The performance of the industrial machines mainly depends on accuracy of their displacement measuring devices. Among the various measuring devices, optical encoders are widely used because of high resolution/cost ratio and small size. Nowadays, the encoders are generally used in the machines which require micron order measurements, for example CNC machines, industrial robots, transfer mechanisms, stages, and so on.
The optical encoder using moire fringes is easy to manufacture compared with other optical encoders, but is very sensitive to the change of fringe space which are caused by tilt or misalignment. The change of fringe space results in quadrature phase shift error of detecting signals and have relatively large influences on the measured values. When two scales which have 200㎛ pitches superposed with 2˚ and the angle of two scales had a tilt of just 0.5 degree, the displacement error reached 16㎛ with its maximum. And it prevents us from improving the resolution of the encoders.
In this paper, newly designed grating, named phase shifted grating, is described to compensate quadrature phase shift error.
In the conventional encoders the detectors are aligned perpendicular to the direction of moire fringes. From the alignment we can receive two 90 degree phase shifted sinusoidal signals. Therefore, the phase of signals is sensitive to the changes of fringe spacing. But by adapting phase-shifted grating instead of previous index scale, we can get 90˚ phase shifted fringes parallel to the direction of moire fringes. Therefore, the phase of signals is insensitive to the changes of fringe spacing in proposed system.
The displacement error of linear encoder using phase shifted grating was experimentally determined to be 3㎛ for tilt of index scale of 0.5˚.
