Patent ID: 8618467

Claim:
An absolute encoder that measures an absolute position of an object, the absolute encoder comprising: a scale portion configured to be movable along with the object, and including a first track having a plurality of first slits formed at a first interval and a second track having a plurality of second slits formed at a second interval different from the first interval; a light source configured to illuminate a light onto the first slits and the second slits; a first detector configured to detect a first signal that is obtained from a light reflected by or transmitted through the first track; a second detector configured to detect a second signal that is obtained from a light reflected by or transmitted through the second track; and a calculator configured to calculate the absolute position of the object based on the first signal and the second signal, wherein the second slits are configured so that a reflected light intensity or a transmitted light intensity of the light illuminated from the light source is different in accordance with a position in a moving direction of the scale portion, and wherein the calculator is configured to: obtain an upper-level signal based on a voltage value of photocurrent obtained by the second detector or an amplitude value of the second signal; obtain a middle-level signal by performing a vernier calculation using the first signal and the second signal; obtain a lower-level signal by performing an arctangent calculation using the first signal; and calculate the absolute position of the object based on the upper-level signal, the middle-level signal, and the lower-level signal.