Abstract:
In the image forming apparatus including a developing device that develops an electrostatic latent image on an image carrying member by using toner, a first toner empty stop condition and a second toner empty stop condition for prohibiting printing because of a lack of the remaining amount in the toner are provided. The first toner empty stop condition and the second toner empty stop condition are switched in response to the state of the device.

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application is a national stage application under 35 USC 371 of International Application No. PCT/JP2009/065499, filed Sep. 4, 2009, which claims the priority of Japanese Application No. 2008-240362, filed Sep. 19, 2008, the contents of which prior applications are incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to an image forming apparatus of an electro photographic system such as a copy machine, a printer, and a facsimile apparatus, particularly, relates to an image forming apparatus that stops a printing detecting a low level of remaining amount in consumable supplies to allow a user to carry out replacement of the consumable supplies. 
     BACKGROUND OF THE INVENTION 
     In the image forming apparatus of the electro photographic system, an electrostatic latent image formed on an image carrying member is actualized with a toner by a developing device to be made into a toner image, the toner image is directly transferred on a recording medium or is transferred on the recording medium via an intermediate transcription member, and finally, this toner image is discharged to the outside together with the recording medium. Therefore, the toner is wasted as the number of printed images is increased. Detecting a toner concentration image transferred on the recording medium or the intermediate transcription member by a toner concentration sensor, if the toner concentration is lowered, the toner is replenished to the developing device from a toner hopper via a sub hopper. 
     When the toner is consumed, the toner state is changed as follows: 
     (1) A toner cartridge becomes vacant. 
     This state is judged by a replenishing amount counter of the toner cartridge. 
     (2) The sub hopper becomes vacant. 
     This state is judged by a remaining amount sensor of the sub hopper. 
     (3) The toner concentration on the recording medium or the intermediate transcription member is lowered. 
     This state is judged by the toner concentration sensor. 
     As a toner empty stop condition to prohibit printing due to toner empty, conventionally, there are two conditions, and each of them has a merit and a demerit, respectively. 
     [Condition 1] Empty Stop Due to Lowering of Toner Concentration (Sub Hopper is Vacant) 
     It is possible to extend interruption of printing. However, when the toner consumption is much, the toner concentration is lowered beyond the scope of assumption. In addition, upon release of empty, replenishing of the sub hopper does not catch up with the toner consumption caused by printing, and the toner concentration is further lowered, leading to a possibility that a trouble caused by abnormal lowering in the toner concentration is generated. In order to avoid this, it is necessary to delay restart of a job by replenishing the toner by compulsion. 
     [Condition 2] Empty Stop Due to Empty of Sub Hopper (Toner Concentration is Normal) 
     It is possible to avoid a risk of lowering of the toner concentration. However, even though there is a possibility of printing yet, a print job is interrupted. 
     In Patent Document 1, it is proposed to output a detection signal with respect to the remaining amount in a developer in a developer container of a cartridge and a caution signal with respect to the state such that the remaining amount in the developer within the cartridge is enough; the remaining amount in the developer has a quantity so as to affect an image quality; and the remaining amount in the developer damages the image quality or the like. In addition, in Patent Document 2, it is proposed to store a parameter for detecting the toner remaining amount that is different for each color to a nonvolatile memory of each cartridge, respectively, and detect the toner remaining amount that is different for each color. However, in any patent document, no condition relating to the toner empty stop is described.
         Patent Document 1: JP Laid-Open Patent Publication No. 2002-268476   Patent Document 2: JP Laid-Open Patent Publication No. 2004-45447       

     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a user friendly image forming apparatus that can carry out the toner empty stop at a right time without lowering a concentration of a toner and can maintain an image quality. 
     In order to solve the problems, a first solution is that in an image forming apparatus comprising a developing device that develops an electrostatic latent image on an image carrying member by using a toner, wherein: 
     a first toner empty stop condition and a second toner empty stop condition for prohibiting printing because the remaining amount in the toner is vacant are provided; 
     the first toner empty stop condition and the second toner empty stop condition are switched in response to the state of the device. 
     In a second solution, the first toner empty stop condition and the second toner empty stop condition are judged by different toner empty stop judgment units, respectively. 
     In a third solution, the second toner empty stop condition is judged after the first toner empty stop condition is judged. 
     In a fourth solution, the image forming apparatus further including: 
     a toner cartridge that contains toner and is replaceably fitted to a body of the image forming apparatus; 
     a sub hopper that is placed between the toner cartridge and the developing device to deliver the toner of the toner cartridge to the developing device; 
     a unit that replenishes the toner from the toner cartridge to the sub hopper; 
     a unit that replenishes the toner from the sub hopper to the developing device; 
     a sub hopper remaining amount detection unit that detects a toner remaining amount in the sub hopper; and 
     a toner concentration sensor that detects the toner concentration in the developing device; wherein: 
     the first toner empty stop condition is judged by the sub hopper remaining amount detection unit depending on the fact that the sub hopper is vacant; and 
     the second toner empty stop condition is judged by the toner concentration detection unit after the sub hopper is vacant depending on the fact that the toner concentration in the developing device is not more than a predetermined value. 
     In a fifth solution, the state of the device when switching the toner empty stop condition between the first toner empty stop condition and the second toner empty stop condition is a color of the toner of the toner cartridge; in the case of the toners of colors of Y, M, and C, the empty stop is judged under the first toner empty stop condition; and 
     in the case of the toner of a color of K, the empty stop is judged under the second toner empty stop condition. 
     In a sixth solution, the state of the device when switching the toner empty stop condition between the first toner empty stop condition and the second toner empty stop condition is a life of the developing device; 
     in the case that the life of the developing device is in the initial stage, the empty stop is judged under the second toner empty stop condition; and 
     in the case that the life of the developing device is in the end stage, the empty stop is judged under the first toner empty stop condition. 
     In a seventh solution, the state of the device when switching the toner empty stop condition between the first toner empty stop condition and the second toner empty stop condition is a charged voltage Vg of the image carrying member or a developing bias voltage Vdc; 
     in the case that the charged voltage Vg or the developing bias voltage Vdc is larger than a predetermined value, under the first toner empty stop condition, the empty stop is judged; and 
     in the case that the charged voltage Vg or the developing bias voltage Vdc is smaller than a predetermined value, under the second toner empty stop condition, the empty stop is judged. 
     In an eighth solution, the state of the device when switching the toner empty stop condition between the first toner empty stop condition and the second toner empty stop condition is genuineness and non-genuineness of the toner cartridge; 
     in the case that the toner cartridge is a genuine one, under the second toner empty stop condition, the empty stop is judged; and 
     in the case that the toner cartridge is a non-genuine one, under the first toner empty stop condition, the empty stop is judged. 
     According to the present invention, since the prohibition of printing condition due to the toner empty is changed depending on the state of the device, it is possible to carry out the toner empty stop at a right time without lowering a concentration of a toner and maintain an image quality, and further, making the apparatus user friendly. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic structure diagram of an image forming apparatus according to the present embodiment. 
         FIG. 2  is a schematic perspective view of a toner cartridge and a sub hopper. 
         FIG. 3  is a schematic structure diagram of the sub hopper. 
         FIG. 4  is a structure diagram of a developing device. 
         FIG. 5  is a front view showing the arrangement of a door of the image forming apparatus. 
         FIG. 6  is a block diagram of toner replenishing control of the image forming apparatus. 
         FIG. 7  is a flow chart of toner empty detection control. 
         FIG. 8  is a flow chart of printing prohibition control upon a toner empty due to a color of a toner. 
         FIG. 9  is a flow chart of printing prohibition control upon the toner empty due to a life of the developing device. 
         FIG. 10  is a flow chart of printing prohibition control upon the toner empty due to Vg/Vdc. 
         FIG. 11  is a flow chart of printing prohibition control upon the toner empty due to genuineness/non-genuineness of a toner cartridge. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, embodiments according to the present invention will be described with reference to the drawings. 
       FIG. 1  is a schematic structure diagram of a color image forming apparatus according to the present invention. The image forming apparatus is formed by an image reading part  1  and an image forming part  2 . The image reading part  1  has a publicly-known configuration that scan a script image to convert it into the image data for reading. In the image forming part  2 , image forming units  3 Y,  3 M,  3 C, and  3 K (they are generically named as an image forming unit  3 ) that form toner images of respective colors, Y (yellow), M (magenta), C (cyan), and K (black) are arranged along an intermediate transfer belt  4 . 
     Each of the image forming units  3 Y,  3 M,  3 C, and  3 K is provided with an image carrying member  5  formed by a photo conductor drum that is arranged in adjacent to the intermediate transfer belt  4 , a charging device  6  that uniformly charges the surface of the image carrying member  5  to form a potential, an exposure device  7  that exposes the surface of the image carrying member  5  on the basis of an image signal to form an electrostatic latent image, and an developing device  8  that actualizes the electrostatic latent image by attaching the toner to the electrostatic latent image on the image carrying member  5  to form toner images of respective colors. 
     The intermediate transfer belt  4  is stretched between a driving roller  9  and a driven roller  10  and can run in an arrow direction. In the inside of the intermediate transfer belt  4 , primary transcription rollers  11  are arranged so as to be opposed to the image carrying members  5  of respective image forming units  3 Y,  3 M,  3 C, and  3 K via the intermediate transfer belt  4 , which form color toner images by transcribing the toner image on the image carrying members  5 . A secondary transcription roller  14  is arranged so as to be opposed to the driving roller  9  of the intermediate transfer belt  4  via the intermediate transfer belt  4 , which transfers a color toner image on the intermediate transfer belt  4  to a sheet P to be fed from a paper feeding unit  12  via a timing roller  13 . On a down stream side in a direction for feeding a sheet of the secondary transcription roller  14 , a fixing device  15  that fixes the color toner image transferred by the secondary transcription roller  14  is arranged. 
     Above the intermediate transfer belt  4 , toner cartridges  16 Y,  16 M,  16 C, and  16 K (they are generically named as a toner cartridge  16 ) and sub hoppers  17 Y,  17 M,  17 C, and  17 K (they are generically named as a sub hopper  17 ) are arranged corresponding to the respective image forming units  3 Y,  3 M,  3 C, and  3 K. 
     As shown in  FIG. 2 , the toner cartridge  16  has spiral springs  18  arranged in its inside so as to be capable of being rotatably driven and contains toners of respective colors therein. The spiral springs  18  of the toner cartridges  16 Y and  16 M for Y and M are rotatably driven by a first stepping motor  19  via a one-way clutch (not illustrated), and the spiral springs  18  of the toner cartridges  16 C and  16 K for C and K are rotatably driven by a second stepping motor  20  via a one-way clutch (not illustrated). If the first stepping motor  19  is normally rotated, the spiral spring  18  of the toner cartridge  16 Y for Y is rotated, and then, the spiral spring  18  of the toner cartridge M for M idly runs. If the first stepping motor  19  is inversely rotated, the spiral spring  18  of the toner cartridge  16 Y for Y idly runs, and the spiral spring  18  of the toner cartridge M for M is rotated. The same is applied to the second stepping motor  20 . In this way, by changing the first stepping motor  19  and the second stepping motor  20  between normal rotation and inverse rotation (hereinafter, they are generically named as a cartridge motor  21 ) to rotate the spiral springs  18  of the toner cartridges  16 Y,  16 M,  16 C, and  16 K for Y, M, C, and K, the toner within the toner cartridge  16  is moved in an arrow direction to be capable of being replenished to the sub hopper  17 . 
     As shown in  FIG. 3 , the sub hopper  17  has a spiral roller  22  and scraping paddles  23  provided in its inside, and they are configured so as to be rotated in conjunction with each other by a stepping motor  24  (hereinafter, referred to as a sub hopper motor). Above the sub hopper  17 , an inlet  25  for accepting the toner from the toner cartridges  16 Y,  16 M,  16 C, and  16 K is placed, and at the bottom of the sub hopper  17 , an outlet  26  for replenishing the toner to the developing device  8  of each color via a path (not illustrated) is placed. In the sub hopper  17 , a sub hopper remaining amount sensor  27  is placed, and the sub hopper  17  is configured so that the toner is scraped toward the sub hopper remaining amount sensor  27  by the scraping paddles  23 . The sub hopper remaining amount sensor  27  is a piezo sensor and, the sub hopper remaining amount sensor  27  detects a toner empty when it is not subjected to a pressure from the toner. The toner which is replenished from the toner cartridges  16 Y,  16 M,  16 C, and  16 K to the sub hopper  17  can be replenished to the developing device  8  by driving the sub hopper motor  24 . 
     As shown in  FIG. 4 , the developing device  8  has an agitating screw  29  and a conveyance screw  30  that convey the developer to the inside of a developer bottle  28  while agitating the developer, and a developer roller  31  that delivers the developer to be conveyed by the conveyance screw  30  to the image carrying members. Within the developer bottle  28 , a toner concentration sensor  32  of a binary developer that is composed of a toner and a carrier is provided. The developer around the toner concentration sensor  32  is agitated by a Mylar sheet  33  that is attached to the agitating screw  29 , a new developer flows around the toner concentration sensor  32  every time the agitating screw  29  is rotated one revolution (one ripple), causing a replacement of the developer. The toner concentration sensor  32  reads a magnetic permeability of iron contained in the carrier to output an analog signal. Accordingly, if an AD converted value (a resolution 10 bit) of the analog signal is large, the toner concentration is low. 
       FIG. 5  indicates a door in front of the image forming apparatus. A toner replenishing door  34  that is opened and closed when replacing the toner cartridges  16 Y,  16 M,  16 C, and  16 K is provided above the image forming apparatus, and a front door  35  that is opened and closed when checking the image forming units  3 Y,  3 M,  3 C, and  3 K is provided below the toner replenishing door  34 . The toner replenishing door  34  intends to replace the cartridges  16 Y,  16 M,  16 C, and  16 K, so that, even if they are opened and closed during printing, the print operation is not stopped. 
       FIG. 6  shows a block diagram of a control system of the image forming apparatus. The control system of the image forming apparatus is configured by an engine control part  41  to control each unit, an MFP controller  42  to control this engine control part  41 , and an operation panel  43 . As for the toner replenishing control, as input means, the toner replenishing door  34 , the front door  35 , the toner concentration sensor  32 , the sub hopper remaining amount sensor  27 , and a toner cartridge memory  44  are connected to the engine control part  41 ; and as output means, the cartridge motor  21 , the sub hopper motor  24 , a Vg/Vdc remote, a Vg/Vdc output setting, and the toner cartridge memory  44  are connected to the engine control part  41 . The engine control part  41  and the MFP controller  42  are connected with each other through communication means. From the MFP controller  42 , a print command is transmitted, and from the engine control part  41 , various kinds of empty information and the like are transmitted. The operation panel  43  is connected to the MFP controller  42  through communication to receive the input of the user and notify the user of various kinds of information. 
     Next, the operation of the image formation according to the image forming apparatus composed of the above-described configuration has been publicly known, and this operation is not directly related to the present invention per se, so that the explanation thereof is herein omitted and the toner replenishing operation will be described as follows: 
     1. Toner Replenishing Control 
     The toner replenishing control is divided into two main controls of the toner replenishing control from the toner cartridge  16  to the sub hopper  17 , and the toner replenishing control from the sub hopper  17  to the developing device  8 . 
     1.1 Toner Replenishing Control from Cartridge to Sub Hopper 
     In the toner replenishing control from the toner cartridge  16  to the sub hopper  17 , due to the output of the sub hopper remaining amount sensor  27  of the sub hopper  17 , the toner is replenished from the cartridge  16  to the sub hopper  17 . 
     In driving of the sub hopper motor  24 , detecting the sub hopper remaining amount sensor  27  in the sub hopper  17  with a period of 200 ms, and driving the cartridge motor  21  depending on its detection result, the toner is replenished from the toner cartridge  16  to the sub hopper  17 . 
     1.2 Toner Replenishing Control from Sub Hopper to Developing Device 
     In the toner replenishing control from the sub hopper  17  to the developing device  8 , the toner is replenished from the sub hopper  17  to the developing device  8  so that the construction of the toner in the developing device  8  (the ratio of the toner with respect to the developer made of the toner and the carrier) becomes a target ratio. 
     This toner replenishing control is divided into the replenishing amount decision control and the replenishing operation control. 
     1.2.1 Replenishing Amount Decision Control 
     Reading an output voltage in the toner concentration sensor  32  within the developing device  8  during rotation of the developer roller  31 , the toner concentration is calculated from a lookup table of the toner concentration with respect to this voltage. From the predetermined target toner concentration and the calculated toner concentration, the toner replenishing amount is decided from the lookup table. 
     1.2.2 Replenishing Operation Control 
     Driving the sub hopper motor  24  of the sub hopper  17  for each color by the toner replenishing amount that is decided by the replenishing amount decision control, the toner is replenished to the inside of the developing device  8 . The replenishing operation control includes the normal toner replenishing control during the printing operation and the compulsory toner replenishing control to be carried out by interruption of printing. In the normal toner replenishing control, the longest time capable of replenishing the toner is rate-controlled by the operational time of the developing device  8  for printing, so that it is not possible to raise the toner concentration to the target toner concentration during printing According to the compulsory toner replenishing control, the toner replenishing mechanism and the developing device  8  are driven until the toner concentration reaches the target toner concentration. 
     2. Toner Empty Detection Control 
     The toner empty state includes four states, namely, “normal”, “nearly empty”, “sub hopper empty”, and “toner empty”. 
     The “normal” is the state other than the following empty states, and this means the state that the toner is sufficiently filled in the toner cartridge  16  and the sub hopper  17 . 
     The “nearly empty” means the state that the toner within the toner cartridge  16  is less although the toner is filled in the sub hopper  17 . 
     The “sub hopper empty” means the state that the toners in the sub hopper  17  and the toner cartridge  16  are empty. 
     The “toner empty” means the state that the toner concentration is further lowered and this makes it impossible to continue printing. This is the state that the toner in the sub hoper  17  is lost further from the “sub hop empty”, the toner in the developing device  8  is also consumed, and the toner concentration is lowered. 
       FIG. 7  shows the toner empty detection control. 
     In step S 1 , upon driving of the cartridge motor  21 , the rotation amount of the cartridge motor  21  is accumulated to estimate the toner replenishing amount that is replenished from the toner cartridge  16  to the sub hopper  17 . If the toner replenishing amount is not more than a predetermined value, the toner empty state is defined as the “normal” state, and if the toner replenishing amount exceeds a predetermined value, the procedure moves to step S 2 . 
     In step S 2 , it is detected whether or not the empty is detected in succession in the predetermined number of times by means of the sub hopper remaining amount sensor in the sub hopper  17 . If the detection of the empty is not more than the predetermined number of times, the toner empty state is defined as the “nearly empty” state such that the toner remaining amount in the toner cartridge  16  is almost empty, and if the toner replenishing amount is detected exceeding the predetermined number of times, the procedure moves to step S 3 . 
     In step S 3 , it is detected by the sub hopper remaining amount sensor  32  whether or not the toner concentration in the developing device  8  being the value of the target toner concentration −1% is detected in thirty times in succession. If the number of times for detection of the target toner concentration is not more than 30, the toner empty state is defined as the “sub hopper empty”, and if the number of times for detection of the target toner concentration exceeds 30, the toner empty state is defined as the “toner empty”. 
     In the case of “nearly empty”, a warning that “toner is to be empty soon” is displayed on the operation panel  43 . 
     In the case of “sub hopper empty” or “toner empty”, a full warning that “the toner is lost” is displayed on the operation panel  43  and the printing is prohibited. 
     When the toner empty state is “normal” and “nearly empty”, the user is capable of replacing the toner cartridge  16  despite the state of ready and waiting. 
     When the toner state is in the “sub hopper empty” and the “toner empty”, the user can replace the toner cartridge  16  during wait, or during print interruption due to printing prohibition during the printing. 
     3. Printing Prohibition Control in Toner Empty 
     The state of the device to prohibit the printing because of the toner empty includes four states as follows: 
     (1) Colors of toner 
     (2) Life of developing device 
     (3) Vg/Dc 
     (4) Genuineness/non-genuineness of cartridge 
     Hereinafter, they will be explained in sequence. 
     3.1 Printing Prohibition Control in Toner Empty Due to Color of Toner 
     In the case of the toners of colors Y, M, and C, by means of the sub hopper remaining amount sensor  27  that are a first toner empty stop judgment unit, the toner empty stop is judged. If the concentrations of the toners Y, M, and C are lowered, the concentration of the color image becomes light and a color shade of a photographic image is changed. Therefore, it is preferable that the toner empty stop is early judged by the sub hopper remaining amount sensor  27  located on the upstream side of the developing device  8 . 
     In the case of the toner of a color K, the toner empty stop is judged by the toner concentration sensor  32  that is a second toner empty stop judgment unit. This reason is as follows. Small amount of lowering of the toner concentration can be allowed because many of the images according to the toner of the color K are character images. In addition, in the case of the toner empty in the toner of the color K, it is preferable that prohibition of printing is delayed as much as possible since not only printing of black and white images but also printing of color images is prohibited. 
       FIG. 8  is a flow chart showing the operation of changing the conditions of the printing prohibition due to the toner empty depending on a color of the toner. 
     In step S 11 , the print operation is started; and in step S 12 , the output of the sub hopper remaining amount sensor  27  is read. If the remaining amount in the sub hopper is available in step S 13 , the flow is ended, and if the remaining amount in the sub hopper is vacant, in step S 14 , the color of the toner is judged. If the color of the toner is Y, M, and C, in step S 15 , the printing is prohibited by the toner empty. If the color of the toner is K, in step S 16 , the output of the toner concentration sensor  32  is read. If the toner concentration exceeds 7%, the flow is ended, and if the toner concentration is not more than 7%, in step S 17 , printing is prohibited due to the toner empty in step S 17 . 
     3.2 Printing Prohibition Control in Toner Empty Due to Life of Developing Device 
     The life of the developing device  8  is to be decided by the rotational agitation time of the agitating screw  29 . This is because, if the rotational agitation time is increased, the developer made of the toner and the carrier (particularly, the carrier) is deteriorated, so that an ability to charge the toner of the carrier is lowered. Therefore, in the image forming apparatus, a life counter for counting the life of the developing device  8  is provided. 
     In the case that the life value of the developing device  8  is in an initial stage, the toner empty stop is judged by the toner concentration sensor  32  that is a second toner empty stop judgment unit. Normally, the toner concentration is controlled to be a concentration with which charging of the toner has the highest stability. In the initial stage, to delay the prohibition of printing has no problem because variation in the toner concentration is small. 
     In the case that the life value of the developing device  8  is in an end stage, the toner empty stop is judged by the sub hopper remaining amount sensor  27  that is a first toner empty stop judgment unit. In the end stage, variation in the toner concentration easily leads to fog and adhesion of the carrier. Therefore, in order to prevent change in the toner concentration as much as possible, the toner empty stop is judged early by the sub hopper remaining amount sensor  27  located on the upstream side of the developing device. 
       FIG. 9  is a flow chart showing the operation of changing the condition of the printing prohibition due to the toner empty depending on the life of the developing device  8 . 
     In step S 21 , the print operation is started, and in step S 22 , the life counter is incremented. The life of the developing device  8  is decided by the agitation time as described above, however, it is assumed that the life of the developing device  8  is decided by the number of sheet to be printed, and the life counter is incremented for each sheet. In step S 23 , the output of the sub hopper remaining amount sensor  27  is read. In step S 24 , if the remaining amount in the sub hopper is available, the flow is ended, and if the remaining amount in the sub hopper is vacant, the count value of the life counter is judged in step S 25 . If the count value is not more than 50K, the printing is prohibited by the toner empty in step S 26 . If the count value is less than 50K, in step S 27 , the output of the toner concentration detection sensor is read. If the toner concentration exceeds 7%, the flow is ended, and if the toner concentration is not more than 7%, in step S 29 , the printing is prohibited by the toner empty. 
     3.3 Printing Prohibition Control in Toner Empty Due to Vg/Vdc 
     It is necessary for a charged voltage (Vg) of the image carrying member  5  formed by the photo conductor drum and a developing bias voltage (Vdc) of the developer roller  31  to increase when the life of the developing device  8  is near the end as well as when the environment of usage and the LD light volume are deteriorated. For example, under a low temperature and a low degree of humidity, the toner charged amount is increased and a developing efficiency is lowered. Therefore, depending on the image stabilization control, the charged voltage (Vg) and the developing bias voltage (Vdc) are increased. As a result, when Vg and Vdc are high, lowering of the toner concentration should be prevented, which become a factor for further increasing Vg and Vdc. 
     That is to say, in the case that Vg and Vdc are increased than predetermined values, by means of the sub hopper remaining amount sensor  27  that is the first toner empty stop judgment unit, the toner empty stop is judged. When Vg and Vdc are high, in order to prevent lowering of the toner concentration, it is preferable that the toner empty stop is early judged by the sub hopper remaining amount sensor  27  located on the upstream side of the developing device. 
     When Vg and Vdc are lower than predetermined values, the toner empty stop is judged by the toner concentration sensor  32  that is the second toner empty stop judgment unit. Change in the toner concentration can be allowed to some extent, so that there is no problem in delaying in prohibition of printing. 
       FIG. 10  is a flow chart showing the operation of changing the conditions of the printing prohibition by the toner empty by Vdc. 
     In step S 31 , the print operation is started, and in step S 32 , the output of the sub hopper remaining amount sensor  27  is read. If the remaining amount in the sub hopper is available in step S 33 , the flow is ended, and if the remaining amount in the sub hopper is vacant, in step S 34 , the Vdc is judged. If Vdc is in the range of −600 V to −900 V, in step S 35 , printing is prohibited by the toner empty. If Vdc is in the range of −599 V to −300V, in step S 36 , the output of the toner concentration sensor  32  is read. If the toner concentration exceeds 7%, the flow is ended, and if the toner concentration is not more than 7%, in step S 37 , the printing is prohibited by the toner empty. 
     3.4 Printing Prohibition Control in Toner Empty Due to Genuineness/Non-Genuineness of Cartridge 
     In the case that the toner cartridge  16  is a genuine one, the toner empty stop is judged by the toner concentration sensor  32  that is the second toner empty stop judgment unit. The genuine toner cartridge  16  has the toner with an excellent quality and can allow change in the toner concentration to some extent, so that there is no problem in delaying in prohibition of printing. 
     In the case that the toner cartridge  16  is a non-genuine one, the toner empty stop is judged by the sub hopper remaining amount sensor  27  that is the first toner empty stop judgment unit. When the toner cartridge  16  is a non-genuine one, the quality of the toner is poorer than that of the genuine one in many cases. Therefore, in order to prevent lowering of the toner concentration, it is preferable that the toner empty stop is early judged by the sub hopper remaining amount sensor  27  located on the upstream side of the developing device. 
       FIG. 11  is a flow chart showing the operation of changing the conditions of the printing prohibition due to the toner empty due to genuineness/non-genuineness of the toner cartridge  16 . 
     In step S 41 , the print operation is started, and in step S 42 , the output of the sub hopper remaining amount sensor  27  is read. In step S 43 , if the remaining amount in the sub hopper is available, the flow is ended, and if the remaining amount in the sub hopper is vacant, it is judged if the toner cartridge  16  is genuine or non-genuine in step S 44 . Whether or not the toner cartridge is a genuine one or a non-genuine one can be judged by reading the information of the toner cartridge memory  44  provided in the cartridge  16 . If the toner cartridge is a non-genuine one, in step S 35 , the printing is prohibited due to the toner empty. If the toner cartridge is a genuine one, in step S 36 , the output of the toner concentration detection sensor is read. If the toner concentration exceeds 7%, the flow is ended, and if the toner concentration is not more than 7%, the printing is prohibited due to the toner empty in step S 37 .