In abnormal operation

friction pair will produce large metal wear debris about tens of micron in size.When the impedance method is used to detect grains

very high sensitivity is necessary. So

a ring electrode capacitive sensor with a differential structure is designed. By replacing the complex signal processing circuit

capacitance-to-digital converter AD7746 is used to convert capacitance sensor output into digital signal directly

simplifying the electronics

and minimizing the interference of distributed and parasitic capacitance. Experiment result shows that the system has high sensitivity with resolution of 0.125 fF.