Adaptable media motor feed system for printing mechanisms

This invention relates to printing mechanisms, particularly those that use stepper motors to advance paper through the mechanisms. The invention also relates to a method of advancing paper through a printing mechanism. The method involves an iterative sequence for controlling a variable speed motor. Before advancing the paper through the printing mechanism with the motor set to run at an initial speed, a sensor detects whether the motor is able to advance the paper. If the motor is unable to advance the paper, the speed of the motor is varied according to the iterative sequence until the paper is able to advance through the printing mechanism. The invention also relates to a printing mechanism that incorporates the use of such an iterative sequence.

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. 
BACKGROUND 
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. 
SUMMARY OF THE INVENTION 
According to one aspect of the present invention, there is provided a 
method of advancing printable media through a printing mechanism using a 
variable speed motor. The method includes the steps of attempting to 
advance the media through the printing mechanism with the motor set to run 
at an initial speed. In a detecting step, it is detected whether the motor 
is able to advance the media through the printing mechanism. Upon 
detecting that the motor is unable to advance the media through the 
printing mechanism with the motor set at the initial speed, repeating the 
two steps of (i) varying the speed of the motor to run at a different 
speed, and (ii) the step of detection until the speed is varied to a speed 
at which the motor is able to advance the media through the printing 
mechanism. 
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. Preferably, 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. 
According to another aspect of the invention, there is provided a printing 
mechanism having a variable speed motor adapted to advance printable media 
through the printing mechanism. The printing mechanism includes a media 
sensor for detecting whether the media is advancing through the printing 
mechanism. The mechanism also includes a motor speed controller adapted to 
vary the speed of the motor in response to an indication from the media 
sensor. Upon receiving indication from the media sensor that the motor is 
unable to advance the media through the printing mechanism when the motor 
is set at a certain speed, the motor speed controller is able to vary the 
speed of the motor to a speed at which the motor is able to advance the 
media through the printing mechanism. 
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 
microprocessor. 
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.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
Referring to the drawings, FIGS. 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 FIGS. 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 FIG. 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 motor 52. 
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 FIG. 3. 
Referring to FIG. 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 FIG. 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 FIG. 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 FIG. 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 FIG. 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 FIG. 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 FIG. 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 12. 
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. 
It is preferable 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: 
______________________________________ 
Slew Speed Torque Margin 
______________________________________ 
1,000 p.p.s. 10% 
900 p.p.s. 14% 
850 p.p.s. 16% 
800 p.p.s. 17% 
750 p.p.s. 18% 
700 p.p.s. 19% 
650 p.p.s. 20% (end of iterative sequence). 
550 p.p.s. 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 
torque margin. 
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 nonvolatile 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 maybe 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 exist. 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. As an example, 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. Alternatively, the stepper motor 52 
might be set to run initially at 1,000 p.p.s., with the speed of the 
stepper motor decreasing at each iterative step, without the intermediate 
test step of dropping to 550 p.p.s. to ascertain the origin of the feed 
failure. 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 by 
battery. 
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.