Figure 1 shows the schematic diagram of the encoder using grating imaging .
The proposed encoder consists of the two gratings. (Although three gratings are needed in the encoder using grating imaging, the first grating is simultaneously used as the third grating in reflection configuration.) Figure 2 shows the schematic diagram of the integrated sensor (index grating part). The light source is a polychromatic incoherent light source (LED). The index grating is fabricated from a silicon wafer and consists of the transmission grating. On each grating line, which is a thin silicon beam, two line photodiodes are installed. Therefore, the scale grating is irradiated through the transmission index grating on which the photodiodes are fabricated.
Figure 3 shows the optical micrograph of the fabricated index grating (80mm pitch). The silicon substrate is etched through so as to see the characters on the paper as shown in Figure 9. The photodiode sensitivity was 21mA/W. Using the fabricated index grating, the displacement signal was examined. The signal varied sinusoidally as a function of the lateral displacement, which was convenient for obtaining a displacement smaller than a single period by interpolation. The period of the signal was equal to an half of the grating pitch. The fabricated index gratings consisted of the two phase-shifted gratings. The two signals can be used for distinguishing the moving direction and for interpolating the displacement smaller than a single period.
Figure 3: Index grating on which line photodiodes are installed.