Impedance frequency characterization of a vibrating wire viscosity sensor with multiharmonic signals

This paper presents a method to estimate the impedance frequency response of a vibrating wire viscosity sensor. The method is based on the application of a multiharmonic signal to the measurement circuit, which includes the sensor. The signal is composed by the harmonics that correspond to the frequencies at which the sensor's impedance is to be estimated. The impedance is determined using least-squares (LS) multiharmonic fitting algorithms to obtain the amplitudes and phases of the harmonics of the impedance current and voltage. The stimulus generation, measurement and estimation procedure is performed by a dedicated developed measurement system. It includes a digital signal processor (DSP) to perform all the calculations, control the programmable gain instrumentation amplifiers (PGIAs) and communicate with the analog-to-digital converters (ADCs). The PGIAs and ADCs, respectively, amplify and acquire the signals across the sensor and a reference impedance which is used to limit and sample the current flowing through the wire of the sensor. An auxiliary processor connected to a digital-to-analog converter manages the stimulus generation. The system is connected by USB to a personal computer (PC) where the measurement results can be stored, interpreted and further processed.