Web Handling Apparatus

Abstract

Web handling apparatus pulls web from a supported roll at a selected speed. A brake is provided for controlling the roll speed and a sensor senses changes in the tension of the web caused by changes in the pulling force of the web. The brake is controlled in accordance with the output of the tension sensor so that the web is maintained substantially under constant tension. Also, the apparatus responds to the instantaneous size of the roll by modulating the braking means so that the drag force imparted by the brake to the web is substantially independent of roll size.

Description

BACKGROUND OF THE INVENTION

This invention relates to web handling apparatus. It relates more particularly to a system for controlling web tension in such apparatus.

The apparatus with which we are concerned here is used to provide web uninterruptedly to machines which consume the web at high speed, a printing press, for example. The apparatus is designed to automatically splice the leading end of a roll of ready web to the trailing end of a depleted roll of running web and to deliver the web continuously in a controlled fashion to the web-consuming machine. In order to accomplish this, the apparatus includes an accumulator situated between the splicer and the web-consuming machine. When the running web is stopped to splice its trailing end to the ready web, there is enough material in the accumulator to supply the needs of the web-consuming machine until the ready web roll is accelerated to running speed after the splice is made.

A conventional machine of this type is disclosed in U.S. Pat. No. 3,305,189 and the present invention will be described in terms of a machine of that type.

Basically, the apparatus includes supports for a pair of web rolls, one of which is running and one of which is at the ready. The running web is conducted into an accumulator and the web material leaving the accumulator travels into the web-consuming machine. The accumulator is comprised of a set of fixed rolls and a movable dancer carrying a second set of rolls. The web is looped between the fixed rolls and the dancer rolls, forming a series of bights. The amount of material in the accumulator is controlled by moving the dancer toward or away from the set of fixed rolls. In other words, as the dancer moves further away from the set of fixed rolls (i.e., upward), the amount of material in the accumulator increases, and vice-versa. In operation, the dancer is biased away from the fixed rolls under constant pressure and is caused to move by changes in the tension in the web.

Usually, web is pulled from the running roll by a pulling roll in the web-consuming machine at a constant rate whose value depends upon the requirements of the web-consuming machine. The running supply roll is braked in a controlled fashion to maintain the proper web tension. In the apparatus illustrated in the above patent, the amount of braking force applied to the running web roll, i.e., the amount of tension imparted to the web, is determined by the position of the dancer in the accumulator which, as pointed out above, reflects web tension.

As the dancer moves toward the set of fixed rolls (i.e., downward) indicating an increase in the web tension, the web braking force is reduced so that web runs into the accumulator at a faster rate, thereby tending to move the dancer up away from the set of fixed rolls. Conversely, if the dancer moves upward, indicating a web tension decrease, the braking force on the running roll is increased to slow down the rate at which web enters the accumulator. This tends to move the dancer downward toward the set of fixed rolls. Thus, during normal operation of the apparatus, the dancer seeks a zero position in an attempt to maintain substantially constant tension in the web. This position is selected so that there is enough web in the accumulator to satisfy the needs of the web-consuming machine when the running roll is stopped for splicing.

These prior system work well at relatively low web speeds. However, at higher speeds, on the order of 1,500 feet per minute, certain problems develop. More particularly, in an attempt to maintain constant web tension at these high speeds, the dancer tends to hunt about its zero point, causing unwanted web tension changes. This is due in large part to the inertia of the dancer and the delay between the time when the dancer issues a command signal to the brake and the time when the brake responds. Also, the running web supply roll is constantly decreasing in diameter so that its moment of inertia is constantly changing. Accordingly, when the brakes are applied, the web tension is affected differently, depending upon the amount of material remaining on the roll. In practice, this effect causes a gradual increase in web tension, with the result that the dancer moves down and reduces the amount of available material in the accumulator. Also, in the prior machines, there is a relatively long time delay between the various events which take place during the splicing cycle, and after splicing there is inadequate control over the refilling of the accumulator. These and other problems make the prior apparatus unsatisfactory for operation at high speeds, particularly when handling the finer webs.

SUMMARY OF THE INVENTION

Accordingly, this invention aims to provide a web tension control system which operates reliably at high web speeds.

Another object of the invention is to provide a web tension control system which maintains close control over web tension immediately before and after a splice.

A further object of the invention is to provide a web tension control system which alleviates the dancer hunting problem.

Yet another object of the invention is to provide a web tension control system which assures that a splice is made only when the running web is at a standstill.

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

Still another object of the invention is to provide a web tension control system which responds quickly to an emergency situation such as a web break.

Yet another object of the invention is to provide a web tension control system which enables the operator to maintain close control over the position of the dancer in the accumulator during all phases of operation of the web supply apparatus, including emergency stops.

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 are exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.

We will describe the present system in conjunction with a web supply apparatus of the type disclosed in the aforesaid patent. The apparatus includes arbors for supporting a roll of running web and a roll of ready web. Both rolls, or more particularly the arbors therefor, are equipped with brakes to vary the speed at which the rolls turn when web is being pulled from them. The running web is fed through an accumulator to the web-consuming machine, i.e., a printing press. In the accumulator, the web is looped between movable dancer rolls and stationary rolls, with the amount of material in the accumulator being determined by the position of the dancer.

During normal operation of the apparatus, the web-consuming machine pulls web out of the accumulator with a force sufficient to maintain the web at a selected, substantially constant velocity, depending upon the requirements of the web-consuming machine. The position of the dancer is continuously measured by a pontentiometer whose resistance is employed as a basis for modulating the braking force applied to the roll of running web to maintain a selected tension in the web. The system also determines dancer velocity at each instant and this measurement is used to further modulate the braking force applied to the supply roll to minimize instabilities and the tendency of the dancer to hunt about its selected zero position point.

Still further, the size of the roll of running web is measured continuously. This is not done by measuring the size of the roll directly by a follower arm. Rather, it is done by obtaining the ratio of the angular velocities of a fixed guide roll ahead of the accumululator and the running roll and employing that measurement to vary the gain of the system to reduce the braking force applied to the running roll as the roll size diminishes. This gives improved braking control for web rolls of all diameters and over a full range of web speeds from thread speed up to full speed emergency stops. This variable gain feature also tends to maintain the dancer at its selected zero position point so that there is no tendency for the dancer to move down, decreasing the web supply in the accumulator as the roll size diminishes.

When the size of the running roll reaches a selected minimum size, the system automatically activates the splice cycle to decelerate the running web, make the actual splice at a time when the web is stopped and acclerate the ready web to running speed. This is accomplished with a minimum delay between these steps so that a good splice is made in a short time. Consequently, the system conserves a maximum amount of web storage during the splice cycle.

The aforesaid controls over web tension and web and dancer movements are achieved substantially entirely by electronic means. Therefore, the system responds very quickly to changing conditions. Moreover, the electronic components which make up the present tension control system are more reliable and require less maintenance than the pneumatic and mechanical elements found in the prior apparatus.

BREIF 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 web supply apparatus incorporating the present invention; and

FIG. 2 is a block diagram partly in schematic form showing the tension control system in the FIG. 1 apparatus in greater detail.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Turning now to FIG. 1 of the drawings, a roll 10 of running web is supported for rotation on a suitable shaft or arbors 12. The web W from roll 10 is conducted through a splicing station 14 and under a guide roll 16 to an accumulator shown generally at 18. The web leaving the accumulator is conducted under a guide roll 22 and thence to the web-consuming machine, ilustratively a printing press 24. During normal operation, a pull roll in press 24 pulls web W with a force which is sufficient to give the web a fixed velocity V.sub.p into the press.

The apparatus also includes a roll 26 of ready web also rotatively supported on a suitable shaft or arbors 28. The leading end of roll 26 is set in the splicing station to await the depletion of the running roll 10. Whereupon it is spliced to the trailing end of the running web as described in the aforesaid patent so that web proceeds uninterruptedly through the accumulator 18 into the press 24. The activation of the splice cycle will be described later.

A conventional tachometer 32 is geared to an arbor 12. The tachometer develops an output signal which is proportional to the velocity of arbors 12 and, hence, roll 10. This signal is coupled to a control console 34 and processed in the manner to be described later. A similar tachometer 36 is geared to an arbor 28 to apply a signal to the console 34 which is proportional to the velocity of roll 26.

A third tachometer 38 is geared to the guide roll 16 and generates an output signal to console 34 which is proportional to the velocity of roll 16. Control console 34 processes the signals from tachometer 38 and from either tachometer 32 or tachometer 36 to develop a control signal which represents the instantaneous size of the roll of running web, i.e., either roll 10 or roll 26.

The accumulator 18 comprises a set of fixed rolls 42 and a set of dancer rolls 44 in a movable dancer 46 situated directly above rolls 42. The rolls 42 and 44 are staggered so that the web W can be looped around the rolls festoon fashion so that an appreciable amount of web can be stored in the accumulator. The amount of web stored in this manner depends upon the distance between the dancer 46 and the fixed rolls 42. As the spacing increases, more web is stored in the accumulator and vice versa.

The dancer 46 carrying the rolls 44 is supported by suitable means such as a cable 52 which extends up vertically and is passed around a pulley 54 positioned above the fixed rolls 42. The cable extends around a second pulley 56 and is attached to a rod 58 of a piston which includes a cylinder 62. The full stroke of the piston causes the dancer 46 to move from its lowermost position adjacent the fixed rolls 42 to an upper position wherein the accumulator stores a maximum amount of web W. In practice, differential pulleys or other means may be employed so that the stroke of the piston can be considerably less than the stroke of dancer 46. A solenoid valve 63 feeds air to or bleeds air from the upper chamber within cylinder 62 (i.e., above the piston). Normally, a constant pressure is applied to the piston which is sufficient to support the dancer 46 above rolls 42.

A potentiometer 66 is geared to pulley 54 so that a full stroke of the dancer 46 varies the resistance of the potentiometer over its complete range. Thus, the instantaneous position of the dancer 46 is reflected by a selected resistance value in potentiometer 46 which is applied to and processed by console 34 as will be described later.

During normal operation, the dancer 46 seeks a zero position Y.sub.O and stores enough web in the accumulator to supply the press 24 when the web input to the accumulator stops during the splicing cycle. At this time, press 24 pulls the web W with a force sufficient to maintain the constant running speed V.sub.P. This force necessary to remove the web at constant velocity is the sum of the inertia force caused by rolls 42 and 44 and the drag force at the running roll, i.e., roll 10. Since the inertia force is substantially constant, if constant web tension is to be maintained, then the drag force at the running roll must be varied as the pulling force by the press varies. This is accomplished at roll 10 by a pneumatically operated disc brake 72 connected by an air line 74 to console 34. A similar brake 76 controls the speed of the ready web roll 26, the fluid for this brake being applied from the console through a fluid line 78.

Thus, during normal operation, changes in web tension are reflected by a movement of the dancer 46 up or down from its zero position Y.sub.O. The instantaneous dancer position is in turn reflected by a given resistance in the potentiometer 66. This causes the console 34 to vary the pressure applied to the brake 72 controlling the running web to increase or decrease the drag force on the running web as necessary to return the dancer to its zero position Y.sub.O.

As will be seen later also, the console processes the resistance value of potentiometer 66 to develop a signal which represents the velocity of the dancer 46. This signal is also used to modulate the drag force applied by brake 72 to obtain even closer control over web tension and the movements of dancer 46. Still more control over web tension and dancer movements is obtained by varying the gain of the closed servoloop in the console in accordance with the size of roll of running web by processing the signals from tachometers 32 and 38. As a result, the system minimizes the hunting movements of the dancer over the full range of web speeds. Also, just the proper amount of braking force is applied to the running web roll for the existent operating conditions.

When the size of the running roll 10 reaches a predetermined minimum diameter, this is detected by console 34 which thereupon initiates the splicing cycle. The console applies a constantly increasing pressure to the break 72 on the running roll so that the roll decelerates at a predetermined rate. By varying the gain of the system in accordance with roll size, as the roll diminishes in size, the dancer can be made to move up, making more storage available at time of splice. Also, as the roll diminishes in size, less storage is required for emergency stops. Accordingly, this system can be programmed so that just enough reserve storage is available in the accumulator to handle an emergency stop proportional to roll diameter.

As soon as the speed of the running web W reaches zero or a selected minimum speed, as measured by the tachometer 38, console 34 emits a control signal to the splicing station 14 to bring the leading edge of the ready web from roll 26 against the trailing edge of the running web W, the ready web having previously been laid into the splicing station 14 and provided with double-faced adhesive as described in the aforesaid patent. The two webs now being spliced together, the control console 34 actuates a knife which cuts the running web immediately behind the splice. Thereupon, the console emits a signal to a conventional eddy current drive 82 associated with the arbors 28 supporting the roll 26 of ready web. Drive 82 accelerates the ready roll for a predetermined time sufficient to bring the ready web substantially up to the web running speed V.sub.P. A similar drive 84 is associated with arbors 12 for accelerating those arbors when they are supporting a roll of ready web.

During the aforesaid splice cycle, the amount of web W entering the accumulator 18 is much less than the amount of material being pulled from the accumulator by press 24. Accordingly, the supply of web stored in the accumulator is reduced, causing the dancer 46 to move down toward the fixed rolls 42. Upon completion of the splicing cycle, console 34 controls the speed of the web entering the accumulator and the movements of the dancer to control the rate at which the accumulator refills with web. In other words, if the supply of web is substantially depleted, the dancer is returned to its zero position Y.sub.O at a faster rate than if there is only a small web deficiency in the accumulator. Since a closed loop servocontrol system is employed, there is much closer control over the movements of dancer 46, particularly during the splice service cycle when the accumulator is being refilled.

As will be described presently, the system also provides means for detecting excessive movements of the dancer 46 which signal some emergency condition such as a web break. In response to these emergency conditions, this system stops the web in a controlled fashion in a minimum of time. All of these factors make the present system preferable to the prior pneumatic tension control systems exemplified in the aforesaid patent.

Referring now to FIG. 2 which shows in greater detail the components in console 34, the present system measures running roll size indirectly by tachometers which measure the angular velocities of the running roll and the fixed speed guide roll 16. This can be done because the following relationship holds:

r.sub.1 = r.sub.2 (.omega..sub.2 /.omega..sub.1) (1)

Where:

r.sub.1 = radius of roll 10

r.sub.2 = radius of roll 16

.omega..sub.1 = angular velocity of roll 10

.omega..sub.2 = angular velocity of roll 16

More particularly, the output voltage of tachometer 32 (or tachometer 36) is applied to an amplifier 92 which filters the signal to eliminate tachometer noise and provides gain so that the output from the amplifier 92 reflects the RPM of the running roll. A similar circuit 94 conditions the output of tachometer 38. The signals from circuits 92 and 94 are applied to an analog divider 96 which divides the latter by the former to develop an output voltage which reflects the instantaneous size of the running roll, e.g., roll 10.

The output of the analog divider 96 is applied to an amplitude comparator 98 which compares this voltage with the voltage from an adjustable splice radius threshold control 102. The voltage output of control 102 represents the running roll size at which it is desired to commence the splice cycle. When the voltage applied to comparator 98 by the divider drops to this value, indicating that the roll of running web is almost depleted, the output of comparator 98 actuates a current driver 104 which energizes a relay 106 to commence the deceleration of the running web in preparation for the splice as described above.

The output of circuit 94 indicative of the speed of the guide roll 16 and, hence, the speed of the web entering the accumulator 18, is applied to a second amplitude comparator 108 where it is compared with the voltage from a velocity threshold control 110. When the speed of roll 16 falls to zero or a selected minimum value at which a satisfactory splice can be formed, comparator 108 develops an output which activates the driver 112 controlling a relay 114. The relay, in turn, initiates the events in the splicing operation described in the aforesaid patent. Thus, the splice is accomplished the instant that the running web stops after commencement of the splicing cycle. The relay 114 also engages the eddy current drive 82 which immediately accelerates the ready web 26 following the splice. Thus, there is minimal delay between the various steps in the splicing cycle.

The output of the analog divider 96 which reflects the instantaneous size of the roll of running web is also used to vary the gain of the control system so that the braking force applied to the roll of running web is varied in accordance with the amount of material on the roll. More particularly, the output of the divider 96 is applied to a multiplier 122 whose other input is a voltage tension command T.sub.c developed as will be described presently by measuring the instantaneous position of dancer 46 (FIG. 1). This is because the tension force on web W due to the application of brake 72 varies linearly with the drag force caused by the brake. However, the drag force does not vary linearly with the pressure applied by the transducer 126 to the brake. Rather, the drag force varies in accordance with the pressure divided by the radius of the running roll 10. In the present system, a linear variation between the commanded tension T.sub.c and resultant paper tension T is achieved by multiplying the commanded tension T.sub.c by the radius r.sub.1 of roll 10 in multiplier 122. The output of multiplier 122 then controls a current driver 124 which drives the pressure transducer 126 operating the brake 72.

When the roll on the lower set of arbors 28 is the running roll, the output of multiplier 122 is applied to a similar current driver 128 which drives a transducer 130 associated with brake 76. Thus, the pressure applied to the brake controlling the running web roll always reflects the size of the roll. For a given tension command, this pressure becomes less as the roll becomes smaller because less drag force is necessary to maintain the same amount of tension in the web.

Still referring to FIG. 2, to develop the tension command T.sub.c, the resistance value of potentiometer 66 is used to measure the position of dancer 47. More particularly, the voltage developed across the potentiometer is applied to an amplifier 150 having a variable offset control 152 which sets the dancer zero position Y.sub.O. The output of the amplifier 150 is a positive or negative voltage reflecting the deviation of the dancer from its zero position Y.sub.O. This voltage is applied by way of a switchable attenuator 154 to a resistor 156 which forms one leg of a summing network shown generally at 158. Attenuator 154 is switched to increase the gain of the circuit to brake the running roll to a halt during an emergency stop situation by a FAST STOP signal applied to its terminal 155 as will be described later.

The voltage across the potentiometer 66 is also applied to a differentiator 162 whose output represents the rate at which the dancer 47 is moving at any given time. This signal is amplified by an amplifier 164 and applied by way of a second switchable attenuator 166 to the summing circuit 158 through the summing resistor 168.

The net voltage at network 158 represents the tension command T.sub.c. This voltage is applied via a normally closed switch 172 and an amplifier 174 to the multiplier 122 as noted above.

During normal operation of the apparatus, switch 172 is closed and the gain of the servoloop is such that as the dancer moves away from its zero position Y.sub.O in response to a change in web tension, a tension command T.sub.c is issued which, when multiplied by the radis r.sub.1 of the running roll, applies just the right braking force to restore the proper tension condition in the web and return the dancer to its zero position. The desired tension T.sub.O can be set by way of an operator-controlled variable resistor 176 in the summing network 158 which is connected to a suitable positive voltage source.

The inclusion of dancer velocity feedback in the servoloop which develops the tension command T.sub.c introduces a damper into the system so that proper selection of position and velocity gains in the servoloop results in a well-behaved system. For example, in a typical apparatus, a 10 pound change in web tension results in a dancer 46 displacement of only one inch. The preferred system is designed with unity gain so that there is minimal overshoot of the dancer 46 when it changes position.

When, for one reason or another, it is desired to brake the running web to a stop very quickly, a FAST STOP signal is applied to terminal 155. This changes the gain of the system so that the system responds as though there were a rapid decrease in web tension by greatly increasing the tension command voltage applied to multiplier 122. This, in turn, results in rapid increase in the brake pressure applied to brake 72 controlling the running web. It is important to note at this point that the braking force applied to the running roll during an emergency stop is still modulated by the size of the running roll. Therefore, more braking force is applied for a large roll than for a smaller roll. Consequently, a roll of any size is brought to a stop in a controlled fashion in a minimum amount of time.

When the running roll is almost depleted, a COMMENCE SPLICE signal from relay 106 opens switch 172 disconnecting the summing network 158 from the amplifier 174. The signal also closes a switch 184 which applies the output of a ramp generator 186 to the amplifier. The voltage ramp from generator 186 causes the transducer 126 to apply a gradually increasing pressure on brake 72 controlling the running web so that the roll stops at a predetermined rate.

When the running web reaches zero speed (or a selected minimal speed), the relay 114 initiates the splicing sequence and then activates the eddy current drive associated with the ready web roll 26 which supplements the acceleration given the ready roll by the pulling force of the web. The eddy current drive is turned off when the ready web reaches some selected percentage of running web speed V.sub.P, usually on the order of 70-90 percent as will be described presently.

As the ready roll 26 is being accelerated, the dancer 46 continues to move down for a time because there is still a net loss of web from the accumulator 18. Then it moves upward as the accumulator refills. Since there is a danger that the dancer might bottom or move up too far, the attenuator 166 in the dancer velocity feedback branch is changed to increase gain and thus increase velocity damping. This limits the acceleration of the dancer during its downward movement and also as it moves upwards during the splice service cycle. This high gain condition is initiated at the beginning of the roll acceleration by a signal from relay 114 applied to attenuator 166 by way of a HOLD circuit 191. Circuit 191 switches the attenuator 166 back to normal after 30-60 seconds. By this time, the dancer is close to its zero position Y.sub.O. Alternatively, the attenuator 166 can be turned on for a selected time interval after cessation of the COMMENCE SPLICE signal.

The point at which the ready web has reached the desired speed is sensed by detecting when the dancer 46 is at rest or has a certain minimum velocity. More particularly, the output of amplifier 164 which represents dancer velocity is applied to a pair of dancer velocity detectors 192 and 194. Detector 192 generates an output signal when the dancer is moving upwards at a rate less than a predetermined magnitude, while detector 194 generates a similar output when the downward velocity of the dancer is less than a predetermined magnitude. An output from both detectors simultaneously thus signifies that the dancer is at rest or is moving at a velocity which is within a certain acceptance band signifying that the ready web is at the desired percentage of running speed V.sub.p. This coincidence is detected by an AND circuit 196 whose output opens a switch 198 which deenergizes a relay 202 controlling the drive 82 associated with the ready web roll 26.

The signal from amplifier 164 representing dancer velocity is also used to detect when an emergency condition exists such as a break in the web W. When the web breaks, there is a drastic decrease in web tension which causes the dancer 46 to move up rapidly away from fixed rolls 42. In the present system, the output of amplifier 164 is applied to an amplitude comparator 204 which also receives an output voltage from a velocity threshold control 206. When the voltage from circuit 164 exceeds the threshold velocity, comparator 204 actuates a driver 208 which closes a relay 210. The closing of the relay 210 can be arranged to brake the running roll to a stop or to control the air pressure applied to cylinder 62 biasing the dancer 46 to slow down the dancer before it reaches its upper and lower limit stops.

The variable gain feature of the present system allows one to control the amount of reserve storage remaining in the accumulator 18 for emergency stops in accordance with roll size. In other words, the amount of necessary reserve storage becomes less as the roll diameter decreases. Therefore, the output of the divider 96 can be applied to vary the zero position control 152 so that the zero position Y.sub.O of the dancer is lowered as the running roll becomes smaller. The accumulator can also operate much fuller during normal operation because dancer hunting is minimized.

Finally, it is important to note that the present system is substantially entirely electronic and therefore it requires a minimum amount of maintenance as compared with prior pneumatic tension control systems. Furthermore, there is less inertia and inherent delays in the present system so that much closer control can be maintained over web and dancer movements. Furthermore, the electrical components consisting in large part of standard operational amplifiers and other stock components, are relatively inexpensive to make.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained and, since certain changes may be made in the above construction without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

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

Claims

We claim:

1. Web handling apparatus comprising

A. means for supporting a roll of running web,

B. means for pulling web from the roll at a selected speed,

C. means for braking the roll,

D. means for sensing changes in the tension of the web as it runs caused by changes in the pulling force on the web,

E. means for controlling the braking means in accordance with the output of the tension change sensing means so as to tend to maintain substantially constant web tension,

F. means responsive to the instantaneous size of the roll, and

G. means for modulating the control of the braking means so that the drag force imparted by the brake to the web is substantially independent of roll size.

2. Web handling apparatus as defined in claim 1 wherein the size responsive means comprise

A. means for measuring the angular velocity of the roll of running web,

B. means for measuring the angular velocity of a fixed diameter roll engaged by the running web,

C. means for processing the two roll velocities to obtain a ratio which reflects the size of the roll of running web.

3. Web handling apparatus as defined in claim 2 wherein

A. the roll velocity measuring means comprise tachometers driven by the rolls, and

B. the processing means comprise a divider which divides the output of the tachometer associated with the fixed diameter roll by the output of the tachometer associated with the roll of running web.

4. Web handling apparatus as defined in claim 1 wherein the means for sensing changes in web tension comprise

A. a deflectible member,

1. having a selected spring constant, and

2. engaged by the running web so that the web moves the member in one direction when its tension increases and moves the member in another direction when its tension decreases, and

B. means for sensing the position of the member to develop an output to control the braking means.

5. Web handling apparatus as defined in claim 1 wherein

A. the deflectible member is a dancer in a web accumulator situated downstream from the roll of running web, and

B. the position sensing means is a potentiometer associated with the dancer which changes its resistance value as the dancer moves in response to changes in web tension.

6. Web handling apparatus as defined in claim 5 and further including

A. means for measuring the instantaneous velocity of the dancer,

B. means for applying the velocity measurement as a damping factor in the brake controlling means so as to increase the braking force on the roll of running web as dancer velocity increases.

7. Web handling apparatus as defined in claim 6 and further including

A. means for actuating the braking means so as to decelerate the roll of running web to a selected minimum speed at a programmed rate, and

B. means for detecting when the web speed is below a predetermined value.

8. Web handling apparatus as defined in claim 7 wherein the web speed detecting means comprise

A. a tachometer associated with a roll engaged by the running web, and

B. means for detecting when the output of the tachometer falls below a predetermined value.

9. Web handling apparatus as defined in claim 7 wherein the actuating means comprise a ramp generator.

10. Web handling apparatus as defined in claim 6 wherein

A. the dancer velocity measuring means comprise means for differentiating a signal reflecting the potentiometer resistance value, and

B. the applying means include a summing network that sums the output of the differentiator with the output provided by the potentiometer.

11. Web handling apparatus as defined in claim 8 and further including

A. means for supporting a roll of ready web, and

B. means for accelerating the ready roll supporting means after the web speed falls below said predetermined value.

12. Web handling apparatus comprising

A. means for supporting a roll of running web,

B. means for supporting a roll of ready web,

C. means for pulling web selectively from said roll at a selected speed,

D. means for braking the rolls,

E. a deflectible member contacting the running web and around which the running web moves,

F. means for sensing the position of the deflectible member, said sensing means emitting a signal when the member deviates from a selected position,

G. means for differentiating the signal from the position sensing means,

H. means for summing the outputs of the position sensing means and the differentiating means, and

I. means for controlling the braking means in accordance with the output of the summing means so as to tend to maintain substantially constant tension in the running web.

13. Web handling apparatus as defined in claim 12 and further including means for modulating the output of the summing means in accordance with the size of the roll of running web so that the braking force applied to the running roll varies inversely with the size of the running roll.

14. Web handling apparatus as defined in claim 13 wherein the modulating means comprise

A. means responsive to the diameter of the running web roll and emitting an output signal in response thereto, and

B. means for multiplying the output of the responsive means by the output of the summing means prior to the application of the latter to the brake control means.

15. Web handling apparatus as defined in claim 14 wherein the running web roll diameter detecting means comprise

A. means for measuring the angular velocity of the running web roll,

B. means for measuring the angular velocity of a fixed diameter roll in contact with the running web,

C. means for dividing the output of the first angular velocity measuring means by the output of the second angular velocity measuring means, and

D. means for multiplying the quotient of said division by the output of the summing means prior to application of the latter to the brake control means.

16. Web handling apparatus as defined in claim 13 further including

A. means for measuring the speed of the running web and delivering an output signal in response thereto, and

B. means responsive to the output of the modulating means for controlling the brake control means so as to brake the running web roll to a selected minimum speed when the running web roll diameter reaches a pre-determined minimum size.

17. Web handling apparatus as defined in claim 16 comprising

A. means for accelerating the roll of ready web after the running web roll reaches said selected minimum speed, and

B. means for disabling the accelerating means after the ready web roll reaches a pre-determined higher speed.

18. Web handling apparatus as defined in claim 17 wherein the disabling means include

A. a first detector means responsive to the output of the differentiating means for emitting an output when the velocity of the deflectible member in one direction from the selected position is less than a minimum value,

B. second detector means responsive to the output of the differentiating means for emitting an output when the velocity of the deflectible member in the opposite direction from the selected position is less than said minimum value, and

C. means for disabling the accelerating means up on the simultaneous occurrence of outputs from the two detector means.

19. Web handling apparatus as defined in claim 18 further including means responsive to the output of the differentiating means for causing the brake control means to brake the running web roll to a stop when the velocity of the deflectable member exceeds a selected maximum value.