U.S. patent number 3,570,735 [Application Number 04/776,486] was granted by the patent office on 1971-03-16 for method and apparatus of guiding moving webs.
This patent grant is currently assigned to GPE Controls, Inc. Invention is credited to Robert S. Kurz.
United States Patent |
3,570,735 |
Kurz |
March 16, 1971 |
METHOD AND APPARATUS OF GUIDING MOVING WEBS
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.
Inventors: |
Kurz; Robert S. (Des Plaines,
IL) |
Assignee: |
GPE Controls, Inc (Morton
Grove, IL)
|
Family
ID: |
25107497 |
Appl.
No.: |
04/776,486 |
Filed: |
November 18, 1968 |
Current U.S.
Class: |
226/3; 83/74;
83/364; 83/367; 225/96.5; 226/18; 242/534.1 |
Current CPC
Class: |
G05D
5/06 (20130101); Y10T 83/148 (20150401); Y10T
83/531 (20150401); Y10T 225/325 (20150401); Y10T
83/536 (20150401) |
Current International
Class: |
G05D
5/00 (20060101); G05D 5/06 (20060101); B65h
023/02 () |
Field of
Search: |
;226/3,15--23
;242/57.1 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Schacher; Richard A.
Claims
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;
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.
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.
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.
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.
Description
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 approach 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 at a 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 FIG. 1 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 10 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 16
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 16 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. Idler 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 idler 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
developed 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 signals 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 46 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 the 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 of 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 summing
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
regulator, shown diagrammatically in FIG. 3. Spring 84 provides a
set point reference working in opposition to coil 82 upon pivotal
jet pipe element 86. Hydraulic fluid from the jet pipe is directed
toward orifices connected to opposite ends of hydraulic cylinder
88, having output shaft 90 mechanically coupled to the guide roll
mechanism to produce the desired influence on the lateral position
of web 52.
According to conventional practice with intermediate web guide
systems, the sensing means would be placed adjacent one or both
edges of the web (depending upon whether edge guide or center guide
was desired) closely adjacent the guide 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 68 so 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 20 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 FIGS. 2 and 3. Two
sensors, labeled S.sub.1 and S.sub.2, are spaced at their
centerlines 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.sub.1 and is displaced from the
centerline of sensor S.sub.2 by a distance x, thereby creating an
angle .alpha. 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 .alpha. is zero.
Applying the examples of FIGS. 4a and 4b to the embodiment of the
invention set forth in connection with FIG. 2, sensors S.sub.1 and
S.sub.2 are considered the respective equivalents of sensors 42 and
44. Since the web edge crosses the null position of sensor S.sub.1
(42) there will be zero signal output from this sensor to signal
conditioning means 40 through line 50. The displacement of the web
edge from the null position of sensor S.sub.2 (44) 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.sub.1
(42), thereby providing an error signal through line 48
proportional to the tangent of angle .alpha. since the distance x
represents the side opposite .alpha. 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.sub.1 (42) and
applied through line 50 to signal conditioning means 40. A signal
equal in magnitude but opposite in polarity will be generated by
sensor S.sub.2 (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.sub.1 (42).
With the web edge disposed as shown in FIG. 4a in the embodiment of
FIG. 3 (sensors S.sub.1 and S.sub.2 analogous to sensors 60 and 62,
respectively) a zero signal would be produced by sensor S.sub.1
(60) and a signal proportional to distance x would be produced by
sensor S.sub.2 (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.sub.1 (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 S.sub.2 (62) will, of
course, remain proportional to x (and therefore to the tangent of
angle .alpha.) 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 sensor-amplifier 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.
4a 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.degree. 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
web and 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.
* * * * *