Patent Publication Number: US-9405212-B2

Title: Image forming apparatus with malfunction detection

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus. 
     2. Description of the Related Art 
     One problem of an image forming apparatus is that the surface potential of a photosensitive drum (photoreceptor) changes due to various factors, which results in a drop in image quality. For example, if the quantity of laser light emitted from a scanner unit drops, the surface potential of the photosensitive drum may become lower than a desired potential, whereby image quality deteriorates. Here a technique to detect the drop in quantity of laser light based on a transfer current amount is known (Japanese Patent Application Laid-Open No. 2012-155075). 
     Various factors may cause a drop in the quantity of the laser light. For example, if the image forming apparatus is continuously used in air in which such micro-foreign matter as dust and chemical substances are floating, the foreign matter enters the apparatus main body and adheres to the laser light source and other optical components inside the scanner unit. The deposition of dust on the surface of such optical components as a reflection mirror and an imaging lens of the scanner unit causes a gradual drop in reflectance and transmittance, whereby the quantity of laser light emitted from the scanner unit drops. On the other hand, if foreign matter adheres to an emission point of a laser element, such as a laser diode, the quantity of laser light drops dramatically. Therefore a technique to accurately determine a factor that causes a drop in the quantity of laser light is demanded. 
     With the foregoing in view, it is an object of the present invention to accurately determine a factor that causes a drop in the quantity of laser light. 
     SUMMARY OF THE INVENTION 
     To achieve the above object, an image forming apparatus according to the present invention has: 
     a light emitting member that includes a first emission portion to which drive current is supplied and from which first laser light is emitted; 
     a photoreceptor to which the first laser light is emitted; 
     a detection portion that detects a value on a surface potential of the photoreceptor; and 
     a determination portion that determines an abnormality of the light emitting member, wherein 
     the detection portion detects, a plurality of times, a value on the surface potential of a portion of the photoreceptor to which the first laser light is emitted, and the determination portion determines whether the light emitting member is in an abnormal state, based on a change amount of the value on the surface potential detected by the detection portion. 
     To achieve the above object, an image forming apparatus according to the present invention has: 
     a first light emitting member that includes a first emission portion emitting first laser light and a second emission portion emitting second laser light, and that emits the first laser light and the second laser light by supply of a common first drive current; 
     a second light emitting member that includes a third emission portion emitting third laser light and a fourth emission portion emitting fourth laser light, and that emits the third laser light and the fourth laser light by supply of a common second drive current; 
     a photoreceptor to which the first laser light and the third laser light are emitted; 
     a light receiving portion that receives the second laser light and the fourth laser light; 
     a light quantity control portion that controls a light quantity of the first laser light, which is emitted to the photoreceptor, based on the light quantity of the second laser light received by the light receiving portion, and controls a light quantity of the third laser light, which is emitted to the photoreceptor, based on the light quantity of the fourth laser light received by the light receiving portion; 
     a detection portion that detects a value on a first surface potential of a portion of the photoreceptor to which the first laser light is emitted, and a value on a second surface potential of a portion of the photoreceptor to which the third laser light is emitted; and 
     a determination portion that determines an abnormality of the light emitting member, wherein 
     the determination portion determines that the light emitting member is in an abnormal state when only one of the value on the first surface potential and the value on the second surface potential, detected by the detection portion, is a first predetermined value or more, and also determines that the light emitting member is in an abnormal state when only one of the value on the first surface potential and the value on the second surface potential, detected by the detection portion, is a second predetermined value or less. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view depicting a general configuration of an image forming apparatus according to this example; 
         FIG. 2  is a schematic diagram depicting a configuration of an image forming processing unit of this example; 
         FIG. 3  is a block diagram depicting a light quantity control portion; 
         FIG. 4  is a perspective view depicting a configuration of a scanner unit; 
         FIGS. 5A to 5C  are diagrams depicting a configuration when a laser light source is packaged in a can package; 
         FIG. 6  is a graph depicting a relationship between an applied voltage and a current value; 
         FIG. 7  is a graph depicting calculation of a drum potential; 
         FIG. 8  is a graph depicting a relationship between a drum potential and the laser light quantity after laser irradiation; 
         FIG. 9  is a flow chart depicting a laser light quantity abnormality determination sequence according to Example 1; 
         FIG. 10  is a diagram depicting a twin beam laser and a PD sensor of Example 2; and 
         FIG. 11  is a flow chart depicting laser light quantity abnormality determination sequence according to Example 2. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Embodiments of the present invention will now be described in detail based on examples with reference to the drawings. The dimensions, materials, shapes and relative dispositions or the like of the components described in the embodiments may need to be appropriately changed depending on the configuration and various conditions of the apparatus to which the present invention is applied. In other words, the scope of the invention is not limited to the following embodiments. 
     With reference to  FIG. 1 , a general configuration of an image forming apparatus according to an example of the present invention (hereafter called “this example”) will be described first.  FIG. 1  is a cross-sectional view depicting a general configuration of the image forming apparatus according to this example. In this example, an electrophotographic laser beam printer will be described as an example of the image forming apparatus. 
     The image forming apparatus  100  includes a paper feeding cassette  101  where recording sheets are set, a pick up roller  102  that picks up paper, a paper feeding roller  103  that feeds and transports paper, a fixing apparatus  104  that fixes a toner image on paper, and a paper ejecting roller  105  that ejects paper. The image forming apparatus  100  also includes an image forming process portion  106  that performs charging, exposure, development, transfer or the like. 
     Paper that is set in the paper feeding cassette  101  is picked up by the pick up roller  102 , and is fed and transported by the paper feeding roller  103 . Then the toner image is transferred to the paper in the image forming process portion  106 , and the toner image is fixed on the paper by the fixing apparatus  104 . Then [the paper] is ejected from the image forming apparatus  100  by the paper ejecting roller  105 . 
     Now, details on the image forming process portion  106  will be described with reference to  FIG. 2 .  FIG. 2  is a schematic diagram depicting the configuration of the image forming process portion of this example. The image forming process portion  106  includes a photosensitive drum  201  as an image bearing member, a charging roller  202 , a developing sleeve  203 , a transfer roller  204 , a charging circuit  205 , a transfer circuit  206 , a scanner unit  207  and a pre-exposure portion  211 . 
     A transfer bias generated by the transfer circuit  206  (voltage applying portion) is applied to the transfer roller  204 . The transfer circuit  206  can change the output bias value and polarity to positive/negative by the control portion  107  that controls the operation sequence of the image forming apparatus. The current detection circuit  210  can detect current A that flows from the transfer circuit  206  to the transfer roller  204 , the photosensitive drum  201 , and a drum earth  209 . 
     In the non-image area, the control portion  107  detects information acquired by the current detection circuit  210  when DC voltage is applied to the transfer roller  204 . The control portion  107  determines the discharge start voltage between the photosensitive drum  201  and the transfer roller  204  based on each of the detected current values, and calculates the surface potential VL on the photosensitive drum  201  (hereafter called “drum potential VL”) using the determination result. The image forming process, including the charging of the photosensitive drum  201 , exposure to light by the scanner unit  207  or the like using the above procedure, is controlled by the control portion  107  that controls the image forming apparatus constituted by a CPU, ASIC or the like. 
     &lt;Scanner Unit&gt; 
     Now the scanner unit of this example will be described with reference to  FIG. 3  and  FIG. 4 .  FIG. 3  is a block diagram depicting a light quantity control portion of the scanner unit that controls the exposure amount of the laser light source.  FIG. 4  is a perspective view depicting the configuration of the scanner unit. The light quantity control portion of the present invention includes a laser driver  303  and the control circuit portion  108  shown in  FIG. 3 . As shown in  FIG. 3 , the laser driver  303  controls the light quantity to be constant while monitoring the light emitting quantity of the laser diode  304  using a PD (photo diode) sensor  305  (light receiving portion). The laser diode  304  is driven by the laser driver  303  in accordance with a video signal  301  from the control circuit portion  108  and a control signal  302  from the control circuit portion  108 , and emits beams (laser light). 
     As shown in  FIG. 4 , the scanner unit  207  includes a laser light source  300  that has a laser diode  304  (see  FIGS. 5A to 5C ) which is a light emitting member, a cylindrical lens  402 , a polygon mirror  403 , an imaging lens  404 , and a reflection mirror  405 . Each optical component is housed in an optical case  401 . The laser light emitted from the laser diode  304  in the laser light source  300  is collected by the cylindrical lens  402  to be a linear beam. The polygon mirror  403  is an example of a rotating polygon mirror, and is rotated in a predetermined direction (arrow S direction) by a scanner motor  406  so as to reflect the laser light during scanning. The scan motor  406  is controlled at a predetermined rotation speed by an acceleration signal/deceleration signal from a speed control portion (not illustrated). 
     The imaging lens  404  is designed to scan the photosensitive drum  201  at a constant speed, and the laser light reflected by the reflection mirror  405  forms a spot on the photosensitive drum  201  and scans in the arrow A direction. By the photosensitive drum  201  rotating in the arrow R direction, an electrostatic latent image is formed on the photosensitive drum  201 . 
     If the image forming apparatus  100  is continuously used in air where micro dust and chemical substances are floating, the dust and chemical substances enter the main body of the image forming apparatus  100 . Although the scanner unit  207  is located inside the image forming apparatus  100 , the micro dust and chemical substances adhere to the optical components or the like inside the scanner unit  207  via an air duct that cools inside the image forming apparatus  100  or the like. If dust is deposited on the surfaces of the reflection mirror  405  and the imaging lens  404 , for example, reflectance and transmittance gradually drop. 
     EXAMPLE 1 
     Example 1 of the present invention will now be described. A package of the laser light source according to Example 1 will be described with reference to  FIGS. 5A to 5C . A can package or a frame package is normally used as a package of the laser light source  300  used for the image forming apparatus  100 .  FIGS. 5A to 5C  are diagrams depicting a configuration when the laser light source is packaged using a can package.  FIG. 5A  is a diagram depicting a can package.  FIG. 5B  is a diagram depicting a laser light source and a PD sensor, where a part of  FIG. 5A  is omitted.  FIG. 5C  is an enlarged view of the laser light source and the PD sensor. 
     In the can package, the laser diode  304  is mounted on a stem (not illustrated), and is sealed by a metal can  502  on which a glass  501  is adhered. Some can packages are open packages which are not sealed and are without glass  501 . In this case, the laser diode  304  is exposed to air. 
     In this example, a laser light quantity abnormality determination, in the case of using the laser light source  300  in a can package without the glass  501 , will be described as an example. The laser diode  304  is created by cleaving end faces on both ends of the resonator, and includes a reflection mirror that transmits a part of the laser light. First laser light that is emitted, passing through the front side reflection mirror of the laser light source  300  (hereafter called “front light”), exposes the photosensitive drum  201  to light. Second laser light that is emitted, passing through the rear side reflection mirror (hereafter called “rear light”), is directed to the PD sensor  305  disposed on the opposite surface. The front light and the rear light are laser light which is emitted by a common drive current supply. 
     In the laser diode  304  used for the laser light source  300 , the laser light is emitted from micro emission points (emission portions). Normally in a laser diode  304  used for the image forming apparatus  100 , a size of the emission point is several μm 2 . Therefore if even one several micro meter sized foreign matter adheres to the front side emission point  304   a  (first emission portion) of the laser diode  304 , the front light is dramatically interrupted, and a desired light quantity may not be acquired on the photosensitive drum  201 , or the spot shape may deform. As a result, image quality drops. 
     If foreign matter adheres to the rear side emission point  304   b  (second emission portion) of the laser diode  304 , the light quantity of the rear light emitted to the PD sensor  305  drops. As mentioned above, the light quantity is controlled to be constant so that the quantity of the light received by the PD sensor  305  becomes constant, hence in this case, the front light quantity becomes higher than the desired light quantity. 
     The laser light emitted from the emission point of the laser diode  304  spreads, and the spot diameter on an optical component in the scanner unit  207  is larger than the size of the emission point. Further, the polygon mirror  403 , the imaging lens  404 , the reflection mirror  405  and the like are sequentially scanned, hence if dust adheres to these optical components, light quantity gently drops in accordance with the amount of the adhering dust. 
     On the other hand, as mentioned above, even one foreign matter adhering to the laser diode  304  dramatically changes the light quantity, since the size of the emission point is small. According to the present invention, the light quantity abnormality (abnormal state) of the laser light source  300  is determined by utilizing the difference of the sensitivity to foreign matter between the laser light source  300  (laser diode  304 ) and other optical components. 
     &lt;Drum Potential Measurement&gt; 
     The measurement of the drum potential will be described in more detail with reference to  FIG. 6 .  FIG. 6  is a graph depicting the relationship between the applied voltage to the transfer roller in a range around the discharge start voltage and the current value flowing through the photosensitive drum. The image forming apparatus of this example has a detection portion (not illustrated) that detects a value on the drum potential (surface potential of the photosensitive drum  201 ). The “value on the drum potential” and “value on the surface potential of the photosensitive drum” are the surface potential of the photosensitive drum  201  itself, or a value correlated to the surface potential (e.g. voltage value of the voltage applied to the transfer roller for acquiring the surface potential, current value of the current flowing to the photosensitive drum and being acquired (detected) by the voltage applying). As shown in  FIG. 6 , current in accordance with the voltage applied to the photosensitive drum  201  flows from the transfer roller  204  until discharge starts (straight line ( 1 )). However, once discharge starts between the photosensitive drum  201  and the transfer roller  204 , current suddenly begins to flow rapidly and exhibits a curve having an inflection point (curve ( 1 )) as shown in the graph. The voltage at the inflection point is regarded as the discharge start voltage. 
     Therefore the discharge current that flows between the photosensitive drum  201  and the transfer roller  204  can be calculated using a Δ value generated by subtracting the straight light ( 1 ) from the curve ( 1 ). Then the voltage when this Δ value reaches a desired current value (e.g. 3 [μA] or −3 [μA]) is determined as the voltage where discharge started. Regarding the discharge characteristics of the photosensitive drum  201 , the potential difference required for discharge differs depending on the difference of the environment and the film thickness of the photosensitive drum. 
     If the surface property of the transfer roller  204  is equivalent to that of the photosensitive drum  201 , then as shown in  FIG. 7 , the potential difference required for starting the discharge becomes a positive/negative symmetric with respect to the drum potential. This characteristic is well known as the discharge phenomenon. If the transfer roller  204  and the photosensitive drum  201  are regarded as having a plane-plane gap, the discharge characteristic of the photosensitive drum  201  is the same as the discharge characteristic of the plane-plane gap, and the drum potential VL can be determined by the following Expression 1. As shown in  FIG. 7 , if VDh is the discharge start voltage on the positive side of the surface potential of the photosensitive drum  201 , and VD 1  is the discharge start voltage on the negative side of the drum potential VL, the drum potential VL is ½ of the total of VDh and VD 1 . In other words, the drum potential VL can be given by the following Expression 1. 
     [Math. 1]
 
 VL =( VDh+VD 1)/2  Expression 1
 
     The drum potential after emitting the laser light can also be determined in the same manner. Bias around the estimated drum potential after emitting the laser light is applied, and the discharge start voltage VL 1  on the negative side of the estimated drum potential after emitting the laser light and the discharge start voltage VLh on the positive side of the estimated drum potential after emitting the laser light, are determined. Then ½ of the total of the determined VL 1  and VLh is determined as the drum potential VL. In other words, the drum potential VL after emitting the laser light can be given by the following Expression 2. 
     [Math. 2]
 
 VL =( VLh−VL 1)/2  Expression 2
 
     &lt;Laser Light Quantity Abnormality Determination Method&gt; 
     Now a laser light quantity abnormality determination method according to this example will be described with reference to  FIG. 8  and  FIG. 9 .  FIG. 8  is a graph depicting the drum potential and laser light quantity after emitting the laser. First the relationship between the laser light quantity and the drum potential will be described. If the quantity of the laser light that the laser light source  300  emits to the photosensitive drum  201  increases, the drum potential changes from −150 V to −100 V, for example. In other words, the absolute value of the drum potential decreases as the laser light quantity increases. 
     The abscissa in  FIG. 8  indicates a number of printed sheets corresponding to the operating time of the image forming apparatus  100 . In this example, a factor that causes a drop in the quantity of the laser light is determined based on the change amount of the drum potential when the drum potential is detected for a plurality of times. In the initial phase of operation, the drum potential when the laser light is emitted at a predetermined light quantity is a desired VLtg. 
     Here the plot of the white circles in  FIG. 8  indicates the state of the drum potential (laser light quantity) that changes by the deposition of foreign matter, such as dust and chemical substance, on the optical components other than the laser diode  304  (contamination of optical components). The plot of the black dots in  FIG. 8  indicates the state of the drum potential (laser light quantity) that changes by deposition of foreign matter on the front side emission point  304   a  of the laser diode  304  (laser failure (front)). The plot of the black triangles in  FIG. 8  indicates the state of the drum potential (laser light quantity) that changes by deposition of foreign matter on the rear side emission point  304   b  of the laser diode  304  (laser failure (rear)). 
     As the plot of the white circles in  FIG. 8  shows, the drum potential (laser light quantity) gradually drops as the contamination of the optical components increases. As the plot of the black dots shows, the drum potential (laser light quantity) suddenly drops if foreign matter adheres to the front side emission point  304   a  of the laser diode  304 .  FIG. 8  shows foreign matter adhering to the front side emission point  304   a  when a number of printed sheets is between X 1  and X 2 . As the plot of the black triangles shows, the drum potential (laser light quantity) suddenly increases if foreign matter adheres to the rear side emission point  304   b  of the laser diode  304 .  FIG. 8  shows foreign matter adhering to the rear side emission point  304   b  when the number of printed sheets is between X 3  and X 4 . In other words, it can be determined that the front side emission point  304   a  is abnormal if the drum potential changes in a negative direction, and that the rear side emission point  304   b  is abnormal if the drum potential changes in a positive direction. The detection timing need not be based on the number of printed sheets, but may be controlled such that detection is performed again at timing X 2  when a predetermined time has elapsed from the timing X 1 . 
     When laser failure occurs, the absolute value of the change amount of the drum potential in a predetermined period (between X 1  and X 2  and between X 3  and X 4  in  FIG. 8 ) is VLs (a predetermined value) or more. In other words, it can be estimated that laser failure occurred if the absolute value of the change amount of the drum potential is VLs or more, that is if the drum potential suddenly changes. On the other hand, if the drum potential gently changes, it can be estimated that failure occurred due to a factor other than laser failure. Thus by measuring the drum potential in accordance with the durability state of the image forming apparatus  100 , [the cause of] a laser light quantity abnormality can be estimated. 
     A control of determining an abnormality of the laser light quantity by the control portion  107  will be further described with reference to  FIG. 9 .  FIG. 9  is a flow chart depicting the laser light quantity abnormality determination sequence according to Example 1. The control portion  107  of the image forming apparatus of this example functions as a determination portion (not illustrated) that determines whether the laser is abnormal, and stores the result as a flag. Voltage is applied to the photosensitive drum  201  from the charging circuit and the transfer circuit (voltage applying portion) via the charging roller  202  and the transfer roller  204 . The image forming apparatus of this example also includes a notification portion (not illustrated) that notifies the user about the failure of each component, such as an abnormality of laser light quantity. 
     First when the laser light quantity abnormality determination sequence is started, the photosensitive drum  201  is rotated (S 901 ), and the photosensitive drum  201  is charged with a charging bias (e.g. −350 V) used for printing (S 902 ). Then the laser light is emitted at a predetermined light quantity (S 903 ), and when the electrostatic latent image formed on the photosensitive drum  201  reaches the transfer roller  204  by the rotation of the photosensitive drum  201 , a predetermined transfer positive bias is applied (S 904 ). 
     With gradually increasing the transfer positive bias, the discharge start voltage VLh on the positive side is determined from the current A that flows from the transfer roller  204  to the ground of the photosensitive drum  201  (S 905 ). In the same manner, a predetermined transfer negative bias is applied (S 906 ), and with gradually decreasing the transfer negative bias, the discharge start voltage VL 1  on the negative side is determined from the current A (S 907 ). Using the above mentioned Expression 2 with VLh and VL 1  determined in S 905  and S 907 , the drum potential VLa after emitting the laser light is calculated (S 908 ). Then the drum potential VLb after emitting the laser light in the previous measurement, which was stored in the storage portion (not illustrated) of the control portion  107 , is read (S 909 ), and VLa of the current measurement result is stored in the storage portion (S 910 ). 
     Then it is checked whether the absolute value of the current measurement result VLa has dropped from the previous measurement result VLb by a predetermined voltage VLs or more, that is, whether the light quantity emitted to the photosensitive drum  201  has increased by a predetermined value or more (S 911 ). If the absolute value has dropped by VLs or more (YES in S 911 ), the control portion  107  determines that the rear side emission point  304   b  (second emission portion) has an abnormality, and stores the laser light quantity abnormality flag, which indicates a drop in the rear light quantity, in the storage portion (S 912 ). The control portion  107  determines that the rear side emission point  304   b  has an abnormality in the following cases. One is a case when a predetermined quantity of the laser light cannot be emitted from the rear side emission point  304   b  even if a predetermined drive current is supplied, because of occurrence of a failure or end of life of the rear side emission point  304   b  itself. The other is a case when the PD sensor  305  cannot receive a predetermined quantity of the laser light from the rear side emission point  304   b  even if a predetermined drive current is supplied, because of foreign matter adhering to the rear side emission point  304   b.    
     If the change amount is smaller than VLs (NO in S 911 ), then it is checked whether the absolute value of the current measurement result VLa has increased from the previous measurement result VLb by a predetermined voltage VLs or more. In other words, it is checked whether the light quantity emitted to the photosensitive drum  201  surface is a predetermined value or less (S 913 ). If the absolute value has increased by VLs or more in the result (YES in S 913 ), the control portion  107  determines that the front side emission point  304   a  (first emission portion) has an abnormality, and stores the laser light quantity abnormality flag, which indicates the drop in front light quantity, in the storage portion (S 914 ). For VLs, potential is determined based on the exposure drop rate when foreign matter adheres to the emission point and is stored in the storage portion in advance. The control portion  107  determines that the front side emission point  304   a  has an abnormality in the following cases. One is a case when a predetermined quantity of the laser light cannot be emitted from the front side emission point  304   a  even if a predetermined drive current is supplied, because of a failure or life of the front side emission point  304   a  itself. The other case is a case when the photosensitive drum  201  cannot be exposed to light at a predetermined light quantity even if a predetermined drive current is supplied, because of foreign matter adhering to the front side emission point  304   a.    
     Then it is determined whether the current measurement result VLa is a value within a first predetermined range (VLt 1  or more and VLth or less in  FIG. 8 ). In concrete terms, it is determined whether the absolute value of the current measurement result VLa is a predetermined voltage VLt 1  or less (S 915 ). If VLt 1  or less (YES in S 915 ), it is determined that the absolute value of the drum potential VLa is low, which is a VL abnormality (S 916 ), and the charging operation is checked next (S 917 ). For VLt 1 , potential is determined based on the exposure amount at which the drum is damaged because the front light quantity is high, and this potential value is stored in the storage portion in advance. 
     If the absolute value of VLa is higher than VLt 1  (NO in S 915 ), it is determined whether the absolute value of the current measurement result VLa is the predetermined voltage VLth or more (S 918 ). If VLa is lower than VLth, the sequence ends (NO in S 918 ), and if VLa is VLth or more (YES in S 918 ), then it is determined that the absolute value of the drum potential is high, which is a VL abnormality (S 916 ), and the charging operation is checked next (S 917 ). For VLth, potential is determined based on the exposure amount at which the printed image quality drops significantly because the front light quantity is low, and this potential value is stored in the storage portion in advance. 
     Now a process of diagnosis to discern from the factors other than the laser light quantity abnormality related to the abnormalities of exposure amount will be described. First in a state where the laser is not emitted, the photosensitive drum  201  is charged with a charging bias (e.g. −350V) (S 917 ). Then, same controls as those are implemented in S 904  to S 908  are implemented, and the drum potential is calculated using the above mentioned Expression 1. If the drum potential is a value in a second predetermined range (e.g. −400V or more, −300V or less), it is determined that the charging circuit is operating without problems as the voltage applying portion (NO in S 919 ), and the transfer operation is checked (S 920 ). If the drum potential is a value outside the second predetermined range (YES in S 919 ), on the other hand, the notification portion notifies the high voltage power supply failure (S 921 ). 
     In the transfer operation check (S 920 ), the photosensitive drum  201  is charged at charging bias 0 V, to check whether the transfer circuit  206  is operating correctly as the voltage applying portion. By sequentially applying a predetermined transfer positive bias and transfer negative bias, it is checked whether the assumed current A, that flows from the transfer roller  204  to the ground of the photosensitive drum  201 , is detected respectively. If the detected current is outside a predetermined current range (that is, if the drum potential is outside the second predetermined range) (YES in S 922 ), the notification portion notifies the high voltage power supply failure (S 921 ). If the detected current is within the predetermined range (that is, if the drum potential is within the second predetermined range) (NO in S 922 ), it is determined that the transfer circuit  206  is operating normally and a failure occurred to the scanner unit  207 . 
     Then it is checked whether the laser light quantity abnormality flag is stored in the storage portion (S 923 ), and if stored (YES in S 923 ), the notification portion notifies the user of the laser light quantity abnormality (S 924 ). If the laser light quantity abnormality flag is not stored (NO in S 923 ), on the other hand, the notification portion notifies the user of a failure of an optical component other than the laser diode  304  (S 925 ). 
     In this example, the previous measurement result is used for VLb, but the same effect can be implemented even if an average value of the measurement results, up to the last measurement time, is used. If the values of VLth and VLt 1  are not one value, but change depending on the operating environment and durability of the image forming apparatus  100 , the laser light quantity abnormality can be determined with even more accuracy. Here a method of calculating the drum potential, from the discharge start voltage based on the current A flowing from the transfer roller  204  to the ground of the photosensitive drum  201 , was described. However, the detection portion may calculate the drum potential based on the current flowing from the charging roller  202  or the developing sleeve  203  to the ground of the photosensitive drum  201 . 
     As described above, in this example, the laser light quantity emitted from the scanner unit  207  is indirectly measured by detecting the drum potential, and the laser light quantity abnormality is detected by checking the change amount of the value related to the drum potential. In other words, in this example it is determined whether the laser diode  304  is abnormal or not based on the change amount of a value related to the surface potential of the photosensitive drum  201 . Thereby the laser light quantity abnormality can be detected. Further, the cause of the laser light abnormality can be detected in detail by discerning whether the abnormality is of the front light quantity or the rear light quantity. If service personnel or the like collect the causes of an abnormality and feed it back to design and development, quality of the image forming apparatus can be improved. 
     If the laser light source  300  generates an abnormal quantity of light that deviates from the desired light quantity, the quality of the print image drops. Moreover, if the front light quantity increases, the photosensitive drum  201  may be damaged. The above mentioned control is an example of determining only a failure, but an abnormality may be determined before a failure occurs if a plurality of thresholds are set for the drum potential. Since the abnormality can be notified to the user before the laser light source  300  completely fails, the downtime of the image forming apparatus  100  due to failure can be reduced. 
     In this example, a can package without glass  501 , where the laser diode  304  is exposed to air, was described as an example, but a can package sealed with glass  501  may be used. In the case of the can package in which the laser diode  304  is sealed by the glass  501 , the laser spot diameter on the glass  501  is about 100 μm, for example. In this case, the portion on the glass  501  where the laser light passes through corresponds to the first emission portion of the present invention. In the case of this configuration, the front light quantity suddenly drops when a foreign matter of about 100 μm or larger, adheres to the laser spot on the glass  501 . 
     EXAMPLE 2 
     Example 2 will now be described with reference to  FIG. 10  and  FIG. 11 . The general configuration of the image forming apparatus  100  of Example 2 is the same as Example 1, except that a multi-beam laser that can emit a plurality of laser beams from the laser diode  304  is mounted on the laser light source  300 . A composing element the same as Example 1 is denoted with a same reference symbol, and description thereof is omitted. In the configuration of Example 2, the laser light quantity abnormality can be determined even with more accuracy than Example 1, by alternately emitting laser beams by the multi-beam laser. In Example 2, a twin-beam laser will be described as an example of the multi-beam laser. 
     &lt;Configuration of Twin-beam Laser&gt; 
     The twin-beam laser used in Example 2 will be described first.  FIG. 10  is a diagram depicting the twin-beam laser and a PD sensor of Example 2. The twin-beam laser has one laser diode  304  in which two resonators are disposed in parallel. The laser light emitted from a front side emission point  304   a   1  (first emission portion) and a front side emission point  304   a   2  (third emission portion) is directed to the photosensitive drum  201  (photoreceptor). Laser light emitted from a rear side emission point  304   b   1  (second emission portion) and a rear side emission point  304   b   2  (fourth emission portion) are received by the PD sensor  305  (light receiving portion). 
     The laser light from the front side emission point  304   a   1  and the rear side emission point  304   b   1  is emitted by a supply of a common first drive current. The laser light from the front side emission point  304   a   2  and the rear side emission point  304   b   2  is emitted by a common second drive current supply. 
     In a standard twin-beam laser used for the image forming apparatus  100 , the interval between resonators, that is the emission points, is about 90 μm. Therefore, even if about a several tens μm sized foreign matter adheres to the end face of the reflection mirror of the laser diode  304 , it is rare that the foreign matter covers two emission points. Therefore the laser light quantity abnormality is determined using the phenomena in which the light quantity of only one of the two emission points drops when a foreign matter adheres to the other emission point. 
     &lt;Laser Light Quantity Abnormality Determination Method&gt; 
     Now a control to determine the laser light quantity abnormality performed by the control portion  107  according to Example 2 will be described with reference to  FIG. 11 .  FIG. 11  is a flow chart depicting the laser light quantity abnormality determination sequence according to Example 2. A step in the flow, the same as Example 1 described in  FIG. 9 , is denoted with a same reference symbol, and description thereof is omitted. 
     A laser is emitted from the laser diode A (LDA) as the first light emitting member on one side of the twin-beam at a predetermined light quantity (S 1101 ). Then the drum potential VL 1 , after emitting the LDA laser, is calculated (S 1102 ). Then it is checked whether VL 2  has been detected by the laser diode B (LDB) as the second light emitting member (S 1103 ), and if not, processing returns to S 1101 , and a laser is emitted from the LDB on the other side of the twin-beam. Then VL 2  is calculated in the same manner for the LDB laser as well (S 1102 ). 
     When calculation of the drum potentials VL 1  and VL 2  ends, it is checked whether only one of the measurement result VL 1  when the LDA laser was emitted, and the measurement result VL 2  when the LDB laser was emitted, is VLhe (first predetermined value) or more (S 1104 ). For VLhe, the potential is determined based on the exposure reduction rate when a foreign matter adheres to one of the front side emission points, and this potential value is stored in advance. If only one of VL 1  and VL 2  is VLhe or more, it is determined that the front side emission point (first emission portion or third emission portion) is abnormal, and a laser light quantity abnormality flag, due to a drop in front light quantity, is stored (S 1105 ). 
     If not, it is checked whether only one of the drum potentials VL 1  and VL 2  is VLle (second predetermined value) or less (S 1106 ). If only one of VL 1  and VL 2  is VLle or less, it is determined that the rear side emission point (second emission portion or fourth emission portion) is abnormal. Then the laser light quantity abnormality flag, due to a drop in rear light quantity, is stored (S 1107 ). Here for VLle, potential is determined based on the exposure increase rate when a foreign matter adheres to one of the rear side emission points, and this potential value is stored in advance. 
     Then it is checked whether both of the drum potentials VL 1  and VL 2  are VLhe or more, or VLle or less (S 1108 ). If the result is YES, it is determined that the abnormality is caused by a factor other than the laser light quantity abnormality due to the adhesion of foreign matter on the laser diode  304 . 
     Just like Example 1, if the laser light quantity abnormality flag is stored (if it is determined that one of the first to fourth emission portions is abnormal), the operation of the charging circuit and the transfer circuit is checked to determine whether the abnormality is due to a factor related to an exposure amount abnormality, and not a laser light quantity abnormality. The method for the operation check is the same as Example 1. If it is determined that the charging circuit and the transfer circuit  206  are operating normally (NO in S 919  and NO in S 922 ), the notification portion notifies the user of the laser light quantity abnormality (S 1109 ). 
     Here if the value of VLhe or VLle is not one value but one that changes depending on the operating environment and durability of the image forming apparatus  100 , then the laser light quantity abnormality can be determined with even more accuracy. By performing control in this way, laser light quantity abnormality can be accurately determined for the image forming apparatus  100  having a multi-beam laser. Further, the laser light quantity abnormality can be determined with even higher accuracy by combining the control of this example with the control of Example 1. For example, the flow of performing the laser failure notification and optical component failure notification described in S 923  to S 925  in  FIG. 9  may be combined with Example 1. 
     In this example, the can package without glass  501  was described. But even in the case of a can package sealed by the glass  501 , the beam spot diameter on the glass is about 90 μm, and the interval between spots is about several hundred μm, and it is rare that the adhesion of foreign matter covers all beams of the multi-beam laser. Therefore even in the case of the can package sealed by the glass  501 , the laser light quantity abnormality can be determined with even higher accuracy than Example 1. 
     According to the present invention, a factor that drops the quantity of laser light can be determined with high accuracy. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2014-143577, filed Jul. 11, 2014, which is hereby incorporated by reference herein in its entirety.