US 3869892 A
A control method and apparatus for maintaining a uniform predetermined thickness of rolled sheet metal in a metal rolling mill where variations in sheet thickness are measured prior to its entry into the mill rolls. As a basic function, signals corresponding to the measured variations are fed forward to control pressure actuator means for the rolls, to provide corrections that eliminate the variations. The actuator means are also controlled by a secondary function that operates independently to slowly return the pressure value of the actuator means toward its original pre-set value as corrections are applied in response to measured variations. These two control functions are added together to form a composite change to the roll pressure actuactor means which controls the roll pressure and prevents it from exceeding its original pre-set value by an amount that would disturb the strip "shape." The variations of the unrolled sheet thickness are measured accurately even though the pass angle seen by the entry gauge is constantly varying.
Description (OCR text may contain errors)
States Patent lligham et a1.
1 Mar. 11, 1975 Primary Examiner-Milton S. Mehr Attorney, Agent, or Firm-Owen, Wickersham & Erickson  ABSTRACT A control method and apparatus for maintaining a uniform predetermined thickness of rolled sheet metal in a metal rolling mill where variations in sheet thickness are measured prior to its entry into the mill rolls. As a basic function, signals corresponding to the measured variations are fed forward to control pressure actuator means for the rolls, to provide corrections that eliminate the variations. The actuator means are also controlled by a secondary function that operates independently to slowly return the pressure value of the actuator means toward its original pre-set value as corrections are applied in response to measured variations. These two control functions are added together to form a composite change to the roll pressure actuactor means which controls the roll pressure and prevents it from exceeding its original pre-set value by an amount that would disturb the strip shape." The variations of the unrolled sheet thickness are measured accurately even though the pass angle seen by the entry gauge is constantly varying.
12 Claims, 3 Drawing Figures CONVERTER PAYOF F TENSiO l l I I I I MAIN DRIVE MOTOR PAYOFF MOTOR SPEED MEASUREMENT REWIND I MOTOR I DIRECTION OF STRIP TRAVEL PAIEIITEIII I I 11% 3 869 as 2 sum 1 o 2 OPERATOR OPERATORS DESIRED STATION PRESSURE E 8 40 2 Q J 3 FEED FORWARD CONTROL SECTION 60 5 I 62 64 66 68 7O MILL z SPEED PAss- ROLL GAP AUTO R LINE TARGET LIMIT RAMP OUTPUT ENTRY DELAY ANGLE CHANGE '"CHKG. RETURN FUNQ FIUNC. CORREC'I'. coMP. FUNC 12 I FEEDBACK CONTROL I 46 I I 44 I L ROLL GAP DIGITAL To ANALOG To CONTROLLER c a f 2 g; R I DIGITAL CONVERTER z 38 Q I I 2 30 2 2o 1; n. I 2 22 '2 34 2s 28 REWIND MAIN DRIVE PAYOFF MOTOR SPEED MOTOR MOTOR MEASUREMENT J FIG.. 1
DIRECTION OF STRIP TRAVEL PAI'ENTEDHARI 1 W5 3,869,892
sum 2 g g THICKNESS I I IE 22 I I2 I Y (a) MEAS. AT ENTRY GAUGE 74 THICKNESS AT ROLL ORT I DELAY BITE IF NO CORR. \76
FROM FEED FWD.
78 AUTOMATlC RAMP OUTPUT or FEED FW RETURN (c) TO ROLL GAP CONTROLLER I I PRESS. TIME CONT.
(d) ROLL PRESSURE A 79 I THICKNESS LEAVING STEADY STATE ROLL am: I ERRoR DUE TO RAMPI I l T a a n l THICKNESS MEAS. I I \I AT EXIT GAUGE FIG 2 I so (RADIATION SOURCE) |5-u-I I-52(DETECTORI 3 FEED FORWARD GAUGE CONTROL SYSTEM FOR A ROLLING MILL In the rolling of sheet-metal to a predetermined thickness the major reduction in thickness is accomplished by the mill rolls which are forced together by some controllable pressure means. Variations in thickness of the sheet metal stock to be rolled must be accounted for during the rolling process in order to obtain a rolled end product of uniform thickness. Measurement of such variations prior to entry of sheet material into the pressure rolls is essential so that feed forward corrections can be made.
One problem heretofore encountered by certain rolling mill control systems related to maintaining the proper cross-sectional shape and thickness of the sheet material. In such mills the rolls are tapered so that the proper cross-sectional shape of material is maintained only when the pressure of the rolls is held within certain limits. Where thickness variations in the supply material required increased roll pressure to control the output sheet thickness, a distortion of the rolls sometimes occurred where the pressure required was excessive. This caused the rolls to bend, thereby resulting in a cross-sectional variation in sheet thickness. Yet, if the pressure of the rolls was excessively diminished, a distortion in the cross-sectional shape in the opposite direction occurred. The present invention solved this problem.
Another problem with prior feed forward control systems was that they did not provide any correction for a varying passline angle through the input gauge. In most rolling mills the material removed from supply rolls passes through a varying passline angle with the horizontal before it enters the rolls and does not pass through either a constant angle or a horizontal path for any appreciable distance before entering the pressure rolls. Thus, in such mills it is impossible or impractical to locate a gauge where the material passline is not varying as the supply roll diameter constantly decreases. The present invention eliminates this problem so that the gauge reading used for the feed forward control is essentially not affected by a varying passline angle.
OBJECTS AND SUMMARY OF THE INVENTION It is, therefore, a general object of the present invention to provide an improved method for controlling the gauge of metal material being rolled to a predetermined thickness from rolled stock of a greater thickness and having random thickness variations.
Another more specific object of the invention is to provide a method as above which measures the sheet thickness of material prior to its entry between pressure rolls even where a varying passline is seen by the entry gauge.
Yet another object of the present invention is to provide a method for controlling the uniform thickness of metal being rolled in a rolling mill wherein the measured entry thickness is utilized to control the pressure of the rolls so as to provide the desired material thick ness despite thickness variations, and more specifically wherein when sudden variations in incoming thickness occur, the roll pressure is abruptly increased when the variation reaches the roll bite to rapidly eliminate the variation in thickness and immediately thereafter the pressure is automatically returned at a relatively slow rate toward the operator's set point, thereby generating a small error in thickness at the output gauge of the mill to cause the feedback controller to compensate by changing payoff tension and/or mill speed.
The apparatus for accomplishing the aforesaid method steps generally comprises a fixed noncontacting radiation gauge of the X-ray or nuclear isotope kind that is mounted near the payoff reel. The strip material being rolled has an angular, varying passline through the entry gauge which generates measurement signals as a result of absorption of nuclear particles in the strip. The measurement signals are generated intermittently at a constant rate (e.g. 20 times per second), and are converted into a measure of the variations in strip thickness.
These signals from the entry gauge are supplied to a high speed feed forward section of a computer system having a series of function blocks with inputs controllable from an operators station having video monitoring facilities. The computer system provides output control signals to a pressure controller which is connected to a pressure actuating system on the mill rolls and it also provides signals to the speed and tension control means on the mills.
The angle of the strip passing through the X-ray gauge at entry affects both the absolute thickness seen by the gauge and also the difference between one reading and the next. The differential of the gauge reading is calculated assuming nominal entry gauge thickness and this value is all that is required for feedforward. The differential entry gauge reading is delayed by a time delay, representing the transport delay of the mill less the time constant of the pressure controller. Delayed changes in entry gauge are used to modify the set point of the pressure controller. The transport delay is modified with mill speed. The gain is modified by strip width and thickness, and also by the alloy being rolled.
Further objects, advantages and features of the present invention will become apparent from the following detailed description taken in conjunction with the accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic and block diagram of a feed forward control system embodying the principles of the present invention;
FIG. 2 is a diagram of my feed forward system showing the action of various parameters during typical operation;
FIG. 3 is a diagrammatic view of the entry gauge for providing measurement signals of strip material with a varying angular passline.
DETAILED DESCRIPTION OF EMBODIMENT General With reference to the drawing, FIG. 1 shows in block diagram form a control system for a metal cold-rolling mill 10 which in broad terms comprises a payoff reel 12 of the metal strip material 14, such as aluminum or copper which is to be rolled to a smaller uniform thickness. The strip is threaded through an entry guage l6 and thence between the bite of a pair of work rolls l8 and 20 and an exit gauge 22 to a rewind reel 24. The latter has a suitable power drive 26 and means to increase tension of the strip between the rewind reel and the work rolls. The payoff reel 12 has a similar drive 28 including means to increase tension between it and the work rolls. The strip engaging work rolls 18 and 20 are pressed together by a pair of back-up rolls 30 and 32 in the conventional manner. The back-up rolls are supported in chocks (not shown) and they are mechanically controllable to vary the gap between the work rolls. One of the work rolls 20 is mechanically connected to a main drive motor 34 having a connected speed measurement device 36 that provides an output equivalent to actual mill speed. A system is provided for moving one work roll toward the other and against the metal strip 14 with the particular pressure necessary to maintain the preselected roller gap. This system, though not shown in detail, is well known to those skilled in the metal rolling art and typically includes a hydraulically powered actuator (not shown) with a preload cylinder 38.
The mill is connected to a feed forward control section 40 of a computer via a computer/mill interface including an analog to digital converter 42 connected to the entry gauge 16. This interface also includes a roll gap controller 44 that controls the actual pressure which the rolls apply to the strip material and hence the thickness of the metal coming out of the roll bite. This controller receives input control signals from a computer system through a digital to analog converter 46 which is part of the computer/mill interface.
As shown in FIG. 1, the computer feed forward control section 40 receives input data from the entry gauge 16 located near the payoff reel 12. This section comprises a series of calculating or functional elements of the computer which process data received from the computer/mill interface and an operators station 48. These functional computer elements compute a new target change for the roll gap controller 44 via the computer/mill interface.
A more'detailed description of the elements at the operators station 48 as well as those comprising the feedforward control section 40 will now be presented.
The Entry Gauge 16 An important aspect of the present feed forward system is that thickness measurements of the strip metal 14 are made as it leaves the payoff reel 12 and are fed forward through the computer system 40 to control small, short term variations of pressure on the mill work rolls l8 and by the roll group control 44. The thickness measurements between the payoff reel 12 and the work rolls are made by the gauge 16 which may, of necessity, be located closely adjacent to the payoff reel where the metal strip being measured passes through the gauge at a varying passline angle. The signal from the entry gauge is read at a high frequency synchronized to the execution rate of the functions in the section 40 of FIG. 1, which is typically 20 times per second.
The gauge itself is preferably of the non-contacting radiation type utilizing either an X-ray or nuclear isotope radiation source. One form of such a gauge is shown in US. Pat. No. 3,757,122.
As shown in FIG. 3, a correction to the thickness measurement for the pass angle of metal through the entry gauge 16 is accomplished by combining the actual measurement of the gauge with an assumption that the actual thickness of the strip is near to target thickness, in the following manner:
For a radiation measuring device such as shown in FIG. 3, the equation relating thickness of material between a radiation source 50 and a detector 52 to the radiation received by the detector is generally known to those familiar with the art as the gauge equation and is expressed as follows:
where T= measured thickness I R measured ratio between the voltage provided by the detector with material present, and the voltage with only air present, subsequently referred to as the ratio.
,u. is a constant depending on the absorption characteristics of the material being measured.
Referring to FIG. 3, which shows material at an angle other than to the source detector time:
T thickness of material which the radiation must penetrate.
T actual thickness of material. By considering a small change in actual thickness A T,,, which results in a small change A T in thickness for the radiation to penetrate, the gauge equation (l) can be differentiated to produce the relationship.
A TM /I M) A RM Where R is the measured ratio for thickness T and A R is a small change in that measured ratio. From basic geometry (FIG. 3)
TA/TM A TA/A TM Combining equations (2) and (3) by basic algebra A TA TA/TM /I M) A RM But, from the basic gauge equation (I),
TA/TM low A/loge RM Now, combining equations (4) and (5):
A T l/uR (log R /log R A R Where R is the ratio R as previously defined if the metal is presented to the gauge perpendicular to the radiation beam; and
R is the ratio R actually measured with the strip at some angle to the radiation beam.
This equation relates a change in actual thickness of the material to a change in measured ratio taken at an angle other than 90 to the gauge.
It it is assumed that R,, is approximately equal to the ratio corresponding to the nominal thickness of metal entering the mill, which may be entered by the operator, then the magnitude of thickness changes of the metal strip entering the mill can be obtained by measuring the ratio R Thus, in the method of the present invention, the input gauge measures the actual thickness (T to obtain the ratio R even though the angle of the strip through the gauge (the passline angle) may The operators station48 normally located within an enclosure near the mill includes suitable apparatus comprising:
a. means for selecting feedforward control on or off;
b. means for entering the nominal entry thickness of the metal being rolled into the mill into the computer system;
c. means for entering the roll pressure target, which the operator desires for good strip shape, into the computer system; and
d. a means for displaying all selected inputs for checking purposes.
These functions are preferably implemented with a video display and keyboard entry station. Since all of the aforesaid apparatus and arrangement of same is well known to those skilled in the art, it is not shown in detail.
The Computer Feed Forward Control Section 40 The feed forward control section 40 of the computer system comprises: a. Transport Delay Function 60.
The instant speed of the mill, measured via the speed measurement device 36 and furnished to the analog to digital converter 42 is supplied to the transport delay function block 60.
Using this speed, the block 60 computes the transport time of the strip from the measurement point to the roll bite. This may be expressed by the following equation:
where d is the distance from the measurement point 16 to the roll bite 18 and v is the strip velocity. In order to make a compensatory adjustment to the roll gap to eliminate the gauge disturbance it takes a finite time for the roll gap controller 38 to move. This time is referred to as the roll gap time constant t To allow for this, the time delay r calculated by block 60 is reduced by this amount and using equation (7) the computation in block 60 is r (d/v) r The voltage received from the entry gauge for the initial correction is delayed by the amount t computed by this transport delay function by setting up a shift register in the computer software in the conventional manner. This shift register is shifted at a fixed execution frequency which is the same as that of the other functions of section 40 in FIG. I, typically 20 times per second. Various values of the 1. are represented by taking a value out of the shift register at a point down the register proportional to r For example, if 2,, 735 milliseconds and the execution time is once every 50 milliseconds the time delay is represented by a value N items down the shift register where The fractional part of N is handled by linear interpolation of the 14th and 15th values in the shift register in the conventional manner. b. Pass Angle Correction Function 62.
The nominal entry thickness is entered into a pass angle function block 62 of the computer by the operator at the operators station 48. Here, this input data is used in conjunction with calibration constant held within the computer to convert the delayed change in entry gauge measurement from the block 62 into a thickness change as described in equation (6) in the previous section on the entry gauge.
For example, ifA R 0.01, R,, 0.6, R 0.5 and p. 0.4 then A TA 1/0.4 0.5 lo 0.6/10 0.5 0.01
0.0368 which is the amount of thickness change that will be entering the roll gap, the units being thousandths of an inch if the units of ,u. are (thousandths of an inch) c. Roll Gap Target Change Computation 64.
From the thickness change input, provided by the pass angle function block 62, a change in roll gap is computed in order to eliminate this thickness change. This is a straightforward computation using previously determined values and calculations relating thickness out of the mill to roll gap target, strip width, strip hardness, and strip thickness.
d. Limit Checking 66.
For operational safety of the mill and to maintain strip quality, it is important that the roll pressure does not depart too far from the value P originally set in by the operator. An excessive change in pressure will cause work roll bending and will alter the shape of the strip. On the other hand the feedforward control section needs to make some pressure change in order to effect a gauge connection. Block 66 therefore checks the absolute value of the difference between the demanded roll gap pressure P and the operators value P and prevents this value from exceeding a preset limit P by restricting the value of P.
Expressed as equations this process is as follows: At
control period n, block 64 generates a required-change- P ix-1 A Pu The value of P" is then limit checked as follows:
9f 1 P'" P l P then P" P'" then P" P P if P" P The value of P is different for different metal thicknesses and' alloys and so the computer selects the ap propriate value from a table based on the actual allo and thickness being rolled. 6. Automatic Ramp Return Function 68.
Up to this point the feedforward control function has calculated a change to the roll gap controller target to completely eliminate changes in entry gauge thickness as the metal is rolled. Now. an automatic ramp return function block 68 compares the roll pressure P desired by the operator (for reasons of good strip shape) with the computed roll gap pressure P" and superimposes upon this computed pressure a change which causes the roll pressure to slowly return or ramp back to the operators desired roll pressure.
This would result in a slowly increasing error in exit gauge of the strip since we are slowly removing the corrective pressure change requested by the feedforward control section 40. However, this gauge error is seen by a standard feedback controller 72 conventionally present in rolling mills, which corrects for the'error by adjusting other mill variables such as speed or tension. The effect of the auto ramp return function 68 is to slowly transfer any long term changes in roll pressure required by changing incoming strip thickness to other variables that also control thickness such as speed or tension. Because the feedback controller 72 does not have an instantaneous response, the effect of the slow ramp return 78 of pressure by block 68, is to cause a small steady state gauge error 79 during the ramp return (See FIG. 2). The ramp is slow enough that this error 79 is negligibly small. To completely eliminate this error it would be possible to send a signal directly from block 68 to the payoff tension or speed used by the feedback controller 72. However, in practice, the size of this gauge error does not require this additional complexity. 1 f. Output Function 70 The output function takes the resultant composite change in roll gap target and outputs it through the digital to analog converter 40 to the roll gap controller 44.
The computation described and embodied by the elements of section 40 of FIG. 1 are typically made 20 times per second. This frequency may vary and is chosen so that a reasonable number of executions are made in the time taken for the strip to travel from the entry gauge to the roll bite.
The aforesaid thickness error caused by the ramping back action is measured by the exit gauge 22 which is connected to the feedback controller 72. This controller receives an input from the analog to digital converter 42 equivalent to actual mill speed, and it supplies feedback outputs to the main drive motor 34 and to the pay-off drive motor 28 which also controls tension of the strip material. Thus, control action taken by the feedback controller 72 substantially removes the thickness error introduced by block 68 described above by changing other mill control values which affect thickness (e.g., pay-off, tension and mill speed control).
The overall combined effect of the feedforward and feedback control systems is illustrated graphically in FIG. 2.
Assume that a sudden change in incoming thickness occurs at the entry gauge 16, as indicated at 74 on the upper trace (a) of the diagram. When this change reaches the roll bite the pressure of the work rolls 18 and 20 is increased abruptly as indicated at 76 of next lower trace (b) to eliminate the change in thickness. Simultaneously, the automatic ramp return function is automatically set up to commence the steady return of roll pressure toward the operators setpoint, as indicated by the ramp 78 of the trace (c). The actual roll pressure change is shown in trace (d) of FIG. 2. This ramping along line 78 generates a small error in thickness out of the mill which causes the feedback controller 72 to compensate in its normal manner by changing the pay-off tension at the control 28 and/or mill speed by the main drive motor 34. Thus, in accordance with the present invention, the initial control action is taken by first changing the roll pressure which is capable of changing at a fast rate. Then, this initial change of roll pressure is slowly transferred to the pay-off tension and mill speed. The resulting variation in strip thickness are shown at the roll bite and at the exit gauge in traces (c) and (f) of FIG. 2. The unique combination of feed forward and feedback control serves to keep the roll pressure of a mill within limits consistent with good strip shape while maintaining a high degree of accuracy on the rolled output thickness of the metal strip material.
To those skilled in the art to which this invention re lates, many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the spirit and scope of the invention. The disclosures and the description herein are purely illustrative and are not intended to be in any sense limiting.
1. A method for controlling the gauge without changing the shape of strip material issuing from a rolling mill that includes a payoff reel, a pair of pressure rolls to produce a desired roll force, pressure control means for varying the pressure of said rolls on the strip material and the gap between them, a takeup reel, an input gauge between said payoff reel and said rolls, tension control means for the strip material, an output gauge and feedback means connected to said output gauge, said method comprising the steps of:
generating a signal at the input gauge for determining a variation in the gauge of material from the payoff reel from a nominal thickness; producing a primary control function causing said pressure control means to operate in response to the variation in gauge detected by said input gauge when the actual gauge variation reaches said pressure rolls, said primary control function operating on said pressure rolls to change their force on the strip at a relatively rapid rate to substantially eliminate the thickness variation;
superimposing a secondary function on said pressure control means immediately after the activation of said primary function, said secondary function causing the changed force of the pressure rolls'on the strip material by said primary control function to commence returning at a relatively slow steady rate toward the original preset level and thereby causing a relatively small variation in target gauge of the material passing from said rolls;
generating an error signal proportional to said small variation at the exit gauge; and
utilizing said error signal to activate a mill control means for eliminating said small variation in gauge.
2. The method as described in claim 1 wherein said error signal is utilized to operate the tension control means to counteract said small variation in gauge.
3. The method as described in claim 1 wherein said error signal is utilized to vary the speed of strip material in the mill to counteract said small variation in gauge.
4. The method as described in claim 1 wherein the signal is generated at the input gauge by measuring the actual thickness of the strip as it passes at a varying angle between the radiation source and detector elements of the entry gauge and using said signal from the input gauge in conjunction with the assumed nominal thickness value of the strip material to compute the actual variation in incoming strip thickness.
5. The method as described in claim 4 wherein said actual variation in incoming strip thickness is determined by the equation T4 (log R /klg R A R where: u a constant depending on the absorption characteristics of the material being measured; R is the ratio between the voltage provided by the detector with material present and the voltage with only air present when the metal is perpendicular to the detectors radiation beam; R is the ratio between the voltage provided by the detector with material present and the voltage with only air present which is actually measured with the strip at an angle to the radiation beam; A R is the current R minus the previous R 6. In a rolling mill including a payoff reel for a strip of sheet material connected to a takeup reel, a pair of pressure rolls for applying a forming force on said strip material and a pressure control means for controlling said pressure rolls, an entry gauge between said payoff reel and said rolls and an exit gauge between said rolls and said takeup reel, a feed forward thickness control system for said mill comprising:
means for utilizing signals from said entry gauge and comparing them with an input proportional to the assumed nominal thickness of the strip material from said payoff reel to produce signals proportional to variations in thickness of the strip material; primary control means for utilizing said signals proportional to said strip thickness variations for operating said pressure rolls control means to cause a relatively rapid increase in pressure by said rolls;
means responsive to activation of said pressure control means for commencing a steady reduction of roll pressure; feedback control means responsive to signals generated by said exit gauge compared with a preselected target gauge for producing an error signal;
and means responsive to said error signals for activating another gauge control means on the mill. 7. The method as described in claim 6 wherein said primary control function is produced by:
computing the transport time of a gauge variation from the measurement point to the roll bite and subtracting from this time a roll gap time constant;
providing a pass angle correction to the nominal entry thickness and using it in conjunction with a calibration constant to convert the delayed change in entry gauge measurement into a thickness change;
computing a change in the roll gap based on the previously computed thickness change modified by characteristics of the strip material; and
checking the computed roll pressure for compliance with operational limits.
8. An apparatus for controlling the gauge and shape of strip material issuing from a rolling mill that includes a payoff reel, a pair of pressure rolls adjustable to a preselected gap to produce a desired target thickness, pressure control means for varying the pressure of said rolls on the strip material and the gap between them, a takeup reel, an input gauge between said payoff reel and said rolls, tension control means for the strip material, an output gauge and feedback means connected to said output gauge, said apparatus comprising:
means for generating a signal at the input gauge for determining a variation in the gauge of material from the payoff reel from a nominal thickness;
means responsive to said signal from said input gauge for producing a primary control function causing said pressure control means to operate when the actual gauge variation reaches said pressure rolls to change their force on the strip at a relatively rapid rate to substantially eliminate the thickness variation; means for superimposing a secondary function on said pressure control means immediately after the activation of said primary function, said secondary function causing the changed force of the pressure rolls on the strip material by said primary control function to commence returning at a relatively slow steady rate toward the original preset level and thereby causing a relatively small variation in target gauge of the material passing from said rolls;
means for generating an error signal proportional to said small variation at the exit gauge; and
means for utilizing said error signal to activate a mill control means for eliminating said small variation in gauge.
9. The apparatus as described in claim 8 including means for utilizing said error signal to operate the tension control means to counteract said small variation in gauge.
10. The apparatus as described in claim 8 including means for utilizing said error signal to vary the speed of strip material in the mill to counteract said small variation in gauge.
11. The apparatus as described in claim 8 wherein said means for generating a signal at the input gauge includes means for measuring the actual thickness of the strip as it passes at a varying angle between the radiation source and detector elements of the entry gauge and means for using said signal from the input gauge in conjunction with the assumed nominal thickness value of the strip material to compute the actual variation in incoming strip thickness.
12. The apparatus as described in claim 8 wherein said means for superimposing a secondary function comprises an automatic ramp return means for generating a signal which moves the pressure control means at a preselected constant rate towards an original preset level until that preset level has been reached.