Note: Descriptions are shown in the official language in which they were submitted.
CA 02553357 2008-06-25
MEDIA REGISTRATION SYSTEMS AND METHODS
BACKGROUND
[0001] Embodiments herein generally relate to media registration/alignment
systems and methods within printers and copiers. Current electronic
registration
systems use a pair of narrow drive nips to control the media alignment during
registration, e.g., see U.S. Patent No. 5,094,442 by Kamprath et al., issued
March 10,
1992, U.S. Patent No. 5,697,609, by Williams et al., issued December 16, 1997,
U.S.
Patent No. 5,697,608, by Castelli et al., issued December 16, 1997, U.S.
Patent No.
5,887,996, by Castelli et al., issued March 30, 1999, U.S. Patent No.
5,678,159, by
Williams et al., issued October 14, 1997, U.S. Patent Application Publication
No.
2003/0146567 published August 7, 2003; U.S. Patent No. 4,971,304 by Lofthus,
issued November 20, 1990; U.S. Patent No. 5,169,140 by Wenthe, Jr., issued
December 8, 1992; U.S. Patent No. 5,219,159 by Malachowski et al, issued June
15,
1993; U.S. Patent No. 5,278,624 by Kamprath et al, issued January 11, 1994;
U.S.
Patent No. 5,794,176 by Milillo, issued August 11, 1998; U.S. Patent No.
6,137,989
by Quesnel, issued October 24, 2000; U.S. Patent No. 6,168,153 Bl by Richards
et
al, issued January 2, 2001; and U.S. Patent No. 6,533,268 B2 by Williams et
al, issued
March 18, 2003. When heavy media, high accelerations, or high drag forces are
present, the surface of the registration nips becomes strained. This strain
has been
demonstrated to cause a media velocity that is different than the ideal roll
surface
velocity, and this results in registration errors. These nip strain errors are
worse with
narrow drive nips, such as those often used in registration systems, but have
also been
observed to cause process registration errors in systems which use relatively
wide
20050081-US-NP 1
CA 02553357 2008-06-25
rollers. New feedback control systems are being developed that enable the
control
system to compensate for this nip strain by measuring actual paper movement.
An
example of such a system is entitled "Print Media Registration Using Active
Tracking
of Idler Rotation", U.S. Patent Application Publication No. 2005/0263,958.
These
systems work well, but add to the cost of the system, which can be an issue in
office
class machines or in systems where multiple registration devices are required.
It is
highly desirable to improve registration system performance without increasing
cost.
SUMMARY
[0002] Methods herein supply a program of intended drive motor
current/voltage levels (current and/or voltage levels) to the drive motor to
establish an
intended velocity of the drive motor and corresponding intended velocity of
the media
moved by the drive roller(s). For example, methods herein align media within
the
drive nip assembly of a printing apparatus by adjusting the intended
current/voltage
levels of the drive motor(s). The intended current/voltage levels are used to
adjust the
intended velocity of the drive motor(s) and associated drive roller(s) so as
to position
or angle the media within the media path of the printing/copying apparatus.
20050081-US-NP 2
CA 02553357 2006-07-25
[0003] However, because of different effects between the drive roller and
media,
the ratio of the velocity of the rollers to the media may not be as expected
from the
intended current/voltage level. In other words, there may be some difference
between the
velocity of the roller and the velocity of the media. This velocity difference
or "velocity
ratio" is caused by the normal interaction of the surfaces of the roller and
media. The
velocity ratio is different than "slippage" which occurs when the maximum
allowable
coefficient of friction between the roller and media is exceeded. After
slippage occurs, it
may be difficult or impossible to establish a relationship between the
velocity of the roller
and media; however, before slippage occurs (before the maximum allowable
coefficient
of friction is exceeded) the embodiments herein establish a relationship
between drive
motor torque (drive motor current/voltage levels) and the velocity ratio.
[0004] Generally, as more current/voltage is applied to the drive motor, the
drive
motor produces more torque, which may increase the interaction forces between
the roller
and media, and may in turn cause the velocity ratio to decrease from an
initial value of
1:1 (unity), when no significant drag or inertial forces are present, to a
ratio that is less
than or greater than one (e.g., 1:0.95, 1:0.90, 1:0.98, 1:1.02 etc.) when drag
or inertial
forces cause the drive force between the rollers and media to increase.
Further, such
change in velocity ratio is generally consistent among different paper types
that may be
handled by a given drive nip assembly (or class or type of drive nip
assembly). Thus, by
only measuring drive motor current/voltage levels, embodiments herein can
determine
the drive force between the drive rollers and media, which can then be used to
detennine
the velocity ratio at any point in time and correct the velocity of the roller
and the
corresponding velocity of the media accordingly, which avoids having to
provide
3
CA 02553357 2008-06-25
additional hardware media sensors, etc. to detect the actual discrepancy
between roller
velocity and media velocity.
[0005] More specifically, method embodiments establish a predetermined
relationship between current/voltage levels and media/drive roller velocity
ratios of
the specific drive nip assembly (or type of drive nip assembly). The
"current/voltage
levels" comprise current and/or voltage levels applied to the drive motor and
provide
an indication of torque being output by the drive motor. The "media/drive
roller
velocity ratios" comprise velocity relationships between the drive roller and
the media
when the media is in contact with the drive roller. Because the predetermined
relationship is based on results of testing one (or one type or class of)
drive nip
assembly, the predetermined relationship is considered to be "associated" with
a given
drive nip assembly.
[0006] The embodiments herein measure current/voltage levels of the drive
motor when the media is in contact with the drive roller so as to determine
the drive
force being output by the drive motor. Then, embodiments herein can reference
the
predetermined relationship between current/voltage levels and media/drive
roller
velocity ratios to determine a difference between the velocity of the drive
roller and
the velocity of the media based on the drive force. Once this velocity
difference is
determined, embodiments herein can change the current/voltage levels being
applied
to the drive motor if the actual velocity of the media is different than the
intended
velocity of the media so as to correct the velocity of the media. Thus, when
referencing the predetermined relationship, embodiments herein produce a
velocity
ratio correction factor. This velocity ratio correction factor calculation can
be done
during any velocity profiles of the drive motor. In addition, the inherent
drag and
inertial forces from the motor and drive system can be calibrated out by
measuring the
20050081-US-NP 4
CA 02553357 2008-06-25
current/voltage levels required to drive the system through a specified
velocity profile
when no media is present in the drive nip assembly.
[0007] Apparatus embodiments herein can include a drive nip assembly that
is adapted to move media within a printing and/or copying apparatus. A drive
motor
is included within the drive nip assembly, and a drive roller is connected to
the drive
motor. Further, a control system is connected to the drive motor. The control
system
allows the intended current/voltage levels to be changed if the actual
velocity of the
drive motor is different than the intended velocity of the drive motor.
[0008] More specifically, the control system establishes a predetermined
relationship between current/voltage levels and media/drive roller velocity
ratios, as
discussed above. After this, the current/voltage levels of the drive motor can
be
measured when the media is in contact with the drive roller to determine a
drive force
on the media. The predetermined relationship between current/voltage levels
and
media/drive roller velocity ratios is referenced to determine the difference
between
the velocity of the drive roller and the velocity of the media. This allows
the control
system to change the current/voltage levels being applied to the drive motor
if an
actual velocity of the media is different than an intended velocity of the
media, so as
to provide correction to the drive nip assembly.
[0009] The control system produces the velocity ratio correction factor when
referencing the predetermined relationship and can calculate the velocity
ratio
correction factor for all velocity profiles of the drive motor. Also, the
control system
is used to calibrate the current/voltage levels required to drive the system
when no
media is present in the drive nip assembly. The control system can repeat this
calibration periodically to compensate for changes in friction over the life
of the
system.
20050081-US-NP 5
CA 02553357 2008-06-25
[0010] Before slippage occurs (before the maximum allowable coefficient of
friction is exceeded) the embodiments herein establish a relationship between
drive
motor torque (drive motor current/voltage levels) and the velocity ratio. The
current/voltage levels of the drive motor can be measured when the media is in
contact with the drive roller to determine a drive force on the media. The
predetermined relationship between current/voltage levels and media/drive
roller
velocity ratios is referenced to determine the difference between the velocity
of the
drive roller and the velocity of the media. This allows the control system to
change
the current/voltage levels being applied to the drive motor if an actual
velocity of the
media is different than an intended velocity of the media, so as to provide
correction
to the drive nip assembly. Thus, by only measuring drive motor current/voltage
levels, embodiments herein can determine the drive force that the rollers are
imparting
on the media, and then calculate the current velocity ratio and correct the
velocity of
the roller and the corresponding velocity of the media accordingly, which
avoids
having to provide additional hardware media sensors, etc. to detect the actual
discrepancy between roller velocity and media velocity.
[O10A] According to an aspect of the present invention, there is provided a
method of controlling a drive motor comprising:
measuring current/voltage levels, comprising at least one of current and
voltage levels, applied to the drive motor connected to a drive roller in a
drive nip
assembly adapted to move media within one of a printing and copying apparatus;
determining a difference between a velocity of said drive roller and a
velocity
of said media based on said current/voltage levels; and
changing said current/voltage levels being applied to said drive motor if an
actual velocity of said media is different than an intended velocity of said
media.
20050081-US-NP 6
CA 02553357 2008-06-25
[O10B] According to another aspect of the present invention, there is provided
a method of controlling a drive roller comprising:
establishing a predetermined relationship between current/voltage levels and
media/drive roller velocity ratios, wherein said current/voltage levels
comprise at least
one of current and voltage levels, applied to the drive motor connected to a
drive
roller in a drive nip assembly adapted to move media within one of a printing
and
copying apparatus, and wherein said media/drive roller velocity ratios
comprise
velocity relationships between said drive roller and said media when said
media is in
contact with said drive roller;
measuring current/voltage levels of said drive motor when said media is in
contact with said drive roller;
referencing said predetermined relationship to determine a difference between
a velocity of said drive roller and a velocity of said media based on said
current/voltage levels; and
changing said current/voltage levels being applied to said drive motor if an
actual velocity of said media is different than an intended velocity of said
media.
[O10C] According to another aspect of the present invention, there is provided
an apparatus for use in moving media within one of a printing and copying
apparatus
comprising:
a drive nip assembly adapted to move media within one of the printing and
copying apparatus;
a drive motor within said drive nip assembly;
a drive roller connected to said drive motor; and
a control system connected to said drive motor,
wherein said control system is adapted to:
20050081-US-NP 6a
CA 02553357 2008-06-25
measure current/voltage levels, comprising at least one of current and voltage
levels, applied to said drive motor;
determine a difference between a velocity of said drive roller and a velocity
of
said media based on said current/voltage; and
change said current/voltage levels being applied to said drive motor if an
actual velocity of said media is different than an intended velocity of said
media.
[O10D] According to another aspect of the present invention, there is provided
an apparatus for use in moving media within one of the printing and copying
apparatus, comprising:
a drive nip assembly adapted to move media within one of a printing and
copying apparatus;
a drive motor within said drive nip assembly;
a drive roller connected to said drive motor; and
a control system connected to said drive motor,
wherein a predetermined relationship exists between current/voltage levels and
media/drive roller velocity ratios, wherein said current/voltage levels
comprise at least
one of current and voltage levels, applied to said drive motor, and wherein
said
media/drive roller velocity ratios comprise velocity relationships between
said drive
roller and said media when said media is in contact with said drive roller,
and
wherein said control system is adapted to:
measure current/voltage levels of said drive motor when said media is in
contact with said drive roller;
reference said predetermined relationship to determine a difference between a
velocity of said drive roller and a velocity of said media based on said
current/voltage;
and
20050081-US-NP 6b
CA 02553357 2008-06-25
change said current/voltage levels being applied to said drive motor if an
actual velocity of said media is different than an intended velocity of said
media.
[0011] These and other features are described in, or are apparent from, the
following detailed description.
20050081-US-NP 6c
CA 02553357 2006-07-25
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] Various exemplary embodiments of the systems and methods are
described in detail below, with reference to the attached drawing figures, in
which:
[0013] FIG. 1 is a graph showing force verses Velocity Ratio curves according
to
embodiments herein;
[0014] FIG. 2 is a schematic representation of drive nip assembly; and
[0015] FIG. 3 is a flow diagram illustrating aspects of embodiments herein.
DETAILED DESCRIPTION
[0016] Embodiments herein use an "electronic" registration control scheme that
compensates for nip-strain induced errors (that occur before the maximum
allowable
coefficient of friction is exceeded) without requiring additional hardware.
The act of
accelerating, translating and deskewing media through baffles generates
inertial and
frictional drag forces that result in nip strain, which in turn causes
velocity ratios with a
value other than unity between the media and drive nip.
[0017] The present inventors have discovered that drive torques applied to the
motors in a registration system are proportional to the drive forces that the
nips exert on
the media. Thus, the embodiments herein provide a control system that
accurately
predicts the velocity ratio of each nip during any given motion profile by
detecting the
current or voltage delivered to the servo motors (after the nip strain curve
for the drive
nips of the system has been previously characterized). Embodiments herein use
the
7
CA 02553357 2006-07-25
required current or voltage applied to the servo or step motor(s) to deduce
the drive force
at the nip(s), and then calculate a real-time correction to the roll velocity
to compensate
for nip-strain. The control system then adjusts the target velocity of the
drive nips so that
the media accurately follows the originally intended velocity profile.
Alternatively,
instead of compensating for the nip strain errors in real time, the velocity
and media
position errors from the calculated nip strain could be tracked and a
correction made near
the end of the registration profile.
[0018] Because of different effects between the drive roller and media, the
ratio
of the velocity of the rollers to the media may not be as expected from the
intended
current/voltage level. In other words, there may be some difference between
the velocity
of the roller and the velocity of the media. This velocity difference or
"velocity ratio" is
caused by the normal interaction of the surfaces of the roller and media. The
velocity
ratio is different than "slippage" which occurs when the maximum allowable
coefficient
of friction between the roller and media is exceeded. After slippage occurs,
it may be
difficult or impossible to establish a relationship between the velocity of
the roller and
media; however, before slippage occurs (before the maximum allowable
coefficient of
friction is exceeded) the embodiments herein establish a relationship between
drive motor
torque (drive motor current/voltage levels) and the velocity ratio.
[0019] Figure 1 illustrates that different types and thicknesses of media
yield the
same or very similar velocity ratio profiles when subjected to the same drag
in the same
drive nip assembly or same type of drive nip assembly. Therefore, Figure 1
illustrates
that the change in velocity ratio can be known if the load is known. For each
type of
drive nip assembly the velocity ratio curves will match very closely. This
type of testing
8
CA 02553357 2006-07-25
can be done during the drive nip assembly design phase or during
manufacturing. If
desired, the curves can be averaged or processed through other statistical
routines to
accommodate specific designer requirements/tolerances, or to be more generally
applied
to broader classes or types of drive nip assemblies. Embodiments herein
observe the load
on the motor (which is directly correlated to the drive force that the roller
imparts on the
media) to produce a correction to the velocity ratio, which can be applied in
real time to
the drive motor and provide accurate positioning of the media within the
printing
apparatus.
[0020] Generally, as more current/voltage is applied to the drive motor, the
drive
motor produces more torque, which may increase the interaction forces between
the roller
and media, and may in turn cause the velocity ratio to change from an ideal
1:1 (unity) to
a ratio that is less than or greater than one (e.g., 1:0.95, 1:0.90, 1:0.98,
1:1.02etc.).
Further, such change in velocity ratio is generally consistent among different
paper types
that may be handled by a given drive nip assembly (or class or type of drive
nip
assembly) and among different velocity profiles that may be applied to a given
drive nip
assembly (or type of drive nip assembly). Thus, by only measuring drive motor
current/voltage levels, embodiments herein can determine the velocity ratio
and correct
the velocity of the roller and the corresponding velocity of all types of
media accordingly,
which avoids having to provide additional hardware media sensors, etc. to
detect the
actual discrepancy between roller velocity and media velocity.
[0021] Thus, the velocity of media in a drive nip is dependent on the drag on
the
media. The ratio of the velocity of the media to the theoretical velocity of
the roller is
less than one when the drag forces act on the media, and can be less than or
greater then
9
CA 02553357 2006-07-25
one due to the combination of drag forces and inertial forces. This can cause
problems in
registration systems, since such systems rely on a predictable media velocity
to achieve
process direction registration, and in many cases, deskew.
[0022] The errors caused by nip strain are largely dependent on the tangential
forces at each nip throughout the registration move. These forces can vary for
each sheet
being registered, depending on a variety of factors: initial registration
errors, acceleration
profiles during the registration move, baffle and/or other paper path
component sheet
drags. Due to this, the forces cannot be "calibrated out" via "learning" or a
set-up
procedure. In many registration systems media is still in an upstream bend
during the
deskew process. Heavy paper and long heavy paper therefore require higher
drive forces,
which results in higher nip strain errors. Large, heavy media that comes in
skewed or
offset in one direction will see different nip strain induced errors than
media skewed or
offset in the opposite direction. The embodiments herein compensate for these
errors
automatically and do not require any knowledge of the media size or weight
being
registered.
[0023] As mentioned above, one way to compensate for these errors is to detect
the position of the sheet using an array of additional sensors or encoders
mounted to the
drive roll idlers and connected to a control system. However, this solution
requires
additional sensing hardware.
[0024] Figure 2 shows a two nip registration device in which the two nips
rollers
204 are driven by separately controlled DC servo motors 200. The skew sensors
212 are
used to detect the skew of the media 206 so that it can be corrected by uneven
usage of
the motors/rollers 200/204 before the media 206 reaches the image transfer
point 210.
CA 02553357 2006-07-25
Input sensors 212 are used to detect the leading edge of the media 206 as well
as its
speed, position, and skew.
[0025] As explained above, the drive torques applied to the motors 200 in a
two-
nip registration system are directly proportional to the drive forces that the
nips 204 exert
on the media 206. With this information, the control system 220 can accurately
know the
velocity ratio of each nip 206 during any given motion profile by detecting
the current or
voltage delivered to the servo motors 200 after the nip strain curve for the
drive nips of
the system has been previously characterized.
[0026] The embodiments herein provide a method of sensing the current or
voltage individually applied to the servo motors, using that value to
calculate a real-time
correction to each different roller velocity to compensate for nip-strain, and
then
adjusting the velocity of the drive nips so that the media accurately follows
the originally
intended profile. The system comprises the drive nip assembly shown in Figure
2 that
has one or more drive rollers 204 and a control system 220 that controls the
voltage or
current to the one or more drive motors 200 so that the motors follow a
prescribed
velocity profile. The control system 220 also uses the voltage or current
applied to the
drive motors 200 to deduce the drive force exerted by the drive rollers 204 on
the media
206, and to provide a correction factor to the prescribed velocity profile
based on the
voltage or current value.
[0027] At least one motor 200, and one drive shaft (gears, etc.) with at least
one
drive nip are used in embodiments herein, although as would be understood by
those
ordinarily skilled in the art, two or more motors 200, drive shafts, etc.
could be used. The
motor(s) 200 can be DC servo motors, step motors, etc. The drive rollers 204
can be
11
CA 02553357 2008-06-25
made from an elastomeric or other similar material. The position and skew of
the lead
edge of the media 206 entering the drive system can be detected using input
sensors
212.
[0028] The control system 220 establishes a predetermined relationship
between current/voltage levels and media/drive roller velocity ratios of the
specific
drive nip assembly (or type of drive nip assembly). The "current/voltage
levels"
comprise current and/or voltage levels applied to the drive motor 200 and
provide an
indication of torque being output by the drive motor 200. The "media/drive
roller
velocity ratios" comprise velocity relationships between the drive roller and
the media
when the media is in contact with the drive roller. Because the predetermined
relationship is based on results of empirical testing of one (or one type or
class of)
drive nip assembly, the predetermined relationship is considered to be
"associated
with" or "unique to" the type of drive nip assembly. The current/ voltage
supplied by
the controller to the motor should have sufficient sensitivity considering the
opposing
drag/ inertial forces. Thus, controller gain/bandwidth must be sufficiently
large to
detect these current/voltage levels.
[0029] The embodiments herein measure current/voltage levels of the drive
motor 200 when the media 206 is in contact with the drive roller 204 so as to
determine the drive force being output by the drive motor 200. Then, the
control
system can reference the predetermined relationship between current/voltage
levels
and media/drive roller velocity ratios to determine the difference between the
velocity
of the drive roller and the velocity of the media (based on the drive force).
Once this
velocity difference is determined, the control system 220 can change the
current/voltage levels being applied to the drive motor 200 if the actual
velocity of the
media is different than the intended velocity of the media (so as to correct
the velocity
of the media).
20050081-US-NP 12
CA 02553357 2006-07-25
[0030] Thus, when referencing the predetermined relationship, embodiments
herein produce a velocity ratio correction factor. This velocity ratio
correction factor can
be applied to all velocity profiles of the drive motor 200. A velocity profile
may, for
example, result in higher forces at the beginning of the movement (when
inertia is higher)
and less forces when the media is partially through the drive nip assembly
(when
maintaining a constant velocity of the media). In one example, embodiments
herein will
automatically apply a larger voltage or current to the motor when high drag
forces or
inertial forces are present. As shown above, this signal is then used to
calculate a
correction factor to the desired velocity profile to compensate for nip strain
errors.
[0031 ] Different velocity profiles are useful for different aspects of media
movement, as would be understood by those ordinarily skilled in the art in
view of this
disclosure. In addition, the current/voltage levels of the drive motor 200 can
be
calibrated when none of the media is present in the drive nip assembly.
Calibration is run
on the drive system when no paper is present, so that the drive torque
inherent to the
system can be subtracted out.
[0032] The correction factor is based on the pre-defined measurement of the
variation of media velocity over a range of drag forces for the drive rollers
used in the
system. The system drive force is calibrated by driving the motors when no
paper is
present, and using the current or voltage readings measured during this
operation to help
deduce the additional drive force exerted on the media during media transport.
The nip
velocity error due to nip strain is corrected on a continuous or frequent
basis, and the
accumulated nip strain error can be corrected just before the media reaches
the image
transfer station. Alternatively, the errors due to the deduced nip strain can
be tracked (but
13
CA 02553357 2006-07-25
not corrected on a continuous basis) and a correction made near the end of the
registration roll velocity profile.
[0033] One exemplary control scheme is shown in flowchart form in Figure 3.
More specifically, in item 300, the arrival of a new sheet of media is sensed.
The input
sensor detects the media's presence and any skew of the media, again using
input sensors
212. Item 302 represents the calculation of the velocity profile which
detennines the
desired velocity (or position) profile form registration of the drive rolls.
This information
is eventually supplied to the controller in item 306 with supplies a control
signal (motor
encoded coded signal) to the current/voltage amplifier (item 312). The
current/voltage is
applied to the "plant" (motor, drives, media drive, rollers, and eventually
media) in item
314. A feedback loop is provided to item 304 from the output of the motors to
correct for
any error that may have occurred to the intended signal being output by item
302.
[0034] Embodiments herein provide an additional feedback loop in items 308 and
310. More specifically, in item 310 a control signal being output by the
controller in item
306 is measured in terms of current and/or voltage. This current/voltage is
then
referenced on a force calibration look-up table or equation which converts in
the
current/voltage into nip the drive forces as shown in item 322. Then, once the
nip drive
forces are known, the nip velocity correction factor (that is based on the nip
strain and
calibration curve shown, for example, in Figure 1, above) is referenced in
item 308.
Thus, item 308 outputs a correction factor that is based on a media/drive
roller velocity
ratio corresponding to the nip drive forces determined in item 310. This
correction factor
is supplied to item 302 so that the velocity profile being output by item 302
can be
14
CA 02553357 2008-06-25
continually adjusted to account for the dynamically changing media/drive
roller
velocity ratio that varies during the interaction between the media and the
nip rollers.
[0035] As shown in Figures 2 and 3, the embodiments herein empirically
establish a predetermined relationship between current/voltage levels and
media/drive
roller velocity ratios of the specific drive nip assembly (or type of drive
nip assembly)
in item 320 (see discussion with respect to Figure 1, above). In addition, for
a given
motor or motor type, the actual force associated with a given current or
voltage
application (draw) can be obtained empirically to create the force calibration
look-up
table shown as item 322. Thus, with the feedback loop input to item 310,
embodiments herein measure current/voltage levels of the drive motor when the
media is in contact with the drive roller so as to determine the drive force
being output
by the drive motor (item 310). Then, embodiments herein can reference the
predetermined relationship between current/voltage levels and media/drive
roller
velocity ratios to determine a difference between the velocity of the drive
roller and
the velocity of the media based on the drive force (item 308). Once this
velocity
difference is determined, embodiments herein can change the current/voltage
levels
being applied to the drive motor if the actual velocity of the media is
different than the
intended velocity of the media so as to correct the velocity of the media in
item 302.
[0036] Thus, when referencing the predetermined relationship, embodiments
herein produce a velocity ratio correction factor that is supplied from item
308 to item
302. Since the velocity ratio correction factor is the same or very similar
for all media
types (or can be averaged, as discussed above) and is based on the force
applied, the
velocity correction factor selected from the look-up table or equation in item
320 can
be
20050081-US-NP 15
CA 02553357 2006-07-25
universally applied to all velocity profiles of the drive motor and all media
types. In
addition, the current/voltage levels of the drive motor can be calibrated when
none of the
media is present in the drive nip assembly in item 320. The desired velocity
profile
defined in box 302 of Figure 3 could function in several ways. It could take
the input
from function 308 and correct the velocity of the drive nips on a continuous
basis.
Alternatively, it could keep track of the velocity, and resulting positional,
errors in the
sheet as a result of the calculated nip strain, but not make a correction to
the nip velocity
profiles until the registration profiles were near completion. Other
variations of these two
control options are also possible, however all make use of the signals sent to
the drive
motors to deduce the nip drive forces and from that the nip strain or velocity
ratio for
each nip. Also note that although the force calibration and nip correction
factor
calculations are shown in separate boxes in Figure 3, these functions could be
combined
and a single conversion directly from motor current or voltage to nip velocity
correction
factor could be performed. Thus, the embodiments herein provide a control
system that
accurately predicts the velocity ratio of each nip during any given motion
profile by
detecting the current or voltage delivered to the servo motors (after the nip
strain curve
for the drive nips of the system has been previously characterized).
Embodiments herein
use the required current or voltage applied to the servo motor(s) to deduce
the drive force
at the nip(s), and then calculate a real-time correction to the roll velocity
to compensate
for nip-strain. The control system then adjusts the target velocity of the
drive nips so that
the media accurately follows the originally intended velocity profile.
[0037] It will be appreciated that the above-disclosed and other features and
functions, or alternatives thereof, may be desirably combined into many other
different
16
CA 02553357 2006-07-25
systems or applications. Also, various presently unforeseen or unanticipated
alternatives,
modifications, variations or improvements therein may be subsequently made by
those
skilled in the art which are also intended to be encompassed by the following
claims.
17
