Digital Processing Method used to Improve the Frequency Measurement Accuracy for Vibrating-Wire Transducers

摘要

This paper is focused on implementation of a digital processing method applied over acquired data obtained from a vibrating wire transducer in normal conditions. The paper carries on the researches from [2] and [3], the main aim being to improve measurement accuracy of transducers with natural frequency around 700Hz. The theoretical analysis of the vibrating wire transducer shows that the output signal is a sum of harmonics: fundamental ω₁ (natural frequency of the vibrating wire) and higher frequency harmonics ω_n. But a FFT analysis of the transducer's output signal shows that, inside the working range, the output signal is strongly affected by noise and that the second degree harmonic is nearly to the high frequency limit. The conditioning circuit studied in [2]-[3] can't extract just fundamental frequency ω₁. To improve circuit performance this paper proposes to use digital processing methods. Theoretically, the transducer's output is a damped oscillations signal, but from processing method point of view this is viewed like a series of numbers represented by the samples of signal periods of oscillation acquired as long as the output signal is higher as a threshold. In our work, the high frequency noises are rejected with low pass FIR filter and the 50Hz noises are rejected using an averaging operator. As a results of the processing, the fundamental frequency is measured with an accuracy less of 0.2Hz.