Design of Signal Processing Circuit for Capacitive Drop Sensor

摘要

In FM method, the capacitive signal is changed through a precision oscillator into the frequency signal, which is measured by the 8254 fast counter controlled by the 89C51 microcomputer and is transmitted to PC according to RS-232. The relative error of frequency measurement is 0025%. DF method is developed to decrease the demand on frequency response of electronic elements and to improve the capability of anti-interference by error compensation. TB method is based on amplitude modulation, in which an active shield-driving circuit is used to ensure the measurement accuracy against the strong impact of shielding cable capacitors and to increase the common mode rejection ratio. Interface circuits, error analysis and experimental result graphs are presented. Excellent linearity between the drop volume and drop growth is achieved.