Dual optical angular encoder

A dual incremental optical angular encoder, comprising:

• a disc bearing marks and two pairs of cells for detecting marks, each pair of cells providing a logic state having of a pair of logic levels allowing a determination of an increment of rotation +1 or −1 when the disc revolves,
• means for comparing at a given instant a sequence of four successive states S0, S1, S2, S3 taken by the first pair of cells, with a sequence of four successive states S′0, S′1, S′2, S′3 taken by the second pair of cells, the last states of these sequences being the states taken at the instant at which the comparison is made, and means for providing an indication of erroneous counting if the sequence S′0, S′1, S′2, S′3 is not equal to S0, S1, S2, S3 or Sx, S0, S1, S2, or S1, S2, S3, Sy, in which Sx represents a prior state of the first pair and Sy is a possible state of the first pair such that the increment in passing from S3 to Sy is not greater than 1 in absolute value.