Thickness detector and image forming apparatus

A thickness detector comprising:

• a roller assembly including:
• a rotator having a plurality of marks different in shape and disposed at different positions on a face of the rotator in a direction of rotation of the rotator;
• and
• an opposing member disposed opposite the rotator, to convey a sheet held in the roller assembly together with the rotator;
• a detector to detect and output an amount of displacement of one of the rotator and the opposing member in a direction of thickness of the sheet;
• and
• a controller including:
• a first acquisition unit configured to acquire first output values for one rotation of the rotator from the detector, the first output values being output in a state in which the sheet is not held in the roller assembly;
• a determining unit configured to determine, based on a value output from the detector, whether the detector has detected one of the plurality of marks of the rotator in a state in which the sheet is held in the roller assembly;
• a second acquisition unit configured to acquire, from the detector, a predetermined number of second output values output after detection of the one of the plurality of marks;
• and
• a calculation unit configured to extract, from the first output values for one rotation of the rotator, output values corresponding to the predetermined number of second output values acquired by the second acquisition unit, based on the value output corresponding to detection of the one of the plurality of marks,
• the calculation unit further configured to calculate a thickness of the sheet based on the second output values and the extracted first output values.