Absolute code of absolute encoder！

The absolute encoder presents important operating differences from
the incremental one: in the incremental encoders the position is
determined by the number of impulses from the zero index, in the
absolute encoders the position is determined by the read of the output
code, which is only for each position inside the turn; consequently the
absolute encoders do not loose the real position when the power supply
is turn 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. The
output code is used to define the absolute position. The most obvious
choice is the binary code which can be easily manipulated from the
external control device for the reading of the position and it is not
necessary to have any particular conversion operations. As the code is extracted directly from the disc (which is in rotation) the syncronization and acquisition of the position at the moment of the variation between one code and another becomes very difficult. If we take,for example two consecutive binary codes as 7 (0111) and 8 (1000) we can note that all the bit of the code change their state; a reading made at the moment of the transition could result completely wrong as is impossible that the variations are instantaneous and contemporary. Due to this problem is used a binary code variant, the Gray code, which
has the particularity that in the passage between the two consecutive codes (even from the last to the first code) only one bit changes its state

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