Patent Publication Number: US-9411278-B2

Title: Image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This patent application claims priority pursuant to 35 U.S.C. §119(a) from Japanese Patent Application No. 2013-193031, filed on Sep. 18, 2013, the entire disclosure of which is incorporated by reference herein. 
     BACKGROUND 
     1. Technical Field 
     Exemplary embodiments of the present invention relate to an image forming apparatus such as a copier, a facsimile machine, a printer, or a multi-function apparatus combining the capabilities of these devices. 
     2. Background Art 
     The image forming apparatuses employing electrophotography, including copiers, facsimile machines, printers, or multi-function apparatuses combining the capabilities of these devices, form an image by fusing a toner image onto a recording medium at a prescribed temperature and pressure, thus fixing the image onto the recording medium. 
     In an image forming apparatus employing electrophotography, conditions for fixing operation such as a set temperature or pressure need to be considered when fusing and fixing the toner image. In particular, to form a quality image, conditions for fixing the toner image vary depending on the type of the recording medium, because image quality is greatly affected by the type, thickness, humidity, smoothness, and coating of the recording medium. 
     Smoothness is measured as follows: A test plate is placed against the surface of the recording medium, and a length of time in which a prescribed amount of air flows between the surface of the recording medium and the test plate is measured in seconds. “Coating” here means that the recording medium is coated or printed with ink or coating material. 
     There is a very high correlation between the smoothness of the recording medium and fixing performance, because a fixing ratio of an image changes depending on a ratio of concavities to convexities in the surface of the recording medium, and in particular in the concave portions of the recording medium. Accordingly, when fixation is performed without the smoothness being considered, a quality image is hardly obtained, and failing to consider the smoothness may cause an abnormal image due to defective fixation. 
     On the other hand, along with recent improvements in the image forming apparatus and diversification of modes of expression, there are now several hundred varieties of recording media. Further, each recording sheet is not the same and is different due to differences of basis weight and thickness, and many brand sheets exist. Accordingly, to create a quality image, the conditions for fixation need to be set precisely for each type and brand of recording media. 
     SUMMARY 
     In one embodiment of the disclosure, there is provided an improved image forming apparatus including paper trays to contain and feed a recording sheet on which a toner image is transferred; status sensors to detect a storage status of the recording sheet contained in the paper trays; a sheet sensor to detect a smoothness in a prescribed area on a surface of the recording sheet; a memory to store detection values of the smoothness detected by the sheet sensor; a fixing device to heat and press the toner image transferred onto the recording sheet and fix the toner image onto the recording sheet; and a control circuit to determine a target fixing temperature of the fixing device based on the detection values stored in the memory. In the image forming apparatus, the control circuit causes the detection values of the recording sheet detected by the sheet sensor to be stored in the memory, causes the sheet sensor to sequentially detect a smoothness of a successive recording sheet and to store a detection value of the smoothness of the successive recording sheet into the memory, determines the target fixing temperature for successive recording sheets depending on the detected smoothness, and resets the detection values stored in the memory to zero when the status sensors detect a change in the storage status of the paper trays. 
     These and other objects, features, and advantages of the present invention will become apparent upon consideration of the following description of preferred embodiments of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional view of an image forming apparatus related to an embodiment of the present invention; 
         FIG. 2  is a schematic front view of an optical sensor according to an embodiment of the present invention; 
         FIG. 3  is a functional block diagram of the image forming apparatus related to the embodiment of the present invention; 
         FIG. 4  is a graph depicting a relation between representative smoothness and corrected temperature according to the embodiment of the present invention; 
         FIG. 5  is a graph depicting a relation of area coverage between normal distribution and standard deviation according to the embodiment of the present invention; 
         FIG. 6  is a graph depicting an example of change between a number of prints and the standard deviation according to the embodiment of the present invention; 
         FIG. 7  is a graph comparing the change of correction temperature for a conventional number of prints and the change of correction temperature for a number of prints according to the present invention; and 
         FIG. 8  is a flowchart illustrating correction of a fixing temperature in a fixing device according to the embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Preferred embodiments of the present invention will be described with reference to accompanying drawings. 
     As illustrated in  FIG. 1 , the image forming apparatus  1  according to the present embodiment employs electrophotography and includes, in order from top to bottom, an original sheet conveyance unit  100 , an image scanner  200 , an apparatus body  300 , and a duplex conveyance unit  400  that is disposed on a side of the main body  300 . 
     The original sheet conveyance unit  100  employs an automatic document feeder (ADF) that automatically and sequentially feeds a topmost sheet from original sheets placed thereon. The ADF can be provided as an option. 
     The original sheet conveyance unit  100  is openably closable relative to the image scanner  200 , and is hinged along the distal end of the image forming apparatus  1 . The original sheet conveyance unit  100  may be formed of a conventional structure. Accordingly, description of the original sheet conveyance unit  100  is omitted. 
     The image scanner  200  is an image scanning device to read both an original sheet being conveyed by the original sheet conveyance unit  100  and a still image of the original placed on a platen of the original sheet conveyance unit  100 . Image data of the original sheet read by the image scanner  200  is output to the apparatus body  300 . The image scanner  200  may be formed of a conventional structure. Accordingly, description of the image scanner  200  is omitted. 
     The apparatus body  300  includes a sheet feeder  10 , an exposure device  20 , an image forming device  30 , an intermediate transfer device  40 , a secondary transfer device  50 , and a fixing device  60  as denoted in an order of image forming process. 
     The sheet feeder  10  is disposed in the bottom of the apparatus body  300 . The sheet feeder  10  includes drawer-type paper trays  11  disposed in two-storied structure in the present embodiment. Specifically, they are an upper paper tray  11 A and a lower paper tray  11 B disposed vertically, step wisely. The paper trays  11  store recording sheets S as recording media. Each of the paper trays  11 A and  11 B is provided with a pair of sheet feed rollers  13 A and  13 B, respectively. Each of the pair of sheet feed rollers  13 A and  13 B disposed at a downstream end and at an upper portion thereof, feeds a topmost sheet from the paper tray  11 A or  11 B to send the fed sheet to a conveyance path  12 . 
     The exposure device  20  is disposed at an upper side of the topmost paper tray  11 A. The exposure device  20  radiates laser beams to the image forming device  30  based on the image data received from an original sheet or fixed original read by the image scanner  200  or the image data received via a PC or a telephone line. 
     The image forming device  30  includes, specifically, image forming units  30   c ,  30   m ,  30   y , and  30   k  for each color of cyan (c), magenta (m), yellow (y), and black (k). The image forming units  30   c ,  30   m ,  30   y , and  30   k  are serially disposed in 4-tandem method. Each of the image forming units  30   c ,  30   m ,  30   y , and  30   k  includes a drum-shaped image carrier  31  that rotates in the clockwise direction as illustrated in  FIG. 1 . Around each image carrier  31 , devices for charging, developing, transferring (i.e., a primary transfer), cleaning, and discharging are disposed to perform each operation in this order. Each image forming units  30   c ,  30   m ,  30   y , and  30   k  is supplied with toner as a developer for each color from toner bottles  32   c ,  32   m ,  32   y , and  32   k.    
     The intermediate transfer device  40  includes an endless intermediate transfer belt  41  that is stretched around a plurality of rollers substantially horizontally and moves to rotate in the counterclockwise direction in the figure. The intermediate transfer device  40  further includes primary transfer devices  42   c ,  42   m ,  42   y , and  42   k  opposed to each image carrier  31  of the image forming units  30   c ,  30   m ,  30   y , and  30   k  with the intermediate transfer belt  41  sandwiched in between. The primary transfer devices  42   c ,  42   m ,  42   y , and  42   k  cause a toner image formed on the image carrier  31  to be transferred to the intermediate transfer belt  41 . 
     The secondary transfer device  50  is disposed on a path of the conveyance path  12  and transfers the toner image as a primarily transferred image formed on the intermediate transfer belt  41  to a recording sheet S as a secondary transfer. 
     The fixing device  60  includes a heat roller  61  and a pressure roller  62 . The heat roller  61  is disposed at a side of the sheet surface and fixes the toner image transferred on the recording sheet S onto the recording sheet S and the pressure roller  62  is disposed at a sheet rear side and presses the recording sheet S against the heat roller  61 . The fixing device  60  according to the present embodiment serves as a fixing means. 
     The fixing device  60  causes the toner image to be fixed onto the recording sheet S by heating and pressurizing the recording sheet S onto which the toner image is secondarily transferred. The apparatus body  300  discharges the recording sheet S after toner fixation from a sheet discharger  14  to a sheet discharge tray  15 . 
     The duplex conveyance unit  400  is used to form images on double sides of the recording sheet S and includes a switchback unit  410  and a reverse unit  420 . In addition, the duplex conveyance unit  400  includes a manual sheet feeder  430  serving as a tray, other than the paper trays  11  that contains recording sheets S to be supplied to the apparatus body  300 . 
     The switchback unit  410  switches an upstream end in the conveyance direction of the recording sheet S the image on one side of which is fixed, with a downstream end of the recording sheet S and conveys the recording sheet S to the reverse unit  420 . The reverse unit  420  re-feeds the recording sheet S to an upstream end of the conveyance path  12  using a path to supply the recording sheet S from the manual sheet feeder  430  to the apparatus body  300 . 
     A sheet sensor  70  to detect media data of the upstream recording sheet S is disposed between the upper sheet feed roller  13 A and the secondary transfer device  50  in the path of the conveyance path  12 . In addition, in the conveyance path  12 , a registration roller pair  80  to adjust a conveyance timing of the recording sheet S is disposed downstream of the sheet sensor  70 . Further, in the conveyance path  12 , a conveyance roller pair  90  to convey the recording sheet S is disposed upstream of the sheet sensor  70 . 
     The sheet sensor  70  is disposed upstream of the registration roller pair  80  and calculates smoothness of the recording sheet S supplied from the paper trays  11  or from the manual sheet feeder  430  to the conveyance path  12 . The sheet sensor  70  detects smoothness of the recording sheet S used for setting fixing conditions including a fixing temperature, which will be described later. The sheet sensor  70  in the present embodiment is employed as a smoothness detection means. 
     Because the sheet sensor  70  is disposed downstream of the conveyance roller pair  90 , the sheet sensor  70  can obtain smoothness of all recording sheets S passing through the conveyance path  12  without providing the sheet sensor  70  at positions corresponding to the paper trays  11 A,  11 B, and the manual sheet feeder  430 , respectively. Further, because the sheet sensor  70  is disposed upstream of the registration roller pair  80 , when the recording sheet S is subjected to the registration process, that is, when the conveyance of the recording sheet S is temporarily stopped, the smoothness of the sheet S is obtained. Accordingly, even when the smoothness is obtained while the sheet S is moving, the accuracy of the obtained smoothness is high. Detailed structure of the sheet sensor  70  will be described later. 
     Examples of recording sheets include, for example, normal paper; coated sheets such as gloss, matt, and art paper; OHP sheets; and embossed sheets. These types of special sheets are increasing in number year by year. Recording materials other than the recording sheet also exist. 
     With contemporary image forming apparatus, setting of fixing conditions is generally performed in accordance with the basis weight of the recording medium. Paper, for example, is classified by basis weight into the following three types: Normal paper having a basis weight of from 60 to 90 grams/m 2 ; medium thickness paper having a basis weight of from 91 to 105 grams/m 2 ; and thick paper having a basis weight of from 106 to 300 grams/m 2 . For each class, fixing temperature and conveyance speed of the recording medium are different. 
     The basis weight of the recording medium is in general specified on the package so that the user can see it. Such basis weight information for setting fixing conditions is input using a control panel provided to a copier, so that the copier recognizes the settings. In the case of a printer, setting is performed by using a printer driver displayed on an attached personal computer (PC) to allow the basis weight information to be included in the printing information, so that the printer recognizes the settings. On the other hand, if the user needs to set the basis weight information manually via the control panel or using the PC, the setting work before printing is bothersome and a desired high-quality image cannot be obtained if erroneously set. 
     Provision of a sensor to detect a thickness of the recording medium that allows the apparatus to automatically select a recording medium and perform image formation has been made to cope with the above problem. In addition, generally, the smoothness of the recording medium is not printed on the package and it is very difficult for the user to obtain the smoothness information. Accordingly, the smoothness of the recording medium has to be obtained by a sensor, for example. 
     As described above, there is a high correlation between the smoothness and the fixation quality. However, the smoothness is measured as the time period in which a prescribed amount of air flows between the surface of the recording medium and the test plate, and therefore, it is difficult to detect the smoothness in a short period of time. Since the smoothness has a high correlation with surface roughness and quantity of reflected light, however, a sensor to measure the surface roughness and the reflected light quantity as an adequate substitute of smoothness is known. 
     As a conventional method of detecting smoothness, a light emitting element (LED) is used, illumination light emitted from the light emitting element (LED) irradiates the surface of the recording medium, and the quantity of reflected light from the surface of the recording medium is obtained, so that the smoothness of the recording medium is obtained from the reflected light quantity. According to this optical detection method, the smoothness can be obtained without contacting the recording medium, and therefore the recording medium is not damaged. 
     In addition, as a method for detecting the smoothness using this type of optical detection method, there is a method of detecting a type of material or level of smoothness of the recording medium based on the quantity of light reflected from the surface of the recording medium and the quantity of light permeating the recording medium. 
     There is also a method in which a light emitting source and two light receiving parts are disposed, light is emitted from the one light emitting source onto the surface of the recording medium, specular reflected light and diffusion reflected light from the light emitting source are received by the two light receiving parts, and the material (smoothness) of the recording medium is detected based on each light quantity by the light receiving parts. 
     The thus-obtained smoothness is, for example, used for setting fixing conditions such as a fixing temperature and image forming conditions. Accordingly, when the image forming apparatus employs the detected smoothness of the recording medium for setting fixing conditions and image forming condition, the smoothness needs to be detected in advance considering a prescribed time required from starting image formation to transfer onto a transfer sheet and until reaching a target fixing temperature, and therefore, a position of the sensor and a timing for detecting the smoothness are particularly important. 
     However, for example, when a paper tray that stores the recording media is replaced with another tray that stores recording media having a different smoothness, because a detected value for the previously-used recording media continues to be used as is, a problem occurs in that it takes a longer time until the detected value is changed to the fixing temperature suitable for the new recording media. 
     To cope with the above problem, one embodiment will be described below. 
     First, in the image forming apparatus  1 , the image scanner  200  reads the original image, and the exposure device  20  writes a latent image for a toner image of each color of the read original image on a surface of the image carrier  31  of each image forming unit  30   c ,  30   m ,  30   y , or  30   k  that is uniformly charged by the charger. 
     Then, in the image forming apparatus  1 , the developing device applies toner of each color to the latent image formed on each image carrier  31  of each image forming unit  30   c ,  30   m ,  30   y , or  30   k , so that the latent image is rendered visible as a toner image. 
     Next, in the image forming apparatus  1 , each toner image formed on the image carrier is sequentially and primarily transferred on the intermediate transfer belt  41  using the primary transfer devices  42   c ,  42   m ,  42   y , and  42   k , so that a desired full-color image is formed on the intermediate transfer belt  41 . 
     On the other hand, either the sheet feed rollers  13 A or  13 B in the two-storied paper trays  11 A,  11 B is selectively rotated so that the recording sheet S is fed out from the corresponding paper trays  11  or the recording sheet S is fed out from the manual sheet feeder  430 . 
     In the image forming apparatus  1 , the recording sheet S fed out from the paper trays  11  or the manual sheet feeder  430  is conveyed to the conveyance path  12 . 
     In the image forming apparatus  1 , the recording sheet S conveyed to the registration roller pair  80  via the conveyance path  12  is conveyed to the secondary transfer position of the secondary transfer device  50  at a matched timing, taken by the registration roller pair  80 , with the toner image formed on the intermediate transfer belt  41 . 
     Herein, in the image forming apparatus  1 , the sheet sensor  70  calculates smoothness of the recording sheet S, and the secondary transfer device  50  transfers the color image on the intermediate transfer belt  41  to the recording sheet S. 
     Then, in the image forming apparatus  1 , the recording sheet S on which the color image is transferred is conveyed to the fixing device  60 , is heated and pressed at a nip portion of the fixing device  60 , so that the color image is fixed onto the recording sheet S. 
     Herein, when the image is to be formed on a backside of the recording sheet S, the image forming apparatus  1  causes a switching claw to switch the conveyance path of the recording sheet S one side of which a color image has been transferred to, so that the recording sheet S is conveyed to the duplex conveyance unit  400 . 
     The switchback unit  410  switches an upstream end in the conveyance direction of the recording sheet S with a downstream end of the recording sheet S, and conveys the recording sheet S to the reverse unit  420 . The reverse unit  420  re-feeds the recording sheet S to an upstream end of the conveyance path  12  using a path to supply the recording sheet S from the manual sheet feeder  430  to the apparatus body  300 . 
     After the recording sheet S has been re-fed, the image forming apparatus  1  causes a color image for the backside of the recording sheet S formed on the intermediate transfer belt  41  to be transferred to the recording sheet S secondarily similar to the case of the surface of the recording sheet S and causes the fixing device  60  to fix the secondarily transferred color image. 
     When the color image has been fixed entirely to the recording sheet S, the image forming apparatus  1  causes the recording sheet S on which the color image has been fixed to be discharged from the sheet discharger  14  onto the sheet discharge tray  15 , and the recording sheet S is stacked thereon. Thus, the image forming operation is terminated. 
       FIG. 2  is a view illustrating a structure of the sheet sensor  70 . 
     The sheet sensor  70  is constructed of a light source  71 , a collimator lens  72 , a specular reflected light sensor  73  serving as an optical sensor, an aperture  74 , and a control circuit  75 . 
     In the present embodiment, the light source  71  is formed of a vertical cavity surface emitting laser (VCSEL). Accordingly, the present light source  71  is more stable than a general light-emitting diode or facet laser diode (LD), can suppress far field pattern (FFP), and can provide a high precision optical system. Here, “FFP” means a beam divergence angle. The light source  71  may be formed of various other light sources such as LEDs other than the vertical cavity surface emitting laser (VCSEL). 
     The collimator lens  72  disposed between the light source  71  and an irradiated surface of the recording sheet S is a converging lens with an aspheric surface. The collimator lens  72  converts laser light beams emitted from the light source  71  into collimated light beams. Herein, “collimate” means to turn the laser beams emitted from the light source  71  into parallel beams neither divergent nor convergent. As a result, the collimated light beam means a laser beam adjusted to a parallel state. 
     The collimator lens  72  adjusts an incident angle of the laser beams emitted from the light source  71  to the recording sheet S and a parallelism of the collimated light beams, so that the sheet sensor  70  can improve the detection sensitivity of the smoothness of the recording sheet S. 
     The specular reflected light sensor  73  is disposed downstream of the reflected light surface of the recording sheet S in the light axis direction of the laser beams emitted from the light source  71 , and is a photodiode to detect reflected specular light beams onto the recording sheet S. 
     The specular reflected light sensor  73  detects the light intensity of the specular light beams reflected from the recording sheet S as a voltage and outputs the detection result in the form of an output signal to the control circuit  75 . 
     The aperture  74  is disposed between the irradiation surface of the recording sheet S and the specular reflected light sensor  73  and controls an incident angle of the reflected light beams incident to the specular reflected light sensor  73 . By providing the aperture  74 , the sheet sensor  70  secures quantity of reflected light beams reflected by the surface of the recording sheet S emitted from the light source  71  and controls the divergent light mixed in the reflected light beams, thereby preventing accuracy in the smoothness detection from decreasing. 
     The control circuit  75  is connected to the specular reflected light sensor  73  and calculates a smoothness of the recording sheet S from the sensor output detected by the specular reflected light sensor  73 . Functions of the control circuit  75  will be described later. 
     With this configuration, the sheet sensor  70  obtains the smoothness of the recording sheet S via operation of the control circuit  75 . 
     The thus-configured sheet sensor  70  detects the light power of the specular reflected light in the specular direction of the laser light beams emitted from the light source  71  to the recording sheet S, so that the smoothness on the surface of the recording sheet S can be detected. The sheet sensor  70  in the present embodiment functions as a smoothness detection means. 
       FIG. 3  is a functional block diagram illustrating architecture of the image forming apparatus  1 . 
     As illustrated in  FIG. 3 , the image forming apparatus  1  includes a central processing unit (CPU)  301 , disposed in the apparatus body  300 , and various elements. The CPU  301  is connected to the various elements via a bus, so that the CPU  301  controls each element and the capabilities of the image forming apparatus  1  can be exerted. 
     The original sheet conveyance unit  100 , the image scanner  200 , and the duplex conveyance unit  400  are connected to the CPU  301  and can be driven or controlled by the CPU  301 . In addition, the CPU  301  is further connected to the paper trays  11 , the conveyance path  12 , the sheet discharger  14 , and the manual sheet feeder  430 , and a drive system of each device is controlled, such that rollers  13 A,  13 B of the paper trays  11 A and  11 B are controlled by the CPU  301 . Further, the CPU  301  is connected to the exposure device  20 , the image forming device  30 , the intermediate transfer device  40 , the secondary transfer device  50 , the registration roller pair  80 , the conveyance roller pair  90 , the fixing device  60 , and the sheet sensor  70 , although all of these devices are not illustrated in  FIG. 3 . Further, the CPU  301  is connected to a memory  302 , a current control circuit  305 , an analog-to-digital (A/D) converter  306 , a voltage detector  307 , and an interface  308 . 
     An empty sensor  16 A detects whether or not the recording sheet S contained in the upper paper tray  11 A is empty and outputs a detection signal to the CPU  301 . A tray sensor  17 A detects whether or not the upper paper tray  11 A is pulled out from the apparatus body  300  and outputs a detection signal to the CPU  301 . 
     Similarly, an empty sensor  16 B that detects whether or not the recording sheet S contained in the lower paper tray  11 B is empty outputs a detection signal to the CPU  301  and a tray sensor  17 B that detects whether or not the lower paper tray  11 B is pulled out from the apparatus body  300  outputs a detection signal to the CPU  301 . 
     Further, an empty sensor  431  detects whether or not the recording sheet S contained in the manual sheet feeder  430  is empty and outputs a detection signal to the CPU  301 . 
     Accordingly, the CPU  301  determines that the state of the paper tray has changed when the empty sensors  16 A,  16 B, and  431  detect that the paper tray is vacant and when the tray sensors  17 A,  17   b  detect that the paper trays  11 A and  11 B are pulled out from the apparatus body  300 . 
     It is noted that the upper paper tray  11 A, the lower paper tray  11 B, and the manual sheet feeder  430  are containers to contain the recording sheet S. In addition, the empty sensors  16 A,  16 B, and  431  and the tray sensors  17 A,  17   b  function as status sensors to detect a storage status of the recording sheet S. Further, because the manual sheet feeder  430  is installed at a side of the duplex conveyance unit  400  when not in use, another sensor to detect whether or not the manual sheet feeder  430  is disposed angled relative to the side of the duplex conveyance unit  400  (as illustrated in  FIG. 1 ) can be provided, similarly to the above-described tray sensors  17 A,  17 B. 
     The fixing device  60  includes a heat source  64  of the heat roller  61 , a heat source control circuit  63 , and a thermistor  66  to detect a temperature of the heat source  64 . The heat source control circuit  63  determines a heat quantity to be supplied to the heat source  64 , that is, a target fixing temperature. 
     Herein, to obtain a high-quality image as described above, the target fixing temperature should be determined considering the smoothness, which has a very high correlation with the fixing quality. Accordingly, the control circuit  75  determines the target fixing temperature set for the heat source control circuit  63  according to the sensor value from the sheet sensor  70  that detects the smoothness of the surface of the recording sheet S. 
     In addition, the fixing device  60  includes an A/D converter  65  that converts an analog value detected by the thermistor  66  into a digital value and sends the converted digital value to the CPU  301  to be processed by the CPU  301 . In addition, the fixing device  60  includes a pressure control circuit  67  that controls pressure of the pressure roller  62  pressing the heat roller  61  and thus a width of the nip portion formed thereby. 
     In addition, because the control circuit  75  of the sheet sensor  70  is connected to the fixing device  60 , a signal sent from the control circuit  75  is received by the fixing device  60 , so that the heat source control circuit  63  and the pressure control circuit  67  are controlled. As such, the control circuit  75  in the present embodiment serves as a means to control the temperature. 
     The memory  302  includes a read-only memory (ROM)  303  and a random access memory (RAM)  304 . The ROM  303  includes program codes and patterns to control fixation that allows the CPU  301  to execute. The RAM  304  temporarily stores detected voltages. 
     The CPU  301  reads the program codes stored in the ROM  303  and loads the data into the RAM  304 . While using the RAM  304  as a data buffer, the CPU  301  executes each program defined by the program codes and controls each element. 
     The current control circuit  305  receives a signal sent from the control circuit  75  of the sheet sensor  70  and controls transfer current values when the secondary transfer device  50  transfers a toner image to a recording sheet S. 
     The A/D converter  306  converts analog voltages detected by the voltage detector  307  into digital values to be processed by the CPU  301 , and sends them to the CPU  301 . 
     The interface  308  serves as an interface for the connection with a data storage  309  such as a hard disk drive and an external communications device  310  such as a personal computer, and thus, image data is transferred from an external device to the image forming apparatus  1 . 
     In the image forming apparatus  1  according to the present embodiment, a representative smoothness (M) is obtained from various smoothness values obtained for each of the plurality of recording sheets S to set a more appropriate target fixing temperature, and the target fixing temperature is corrected depending on the representative smoothness (M). 
     The representative smoothness (M) is used to determine a corrected temperature from a correction temperature list relative to the prescribed representative smoothness (M) as illustrated in  FIG. 4 . In actuality, an increase or decrease in the correction temperature is set as the target set temperature relative to the current target fixing temperature of the fixing device  60 . 
     When any of the empty sensors  16 A,  16 B,  431  and the tray sensors  17 A,  17 B detects a change in the status of the recording sheet S, the detection value stored in the memory  302  is reset and the status returns to an original state. Specifically, when the detection value is reset, the increase or decrease in the correction temperature is set to substantially zero ‘0’. 
     Thus, in the present embodiment, the target fixing temperature of the fixing device  60  is obtained as follows: First, a previous detection value and a next detection value of the recording sheet S detected by the sheet sensor  70  are sequentially stored in the memory  302 , so that an average smoothness (m) is obtained. Next, a representative smoothness (M) is calculated using the average smoothness (m) and a standard deviation (σ), and an increase and decrease value is determined based on the representative smoothness (M). 
     Herein, as illustrated in  FIG. 5 , a coefficient A of the standard deviation (σ) used in calculating the representative smoothness (M) is preferably 3, based on ±δ3 in which 99.7% of area in a normal distribution based on the average smoothness (m) can be covered. 
     In addition, a number of prints B of the reset timing to correct the representative smoothness (M) in a calculation formula may be an 8th sheet as illustrated in  FIG. 6  in which the standard deviation becomes stable; however, because the greater number is more preferable from the view of stability, a 10th sheet is suitable. 
     Accordingly, as illustrated in  FIG. 7 , compared to a case in which a correction is not performed based on the representative smoothness (M), the increase and decrease value of the correction temperature can be more stable when the correction based on the representative smoothness (M) is performed. 
     Accordingly, an appropriate target fixing temperature to which temperature correction is applied can be obtained, so that the fixing operation at the target fixing temperature can be performed quickly, and the temperature control relating to the fixing process can be performed effectively. 
     More specifically, the target fixing temperature of the fixing device  60  is determined for the following recording sheet S in response to the detected smoothness. 
     Next, with reference to  FIG. 8 , a determination of the target fixing temperature according to the present embodiment will be described. 
     In Step S 1 , the CPU  301  controls the heat source control circuit  63  with a preset initial value, that is, a target fixing temperature set as an initial value based on a sheet thickness and a smoothness of an A4 regular sheet, that is regularly used as a recording sheet S, and causes the heat source control circuit  63  to perform an image forming process. 
     Next, in Step S 2 , the CPU  301  detects a storage status of the recording sheet S contained in paper trays  11  or a manual sheet feeder  430 , and in particular, a paper tray  11 A or  11 B selected by a user or automatically by the CPU  301 . 
     Specifically, each empty sensor  16 A,  16 B,  431  or the tray sensor  17 A,  17 B performs detection to detect whether or not there is a possibility that the smoothness of the recording sheet S has changed due to, for example, a replacement of the recording sheet S. 
     Herein, when it is detected that the recording sheet S is not replaced (NO) and that the image forming process is performed, the CPU  301  controls the heat source control circuit  63  such that the image forming process is performed with the same target fixing temperature as in the previous image forming process. 
     On the other hand, when it is detected that the recording sheet S is replaced (YES) and that the image forming process is performed, the CPU  301  resets the target fixing temperature set in the previous image forming process and the detection value stored in the memory  302  and controls the heat source control circuit  63  with an initial value. 
     Note that the present storage status change is, if detected by the empty sensor  16 A,  16 B, or  431 , recognized as an interrupt signal, resetting is not always performed in the timing of Step S 2 . In addition, when the empty sensor  16 A,  16 B, or  431  detects a storage state change, there are many cases in which a same type of recording sheet S is replenished. In such a case, there is no need of resetting the initial value. 
     Then, in Step S 3 , the CPU  301  starts conveyance of the recording sheet S, causes the registration roller pair  80  to correct skew of a previous recording sheet S 1  and the sheet sensor  70  to detect the smoothness at a time of registration such as a secondary transfer timing adjustment. 
     In Step S 4 , the CPU  301  calculates an average smoothness (m) based on the detection value stored in the memory  302 . Herein, when resetting to an initial value is not performed in Step S 2 , even though the recording sheet S is for the 1st image forming process, the memory  302  includes detection values stored in the past before the previous time. 
     As a result, in Step S 4 , the CPU  301  obtains an average smoothness (m) according to a formula 1 and stores the obtained value in the memory  302 , wherein “m” is the average smoothness of the detection value stored in the memory  302 , “Xi” is a variable stored in the memory  302 , “N” is a number of recording sheets S supplied, and “σ” is the standard deviation. 
     
       
         
           
             
               
                 
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     In addition, in Step S 3 , the CPU  301  calculates a standard deviation (σ) by a dispersion formula 2 and stores the obtained value in the memory  302 . 
     
       
         
           
             
               
                 
                   
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     In Step S 5 , the CPU  301  controls the heat source control circuit  63  to change the target fixing temperature using the average smoothness (m) and the standard deviation (σ). 
     Herein, for example, in the case of using a regular sheet having a high smoothness, because the change in the detected value detected by the sheet sensor  70  is small, the average smoothness (m) does not change drastically. As a result, in such a case, the CPU  301  controls the heat source control circuit  63  to change the target fixing temperature using the average smoothness (m) or its approximation. 
     By contrast, for example, when a recording sheet S having a large surface roughness is used, change in the detection value detected by the sheet sensor  70  is large, so that the average smoothness (m) changes a lot. When the variations in the smoothness are large, the target fixing temperature varies for each recording sheet S, so that the temperature control is unstable. As a result, in such a case, the CPU  301  controls the heat source control circuit  63  to change the target fixing temperature using the standard deviation (σ) or its approximation. 
     Next, in Step S 6 , the CPU  301  determines whether or not the number (N) of prints used in calculating the average smoothness (m) reaches a prescribed number B, for example, 10 sheets. 
     When the CPU  301  determines that the number (N) of prints does not reach the prescribed number B (10 sheets) (NO in S 6 ), the CPU  301  does not perform correction of the target fixing temperature (Step S 7 ), and the process moves to a Step S 11  with the target fixing temperature updated in Step S 5 . 
     On the other hand, when the CPU  301  determines that the number (N) of prints reaches the prescribed number B (10 sheets) (YES in Step S 6 ), referring to the average smoothness (m) and the standard deviation (σ) stored in the memory  302  (Step S 8 ), the CPU  301  calculates a representative smoothness (M) by a formula M=m−Aσ, in which M is the representative smoothness and A is a rational number (in Step S 9 ). 
     In Step S 10 , the CPU  301  outputs a correction value according to the representative smoothness (M) to the control circuit  75 . As a result, the control circuit  75  outputs a control signal obtained by adding a correction value to or subtracting a correction value from the target fixing temperature, to the heat source control circuit  63  and controls it, and the CPU  301  repeats the above routine until the end of the image forming process (Step S 11 ). 
     As described heretofore, the image forming apparatus according to the present embodiment includes the paper trays  11 ,  430  to contain the recording sheet S and feed it, the empty sensors  16 A,  16 B, and  431  or the tray sensors  17 A and  17 B to detect the storage status of the recording sheet S contained in the paper trays  11 ,  430 , the sheet sensor  70  to detect the smoothness in a prescribed area on the surface of the recording sheet S, the memory  302  to store the detection value of the smoothness detected by the sheet sensor  70 , the fixing device  60  to heat and press the toner image transferred onto the recording sheet S and fix it onto the recording sheet S, and the control circuit  75  to determine the target fixing temperature of the fixing device  60  based on the detection values stored in the memory  302 . The control circuit  75  stores the detection value of the previous recording sheet S detected by the sheet sensor  70  to the memory  302 , and causes to sequentially detect a smoothness of the successive recording sheets S and to store the detection value to the memory  302 . The control circuit  75  determines the target fixing temperature for the successive recording sheets S depending on the detected smoothness. The sensors  16 A,  16 B,  431 ,  17 A, and  17 B each detect a change in the storage status of the paper trays  11 ,  430  and the detection value stored in the memory  302  is reset to zero, thereby reducing the time to change the target fixing temperature of the fixing device  60  for the recording sheet S in a case in which it is forecasted that the smoothness of the recording sheet S changes. 
     In the above embodiment, a case in which the recording sheet S 1  to S n  are fed from a same paper tray (for example, the paper tray  11 A) in the image forming process relative to a same print job has been described, but the present invention is not limited thereto. For example, the present embodiment of the invention can be applied to an image forming process of a mixed mode related to a series of print jobs using a copier function and different sizes (A4 and A3) of recording sheets S, that is, fed from different paper trays (for example, the paper trays  11 A and  11 B). In such a mixed mode, the CPU  301  switches the paper trays  11 A and  11 B depending on the change of the sizes of the originals detected by the original sheet conveyance unit  100 , and therefore, the detection value can be reset at the time of switching. 
     According to the image forming apparatus as disclosed herein, there is such an effect that the time to change the target fixing temperature of the fixing device can be reduced when it is prospected that the smoothness of the recording sheet will change, and further, the present invention may be applied to a copier, a facsimile machine, a printer, and a multifunction apparatus using the capabilities of the above devices in combination. 
     Additional modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described herein.