1. Field of the Invention
The present disclosure relates to an image forming apparatus that a sheet material.
2. Description of the Related Art
A small and inexpensive stepping motor is often used as a drive source for a sheet feeding/conveying system in an image forming apparatus. The stepping motor is generally driven by a constant current chopper control system. While the stepping motor can contribute to achieving a compact and inexpensive structure, the stepping motor frequently causes a step-out phenomenon in which the rotation of a rotor cannot be synchronized with the input of a pulse signal. The step-out phenomenon occurs, for example, in an overload state to the pulse rate of the pulse output to the stepping motor from a drive circuit.
An image forming apparatus that performs multifarious types of image formation needs to support various kinds of sheets, such as plain paper and thick paper. Therefore, the required torque of the stepping motor varies significantly depending on the kind of sheets in use in some cases. With regard to the torque for causing a sheet to enter between conveying rollers made of a sponge material and disposed along a sheet conveying path, for example, the torque for thick paper (200 g/cm) becomes twice as high as the torque for plain paper (80 g/cm) in some cases. The torque of the stepping motor is determined by the value of the drive current. Therefor, the selection of the stepping motor and the selection of the drive current that determines the torque are determined on the assumption of using thick paper which faces severer conditions.
However, plain paper is predominantly used in general offices or the like. Therefore, the constant use of the drive current set for thick paper causes more than necessary power consumption and noise generation. Thus, a technique to solve this problem is proposed (see Japanese Patent Application Laid-Open No. 2001-322734). According to the technique described in Japanese Patent Application Laid-Open No. 2001-322734, the drive current value is set for plain paper in a normal operation mode, and is set for thick paper only when a user operates to set thick paper.
Recently, there is a proposal of a technique of optimizing the drive current based on the relationship between the maximum output torque of a stepping motor and the level of the drive current (see US 2011/0229235). The technique disclosed in US 2011/0229235 calculates an estimated load torque at the time of conveying a first sheet. The estimated load torque is used to determine a current value corresponding to the level of the target load torque at the time of conveying second and subsequent sheets.
The technique disclosed in Japanese Patent Application Laid-Open No. 2001-322734 is effective in preventing a step-out phenomenon. However, this technique still faces the problem when actual sheets in use differ from the set sheets, for example, when a sheet cassette for plain paper is removed and thick paper is fed in this sheet cassette.
The technique disclosed in US 2011/0229235 independently controls a plurality of stepping motors, thus complicating the control circuit when a plurality of stepping motors is used.