Absolute encoder that detects an absolute position of an object

A method of measuring an absolute position of an object using an absolute encoder, 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,
• wherein the method comprises the steps of:
• obtaining an upper-level signal based on a voltage value of photocurrent obtained by the second detector or an amplitude value of the second signal;
• obtaining a middle-level signal by performing a vernier calculation using the first signal and the second signal;
• obtaining a lower-level signal by performing an arctangent calculation using the first signal;
• and
• calculating the absolute position of the object based on the upper-level signal, the middle-level signal, and the lower-level signal.