Study On Control Method Based On FPGA For Surface Topography Interferometry

The high-precision and automated surface topography measurement has a very important effect in the field of precision measurement, product manufacturing, biomedical, electronic manufacturing. In recent years many optical measurement methods of micro surface topography were already proposed and well developed. There are some methods applied popularly, for instance, white-light phase stepping interference technology, traditional phase stepping interference technology and multi-wavelength interference technology. This paper has studied the control of motion and image acquisition based on the Field-Programmable Gate Array, which around the system of surface topography interferometry combination of wavelength switch and phase shift scanning.The main research contents as following:Firstly, discuss the principle of interference measurement system based on the combination of wavelength rotation and phase shift scanning, realize the drive of piezoelectric ceramic for phase shift, upper computer communicates with the FPGA via serial port RS232. According FPGA sends out square wave signal which outputs analogue quantity and then drives the piezoelectric ceramic via integrator amplifier, and the whole drive is conducted under open-loop control mode. Finally, calculate PZT actuating stepping distance by fitting algorithm for the ellipse。Then, the wavelength rotation system has been designed. According driving stepping motor to pull the disk with optical filter in motion based on FPGA so as to realize wavelength rotation. The upper machine program based on Labview, and conduct orderly control on the PZT and wavelength rotation.Thirdly, driving method of two-dimension work platform is expounded. The two-dimension work platform respectively coordinates with fine lead screw for the drive via a stepping motor in X axis and Y axis, and performs accurate location by the signal feedback by the grating.Finally, wavelength rotation, PZT actuating and two-dimension work platform driven is discussed. By coordinated control of three modules, so that realize the result in which wavelength light source is controlled to switch by turn every time the PZT is driven in one period. Meanwhile, externally trigger CCD via FPGA I/O, and acquiring the interference image every time while the PZT is driven for one step. And the interference images are saved in the computer and realize the measurement in larger scope via the motion of two-dimension work platform.