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Claims  |
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What is claimed is:
1. A control system to register the angular position of a rotatable member
with respect to a moving series of indicia, said rotatable member having a
homing position, comprising,
means for scanning the indicia at a preselected sample rate to detect a
selected one of said indicia and producing a first signal indicative of
the presence of said selected indicia,
means for sensing the angular position of the rotatable member and
producing a second signal indicative of that angular position,
means for comparing coincident ones of said first and second signals to
produce an error signal whose magnitude and sign measure the magnitude and
direction of any misregistration between the position of the selected
indicia and a preselected angular position set point of the rotatable
member,
electronic means for processing said error signal to produce a correction
signal comprising means for producing a gain that is proportional to the
magnitude of the error signal, means for producing a gain that reflects
the integrated error over a preceding number of said error signals, means
for separately adjusting the proportional gain, the integral gain, and an
overall gain which is the sum of the proportional and integral gains, and
means for zeroing said integral gain when 1) the signs of said integral
gain and said proportional gain differ and (ii) the proportional gain
exceeds a preset percentage of the integral error band, and
mechanical means responsive to said correction signal for continuously
adjusting the mutual displacement of said selected indicia and said
rotatable member to bring them into, and maintain, register.
2. The register control system of claim 1 further comprising means for
determining the speed of travel of selected indicia, and wherein said
overall gain adjusting varies as a function of said indicia travel speed.
3. The register control system of claim 1 wherein said angular position
sensing means comprises an angular position transducer that produces
multiple equally spaced position pulses with each revolution of the
rotatable member, and further comprising digital electronic means for
interpolating the angular position of said scanning and homing pulses
between said position pulses.
4. The register control system of claim 3 wherein said digital electronic
means includes a high frequency clock, and means for counting clock pulses
between the each of said scanner and homing pulses and the immediately
preceding and following transducer position pulses.
5. The register control system of claim 1 further comprising means for
displaying for each revolution of said rotatable member,
a vector arrow whose length represents the angular distance between
successive homing pulses defined as the head of the arrow and whose
direction represents the direction of indicia travel,
a first marker displayed along said arrow at a position along said arrow
corresponding to the angular position of the selected indicia with respect
to that of the homing pulse, and
a second marker displayed along said arrow indicative of the angular
position of said set point with respect to said homing pulse,
the displayed linear distance between said selected indicia first marker
and said preselected angular position second marker providing a real-time
analog representation of any said misregistration.
6. The register control system of claim 1 further comprising means for
activating said first signal producing means intermittently during each
revolution of said rotatable member thereby creating an inspection zone,
and means for moving the inspection zone to correspond to movement of said
selected indicia.
7. The register control system of claim 1 wherein said selected indicia are
a series of registration marks on a preprinted web and said rotatable
member is a function cylinder that acts on the web.
8. A control process for registering the angular position of a rotatable
member with respect to a moving series of indicia, said rotatable member
having a homing position, comprising,
scanning the indicia at a preselected sample rate to detect a selected one
of said indicia,
producing a first signal responsive to said scanning indicative of the
presence of said selected indicia,
sensing the angular position of the rotatable member,
producing a second signal responsive to said sensing indicative of that
angular position,
comparing coincident ones of said first and second signals to produce an
error signal whose magnitude and sign measure the magnitude and direction
of any misregistration between the position of the selected indicia and a
preselected angular position set point of the rotatable member,
electronically processing said error signal to produce a correction signal
comprising:
(i) producing a gain that is proportional to the magnitude of the error
signal,
(ii) producing a gain that reflects the integrated error over a preceding
number of said error signals,
(iii) separately adjusting the proportional gain, the integral gain and an
overall gain which is the sum of the proportional and integral gains, and
(iv) zeroing said integral gain when 1) the signs of said integral gain and
said proportional gain differ and (ii) the proportional gain exceeds a
preset percentage of the integral error band, and
continuously mechanically adjusting the mutual displacement of said
selected indicia and said rotatable member in response to said correction
signal to bring them into, and maintain, register.
9. The register control process of claim 8 further comprising determining
the speed of travel of selected indicia, and varying said overall gain as
a function of said indicia travel speed.
10. The register control process of claim 8 wherein said angular position
sensing comprises producing multiple equally angularly spaced position
pulses with each revolution of the rotatable member, and digitally
interpolating the angular position of said scanned and homing pulses
between said position pulses.
11. The register control process of claim 10 wherein said digital
interpolating includes counting clock pulses between each of said scanned
and homing pulses and the immediately preceding and following position
pulses.
12. The register control process of claim 8 further comprising the step of
displaying for each revolution of said rotatable member
a vector arrow whose length represents the angular distance between
successive homing positions defined as the head of the arrow and whose
direction represents the direction of indicia travel,
a first marker displayed along said arrow whose position along said arrow
corresponds to the angular position of the selected indicia with respect
to said homing pulse,
a second marker displayed along said arrow indicative of the angular
position of a set point with respect to said homing pulse,
the displayed linear distance between said selected indicia marker and said
preselected angular position marker providing a real-time analog
representation of any said misregistration.
13. The register control process of claim 8 further comprising the steps of
activating said first signal producing intermittently during each
revolution of said rotatable member thereby creating an inspection zone
during said activation, and moving the inspection zone to correspond with
movement of said selected indicia.
14. The register control process of claim 8 wherein said selected indicia
are a series of registration marks on a pre-printed web and said rotatable
member is a function cylinder that acts on the web.
15. A process to control asynchronous misregistration between the angular
position of a rotatable member and a moving series of indicia, said
rotatable member having a homing position, the process scanning said
indicia to detect a selected one of said indicia, producing a first signal
in response to said detecting indicative of the presence of said selected
indicia, sensing the angular position of the rotatable member, producing a
second signal in response to said sensing indicative of that angular
position, comparing coincident ones of said first and second signals to
produce an error signal whose magnitude and sign measure the magnitude and
direction of any misregistration between the position of the selected
indicia and a preselected angular position of the rotatable member,
processing said error signal to produce a correction signal, and
continuously adjusting the mutual displacement of said selected indicia
and said rotatable member in response to said correction signal to bring
them into register, comprising
activating said first signal producing intermittently during each
revolution of said rotatable member to produce an inspection zone that
precedes and includes said selected indicia, and
moving said inspection zone in coordination with the asynchronous movement
of said selected indicia with respect to said homing pulse.
16. The register control process of claim 15 wherein each said inspection
zone begins generally midway between the two most widely spaced of said
indicia in a given revolution of said rotatable member and terminates upon
detection of one of said selected indicia.
17. A high accuracy system for measuring the angular position of a
rotatable member comprising an angular position transducer that produces
multiple equally angularly spaced first pulses with each revolution of the
rotatable member, and digital electronic means for interpolating the
angular position of the rotating member at a given angular position
between two adjacent ones of said first pulses using measurements of a
time interval .DELTA.T1 for the rotating member to rotate from a first one
of said adjacent first pulses to said given angular position and interval
.DELTA.T2 for the rotatable member to rotate from said given angular
position to the second of said adjacent first pulses.
18. The high accuracy measurement system of claim 17 wherein said given
angular position is identified by a second pulse, wherein said digital
electronic means includes a high frequency clock, means for separately
counting pulses emitted by said clock during said time intervals .DELTA.T1
and .DELTA.T2, and means for forming a ratio of said counts.
19. The high accuracy measurement system of claim 18 further including
means for measuring the time intervals between two adjacent pairs of first
transducer pulses where one of said pairs precedes said interval .DELTA.T1
and the others of said pairs follows the interval .DELTA.T2, and means for
averaging these preceding and following intervals to measure the interval
therebetween that includes said given angular position.
20. A real-time, two-dimensional, analog display of a system that controls
the register between the angular position of a rotatable member and a
moving series of selected indicia, said rotatable member having a homing
position, and said system scanning the selected indicia to detect said
selected indicia and producing a first signal indicative of the presence
of said selected indicia, sensing the angular position of the rotatable
member and producing a second signal indicative of that angular position,
and comparing coincident ones of said first and second signals to produce
an error signal whose magnitude and sign measure the magnitude and
direction of any misregistration between the position of the selected
indicia and a preselected set point angular position of the rotatable
member, processing said error signal to produce a correction signal,
comprising,
a vector arrow whose length represents the angular distance between
successive homing positions defined as the head of the arrow and whose
direction represents the direction of indicia travel,
a first marker displayed along said arrow whose position along said arrow
corresponds to the angular position of the selected indicia with respect
to that of said homing pulse, and
a second marker displayed along said arrow indicative of the angular
position of said set point with respect to said homing pulse,
wherein the displayed linear distance between said first and second markers
provide a real-time analog representation of any said misregistration.
21. The real-time, two-dimensional, analog display of claim 20 wherein said
first signal producing is activated periodically during each revolution of
said rotatable member and further comprising a bar arrayed along said
vector arrow whose length and position with respect to the arrow
corresponds to an inspection zone where said first signal producing is
activated.
22. The real-time, two-dimensional, analog display of claim 21 wherein the
position of said markers and inspection zone are redisplayed with each
revolution of said rotatable member to produce an analog visual
representation of the register correction. |
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Claims |
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Description |
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BACKGROUND OF THE DISCLOSURE
This invention relates in general to feedback control systems, and more
particularly to a method and apparatus to control the operation of
equipment acting on a moving web in coordination with the location of
printed material on the web.
In general, it is well known to use feedback loops to control an operation
in response to one or more sensed inputs. In the printing industry it is
necessary to coordinate the position of a rapidly moving web of paper with
the operation of printing cylinders. A common approach is to print a
series of registration marks along the web, and then scan the marks. A
basic approach introduced by Hurletron is to scan the web for brief
intervals in an inspection zone centered on the anticipated position of
the mark. A lead inspection zone precedes the mark, followed by a dead
zone and a lag inspection zone. Sensed misregistrations in the dead zone
are ignored to avoid hunting. Also, the sensed mark is validated; the same
mark must be seen in the inspection zone for a predetermined number of
successive scans. This control arrangement works well in a printing press,
but it works well because the position of a mark does not move relative to
the controlled cylinder. It is synchronous with the cylinder. Where the
operation is asynchronous, as where there is a cumulative error or an
out-of-register splice, the control system does not see the same mark the
required number of successive times. No mark is validated, and the system
loses control.
In off-press web finishing, e.g. in folding, perforating, gluing, and
cutting a pre-printed web, the location of the marks is often not
well-determined; they are not synchronous with the cylinder. As a result,
conventional on-press controls do not function well for off-press
applications, or more generally, for applications where the position of
the mark can change significantly, whether due to localized variations or
to cumulative variations in the length of the impressions printed on the
web. These variations occur because once the web is released from the
tight control of the press, variations in web tension due to printing,
heating, chilling, handling and atmospherics cause corresponding
variations in the dimensions of the web, and hence of the impressions
printed on the web. These variations are particularly acute when the
printed web is rewound, stored, and then unwound at a later time to be run
through a finishing line. They tend to make impressions run consistently
longer or shorter than their initial printed length.
U.S. Pat. Nos. 5,129,568 and 5,224,640 to Fokos et al. disclose an off-line
web finishing system which overcomes many of the problems of prior art
systems that maintain registration by stretching the web, introducing
variations in the path length, phasing the operation of the function
cylinders, or some combination of these approaches. These prior art
systems and their deficiencies are detailed in these patents. With
cumulative (synchronization) error, the phasing gears usually are not able
to keep up with the error since the error may be greater than the maximum
correction rate possible without introducing over correction or hunting.
With path length changes, compensating rolls or equivalent structures soon
reach their operational limits and cannot accommodate further repeated
errors of the same type. Moreover, the dynamic response of known systems
often results in system instabilities such as hunting.
The Fokos et al. patents solve these problems in the way machinery in the
line is driven. In one form, these patents teach using 1) two drive
shafts, a main shaft for web transport and a secondary shaft for the
function cylinders, and 2) a continuous ratio adjustment between the speed
of operation of the lineshaft to correct for cumulative error. One line
drives the other line through a variable transmission or the like to
produce the continuous ratio adjustment. Conventional phasing gears at
each function cylinder can provide additional adjustment to deal with
localized errors. These patents also disclose independent drive motors at
each function cylinder, also operated with continuous ratio adjustments
with respect to the web transport. While these drive systems provide
significant performance advantages over the prior art, they nevertheless
have certain drawbacks.
First, transmissions, lineshafts, and other equipment connected between the
shafts and driven cylinders introduce some degree of play, which is in
itself a source of misregistration. Second, the dynamic response of these
mechanical systems is limited in part by the mass of the components and
any play or resilience in the system. Third, there is additional cost for
a second lineshaft and its installation. Fourth, these systems with known
electronic controls do not respond well to massive errors, as occur in
connection with splices. The entire line adjusts as soon as a splice is
detected at the head of the line. Good impressions downstream are
processed out of registration, and continue to be processed out of
register, while the system regains synchronous operation. Fifth, the
system does not accommodate well to large changes in the line speed, e.g.
operation at 500 and 1,000 feet per minute (fpm). The problem is even
worse if the speed ranges from manual mode set-up speeds of under 100 fpm
to normal line operating speed of 1,000 to 2,000 fpm in an automatic mode.
Scanning rates and dynamic system responses that produce satisfactory
results at one speed do not function as well at a much different speed.
The flexibility of known systems is thus constrained. Sixth, the angular
position of the function cylinders in which they act on the web must be
initialized into a synchronous start with the registration marks. Seventh,
after the initial synchronization the operator has a display of the
instantaneous registration error, but there is no indication of the
history of the correction process, e.g. its momentum. Nor does any known
display assist the operator in finding a registration mark if it is lost,
as due to a splice.
In control systems, it is known to use proportional (P) controls, that is,
controls where the degree of correction is varied in proportion with the
magnitude of a sensed error. A large sensed error produces a larger
correction than a small sensed error to hasten the return of the system to
a desired condition, e.g. in register. Integral (I) control is also known,
where the correction responds to the measured average or integrated error
over some preselected prior period of operation. Derivative control is
also well known and often used in combination with PI control to form a
class of control, PID. The derivative control senses and responds to the
rate at which a correction occurs.
As applied to web registration control where there is cumulative error,
none of these forms of control have been successful heretofore.
Proportional control with cumulative error occurring at high speeds
requires large proportional gains. A high gain is needed to deal with the
comparatively large recurrent errors and to achieve the necessary register
accuracy. But the high gain causes the correction to overshoot the set
point (where the system is in register). Hunting occurs as the system
oscillates about the set point, or while the system looks for a
registration mark which has been over-corrected to a degree that it falls
out of the preset inspection zone. Integral controls, in turn, when
responding to cumulative errors of the same type, e.g. impressions that
are consistently printed long or short, tend to develop a momentum that
causes the error correction to overshoot a set point and then oscillate
about it, or lose it entirely. The system may settle on the set point, but
only after a fairly lengthy interval during which time thousands of feet
of printed web are processed out of register and must be scrapped. PID
controls have not solved these fundamental problems since they do not
automatically accommodate for this change in the error sampling rate that
is inherent in register control systems.
U.S. Pat. No. 4,994,975 to Minschart describes a system for off-line web
finishing that automatically initializes synchronization between register
marks and a processing machine. It selects a registration mark and stores
in memory a digital sequence that describes at least one characteristic of
the mark. Subsequently detected signals continue to be analyzed and stored
in the same manner. Periodically the stored contents of memory are
analyzed to locate the selected mark. Finally, the deviation of this
selected mark is calculated from 1) a set point position and 2) the
position of an operating element of an associated one of the processing
machines.
While Minschart recites the use of proportional (P), proportional-integral
(PI) and proportional-integral-differential (PID) controllers to produce
error correction signals, all three forms are described as suitable and of
well-known design. The control advantages purportedly derive from the
computer analysis of memory-stored marks to one another and to the
position of the machine. This control arrangement does not address or
solve the hunting, and validation problems noted above with respect to
conventional P or PI systems. The system also requires repeated pattern
recognition, e.g. an analysis of a mark by its width, direction of travel
and shape. This analysis requires substantial signal processing and
capability and is sensitive to irregularities in the printing of the marks
or other printed indicia. Moreover, the accuracy of the control remains
limited by the ability of an encoder to determine at any instant the
precise angular position of the machine element carrying out the process.
It is therefore a principal object of this invention to provide a control
system (method and apparatus), particularly one for off-line web
finishing, that corrects for both localized and cumulative errors, and is
highly stable even with substantial changes in line speed such as
variations of a factor of twenty or more.
Another object is to provide a control system which has the foregoing
advantages and which can operate with an asynchronous start and which can
quickly and reliably accommodate splices, cumulative error, or other
asynchronous operating conditions, during operation.
A further object is to provide a system that is highly flexible to
accommodate i) different types of web finishing systems such as single or
dual shaft drives, variable ratio or fixed ratio systems and (ii) the
control of different processes such as registration, cutoff length
control, and infeed tension control, and (iii) different modes of control,
e.g. manual or automatic.
Yet another object is to avoid the need for a dead zone around the set
point.
Still another object is to provide a control system with all of the
foregoing advantages that significantly reduces capital cost for hardware
(e.g. position transducers, in-register splicers, dual lineshafts) as well
as hardware installation time and set up time.
Yet another object of the invention is to provide a system with a graphic
display that provides a convenient, real-time, analog display of the
correction process.
A further object is to provide an arrangement for greatly enhancing the
accuracy of angular position information from an encoder or other standard
angular position transducer.
SUMMARY OF THE INVENTION
When used to control the register of function machinery acting on a printed
web, an electronic controller produces a registration error signal by
comparing signals that measure (i) the location of registration marks on
the web that correlate with the location of the printed material, e.g.
impressions on the web, and (ii) the angular position of a function
cylinder acting on the web. The angular position is preferably determined
by an encoder or the like that produces a homing pulse. The controller
introduces a proportional gain and an integral gain to the error signal.
These gains are scaled to control "take over" point of the integral gain
with respect to the proportional gain to operate in a stable manner. The
controller zeroes the integral gain if (i) the integral and proportional
errors differ in sign, as when the integral error cross the set point, and
(ii) the proportional error exceed a preset value, e.g. .+-.5% of the
integral error band. The proportional and integral gains are added
algebraically to form an overall system gain that is output as a
correction signal to a servo-controlled D.C. motor, or an equivalent drive
device, that controls the phase of the function cylinder with respect to
the printed pattern on the web. The overall gain is variable as a function
of the web speed to make the controls substantially insensitive to the
line speed. With a digital controller, the output signal is preferably
initially pulse width modulated. In the preferred form, a D to A circuit
produces a corresponding analog DC voltage that is applied to the controls
for the D.C. motor. The proportional gain and integral gain are separately
variable, as is the overall gain.
The controller attaches an inspection zone to a selected registration mark,
or like indicia such as shaft angular position, whose position is being
controlled. Thus, if there is cumulative error or some other condition
producing an asynchronous condition, the controller moves the inspection
zone to follow the selected mark. The inspection zone preferably is set to
begin half way between the two most widely spaced registration marks. The
inspection zone terminates when the selected mark is detected, in this
instance the following mark of the two most widely spaced.
To enhance the accuracy of the encoder, or the like, a high frequency clock
signal is counted between encoder pulses. The time intervals between a
measured point and the preceding and following pulses, given a fixed clock
rate, form a time interval ratio that interpolates the angular position of
the function cylinder between the pulses. The clock rate is selected in
coordination with system parameters such as the line speed. The rate
should produce the needed degree of resolution and accuracy at line speeds
ranging from very slow, e.g. 20 to 30 impressions per minute during set up
operation in a manual or automatic mode, to 1,000 to several thousand feet
per minute during production operation in a fully automatic mode.
To enhance the operator control, the system includes an LCD display or the
like with an analog vector arrow displayed in real time whose length
corresponds to the actual impression length, and whose direction reflects
the direction of travel of the web. The homing point on the encoder is
defined as the head of the arrow. Sensed registration marks, a set point,
and an inspection zone are displayed along the arrow with their position
on the arrow corresponding to their actual position along the length of
web under inspection during that revolution of the function cylinder.
Error corrections are displayed as the linear distance along the arrow
between the set point and a selected registration mark. The inspection
zone is displayed, in the preferred form, as a bar extending under the
arrow from a start point half way between the two most widely spaced marks
to the selected mark. The actual movement of the mark and the attached
zone on the display shows in a graphic, real-time form the direction and
speed of the register correction. The display preferably also includes
digital readouts of parameters such as the line speed, set point, the
error expressed in mils from the set point, the mode of operation--e.g.,
manual or automatic, and the rate in percent of the correction being
applied. A keyboard associated with the display inputs changes of the
operating parameters.
These and other features and objects will be more readily apparent from the
following detailed description which should be read in light of the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a simplified view in side elevation of an off-line web finishing
system operated with a control system according to the present invention;
FIG. 2 is a top plan view of the printed web shown in FIG. 1;
FIGS. 3A and 3B are schematic views in side elevation of a rotary cutter
rotating in coordination with the moving web shown in FIGS. 1 and 2;
FIG. 4 is a highly simplified schematic view in side elevation of the
rotary cutter shown in FIG. 1 with the cutter driven by a second drive
shaft coupled to a main lineshaft through a variable ratio harmonic drive;
FIG. 5 is a high level schematic block diagram of a control system
according to the present invention for use on the line shown in FIGS. 1-4;
FIG. 6 is a more detailed block diagram of the electronic control system
shown in FIG. 5;
FIG. 7A is an equivalent system diagram of the feedback control loop of the
present invention at a line operating of speed 1,000 fpm;
FIG. 7B is a diagram corresponding to FIG. 4A at a line operating speed of
100 fm;
FIG. 8-1 to 8-3 is a flow chart showing the operation of the control system
shown in FIG. 6;
FIG. 9 is a view of the real-time analog display according to the present
invention during operation of the finishing and control systems of FIGS.
1-8; and
FIG. 10 is a Bode diagram of system gain at 100 and 1000 fpm as a function
of frequency.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIGS. 1-4 show an off-line web finishing system 10, a principal application
of the control system of the present invention. Most of the following
description will describe the invention operating in the context of
off-line web finishing. A web 12 previously printed with a series of
impressions 14 (FIG. 3) is unwound from a roll 16 and fed through the
finishing line (FIG. 1). The line performs multiple functions on the web,
usually more than twenty, and delivers a processed product, such as a
signature used to form a magazine, to a delivery conveyor 18 at the end of
the line. The impressions have a repeat length L (FIG. 2) along the
longitudinal axis of the web which typically corresponds to the
circumference of a print cylinder, 630 mm being a common value. Because of
the elastic and environmentally sensitive nature of paper, the repeat
length of the impressions 14 can, and usually will, vary from the expected
length. FIG. 2 shows a cumulative error where the impressions are each
printed long. The transverse dashed lines 20 illustrate where a finishing
function, such as the operation of a rotary cutter, will fall on the web
in the absence of correction. While the problem as illustrated in FIG. 2
is exaggerated, it clearly demonstrates how cumulative errors of the same
type (a long or short repeat length) can rapidly lead to a cut 20a within
an impression, not between impressions as shown at 20b. A web cut at 20a
is not usable. Besides the cumulative errors, the paper may expand or
contract locally in a highly unpredictable manner resulting in localized
and rapidly changing positional errors that can also be of a sufficient
magnitude to result in an operation being performed on the web so as to
destroy the product.
FIGS. 3A and 3B illustrate in a simplified manner the timing between the
operation of a function cylinder, here a rotary cutter 22, and the web. In
FIGS. 3A and 3B dashed lines 24 represent the location of registration
marks on the web. There are typically several marks per impression. The
control system sees all of the marks, but controls the position of one
selected mark. As is well known, the mark is not necessarily an actual
mark; it can be any feature on the web that denotes position, for example,
a white space between printed areas. In non-printed web applications, such
as the control of the angular position of a cylinder, the mark can be a
magnet secured on the cylinder or a homing point on an encoder mounted on
the cylinder shaft. The web in FIGS. 3A, 3B moves in the direction of
arrow 26. In FIG. 3A a blade 22a is rotating toward a cutting position
where it impacts on the web for an instant. In FIG. 3B the blade has
rotated in conjunction with an advance of the web to cut the web at point
C. This illustrates a misregister since the cut occurs ahead of the
desired location on the web.
The system 10 begins wit | | |
