Reducing tilt errors in moiré linear encoders using phase-modulated grating

A new hardware compensation method reducing displacement measurement errors, caused by tilt of the index scale in a moiré linear encoder, has been developed. In conventional moiré linear encoders, the detectors are aligned perpendicular to the line of the moiré fringes and this structure is very sensitive to an unwanted tilt of the gratings. In this article, a newly designed grating, called a phase-modulated grating, is developed to compensate for nonorthogonal errors. By using the phase-modulated grating instead of a conventional index, it is possible to reduce nonorthogonal errors of the moiré linear encoder.