Abstract:
The continuous weighing meter [according to the invention has] having a conveyor belt [( 1 )] driven by a motor [( 3 )] and running over a guide roller ( 4 ), which is loaded with bulk material via an input funnel [(6)] from a feed arrangement [( 9 )]. In addition to a first weighing arrangement [( 11 )], which determines the gross loading of the loaded conveyor belt [( 1 )], the continuous weighing meter has a second weighing arrangement [( 15 )], which determines the tare loading of the empty, but possibly dirty and inhomogeneous conveyor belt [( 1 )]. The weighing distances have the lengths S B  [( 11 )] or S T  [( 15 )] and are separated by a distance L between homologous points. The weighing results are converted to digital form in evaluation [equipments ( 22, 21 )] units and taken to a computer [( 25 )] which calculates the net loading of the conveyor belt [( 1 )]. Additionally the computer [( 25 )] monitors the running of the belt for slip using increment transmitters [( 17, 23 )] and associated counters [( 18, 24 )] and generally controls the feed arrangement [( 9 )].

Description:
TECHNICAL FIELD 
     The present invention relates to a continuous weighing meter for bulk materials with zero adjustment control. 
     BACKGROUND OF THE INVENTION 
     Several processes and devices for the monitoring and recalibration of the zero point of conveyor scales are known. WO 91/14927 (D1) and WO 95/29390 (D2) may be cited as representing the large number of descriptions, which are part of the state of the technology. The aim of the aforementioned processes and devices and also of the present invention is to increase the accuracy of the determination of the flow of material by a continuous weighing meter. In WO 91/14927, the supply of bulk material to be measured is periodically interrupted to determine the zero point of the weighing returned by the empty belt, while in WO 95/29390, two weighing stations are provided downstream from the despatching station and, within certain tolerances, the two weighing results are applied to form an average. 
     Both in GB 358 786A and also in FR 2 129 807A, methods and devices are disclosed, with which the belt is weighed twice, indeed with a first weighing device on its upper side together with the bulk material placed on it, and with a second weighing device on its returning underside. In GB 358 786A, purely mechanical means are provided to form the difference between the two weights. 
     The disadvantage of this method and the corresponding devices lies first in that any material adhering to the belt distorts the second weighing on its underside. Furthermore, the weighing results are not correct if bulk material first adheres to the returning part of the belt, but then in the passage between the guide roller and the second weighing however is not present, or if bulk material falls from the upper side of the belt onto the returning part. Apart from this, in accordance with these publications, neither the position nor the speed of the belt is known. The speed of the belt can therefore be changed in a non-suitable manner, for example as a function of the difference between the target and actual values of the flow of the material, which makes both the maintenance of a desired flow of material and the use of spliced or non-homogeneous belts more difficult or entirely impossible. Furthermore, slip in the belt can neither be established nor compensated using the methods presented above. 
     In U.S. Pat. No. 2,997,205, a method and a device are presented, with which the belt is weighed twice on its upper side, in a first weighing device without the bulk material placed upon it, and then in a second weighing device together with the bulk material placed upon it. Here also, however, the exact time of running of a determined point on the belt between the two weighing devices is neither known nor of significance. 
     In none of the devices or methods mentioned above is the exact distance between the two weighing devices, measured along the direction of travel of the belt, of significance. 
     The periodic interruption of the supply obviously permits the calculation of the possibly varying zero point. This is however troublesome for many applications, since the despatch to another processing station of the material being weighed cannot be interrupted repeatedly without consequence. Added to this, the last calculated zero point remains stored for the period between two such interruptions. In the case of problematical bulk materials such as chocolate solids, flour and other partly sticky substances, the zero point can vary relatively quickly. The process described in WO 95/29390 does not contribute anything to the absolute accuracy of the weighing when residues of the materials mentioned—possibly even on the underside of the conveyor belt—build up slowly and remain there. 
     The aim which is to be addressed by the present invention, is the production of a continuous weighing meter, with which the zero point can be continuously and permanently determined and is always up to date and available for the processing of the gross weighing. 
     SUMMARY OF THE INVENTION 
     A continuous weighing meter for bulk material, constructed in accordance with the present invention, includes bulk material input means for supplying bulk material, a conveyor belt onto which the bulk material is deposited, and means for moving the conveyor belt. This continuous weighing meter also includes first weighing means positioned downstream from the bulk material input means and having a first force measuring cell over which the conveyor belt passes and first evaluation equipment for measuring the gross loading of the conveyor belt. This continuous weighing meter further includes second weighing means positioned upstream from the bulk material input means and having a second force measuring cell over which the conveyor belt passes and second evaluation equipment for measuring the tare loading of the empty conveyor belt. Also included in this continuous weighing meter are means for determining the running speed of the conveyor belt and a computer having a central processing unit and computing and control programs and responsive to the gross loading measurement of the conveyor belt, the tare loading measurement of the empty conveyor belt, and the determination of the running speed of the conveyor belt for determining the bulk material load on the conveyor belt during a period of time determined by the distance between the first force measuring cell and the second force measuring cell divided by the running speed of the conveyor belt. 
     The invention will be described in conjunction with the enclosed drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of the continuous weighing meter according to the invention, 
     FIG. 2 is a block schematic of the electrical equipment. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In FIG. 1, the continuous weighing meter is shown in schematic form. A conveyor belt  1  is driven by an electric motor  3  via a drive roller  2  and runs over a guide roller  4 . The guide roller  4  is also formed as a tensioning roller in a known manner. This construction is well known and so configured that even running of the conveyor belt  1  can be produced. Further, the positions of the drive roller  2  and the guide roller  4  can be interchanged. 
     An input funnel  6  guides the mass stream of a bulk material  7  to be metered onto the conveyor belt  1 . The bulk material  7  is delivered to the input funnel in a known manner, using for instance a feed screw, vibrator, a conveyor belt or other known conveying method. These known and possible conveying methods are represented by the block bearing reference number  9 . Direct delivery from a silo by the conveyor belt also is a possibility, in which case the conveyed quantity is controlled only by the speed of the conveyor belt  1 . If a feed arrangement  9  is included, then this is also controlled as well as the belt speed. 
     In the direction of flow of the conveyor belt  1 , after the input funnel  6  there is a weighing arrangement  11 , comprising a force measuring cell  12 , which measures the force on a rod  13  stretching across the entire width of the conveyor belt  1 . The actual measurement distance, designated here by the letter S and at whose centre the rod  13  is positioned, is limited by two further rods  14 . The relationship between the force measured by the measurement cell  12 , the length of the measurement distance, the speed of the belt VB and the flow of material m is known and does not need to be discussed here. The flow of material m, determined in this manner, is in any case a gross value, since the weight of the conveyor belt  1  is continuously weighed in with it. Added to this, there are influences of the belt tightness, since, for instance, the rod  12  lies somewhat higher than the two rods  14 . The gross value m has thus to be cleaned up by a tare value, which is given by the measurement of the empty conveyor belt  1 . 
     Now instead of periodically interrupting the flow of the bulk material  7 , to obtain the tare value, which remains stored until the next determination, according to the arrangement shown in FIG. 1, a second weighing arrangement  15  is provided, preferably fully identical to the weighing arrangement  11 . This second weighing arrangement  15  is positioned before the input funnel  6  in the direction of the movement of the conveyor belt  1  and comprises similarly two rods  14  defining the measurement distance S and a weighing force accepting rod  13 , which for its part impacts on a force measurement cell  16 . The signals generated by the force measurement cells  12 ,  16  are each converted into digital weight signals by evaluation units  21 ,  22  which are conducted to a computer  25 . 
     The interaction between the two weighing arrangements  11  and  15  is explained first in summary, bringing into consideration other elements shown in FIG. 1, and then in greater detail using FIG.  2 . 
     On the motor  3  or on a drive connected to the motor not separately shown, an increment transmitter  17  is connected, with the aid of which a digital signal is generated in a counter  18 , corresponding to the speed of rotation of the drive roller  2 . Additionally, an increment transmitter  23 , similar to the increment transmitter  17 , can measure the rotational speed of the guide roller  4 . The signals of the increment transmitter  23  are then converted into a digital signal, corresponding to the speed of rotation of the guide roller  4 , in a second counter  24 . In so far as the conveyor belt  1  exhibits no slip on the drive roller  2 , the signals generated by the counters  17  and  23  and passed to the computer  25  are of the same amplitude; a difference between the two signals means that there is slip in the conveyor belt  1  in which case the computer  25  generates a corresponding signal, which for example can be used for automatic stoppage of the FIG. 1 equipment. 
     From the foregoing, the time required for a particular point on the conveyor belt  1  to pass over the measurement distance S and the time expiring for the same point to pass over the distance L between the two rods  13  lying in the middle of the weighing arrangements  11 ,  15  can be determined. For the general case, where the weighing arrangements  11 ,  15  are not identically constructed, the measurement distance of the weighing arrangement  11  is designated S B  and that of the weighing arrangement  15  is designated S T . 
     A weighing process for each of the two force measurement cells  12 ,  16  requires a certain, typically short, time interval, which may be to develop an average, or it may be, as in the case of string force measurement cells, for systematic reasons. In the stated time interval, the conveyor belt  1  moves forward past the weighing arrangement  11  by a certain distance S B /kB under the assumption that in the time during which the conveyor belt  1  covers the distance S B , kB weighings are undertaken. The weighing arrangement  15  has the measurement distance S T  and similarly a certain, possibly differing, time interval for a weighing, so that in the same time, when the weighing arrangement  11  performs kB weighing operations, kT weighings are available from the weighing arrangement  15 . It is expedient that kB=kT. As discussed further below, however, it is possible that kT=kB/h; i.e. the weighing arrangement  15  works with a lower time resolution than weighing arrangement  11 . If further S B ? S T , then, for the calculation of the tare loading by the computer, the measurement distance S T  of the weighing arrangement  15  and the number kT.S T /S B  completed by the travel of the conveyor belt  1  over this measurement distance are brought in. Analogous to this consideration, during the time in which the conveyor belt  1  passes over the distance L, j weighing operations occur in the weighing arrangement  11 . 
     FIG. 2 shows in block schematic form, how the different digital signals are processed. The computer is shown in FIG. 1 as a box identified by the reference  25 . It comprises a central processing unit  30 , which undertakes all the arithmetic and logic operations and also contains the control program in file. An input/output unit  32  is connected to the central processing unit  30  by data feeder lines  36 ,  37 , via which the control quantities, such as belt speed, bulk flow, on and off commands and, if provided in the program, limit values (for instance for target deviation and maximum tare value) can be input. Inputs are effected via the input/output unit  32  using a schematically represented keyboard  35 ; output values are shown by numerical magnitudes and operating conditions on a visual display unit  34 . The input values covering motor revolutions ( 18 ), belt running speed ( 24 ) and gross weighing ( 22 ) are fed in directly to the central processing unit  30  over bidirectional data lines  38 ,  39 ,  40 ; bidirectional because the central processing unit  30  outputs at least the central timing for the elements referenced as  18  to  24 . The evaluating unit for tare weighing, that is for the weighing arrangement  15 , is connected via a further bidirectional line  41  to a shift register  31 . This includes, for example, j counting stages, whose content is shifted by one stage forward at each pulse with the reasonable assumption, that the weighing arrangements  11 ,  15  carry out their weighing operations at the same clock speed. The result output from the last stage with the number j is processed with the result of the gross weighing of weighing arrangement  11  in the central processing unit  30  to give a net result; the entry read into the counting stage with the number  1  is the current weighing of weighing arrangement  15 . It is within the scope of the invention, that the number of counting stages j might be reduced by a factor h with a simultaneous reduction of the shift timing by the same factor h. This has the consequence that a certain tare result of the weighing arrangement  15  is used for h gross weighings. This can be indicated when the positional and time variations of the tare result is small. 
     Instead of a shift register  31 , the use of an addressable RAM with j numbered store locations is similarly within the scope of the invention, whereby the reduction to j/h store locations, as previously mentioned, is similarly included within the scope of the invention. In this arrangement, at each weighing, gross as well as tare, the corresponding store location is addressed and at the same time the address selector is advanced by one number. 
     The running speed of the belt (Motor  3 ) and the feed arrangement  8  controlling the flow of the bulk material  7  can be controlled by the process program in the central processing unit; processes and devices for this purpose are known; likewise for the processing of the data from the devices designated  17  to  24 . 
     The advantage of the application described here of the second weighing cell  15  evaluating the running tare value is that a tare weighing, which is currently determined, is available for every gross weighing and also periodical variations of the tare, such as inhomogeneities of the conveyor belt  1 , and also irregularities such as dirtying of the belt (including the build-up of incrustations) are equally captured and considered. On the one hand, this obviates the periodic empty running of the conveyor belt  1 , and, on the other hand, one is not working with an average tare value, but with a multiplicity of individual and currently updated values. 
     The measures under the invention also increase the availability of the feed mechanism, which forms the decisive part of the continuous weighing meter. 
     A further advantage of the continuous weighing meter according to the invention is that spliced conveyor belts can be used instead of circular woven or otherwise homogeneous products. Spliced conveyor belts exhibit an inhomogeneity at the spliced section, which cannot be taken into account using stored tare values. 
     Using the arrangement according to the invention, mass inhomogeneities, irrespective of their cause, are effectively subtracted and thereby the resolution and accuracy of the continuous weighing meter is increased with simultaneous reduction of the cost of the conveyor belt as a welded part. 
     While in the foregoing there have been described preferred embodiments of the present invention, it should be understood by those skilled in the art that various modifications and changes can be made without departing from the true spirit and scope of the present invention.