Keiji Matsui

Optical encoder using a slit-width-modulated grating

Abstract In this paper we propose a simple optical encoder using slit-width-modulated gratings. The distortion of the encoder signal (harmonic noise) is reduced by using the slit-width-modulated gratings to obtain a sinusoidal curve. The intensity of the light transmitted through the superimposed gratings was investigated by the Fresnel diffraction theory. The Fourier coefficient of the gratings was analysed in order to reduce the distortion of the encoder signal. For practical use, the slit width of the gratings was designed to reduce the third and fifth harmonics of the encoder signal. In the experiment, the slit-width-modulated gratings were fabricated by optical lithography. It was confirmed that the signal distortion was reduced and virtually independent of the air gap between the two gratings. Since the interpolation error is considerably suppressed, the proposed technique will be useful in precision machining.

Pitch-modulated phase grating and its application to displacement encoder

Abstract A binary phase grating with modulated pitch is investigated for a simple displacement encoder. The grating consists of a binary phase grating to eliminate the zeroth-order diffraction, and the pitch of the grating is modulated to compensate the higher harmonics of the encoder displacement signal. Therefore, an undistorted sinusoidal signal as a function of displacement is obtained by simply superimposing a conventional binary grating on the pitch-modulated phase gratings for any air gap between the gratings. The characteristics of the proposed gratings and the encoder signal are investigated by the Fresnel diffraction theory. The proposed grating has been fabricated lithographically, and the signal was examined experimentally. Considering these results, the proposed technique can suppress interpolation error and will be useful for an encoder in precision machining.

Compact optical encoder using modulated-pitch phase grating: suppression of harmonic noise and contrast change

In this paper, we investigate the combination of two gratings for a compact optical displacement encoder. The modulated pitch gratings are considered to obtain the sinusoidal signal as a function of displacement. In addition, the phase grating with the pitch modulation is examined to suppress the contrast change of the encoder signal as a function of the gap between the index and main scales. By these investigations, we find some combinations of gratings to satisfy the above requirements.