Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
1998-12-23
2001-07-24
Le, N. (Department: 2861)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
C347S246000, C250S205000
Reexamination Certificate
active
06266078
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image forming apparatus for forming image information on an image bearing member using laser beams from a plurality of laser optical sources. More particularly, the invention relates to an image forming apparatus, such as a digital copier, a laser-beam printer, a facsimile apparatus or the like, in which laser beams subjected to optical modulation from a plurality of laser light sources are guided onto an image bearing member, such as a photosensitive member, an electrostatic recording medium or the like, and image information comprising, for example, an electrostatic latent image is formed on the surface of the image bearing member.
2. Description of the Related Art
Conventionally, in a laser driving circuit of image forming apparatuses of this type, in order to protect a laser from being destructed, the output of the laser beam is monitored, and the driving current for the laser is limited or interrupted when the output exceeds a specified value.
A specific control method in the above-described approach will now be described with reference to FIG.
1
.
FIG. 1
is a block diagram illustrating the configuration of a conventional 1-beam laser driving circuit. In
FIG. 1
, there are shown a laser chip
510
, a laser
511
, a PD (photodiode) sensor
513
, a pulse current source
521
, a bias current source
523
, a switch
531
for a pulse, a switch
533
for the laser, a PWM (pulse-width modulation) circuit
535
, a current-to-voltage converter
541
, an amplifier
542
, a sequence controller
543
, a comparator
545
and an APC (automatic power control) circuit
550
.
Conventionally, in an image forming apparatus of this type, as shown in
FIG. 1
, the laser chip
510
comprising the single laser
511
and the PD sensor
513
is used. By using the two current sources, i.e., the bias current source
523
and the pulse current source
521
, for the laser
511
, the emission characteristics of the laser
511
are improved. In order to stabilize the emission of the laser
511
, the amount of a bias current is automatically controlled by effecting feed-back to the bias current source
523
using an output signal from the PD sensor
513
. That is, the output signal from the PD sensor
513
is input to the current-to-voltage converter
541
, and is then amplified by the amplifier
542
. The output of the amplifier
542
is input to the APC circuit
550
, and is then supplied to the bias current source
523
as a control signal. This circuitry is called APC circuitry, which is now generally used as circuitry for driving a laser. The laser has a temperature characteristic such that as the temperature is higher, the amount of current for obtaining a constant amount of light is larger. In addition, since the laser performs self-heating, a constant amount of light cannot be obtained only by supplying a constant current. This phenomenon greatly influences image formation. In order to solve such a problem, the above-described APC circuirtry has been devised. By adopting this circuitry, it is possible to obtain a constant amount of light without being influenced by the ambient temperature of the laser, and to inexpensively realize appropriate image formation. The output of the PD sensor
513
is used not only for stabilizing the output of the laser, but also for protecting the laser from being destructed. The laser is very easily destructed. If the laser is used in a condition slightly exceeding a rating, it will be degraded or destructed in a short time. In a circuit for protecting the laser, a limiting value LIM which is slightly larger than a reference value for an optical output usually used in image formation is set. As shown in
FIG. 1
, the output of the PD sensor
513
is compared with the limiting value LIM using the comparator
545
. When the output of the PD sensor
513
exceeds the limiting value LIM, a warning signal indicating abnormal emission is output. While the warning signal is output, current supply to the laser is limited or interrupted using the switch
533
for the laser comprising a transistor switch or the like.
In digital image forming apparatuses, there is a strong demand to increase an image forming speed. In order to reply to such a demand, image forming apparatuses in which a laser driving circuit using a multiple lasers is provided have been proposed. The above-described protection circuit is also indespensable in such an apparatus, and it is requested to protect degradation or destruction of a plurality of lasers. In this case, a configuration as shown in
FIG. 2
which is similar to the configuration shown in
FIG. 1
is used.
FIG. 2
is a block diagram illustrating the configuration of a conventional twin-beam laser driving circuit. In
FIG. 2
, there are shown a multilaser chip
610
, an A laser
611
, a B laser
612
, a PD sensor
613
, an A pulse current source
621
, a B pulse current source
622
, an A bias current source
623
, a B bias current source
625
, a switch
631
for an A pulse, a switch
632
for a B pulse, a switch
633
for the A laser, a switch
634
for the B laser, a PWM circuit
635
for the A laser, a PWM circuit
636
for the B laser, a current-to-voltage converter
641
, an amplifier
642
, a sequence controller
643
, a comparator
645
, and a time-division APC circuit
650
.
In
FIG. 2
, the switch
631
for the A pulse and the switch
632
for the B pulse switch on/off the A pulse current source
621
and the B pulse current source
622
in accordance with an output signal from the PWM circuit
635
for the A laser and an output signal from the output from the PWM circuit
636
for the B laser, respectively; The amplifier
642
amplifies the output of the PD sensor
613
converted into a voltage by the current-to-voltage converter
641
. The comparator
645
compares the output of the PD sensor
613
converted into the voltage and amplified by the amplifier
642
with the limiting value LIM for the optical output, and outputs a warning signal indicating abnormal emission when the output of the PD sensor
613
exceeds the limiting value LIM. While the warning signal is output, current supply to the A laser
611
and the B laser
612
is limited or interrupted using the switch
633
for the A laser and the switch
634
for the B laser, each comprising a transistor switch or the like, respectively.
However, in a multilaser configuration in which a plurality of lasers are provided in a chip, a single PD sensor is provided commonly for the plurality of lasers because of restriction in the configuration of the chip. Hence, in the above-described conventional protection circuit, only the sum of optical detection signals for the plurality of lasers is known when, for example, both the A laser and the B laser are simultaneously lit. As a result, even if the optical output of one laser exceeds the limiting value, current supply for that laser is not limited or interrupted unless the sum of the optical outputs of the lasers exceeds the sum of the limiting values. Furthermore, it is impossible to specify the laser which is degraded or destructed.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an image forming apparatus which includes a driving circuit for a multilaser chip having a plurality of laser light sources and which can limit or interrupt the emission of the laser light sources based on a result of comparison of the detected optical output for each the laser light source with a corresponding limiting value during time-division control.
According to one aspect, the present invention relates to an image forming apparatus including a plurality of laser light sources, light-amount control means for controlling an optical output of a laser beam from each of the plurality of laser light sources to a constant amount of light by performing time division processing, and light detection means for detecting optical outputs of the plurality of laser light sources. The light detection means includes sensor units whose number is
Kataoka Tatsuhito
Kawakami Takayuki
Koga Katsuhide
Nakayama Kazumi
Nakayama Tomobumi
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Le N.
Nakayama Kazumi
Nguyen Lamson D.
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