Digital mass flow control system

Abstract

The throughput of a conveyor belt is controlled by a motor whose speed is established by the instantaneous state of an up/down counter. The counter is incremented upwardly by setpoint pulses representative of the desired mass flow rate, and downwardly by feedback pulses from a rate multiplier representative of the actual mass flow rate. When the actual mass flow rate is equal to the setpoint flow rate, the average number of setpoint pulses will equal the average number of feedback pulses with the result that the state of the counter will be stable with time. Speed control means for the motor maintains the belt speed at a level dependent upon the state of the counter. A variation between the actual and desired mass flow rates is manifested by a change in the average number of feedback pulses which results in a change in the state of the counter. The resultant change in the motor speed and hence the belt speed restores the average number of feedback pulses to equality with the setpoint pulses thus restoring stability to the counter and restoring the actual mass flow rate to the setpoint rate.

What is claimed is: .[.1. A digital volumetric flow control system for use with a motor driving a conveyor belt having means for feeding material onto the belt at a constant rate, said system comprising: 
     
        signal to an analog signal that establishes the motor speed..]. 8. A digital mass flow control system according to claim .[.3 including.]. .Iadd.26 wherein said first digital means includes .Iaddend.weighing means associated with said conveyor belt for producing a digital signal representative of the instantaneous mass of material on said conveyor belt, encoder means associated with said conveyor belt for producing a digital signal representative of the instantaneous rate of movement of said conveyor belt, said first digital means being responsive to the digital signals of said weighing means and said encoder means for producing said digital signal representative of the instantaneous actual 
     
     
        mass flow rate of material on said conveyor belt. 9. A digital mass flow system according to claim 8 wherein said encoder means produces a pulse train whose instantaneous frequency is representative of the instantaneous 
     
     
        speed of said conveyor belt. 10. A digital mass flow system according to claim 9 wherein said encoder comprises a gear composed of a magnetic material and being provided with a plurality of teeth, said gear being mounted upon the shaft of said motor driving said conveyor belt and a magnetic pickup being mounted in proximity to said teeth of said gear. 
     
     
            A digital mass flow control system according to claim 9 wherein said weighing means includes a transducer for producing an analog signal representative of the instantaneous mass of material on said belt, and an analog-to-digital converter having a storage register whose contents constitute a digital representation of the instantaneous mass of material 
     
     
        on said belt. 12. A digital mass flow control system according to claim 11 wherein said first digital means includes a rate multiplier having a pulse input terminal and level input terminals, said pulse input terminal being connected to the output of said encoder means, and said level input terminals being connected to said storage register of said converter, the output of said rate multiplier being a train of feedback pulses whose 
     
     
        average frequency is representative of the actual mass flow rate. 13. A digital mass flow control system according to claim 12 wherein said 
     
     
        converter is a digital voltmeter. 14. A digital mass flow control system according to claim 12 including means for scaling the output of said rate multiplier for producing a scaled digital signal in the form of a pulse train, each pulse of which represents a discrete increment of mass delivered by said conveyor belt, and a counter for counting the pulses produced by said scaling means whereby the state of said counter 
     
     
        represents the total mass of material passed by said conveyor belt. 15. A digital mass flow control system according to claim 14 wherein means are provided to adjust the contents of said storage register of said converter to zero for a prdetermined input to said converter to provide a tare 
     
     
        control. 16. A digital mass flow control system according to claim 15 wherein said counter for counting the pulses produced by said scaling means ia a bi-directional counter which receives a countup or a countdown 
     
     
        signal from said converter means. 17. A digital mass flow control system according to claim 12 including a mass flow rate counter means, said mass flow rate counter means including a counter for repetitively counting the pulses at said output of said rate multiplier for predetermined periods of time, a storage register for storing the contents of said counter at the end of each predetermined period of time, and display means for displaying 
     
     
        the count stored in said storage register. 18. A digital mass flow indicator system for use with a motor driven conveyor belt having means for feeding material onto the belt, comprising: weighing means associated with said conveyor belt for producing a digital signal representative of the instantaneous mass of material on said conveyor belt;   encoder means associated with said conveyor belt for producing a digital signal in the form of a pulse train whose instantaneous frequency is representative of the instantaneous rate of movement of said conveyor belt; .Iadd.   digital rate .Iaddend.multiplier means responsive to the digital signals produced by said weighing means and said encoder means for producing a digital signal in the form of a pulse train.Iadd., each pulse in said train representing an instantaneous increment of mass on said belt, the frequency of said pulse train representing instantaneous mass flow.Iaddend.; and   means for scaling the pulse train digital signal produced by said multiplier means to produce a digital signal pulse train in which each pulse is representative of an increment of mass delivered by said conveyor   
     
     
        belt .Iadd.in predetermined dimensions. .Iaddend. 19. A digital mass flow indicator system according to claim 18 wherein said weighing means includes a transducer for producing an analog signal representative of the instantaneous mass of material on said belt, and an analog-to-digital converter having a storage register whose contents constitute a digital 
     
     
        representation of the instantaneous mass of material on said belt. 20. A digital mass flow indicator system according to claim 19 wherein said multiplier means is a rate multiplier having a pulse input terminal and level input terminals, said pulse input terminal being connected to the output of said encoder means, and said level input terminals being connected to said storage register of said converter, the output of said multiplier being a digital signal representative of the instantaneous 
     
     
        actual mass flow rate of material on said conveyor belt. 21. A digital mass flow indicator system according to claim 20 wherein said converter is 
     
     
        a digital voltmeter. 22. A digital mass flow indicator system according to claim 20 including a counter for counting the pulses produced by said scaling means whereby the state of said counter represents the total mass 
     
     
        of material passed by said conveyor belt. 23. A digital mass flow indicator system according to claim 22 whwerein means are provided to adjust the contents of said storage register of said converter to zero for 
     
     
        a predetermined input to said converter to provide a tare control. 24. A digital mass flow indicator system according to claim 23 wherein said counter for counting the pulses produced by said scaling means is a bi-directional counter which receives a countup or a countdown signal from 
     
     
        said converter means. 25. A digital mass flow indicator system according to claim 20 including a mass flow rate counter means, said mass flow rate counter means including a counter for repetitively counting the pulses at said output of said rate multiplier for predetermined periods of time, a storage register for storing the contents of said counter at the end of each predetermined period of time, and display means for displaying the count stored in said storage register. .Iadd. 26. A digital mass flow control system for use with a motor driving a conveyor belt having means for feeding material onto the belt, said system comprising: first digital means for producing a digital signal in the form of a train of feedback pulses whose frequency is representative of the actual mass flow rate of material on said conveyor belt, each of said feedback pulses representing an increment of mass;   set point means for generating a digital signal in the form of a train of pulses representative of the desired mass flow rate;   second digital means responsive to the feedback pulses from said first digital means and set point pulses from said set point means for producing a digital control signal representative of the speed at which said motor must drive the belt in order to establish equality between the desired and the actual mass flow rate, said second digital means canceling a set point pulse for each feedback pulse or vice versa, and being operative to store any excess set point or feedback pulses not canceled; and   speed control means responsive to said digital control signal for controlling the speed of said motor, said speed control means including means for disabling said motor until said digital control signal reaches   
     
     
        some predetermined value greater than one. .Iaddend..Iadd. 27.  A digital mass flow control system in accordance with claim 26 wherein said speed control means is a servo-amplifier responsive to said second digital means for converting said digital signal to an analog signal that establishes the motor speed and wherein said servo-amplifier is biased so that current flow to the motor begins when said digital control signal reaches some predetermined value greater than one. .Iaddend..Iadd. 28. A digital mass flow control system according to claim 26 wherein the digital signal produced by said set point means is a train of set point pulses whose average frequency is representative of the desired mass flow rate, and wherein said second digital means includes an up/down digital counter having a terminal to which applied pulses cause the counter to count upwardly and a terminal to which applied pulses cause the counter to count downwardly, said feedback and set point pulses being applied to respective terminals of said up/down counter. .Iaddend. .Iadd. 29. A digital mass flow control system in accordance with claim 28 wherein said up/down digital counter is an N stage binary counter. .Iaddend..Iadd. 30. A digital mass flow control system in accordance with claim 29 wherein said speed control means is a current output servo-amplifier and wherein said means for disabling said motor until said digital control signal reaches some predetermined value greater than one includes gating means for gating only the higher order stages of said up/down digital counter to the input of said servo-amplifier. .Iaddend..Iadd. 31. A digital mass flow control system in accordance with claim 28 wherein said means for disabling said motor until said digital control signal reaches some predetermined value greater than one includes gating means for disabling an output of the instantaneous state of said up/down counter below said predetermined count greater than one and enabling an output of the instantaneous state of said up/down counter above said predetermined count greater than one. .Iaddend..Iadd. 32. A digital volumetric flow control system for use with a motor driving a conveyor belt having means for feeding material onto the belt at a substantially constant rate, said system comprising: first digital means for producing a digital signal in the form of a train of feedback pulses whose frequency is representative of the speed of said conveyor belt, each of said feedback pulses representing an increment of movement of said conveyor belt;   set point means for generating a digital signal in the form of a train of pulses representative of the desired volumetric flow rate;   second digital means responsive to the feedback pulses from said first digital means and set point pulses from said set point means for producing a digital control signal representative of the speed at which said motor must drive the belt in order to establish equality between the desired and actual volumetric flow rate, said second digital means canceling a set point pulse for each feedback pulse or vice versa, and being operative to store any excess set point or feedback pulses not canceled; and   speed control means responsive to said control signal for controlling the speed of said motor, said speed control means including means for disabling said motor until said digital control signal reaches some   
     
     
        predetermined value greater than one. .Iaddend..Iadd. 33.  A digital volumetric flow control system in accordance with claim 32 wherein said speed control means is a current output servo-amplifier responsive to said second digital means for converting said digital control signal to an analog signal that establishes the motor speed and wherein said servo-amplifier is biased so that current flow to the motor begins when said digital control signal reaches some predetermined value greater than one. .Iaddend..Iadd. 34. A digital volumetric flow control system in accordance with claim 31 wherein said second digital means comprises an up/down counter, said up/down counter being counted up in response to set point pulses and being counted down in response to feedback pulses. .Iaddend..Iadd. 35. A digital volumetric flow control system in accordance with claim 34 wherein said up/down counter is an N stage binary counter. .Iaddend. .Iadd. 36. A digital volumetric flow control sytem in accordance with claim 35 wherein said speed control means is a servo-amplifier responsive to said digital control signal for converting said digital control signal to an analog signal that establishes the motor speed and wherein said means for disabling said motor until said digital control signal reaches some predetermined value greater than one includes gating means for gating only the higher order stages of said binary counter to the input of said servo-amplifier. .Iaddend..Iadd. 37. A digital volumetric flow control system in accordance with claim 34 wherein said digital control signal reaches some predetermined value greater than one includes gating means for disabling an output of the instantaneous state of said up/down counter below said predetermined count greater than one and enabling an output of the instantaneous state of said up/down counter above said predetermined count greater than one. .Iaddend..Iadd. 38. A digital mass flow control system according to claim 8 including means for selectively bypassing said weighing means to cause said first digital means to be directly responsive to said encoder means, whereby said first digital means produces a digital signal representative of the instantaneous volumetric flow rate of material on the conveyor belt. .Iaddend..Iadd. 39. A digital material flow control system for use with a motor driving a conveyor belt having means for feeding material onto the belt, said system comprising: encoder means associated with said conveyor belt for producing a first digital signal representative of the instantaneous rate of movement of said conveyor belt;   weighing means associated with said conveyor belt for producing a second digital signal representative of the instantaneous mass of material on said conveyor belt;   first digital means responsive to said first and second digital signals for producing a third digital signal in the form of a train of feedback pulses whose frequency is representative of the actual rate of mass flow of material on said conveyor belt, each of said feedback pulses representing an increment of mass;   set point means for generating a fourth digital signal in the form of a train of pulses whose frequency is representative of the desired flow rate;   up/down counter means having up and down input terminals for producing a digital control signal representative of the speed at which said motor must drive the belt in order to establish equality between the desired and the actual flow rate, said up/down counter means counting up one count for each pulse applied to said up input terminal and counting down one count for each pulse applied to said down input terminal;   means for applying said fourth digital signal to said up input terminal;   means for selectively applying either said first or said third digital signals to said down input terminal of said up/down counter means whereby said material flow control system operates as a volumetric flow control system when said first digital signal is applied to said down input terminal and as a mass flow control system when said third digital signal is applied to said down input terminal; and   speed control means responsive to said digital control signal for   
     
     
        controlling the speed of said motor. .Iaddend..Iadd. 40.  A digital material flow control system in accordance with claim 39 wherein said weighing means includes a load cell associated with said conveyor belt for producing an analog signal representative of the instantaneous mass of material on said conveyor belt and a digital volt meter responsive to said analog output signal of said load cell for producing said second digital signal. .Iaddend..Iadd. 41. A digital material flow control system in accordance with claim 39 wherein said first digital means comprises a binary rate multiplier. .Iaddend..Iadd. 42. A digital material flow control system in accordance with claim 39 wherein said means for selectively applying either said first or said third digital signals to said down input terminal of said up/down counter is a single pole double throw switch. .Iaddend..Iadd. 43. A digital material flow control system in accordance with claim 39 wherein said speed control means includes means for disabling said motor until said digital control signal reaches some predetermined value greater than one. .Iaddend..Iadd. 44. A digital material flow control system in accordance with claim 43 wherein said speed control means is a current output servo-amplifier responsive to said up/down counter for converting said digital control signal to an analog signal that establishes the motor speed and wherein said servo-amplifier is biased so that current flow to the motor begins when said digital control signal reaches some predetermined value greater than one. .Iaddend..Iadd. 45. A digital material flow control system in accordance with claim 43 wherein said means for disabling said motor until said digital control signal reaches some predetermined value greater than one includes gating means for disabling an output of the instantaneous state of said up/down counter below said predetermined count greater than one and enabling an output of the instantaneous state of said up/down counter above said predetermined count greater than one. .Iaddend.