Patent Publication Number: US-7903986-B2

Title: Reuse method and a reusable device for an image forming apparatus having a first process linear velocity and a second image processing apparatus having a second process linear velocity

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is based on and claims priority from Japanese Patent Application No. 2008-122295, filed on May 8, 2008 in the Japan Patent Office, the entire contents of which are hereby incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Exemplary aspects of the present invention relate to a reuse method and an image forming apparatus, and more particularly, to a reuse method and an image forming apparatus for efficiently reusing a reusable device and a sensor in another image forming apparatus. 
     2. Description of the Related Art 
     Related-art image forming apparatuses, such as copiers, facsimile machines, printers, and multifunction devices having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording material (e.g., a sheet) based on image data using electrophotography. 
     For example, when an electrostatic latent image is formed on a surface of a photoconductor, serving as an image carrier, a development device develops the electrostatic latent image with a developer (e.g., a two-component developer) into a visible toner image. The two-component developer includes toner and carrier. The development device stirs the toner and the carrier to charge them by friction. Then, when the charged toner adheres to an electrostatic latent image formed on an image carrier, a toner image is formed as a visible image. 
     In order to form a toner image on the image carrier, as the development device supplies toner to the image carrier, the amount of toner remaining in the development device decreases, so that a ratio between the amount of toner and the amount of carrier in the developer changes from an original state. However, for good imaging quality, it is important to maintain a constant ratio between the amount of toner and the amount of carrier, which ratio is also hereinafter referred to as toner density. Therefore, one related-art image forming apparatus includes a toner density sensor to monitor the toner density. When the toner density sensor detects that the toner density falls below a threshold density, fresh toner is supplied to the development device so as to maintain a predetermined toner density. 
     The toner density sensor can be a magnetic sensor, which detects changes in toner density by detecting changes in magnetic permeability of the developer. However, since magnetic sensors in general tend to be highly sensitive, and tend to be affected by errors in the manufacture of components of the sensor and the like. Consequently, each magnetic sensor outputs a slightly different reading from any other, that is, handles the relation between toner density and output voltage differently. 
     In order to prevent such variations in accuracy of the magnetic sensor, the image forming apparatus performs an initial adjustment of a control voltage of the magnetic sensor before use of the development device. The new development device initially stores developer having a predetermined toner density of 5%, for example. While the development device stirs the developer, the magnetic sensor detects toner density. The image forming apparatus adjusts the control voltage of the magnetic sensor such that the output voltage of the magnetic sensor becomes a voltage of 3 V, for example, when the predetermined toner density is 5%. Having thus calibrated the relation between the toner density and the output voltage, thereafter, fresh toner is added to the development device to increase the toner density to, for example, 7%, or a level that is appropriate for good image formation. 
     The new development device initially has a toner density of 5%, that is, lower than the 7% appropriate for image formation, because typically toner stored in the development device at the beginning of use is not electrically charged and thus easily scatters when the developer is stirred. As the amount of toner stored in the development device increases, the toner density also increases. Therefore, the initial toner density in the new development device is purposely set low in advance, thereby reducing scattering of toner in initial stirring of the developer. Then, the toner is charged by stirring, and toner density is increased by adding more toner. 
     This matter of toner density and its control becomes important when it comes to attempting to recycle components of the image forming apparatus. Such recycling first requires a brief discussion of the structure of a typical image forming apparatus, which now follows. 
     Typically, related-art image forming apparatuses using electrophotography include a photoconductor carrying a toner image, a charger charging a surface of the photoconductor, an exposure device exposing the charged surface of the photoconductor to form an electrostatic latent image, and a development device supplying toner to the electrostatic latent image formed on the surface of the photoconductor to form a toner image thereon. 
     Each of the above devices has a different service life from any other. Thus, for example, the photoconductor has a shorter service life than that of the development device. Therefore, when the photoconductor reaches the end of its life, the development device and the toner density sensor can still be used in another image forming apparatus. 
     However, as any given image forming apparatus has a process linear velocity different from that of any other image forming apparatus, the calibration of the toner density sensor for one development device, that is, the adjustment of the relation between the toner density sensor and the output voltage determined at the process linear velocity of the image forming apparatus which has used the development device and the toner density sensor, cannot usually be directly applied to another image forming apparatus without some adjustment. In other words, the toner density sensor needs to be calibrated again to set the correct, predetermined relation between the toner density and the output voltage for any given development device of any given image forming apparatus. 
     However, in order to adjust the control voltage of the toner density sensor, the toner density in the development device needs to be precisely known. Since the development device has already been used, the toner density in the development device differs from the initial toner density (5%), and it is difficult to know an exact toner density in the development device. Therefore, when the development device and the toner density sensor are reused, the toner density sensor cannot precisely detect the toner density. 
     Accordingly, there is a need for a technology capable of providing a method of reusing a development device and a toner density sensor as described above. 
     BRIEF SUMMARY OF THE INVENTION 
     This specification describes a reuse method according to illustrative embodiments of the present invention. In one illustrative embodiment of the present invention, the reuse method reuses a reusable device and a sensor of a first image forming apparatus having a first process linear velocity in a second image forming apparatus having at least one second process linear velocity different from the first process linear velocity. The reuse method includes installing the reusable device and the sensor in the first image forming apparatus, measuring output of the sensor at the second process linear velocity of the second image forming apparatus when the first image forming apparatus switches from the first process linear velocity to the second process linear velocity of the second image forming apparatus in an initial state before starting to use the reusable device, storing information on the output of the sensor at the second process linear velocity of the second image forming apparatus, removing the reusable device and the sensor from the first image forming apparatus and installing the reusable device and the sensor in the second image forming apparatus, reading the stored information, and adjusting the output of the sensor to correspond to the second process linear velocity of the second image forming apparatus based on the read information. 
     This specification further describes an image forming apparatus according to illustrative embodiments of the present invention. In a further illustrative embodiment of the present invention, the image forming apparatus switches from a first process liner velocity to at least one second process linear velocity, and includes a reusable device, a sensor, a measurement device, and a storage device. The sensor detects a state of the reusable device at a first process linear velocity. The measurement device measures output of the sensor at least one second process linear velocity in an initial state before starting to use the reusable device. The storage device stores information on the output of the sensor at the second process linear velocity in the initial state. 
     This specification further describes an image forming apparatus according to illustrative embodiments of the present invention. In a further illustrative embodiment of the present invention, the image forming apparatus operates at least one process linear velocity, and includes a reusable device, a sensor, a storage device, a reader, and an adjuster. The reusable device is installed in the image forming apparatus for reuse. The sensor detects a state of the reusable device. The storage device stores information on output of the sensor at the process linear velocity in an initial state before starting to use the reusable device. The reader reads the information stored by the storage device. The adjuster adjusts the output of the sensor to correspond to the process linear velocity based on the information read by the reader. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the invention and the many attendant advantages thereof will be more readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a schematic view of an image forming apparatus according to an illustrative embodiment of the present invention; 
         FIG. 2  is a schematic sectional view of the image forming device  1 Y included in the image forming apparatus shown in  FIG. 1 ; 
         FIG. 3  is a circuit diagram of a toner density sensor included in the image forming device  1 Y shown in  FIG. 2 ; 
         FIG. 4  is a schematic diagram of a source circuit included in the image forming apparatus shown in  FIG. 1 ; 
         FIG. 5A  is a graph of a waveform output from an oscillator included in the toner density sensor shown in  FIG. 2 ; 
         FIG. 5B  is a graph of a waveform output from a resonator circuit included in the toner density sensor shown in  FIG. 2 ; 
         FIG. 5C  is a graph of a waveform output from an inverting amplifier of a phase comparator included in the toner density sensor shown in  FIG. 2 ; 
         FIG. 5D  is a graph of a waveform output from a comparator of the phase comparator included in the toner density sensor shown in  FIG. 2 ; 
         FIG. 5E  is a graph of a waveform output from a smoothing circuit included in the toner density sensor shown in  FIG. 2 ; 
         FIG. 6  is a schematic block diagram of a first image forming apparatus including the development device and the toner density sensor shown in  FIG. 2 ; 
         FIG. 7A  is a schematic block diagram of a second image forming apparatus before installing the development device and the toner density sensor shown in  FIG. 2 ; 
         FIG. 7B  is another schematic block diagram of the second image forming apparatus mounted with the development device and the toner density sensor shown in  FIG. 2   
         FIG. 8  is a schematic block diagram of a second image forming apparatus according to another example embodiment; 
         FIG. 9  is a schematic block diagram of a first image forming apparatus according to another example embodiment; 
         FIG. 11A  is flowchart of a reuse method using the first image forming apparatus shown in  FIG. 6  and the second image forming apparatus shown in  FIG. 7A ; 
         FIG. 10B  is a flowchart of succeeding processes of the reuse method shown in  FIG. 10A ; 
         FIG. 11A  is a flowchart of a reuse method using the first image forming apparatus shown in  FIG. 6  and the second image forming apparatus shown in  FIG. 8 ; 
         FIG. 11B  is a flowchart of succeeding processes of the reuse method shown in  FIG. 11A ; 
         FIG. 12A  is a flowchart of a reuse method using the first image forming apparatus shown in  FIG. 9  and the second image forming apparatus shown in  FIG. 7A ; 
         FIG. 12B  is a flowchart of succeeding processes of the reuse method shown in  FIG. 12A ; and 
         FIG. 13  is a graph illustrating a relation between a process linear velocity and an output voltage of the toner density sensor shown in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In describing illustrative embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this specification is not intended to be limited to the specific terminology so selected, and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner and achieve a similar result. 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, in particular to  FIG. 1 , an image forming apparatus  200  according to an illustrative embodiment of the present invention is described. 
       FIG. 1  is a schematic view of the image forming apparatus  200 . The image forming apparatus  200  includes image forming devices  1 Y,  1 M,  1 C, and  1 K, an exposure device  8 , a transfer conveyance belt device  9 , a feeding device  13 , a fixing device  18 , discharge rollers  20 , a discharge device  21 , and a controller  300 . The image forming devices  1 Y,  1 M,  1 C, and  1 K include photoconductors  2 Y,  2 M,  2 C, and  2 K, respectively. The transfer conveyance belt device  9  includes a conveyance belt  11 , transfer bias rollers  12 Y,  12 M,  12 C, and  12 K, a belt cleaner  19 , and a pair of registration rollers  17 . The feeding device  13  includes a paper tray  14 , a separation roller  15 , and a pair of feed rollers  16 . The fixing device  18  includes a fixing roller  18 A, a heating roller  18 B, a fixing belt  18 C, and a pressing roller  18 D. 
     The image forming apparatus  200  may be a copier, a facsimile machine, a printer, a plotter, a multifunction printer having at least one of copying, printing, scanning, plotter, and facsimile functions, or the like. According to this illustrative embodiment, the image forming apparatus  200  forms a full color toner image by superimposing yellow, magenta, cyan, and black toner images on each other on the conveyance belt  11 . However, it is to be noted that the image forming apparatus  200  is not limited to the full color image forming apparatus and may form a color and/or monochrome image with other structure. 
     The image forming devices  1 Y,  1 M,  1 C, and  1 K form yellow, magenta, cyan, and black toner images, respectively, with developer (e.g., toner) corresponding to color separation components of a color image. 
     The image forming devices  1 Y,  1 M,  1 C, and  1 K have the same structure, except that they store different color toner. 
     The exposure device  8  is provided above the image forming devices  1 Y,  1 M,  1 C, and  1 K, and forms an electrostatic latent image on each surface of the photoconductors  2 Y,  2 M,  2 C, and  2 K. The transfer conveyance belt device  9  is provided below the image forming devices  1 Y,  1 M,  1 C, and  1 K. 
     The endless conveyance belt  11  is wrapped around a plurality of rollers including a driving roller, a driven roller, and the like. The transfer bias rollers  12 Y,  12 M,  12 C, and  12 K oppose the photoconductors  2 Y,  2 M,  2 C, and  2 K via the conveyance belt  11  to form respective transfer nips therebetween. 
     The feeding device  13  is provided in a lower portion of the image forming apparatus  200 . The paper tray  14  stores recording materials P (e.g., print sheets, OHP (overhead projector) films, or the like). The separation roller  15  separates one sheet of recording material P from other recording materials P stored in the paper tray  14 . The feed roller  16  feeds the recording material P separated by the separation roller  15 . 
     The pair of registration rollers  17  is provided below and to the right of the transfer conveyance belt device  9 , and temporarily stops the conveyed recording material P. The fixing device  18  is provided above and to the left of the transfer conveyance belt device  9 . The fixing belt  18 C is wrapped around the fixing roller  18 A and the heating roller  18 B. The pressing roller  18 D opposes and presses the fixing roller  18 A to form a fixing nip therebetween. 
     The discharge roller  20  and the discharge device  21  are provided in an upper portion of the image forming apparatus  200 . The discharge roller  20  discharges a recording material P to the outside of the image forming apparatus  200 . The discharge device  21  stores the recording material P discharged by the discharge roller  20 . 
     Referring to  FIG. 2 , a description is now given of a structure of the image forming device  1 Y. It is to be noted that the image forming devices  1 M,  1 C, and  1 K have a structure equivalent to that of the image forming device  1 Y. 
       FIG. 2  is a schematic sectional view of the image forming device  1 Y. The image forming device  1 Y includes a process unit  10 Y. The process unit  10 Y includes the photoconductor  2 Y, a charging roller  3 Y, a development device  4 Y, a cleaner  5 Y, and a discharge lamp  6 Y. The development device  4 Y includes a development case  40 , a development roller  41 , a doctor blade  42 , a first conveyance screw  43 , a second conveyance screw  44 , a pump  45 , a toner density sensor  7 Y, and a connector  28 . The cleaner  5 Y includes a cleaning blade  50  and a cleaning brush roller  51 . 
     The process unit  10 Y is detachably attached to a body of the image forming apparatus  200 . The photoconductor  2 Y, serving as an image carrier, carries a toner image. The charging roller  3 Y charges the photoconductor  2 Y. The development device  4 Y supplies yellow toner to the photoconductor  2 Y. The cleaner  5 Y cleans a surface of the photoconductor  2 Y. The discharge lamp  6 Y discharges the photoconductor  2 Y. 
     The development case  40  stores developer including toner and carrier. The development roller  41  carries the developer. The doctor blade  42  controls thickness of the developer carried by the development roller  41  so as to maintain a uniform thickness of the developer. The toner density sensor  7 Y is provided in the development case  40 , and detects density of the yellow toner. For example, the toner density sensor  7 Y is a magnetic sensor for detecting a change in magnetic permeability of the developer. 
     Operation of the image forming apparatus  200  is described with reference to  FIGS. 1 and 2 . 
     When the charging roller  3 Y depicted in  FIG. 2  uniformly charges the surface of the photoconductor  2 Y to a high electrical potential, the exposure device  8 Y depicted in  FIG. 1  emits a laser beam L that is directed onto the surface of the photoconductor  2 Y based on image data, so that electrical potential of the radiated portion of the surface of the photoconductor  2 Y decreases, thereby forming an electrostatic latent image thereon. 
     As illustrated in  FIG. 2 , when the first conveyance screw  43  and the second conveyance screw  44  of the development device  4 Y stir and convey the developer, the developer is charged by friction, so that the charged developer is carried on a surface of the rotating development roller  41 . Then, the doctor blade  42  equalizes the thickness of the developer carried by the development roller  41 . Thereafter, yellow toner carried by the development roller  41  adheres to the electrostatic latent image formed on the photoconductor  2 Y at a development area in which the development roller  41  opposes the photoconductor  2 Y, thereby forming a visible yellow toner image on the surface of the photoconductor  2 Y. As with the image forming device  1 Y, the image forming devices  1 M,  1 C, and  1 K form magenta, cyan, and black toner images on the photoconductors  2 M,  2 C, and  2 K, respectively. After the yellow toner image is transferred onto a transfer material P, the cleaner  5 Y cleans the surface of the photoconductor  2 Y, and the discharge lamp  6 Y discharges the surface of the photoconductor  2 Y. 
     The separation roller  15  of the feeding device  13  depicted in  FIG. 1  rotates to separate one sheet of recording material P from other recording materials P stored in the paper tray  14 . When the feed roller  16  feeds the separated recording material P to the pair of registration rollers  17 , the pair of registration rollers  17  stops the recording material P. 
     After the yellow toner image is carried on the surface of the photoconductor  2 Y, the pair of registration rollers  17  resumes rotating to feed the recording material P to the conveyance belt  11 . As the conveyance belt  11  rotates, the recording material P is conveyed to the transfer nip formed between the transfer bias roller  12 Y and the photoconductor  2 Y carrying the yellow toner image. The transfer bias roller  12 Y is supplied with a transfer bias at the transfer nip, thereby electrostatically transferring the yellow toner image formed on the photoconductor  2 Y to the recording material P. 
     Similarly, the magenta, cyan, and black toner images formed by the image forming devices  1 M,  1 C, and  1 K are transferred and superimposed on the recording material P. 
     When the recording material P bearing the respective color toner images is conveyed to the fixing device  18  depicted in  FIG. 1 , the recording material P is sandwiched between the fixing roller  18 A and the pressing roller  18 D and supplied with heat and pressure, thereby fixing a full color toner image on the recording material P. Then, the discharge rollers  20  discharge the recording material P to the discharge device  21 . After toner image formation, when the toner density sensor  7 Y, serving as a component state sensor, detects that the toner density decreases to less than a predetermined density, fresh yellow toner is supplied to the development device  4 Y. 
     Referring to  FIG. 3 , a description is now given of a circuit configuration of the toner density sensor  7 Y. 
       FIG. 3  is a circuit diagram of the toner density sensor  7 Y. As illustrated in  FIG. 3 , the toner density sensor  7 Y includes an oscillator  100 , a resonator circuit  110 , a phase comparator  120 , a smoothing circuit  130 , an amplifier  140 , and a memory chip (an IC (integrated circuit) chip)  150 . The oscillator  100  includes an oscillator element  101 . The resonator circuit  110  includes a resistor R 3 , a first coil L 1 , a second coil L 2 , and condensers C 1 , C 2 , and C 3 . The phase comparator  120  includes an inverting amplifier IC 2 - 2  and a comparator IC 2 - 3 . The smoothing circuit  130  includes an Op-Amp (operational amplifier) IC 1 - 1 . The amplifier  140  includes an Op-Amp IC 1 - 2 . 
     The memory chip  150  is provided on the same substrate as that of the toner density sensor  7 Y. The memory chip  150  is a nonvolatile memory capable of storing information when not powered. The memory chip  150  stores a production lot and usage of a component (the photoconductor  2 Y, the development device  4 Y, the toner density sensor  7 Y, or the like) installed in the process unit  10 Y depicted in  FIG. 2 . The memory chip  150  updates such information by communicating with the image forming apparatus  200  via the connector  28  depicted in  FIG. 2 . Alternatively, the memory chip  150  may include an antenna for wirelessly transmitting and receiving information. 
     Referring to  FIGS. 4 and 5A ,  5 B,  5 C,  5 D, and  5 E, a description is now given of a circuit for supplying a driving source to each circuit of the toner density sensor  7 Y.  FIG. 4  is a schematic diagram of the source circuit. The toner density sensor  7 Y further includes a step-down circuit  170 . The image forming apparatus  200  further includes a driving source supplier  160 .  FIGS. 5A ,  5 B,  5 C,  5 D, and  5 E illustrate respective waveforms of a voltage output from the respective circuits. 
     Before the driving source supplier  160  supplies a voltage of about 12 V to each circuit via the connector  28 , the step-down circuit  170  decreases the voltage of about 12 V down to about 5 V to be supplied to the oscillator  100 , the phase comparator  120 , and the memory chip  150 , respectively, whereas a voltage of about 12 V is supplied to the Op-Amp IC 1 - 1  of the smoothing circuit  130  and the Op-Amp IC 1 - 2  of the amplifier  140 , respectively. 
     The oscillator  100  oscillates at a frequency of about 4 MHz using the oscillator element  101  depicted in  FIG. 3  made of crystal, ceramics, or the like, and is supplied with the voltage of about 5 V decreased by the step-down circuit  170 . The oscillator  100  converts the voltage of about 5 V into a voltage V 1  having a rectangular waveform of about 4 MHz, as illustrated in  FIG. 5A , to be output to the resonator circuit  110  depicted in  FIG. 3 .  FIG. 5A  illustrates a waveform of the output voltage V 1  from the oscillator  100  to the resonator circuit  110 . 
     As illustrated in  FIG. 3 , the resistor R 3  and the first coil L 1  form a first resonator circuit. The second coil L 2 , which forms a second resonator circuit, is combined with the first coil L 1  with a magnetic binding coefficient k. Since the condensers C 1 , C 2 , and C 3  are shared between the first resonator circuit and the second resonator circuit, the first resonator circuit and the second resonator circuit have the same resonance characteristics. The second coil L 2  opposes the first coil L 1  to form a resonance point. The output voltage V 1  from the oscillator  100  is input to the first coil L 1  via the resistor R 3 , thereby increasing input impedance at the resonance point. In addition, the resistor R 3  prevents unstable oscillation of the oscillator  100  due to the influence of the resonance circuit  110 . It is to be noted that self-inductances of the first coil L 1  and the second coil L 2  are 8.15 μH. 
       FIG. 5B  illustrates waveforms of a voltage V 2 . The second coil L 2  outputs the voltage V 2  canceling out the voltage V 1  input to the first coil L 1  at the resonance point. Due to magnetic permeability of a developer  111  provided in the vicinity of the first coil L 1  and the second coil L 2 , mutual inductance between the first coil L 1  and the second coil L 2  varies, so that the output voltage V 2  output from the second coil L 2  varies. 
     The magnetic permeability of the developer  111  varies according to a mixture ratio between magnetic carrier and non-magnetic toner. More specifically, when toner density is low, the magnetic permeability of the developer  111  increases, and when toner density is high, the magnetic permeability of the developer  111  decreases. As illustrated in  FIG. 5B , the voltage V 2 output from the second coil L 2  of the second resonance circuit has a sine wave. A solid line in  FIG. 5B  represents a waveform when the toner density of the developer  111  has an appropriate value, and a broken line in  FIG. 5B  represents a waveform when the toner density is smaller than the appropriate value. Therefore, as the toner density of the developer  111  varies, mutual impedance at the resonance point varies, thereby generating a phase difference between the waveforms of the voltage V 2  as indicated by the solid line and the broken line as described above. 
       FIG. 5C  illustrates waveforms of the voltage output from the inverting amplifier IC 2 - 2 . The voltage V 2  output from the second coil L 2  of the second resonance circuit (sine wave) is input to the phase comparator  120 . The inverting amplifier IC 2 - 2  of the phase comparator  120  inverts and amplifies the input sine wave. The comparator IC 2 - 3  of the phase comparator  120  compares an output voltage V 3  output from the inverting amplifier IC 2 - 2  and the output voltage V 1  output from the oscillator  100 . The inverting amplifier IC 2 - 2  inputs a direct current voltage from a source circuit, not shown, and the alternating voltage V 2  output from the second coil L 2  to the phase comparator  120  to perform an XOR operation, and outputs a rectangular waveform as illustrated in  FIG. 5C . 
       FIG. 5D  illustrates waveforms of an output voltage V 4  output from the phase comparator  120 . The comparator IC 2 - 3  inputs the output voltage V 1  from the oscillator circuit  120  and the output voltage V 3  from the inverting amplifier IC 2 - 2  to perform an XOR operation, and outputs a phase component as illustrated in  FIG. 5D . 
     As illustrated in  FIG. 5D , an interval of on-state of the output waveform of the voltage V 4  when the toner density is low as indicated by a broken line is longer than that of the output waveform of the voltage V 4  when the toner density is appropriate as indicated by a solid line. 
       FIG. 5E  illustrates waveforms of an output voltage V 5  output from the Op-Amp IC 1 - 1 . The phase comparator  120  inputs the output voltage V 4  to the smoothing circuit  130 . The Op-Amp IC 1 - 1  of the smoothing circuit  130  outputs flat waveforms as illustrated in  FIG. 5E , which are average values of the waveforms as indicated in  FIG. 5D . A solid line represents an output voltage V 5-1  when the toner density is appropriate, and a broken line represents an output voltage V 5-2  when the toner density is smaller than the appropriate value. Since the interval of on-state of the output waveform of the voltage V 4  when the toner density is low as indicated by a broken line depicted in  FIG. 5D  is longer than that of the output waveform of the voltage V 4  when the toner density is high, the output voltage V 5-2  when the toner density is smaller than the appropriate value is greater than the output voltage V 5-1  when the toner density has an appropriate value. 
     The amplifier  140  amplifies the output voltage V 5  output from the smoothing circuit  130 . The output voltage V 5  has a difference of about 0.5 V even when the toner density has a maximum difference. Therefore, the amplifier  140  amplifies a voltage difference between a control voltage V cont  and the output voltage V 5  output from the smoothing circuit  130  fourfold, thereby obtaining an output voltage V out  of the toner density sensor  7 Y. 
     Each toner density sensor  7 Y has variations in a relation between toner density and output voltage (detected output), due for example to errors in the manufacture of components of the toner density sensor  7 Y. Thus, before use of the development device  4 Y (or the process unit  1 Y), the control voltage V cont  of the toner density sensor  7 Y is calibrated such that a relation between the toner density and the output voltage has a predetermined relation. A method of calibration of the control voltage V cont  is described below. 
     Developer in the development device  4 Y has an initial toner density of about 5%. The memory chip  150  of the toner density sensor  7 Y stores a reference output voltage of about 3 V for the predetermined toner density of about 5% and determines whether or not the output voltage V out  of the toner density sensor  7 Y is the reference output voltage of about 3 V. When the output voltage V out  of the toner density sensor  7 Y is not the reference output voltage of about 3 V, the control voltage V cont  is adjusted such that the output voltage V out  of the toner density sensor  7 Y reaches the reference output voltage of about 3 V. Then, the memory chip  150  rewrites the value of the control voltage V cont  stored in advance in the memory chip  150  to an adjusted value of the control voltage V cont . Thereafter, the development device  4 Y stirs the developer to increase the amount of charged toner in the development device  4 . Then, based on the rewritten value of the control voltage, toner is added to the developer such that the output voltage V out  of the toner density sensor  7 Y reaches a target voltage (output voltage of about 2.2 V when the toner density is 7%). 
     Referring to  FIGS. 6 ,  7 A, and  7 B, a description is now given of a reuse system for reusing the development device  4 Y and the toner density sensor  7 Y included in the image forming apparatus  200  for another image forming apparatus having a different process linear velocity. 
       FIG. 6  is a schematic block diagram of an image forming apparatus  200 A.  FIGS. 7A and 7B  are schematic block diagrams of an image forming apparatus  200 B. The image forming apparatus  200 A including a development device  4 Y and a toner density sensor  7 Y before reuse (or reproduced) is called a first image forming apparatus. The image forming apparatus  200 B reusing the development device  4 Y and the toner density sensor  7 Y is called a second image forming apparatus. 
     As illustrated in  FIG. 6 , the image forming apparatus  200 A includes the process unit  10 Y including the development device  4 Y, the toner density sensor  7 Y, a storage device  29 , and a measurement device  30 . It is to be noted that the image forming apparatus  200 A has a structure equivalent to that of the image forming apparatus  200  depicted in  FIG. 1 , and the process unit  10 Y has a structure equivalent to that of the process unit  10 Y depicted in  FIG. 2 . Therefore, the process unit  10 Y includes the photoconductor  2 Y, the charging roller  3 Y, and the like. 
     The storage device  29  is the memory chip  150  depicted in  FIG. 3  of the toner density sensor  7 Y. The measurement device  30  measures an output voltage V out  of the toner density sensor  7 Y. The storage device  29  stores information on the output voltage V out  of the toner density sensor  7 Y measured by the measurement device  30 . The image forming apparatus  200 A switches from a predetermined process linear velocity in image formation to another process linear velocity. 
       FIG. 7A  illustrates a state of the image forming apparatus  200 A before installation of the development device  4 Y and the toner density sensor  7 Y.  FIG. 7B  illustrates the image forming apparatus  200 B in which the development device  4 Y and the toner density sensor  7 Y are installed in a process unit  10 Y′. The storage device  29  and the measurement device  30  are installed in the process unit  10 Y′. As illustrated in  FIGS. 7A and 7B , the image forming apparatus  200 B includes a reader  31  and an adjuster  32 . 
     The reader  31  reads the information stored in the storage device  29  depicted in  FIG. 6 . The adjuster  32  adjusts the output voltage V out  of the toner density sensor  7 Y based on the information read by the reader  31 . More specifically, the adjuster  32  adjusts the output voltage V out  by adjusting the control voltage V cont . 
       FIG. 8  illustrates an image forming apparatus  200 B′ according to another illustrative embodiment. The image forming apparatus  200 B′, serving as a second image forming apparatus, includes the reader  31 , the adjuster  32 , and an extractor  34 . 
     The extractor  34  extracts specific information from the information read by the reader  31 . The remainder of the configuration of the image forming apparatus  200 B′ is equivalent to that of the image forming apparatus  200 B depicted in  FIGS. 7A and 7B . 
       FIG. 9  illustrates an image forming apparatus  200 A′ according to yet another illustrative embodiment. The image forming apparatus  200 A′, serving as a first image forming apparatus, includes the storage device  29 , the measurement device  30 , and a computing device  33 . The computing device  33  calculates a relational expression representing a relation between a process linear velocity and the output voltage V out . The remainder of the configuration of the image forming apparatus  200 A′ is equivalent to that of the image forming apparatus  200 A depicted in  FIG. 6 . 
     Referring to  FIGS. 10A and 10B , a description is now given of a method of reusing the development device  4 Y and the toner density sensor  7 Y.  FIG. 10A  is a flowchart of the reuse method using the image forming apparatus  200 A depicted in  FIG. 6  and the image forming apparatus  200 B depicted in  FIGS. 7A and 7B .  FIG. 10B  is a flowchart of succeeding processes of the reuse method. 
     In step S 1 , when the development device  4 Y and the toner density sensor  7 Y are installed in the image forming apparatus  200 A, serving as a first image forming apparatus, the controller  300  depicted in  FIG. 1  orders calibration of the control voltage C cont  of the toner density sensor  7 Y. In step S 2 , the controller  300  orders the image forming apparatus  200 A having developer in the initial state to switch from a predetermined process linear velocity to a process linear velocity of the image forming apparatus  200 B. In step S 3 , the measurement device  30  of the image forming apparatus  200 A depicted in  FIG. 6  measures the output voltage V out  of the toner density sensor  7 Y at the velocity of the image forming apparatus  200 B. That is, the output voltage V out  of the toner density sensor  7 Y at the velocity of the image forming apparatus  200 B is measured before adjustment of the toner density in the developer to a toner density of about 7%, which is a level that is appropriate for image formation in step S 5 . 
     More specifically, in step S 2 , when the development device  4 Y of the image forming apparatus  200 A stirs developer at a velocity corresponding to the process linear velocity of the image forming apparatus  200 B, the toner density sensor  7 Y measures toner density of the stirred developer. Then, in step S 3 , the measurement device  30  measures the output voltage V out  of the toner density sensor  7 Y. During measurement of the output voltage V out  of the toner density sensor  7 Y, since toner is not supplied from the development device  4 Y to the photoconductor  2 Y, the toner density in the development device  4 Y maintains the initial toner density of about 5%. 
     When the toner density sensor  7 Y detects the same toner density at different process linear velocities (e.g., rates of stirring developer, or the like), output voltage V out  of the toner density sensor  7 Y varies. More specifically, when the process linear velocity is high, the output voltage V out  of the toner density sensor  7 Y when detecting the toner density decreases. Conversely, when the process linear velocity is low, the output voltage V out  of the toner density sensor  7 Y increases. 
     Therefore, by measuring the output voltage V out  of the toner density sensor  7 Y at the process linear velocity of the image forming apparatus  200 B, the measurement device  30  obtains the relation between the process linear velocity of the image forming apparatus  200 B and the output voltage V out  of the toner density sensor  7 Y. Then, as illustrated in  FIG. 10A , in step S 4 , the storage device  29  of the image forming apparatus  200 A depicted in  FIG. 6  stores the information obtained by the measurement device  30 . Thereafter, toner is added to the development device  4 Y to adjust the toner density in the developer to about 7%, which is a level that is appropriate for image formation in step S 5 , and the development device  4 Y is ready for use. 
     In step S 11 , the development device  4 Y and the toner density sensor  7 Y are installed in the image forming apparatus  200 B serving as a second image forming apparatus. Also, the storage device  29  is installed in the process unit  10 Y′. 
     In step S 12 , when the process unit  10 Y′ is installed in the image forming apparatus  200 B, the reader  31  depicted in  FIG. 7B  included in the image forming apparatus  200 B reads from the storage device  29  the relation between the process linear velocity of the image forming apparatus  200 B and the output voltage V out  of the toner density sensor  7 Y. In step S 13 , based on that relation, the adjuster  32  depicted in  FIG. 7B  adjusts the output voltage V out  of the toner density sensor  7 Y to a value corresponding to the process linear velocity of the image forming apparatus  200 B. More specifically, the adjuster  32  adjusts the control voltage such that the output voltage V out  of the toner density sensor  7 Y corresponds to the process linear velocity of the image forming apparatus  200 B and rewrites the control voltage stored in the storage device  29  into the adjusted control voltage. 
     Therefore, since the output voltage V out  of the toner density sensor  7 Y is adjusted to correspond to the process linear velocity of the image forming apparatus  200 B, the toner density sensor  7 Y can properly detect toner density when reused for the image forming apparatus  200 B. 
     Referring to  FIGS. 11A and 11B , a description is now given of a method of reusing the development device  4 Y and the toner density sensor  7 Y.  FIG. 11A  is a flowchart of the reuse method using the image forming apparatus  200 A depicted in  FIG. 6  and the image forming apparatus  200 B′ depicted in  FIG. 8 .  FIG. 11B  is a flowchart of succeeding processes of the reuse method. 
     In step S 21 , the controller  300  depicted in  FIG. 1  orders calibration of the control voltage of the toner density sensor  7 Y. In step S 22 , the controller  300  orders the image forming apparatus  200 A having developer in the initial state to switch to a plurality of process linear velocities. The multiple process velocities are process velocities of an image forming apparatus which may reuse the development device  4 Y and the toner density sensor  7 Y. In step S 23 , when the image forming apparatus  200 A switches to each process velocity, the measurement device  30  measures an output voltage V out  of the toner density sensor  7 Y at each process velocity, thereby obtaining the relation between each of the plurality of process velocities and each output voltage of the toner density sensor  7 Y. In step S 24 , the storage device  29  stores information obtained by the measurement device  30 . In step S 25 , toner is added to the development device  4 Y, so as to adjust the toner density to a density of about the 7% appropriate for image formation. 
     In step S 31 , the development device  4 Y, the toner density sensor  7 Y, and the storage device  29  included in the image forming apparatus  200 A are installed in the image forming apparatus  200 B′. In step S 32 , the reader  31  depicted in  FIG. 8  reads out the relation between each of the plurality of process velocities and each output voltage of the toner density sensor  7 Y. Then, in step S 33 , the extractor  34  depicted in  FIG. 8  extracts specific information from the information read by the reader  31 . More specifically, the extractor  34  extracts the relation between a process linear velocity of the image forming apparatus  200 B′ and output voltage V out  of the toner density sensor  7 Y. In step S 34 , based on that relation, the adjuster  32  adjusts the output voltage of the toner density sensor  7 Y to a value corresponding to the process linear velocity of the image forming apparatus  200 B′. In order to reuse the development device  4 Y and the toner density sensor  7 Y for another image forming apparatus having a different process velocity, the extractor  34  reads out the relation between a process linear velocity of the image forming apparatus and output voltage V out  of the toner density sensor  7 Y. 
     According to this illustrative embodiment, the output voltage V out  of the toner density sensor  7 Y can be adjusted to correspond to one process linear velocity selected from the plurality of process liner velocities. Therefore, the development device  4 Y and the toner density sensor  7 Y can be reused for an image forming apparatus arbitrarily selected from a plurality of image forming apparatuses having different process linear velocities. That is, the development device  4 Y and the toner density sensor  7 Y can be reused for a wide variety of image forming apparatuses. 
     Referring to  FIGS. 12A and 12B , and  13 , a description is now given of a method of reusing the development device  4 Y and the toner density sensor  7 Y.  FIG. 12A  is a flowchart of the reuse method using the image forming apparatus  200 A′ depicted in  FIG. 9  and the image forming apparatus  200 B depicted in  FIG. 7A .  FIG. 12B  is a flowchart of succeeding processes of the reuse method.  FIG. 13  is a graph illustrating a relation between a process linear velocity and an output voltage of the toner density sensor  7 Y. 
     In step S 41 , the controller  300  depicted in  FIG. 1  orders calibration of the control voltage of the toner density sensor  7 Y. In step S 42 , the controller  300  orders the image forming apparatus  200 A′ having developer in the initial state to switch to a plurality of process linear velocities. The plurality of process velocities may be a process velocity of an image forming apparatus which may reuse the development device  4 Y and the toner density sensor  7 Y or a process linear velocity of another image forming apparatus. In step S 43 , when the image forming apparatus  200 A′ switches to each process velocity, the measurement device  30  measures output voltage V out  of the toner density sensor  7 Y at each process velocity, in step S 43 . Since no toner is consumed by the development device  4 Y when the output voltage V out  is measured, frequent measurement of the output voltage of the toner density sensor  7 Y may cause degradation of toner provided in the development device  4 Y. Therefore, the output voltage of the toner density sensor  7 Y is preferably measured about 2 to 5 times, for example. 
     In step S 44 , based on a result of measurement of the output voltage, the computing device  33  depicted in  FIG. 9  calculates a relational expression representing a relation between the process linear velocity and the output voltage of the toner density sensor  7 Y. More specifically, as illustrated in  FIG. 13 , the computing device  33  plots output voltage values of the toner density sensor  7 Y corresponding to each process linear velocity as indicated by dots a 1 , a 2 , a 3 , and a 4 , and calculates a relational expression representing an approximate curve line b. In step S 45 , the storage device  29  stores the relational expression calculated by the computing device  33 . 
     Since the relational expression representing the relation between the process linear velocity and the output voltage is primary or polynomial based on a value of the process linear velocity, the computing device  33  may selectively switch between a mode of calculating a primary expression and a mode of calculating a polynomial expression. 
     As illustrated in  FIG. 12B , in step S 51 , the development device  4 Y, the toner density sensor  7 Y, and the storage device  29  of the image forming apparatus  200 A′ are installed in the image forming apparatus  200 B. In step S 52 , the reader  31  depicted in  FIG. 7B  reads a relational expression of a relation between a process linear velocity and an output voltage of the toner density sensor  7 Y. Then, in step S 53 , based on the relational expression, the adjuster  32  adjusts the output voltage of the toner density sensor  7 Y to a value corresponding to the process linear velocity of the image forming apparatus  200 B. For example, when the image forming apparatus  200 A′ depicted in  FIG. 9  generally uses a process linear velocity of 230 mm/sec, and the image forming apparatus  200 B depicted in  FIG. 7B  uses a process linear velocity of 154 mm/sec, the difference in output voltage ΔVt of the toner density sensor  7 Y at the two different process linear velocities is easily known by using the approximate curve line b depicted in  FIG. 13 . By subtracting the difference in output voltage ΔVt from the output voltage of the toner density sensor  7 Y, the output voltage of the toner density sensor  7 Y can be adjusted to correspond to the process linear velocity of the image forming apparatus  200 B. Therefore, when the toner density sensor  7 Y is reused in the image forming apparatus  200 B, the toner density sensor  7 Y can properly detect toner density. 
     According to this illustrative embodiment, the output voltage of the toner density sensor  7 Y can be adjusted to correspond to various process linear velocities, so that various types of image forming apparatuses can reuse the development device  4 Y and the toner density sensor  7 Y. 
     According to this illustrative embodiment, since the output voltage of the toner density sensor  7 Y can be adjusted to correspond to a process linear velocity of an image forming apparatus to reuse the toner density sensor  7 Y, the toner density sensor  7 Y can properly detect toner density. 
     As can be appreciated by those skilled in the art, although the present invention has been described above with reference to specific illustrative embodiments the present invention is not limited to the specific embodiments described above, and various modifications and enhancements are possible without departing from the scope of the invention. Although the image forming apparatus  200  depicted in  FIG. 1  uses a direct transfer method in which a toner image is directly transferred to a recording material, the image forming apparatus  200  may use an indirect transfer method in which a toner image is transferred to the recording material via a belt member, e.g., an intermediate transfer belt. In addition, according to this illustrative embodiment, the reuse method and the reuse system can be applied not only to reusing the development device  4 Y and the toner density sensor  7 Y, but also to a combination of an image carrier or the like and a detector such as a photosensor provided in the image carrier. It is therefore to be understood that the present invention may be practiced otherwise than as specifically described herein. For example, elements and/or features of different illustrative embodiments may be combined with each other and/or substituted for each other within the scope of the present invention.