US3253744A

US3253744A - Electrical control system for grinding mill

Info

Publication number: US3253744A
Application number: US398450A
Authority: US
Inventors: Arthur R Macpherson; Robert C Meaders
Original assignee: Nordberg Manufacturing Co
Current assignee: Nordberg Manufacturing Co
Priority date: 1964-09-22
Filing date: 1964-09-22
Publication date: 1966-05-31
Anticipated expiration: 1983-05-31

Description

May 31, 1966 A. R. MacPHERsoN ETAL 3,253,744

ELECTRICAL CONTROL SYSTEM FOR GRINDING MILL Filed Sept. 22, 1964 3 Sheets-Sheet l May 31, 1966 A. R. MacPHERsoN ETAL 3,253,744

ELECTRICAL CONTROL SYSTEM FOR GRINDING MILL May 31, 1966 A. R. M-LICPHERSON ETAL 3,253,744-

I'ILECTRICAL CONTROL SYSTEM FOR GRINDING MILL i-ilcd Sept. 22, 1964 3 Sheets-Sheet 3 United States Patent O 3,253,744, ELECTRICAL CONTROL SYSTEM FOR GRINDING NULL Arthur R. MacPherson, Montreal, Quebec, Canada, and

Robert C. Meaders, Miiwaukee, Wis., assignors to Nordberg Manufacturing Company, Milwaukee, Wis., a

corporation of Wisconsin Filed Sept. 22, 1964, Ser. No. 398,450 16 Claims. (Cl. 222-56) This invention relates to a control system for controlling the rate 'of feed to a grinding mill, and has application to a dual-chambered grinding mill, and is a continuation in part of application Serial No. 317,605, iiled October 2l, 1963, now abandoned.

A primary purpose of the invention is a control system and method of the type described in which the feed is varied in accordance with the weight of the charge in the mill.

Another purpose is a method and apparatus of the type described, suitable for use in a dual-chambered grinding mill, in which the weight of each chamber has components determined by the charge in both chambers.

Another purpose is a simple and reliably operable electri'cai control system -for use with a rotating grinding mill.

Another purpose is a control system and apparatus of the type described which is eifective to control the rate of feed rto both chambers of a dual-chambered material reduction mill.

Another purpose is a control system of the type described in which both the charge in the mill and the rate of feed to the mill are monitored to provide control data.

Another purpose is a material reduction mill control system utilizing a power override to provide an additional monitor of the charge in the mill.

Another purpose is a method of controlling a material reduction mill in which the charge in the mill is utilized in regulating the desired rate of feed to the mill.

Another purpose is a method of varying the feed rate to a material reduction mill automatically so as to maintain a predetermined weight of charge in the mill.

Another purpose is a method of varying the 'feed in a dual chambered material reduction mill to either chamber so as to maintain diierent charges in each grinding chamber.

Another purpose is a method of controlling the feed and/or charge so as to maintain the maximum throughput or grinding capacity to either or both grinding compartments and the mill as a whole.

Other purp-oses will appear in the ensuing specification, drawings and claims.

The invention is illustrated diagrammatically in the following drawings wherein:

FIGURE 1 is a diagrammatic illustration of a mill of the type described.

FIGURE 2 is a schematic illustration of the control system suitable for use with the mill in FIGURE l.

FIGURE 3 is Ia more detailed electrical schematic illustrating the means for controlling the rate of feed to the mill in FIGURE l, and

FIGURE 4 is `a diagrammatic illustration of a further form of control system.

The invention should not be limited to any particular type of grinding mill or material reduction mill, although the invention has lapplication with a mill of the type shown in co-pending application Serial No. 229,334, tiled October 9, 1962. This mill has a pair of grinding chambers separated by a central discharge area, which is illustrated at 10 in FIGURE 1. The two chambers are illustrated at 12a and 12b, and they may be generally similar in size and shape. The mill may be supported on its trunnions 14a and 14b, with the trunnions being mounted on suitable strain gauges 16a 'and 16b, which are in turn positioned on concrete supports or the like 18a and 18b. Feed troughs 20a and 20b may be positioned at each end of the mill for introducing material to be ground into the chambers. The present control system may fbe used with a -so-called autogenofus grinding mill in which the charge in the mill grinds itself. The oontrol system may also be used with the more conventional ball or rod mills, in which steel balls or rods have been added to aid in material reduction.

The strain gauges 16a and 16b illustrated in FIGURE l may be of a conventional type and provide a resist- Iance or impedance variable with the superimposed weight on the gauge. 'Ilhe placement of the strain gauges may vary, and it is satisfactory to place the gauges at the trunnion supports. When the strain gauges are so positioned, the total weight superimposed on each gauge is made [up of two components, one component ybeing from the chamber directly adjacent the strain gauge, and the other component being caused =by the opposite chamber. This placement of the gauges provides a ratio of 3:1 between the weight component caused by the charges in the chambers. For example, the strain gauge at the lefthand side of FIGURE l may have a weight superimposed on it made up three-fourths by the charge in the left-hand chamber and one-fourth of the charge in the right-hand chamber. 'Ill-le opposite strain gauge will have a corresponding weight relationship. 'Ilhe invention should not be limited to this particular weight ratio as fed to motors 24a and 24b which are utilized to control4 the rate of feed to the feed chutes 20a and 20b. Normally, suitable conveyors will be positioned above the feed chutes and the motors 24a and 24b will drive these l conveyors. The rate at which the conveyors are driven may determine the rate at which material is supplied through the feed chutes and into the grinding chambers, or the feed rate may be determined by vibrating feeders or lche like.

Suitable potentiometers, or rheostats, or variable resistances are indicated at 26a and 261:. Assuming a 3:1 ratio between the Weight components as described above, each potentiometer may have two variable resistances with one being three times as large as the other. The larger lor primary resistance component of potentiometer 26a will be fed to bridge 22a and the smaller component will be fed to bridge 22h. In like manner the two resistance components of potentiometer 2Gb will be utilized in bridges 2221 and 22a. yIn effect, the smaller resistance component from each potentiometer is used to cancel out or balance the smaller or minor weight,

component in the strain gauge impedance. In other words, strain gauge 16a will have a weight component due to the charge in each chamber. The weight component from the right-hand chamber will be the smaller or minor component. The smaller impedance from potentiometer 2Gb will be used to cancel out the weight component from the right-hand chamber and so permit a balance between the setting of potentiometer 26a and that portion of the impedance from strain gauge 16a which represents the weight in the left-hand chamber. In some applications, iixed resistance may be used, rather than variable resistances.

FIGURE 3 illustrates a bridge circuit which may be utilized at 22a or 2211 in the circuit of FIGURE 2. A suitable power supply 28 supplies a D.C. voltage between

terminals

30 and 32 of the bridge circuit. One arm-of the bridge contains impedance 34, and assuming this is the left-hand bridge of FIGURE 2, this impedance will be equal to the large impedance of potentiometer 26a. Impedance 36 will be equal to the minor component or the feedback component from potentiometer 26b. Impedance 38 will be equal to three-fourths of the reading of strain gauger 16 assuming that the 3:1 ratio between weight components, as specified above, is used. Impedance 40 will be equal to one-fourth of the reading of strain gauge a.

Impedances

38 and 40 together Will equal the combined impedance provided by strain gauge 16a.

Terminals 42 and 44 provide the output from the bridge circuit. These two terminals are connected through suitable lines to. an amplifier indicated generally at 46 with the amplifier being connected to a directional sensitive relay 48. The relay 48 may include normally open contacts 50 and normally closed contacts 52. If the direction of the output from a bridge circuit indicates an excess of charge in a chamber, contacts 52 will open, adding resistance 54 in circuit with resistance 56 and drive motor 58. If the charge is too low, contacts 50 will close, removing resistance 56 to speed up motor 58.

FIGURE 4 illustrates a modified form of control system. A material reduction mill 60 may be a dual-chambered mill with chambers 62a and 62h. Trunnions 64a and 64b at each end of the mill may lbe supported in any satisfactory manner, with the supports each including a plurality, in this case three, electric strain gauges or the like 66a and 6617. As each side of the mill has an identical control system, only one-half or one portion of the control system will be described hereinafter. It should be understood however that each portion of the control system will operate in a similar manner.

The electric signals from strain gauges 66a are carried by wires 68 to a combination power supply and computation circuit 70. Volt meters or the like 72 may 4be connected to the unit 70 so that the current from any one of the strain gauges may be read to give an indication of the load thereon. The unit 70 is effective to supply power to the strain gauges so that the strain gauges, which may be Variable resistances, may then provide electric signals indicative of the super-imposed loads. The unit 70 will take the signals from the various strain gauges and provide -outputs which are indicative of the charge or load in each of the mill chambers. This may be accomplished by circuits of the type illustrated in FIGURES A2 and 3, or by other types of comparable circuits. Each chamber will have a load component in the other chamber, and the circuit of unit 70 is eifective to provide output signals representing only the charge in a particular chamber.

The output from unit 70 is carried by wire 74 to a control instrument 76 whichmay be a periodic error integrating controller of the type man-ufactured by the Ramsey Engineering Company, of St. Paul, Minnesota. The controller 76 will be set at a predetermined weight or charge which is the desired charge in the mill for optimum operation. The unit 76 will compare4 the desired charge, which may be in the form of an electric signal or may be'otherwise, with the signal received over line 74 which is indicative of the actual load in the mill chamber. The controller 76 will integrate or average the diiference or error signal for a predetermined but adjustable period of time. If at the end of the sample interval, the average error signal is zero, then the controller will have no output. If a positive or negative error integral exists at the end of the sample interval,

then t-he controller will produce a series of step increaseor decrease signals, proportional to the magnitude of the error integral.

The output from controller 76 will be passed over wire 78 to an automatic set point changer 80 of the type manufactured by Ramsey Engineering Company, of St. Paul, Minnesota. The set point changer 80 controls the rate of feed to the mill chambers as described hereinafter. Initially, this unit is set for a predetermined rate of feed which will give the desired charge or load in the mill chamber. In the event there is an error signal `from the controller 76, then the set rate of feed at unit 80 will be changed by the error signal, The unit 80 normally provides an output over line 82 to a second integrating controller 84 which is effective to compare the set rate of feed from the unit 80 with the actual measured rate of feed from the conveyor scale measuring system 86.

The unit 86 is effective to weigh the material being fed over conveyor 88 and through a feed chute 90 to the mill chamber. The signal from weighing unit 86 is compared or integrated in controller 84 with the signal from the set point changer 80. Again, the error signal is integrated over a period of time so that minute or brief changes in the rate of feed, which do not continue, will not affect the overall rate of feed. The output from controller 84 is fed over a line 92 to a motorized rheostat assembly 94 which is effective to control the rate of-supply of air or some other operating fluid from line 96 to line 98. The air supply through line 98 controls a feeder assembly 100 which determines the amount of material fed'from hopper 102 onto conveyor 88. The feeder 100 is pneumatically or hydraulically operated to control the volume of material discharged from the hopper `onto the conveyor. The invention should not be limited to la pneumatic or hydraulic system, as other control means, for example electric, may be practical.

It is desirable to provide a further control for the overall system. A thermal converter 104 monitors the load on the motors (not shown) driving the mill 60. If the load on the motors is excessive, or is above a predetermined set point, the thermal converter 104 will provide a signal over line 106 and line 108 to the set point changer 80. The set point changer will have its feed rate reduced until the load on the motors is again within limits. Normally the set point changer -80 will reduce its load in steps, although it would be a continuous type of variation.

The use, operation and function of the invention are as follows:

In the form of FIGURES l, 2 and 3, when the charge in each chamber of the mill is equal to the charge which has been determined to provide optimum grinding, impedances 38 Iand 40 will be balanced by

impedances

34 and 36 and there will be no output from either bridge circuit and no signal will be supplied to either of the amplifiers. However, if the charge in the left-hand chamber becomes too great, for any one of a number of reasons, impedance 38 will be greater than impedance 34, which is its balancing impedance, and there will be an output from the bridge to the amplifier 46. Contacts 52 will open and motor 58 will be slowed down to ldecrease the rate of feed to the left-hand chamber. This condition will continue until the charge in the left-hand chamber is sufficient to provide a balancing of impedances. The operation of the bridge circuit for the righthand chamber is exactly the same. When the charge in a particular chamber is either too great or too small, motor 58 will be speeded up or slowed down to change the rate of feed to the chamber and this condition will continue until the charge in the chamber provides equal irnpedances or until the charge is at its predetermined optimum level. The direction or polarity of the output from the bridge circuit determines whether contacts 50 or 52 will be operated.

The charge in the mill may be made up of the material being ground as well as any grinding media, for example balls or rods. As described herein, e-ach of the strain gauge measuring means provides an impedance or a resistance which is variable in accordance with the weight superimposed upon it. By the particular placement of the strain gauges there is a 3:1 ratio between weight components from the two chambers. It should be realized that other positions of the strain gauges will provide other weight ratios. Also, the invention should not be limited to providing impedances which vary in accordance with the weight superimposed upon a strain gauge. What is important is to vary an elect-rical parameter in accordance with Weight. -A voltage may vary, or a current may vary.

In the form shown in FIGURE 4, there is 'an electric signal developed which is indicative of the charge in each of the mill chambers. Each of these signals isutilized in a system for controlling the charge to a particular chamber. The control system consists of two integrating or averaging units, one of which develops an error signal relating the predetermined or set charge in the mill and the actual charge in the mill, while the other develops an error signal by comparing the actual rate of feed with the set rate of feed. The first of these controllers or integrating units provides an output signal which is indicative of the difference, over a predetermined sampling interval, of the actual charge in the mill against the set charge and this output signal is utilized in changing the set or desired rate of feed to the mill. Both the rate of feed to the mill and the actual charge in the mill are monitored and the two monitoring signals are together utilized to control the rate of feed. The power override of FIGURE 4 provides additional control. When the power of the mill drive is excessive, indicating a possible charge overload, the set rate of feed will be decreased to bring mill drive motor operation within limits.

Although the invention has been described in connection with a dual-chambered grinding mill, particularly an autogenous grinding mill, the invention obviously has other applications. The mill need not be autogenous, but may utilize balls, rods or other grinding media. Also, the particular control circuits may be used on a mill having oneY grinding chamber or more than two grinding chambers.

Whereas the preferred form of the invention has been shown and described herein, it should be realized that there are many modifications, substitutions and alterations thereto within the scope of the following claims.

We claim:

1. A method of controlling the yfeed to a dual-chambered material reduction mill including the steps of weighing one chamber, said weight being made up by a major component from the charge in the chamber being weighed and a minor component from the charge in the other chamber, varying an electrical impedance in accordance with said weight, removing from the impedance that portion caused by the charge in the other chamber, providing a reference electrical impedance variable with the desired charge in the chamber being weighed, compa-ring said reference impedance and the adjusted Weight impedance and varying the rate of feed to the chamber being weighed until said impedances are equal.

2; In a system for controlling the material feed to an integral dual-chambered material reduction mill, variable electrical impedance `strain gauge means for supporting each chamber of the mill, means for comparing the impedance of the strain gauge means supporting each chamber with a reference electrical impedance, and means for varying the feed to each chamber of the mill in accordance with any difference between its associated strain gauge impedance and the reference impedance.

3. The system of claim 2 further characterized in that each strain gauge impedance has a component caused bythe weight of the material in each chamber of the mill, and means for cancelling out that component of a vstrain gauge impedance caused by the Weight of the material in the opposite chamber in the mill.

4. The system of claim 2 further characterized in that said reference impedances are variable.

v5. In a system for controlling the material feed to an integral du-al-chambered material reduction mill, variable electrical imped-ance strain gauge means for supporting each chamber of the mill, each strain gauge impedance having a component due to the weight of the material in each chamber of the mill, a pair of reference impedances, one `for ea-ch chamber, means for comparing each strain gauge impedance and a reference impedance including a pai-r of bridge circuits, each bridge circuit including a str-ain gauge impedance, the reference impedance associated with a particular chamber, and a portion of the other reference impedance, and means for varying the feed to each chamber of the mill in accordance with the output from said bridge circuits.

6. A method of controlling the feed to a material reduction mill including the steps of developing an electric signal indicative of the charge in the mill, comparing Said `signal with a reference representing the desired charge in the mill to produce an electric signal representing the difference between the actual and desired charge in the mill, developing an electric signal representa-tive of the rate of feed of material to the mill, comparing said lastnamed signal with a reference representing the desired rate ofv feed to the mill to produce an electric signal repl resenting the difference between the actual and th'e desired rate of feed to the mill, utilizing the signal represting the difference between the lactual and desired charge in the mill to control the desired rate of feed `to the mill, and utilizing the signal representing the difference between the actual and desired rate of feed to the mill to control the rate of feed to the mill.

7. The method of claim 6 further characterized in that the step of comparing the signa-l representing the actual char-ge in the mill with the reference occurs over a predetermined interval, with the difference between the signal and the reference over this interval being averaged.

S. The method of claim 6 further characterized in that the step of comparing the signal representing the actual rate of feed with the reference occurs over a predetermined interval, with the difference between the s ignal and the reference 'being averaged over this linterval.

9. The method of claim 6 further characterized by and including the step of monitoring the load on the mill drive to provide an electric signal to vary the reference of the mill rate of feed when the power consumption of the mill drive exceeds a predetermined level.

10. A method of controlling the feed to a material lreduction mill lhaving a plurality of chambers including the `steps of developing a plurality of electric s-ignals, each indicative of the charge in a chamber, comparing each of said signals with a reference representing the desired charge in each chamber to produce a plurality of output signals each of which represents the difference between the actual and desired charge for a particular mill chamber, developing a plurality of electric signals, each representative of the rate -of feed of material to a mill chamber, comparingeach of said las-t-named signals with a reference representing the desired rate of feed'to each mill chamber to produce a plurality of electric signals each of which represents the difference between the actual and desired rate of feed to a mill chamber, utilizing the signals representing the difference between the actual and desired charge in the mill chambers to control the desired rate of feed to each mill chamber, and utilizing the signals 4representing the difference 'between the actual and desired rate of feed to each mill chamber to con-trol the rate of feed to the mill chambers.

11. In a system of control for a material reduction mill, means for developing an electric signal indicative of the charge in the mill, means for comparingsaid signal with a reference representing the desired charge in the mill to produce an output signal representing the difference between the actual and desired charge in the mill, a material feed system for the mill, means for developing an electric signal representative `of the rate of feed of material to the mill, means for comparing said last-named signal with a reference represen-ting the desired rate Iof feed to the mill to produce an electric signal representing the difference between the actual and the desired rate of feed to the mill, means for utilizing the signal representing the difference between Ithe actual and desired charge in the mill to control the `desired rate of feed to the mill, and means for utilizing the signal representing the diiference between the actual and desired rate of feed to the mill to control the material feed system.

12. The system of claim 11 further characterized in that the means for comparing the signal representing the charge in the mill with a reference includes means for integrating the resultant difference over a predetermined interval.

13. The system of claim 11 further characterized in that the means for comparing the signal representing rate of feed to the mill with a reference includes means for integrating the resultant difference over a predetermined interval.

14. The system of claim 11 further characterized by and including means for monitoring power consumption in the mill drive, and means, cooperating with said monitoring means, for changing the desired rate of feed to the mill when powe-r consumption exceeds a predetermined level. l

15. Th-e system of claim 11 further characterized in that the material feed system is pneum-atically operated.

16. A control system for a material reduction mill having a plurality of chambers, means for developing a plural-ity of electric signals, each `indicative of the charge in a mill chamber, means for comparing each signal with a reference yrepresenting the desired charge in each chamber to produce a plurality of 'output signals each representing the difference between the actual and desired charge for a particular mill chamber, a material feed system for each mill chamber, means for developing a plurality of electric signals each representative of the rate of feed to a mill chamber, means for comparing each of said lastnamed signals with a reference representing the desired rate of feed to each mill chamber to produce a plurality of output signals each representing the difference between the actual and desired rate of feed to a particular chamber, means for utilizing the signals representing the difference between the actual and desired charge in the mill chambers to con-trol the desired rate of lfeedto each mill chamber, and means for utilizing the signals representing the difference between the actual and desired rate of feed to the mill chambers.

References Cited by the Examiner UNITED STATES PATENTS 2,764,360 9/1956 Podszus 241-34 2,766,939 10/ 1956 Weston.

FOREIGN PATENTS 227,782 9/ 1959 Australia. 1,160,190 7/1958 France. 1,131,974 5/1962 Germany.

,RAPHAEL M. LUPO, Primary Examiner. HADD s. LANE, Examiner.

Claims

1. A METHOD OF CONTROLLING THE FEED TO A DUAL-CHAMBERED MATERIAL REDUCTION MILL INCLUDING THE STEPS OF WEIGHING ONE CHAMBER, SAID WEIGHT BEING MADE UP BY A MAJOR COMPONENT FROM THE CHARGE IN THE CHAMBER BEING WEIGHED AND A MINOR COMPONENT FROM THE CHARGE IN THE OTHER CHAMBER, VARYING AN ELECTRICAL IMPEDANCE IN ACCORDANCE WITH SAID WEIGHT, REMOVING FROM THE IMPEDANCE THAT PORTION CAUSED BY THE CHARGE IN THE OTHER CHAMBER, PROVIDING A REFERENCE ELECTRICAL IMPEDANCE VARIABLE WITH THE DESIRED CHARGE IN THE CHAMBER BEING WEIGHED, COMPARING SAID REFERENCE IMPEDANCE AND THE ADJUSTED WEIGHT IMPEDANCE AND VARYING THE RATE OF FEED TO THE CHAMBER BEING WEIGHED UNTIL SAID IMPEDANCE ARE EQUAL.