Abstract:
A light beam scanner switches between a first mode in which a laser is turned ON to emit a light beam every surface period of a rotary polygon to detect the light beam, and a second mode in which the laser is turned ON to emit the light beam with a long period specified with an integral multiple of two or more of the surface period of the rotary polygon to detect the light beam, for formation of an image and non-formation of an image, thereby improving a life of the laser.

Description:
[0001]     This application claims priority from Japanese Patent Application Nos. 2003-196347 filed on Jul. 14, 2003 and 2004-189911 filed on Jun. 28, 2004, which are hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a technique for improving lighting-up control for a laser.  
         [0004]     2. Related Background Art  
         [0005]     In a laser beam printer apparatus or the like for recording an image by an electrophotographic process, it is necessary to scan a photosensitive member with a laser beam to form an image. In particular, the stable rotating speed control for a rotary polygon mirror for deflecting a laser beam is important.  
         [0006]     In general, in order to perform constant speed rotation control for a rotary polygon mirror driving motor, there is a method (PLL control method) for controlling a voltage applied to an IC for driving a rotary polygon mirror driving motor so that a phase difference between an oscillation frequency to become a reference, and an actually measured rotational frequency of the rotary polygon mirror driving motor falls within a predetermined range.  
         [0007]     The technique for detecting a rotational frequency (rotating speed) of a rotary polygon mirror driving motor is mainly classified into two techniques. One of them is a technique that utilizes a driving magnet (hereinafter, referred to as “a magnet” for short) magnetized in a multi-pole style and provided in a rotary member (rotary polygon mirror driving motor or rotary polygon mirror), and hall elements or sinking comb-shaped electric wires (FG sensors) provided on a printing wiring board. With this technique, since the rotation of the rotary member causes the magnet to rotate, a voltage pulse synchronized with the rotation of the rotary polygon mirror driving motor is obtained when magnetic poles of the magnet cross a pattern (FG pattern) of the hall elements or the sinking comb-shaped electric wires.  
         [0008]     With this technique, however, it is difficult to carry out the constant speed control for the rotary polygon mirror driving motor with high accuracy due to a limit to the accuracy of division of the magnet, the accuracy of positions between the hall elements, and the accuracy of division of the FG pattern; lack of uniformity of the magnetization of the magnet (the dispersion in the magnetic forces between the magnetic poles, the condition of division between the magnetic poles, and the like); the fluctuation of the magnetic force of the magnet resulting from a change in temperature; and the like.  
         [0009]     On the other hand, the other technique utilizes a sensor (BD sensor) provided inside an optical scanner for detecting a scanning timing for a light beam in order to control the writing position of an image. Since this BD sensor is not influenced by the fluctuation in magnetic force due to a magnet, the constant speed rotation control for the rotary polygon mirror driving motor can be carried out with high accuracy.  
         [0010]     Such an apparatus for stably controlling a rotating speed is disclosed in Japanese Patent Application Laid-Open No. H09-183251.  
         [0011]     However, in order to make a light beam incident to the BD sensor, it is necessary to forcibly light up a laser for emitting the light beam. Now, as for a timing at which the laser is forcibly lighted up in order to make the light beam incident to the BD sensor, the continuous light emission has to be started after a light beam used in scanning passes through an effective image area and before the light beam reaches a light receiving portion of the BD sensor. However, there is a problem that light reflected by a part of a writing optical system, refracted at a corner of an optical part, or reflected at a corner of a polygon mirror during forcible lighting-up, i.e., so-called flare light, reaches a photosensitive drum to write an unnecessary image. In addition, it is not preferable that a photosensitive member for image formation is needlessly exposed with a laser beam, and it is also not preferable that a laser is needlessly lighted up to shorten a laser lighting-up life. From a viewpoint of preventing these situations, it is desirable that a time period required to forcibly light up a laser to emit a light beam is as short as possible, and it is also desirable that a timing of starting to forcibly light up the laser to emit the light beam is as late as possible. Accordingly, various means for lighting up a laser in consideration of a timing of passing through a detection portion have been taken.  
         [0012]     However, the laser is forcibly lighted up to emit the light beam every period during rotation of the polygon mirror, and this becomes a large factor for shortening a laser lighting-up life because a time zone for forcible lighting-up, if accumulated, always occupies about 5% of a time period during the rotation control as compared with the case of data of an image area to be drawn.  
         [0013]     Thus, if the rotation of the polygon mirror is stopped during stand-by, then the lighting-up life can be saved to some degree. However, a reactivation time of the polygon mirror exerts an influence on delay of the next first print out time to delay the first print out time. In addition, if the beam detection timing is lost once, then it becomes necessary to expose an image area of a photosensitive member in order to find out the detection timing again. In particular, it is not preferable that such exposure is always carried out right before the start of the image formation. For these reasons, the means for stopping the rotation of the polygon mirror may not always be taken in some cases.  
       SUMMARY OF THE INVENTION  
       [0014]     It is an object of the present invention to provide a light beam scanner which is capable of carrying out rotation control so as to avoid lighting-up of a laser every surface period of a rotary polygon to detect a light beam, thereby improving a life of the laser.  
         [0015]     In order to attain the object described above, according to one aspect of the invention, a light beam scanner includes: a light beam generating portion; a beam detecting portion for detecting the light beam; a rotary polygon for deflecting and scanning the light beam; and a rotation controlling portion for controlling rotation of the rotary polygon on the basis of detection of the light beam by the beam detecting portion. The light beam scanner has: a first mode in which the light beam generating portion is turned ON to generate the light beam every surface period of the rotary polygon to detect the light beam; and a second mode in which the light beam generating portion is turned ON to generate the light beam with a long period specified with an integral multiple of 2 or more of the surface period of the rotary polygon to detect the light beam. In the light beam scanner, the rotation controlling portion, during non-formation of an image, controls the rotation of the rotary polygon so as to avoid the first mode.  
         [0016]     Further, in order to attain the object described above, according to another aspect of the invention, a light beam scanner includes: a light beam generating portion; a beam detecting portion for detecting the light beam; a rotary polygon for deflecting and scanning the light beam; and a rotation controlling portion for controlling rotation of the rotary polygon on the basis of detection of the light beam by the beam detecting portion. The light beam scanner has: a first mode in which the light beam generating portion is turned ON to generate the light beam every surface period of the rotary polygon to detect the light beam; and a second mode in which the light beam generating portion is turned ON to generate the light beam with a long period specified with an integral multiple of 2 or more of the surface period of the rotary polygon to detect the light beam. In the light beam scanner, the rotation controlling portion, during formation of an image, controls the rotation of the rotary polygon in the first mode, and during non-formation of an image, controls the rotation of the rotary polygon in the second mode.  
         [0017]     The above and other objects, features, and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  is a schematic perspective view of an image forming portion in an image forming apparatus according to an embodiment of the present invention;  
         [0019]      FIG. 2  is a block diagram explaining a control circuit of a scanner motor;  
         [0020]      FIG. 3  is a diagram showing states of output signals of a rotating speed controlling unit;  
         [0021]      FIG. 4  is a timing chart when the rotating speed controlling circuit is in operation;  
         [0022]      FIG. 5  is a schematic circuit diagram of an integration/driving circuit of the rotating speed controlling circuit; and  
         [0023]      FIG. 6  is a timing chart explaining an operation of a laser control portion. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]     The present invention will now be described in detail below with reference to the accompanying drawings showing a preferred embodiment thereof. In the drawings, elements and parts, which are identical throughout the views, are designated by identical reference numeral, and duplicate description thereof is omitted.  
         [0025]      FIG. 1  is a schematic perspective view explaining a construction of a main portion in an image forming apparatus (laser beam printer) according to an embodiment of the present invention.  
         [0026]     An image signal (VDO signal)  101  is inputted to a laser unit  102  to be subjected to ON/OFF-modulation to thereby output a laser beam  103 . A scanner motor  104  constantly rotates a rotary polygon mirror  105 . A laser beam  107  deflected by the rotary polygon mirror  105  is focused on a photosensitive drum  108  as a surface to be scanned by an imaging lens  106 . Thus, the surface of the photosensitive drum  108  is horizontally scanned with the laser beam  107  modulated with the image signal  101  (the scanning in a main scanning direction). In addition, the laser beam with which the surface of the photosensitive drum  108  has been horizontally scanned is applied to a photoelectric conversion element  109 , which generates in turn a horizontal synchronous signal (hereinafter, referred to as “BD signal” for short). The BD signal is transmitted through a group line  110  to be guided to a control circuit  111 . Here, a signal, which is obtained by frequency-dividing the BD signal by the number of surfaces of the polygon mirror  105 , corresponds to a motor synchronous signal for the scanner motor  104 . A latent image, which is formed on the photosensitive drum  108  with the modulated laser beam  107 , is visualized in the form of a toner image by a developing device (not shown), and the toner image is then transferred onto a recording paper  112 .  
         [0027]      FIG. 2  is a block diagram explaining a control circuit for the scanner motor.  
         [0028]     A power supply voltage (e.g., +24 V: not shown), a GND voltage (not shown), a /ACC signal  123  used to apply acceleration, and a /DEC signal  124  used to apply deceleration are inputted from a rotating speed controlling circuit  234  to the scanner motor  104 . Here, a symbol “/” represents negative logic. Three commands, i.e., an acceleration command, a deceleration command and a speed holding command are transmitted in the form of the /ACC signal  123  and the /DEC signal  124 . After a /BD signal  110  is shaped into a rectangular wave having a fixed length in a wave-shaping portion  231 , the resultant signal is frequency-divided by the number of surfaces of the polygon mirror  105  in a 1/(number of polygon surfaces) frequency division circuit  232  to obtain a BDN signal  113 . Then, the rotating speed controlling circuit  234  detects a rotating speed using the BDN signal  113  to automatically operate a next acceleration/deceleration command. Here, the rotating speed controlling circuit  234  frequency-divides the /BD signal  110  by the number of surfaces for one revolution of the polygon mirror  105  in the frequency division unit  232  because the BDN signal  113  is prevented from being influenced by the dispersion in surfaces of the polygon mirror. The control is carried out by referring constantly to the /BD signal  110  from a certain surface of the polygon mirror using the BDN signal  113 .  
         [0029]     A UBL signal  251  is a forcible lighting-up signal, for the laser  102 , which is used to detect the /BD signal  110 . After a lapse of a predetermined time period with the /BD signal  110  as a reference, a laser lighting-up controlling circuit  250  outputs the UBL signal  251 , and after detecting the /BD signal  110 , subsequently lights out the laser  102 .  
         [0030]      FIG. 3  is a diagram showing states of speed control commands issued from the rotating speed controlling circuit  234 , and  FIG. 4  is a timing chart when the rotating speed controlling circuit  234  is in operation.  
         [0031]     In a state until a rotating speed of the scanner motor  104  is increased up to a specified rotating speed, i.e., in an acceleration state, control commands, as in a time area of the acceleration state shown in  FIG. 4 , have two control commands, i.e., an acceleration command (/ACC=L (low level), and /DEC=H (high level)), and a rotation hold command (/ACC=/DEC=H). These command signals are generated from a target period count value and a comparison circuit. Then, the command signal is transmitted as the acceleration command to the scanner motor  104  in correspondence to only a time period by which a period of the BD signal  110  is longer than that in forming an image. That is to say, the acceleration command is more frequently issued as the rotating speed is lower, and the acceleration command is less frequently issued as the rotating speed approaches the specified rotating speed.  
         [0032]     On the other hand, when the rotating speed of the scanner motor  104  is higher than that in forming an image, the control commands, as shown in a time area in the deceleration state of  FIG. 4 , have two control commands consisting of the deceleration command (/ACC=H, and /DEC=L) and the rotation hold command (/ACC=/DEC=H). A rate of the deceleration command occupied in the speed hold state is larger as the rotating speed is higher, and finally the state (rotating speed) changes into the speed hold state (the rotating speed in formation of an image).  
         [0033]     In this speed hold state, the rotation is controlled in accordance with the rotation hold command (/ACC=/DEC=H). In actuality, in order to compensate for the minute acceleration/deceleration due to the noises, the driver dispersion or the like, the acceleration/deceleration command having about 1 to about several pulses is sparsely outputted to carry out the control so that the rotating speed is hardly changed. The foregoing is the description of the control circuit for generating the scanner motor control signals.  
         [0034]      FIG. 5  is a schematic circuit diagram showing a configuration of an integration circuit of a scanner motor driver IC provided inside the scanner motor  104 .  
         [0035]     The integration circuit includes constant current circuits  140  and  141 , switching elements  142  and  143 , a capacitor  145 , and an amplifier  144 . The constant current circuits  140  and  141 , and the switching elements  142  and  143  constitute a charge and discharge circuit for the capacitor  145 . Upon reception of an /ACC signal  123  at L in the switching element  142 , the switching element  142  is turned ON to charge the capacitor  145  with a current from the constant current circuit  140 . In addition, upon reception of a /DEC signal  124  at L in the switching element  143 , the switching element  143  is turned ON to discharge a current set by the constant current circuit  141  from the capacitor  145 . Consequently, a voltage developed across the capacitor  145  is increased or decreased in proportion to an ON-time of the /ACC signal  123  or the /DEC signal  124 . This voltage is applied to a driving portion (not shown) through the amplifier  144  in the after stage. The driving portion supplies a current proportional to this voltage value to the scanner motor to rotate the scanner motor. When the rotating speed of the scanner motor is lower than the specified rotating speed, the voltage developed across the capacitor  145  is increased to accelerate the scanner motor. Conversely, when the rotating speed of the scanner motor is higher than the specified rotating speed, the voltage developed across the capacitor  145  is decreased to decelerate the scanner motor. Finally, the rotating speed of the scanner motor is stabilized at the target rotating speed.  
         [0036]      FIG. 6  shows a relationship between the lighting-up control for the laser and the rotation period detection signal /BD  110 .  
         [0037]     In  FIG. 6 , a UBL signal  251  represents a forcible lighting-up command for detection of the /BD signal  110 . After a lapse of a predetermined time period of t0 with the /BD signal  110  detected last time as a reference, the forcible lighting-up command is issued to forcibly light up the laser, and after detection of the /BD signal  110 , the laser is subsequently lighted out. The laser lighting-up controlling circuit  250  operates so as to switch an output state of the UBL signal shown in  FIG. 6  over to another output state in accordance with an image forming portion state  242 .  
         [0038]     In  FIG. 6 , STATE A is a state during stop of the rotation.  
         [0039]     STATE D is a state right before an image is drawn.  
         [0040]     STATE E is a state when an image is being drawn.  
         [0041]     Operations in the STATE D and STATE E will hereinafter be described.  
         [0042]     The 1/(number of polygon surfaces) frequency division circuit  232  is controlled so as to carry out the frequency division, and a 1/(hold rotating speed) frequency division circuit  244  is controlled so as not to carry out the frequency division. The laser lighting-up controlling circuit  250  operates with an output  240  of the wave-shaping portion  231  as a reference, and the rotating speed controlling circuit  234  operates with the BDN signal  113  as a detection signal. A time period of (t0+t1) is set as a target period in the rotating speed controlling circuit  234 . The forcible lighting-up command based on the UBL signal is issued after a lapse of a time period of t0 timed from the last detection of the BD signal. t1 is a lighting-up margin time period, and thus is a time period, which is provided for detection of the BD signal in advance in anticipation of the fluctuation in the BD period due to the rotation fluctuation of the polygon mirror and the dispersion in mirrors. The adjustment for stabilization of the amount of light of the laser of this apparatus is carried out while the UBL signal is in a valid state. Hence, that adjustment is carried out right before an image is drawn, whereby the rotating speed state and the laser light beam amount stabilization state equal to those when an image is being drawn are obtained.  
         [0043]     An operation in STATE C will hereinafter be described.  
         [0044]     The STATE C is a state when the rotation is stable and right before the stabilization thereof. At this time, the 1/(number of polygon surfaces) frequency division circuit  232  is controlled so as not to carry out the frequency division, and the 1/(hold rotating speed) frequency division circuit  244  is also controlled so as not to carry out the frequency division. The laser lighting-up controlling circuit  250  and the rotating speed controlling circuit  234  operate with the BDN signal  113  as a detection signal. A time period of (t0C+t1) is set as a target period in the rotating speed controlling circuit  234 . The forcible lighting-up command based on the UBL signal is issued after a lapse of a time period of t0C timed from the last detection of the BD signal. t1 is a lighting-up margin time period, and thus is a time period, which is provided for detection of the BD signal in advance in anticipation of the fluctuation in the BD period due to the rotation fluctuation of the polygon mirror and the dispersion in mirrors. A timing at which the BD signal is detected corresponds to a surface, which is used as a reference by the rotation controlling portion.  
         [0045]     An operation in STATE B will hereinafter be described.  
         [0046]     The STATE B is a state when stand-by rotation is held. At this time the 1/(number of polygon surfaces) frequency division circuit  232  is controlled so as not to carry out the frequency division, and the 1/(hold rotating speed) frequency division circuit  244  is controlled so as to carry out the frequency division with a predetermined value. The laser lighting-up controlling circuit  250  and the rotating speed controlling circuit  234  operate with an output signal  245  of the 1/(hold rotating speed) frequency division circuit  244  as a detection signal. A time period of (t0B+t1) is set as a target period in the rotating speed controlling circuit  234 . The forcible lighting-up command based on the UBL signal  251  is issued after a lapse of a time period of t0B timed from the last detection of the BD signal. t1 is a lighting-up margin time period, and thus is a time period, which is provided for detection of the BD signal in advance in anticipation of the fluctuation in the BD period due to the rotation fluctuation of the polygon mirror and the dispersion in mirrors. A timing at which the BD signal is detected corresponds to a surface, which is used as a reference by the rotation controlling portion.  
         [0047]     A frequency division value obtained from the 1/(hold rotating speed) frequency division circuit  244  is determined depending on the rotational stability in the open control for the polygon mirror. The frequency division circuit  244  includes an integration circuit corresponding to the integration circuit of  FIG. 5  as described above. Also, even when the rotation hold control is continued to be held, these systems can hold the rotating speed to some degree, including the inertia of the polygon mirror and the motor. A long period as the lowest limit necessary for detection of the rotation state is set on the basis of the above, whereby the reduction of the number of times of the forcible lighting-up of the laser specific to the present invention is attained as much as possible. That reduction is attained on condition that when the stand-by rotation is held, the rotation accuracy of the polygon motor is unnecessary, and for the purpose of shortening a first print out time, the rotation accuracy in stand-by can be recovered up to the rotation accuracy in drawing of an image as quickly as possible.  
         [0048]     While in the above explanation, there has been described the specific example in which the control for the rotary polygon is carried out on the one revolution-basis, the present invention is not intended to be limited thereto. For example, the present invention can also be applied to other various cases where rotations corresponding to integral multiples of the number of surfaces of a polygon mirror are bases for the control. As a result, there is an effect that a time period required for the laser light emission when no image is drawn can be remarkably shortened by the means taken in the present invention. In addition, the control for the operation from stop of the rotary polygon to activation thereof has not been described, because it is not related to the substance of the present invention. Therefore, its details are omitted here. For example, the invention relating to activation, which has conventionally been proposed, may be included in the constitution of the present invention.  
         [0049]     Heretofore, the laser is needlessly forcibly lighted up mainly in non-formation of an image such as in stand-by to shorten the life of the laser. However, as described above, according to this embodiment, it is possible to provide the laser printer in which the holding of stand-by rotation maintaining the first print out time is compatible with the lengthening of the life of the laser by effectively utilizing the control in the STATE B and STATE C.  
         [0050]     The present invention is not limited to the apparatus of the above-mentioned embodiment, and hence may also be applied to a system constituted by a plurality of apparatuses and instruments, or an apparatus including one instrument. It is to be understood that the present invention is implemented even when a medium such as a storage medium, which stores therein a program code of a software for realizing the function of the above-mentioned embodiment is supplied to a system or an apparatus, and a computer (or a CPU or an MPU) of the system or the apparatus reads out the program code stored in the medium such as a storage medium to execute the program code.  
         [0051]     In this case, the program code itself read out from the medium such as a storage medium realizes the function of the above-mentioned embodiment, and hence the medium such as a storage medium, which stores therein the program code constitutes the present invention.  
         [0052]     As for the medium such as a storage medium, for supplying the program code, for example, there may be used a floppy (registered trademark) disc, a hard disc, an optical disc, a magneto-optical disc, a CD-ROM, a CD-R, a CD-RW, a DVD-ROM, a DVD-RAM, a DVD-RW, a DVD+RW, a magnetic tape, a nonvolatile memory card, a ROM, a download process made through a network, or the like.  
         [0053]     In addition, it is to be understood that the present invention includes not only a case where the program code read out by the computer is executed to realize the function of the above-mentioned embodiment, but also a case where an OS or the like running on a computer executes a part of or all of an actual control processing in accordance with an instruction of the program code, and the function of the above-mentioned embodiment is realized through such a processing.  
         [0054]     Moreover, it is to be understood that the present invention includes a case where after a program code read out from a medium such as a storage medium is written to a memory provided in a function expanding board inserted into a computer or a function expanding unit connected to a computer, a CPU or the like provided in the function expanded board or the function expanded unit executes a part of or all of an actual control processing in accordance with an instruction of the program code, and the function of the above-mentioned embodiment is realized through such a processing.