Abstract:
A fixing apparatus includes an induction coil for generating a magnetic field upon electric power supply thereto; a fixing rotatable member for induction heat generation by the magnetic field generated by the induction coil; temperature detector for detecting a temperature of the fixing rotatable member; determining unit for determining an electric power value to be supplied to the induction coil on the basis of a detected temperature by the temperature detector; and a controller for controlling the electric power supply to the induction coil on the basis of the electric power value determined by the determining unit. The controller substantially continues to supply the electric power while the electric power value determined by the determining unit is higher than a predetermined value, and effects an ON/OFF control of rendering on and off the electric power supply when the electric power value is lower than the predetermined value.

Description:
FIELD OF THE INVENTION AND RELATED ART 
     The present invention relates to a fixing device usable with an image forming apparatus such as a copying machine or a printer using an electrophotographic process, an electrostatic recording process or the like. In conventional image forming apparatuses, a toner image is formed in an image forming station through an indirect (transfer) type or direct type electrophotographic process, electrostatic recording process, magnetic recording process or another known process; and the formed toner image is transferred onto a recording material such as a transfer sheet, recording material, photosensitive paper electrostatic recording paper or the like; and the toner image is fixed on the recording material by a heat fixing device of a heating roller type into a permanent image. 
     The fixing device or the heating roller type comprises a fixing roller containing therein a heater (heating roller) and a pressing roller press-contacted thereto for form a nip, into which the recording material is introduced and through which the recording material is passed, by which the toner image on the recording material is fixed on the surface of the recording material by heat and pressure. 
     Recently, the demands with respect to the heat fixing device is (1) high speed start-up and (2) unnecessity for stand-by temperature control, in terms of the convenience and reduction of energy consumption. 
     In order to accomplish the high speed start-up, it would be considered to reduce the time required for temperature rise up to a desired temperature by reducing the thermal capacity by reducing the thickness of the heat roller, and by using an IH (electromagnetic induction heating type) as a heat source. 
     A temperature control system in the IH system, it would be considered to use ON-OFF control system which has been used for a temperature control for a halogen lamp (heat source). However, use of the ON-OFF control system with the heat roller having a small thermal capacity results in abrupt temperature changes occurring, and therefore, the proper control is difficult. 
     Then, a feed-back type is being considered wherein electric power supplied to the heat source used in the IH type system is linearly controlled in accordance with a result of temperature measurement of the fixing device (FIG.  8 ). In FIG. 8, the FIG.  8  represents an electric power value Po supplied to the heat source, and the abscissa represents a temperature T. When the temperature of the fixing device is Ta, the electric power Pa is supplied to the heat source. 
     Generally, the relation between the electric power supplied to an induction coil and the electric power efficiency is as shown in FIG. 9 in the IH system. In FIG. 9, the ordinate represents an electric power efficiency η, and the abscissa represents an electric power Po supplied to the heat source. When the electric power of 200W is supplied to the induction coil, the electric power efficiency is approx. 88%, but when the electric power not more than 200W is supplied, the electric power efficiency is significantly lower than that. 
     The fixing temperature in the image forming operation is generally not higher than 210° C. When the electric power of 200W is supplied to the coil constituting the IH type voltage source, the temperature may exceed 210° C. 
     However, if the range not higher than 200W is used, the electric power efficiency is not good, as described above, and therefore, doing so is not advantageous from the standpoint of energy conservation. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a principal object of the present invention to provide a fixing device of an electromagnetic induction heating type wherein the electric power efficiency is high. 
     It is another object of the present invention to provide a fixing device wherein an electric power value exhibiting a low electric power efficiency is not used. 
     According to an aspect of the invention, there is provided a fixing apparatus comprising an induction coil for generating a magnetic field upon electric power supply thereto; a fixing rotatable member for induction heat generation by the magnetic field generated by said induction coil; temperature detecting means for detecting a temperature of said fixing rotatable member; determining means for determining an electric power value to be supplied to said induction coil on the basis of a detected temperature by said temperature detecting means; and control means for controlling the electric power supply to said induction coil on the basis of the electric power value determined by said determining means, wherein said control means substantially continues to supply the electric power while the electric power value determined by said determining means is higher than a predetermined value, and effects an ON/OFF control of rendering on and off the electric power supply when the electric power value is lower than the predetermined value. 
     These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram illustrating a device of a first embodiment of the present invention. 
     FIG. 2 is a sectional view illustrating a structure of a laser beam printer shown in FIG.  1 . 
     FIG. 3 is a sectional view showing a structure of a fixing device  251  of FIG.  2 . 
     FIG. 4 is a flow chart showing an example of a control program stored in a ROM  103  shown in FIG. 1 in the first embodiment. 
     FIG. 5 is a graph of a relation between the temperature and the electric power supply, showing an example or switching of the control mode. 
     FIG. 6 is a flow chart showing an example of a control program stored in a ROM  103  shown in FIG. 1 in the second embodiment. 
     FIG. 7 is a graph of a relation between the temperature and the electric power supply, showing an example of switching of the control mode. 
     FIG. 8 is a graph of a relation between the temperature and the electric power supply in a conventional temperature control. 
     FIG. 9 is a graph of an example of a relation between the electric power supplied to the induction coil and the electric power efficiency. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The embodiments of the present invention will be described in conjunction with the accompanying drawings. 
     First Embodiment 
     FIG. 1 is a flow chart of a system according to a first embodiment of the present invention. In this embodiment, the apparatus is a laser beam printer  1  using an image transfer type electrophotographic process. In the laser beam printer  1 , there are provided a CPU 101 , a ROM  102 , a RAM  103 , an image memory  104 , an input/output portion  105 , an image formation station interface  106 , an input/output portion  108  and an electric power supply portion  110  which are mutually connected through a system bus. 
     The ROM  102  stores a control program and the like. The CPU 101  controls various parts of the laser beam printer  1  in accordance with the control program stored in the ROM  102 . The RAM  103  is used as a work area of the CPU 101 . 
     The input/output portion  105  converts the PDL (page description language) inputted from the computer to bit map data. The image memory  104  functions to stores the processed image data. The image formation station  107  is connected to an image formation station I/F, and functions to forms a latent image in accordance with the image data, and develop the latent image, and then, transfers toner image provided by the development onto the recording material. 
     The electric power supply portion  110  functions to supply a high oscillation current from a high oscillation switching voltage source of 10 kHz-100 kHz to an induction coil  111   c  for induction beating of the fixing roller  111 . The electric power supply portion  110  is supplied with an electric power value setting signal and an electric power supply ON/OFF from the CPU 101 . The temperature sensor  109  is effective to detect the temperature of the fixing roller  111 . The input/output portion  108  is connected with a temperature sensor  109 . 
     FIG. 2 illustrate a structure of a laser beam printer  1  of FIG.  1 . In FIG. 2, designated by  231  is an electrophotographic photosensitive member in the form of a rotatable drum, as is rotated in a direction indicated by an arrow in FIG. 2 at a peripheral speed (process speed). Designated by  232  is a charging roller for electrically charging the electrophotographic photosensitive member  231  to a predetermined potential in a predetermined polarity. Designated by  232  is a laser scanner which functions to effect laser beam scanning exposure L of the electrophotographic photosensitive member  231  corresponding to the image information pattern. By the scanning exposure, an electrostatic latent image is formed on the surface of the electrophotographic photosensitive member  231  in accordance with the intended image information pattern. 
     Reference numeral  234  designates a developing device functioning to develop the electrostatic latent image on the surface of the electrophotographic photosensitive member  231 . By the developing operation, the toner image is formed on the surface of the electrophotographic photosensitive member  231 . The developing method may be a jumping developing method, two component developing method or the like in combination of an image exposure and a reverse development. 
     Designated by  235  is a transfer roller which forms a transfer nip  236  between the electrophotographic photosensitive member  231 . The recording material (transfer material) is fed into the nip at a predetermined timing from a sheet feeder  37  so that toner image is transferred from the surface of the electrophotographic photosensitive member  231  thereonto. The recording material  213  now having the transferred toner image is separated from the surface of the electrophotographic photosensitive member  231  and is fed into the fixing device  251  by way of the feeding guide  8 . The fixing device  251  functions to fix the toner image on the recording material  213  by heat and pressure. 
     Designated by  244  is a pair of feeding rollers provided to feed the recording material  213  on which the image has been fixed, to the sheet discharging path  243  from the fixing device  251 . Reference numeral  245  designates a pair of discharging roller which functions to discharge the recording material  213  fed by the feeding rollers onto a sheet discharge tray  246 . 
     Reference numeral  237  designates a sheet feeding cassette for accommodating the recording material  213 . Reference numeral  238  designates a sheet feeding roller  238  cooperating with an unshown separating member (singling out) to separate one by one the recording materials  213  from the sheet feeding cassette  237 . Reference numeral  239  designates a pair of feeding rollers which functions to feed the recording material  213  separated from the sheet feeding cassette  237  into the transfer nip  236  through a sheet path  241  at a predetermined control timing. 
     Designated by  240  is a top sensor provided in the sheet path  241  to detect the leading end of the recording material  213 . 
     Reference numeral  242  designates a cleaner functioning to remove deposited contamination suchh as untransferred toner, paper dust or the like remaining on the surface of the electrophotographic photosensitive member  231  after the image transfer. The electrophotographic photosensitive member  231  having been subjected to the surface cleaning operation is repeatedly used for the image formation. 
     FIG. 3 shows a structure of the fixing device  251  shown in FIG.  2 . In FIG. 3, reference numerals  109 ,  111 ,  111   c  designate the same elements as with FIG.  1 . The fixing device  251  includes a fixing roller  111  and a pressing roller  112  which are arranged substantially vertically, and arc press-contacted to each other. The pressing roller  112  is press-contacted with a predetermined pressure to a lower surface portion of the fixing roller  111  by an unshown pressing mechanism including a spring applying a spring force to a rotational axis of the fixing roller  111 , by which a fixing nip (press-contact nip) N is formed. The pressing roller  112  is urged by approx. 30 kg, and the fixing nip N has a width or approx. 6 mm. The urging force is variable to change the width of the nip. 
     When the fixing roller  111  is rotated, the pressing roller  112  is driven by the fixing roller  111  because of a frictional force at the fixing nip N with the rotational driving of the fixing roller  111 . 
     The fixing roller  111  includes a surface parting layer (electroconductive cylindrical roller) as a base and a surface parting layer. The core metal cylinder is made of steel having an electroconductivity and having an outer diameter of 40 mm and a thickness of 0.7 mm. The surface parting layer is provided to enhance a parting property of the surface of the core metal cylinder, and an example of which is a PTFE (polytetrafluoroethylene) or PFA (perfluoroslkoxy) or the like layer having a thickness of 10-50 μm. In order to improve the fixing property and to reduce the surface temperature non-uniformity of the fixing roller  111 , an elastic layer of silicone rubber having a thickness of 20-500 μm, for example, may be provided between the core metal cylinder and the surface parting layer. 
     In the space in the core metal cylinder, there is provided an induction coil assembly  314  which comprises an induction coil  111   c , a coil holder  305 , a core (core of the magnet)  307  and a stay  306 . 
     The coil holder  305  is in the form of a semicircular trough of heat resistive resin material such as PPS (polyphenylenesulfide), PEEK (polyethelethelketon), phenolic resin. The induction coil  111   c  is formed by 6 turns of Litz wire of 50-150 twisted yarns. The number of winding is not limited to 6, but may be 4-10 turns. The induction coil  111   c  is placed face down adjacent to the inner surface of the core metal cylinder, and the opposite end portions of the stay  306  is fixed between unshown fixing unit frames. The core  307  is provided inside the coil holder  5  in the form having a T-shaped cross-section, and is made integral into the induction coil assembly  314 . 
     The induction coil  111   c  generates alternating magnetic flux by a high frequency AC current supplied from the electric power supply portion  110  (FIG.  1 ), the magnetic field induced by the AC current generates eddy currents in the thickness of the core metal cylinder to produce joule heat which heats the fixing roller  111  to raise the temperature of the fixing roller  111 . 
     The pressing roller  112  comprises a hollow core metal  311  and a surface parting property heat resistive rubber layer formed on the outer surface thereof or an elastic layer  312  which is a sponge layer functioning also as a heat insulation layer between the hollow core metal  311  and the surface. 
     Designated by  310  is a separation claw for separating the recording material having passed through the fixing nip N. 
     The heat generation in the fixing roller  111  may be increased by increasing the number of windings of the induction coil  111   c , using, for the core  307 , ferrite, permalloy or the like having a high magnetic permeability and a low remanent magnetic flux density, or by raising the frequency of the AC current. 
     FIG. 4 is a flow chart showing an example of a control program stored in the RAM  103  shown in FIG.  1 . The fixing device  251  is set such that fixing target temperature Tt is exceeded when the induction coil  111   c  is continuously supplied with an electric power of 200W which is 88% of the power at which the electric power efficiency abruptly lowers. 
     The surface temperature of the fixing roller  111  (roller temperature T) is roller temperature T is measured by the temperature sensor  109  (S 01 ), and an electric power value Po of the electric power to be supplied to the induction coil  111   c  is determined on the basis or the temperature detected by the temperature sensor  109  (S 02 ). The comparison is made between the electric power value Po to be supplied to the induction coil  111   c  and a predetermined electric power value which is 200W in this embodiment (S 03 ). If the electric power value Po is larger than 200W, the electric power of Po continues to be supplied without shutting the electric power supply to the induction coil  111   c  from the electric power supply portion  110  (S 04 ). Thereafter, the operation returns to the step S 01 , and the same steps are repeated. 
     On the other hand, the electric power value Po determined at the step S 02  is not higher than 200W (S 03 ), the comparison is further made between the roller temperature T detected by the temperature sensor  109  and the target temperature Tt. When the roller temperature T is not higher than the target temperature Tt, the electric power of 200W is supplied to the induction coil  111   c  from the electric power supply portion  110  (S 06 ). Then, the operation returns to S 01 . 
     On the other hand, when the roller temperature T is not lower than the target temperature Tt (S 05 ), the electric power supply from the electric power supply portion  110  to the induction coil  111   c  is stopped (S 07 ). Then, the operation returns to S 01 . 
     By the temperature control for the fixing roller  111  described in the foregoing, when the electric power to be supplied to the induction coil  111   c  is not lower than 200W, a linear control in which the electric power determined on the basis of the roller temperature T is supplied, and when it is necessary to supply the electric power not more than 200W, the electric power supply of 200W is on-off-controlled. In this manner, the temperature control does not use the electric power value not more than 200W which leads to a low electric power efficiency. FIG. 5 shows an example of a relation between the temperature and the electric power supply and an example of switching of control mode. 
     Second Embodiment 
     This embodiment is different from the first embodiment in the temperature control method for the fixing roller  111 . 
     FIG. 6 is a flow chart showing an example of a control program stored in a ROM  103  shown in FIG. 1 in the second embodiment. By the temperature sensor  109 , the surface temperature of the fixing roller  111  (roller temperature T) is measured (S 11 ), and the electric power value Po to be supplied to the induction coil  111   c  is determined on the basis of the temperature detected by the temperature sensor  109  (S 12 ). The detected roller temperature T is compared with a predetermined temperature value Tb which is lower than the target temperature Tt (S 13 ). 
     When the roller temperature T is lower than the predetermined temperature value Tb, the electric power of electric power value Po is supplied to the induction coil  111   c  from the electric power supply portion  110  (S 14 ). 
     On the other hand, when the roller temperature T is not lower than the predetermined temperature Tb, the comparison is made between the target temperature Tt and the roller temperature T (S 15 ). When the roller temperature T is not lower than the target temperature Tt, the electric power supply from the electric power supply portion  110  to the induction coil  111   c  is stopped (S 17 ). On the other hand, when the roller temperature T is lower than the target temperature Tt, the electric power of 200W is supplied to the induction coil  111   c . FIG. 7 shows an example of a relation between the temperature and the electric power supply and an example of switching of control mode. 
     The present invention is applicable to a system comprising a plurality of piece of equipment, or to an apparatus of a single equipment. 
     The present invention is implemented by a computer (or CPU or MPU) reading and executing the program code stored in a memory medium which stores the program code of software for executing the function of any one of the foregoing embodiments. During the linear control mode operation, a phase control or wave number control may be carried out to provide the determined electric power. Kimari2 
     While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the purpose of the improvements or the scope of the following claims.