Abstract:
A fixation device includes a first conveyance member, a first press member facing the first conveyance member, a second press member facing the first conveyance member, and a support mechanism supporting the first and second press members and capable of switching whether or not to press the first and second press members against the first conveyance member.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority based on 35 USC 119 from prior Japanese Patent Application No. 2010-210572 filed on Sep. 21, 2010, entitled “FIXATION DEVICE AND IMAGE FORMATION APPARATUS”, the entire contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present disclosure relates to a fixation device and an image formation apparatus such as an electrophotographic printer, a copier, a facsimile, or the like. 
         [0004]    2. Description of Related Art 
         [0005]    For conventional image formation apparatuses, there is a known technique to change pressure between a press roller and a fixation roller in a fixation device according to printing conditions in the process to transfer a developer corresponding to a print image to a recording sheet and fuse the developer to the recording sheet by heat and pressure (see Japanese Patent Application Publication No. 2009-294331). 
       SUMMARY OF THE INVENTION 
       [0006]    In the conventional image formation apparatus, the fixation device may include a press pad used as an auxiliary press member such that the unrotatable press pad and the rotatable press roller are simultaneously pressed against the fixation roller, in order to increase the contact area between the fixation device and the recording sheet. If the fixation roller or the press roller is rotated while heating even when no sheet is conveyed between the fixation roller and the press roller, the press pad is gradually degraded, which may cause failures including increasing torque due to friction and insufficient charge. 
         [0007]    A first aspect of the invention is a fixation device including: a first conveyance member; a first press member facing the first conveyance member; a second press member facing the first conveyance member; and a support mechanism supporting the first and second press members and capable of switching between whether or not to press the first and second press members against the first conveyance member. 
         [0008]    A second aspect of the invention is an image formation apparatus including: the fixation device according to the first aspect; and a controller configured to control the support mechanism to switch between whether or not to press the first and second press members against the first conveyance member. 
         [0009]    A third aspect of the invention is a fixation device including: a first conveyance member; a first press member facing the first conveyance member; a second press member facing the first conveyance member; and a controller configured to switch between whether or not to press the first and second press members against the first conveyance member. 
         [0010]    According to one of the aspects, the first press member or the second press member can be pressed against the first conveyance member only when a sheet is fed to the fixation device. This allows the first press member or the second press member to come into contact directly or indirectly with the first conveyance member only during a minimum period of time, thus preventing degradation such as abrasion. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIGS. 1A and 1B  are schematic configuration views showing a fixation device of Embodiment 1. 
           [0012]      FIG. 2  is a schematic configuration view showing an image formation apparatus of Embodiment 1. 
           [0013]      FIG. 3  is a schematic block diagram showing a print controller of the image formation apparatus of Embodiment 1. 
           [0014]      FIGS. 4A and 4B  are external views showing a press pad of Embodiment 1. 
           [0015]      FIG. 5  is a view showing a support frame of the press pad of Embodiment 1. 
           [0016]      FIGS. 6A and 6B  are schematic configuration views showing a release mechanism of Embodiment 1. 
           [0017]      FIGS. 7A and 7B  are views showing an operation of a pad lever in Embodiment 1. 
           [0018]      FIGS. 8A and 8B  are views showing an operation of a roller lever in Embodiment 1. 
           [0019]      FIGS. 9A and 9B  are diagrams showing nip modes of press members at rotational angles of a camshaft. 
           [0020]      FIG. 10  is a flowchart showing an operation of the image formation apparatus of Embodiment 1. 
           [0021]      FIG. 11  shows time charts of a printing operation in Embodiment 1. 
           [0022]      FIG. 12  is a view showing the printing operation of the image formation apparatus and an operation of the fixation device in Embodiment 1. 
           [0023]      FIGS. 13A and 13B  are schematic configuration views showing a fixation device of Embodiment 2. 
           [0024]      FIG. 14  shows time charts showing a problem of the printing operation of Embodiment 1. 
           [0025]      FIG. 15  shows time charts of a printing operation of Embodiment 2. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0026]    Descriptions are provided below for embodiments based on the drawings. In the respective drawings, the same constituents are designated by the same reference numerals and duplicate explanation concerning the same constituents is omitted. All of the drawings are provided to illustrate the respective examples only. 
       Embodiment 1 
     (Configuration of Embodiment 1) 
       [0027]      FIG. 2  is a schematic configuration view showing an image formation apparatus of Embodiment 1. 
         [0028]    An image formation apparatus  10  includes sheet feeder  11 , a conveyance unit ( 15   a ,  15   b ,  17   a , and  17   b , for example), image formation unit  20 , fixation device  40 , a sheet ejection mechanism, and stacker  53 . Sheet feeder  11  feeds recording media (for example, recording sheets)  100 . The conveyance unit, or a recording medium conveyance unit, is configured to convey recording sheets  100 . Image formation unit  20  forms a toner image as a developer image on each recording sheet  100 . Fixation device  40  serves as a fixer or a fuser and is configured to fix the toner image on each recording sheet  100 . The sheet ejection mechanism constitutes the recording medium conveyance unit ejecting recording sheets  100 . Stacker  53  accommodates ejected recording sheets  100 . Image formation apparatus  10  further includes: sheet conveyance motor  19  (shown in  FIG. 3 , described later) configured to rotate rollers; a clutch turning on/off power transmission to the rollers on medium conveyance path  101 ; image formation unit power supply  65  shown in  FIG. 3 , described later; and a low-voltage power supply configured to supply 5V direct current and 24V direct current to the circuit and motors. 
         [0029]    Sheet cassette  110  stores recording sheets  100  therein. Sheet feeder  11  includes: sheet cassette  110  loaded at the bottom of image formation apparatus  10 ; recording sheets  100  stored in sheet cassette  110 ; sheet sensor  111  configured to determine the presence of recording sheets  100  in sheet cassette  110 ; pickup roller  12  configured to separate and pick up recording sheets  100  one by one from sheet cassette  110  in conjunction with a strap for separation; hopping sensor  13  configured to determine whether the recording sheets  100  are being fed; sheet feed roller  14   a ; and retard roller  14   b.    
         [0030]    Sheet cassette  110  stores the plurality of recording sheets  100  and is detachably loaded at the bottom of image formation apparatus  10 . Recording sheets  100  may be sheets of quality paper, recycled paper, glossy paper, mat paper, OHP (overhead projector) film, or the like and have a predetermined size. Images are recorded on recording sheets  100 . 
         [0031]    Pickup roller  12  rotates in pressure contact with recording sheets  100 . Hopping sensor  13 , sheet feed roller  14   a , and retard roller  14   b  are located downstream of pickup roller  12  in medium conveyance path  101 . Sheet feed roller  14   a  and retard roller  14   b  face each other so as to sandwich each recording sheet  100 . 
         [0032]    The conveyance unit includes conveyance roller  15   a , pinch roller  15   b , resist roller  17   a , and pinch roller  17   b.    
         [0033]    The conveyance roller  15   a  and pinch roller  15   b  are opposed to each other downstream of sheet feed roller  14   a  and retard roller  14   b  in medium conveyance path  101  so as to sandwich each recording sheet  100 . Conveyance roller  15   a  is driven by sheet conveyance motor  19  shown in  FIG. 3 , later described, and pinch roller  15   b  rotates together with conveyance roller  15   a.    
         [0034]    Resist roller  17   a  and pinch roller  17   b  are opposed to each other downstream of conveyance roller  15   a  and pinch roller  15   b  in medium conveyance path  101  so as to sandwich each recording sheet  100 . Resist roller  17   a  is driven by sheet conveyance motor  19  shown in  FIG. 3 , later described, and pinch roller  17   b  rotates together with resist roller  17   a.    
         [0035]    Print start position sensor  18  as a recording medium detector is provided downstream of resist roller  17   a  and pinch roller  17   b  in medium conveyance path  101 . 
         [0036]    Image formation unit  20  includes image forming unit  22 , transfer roller  21 , and light emitting diode (hereinafter, referred to as an LED) head  25  as an exposure device. LED head  25  is attached to image forming unit  22  and projects light corresponding to image information onto the surface of photoreceptor drum  23 . Furthermore, image forming unit  22  is separated into developer container (hereinafter, referred to as a toner cartridge)  55  positioned in an upper part and image forming unit body  22   a  positioned in a bottom part of image formation unit  20 . 
         [0037]    Image forming unit body  22   a  includes: photoreceptor drum  23  configured to carry an electrostatic latent image based on the image information; charge roller  24  as a charge member configured to charge photoreceptor drum  23 ; development roller as a developer supporter configured to develop the electrostatic image on the surface of photoreceptor drum  23  with toner as a developer; supply roller  27  as a supply member configured to supply the toner to development roller  26 : development blade  28 ; and cleaner  29  configured to scrape toner remaining on photoreceptor drum  23 . Charge roller  24 , development roller  26 , and cleaner  29  are pressed against photoreceptor drum  23  with predetermined amounts of contact. Development blade  28  and supply roller  27  are pressed against development roller  26  with predetermined amounts of contact. 
         [0038]    Image formation unit  20  includes a development unit configured to develop a toner image on each recording sheet  100 , for example. 
         [0039]    Photoreceptor drum  23  has a circular cylindrical shape and is rotatably supported. Photoreceptor drum  23  includes a photoreceptor layer on a conductive supporter made of aluminum or the like. The photoreceptor layer includes a photoconductor layer and a charge conveyance layer. Photoreceptor drum  23  is provided so as to abut on charge roller  24 , transfer roller  21 , and development roller  26 . Furthermore, photoreceptor drum  23  is provided so as to come into contact with the tip end of the cleaner  29 . Photoreceptor drum  23 , serving as an image carrier, is rotatable in a direction indicated by an arrow in the drawing. Photoreceptor drum  23  is configured to retain charges and carry a toner image on the surface thereof. Hereinafter, the constituent components of image formation unit  20  are described sequentially in the rotational direction of photoreceptor drum  23 . 
         [0040]    Charge roller  24  includes a conductive metallic shaft covered with semiconductive rubber such as silicon rubber and has a circular cylindrical shape. Charge roller  24  is rotatably supported in pressure contact with photoreceptor drum  23 . Charge roller  24  is charged by image formation unit power supply  65  shown in  FIG. 3 , later described, and rotates in pressure contact with photoreceptor drum  23  to apply a predetermined voltage to photoreceptor drum  23 . Photoreceptor drum  23  therefore uniformly stores charges in the surface. 
         [0041]    LED head  25  includes a plurality of LEDs, a lens array, and an LED driving device and is provided above photoreceptor drum  23 . LED head  25  projects light corresponding to image information onto the surface of photoreceptor drum  23  to form an electrostatic latent image on the surface of photoreceptor drum  23 . 
         [0042]    Supply roller  27  is formed so as to cover a conductive metallic shaft and has a circular cylindrical shape. Supply roller  27  is provided so as to abut on development roller  26 . Supply roller  27  has a voltage applied by image formation unit power supply  65  shown in  FIG. 3 , later described, and comes into pressure contact with development roller  26  to supply toner to development roller  26 . 
         [0043]    Development roller  26  includes a conductive metallic shaft covered with a semiconductive urethane rubber material or the like and has a circular cylindrical shape. Development roller  26  is provided so as to abut on supply roller  27  and photoreceptor drum  23  and to come into contact with the edge of development blade  28 . Development roller  26  has a voltage applied by image formation unit power supply  65  shown in  FIG. 3 , later described, and attaches toner to the electrostatic latent image formed on the surface of photoreceptor drum  23  to form a toner image. The electrostatic latent image is thus developed. 
         [0044]    Development blade  28  as a developer layer controller is made of stainless steel or the like and has a plate shape. Development blade  28  is provided so as to have an edge in contact with the surface of development roller  26 . Development blade  28  scrapes excess toner exceeding a certain amount on the surface of development roller  26  to control the thickness of toner attached on the surface of development roller  26  so as to always have a uniform thickness. 
         [0045]    Cleaner  29  as a cleaner is made of a rubber member or the like and has a plate shape. Cleaner  29  is provided so as to have an end in contact with the surface of photoreceptor drum  23 . Cleaner  29  scrapes toner remaining on photoreceptor drum  23  for cleaning after the toner image formed on photoreceptor drum  23  is transferred to recording sheets  100 . 
         [0046]    Fixation device  40  includes fixation roller  41 , serving as a fuser member or a first conveyance member, and press roller  42 , serving as a first press member. Fixation device  40 , serving as a fixer or a fuser, is configured to press and heat recording sheets  100  for fusing of toner images. 
         [0047]    The sheet ejection mechanism includes ejection sensor  50  as a recording medium detector, ejection rollers  51   a  and  51   b , and ejection rollers  52   a  and  52   b . Ejection sensor  50  is provided downstream of fixation device  40  in medium conveyance path  101 . Ejection rollers  51   a  and  51   b  are opposed to each other downstream of ejection sensor  50  in medium conveyance path  101  so as to sandwich each recording sheet  100 . Ejection rollers  52   a  and  52   b  are also opposed to each other downstream of ejection sensor  50  in medium conveyance path  101  so as to sandwich each recording sheet  100 . Ejection rollers  51   a ,  51   b ,  52   a , and  52   b  are driven by sheet conveyance motor  19  shown in  FIG. 3 , later described. 
         [0048]      FIG. 3  is a schematic block diagram showing a print controller of the image formation apparatus of Embodiment 1. 
         [0049]    Image formation apparatus  10  includes print controller  60 , LED head  25  as the recording light projection member; image formation unit  20  configured to form a toner image corresponding to recording light; image formation unit power supply  65  configured to apply high voltage to image formation unit  20 ; sheet conveyance motor  19  configured to drive a sheet conveyance unit for conveying recording sheets  100 ; sheet conveyance motor power supply  66  configured to supply electric power to sheet conveyance motor  19 ; fixation motor  44  as a drive source configured to drive release mechanism  80  shown in  FIG. 1B , later described; fixation motor power supply  67  configured to supply electric power to fixation motor  44 ; print start position sensor  18 ; ejection sensor  50 ; heater  43  as a heater configured to heat fixation roller  41 ; heater power supply  68  configured to supply electric power to heater  43 ; and thermistor  45  as a temperature measurement unit measuring temperature of fixation device  40 . 
         [0050]    Print controller  60  includes motor controller  61 , sheet position detector  62 , and heating controller  63 . Print controller  60  is connected to LED head  25 , image formation unit power supply  65 , sheet conveyance motor power supply  66 , fixation motor power supply  67 , print start position sensor  18 , ejection sensor  50 , heater power supply  68 , and thermistor  45 . Image formation unit power supply  65  is further connected to image formation unit  20 . Sheet conveyance motor power supply  66  is further connected to sheet conveyance motor  19 . Fixation motor power supply  67  is further connected to fixation motor  44 . Heater power supply  68  is connected to heater  43  incorporated in fixation device  40 . 
         [0051]    Motor controller  61  is connected to sheet conveyance motor power supply  66  and motor power supply  67 . Motor controller  61  controls the driving of sheet conveyance motor  19  and fixation motor  44 . 
         [0052]    Sheet position detector  62  is connected to print start position sensor  18  and ejection sensor  50 . Sheet position detector  62  detects the position of each recording sheet  100 . 
         [0053]    Heating controller  63  is connected to heater power supply  68  and thermistor  45 . Heating controller  63  detects temperature of thermistor  45  and controls heater power supply  68  to heat and control fixation roller  41  to a constant temperature based on the result of detection. 
         [0054]      FIGS. 1A and 1B  are schematic configuration views showing the fixation device of Embodiment 1.  FIG. 1A  shows a radial sectional view of fixation device  40 , and  FIG. 1B  shows a longitudinal sectional view of fixation device  40 . 
         [0055]    As shown in  FIG. 1A , fixation device  40  includes: fixation roller  41 , serving as a fuser member or a first conveyance member, configured to supply heat to recording sheets  100  and convey recording sheets  100 ; heater  43  as a heater configured to heat fixation roller  41 ; fixation belt  46 , serving as a second conveyance member, configured to press recording sheets  100  and convey recording sheets  100 ; press roller  42 , serving as the first press member, configured to press fixation belt  46 ; press pad  70  serving as a second press member; and thermistor  45  as a temperature detecting member configured to detect surface temperature of fixation roller  41 . 
         [0056]    Recording sheets  100  pass through the nip portion formed between fixation roller  41  and fixation belt  46  from right to left in the drawing to be pressed and heated. The toner image formed on the top surface of each recording sheet  100  is thus developed. 
         [0057]    Fixation belt  46  is laid on press roller  42  and press pad  70 . Press roller  42  presses fixation roller  41  through fixation belt  46 . 
         [0058]    Press pad  70  is provided upstream of press roller  42 . The edge of press pad  70  is positioned near the nip portion between press roller  42  and fixation roller  41  and presses fixation roller  41  through fixation belt  46 . 
         [0059]    Heater  43  is provided within fixation roller  41  in a non-contact position with the same. Thermistor  45  is in contact with the surface of fixation roller  41  and measures the surface temperature thereof. Press roller  42  and press pad  70  press fixation roller  41  through fixation belt  46  to form the nip portion. 
         [0060]    Thermistor  45  as the temperature measurement unit is a device having resistance varying with temperature. Print controller  60  measures the resistance value to obtain the temperature of thermistor  45 . In Embodiment 1, thermistor  45  is a device having a characteristic of resistance decreasing as the temperature increases. 
         [0061]    Heater  43  is a heat source such as a halogen heater, for example. When electric power is supplied to a heat generator incorporated in heater  43 , heater  43  generates heat and transmits the generated heat to the inner surface of fixation roller  41 . The voltage applied to heater  43  is 100 V, and the output of heater  43  is 800 W. 
         [0062]    As shown in  FIG. 1B , fixation device  40  further includes release mechanism  80 . Release mechanism  80 , serving as a switch mechanism or a support mechanism, transmits a driving force from fixation motor  44  (see,  FIG. 3 ) to fixation roller  41  for driving the same and controls separation and contact of fixation roller  41 , press roller  42 , and press pad  70 . Release mechanism  80  includes: fixing gear  81 ; cam gear  85  configured to operate with fixing gear  81  through a group of gears; camshaft  86  fixed at the center of cam gear  85 ; roller release cams  88 - 1  and  88 - 2  fixed at both ends of camshaft  86 ; pad release cams  87 - 1  and  87 - 2  fixed inside of the roller release cams  88 - 1  and  88 - 2 ; press pad levers  97 - 1  and  97 - 2  pressed by pad release cams  87 - 1  and  87 - 2 ; press roller levers  98 - 1  and  98 - 2  pressed by roller release cams  88 - 1  and  88 - 2 ; and frame  89  covering the aforementioned members. 
         [0063]    Press roller  42  is provided under fixation roller  41 . Provided to the left of press roller  42  are press pad lever  97 - 1 , not fixed to press roller  42 , and press roller lever  98 - 1 , fixed to the shaft of press roller  42 . Provided to the right of press roller  42  are press pad lever  97 - 2 , not fixed to press roller  42 , and press roller lever  98 - 2 , fixed to the shaft of press roller  42 . 
         [0064]    Fixation roller  41  has a circular cylindrical shape with a diameter of 30 mm. Fixation roller  41  includes a core tube as a base member made of an iron pipe and an elastic layer covering the core tube. The elastic layer is made of silicon rubber and has a thickness of 1 mm. Fixation roller  41  is driven and rotated by fixation motor  44 . When fixation motor  44  rotates forward, fixation roller  41  rotates forward in the direction that recording sheets  100  are conveyed. When fixation motor  44  reverses, fixation roller  41  rotates opposite to the direction that recording sheets  100  are conveyed. Between the fixation motor  44  and fixation roller  41 , for example, a one-way gear, a clutch, or the like may be provided so that fixation roller  41  does not rotate when fixation motor  44  reverses. 
         [0065]    Press roller  42  has a circular cylindrical shape with a diameter of 20 mm. Press roller  42  includes a core tube as a base member made of an iron metallic solid shaft and an elastic layer covering the core tube. The elastic layer is made of heat-resistant porous sponge and has a thickness of 1 mm. Both ends of press roller  42  are rotatably supported on press roller levers  98 - 1  and  98 - 2 . Press roller  42  is in contact with fixation belt  46  and rotates together with driven fixation belt  46 . Press roller levers  98 - 1  and  98 - 2  are supported so as to rotate around rotational shaft  91  shown in  FIG. 7 , described later. 
         [0066]    Frame  89  supports the both ends of fixation roller  41  so that fixation roller  41  freely rotates. Press roller levers  98 - 1  and  98 - 2  are placed within frame  89  and support press roller  42  so that press roller  42  freely rotates. Press roller levers  98 - 1  and  98 - 2  are provided with holes so as to be prevented from interfering with fixation roller  41  when rotating around rotational shaft  91  shown in  FIG. 7 , described later. 
         [0067]    Press pad levers  97 - 1  and  97 - 2  are provided between press roller levers  98 - 1  and  98 - 2  and support not-shown press pad  70 . Press pad levers  97 - 1  and  97 - 2  are provided with holes so as to be prevented from interfering with fixation roller  41  and press roller  42  when rotating around rotational shaft  91  shown in  FIG. 7 , described later. 
         [0068]      FIGS. 4A and 4B  are external views of the press pads in Embodiment 1.  FIG. 4A  shows an entire exterior of press pad  70 , and  FIG. 4B  shows the edge of press pad  70 . 
         [0069]    Press pad  70  includes base member  71  and rubber layer  72 . Base member  71  has a dogleg shape with a central part protruding in the short-side direction. Base member  71  has a wedge-shaped end at the top. 
         [0070]    Rubber layer  72  is formed on the wedge-shaped end of base member  71 . Rubber layer  72  is configured to come into contact with fixation belt  46  and press fixation belt  46  to form the nip portion between fixation roller  41  and press pad  70 . 
         [0071]    Rubber layer  72  includes surface layer  73  and elastic layer  74  as shown in  FIG. 4B . 
         [0072]    Base member  71  is made of a metallic material such as aluminum, iron, or stainless steel in order to keep a certain degree of rigidity. Surface layer  73  is made of a resin material having high heat resistance and low surface frictional resistance, such as silicon resin or fluorine resin. Elastic layer  74  is made of a rubber material having high heat resistance, such as silicon rubber, silicone rubber sponge, or fluorine rubber. 
         [0073]      FIG. 5  is a view showing a support frame of the press pad of Embodiment 1. Press pad  70  is supported by support frame  75  having a square U-shape. Support frame  75  is engaged with central part of base member  71  protruding in the short-side direction and supports the lower part of base member  71 . Between the bottom of press pad  70  and support frame  75 , a plurality of pad springs  76  pressing press pad  70  in the longitudinal direction are provided. 
         [0074]      FIGS. 6A and 6B  are schematic configuration views showing the release mechanism of Embodiment 1.  FIG. 6A  is a side view of release mechanism  80 , showing a driving force transmission path from the fixation motor, and  FIG. 6B  is a front view of release mechanism  80 , showing a driving force transmission path to the pressure levers. 
         [0075]    As shown in  FIG. 6A , release mechanism  80  includes: fixing gear  81 ; reverse gear  82 ; fixation motor drive gear  83 ; one-way gear  84  in a gear train transmitting driving force in one rotational direction; cam gear  85  which is connected to one-way gear  84  and rotates only in one direction; camshaft  86  which is fixed to central part of cam gear  85  and rotates together with cam gear  85 . 
         [0076]    Reverse gear  82  is provided under fixing gear  81 . Fixation motor drive gear  83  is provided to the right of reverse gear  82  in the drawing. One-way gear  84  is provided above fixation motor drive gear  83 . Cam gear  85  is provided above one-way gear  84 . 
         [0077]    As shown in  FIG. 6B , release mechanism  80  further includes pad release cams  87 - 1  and  87 - 2 , roller release cams  88 - 1  and  88 - 2 , press pad levers  97 - 1  and  97 - 2 , and press roller levers  98 - 1  and  98 - 2 , which are accommodated in frame  89 . 
         [0078]    Press pad levers  97 - 1  and  97 - 2  and press roller levers  98 - 1  and  98 - 2  are fixed to camshaft  86  and are positioned at both longitudinal ends of fixation device  40 . 
         [0079]    Pad release cams  87 - 1  and  87 - 2  and roller release cams  88 - 1  and  88 - 2  are opposed to spring  90  shown in  FIG. 7 , described later, with press pad levers  97 - 1  and  97 - 2  and press roller levers  98 - 1  and  98 - 2  interposed therebetween, respectively. 
         [0080]    Camshaft  86  is rotatably supported. When cam gear  85  rotates, camshaft  86 , pad release cams  87 - 1  and  87 - 2 , and roller release cams  88 - 1  and  88 - 2  rotate. When pad release cams  87 - 1  and  87 - 2  rotate to a predetermined angle, pad release cams  87 - 1  and  87 - 2  press press pad levers  97 - 1  and  97 - 2 , respectively. When roller release cams  88 - 1  and  88 - 2  rotate to a predetermined angle, roller release cams  88 - 1  and  88 - 2  press press roller levers  98 - 1  and  98 - 2 , respectively. 
       (Operation of Embodiment 1) 
       [0081]    The printing operation of image formation apparatus  10  is described based on  FIG. 2 . 
         [0082]    Recording sheets  100  are conveyed along medium conveyance path  101  from the upstream side to the downstream side. Sheet cassette  110  is most upstream, and stacker  53  is most downstream. 
         [0083]    Image formation apparatus  10  is connected to a higher level system by wire or wirelessly. When print data is transferred from the higher level system and a print instruction is received, a pickup motor (not shown) rotates pickup roller  12  to separate the plurality of recording sheets  100  one by one and transmit the same to the downstream side in medium conveyance path  101 . Hopping sensor  13  on the way detects whether pickup roller  12  normally feeds each recording sheet  100 . If pickup roller  12  does not normally feed recording sheet  100 , the sheet feeding operation is performed again. At substantially the same time as the start of the sheet feeding operation, image formation unit  20  starts rotation of the rollers and thereby rotates photoreceptor drum  23  more than one turn until recording sheet  100  reaches photoreceptor drum  23 . 
         [0084]    When sheet conveyance motor  19  rotates sheet feeding roller  14   a , the rotation of sheet feed roller  41  rotates retard roller  14   b , which is in contact with sheet feeding roller  14   a , together. Recording sheet  100  conveyed from pickup roller  12  is conveyed by sheet feeding roller  14   a  and retard roller  14   b  sandwiching recording sheet  100  to conveyance roller  15   a  and pinch roller  15   b  which are positioned downstream in medium conveyance path  101 . 
         [0085]    Recording sheet  100  is tilted when reaching conveyance roller  15   a  and pinch roller  15   b  because of separation by pickup roller  12  and paper feeding roller  14   b . Recording sheet  100  is struck on conveyance roller  15   a  and pinch roller  15   b  which are not rotating and is thus brought into an untilted position. After recording sheet  100  is struck, conveyance roller  14   a  is connected to power by a clutch to rotate. 
         [0086]    Recording sheet  100  is further conveyed by conveyance roller  15   a , pinch roller  15   b , resist roller  17   a , pinch roller  17   b , and the leading edge thereof turns on print start position sensor  18 . After a certain period of time since print start position sensor  18  is turned on, LED head  25  starts the exposure to form an electrostatic latent image on photoreceptor drum  23 . 
         [0087]    Photoreceptor drum  23  of image formation unit  20  rotates clockwise as shown in the drawing, and the surface of photoreceptor drum  23  is uniformly charged by charge roller  24  at first. Photoreceptor drum  23 , uniformly charged, is exposed to light based on the image information received from the higher level system by LED head  25 . On photoreceptor drum  23 , an electrostatic latent image is thus formed. Photoreceptor drum  23  with the electrostatic latent image formed thereon is subjected to development by supply roller  27  and development roller  26 . A toner image is thus formed. Photoreceptor drum  23  with the toner image developed, sandwiches and conveys recording sheet  100  in conjunction with transfer roller  21 . Moreover, transfer roller  21  is supplied with a voltage of about +3000 V to attract toner attached on photoreceptor drum  23  close to recording sheet  100 , thus transferring the toner image to recording sheet  100 . Recording sheet  100  with the toner image transferred thereon is positively charged by transfer roller  21 . Recording sheet  100  is transmitted to fixation device  40  for fusing of the toner image. The toner remaining on photoreceptor drum  23  is scraped off by cleaner  29  and is then accommodated in a waste toner container of toner cartridge  55  by a collecting mechanism (not shown). 
         [0088]    Recording sheet  100  with the toner image transferred thereon is sandwiched and conveyed through the nip portion which is pressed to be formed by fixation roller  41  and fixation belt  46  (not shown) in fixation device  40 . At the nip portion, recording sheet  100  is heated by fixation roller  41  and is pressed by the energizing force of press pad  70  and press roller  42 , shown in  FIG. 1  and as described above. The toner therefore melts, and the toner image is fused. 
         [0089]    Recording sheet  100  with the toner image fused thereon turns on ejection sensor  50  and is then conveyed by rotating ejection rollers  51   a  and  51   b  and ejection rollers  52   a  and  52   b . Conveyed recording sheet  100  is then ejected to stacker  53 . 
         [0090]      FIGS. 7A and 7B  are views showing an operation of the pad levers of Embodiment 1. The pad levers and pad release cams are representatively referred to as pad lever  97  and pad release cam  87  in the following description. 
         [0091]    In  FIG. 7A , press pad lever  97  supports press pad  70  through support frame  75  and pad spring  76  so that press pad  70  freely expands and contracts. Press pad lever  97  is pressed by spring  90  in a direction indicated by arrow C 1 . Press pad lever  97  presses press pad  70  upward through rotational shaft  91  by the pressing force of spring  90 . Press pad  70  presses fixation belt  46  (not shown) to press fixation roller  41 . Pad release cam  87  is fixed to camshaft  86 . In  FIG. 7A , pad release cam  87  is positioned at such an angle that pad release cam  87  is not in contact with contact portion A of press pad lever  97 . In  FIGS. 7A and 7B , distance L 1  is larger than distance L 2  (L 1 &gt;L 2 ). 
         [0092]    A reverse driving force of fixation motor  44  is transmitted through fixing gear  81 , reverse gear  82 , fixation motor drive gear  83 , one-way gear  84 , and cam gear  85 , which are shown in  FIG. 6A  described above, to camshaft  86  to rotate pad release cam  87  connected to camshaft  86  in a direction indicated by arrow D 1 . Forward driving force of fixation motor  44  is not transmitted to camshaft  86  because one-way gear  84  rotates in an idle manner. 
         [0093]    Fixation motor  44  is a stepping motor and has a characteristic of its rotational angle varying according to the number of pulses inputted from print controller  60 . Fixation motor  44  controls the rotational angle through the number of pulses inputted to fixation motor  44 . 
         [0094]    In a nip state of fixation roller  41  and press pad  70  (shown in  FIG. 7A ), press pad lever  97  receives pressure from spring  90  provided between press pad lever  97  and frame  89  and therefore rotates around rotational shaft  91 . Expandably supported press pad  70  receives pressure from press pad lever  97  to press fixation roller  41  upward through fixation belt  46 . 
         [0095]    As shown in  FIG. 7B , pad release cam  87 , rotated in the direction indicated by arrow D 1 , presses press pad lever  97  against spring  90  to release press pad  70  supported by press pad lever  97  in a direction indicated by arrow F 1 . 
         [0096]      FIGS. 8A and 8B  are views showing an operation of the roller levers of Embodiment 1. The roller levers and roller release cams are representatively referred to as roller lever  98  and roller release cam  88  in the following description, respectively. 
         [0097]    In a similar manner to pad release cam  87  shown in  FIGS. 7A and 7B , reverse driving force of fixation motor  44  rotates roller release cam  88  connected to camshaft  86  in a direction indicated by arrow D 2 . 
         [0098]    In  FIG. 8A , press roller lever  98  supports press roller  42  so that press roller  42  freely rotates. Press roller lever  98  is pressed by spring  90  in a direction indicated by arrow C 2 . Press roller lever  98  presses press roller  42  upward through rotational shaft  91  by the pressing force of spring  90 . Press roller  42  presses fixation belt  46  (not shown) and therefore presses fixation roller  41 . On camshaft  86 , roller release cam  88  is fixed. In  FIG. 8A , roller release cam  88  is positioned at such an angle that roller release cam  88  is not in contact with contact portion B of press roller lever  98 . In  FIGS. 8A and 8B , distance L 3  is larger than distance L 4  (L 3 &gt;L 4 ). 
         [0099]    As shown in  FIG. 8B , roller release cam  88 , rotated in a direction indicated by arrow D 2 , presses press roller lever  98  in a direction indicated by arrow E 2  against spring  90  and therefore releases press roller  42  supported by press roller lever  98  in a direction indicated by arrow F 2 . 
         [0100]      FIGS. 9A and 9B  are diagrams showing nip modes of the press members (press roller  42  and press pad  70 ) for a rotational angle of the camshaft  86 . The graph of  FIG. 9A  shows the nip modes of press pad  70  and press roller  42  for the rotational angles of the camshaft  86 . Herein, when the rotational angle of camshaft  86  is 0 degree, each press member (press roller  42  and press pad  70 ) is in a full nip state. In  FIG. 9B , the rows individually show the nip modes of press pad lever  97  and press roller lever  98 , and the columns show lever operations at each nip mode. 
         [0101]    As shown in  FIG. 9A , when the rotational angle of camshaft  86  increases from 0 to 60 degrees, press pad  70  starts to be released. When the rotational angle thereof is between 60 and 120 degrees, the press pad  70  is released, and press roller  42  is in a nip state. Hereinafter, such a state is called mode B (roller nip). The column of mode B (roller nip) of  FIG. 9B  shows the positions of press pad  70  and press roller  42  in mode B (roller nip). 
         [0102]    As shown in  FIG. 9A , when the rotational angle of camshaft  86  increases from 120 to 180 degrees, press pad  70  remains released, and press roller  42  starts to be released. When the rotational angle thereof is between 180 and 240 degrees, press pad  80  and press roller  42  are both released. Such a state is called mode A (released) hereinafter. The column of mode A (released) in  FIG. 9B  shows the positions of press pad  70  and press roller  42  in mode A (released). 
         [0103]    As shown in  FIG. 9A , when the rotational angle of camshaft  86  increases from 240 to 300 degrees, press pad  70  and press roller  42  are both shifted to the nip states. When the rotational angle thereof is between 300 and 360 degrees, press pad  80  and press roller  42  are both in the nip states. Such a state is called mode C (full nip) hereinafter. The column of mode C (full nip) in  FIG. 9B  shows the positions of press pad  70  and press roller  42  in mode C (full nip). 
         [0104]      FIG. 10  is a flowchart showing an operation of the image formation apparatus of Embodiment 1. 
         [0105]    In step S 1 , print controller  60  waits for a print instruction from a higher level apparatus or an external apparatus. 
         [0106]    In step S 2 , print controller  60  controls fixation motor  44  through motor controller  61  and causes only press roller  42  to press fixation roller  41  through release mechanism  80 . In short, print controller  60  brings the press members (press roller  42  and press pad  70 ) into mode B (roller nip). 
         [0107]    In step S 3 , heating controller  63  detects the current temperature Tup of fixation roller  41  through thermistor  45  and sets a control setting temperature Tsp. Heating controller  63  starts to control the temperature of fixation roller  41  and thereby controls setting temperature Tsp through heater power supply  68 . 
         [0108]    In step S 4 , print controller  60  sets a lower temperature limit Tlimit and an upper temperature limit T 2  of thermistor  45 . Print controller  60  compares current temperature Tup with a printable temperature range determined by the lower and upper temperature limits Tlimit and T 2 . Print controller  60  waits until current temperature Tup reaches the printable temperature range. 
         [0109]    Print controller  60  previously stores the setting values including lower and upper temperature limits Tlimit and T 2  and control setting temperature Tsp. The setting values are experimentally obtained temperatures. 
         [0110]    In step S 5 , print controller  60  starts to convey recording sheet  100  through motor controller  61 . In step S 6 , print controller  60  waits until sheet position detector  62  and print start position sensor  18  detect that recording sheet  100  reaches a predetermined position. 
         [0111]    In step S 7 , print controller  60  controls the fixation motor  44  through motor controller  61  to cause both press roller  42  and press pad  70  to press fixation roller  41  (mode C (full nip)). 
         [0112]    In step S 8 , print controller  60  continues printing and detects through sheet position detector  62  and ejection sensor  50 , whether printing is finished. The print controller  60  determines that recording sheet  100  passes by the ejection sensor  50  and is ejected to stacker  53  if a predetermined period of time elapses after the ejection sensor  50  is turned on and then off. The print controller  60  then determines that the printing (print) is finished. 
         [0113]    In step S 9 , print controller  60  controls fixation motor  44  through motor controller  61  and causes both press roller  42  and press pad  70  to be separated from fixation roller  41  (mode A (released)). Press roller  42  and press pad  70  then stop being pressed, and the pressure thereof is removed. The operation of  FIG. 10  is thus terminated. 
         [0114]    The operation of  FIG. 10  is repeated for each print to release the pressure of press pad  70  before and after each recording sheet  100  passes through fixation device  40 . 
         [0115]      FIG. 11  shows time charts ((a) to (g)) of the printing operation of Embodiment 1. 
         [0116]      FIG. 11(   a ) shows the temperatures of fixation roller  41  and fixation belt  46 .  FIG. 11(   b ) shows the heater control state.  FIG. 11(   c ) shows the drive state of fixation motor  44 .  FIG. 11(   d ) shows the nip mode of release mechanism  80 .  FIG. 11(   e ) shows whether the sheet conveyance unit is conveying recording sheet  100 .  FIG. 11(   f ) shows the state of print start position sensor  18 .  FIG. 11(   g ) shows the state of ejection sensor  50 . 
         [0117]    Time A 0  is a time when the image formation apparatus  10  receives a print request. At time A 0 , recording sheet  100  is not conveyed yet as shown in  FIG. 11(   e ). Print controller  60  turns on heater  43  to start the heating control as shown in  FIG. 11(   b ) and drives fixation motor  44  in reverse as shown in  FIG. 11(   c ). As shown in  FIG. 11(   d ), the nip mode of release mechanism  80  is mode A (released). 
         [0118]    Time A 1  is a time when the nip mode of release mechanism  80  becomes mode B (roller nip) after time A 0  as shown in  FIG. 11(   d ). At time A 1 , print controller  60  drives fixation motor  44  forward as shown in  FIG. 11(   c ). Print controller  60  controls heater  43  while rotating fixation roller  41  to increase the temperatures of fixation roller  41  and press roller  42 . 
         [0119]    Time A 2  is a time when the temperature of fixation roller  41  reaches the printable temperature range as a first temperature range after time A 1 . The printable temperature range is a range of temperature of fixation roller  41  at which toner can be fused to recording sheets  100 . The printable temperature range is defined by lower and upper temperature limits Tlimit and T 2 . For example, lower and upper temperature limits Tlimit and T 2  are 160 and 200° C., respectively. 
         [0120]    If the current temperature is higher than upper temperature limit T 2 , heating controller  63  stops the power supply from heater power supply  68  to heater  43  to reduce the temperature of fixation roller  41  for cooling down. 
         [0121]    If the current temperature is lower than lower temperature limit Tlimit, heating controller  63  starts the power supply from heater power supply  68  to heater  43  to increase the temperature of fixation roller  41  for warming up. 
         [0122]    At time A 2 , print controller  60  drives fixation motor  44  in reverse as shown in  FIG. 11(   c ) and brings release mechanism  80  into mode C (full nip) as shown in  FIG. 11(   d ). At the same time, print controller  60  starts sheet conveyance by the sheet conveyance unit and then starts the image forming operation. 
         [0123]    Time A 3  is a time when release mechanism  80  becomes mode C (full nip) after time T 2 . At time A 3 , print controller  60  drives fixation motor  44  forward as shown in  FIG. 11(   c ). 
         [0124]    Time A 4  is a time when ejection sensor  50  is turned on from off after time A 3  and when the trailing edge of recording sheet  100  passes by the ejection sensor  50 . At this time, print controller  60  stops the heater control as shown in  FIG. 11(   b ). 
         [0125]    Time A 5  is a time when a predetermined period of time elapses after time A 4 . At this time, print controller  60  determines that recording sheet  100  is ejected from image formation apparatus  10 . Print controller  60  reverses fixation motor  44  as shown in  FIG. 11(   c ) and changes the nip mode of release mechanism  80  to mode A (released) as shown in  FIG. 11(   d ). Print controller  60  then terminates sheet conveyance by the sheet conveyance unit as shown in  FIG. 11(   e ). 
         [0126]      FIG. 12  (( a ) to ( j )) shows the printing operation of the image formation apparatus and the operation of the fixation device in Embodiment 1. 
         [0127]      FIG. 12(   a ) shows the printing operation of image formation apparatus  10  at time A 0 . In  FIG. 12(   a ), sheet conveyance is not started yet. 
         [0128]      FIG. 12(   b ) shows the operation of fixation device  40  at time A 0 . Fixation device  40  is in mode A (released). 
         [0129]      FIG. 12(   c ) shows the printing operation of image formation apparatus  10  at times A 1  and A 2 . The sheet conveyance is not started yet. 
         [0130]      FIG. 12(   d ) shows the operation of fixation device  40  at times A 1  and A 2 . Fixation device  40  is in mode B (roller nip). 
         [0131]      FIG. 12(   e ) shows the printing operation of image formation apparatus  10  at time A 3 . The leading edge of recording sheet  100  reaches the tip of print start position sensor  18 . This position is a predetermined position at which press pad  70  is pressed against fixation roller  41 . 
         [0132]      FIG. 12(   f ) shows the operation of fixation device  40  at time A 3 . Fixation device  40  is in mode C (full nip). 
         [0133]      FIG. 12(   g ) shows the printing operation of image formation apparatus  10  at time A 4  after recording sheet  100  gets out of fixation device  40 . 
         [0134]      FIG. 12(   h ) shows the operation of fixation device  40  at time A 4 . Fixation device  40  is in mode C (full nip). 
         [0135]      FIG. 12(   i ) shows the printing operation of image formation apparatus  10  at time A 5 . Recording sheet  100  is ejected to stacker  53 . 
         [0136]      FIG. 12(   j ) shows the operation of fixation device  40  at time A 5 . Fixation device  40  is in mode A (released). 
       (Effect of Embodiment 1) 
       [0137]    According to fixation device  40  and image formation apparatus  10  of Embodiment 1, press pad  70  presses fixation belt  46  only when each sheet goes through fixation device  40 . This allows press pad  70  to be brought into contact with fixation belt  46  only for a minimum period of time, thus preventing degradation due to abrasion and the like. 
       Embodiment 2 
     (Configuration of Embodiment 2) 
       [0138]      FIGS. 13A and 13B  are schematic configuration views showing a fixation device of Embodiment 2. The same elements as those in  FIG. 1A  showing Embodiment 1 are given the same reference numerals. 
         [0139]    Fixation device  40 A of Embodiment 2 is the same as the fixation device  40  of Embodiment 1 except that fixation device  40 A includes press pad  70 A having a shape different from that of press pad  70  of Embodiment 1. 
         [0140]      FIG. 13A  shows that press pad  70 A and press roller  42  both press fixation roller  41  through fixation belt  46 . 
         [0141]      FIG. 13B  shows that only press roller  42  presses fixation roller  41  through fixation belt  46 . Press pad  70 A is released, and the lower end of press pad  70 A is in contact with fixation belt  46 . 
       (Operation of Embodiment 2) 
       [0142]    The printing operation of image formation apparatus  10 A of Embodiment 2 is the same as that of image formation apparatus  10 A of Embodiment 1 shown in  FIG. 2 . 
         [0143]      FIG. 14  (( a ) to ( g )) shows time charts presenting a problem of the printing operation of Embodiment 1. The same elements as those of  FIG. 11  are given the same reference numerals. 
         [0144]      FIG. 14  shows a case where print is started with press pad  70  cooled to room temperature in Embodiment 1. 
         [0145]    Time A 0  is a time where image formation apparatus  10  receives a print request. At time A 0 , the temperatures of fixation roller  41 , fixation belt  46 , and press pad  70  are lower than those in the case of  FIG. 11(   a ). 
         [0146]    Press pad  70  is not in contact with fixation belt  46  between times A 0  and A 3 . The temperature of press pad  70  therefore remains low as shown in  FIG. 14(   a ). 
         [0147]    At time A 3 , press pad  70  comes into contact with fixation belt  46 . The temperature of press pad  70  is greatly different from the temperature of fixation belt  46 . Accordingly, as shown in portion B of  FIG. 14(   a ), press pad  70  receives heat to greatly increase its temperature, and fixation belt  46  greatly decreases in temperature. As the temperature of fixation belt  46  decreases, the temperature of fixation roller  41  in contact with fixation belt  46  greatly decreases similarly. 
         [0148]    Fixation roller  41  is formed of the members with high heat capacities, such as the core tube and elastic layer. Accordingly, it takes a certain time for heat generated by heater  43  to reach the surface of fixation roller  41  and increase the temperature of fixation roller  41  to the printable temperature again. Fixation roller  41  does not reach the printable temperature yet when recording sheet  100  reaches the fixation device. This can cause insufficient fusing due to a short supply of heat to recording sheet  100 . 
         [0149]    Accordingly, press pad  70 A of Embodiment 2 has a different configuration from that of press pad  70  of Embodiment 1, and a part of press pad  70 A other than the nip portion is brought into contact with fixation belt  46  to previously increase its temperature. 
         [0150]      FIG. 15  (( a ) to ( g )) shows time charts of the printing operation in Embodiment 2. The same elements as those in  FIGS. 1 and 14  are given the same reference numerals. 
         [0151]    Between time A 0  and time A 3  of Embodiment 2, press roller  42  presses and heats fixation belt  46  during the warming up process. At this time, the lower part of press pad  70 A other than the nip portion is in contact with fixation belt  46  which is therefore heated. 
         [0152]    At time A 3 , both of press pad  70 A and press roller  42  pressurize the nip portion. Since the temperature of press pad  70 A is increased enough, press pad  70 A does not greatly decrease the temperature of fixation belt  46  and the surface temperature of fixation roller  41 . It is therefore possible to prevent insufficient fusing due to the short supply of heat to recording sheet  100  and the like. 
       (Effect of Embodiment 2) 
       [0153]    According to fixation device  40 A and image formation apparatus  10 A of Embodiment 2, it is possible to prevent fixation belt  46  from decreasing in temperature due to the press pad  70 A with low temperature after fixation belt  46  is brought into contact with press pad  70 A in the case where the warming up and print are started with fixation device  40 A cooled to room temperature. It is therefore possible to prevent printing failure. 
       (Modifications) 
       [0154]    The invention is not limited to the aforementioned embodiments and can be used and modified in various ways. Such applications and modifications are shown in the following (a) to (f), for example. 
         [0155]    (a) Fixation devices  40  and  40 A and image formation apparatuses  10  and  10 A of Embodiments 1 and 2 are applied to a printer, but the invention is not limited to this. Fixation devices  40  and  40 A and image formation apparatuses  10  and  10 A of Embodiments 1 and 2 can be applied to copiers, facsimiles, and multifunction printers. 
         [0156]    (b) In fixation devices  40  and  40 A and image formation apparatuses  10  and  10 A of Embodiments 1 and 2, heater  43  is placed in non-contact with fixation roller  41 , but the invention is not limited to this. Heater  43  may be placed in contact with fixation roller  41 . 
         [0157]    (c) In fixation devices  40  and  40 A and image formation apparatuses  10  and  10 A of Embodiments 1 and 2, thermistor  45  is placed in contact with the surface of fixation roller  41  so as to sense the temperature thereof. However, the invention is not limited to this. Thermistor  45  may be placed in non-contact with fuser roller  41  to sense the temperature. 
         [0158]    (d) In image formation apparatuses  10  and  10 A of Embodiments 1 and 2, the sheet conveyance position is detected by print start position sensor  18 , but the invention is not limited to this. The press state may be changed a predetermined period of time after print start position sensor  18  detects the leading edge of each sheet. Alternatively, instead of using print start position sensor  18 , the press state may be changed a predetermined period of time after sheet conveyance motor  19  starts sheet conveyance. 
         [0159]    (e) According to fixation device  40 A and image formation apparatus  10 A of Embodiment 2, press pad  70 A is brought into contact with fixation belt  46  to be heated in case warming up or print is started with fixation device  40 A cooled to room temperature. However, the invention is not limited to this. If the temperature measured by thermistor  45  is low in a second temperature range, press pad  70 A may be heated with press pad  70 A and press roller  42  set to mode C (full nip) to prevent printing failure. The second temperature range is a range of temperature from −10 to 30° C., for example. Furthermore, press pad  70 A and press roller  42  may be set to mode C (full nip) to be heated if it is detected that the current print is the first print after image formation apparatus  10 A is powered on. 
         [0160]    (f) In fixation devices  40  and  40 A and image formation apparatuses  10  and  10 A of Embodiments 1 and 2, the first temperature range is a printable temperature range where toner can be appropriately fused to recording sheets  100 . However, the invention is not limited to this. The first temperature range may be a range of temperature which can increase to the printable temperature range while each recording sheet  100  is conveyed from sheet cassette  110  to fixation device  40  or  40 A. 
         [0161]    The invention includes other embodiments in addition to the above-described embodiments without departing from the spirit of the invention. The embodiments are to be considered in all respects as illustrative, and not restrictive. The scope of the invention is indicated by the appended claims rather than by the foregoing description. Hence, all configurations including the meaning and range within equivalent arrangements of the claims are intended to be embraced in the invention.