Image forming apparatus

An image forming apparatus has a document scanning unit which can be shifted while illuminating an original and which is shifted backwardly at a speed faster than a speed when said document scanning unit is shifted forwardly to scan the original, and a control unit which lowers the electric power supplied to an image fixing means when the document scanning unit is shifted backwardly.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to an image forming apparatus such as a
 copying machine and the like.
 2. Related Background Art
 Conventionally, in image forming apparatuses such as copying machines, a
 fixing device including a heat pressing roller has been used as a fixing
 means for fixing an image to a transfer sheet. Since the fixing device
 including such a heat pressing roller utilizes a heat source having great
 consumption of electricity such as a halogen heater, it is very difficult
 to reduce consumption of electricity of the entire image forming
 apparatus.
 Particularly, continuous copying operations are effected by using a copying
 machine having a fluorescent lamp as an illumination light source of an
 original scanning portion, since consumption of electricity becomes great
 during back scanning for returning an original scanning body from a scan
 finishing position to a home position, maximum consumption of electricity
 of the machine may not be suppressed within a desired range. The reason is
 that it takes a long time for the fluorescent lamp to stabilize a light
 amount after the lamp is turned ON so that the fluorescent lamp cannot be
 turned OFF during the back scanning, and a shifting speed of the original
 scanning body in the back scanning is faster than in forward scanning,
 with the result that consumption of electricity of a drive motor during
 the back scanning becomes great.
 Due to such great consumption of electricity in the back scanning of the
 original scanning body, the consumption of electricity of the entire
 machine may not be suppressed within the desired range.
 SUMMARY OF THE INVENTION
 The present invention aims to eliminate the above-mentioned conventional
 drawback, and an object of the present invention is to provide an image
 forming apparatus which can suppress consumption of electricity of the
 apparatus when a document scanning unit is shifted backwardly.
 Another object of the present invention is to provide an image forming
 apparatus comprising a document scanning unit which can be shifted while
 illuminating an original and which is shifted backwardly at a speed faster
 than a speed when the document scanning unit is shifted forwardly to scan
 the original, an image fixing means for fixing an image on a recording
 material, and a control means for controlling an electric power supplied
 to the image fixing means, and the control means lowers the electric power
 supplied to the image fixing means when the document scanning unit is
 shifted backwardly.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 The present invention will now be fully explained with reference to
 accompanying drawings.
 First Embodiment
 FIG. 1 shows an example of an image forming apparatus according to the
 present invention.
 An original is illuminated by a light source 2 (fluorescent lamp in this
 example), and light reflected from the original is converted into an image
 signal by a photo-electrical converting element such as CCD. The image
 signal is appropriately altered by a reader controller 6 and then is sent
 to a printer portion 3.
 In the printer portion 3, on the basis of the image signal sent from the
 reader controller 6, a photosensitive drum 7 is subjected to image
 exposure to form an electrostatic latent image.
 The electrostatic latent image is visualized with toner by a developing
 unit 8, and, thereafter, the visualized toner image is primarily
 transferred onto an intermediate transfer drum 9.
 Then, the toner image is transferred onto a recording material at a
 secondary transferring station 10, and the recording material on which the
 toner image was born is brought, by a transfer belt 11, to a fixing device
 4, where the toner image is fixed to the recording material by heat and
 pressure.
 In this case, the reader controller 6 sends an interruption signal to a
 controller 31 the printer portion 3 in synchronous with a back scanning
 operation for returning the light source 2 to an initial position for the
 original scanning.
 In controller 31 of the printer portion 3, on the interruption signal sent,
 a way for controlling a temperature of the fixing device 4 is altered.
 The fixing device 4 comprises a fixing roller (upper roller having an outer
 diameter of 46 mm, a rubber thickness of 2.1 mm and a PFA tube having
 surface layer thickness of 50 .mu.m) and a pressure roller (lower roller
 having an outer diameter of 46 mm, a rubber thickness of 2.3 mm and a PFA
 tube having surface layer thickness of 50 .mu.m), and halogen heaters 5
 (both 550 W) as heating sources are incorporated within the rollers.
 In the illustrated embodiment, as the way for controlling the temperature
 of the fixing device, a method in which only the heater of the upper
 roller among the heaters of the fixing device 4 is turned ON/OFF is used.
 This is shown in FIG. 3.
 After original scanning (forward scanning) for about two seconds, the light
 source is returned to the home position (back scanning) for about 0.5
 second.
 Since the speed of the back scanning is faster than the speed of the
 forward scanning by about four times, consumption of electricity of the
 original reading apparatus becomes maximum during the back scanning (since
 the fluorescent lamp cannot be turned OFF during the series of image
 forming operations, as mentioned above).
 More specifically, the consumption of electricity becomes greater in the
 back scanning than in the forward scanning by about 65 W.
 In this case, as shown in FIG. 3, by turning OFF only the upper heater,
 peak of consumption of electricity can be reduced.
 More specifically, in the back scanning, the consumption of electricity of
 550 W corresponding to one of heaters in the illustrated embodiment can be
 reduced.
 Further, in comparison with the case where the lower heater is not turned
 OFF in the back scanning, since the peak of consumption of electricity
 becomes that in the scanning, maximum consumption of electricity can be
 reduced by 65 W.
 When such fixing temperature control is effected, in the back scanning,
 heat amount applied to the fixing device is reduced to half.
 However, in effect, since the time of back scanning is shorter than the
 time of forward scanning by about 1/4 and the heat amounts of the fixing
 roller and the pressure roller, even if the heaters are not turned ON
 instantaneously, abrupt reduction in temperature does not occur, with the
 result that great influence is not affected to performance of the fixing
 device (fixing ability and fixing offset).
 Second Embodiment
 In a second embodiment of the present invention, as a way for altering
 temperature control of the fixing device 4, the heaters 5 are turned ON
 alternately with short intervals. The others are the same as those in the
 first embodiment.
 This is shown in FIG. 4.
 Also in this case, in the back scanning, the consumption of electricity of
 550 W corresponding to one of heaters can be reduced.
 Further, since timing of peak becomes that in the scanning, peak of
 consumption of electricity can be reduced by 65 W.
 Third Embodiment
 FIG. 2 shows another example of an image forming apparatus according to the
 present invention.
 An image is formed in the same manner as those in the first and second
 embodiments, except for the fact that the originals can automatically be
 exchanged by an original exchanging device 12.
 In this case, the signal is sent from the reader controller 6 to the
 printer portion 3 in synchronous with the exchange of original. The signal
 is also in synchronous with the back scanning of the light source due to
 inherent feature of the image forming apparatus.
 In the exchange of original (in the back scanning), in addition to 65 W
 described in connection with the first embodiment, 130 W corresponding to
 the original exchanging device is added, so that consumption of
 electricity of about 195 W in total is increased in comparison with the
 forward scanning.
 On the basis of the signal sent, the printer portion 3 alters the
 temperature controlling method of the fixing device 4.
 In this example, as the altering method, only one of the heaters 5 of the
 fixing device 4 is turned ON.
 This is shown in FIG. 5.
 Thus, by turning ON only the upper heater in exchange of original, peak of
 consumption of electricity can be reduced.
 More specifically, in the back scanning, the consumption of electricity of
 550 W corresponding to one of heaters in the illustrated embodiment can be
 reduced.
 Further, since timing of peak becomes that in the scanning, peak of
 consumption of electricity can be reduced by 195 W.
 Fourth Embodiment
 In a fourth embodiment of the present invention, an image is formed in the
 same manner as that in the third embodiment, except for the fact that the
 heaters 5 are turned ON alternately with short intervals as a way for
 altering temperature control of the fixing device 4.
 This is shown in FIG. 6.
 Also in this case, in the back scanning, the consumption of electricity of
 550 W corresponding to one of heaters can be reduced.
 Further, since timing of peak becomes that in the scanning, peak of
 consumption of electricity can be reduced by 195 W.
 Results of the first to fourth embodiments are summarized in FIG. 7.
 As mentioned above, in response to the signal from the reader controller in
 synchronous with the back scanning of the original reading device, by
 altering the turning ON of the heaters of the fixing device of the
 printer, peak of consumption of electricity of the image forming apparatus
 can be reduced.
 Furthermore, in response to the signal from the reader controller in
 synchronous with the exchange of original of the original exchanging
 device, by altering the turning ON of the heaters of the fixing device of
 the printer, peak of consumption of electricity of the image forming
 apparatus can be reduced.
 The present invention is not limited to the above-mentioned embodiments,
 but various alterations can be made within the scope of the invention.