1. Field of the Invention
The present invention relates to an image forming apparatus comprising a proximity charger to be impressed with a superimposed voltage of a DC voltage and an AC voltage.
2. Description of Related Art
Recently, for charging in an image forming apparatus, a proximity charging method is mainly adopted. In the proximity charging method, for example, a roller-type charger is provided in proximity to the surface of a photoreceptor drum so as to be in contact or out of contact with the surface of the photoreceptor drum. A superimposed voltage of a DC voltage and an AC voltage is applied to the charger so that the charger can charge the surface of the photoreceptor drum uniformly.
It is known that the charged potential Vs of the surface of the photoreceptor drum and the peak-to-peak voltage Vpp of the AC voltage Vac have a relationship as illustrated in FIG. 8. While the peak-to-peak voltage Vpp is within a range from a charging start voltage Vth to a voltage 2×Vth, the charged potential Vs is substantially proportional to the AC voltage Vac. Here, the charging start voltage value Vth is a voltage value that permits the charger to start charging the photoreceptor drum, and the voltage value Vth is defined by the DC voltage Vdc. The charging start voltage Vth is determined depending on the characteristics of the photoreceptor drum and other factors. In the case of FIG. 8, the voltage value Vth is 800V, and the voltage value 2×Vth is 1600V.
After the peak-to-peak voltage Vpp becomes above the value 2×Vth, the charged potential Vs of the surface of the photoreceptor drum is saturated and substantially kept constant at Vs0. Therefore, in order to charge the surface of the photoreceptor drum to have a uniform charged potential Vs, it is necessary to apply a superimposed voltage obtained by superimposing an AC voltage Vac having a peak-to-peak voltage Vpp greater than 2×Vth on a DC voltage Vdc to the charger. In this regard, the charged potential Vs0 depends on the DC voltage Vdc of the superimposed voltage.
Meanwhile, in an image forming apparatus, the amount of discharge from a charger is required to be constant regardless of changes in environmental conditions, variations in the resistance of the charger due to manufacturing errors, etc. so as to charge a photoreceptor drum uniformly without causing deterioration of the photoreceptor drum, poor-quality image formation, etc. For this purpose, conventionally, an image forming apparatus comprises a measuring device that measures the alternating current flowing in the charger via the photoreceptor drum, and a controller.
The measuring device measures values of the alternating current while no sheets are fed in the image forming apparatus. Specifically, the measuring device measures values of the alternating current flowing in the charger when alternating voltages Vac having different peak-to-peak values Vpp respectively, all of which are less than 2×Vth, are applied to the charger sequentially. In a similar way, the measuring device measures the values of alternating current flowing in the charger when alternating voltages Vac having different peak-to-peak voltages Vpp respectively, all of which are equal to or greater than 2×Vth, are applied to the charger. In this specification, a range in which the peak-to-peak voltage Vpp is less than 2×Vth is referred to as a forward discharge range, in which charge transfers only from the charger to the photoreceptor drum (that is, mono-directional charge transfer occurs), and a range in which the peak-to-peak voltage Vpp is equal to or greater than 2×Vth is referred to as a reverse discharge range, in which charge transfers from the charger to the photoreceptor drum and from the photoreceptor drum to the charger alternately (that is, bi-directional charge transfer between the charger and the photoreceptor drum occurs).
From the values of the alternating current collected by the measuring device, the controller determines a peak-to-peak voltage Vppi of the alternating voltage Vaci to be used as a component of the charging voltage in a printing process. In this specification, such a control process is referred to as a first charging voltage determination process.
A specific example of the first charging voltage determination process will hereinafter be described with reference to FIG. 9. The controller obtains values Iac1-Iac3 of the alternating current flowing in the charger when AC voltages Vac1-Vac3 are applied to the charger in the forward discharge range, and from the alternating current values Iac1-Iac3, the controller derives a characteristic line L1 indicating alternating current values with respect to the applied AC voltage in the forward discharge range by direct approximation. In a similar way, the controller derives a characteristic line L2 indicating alternating current values with respect to the applied AC voltage in the reverse discharge range. The controller determines the point of intersection between the characteristic lines L1 and L2 as the alternating voltage Vaci to be used as a component of a superimposed charging voltage in a printing process.
When the alternating current value Iac is determined by the first charging voltage determination process, non-uniformity of the film thickness of the photoreceptor drum is taken into consideration in some cases. More specifically, while the photoreceptor drum is rotated once, the controller obtains the alternating current values Iac at a predetermined number of places different from each other in the circumferential direction. The controller determines the average of the measured alternating current values Iac as the alternating current value Iac achieved by application of the alternating voltage Vac to the charger.
There are other ways of deriving a peak-to-peak voltage Vpp (see, for example, Japanese Patent Laid-Open Publication No. 2009-086108).
Meanwhile, a roller-type charger is likely to cause more abrasion of the photoreceptor film, as compared to a corona-discharge-type charger. In a recent image forming apparatus, also, in order to remove discharge products and the like adhering to the photoreceptor film, the photoreceptor film is scraped as needed. In a case in which a roller-type charger is used in such an image forming apparatus, it is important to use a photoreceptor having a thick photoreceptor film and to minimize the amount of abrasion per a predetermined number of rotations of the photoreceptor.
In the first charging voltage determination process, an AC voltage Vaci that is the point of intersection between the characteristic lines L1 and L2 is derived from the difference in slope between the characteristic lines L1 and L2. However, the inventors found out by an experiment that there are cases in which the AC voltage Vaci determined by the first charging voltage determination process is not proper, depending on the photoreceptor film thickness and/or the ambient temperature. For example, when the ambient temperature is low or when the photoreceptor film is thick, the difference in slope between the characteristic lines L1 and L2 is small, and the AC voltage Vaci derived from the slope difference is likely to shift to a lower side. If a charging voltage including an AC voltage Vaci lower than a proper value is used in a printing process or the like, toner fogging may occur.