US3570735A

US3570735A - Method and apparatus of guiding moving webs

Info

Publication number: US3570735A
Authority: US
Inventors: Robert S Kurz
Original assignee: GPE Controls Inc
Current assignee: GPE Controls Inc
Priority date: 1968-11-18
Filing date: 1968-11-18
Publication date: 1971-03-16
Anticipated expiration: 1988-03-16
Also published as: FR2024881A1; GB1214757A

Abstract

A method and apparatus for controlling the lateral position of a moving strip or web by sensing both the instantaneous lateral position of at least one edge of the web and the instantaneous angle between the actual path of travel of the web and the desired or ideal path. Appropriate signals commensurate with lateral position and angle are developed and combined to provide a control signal which sets the rate at which the web moves or ''''walks'''' laterally toward the desired position. The sensors may be positioned in proximity to the point at which control is desired without the usual sacrifice of stability associated with such positioning.

Description

United States Patent Robert S. Kurz Des Plaines, 111. 776,486

Nov. 18, 1968 Mar. 16, 1971 GPE Controls, Inc Morton Grove, Ill.

Inventor Appl. No. Filed Patented Assignee .METHOD AND APPARATUS OF GUIDING Primary Examiner-Richard A. Schacher Attorney-Norman A. Witt ABSTRACT: A method and apparatus for controlling the lateral position of a moving strip or web by sensing both the in-. stantaneous lateral position of at least one edge of the web and the instantaneous angle between the actual path of travel of the web and the desired or ideal path. Appropriate signals commensurate with lateral position and angle are developed and combined to provide a control signal which sets the rate at which the web moves or walks laterally toward the desired position. The sensors may be positioned in proximity to the point at which control is desired without the usual sacrifice of stability associated with such positioning.

PATENTEH m1 619?! SHEET 2 BF 2 FIG. 4b

INVENTOR.

QLQLqSR METHOD AND APPARATUS F GUTDING MOVING WEBS This invention relates to lateral position control of moving webs and, more specifically, to an improved method and apparatus for effecting control of the lateral position of a longitudinally traveling web at a specified location along the line of travel thereof.

In operations involving moving webs it is often necessary to effect relative lateral movement of the web and some portion of the apparatus through which it is moving in order to achieve the desired results. in some applications the object is to maintain one edge of the web in a particular lateral position, such applications being commonly known as edge guide, while in other applications the center line of the web is maintained in a desired lateral position, such applications being commonly termed center guide. in general, the web lateral position is maintained by a closed loop servosystem having means for sensing and developing a signal commensurate with the instantaneous position of an edge or the center of the web in relation to some reference position, and effecting relative lateral movement of the web and reference in a direction tending to correct the error.

One of the most common means for effecting the required physical movement is a hydraulic cylinder actuated by fluid flow to the proper end of the cylinder from a suitable hydraulic controller responsive to the signals developed by the sensors. Since the controller and actuator have no fixed reference position, being positioned entirely in response to the error signal applied, such systems are known in the art as floating control. The flow rate of fluid to the cylinder is equal to the gain of the controller (which may be thought of as a hydraulic amplifier) times the magnitude of the applied error signal. Thus, the cylinder rod moves at a speed proportional to the error since the controller integrates the error applied to it into a flow rate to the cylinder.

it is important that gain always be maximized, while maintaining stability of the system, in order to achieve the best possible control accuracy at the sensor. For this reason, the sensing means is located in proximity to the effective point where web lateral movement takes place so that essentially instantaneous feedback may be implemented, thereby maximizing control accuracy as related to the sensing means. However, the point at which control accuracy is desired (i.e., where the relative positions of the reference and the web edge or centerline are to maintained in order to achieve the desired results) must be at some point other than the effective point of web movement; that is, the web is always shifted laterally, relative to the reference, at some point in its travel in order to influence the lateral position at another point along the line of travel where control accuracy is desired. A change in web lateral position at the latter point is thus longitudinally displaced from the sensor position and its is possible that an error may exist at the critical point of control accuracy although no error exists at the sensor. This has generally been regarded as a necessary evil, however, since 'to move the sensing means toward the point of desired control accuracy will decrease instantaneous feedback, thereby necessitating an overall decrease in the control gain and its error removal ability in order to maintain stability.

The present invention represents a significant departure in control technique from the prior art systems described above by providing an additional feedback which allows relocation of the sensors to the point where control is desired without sacrificing gain or stability. This additional feedback is in the form of a signal commensurate at least in part with the instantaneous angle between the actual and desired path of travel of the web edge or centerline. The tangent of this angle times the speed at which the web is traveling is the rate at which the web moves or walks laterally at a point where it is frictionally restrained against lateral movement, such as a fixed idler or process roll. If the control is such that it causes the web to appreach the process roll at an angle whose tangent is proportional to the lateral error in position then the control error integrating action is transferred from the hydraulic cylinder to the lateral walking rate of the web at the idler roll. Thus, the nature of the control with respect to the hydraulic cylinder is changed from integrating to positioning since there is a discrete position of the cylinder rod associated with any given angle at which the web should be set to approach the process roll.

The principle object of the present invention is to provide a closed loop control system for maintaining a desired relationship between the lateral position of a moving web and a reference position with greater accuracy ata fixed point than is commonly attainable according to present practices.

Further objects of the invention are: To provide a novel method for rearranging web position control hardware to achieve greater accuracy; to provide a web edge guide system wherein an error in edge position is corrected by setting the rate at which the web walks toward the correct position; to provide a method of web lateral position control by sensing the angle at which the web is disposed with respect to the desired line of travel and developing a control signal commensurate with the tangent of such angle; and, to provide a method of controlling web lateral position which allows placing the sensing means in close proximity to the point where position control is desired without sacrificing stability of the control loop.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others thereof, which will be exemplified in the method hereinafter disclosed, and the scope of the invention will be indicated in the claims.

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 showing of a typical example of one application of conventional web lateral position control;

FIG. 2 is a similar diagrammatic illustration utilizing the method of the present invention in the application of FIG. 1;

FIG. 3 is a diagrammatic showing, in somewhat greater detail, of another application of web position control utilizing the present invention; and

FIGS. 4a and 4b are enlarged, fragmentary, diagrammatic showings of two examples of positional relationship between web and sensors.

The diagram of FIG. 1 is intended to show the basic system of a conventional web position control system wherein one edge of the web is maintained in a desired relationship to a reference position. in FIG. 1 the edge control system is applied in a typical manner to provide edge alignment of the coil on which the web is wound at the end of its travel. This application is usually termed windup or tension reel" control and provides a good indication of the effectiveness of the system since the side of the wound coil shows very clearly any misalignment of the edge. Although. improvement of control accuracy in tension reel applications is one of the major advantages of the invention, it will be apparent from the ensuing description that the invention may be applied in other edge guide, and center guide applications as well.

in FlG. l the reference numeral 10 denotes an elongated strip or web of material being longitudinally transported in a direction indicated by the arrow by suitable drive means (not shown). Web 10 is wound into a coil 12 at the end of its travel after passing over fixed idler roll 14. it is desired that edge 16 of web ill be laterally positioned when wound on coil 12 so that the end of the coil formed by edge 16 will be perfectly flat. Thus, the edge of the coil comprises the reference position relative to which edge i6 is moved in order to achieve the desired result.

The control system provided to insure such alignment of edge 16 includes sensor 18, signal conditioning means 20, controller 22, and actuator 24. These elements are shown in block form, being components of conventional web control systems in wide commercial use. Sensor 18 may comprise any suitable means for developing a signal having a magnitude commensurate with the lateral deviation of edge i6 from a desired position, such means including pneumatic, photoelectric, mechanical, as well as many others. The signal from sensor 18 is suitably modified by signal conditioning means 20 to provide a usable error signal input to controller 22. The latter may comprise, for example, a conventional jet pipe hydraulic relay regulator which serves to control movement of actuator 24 in the form of a hydraulic cylinder at a rate of speed proportional to the error signal. Actuator 24 is arranged to effect reciprocal movement of coil 12 in a direction transverse to the direction of travel of web 10. ldler roll 14 serves to restrain the web against lateral movement at a point some distance upstream from coil 12. Since in this application the reference position (end of coil 12) moves laterally with respect to the direction of web travel, sensor 18 is mounted upon rigid arm 26 which is connected to actuator 24 so that sensor 18 is moved along with coil 12 and remains in fixed relation to the reference position.

The object of this system is to move coil 12 so as to maintain alignment of the end thereof with edge 16, as the latter approaches the coil, by developing an error signal commensurate with the difference between the lateral position of edge 16 in advance of idler roll 14 and the reference edge of coil 12. in other words, the reference position is moved so as to be ideally at the proper lateral position as instructed by the sensors in accordance with the approaching web edge position. In this respect tension reel applications present a somewhat special case since in all other conventional applications the normal practice is to shift the web laterally at some point upstream of a stationary reference at which the web is frictionally restrained against lateral movement; in the case of tension reel, as previously explained, the reference is shifted laterally while frictionally restraining the web at some point upstream. However, it should be noted that both cases are mathematically identical for purposes of analyzing control performance and the shift in web alignment and reference position is entirely relatively although the sensing means must, of course, remain fixed with respect to the reference. Thus, it may be said that lateral movement of the web relative to the coil takes place at the idler roll, whereby it is at this point that the relative movement or control is effected, but the coils is the point where lateral control is desired.

Turning now to FIG. 2, a similar system of tension reel control is shown, suitable for carrying out such control according to the method of the present invention. Web 28 is wound on coil 30 after passing over fixed idier roll 32 with edge 34 maintained in fixed lateral alignment relative to the end of coil 30 by moving the coil laterally in exactly the same way as in the prior art example. That is, coil 30 is moved by actuator 36 under the control of controller 38 in response to an input from signal conditioning means 40 which operate on the signal developed by suitable sensing means. In the present example, however, it will be noted that the sensing means comprises two

edge sensors

42 and 44, rather than only one, and that the sensing means is positioned in proximity to the windup coil rather than the idler roll. Again, the sensors are mounted on rigid arm 46 for movement by actuator 36 so as to remain in fixed relationship to the reference, i.e., to the end of coil 30.

A basic principle of the present invention is developing an error signal which includes a component commensurate with the deviation of the instantaneous line of travel of the controlled edge from the ideal or desired line of travel, as opposed to providing an error signal indicating only the deviation of the instantaneous lateral position of one point on the edge. In other words, the component of the signal providing the additional feedback is commensurate with the angle between the actual and desired lines of travel (at the sensor location) of the controlled edge. This angle is sensed, and the signal developed, by positioning two separate sensors at spaced points along edge 34 and combining the edge position signals deveioped thereby to produce a resultant error signal. The null setpoint of

sensors

42 and 44 is, of course, so calibrated that a zero resultant signal is produced when edge 34 is disposed in exact alignment with the end of coil 30.

The signais produced by

sensors

42 and 44 are preferably opposite in polarity for a web edge deviation in the same direction. That is, if the signal from sensor 42 becomes higher in positive value as a greater area of the sensor is uncovered by the web, then the signal from sensor 44 should increase in a negative direction as the web uncovers more of that sensor. The signals are summed at junction #56 and applied through line 48 to signal-conditioning means 40. The latter are preferably constructed and arranged to cause the magnitude of the signal supplied to controller 38 to be in direct proportion to the tangent of the angle between the sensed line of edge 34 and the ideal line thereof which, as previously stated, would produce a zero resultant signal from the sensing means. In this way, the correction factor applied is greater as the error becomes greater. A separate signal from sensor 42 is supplied through line 50 to signal-conditioning means 40. This signal is commensurate with the deviation of the instantaneous position of the edge of web 34 from the null or reference position at sensor 42, as in the single sensor of the prior art example. Of course, a third sensor could be used in the present system to provide the edge position signal through line 50, if such arrangement were desireable, with

sensors

42 and 44 being utilized to produce only the angle signal. The signals provided through

lines

48 and 50 are combined by signal conditioning means 40 to provide an error signal to controller 38. The resultant flow rate from the controller causes movement of actuator 36, and thereby coil 30, to a position which will cause the angle at which edge 34 approaches coil 30 to be commensurate with the deviation between actual and desired edge position at the sensing means. Movement of coil 30 in a direction transverse to the direction of travel web 28 tends to cause a relative lateral walking action of the web with respect to the coil. Since he walking rate of the web with respect to the coil is in linear proportion to the tangent of the aforementioned angle, the walking rate will be set in accordance therewith. Thus, the control signal sets the walking rate of the web and makes this factor the error integrator of the system.

An example of the application of the present invention to a typical web guide system intermediate of the line of travel of the web (as opposed to tension reel) is shown in FIG. 3. This figure also includes a more detailed showing of a complete system for implementing the invention. Web 52 passes through a conventional guide roll arrangement consisting of a pair of rigidly attached rolls 54 and 56, suitably mounted for lateral and pivotal movement according to well known practice for influencing the lateral movement of web 52. Fixed idler roll 58 is provided to frictionally restrain lateral movement at a desired point downstream in the travel of web 52 from the guide roll arrangement. A pair of

sensors

60 and 62 are positioned adjacent edge 64 0f web 52 at a station near idler roll 58. In this

example sensors

60 and 62 are assumed to be of the photoelectric type, wherein a light source is positioned on the opposite side of web 52 and impinges upon the photosensitive surfaces of the sensors in accordance with the degree to which opaque web 52 is interposed therebetween. Hence the signals produced by the sensors, e.g., the current output from one or more solar cells, will be in proportion to the lateral position of edge 64 at the sensor station.

Sensors

60 and 62 are connected to

amplifiers

66 and 68, respectively, whereby the voltage on the output side of these amplifiers will be proportional to the current output of the solar cells. Conventional means are provided at 70 and 72 for independently adjusting the gain of

amplifiers

66 and 68, respectively. Outputs of the two amplifiers are summed via :mming resistors 74 and 75 and the combined signal supplied as in input to a third amplifier 76. The gain of amplifier 76 is established by the ratio of fixed resistances 78 and 80.

The output of amplifier 76 serves as the control signal input to moving coil 82 of a conventional jet pipe hydraulic relay guide or center guide was desired) closely adjacent the guide a roll mechanism rather than in the vicinity of the idler roll. Such an arrangement would provide the instantaneous feedback regarding the effect on web position of guide roll movements, as explained earlier. The object of the present invention, however, is to establish the rate at which the web walks laterally across idler roll 58 in response to a lateral shifting movement by the guide roll mechanism. This is done, as in the previously described embodiment, by utilizing-a control signal commensurate with the angle between the actual and desired lines of travel of edge 64. Since the walking rate of the web is the error integrator, a relatively large total positioning gain may be utilized for the system,.while avoiding instability.

FIG. 3 also illustrates an alternate method of taking into account the lateral position of edge 64, as well as the angle. This may be accomplished by adjusting the gain of amplifiers 66 and bdso that the signal from one sensor is weighted more heavily than the signal from the other sensor. The same thing could be accomplished, of course, by proper selection of the values of summing resistors 74 and 75. The differential weighting of the signals from

sensors

60 and 62 in this manner produces a resultant control signal which properly takes into account the position of edge 64 as well as the angle at which it is disposed relative to the ideal line of travel. A typical example of the magnitude of weighting contemplated is on the order of percent greater value for the signal from one sensor for an identical deviation of web edge 64.

The diagrammatic examples of FIGS. 4a and 4b may aid in a more complete understanding of the operation of certain aspects of the invention, as shown in the applications of both H68. 2 and 3. Two sensors, labeled S, and 5,, are spaced at theircenterlines by a distance d The sensors are adapted to produce appropriate signals, in accordance with known techniques, in accordance with the amount of the sensor surface which is covered by the moving web. The signals from the sensors relative to the amount of web coverage are so calibrated that zero output, i.e., no signal, is produced when exactly one-half of the sensor surface is covered by the web. The ideal line of travel of the illustrated edge of the web (in terms of the ideal final result) is indicated by the dashed line, covering half of each sensor, thereby producing zero output or error signal. In FIG. 4a the web extends directly over the centerline of sensor S, and is displaced from the centerline of sensor S, by a distance x, thereby creating an angle a between the actual and ideal lines of travel of the web edge having a tangent equal to ('x /d). In FIG. 4b the web edge is displaced by an equal distance (x) from the half-covering position of each sensor, whereby the edge is parallel to the path of ideal or desired travel and angle a is zero.

Applying the examples of FIGS. 4a and db to the embodiment of the invention set forth in connection with FIG. 2, sensors S, and S are considered the respective equivalents of

sensors

32 and 44. Since the web edge crosses the null position of sensor S, (42 there will be zero signal output from this sensor to signal conditioning means through line 50. The displacement of the web edge from the null position of sensor S (at by distance x will produce a signal proportional to this distance which will be summed at junction 46 with the zero signal from sensor S, (42), thereby providing an error signal through line 48 proportional to the tangent of angle or since the distance x represents the side opposite a and the distance d, representing the side adjacent, is always the same.

When the web edge is disposed as in FIG. 4b, a signal commensurate with distance x will be generated by sensor S, (62) andapplied through line 50 to signal conditioning means 40. A signal equal in magnitude but opposite in polarity will be generated by sensor S (44) since the web is displaced by an identical distance in the same direction. Hence the signals from the two sensors will cancel one another when summed at junction 46; the signal indicating the angle between actual and desired paths of travel of the web edge will be zero, and the error signal will be proportional only to the deviation between actual and desired positions of the edge of the web at sensor S, (42).

. With the web edge disposed as shown in FIG. 4a in the embodiment of'FIG. 3 (sensors S, and S analogous to sensors and 62, respectively) a zero signal would be produced by sensor S, (60) and a signal proportional to distance x would be produced by sensor S (62). The signal from each sensor is multiplied by the gain of its respective amplifier, and the'outputs of the two amplifiers are summed to provide the resultant error signal. Since-the signal'from sensor S, (60) is zero with the web edge disposed at the null point, this will remain zero regardless of the gain of amplifier 66. The signal form sensor 8, (62) will, of course, remain proportional to x (and therefore to the tangent of angle a) when multiplied by the gain of amplifier 68. When the web edge is disposed as in FIG. 4b the signals from the two sensors will be equal in magnitude and opposite in polarity, as in the previous example, but they'are multiplied by two different gains before being summed. Hence, there will be a resultant signal, based on the sensoramplifier combination with the largest gain, based on the positional deviation of the web edge (zero, angle error) from the ideal. Operation of various embodiments of the invention for web edge dispositions other than those shown in FIGS. 40 and 4b should be apparent from the foregoing explanation.

The integrating control systems of the prior art operate to set the rate of change of actuator (cylinder rod) position in proportion to the magnitude of the error between process and setpoint (i.e., the null point of the sensor); as long as an error exists the actuator will continue to move to correct that error. The positioning control method of the present invention will cause the actuator to assume a position which sets the web at an angle (between the points of lateral movement and of. frictional restraint against lateral movement) causing it to walk laterally toward the correct position at a rate proportional to the magnitude of the error. A positioning control does not have the 90 phase lag inherent in an integrating system. It is thus apparent that the control technique of the present invention, based on the angle-sensing and feedback method described herein, although carried out with conventional web guide hardware and involving relatively simple rearrangements and modifications thereof, represents a fundamentally different approach to the problem.

From the foregoing description, it is also apparent that the invention may also be applied in center guide applications. For example, the center line position may be established by summing signals from two sensors positioned adjacent opposite sides of the weband combined with a signal commensurate with the angle of one edge as explained above. As an additional alternative, two sensors could be positioned adjacent each web edge with the signals processed in two networks similar to that shown in FIG. 3 and ultimately combined to provide a single control signal commensurate with both the position and angle of the web center line.

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 carrying out the above method without departing from the scope of the invention, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

i claim:

1. A method of controlling the lateral position of a moving web at a given location along the path of travel thereof relative to a reference position said method comprising:

a. positioning sensing means in fixed relation to said reference position and in such relation to said web as to sense both positional angular deviation of the path of travel of a longitudinal section of said web from a line representing the path of travel of said section required in order to maintain a desired lateral position of said web at said given location;

. sensing the path of travel of said web section; and

c. effecting relative lateral movement of said web pursuant to the path of travel sensed and said reference position at points spaced along the line of travel of said web to a position wherein said web is urged toward lateral movement at said given location in a direction tending to correct said deviation and at a rate proportional to the summation of said positional and angular deviation.

2. The method according to claim 1 wherein said sensing means are positioned in close proximity to said given location.

3. The method according to claim 2 wherein said reference position, and therefore said sensing means, remain stationary, said relative lateral movement being effected by physically shifting said web laterally at a point spaced along said line of travel from said reference position.

4. The method according to claim 3 wherein said'web is frictionally restrained against lateral movement at said given location.

5. The method according to claim 2 wherein said reference position, and therefore said sensing means, is movable, said relative lateral movement being effected by physically shifting said reference position and moving said web laterally therewith, while frictionally restraining said web against lateral movement at a point spaced along said line of travel from said reference position.

6. The method according to claim 1 wherein said sensing means comprise a pair of web edge position sensors positioned at spaced points along one edge of said web and each adapted to produce a signal commensurate with the lateral position of the web edge adjacent thereto.

7. The method according to claim 6 wherein said one edge comprises said longitudinal section and said signals commensurate with lateral position are combined to provide a resultant signal commensurate with said angular deviation.

8. The method according to claim 6 wherein said one edge comprises said longitudinal section and said sensors are calibrated to produce zero output when the path of travel of said web edge coincides with said line of required travel.

9. The invention according to claim 8 wherein said signals from each of said sensors are proportional in magnitude to the amount of deviation of said edge from said required line of travel and opposite in polarity for deviations on opposite sides of said line, and said signals from the two sensors are opposite in polarity from one another for deviations of said edge on the same side of said line.

10. The invention according to claim 9 wherein said signals from the two sensors are multiplied by different gain factors and are then summed to provide a resultant signal which is thereby commensurate with both positional and angular deviation of said edge from said line at said sensing means.

1 l. The method of controlling the lateral position of a moving web at a given location along the path of travel thereof relative to a reference position, said method comprising the steps of: sensing angular deviation of the web from a desired path of travel and generating a signal therefor, sensing the positional deviation of the web from a desired lateral position and generating a signal therefor, combining the angular deviation and positional deviation signals to produce an error signal, and effecting lateral movement of said web and said reference position in response to said error signal to correct the deviation.

12. In an apparatus for controlling the lateral displacement of a longitudinal moving web at a given location along the path of travel thereof relative to a reference position: means for developing an error signal that includes a component commensurate 'with the deviation of the instantaneous path of travel of the web from the desired path of travel, said errorsignal-developing means comprising means for sensing the angular deviation of the web from a desired path of travel and for developing an angular deviation signal, means for sensing the positional deviation of the web from a desired lateral position and for developing a positional deviation signal, and means summing the angular and positional deviation signals to produce said error signal, and means responding to said error signal to effect lateral movement of the web to correct the deviation at a rate proportional to said error signal.

13. The combination defined in claim 12, wherein said angular deviation sensing means includes first and second sensor means developing signals which are summed to provide the angular deviation signal.

14. The combination defined in claim 13, wherein the signal of one of said first and second sensors defines the positional deviation signal.

15. The combination defined in claim 14, wherein said first and second sensor means include a pair of web edge position sensors positioned at spaced points along one edge of the web and each adapted to produce a signal commensurate with the lateral position of the web edge adjacent thereto.

Claims

1. A method of controlling the lateral position of a moving web at a given location along the path of travel thereof relative to a reference position said method comprising: a. positioning sensing means in fixed relation to said reference position and in such relation to said web as to sense both positional angular deviation of the path of travel of a longitudinal section of said web from a line representing the path of travel of said section required in order to maintain a desired lateral position of said web at said given location; b. sensing the path of travel of said web section; and c. effecting relative lateral movement of said web pursuant to the path of travel sensed and said reference position at points spaced along the line of travel of said web to a position wherein said web is urged toward lateral movement at said given location in a direction tending to Correct said deviation and at a rate proportional to the summation of said positional and angular deviation.

4. The method according to claim 3 wherein said web is frictionally restrained against lateral movement at said given location.

11. The method of controlling the lateral position of a moving web at a given location along the path of travel thereof relative to a reference position, said method comprising the steps of: sensing angular deviation of the web from a desired path of travel and generating a signal therefor, sensing the positional deviation of the web from a desired lateral position and generating a signal therefor, combining the angular deviation and positional deviation signals to produce an error signal, and effecting lateral movement of said web and said reference position in response to said error signal to correct the deviation.

12. In an apparatus for controlling the lateral displacement of a longitudinal moving web at a given location along the path of travel thereof relative to a reference position: means for developing an error signal that includes a component commensurate with the deviation of the instantaneous path of travel of the web from the desired path of travel, said error-signal-developing means comprising means for sensing the angular deviation of the web from a desired path of travel and for developing an angular deviation signal, means for sensing the positional deviation of the web from a desired lateral position and for developing a positional deviation signal, and means summing the angular and positional deviation signals to produce said error signal, and means responding to said error signal to effect lateral movement of the web to correct the deviation at a rate proportional to said error signal.