Image forming apparatus including position detector

A position detector, comprising:

• a scale attached to a movable member and including a plurality of optical marks formed in line at a substantially constant interval;
• an image sensor configured to capture an image of at least two of the plurality of optical marks and the interval between the at least two optical marks at a time, the image sensor further configured to be larger than the width of two of the plurality of optical marks and the interval between the two optical marks;
• a signal processor configured to determine a coordinate of an optical mark with respect to a reference position using the data captured by the image sensor;
• and
• a position computing unit configured to compute a distance traveled by the optical mark based on a number of the optical marks that pass the reference position and the coordinate of the optical mark with respect to the reference position and to determine the value of the interval between the at least two optical marks based on the image of the at least two optical marks.

A travel detector for detecting a rotation speed and a position of a rotationally movable member, comprising:

• a scale attached to a rotationally movable member and including a plurality of optical marks formed in line at a substantially constant interval;
• an image sensor configured to capture an image of at least two of the plurality of optical marks and an interval between the two optical marks at a time, the image sensor further configured to be larger than the width of two of the plurality of optical marks and the interval between the two optical marks;
• a signal processor configured to determine a coordinate of an optical mark with respect to a reference position using the data captured by the image sensor;
• and
• a position computing unit configured to compute a distance traveled by the optical mark based on a number of the optical marks that pass the reference position and the coordinate of the optical mark with respect to the reference position and to determine the value of the interval between the at least two optical marks based on the image of the at least two optical marks.