Droplet Capacitive Signal Processing Method Based On Microcontroller

Fiber-Capacitive Drop Analyzer (FCDA) can produce volume-based droplet fingerprint by monitoring the capacitive signal and the optical signal during the droplet growth. The droplet fingerprint is a comprehensive reflection of the liquid physical and chemical properties, which can be used for liquid identification. The purpose of this thesis is to design the capacitive signal processing circuit and the software processing program for capacitance drop sensor in the FCDA.In this paper, the working principle of capacitance drop sensor is introduced, and the importance of capacitance sensor in FCDA is analyzed. We analyze the linear relationship between the capacitance sensor and droplet volume by a simple mathematical model. Then we compare the characteristics of several main capacitance measurement circuits, and combined with capacitive sensor measure demands to select an appropriate capacitance measurement method.In this paper, two kinds of capacitance measurement circuits are designed:frequency modulation capacitance measurement circuit and AD7151capacitance measurement circuit. The frequency modulation method uses oscillating circuit, which can transform the capacitive signal into a frequency signal. Through the frequency measurement circuit and analog to digital conversion circuit, the measuring result is imported to the single chip microcomputer (SCM). AD7151is a versatile capacitance measurement integrated chip, which can directly measure capacitance of the transducer. Using I2C interface, AD7151can achieve communication with SCM.Capacitive signal software processing program includes filter, subsection, interpolation and normalization. In this paper, we design zero-phase Butterworth filter for signal filtering, and we combine neighborhood comparing method with threshold detection to divide the signals into segments. Then two different styles of normalizations, respectively based on time dividing and capacitive dividing, are compared and reached.At the end of this paper, the property of capacitance measurement circuit and software program is verified through experiments, and the droplet fingerprint is acquired. The.problems encountered in experiments are analyzed, and some improvement suggestions are provided.