US3604646A

US3604646A - Mass rate control system for paper stock refiners

Info

Publication number: US3604646A
Application number: US883910A
Authority: US
Inventors: Marion A Keyes; John A Gudaz
Original assignee: Beloit Corp
Current assignee: Beloit Corp
Priority date: 1969-12-10
Filing date: 1969-12-10
Publication date: 1971-09-14
Anticipated expiration: 1988-09-14

Abstract

From consistency and flow data a signal is provided which is representative of the kilowatt hours per ton required to process the fluid paper stock passing through a refiner. This signal is compared with a desired set point and any difference is utilized as the set point for a power control loop. The function of the power control loop is to adjust the spacing of a longitudinally movable beater element contained in the refiner with respect to its stationary beater element. In this way, the power required by the drive motor that rotates the longitudinally movable beater element is maintained substantially uniform. The above action involves feedforward and feedback techniques. Additional feedforward correction is made for deviations in a preselected property, such as freeness, average fiber length and the like, of the incoming stock.

Description

United States Patent lnventors Appl. No. Filed Patented Assignee MASS RATE CONTROL SYSTEM FOR PAPER STOCK REFINERS 2,727,694 12/1955 Helmick,.lr.etal 3,078,051 2/1963 Patterson Primary Examiner-Granville Y. Custer, .I r. AttrneyDugger, Peterson, Johnson & Westman ABSTRACT: From consistency and flow data a signal is provided which is representative of the kilowatt hours per ton required to process the fluid paper stock passing through a refiner. This signal is compared with a desired set point and 14Cl 'ms lDrawin Fi g g any difference is utilized as the set point for a power control [52] US. Cl 241/37, loop The f ti f the power Conn-0| loop is to adjust the 241/36 spacing of a longitudinally movable beater element contained [51] lltl. Cl B026 7/14 i h refiner i h respect t it t ti b t l t [50] Field of Search 241/37, 63, this w the power required by the drive motor that rotates 64, 36; 162/253 254 the longitudinally movable beater element is maintained substantially uniform. The above action involves feedforward and [56] References and feedback techniques. Additional feedforward correction is UNITED STATES PATENTS made for deviations in a preselected property, such as free- 1,815,155 7/1931 Lewellen et al 241/37 ness, average fiber length and the like, of the incoming stock.

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PATENTED SEP] 4 l97l DESIRED TON SET POINT e 19 3 R] A EM 5 2 M E a v; a 8 T E 3 M 3 w E m 2 w .H wwu w n m w T- mmp U. h w h h z H WM U 0 W R on IP-rift H EC 6 VI .ll v 0. m n\ nn .ufl |uU H I- I E] U R R m W TM mw 2 we L 0 mm; m wk 9. m. Z W .m mm J mm) J mm m k Mm 5r T c1 4 I: .v PM T T z 2| M w 0 0 /6 0 m0 K m 8 5 z 8 :w. Pc C C 2 5 K X c r umwwm r 2 0 01. L H 5 5 00 T m b RPM n. 2 5 5 SE 5 EWEKGY/ B ONE 0/? Y MASS RATE CONTROL SYSTEM FOR PAPER STOCK REFINERS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to refiners for processing fluid paper stock, and pertains more particularly to a control system utilizing consistency and flow information derived from the fluid stock passing through the refiner, as well as operating data relating to a selected property of the incoming stock.

2. Description of the Prior Art Reference is made to U.S. Pat. No. 3,309,031, issued Mar. 14, I967 to Richard F. McMahon et al. for MATERIAL WORKING APPARATUS, which patent is owned by the present assignee. The control system described and claimed in said McMahon et al. patent hasperformed exceedingly well. However, the system set forth in said McMahon et al. patent is only flow sensitive. In certain respects, the alluded-to prior art system has been too precise in its operation and certain difficulties have arisen, particularly where the delta T (AT) transmitter is subjected to considerable vibration.

SUMMARY OF THE INVENTION Oneof the primary objects of the instant invention is to provide a control system for paper stock refiners that will be responsiveto both flow and consistency. From the flow and consistency data, a calculation is made that provides the basis for an accurate positioning of the axially shiftable beater element so that the power requirements of the drive motor that rotates this beater element willbe kept relatively uniform.

Another object is to provide a system of the foregoing character that utilizes feedforward and feedback control principles.

Anotherobject of the invention is to constantly adjust the systemto compensate for variations in a particular stock property, such as freeness or fiber length.

Quite briefly, the present invention takes a set point indicative of the desired number of kilowatt hours per ton which should be applied to a ton of fluid .stock flowing through a refiner and compares this set point with a signal representative of the calculated kilowatt hours per ton that has been derived from the consistency and flow data taken from the fluid stock passing through the refiner. Any difference between the two signals is utilized to provide an adjusted set point for a power control loop which causes the movable beater element of the refiner. to be positioned so that the load imposed upon the drive motor that rotates the movable beaterelement is maintained substantially constant. However, where a particular stock property varies sufficiently, the system corrects for the variation by immediately modifying the local set point for the power control loop.

BRIEF DESCRIPTION OF THE DRAWING The sole FIGURE exemplifying the invention is for the most part in block form but the refining apparatus is shown diagrammatically.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring .now in detail to the drawing, it should beexplained atthe outset that the circuitry constituting the invention may be operated in either of two modes, each of which will become manifest asthe description progresses, and that each mode can be promptly updated for a change in stock property which if allowed to continue would result in a worsening of the power stabilization.

Before describing the control systernit will be well to refer to typical refining apparatus with which the invention may be effectively; associated. Accordingly, a refiner has been designated generally by the reference numeral 10. In the form depicted the refiner is of the so-called disc variety and inconical plug-type refiner, such refiner generally being'known as a Jordan refiner, in which a rotatable conical plug is moved axially with respect to a complemental shell in which it is contained.

As illustrated, the refiner 10 in the present situation has a drive shaft 16 connected with the rotary element 14 and by means of an electric drive motor 18 the shaft 16 is rotated 56 as to produce the desired rotation of the element 14. Since the present invention is regarded as an improvement over 'said McMahon et al. patent, reference may be made to this patent for further explanation as far as the construction of the refiner 10 is concerned. In the McMahon et al. patent, a pneumatic motor is referred to as positioning the movable beater element with respect to the stationary element; in the present situation, it is planned that an electric motor 20 will be utilized. Such motor may function in the form described in U.S. Pat. No. 1,933,814 issued on Nov. 7, 1933 to Darcy E. Lewellen et al. for STOCK CONSISTENCY CONTROL. Although not of recent innovation, the said Lewellen et al. patent is pertinent because it deals with a Jordan refiner and very succinctly shows a motor for rotating a threaded shaft that is employed for positioning the conical plug with respect to the encompassing shell.

As far as the present drawing is concerned, it will be discerned that the fluid stock enters the refiner 10 through an inlet conduit 22 and is discharged through an outlet conduit 23. In contradistinction to the aforesaid McMahon et al. patent, no attempt is herein made to determine the temperature differential between the stock entering the refiner 10 through the conduit 22 and the temperature leaving the refiner via the conduit 23. Instead, a consistency transmitter 24 is associated with the conduit 22, although the consistency of the fluid stock could be determined after the refining process has been completed, then being associated with the conduit 23. A correction factor would be introduced in order to compensate for any consistency change occasioned by the refinement that has transpired. Also utilized in the present control system is a flow transistor 26 that provides a signal indicative of the rate of flow of the fluid stock.

From the information derived from the transmitters 24 and 26, a calculator 28 computes the tonnage rate, doing so on a tons-per-hour basis by multiplying the signals from the trans mitters 24 and 26 together and also by an appropriate constant which will result in the units being on the basis'of tons per hour. Hence, the calculator 28 provides a signal that is representative of the massstock rate and more specifically tons per hour.

A second calculator 30 is utilized for computing a signal that is indicative of the difference between a representative kilowatt value, which will be referred to more in detail hereinafter, and the actual kilowatts required by the motor 18 when the refiner 10 is operating under no-load conditions, that is without stock flowing therethrough. What the calculator 30 does is to accept the output signal from the calculator 28 and divides or computes a ratio between the representative kilowatt value minus the no-load kilowatt value and the tonsper-hour measure obtained from the calculator 28. More 1 second input terminals 32a, 32b and an output terminal 32c,- A

set point indicative of the amount of energy that desirably should be applied to a given ton of fiber stock (desired KWH per bonedry ton) contained in the fluid stock flow entering the refiner through the conduit 22 is compared with the calculated kilowatt-hours-per-ton signal derived from the calculator 30, these signals being applied via the terminals 32a and 32b respectively, and any difference appears as an error signal on the output line 32c. The error signal fed over the output line 320 is delivered to a kilowatt-hours-per-ton controller 34 that provides an adjusted set point for a power control loop now to be described.

Hence, the signal provided by the controller 34 is utilized in a power control loop, being impressed on a summing junction or comparator circuit 36 through an input terminal 36a. Additionally, the comparator 36 has a second input terminal 36b and an output terminal 360. The power control loop further includes a power transmitter 38 of the wattmeter type which is electrically connected to the drive motor 18 so as to provide a signal at the input terminal 36b which is representative of the actual power being used by said motor so that it can be compared with the set point signal arriving via the input terminal 360, this power signal being forwarded over a line 37 connecting between the transmitter 38 and the terminal 36b. It is any difference between these last-mentioned signals that is reflected as an error signal at the output terminal 36c. The terminal 36c is connected directly to controller 40 that forwards an appropriate signal reflective of the error to a pulse duration power controller 42 having a deadband determined by selected limits doing so through a summing junction or comparator circuit 44 having

input terminals

44a, 44b and an output terminal 44c. The 44b is included in a feedforward loop yet to be described. However, with the information given up to this point in mind, it will suffice to state that whenever the error signal from the comparator 36 is within prescribed upper and lower limits, the controller 42 will take no corrective action.

More specifically, it will be noted that the controller 42 is connected to the control motor that functions to position the rotatable refining element 14 with respect to the stationary element 12. However, when the error signal delivered via the terminal 36c is above or below the selected limits, the controller 42 then produces a pulse signal having a duration or span proportional to the difference or error sensed between the input signal applied via the points 36a and 36b. Of course, the signal arriving via the terminal 44b may alter the situation, as will presently be made clear.

Earlier herein it was indicated that the calculator 30 accepts a no-load kilowatt signal over line 30b. Now that the power transmitter 38 has been referred to it can be explained that a line 45 connects directly to the power line 37 and extends to a first selector switch 46 that has been embodied in the drawing solely for the purpose of enabling a visual understanding of how the input signals to the calculator 30 are processed. In this regard, the switch 46 comprises a movable contact arm 46a engageable with either of two fixed contacts 46b or 46a. The contact 46b is attached to the line 45 and the contact 460 is dead, being an off contact. The arm 46a is attached to a third contact belonging to the switch 46 and it is this last-mentioned contact that is connected to the input line 30b of the calculator 30.

Thus, to introduce the no-load kilowatt signal into the calculator 30, where it is stored for use in computing the output signal or ratio of the difference between the actual kilowatts and the no-load kilowatts, the arm 46a is moved from its depicted position into engagement with the contact 46b. To determine the no-load or ineffective portion of power that does not contribute to refining, the beater element 14 is backed away from the stationary element 12 to a point where no refining is being realized. Since this is the mechanical no load state, the power transmitter forwards a signal representing the electrical power used by the motor 18.

From the foregoing, it is to be noted that both feedforward and feedback techniques are employed to reduce system maladjustments. In this regard, feedforward action is provided by the power-per-unit-mass loop that is responsive to flow and consistency disturbances and feedback action by the power control loop that is responsive to actual power changes.

Inasmuch as two modes or methods of operation are con templated when practicing the invention, attention is now directed to the presence of a second switch 48 that will serve to physically indicate what transpires, the switch 48 having a movable arm 48a and fixed

contacts

48b, 48c and 48d. When the switch arm 48a bridges the

contacts

48b and 48d, the set point provided by the controller 34 is not only applied to the input terminal 36a of the comparator 36 but is also applied to the input line 30c of the calculator 30. Hence, the calculated kilowatt signal that is utilized in computing the ratio signal to"? be fed to the comparator 32 is derived directly from the controller 34. In this instance, the set point signal introduced by way of the input line 300 has the no-load kilowatt signal, which is entered via the line 30b, subtracted therefrom. It is, therefore, the difference between the calculated kilowatt signal and the no-load kilowatt signal that is ratioed with the tons-perhour signal arriving from the calculator 28 to furnish the input applied to the terminal 32b of the comparator 32.

On the other hand, when the switch arm 48a bridges the contacts 480 and 48d, the actual kilowatt signal existing at any given moment is forwarded from the power transmitter 38, which is the same signal impressed on the input terminal 36b of the comparator 36, over the input line 300 to the calculator 30. Hence, in the latter situation or second mode, the kilowatt hours per ton or work per unit mass is the same as in the earlier described arrangement except that instead of using the power set point from the controller 34, a measured power value is utilized in the calculation performed by the calculator 30. In actual practice, the switch 46 would not normally be used inasmuch as one or the other of the operational modes would be selected in advance and used rather than switching between the two. However, the switch 48 serves as a facile means, as does the switch 46 with respect to the no-load data, for illustrating the two ways in which the control system herein described can function. In actual practice the system would be programmed for one mode or the other and would not make use of switches as such.

Inasmuch as certain properties of the stock entering the refiner 10 through the inlet conduit 22 will affect the operation of that portion of the system described up to this point, provision is made for effecting a prompt change to take care of the variation in stock property that has occurred. Since the specific stock property or characteristic is unimportant, such property will simply be denoted as Z. The property might well be concerned with fiber dimensions, their flexibility, their freedom of movement or any other characteristic that should not be allowed to deviate from an acceptable norm. Z,, will therefore be employed to denote the measured stock property, a stock property transmitter forwarding an appropriate signal indicative of the property Z. The desired stock property will be denoted as Z,, and a set point signal representative thereof applied to a summing junction or comparator 52. More specifically, the set point signal is supplied via an input terminal 52a and the signal from the transmitter 50 via a second input terminal 52b, any difference or error between the input signals appearing at the output terminal 52c of the comparator 52.

Since the purpose of the feedforward loop currently being described is to introduce into the overall system a corrective action which is a function of the incoming stock property 2, it will be necessary to provide a control signal that will possess the proper weight or influence. Consequently, a simple calculator 54 comprising a multiplier 54a and adder 54b will visually portray what can be employed, although in actual practice all of the computations, that is those performed by the calculators 28 and 34 as well, could be handled by a single computer or each with a suitable computer/controller. Hence, the difference signal (Z, Z,,,) at the output terminal 52c might be multiplied by a constant K, and the product added to a constant K to provide the requisite control. Although a conventional computer/controller could be employed, as indicated above, a controller 56 has been shown, the controller 56 receiving the output from the calculator 54 and supplying the previously alluded-to input terminal 44b with an appropriate signal. Any predetermined difference between the signals arriving via the two

terminals

44a, 44 b will influence the action of the deadband controller 42 that controls the beater adjusting motor 20.

In view of the detailed description that has been given, coupled signal a great degree with the operational sequence that occurs, an elaborate description of the operation need not be presented. However, it should be remembered that a set point is provided for the power control loop, this being the signal applied to the input terminal 36a, that is derived from actual operational data involving both flow and consistency. It will be appreciated that the flow and consistency determinations are made in a kilowatt-hours-per-ton control loop which includes the

calculators

28, 30, the comparator 32 and the controller 34. Any difference between such a calculated set point signal and the actual power signal from the transmitter 38 is utilized in the control of the motor 20, by way of the controller 40 and the deadbanded power controller 42; which action determines the spacing between the

refining elements

12 and 14. In this way, the power requirements of the motor 18 are maintained at a substantially uniform and desired level, any fluctuations or variations being rather minimal even where the consistency of the fluid stock entering through the conduit 22 and its rate of flow may vary quite widely.

However, when the measured stock property signal Z,, deviates or shifts sufficiently from the desired stock property signal Z,,, a relatively rapid adjustment is made of the beater element 14 with respect to the element 12 so that compensation is effected through the controller 56 without the delay that would otherwise occur only through the feedback controller 40. Therefore, it will be appreciated that a second feedforward loop is provided through the agency of the transmitter 50 and the controller 56 associated therewith, this loop being in addition to the feedforward loop which includes the transmitter 24, 26' and the controller 34.

I claim:

1. In combination with a refiner for processing fluid paper stock which includes a pair of relatively rotatable and axially movable refining elements, a drive motor for relatively rotating said elements and a control motor for axially shifting one of said elements with respect to the other, a control system comprising a power control loop responsive to the actual power consumed by said drive motor to operate said control motor in accordance with any difference between a signal representative of the actual power consumed by said drive motor and a power set point signal, and an energy-per-unitmass control loop responsive to rate of flow and consistency data derived from the fluid stock passing through said refiner to provide a calculated energy per unit mass signal which is compared with a set point signal indicative of a desired energy per unit mass so as to provide said power set point signal in accordance with any difference therebetween, whereby the power required by said drive motor is relatively uniform.

2. The combination of claim 1 in which said energy-perunit-mass control loop includes first calculating means responsive to said rate of flow and consistency data for providing a signal representative of tons per hour of stock flowing through said refiner, second calculating means for providing a signal that is the ratio of the difference between the no-load power value required by said refiner for the particular flow condition and a power value with respect to the value of said tons-perhour signal, and means for comparing the value of said ratio signal with said set point signal that is indicative of the desired energy per unit mass to provide said power set point signal.

3. The combination of claim 2 in which the value of said power set point signal constitutes the power value, the no-load power value being subtracted from the value of said power set point signal by said second calculating means to provide said difference.

4. The combination of claim 2 in which the value of the actual power consumed by said drive motor constitutes the power value, the no-Load power value being subtracted from the value of the actual power by said second calculating means to provide said difference.

5. The combination of claim 1 including a stock property loop responsive to the value of a given stock property for modifying the amount of actual power supplied to said drive motor in accordance with the difference between the measured value of said given stock property and the value of a desired value of said given stock property.

6. In combination with a refiner for processing fluid paper stock which includes a pair of relatively rotatable and axially movable refining elements, a drive motor for relatively rotating said elements and a control motor for axially positioning one of said elements with respect to the other, a control system comprising a power control loop including a power transmitter for providing a signal which is a measure of the actual power being consumed by said drive motor, comparator means for comparing said actual power signal with a power set point signal to provide an error signal representative of any difference therebetween, means responsive to said error signal for operating said control motor in accordance with the value of said error signal so as to position said one refining element axially with respect to said other refining element, means providing a signal which is a measure of the flow rate of fluid stock through said refiner, means providing a signal which is a measure of the consistency of said fluid stock, first calculating means for computing a mass stock rate signal from said flow and consistency signals, second calculating means for computing a signal representative of energy per unit mass, said energy-per-unit-mass signal being derived from a ratio of the difference between first and second signals with respect to said mass stock rate signal wherein said second signal is representative of the power required by said refiner under no-load conditions, comparator means for comparing said energy per unit mass signal with a set point signal indicative of a desired energy per unit mass to provide an error signal in accordance with any difference therebetween, and means responsive to said last-mentioned error signal for providing said power set point signal,

7. The combination of claim 6 in which said first signal corresponds to said power set point signal.

8. The combination of claim 6 in which said first signal corresponds to said signal which is a measure of the actual power being consumed by said drive motor.

9. The combination of claim 6 including the means responsive to a measured stock property for modifying the value of said first-mentioned error signal in accordance with any difference between said measured stock property and the set point representing a desired stock property.

10. In combination with a refiner for processing fluid paper stock which includes a pair of relatively rotatable and axially movable refining elements, an electric drive motor for relatively rotating said elements and an electric control motor for axially positioning one of said elements with respect to the other, a control system comprising a power control loop including a power transmitter for providing a signal which is a measure of the actual power being consumed by said drive motor, first comparator means for comparing said actual power signal with a power set point signal to provide a first error signal representative of any difference therebetween, a deadbanded pulse duration power controller responsive to said error signal for furnishing pulses to said control motor having a duration proportional to the value of any such error signal so as to position said one refining element with respectto said other refining element in accordance with the length of pulses supplied by said controller, means providing a signal which is a measure of the flow rate of fluid stock through said refiner, means providing a signal which is a measure of the consistency of said fluid stock, first calculating means for computing a signal representative of tons per hour of stock flowing through said refiner, second calculator means having first, second and third input lines, said first input line being connected to said first calculating means, said second input line introducing into said second calculating means a given kilowatt signal and said third input line introducing into said second calculating means a signal representative of the kilowatts required by said refiner under no-load conditions, said second calculating means ratioing the difference between said signals introduced via said second and third input lines with respect to the signal introduced via said first input line to provide an output signal representative of calculated kilowatt hour per ton, second comparator means for comparing said calculated kilowatt-hours-per-ton signal with a set point signal representative of a desirednumber of kilowatt hours per ton to provide a second error signal in accordance with any difference therebetween, and a kilowatt-hours-per-ton controller responsive to said last-mentioned error signal for providing said power set point signal.

11. The combination of claim 10 in which said power set point signal constitutes said given kilowatt signal introduced via said second input line to said second calculating means.

12. The combination of claim 10 in which said actual power signal constitutes said given kilowatt signal introduced into said second calculating means via said second input line.

13. The combination of claim 10 including a third comparator means disposed between said first comparator means and said deadbanded pulse duration power controller for comparing said first error signal with a stock property signal having a value representative of any change that said stock property signal has deviated from a desired value so as to provide a third error signal in accordance with the difference between said stock property signal and said first error signal, said third error signal controlling said deadbanded pulse duration power controller in accordance with the value thereof.

14. The combination of claim 13 including a stock property transmitter for providing a signal which is the measure of the stock property of stock flowing through said refiner, and a fourth comparator means for comparing said measured stock property signal with a set point signal representative of the stock property that is desired so as to produce said third error signal.

mg UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,604,646 Dated September 14, 1971 Inventor) Marion A. Keyes, IV and John A. Gudaz It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 42, "transistor" should be --transmitter--. .1 Column 5, line 9, after "pled" delete "signal" and insert -to--.

I Signed and sealed this 29th day of February 1972.

(SEAL) Atte'st:

EDWARD M.FLETCHER,JH. ROBERT GOT TSCHALK Attesting Officer Commissioner of Patents

Claims

1. In combination with a refiner for processing fluid paper stock which includes a pair of relatively rotatable and axially movable refining elements, a drive motor for relatively rotating said elements and a control motor for axially shifting one of said elements with respect to the other, a control system comprising a power control loop responsive to the actual power consumed by said drive motor to operate said control motor in accordance with any difference between a signal representative of the actual power consumed by said drive motor and a power set point signal, and an energy-per-unit-mass control loop responsive to rate of flow and consistency data derived from the fluid stock passing through said refiner to provide a calculated energy per unit mass signal which is compared with a set point signal indicative of a desired energy per unit mass so as to provide said power set point signal in accordance with any difference therebetween, whereby the power required by said drive motor is relatively uniform.

2. The combination of claim 1 in which said energy-per-unit-mass control loop includes first calculating means responsive to said rate of flow and consistency data for providing a signal representative of tons per hour of stock flowing through said refiner, second calculating means for providing a signal that is the ratio of the difference between the no-load power value required by said refiner for the particular flow condition and a power value with respect to the value of said tons-per-hour signal, and means for comparing the value of said ratio signal with said set point signal that is indicative of the desired energy per unit mass to provide said power set point signal.

6. In combination with a refiner for processing fluid paper stock which includes a pair of relatively rotatable and axially movable refining elements, a drive motor for relatively rotating said elements and a control motor for axially positioning one of said elements with respect to the other, a control system comprising a power control loop including a power transmitter for providing a signal which is a measure of the actual power being consumed by said drive motor, comparator means for comparing said actual power signal with a power set point signal to provide an error signal representative of any difference therebetween, means responsive to said error signal for operating said control motor in accordance with the value of said error signal so as to position said one refining element axially witH respect to said other refining element, means providing a signal which is a measure of the flow rate of fluid stock through said refiner, means providing a signal which is a measure of the consistency of said fluid stock, first calculating means for computing a mass stock rate signal from said flow and consistency signals, second calculating means for computing a signal representative of energy per unit mass, said energy-per-unit-mass signal being derived from a ratio of the difference between first and second signals with respect to said mass stock rate signal wherein said second signal is representative of the power required by said refiner under no-load conditions, comparator means for comparing said energy per unit mass signal with a set point signal indicative of a desired energy per unit mass to provide an error signal in accordance with any difference therebetween, and means responsive to said last-mentioned error signal for providing said power set point signal.

10. In combination with a refiner for processing fluid paper stock which includes a pair of relatively rotatable and axially movable refining elements, an electric drive motor for relatively rotating said elements and an electric control motor for axially positioning one of said elements with respect to the other, a control system comprising a power control loop including a power transmitter for providing a signal which is a measure of the actual power being consumed by said drive motor, first comparator means for comparing said actual power signal with a power set point signal to provide a first error signal representative of any difference therebetween, a deadbanded pulse duration power controller responsive to said error signal for furnishing pulses to said control motor having a duration proportional to the value of any such error signal so as to position said one refining element with respect to said other refining element in accordance with the length of pulses supplied by said controller, means providing a signal which is a measure of the flow rate of fluid stock through said refiner, means providing a signal which is a measure of the consistency of said fluid stock, first calculating means for computing a signal representative of tons per hour of stock flowing through said refiner, second calculator means having first, second and third input lines, said first input line being connected to said first calculating means, said second input line introducing into said second calculating means a given kilowatt signal and said third input line introducing into said second calculating means a signal representative of the kilowatts required by said refiner under no-load conditions, said second calculating means ratioing the difference between said signals introduced via said second and third input lines with respect to the signal introduced via said first input line to provide an output signal representative of calculated kilowatt hour per ton, second comparator means for comparing said calculated kilowatt-hours-per-ton signal with a set point signal representative of a desired number of kilowatt hours per ton to provide a second error signal in accordance with any difference therebetween, and a kilowatt-hours-per-ton controller responsive to said last-mentioned error signal for providing said power set point signal.