Patent ID: 6118463
Filing Date: 2000-09-12
Classification: H04N

Abstract:
An image forming apparatus comprising:a transport mechanism transporting a recording medium on a belt which is transported at a constant speed in a transport direction;a plurality of electrostatic recording units arranged in the transport direction of the recording medium and transferring toner images of different colors;a positional error detecting part using the toner image of a reference color transferred by one of the electrostatic recording units as a reference image, and relatively detecting positional error information related to positional errors of the toner images of remaining colors transferred by the remaining electrostatic recording units with respect to the reference image; anda positional error correcting part relatively correcting the positional errors of the toner images transferred by the remaining electrostatic recording units based on the positional error information,said positional error detecting part comprising:first means for controlling said electrostatic recording units to transfer at least two resist marks on the belt at a scan starting end and a scan terminating end of the belt in a main scan direction which is perpendicular to the transport direction of the recording medium, with respect to each of the colors; andsecond means, including a sensor detecting the resist marks, for detecting positional error quantities of the resist marks of each of the remaining colors with respect to the resist marks of the reference color based on detection times of the resist marks of the reference color and of the remaining colors,wherein each of the resist marks has a first straight line parallel to a main scan direction which is perpendicular to the transport direction of the recording medium and a second straight line included with respect to the main scan direction and a sub scan direction which is perpendicular to the main scan direction,wherein said second means measures a first time from a detection time of the first line to a detection time of the second line of one resist mark of the reference color, and a second time from a detection time of the first line to a detection time of the second line of each of the remaining colors, and detects an error quantity in the main scan direction of each of the remaining colors based on a difference between the first time and the second time, andwherein said positional error detecting part sets an intersecting angle .theta. of the second straight line of the resist mark with respect to the first straight line to a value satisfyingtan.theta..ltoreq.(v.multidot.T/S)where v denotes a transport speed of the belt in (mm/sec), S denotes a detection accuracy of the sensor in (.mu.m), and T denotes a sampling period of a detection signal output from the sensor in (sec).