Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
2002-02-19
2004-01-27
Pham, Hai (Department: 2861)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
C347S130000
Reexamination Certificate
active
06683638
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a print head of the dynamically driven type, and an image forming apparatus employing the print head.
2. Description of the Related Art
The present invention is applicable to, for example, an electrophotographic printer in which a charged photosensitive drum is selectively illuminated to form an electrostatic latent image, which is developed by application of toner, then transferred to paper and fused onto the paper. To understand the problem addressed by the present invention, it is useful to understand the control circuits of this type of printer in some detail, so a description will be given at this point with reference to
FIG. 1
, which is a block diagram of the control circuits of an electrophotographic printer employing light-emitting diodes (LEDs) for illumination, and
FIGS. 2 and 3
, which are timing diagrams illustrating the printing operation.
The printing control unit
1
in
FIG. 1
is a computing device comprising a microprocessor, read-only memory (ROM), random-access memory (RAM), input-output ports, timers, and other facilities. Upon receiving signals SG
1
, SG
2
, etc. from a higher-order controller (not visible), the printing control unit
1
generates signals that control a sequence of operations for printing dot-mapped data. The data are provided in signal SG
2
, which is sometimes referred to as a video signal because it supplies the dot-mapped data one-dimensionally.
The printing sequence starts when the printing control unit
1
receives a printing command from the higher-order controller by means of control signal SG
1
. First, a temperature (Temp.) sensor
160
is checked to determine whether the fuser
161
is at the necessary temperature for printing. If it is not, current is fed to a heater
161
a
to raise the temperature of the fuser
161
.
When the fuser
161
is ready, the printing control unit
1
commands a motor driver
162
to drive a develop-transfer process motor (PM)
163
, activates a charge signal SGC to turn on a charging power source
164
, and thereby applies a voltage to a charging unit
165
that negatively charges the surface of a photosensitive drum (not visible).
In addition, a paper sensor
166
is checked to confirm that paper is present in a cassette (not visible), and a size sensor
167
is checked to determine the size of the paper. If paper is present, another motor driver
168
drives a paper transport motor (PM)
169
according to the size of the paper, first in one direction to transport the paper to a starting position sensed by a pick-up sensor
170
, then in the opposite direction to transport the paper into the printing mechanism.
When the paper is in position for printing, the printing control unit
1
sends the higher-order controller a timing signal SG
3
(including a main scanning synchronization signal and a sub-scanning synchronization signal) as shown in FIG.
2
. The higher-order controller responds by sending the dot data for one page in the video signal SG
2
. The printing control unit
1
sends corresponding dot data (HD-DATA) to an LED head
2
in synchronization with a clock signal (HD-CLK). The LED head
2
comprises a linear array of LEDs for printing respective dots.
After receiving data for one line of dots in the video signal SG
2
and sending the data to the LED head
2
, the printing control unit
1
sends the LED head
2
a latch command by means of a load signal (HD-LOAD), causing the LED head
2
to store the print data (HD-DATA), then sends the LED head
2
a strobe signal (HD-STB-N), causing the LED head
2
to illuminate the negatively-charged photosensitive drum according to the stored print data (HD-DATA), thereby forming an electrostatic latent image made up of dots with higher potentials than their surrounding areas. In the developer (not shown), negatively charged toner particles are electrically attracted to these dots, so that a toner image is formed.
FIG. 2
illustrates these operations for three consecutive lines (lines N−1, N, N+1).
FIG. 3
shows the above sequence of signals in more detail, assuming that there are four thousand nine hundred ninety-two (4992) dots per line, which is suitable for printing six hundred dots per inch (600 dpi) on A4-size paper. To speed up the printing process, the strobing of one line (e.g., line N−1) may proceed in parallel with the transfer of data for the next line (e.g., line N), and the fastest possible clock rate may also be used.
Rotation of the photosensitive drum brings the toner image to a transfer unit
171
. A high-voltage transfer power source
172
turned on by control signal SG
4
supplies a positive voltage to the transfer unit
171
, whereby the toner image is transferred onto paper as the paper passes between the photosensitive drum and the transfer unit.
The paper bearing the transferred toner image is transported to the fuser
161
. When the paper meets the fuser
161
, the toner image is fused onto the paper by a combination of pressure and heat generated by the heater
161
a.
Finally, the printed sheet of paper passes an exit sensor
173
and is ejected from the printer.
The printing control unit
1
controls the high-voltage transfer power source
172
according to the information detected by the size sensor
167
and pick-up sensor
170
so that voltage is applied to the transfer unit
171
only while paper is passing between the transfer unit
171
and photosensitive drum. When the paper passes the exit sensor
173
, the printing control unit
1
turns off the high-voltage charging power source
164
and halts the developer-transfer process motor
163
.
When a series of pages are printed, the above operations are repeated.
FIG. 4
is a simplified schematic drawing showing the conventional circuit structure of the LED head
2
. The print data signal HD-DATA and clock signal HD-CLK are received by a shift register comprising, for example, four thousand nine hundred ninety-two flip-flops FF
1
, FF
2
, . . . , FF
4992
. The load signal HD-LOAD is received by a corresponding number of latches LT
1
, LT
2
, . . . , LT
4992
, which latch the data output by the shift-register flip-flops when the HD-LOAD signal is active (high). The strobe signal HD-STB-N is supplied to a circuit comprising an inverter G
0
, NAND gates G
1
, G
2
, . . . , G
4992
, and switching elements (transistors) TR
1
, TR
2
, . . . , TR
4992
which are interconnected to drive a linear array of light-emitting diodes LD
1
, LD
2
, . . . , LD
4992
when the strobe signal HD-STB-N is active (low), provided the print data output from the corresponding latches are high (indicating black dots). The transistors TR
1
, TR
2
, . . . , TR
4992
operate as an array of driving elements, while the LEDs LD
1
, LD
2
, . . . , LD
4992
operate as an array of driven elements. The power source of the current that drives the LEDs is denoted VDD.
The output signals from the latches are also supplied to a plurality of memory circuits M
1
, M
2
, . . . , M
4992
that store compensation data for the LEDs. Several bits of compensation data (b
0
-b
5
) are output for each LED. These bits control further transistors that supply additional current to the LEDs LD
1
, LD
2
, . . . , LD
4992
to compensate for LED-to-LED variations in the electrical characteristics of the LEDs themselves and their main driving transistors TR
1
, TR
2
, . . . , TR
4992
.
Incidentally, in the flip-flops FF
1
, FF
2
, . . . in
FIG. 4
, D
1
, D
2
, . . . are data input terminals and Q
1
, Q
2
, . . . are data output terminals. In the latches LT
1
, LT
2
, . . . , D is a data input terminal, Q is a data output terminal, and G is a control signal input terminal.
The memory compensation data circuits M
1
, M
2
, . . . , M
4992
are necessary for the following reason. In a printer employing the LED head in
FIG. 4
, all of the LEDs LD
1
, LD
2
, . . . , LD
4992
are switched on for the same length of time, which is determined by the strobe signal HD-STB-N. Thus if these LEDs, or their driving transistors TR
1
Oki Data Corporation
Pham Hai
Rabin & Berdo P.C.
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