Belt driving device and image forming apparatus

A belt driving device installed in an image forming apparatus, wherein the image forming apparatus comprises:

• plural image carriers each configured to carry a visible image;
• a visible image forming unit configured to form the visible images on the corresponding image carriers;
• an image forming control unit configured to control the visible image forming unit;
• a transfer unit configured to transfer the visible images on the image carriers so as to be superposed on a surface of a belt member having an endless form configured to endlessly move, or on a recording member held on the surface;
• and
• a contact/separation unit configured to cause the surface of the belt member to contact/separate from at least one of the image carriers, or to cause the belt member to contact/separate from an opposing member facing the belt member, wherein the belt driving device comprises:
• a driving rotary body configured to endlessly move the belt member by a rotational drive thereof, while having an inner loop of the belt member stretched around the driving rotary body;
• a subordinate rotary body configured to be rotated by the endless movement of the belt member while being in contact with the belt member;
• a first detecting unit configured to detect a driving speed at which the driving rotary body is driven;
• a second detecting unit configured to detect rotational angular displacement or a rotational angular speed of the subordinate rotary body;
• and
• a belt driving control unit configured to determine a reference timing of a preceding rotation of the belt member and a speed variation pattern of the belt member during the preceding rotation based on detection data obtained by the first detecting unit in the preceding rotation and detection data obtained by the second detecting unit in the preceding rotation, and to control, during a succeeding rotation which is continuously performed after the preceding rotation, the driving speed of a driving source configured to drive the driving rotary body, based on the reference timing and the speed variation pattern, wherein:
• during a rotation of the belt member after a contact-state-changing rotation, the belt driving control unit controls the driving speed of the driving source based on a reference timing determined in a rotation of the belt member before the contact-state-changing rotation, instead of a reference timing determined in the contact-state-changing rotation, wherein the contact-state-changing rotation corresponds to
• a rotation of the belt member in which a number of the image carriers contacting the belt member changes, or
• a rotation of the belt member in which the opposing member that has been separated from the belt member comes in contact with the belt member, or
• a rotation of the belt member in which the opposing member that has been in contact with the belt member separates from the belt member.