Abstract:
An image forming apparatus including a photosensitive body, a charger configured to charge the photosensitive body, an exposure unit configured to expose the photosensitive body charged by the charger, a developer transport section configured to cause relative movement with respect to the photosensitive body for transporting developer therebetween, a current measurement section configured to measure a current flowing between the photosensitive body and the developer transport section, and a determination unit configured to compare a current with a threshold, the current measured by the current measurement section when the photosensitive body and the developer transport section move relatively to each other and a first area of the photosensitive body that needs to be exposed faces the developer transport section, and determine whether an exposure of the photosensitive body is proper based on a result of the comparison.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The application claims priority from Japanese Patent Application No. 2009-179445 filed on Jul. 31, 2009. The entire content of this priority application is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to an image forming apparatus and to an exposure control method of controlling exposure in the image forming apparatus. 
     BACKGROUND 
     An image forming apparatus having a current measurement system for measuring an electrical current in a transfer unit is known. It measures an output current in a condition that the transfer unit is not in operation and that in a different condition. Then, it performs a comparison between the measured output currents. It determines whether the transfer unit is provided, or whether an abnormal operation in switching between standby and contact positions occurs based on the result of the comparison. 
     In an image forming apparatus, an improper exposure may occur. In this case, a photosensitive body does not get sufficient exposure. However, in the above known image forming apparatus, such an improper exposure is not seriously discussed. It only discloses how to determine whether the transfer unit is provided, or whether an abnormal operation occurs in switching between the standby and contact positions. 
     SUMMARY 
     There is a need in the art to provide an image forming apparatus capable of detecting an improper exposure and an exposure control method of controlling the exposure if the improper exposure is detected. 
     An image forming apparatus according to an aspect of the invention includes a photosensitive body, a charger, an exposure unit, a developer transport section, a current measurement section, and a determination unit. The charger charges the photosensitive body. The exposure unit exposes the charged photosensitive body. The developer transport section causes relative movement with respect to the photosensitive body for transporting developer therebetween. The current measurement section measures a current that flows between the photosensitive body and the developer transport section. It measures a current when the photosensitive body and the developer transport section make relative movements and a first area of the photosensitive body that needs to be exposed faces the developer transport section. The determination means determines whether the exposure is proper based on a comparison of the current measured by the current measurement section with a threshold. 
     If the photosensitive body is properly exposed, the number of electrons charged on the photosensitive body is sufficiently reduced. As a result, the current measured by the current measurement section when the area (first area) of the photosensitive body faces the developer transport section becomes lower. If the photosensitive body is not properly exposed, that is, an improper exposure occurs, the number of electrons charged on the photosensitive body is not sufficiently reduced. As a result, the current measured by the current measurement section when the area of the photosensitive body faces the developer transport section does not become lower. Namely, the improper exposure can be detected by comparing the measured current with the threshold. 
     An exposure control method of controlling exposure in the image forming apparatus according to an aspect of the present invention includes charging the photosensitive body by the charger, exposing the charged photosensitive body to light emitted from the exposure unit, measuring a current flowing between the photosensitive body and the developer transport section under a condition that an area (first area) of the photosensitive body that needs to be exposed faces the developer transport section, determining whether the exposure is proper based on a comparison of the current with a threshold, and controlling the exposure unit based on a result of the determination. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Illustrative aspects in accordance with the present invention will be described in detail with reference to the following drawings wherein: 
         FIG. 1  is a side sectional view illustrating the general construction of a printer according to one of the illustrative aspects of the invention; 
         FIG. 2  is a schematic diagram illustrating electrical circuits on a circuit board and printer components relative to the electrical circuits; 
         FIG. 3  is a timing chart illustrating variations in voltages and currents according to time; 
         FIG. 4  is a flowchart illustrating an improper exposure detection process; and 
         FIG. 5  is a schematic diagram illustrating a developer transport section according to the illustrative aspect of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     &lt;Illustrative Aspect&gt; 
     An illustrative aspect of the present invention will be explained with reference to  FIGS. 1 through 4 . 
     1. General Construction of Printer 
       FIG. 1  is a side sectional view illustrating the general construction of a color laser printer  1 , which is an example of an image forming apparatus of the present invention and hereinafter referred to as the printer. 
     In the following description, the right side of  FIG. 1  corresponds to the front side of the printer  1 . Further, printer components provided for respective colors generally have the same structural configuration, and thus the name thereof with a representative number is used in descriptions of those components unless they need to be explained separately. For example, the transfer roller  14  represents the transfer rollers  14 K,  14 Y,  14 M,  14 C for colors black, yellow, magenta and cyan, respectively. 
     The printer  1  includes a main casing  2  and a paper feed tray  4  at the bottom of the main casing  2 . Sheets  3  (an example of recording media) are stacked in the paper feed tray  4 . A pickup roller  5  is provided above the front end of the paper feed tray  4 . A pair of registration rollers  8  is provided above the pickup roller  5 . The top sheet  3  in the paper feed tray  4  is picked up by the pickup roller  5  as it rotates and is passed to the registration rollers  8 . The registration rollers  8  perform a skew correction for the sheet  3  as necessary and pass the sheet  3  onto a belt unit  11  in an image forming unit  10 . 
     The image forming unit  10  includes the belt unit  11 , a scanner unit  19 , a process unit  20 , a fuser unit  31  and a circuit board  34 . 
     The belt unit  11  is configured such that a belt  13  is stretched and looped over belt rollers  12 , one of which is arranged in the front and the other in the rear. The belt  13  rolls counter-clockwise as the rear belt roller  12  revolves and the sheet  3  on the top surface of the belt  13  is passed to the rear. 
     Inside a loop of the belt  13 , the transfer roller  14  is arranged on an opposite side of the belt  13  from a side where a photosensitive drum  28  is arranged in the process unit, which will be explained later. The photosensitive drum  28  is an example of a photosensitive body of the present invention. The transfer roller  14  is an example of a transfer unit that is an object for the current measurement and included in the developer transport section of the present invention. It is prepared by covering a metal roller shaft with a rubber having conductive properties. The transfer roller  14  is pressed against the photosensitive drum  28  so that the sheet  3  is sandwiched between the transfer roller  14  and the photosensitive drum  28  when it is passed through on the belt  13 . 
     The scanner unit  19 , which is an example of an exposure unit of the present invention, includes an optical system (not shown). The optical system is configured to apply different colors of laser beams L to the surfaces of the respective photosensitive drums  28 . A polygonal mirror (not shown) guides laser beams emitted from laser diodes (LDs)  33  toward the respective photosensitive drums  28 . 
     The process unit  20  includes a frame  21  that can be pulled out of the main casing  2 , and four removable developer cartridges  22  ( 22 K,  22 Y,  22 M and  22 C) placed in the frame  21 . The developer cartridges  22  are provided for four different colors of developer. In this example, black, yellow, magenta and cyan developer cartridges are arranged in this order from an upstream to the downstream of the sheet feed path. At the bottom of the frame  21 , the photosensitive drum  28  and charger  29  are provided for the developer cartridge  22 . The charger  29  is an example of a charger of the present invention. 
     The developing cartridge  22  includes a toner container  23 , a feed roller  24 , a developing roller  25  and a layer thickness control blade  26 . The feed roller  24 , the developing roller  25  and the layer thickness control blade  26  are arranged in the lower portion of the developing cartridge  22 . The developing roller  25  is an example of a developer transport section that is not an object for the current measurement. The toner container  23  of the developing cartridge  22  contains positive charge toner particles in the corresponding color. The positive charge toner in each color is an example of developer. 
     The toner particles ejected from the toner container  23  are passed to the developing roller  25  by the feed roller  24  as it revolves, and positively charged due to triboelectricity produced between the feed roller  24  and the respective developing roller  25 . The developing roller  25  starts revolving when a developing bias is applied. As the developing roller  25  revolves, the toner particles passed thereon goes through between the layer thickness control blade  26  and the developing roller  25 . As a result, the toner particles are more positively triboelectrically charged, and a thin layer of the toner particles with an even thickness is formed on the developing roller  25 . The photosensitive drum  28  includes a metal drum body that is grounded and the outer surface thereof is covered with a positive charge photosensitive layer, which may be made of polycarbonate. 
     The charger  29  is a scorotron charger and includes a discharge wire  29   a  and a grid  29   b . The discharge wire  29   a  is arranged at a predetermined distance away from the photosensitive drum  28  such that it faces the photosensitive drum  28 . The grid  29   b  is arranged between the discharge wire  29   a  and the photosensitive drum  28 . It is configured to control the electric discharge from the discharge wire  29   a  to the photosensitive drum  28 . In the charger  29 , a high voltage is applied to the discharge wire  29   a  to induce corona discharge so that a current from the discharge wire  29   a  to the grid  29   b  remains constant. Namely, the surface of the photosensitive drum  28  is positively charged at an even level by maintaining the grid voltage constant. 
     The fuser  31  includes a heat roller  31 A, which includes a heat source, and a pressure roller  31 B, which is configured to press the sheet  3  against the heat roller  31 A. It thermally fixes the toner image transferred on the surface of the sheet  3 . 
     2. Image Forming Process 
     During image formation, the photosensitive drum  28  revolves counter-clockwise and the surface thereof is positively charged at an even potential (e.g., at +800 V) by the charger  29  according to the revolution. A high-speed raster line of the laser beam is emitted from the scanner  19  and the positively charged area of the photosensitive drum is exposed to light of the laser beam. As a result, an electrostatic latent image that corresponds to an image to be printed on the sheet  3  is formed on the surface of the photosensitive drum  28 . The exposed area of the surface of the photosensitive drum  28  is charged at +200 V, for example. 
     The developing roller  25  holds the positively charged toner particles on the surface thereof. As the developing roller  25  revolves, the positively charged toner particles touch the photosensitive drum  28  and cling to the area where the electrostatic latent image is formed. As a result, the electrostatic latent image becomes visible. Because the exposed area on the surface of the photosensitive drum  28  has a potential lower than the developing bias (of about +400 V) that is applied to the developing roller  25 , the toner particles are held in the area in a form of a toner image (a developing image) 
     A negative transfer voltage (of about −3000 V) is applied to the transfer roller  14 . The sheet  3  is passed through between the photosensitive drum  28  and the transfer roller  14 . When it passes through a transfer point (a transfer nip of the transfer drum  14 ), the toner image on the surface of the photosensitive drum  28  is transferred onto the sheet  3  due to the negative transfer voltage. The sheet  3  on which the toner image is transferred is passed to the fuser  31  and the toner image is thermally fixed. 
     The sheet  3  on which the toner image is thermally fixed is transferred from the fuser  31  to an upper area of the printer  1  and ejected onto a paper receiving tray provided on the top surface of the main casing  2 . 
     3. Electrical Configuration of Printer 
       FIG. 2  is a schematic diagram illustrating configurations of the electrical circuit  50  formed on the circuit board  34  and the printer components related to the electrical circuit  50 . The electrical circuit  50  includes a CPU  60 , a ROM  61  and a RAM  62 . It further includes a charge voltage supply circuit  51 , an LD drive circuit  52 , a developing bias supply circuit  53 , a motor drive circuit  54 , a transfer voltage supply circuit  55  and a transfer current detection circuit  56 . The CPU  60  is an example of determination unit, current control section, improper exposure detection execution section, light intensity control section or separation control section. The LD drive circuit  52  is an example of a light intensity control section. The transfer voltage supply circuit  55  is an example of a current control section and a voltage control section. The transfer current detection circuit  56  is an example of current measurement section, current detection circuit, current control section and current control sections. 
     The ROM  61  stores operation programs. The CPU  60  performs overall control of the printer  1  by executing those operation programs. The RAM  62  stores image data used for the printing process. 
     The charge voltage supply circuit  51  generates a charge voltage Vcgw that is applied to the discharge wire  29   a  of the charger  29  and a grid voltage Vcgg that is applied to the grid  29   b  of the charger  29 . 
     The LD drive circuit  52  generates an LD drive current Id that is supplied to the LD  33  for illuminating the surfaces of the photosensitive drum  28  with the laser beam L from the LD  33  at a predetermined level (i.e. with a predetermined amount of the laser) according to the control performed by the CPU  60 . The developing bias supply circuit  53  generates a developing bias Vdev (the bias voltage) that is applied to the developing roller  25 . 
     The motor drive circuit  54 , which is an example of a separation control section, is connected to a motor  25   a  that is provided for bringing the developing roller  25  pressed against or separating it from the photosensitive drum  28 . The developing roller  25  is installed so as to be movable in a direction toward the photosensitive drum  28  until it is pressed against the photosensitive drum  28  and in a direction away from the photosensitive drum  28 . During the printing operation of the printer  1 , the CPU  60  controls the motor drive circuit  54  to drive the motor  25   a  so that the developing roller  25  is pressed against the photosensitive drum  28  to make the toner particles cling to the photosensitive drum  28 . In improper exposure detection mode, which will be explained later, the CPU  60  controls the motor drive circuit  54  to drive the motor  25   a  so as to separate the developing roller  25  from the photosensitive drum  28  and restrict a current flowing from the photosensitive drum  28  to the developing roller  25 . 
     The CPU  60  controls the transfer voltage supply circuit  55  to generate a transfer voltage Vt that is applied to the transfer roller  14 . The transfer voltage Vt is an example of a bias voltage. 
     The transfer current detection circuit  56  detects a transfer current It that is generated when the transfer voltage Vt is applied. The CPU  60  performs constant current control to regulate the transfer current It to a predetermined level based on a detection signal (a feedback signal) sent by the transfer current detection circuit  56 . When the transfer voltage supply circuit  55  is deactivated, the transfer current detection circuit  56  also detects an inflowing current Ir that flows from the charged photosensitive drum  28  to the transfer current detection circuit  56  via the belt  13  and the transfer roller  14 . 
     4. Timing of Voltage Application and Current Feed 
       FIG. 3  is a timing chart that illustrates timing of voltage application and current feed, and also timing of the current flowing from the photosensitive drum to the transfer roller. The voltages and the current explained above vary as in this timing chart. How the LD drive current Id is supplied differs depending on situations in which the exposure is proper or not. The following section describes how the LD drive current Id is supplied when the exposure is proper. 
     First, the timing of voltage application and current feed will be explained. The CPU  60  controls the charge voltage supply circuit  51  to start application of the charge voltage Vcgw to the discharge wire  29   a  and application of the grid voltage Vcgg to the grid  29   b  at time T 1  when a predetermined time has elapsed since the printer  1  is turned on. When the charge voltage Vcgw and the grid voltage Vcgg reach thresholds at T 2 , the CPU controls the main motor drive circuit (not shown) to rotate the main motor so that the photosensitive drum  28  starts revolving. 
     At T 2 , the main motor starts revolving. At T 3 , the first charged area of the photosensitive drum  28  completely passes through an exposure point P (see  FIG. 2 ) at which the laser beam L from the LD  33  is focused. The CPU  60  remains on standby during the period between T 2  and T 3 . At T 3 , the CPU  60  controls the LD drive circuit  52  to start supply of the LD drive current Id for the improper exposure detection. Since operations in a condition that the exposure is proper are being discussed here, the LD drive circuit  52  should continuously supply the LD drive current Id to keep the LD  33  turned on. The supply of the LD drive current Id continues until a predetermined time elapses at T 4 . 
     At T 4 , the CPU  60  stops the supply of the LD drive current Id from the LD drive circuit  52  to turn the LD  33  off, and then goes on standby until a print request is input by a user of the printer  1 . When the print request is input at T 9 , the CPU  60  switches the supply of the LD drive current Id between on and off (only the case that the supply remains on is shown in  FIG. 3 ) based on the image data on the image to be printed. Namely, the photosensitive drum  28  is exposed according to the image data and an electrostatic latent image corresponding to the image data is formed on the photosensitive drum  28 . The supply of the LD drive current Id continues until a complete shape of the electrostatic latent image is formed at T 14 . 
     The exposure of the photosensitive drum  28  starts at T 9  and an area of the photosensitive drum  28  that is firstly exposed to the light by the exposure reaches a point where it faces the developing roller  25  shortly after T 11 . The CPU  60  controls the developing bias supply circuit  53  to start the application of the developing bias Vdev at T 10 , which is earlier than T 11 , so that the developing bias Vdev rises to a proper level at the time of T 11 . The developing bias Vdev is continuously regulated to a constant level until the entire electrostatic latent image on the photoconductive drum  28  becomes visible at T 15 . 
     The first exposed area of the photosensitive drum  28  reaches a point where it faces the transfer roller  14  shortly after T 13 . The CPU  60  controls the transfer bias supply circuit  55  to start the application of the transfer bias Vt at T 12 , which is earlier than T 13 , so that the transfer bias Vt rises to a sufficient level at the time of T 13 . The transfer bias Vt is continuously regulated to a constant level until the entire toner image held by the photosensitive drum  28  is transferred onto the sheet  3  at T 16 . 
     Next, the timing at which the inflowing current flows from the photosensitive drum  28  to the transfer roller  14  will be explained. After the charging has started at T 1 , the first charged area of the photosensitive drum  28  reaches a point where it faces the transfer roller  14  at T 5 . 
     When the first charged area of the photosensitive drum  28  has reached the point where it faces the transfer roller  14 , the electric charge on the surface of the photosensitive drum  28  moves to the transfer roller  14  via the belt  13 , that is, a current flows from the photosensitive drum  28  to the transfer roller  14 . The inflowing current Ir rises up to a certain level and then remains at that level. 
     When the first exposed area of the photosensitive drum  28  has reached at the point where it faces the transfer roller  14  at T 7 , the inflowing current Ir falls because the electric charge is reduced by the exposure and remains low until the first exposed area passes the point at T 8 . The exposure is stopped at T 4 . When an area that has passed the exposure point P after T 4  reaches the point where it faces the transfer roller  14 , the inflowing current rises back to the previous level and remains at that level. 
     5. Determination Process in Improper Exposure Detection 
     An improper exposure is a condition that the photosensitive drum  28  is not properly exposed. Causes of the improper exposure include an improper laser beam level, an improper charge level on the photosensitive drum  28  and broken harnesses. If the LD  33  or the LD drive circuit  52  becomes defective or deteriorates, the proper level of the laser beam cannot be achieved. If the charge voltage supply circuit  51  or the charger  29  becomes defective or deteriorates, the photosensitive drum  28  is not properly charged. Moreover, the photosensitive drum  28  is not properly charged if it itself deteriorates. These causes are only some examples and improper exposure may result from other causes. 
     When the improper exposure occurs, the photosensitive drum  28  is not properly exposed and the electric charge on the surface thereof is not sufficiently reduced. Therefore, the inflowing current Ir does not fall sufficiently even when the first exposed area of the photosensitive drum  28  reaches the point where it faces the transfer roller  14  at T 7  as illustrated in  FIG. 3 . The CPU  60  detects the improper exposure by comparing a current detected (or measured) in the period between T 7  and T 8  with a threshold. 
       FIG. 4  is a flowchart of the determination process in the improper exposure detection. When the printer  1  is turned on, the CPU  60  enters improper exposure detection mode before starting the image forming process. The determination process starts when the CPU  60  enters improper exposure detection mode. 
     In step S 101 , the CPU  60  drives the motor  25   a  to separate the developing roller  25  from the photosensitive drum  28  so that a current does not flow between the photosensitive drum  28  and the developing roller  25 . 
     In step S 102 , the CPU  60  controls the charge voltage supply circuit  51  to apply the charge voltages (the charge voltage Vcgw, the grid voltage Vcgg) to the charger  29  (at T 1  in  FIG. 3 ). As a result, the charging of the photosensitive drum  28  starts. 
     In step S 103 , the CPU  60  drives the maim motor to start the rotation of the photosensitive drum  28  (at T 2 ). 
     In step S 104 , the CPU  60  remains on standby until the first charged area of the photosensitive drum  28  reaches the point where it faces the transfer roller  14 . The CPU  60  starts a timer (not shown) at T 1  at which the application of the charge voltages to the charger  29  starts. When a predetermined time (a period between T 1  and T 5 ) has elapsed, the CPU  60  assumes that the first charged area of the photosensitive drum  28  reaches the point where it faces the transfer roller  14 . To determine other points of timing, it also uses the timer to determine elapsed time and determine the timing based on the elapsed time. 
     In step S 105 , the inflowing current Ir starts flowing from the charged area of the photosensitive drum  28  to the transfer roller  14  via the belt  13  when the first charged area of the photosensitive drum  28  reaches the point where it faces the transfer roller  14  (at T 5 ). The inflowing current Ir rises to a constant level at T 6 . The CPU  60  controls the transfer current detection circuit  56  and determines the value of the inflowing current Ir during the period between T 6  and T 7 . Namely, the CPU  60  measures the second flowing current Ir 2  that flows into the transfer roller  14  when the second area of the photosensitive drum  28  faces the transfer roller  14 , where the second area is an area that is not exposed on the surface of the photosensitive drum  28 . The second inflowing current Ir 2  is an example of a current measured by the current measurement unit when-the second area of the photosensitive body, which is an unexposed area of the photosensitive body, faces the developer transport section. 
     In step S 106 , the CPU  60  controls the LD drive circuit  52  so that the photosensitive drum  28  is exposed (between T 3  and T 4 ). If the exposure step of S 106  is performed prior to step S 105 , a step in which the value of the inflowing current Ir 1  from the first area of the photosensitive drum  28  is determined (step S 108 , which will be explained later) can be performed immediately after step S 105  and before step S 106 . The first area of the photosensitive drum  28  is an area that needs to be exposed on the surface of the photosensitive drum  28 . 
     In step S 107 , the CPU  60  remains on standby until the photosensitive drum  28  rotates and the area thereof needs to be exposed reaches the point where it faces the transfer roller  14 . “The area thereof needs to be exposed (first area)” refers to an area that is actually exposed by the exposure unit when the exposure is proper. The reason why the area is expressed as “the area needs to be exposed” instead of “the exposed area” is that it may not be exposed at all when improper exposure occurs. Namely, “the area needs to be exposed” is a target area of the exposure performed by the exposure unit whether the improper exposure occurs. 
     In step S 108 , the inflowing current Ir 1  flows from the first area of the photosensitive drum  28  to the transfer roller  14  via the belt  13  when the first area reaches the point where it faces the transfer roller  14  (at T 7 ). The CPU  60  controls the transfer current detection circuit  56  and determines the value of the inflowing current Ir during the period between T 7  and T 8 . Namely, the CPU  60  measures the first inflowing current Ir 1  flowing into the transfer roller  14  when the first area of the photosensitive drum  28  faces the transfer roller  14 . The first inflowing current Ir 1  is an example of a current measured by the current measurement section when the area of the photosensitive body, which is an area of the photosensitive body that needs to be exposed, faces the developer transport section. 
     In step S 109 , the CPU  60  calculates a difference between the first and the second inflowing currents measured in step S 105  and step S 108 , respectively, compare the difference with the second threshold, and determines whether the exposure is proper based on the result of the comparison. By comparing the difference with the second threshold, chances of false detection of the improper exposure due to environmental factors, such as ambient temperature and humidity, during the measurement can be reduced. If the exposure is proper, the difference should be substantially the same because the environmental factors affect the value of the currents flowing from the first area and the first area at the same level. Namely, by comparing the difference between the first inflowing current and the first inflowing current with the threshold, the improper exposure is properly detected without affected by the environmental factors. 
     If the difference is lower than the second threshold, the CPU  60  determines that the exposure is improper and proceeds to step S 110 . If the difference is equal to or higher than the second threshold, the CPU  60  determines that the exposure is proper and proceeds to step S 117 . 
     In step S 110 , the CPU  60  compares the difference with the third threshold that is lower than the second threshold. If the difference is lower than the third threshold, the CPU  60  determines that a printer component that affects the exposure of the photosensitive drum  28  is defective, that is, the printer component does not function at all or its performance is reduced due to deterioration. 
     The printer component that affects the exposure of the photosensitive drum  28  is such as the LD drive circuit  52 , the LD  33 , the charge voltage supply circuit  51 , the charger  29  and the harnesses. If the LD  33  becomes defective, for example, the first area of the photosensitive drum  28  is not properly exposed. As a result, the electrical charge is not reduced as much as it should be by the exposure and the difference between the first and the second inflowing currents (i.e., the currents flowing from the area that should be exposed and from the area should not be exposed) is equal to or close to zero. 
     If the charger  29  becomes defective, it cannot charge the photosensitive drum  28  to a proper level. As a result, the first inflowing current Ir 1  does not vary largely from the second inflowing current Ir 2  and thus the difference between them is equal to or close to zero. 
     If the photosensitive drum  28  becomes defective (or deteriorated in this case), it cannot be properly charged. As a result, the electrical charge is not reduced as much as it should be by the exposure and the difference in the first and the first inflowing currents Ir is equal to or close to zero. 
     By comparing the difference with the third threshold, the malfunctions of the printer components can be detected. When a printer component other than the ones that described above becomes defective, the malfunction may affect the exposure of the photosensitive drum  28 . If the improper exposure occurs due to the malfunction, the difference in the currents also becomes equal to or close to zero and thus the malfunction can be detected. When the malfunction is detected, the CPU  60  proceeds to step S 111 . If the malfunction is not detected, the CPU  60  proceeds to step S 112 . 
     In step S 111 , the CPU  60  reports the malfunction, for example, by displaying a message indicating the malfunction on a display screen of the printer  1 , by providing audio information, or by sending email to an administrator of the printer  1 . 
     In step S 112 , the CPU  60  controls the LD drive circuit  52  to increase the amount of laser light emitted from the LD  33  by one step and to expose the photosensitive drum  28  to the increased intensity of light. Although the amount of increase per step can be set to any amount, it should be set to a small amount because the total amount of the light may largely exceed a proper level if the amount of increase per step is set to a large amount. 
     In step S 113 , the CPU  60  remains on standby until the photosensitive drum  28  revolves and the area thereof that needs to be exposed in step S 112  reaches the point that it faces the transfer roller  14 . When the area reaches the point where it faces the transfer roller  14  in step S 114 , a signal that indicates the first inflowing current Ir 1  is output from the transfer current detection circuit  56  and it is input to the CPU  60 . 
     In step S 115 , the CPU  60  determines whether the exposure is proper in the same manner as step S 109 . If the exposure is improper, the CPU  60  proceeds to step S 116 . If the exposure is proper, the CPU  60  proceeds to step S 117 . 
     In step S 116 , the CPU  60  determines whether the number of times that the intensity of light emitted from the LD  33  is increased exceeds the limit, or whether the light intensity reaches the upper limit. If at least one of results of the determinations is yes, the CPU  60  determines that a malfunction occurs, and proceeds to step S 111 . If both of them are no, the CPU  60  returns to step S 112  and repeat the steps. 
     In step S 117 , the CPU  60  starts the image forming process. 
     6. Effect of Illustrative Aspect 
     The printer  1  of this illustrative aspect can detect the improper exposure of the photosensitive drum  28  based on the comparison of the first inflowing current with the threshold. 
     Further, the inflowing current Ir is measured for the improper exposure detection while the constant current control, which regulates the transfer voltage Vt applied to the transfer roller  14  to a constant level, is deactivated. If the constant current control is activated, the current is quickly returned to the original level even when the inflowing current Ir is present. Therefore, the inflowing current Ir is not measured precisely. By measuring the inflowing current Ir while the constant current control is deactivated, variations in the current continue for a certain period of time. Thus, the inflowing current Ir is more easily measured (or detected). 
     The transfer current detection circuit  56  is used for measurement of the inflowing current Ir. The transfer current detection circuit  56  is included in the current control section (CPU  60 , transfer voltage supply circuit  55  and transfer current detection circuit  56 ) for the constant current control that regulates the transfer voltage Vt to the constant level. Namely, extra printer components are not required for the measurement of the inflowing current Ir and thus the number of parts of the printer  1  does not increase. 
     The difference between the first and the first inflowing currents is compared with the second threshold. Therefore, the improper exposure is reliably detected regardless of the environmental factors in the inflowing current measurement. 
     If the difference is lower than the third threshold, which is lower than the second threshold, a malfunction of the exposure unit is determined, that is, the exposure unit is not practically functioning. 
     If the improper exposure is detected, the LD  33  is controlled so as to increase the intensity of light emitted from the LD  33 . Therefore, an impact of the improper exposure can be reduced. 
     If the improper exposure is detected, the intensity of light emitted from the LD  33  is increased such that the difference between the first and the second inflowing currents is equal to or higher than the second threshold. If the difference is equal to or higher than the second threshold, the exposure is considered as proper. Therefore, an impact of the improper exposure can be reduced by increasing the intensity of light so that the difference is equal to or higher than the second threshold. 
     &lt;Another Illustrative Aspect&gt; 
     Next, another illustrative aspect of the present invention will be explained with reference to  FIG. 5 . 
     In this aspect, a cleaning section is added to the printer  1  of the illustrative aspect described above and other configurations are the same. The same printer components as those in the previous illustrative aspect are indicated by the same symbols and will not be explained. 
     The cleaning section includes cleaning rollers  65  and a cleaning voltage supply circuit (not shown). Each cleaning roller  65  is arranged in a location ahead of the corresponding transfer roller  14  and behind the corresponding charger  29  in the rotation direction of the photosensitive drum  28 . It is pressed against the transfer roller  14  by a pressing member (not shown). The cleaning voltage supply circuit is configured to apply a bias voltage to the cleaning roller  65 . After the transfer of an image onto the sheet  3  by the transfer roller  14  is complete, the bias voltages are applied to the cleaning roller  65 , and residues, such as paper and toner residues, on the photosensitive drum  28  are collected temporarily by the cleaning roller  65 . 
     The developing roller  25  is an example of the transfer unit that is not an object for the current measurement. The cleaning roller  65  is also an example of the transfer unit that is not an object for the current measurement. The transfer roller  14  is an example of the transfer unit that is an object for the current measurement. 
     When the CPU  60  measures the inflowing current Ir for the improper exposure based on a signal from the transfer current detection circuit  56 , the developing roller  25  is separated from the photosensitive drum  28  while keeping the cleaning roller  65  pressed against the photosensitive drum  28 . Because the cleaning roller  65  does not face the first area of the photosensitive drum  28  before the first area reaches the point where it faces the transfer roller  14 , it does not affect the accuracy of the inflowing current measurement. By keeping the cleaning roller  65  pressed against the developing roller  25 , a separation control mechanism for separating a transfer unit that is not an object for the current measurement from the photosensitive drum  28  can be simplified. 
     &lt;Other Illustrative Aspects&gt; 
     The present invention is not limited to the aspect explained in the above description made with reference to the drawings. The following aspects may be included in the technical scope of the present invention, for example. 
     (1) In the above aspect, the improper exposure is detected based on the inflowing current Ir flowing from the photosensitive drum  28  to the transfer roller  14 . However, it may be detected based on an inflowing current flowing from the photosensitive drum  28  to the developing roller  25 , that is, to a transfer unit that is an object for the inflowing current measurement. It may be also detected based on an inflowing current flowing from the photosensitive drum  28  to the cleaning roller  65 . In this case, the developing roller  25  and the transfer roller  14  are separated from the photosensitive drum  28  to restrict current flow between the photosensitive drum  28  and the feed parts that are not objects for the measurement, that is, the developing roller  25  and the transfer roller  14 . Thus, the value of the inflowing current can be accurately measured. 
     (2) In the above aspect, when the improper exposure is detected, the intensity of light emitted from the LD  33  is increased. However, the developing bias Vdev (the bias voltage) applied to the developing roller  25  may be varied instead of or in addition to the increase in the intensity of light emitted from the LD  33  so as to increase the amount of developer transported to the photosensitive drum  28 . 
     (3) In the above aspect, the difference between the first and the second inflowing currents is compared with the second threshold and whether the exposure is proper is determined based on the result of the comparison. However, it may be determined based on a result of comparison between the first inflowing current and the first threshold. In this case, the exposure is determined as improper if the first inflowing current is equal to or higher than the first threshold. If the exposure is determined as improper, the intensity of light is increased to maintain the inflowing current lower than the first threshold. 
     (4) In the above aspect, the current flowing from the area of the photosensitive drum  28  that is not exposed (i.e., the second flowing current) is measured first and then the current flowing from the area of the photosensitive drum  28  that needs to be exposed (i.e., the first flowing current) is measured. However, the first flowing current may be measured first and then the second flowing current may be measured. In this case, the first area of the photosensitive drum  28  returns to a point where the LD  33  charges the photosensitive drum  28  (point R in  FIG. 2 ) faster in comparison to the case that current flowing from the second area is measured first. Therefore, exposed points on the photosensitive drum  28 , where electrical potential is lower than unexposed points on the photosensitive drum  28 , can be recovered faster and thus a start of the image forming process is not interfered. 
     (5) In the above aspect, the printer  1  enters improper exposure detection mode when it is turned on. However, it may be configure to enter improper exposure detection mode at a certain interval under the condition that the image forming process is not performed. Alternatively, it may be configured to enter improper exposure detection mode upon a request input from the outside. 
     (6) In the above aspect, a color laser printer is used as an example of an image forming apparatus. However, an image forming apparatus of the present invention is not limited to a color laser printer, but rather may be a monochrome laser printer, a color LED printer or a monochrome LED printer. Further, it may be a multi-function machine having a facsimile function, a copier function, and the like.