Design of data acquisition system for a laser interferometer displacement equipment

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Dr. Bognár György
Department of Electron Devices

During my thesis work, my purpose was to update and develop the departmental laser interferometer displacement equipment. The measurement is based on the phenomenon of the interferometry. With this method, the displacement can be measured in nanometer range. The whole equipment is extremely sensitive, any mechanical disturbance can cause measurement errors, thus the whole system is placed on a vibration-isolated table. During my work, I focused on eliminating as much disturbance as possible, and on raising the sampling frequency of the system.

The equipment is PC independent at the moment, so the fan of the computer cannot induce errors. The impulses of laser interferometer measurement board are processed by an Altera DE2 (development and education) board, exactly by its Cyclone II FPGA. The data acquisition logic is implemented into this device, which has to detect the impulses from the inputs. These data, and the state of a timer are stored in registers, thus the result can be shown on an LCD display, and saved to an SD card.

The FPGA is also responsible for controlling the data storage. The implemented hardware is described in verilog hardware description language. After the successful initialization, the results are stored in a text file. These data can be presented by a computer and a card reader, which is totally independent from the delicate measurement system. This way the connection between the measurement system and the outside world can be minimised.

The communication with the SD card is solved through SPI (serial peripheral interface), the card is formatted to FAT file system, the state of the inner registers are saved thousand times a second. My thesis work contains an introduction about the interferometry, the measurement system and the measurement board. The working principals of the LCD display and the SD card are also mentioned, and a detailed description about my design is given. The correct operation is confirmed and validated by the tests carried out, and by their results.


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