Research On A Micro-process System Towards LDGI (Linear Diffraction Grating Interferometer)

Nowadays ultra-precision machining technology has tiptoed into nano field(0.1-100nm), which calls for a higher resolution about measurement instruments.Therefore, Coordinate Measuring Machines (CMMs) with nano-level resolution iscoming into being. The key technology in CMM includes nano sensor and signalsubdivision, which are discussed deeply in this article to catch the essence of them.A high-resolution displacement sensor LDGI (Linear Diffraction GratingInterferometer) based on the grating Doppler effect is imported in this study.1diffraction beams were produced through the holographic gratings whose frequencyis related to the velocity of the stage. The two beams are interfered after a series ofpolarization and split effects. Optical path difference of two beams is removed andhigher grade diffraction beams’ interference are avoided by several generationdevelopment. High frequency signals in interference are eliminated as a result. ThePDs(Photoelectric Detectors) in LDGI are then introduced to transform lightintensity into electric signal for subdivision afterwards.In this article pulse counter and phase subdivision are conbined to realizenano-displacement measurement. In this method DSP (Digital Signal Processor)and CPLD (Complex Programmable Logic Device) cooperated to finish subdivision,which can get the resolution below1nm. The sine and cosine signals from LDGIare transmitted to the counter module and ADC(Analog Digital Converter) at thesame time. The counter module realized big count while ADC small count. As theprocess core, DSP planned the whole displacement with the two count while CPLDcontrolled the peripherals and data transmission. Moreover, this method was testedwith the contrast of the reading of laser interferometer towards the samedisplacement. The standard deviation in the test for the method is below15nm.This method used hardware counter to process the displacement that didn’t needsubdivision. It can save processor consumption effectively and improve thefrequency response. The anti-DC drift module also increased the suitability of thesystem.The research rooted in “Micron/nano three-dimensional measurement technology and equipment” sponsored by National High-tech Research andDevelopment Project (863project). The research has some practical value.