1. The principle of the incremental measuring system is the scanning of a line pattern on a glass or plastic disc. The states of the line pattern transparent or not transparent are converted into electronic pulses by an opto-electronic unit (e.g. transparent = 5V, not transparent = 0V). The analysis of the signals is performed in an evaluation unit by counting up or down with each pulse. The current count is stored in digital form and is instantly available for evaluation.

2. The position is determined by the number of impulses from the zero index.

Absolute Encoder :

1. Every position of the measurement range/angle is identified by a definite code on a glass or plastic disc. This code is represented on the disc in the form of light and dark regions within different tracks. This combination relates to an absolute numerical value. Thus, the position value is always directly available, counters are not necessary. In addition it is not possible to get continuously invalid values caused by interferences or loss of the supply voltage. Movements which are done while the system is turned off are immediately measured after the system is powered up.

2. The position is determined by the read of the output code, which is only for each position inside the tum; consequently the absolute encoders do not loose the real position when the power supply is tum off (even if shifted). To a following power up (thanks to the direct coding on the disc) the position is up to date and available and it is not necessary, as for the incremental encoders, to seek the zero index.

Conclusion:

One main disadvantage of incremental encoders when used as a position sensor, is that they require external counters to determine the absolute angle of the shaft within a given rotation. If the power is momentarily shut off, or if the encoder misses a pulse due to noise or a dirty disc, the resulting angular information will produce an error. One way of overcoming this disadvantage is to use absolute position encoders.

One main advantage of an absolute encoder is its non-volatile memory which retains the exact position of the encoder without the need to return to a “home” position if the power fails. Most rotary encoders are defined as “single-turn” devices, but absolute multi-turn devices are available, which obtain feedback over several revolutions by adding extra code disks.