Abstract:
An image forming apparatus has an image forming unit together with a fixing unit which uses two rotating members to form a nip in which the recording material with an unfixed image is fed. A driving unit drives the rotatable fixing members and a flywheel is used for increasing an inertial force of that drive, there also being a connecting device which selectively connects the flywheel with the driving unit together with a switching device that switches operation of that connecting device in accordance with the type of recording material being used or the density of the image to be printed.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an image-forming apparatus using an electrophotographic system or an electrostatic recording process, and in particular relates to an image-forming apparatus such as a copying machine, a printer, and a facsimile machine, or to a multi-function machine having a plurality of these functions.  
         [0003]     2. Description of the Related Art  
         [0004]     An image-forming apparatus, such as a copying machine, a printer, and a facsimile machine, have widely used a method in which an unfixed toner image is formed on a recording material (a paper member such as transfer paper, photographic paper, dielectric-coated paper, and printing paper) formed thereon by a transfer system (indirect system) or a direct system with an appropriate image-forming mechanism, such as an electrophotographic system, an electrostatic recording system, and a magnetic recording system; then, the unfixed toner image is thermally fixed.  
         [0005]     Recently, in such an image-forming apparatus, various kinds of recording material has been required in addition to so-called copy paper. As an example, there is cardboard: in order to fix an image on cardboard you require large thermal capacity, and a pressurizing force applied to a fixing roller is an important condition as well as thermal capacity applied to the recording material, so that the pressurizing force also needs to be increased.  
         [0006]     However, as a negative effect of the increased pressure, the drive load violently increases at the moment when the leading edge of the cardboard is introduced into the nip between the fixing rollers. As a result, rotational variations of the fixing rollers are liable to be produced because of the rotational variation of a motor itself and the accumulation of micro-deflections of components of a drive system produced if the drive transfer path to the fixing rollers.  FIG. 18  is a graph showing rotational variations of a fixing roller  4   a  plotting time t in abscissa and peripheral speed V of the fixing roller  4   a  in ordinate, and symbol A denotes the rotational variations when the cardboard is introduced into the nip of the fixing roller  4   a . As shown in the drawing, the peripheral speed is reduced by the violently increased load. When the fixing roller  4   a  rides out the leading edge, the load is decreased; however, the peripheral speed is increased higher than a steady speed by the reaction of the drive system, which is deflected when the load is increased.  
         [0007]     Then, the rotational variations are attenuated so that the steady speed is returned: at this time, the following problems arise.  
         [0008]     Japanese Patent Laid-Open No. 6-318009 discloses that in order to prevent the speed of a fuser from varying when paper runs into the fuser, the drive system of the fuser may be provided with a flywheel.  
         [0009]     Japanese Patent Laid-Open No. 2000-19798 discloses a magnetic clutch is provided for selectively connecting between a drive system of a fuser and a flywheel, so that when a recording material is jammed, the connection between the drive system of the fuser and the flywheel is released so as to reduce a period of time required for stopping the fuser.  
         [0010]     A full-color image-forming apparatus is provided with a device for applying silicone oil as release agent on a fixing roller in order to improve releasability of a transfer agent from the fixing means and to prevent jamming.  
         [0011]     In such a device, since it is driven by the same drive source as that of the fixing roller  4   a , when the fixing roller varies in rotation, an oil applying roller  40  also varies in rotation. At this time, oil coating unevenness may be produced, resulting in uneven brightness and streaks of images. An example of such a device will be described below.  
         [0012]     Referring to  FIG. 15 , the fixing roller  4   a  and the pressure roller  4   b , which are for fusing the recording material in the nip by pressurizing and conveying it, are made of an aluminum core having elastic silicone rubber fixed on the surface; inside each core, halogen heaters  45   a  and  45   b  are arranged as heating sources; and thermistors (not shown) come in contact with on the respective surfaces of the silicone rubber for detecting temperatures of the surfaces. The controller in the body controls the electric power supply to the halogen heaters by comparing the temperature of each thermistors with a pre-established temperature so as to turn on the halogen heaters  45   a  and  45   b  via an AC driver whenever the temperature is lower than the established one while turning off electric power supply whenever the temperature is higher than the established one, thereby controlling the temperatures of the fixing roller  4   a  and the pressure roller  4   b  to maintain them constant.  
         [0013]     In the device shown in the drawing, part of a first pumping roller  43  is dipped in release oil O contained in an oil pan  65  while a second pumping roller  42  rotates adjoining the first pumping roller  43 . A third pumping roller  40  is adjacent to the second pumping roller  42 . The second pumping roller  42  is rotated by a drive source so as to transmit the driving force to a coating roller  40  for applying the release oil O via the third pumping roller  41  and the first pumping roller  43 . The second pumping roller  42  also serves as the rotational center for urging the coating roller  40  to the fixing roller  4   a  by a pressure spring  38 . In such a manner, the coating roller  40  applies the release oil O sequentially pumped by each of the pumping rollers on the surface of the fixing roller  4   a . A metering blade  44  for restricting the release agent made of an elastic body, such as fluoride rubber, and urged by a spring  49  so as to constantly maintain a predetermined pressure is arranged in contact with the coating roller  40 , thereby defining the amount of the oil on the coating roller  40  at a predetermined value. The oil on the coating roller  40  restrictedly applied thereon is transferred to the fixing roller  4   a . The residual oil on the coating roller  40  is removed outside the oil pan  65  by a cleaning blade  39  together with toner and paper dust stuck on the fixing roller  4   a . This residual oil is recovered to an oil tank (not shown), which will be described later, via a filter for recycling.  
         [0014]     Details will be described with reference to FIGS.  15  to  17 . As described above, the oil transferred to the pumping roller  41  from the pumping roller  43  and the oil pan  65  is restricted by the metering blade  44  on the coating roller  40 . The restricted oil forms a minute oil sump D between the edge of the metering blade  44  and the coating roller  40  along the entire longitudinal region during the steady rotation. The oil sump D is formed to keep the balance between the amount of oil scraping through the metering blade  44 , a surface tension, an oil viscosity, and the gravity. However, if vibration, such as rotational variations mentioned above, is applied, the oil sump D gets out of balance so that the oil pours out. The poured out oil is transferred on the recording material in a streak pattern via the fixing roller  4   a , and the length of the streak from the leading edge of the recording material is equivalent to distance L between the metering blade  44  and the nip of the fixing roller  4   a  ( FIG. 16 ). The oil streak is not noticeable in images if the amount is small; however, if the amount is increased, the oil streak affects images as uneven brightness and color shading, and this effect deteriorates in proportion to the intensity of the vibration, such as the rotational variations.  
         [0015]     However, when the flywheel is provided in the driving system of the fuser in order to suppress the rotational variations due to the recording material running into the nip of the fuser, since the flywheel must be rotated even for fixing images on thin paper having small rotational variation when running into the fuser, vibration due to the decentration of the flywheel and dispersion of the weight may be generated, deteriorating banding.  
         [0016]     In a small sized device with a small distance between the transfer and the fixing, the vibration also may especially affect the vicinity of an engine through a frame. Although the vibration is reduced if the flywheel and the drive transfer system are manufactured with high precision, this device has a defect of high cost including assembling requiring adjustment. When the fuser is started in a state that the flywheel is connected thereto, since the inertial load due to the flywheel is large, a motor larger than necessary must be used especially in using a stepping motor as a drive source.  
       SUMMARY OF THE INVENTION  
       [0017]     It is an object of the present invention to provide an image-forming apparatus capable of suppressing speed variations of fixing means when a recording material runs into a nip as well as suppressing reduction in life of driving means.  
         [0018]     It is another object of the present invention to provide an image-forming apparatus capable of preventing to wrongly detect the driving means as an abnormal rotational state when the driving means is connected to inertial means.  
         [0019]     In a first aspect of the present invention, an image-forming apparatus includes fixing means for fixing an image formed on a recording material at a nip; driving means for driving the fixing means; inertial means for increasing an inertial force; connecting means for selectively connecting between the driving means and the inertial means; and switching means for switching an operation of the connecting means in accordance with a kind of the recording material.  
         [0020]     In a second aspect of the present invention, an image-forming apparatus includes fixing means for fixing an image formed on a recording material at a nip; driving means for driving the fixing means; inertial means for increasing an inertial force; connecting means for selectively connecting between the driving means and the inertial means; detecting means for detecting abnormal rotation of the driving means; and stopping means for stopping the driving means when the abnormal rotation is detected, wherein when the connecting means is operated, the stopping means cancels a signal from the detecting means.  
         [0021]     In a third aspect of the present invention, an image-forming apparatus includes fixing means for fixing an image formed on a recording material at a nip; driving means for driving the fixing means; inertial means for increasing an inertial force; connecting means for selectively connecting between the driving means and the inertial means; and switching means for switching an operation of the connecting means in accordance with a density of an image to be formed on the recording material.  
         [0022]     Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]      FIG. 1  is a drive drawing of a fuser according to the present invention.  
         [0024]      FIG. 2  is a front view of an entire apparatus according to the present invention.  
         [0025]      FIG. 3  is a front view of the fuser according to the present invention.  
         [0026]      FIG. 4  is a drawing showing oil circulation of the fuser according to the present invention.  
         [0027]      FIGS. 5A and 5B  are graphs showing rotational variations of a fixing roller.  
         [0028]      FIG. 6  is a block diagram showing an image processor according to the present invention.  
         [0029]      FIG. 7  is a block diagram showing an image memory and peripheral devices.  
         [0030]      FIG. 8  is a block diagram showing an external I/F processor and the peripheral devices.  
         [0031]      FIG. 9  is a drawing showing an operation unit.  
         [0032]      FIG. 10  is a block diagram of an operational control unit.  
         [0033]      FIG. 11  is a flowchart of connection control of a flywheel.  
         [0034]      FIG. 12  includes setting tables for the flywheel and a fixing speed.  
         [0035]      FIG. 13  is a drawing showing another embodiment.  
         [0036]      FIG. 14  is a drawing showing another embodiment.  
         [0037]      FIG. 15  is a drawing showing a conventional example.  
         [0038]      FIG. 16  is a drawing showing a conventional example.  
         [0039]      FIG. 17  is a drawing showing a conventional example.  
         [0040]      FIG. 18  is a graph showing rotational variations of a peripheral speed of a fixing roller in a conventional example without the flywheel. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0041]     The present invention will be described more specifically by the following embodiments. Embodiments below are examples of the best mode for carrying out the invention; however, the present invention is not limited to these embodiments.  
         [heading-0042]     [First Embodiment] 
         [0043]     An embodiment of an electrophotographic copying machine using a recording material conveying device with the above-mentioned unit used therefor will be described below.  FIG. 2  is a schematic sectional view of the copying machine according to the first embodiment.  
         [0044]     First, referring to  FIG. 2 , a schematic structure of the copying machine will be described. In the copying machine, image information read by a reader unit  1  having a scanning optical system is photo-electrically converted and transmitted to an image-forming unit  2 , so that in the image-forming unit  2 , an image is formed on a recording material S supplied by a feeding unit  3 . The recording material S having the image formed thereon is conveyed to a fuser  4  in which a transferred image is fixed by heat and pressure applied thereto. Since a series of electrophotographic processes is known, detailed description is omitted.  
         [heading-0045]     (Reader Unit)  
         [0046]     A document D placed on a document table glass  11  is irradiated to light by a scanning optical system including a light source and a reflection mirror group. The reflected light is focused on a CCD (charge coupled device)  14  via a reducing lens  13 . The image information is transferred to a memory after photo-electrical conversion and A/D (analog to digital) conversion. The maximum document size is LTR (letter size) or A3 (Japanese Industrial Standard).  
         [heading-0047]     (Feeding Unit)  
         [0048]     In a lower portion of the copying machine, a paper cassette  31  having recording materials S accommodated therein is detachably mounted. On standby, a pick-up roller  32  is spaced from the surface of the recording material S by a turned-on solenoid (not shown) connected to the pick-up roller  32 . When the recording material is fed, the pick-up roller  32  is brought into contact on the surface of the recording material by the turned-off solenoid. Then, the top recording material is fed by the rotating pick-up roller  32 . The pick-up roller  32  is driven by a conveying roller  33  through a timing belt.  
         [0049]     The picked-up recording material is introduced into the nip between the conveying roller  33  and a retard roller  34 . The conveying roller  33  is rotated in a conveying direction of the recording material while the retard roller  34  is rotated in a direction opposite to the conveying direction via a torque limiter (not shown). Since only the leading edge of a first sheet exists in the nip, the torque limiter is overcome by a frictional force between the recording material and the conveying roller  33 , so that the retard roller  34  is rotated in the conveying direction. Then, when recording materials overlapping one another reach the nip, a frictional force between first and second recording materials is overcome by the torque limiter, so that the retard roller  34  is rotated in the direction opposite to the conveying direction. Thus, only the top sheet is separated and conveyed. Even if a plurality of the recording materials S are temporarily picked up, only the top sheet is separated to proceed in the same way.  
         [0050]     By such feeding operation, placed recording materials can be fed one by one.  
         [heading-0051]     (Conveying Unit)  
         [0052]     The recording material fed by the feeding unit  3  is once stopped at its leading edge by a resist roller  22 , and then is again fed in conformity with images formed by the image-forming unit  2 , so that the images are transferred in a transfer unit. The resist roller  22  is rotated by a stepping motor (not shown), and the rotation is controlled by a controller in the apparatus body.  
         [heading-0053]     (Image-Forming Unit)  
         [0054]     A laser beam having image information read by the reader unit  1  is emitted from a laser beam source  21 . The laser beam is scanned in a generating direction of a photosensitive drum  23  by the rotation of a polygon mirror  20  so as to form latent images on the drum surface electrically charged by a charger  24  in advance. The latent images are developed by a developer unit  25  arranged around the photosensitive drum  23 , and then, toner images are transferred on the recording material S by a transfer charger  26 . The residual toner on the drum surface after the image transfer is removed by a cleaner  27 .  
         [0055]     In this apparatus, the same image-forming units are sequentially arranged for cyan, magenta, yellow, and black, and read images are separated into the respective colors at each unit so as to form color images by multiple transfer.  
         [heading-0056]     (Fuser)  
         [0057]     The recording material S having toner images transferred thereon in the image-forming unit  2  is introduced to the fuser  4 , so that the toner images are fixed on the recording material S by the heat and pressure applied thereto during passage between a fixing roller  4   a  and a pressure roller  4   b.    
         [0058]     Details of the fuser will be described with reference to  FIG. 1 .  FIG. 1  is a sectional view of the fuser according to the first embodiment of the present invention.  
         [0059]     Referring to  FIG. 1 , the fixing roller  4   a  and the pressure roller  4   b , which are for fixing the recording material in the nip by pressurizing and conveying it, are made of an aluminum core having elastic silicone rubber fixed on the surface; inside each core, halogen heaters  45   a  and  45   b  are arranged as heating sources; and thermistors  45   a  and  45   b  come in contact with on the respective surfaces of the silicone rubber for detecting temperatures of the surfaces. The controller in the apparatus body controls the electric power supply to the halogen heaters by comparing the temperature of each thermistors with a pre-established temperature so as to turn on the halogen heaters  45   a  and  45   b  via an AC driver if it is lower than the pre-established temperature while turning off the halogen heaters  45   a  and  45   b  if it is higher than the pre-established temperature, thereby controlling the temperatures of the fixing roller  4   a  and the pressure roller  4   b  constant.  
         [0060]     In the device shown in  FIG. 3 , part of a first pumping roller  43  is dipped in release oil O contained in an oil pan  65  while a second pumping roller  42  rotates adjoining the first pumping roller  43 . A third pumping roller  40  is adjacent to the second pumping roller  42 . The second pumping roller  42  is rotated by a drive source so as to transmit a driving force to a coating roller  40  for applying the release oil O via the third pumping roller  41  and the first pumping roller  43 . The second pumping roller  42  also serves as the rotational center for urging the coating roller  40  to the fixing roller  4   a  by a pressure spring  38 . In such a manner, the coating roller  40  applies the release oil O sequentially pumped by each of the pumping rollers on the surface of the fixing roller  4   a . A metering blade  44  made of an elastic body, such as fluoride rubber, for restricting the release agent and urged by a spring  49  so as to constantly maintain a predetermined pressure is arranged in contact with the coating roller  40 , thereby defining the amount of the oil on the coating roller  40  at a predetermined value. The oil on the coating roller  40  restrictedly applied thereon is transferred to the fixing roller  4   a . The residual oil on the coating roller  40  is removed outside the oil pan  65  by a cleaning blade  39  together with toner and paper dust stuck on the fixing roller  4   a . This residual oil is recovered to an oil tank, which will be described later, via a filter for recycling.  
         [0061]     An oil application unit will be described in detail. The passing amount of the oil O pumped up by the first pumping roller  43  is restricted to some extent with a gap to the second pumping roller  42 . The gap between the first and second pumping rollers generally has a predetermined value (about 0.1 mm to 0.3 mm), with which the oil passing amount is determined. The silicone oil O having passed through this gap is pumped to a gap between the third pumping roller  41  and the second pumping roller  42 . The second pumping roller  42  has grooves with a depth of about 0.1 mm formed on the surface, and the oil O remaining in the grooves is transferred to the third pumping roller  41 . The third pumping roller  41  comes in contact with the coating roller  40  so as to slightly invade it, and has a small surface roughness of Ra 3.2 so as to avoid damage to the coating roller  40  with the silicone rubber surface. The material of each pumping roller is a metal. Then, the oil O transferred from the third pumping roller  41  is conveyed to the metering blade  44  for restricting the release agent along the surface of the coating roller  40 . The coating roller  40  is made up of a silicone rubber surface and a formed silicone sponge internal surface, which are fixed on a metallic core.  
         [0062]     The metering blade  44  is rotatable about a rotational axis, and is urged to the coating roller  40  at a predetermined pressure by the spring  49 . Thus, the passing amount of the oil on the coating roller  40  is restricted to have a desired value by passing through a contact gap to the metering blade  44 . The oil O is applied at an optimum film thickness to the fixing roller  4   a  via the nip to the fixing roller  4   a.    
         [0063]     The length of the second pumping roller  42  is larger than the distance between flanges  43   a  for securing a gap arranged on both sides of the first pumping roller  43 . The length of the third pumping roller  41  is smaller than those of the second pumping roller  42  and the oil coating roller  40 , and both ends of the third pumping roller  41  are tapered so as to avoid damage to the surface rubber of the coating roller  40 . The length of the coating roller  40  is 301 mm in consideration of a positional allowance ±2 mm of paper with a maximum document A3 size of 297 mm; and the length of the metering blade  44  is increased longer than that of the coating roller  40  so as to restrict the full surface.  
         [0064]     A thermistor  48  is arranged in the oil, and an oil temperature is detected by a controller so as to control the temperature at a predetermined value by passing electric current through a surface heater H via an AC driver. Hence, the fusing stability during machine startup can be ensured, and gloss variation due to a variation in the amount applied of oil can be prevented.  
         [0065]      FIG. 4  is a side view of the oil pan  65 : a float  50  is floating on the oil O contained in the oil pan  65 ; the float  50  is rotatably supported by a shaft  51  so as to move up and down in accordance with an oil level, and a flag  52  arranged so as to oppose the float  50  also rotates simultaneously. Thus, if the amount of the oil O is reduced by consumption, a sensor  53  cannot detect the flag  52 , so that an oil pump  54  is driven by the body controller detecting that signal. At this time, the oil pump  54  pumps up oil into the oil pan  65  from the oil tank  55  arranged below the pump. If the sensor  53  detects the flag  52 , the oil pump  54  is stopped operating so as to maintain the oil level of the oil pan  65  constant. The oil containing offset toner and paper dust scraped by the cleaning blade as described above drops into a recovery pan  60 . The dirt in the recovered oil is removed with a filter  61 , and the oil is again returned to the oil tank  55  for recycling use.  
         [0066]      FIG. 1  is a drawing showing a drive system according to the embodiment of the present invention: a drive unit  9  in the body is structured to have a drive output from a drive motor unit including a motor  90  and a gear box  91  to a coupling  92  via a reduction gear train. The fuser can be pulled out of the body toward oneself (in arrow direction in the drawing) for jam disposal and maintenance, and is connected to the body drive unit  9  via a coupling  93 . The drive force transmitted from the coupling is transferred to the fixing roller  4   a  via gears  94  and  95 . The gear  94  is mated with a back-stop one-way gear  96  so that the fixing roller  4   a  cannot be rotated in a direction opposite to the conveying direction by an external force from a user, etc. The pressure roller  4   b  basically follows the fixing roller  4   a , and it is driven to have a peripheral speed slightly smaller than that of the fixing roller  4   a  in case of slippage between the rollers. In order to prevent poor rotation due to the speed difference between the rollers, the speed difference is absorbed by using a one-way gear  97 , which is locked in the driving direction, so that the one-way gear  97  idles in the following state of the fixing roller  4   a . To the fixing roller  4   a , a hand knob  98  is connected for jam disposal. From the gear  94 , the drive force is transferred to the oil application unit so as to drive the coating roller  40 .  
         [0067]     On the other hand, a flywheel  103  is provided with gears  100  and  101  therebetween for increasing an inertial force. The flywheel  103  is controlled to have switched connection/disconnection by a magnetic clutch  102  connected between the flywheel  103  and the gear  101  under the control of the body controller. The number of teeth of the gear  101  is smaller than that of the gear  100 , so that the flywheel  103  is driven to have an increased speed. Since the inertial energy of a flywheel increases in proportion to the square of an angular velocity, the flywheel can be miniaturized.  
         [0068]     A control method according to the embodiment will be described below.  
         [0069]      FIG. 6  is a block diagram showing the internal structure of an image-forming section  170 , and components connected to an image memory  130 .  
         [0070]     First, the flow of the printing process of scanned images will be described: a document image focused on the CCD  109  via the lens  13  is converted into an analog electrical signal by the CCD  14 ; The converted image information enters an analog signal processor  300  so as to be A/D (analog-to-digital) converted in an AD-SH processor  301  after being corrected for sample-and-hold and dark level; and furthermore, shading correction is performed on the digitized signal. In the shading correction, dispersion in the CCD  109  for each pixel and positional dispersion in the light quantity due to light-distribution characteristics of a document illumination lamp  12  are corrected.  
         [0071]     Then, in an RGB line-to-line correction unit  302 , RGB line-to-line correction is executed. At one time, since light rays entered in respective RGB light receiving units of a CCD sensor  109  are displaced with each other on the document corresponding to the positional relationship of the respective RGB light receiving units, RGB signals are synchronized at this time.  
         [0072]     Then, in an input masking unit  303 , an input masking processing is carried out so as to convert luminance data into density data. Original RGB values produced from the CCD  109  are affected by a color filter attached to the CCD  109 , so that the effect is corrected for converting the values into pure RGB values.  
         [0073]     Thereafter, in a variable power unit  304 , images are vari-focused at a desired variable power rate, so that the vari-focused image data are transferred to the image memory  130  for storing.  
         [0074]     Computer image data is also entered into image memory  130  from external I/F processor  140 .  
         [0075]     In the course of printing the stored images: first, image data are transferred to a y corrector unit  305  from the image memory  130 ; and in the y corrector unit  305 , in order to make the output correspond to a density value established in an operation unit  203 , original density data are converted into density data corresponding to a desired output-density value based on a look-up table (LUT) prepared in consideration of printer characteristics.  
         [0076]     Then, the density data is transferred to a binarization unit  306 . In the binarization unit  306 , multiple-valued density data are binarized. In the multiple-valued density data, 8 bits of density data, for example, have any one of density values of 0 to 255; however, by the binarization, the density has only two values of 0 and 255. That is, in order to express the density of one pixel, 8 bits of data have been required, whereas, by the binarization, one bit of data is sufficient. As a result, the memory capacity required for storing image data is reduced; however, on the other hand, image gradation varies from the original gray scale of 256 to a gray scale of two, so that the quality of image data with a large number of half-tone images, such as picture images, may deteriorate as a result of binarization.  
         [0077]     Then, pseudo-halftone image expression by the binarized data becomes important. An error diffusion technique is incorporated here for that expression. In this technique, if the density of one image is larger than a threshold value, the image has a density value of 255 while if it is smaller than the threshold value, the image has a density value of zero, so that after the binarization, a difference between actual density data and the binarized density data is obtained as an error signal so as to distribute the error signal to circumferential pixels. The error distribution is executed by multiplying a predetermined weighting coefficient on a matrix by an error due to the binarization so as to add the product to circumferential pixels. Consequently, the average density value of the entire images is maintained so that the pseudo-halftone image can be expressed by the binarization.  
         [0078]     The binarized density data are transferred to a smoothing unit  307  in a printer section  2 . In the smoothing unit  307 , the data are complemented so that ends of the binarized image line are smooth, and the complemented image data are produced to an exposure control unit  120 . The exposure control unit  120  controls the irradiation timing from the laser beam source  21  on the respective color photosensitive drums  23  so as to form latent images of the image data.  
         [0079]     An external I/F processor  140  is an interface for transferring image data in the image memory  130  to a desired computer via a network.  
         [0080]      FIG. 7  is a block diagram showing an internal structure of the image memory  130 , and peripheral devices.  
         [0081]     The image memory  130  is composed of a page memory  401 , a memory controller  402 , a compression/expanding unit  403 , and a hard disk  404 .  
         [0082]     The image data supplied from the external I/F processor  140  and the image-processing section  170  to the image memory  130  are written in the page memory  401  by the memory controller  402 , and then, are fed to the printer section  2  or stored in the hard disk  404 . When the image data are stored in the hard disk  404 , the image data are compressed in the compression/expanding unit  403 , and are written in the hard disk  404  as compressed data. The image data stored in the memory controller  402  or the hard disk  404  are also read out to the page memory  401 . At this time, the compressed data read out of the hard disk  404  is expanded through the compression/expanding unit  403  so as to write the expanded image data in the page memory  401 . The memory controller  402  generates a DRAM refresh signal for feeding the signal to the page memory  401 , and also adjusts the access to the page memory  401  from the external I/F processor  140 , the image-processing section  170 , and the hard disk  404 . Furthermore, the memory controller  402  controls the determination of a writing address to the page memory  401 , a reading address from the page memory  401 , and a reading direction in accordance with the instruction of a CPU  201 . By these processions, the CPU  201  can control the producing function to the printer section  2  via the image-processing section  170 , the function producing part of images by cutting only the part thereof, and the rotating function of images after a plurality of document images are laid out in the page memory  401 .  
         [0083]     With regard to a sort mode, for a bundle of documents, a process in that images are readout in the order of being recorded in the image memory  130  so as to print them is repeated multiple times. By such a control process, even a finisher having a small number of bins can serve the same function as that of a finisher having a large number of bins.  
         [0084]      FIG. 8  is a block diagram showing the internal structure of the external I/F processor  140 , and peripheral devices. The external I/F processor  140  brings in image data from the reader unit  1  via the image memory  130  so as to be supplied to external computers or external facsimile machines through a network or telephone lines. Also, by the external I/F processor  140 , image data fed from external computers or external facsimile machines over a network or telephone lines are produced to the printer section via the image memory  130  and the image-processing section  170  so as to form images.  
         [0085]     The external I/F processor  140  is composed of a core section  506 , a facsimile section  501 , a hard disk  502  for storing communication image data of the facsimile section  501 , a computer interface section  503  for connecting external computers  190 , a format section  504 , and an image memory  505 .  
         [0086]     The facsimile section  501  is connected to a public circuit through a modem (not shown), thereby receiving facsimile communication data from the public circuit and sending facsimile communication data to the public circuit. The facsimile section  501  achieves facsimile functions of sending facsimile data at a designated time and sending image data corresponding to an inquiry through a specific password by using facsimile images stored in the hard disk  502 .  
         [0087]     Thus, after images are first supplied to the facsimile section  501  through the image memory  130  from the reader unit  1  so as to store them in the facsimile hard disk  502 , facsimile data can later be sent without using the reader unit  1  and the image memory  130  as facsimile functions.  
         [0088]     The computer interface section  503  is an interface for communicating data to external computers  190 , and includes a local area network (LAN), a serial I/F, a SCSI-I/F (small computer systems interface), and a Centronics I/F for inputting printer data. Through the computer interface unit  503 , external computers  190  are informed of states of the printer section  2  and the reader unit  1 . Alternatively, under instructions from an external computer  190 , images readout in the reader unit  1  are transferred to the external computer  190 .  
         [0089]     The computer interface section  503  also receives print image data from external computers  190 . At this time, since the print image data supplied from the external computers  190  are described under a dedicated printer code, in the format unit  504 , the supplied data code is converted into raster image data capable of forming images in the printer section  2 . The converted raster image data are developed in the image memory  505  by the format section  504 . On the other hand, when image data are sent to external computers  190  via the computer interface unit  503 , the image format unit  504  transforms the gray level of print image data supplied from the image memory  130  and also converts the print image data into an image format recognizable by the external computers  190  in the image memory  505 .  
         [0090]     The image memory  505  is also used in sending (network scanning function) image data from the reader unit  1  to external computers  190  in addition to be used as a memory developing the raster image data in the format unit  504  in such a manner. That is, when images from the reader unit  1  are sent to external computers  190  via the computer interface unit  503 , image data supplied from the image memory  130  are once developed in the image memory  505  so as to convert them here into a data format transmissible to external computers  190 , and then, the image data is fed from the computer interface unit  503  to the external computers  190 .  
         [0091]     The core section  506  controls data transfer executed among the facsimile unit  501 , the computer interface unit  503 , the format unit  504 , and the image memory  505 , and the image memory  130 . Thereby, even when a plurality of image outputs are connected to the external I/F processor  140 , or when there is only one image transfer path to the image memory  130 , exclusion control and priority control are performed under the administration by the core section  506 , so that images are appropriately outputted.  
         [0092]      FIG. 9  is a schematic view showing a structure of the operation unit  203  of the image-forming apparatus.  
         [0093]     Referring to the drawing, on a display  3001 , various messages, such as operating states of the apparatus and operating instructions to a user, and operating procedures are displayed.  
         [0094]     The surface of the display  3001  is a touch panel that serves as a selector key by touching thereon. A ten-key pad  3002  is a numerical pad for inputting numerals. By pushing the copy key  3003 , copying operation is started.  
         [0095]     With the operation unit, a user can specify the paper kind of the recording material to be used, the paper thickness, and copying density. When using as a printer, a user can specify the paper kind, the paper thickness, and copying density through an external computer  190 .  
         [0096]      FIG. 10  is a block diagram of an operation control unit of the image-forming apparatus. A CPU  201  basically controls the image-forming apparatus, and has an ROM  206  having control programs written therein, a work processing RAM  205 , and an input/output port  204  connected to the CPU  201  through an address bus and a data bus. Part region of the RAM  205  is a backup RAM in that data cannot be wiped out even if the power supply is turned off. To the input/output port  204 , various work load devices controlled by the image-forming apparatus, such as a motor and a clutch, and input devices to the image-forming apparatus  100 , such as a sensor for detecting a position of paper, are connected.  
         [0097]     The CPU  201  sequentially controls an input/output through the input/output port  204  in accordance with control programs of the ROM  206  so as to execute the image-forming process.  
         [0098]     The operation unit  203  is also connected to the CPU  201  that controls the display and key input portion of the operation unit  203 . As described above, a user instructs the CPU  201  to switch the image-forming operation mode and switch the display through the key input, and the CPU  201  displays operation states of the image-forming apparatus  100  and the operation mode established by the key inputting on the display of the operation unit  203 .  
         [0099]     To the input/output port  204 , an exposure controller for controlling to turn on the laser  21  for forming latent images on the drums from image data as described above, a high pressure controller, such as the electric charger  24  and the transfer charger  26 , a stepping motor controller for controlling a stepping motor for feeding and conveying recording materials, a fuser motor (MTR), a solenoid (SL), and various kinds of I/O controller for controlling sensors, such as a photo-interrupter, are connected. Also, the CPU exchanges data of various AD signals, such as a temperature/humidity sensor, a paper thickness sensor, and a rotational speed sensor of the fuser motor, so as to control the high voltage output of the charger, for example, in accordance with the data.  
         [0100]     The connection control of the magnetic clutch  102  for the flywheel  103  will now be described.  FIG. 11  is a control flowchart of the embodiment.  
         [0101]     Upon starting page print, the fuser motor  90  is started at a quarter of the fixing speed of normal paper according to a speed setting table.  
         [0102]     By the input from a user or by an automatic detection, the kind and the thickness of the recording material is specified so as to optimize image-forming conditions, such as the output of the charger, and to determine the connection state of the flywheel.  
         [0103]      FIG. 12  includes setting tables for the flywheel, in which, depending on the image density and the kind of paper, the flywheel is established whether it is connected or not.  
         [0104]     The solid-color image density (maximum density) for each color is designated as 100%, i.e., up to 400% because of four colors, and the density data are transferred from the image-forming unit  170  to the CPU  201 .  
         [0105]      FIGS. 5A and 5B  include graphs showing rotational variations of the fixing roller, plotting time in abscissa and fixing speed in ordinate. Symbol HY represents variations of the fixing speed when heavy cardboard digs into the fixing nip without the flywheel connected thereto; symbol NL represents the behavior of normal paper; symbols Ln and Lh denote limits that the shock during the digging affects images. The image defect does not appear on normal paper even without the flywheel.  
         [0106]     Thus, in the case of thin paper and normal paper with a basis weight less than 150 g/m 2  in that the fixing speed is high and the shock is small when the recording material runs into the fixing nip, the state that the flywheel is released and not connected over the entire image density range is maintained.  
         [0107]     On the other hand, in the case of heavy cardboard with a basis weight more than 150 g/m 2  in that the fixing speed is slow and the shock is large when the recording material runs into the fixing nip, the flywheel is connected over the entire image density range.  
         [0108]     In the case of cardboard with an intermediate thickness between the normal paper and the heavy cardboard and having a basis weight less than 150 g/m 2 , the flywheel is connected in a density range more than 100% because only the shock of high density images is conspicuous.  
         [0109]     In an OHP sheet (over head projector sheet), which is an optically transparent resin sheet, the flywheel is released over the entire image density range.  
         [0110]     In the case that the flywheel is connected according to a flywheel setting table, the magnetic clutch  102  is turned on after the fixing roller is started, and then, the fixing roller is speeded up to each speed.  
         [0111]     In such a manner, by connecting the flywheel at low speed, the load applied to the driving system (gears) due to impact during connection of the flywheel is alleviated.  
         [0112]     In order to secure power fusing, the fixing speed is established as follows: where the rotational speed of normal paper or thin paper denotes V, the fixing speed for cardboard is ⅔V; ½V for heavy cardboard; ⅓V for the OHP sheet; and ¼V for pre-revolution and post-revolution.  
         [0113]     Upon finishing the printing and discharging recording materials outside the machine, the speed of the fuser motor  90  is changed to the post-revolution speed according to the speed setting table, and then, the magnetic clutch  102  is turned off so as to release the flywheel. Upon completing the post-revolution, a series of page print controls is finished.  
         [0114]     If an abnormal situation, such as error and jam, occurs, the flywheel is released, and then, the fuser motor  90  is stopped (electric power supply to the fuser motor is stopped), so as to avoid damage of the body by reducing an inertial force and the time until the fixing roller stops. If the flywheel is released at the same time the fuser motor is stopped, the stop time can be further reduced effectively.  
         [0115]     According to the embodiment, the revolving speed (the number of revolution per unit time) of the fuser motor  90  (DC motor) is detected by the revolving speed detector ( FIG. 10 ), and the body controller (CPU) controls drive of the fuser motor in accordance with the detected revolving speed.  
         [0116]     If the detected revolving speed of the fuser motor varies from a predetermined speed, the controller determines that an abnormal situation occurs in the fuser motor so as to produce an error signal and to display error occurrence on the display of the operation unit. At this time, a stop signal is produced together with the error signal so as to forcedly stop the fuser motor.  
         [0117]     The error signal may be sent to a personal computer network connected to the image-forming apparatus with an LAN cable so as to display the fuser motor error on a monitor of the personal computer.  
         [0118]     During operation of the magnetic clutch, i.e., when the flywheel is connected and released, by ignoring the signal from the revolving speed detector for two seconds, error detection is prevented in that the abnormal revolving speed occurs although it is not generated.  
         [0119]     In such a manner, the error detection due to variations of the revolving speed in the fuser motor when the flywheel is connected and released is avoided so as to successively control the fuser motor so that the fuser motor approaches the fixing speed as soon as possible.  
         [0120]     According to the embodiment, the color printer is exemplified; alternatively, a monochrome printer may also incorporate the invention, and in particular the invention is effectively incorporated to a printer where the distance between an image-forming unit and a fuser is small.  
         [heading-0121]     [Second Embodiment] 
         [0122]     In a second embodiment, the flywheel is provided adjacent to a fixing roller gear. If there is a space for arranging the flywheel above the unit, this embodiment is more advantageous for reduced rotational variations due to a shock because the number of drive trains is small so that effects of deflection and backlash are smaller. Like reference numerals designate like portions with the same functions.  
         [0123]     With the fixing roller gear  95 , an adjacent gear  104  with a small number of teeth is mated so as to drive and increase the flywheel  103  in speed. In the same way as in the first embodiment, the magnetic clutch  102  can be connected to control it. Since the number of revolution of the flywheel is smaller than that of the first embodiment, the flywheel with a larger diameter is used for having the same effect.  
         [0124]     The control is the same as that of the first embodiment.  
         [heading-0125]     [Third Embodiment] 
         [0126]     A third embodiment is another example where the flywheel is provided adjacent to the fixing roller gear. If there is a space for arranging the flywheel and the motor above the unit, this embodiment is more advantageous for reduced rotational variations due to a shock because the number of drive trains is small and furthermore, effects of deflection and backlash are smaller. With reference to  FIG. 13 , like reference numerals designate like portions with the same functions, as in the first embodiment.  
         [0127]     With the fixing roller gear  95 , an adjacent gear  94  with a small number of teeth is mated, and the flywheel  103  is fitted to a drive shaft  105  of the drive gear  94 . In the same way as in the first embodiment, the magnetic clutch  102  can be connected, and during connection, an inertial load is applied to the drive shaft  105 . Since the number of revolutions of the flywheel is smaller than that of the first embodiment, the flywheel with a larger diameter is used for having the same effect.  
         [0128]     As described above, according to the present invention, by selectively connecting the inertial means, while a shock when the recording material runs into the fixing nip is prevented from affecting image quality, reduction in life of the drive system can be suppressed. Also, since a large sized device and components with high accuracy are not necessary, cost reduction is enabled.  
         [0129]     While the present invention has been described with reference to what are presently considered to be the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. On the contrary, the invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.  
         [0130]     This application claims priority from Japanese Patent Application No. 2003-331932 filed Sep. 24, 2003, which is hereby incorporated by reference herein.