Patent Publication Number: US-7720399-B2

Title: Image forming apparatus controlling transfer output in accordance with variation of environment conditions

Description:
BACKGROUND OF THE INVENTION 
   This invention relates to an image forming apparatus that forms an image using electrophotographic technique. 
   An image forming apparatus that forms a color image using electrophotographic technique includes a plurality of photosensitive drums and corresponding transfer units. The photosensitive drums and transfer units face each other and are disposed along a feeding belt for feeding a recording medium. Toner cartridges are provided for supplying toners of respective colors to the respective photosensitive drums. A latent image is formed on the surface of each photosensitive drum by means of a charging roller and an exposure unit disposed on the circumference of the photosensitive drum. The latent image is developed by the toner, and a toner image is formed on the surface of each photosensitive drum. 
   When the feeding belt moves, a recording medium is fed through between the respective photosensitive drums and the transfer units. Each transfer unit is applied with a transfer output (i.e., a transfer bias), and the recording medium is applied with an electric charge opposite in polarity to the toner image formed on the surface of the photosensitive drum. The toner image is transferred from the surface of the photosensitive drum to the recording medium. Thereafter, the recording medium is fed to a fixing unit. The fixing unit applies heat and pressure to the toner image, so that the toner image is fixed to the recording medium. 
   Conventionally, a “differential constant current controlling method” is known as a method for controlling the transfer output applied to the transfer unit. In the differential constant current controlling method, a target current flowing through a transfer unit is predetermined, and the transfer output applied to the transfer unit is determined while detecting the feedback current so that the feedback current becomes equal to the target current. Further, the transfer output is controlled in accordance with the preliminarily detected electric resistance of the transfer unit. 
   Therefore, Japanese Laid-Open Patent publication No. 2000-235316 discloses a technique in which an amount of variation (i.e., variation with time) of the electric resistance of the feeding belt is detected, and the transfer output is controlled according to the detected amount of variation. 
   However, in the above described technique, the detection of the variation of the electric resistance of the feeding belt is performed irrespective of whether there is a variation of the temperature or not. Therefore, there is a problem that it takes a long time to start printing operation. 
   Accordingly, it is demanded to develop an image forming apparatus capable of controlling the transfer output quickly and correctly in accordance with the variation of the environmental condition such as temperature, humidity or the like. 
   SUMMARY OF THE INVENTION 
   The present invention is intended to solve the above described problems, and an object of the present invention is to provide an image forming apparatus capable of controlling the transfer output quickly and correctly in accordance with the variation of the environmental condition such as temperature, humidity or the like. 
   The present invention provides an image forming apparatus including: 
   an image bearing body that bears a developer image; 
   a transfer member for transferring the developer image to a recording medium; 
   a storing section in which a stored temperature value and a stored electric resistance value of the transfer member are preliminarily stored; 
   a temperature detecting section that detects a temperature of the transfer member; 
   a calculating section that calculates a temperature variation value representing a variation between a detected temperature value detected by the temperature detecting section and the stored temperature value stored in the storing section; 
   a comparing-and-determining section that compares the temperature variation value and a predetermined temperature variation threshold and determines whether the temperature variation value is less than the temperature variation threshold or not; 
   an output control section that determines a transfer output based on the stored electric resistance value stored in the storing section, in the case where the comparative determining section determines that the temperature variation value is less than the temperature variation threshold, and 
   an output applying section that applies the transfer output to the transfer member. 
   With such an arrangement, when the variation value of the temperature (between the detected temperature value and the stored temperature value of the transfer member) is less than the predetermined temperature variation threshold, the transfer output having been previously determined is applied to the transfer member, and then the image formation is performed. In such a case, the determination process of the transfer output can be omitted, and therefore the process time can be shortened. Further, the transfer output can be controlled without detecting the electric resistance of the transfer unit by applying an output to the transfer member. 
   Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the attached drawings: 
       FIG. 1  is a block diagram showing a functional configuration of a printer according to the embodiment of the present invention; 
       FIG. 2  is a schematic view showing the printer according to the embodiment of the present invention; 
       FIG. 3  is an enlarged view of a cyan image forming unit of the printer of  FIG. 2  and its surroundings; 
       FIG. 4  is a schematic view showing a position of a belt temperature detecting sensor; 
       FIG. 5  is an illustrative view showing an example of an environment detection table; 
       FIG. 6  is an illustrative view showing an example of a threshold storing section; 
       FIG. 7  is a schematic view showing a transfer circuit; 
       FIG. 8  is an illustrative view showing an example of a transfer voltage table; 
       FIG. 9  is an illustrative view showing the relationship between the detected electric resistance value and the transfer voltage output value of the transfer roller; 
       FIG. 10  shows an image forming area on a recording medium; 
       FIG. 11  shows the result of evaluation of image quality; 
       FIG. 12  is an illustrative view showing the relationship between the detected temperature value of the transfer belt and the detected electric resistance value of the transfer roller; 
       FIG. 13  is a flow chart showing a printing process starting operation of the printer according to the embodiment of the present invention; 
       FIG. 14  is a flow chart showing a printing process restarting operation of the printer according to the embodiment of the present invention; 
       FIG. 15  is a schematic view showing a configuration of a modification of the embodiment of the present invention; and 
       FIG. 16  is an illustrative view showing a position of a belt temperature detecting sensor according to the modification of the embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Hereinafter, an embodiment of the present invention will be described with reference to the attached drawings. 
     FIG. 2  is a schematic view showing a configuration of a printer according to the embodiment of the present invention. 
   In  FIG. 2 , a recording medium storing cassette  19  is provided on the lower part of a printer  10  as an image forming apparatus. The recording medium storing cassette  19  stores a plurality of recording media  12 . The printer  10  includes a hopping roller  20  that feeds the recording medium  12  out of the recording medium storing cassette  19  sheet by sheet, and first and second registration rollers  21  and  22  that further feed the recording medium  12  along a feeding path. 
   Along the feeding path of the recording medium  12 , four image forming units  11 K,  11 Y,  11 M and  11 C are disposed in this order in the feeding direction of the recording medium  12 . 
   The image forming unit  11 K includes a photosensitive drum  13 K as an image bearing body having a surface on which a black image can be formed. Similarly, the image forming unit  11 Y includes a photosensitive drum  13 Y on which a yellow image can be formed, the image forming unit  11 M includes a photosensitive drum  13 M on which a magenta image can be formed, and the image forming unit  11 C includes a photosensitive drum  13 C on which a cyan image can be formed. 
     FIG. 3  is an enlarged view showing the image forming unit  11 C (i.e., the cyan image forming unit) and its surroundings. 
   As shown in  FIG. 3 , the cyan image forming unit  11 C includes a charging roller  14 C, an LED head  15 C and a developing roller  16 C disposed around the photosensitive drum  13 C. The charging roller  14 C uniformly charges the surface of the photosensitive drum  13 C, the LED head  15 C forms a latent image on the surface of the photosensitive drum  13 C, and the developing roller  16 C develops the latent image to form a toner image. Further, a sponge roller  17 C is urged against the developing roller  16 C, which causes the cyan toner to adhere to the surface of the developing roller  16 C. 
   A transfer roller  18 C as a transfer unit (i.e., a transfer member) is disposed outside the image forming unit  11 C. The transfer roller  18 C faces the photosensitive drum  13 C with the feeding path disposed therebetween. 
   The image forming units  11 K,  11 Y and  11 M have the similar configurations as the image forming unit  11 C. 
   Along the feeding path of the recording medium  12 , a transfer belt  23  (i.e., a feeding member) is stretched around a driving roller  24  and a driving auxiliary roller  25 . The transfer belt  23  is composed of a semi-conductive plastic film having high electric resistance, and has a seamless and endless form. When the driving roller  24  is rotated by a belt motor  58  (described later), the transfer belt  23  is driven in the direction indicated by an arrow shown in  FIGS. 2 and 3 . A cleaning blade  26  is disposed in contact with the surface of the lower part of the transfer belt  23 . When the transfer belt  23  is driven, the cleaning blade  26  scrapes the toner debris or the like from the surface of the transfer belt  23 . 
   Further, a belt temperature detecting sensor  27  as a temperature detecting section is disposed in contact with the surface of the lower part of the transfer belt  23  (see  FIG. 4 ). 
     FIG. 4  is a schematic view showing the position of the belt temperature detecting sensor  27  according to the embodiment. 
   The belt temperature detecting sensor  27  is composed of a thermistor for detecting the temperature of the transfer belt  23 . In order to prevent the abrasion of the surface of the transfer belt  23  (that may cause defective transferring) due to the sliding contact with the transfer belt  23 , and to prevent the toner from adhering to the belt temperature detecting sensor  27  itself, the belt temperature detecting sensor  27  is disposed on a portion of the transfer belt  23  that does not contact the recording medium  12 . In this embodiment, the belt temperature detecting sensor  27  is disposed at the lower end portion of the transfer belt  23  and at the downstream end in the feeding direction (indicated by an arrow in  FIG. 4 ) of the recording medium  12  as shown in  FIG. 4 . 
   The recording medium  12  is fed by the second registration roller  22 , and is placed on the upper surface of the transfer belt  23 . When the transfer belt  23  is driven to move, the recording medium  12  is first fed through between the photosensitive drum  13 K and the transfer roller  18 K. In this state, the transfer roller  18 K is applied with a transfer output (i.e., a transfer bias), and the black toner image formed on the surface of the photosensitive drum  13 K is transferred to the surface of the recording medium  12 . Subsequently, the recording medium  12  is fed through between the respective photosensitive drums  13 Y,  13 M and  13 C and the transfer rollers  18 Y,  18 M and  18 C, and the toner images of the respective colors are transferred to the recording medium  12 . Then, the recording medium  12  to which the toner images of four colors have been transferred is fed to a fixing unit  28  by the transfer belt  23 . 
   The fixing unit  28  includes heat rollers  29   a  and  29   b  and a fixing unit temperature detecting sensor  30 . The fixing unit temperature detecting sensor  30  includes a thermistor that detects the temperature of the heat rollers  29   a  and  29   b . The recording medium  12  fed into the fixing unit  28  is heated and pressed by the heat rollers  29   a  and  29   b , and the toner images of the respective colors are fixed to the recording medium  12 . Then, the recording medium  12  to which toner images of four colors have been fixed is fed to an ejection opening by ejection rollers  31 . 
   Next, the control system of the printer  10  will be described. 
     FIG. 1  is a block diagram showing a functional configuration of the printer according to the embodiment of the present invention. 
   A host interface section  32  has a function to interface with a host device (not shown) at the physical layer, and is composed of a connector, a communication chip or the like. The host interface section  32  receives a command to perform a printing operation, an image data to be printed or the like from the host device, and sends the same to a command/image processing section  33 . 
   The command/image processing section  33  interprets the command received from the host device via the host interface section  32 , and expands the image data into a bitmap data. The command/image processing section  33  includes a micro processor, RAM and the like. The command interpreted by the command/image processing section  33  is sent to a printing control section  35 . The image data expanded by the command/image processing section  33  is sent to an LED head interface section  34 . 
   The LED head interface section  34  has a function to process the image data (received from the command/image processing section  33 ) in accordance with the interface of the respective LED heads  15 K,  15 Y,  15 M and  15 C. The LED head interface section  34  is composed of a semicustom LSI, RAM and the like. 
   A motor control section  55  controls and drives a hopping motor  56 , a registration motor  57 , the belt motor  58 , a drum motor  59 , a heater motor  60  and the like. 
   The hopping motor  56  functions as a driving unit for driving the hopping roller  20 . The registration motor  57  drives the first and second registration rollers  21  and  22 . The belt motor  58  drives the driving roller  24  to thereby move the transfer belt  23 . The drum motor  59  drives the photosensitive drums  13 K,  13 Y,  13 M and  13 C of the respective image forming units  11 K,  11 Y,  11 M and  11 C. The heater motor  60  drives the heat rollers  29   a  and  29   b  of the fixing unit  28 . 
   A fixing unit temperature control section  61  controls the temperature of the fixing unit  28  based on the temperature of the heat rollers  29   a  and  29   b  detected by the fixing unit temperature detecting sensor  30 . 
   Heaters  62  composed of halogen lamps are provided in the heat rollers  29   a  and  29   b  shown in  FIGS. 2 and 3 . The heaters  62  are supplied with electric power from an electric power supply section (not shown) controlled by the fixing unit temperature control section  61 , and heat the heat rollers  29   a  and  29   b.    
   An environmental-temperature detecting sensor  36  (i.e., an environmental-temperature detecting section) composed of a thermistor that detects the temperature in the printer  10  as a detected environmental-temperature value t. An environmental-humidity detecting sensor  37  (i.e., an environmental-humidity detecting section) detects the humidity in the printer  10  as a detected environmental-humidity value h. In this embodiment, the environmental-temperature detecting sensor  36  and the environmental-humidity detecting sensor  37  are mounted on a high voltage board (not shown) provided on the side of the printer  10 . 
   An environment detecting section  38  monitors the inputs from the environmental-temperature detecting sensor  36  and the environmental-humidity detecting sensor  37 , and obtains an environment detection value E based on the respective input values and an environment detection table  39  described later. The environment detecting section  38  sends the environment detection value E to the printing control section  35  described later. 
     FIG. 5  is an illustrative view showing an example of the environment detection table. 
   In the environment detection table  39 , the environment detection values E are stored corresponding to respective ranges of the detected environmental-temperature value t and the respective ranges of the detected environmental-humidity value h as shown in  FIG. 5 . The environment detection value E is a numeric value that represents the environmental condition of the transferring in the printer  10 . The environment detection value E is used for setting the temperature variation threshold (described later) in a belt temperature determining section  40 , for controlling the transfer voltage in a transfer voltage control section  47 , or the like. 
   For example, when the detected environmental-temperature value t is 12° C. and the detected environmental-humidity value h is 20%, the environment detecting section  38  chooses and obtains the environment detection value E of 8 (E=8) in accordance with the environment detection table  39 . When the detected environmental-temperature value t is 22° C. and the detected environmental-humidity value h is 40%, the environment detecting section  38  obtains the environment detection value E of 5 (E=5). When the detected environmental-temperature value t is 28° C. and the detected environmental-humidity value h is 70%, the environment detecting section  38  obtains the environment detection value E of 1 (E=1). 
   The belt temperature determining section  40  includes a calculating section  41 , a threshold storing section  42 , and a comparing-and-determining section  43 . The belt temperature determining section  40  has a function to control the belt temperature detection sensor  27  to detect the temperature of the transfer belt  23 . 
   The calculating section  41  calculates a temperature variation value ΔT=|T−To| representing the amount of variation of the temperature of the transfer belt  23  based on the temperature (i.e., the detected temperature value T) of the transfer belt  23  detected by the belt temperature detecting sensor  27  and the temperature (i.e., the stored temperature value T O ) of the transfer belt  23  stored in a storing section  53  described later. 
   The threshold storing section  42  stores temperature variation threshold ΔT th  representing a threshold of the temperature variation value ΔT. 
     FIG. 6  is an illustrative view showing the configuration of the threshold storing section  42 . 
   In the threshold storing section  42 , temperature variation thresholds ΔT th  are stored corresponding to respective ranges of the environment detection value E. 
   For example, the threshold storing section  42  stores the temperature variation threshold ΔT th  of 10° C. corresponding to the range of the environment detection value E from 1 to 2. The threshold storing section  42  stores the temperature variation threshold ΔT th  of 8° C. corresponding to the range of the environment detection value E from 3 to 6, and the threshold storing section  42  stores the temperature variation threshold ΔT th  of 5° C. corresponding to the range of the environment detection value E from 7 to 8. 
   The setting of the temperature variation threshold ΔT th  to be stored in the threshold storing section  42  will be described later. 
   The comparing-and-determining section  43  chooses (and sets) the temperature variation threshold ΔT th  from the threshold storing section  42  according to the environment detection value E detected by the environment detecting section  38 . The comparing-and-determining section  43  compares the temperature variation threshold ΔT th  and the temperature variation value ΔT calculated by the calculating section  41 , and determines whether the temperature variation value ΔT is less than the temperature variation threshold ΔT th . The determination result of the comparing-and-determining section  43  is notified to the printing control section  35 . 
   For example, in the case where the environment detecting section  38  detects the environment detection value E of 1, the comparing-and-determining section  43  chooses (and sets) the temperature variation threshold ΔT th  of 10° C. based on the threshold storing section  42 . Then, the comparing-and-determining section  43  determines whether the temperature variation value ΔT (calculated by the calculating section  41 ) is less than 10° C. or not. 
   A high-voltage control section  44  is composed of a micro processor or a custom LSI. The high-voltage control section  44  controls a charge voltage control section  45 , a developing voltage control section  46  and a transfer voltage control section  47  so as to control charge voltages, developing voltages and transfer voltages for the respective image forming units  11 K,  11 Y,  11 M and  11 C. 
   The charge voltage control section  45  controls the supply (and the stoppage of supply) of the charge voltages applied to the charging rollers  14 K,  14 Y,  14 M and  14 C. 
   The developing voltage control section  46  controls the supply (and the stoppage of supply) of the developing voltages applied to the developing rollers  16 K,  16 Y,  16 M and  16 C. 
   The transfer voltage control section  47  controls the supply (and the stoppage of supply) of the transfer voltages applied to the transfer rollers  18 K,  18 Y,  18 M and  18 C, and includes an output control section  48 , an output applying section  49  and an electric resistance detecting section  50 . 
     FIG. 7  is a schematic view showing a transfer circuit. 
   Transfer voltage power sources  51 K,  51 Y,  51 M and  51 C have a function as the transfer voltage control section  47 , and are respectively connected to the transfer rollers  18 K,  18 Y,  18 M and  18 C as shown in  FIG. 7 . In this embodiment, the transfer voltage power sources  51 K,  51 Y and  51 M are composed of constant-voltage power sources capable of outputting voltages of up to 5 kV. The transfer voltage power source  51 C is composed of a constant-voltage power source capable of outputting voltage of up to 7 kV. 
   The output control section  48  calculates the output values of the transfer voltages (i.e., transfer voltage output values V) to be applied to the respective transfer rollers  18 K,  18 Y and  18 M and  18 C based on the electric resistances (i.e., stored electric resistance values I O ) of the transfer rollers  18 K,  18 Y,  18 M and  18 C stored in the storing section  53  described later, and notifies an output applying section  49  of the transfer voltage output values V. In the calculation of the transfer voltage output value V by the output control section  48 , printing information notified by the command/image processing section  33  to the printing control section  35  and the environment detection value E detected by the environment detecting section  38  are used as well as the above described stored electric resistance value I O , and a transfer voltage table  52  is referred. 
     FIG. 8  is an illustrative view showing an example of the transfer voltage table. 
   As shown in  FIG. 8 , the transfer voltage table  52  stores transfer voltage table value V, corresponding to the printing information and the environment detection value E. The printing information stored in the transfer voltage table  52  includes medium-type information (i.e., information of the type of the recording medium  12  used in the printing process) as medium-specification information notified by the printing control section  35  via the high-voltage control section  44 , and medium-thickness information (i.e., information of the thickness of the recording medium  12 ) which is also described as medium-weight information. The printing information further includes color information of the toners used in the respective image forming units  11 K,  11 Y,  11 M and  11 C. 
   In this embodiment, the transfer voltage table  52  stores the transfer voltage table value V 1  corresponding to the respective combinations of the color information “K”, “Y”, “M” and “C” of the toners used in the image forming units  11 K,  11 Y,  11 M and  11 C and the environment detection values E ranging from 1 to 8, in association with the respective medium information including the medium-type information (“usual paper”) and the medium-thickness information (“thick paper”). For example, if the environment detection value E is 1, the transfer voltage table value V 1  stored in the transfer voltage table  52  is 2.49 kV for the transfer roller  18   k  forming black image (i.e., the color information “K”) corresponding to the medium-type information of “usual paper” and the medium-thickness information of “thick paper”. 
   The output control section  48  refers to the above described transfer voltage table  52  and obtains the transfer voltage table value V 1 . The output control section  48  calculates the transfer voltage calculation value V 2  based on the stored electric resistance value I O  stored in the storing section  53  described later. Then, the output control section  48  adds the transfer voltage table value V 1  and transfer voltage calculation value V 2 , and obtains the transfer voltage output value V=V 1 +V 2 . The calculated voltage output value V is notified to the output applying section  49 . 
   The output applying section  49  applies the outputs from the high-voltage transformer provided in the respective transfer power sources  51 K,  51 Y,  51 M and  51 C to the respective transfer rollers  18 K,  18 Y,  18 M and  18 C via the electric resistance of 100 MΩ. 
   The electric resistance detecting section  50  detects the electric resistances of the respective transfer rollers  18 K,  18 Y,  18 M and  18 C (i.e., the detected electric resistance values). To be more specific, the electric resistance detecting section  50  detects the currents flowing through the respective transfer rollers  18 K,  18 Y,  18 M and  18 C as the detected electric resistance values I of the transfer rollers  18 K,  18 Y,  18 M and  18 C while applying constant transfer voltages to the respective transfer rollers  18 K,  18 Y,  18 M and  18 C by means of the output applying section  49 . The detected electric resistance values I are sent to the printing control section  35  via the high-voltage control section  44 . 
   The storing section  53  stores the temperature of the transfer belt  23  as the stored temperature value To, and stores the electric resistances of the respective transfer rollers  18 K,  18 Y,  18 M and  18 C as the stored electric resistance values I o . 
   An updating section  54  is controlled by the printing control section  35 , and has a function to update the stored temperature value T o  and the stored electric resistance value I O  respectively to the detected temperature value T detected by the belt temperature detecting sensor  27  and the detected electric resistance value I detected by the electric resistance detecting section  50 . 
   The printing control section  35  has a function to control the respective parts of the printer  10  based on the command received from the command/image processing section  33 . 
   Next, the setting of the temperature variation threshold ΔT th  to be stored in the threshold storing section  42  will be described. 
     FIG. 9  is an illustrative view showing the relationship between the detected electric resistance value I and the transfer voltage output value V of the transfer roller. 
   The relationship shown in  FIG. 9  is obtained by the following printing test. First, solid images (100% image) of black (k), yellow (Y), magenta (M) and cyan (C) are respectively formed on the recording media of A4 size (297 mm×210 mm). An image forming area A (289 mm×202 mm) is defined on the surface of each recording medium as shown in  FIG. 10 . Then, the respective recording media are visually observed, and the image quality is evaluated.  FIG. 11  shows the evaluation result of image quality. In  FIG. 11 , a mark “x” indicates that a defective image is observed, i.e., non-printed white spots appear in the image or the image exhibits a reduced color density. A mark “o” indicates that the above described defective image is not observed, i.e., an excellent image is formed. In  FIG. 11 , the detected electric resistance value I of the transfer roller is varied as 4.4 μA, 6.4 μA and 7.6 μA. For each detected electric resistance value I of the transfer roller, the transfer voltage output value V is varied in four ways. 
   The experimental result shown in  FIG. 11  is expressed in the form of a graph showing the relationship between the detected electric resistance value I and the transfer voltage output value V as shown in  FIG. 9 . In  FIG. 9 , the meanings of the marks “x” and “o” are the same as those shown in  FIG. 11 . 
   In  FIG. 9 , a marked area indicates an excellent-transfer area in which an excellent image is formed (i.e., an excellent transforming is performed). In the case where the environment detection value E is 7 or 8, the relationship between the detected electric resistance value I (μA) and the transfer voltage output value V (kV) is generally expressed as the following equation (1).
 
 V=−αI+β   (1)
 
   Here, α and β can take values respectively in the following ranges:
     0.09≦α≦0.15, and   3.7≦β≦4.2.   

   In this embodiment, the following equation (2) is employed (α=0.123, β=3.86):
 
 V=− 0.123 I+ 3.86  (2)
 
   The solid line shown in  FIG. 9  corresponds to the above described equation (2). 
   From the result shown in  FIG. 9 , it is understood that the excellent-transfer range ΔV of the transfer voltage output value is 200V (ΔV=200 V). 
   It has been proved that, for example, when the power is turned ON after the printer is left for 6 hours or more under the environment of low temperature and low humidity (i.e., on condition that the environment detection value E is 7 or 8) so that the environment is stabilized, the detected electric resistance value I is 4.4 μA on average. When the initial detected electric resistance value I is set to be 4.4 μA, it is understood that the maximum detected electric resistance value I (corresponding to the excellent-transfer range ΔV of the transfer voltage output value of 200V) is 6.4 μA from  FIG. 9 . Therefore, the variation amount ΔI of the detected electric resistance value I of the transfer roller is found to be 2 μA (ΔI=2 μA). 
     FIG. 12  is an illustrative view showing the relationship between the detected temperature value T of the transfer belt and the detected electric resistance value I of the transfer roller. 
   In  FIG. 12 , the detected electric resistance values I of the transfer roller  18 K are indicated by mark “□”, and the detected electric resistance values I of the transfer roller  18 C are indicated by mark “Δ” for the detected temperature values T of the transfer belt  23 . The solid line indicates the correlation between the detected temperature value T and the detected electric resistance values I of the transfer rollers  18 K and  18 C. The detected electric resistance values I of the respective transfer rollers  18 K and  18 C are in proportion to the detected temperature value T of the transfer belt  23 . In  FIG. 12 , when the detected electric resistance values I of the transfer rollers  18 K and  18 C vary by the variation amount ΔI of 2 μA, the detected temperature value T of the transfer belt  23  varies by 5° C. Therefore, the variation amount of the detected temperature value T of the transfer belt  23  corresponding to the excellent-transfer range shown in  FIG. 9  is less than or equal to 5° C. 
   Based on the above described result, the temperature variation threshold ΔT th  corresponding to the environment detection value E ranging from 7 to 8 is determined as 5° C. (i.e., ΔT th =5° C.). Similarly, the temperature variation threshold ΔT th  corresponding to the environment detection value E ranging from 3 to 6 is determined as 8° C. (i.e., ΔT th =8° C.), and the temperature variation threshold ΔT th  corresponding to the environment detection value E ranging from 1 to 2 is determined as 10° C. (i.e., ΔT th =10° C.), as shown in  FIG. 6 . 
   Next, the operation of the printer  10  in the case where the printing process is performed while controlling the transfer voltage will be described with reference to  FIG. 13 . 
   First, the operation of the printer  10  after the power activation or prior to the correction of a color shift (i.e., the printing process starting operation) will be described with reference to  FIG. 13 . 
     FIG. 13  is a flow chart showing the printing process starting operation of the printer according to the embodiment of the present invention. 
   In the printer  10 , after the power is turned on or before the correction of a color shift is performed, the printing control section  35  instructs the high-voltage control section  44  to detect the electric resistances of the respective transfer rollers  18 K,  18 Y,  18 M and  18 C. The high-voltage control section  44  controls the transfer voltage control section  47  so that the electric resistance detecting section  50  detects the electric resistances of the respective transfer rollers  18 K,  18 Y,  18 M and  18 C. The electric resistance detecting section  50  detects the electric resistances of the respective transfer rollers  18 K,  18 Y,  18 M and  18 C (as the detected electric resistance values), and then sends the detected electric resistance values to the printing control section  35  via the high-voltage control section  44 . Then, the printing control section  35  controls the updating section  54  so that the storing section  53  stores these detected electric resistance values as the stored electric resistance values I KO , I YO , I MO  and I CO  (step S 101 ). 
   Further, the printing control section  35  instructs the belt temperature determining section  40  to detect the temperature of the transfer belt  23 . With this instruction, the belt temperature determining section  40  causes the belt temperature detecting sensor  27  to detect the temperature of the transfer belt  23 . The belt temperature detecting sensor  27  detects the temperature of the transfer belt  23  as the detected temperature value, and sends the detected temperature value to the printing control section  35  via the belt temperature determining section  40 . The printing control section  35  then controls the updating section  54  so that the storing section  53  stores the detected temperature value as the stored temperature value T O  (step S 101 ). 
   Next, the printer  10  enters a state of waiting for the print data, or the printer  10  completes the correction of the color shift and then enters a state of waiting for the print data (step S 102 ). 
   In the state of waiting for the print data, when the host interface section  32  receives the print data (step S 102 ), the instruction (command) to start printing is sent to the printing control section  35  via the command/image processing section  33 . 
   On the receipt of the instruction, the printing control section  35  controls the environment detecting section  38  to detect the environment detection value E (step S 103 ). The environment detecting section  38  detects the environment detection value E based on the respective detected values detected by the environmental-temperature detecting sensor  36  and the environmental-humidity detecting sensor  37  and the environment detection table  39 , and notifies the printing control section  35  of the environment detection value E. 
   When the printing control section  35  receives the notification of the environment detection value E, the printing control section  35  reads the stored temperature value T O  from the storing section  53 . Further, the printing control section  35  notifies the belt temperature determining section  40  of the stored temperature value T O  and the environment detection value E, and instructs to perform the detecting process of the temperature of the transfer belt  23  and the comparing-and-determining process. 
   On the receipt of the instruction, the belt temperature determining section  40  causes the belt temperature detecting sensor  27  to detect the detected temperature value T of the transfer belt  23  (step S 104 ). Then, the calculating section  41  calculates the temperature variation value ΔT=|T−T O  based on the detected temperature value T having been detected and the stored temperature value T O  having been notified. 
   Next, the comparing-and-determining section  43  chooses (and sets) the temperature variation threshold ΔT th  under the environmental condition inside the printer  10  based on the threshold storing section  42  ( FIG. 6 ) and the environment detection value E notified by the printing control section  35 . Then, the comparing-and-determining section  43  compares the temperature variation threshold ΔT th  and the temperature variation value ΔT calculated by the calculating section  41 , and determines whether the temperature variation value ΔT is greater than or equal to the temperature variation threshold ΔT th  (step S 105 ). 
   When the comparing-and-determining section  43  determines that the temperature variation value ΔT is greater than or equal to the temperature variation threshold ΔT th  (YES in step S 105 ), the belt temperature determining section  40  notifies the printing control section  35  of the determination result and the detected temperature value T. 
   When the printing control section  35  receives the notification from the comparing-and-determining section  43 , the printing control section  35  determines whether there is any print data with which the printing process is being performed (step S 106 ). When the printing control section  35  determines that there is no print data with which the printing process is being performed, or when the printing process with the print data is completed, the printing control section  35  instructs the high-voltage control section  44  to detect the electric resistances of the transfer rollers  18 K,  18 Y,  18 M and  18 C. The high-voltage control section  44  controls the transfer voltage control section  47  so that the electric resistance detecting section  50  detects the electric resistances of the transfer rollers  18 K,  18 Y,  18 M and  18 C as the detected electric resistance values I K , I Y , I M  and I C  (step S 107 ). The detected electric resistance values I K , I Y , I M  and I C  detected by the electric resistance detecting section  50  are sent to the printing control section  35  via the high-voltage control section  44 . 
   Next, the printing control section  35  controls the updating section  54  to update the stored electric resistance values I KO , I YO , I MO  and I CO  respectively to the detected electric resistance values I K , I Y , I M  and I C , and to update the stored temperature value T O  the detected temperature value T (step S 108 ). 
   Further, the printing control section  35  notifies the high-voltage control section  44  of the environment detection value E and the medium information, and causes the high-voltage control section  44  to calculate the transfer outputs to be applied to the respective transfer rollers  18 K,  18 Y,  18 M and  18 C. The high-voltage control section  44  controls the transfer voltage control section  47  to cause the output control section  48  to calculate the transfer voltage output values. The output control section  48  refers to the transfer voltage table  52  ( FIG. 8 ), and obtains the transfer voltage table, values V K1 , V Y1 , V M1  and V C1  for the transfer rollers  18 K,  18 Y,  18 M and  18 C based on the environment detection value E and the medium information. The output control section  48  further calculates the transfer voltage calculated values V K2 , V Y2 , V M2  and V C2  based on the stored electric resistance values I K , I Y , I M  and I C . Further, the output control section  48  adds the transfer voltage table values V K1 , V Y1 , V M1  and V C1  and the transfer voltage calculated values V K2 , V Y2 , V M2  and V C2 , to obtain the transfer voltage output values V K , V Y , V M  and V C  to be applied to the respective transfer rollers  18 K,  18 Y,  18 M and  18 C (step S 109 ). 
   Then, the printing control section  35  controls to start the printing process (step S 112 ). The output applying section  49  is controlled by the high-voltage control section  44 , and applies the transfer voltages of the transfer voltage output values V K , V Y , V M  and V C  to the respective transfer rollers  18 K,  18 Y,  18 M and  18 C for transferring the toner images to the recording medium  12 . With this, the print starting process is completed. 
   In the above described step S 105 , if the comparing-and-determining section  43  determines that the temperature variation value ΔT is less than the temperature variation threshold ΔT th  (NO in the step S 105 ), the printing control section  35  notifies the high-voltage control section  44  of the stored electric resistance values I KO , I YO , I MO  and I CO , the environment detection value E and the medium information, and causes the high-voltage control section  44  to calculate the transfer outputs to be applied to the respective transfer rollers  18 K,  18 Y,  18 M and  18 C. The output control section  48  obtains the transfer voltage table values V K1 , V Y1 , V M1  and V C1  from the transfer voltage table  52  based on the environment detection value E and the medium information. Further, the output control section  48  adds the transfer voltage table values V K1 , V Y1 , V M1  and V C1  to the transfer voltage calculated values V K2 , V Y2 , V M2  and V C2  for the transfer rollers  18 K,  18 Y,  18 M and  18 C having been calculated based on the stored electric resistance values I K0 , I Y0 , I M0  and I C0  (i.e., the transfer voltage calculated values having been calculated in the previous printing process) so as to obtain the transfer voltage output values V K , V Y , V M  and V C  (step S 109 ). Then, the printing control section  35  controls to start the printing process (step S 112 ). 
   Next, the operation of the printer  10  (i.e., the printing process restarting operation) in the case where the printing process is interrupted and a cleaning process of the charging rollers  14 K,  14 Y,  14 M and  14 C is performed will be described with reference to  FIG. 14 . 
     FIG. 14  is a flow chart showing the printing process restarting operation of the printer according to the embodiment of the present invention. 
   In the printing process of the printer  10 , when the number of times of the transferring to the recording medium  12  exceeds the predetermined number (for example, 100 pages), the printing control section  35  interrupts the printing process and performs the cleaning process of the charging rollers  14 K,  14 Y,  14 M and  14 C (step S 110 ). 
   When the cleaning process is completed, the printing control section  35  receives a notification of termination, and performs the respective processes (steps S 103  to S 109 ) from the detection of the environment detection value E (step S 103 ) to the calculation of the transfer voltage output value (step S 109 ) as is the case with the printing process staring operation ( FIG. 13 ). 
   Then, the printing control section  35  restarts the printing process based on the calculated transfer voltage output value (step S 112 ). The output applying section  49  controls the high-voltage control section  44  to apply the transfer voltages to the respective transfer rollers  18 K,  18 Y,  18 M and  18 C for transferring the toner images to the recording medium  12 . With this, the printing process is restarted. 
   As described above, the printer according to this embodiment determines whether the transfer output is appropriate or not based on the variation amount of the temperature of the transfer belt. Only when the printer determines that the variation amount is not appropriate, the printer detects the electric resistances of the transfer rollers and applies the transfer outputs to the transfer rollers based on the electric resistances. Accordingly, it becomes possible to control the transfer output based on the variation of the electric resistance values, and to omit the time required for ineffectual detecting process, with the result that the processing time can be shortened. Further, the threshold (set corresponding to the variation amount of the temperature) can be changed according to the environmental condition in the printer, and the transfer output can be corrected, with the result that an excellent image formation can be performed by means of the optimum transferring output. 
   Further, in a conventional controlling system, for example, the transfer voltage output value of 3.32 kV is applied at the detected electric resistance value of 4.4 μA in  FIG. 9 . If the detection of the transfer current is not performed (for example, during the continuous printing operation), the transfer voltage output value V is not corrected, and therefore the transfer voltage output value V may be out of the excellent-transfer area (see, a point P in  FIG. 9 ). As a result, there is a problem that a defective image may be formed. For example, non-printed white spots appear in the image or the image exhibits a reduced color density. 
   In contrast, according to the controlling system of this embodiment, when it is determined that the transfer output is not appropriate (based on the variation amount of the temperature of the transfer belt), the electric resistance value of the transfer roller is detected to thereby correct the transfer voltage output value. Therefore, it is possible to apply the transfer voltage output value V of 2.92 kV in the excellent-transfer range at the detected electric resistance value I of 7.6 μA. Therefore, according to this embodiment, it becomes possible to prevent the occurrence of the above described defective image. 
   In this embodiment, the belt temperature detecting sensor  27  for detecting the temperature of the transfer belt  23  is disposed on the position shown in  FIGS. 2 and 4 . However, the present invention is not limited to this example. 
     FIG. 15  is a schematic view showing the configuration of the printer according to the modification of the embodiment of the present invention.  FIG. 16  is an illustrative view showing the position of the belt temperature detecting sensor according to the modification of the embodiment of the present invention. 
   In the printer  70  according to the modification, the belt temperature detecting sensor  72  is disposed in contact with the inner surface of the transfer belt  71  at the lower part of the transfer belt  71  as shown in  FIG. 15 . Further, the belt temperature detecting sensor  72  is disposed at the center portion in the direction of the rotation axis of the driving roller  24  as indicated by a dashed line in  FIG. 16 . The belt temperature detecting sensor  72  is disposed on a portion of the transfer belt  23  that does not contact the recording medium  12 . Therefore, it becomes possible to prevent the defective transferring due to the abrasion of the transfer belt  71  and the adhesion of the toner to the belt temperature detecting sensor  72 . Further, since the belt temperature detecting sensor  72  is disposed at the center portion, unbalanced temperature detection can be prevented. 
   In the above description, the present invention is applied to the printer. However, the present invention is applicable to a facsimile, a copier, a color printer or the like. 
   In the above description, the electric resistance value of the transfer roller (the transfer unit) is expressed in “μA” because the electric resistance value can be evaluated by detecting the current flowing through the transfer roller while applying a predetermined voltage to the transfer roller (see the description of the electric resistance detecting section  50 ). 
   While the preferred embodiments of the present invention have been illustrated in detail, it should be apparent that modifications and improvements may be made to the invention without departing from the spirit and scope of the invention as described in the following claims.