Patent ID: 7784892

Claim:
A printer, comprising: a scale provided with a plurality of marks or slits arranged in a first direction such that a distance between centers of adjacent marks or slits in the first direction assumes a first length; an encoder, opposing the scale and comprising: a photo emitter, operable to emit light; and a plurality of photo detectors, each of which has a light receiving region adapted to receive the light emitted from the photo emitter and transmitted by way of the marks or slits, and is operable to output a detection signal in accordance with a quantity of the light received by the light receiving region, so that the detection signal has a first cycle corresponding to the first length; a control signal generator, operable to generate a control signal having a second cycle which is (½ n1 ) of the first cycle; a motor; and a controller, operable to estimate a rotation speed of the motor based on a third cycle which is defined by subsequent (2 n1 ) second cycles, wherein: n 1 is an integer no less than one, and the photo detectors are arranged in a second direction perpendicular to the first direction while being shifted in the first direction by a second length which is not an integral multiple of the first length.