Field of Invention
This invention relates to printing mechanisms and relates particularly,
but not exclusively, to those that use stepper motors to advance printable media
through the mechanism. The invention also relates to a method of advancing printable
media through a printing mechanism.
A variety of different printing mechanisms incrementally advance print
media through the mechanism to receive an image. These print mechanisms include,
for example, electrophotographic ("laser") and inkjet printers, plotters, facsimile
machines, cameras, and the like, which may be used in business, industry, home,
or other environments. For the purposes of illustration, the term "printer" will
be used herein, in the embodiment of an inkjet printer to explain the concepts
of this invention. While a variety of different print media may be used, such as
paper, transparencies, foils, fabric, card-stock and the like, for convenience,
the term "paper" is used herein for convenience.
Motors, such as stepper motors, have been used to advance paper through
the paper feeder mechanism of printing mechanisms, such as ink-jet printers. In
these earlier printing mechanisms, the motors operate at a relatively constant
rotational speed for the entire operational life of the printing mechanism.
In such motors, the higher the rotational speed ("slew speed") at
which the motor is set to operate, the faster the paper is able to be fed through
the paper feeder mechanism of the printing mechanism. There is, however, an upper
limit to the speed at which the motor is able to advance the paper through the
printing mechanism. The torque produced by a stepper motor varies inversely with
the rotational speed of the axle of the motor, so when stepper motors operate at
higher rotational speeds, the torque output is lower. Hence, each stepper motor
is characterized by an optimum rotational speed, above which the motor is more
likely to stall when it cannot generate sufficient torque to advance the paper.
However, these earlier printing mechanisms are generally not set to operate within
the range of this optimum rotational speed, even when the printing mechanisms are
new, because the printing mechanisms may be unable to maintain such high performance
for the life-span of the printing mechanism. This is due to the fact that as a
stepper motor ages with use, typically the amount of torque produced by the motor
at a particular rotational speed gradually decreases with time.
In anticipation of the inevitable decrease in motor performance, the
motors in these earlier printing mechanisms have been set to operate at relatively
constant rotational speeds that are well below the optimum capability of the new
motors. Setting the new motors to operate well below their optimum rotational speed
ensures that the motors will continue to provide adequate torque to advance the
paper, even when the motors are older. Thus, in these earlier printing mechanisms,
the optimum capability of the new motors are generally not utilized fully.
Furthermore, these earlier printing mechanisms are often designed
with more powerful motors than are actually required, so that the motors, set to
operate at a constant intermediate speed, will be able to provide adequate torque
over a longer operational life. Unfortunately, an increase in the power capacity
of a motor tends to be associated with increases in size, weight and cost of the
product. Compactness of design and portability are therefore sacrificed to increase
the longevity of the printing mechanisms. The accompanying increase in the power
consumption of the motor is also a disadvantage especially in portable printing
mechanisms which are battery powered.
Patent Abstracts of Japan, Vol. 017, No. 071 (E-1319) (JP-A-04 275097)
describes a system for controlling a paper feed motor by providing means for detecting
step-out of the motor and reducing the motor speed to a value at which step-out
no longer occurs.
Patent Abstracts of Japan, Vol. 010, No. 291 (M-522) (JP-A-61 106347)
describes a control system for varying the speed of a printing paper supply motor
in dependence on the type of paper being supplied. Paper supply sensors are used
in the control system.
Summary of the Invention
In accordance with a first aspect of the present invention there is
provided a method of advancing printable media through a printing mechanism using
a variable speed motor, as defined in Claim 1.
In accordance with a second aspect of the present invention there
is provided a printing mechanism having a variable speed motor adapted to advance
printable media through the printing mechanism, as defined in Claim 6.
In accordance with a third aspect of the present invention there is
provided a method of advancing printable media through a printing mechanism using
a variable speed motor, as defined in Claim 9.
In an illustrated embodiment, the different speed may be slower than
the initial speed of the motor. Alternatively, the different speed may be faster
than the initial speed of the motor. Preferably, however, the initial speed is
proximate the highest speed at which the motor, when it is new, is able to consistently
advance media through the printing mechanism without stalling. According to the
invention as claimed in claims 1 and 6, upon initially detecting that the motor
is unable to advance the media through the printing mechanism and before repeating
the two steps, there is a step of lowering the speed of the motor to a low speed,
below the different speed, at which low speed the motor is certain to be able to
advance the media through the printing mechanism.
Preferably, before the step of detection, there is the step of running
the motor for a limited time period to provide an opportunity for any media to
be advanced through the printing mechanism. Preferably, each time the speed of
the motor is varied, the speed decreases each time by a constant interval of speed.
Preferably, the motor is a stepper motor, and the speed of the motor is varied
by a micro-processor.
In the illustrated embodiment, preferably the motor speed controller
is able to repeatedly vary the speed of the motor until the media sensor detects
that the speed of the motor has been varied to a speed at which the motor is able
to advance the media through the printing mechanism. Preferably, the media sensor
comprises an optical-mechanical mechanism which, upon the mechanism engaging with
the media, is able to indicate to the motor speed controller that the media is
able to advance through the printing mechanism. Preferably, the motor speed controller
comprises a micro-processor.
An object of this invention is to optimize the performance of a printing
mechanism that uses a variable speed motor to advance the printable media through
the printing mechanism, and/or to allow the printing mechanism to function as effectively
with a less powerful motor.
Brief Description of the Drawings
In order that the invention might be more fully understood, a preferred
embodiment of the invention will be described, by way of example only, with reference
to the accompanying drawings, in which:
Detailed Description Of A Preferred Embodiment
- Figure 1 is a front perspective view illustrating a printer and sheet feeder
in an operational mode;
- Figure 2 is a rear perspective view illustrating the printer and sheet feeder
of Figure 1;
- Figure 3 is a right side view of the printer and sheet feeder in the operational
mode of Figures 1 and 2;
- Figure 4 is a view similar to Figure 3 with portions broken away to illustrate
the paper feed path of the paper feeder mechanism;
- Figure 5 is a partial view of the printer and sheet feeder of Figure 4 with
different portions broken away to illustrate the linkage between the paper feeder
motor (stepper motor) in the printer and the paper feed mechanisms in the sheet
- Figure 6 is a partial perspective view of the printer of Figure 1 with an access
door being opened.
- Figure 7 is a block diagram illustrating various components found in the printer
of Figure 1; and
- Figure 8 is a schematic diagram of a media sensor used in the printer of Figure
1. (Figure 8 is not drawn to scale).
Referring to the drawings, Figures 1, 2 and 3 are illustrations of
an embodiment of a printing mechanism, here shown as an inkjet printer 10, constructed
in accordance with the present invention, which may be used for printing for business
reports, correspondence, desktop publishing, and the like, in an industrial, office,
home or other environment. A variety of inkjet printing mechanisms are commercially
available. For instance, some of the printing mechanisms that may embody the present
invention include plotters, portable printing units, copiers, cameras, video printers,
and facsimile machines, to name a few. For convenience, the concepts of the present
invention are illustrated in the environment of an inkjet printer 10.
The print media may be any type of suitable sheet material, such as
paper, card-stock, transparencies, mylar, and the like, but for convenience, the
illustrated embodiment is described using paper as the print media.
The printer 10 includes a casing 172 which is best seen in Figures
4 and 6. The casing 172 houses a mechanical printer structure, such as a carriage
assembly (not visible) which reciprocates along a guide rod (also not shown) to
carry one or more inkjet printhead cartridges 182, such as cartridge or "pen".
The carriage assembly is driven by a carriage motor 80. The cartridge 182 emits
ink droplets through an inkjet printhead 182b onto the paper as the paper passes
beneath the reciprocating cartridge. Typically, the cartridge moves laterally to
print a single swath across the paper, the paper advances, another lateral swath
is printed, and so forth until an entire printed sheet emerges from the printer.
The casing 172 also houses a paper-handling system which, in the present
embodiment, is in the form of a paper sheet feeder 12, as well as an electronic
circuits that control the printer 10. The electronic circuitry contains a micro-processor
51 that influences the operation of the printer. A user is able to alter parameters
that are used by the micro-processor 51 by entering information and selections
through an input/output (I/O) interface 54. Referring to Figure 7, signals from
the microprocessor are directed to drivers which in turn control the operation
of various components of the printer 10. In particular, a printhead driver 182a
is used to drive the inkjet printhead 182b. A carriage motor driver 80a is used
to drive a carriage motor 80. A stepper motor driver 52a is used to drive a stepper
The printer 10 may be connected to a computer (not shown) by a cable,
and it is typically the computer that instructs the printer 10 to print an image
on a sheet of media, such as paper 235 of which an uppermost sheet of paper 235a
will be the first to be drawn into the printer 10.
The printer 10 comprises the sheet feeder 12 illustrated in Figure
3. Referring to Figure 4, a stack of paper 235 is loaded into the sheet feeder
12. The leading edge of the upper sheet of paper 235a from the stack 235 is positioned
beneath a pinch roller 230. The pinch roller rotates about shaft 231.
Referring to Figure 7, a variable speed motor is contained within
the printer 10. In the embodiment, the variable speed motor is in the form of the
stepper motor 52, although it is conceivable that the invention may be adapted
for use with other variable speed motors.
Referring to Figure 5, the stepper motor 52 drives a gear 232 which
engages with a corresponding gear 234 in the sheet feeder 12. The corresponding
gear 234 is connected by a conventional gear train 233 (partially visible) to the
shaft 231 for driving the pinch roller 230. The stepper motor 52 is driven in accordance
with the present embodiment, to be further described, in order to advance paper
235 from the sheet feeder 12, one sheet at a time, through the printer 10.
In order to advance paper through the printer 10, the uppermost sheet
235a in the stack of paper 235 is grabbed by the pinch roller 230. The paper is
advanced past slots 228, 22, and then directed beneath the ink-jet cartridge 182,
as the cartridge traverses the width of the paper. The printed sheet emerges from
a paper exit slot 30 on the other side of the printer.
As the paper enters the casing 172 near the path of the ink-jet cartridge
182, a media sensor detects the entry of the leading edge 71 of the paper 235a
into the casing 172.
Referring to Figure 8, in the present embodiment, the media sensor
is in the form of an optical-mechanical sensing mechanism 70. Part of the mechanism
consists of a shaped collar member 73 that is hooked loosely about the axle of
a driver roller 78 which is also driven by the stepper motor 52. This loose connection
allows the collar member 73 to swivel independently about the axle of the drive
roller 78. The collar member 73 swivels in a plane that is substantially normal
to the axle of the drive roller. The upper portion of the collar member is provided
with an upstanding tag 74, while the lower portion is provided with an elongated
depending tag 75. Hence, the upstanding tag 74 and the depending tag 75 both swivel
about the axle of the drive roller in tandem. The collar member is maintained in
an equilibrium disposition (as illustrated in Figure 8) by means of a spring (not
shown) attached to depending tag 75. The spring resists any swiveling motion of
the collar member in a clockwise direction (as indicated by the circular arrow
in Figure 8).
Another part of the optical-mechanical sensing mechanism 70 comprises
an opto-coupler 76 (shown partially in outline) which includes a slot 77. A beam
of light is directed from one end of the slot to the other, traveling across the
slot 77. However, when the collar member is in the equilibrium position, as illustrated
in Figure 8, the depending tag 75 is positioned within the slot 77, so as to block
the light path of the beam of light that is intended to shine from one side of
the slot to the other.
The upstanding tag 74 of the sensing mechanism 70 is positioned in
the path of the paper feed. As the leading edge 71 of the paper contacts the upstanding
tag 74, the tag is urged forwardly so as to rotate clockwise around the axle of
the drive roller. Since the depending tag 75 moves in tandem with the upstanding
tag 74, the forward clockwise movement of the upstanding tag causes the depending
tag 75 to move out of the way of the light path of the beam of light, A-A. The
upstanding tag 74 remains forwardly urged by the paper as the paper advances along
the paper feed path through the printer 10. This forward urging of the upstanding
tag 74 effectively causes light to shine from one side of the slot 77 to the other
to complete an optical light circuit contained within the opto-coupler 76. This
causes the circuit to send an appropriate signal 50 to the micro-processor 51 indicating
that the paper has been able to successfully enter the printer from the sheet feeder
The invention, however, is not restricted to this particular form
of media sensor in the form of the optical-mechanical sensing mechanism 70. Alternative
media sensors, such as purely optical devices, including photosensors, or purely
mechanical sensors, may be used to detect the entry of paper into the casing.
It is not essential that the detection be limited to the entry of
paper into the casing. It is preferred that the sensor be able to detect that the
printable media has been successfully grabbed by the pinch roller 230, or equivalent
mechanism, to such an extent that the paper is ensured of reaching the printing
mechanism of the printer. Hence, the actual position of the media sensor in the
design of the printer may be varied, so long as the media sensor is able to perform
the foregoing function.
The speed of rotation of the shaft 231 ("slew speed"), which is driven
by the stepper motor 52, may be varied according to the instruction sent to the
motor by a motor speed controller. In the present embodiment, the micro-processor
51 performs the function of the motor speed controller. The micro-processor 51
is controlled by a computer program which may be stored in the computer circuitry.
However, it is within the scope of the invention for a variable speed motor to
be controlled by some mechanical or other effective means, such as a host computer
coupled to the printer. As another example, a form of media sensor might be used
to selectively trigger a range of relays, each adapted to cause the stepper motor
52 to operate at a different speed. Hence, the motor speed controller used to control
the stepper motor 52 need not be restricted to a micro-processor and computer program.
Initially, the stepper motor 52 is set at a high slew speed (measured
in pulses per second, "p.p.s."), causing the pinch roller 230 to rotate at a high
rotational rate. Preferably, this initial rotation rate should not be set at the
absolute highest rate possible, since this would leave little margin for error,
due to variation in the performance of commercially available stepper motors. In
the present embodiment, the new stepper motor 52 has the ability to advance paper
at a rate faster than 1,000 p.p.s. Nevertheless, it has been found that performance
above 1,000 p.p.s. may be erratic with little margin for error, and so from a design
viewpoint, 1,000 p.p.s. has been selected as an upper limit at which most new stepper
motors are able to consistently advance the paper through the printer without stalling.
The stepper motor 52 first of all attempts to advance the paper through
the printer 10 while running at the high initial speed of 1,000 p.p.s. The stepper
motor rotates for a predetermined checking period, for example, in the range of
2,000 to 5,000 steps. In the present embodiment, the stepper motor rotates for
3,300 steps which is equivalent to about seventy revolutions of the axle of the
stepper motor. In the stepper motor used in the embodiment, 1 step = 7.5 degrees
of rotation. However, this is a characteristic of the particular motor selected
for the embodiment, and is not necessarily a constant equation for every motor.
If the amount of torque generated by the stepper motor 52, running at a slew speed
of 1,000 p.p.s., is sufficient to advance the paper, then the sensing mechanism
70 would be reasonably expected to detect the paper within this time period taken
for the rotation of the 3,300 steps. If the sensing mechanism 70 detects that the
paper has advanced successfully through the printer within this checking period,
the motor is then set to run at this maximum slew speed of 1,000 p.p.s., and continues
to do so until a sheet of paper cannot be advanced at this speed. Hence, the printer
is able to function at this high rotational speed of 1,000 p.p.s., at least while
the motor is relatively new, in contrast to earlier designs, which were limited
to an intermediate speed, i.e. 500 p.p.s., for the entirety of their lifespan.
However, when the sensing mechanism 70 does not detect any paper entering
the printer during the 3,300 step checking period, it could be due to the inability
of the stepper motor 52 to provide sufficient torque when running at the maximum
initial rotational speed of 1,000 p.p.s. Alternatively, the lack of detection of
any paper could simply be due to the absence of paper in the sheet feeder 12.
The invention as claimed in claims 1 and 6 is able to test whether
the paper feed failure is due to insufficient motor torque, or due to the fact
that there is no paper in the sheet feeder. To perform this test, the micro-processor
51 instructs the stepper motor 52 to lower the rotational speed of the stepper
motor to a lower level at which paper advancement would be certain to succeed,
in this instance, a slew speed of 550 p.p.s. If the sensing mechanism 70 detects
that paper advancement is successful at this low speed of 550 p.p.s., it is assumed
that the stepper motor 52 was unable to provide sufficient torque when running
initially at the higher speed of 1,000 p.p.s. This fact may be stored in non-volatile
memory 60 of the circuitry. Consequently, when the next sheet of paper is to be
printed, the micro-processor 51 instructs the stepper motor 52 to commence rotation
at a lower initial speed of 900 p.p.s. rather than the maximum speed of 1,000 p.p.s.
The same iterative sequence is performed. If the paper advances successfully at
900 p.p.s., then the printer 10 continues to advance paper at the speed of 900
p.p.s. until a further failure occurs. However, when the sensing mechanism 70 detects
a failure to advance the paper at 900 p.p.s., the sensing mechanism provides an
indication to the micro-processor 51, which in turn instructs the stepper motor
52 to drop once again to the lower level of 550 p.p.s. to ascertain the origin
of the failure, as described above (no paper or failure to pick). If advancement
is successful at 550 p.p.s., the printer attempts to advance the next sheet of
paper at a sequentially lower speed, such as 850 p.p.s., and so forth.
In the present embodiment, the iterative sequence proceeds as described,
according to the following sequence:
20% (end of iterative sequence)
22% (test speed)
The above figures have been ordered at intervals of 50 p.p.s., although
the invention is not limited to the above sequence, increments or proportions,
of values. For example, rather than 5% speed reduction steps, the reductions may
be at 10%, 20% or other suitable intervals, depending upon the particular implementation.
In the above table, the rotational speed of the stepper motor 52 has
been expressed in terms of the slew speed of the motor, measured in pulses per
second (p.p.s.). The torque produced by the stepper motor 52 at a particular slew
speed has been expressed in terms of torque margin, which is proportional to torque.
The torque margin is a measure of the difference between the actual torque generated
at the particular slew speed, and the torque at which the motor is expected to
stall. Hence, when a stepper motor runs with a very small torque margin, there
is a higher likelihood of stalling, as compared to when the motor runs with a higher
In the illustrated embodiment, once the slew speed of the stepper
motor 52 decreases to 650 p.p.s., the printer will no longer rely on the iterative
sequence. Hence, if the stepper motor fails to advance paper when running at 650
p.p.s., then the printer will indicate that it is out of paper, and will not proceed
with the printing job.
At any point in the iterative sequence of the embodiment, the prevailing
speed at which the stepper motor 52 is set to run is stored preferably in non-volatile
memory 60 in the circuitry of the printer 10. When the printer is turned off, this
information in the non-volatile memory 60 is retained. When the printer is turned
on again, the iterative process starts with the stepper motor 52 set to run at
the initial slew speed of 1,000 p.p.s. However, if failure this time occurs at
the speed of 1,000 p.p.s., rather than lowering the speed to the next incremental
level of 900 p.p.s., the speed of the stepper motor is lowered straight down to
the speed value stored in the non-volatile memory 60.
There may be instances where the performance of the motor or printer
is modified or enhanced, for example, as a result of repairs or replacement with
new parts. When such improvements have been made, for example in the present embodiment,
the stepper motor 52 may no longer need to be slowed down to the lower operational
speed that has been stored in non-volatile memory 60. Hence, when the enhanced
printer is first turned on, the stepper motor 52 commences operation at the initial
slew speed of 1,000 p.p.s. If no feed failures occur at 1,000 p.p.s. due to the
enhancements, then the lower operational speed stored in the memory 60 is changed
back to the upper value of 1,000 p.p.s. It is assumed that the conditions that
had required the printer to operate at the lower speed no longer east. In this
manner, the printer using the iterative sequence of the embodiment is able to respond
even to positive improvement in motor performance, as well as being able to compensate
for negative degradation of motor performance over time.
It is important to note that the claimed invention in its broadest
aspect is not limited to the iterative sequence described above. Although the stepper
motor 52 in the embodiment is set initially at its optimum rotation speed and subsequently
lowered progressively, the invention may use a variety of other sequences, particularly
when the variable speed motor is capable of being controlled iteratively to optimize
the performance of the motor at any point in time. According to the invention as
claimed in claim 9, the stepper motor 52 might be set initially at its lowest
rotation speed, which may then be increased progressively with each subsequent
sheet of paper, until the motor fails to advance the paper. There are any number
of possible iteration sequences that would allow the stepper motor to arrive at
an optimum speed depending on the prevailing operational parameters.
An advantage of the embodiment is that at each point in the operational
life of the printer 10, the stepper motor 52 is made to run the optimum speed at
which the motor, at that point, is capable of functioning consistently without
stalling. As an example, a new printer is able to advance paper initially at a
speed of, say, 1,000 p.p.s. This may decrease gradually over the lifetime of the
printer down to a rate of say, 550 p.p.s. Hence, a printer incorporating the iterative
control sequence of the embodiment would be able to take advantage of the high
motor speed of 1,000 p.p.s., at least while the motor is new. This is in contrast
to earlier printers which, for the entire operational life of the printer, are
set to operate at a low constant speed of rotation of, say, 550 p.p.s. which is
done to ensure that the motor will be capable of maintaining this lower rotational
speed for the lifetime of the printer. Therefore, another advantage of the present
invention is that the performance of the printer is optimized, at least during the
early part of the printer's lifetime. As an example, it has been found that a new
printer, constructed according to the embodiment, advanced paper almost twice as
fast as compared to an earlier model of the printer that does not make use of the
iterative control sequence of the embodiment.
Another advantage of the invention is that it permits the use of smaller
motors in printing mechanisms. Since, according to this invention, it is possible
to run the motor closer to its optimum capability, it is no longer necessary to
use a larger motor set to run at medium capability. A smaller motor running close
to its optimum speed would provide similar performance as a larger motor running
at medium performance. The ability to use smaller motors allows for the design
of lighter and more economical printers. Smaller motors also require less power,
which is especially useful for the design of portable printers which may be powered
Furthermore, there are other factors, apart from the age of the motor,
that may influence the ability of the stepper motor 52 to advance the paper. The
thickness of the paper or printable media, and even ambient temperature, can influence
the performance of the stepper motor 52. The iterative control sequence allows
the stepper motor 52 to run at the optimum rotational speed suitable for the prevailing
operational parameters; such as motor age, ambient conditions and/or the characteristics
of the printable media.
Without this ability to iteratively vary the speed of the motor 52,
the printer 10 would have to operate with the motor set to rotate at a constant
speed. Since the stepper motors of earlier printers are conservatively set to run
well below maximum performance, so that the motors will be able to perform for
a longer operational lifetime, the optimum capability of stepper motors in these
earlier printers was never utilized fully.