US Patent 7687766 Out-of-round coder

Device for detection of out-of-round on an angle sensor including a disc supporting angle coding means rigidly attached to a rotating shaft and at least one fixed optical system including a coherent light source emitting a beam interfering with the angle coding means of the support disc in order to code its angular position, means for collimation of the said beam before interference and a photodetector of the modulated beam after interference. In accordance with one possibility, it includes two optical systems arranged 90° from each other.