1. Field of the Invention
The present invention relates to an image heating apparatus which heats an unfixed image on a recording medium, and more particularly, to an image heating apparatus effectively applicable to a fixing apparatus for an image formation apparatus such as a copier, facsimile and printer based on an electrophotography scheme or electrostatic recording scheme.
2. Description of the Related Art
In recent years, attention is being given to a fixing apparatus, from the standpoints of energy saving and ease of use, capable of heating an image heating body for heating an unfixed image on a recording medium up to a target temperature in a short time and making a quick start.
As an image heat generation section for this type of fixing apparatus, one using an image heating apparatus based on an induction heating (IH) scheme is known. This image heating apparatus generates an eddy current by causing a magnetic field generated by an induction heating apparatus to act on an image heating body and heats an unfixed image on a recording medium such as transfer paper and OHP (Over Head Projector) sheet with Joule heat of the image heating body caused by this eddy current.
Compared to an image heating apparatus using a halogen lamp as a heat source of a heat generation section that heats the image heating body, this IH-scheme image heating apparatus has an advantage of having high heat generating efficiency and being capable of increasing a fixing speed. Furthermore, the image heating apparatus using a thin sleeve or belt, etc., as the image heating body has a small heat capacity of the image heating body, can heat this image heating body in a short time and improve rising response considerably.
The fixing apparatus using this type of image heating apparatus keeps maximum power output so as to make a warm-up time as short as possible and controls the temperature of the image heating body so as to tolerate a certain degree of overshoot with respect to a target temperature and shorten a time required to reach the target temperature. A tolerance of the overshoot with respect to this target temperature is approximately 5° C. This is because if the overshoot exceeds the target temperature by 10° C. or more, a luster variation occurs on the fixed image.
In such a fixing apparatus, there is only a certain degree of overshoot when the temperature of the image heating body is low, but when the temperature of the image heating body increases from a state in which the temperature is relatively high, there is a problem that an excessive power output is required, causing an excessively large overshoot.
For this reason, this type of fixing apparatus causes a large overshoot of the image heating body, producing an offset or producing a luster variation during printing of a first sheet.
Therefore, a method for this type of conventional fixing apparatus to change a power supply output of the image heating apparatus according to the temperature of the image heating body is proposed in a Patent Document (Unexamined Japanese Patent Publication No. 2001-222191) etc.
That is, the fixing apparatus disclosed in the Patent Document is provided with a temperature detection section made up of a thermistor in the vicinity of the image heating body to detect the temperature of the image heating body and select optimum power from among 700 W to 1300 W according to the temperature of the image carrier. More specifically, a control circuit of the main body given the output of the thermistor selects any one level of the power based on the detected temperature of the image heating body and outputs a power control signal to an IH control circuit.
On the other hand, the IH-scheme image heating apparatus normally controls power supplied to a heat source using values calculated based on a predetermined control rule in accordance with the detected temperature of the temperature detection section which contacts the image heating body or which is placed in the vicinity thereof and thereby keeps the image heating body to a predetermined fixing temperature (target temperature).
As the above described control rule, PID (Proportional, Integral, Derivative) control including PI control and PD control is used. This PID control performs control by not only causing the amount of operation of the power control section to be proportional to a deviation between the detected temperature of the temperature detection section and the target temperature of the image heating body based on the trend of increase/decrease of the deviation but also taking into account elements proportional to the integral of the deviation or elements proportional to the derivation of the deviation.
Furthermore, the temperature information from the temperature detection section is sampled in a certain period (sampling period) and incorporated into the control rule of PID control.
When the temperature of the entire apparatus of such an image heating apparatus is low, it is preferable to heat the image heating body through PID control with such a setting that causes the image heating body to overshoot the target temperature to a certain degree because the warm-up time is shortened in this way.
However, if the ambient temperature of the image heating apparatus is already high, when the image heating body is heated next, heating the image heating body through PID control with the same setting as that when the temperature of the entire apparatus is low increases the temperature rising rate of the image heating body and increases the overshoot.
Furthermore, the magnetic characteristic of this type of image heating apparatus changes as the temperature of the image heating body increases, and therefore when the image heating body is heated through the PID control with the same setting as that at the time of low temperature, there is a problem that it is hard to enter the output when the temperature is high.