Economic high resolution fringe counting for heterodyne interferometers using FPGA technology

Simple FPGA PCI boards are capable of capturing digital signals up to 40 MHz and analogue signals up to 200 kHz, making them suitable to replace whole machine controllers of multi-axis measuring machines. Unfortunately capturing a 4 MHz heterodyne signal (beat frequency) at a 40 MHz rate will result in a poor phase interpolation. We present a statistical approach to the phase evaluation while maintaining a reasonably high data rate and synchronicity with acquired analogue signals. Sub-nanometre resolution is achieved by averaging counter values sampled asynchronously to the beat frequency. The implemented FPGA program allows signal processing for three simultaneously captured interferometer positions with synchronized 8 channels A/D in- and output, interpolated A-B encoder output signals (usable for motion controller boards) and high speed DMA data transfer to the PC memory all on one PCIe board. Fringe counting works up to 6 MHz covering a speed range of more than ±0.4 m/s. The implemented 64 bit position counters cover measurements up to several meters with sub nm resolution