Abstract:
A laser scanning apparatus, can suppress the unnecessary emission of a laser beam for a photosensitive drum, can prevent the degradation of the photosensitive drum, and can prolong the life time of a laser, using simple configuration. An image forming apparatus includes this laser scanning apparatus, and a method for starting the laser scanning apparatus. A laser scanning apparatus performing exposure with a laser beam predicts a time elapsing until completion of start on the basis of initial start conditions, shuts off the laser beam during the time predicted, emits a laser beam after the time predicted elapses, and detects completion of start.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a laser scanning apparatus suitable for an image forming apparatus such as a printer and a copying machine with an electrophotographic system respectively, and in particular, to the emission control of a laser beam at the time of starting the laser scanning apparatus. 
     2. Related Background Art 
     Conventionally, in a laser beam printer which drives a rotary polygon mirror on the basis of a horizontal synchronizing signal (hereinafter, this is called a BD signal), for example, a method of performing configuration and control as disclosed in Japanese Patent Application Laid-Open No. 08-183198 is known. 
     That is, the rotation control of the rotary polygon mirror is performed by measuring a cycle of a horizontal synchronizing signal between adjacent BD signals, and determining the emission timing of a laser beam for acquiring a next BD signal every horizontal synchronization from the BD cycle measured. 
     Another method is also known, the method where this timing control is performed during start to the steady rotation of a rotary polygon mirror as well as the steady rotation so as to prevent the degradation of a photosensitive drum by not irradiating a photosensitive drum surface with a laser beam as much as possible, and to prolong the life time of a laser by shortening its lighting time. 
     However, it is necessary to change the lighting timing of the laser in real time in the above-described conventional example, even when a BD cycle greatly differ from the steady rotation not only under steady rotation but also under start. 
     Thus, this means that it is necessary to compute the optimal timing in real time every input of the BD signal so as not to emit the laser beam on a photosensitive drum surface. Hence, there is a problem that constitution and control become complicated. 
     SUMMARY OF THE INVENTION 
     The present invention is made under such circumstances. An object of the present invention is to provide a laser scanning apparatus that can prevent the degradation of a photosensitive drum and can prolong the life time of a laser by suppressing the unnecessary emission of a laser beam to the photosensitive drum with the simple configuration for timing control, an image forming apparatus using this laser scanning apparatus, and a method of starting the laser scanning apparatus. 
     The laser scanning apparatus of the present invention is characterized by comprising a rotary polygon mirror for scanning a laser beam, synchronizing signal generating means for detecting the laser beam scanned by the rotary polygon mirror and generating a synchronizing signal, measuring means for measuring a cycle of the synchronizing signal generated by the synchronizing signal generating means, laser emission control means for making a laser beam emitted in order to acquire a synchronizing signal for next scan, start speed measuring means for measuring increasing speed of rotation of the rotary polygon mirror from the cycle of the synchronizing signal measured by the measuring means in an early stage of starting rotation of the rotary polygon mirror, start time prediction means for predicting start time required for the rotary polygon mirror to attain steady rotation on the basis of information from the start speed measuring means, and control means for making a laser beam not emitted until the time predicted by the start time prediction means elapses from measurement completion by the start speed measuring means, making the laser beam emitted after the predicted time elapses, and operably controlling the laser emission control means when the cycle of the synchronizing signal measured by the measuring means reaches a predetermined cycle. 
     Other objects, configuration, and effects of the present invention will become apparent from the following detailed description taken in connection with the accompany drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a configuration of a first embodiment; 
     FIG. 2 is a timing chart of signals in a horizontal scanning direction; 
     FIG. 3 is a block diagram showing a configuration of an unblanking signal generating circuit etc.; 
     FIG. 4 is a flow chart that shows the start control timing of a scanner motor; 
     FIG. 5 is a timing chart showing the change of the rotation number of a scanner motor and the change of an unblanking signal etc.; 
     FIG. 6 is a flow chart showing the start control of a scanner motor in a second embodiment; and 
     FIG. 7 is a timing chart showing the change of the rotation number of a scanner motor and the change of an unblanking signal etc. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described with embodiments of a laser beam printer in detail. In addition, the present invention can be applied to not only the form of an apparatus but also the form of a method of starting a laser scanning apparatus by being backed by the explanation of the embodiments. 
     (Embodiment 1) 
     FIG. 1 shows the configuration of a “laser beam printer” of a first embodiment. The image formative operation of the laser beam printer will be described on the basis of FIG.  1 . An image signal (VDO signal)  101  is inputted into a laser unit  102 . The above-described laser unit  102  generates a laser beam  103  ON/OFF-modulated. A scanner motor  104  makes a rotary polygon mirror  105  steadily rotate. An image formation lens  106  makes a laser beam  107 , which is deflected by the rotary polygon mirror  105 , focused on a photosensitive drum  108  that is a scan plane. It is assumed in this embodiment that a rotary polygon mirror with six planes is used. 
     Therefore, the laser beam  107  modulated by the image signal  101  is horizontally scanned (scanning in the direction of the horizontal scanning) on the photosensitive drum  108 . A beam detection hole  109  admits the laser beam  107  from a slit-like incident hole. The laser beam  107  entering from this incident hole  109  is led to a photoelectric transducer  111  through the inside of an optical fiber  110 . The laser beam transformed into an electric signal by the photoelectric transducer  111  serves as a horizontal synchronizing signal BD after being amplified by an amplifier (not shown). A latent image formed on the photosensitive drum  108  becomes a toner image visualized by a development unit (not shown), which is transferred on transfer paper  112  by a transferring unit (not shown). 
     Next, control signals for image formation will be described with using FIG.  2 . 
     An image is formed on transfer paper  121 . A toner image is formed on this transfer paper  121 . Nevertheless, lest the toner image formed should protrude from the transfer paper  121  due to the deviation of the transfer paper  121  etc., an area (image formation area)  122  which can be exposed by the laser beam is provided. Moreover, an image controller (not shown) outputs a picture signal  126 . In many cases, each image controller is a controller, being different from a control unit handling control signals such as a BD signal, or an external computer. In such configuration, also in order to make a photosensitive member not exposed even if the image controller turns on an image signal in a non-image area, the image formation area  122  is provided. Therefore, this image formation area  122  will have the size respectively different in accordance with the size of the transfer paper  121 . 
     Next, an image formation signal at the time of forming an image equivalent to one horizontal scanning  123  on the transfer paper  121  will be described. The BD signal  124  is a synchronizing signal in the direction of the horizontal scanning that is described previously, and other signals are generated with synchronizing with this BD signal. 
     A mask signal  125  is turned on or off according to the mask area  122  on the transfer paper  121 . Owing to this, the image signal  126  having image information is forbidden, and the exposure out of the image formation area  122  is also forbidden. 
     A BD allowance signal  127  permits the input of the BD signal. This signal masks the BD signal  124  in order to make a BD signal not received in a predetermined period from the previous BD signal. This prevents the horizontal synchronization from shifting due to noise. 
     An unblanking signal  128  is a timing signal for making the laser beam  107  forcibly turned on when the laser beam  107  scans the detection hole  109  for the BD signal. Moreover, when the unblanking signal  128  makes the laser beam turned on, sampling and adjustment of the laser quantity is performed for adjusting the laser quantity to a reasonable level. 
     FIG. 3 shows the configuration of an unblanking signal generating circuit and the like. A laser lighting timing control circuit  131  is a timing generation circuit of turning on or off the laser with synchronizing with the BD signal in the predetermined timing from the BD period when the scanner motor  104  is rotating at a predetermined rotation number. For example, supposing that the BD cycle at the time of a predetermined rotation number is 500 μs, the timing generation circuit resets an internal timer when a BD signal is input, turns off the laser at 5 μs after the internal timer is reset, and turns on the laser at 450 μs after the internal timer is reset. 
     This laser lighting timing control circuit  131  performs the control of not emitting the laser beam to the photosensitive drum  108  and turning on the laser in the vicinity of the timing when the BD signal can be acquired. 
     Moreover, the laser lighting timing control circuit  131  can be enabled or disabled with a signal  137  from a CPU 132 . The CPU 132  controls the paper feed and the like in the laser beam printer. This CPU 132  can also output the signal  136  for forcibly turning on the laser. 
     That is, the laser lighting timing control circuit  131  (signal  135 ) and the CPU  132  (signal  136 ) can generate the unblanking signal. A signal generated by synthesizing them by an OR circuit  134  turns into the unblanking signal. 
     A BD cycle measuring circuit  133  measures a cycle from a certain BD signal pulse to the following BD signal pulse, and outputs the result to the CPU  132 . Owing to this, the CPU  132  can detect the BD cycle, i.e., the rotation number of the scanner motor  104  in real time. 
     Moreover, the motor roll control circuit  138  controls the rotation number of the scanner motor  104  on the basis of this BD cycle. If the BD cycle measured in the BD cycle measuring circuit  133  is slower than a predetermined cycle, the motor roll control circuit  138  outputs a signal for accelerating the scanner motor  104 , and if faster than the predetermined cycle, the motor roll control circuit  138  outputs a signal for making the scanner motor  104  slow down. 
     In the laser beam printer configured as described above, as shown in FIGS. 4 and 5, the start control of the unblanking signal is performed. 
     FIG. 4 is a flow chart showing the start control of the scanner motor. FIG. 5 is a timing chart showing the change of the rotation number of the scanner motor  104  and the change of the unblanking signal. 
     The start control (control in the flow chart shown in FIG. 4) of the scanner motor  104  is started at the same time when the rotation control of the scanner motor  104  is started. When started, a 1.0-sec wait is first performed (see step  401 ; in the figure, step  401  is abbreviated to S 401 . This manner is applied to all steps in this specification and drawings. This is a wait for predicting the start time of the rotation number, described later, with more sufficient precision. After the wait, the CPU  132  outputs the signal  136  to forcibly turning on the laser (step  402 , timing T 501 ). Then, the BD cycle measuring circuit  133  measures the BD cycle, and fetches the data into the CPU  132  to assign the data to variable S 1  (step  403 ). After waiting for such a time that the increase of the rotation number of the scanner motor  104  can be measured, that is, 0.5 sec in this embodiment (step  404 ), the BD cycle measuring circuit  133  measures a BD cycle again, and fetches the data into the CPU  132  to assign the data to variable S 2  (step  405 ). Then, by turning off the signal  136 , the unblanking signal is turned off for the laser to be turned off (step  406 , timing T 502 ). 
     A time elapsing until reaching a predetermined rotation number is predicted from the rotation number S 1  of the scanner motor after 1.0 sec from the start of the scanner and the rotation number S 2  after 1.5 sec. In this embodiment, the time is predicted in direct proportion from rotation numbers S 1  and S 2 . For example, suppose that the predetermined rotation number is 20000 rpm, S 1 =2.50 ms, and S 2 =1.67 ms. Since this is the BD cycle of a hexahedron, the rotation number of the scanner motor  104  are 4000 rpm at the timing T 501  and 6000 rpm at the timing T 502 . An increasing part of the rotation number during 0.5 sec in this time interval is 2000 rpm. Supposing the rotation number increases by this rate, the rotation number will become at 20000 rpm after 3.5 sec from timing T 502 . 
     Then, a wait is performed until the predicted start time (step  408 ). For example, the wait is performed for the above-described predicted time, 3.5 sec. When this time elapses, the signal  136  is turned on and the laser is made to forcibly turn on (step  409 , timing T 503 ). Then, the CPU 132  monitors the BD cycle measured in the BD cycle measuring circuit  133  about whether the BD cycle reaches the predetermined cycle (step  410 ). When reaching the predetermined rotation number, the CPU  132  outputs the signal  137  to enable the laser lighting timing control circuit  131  (step  411 ). Simultaneously, the signal  136  is turned off and the forcible lighting of the laser is terminated by the CPU  132  (step  412 , timing T 504 ). Owing to this, it becomes possible to output the unblanking signal only in the vicinity of the timing when the BD signal can be fetched with synchronizing with the BD signal as shown in the FIG.  2 . Then, the start control of the scanner motor  104  is terminated. 
     As described above, it is possible in this embodiment to simplify the configuration for the timing control for acquiring a BD signal. At the same time, it is possible to suppress the emission of a laser beam to a photosensitive drum surface at the time of starting to the minimum, and to suppress the emission of the laser beam beyond the need for the photosensitive drum. Hence, the degradation of the photosensitive drum can be prevented, and the lifetime of a laser can be prolonged. 
     (Embodiment 2) 
     A “laser beam printer” which is a second embodiment will be described with using FIGS. 6 and 7. 
     Since the hardware configuration of the laser beam printer in this embodiment is the same as that of the first embodiment, FIGS. 1 to  3 , and description thereof will be also used in this embodiment for the description to be omitted here. 
     In this embodiment, as shown in FIGS. 6 and 7, the start control of an unblanking signal is performed. 
     FIG. 6 is a flow chart showing the start control of the scanner motor  104 . FIG. 7 is a timing chart showing the change of the rotation number of the scanner motor  104  and the change of the unblanking signal. 
     The start control of the scanner motor  104  (control in the flow chart shown in FIG. 6) is started at the same time when the rotation control of the scanner motor  104  is started. After the start, first, a 1.0-sec wait is performed (step  601 ). After the wait, the CPU  132  outputs the signal  136  to forcibly turning on the laser (step  602 , timing T 701 ). Then, the BD cycle measuring circuit  133  measures a BD cycle, and fetches the data into the CPU  132  to assign the data to variable S 1  (step  603 ). Further, by turning off the signal  136 , the unblanking signal is turned off for the laser to be turned off (step  604 , timing T 702 ). Therefore, the laser is turned on only for the very short time when the BD cycle can be acquired. 
     It is confirmed whether the cycle of variable S 1  reaches a predetermined BD cycle (Step  605 ). If not reaching the predetermined cycle, the steps are repeated from the 1.0-sec wait at step  601 . 
     Thus, until the rotation of the scanner motor  104  reaches the predetermined rotation number, the laser is turned on only for the slight time every 1.0 sec. 
     When the scanner motor  104  reaches the predetermined rotation number, the CPU  132  outputs the signal  137  to enable the output of the signal  135  in the laser lighting timing control circuit  131  (step  606 ). Hence, an unblanking signal  128  is outputted only in the vicinity of the timing when the BD signal can be acquired with synchronizing with the BD signal  124  as shown in the FIG.  2 . Then, the start control of the scanner motor  104  is terminated. 
     As described above, in this embodiment, even if it is difficult to predict the start time of a scanner motor, it is possible to simplify the configuration for timing control for acquiring a BD signal. At the same time, it is possible to suppress the emission of a laser beam to a photosensitive drum surface at the time of starting to the minimum, and to suppress the unnecessary emission of the laser beam for the photosensitive drum. Hence, the degradation of the photosensitive drum can be prevented, and the lifetime of a laser can be prolonged. 
     As described above, according to this embodiment, it is possible with using simpler configuration to suppress the unnecessary emission of a laser beam for a photosensitive drum, to prevent the degradation of the photosensitive drum, and to prolong the lifetime of a laser. 
     While several preferred embodiments have been described above, it is to be understood that changes and variations may be made without departing from the sprit on scope of the following claims.