his offset is caused by the time delay in the transmitter and receiver elements and amplification circuits.Subtracting this offset from the average value,Fig4.1(b)shows the residual elror of the distance measurement.The standard deviation of these residual error is =0.78 mm. РFig4.1 Results of distance measurementРIt is clear from this result that there is a large digitizing error from the A/D conversion(200 kHz).The interval time of A/D sampling is 5.0s,which corresponds to about 1.7 mm in propagation distance.Then,the maximum digitizing error corresponds to about 0.86 mm.it is estimated that the major part of this residual error is derived from this digitizing error.Therefore it would be possible to reduce the distance measurement error by using a faster analog input board in the future.
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