Displacement detector and image forming device having the same

An image forming device comprising:

• a conveying unit configured to convey a sheet;
• an image forming unit with a rotatable image carrier, configured to produce a toner image on the image carrier, and transfer the toner image from the image carrier onto the sheet that the conveying unit conveys;
• a monitor unit including a displacement detector configured to determine a displacement or rotation angle of a target, which is a sheet conveyed by the conveying unit or the image carrier, the monitor unit configured to use the displacement detector to monitor the displacement or rotation angle of the target;
• and
• a controller unit configured to, based on the displacement or rotation angle that the monitor unit monitors, control the conveying unit or the image forming unit, wherein
• the displacement detector comprises:
• an emission unit configured to emit a variable amount of laser light to a predefined range in a space in which the target moves;
• a detection unit configured to set a capture region in the predefined range and detect a distribution of amounts of light reflected from the capture region;
• a calculation unit configured to make the emission unit repeatedly emit laser light, make the detection unit repeatedly detect a distribution of amounts of reflected light, and based on a shift of speckle pattern indicated by a difference between the distributions of amounts of reflected light detected at different times, calculate a displacement of the target;
• and
• a correction unit configured to measure a speckle contrast from one of the distributions of amounts of reflected light and, based on an error between the measured value and a reference value, correct the amount of laser light to be emitted, the correction unit setting the reference value to the value of a speckle contrast in a case in which the amounts of light reflected from the capture region have a maximum no more than an upper limit of a detectable range of the detection unit and a minimum no less than a lower limit of the detectable range.