Patent ID: 7825368

Claim:
An absolute position length-measurement type encoder comprising: a scale having an incremental track formed therein with incremental patterns including first light and dark patterns formed at equal intervals in first periods, an absolute track formed therein with absolute patterns representing absolute positions, and a reference position track formed therein with reference position patterns including second light and dark patterns formed at equal intervals in second periods longer than the first periods; a light source for emitting a measurement light to the scale; a photodetector for receiving the measurement light reflected at or transmitted through the scale; and a signal processing circuit for processing a received-light signal of the photodetector to detect an absolute position of the scale, wherein the signal processing circuit conducts a first judgment about which one of the second periods the scale is located in based on an absolute position signal obtained from the absolute patterns, and then conducts a second judgment about which one of the first periods the scale is located in based on the first judgment and detects an absolute position of the scale based on a result of the first judgment, a reference position signal obtained from the reference position patterns, and a relative position signal obtained from the incremental patterns based on a result of the second judgment.