System for precisely controlling discharge rates of loss-in-weight feeder systems

{
  "publication_number": "US5103401A",
  "country": "US",
  "kind": "A",
  "title": "System for precisely controlling discharge rates of loss-in-weight feeder systems",
  "abstract": "System for controlling the discharge rate of a material feeder. A weight scale connected to the material is sampled and the samples input to a digital processor. The digital processor determines for consecutive time intervals during discharge of said material, a weight loss over the time interval. The weight loss for the interval is compared with a desired weight loss which is calculated from a desired setpoint material discharge rate. The resulting error is averaged with subsequent errors to form a control signal for the material feeder. The calculation of errors over each subsequent interval increases the resolution of the control signal, increasing the ability to control discharge rates to compensate for changes in material density which otherwise affect discharge rates.",
  "claims": [
    "1. A system for controlling the amount of material discharged from a feeder comprising: a weight scale for measuring the weight of a supply of material being discharged from a feeder; a digital processor for periodically sampling a signal from said weight scale, and digitizing said signal; digital filter means for filtering said digitized signal, and providing an array of continuously updated average weight values, including an oldest and newest average weight values which are a moving average of said digitized signal; means for providing a desired digital weight loss signal over the time interval represented by average weight values of said array; means for providing a loss in weight signal based upon said array of average weight loss values; means for deriving an error signal based upon said loss in weight signal and said desired weight loss signal; second digital filter means for providing an average of a plurality of said error signals; and, means for providing a proportional integral control signal in response to said error signal for controlling the discharge rate from said feeder.",
    "2. The system for controlling the amount of material discharged from a feeder according to claim 1 wherein said first digital filter means provides said oldest and newest weight averages to said means for providing a loss in weight signal which computes said loss in weight signal as a function of said oldest and newest weight averages.",
    "3. The system of claim 1 wherein said error signal is derived from an array of error signals comprising consecutive measured error signals having an oldest error value and a newest error value, each error value being proportional to the difference between said oldest value of average weight, the newest value of average weight and said desired weight loss.",
    "4. The system of claim 3 wherein said error signal comprises a running average of said array of error values.",
    "5. A system for controlling the amount of material discharged from a feeder having a selectable mass discharge rate comprising: a weight scale means for measuring the weight of said feeder and supplying at periodic intervals a digital weight value; a digital filter providing an array of such digital weight values, and providing an average weight value for said array; means for providing a reference rate of mass discharge for said feeder; loss in weight determining means for providing from a plurality of consecutive average weight values a plurality of loss of weight values; error determination means for determining from each consecutive loss of weight values and said reference rate of mass discharge an array of error values, and an average error value from said array of error values; and, proportional control means for supplying a control signal to said feeder for controlling said feeder discharge rate in a direction to reduce said average error value.",
    "6. A system for controlling the amount of material discharged from a feeder according to claim 5 wherein said reference means comprises means for generating a weight loss from said desired mass discharge rate.",
    "7. A system for controlling the amount of material discharged from a material feeder having a controllable discharge rate comprising: a weight scale means measuring the weight of said material feeder, and providing a digital signal representing the weight of said feeder; a digital processor connected to receive said digital signal, and connected to receive as an input a desired discharge rate for said feeder, said digital processor being programmed to execute the steps of: determining a predicted loss in weight over a plurality of consecutive time intervals at said desired discharge rate; comparing at each time interval the predicted loss in weight and the actual loss in weight of said material feeder; deriving an error between said predicted loss in weight and said actual loss in weight for each of said consecutive intervals; and, deriving a control signal from said error signal for controlling said feeder discharge rate.",
    "8. The system for controlling the amount of material discharged from a material feeder having a controllable discharge rate according to claim 7 comprising means for accumulating a plurality of said loss in weight errors, and averaging said plurality of errors over a plurality of said intervals.",
    "9. The system according to claim 7 wherein said digital processor accumulates a plurality of actual loss in weight measurements and produces an average of said actual loss in weight measurements.",
    "10. The system according to claim said processor updates said plurality of loss in weight measurements by discarding the oldest measurement each consecutive interval and adding a new loss of weight measurement."
  ],
  "description_excerpt": "The present invention relates to gravimetric weight loss feeding apparatus for controlling the discharge of dry materials. Specifically, the present invention comprises a control system which controls the material discharge rate to compensate for changes in material density.\n\nLoss in weight feeder systems are in general use in industrial processes requiring precise metering of dry materials which are difficult to precisely meter. These systems control material discharge by weighing the volume of material being discharged and comparing the weight against a set point weight which decreases over time. The resulting error from setpoint weight is utilized as a feedback signal to increase or decrease the discharge rate of materials.\n\nSystems of this type are exemplified in U.S. Pat. Nos. 4,579,252, 4,301,510 and U.S. Pat. No. Reissue 27,115. These systems, known generally as \"gravimetric\" systems, control discharge rates with respect to a setpoint rate of discharge.\n\nThe difficulty in obtaining a higher degree of control over discharge rate results from the nature of the material. Many dry materials experience a change in density which will naturally result in a steep rate change in discharge rate. These changes in material consistency result in a change in discharge rate which is beyond the response capability of loss in weight control systems.\n\nOther anomalies which detract from system performance include vibration of the weighing scale, which introduces a quantum of noise into the weight signal, as well as sensor drift.",
  "cpc": [
    "G01G 13/29",
    "G05D 7/0623"
  ],
  "ipc": [
    "G01G 13/20",
    "G01G 13/29",
    "G05D 7/06"
  ],
  "assignees": [
    "Merrick Industries Inc"
  ],
  "inventors": [
    "Norman R. Johnson"
  ],
  "filing_date": "1989-11-21",
  "publication_date": "1992-04-07",
  "grant_date": "1992-04-07",
  "priority_date": "1989-11-21",
  "application_number": "US-43998589-A",
  "family_id": "23746948",
  "cited_by_count": 45,
  "citations": [
    "US4164260A",
    "US4301510A",
    "US4754822A",
    "US4882784A"
  ]
}