A light beam interrupted by a rotating code wheel is the essence of an optical shaft encoder.
The light produced by a LED is transmitted through a code disc with an alternating opaque and transparent pattern (i.e. slits and bars); the code disc is fixed on the motor shaft. Small sensors like photo-transistors or photo-diodes measure the light modulation. To allow for higher resolution at a given diameter than that achievable by a simple direct beam interruption method, a mask or phase plate is placed in the light path above the photo-detectors.
The light from the LED can only reach the detectors when the code wheel slits are aligned with the slits on the mask disc, and, since the code wheel is rotating, the detector received alternating periods of light and dark. The position of the sensors or the form of the mask enables two phase shifted signals called channel A and B to be generated.
Digital encoder provides two signals in TTL or CMOS compatible form. These are square wave signals with an ideal 50% duty cycle. The number of lines on the disk gives the resolution of the encoder – this results in two series of pulses each equal to the number of slots on the code disk.
More information is available with encoders having a third signal know as an index pulse. This signal appears once per shaft revolution and its ideal width is 90ºe (electrical degrees) and can be used for initialization purposes.
Magnetic encoders
The magnetic flux variations of a low-inertia multi-polar magnetic disk are detected by two solid state sensors (Hall effect sensors) and transmitted to the controller. They generate two TTL-CMOS compatible output signals with 90ºe phase shifted two channels.
The Micro-Drives magnetic encoder product range includes:
Series MDE2-16 – a low resolution encoder mounted on the rear shaft of DC-Micromotors.
