Abstract
A bar code ruler has been used in digital levelling for automatic reading of a levelling staff. We propose here a new periodic bar code that consists of three sinusoidal patterns encoded in adjoining bars with the structure of a uniform bar-centre distance. Its positioning algorithm can be implemented in two steps: first is recognition of the bar code in the frequency domain by detecting a basic spectral line and its phase; the second is recognition of a code word based on calculation of the phases of three patterns via discrete Fourier transformation (DFT). The phase difference method is effective in spectral line detection for correcting the error caused by the discreteness of the spectrum. And the correct code word recognition rate has been increased by increasing the digital frequencies of the three sinusoidal patterns. From comparative experimental results, we can see that the new bar code is superior in code word recognition to the double sinusoidal bar code; and the measurement precision of sight stadia increased about 10% after correcting the spectrum. The precision of the vertical relative displacement measurement at 100 m is 0.295 mm, which meets practical needs.
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