U.S. patent application number 15/899824 was filed with the patent office on 2018-09-27 for transport control method, a transport apparatus, and a printing apparatus.
The applicant listed for this patent is SCREEN HOLDINGS CO., LTD.. Invention is credited to Shoji Kakimoto, Osamu Morizono.
Application Number | 20180273330 15/899824 |
Document ID | / |
Family ID | 63581369 |
Filed Date | 2018-09-27 |
United States Patent
Application |
20180273330 |
Kind Code |
A1 |
Kakimoto; Shoji ; et
al. |
September 27, 2018 |
TRANSPORT CONTROL METHOD, A TRANSPORT APPARATUS, AND A PRINTING
APPARATUS
Abstract
A transport control method for use in transporting a medium in a
predetermined direction by an upstream drive roller and a
downstream drive roller, to operate the downstream drive roller by
using PID control based on a detection value of a tension sensor
disposed downstream of the upstream drive roller and upstream of
the downstream drive roller for detecting tension. The transport
control method includes a stable state determining step for
determining, at time of the operation, whether a difference between
the detection value of the tension sensors and a target value is
within a stable width to make a stable state which maintains a
stability time; and a gain decreasing step for decreasing a gain of
the PID control to be less than an initial value when the
difference is determined to be in the stable state.
Inventors: |
Kakimoto; Shoji; (Kyoto,
JP) ; Morizono; Osamu; (Kyoto, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SCREEN HOLDINGS CO., LTD. |
Kyoto |
|
JP |
|
|
Family ID: |
63581369 |
Appl. No.: |
15/899824 |
Filed: |
February 20, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B65H 20/02 20130101;
B65H 2801/21 20130101; B65H 2515/31 20130101; B65H 23/188 20130101;
B65H 2801/12 20130101; B65H 23/1806 20130101; B65H 2301/4165
20130101; B65H 23/044 20130101; B65H 2557/2644 20130101; B65H
2801/06 20130101 |
International
Class: |
B65H 23/18 20060101
B65H023/18; B65H 20/02 20060101 B65H020/02 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 21, 2017 |
JP |
2017-054419 |
Claims
1. A transport control method for use in transporting a medium in a
predetermined direction by an upstream drive roller disposed in an
upstream position in a transport direction of the medium, and a
downstream drive roller disposed in a downstream position in the
transport direction, to operate the downstream drive roller by
using PID control based on a detection value of a tension sensor
disposed downstream in the predetermined direction of the upstream
drive roller and upstream of the downstream drive roller for
detecting tension of the medium, the transport control method
comprising: a stable state determining step for determining, at
time of the operation, whether a difference between the detection
value of the tension sensor and a target value is within a stable
width to make a stable state which maintains a stability time; and
a gain decreasing step for decreasing a gain of the PID control to
be less than an initial value when the difference is determined to
be in the stable state.
2. The transport control method according to claim 1, further
comprising an initial value setting step for setting the gain to
the initial value when the upstream drive roller starts to be
driven from a stopped state, and when the upstream drive roller
starts to be decelerated toward the stopped state.
3. The transport control method according to claim 1, further
comprising a gain increasing step for increasing the gain when,
after the gain decreasing step, the difference is determined to
have deviated from the stable width, or although within the stable
width, is in a nonstable state incapable of maintaining the
stability time.
4. The transport control method according to claim 2, further
comprising a gain increasing step for increasing the gain when,
after the gain decreasing step, the difference is determined to
have deviated from the stable width, or although within the stable
width, is in a nonstable state incapable of maintaining the
stability time.
5. The transport control method according to claim 1, wherein the
stable state determining step has the stable width in at least two
types.
6. The transport control method according to claim 1, wherein the
gain decreasing step has a minimum gain set as a lower limit in
decreasing the gain to be less than the initial value.
7. The transport control method according to claim 1, wherein the
gain increasing step has a maximum gain set as an upper limit in
increasing the gain.
8. The transport control method according to claim 2, wherein the
gain increasing step has a maximum gain set as an upper limit in
increasing the gain.
9. The transport control method according to claim 1, wherein the
gain is a proportional gain.
10. A transport apparatus for transporting a medium in a
predetermined direction, comprising: an upstream drive roller
disposed in an upstream position in a transport direction of the
medium; a downstream drive roller disposed in a downstream position
in the transport direction; a tension sensor disposed downstream in
the predetermined direction of the upstream drive roller, and
upstream of the downstream drive roller, for detecting tension of
the medium; a drive control unit for operating the downstream drive
roller by using PID control based on a detection value of the
tension sensor; a stable state determining unit for determining, at
time of the operation, whether a difference between the detection
value of the tension sensor and a target value is within a stable
width to make a stable state which maintains a stability time; and
a gain adjusting unit for decreasing a gain of the PID control to
be less than an initial value when the stable state determining
unit has determined that the difference is in the stable state.
11. The transport apparatus according to claim 10, wherein the gain
adjusting unit sets the gain to the initial value when the drive
control unit starts to drive the upstream drive roller from a
stopped state, and when the drive control unit starts to decelerate
the upstream drive roller toward the stopped state.
12. The transport apparatus according to claim 10, wherein the gain
adjusting unit increases the gain when, after the gain adjusting
unit decreased the gain, the stable state determining unit
determines that the difference has deviated from the stable width,
or although within the stable width, is in a nonstable state
incapable of maintaining the stability time.
13. The transport apparatus according to claim 11, wherein the gain
adjusting unit increases the gain when, after the gain adjusting
unit decreased the gain, the stable state determining unit
determines that the difference has deviated from the stable width,
or although within the stable width, is in a nonstable state
incapable of maintaining the stability time.
14. The transport apparatus according to claim 10, wherein the
stable state determining unit has the stable width in at least two
types.
15. The transport apparatus according to claim 10, wherein the gain
adjusting unit has a minimum gain set as a lower limit in
decreasing the gain to be less than the initial value.
16. The transport apparatus according to claim 10, wherein the gain
adjusting unit has a maximum gain set as an upper limit in
increasing the gain.
17. The transport apparatus according to claim 11, wherein the gain
adjusting unit has a maximum gain set as an upper limit in
increasing the gain.
18. The transport apparatus according to claim 10, wherein the gain
is a proportional gain.
19. A printing apparatus for performing printing while transporting
an elongate printing medium in a predetermined direction,
comprising: a printing station for printing on the printing medium
in a printing area disposed along a transport path of the printing
medium; an upstream drive roller disposed upstream of the printing
area; a downstream drive roller disposed downstream of the printing
area; a tension sensor disposed downstream in the predetermined
direction of the upstream drive roller, and upstream of the
printing area, for detecting tension of the printing medium; a
drive control unit for operating the downstream drive roller based
on a detection value of the tension sensor and by using PID
control; a stable state determining unit for determining, at time
of the operation, whether a difference between the detection value
of the tension sensor and a target value is within a stable width
to make a stable state which maintains a stability time; and a gain
adjusting unit for decreasing a gain of the PID control to be less
than an initial value when the stable state determining unit has
determined that the difference is in the stable state.
20. The printing apparatus according to claim 19, wherein the gain
adjusting unit sets the gain to the initial value when the upstream
drive roller starts to be driven from a stopped state, and when the
upstream drive roller starts to be decelerated toward the stopped
state.
Description
BACKGROUND OF THE INVENTION
(1) Field of the Invention
[0001] This invention relates to a transport control method, a
transport apparatus, and a printing apparatus for transporting an
elongate printing medium in a predetermined direction.
(2) Description of the Related Art
[0002] Conventionally, a printing apparatus having this type of
transport apparatus includes a paper feeder, a printing station, a
takeup roller and the transport apparatus (see Japanese Unexamined
Patent Publication No. 2014-24266 (FIG. 1), for example).
[0003] The above transport apparatus includes a first drive roller,
a second drive roller, a third drive roller, and a fourth drive
roller. The first drive roller is disposed downstream of the paper
feeder, which supplies elongate printing paper, and has a nip
roller for feeding the printing paper from the paper feeder. The
second drive roller has a nip roller for feeding the printing paper
sent by the first drive roller into a printing area directly under
the printing station. The third drive roller (also called a heat
roller) dries the printing paper printed at the printing station
and wound at a large winding angle thereon, and sends the printing
paper downstream. The fourth drive roller has a nip roller for
sending the printing paper dried by the third drive roller on to
the takeup roller. The transport apparatus further includes a first
tension sensor disposed downstream of the first drive roller for
detecting the tension of the printing paper sent from the first
drive roller, a second tension sensor disposed downstream of the
second drive roller and upstream of the printing station for
detecting the tension of the printing paper in a position upstream
of the printing station, and a third tension sensor disposed
upstream of the fourth drive roller for detecting the tension in a
position upstream of the fourth drive roller.
[0004] The transport apparatus of such construction employs a
transport control method which, while driving the second drive
roller at a constant transport speed, controls transport of the
printing paper by operating the other drive rollers. Specifically,
the first drive roller is operated so that the first tension sensor
will attain a target value. The third drive roller is operated so
that the second tension sensor will attain the target value.
Further, the fourth drive roller is operated so that the third
tension sensor will attain the target value. Note that the fourth
drive roller is made to reflect speed variations of the third drive
roller operated in accordance with the second tension sensor,
thereby to reflect tension variations occurring in the printing
area. The above operations are carried out under PID (Proportional
Integral Differential) control. Its gain is fixed, for example, to
100% for a period from a starting point to a stopping point of
printing paper transport.
[0005] However, the conventional example with such construction has
the following problem.
[0006] With the conventional transport control method, there occurs
large variations in the control amount for the first, third and
fourth drive motors at a printing speed based on a substantially
constant transport speed, which poses a problem of easily causing
paper transport irregularities. Such transport irregularities
influence a tense condition of the printing paper in the printing
area, which is detrimental to print quality. It is therefore
desirable to minimize transport irregularities.
[0007] So the inventors herein have tried a transport control with
the gain in PID control lowered from 100% to 30%. It has been found
that such lowering of the parameter to 30% is effective to check
variations in the control amount at the printing speed with a
substantially constant transport speed. However, the above measure
has entailed a different problem that the tension varies
extensively at a time of transport speed acceleration from a
stopped state to the printing speed. An excessive variation in
tension will apply load in the transport direction to the printing
paper, thereby causing damage to the printing paper. It is
therefore desirable to minimize variations. Thus, it is unrealistic
to conduct transport controls by reducing the gain in PID
control.
SUMMARY OF THE INVENTION
[0008] This invention has been made having regard to the state of
the art noted above, and its object is to provide a transport
control method, a transport apparatus, and a printing apparatus
which can inhibit transport irregularities, while suppressing
tension variations at the time of acceleration, by rendering a gain
variable according to a stability level of tension control.
[0009] The above object is fulfilled, according to this invention,
by a transport control method for use in transporting a medium in a
predetermined direction by an upstream drive roller disposed in an
upstream position in a transport direction of the medium, and a
downstream drive roller disposed in a downstream position in the
transport direction, to operate the downstream drive roller by
using PID control based on a detection value of a tension sensor
disposed downstream in the predetermined direction of the upstream
drive roller and upstream of the downstream drive roller for
detecting tension of the medium, the transport control method
comprising a stable state determining step for determining, at time
of the operation, whether a difference between the detection value
of the tension sensor and a target value is within a stable width
to make a stable state which maintains a stability time; and a gain
decreasing step for decreasing a gain of the PID control to be less
than an initial value when the difference is determined to be in
the stable state.
[0010] According to this invention, when the stable state
determining step determines that the difference between the
detection value of the tension sensor and the target value is
within a stable width to make a stable state which maintains a
stability time, the gain decreasing step decreases the gain of PID
control to be less than the initial value. Therefore, when tension
control is not stable such as at a time of acceleration, PID
control is carried out with the gain at the initial value. Only
when tension control is stable, PID control is carried out with
sensitivity lowered by a small gain. As a result, when tension
control is unstable as at the time of acceleration, for example,
the control amount becomes large. The control amount becomes small
when tension control is stable as in a constant speed state. Thus,
transport irregularities of the medium can be inhibited while
inhibiting tension variations at the time of acceleration.
[0011] In this invention, it is preferred that an initial value
setting step is executed to set the gain to the initial value when
the upstream drive roller starts to be driven from a stopped state,
and when the upstream drive roller starts to be decelerated toward
the stopped state.
[0012] The tension is unstable at times of acceleration and
deceleration. By performing the control with the initial value
instead of a small gain, therefore, the control amount is increased
to be able to inhibit tension variations at the times of
acceleration and deceleration.
[0013] In this invention, it is preferred that a gain increasing
step is executed to increase the gain when, after the gain
decreasing step, the difference is determined to have deviated from
the stable width, or although within the stable width, is in a
nonstable state incapable of maintaining the stability time.
[0014] The gain is enlarged to increase the control amount at the
time of nonstable state. This can stabilize tension even at times
of large tension variations.
[0015] In this invention, it is preferred that each of the stable
state determining step and the gain decreasing step has the stable
width in at least two types.
[0016] By determining the stable state of tension with two types of
stable width, when decreasing the gain, a clearly defined gain
adjustment can be made according to tension variations.
[0017] In this invention, it is preferred that the gain decreasing
step has a minimum gain set as a lower limit in decreasing the gain
to be less than the initial value.
[0018] Setting the minimum gain as a lower limit can avoid an
excessively small gain lowering sensitivity too much, which would
cause an inconvenience in tension adjustment.
[0019] In this invention, it is preferred that the gain increasing
step has a maximum gain set as an upper limit in increasing the
gain.
[0020] Setting the maximum gain as an upper limit can avoid an
excessively large gain raising sensitivity too high, which would
cause an inconvenience in tension adjustment.
[0021] In this invention, it is preferred that the gain is a
proportional gain.
[0022] In transport control of an elongate printing medium, the
control can be performed well only by adjusting the proportional
gain.
[0023] In another aspect of this invention, there is provided a
transport apparatus for transporting a medium in a predetermined
direction, comprising an upstream drive roller disposed in an
upstream position in a transport direction of the medium; a
downstream drive roller disposed in a downstream position in the
transport direction; a tension sensor disposed downstream in the
predetermined direction of the upstream drive roller, and upstream
of the downstream drive roller, for detecting tension of the
medium; a drive control unit for operating the downstream drive
roller by using PID control based on a detection value of the
tension sensor; a stable state determining unit for determining, at
time of the operation, whether a difference between the detection
value of the tension sensor and a target value is within a stable
width to make a stable state which maintains a stability time; and
a gain adjusting unit for decreasing a gain of the PID control to
be less than an initial value when the stable state determining
unit has determined that the difference is in the stable state.
[0024] According to this invention, when the drive control unit
operates the downstream drive roller by PID control, the stable
state determining unit determines whether or not the difference
between the detection value of the tension sensor and the target
value is in a stable width to make a stable state which maintains a
stability time. When the stable state is established, the gain
adjusting unit makes the gain of PID control smaller than an
initial value. Therefore, when tension control is not stable such
as at a time of acceleration, PID control is carried out with the
gain at the initial value. Only when tension control is stable, PID
control is carried out with sensitivity lowered by a small gain. As
a result, when tension control is unstable as at the time of
acceleration, for example, the control amount becomes large. The
control amount becomes small when tension control is stable as in a
constant speed state. Thus, transport irregularities of the medium
can be inhibited while inhibiting tension variations at the time of
acceleration.
[0025] In a further aspect of this invention, there is provided a
printing apparatus for performing printing while transporting an
elongate printing medium in a predetermined direction, comprising a
printing station for printing on the printing medium in a printing
area disposed along a transport path of the printing medium; an
upstream drive roller disposed upstream of the printing area; a
downstream drive roller disposed downstream of the printing area; a
tension sensor disposed downstream in the predetermined direction
of the upstream drive roller, and upstream of the printing area,
for detecting tension of the printing medium; a drive control unit
for operating the downstream drive roller based on a detection
value of the tension sensor and by using PID control; a stable
state determining unit for determining, at time of the operation,
whether a difference between the detection value of the tension
sensor and a target value is within a stable width to make a stable
state which maintains a stability time; and a gain adjusting unit
for decreasing a gain of the PID control to be less than an initial
value when the stable state determining unit has determined that
the difference is in the stable state.
[0026] According to this invention, when the drive control unit
operates the downstream drive roller by PID control, the stable
state determining unit determines whether or not the difference
between the detection value of the tension sensor and the target
value is in a stable width to make a stable state which maintains a
stability time. When the stable state is established, the gain
adjusting unit makes the gain of PID control smaller than an
initial value. Therefore, when tension control is not stable such
as at a time of acceleration, PID control is carried out with the
gain at the initial value. Only when tension control is stable, PID
control is carried out with sensitivity lowered by a small gain. As
a result, when tension control is unstable as at the time of
acceleration, for example, the control amount becomes large. The
control amount becomes small when tension control is stable as in a
constant speed state. Thus, transport irregularities of the
printing medium can be inhibited while inhibiting tension
variations at the time of acceleration. As a result, the quality of
printing on the printing medium by the printing station can be
improved.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] For the purpose of illustrating the invention, there are
shown in the drawings several forms which are presently preferred,
it being understood, however, that the invention is not limited to
the precise arrangement and instrumentalities shown.
[0028] FIG. 1 is a schematic view showing an entire inkjet printing
system having a transport apparatus according to an embodiment;
[0029] FIG. 2 is a schematic view showing a control relationship of
a first to a fourth drive rollers;
[0030] FIG. 3 is a graph showing a relationship between a command
value to the second drive roller, gain, and tension as matched with
transport distances of web paper WP;
[0031] FIG. 4 is a flow chart showing an example of control;
[0032] FIG. 5 is a flow chart showing the example of control;
[0033] FIG. 6 is a graph showing variations in tension in the
inkjet printing system according to the embodiment;
[0034] FIG. 7 is a graph showing variations in command value in the
inkjet printing system according to the embodiment;
[0035] FIG. 8 is a graph showing variations in tension with a gain
set to 100% in an inkjet printing system according to a
conventional example;
[0036] FIG. 9 is a graph showing variations in command value with
the gain set to 100% in the inkjet printing system according to the
conventional example;
[0037] FIG. 10 is a graph showing variations in tension with the
gain set to 30% in the inkjet printing system according to the
conventional example; and
[0038] FIG. 11 is a graph showing variations in command value with
the gain set to 30% in the inkjet printing system according to the
conventional example.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0039] An embodiment of this invention will be described
hereinafter with reference to the drawings.
[0040] FIG. 1 is a schematic view showing an entire inkjet printing
system having a transport apparatus according to this
embodiment.
[0041] An inkjet printing system 1 according to this embodiment
includes an inkjet printing apparatus 3, a paper feeder 5 and a
takeup roller 7.
[0042] The inkjet printing apparatus 3 performs printing on
elongate web paper WP. The paper feeder 5 holds a roll of web paper
WP to be rotatable about a horizontal axis, and unwinds the web
paper WP from the roll of web paper WP to feed it to the inkjet
printing apparatus 3. The takeup roller 7 winds up the web paper WP
printed by the inkjet printing apparatus 3 about a horizontal axis.
Regarding the side from which the web paper WP is fed as upstream
and the side to which the web paper WP is discharged as downstream,
the paper feeder 5 is disposed upstream of the inkjet printing
apparatus 3 while the takeup roller 7 is disposed downstream of the
inkjet printing apparatus 3.
[0043] The inkjet printing apparatus 3 corresponds to the "printing
apparatus" in this invention. The web paper WP corresponds to the
"printing medium" and the "medium" in this invention.
[0044] The inkjet printing apparatus 3 includes a first drive
roller M1 in an upstream position thereof for taking in the web
paper WP from the paper feeder 5. The web paper WP unwound from the
paper feeder 5 by the first drive roller M1 is transported
downstream toward the takeup roller 7 along a plurality of
rotatable transport rollers 11.
[0045] An edge position controller 15 is disposed downstream of the
first drive roller M1. When the web paper WP wanders off in
directions perpendicular to a transport direction, the edge
position controller 15 will automatically adjust and control the
web paper WP to be transported to a right position.
[0046] A second drive roller M2 is disposed downstream of the edge
position controller 15. The web paper WP fed downstream by the
second drive roller M2 has the transport direction changed by a
transport roller 11 disposed downstream of the second drive roller
M2, to advance along a transport path to a printing area PA where
printing is done. This transport roller 11 has a rotary encoder 13
mounted thereon. The printing area PA has a plurality of transport
rollers 11 arranged along the transport path of the web paper
WP.
[0047] A printing station 19 is disposed above the printing area
PA. The printing station 19 in this embodiment includes four inkjet
heads 19a-19d, for example. The inkjet head 19a in the most
upstream position, for example, dispenses ink droplets of black
(K), the next inkjet head 19b ink droplets of cyan (C), the next
inkjet head 19c ink droplets of magenta (M), and the next inkjet
head 19d ink droplets of yellow (Y). The inkjet heads 19a-19d are
arranged separately at predetermined intervals in the transport
direction.
[0048] The web paper WP printed in the printing area PA has the
transport direction changed by a downstream transport roller 11. A
third drive roller M3 is disposed ahead. The third drive roller M3
winds the web paper WP at a large winding angle, and contacts the
web paper WP to dry the ink droplets on the web paper WP. This
third drive roller M3 has a built-in heater, and is also called a
heat drum.
[0049] The web paper WP dried by the third drive roller M3 is sent
by a fourth drive roller M4 to the takeup roller 7, while having
its direction changed by a plurality of transport rollers 11. An
inspecting unit 23 is disposed upstream of the fourth drive roller
M4. The inspecting unit 23 inspects the web paper WP printed at the
printing station 19. The takeup roller 7 takes up in a roll form
the web paper WP inspected by the inspecting unit 23.
[0050] The first drive roller M1, second drive roller M2 and fourth
roller M4 described above individually have nip rollers 25
rotatably attached. A carrying force to the web paper WP is applied
by the nip roller 25 pinching the web paper WP with each drive
roller. The pressing force of each nip roller 25 is applied by an
air cylinder (not shown), for example. The nip rollers 25 are
formed of an elastic material such as rubber, for example.
[0051] A first tension sensor TP1 is disposed downstream of the
first drive roller M1 and upstream of the edge position controller
15. A second tension sensor TP2 is disposed downstream of the
second drive roller M2 and upstream of the printing area PA. A
third tension sensor TP3 is disposed downstream of the third drive
roller M3 and upstream of the fourth drive roller M4. The first to
third tension sensors TP1-TP3 successively detect current tension
applied to the web paper WP, and output detection values of the
tension.
[0052] The inkjet printing apparatus 3, paper feeder 5 and takeup
roller 7 are operable under overall control of a main controller
49.
[0053] The main controller 49 includes a control unit 51 and a
storage unit 57. The control unit 51 is constructed of a CPU and
other components. The control unit 51, upon instructions by the
operator to start printing, controls transportation of the web
paper WP by giving the second drive roller M2 a command value
indicating a transport speed, and giving the first drive motor M1,
third drive motor M3 and fourth drive motor M4 control amounts
based on PID control as described in detail hereinafter. The
control is carried out with reference to the command value given to
the second drive roller M2, to realize a printing speed provided by
the transport speed at the time of printing which meets printing
conditions set beforehand by the operator. The control unit 51
determines the transport speed and transport distances of the web
paper WP based on output signals of the rotary encoder 13. The
printing conditions are conditions relating to print quality, such
as the transport speed of the web paper WP and each target value of
tension in each part applied to the web paper WP, for example. The
storage unit 57 stores beforehand two types of stable width, a
stability time, a target value of tension, an initial value of
gain, a decrease amount of gain, an increase amount of gain, two
types of minimum gain, and a maximum gain to be described
hereinafter.
[0054] Reference is now made to FIG. 2. FIG. 2 is a schematic view
showing a control relationship of the first to fourth drive
rollers.
[0055] The control unit 51 carries out PID control based on a
difference between a target value of tension to be applied to the
web paper W in the location of the first tension sensor TP1 and a
detection value of the first tension sensor TP1, and gives a
control amount to the first drive roller M1 to make the detection
value equal to the target value. The control unit 51 carries out
PID control based on a difference between a target value of tension
to be applied to the web paper W in the location of the second
tension sensor TP2 and a detection value of the second tension
sensor TP2, and gives a control amount to the third drive roller M3
to make the detection value equal to the target value. Similarly,
the control unit 51 carries out PID control based on a difference
between a target value of tension to be applied to the web paper WP
in the location of the third tension sensor TP3 and a detection
value of the third tension sensor TP3, and gives a control amount
to the fourth drive roller M4 to make the detection value equal to
the target value. Further, the control unit 51 preferably performs
control by adding to the above control amount for the fourth drive
roller M4 an adjustment value based on an amount of change in the
rotating speed of the third drive roller M3.
[0056] The second drive roller M2 described above corresponds to
the "upstream drive roller" in this invention. The third drive
roller M3 corresponds to the "downstream drive roller" in this
invention. The second tension sensor TP2 described above
corresponds to the "first tension sensor" in this invention.
[0057] The above control unit 51 carries out PID control based on
differences between detection values and target values, and for
this purpose the initial value is set to 100%. In this embodiment,
control is carried out based on only PI of PID control, and the
gain adjusted when the state is determined stable is only
proportional gain (P) as described hereinafter.
[0058] The control unit 51 described above corresponds to the
"drive control unit", "stable state determining unit" and "gain
adjusting unit" in this invention.
[0059] Reference is now made to FIG. 3. FIG. 3 is a graph showing a
relationship between the command value to the second drive roller
M2, gain and tension as matched with transport distance of the web
paper WP.
[0060] The control of the command value to the second drive roller
M2, namely the transport speed used as the reference, is a control
for making a converted speed based on the above rotary encoder 13
constant. The command value to the second drive roller M2 is
therefore zero for distance zero (which corresponds also to time),
which command value provides printing speed SP at distance d1. The
command value given begins deceleration from the printing speed SP
at distance d18 where the printing ends, and reduces the transport
speed to zero at distance d19.
[0061] On the other hand, the control amounts to the first drive
roller M1, third drive roller M3, and fourth drive roller M4 vary
as shown in a vertically oscillating dotted line in the graph of
the command value to the second drive roller M2, for example.
Specifically, the control unit 51 outputs the command value
according to a difference between each of the detection values of
tension by the first to third tension sensors TP1-TP3 and the
target value, and the proportional gain. When the control unit 51
determines the differences, it is preferred to make the detection
values of tension into moving average deviations. This can suppress
disturbance of the control caused by temporary variations of
tension due to the influence of noise or external factors.
[0062] Adjustment of the proportional gain by the control unit 51
noted above will be described. Here, the storage unit 57 is assumed
to have stored therein beforehand: stable width A=.+-.500 g, stable
width B=.+-.1000 g, stability time ST=5 seconds, initial value of
gain=100%, decrease amount of gain G1=20%, increase amount of gain
G2=20%, minimum gain Gmin1=20%, minimum gain Gmin2=60%, and maximum
gain Gmax=100%. These values such as of stable width A are given
only by way of example, and can be set variously according to
objects to be transported, transport routes, characteristics of the
rollers, ambient environment of the apparatus, and so on.
[0063] Stable width A and stable width B provide ranges of
variation of tension for determining whether or not the tension is
in a stable state. Tension is determined to be in the stable state
when difference .DELTA.ts between a detection value and a target
value of tension is within these stable widths A and B and this
state maintains the next stability time ST. Stability time ST=5
seconds provides a time for determining whether or not difference
.DELTA.ts is in the stable state. The initial value of gain
provides a gain at the time PID control is started, a gain at the
time of increasing transport speed from stopped state to printing
speed, and a gain the time of stopping transport speed from
printing speed. The decrease amount of gain G1 provides an amount
of gain subtracted from a current gain when stable state is
determined. The increase amount of gain G2 provides an amount of
gain added to a current gain when a nonstable state is determined.
Minimum gain Gmin1 provides a lower limit to a gain resulting from
a subtraction made when a stable state is determined in stable
width A. Minimum gain Gmin2 provides a lower limit to a gain
resulting from a subtraction made when a stable state is determined
in stable width B. Maximum gain Gmax provides an upper limit to a
gain resulting from an addition.
[0064] In the above example, only the same decrease amount of gain
G1 is subtracted when a stable state is determined, whether the
determination is based on the stable width A or the determination
is based on the stable width B. However, since the stable state
determined based on the stable width A is higher in stability, the
gain may be decreased by a decrease amount of gain G2 having a
larger value than the decrease amount of gain G1 when the stable
state is determined based on stable width A. Since the gain can
thereby be made small quickly, tension control can be performed
with increased stability when the stable state is continued.
[0065] In the following description, the tension and gain in FIG. 3
may represent any combination of the first tension sensor TP1 and
first drive roller M1, the second tension sensor TP2 and third
drive roller M3, or the third tension sensor TP3 and fourth drive
roller M4.
[0066] In FIG. 3, from distance 0 to distance d3 the gain is
maintained at the initial value which is 100%. Assume here that, in
the stability time ST from distance d2 to distance d3, difference
.DELTA.ts between target value TG of tension and the detection
value of tension does not fit in the stable width A, but fits in
the stable width B. The tension is therefore determined to be in a
stable state. So the decrease amount of gain G1 (=20%) which is a
decrease value at the time of stable width B is subtracted from the
current gain (=100%), to decrease the gain to 80%. Assume that the
tension in the stability time ST from distance d4 to distance d5
fits in the stable width B. Since the tension is therefore
determined to be in a stable state, the decrease amount of gain G1
(=20%) is subtracted from the current gain (=80%), to decrease the
gain to 60%.
[0067] Assume that, in the stability time ST from distance d6 to
distance d7, difference .DELTA.ts fits in the stable width A
smaller than the stable width B. Since the tension is therefore
determined to be in a stable state, the decrease amount of gain G1
(=20%) which is the decrease value at the time of stable width A is
subtracted from the current gain (=60%), to decrease the gain to
40%. Assume that, in the stability time ST from distance d8 to
distance d9, difference .DELTA.ts fits in the stable width A. Then,
the decrease amount of gain G1 (=20%) is subtracted from the
current gain (=40%), to decrease the gain to 20%. Here, even if the
stable width A can be further maintained for another stability time
ST, the gain will not be further reduced since restriction is set
by the lower limit of minimum gain Gmin1=20%. This can avoid the
inconvenience of PID control being destabilized by an excessively
small gain.
[0068] Assume that, in the stability time ST from distance d10 to
distance d11, difference .DELTA.ts has deviated from the stable
width A and has further shifted to the stable width B. In this
case, whether the stability time ST is maintained is not
questioned. It is adequate to check whether or not difference
.DELTA.ts of tension exceeded stable width A. That is, it is
determined whether the control has begun to be unstable. Since the
tension is therefore determined to be in a nonstable state, the
gain increase amount G2 (=20%) is added to the current gain (=20%),
to increase the gain to 40%. Assume that, in the stability time ST
from distance d12 to distance d13, tension is outside the stable
width B. Since the tension is therefore determined to be in a
nonstable state, the gain increase amount G1 (=20%) is added to the
current gain (=40%), to increase the gain to 60%. Assume that, in
the stability time ST from distance d15 to distance d16, difference
.DELTA.ts fits in the stable width A. Since the tension is
therefore determined to be in a stable state, the decrease amount
of gain G1 (=20%) which is the decrease value at the time of stable
width A is subtracted from the current gain (=60%), to decrease the
gain to 40%. Assume that, in the stability time ST from distance
d16 to distance d17, difference .DELTA.ts fits in the stable width
B. Since the tension is therefore determined to be in a stable
state, the decrease amount of gain G1 (=20%) is subtracted from the
current gain (=40%), to decrease the gain to 20%.
[0069] When the control unit 51 finishes the printing and shifts
the transport speed from printing speed to stopped state, maximum
gain Gmax=100% is set irrespective of the amount of a current
gain.
[0070] The second drive roller M2, third drive roller M3, second
tension sensor TP2, and control unit 51 correspond to the
"transport apparatus" in this invention.
[0071] Next, a control flow by the control unit 51 will be
described with reference to FIGS. 4 and 5. FIGS. 4 and 5 are a flow
chart showing an example of control.
Step S1
[0072] The control unit 51 sets the gain to the initial value
(=100%). The initial value need not necessarily be 100%, but should
preferably be a maximum value of gains to be adjusted.
Step S2
[0073] The control unit 51 repeatedly checks whether acceleration
is completed, and moves to the next process in step S3 upon
completion of acceleration.
Step S3
[0074] The control unit 51 branches the process based on whether
deceleration is started or not. When deceleration is started, the
gain is set to the initial value (=100%).
Step S5
[0075] Difference .DELTA.ts between the detection value of tension
and the target value is calculated. The detection values of tension
at the time of calculating difference .DELTA.ts, preferably, are
moving average deviations for the reason noted hereinbefore.
[0076] Steps S6-S9 carry out processes for lowering the gain when
difference .DELTA.ts of tension is within the stable width A to
maintain a stability time ST.
Step S6
[0077] Checking is made whether or not difference .DELTA.ts is
within the stable width A and this state maintains the stability
time ST, and the process branches according to the result. When
difference .DELTA.ts is within the stable width A to make a stable
state which maintains the stability time ST, the process moves to
step S7. Otherwise it is a nonstable state, and the process moves
to step S2.
Step S7
[0078] When step S6 indicates a stable state in which difference
.DELTA.ts is within the stable width A and this state maintains the
stability time ST, the gain decrease amount G1 is subtracted from a
current gain to make a new gain.
Step S8
[0079] The gain resulting from the subtraction is compared with the
minimum gain Gmin1, and the process branches according to a result
of the comparison. When the new gain is larger than minimum gain
Gmin1, the process branches to step S2. When the new gain is
smaller than minimum gain Gmin1, the process moves to step S9.
Step S9
[0080] When the gain resulting from the subtraction is smaller than
minimum gain Gmin1, the gain is set to minimum gain Gmin1 and the
gain is fixed irrespective of the arithmetic result. This can avoid
an excessively small gain lowering sensitivity too much, which
would cause an inconvenience in tension adjustment.
[0081] Steps S10-S13 carry out processes for lowering the gain when
difference .DELTA.ts of tension is within the stable width B to
maintain a stability time ST. A gain decrease amount G1a larger
than the gain decrease amount G1 may be used to increase the
decrease amount in the case of stable width A.
Step S10
[0082] Checking is made whether or not difference .DELTA.ts is
within the stable width B and this state maintains the stability
time ST, and the process branches according to the result. When
difference .DELTA.ts is within the stable width B to make a stable
state which maintains the stability time ST, the process moves to
step S11. Otherwise, the process moves to step S14.
Step S11
[0083] When step S10 indicates a stable state in which difference
.DELTA.ts is within the stable width B and this state maintains the
stability time ST, the gain decrease amount G1 is subtracted from a
current gain to make a new gain.
Step S12
[0084] The gain resulting from the subtraction is compared with the
minimum gain Gmin2, and the process branches according to a result
of the comparison. When the new gain is larger than minimum gain
Gmin2, the process branches to step S2. When the new gain is
smaller than minimum gain Gmin2, the process moves to step S13.
Step S13
[0085] When the gain resulting from the subtraction is smaller than
minimum gain Gmin2, the gain is set to minimum gain Gmin2 and the
gain is fixed irrespective of the arithmetic result. This can avoid
an excessively small gain lowering sensitivity too much, which
would cause an inconvenience in tension adjustment. The reason for
minimum gain Gmin2>minimum gain Gmin1 is that the stable width B
is wider than the stable width A and has larger tension variations
than the stable width A, and therefore preferably has a larger gain
than in the stable state within the stable width A.
[0086] Steps S14-S17 confirm a shift of difference .DELTA.ts from
stable width A to stable width B. In other words, these steps check
whether or not the tension control has been disturbed.
Step S14
[0087] Checking is made whether or not difference .DELTA.ts has
shifted from stable width A to stable width B or has shifted
outside stable width B, and the process branches according to the
result. When it has shifted, the process moves to step S15.
Otherwise the process moves to step S18.
Step S15
[0088] The gain increase amount G2 is added to the current
gain.
Step S16
[0089] The gain resulting from the addition is compared with the
maximum gain Gmax, and the process branches according to a result
of the comparison. When the new gain is larger than the minimum
gain Gmax, the process branches to step S17. When the new gain does
not exceed the maximum gain Gmax, the process moves to step S2.
Step S17
[0090] The gain is fixed to the maximum gain Gmax. Setting the
maximum gain Gmax as an upper limit can avoid an excessively large
gain raising sensitivity too much, which would cause an
inconvenience in tension adjustment.
[0091] Steps S18-S21 confirm a shift of difference .DELTA.ts beyond
stable width B. In other words, these steps check whether or not
the tension control has been further disturbed.
Step S18
[0092] Checking is made whether or not difference .DELTA.ts has
shifted beyond the stable width B and this state maintains
stability time ST, and the process branches according to the
result. When difference .DELTA.ts is outside the stable width B to
make a nonstable state which maintains the stability time ST, the
process moves to step S19. Otherwise the process moves to step
S2.
Step S19
[0093] The gain increase amount G2 is added to the current
gain.
Step S20
[0094] The gain resulting from the addition is compared with the
maximum gain Gmax, and the process branches according to a result
of the comparison. When the new gain is larger than the minimum
gain Gmax, the process branches to step S21. When the new gain does
not exceed the maximum gain Gmax, the process moves to step S2.
Step S21
[0095] The gain is fixed to the maximum gain Gmax.
[0096] According to this embodiment, the control unit 51, when
operating the first drive roller M1, third drive roller M3, and
fourth drive roller M4 by PID control, determines whether or not
differences .DELTA.ts between the detection values of the
first-third tension sensors TP1-TP3 and the target value are in the
stable width A or B to make a stable state which maintains a
stability time ST. When the stable state is established, the gain
of PID control is made smaller than an initial value. Therefore,
when tension control is not stable such as at a time of
acceleration, PID control is carried out with the gain at the
initial value. Only when tension control is stable, PID control is
carried out with sensitivity lowered by a small gain. As a result,
when tension control is unstable as at the time of acceleration,
for example, the control amount becomes large.
[0097] The control amount becomes small when tension control is
stable as in a constant speed state. Thus, transport irregularities
of the web paper WP can be inhibited while inhibiting tension
variations at the time of acceleration. As a result, the quality of
printing on the web paper WP by the printing station 19 can be
improved.
[0098] The gain is set to the initial value when starting drive
from a stopped state to the printing speed and when starting
deceleration from the printing speed to a stopped state. This can
inhibit tension variations at the times of acceleration and
deceleration. Further, the gain is enlarged to increase the control
amount at the time of nonstable state. This can stabilize tension
even at times of large tension variations.
[0099] A comparison is now made between the foregoing embodiment
and a conventional example with reference to FIGS. 6 to 11.
[0100] FIG. 6 is a graph showing variations in tension in the
inkjet printing system according to the embodiment. FIG. 7 is a
graph showing variations in the command value in the inkjet
printing system according to the embodiment. FIG. 8 is a graph
showing variations in tension with a gain set to 100% in an inkjet
printing system according to the conventional example. FIG. 9 is a
graph showing variations in the command value with the gain set to
100% in the inkjet printing system according to the conventional
example. FIG. 10 is a graph showing variations in tension with the
gain set to 30% in the inkjet printing system according to the
conventional example. FIG. 11 is a graph showing variations in the
command value with the gain set to 30% in the inkjet printing
system according to the conventional example.
[0101] In this embodiment, as shown in the area enclosed by a
dotted line in FIG. 6, variations in tension are inhibited also in
an acceleration process in which transport speed is increased from
a stopped state to the printing speed. As shown in the area
enclosed by a dotted line in FIG. 7, it will be seen that
variations in the control amount particularly to the third drive
roller M3 are inhibited to inhibit transport irregularities.
[0102] In the conventional example having the gain set to 100%, on
the other hand, there occur little tension variations as shown in
FIG. 8. However, as shown in the area enclosed by a dotted line in
FIG. 9, in particular, it will be seen that variations in the
control amount to the third drive roller M3 are large.
[0103] In the conventional example having the gain set to 30%, as
shown in the area enclosed by a dotted line in FIG. 11, variations
in the control amount to the third drive roller M3 are inhibited.
However, as shown in the area enclosed by a dotted line in FIG. 10,
it will be seen that tension variations are large in the
acceleration process in which transport speed is increased from
stopped state to printing speed.
[0104] These graphs of tension, command value, and control amount
show that this embodiment produces advantageous effects compared
with the conventional example.
[0105] This invention is not limited to the foregoing embodiment,
but may be modified as follows:
[0106] (1) In the foregoing embodiment, the gain is reduced a
plurality of times when a stable state is indicated. When a stable
state is indicated once, the gain may be reduced only once.
[0107] (2) The foregoing embodiment provides two types of stable
width. This invention is not limited to this. For example, the
stable width may be provided in one type, or in three or more
types.
[0108] (3) In the foregoing embodiment, only the proportional gain
is adjusted. This invention is not limited to this. For example,
depending on what is transported, integral gain (I) and/or
differential gain (D) may be adjusted besides the proportional gain
(P).
[0109] (4) The foregoing embodiment has been described taking the
inkjet printing apparatus 3 as an example of printing apparatus.
This invention is not limited to the inkjet printing apparatus 3.
For example, any printing mode will do as long as a printing
apparatus performs printing while transporting an elongate printing
medium.
[0110] (5) The foregoing embodiment has been described taking for
example the transport path of the web paper WP constructed as shown
in FIG. 1. This invention is not limited to such construction.
[0111] (6) The foregoing embodiment has been described taking the
web paper WP as an example of printing medium and medium. This
invention is not limited to such a printing medium or medium. This
invention is applicable also to a printing medium and medium such
as film, for example.
[0112] This invention may be embodied in other specific forms
without departing from the spirit or essential attributes thereof
and, accordingly, reference should be made to the appended claims,
rather than to the foregoing specification, as indicating the scope
of the invention.
* * * * *