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High speed and high precision displacement measurement is hard to be achieved because of the restriction of displacement sensors: the maximum allowable moving speed is in contradiction to the measuring accuracy. In order to enable the high precision displacement measurement of the ending to achieve high speed moving course, a high speed and high precision displacement measurement method based on double grating scales is presented. Based on the analysis of measuring theory of grating scale, the method of signal's switch and merge of double grating scales is discussed. By using the analysis of the subdivision error of Moire fringe, the error of the count impulse's switch process is decreased. The signal process circuit is designed by using the programmable logic device-FPGA. And through the logical and timing simulation by the design software of FPGA, the feasibility of method is proved. The experimental results verify that the method is effectively used to control a macro/micro driven system.