The principle of grating length measurement is to turn the light emitted by the light source into parallel light through the telescopic system and shine on the transmission scale grating and indicator grating with opposite grating lines. When the scale grating moves along the vertical direction of the grating line relative to the fixed indicator grating, the moire fringes with alternating light and dark will move along the grating line. When two grating rulers move relative to each other by a grating distance, the light intensity of moire fringes changes periodically, sweeping four photoelectric elements placed in front of the indicator grating, thus generating two sinusoidal electrical signals, which are processed by the circuit system and become pulse signals. Because the measuring rod of the instrument is rigidly connected with the scale grating, the relationship between the displacement L of the measuring rod and the pulse signal can be expressed by the following formula.
L=A？ N=
Where a is pulse equivalent; N-way subdivision ability; K- grating linear density; N- number of pulses.
Pulse equivalent is the equivalent displacement of each pulse. Let n= 100, k= 100 and n= 1, then
L= × 1=0？ 1μm
The above formula shows that when using 100 wire grid, after the electrical signal is subdivided by 100, the measuring rod moves by 0? 1μm, the circuit will send out electric pulses. The measured size can be accurately obtained by recording the pulse number with a microcomputer.