1. Field of the Invention
The present invention relates to an image forming apparatus as a copier, a printer, a facsimile machine, or a compound machine thereof, and particularly to an image forming apparatus including a fixation device for fixing a toner image formed by an electrophotographic system to a transfer sheet.
2. Description of the Related Art
Recently, it has been made possible to obtain a large amount of image information and the like owing to a development of a communication network such as the Internet. In order to process such information, an image forming apparatus as a copier, a printer, a facsimile machine, or a compound machine thereof has been required more to have performance capable of forming a large amount of high-quality color images and the like at a high speed.
It is frequent that an image forming apparatus which forms an image by an electrophotographic system is used as such an image forming apparatus. In the above-described image forming apparatus, there is provided a fixation device for developing, by a toner, an electrostatic latent image formed based on the image information, and fixing an emerging toner image to a transfer sheet.
As the fixation device, there are known one which includes a fixing roller as a fixing member, and a pressure roller, each having a heater therein, and is constructed to form a pressure contact portion by bringing the fixing roller and the pressure roller into pressure contact with each other, one which is constructed to form a pressure contact portion by bringing a fixing roller member and a pressure roller into pressure contact with each other, in which the fixing roller member is formed by extending a fixing belt between a heating roller having a heater therein and a fixing roller, and the pressure roller is a pressure member also having a heater therein, and the like. In any of the above-described fixation devices, the transfer sheet to which the toner image is transferred is inserted into the pressure contact portion of the fixing roller and the pressure roller. Then, the toner is heated and pressurized by the pressure contact portion, is thus fused and adhered to the transfer sheet. Thereby, the toner is fixed thereto.
In order to fix the toner image formed on such a transfer material at a high speed by the fixation device with the construction as described above, for example, it is considered to raise a fixing temperature of the fixing member and to shorten a time necessary for the transfer material to pass through the pressure contact portion.
In order to raise the fixing temperature in a short time, it is considered to provide a high power heater in the fixation device, to increase the heater in amount and number, and so on. However, large power consumption is required when it is attempted to raise the fixing temperature in a short time, to increase the number and amount of the heater, and so on. Accordingly, there has been a problem in that, when it is attempted to provide a high speed and high image quality image forming apparatus, it is necessary to increase the power consumption of the entire image forming apparatus.
In this connection, it is necessary to control power consumption of the fixation device under operation in order to restrict the power consumption of the entire image forming apparatus within predetermined power consumption. For example, the following technologies have been proposed. In the first technology, priorities are given to an upper heat roller (a fixing roller) and a lower heat roller (a pressure roller), target temperatures are determined for the individual rollers, the heat roller having higher priority is operated first-based on the target temperature, the heat roller having lower priority is operated during a stop of energization of the heat roller having the higher priority, and both heat rollers are thus alternately operated so as not to be made to turn on simultaneously (for example, refer to JP-Tokukaihei-9-311580A). In the second technology, during a period from transfer of one image to an intermediate transfer body to transfer of the next image thereto, for example, the heat roller of either the fixing roller or the pressure roller is energized, and the two heat rollers are not energized simultaneously (for example, refer to JP-Tokukai-2000-19926A). In the third technology, the two heat rollers are alternately energized at a constant cycle (for example, refer to JP-Tokukai-2004-70056A). Moreover, in the fourth technology, in order to control the power consumption in conjunction with other devices than the fixation device, a dehumidifying heater for the transfer sheet is operated while the energization of the heater of the fixation device is being stopped, and the power consumption of the image forming apparatus is thus to be controlled (for example, refer to JP-Tokukai-2003-58024A).
However, in any of the constructions described in the above conventional technologies, the two heaters were always controlled to be energized based on the preset priorities while the image forming apparatus was being operated.
Specifically, the constructions described in JP-Tokukaihei-9-311580A, JP-Tokukai-2000-19926A, and JP-Tokukai-2003-58024A relate to the energization control for the two heaters in the fixation device, and the construction described in JP-Tokukai-2003-58024A relates to the energization control for the two heaters which are the dehumidifying heater and the heater of the fixation device. In any of the constructions, the priorities are preset for the two heaters, the heater having the higher priority is energized first, and the heater having the lower priority is energized while the energization of the heater having the higher priority is being stopped. In such a way, the control is always made so that the two heaters cannot be energized simultaneously, and the power consumption of the image forming apparatus under operation is thus controlled within the range of the predetermined power consumption.
Accordingly, the number of heaters for which the energization is controlled is increased to three or more, for example, in the case of providing the heaters to the following rollers for the purpose of fixing the toner image formed on the transfer material at the high speed. The rollers are the heat roller which heats the fixing belt of the fixation device, the pressure roller which forms the pressure contact portion abutting on the fixing belt and inserting the transfer material therethrough, an external heat roller provided so as to supply heat to the heat roller by abutting on the pressure roller as its surface temperature is less prone to rise owing to an elastic layer provided on the surface thereof, and the like. Moreover, when the operation state of the image forming apparatus is changed, for example, to warming up (WU), immediately after the start of paper feeding, under paper feeding, and standby, and in the case where it is required to perform the energization control for the heaters so as to make it possible to perform the optimum temperature control for the fixation device, which corresponds to the respective operation states, the heaters each having higher priority, for which it is necessary to preferentially perform the energization, are different depending on the respective operation states. Hence, there has been a problem in that it is impossible to perform the optimum temperature control for the fixation device in response to the operation state of the image forming apparatus within the range of the predetermined power consumption only by always making the control based on the preset priorities to avoid simultaneous energization of the two heaters.