| 摘要 |
PROBLEM TO BE SOLVED: To accurately perform measurement regardless of an aperture, fluid density, and flow velocity by changing the resolution of aΣΔA/D converter according to usage conditions, and by selecting only a frequency within a specific range from the output for outputting. SOLUTION: An AC charge signal being detected by a piezoelectric element 21 is converted into an AC signal by a charge converter 2, and is subjected to A/D conversion by aΣΔA/D converter 25. For a converted eddy signal, a digital filter 26 where a band pass filter is composed selects only a frequency within a specific range, and eliminates a noise component for outputting. In this case, theΣΔA/D converter 25 inputs an eddy signal Vin of an AAF 24 by a differential amplifier 30, and a D/A converter 33 inputs the signal of a comparator 32 passing through an integrator 31 and a PWM signal being selected by a microprocessor 27 in the case of input with specific conditions and outputs±Δsignal. In this case, the maximum value of step size width±Δis set according to setting conditions such as an aperture, fluid density, and flow velocity, thus obtaining fixed resolution without depending on the aperture, the fluid density, or the like.
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