Sheet transport unit for recording systems

In a sheet transport unit for recording systems which uses a pulse motor to feed a recording sheet quantitatively and intermittently, multiple different current apply intervals are specified within the stop period during intermittent sheet feed, so that a maximum amount of current will be applied to the first interval immediately after the drive stops. Thereby, the pulse motor is never be damaged due to its temperature rise despite a load variation and the load variation can be absorbed enough to ensure high positioning accuracy.

BACKGROUND OF THE INVENTION 
This invention relates to a recording system which feeds a recording sheet 
intermittently by a given pitch using a pulse motor to thereby form images 
on it, more particularly to a pulse motor driving system used as a driving 
source for the recording sheet transport mechanism in the recording 
system. 
RELATED BACKGROUND ART 
The recording system for a printer, facsimile, or copying machine has such 
a configuration that the energy generating unit for imaging elements of a 
recording head is driven based on recording data to record images on 
paper, plastic sheets, or any other recording medium. 
This type of recording system is usually of a print type to record one 
whole line, as a unit page print type to record one whole page, as a unit 
or of a serial type to record data while moving the carriage equipped with 
a recording head over a recording sheet, which then feeds the recording 
sheet by a given pitch when recording of one line completes to be ready 
for recording of the next line. 
In the serial-type recording system or a recording system which feeds a 
recording sheet by a given pitch or amount and intermittently, the 
accuracy in recording sheet feed greatly affects image quality. Therefore, 
a pulse motor ensuring excellent positioning accuracy is generally 
employed as a driving source to transport (feed) recording sheets. 
In addition, a member related to the sheet feed accuracy such as; a driving 
transfer means for sheet transport units or a feed roller which directly 
touches recording sheets to transport them, must permit high accuracy. 
The pulse motor for sheet feed is often used not only to feed sheets but 
also to drive any load including a suction recovery pump located at the 
exit of an ink jet recording system. 
However, if a mechanism which the sheet feed pulse motor drives in addition 
to sheet feed is prone to a great load variation, the load variation 
causes the pulse motor not to stop accurately and deviates the pulse motor 
from its specific stop position (angle). This causes such a critical 
technical problem as a sheet feed pitch error, thus deteriorating image 
quality. 
To resolve that adverse effect of the load variation, the mechanism having 
a load variation should be driven by another driving source. 
Alternatively, a clutch is interposed between the sheet transport 
mechanism and the mechanism whose load varies. These measures require 
installation of an extra motor or an additional clutch, eventually 
increasing the cost or size of a system. The measures are, therefore, 
unfavorable. 
In another method, a pulse motor is energized in its stop period which is 
in between two successive sheet feeding or advancing operations and in 
which no recording is effected during intermittent sheet feed in order to 
retain a holding torque, of the pulse motor whereby the pulse motor is 
used as a dedicated sheet feed pulse motor for recording systems (Japanese 
Laid-Open Patent Application No. 54-49026). 
However, this kind of methods allows electric energy applied during the 
stop period of a pulse motor to be fully converted into heat energy. This 
accelerates temperature rise in the pulse motor and eventually causes a 
damage due to a burnout. 
SUMMARY OF THE INVENTION 
This invention aims to solve the aforementioned drawbacks, offering a sheet 
transport unit for recording systems to ensure high positioning accuracy 
despite a load variation. 
The other purposes of this invention will be duly revealed through more 
specific embodiments mentioned later.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An embodiment of this invention is described below with reference to the 
drawings. 
FIG. 1 shows a sheet transport system for recording systems. FIG. 2 shows a 
perspective view of the sheet transport system in FIG. 1. 
In FIGS. 1 and 2, only a top sheet of recording sheets 2 (paper, thin 
plastic plate, or any other recording medium) loaded in a sheet feed 
cassette is picked up by rotating a sheet feed roller 3 and fed to the 
interspace between sheet feed guides 4 and 5. 
The sheet feed roller 3 halts at the illustrated position after a single 
rotation. Then, transporting force for the recording sheet 2 quenches. At 
this time, the tip of the recording sheet 2 is sandwiched between a lower 
transport roller 9 which a pulse motor 8 drives and an upper transport 
roller 10 which is pressed toward the lower transport roller and follows 
the rotation thereof. Thereafter, recording sheet 2 is restricted in its 
feed pitch by a pair of these rollers 9 and 10. 
The lower transport roller 9 is driven by the pulse motor 8 via a belt 6 
and a pulley 7. 
The recording sheet 2 is further fed and then stopped when its tip is 
caught between a lower sheet discharge roller 11 and an upper sheet 
discharge roller 12. 
The upper sheet discharge roller 12 is held down to the lower sheet 
discharge roller 11 by means of a spring unshown. 
Here, the lower sheet discharge roller 11 is coupled with the lower 
transport roller 9 via gears 14, 15, and 16. The lower sheet discharge 
roller 11 is set to rotate faster than the lower transport roller 9 by a 
given percentage (for example, 2%). 
Thus setting the rotation speed, the recording sheet 2 always remains 
properly tensioned at a platen 17. 
In this state, a carriage 19 having a recording head 18 scans along a rail 
20 in arrow B direction, whereby recording for a single line is performed. 
When recording of a single line completes, the transport roller 9 is 
rotated in arrow A direction so that the recording sheet 2 will be fed by 
a given pitch to be ready for recording of the next line. This series of 
operations is repeated. 
When recording of the entire recording sheet 2 completes, the recording 
sheet 2 is discharged onto a sheet discharge tray 21 by a pair of sheet 
discharge rollers 11 and 12. 
In the meantime, an ink exit of the recording head 18 may clog and fail to 
supply ink for some portions. This clogging can be recovered (cleared) by 
a suction recovery mechanism. To be more specific, an exit surface 18A of 
the recording head 18 is sealed with a cap 22. Then, a pump 24 is used to 
suck inside of the cap 22 via a tube 23. Thus, clogging of each exit is 
cleared (recovered). This is the suction recovery mechanism. 
The suction pump 24 is driven when pushed by a cam 25 fixed at the end of 
the lower transport roller 9. 
Thereby, a load torque applied to the pulse motor 8 greatly varies 
depending on whether the cam 25 is pushing the pump 24 or not. 
With this invention, in a sheet transport unit for a recording system 
adopting a pulse motor 8 for feeding a recording sheet 2 intermittently, 
by a given amount each time, multiple different current apply intervals 
are specified within the stop duration which is in between two successive 
sheet feedings of the intermittent sheet feed and in which no recording is 
effected so that a maximum amount of current will be applied to the same 
phase as that in which the motor drive stops during the first interval 
immediately after the motor drive stops. Thereby, even if a load to the 
pulse motor 8 varies with on or off of the pump 24, temperature rise in 
the pulse motor 8 can be suppressed and the load variation can be 
absorbed. Thus, high positioning accuracy can be ensured despite a load 
variation. 
In a typical embodiment of this invention, when the pulse motor 8 stops 
during intermittent and a given pitch or amount of feed of a recording 
sheet 2 or when the pulse motor stops after the recording sheet 2 is fed 
by a given pitch at a time of line feed, a given amount of current is 
applied to the same phase as that in which the motor drive stops during a 
given period of time immediately after the drive stops. This helps 
increase in a holding torque of the pulse motor 8. Thereby, the pulse 
motor 8 always stops rotating at the same position. 
Moreover, after a given time has elapsed within the stop period of the 
pulse motor 8, or after rotation of the pulse motor has stopped and 
oscillation of a transport roller 9 or any other inertial load has 
attenuated, current is disconnected. This successfully prevents such a 
fault that the pulse motor 8 itself is heated up and eventually broken due 
to the temperature rise. 
FIG. 3 shows a circuit to control rotation of the above pulse motor 8. 
In FIG. 3, 26 represents a microcomputer to control rotation or stoppage, 
rotation rate, rotation speed, and driving current for the pulse motor 8. 
The microcomputer 26 incorporates a timer T and outputs control signals to 
a pulse motor driving IC 27 as well as driving current control elements 
(Tr1 and Tr2) 33 and 34. 
The pulse motor driving IC 27 detects currents flowing phases A, A, B, and 
B of the pulse motor 8 using voltages across current detecting resistors 
R.sub.SA and R.sub.SB) 28 and 29. Then, the currents flow until the 
voltages become equal to comparison voltage V.sub.REF generated through 
voltage dividing resistors (R1 and R2) 30 and 31. Thus, chopping is done 
to control constant current. 
The SLA7024M of Sanken Electric Co., Ltd. may be used as the pulse motor 
driving IC 27. 
The comparison voltage V.sub.REF varies with on's or off's of driving 
current control elements (Tr1 and Tr2) 33 and 34 according to the 
following expressions: When both Tr1 and Tr2 are off; 
##EQU1## 
When Tr1 is on and Tr2 is off; 
##EQU2## 
When Tr1 is off and Tr2 is on; 
EQU V.sub.REF .perspectiveto.0 
Under the control of the comparison voltages given by the above expression 
(1), (2), and (3), the driving currents (1), (2), and (3) get smaller in 
that order. That current values are changeable. 
FIG. 4 shows the waveforms of signals INA, INA, INB, and INB sent from the 
microcomputer 26 to the pulse motor driving IC 27. The waveforms of the 
currents flowing through the phases of the pulse motor 8 (for two-phase 
exciting mode) are also shown graphically. 
FIGS. 5A and 5B are flowcharts demonstrating sheet feed control operations 
by the microcomputer 26. 
In FIG. 5A, the system enters a stand-by routine at a step S200. Upon 
receipt of a sheet feed command at a step S201, it rotates the pulse motor 
8 to feed sheets at a step S202. 
With a recording command for one line received at a step S203, the system 
proceeds to a step S204, and then rotates a carriage motor unshown and 
drives a recording head 18 to record data. 
At a step S205, it is determined whether the recording operation is to be 
performed on the last line. If it is not on the last line, the system 
proceeds to a step S206. Thereby, the pulse motor 8 is rotated for a 
single line to feed a recording sheet 2 for recording of a single line. 
A driving current control element Tr1 is set to on at a step S207 to change 
the stoppage current, so that a certain torque will be applied (a given 
holding torque will be generated) immediately after the pulse motor 8 is 
stopped after completing sheet feed. The stoppage current at this time is 
provided to the same phase as that in which the motor drive has stopped. 
In addition, at a step S208, a stoppage current timer T is set so that the 
stoppage current will flow for a certain duration within the stop period 
of the pulse motor 8. 
Thereafter, the system returns to the step S203 and determines if the next 
recording command is found. If it is found, the above operations are 
repeated. 
FIG. 5B shows an interruption routine 300 performed at intervals of a 
certain duration for the stoppage current timer T. 
In FIG. 5B, the stoppage current timer T set as previously mentioned checks 
at the step S301 if the timer value becomes nil and reduces the value at a 
step S302 until it becomes nil. When the value becomes nil, the driving 
current control element Tr2 is set to on at a step S303 so that pulse 
motor current will be nil. 
In FIG. 5A, when it is confirmed at a step S205 that recording of the last 
line has completed, the system proceeds to a step S209 and rotates the 
pulse motor 8 by a given pitch to discharge a sheet. Then, the system 
proceeds to a step S210 for a recovery routine. Then it returns to the 
step S201 and waits for the next sheet feed and recording commands. 
According to the embodiment described above, when the pulse motor 8 which 
feeds a recording sheet 2 a given pitch or amount and intermittently stops 
during intermittent sheet feed, multiple different current applying 
intervals are specified so that a given amount of current will be applied 
during the first interval immediately after the drive stops. Even if a 
load to the pulse motor 8 varies due to on or off of an ink recovery unit 
for which the motor also works, the load variation can be absorbed without 
damaging to the pulse motor 8 due to the temperature rise. This has 
embodied a sheet transport unit which ensures high positioning accuracy 
even if a load variation occurs. 
In the above embodiment, this invention is adopted in a sheet transport 
unit for bubble-set type ink jet recording systems. Herein, the bubble-set 
type ink jet recording system is an ink jet recording system in which 
heating elements are installed along a recording fluid path within a 
recording head 18 to bring about a state change in the recording fluid or 
create bubbles in the recording fluid using heat energy, whereby fluid 
drops produced with pressure of the bubbles are fused on recording sheets 
for recording. This invention is also applicable to the sheet transport 
unit for an ink jet recording system using electromechanical energy 
conversion elements, thermal recording system, wire-dot type recording 
system, laser-beam type recording system or any other recording system 
using any type of recording head. 
In the above embodiment, the stop period of the pulse motor 8 which is 
between two successive sheet feeding operations is divided into two 
intervals, namely; current apply and non-apply intervals. In the later 
current non-apply interval, current should not necessarily be made nil. 
That is, a very small amount of current may flow in such a way that the 
temperature of the pulse motor 8 will not be a hindrance. 
In another embodiment, the aforementioned stop period is divided into three 
or more intervals. Then, a maximum amount of current is applied during the 
interval immediately after the pulse motor stops and then the current is 
reduced gradually in the subsequent intervals. This method is also 
feasible and has proved equally effective. 
As the above description has clarified, this invention yields the following 
advantages: In a sheet transport unit for recording systems which uses a 
pulse motor to feed a recording sheet by a given pitch or amount each time 
intermittently, multiple different current applying intervals are 
specified within the stop period during intermittent sheet feed so that a 
maximum amount of current will be applied during the first interval 
immediately after the drive stops. Thereby, even a unit including a factor 
of great load variation can ensure satisfactory accuracy in quantitative 
feed of a recording sheet and permit excellent image quality. In addition, 
since current is disconnected after the drive stops completely, 
temperature rise in a pulse motor is subdued in a practical level. This 
prevents a burnout or any other fault due to the temperature rise.