An encoder for detecting a forward/reverse movement of a rotatable object includes a rotating shaft that is supported on a base and rotates in accordance with the movement of the subject, two rotating disks fixed to the rotating shaft, and two photocouplers mounted on the base which are associated with the disks. Each of said rotating disks is formed with a predetermined light modulating pattern to vary the intensity of light along a circumferential direction. Each of the photocouplers emits a light toward the rotating disk and detects the light modulated by the pattern on the rotating disk. The angular relationship between the rotating disks is selected such that the output signals from the photocouplers due to the rotation of the rotating shaft have a predetermined phase difference.
Dual sensor encoder for detecting forward/reverse rotation having light modulating patterns with a predetermined phase difference