US3749331A

US3749331A - Tension reference signal generation means for reel drives

Info

Publication number: US3749331A
Application number: US00203286A
Authority: US
Inventors: R Nedreski
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1971-11-30
Filing date: 1971-11-30
Publication date: 1973-07-31
Anticipated expiration: 1990-07-31
Also published as: IT971323B; JPS4863160A

Abstract

Static circuit means is provided for generating a tension reference signal in response to an input signal proportional to the diameter of strip material on a reel, the tension reference signal having a magnitude defined by T A + BD where T is the magnitude of the reference signal, D is the magnitude of the diameter signal, and A and B are constants having an interrelationhip defined by B R - RA/10 where R is the buildup ratio of the reel, the build-up ratio being the ratio of the maximum (full) reel diameter to the minimum (empty) reel diameter.

Description

United States Patent 1 91 Nedreski 111 3,749,331 1 July 31, 1973 TENSION REFERENCE SIGNAL GENERATION MEANS FOR REEL DRIVES [52] US. Cl 242/75.51, 242/7547, 318/6 [51] Int. Cl... B65h 25/04, 1365b 23/20, B6511 25/22 [58] Field of Search 242/755, 75.51,

[5 6] References Cited 6/1965 Haley ..242/7s.s 3/1966 Barrett 242/7551 Primary Examiner-George F. Mautz Assistant Examiner-Edward J. McCarthy Attorney-James C. Davis, Jr. et a1.

[57] ABSTRACT Static circuit means is provided for generating a tension reference signal in response to an input signal proportional to the diameter of strip material on a reel, the tension reference signal having a magnitude defined by T= A BD where T is the magnitude of the reference signal, D is the magnitude of the diameter signal, and

A and B are constants having an interrelationhip de- UNITED STATES PATENTS fined by B R RA/lO where R is the buildup ratio 2,462,233 j 2/1949 Stultz 242/7551 of the reel, the build-up ratio being the ratio of the 1939 y- 217 5 maximum (full) reel diameter to the minimum (empty) 2,874,917 reel diameter 2,043,208 6/1936 Bohn.... 242/7551 2,765,989 10/1956 Pell.-..... 242/7551 12 Claims, 2 Drawing Figures POWER I 40 CONVERSION POWER i l l CONVERSION AL. LC. 0 l L PATENIED M31 CONVERSION POWER CONVERSION REF-l l N VIENTOR. ROBERT J. NEDRESKI FIG. 2

Hts ATTORNEY BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to tension control in strip material handling systems in which strip material is wound upon a reel driven by a variable speed motor and, more particularly, to improved static means for generating a tensionreference signal related in a predetermined linear manner to the build-up ratio of the reel and variations in reel diameter.

2. Description of the Prior Art In the process of certain sheet material, such as sheet steel, the material is commonly handled and worked in strip form. It is often desirable that the material be coiled on a reel driven by a variable speed motor, typically a d-c motor. For uniformity of quality of the strip material, it is essential that the tension under which the strip material is wound onto and off of the reel be precisely controlled. It has heretofore been widely recognized in the field that it; is desirable that the tension under which the strip material is wound onto or off of the reel be varied as-a function of reel'diameter. More specifically, it has been recognized for reasons such as those pointed out in US. Pat. No. 3,241,785, granted on Mar. 22, 1966, to E. F. Barret, that the tension is preferably reduced with increasing diameter of the strip material on the reel. This can be conveniently accomplished .in practice by generating a reference signal as a function of reel diameter and usingthe reference signal as .the primary armature current control input to the motor driving the reel. Heretofore, reference signals which vary as a function of reel diameter have been generated by various mechanical and electro mechanical devices.

In accordance with the present invention, it has been I determined that optimum strip tension is attained when the reference signal is a linear function of the reel diameter. More specifically, it has been established that optimum strip' tension is attained when the magnitud Tot the reference signal is defined by: v I

where!) has a-magnitude proportional to the reel diameter andv Ajand B are constants having an .interrelationshipdefined'by;

' j B R a 11 where-R is the build-up ratio of the reel. The build-up v 'A still further object is to provide the foregoing objects by static means of relatively simple and uncomplicated form.

Briefly stated, in carrying out the invention in one form, an improved reference signal generating means is provided for a strip material handling system including a reel upon which strip material is wound, a variable speed motor, and armature current control means responsive to control signals including the reference signal. The reference signal generating means includes static circuit means for producing a signal related in a predetermined linear manner to the build-up ratio of the reel and to variations in the reel diameter. The signal generating means of this invention also includes means for'generating a diameter signal proportional to the diameter of the strip material coiled on the reel. The static circuit means is responsive to the diameter signal to generate the reference signal defined by:

where T is the magnitude of the reference signal, D is the magnitude of the diameter signal, and A and B are constants having an interrelationship defined by:

where R is the build-up ratio of the reel. By a further aspect of the invention, the static circuit means is effective to maintain the constant A equal to or greater than By a still further aspect of the invention, the static circuit means includes amplifier means coupled to first and second sources of potential for generating in response thereto a signal proportional to the constant B, constant A being proportional to the voltage of the second source. A static multiplier is coupled to the diameter signal generation means and the amplifier for producing a signal proportional to the product BD, and a summing circuit is coupled to both the multiplier and the second source for producing the reference signal.

BRIEF DESCRIPTION OF THE DRAWINGS While the novel features of-this invention are set forth with particularity in the appended claims, the intailed description taken in conjunction with the drawratio'is the ratio of the maximum '(full) reel diameter v to'the minimum (empty) reel diameter. Furthermore,

to assure that tension does not increase with increasing reel diameter, the constant A must be equal to or greater than 10.

SUMMARY OF THE INVENTION It is therefore an object of this invention to provide means for generating a reference signal which varies in accordance with the foregoing equations.

Another object of this invention is to provide static means for generating an armature current reference signal for use in reel drives for strip material handling systems.

Yet another object is to provide the foregoing objects by static means suitable for use in a wide range of reel drive applications.

ings, in which: I

FIG. 1 is a schematic view of a strip material handling system having a reel drive in which tension is precisely regulated in accordance ,with this invention; and

FIG. 2 is a circuit diagram of the static reference signal generation apparatus of thisinvention.

DETAILED DESCRIPTION ratus and a speed feedback signal from a'tachometer 20 driven by the motor 16. As is well known in the art, the

control apparatus 18 operates in response to the reference signal REF-l and the feedback signal from the tachometer 20 to control the rate at which electric power from an 21-0 source is transformed to d-c power by power conversion apparatus 22 and supplied to the armature and field windings 24 of the motor 16.

The strip of sheet material is, depending upon the direction in which the strip 10 is driven by the

rolls

12 and 14, either wound onto or off of a reel 30, which is driven by a variable speed d-c'motor 32. It is desirable that the reel 30 be driven by the motor 32 such that the tension in the strip 10 is precisely determined by the magnitude of a reference signal REF-2 supplied to control apparatus 34. In many strip handling applications, the reference signal REF-2 is preferably reduced as the strip material builds up on the reel so that the actual tension in the strip is reduced with increasing reel diameter. As pointed out above, the present invention is concerned with static circuit means for generating the reference signal REF-2 as a precise linear function of reel diameter and build-up ratio. Before turning attention to the static circuitry of thisinvention, it will be well to review briefly the manner by which actual strip tension is regulated in accordance with the references signal REF-2.

Let it initially be assumed that the motor 32 is being operated in the constant horsepower range under substantially steady state conditions (the line speed of the strip 10 is substantially constant). Let it also be assumed that windage and friction losses are low relative to the torque providing the strip tension. Under these conditions, the armature current supplied to the motor 32 is directly proportional to motor torque and strip tension. By utilizing a current feedback signal derived from the voltage drop across a resistor 36 in the armature circuit, the armature current and therefore the tension of the strip 10 can be regulated by the reference signal REF-2. When the actual current, and therefore strip tension, is equal to that called for by the reference signal REF-2, the feedback signal on line 38 to the control apparatus 34 will be equal to the reference signal REF-2, and the power conversion apparatus 40 will continue to transfer electric power from an we source to the armature and field 42 of the motor 32 at a rate sufficient to maintain the scheduled tension. If, however, the actual tension should increase, the negative feedback signal on line 38 will similarly increase, and the control apparatus 34 will respond by reducing the armature current supplied by the power conversion apparatus 40 so as to thereby reduce the actual tension. Similarly, if the actual tension should decrease, the negative feedback signal will decrease and the control apparatus 34 will respond by increasing the armature current so as to increase the actual tension to that calledfor by the reference signal REF-2.

During periods of strip acceleration or deceleration, to assure that the strip tension is regulated in accordance with reference signal REF-2, an inertia compensation signal LC. is also supplied to the control apparatus 34. The magnitude of the signal LC. is such that the motor 32 supplies additional torque (and armature current) during periods of acceleration and less torque (and armature current) during periods of deceleration. Suitablemeans for generating the inertia compensation signal is disclosed and claimed by copending US. Pat. application Ser. No. 203,285, entitled Static Means for Generating Inertia Compensation Signals in Reel Drives, and filed concurrently herewith in the name of R. .I. Nedreski on Nov. 30, I971, and assigned to the assignee of this invention. The inertia compensation signal generating means of that invention includes solid state multiplying and dividing devices which operate on reel diameter and strip acceleration signals to produce the inertia compensation signal. The REF-2 signal, the negative current feedback signal on line 38, and the inertia compensation signal I.C. provide a net signal to the control apparatus 34 having a magnitude sufficient to accurately maintain the tension called for by the reference signal REF-2 during periods of strip acceleration and deceleration.

The novel apparatus of this invention for generating the tension reference signal REF-2 will now be described. The signal generation apparatus includes means for generating a signal proportional to the actual reel diameter. One form of such apparatus is illustrated by FIG. 1. This apparatus includes a lever 56 having a roller 58 on one end thereof, the roller 58 riding on the outer surface of the coil of strip material on the reel 30 so that the position of the lever 56 provides a mechanical indication of the diameter of the strip material on the reel 30. Suitable electrical means, such as potentiometer, is provided within block 60 to convert the position of the lever 56 into an'electrical signal directly proportional to the diameter of the coil of strip material. The electrical signal generated by the electrical apparatus of block 60 is supplied over line 62 to signal generating network or circuit 64. The exact nature of the apparatus utilized to generate the diameter signal is, of course, not significant with respect to this invention since the signal can be generated in many ways. For example, the electromechanical approach just described could be replaced by an optical arrangement in which the diameter of the coil is sensed optically by photoelectric cells or the like. Other approaches for generating a diameter signal directly proportional to the coil diameter will readily occur to those skilled in the art.

Referring now to FIG. 2, the novel and completely static signal generating circuit or network 64 will bedescribed. The circuit 64 includes, as shown, a solid state multiplying element 68 which receives at one multiplication input 70 the diameter signal on line 62. Solid state multiplying elements such as element 68 are wellknown to those skilled in the art and are commercially available. In one embodiment of the present invention, the element 68 was obtained from Motorola Semiconductor Products and was identified by the manufacturer as Model MCl595L.

As pointed out previously, the function of the circuit 64 is to receive the diameter signal on lines 62 and to generate in response thereto a tension reference signal REF-2 having a magnitude defi ned by;-

where T is the magnitude of the tension reference signal REF-2, D is the magnitude of the diameter signal, and A and B are constants having an interrelationship defined by:

where R is the build-up ratio. The elements comprising the circuit 64 are selected and arranged so as to maintain the constant A at a magnitude equal to or greater than 10 for reasons which will become apparent as this description proceeds.

The circuit 64 will now be described in detail with reference to FIG. 2. in addition to the static multiplying element, 68, to which the diameter signal is supplied at input 70, the network 64 includes a pair of

operational amplifiers

72 and 74, a pair of potentiometers 76 and 78, a rheostat 80, and

resistors

82, 84, 86, 88, 89, 90 and 91. As illustrated, resistor 82 is connected between a source 92 of positive d-c potential and the input 94 of the amplifier 72, and resistor 84 is connected be tween the input 94 and the output 96 of the amplifier 72. The rheostat 80, the potentiometer 76, and the resistor 86 are connected in series between a source 100 of negative d-c potential and a common or neutral point 102. Wiper arm and point 104 of the potentiometer 76 is connected through the resistor 88 to the input 94 of amplifier 72 and through the resistor 89 to the input 106 of amplifier 74. The output 96 of the amplifier 72 is connected to a multiplying input 108 of the multiplying element 68, and the product output 110 of the multiplying element 68 is connected throughvthe resistor 90 to the input 106 of the amplifier 74. The amplifier 74 is paralleled by resistor 91 connected between its input 106 .and its output 112. The output 112 of the amplifier 74 is connected by the potentiometer 78 to common or neutral point 102, with the wiper arm and point 114 of the potentiometer78 connected to line 116 over which the tension reference signal REF-2 is supplied'to the control apparatus 34.

The rheostat 80 and the potentiometer 76 are initially adjusted to produce at point 104 a fixed negative voltage proportional to the magnitude of the constant A. The resistor 86 is selected such that the voltage at 104 cannot fall below a predetermined level corresponding to the minimum acceptable value, 10, of constant A in the above equations. The rheostat 80 is adjusted, preferably during the initial drive system installation, to limit the maximum value of the constant A, and the potentiometer 76 is adjusted by the operator to establish the actual value of the constant A within the range established by the rheostat' 80 and the resistor 86. Oncethe potentiometer 76 has been adjusted, the voltage at 104 and the value of the constant A in the above equations are fixed. The negative voltage at 104 is operated on by amplifier 74 and its

gain setting resistors

89 and 91 to produce a positive signal at the output 1 12 of the amplifier 74 proportional to the constant A. The voltage at 104 is simultaneously operated on by amplifier 72 and its gain-setting

resistors

84 and 88 to produce a positive signal at output 96proportional to RA/lO. Since thevoltage at 104 is'proportional to the constant-A,

resistors

84 and 88 are selected to provide a gain proportional to one-tenth of the build-rip ratio of the driven reel. if the resistance of resistor 88 is adjustable as illustrated, the network 64 may be used for reel drives having different build-up ratios. Alternatively, a fixed value resistor 88 may be selected for a particular reel drive having a fixed build-up ratio.

In a similar manner, resistor 82 is selected along with resistor 84 such that the amplifier 72 operates on the positive voltage of the source 92 to produce a negative signal at its output 96 proportional to the build-up ratio, R. It will be shown that the gain of the amplifier 72 relative to the signal supplied by the source 92 is preferably RVllDV where R is the build-up ratio, V is the voltage of the source 92,-and V' is the voltage at 104 corresponding to A equal to 10. It will be seen that a net signal proportional to -R RA/ I0 is thus generated and supplied to the input 108 of the multiplier 68. This net signal will be seen from the foregoing equations to be proportional to B. in the multiplier 68, the signal proportional to the actual reel diameter, supplied to input 70, is multiplied by the signal proportional to -B, supplied to input 108, to produce at output 110 a signal proportional to BD. Resistor 90 is selected relative to resistor 91 such that the amplifier 74 operates on the signal proportional to BD to produce at its output 112 a signal proportional to BD. Since the amplifier 74 also produces a signal proportional to the constant A, it will be seen that a signal proportional to A BD is produced at the output 112 of the amplifier 74. Once the potentiometer 78 is adjusted to provide a base output level, variations in the diameter signal supplied to the input of multiplier 68 will produce at the wiper arm 114 of the potentiometer 78 a signal which varies in accordance to the above equations, namely T= A BD'and B R RA/lO.

multiplier 68 varies between 2.5 volts when the reel is' empty and [0.0 volts when the reel is full. Let it further be assumed that the source 92 of positive potentialhas a voltage of 20 volts, that the'source 100 of negative potential has a voltage of 20 volts, and that a taper of 2.5 to 1 is desired. In other words, let it be assumed that it is desired that the strip tension at the minimum reel diameter be 2.5 times as great as the tension at the maximum reel diameter. In such a case, the elements comprising the circuit 64 may be selected and adjusted such that:

the voltage at 104 is 12.0 volts;

the gain established by

resistors

84 and 88 is 0.4;

the gain established by

resistors

84 and 82 is 0.2;

the gain established by

resistors

91 and 89 is 1.0;

the gain established by resistors 91 and is. 1.0;

the voltage at 114 is 10.0 volts when the diameter signal at input 70 is 2.5 volts; and

a REF-2 signal of 10.0 volts will produce the actual strip tension desired at the minimumreel diameter. With the elements selected and adjusted in thespecified manner, the voltage at 114 and thus the REF-2 signal will vary linearly between lOLO volts at the minimum reel diameter and 4.0 volts at the maximum reel diameter, thus providing the desired 2.5 to l taper.

If, however, it is desired to provide uniform tension in the strip material, it is merely necessary toadjust'the' potentiometer 76 to provide a voltage of 10.0 volts at 104. This voltage, which is proportional to a constant A equal to 10, will assure a constant voltage at 114 of 10 volts since the B input signal to input 108 of the multiplier 68 became and remains zero when the voltage at 104 is 10.0 volts and the gain of the amplifier 72 is maintained at the levels specified above.

In the foregoing description, it has been stated that various signal levels are proportional to certain eonstants and variables. it will, of course, be obvious to those skilled in the art that certain consistencies must be maintained if the circuit 64 is to work in the manner described. For example, it is essential that the proportionality constants be known and utilized in relating the various circuit elements. For example, if the voltage at point 104 were 1 volt and this was selected to be proportional to A 10, it would be necessary to increase the voltage at source 92 proportionally or make a corresponding change in the gain established by the.

resistors

88 and 82. Generally, assuming that the voltage of the source 92 is fixed, the relative gains established by the

resistors

88 and 84 and the

resistors

82 and 84, respectively, should be R/lO and RVllOV where V is the voltage of the source 92 and V is the voltage at 104 corresponding to A equal to 10.

Fromthe foregoing, it will be seen that this invention provides means for generating a tension reference signal for use in reel drive applications. The signal generation means of this invention includes means for generating a diameter signal proportional to the diameter of strip material on a reel and static circuitry responsive to the diameter signal to generate a reference signal with a magnitude defined by:

where T is the magnitude of the reference signal, D is themagnitude of the diameter signal, and A and B are constants havingan interrelatinship defined by:

where R is the build-up ratio of the reel. Furthermore, as taught herein, the static circuit means is effective to maintain the constant A equal to or greater than 10.

While the invention has been shown and described herein with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form, details, and application may be made therein without departing from the spirit and scope of the invention. For example, the elements comprising the signal generation circuit 64 can take on formsdiffering from those illustrated and can be combined in various ways to provide the static arithmetic operations described herein. Furthermore, although the invention has been illustrated in d-c reel drive system, it is equally applicable to a-c drive systems. Accordingly, it is intended that all such modifications and changes be included within the scope of the appended claims;

What is claimed as new and is desired to secure by Letters Patent of the United States is: 1. In strip material handling apparatus including a reel upon which'strip material may be wound and unwound, the reel having a build-up ratio R equal to the ratio-of the maximum (full) reel diameterto the minimum (empty) reel diameter, variable speed motor drive means for driving the reel, and motor control means responsive to control signals including a reference signal for varying the tension under which the strip material is wound onto the reel in accordance with variations in the reference signal, reference signal generation means comprising:

means for generating a diameter signal proportional to the diameter of the reel, and static circuit means coupled to said diameter signal generation means for receiving said diameter v signal therefrom, said static circuit means being responsive to said diameter signal to generate a reference signal having a magnitude defined by:

p a R RA/lO where R is the build-up ratio.

2. Reference signal generation means as defined by claim 1 in which said static circuit means includes means for maintaining the constant A equal to or greater than 10, whereby said reference signal remains constant or decreases with increases in the diameter'of strip material on the reel.

3. Reference signal generation-means as defined by claim 1 in which said static circuit means further comprises:

amplifying means coupled to a first source of potential and a second source ofv potential proportional v to the constant A and responsive thereto to genermeans and being responsive thereto to generatesaid reference signal.

4. Reference signal generation means as defined by claim 3 in which said amplifying means operates on signals received from said first source of potential and said second source of potential with relative gains I of RV] 10V and R/l0, respectively, where V is the voltage of said first source of potential and V is the voltage of said second source corresponding to the constant A equal to 10.

5. Reference signal generation means as defined by claim 4 in which'said amplifyingmeans includes means for adjusting the gains of said amplifying means sofas to accommodate variations in build-up ratioandwoltage of said first source of potential.

6. Reference signal generation means as defined by I claim 5 in which said static circuitineansjneltrdes means for maintaining the constant A equalatofor greater than 10, whereby said reference signal remains constant or decreases with increases in the'dianieter of I strip material on the-reel.

7. Reference signal generation asdef'lned claim 3 further comprising said secondsource'o r tial, the voltage of said second source of potential being sufficient to maintain the constant A equal toqpgreate than 10, whereby said reference signal remains cons tant or decreases with increases in the diameterof strip material or the reel. "e."

8. Reference signal generation means as defined 2 claim 7 further comprising means for adjusting they oil age of said second source of potential so as to vary the rate at which strip tension changes with respect to reel diameter.

9. Reference signal generation means as defined by claim 8 wherein said summing means provides amplification of signals received from said multiplication means and said second source of potential. I

10. Reference signal generation means as defined by claim 9 in which said amplifying means operates on sig-- nals received from said first source of potential and said second source of potential with'gains of RV! 10V and R/lO, respectively, where V is the voltage of said first 12. Reference signal generation means as defined in claim 11 in which said static circuit includes means for maintaining the constant A equal to or greater than l0, whereby said reference signal remains constant or decreases with increases in the diameter of strip material onthe reel.

Claims

1. In strip material handling apparatus including a reel upon which strip material may be wound and unwound, the reel having a build-up ratio R equal to the ratio of the maximum (full) reel diameter to the minimum (empty) reel diameter, variable speed motor drive means for driving the reel, and motor control means responsive to control signals including a reference signal for varying the tension under which the strip material is wound onto the reel in accordance with variations in the reference signal, reference signal generation means comprising: means for generating a diameter signal proportional to the diameter of the reel, and static circuit means coupled to said diameter signal generation means for receiving said diameter signal therefrom, said static circuit means being responsive to said diameter signal to generate a reference signal having a magnitude defined by: T A + BD where T is the magnitude of the reference signal, D is the magnitude of the diameter signal, and A and B are constants having an interrelationship defined by: B R - RA/10 where R is the build-up ratio.

2. Reference signal generation means as defined by claim 1 in which said static circuit means includes means for maintaining the constant A equal to or greater than 10, whereby said reference signal remains constant or decreases with increases in the diameter of strip material on the reel.

3. Reference signal generation means as defined by claim 1 in which said static circuit means further comprises: amplifying means coupled to a first source of potential and a second source of potential proportional to the constant A and responsive thereto to generate at an output thereof a signal proportional to the constant B, multiplication means having a first input coupled to receive the diameter signal and a second input coupled to receive the output of said amplifying means and being responsive thereto to generate at an output thereof a signal proportional to the product BD, and summing means coupled to said second source of potential and to said output of said multiplication means and being responsive thereto to generate said reference signal.

4. Reference signal generation means as defined by claim 3 in which said amplifying means operates on signals received from said first source of potential and said second source of potential with relative gains of RV''/10V and R/10, respectively, where V is the voltage of said first source of potential and V'' is the voltage of said second source corresponding to the constant A equal to 10.

5. Reference signal generation means as defined by claim 4 in which said amplifying means includes means for adjusting the gains of said amplifying means so as to accommodate variations in build-up ratio and voltage of said first source of potential.

6. Reference signal generation means as defined by claim 5 in which said static circuit means includes means for maintaining the constant A equal to or greater than 10, whereby said reference signal remains constant or decreases with increases in the diameter of strip material on the reel.

7. Reference signal generation means as defined by claim 3 further comprising said second source of potential, the voltage of said second source of potential being sufficient to maintain the constant A equal to or greater than 10, whereby said reference signal remains constant or decreases with increases in the diameter of strip material or the reel.

8. Reference signal generation means as defined by claim 7 fUrther comprising means for adjusting the voltage of said second source of potential so as to vary the rate at which strip tension changes with respect to reel diameter.

9. Reference signal generation means as defined by claim 8 wherein said summing means provides amplification of signals received from said multiplication means and said second source of potential.

10. Reference signal generation means as defined by claim 9 in which said amplifying means operates on signals received from said first source of potential and said second source of potential with gains of RV''/10V and R/10, respectively, where V is the voltage of said first source of potential and V'' is the voltage of said second source corresponding to the constant A equal to 10.

11. Reference signal generation means as defined by claim 10 in which said amplifying means includes means for adjusting the gains of said amplifying means so as to accommodate variations in build-up ratio and voltage of said first source of potential.

12. Reference signal generation means as defined in claim 11 in which said static circuit includes means for maintaining the constant A equal to or greater than 10, whereby said reference signal remains constant or decreases with increases in the diameter of strip material on the reel.