVE CONVEYOR ANGLE               Load On              M.V. with Weigh Load on Calibration                         Cell       Load on M.V. with                                                     ZeroTest  Bridge       Calibration               Member Above                         Input Tare Calibration                                            Load Above                                                     TareRun No. Angle Pins    Load Cell Volts M.V. Pins    Load Cell                                                     C.F.__________________________________________________________________________1      0°       18#               15.05 2.504                                    13.4622      0°    18#       15.05 2.500        10.351   1.3963      0°       36#               15.05 2.502                                    24.4104      0°    36#       15.05 2.504        18.125   1.4025      0°       54#               15.05 2.505                                    35.3406      0°    54#       15.05 2.500        25.970   1.4017      0°       72#               15.05 2.503                                    46.3208      0°    72#       15.05 2.502        34.100   1.387*                                            Average                                                     1.4009      5°       18#               15.05 2.508                                    13.39810     5°    18#       15.05 2.508        10.488   1.36511     5°       36#               15.05 2.505                                    24.3312     5°    36#       15.05 2.507        18.495   1.36513     5°       54#               15.05 2.508                                    35.1814     5°    54#       15.05 2.499        26.56    1.358*15     5°       72#               15.05 2.508                                    46.2516     5°              15.05 2.502        34.60    1.363                                            Average                                                     1.36417    10°       18#               15.05 2.510                                    13.30518    10°    18#       15.05 2.506        10.651   1.32519    10°       36#               15.05 2.504                                    24.0620    10°    36#       15.05 2.492        18.778   1.32321    10°       54#               15.05 2.503                                    35.0022    10°    54#       15.05 2.499        27.20    1.316*23    10°       72#               15.05 2.503                                    45.8524    10°    72#       15.05 2.505        35.19    1.326                                            Average                                                     1.32525    15°       18#               15.05 2.506                                    13.08726    15°              15.05 2.506        10.763   1.28127    15°       36#               15.05 2.503                                    23.7028    15°              15.05 2.497        18.983   1.28529    15°       54#               15.05 2.497                                    34.3530    15°              15.05 2.500        27.55    1.272*31    15°       72#               15.05 2.503                                    45.0932    15°              15.05 2.500        35.72    1.282                                            Average                                                     1.28333    20°       18#               15.05 2.503                                    12.78334    20°    18#       15.05 2.507        10.845   1.234*35    20°       36#               15.05 2.507                                    23.1136    20°    36#       15.05 2.507        19.108   1.24137    20°       54#               15.05 2.508                                    33.5738    20°    54#       15.05 2.503        27.52    1.24239    20°       72#               15.05 2.506                                    43.9340    20°    72#       15.05 2.509        35.89    1.241                                            Average                                                     1.24141    25°       18#               15.05 2.505                                    12.40342    25°              15.05 2.504        10.775   1.19743    25°       36#               15.05 2.506                                    22.4144    25°              15.05 2.503        19.09    1.20045    25°       54#               15.05 2.498                                    32.4246    25°              15.05 2.502        27.42    1.20047          72#               15.05 2.498                                    42.6548                            15.05 2.499        35.75    1.207*                                            Average                                                     1.199__________________________________________________________________________  0°           5° 10°                               15°                                        20°                                                 25° ##STR1##   ##STR2##            ##STR3##                      ##STR4##                                ##STR5##                                         ##STR6##                                                  ##STR7##__________________________________________________________________________ 
    
     
                                           TEST 2__________________________________________________________________________NEGATIVE CONVEYOR ANGLE          Load On        M.V. withWeigh    Load on          Calibration                  Cell   Load on                               M.V. with                                      ZeroTest Bridge    Calibration          Member Above                  Input                      Tare                         Calibration                               Load Above                                      TareRun No.Angle    Pins  Load Cell                  Volts                      M.V.                         Pins  Load Cell                                      C.F.__________________________________________________________________________1     0°    18#           15.05                      2.503                         6.8972     0°          18#     15.05                      2.506    5.648  1.3983     0°    36#           15.05                      2.501                         11.2924     0°          36#     15.05                      2.503    8.745  1.408*5     0°    54#           15.05                      2.503                         15.6886     0°          54#     15.05                      2.506    11.963 1.3947     0°    72#           15.05                      2.504                         20.1008     0°          72#     15.05                      2.504    15.071 1.400                               Average =                                      1.3979    5°    18#           15.05                      2.500                         6.89110   5° 18#     15.05                      2.505    5.519  1.45711   5°    36#           15.05                      2.506                         11.26012   5° 36#     15.05                      2.508    8.580  1.44213   5°    54#           15.05                      2.508                         15.63314   5° 54#     15.05                      2.506    11.575 1.44715   5°    72#           15.05                      2.506                         19.98616   5° 72#     15.05                      2.508                               Average --                                      1.44917   10°    18#           15.05                      2.505                         6.83718   10°          18#     15.05                      2.508    5.397  1.49919   10°    36#           15.05                      2.508                         11.15920   10°          36#     15.05                      2.501    8.256  1.503*21   10°    54#           15.05                      2.504                         15.44422   10°          54#     15.05                      2.507    11.203 1.48823   10°    72#           15.05                      2.507                         19.75224   10°          72#     15.05                      2.509    14.109 1.487                               Average --                                      1.491__________________________________________________________________________        0°  5°  10° ##STR8##         ##STR9##                    ##STR10##                               ##STR11##__________________________________________________________________________ 
    
     These tests were conducted using a 100 pound load cell (for Test 1) and a 250 pound load cell (for Test 2), a 30 inch conveyor belt, and a 35 degree troughing idler. First, the belt scale 10 is placed in a level position, i.e. a conveyor angle of 0 degree, and the calibration weight 40 is placed on the weight arms 18 and resting against the calibration pins 38 to record the load cell output. Then, the same calibration weight 40 is placed on the calibration member 42 above the load cell 26 and the load cell output is again recorded. This procedure is repeated for the belt scale 10 inclined at various positive and negative angles. in this manner, a set of nonlinear curves relating calibration factor to conveyor angle is generated for positive and negative conveyor angles. Accordingly, a zero tare calibration factor or base calibration factor is defined as the quotient of the output of the load cell when the calibration weights 40 are placed on the calibration pins 38 divided by the output of the load cell when the calibration weights 40 are positioned above the load cell 26. During the tests approximately 15.0 volts is applied to the input of the load cell 26 whereby the load cell has an output of 3 mv per volt. Further, prior to the tests a tare was applied to the load cell until an output of approximately 2.5 mv is generated. This tare voltage is subsequentially substracted from the output generated by the loaded cell, e.g. during the first test run the tare (2.504 mv) is substracted from the output with the load on the calibration pins (13.462 mv) which, in turn, is divided by the tare (2.500 mv) substracted from the output with the load above the load cell (10.351 mv) which equals a zero tare calibration factor (1.396). 
     Each zero tare calibration factor is similarly determined for different calibration weights and an average value is arrived at (one odd reading from each group was rejected from further analysis) as indicated by the asterisk in Test 1 and 2. 
     Accordingly, a multiplier of the base (zero degree) calibration factor is plotted as a set of nonlinear curves 46 and 48 for positive and negative weigh bridge angles respectively. These curves 46 and 48 can readily be compared to curves 52 and 54 for (1/cos of an angle) for positive conveyor angles and (cos of an angle) for negative conveyor angles, respectively. 
     The method for calibrating a weigh scale of the present invention results in the accurate determination of the weight of material transported on a belt scale, since, the belt scale, when calibrated in actual field use, while running empty, need only have the calibration weights 40 placed on the calibration pins 38 so that the load cell output is adjusted to be equal to the force directly above the load cell. This output signal is directly proportional to the force of the calibration weights multiplied by the multiplier of the base calibration factor.