Measuring grating reading head form

　　A direct-reading grating reading head is a reading head that uses a vertical illumination beam to obtain a moiré fringe signal, also known as a vertical-incidence grating reading head. The figure shows two typical optical path layouts. Figure 1 (a) is a transmission reading head, and Figure 1 (b) is a reflection reading head.

　　This type of reading head is represented by the silicon photocell receiving system, which is widely used due to its simple and compact structure, stable performance of silicon photocells and easy installation.

　　The direct reading head is suitable for photographic black and white gratings (i.e. amplitude gratings) with a grating pitch greater than 0.01 mm. For black and white gratings with a pitch of 25 to 100 line pairs/mm, the gap d between the main grating and the indicator grating can be a small gap [d = (3 to 5) ω, or a large gap (d = ω2/λ). ​​For gratings with less than 25 line pairs/mm, the gap should be adjusted according to the waveform of the required photoelectric signal. Increasing the grating gap and widening the beam can make the output waveform closer to sine.

　　1-light source 2-condenser 3-objective lens port-photocell 5-main grating 6-indicator grating 7-field mirror 8-reflector

　　Figure 1 Direct-reading grating reading head

　　In recent years, the application of reflection gratings made on steel strips or steel substrates has increased, because the reflection steel strip grating has the advantages of being durable and having a linear expansion coefficient similar to that of the workpiece. For reflection gratings, the optical path layout in the form of Figure 1 (b) should be used.

　　The spectroscopic reading head is suitable for the phase grating system. Its typical optical path is shown in Figure 2. Now take the transmission optical path in Figure 2 (a) as an example to explain its working principle. The light emitted from the point (or line) light source 1 is formed into parallel light through the condenser 2 to illuminate the main grating 3 and the indicator grating 4, and the two gratings produce multi-level diffraction. If a double-level blazed grating is used, the 0th and 1st order diffraction beams are emitted from the main grating, and then diffracted by the indicator grating to obtain four wavefronts (0, 1), (0, 0) and (1, 0), (1, 1). The two mutually parallel 1st order wavefronts (0, 1) and (1, 0) are converged by the condenser 5 and the slit 6, and received by the photoelectric tube 7 and converted into grating electrical signals.

　　1-light source 2-condenser 3-primary light shed 6-slit 7-photoelectric tube 8-light separation prism

　　Figure 2 Spectroscopic reading head

　　The function of the slit (aperture) 6 is to block all diffraction except (0, 1) and (1, 0) to improve the contrast of the output signal. In order to overcome the measurement error caused by the gap change between the two gratings, the optical path should be arranged with the minimum deflection angle, that is, -β=a. Since the influence of diffraction can be greatly weakened by the aperture, this type of reading head can use a larger grating gap.