Patent ID: 7773914

Claim:
An image forming apparatus, comprising: a rotating member having open end portions in an axial rotation direction of the rotating member; a motor configured to rotate the rotating member; a marking member configured to mark a rotational position of the rotating member fixedly disposed at one of the open end portions of the rotating member, the marking member having a primary portion and a secondary portion along a circumference of the marking member, the marking member concentrically rotating with the rotating member on a rotational path of the circumference of the marking member; a mark sensor configured to perform a mark detecting operation for detecting the primary portion and the secondary portion of the marking member, and to output a primary signal when detecting the primary portion and a secondary signal when detecting the secondary portion; a position sensor configured to determine the rotational position of the rotating member based on a time at which the position sensor starts receiving one of the primary and secondary signals generated during an initial time period of the mark detecting operation performed by the mark sensor; and a motor controller configured to control the motor based on a determination result obtained by the position sensor to make the rotational position of the rotating member in agreement with a target position at a predetermined time, wherein the primary portion of the marking member includes a plurality of primary portions and the secondary portion of the marking member includes a plurality of secondary portions, wherein the plurality of primary portions and the plurality of secondary portions are arranged alternately on a same circumference, wherein the number of the plurality of primary portions is equal to the number of the plurality of secondary portions, wherein one of the plurality of primary portions and the plurality of secondary portions have different lengths from each other in a rotating direction of the marking member and the other of the plurality of primary portions and the plurality of secondary portions have equal lengths to each other in a rotating direction of the marking member, and wherein the position sensor determines the rotational position of the rotating member based on primary reception times at which the position sensor starts receiving a signal corresponding to a first detected length of one of (1) the one of the plurality of primary portions and the plurality of secondary portions having different lengths, and (2) the other of the plurality of primary portions and the plurality of secondary portions having equal lengths, and at which the position sensor starts receiving another signal corresponding to a second detected length of another one of (1) the one of the plurality of primary portions and the plurality of secondary portions having different lengths, and (2) the other of the plurality of primary portions and the plurality of secondary portions having equal lengths.