In robotics, this is a device used to generate pulses as a wheel turns, which can be used to measure velocity.

A very popular sensor device used in mobile robotics. It reads wheel and axis motion and/or position, and translate it into digital form.

A feedback device in the robot manipulator arm that provides current position (and orientation of the arm) data to the controller. A beam of light passes through a rotating code disk that contains a precise pattern of opaque and transparent segments on its surface. Light that is transmitted through the disk strikes photo-detectors, which convert the light pattern to electrical signals. See Feedback, Closed-loop Control and Feedback Sensor.