Web tension control system

Abstract

The present system maintains and controls the tension or speed of a moving web over a preselected range of web speeds in the vicinity of the prime mover speed by means of a web draw roll driven by controlled overspeed and underspeed clutches operating off the prime mover. A balanced, undamped, low inertia, preloaded dancer assembly contacts the web and web tension upsets are reflected in movements of the dancer assembly. Such movements are detected by a transducer and applied to an electronic controller which provides a continuous, proportional output signal to either the overspeed or underspeed clutch. The clutch applies a drive torque or holdback torque component to the draw roll which changes the web tension as needed to reposition the dancer assembly to its reference position.

Description

BACKGROUND OF THE INVENTION

This invention relates to a system for controlling the tension or speed in a continuous web moving to or from web processing apparatus.

Web tension may vary because of variations in the modulus of elasticity of the web along its length, changes in web thickness, speed of the web-consuming machine and for other reasons.

In many instances, then, it is necessary to control the tension in a moving web as it is drawn into or out of a printing press, embosser, coater or other such machine through which the web must move under controlled tension to be processed properly.

A number of systems have been devised to provide this tension control and they often include a floating dancer assembly in contact with the moving web which changes position in response to web tension changes. Dancer position is then used to control the speed of a driven roll in contact with the web. The dancer is normally preloaded so that, in operation, it tends to assume a selected reference position which is indicative of the desired web tension. In the case of the infeed device, any increase in tension causes the dancer to move in one direction from its reference position. This movement initiates an increase in the speed of the driven roll which tends to decrease the tension in the web so that the dancer returns to its reference position. On the other hand, a decrease in web tension causes the dancer to move in the opposite direction from its reference position, producing a decrease in the speed of the driven roll so that web tension increases sufficiently to return the dancer to its reference position. Thus, any web tension upsets are reflected in movements of the dancer which, in turn, control the speed of the driven roll to compensate for the tension upsets.

The outfeed device operates in a comparable way to maintain uniform tension in web exiting such processing apparatus.

There are several known types of floating dancer tension control devices. One type, shown in U.S. Pat. No. 3,087,663, requires a separate regulated d.c. motor driving a differential drive system to control the speed of the driven roll to maintain proper web tension. The added motor and differential increase the initial cost of the system and its space requirements.

In another type of system we are aware of, the speed of the draw roll is changed by over and underspeed clutches operating off the main line shaft. The clutches are controlled by limit switches which sense extreme positions of the dancer. In response to a web tension increase, the dancer moves up until it trips one switch which then actuates the overspeed clutch, resulting in an increase in the speed of the draw roll which lessens web tension so that the dancer moves down until it trips the other limit switch. This actuates the underspeed clutch, resulting in a decrease in speed of the draw roll and an increase in web tension, causing the dancer to travel upwards again, and so on.

While such systems work fairly well at slow web speeds, their performance at high speeds, i.e. on the order of 2,000 fpm and more, is not satisfactory. When the dancer roll reaches the ends of its path of travel, which may be as long as 10 inches, it must immediately reverse direction. This is reflected in a change in the pressure in the dancer loading cylinder. At fast web speeds, these pressure pulses upset the stability of the system unless a large and expensive surge tank or accumulator is incorporated into the system.

Additional instabilities are introduced into that type of prior infeed because it cannot compensate for changes in web tension on the downstream side of the infeed. Even worse, it will translate any web speed change on the downstream side into an unwanted tension change. Also, if the web slips on the draw roll as little as 0.1%, control problems arise.

A variation of that type of infeed arrangement is shown in U.S. Pat. No. 3,659,767. There, dancer position is used to control a variable ratio transmission or variator coupled to the draw roll. While its performance at high speeds is better than the arrangement just described, it still is not entirely satisfactory. Stability problems still arise because of load cylinder pulsing and the infeed is difficult to maintain in fine tune. Further, the transmission is quite large and expensive. Also, the transmission, being composed of many mechanical parts which must be moved to change drive speed, responds too slowly. Further, the system tends to control web speed over a particular narrow range of, say, one-half percent. This causes various parts in the transmission to become worn or develop flats so that the system can no longer maintain continuous control over speed. Rather, it must, in effect, over-compensate in order to make the transmission respond beyond that very narrow speed range.

Thus, for these reasons and others, prior tension control apparatus which sense web tension and respond by changing the speed of the draw roll to restore the proper tension value do not operate entirely satisfactorily in many applications.

SUMMARY OF THE INVENTION

Accordingly, the present invention aims to provide a web tension control system which maintains accurate control over the tension in a moving web over a wide range of web speeds, e.g. from 50 fpm to 2,500 fpm and higher.

A further object of the invention is to provide a web tension control system which is relatively inexpensive to make, maintain and operate.

Another object is to provide a web tension control system which corrects for even small tension upsets.

A further object of the invention is to provide a system of this general type which maintains uniform web tension over a wide range of tensions.

A further object is to provide a web tension control system which can compensate for downstream changes in web tension and speed.

Yet another object of the invention is to provide a system of this type which is easily constructed from standard electromechanical components.

A further object of the invention is to provide a web tension control system which is quite stable in operation even at high web speeds.

Yet another object is to provide a system of this type which is relatively efficient as compared with those requiring separate control motors because energy is coupled back into the mechanical drive system rather than being dissipated.

Other objects will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the features of construction, combination of elements and arrangement of parts which will be exemplified in the construction hereinafter set forth and the scope of the invention will be indicated in the claims.

For purposes of discussion, we will describe the present system in terms of an infeed controlling the tension of web entering a press, for example. It should be understood, however, that it is equally applicable as an outfeed device and, indeed, in any application where it is desirable to maintain uniform tension and/or speed in a moving web.

Briefly, web from a roll or other source passes over a driven roll and thence under a guide roll to the press which normally runs at a uniform speed. A balanced, low inertia, undamped, floating roll dancer assembly is positioned in the web path between the driven roll and guide roll. The dancer assembly is comprised of a pair of parallel rolls which engage the web on opposite sides thereof and which are spaced from each other in the direction of web travel. The rolls are rotatively mounted in a frame which, in turn, pivots on low friction bearings about an axis which is parallel to the axes of rotation of the rolls.

The dancer assembly is suitably preloaded so that it assumes a position of maximum web storage. During operation of the system, when the web is under tension, the dancer assembly tends to assume a reference position midway between its maximum and minimum storage positions.

Any tension change in the web downstream from the driven roll causes the dancer assembly to move from its reference position to accommodate the tension change. These dancer movements give rise to corresponding electrical signals which cause a controller to vary the torque coupling in a pair of variable slip over and underspeed clutches connected between the machine line shaft and the driven roll.

Since the dancer assembly is balanced, no preloading is required to compensate for the weight of the dancer assembly. This is important when it is necessary to process web under low tension because ten pounds of loading pressure just to raise the dancer may represent as much as fifty pounds of web tension. Also, the assembly's small mass and low friction mounting means that the dancer has relatively low mechanical inertia. Thus, it can change position very quickly in response to a web tension change to develop the proper corrective signal for the controller.

Also, as will be described later, the dancer assembly is specially designed so that its movements do not affect the relationship between the torques applied to the assembly by the tensioned web and the loading device.

The controller develops electrical signals corresponding to both dancer position and dancer velocity. These signals are summed and the resultant signal is applied to a lag circuit which compensates for the small amount of mechanical inertia in the system. The output of the lag circuit is applied to continuously control the overspeed and underspeed clutches modulating between them, depending upon whether drive torque or holdback torque must be applied to the draw roll in order to restore the proper web tension that will cause the assembly to return to, and remain in, its reference position.

In other words, if there is a momentary tension increase in the web, the dancer assembly pivots so as to shorten the web path between the driven roll and the guide roll. This dancer movement produces a corresponding electrical signal which causes the controller to increase the torque coupling in the overspeed clutch, thereby increasing the driving torque on the driven roll. The tension in the web beyond the driven roll is thus decreased, as is the torque component on the dancer assembly due to web tension, sufficiently to return the dancer assembly to its reference position.

Conversely, when the web suffers a tension decrease, the dancer assembly pivots so as to lengthen the path of the web between the driven roll and the guide roll. This dancer movement, in turn, causes the controller to increase the torque coupling in the underspeed clutch so that the necessary holdback torque is applied to the driven roll to increase web tension just enough to return the dancer assembly to its reference position. In other words, the present system senses the tension in the web and responds to a change therein by continuously modulating the torque on the driven roll, rather than its speed, to effect tension correction. This torque control is maintained over a predetermined range of web speeds in the vicinity of the speed of the line shaft, e.g. .+-. 4%. The system can tolerate web slippage, web elongation, and even downstream web speed changes over this preselected speed range. Any energy savings during torque modulation is coupled back into the machine line shaft so that the rating of the drive motor can be kept to a minimum.

The controller responds to both dancer position and rate so that it exercises very fast and continuous control over the torque applied to the driven roll. Any change in that torque is reflected almost immediately (i.e. five Hz or less) in a corrective change in web tension that will tend to return the dancer to its reference position.

Moreover, the subject dancer assembly in conjunction with the electronic controller and over and underspeed drives constitutes a true integrating type of system. As a consequence, even a small change in dancer position due to a small tension upset results in a corrective torque component being applied to the driven roll. Finally, since the system responds quickly and is quite stable and exercises control only over the relatively short preselected web speed range, the excursions of the dancer assembly about its reference position are very short (e.g. 1/16 inch) and web storage provided by only the two offset dancer rolls is quite adequate even at high web speeds.

Thus, the present arrangement provides continuously variable torque control over the draw roll up to the full rating of the clutches to maintain a constant trim on web tension from a web-up speed as low as 50 fpm to normal operating speeds as high as 2500 fpm. Actually, the tension can be maintained to an accuracy of as high as 1% and even lower over the entire web speed range.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings, in which:

FIG. 1 is a diagrammatic view of the web tension control system embodying the principles lf this invention; and

FIG. 2 is a schematic diagram showing in greater detail certain elements of the FIG. 1 system.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1 of the drawings, since this is a balanced system, the web can travel in either direction through the system. However, in the infeed application specifically shown, the web W is traveling from left to right. Web is trained around a driven draw roll 10 and thence passed through a dancer assembly indicated generally at 12. From there, it is trained under a guide roll 14 and drawn into the web-consuming machine (not shown). In response to tension changes in the web W, the dancer assembly 12 moves from its reference position to lengthen or shorten the web path as necessary to maintain uniform tension in the web.

The dancer motion is detected by a control section shown generally at 16 which responds by applying a drive torque or a holdback torque to the draw roll 10. That, in turn, decreases or increases the tension in the web W entering the dancer assembly as needed to return the dancer assembly to its reference position.

The draw roll 10 has a shaft 10a which is coupled to the control section 16. Web slippage relative to roll 10 is minimized by means of a nip roll 18 which is supported via its shaft 18a at the ends of a pair of elongated end plates 22. The opposite ends of plates 22 are connected by pivots 24 to the machine frame (not shown). Nip roll 18 is pressure loaded against roll 10 by suitable means illustrated herein as pressure cylinders 26 acting between end plates 26 and the machine frame.

The dancer assembly 12 is comprised of a pair of parallel rolls 32 and 34 which are spaced apart in the direction of web travel and engage web W on opposite sides thereof. The opposite ends of rolls 32 and 34 are journaled in a pair of end plates 36a and 36b. A pair of stub shafts 38a and 38b project laterally from the centers of plates 36a and 36b, respectively, and these shafts are journaled in a pair of bearing assemblies 42a a 42b, respectively.

The components of the dancer assembly 12 are arranged with respect to the draw roll 10 and guide roll 14 so that it is a completely balanced system. In other words, when the assembly is in its neutral position, the stretch of web between draw roll 10 and roll 32 is parallel to the stretch of web between roll 34 and guide roll 14. Both of these are perpendicular to the plane defined by the dancer rolls when the assembly is in its reference (e.g. horizontal) position. When the dancer assembly pivots on its bearing assemblies 42a, 42b in one direction or the other from its reference position, it either lengthens or shortens the path of the web W between rolls 10 and 14. Since the dancer is balanced, no loading is required to compensate for the weight of the dancer and, therefore, the system can operate at very low web tensions.

The dancer assembly is biased toward its web storage position by suitable means illustrated herein as a loading cylinder 44. The end of cylinder 44 is provided with an ear 46 which is connected by a pivot 48 to an ear 52 attached to the machine frame. The loading cyclinder shaft 44a is pivotally connected at 49 to one of the dancer assembly end plates, i.e. plate 36a. The pivotal connection 49 between shaft 44a and the end frame is situated outboard of shaft 38a so that the cylinder can impart an appreciable torque to the dancer assembly.

Fluid under pressure is fed to cylinder 44 by way of a pressure regulator 50 and the cylinder pressure is adjusted so that the torque exerted by the cylinder on the dancer assembly just offsets the opposing torque due to the desired tension in web W so that the dancer assembly tends to assume a reference position as seen in FIG. 1. Of course, if need be, a duplicate loading cylinder can be stalled at the opposite side of the assembly, i.e. at plate 36b, to minimize racking of the assembly. Also, the loading cylinder 44 could just as well be arranged to push down on plate 36a (and/or 36b) on the other side of bearing assembly 42a, (42b).

The present dancer assembly is specially designed so that no changes in web tension arise because of changes in the relationship between the torques on the assembly due to web tension and cylinder 44 at different angular positions of the assembly. More particularly, one of the shortcomings of many prior dancer roll systems stems from the fact that as the dancer assembly moves from its reference position, the forces on the dancer due to web tension and dancer loading vary as a function of the angle to which the dancer assembly pivots due to a tension upset. Consequently, in those prior systems, the dancer assembly itself introduces unwanted tension changes in the web which adversely affect the stability of the overall system. The dancer assembly shown in FIG. 1 maintains constant web tension with a constant loading force from cylinder 44 through all positions of the dancer assembly and there is no load cylinder pulsing when the dancer changes direction. This is accomplished by arranging the loading cylinder 44 and its shaft 44g or other loading devices so that the force is always applied parallel to the stretches of web W entering and leaving assembly 12. This condition prevails provided the triangle defined by points A, B and C in FIG. 1 is geometrically similar to the triangle defined by points A, D and E in that figure. It can easily be shown mathematically that as long as the above similarity persists, the force on the dancer assembly from cylinder 44 will equal the product of a constant and the force on the dancer due to web tension for all deviations of the dancer assembly from its neutral position.

Still referring to FIG. 1, any rotation of the dancer assembly on shafts 38a and 38b is sensed by a suitable detector illustrated herein as a potentiometer 56 which is connected electrically to the control section 16. More specifically, the potentiometer 56 applies a signal to an electronic controller 58 in section 16 which corresponds to the deviation of the dancer assembly from its illustrated reference position. The controller 58, in turn, provides an output to a variable slip overspeed clutch 62 or a variable slip underspeed clutch 64 coupled to draw roll 10, depending upon whether drive torque or holdback torque should be applied to the roll to correct web tension.

The clutches 62 and 64 are driven by the machine line shaft 66. For this, a toothed pulley 68 rotating with the line shaft 66 is connected by a timing chain 72 to a toothed pulley 74 affixed to the input shaft 62a of clutch 62 and also to a toothed pulley 76 connected to the input shaft 64a of clutch 64. The pulley 74 has fewer teeth than pulley 68 and pulley 76 has correspondingly more teeth than pulley 68 to obtain the desired speed differential between the two clutches over the desired speed range of, say .+-.4% of the line shaft speed. The two clutches also have output shafts 62b and 64b which carry toothed pulleys 78 and 82, respectively, which are connected by a timing chain 84 to a toothed pulley 88 on the end of the draw roll shaft 10a. The clutches are set to magnetically couple a selected torque to the draw roll 10 as needed to maintain the dancer assembly in its reference position. The torque coupling changes required to compensate for web tension variations are made by controlling the current in the clutch coils. A variable slip magnetic particle clutch suited for this purpose is manufactured by W. J. Industries Inc. and the Vickers Division of Sperry Rand Corp. although other comparable electric, pneumatic or friction clutches can also be used, as long as they permit continuously varying control over the torque applied to the draw roll.

The dancer assembly 12 and control section 16 together function as an error position integrator so that the infeed responds to even very small tension changes. Also, the control section 16 includes feedback between the clutches and the controller and within the controller itself to keep instabilities within the system to a minimum. As a consequence, the infeed maintains very close control over web tension i.e. within 1% of the desired tension. Also, the excursions of the dancer assembly 12 from its reference position are very small, i.e. on the order of 1/16 inch during normal operation and only 1 to 2 inches during an emergency stop situation. Since the dancer and overall system have very fast response and only controls web speed within .+-.4% of line speed, the dancer assembly does not require much storage capacity even at web speeds in excess of 2500 fpm.

Turning now to FIG. 2, the controller 58 contributes substantially to the basic stability of the overall system. It includes a regulated, dual polarity power supply 82 connected to the potentiometer 56. The output of the potentiometer, taken from the wiper 56a thereof, is applied to a d.c. amplifier 84 and to a differentiator 86.

When the torque applied to the dancer assembly 12 (FIG. 1) by the loading cylinder 44 exactly offsets the torque due to the desired web tension and the dancer assembly assumes its horizontal reference position,, the voltage at the potentiometer 56 is 0 volts. However, when the dancer assembly moves in response to a tension change in the web, a signal is applied to amplifier 84 and differentiator 86.

The output of the amplifier is a signal corresponding to the position of the dancer, while the output of the differentiator is proportional to the dancer velocity. These two signals are summed and applied to a lag circuit 88 illustratively consisting of a high gain amplifier 89 and an RC circuit 92 connected between the amplifier 89 input and output. The lag circuit is basically an integrator and provides a negative phase shift to compensate for the mechanical inertia that remains in the infeed system.

The circuit 88 output is applied directly to a diode 94 and by way of an inverter 96 to a diode 98, both diodes being arranged to pass only positive going signals. When the dancer assembly moves from its reference position, a positive going signal is applied either to diode 94 or diode 98, depending upon the direction of dancer movement. For purposes of discussion, we will assume that an increase in web tension which causes the dancer assembly 12 to rotate clockwise in FIG. 1 produces a positive going signal at diode 94.

The output of diode 94 is applied by way of a voltage regulator 102 to an overspeed firing circuit 104. The output of circuit 104 is used to trigger a pair of SCR's 106 and 108 connected to pass current during alternate half cycles from a center tapped transformer 110 to a series circuit consisting of the coil 62c of overspeed clutch 62 and a resistor 112. The output of the regulator 102 determines the firing angle of the SCR's and therefore controls the current in the clutch coil. A diode 111 is connected across coil 62c to accomodate back EMF due to a collapsing field in coil 62c. The voltage at the junction of coil 62c and resistor 112 which is proportional to the current through the coil 62c is fed back and summed with the signal from diode 94. Thus, the signal applied by voltage regulator 102 to firing circuit 104 makes the current through the clutch coil 62c at all times proportional to the output of the lag circuit 88. Thus, very fast control is exercised over the overspeed clutch 62 in response to dancer movements due to a web tension increase.

Still referring to FIG. 2, similar circuitry follows the diode 98 to control the underspeed clutch 64. Thus, the diode 98 output is applied via a voltage regulator 114 to an underspeed firing circuit 116 whose output is used to trigger a pair of SCR's 118 and 122. These SCR's are arranged to conduct current from a transformer 124 during alternate half cycles to a series circuit consisting of the underspeed clutch coil 64c and a resistor 126, with a diode 127 in parallel with the coil. The output of the lag circuit also controls the firing angle of these SCR's and thus the current through coil 64c. A voltage proportional to the current through the coil 64c appears at the junction of that coil and the resistor 126 and is fed back to the input of regulator 114 where it is summed with the signal from diode 98. Thus, the signal applied to the firing circuit 116 controls the current through the underspeed clutch coil 64c so that it is always proportional to the output of the lag circuit 88. Resultantly, very close control is maintained over the torque coupling between the underspeed clutch 64 and the draw roll 10 in response to counterclockwise dancer movement indicative of a web tension decrease.

In summary, then, the present system controls web tension by sensing web tension changes using a balanced, specially loaded, undamped dancer having low mechanical inertia and thus fast response to tension upsets. The dancer motion is sensed and a signal is applied to a controller which continuously develops proportional control signals for the continuously controllable overspeed and underspeed clutches. Thus, these clutches are modulated so as to apply just the proper torque component to the draw roll to maintain the dancer in its reference position.

Since the clutches are set to always drive the draw roll at speeds slightly less than or slightly more than line shaft speed, the excursions of the dancer are quite small, e.g. 1/16 inch, and system operation is quite stable. Also, there is no dancer loading cylinder pulsing problem. Thus, web tension can be controlled with great accuracy over the full range of web speeds of from 50 fpm to 2500 fpm. As noted above, in a typical case, the clutches are set to run 4% underspeed and 4% overspeed and this amount may vary in different applications. As a general rule, however, it has been found that if the speed difference is much less than .+-.2% of line shaft speed, tension upsets take a fairly long time to correct. Also, if the difference is much more than 10% of shaft speed, there is relatively large energy dissipation incident to tension control.

The present system can accommodate downstream web speed changes because the clutches operate off the main line shaft, as does the downstream web-consuming machine so they compensate for any such change. Further, any speed upsets are not translated into web tension upsets as is the case with some prior infeeds.

The present infeed is much less expensive than prior systems, costing only about one-half as much as the type shown in U.S. Pat. No. 3,087,663 and on the order of one-third less than the type depicted in said U.S. Pat. No. 3,659,767 and its mechanical clutch-type drive is only on the order of one-third the size of the differential drives and motors in those prior infeeds, yet it is easier to operate and control.

It will thus be seen that the objects set forth above are efficiently attained. It should also be understood that certain changes may be made in the above description without departing from the scope of the invention. For example, using this equipment, one can control draw roll speed to impart a precise amount of elongation to a web by maintaining a precise underspeed torque. This is useful, for example, to orient polyethylene. As another example, in those applications such as commercial printing where the tension in the web downstream from the infeed is always greater than the tension in the web upstream therefrom, the overspeed clutch 62 and related control elements may be omitted. Conversely, in outfeed applications where the upstream web is under greater tension than the downstream web, the underspeed components of the system may be dispensed with.

Also, the same principle can be applied to accurately control web speed. This involves sensing web speed changes instead of tension using a roll tachometer as the input to the controller.

It will also be understood that the following claims are intended to cover all of the generic and specific features herein described.

Claims

I claim:

1. A system for controlling tension in a moving web comprising

A. a draw roll for engaging said web,

B. a guide roll spaced from and parallel to the draw roll for engaging said web,

C. a balanced, undamped, low inertia dancer assembly positioned between the draw roll and the guide roll, said dancer assembly including

1. a frame, and

2. a pair of spaced-apart dancer rolls rotatively supported by the frame so that their axes are parallel to the draw and guide roll axes and for engaging opposite sides of said web,

D. means for pivotally mounting the frame so that it can pivot about an axis midway between and parallel to the dancer rolls in response to changes in the tension of the web,

E. means for applying a torque to the dancer assembly so as to offset the torque applied thereto by a selected tension in the web so the assembly tends to assume a reference position, said torque-applying means applying torque by exerting a force on said assembly which is directed along a line which remains parallel to said entering and leaving web paths as the dancer assembly deviates from its reference position,

F. means for detecting movement of the dancer assembly away from its reference position in response to a change in web tension and producing an output signal in response thereto, and

G. means responsive to said output signal for applying the proper torque correction to the draw roll to change the tension in the web such that the dancer assembly tends to return to its reference position.

2. The system defined in claim 1 wherein the mounting means include bearing units which allow the assembly to pivot with minimum frictional losses.

3. The system defined in claim 1 wherein the draw roll, guide roll and dancer rolls are arranged so that the web paths entering and leaving the dancer assembly are parallel.

4. The system defined in claim 3 wherein said entering and leaving web paths are perpendicular to the plane defined by the dancer rolls when the dancer assembly is in its reference position.

5. The system defined in claim 1 and further including

A. a nip roll, and

B. means for mounting the nip roll so as to form a nip with the draw roll to minimize slippage of web on the draw roll.

6. The system defined in claim 1 wherein the torque applying means comprises

A. a pressurizable loading cylinder having a movable shaft,

B. a stationary support,

C. means for pivotally connecting the loading cylinder and its shaft between the frame and the support so that when the cylinder is pressurized, the frame is pivoted about its axis so as to lengthen the web path between the draw roll and the guide roll, and

D. means for pressurizing the cylinder so as to maintain the assembly in its reference position at said selected web tension.

7. The system defined in claim 6 wherein the cylinder and its pivotal connections to the assembly are arranged and adapted so that the axis of the cylinder always remains parallel to the entering and leaving web paths as the assembly deviates from its reference position in response to web tension changes.

8. The system defined in claim 1 wherein the detecting means include a potentiometer whose resistance changes as the assembly deviates from its reference position.

9. The system defined in claim 1 wherein the torque correction applying means comprise

A. rotary drive means,

B. one of a controllable overspeed and underspeed clutch coupled between the drive means and the draw roll, and

C. an electronic controller responsive to the output of the detecting means to provide control signals for the clutches so that said proper torque correction is applied to the draw roll.

10. The system defined in claim 9 wherein

A. the other of the overspeed and underspeed clutch is also coupled between the drive means and the draw roll,

B. the clutches are current controlled, and

C. the controller comprises

1. a first current regulator responsive to the output of the detecting means for controlling the current in the overspeed clutch,

2. a second current regulator responsive to the output of the detecting means for controlling the current in the underspeed clutch, and

3. means for coupling the output of the detecting means alternatively to the overspeed and underspeed current regulators so that said proper torque correction is applied to the draw roll.

11. The system defined in claim 10 wherein the coupling means include

A. means for amplifying the output of the detecting means,

B. means for differentiating the output of the detecting means, and

C. means for summing the outputs of the amplifying and differentiating means so that the signal applied to the current regulators reflects the instantaneous position and velocity of the dancer assembly.

12. The system defined in claim 11 wherein the coupling means also include a lag circuit connected between the summing means and the current regulators.

13. The system defined in claim 10 and further including

A. means for sensing the current in the overspeed clutch and developing a first signal in response thereto,

B. means for sensing the current in the underspeed clutch and developing a second signal in response thereto,

C. means for summing the first signal at the input of the first current regulator, and

D. means for summing the second signal at the input of the second current regulator, so as to provide negative feedback to more closely regulate the currents in the overspeed and underspeed clutches.

14. A system for maintaining uniform tension in a moving web comprising

A. a draw roll for engaging the web,

B. a dancer assembly positioned adjacent the draw roll, said dancer assembly including

1. at least one dancer roll rotatively supported parallel to the axis of the draw roll and for engaging the moving web,

2. means for mounting the dancer roll so that it can pivot about a pivotal axis which is parallel to the axis of the draw roll in response to changes in the tension of the web engaged by the dancer roll, and

3. means for applying a torque to the dancer assembly at said pivotal axis so as to offset the torque applied thereto by a selected tension in the web engaged by the dancer roll so that the assembly tends to assume a reference position about said pivotal axis,

C. means for detecting movement of the dancer assembly from its reference position in response to a change in web tension and producing an output signal in response thereto,

D. drive means connected to the draw roll for applying tension to the web,

E. a controllable, variable slip overspeed clutch connected to the drive means to provide a drive torque to the drive means,

F. a controllable, variable slip underspeed clutch connected to the drive means to provide a holdback torque to the drive means, and

G. an electronic controller connected between the detecting means and the clutches for controlling the clutches in accordance with the output signal from the detecting means so that the draw roll controls the tension in the web as needed to maintain the dancer assembly in its reference position.

15. The system defined in claim 14 wherein the controller is comprised of

A. a position amplifier connected to amplify the output signal from the detecting means,

B. a differentiator connected to differentiate the output signal from the detecting means,

C. means for summing the outputs of the amplifier and differentiator so as to produce a signal representing the instantaneous position and velocity of the dancer assembly,

D. means for controlling the slippage in said clutches in accordance with the output of the summing means so that the torque applied by the clutches to the drive means is continuously trimmed.

16. The system defined in claim 15 and further including an adjustable lag circuit connected between the summing means and the controlling means for imparting a negative phase shift to the signal from the summing means.

17. The system defined in claim 15 wherein the controlling means include means for applying the output signal from the summing means alternatively to said clutches.

18. A system for controlling tension in a moving web comprising

A. a driven roll for engaging the web,

B. a prime mover,

C. a continuously controllable clutch whose torque coupling varies in response to a control signal connected between the prime mover and the driven roll, said clutch being arranged to operate at maximum torque coupling at a speed slightly different from the speed of the prime mover,

D. means for sensing a tension change in the web and developing a control signal corresponding thereto, and

E. means for coupling the control signal to the clutch to vary the torque coupling from the prime mover to the driven roll to produce a compensating tension change in the web.

19. The system defined in claim 18 wherein

A. the clutch is current-controlled, and

B. the coupling means include a current regulator responsive to said control signal for controlling the current in the clutch.

20. The system defined in claim 19 and further including

A. a second current controlled clutch,

B. a second current regulator responsive to the output of the sensing means for controlling the current in the second clutch,

C. means for applying said control signal alternatively to the first and second current regulators so that the proper torque is coupled to the driven roll.

21. The system defined in claim 18 wherein said slightly different speed is within the range of .+-. 2% to 10% of the speed of said prime mover.