Detection dot optimization manner null of capacitance sensor for paper leaf inspection

摘要

PURPOSE: To control the resonance characteristic of a tuning circuit at optimum sensitive position every time just before carrying paper sheets and always stabilize sensor function by inserting a varicap in series in the tuning circuit detecting capacity variation. CONSTITUTION: In a carrying path of paper sheets 2 containing thread 4, electrode plates 8a, 6a are arranged face to face forming a detection capacitor part C. A capacitor part C forms the tuning circuit 40 of an electrostatic sensor circuit 120 together with an inductance element L. The tuning circuit 40 is driven with sine waves from an oscillation circuit 20 and the detection output is input in a tuning amplifier circuit 50. Just before starting operation, a control means (MPU) 82 changes the control voltage of varicaps VC1, VC2 in turn, and stores the output variation of the tuning amplifier circuit 50. The control voltage showing a maximum output variation in the output variations is impressed to the varicaps VC1, VC2. Then, by continuously carrying the paper sheets 2, the existence of thread 4 in each paper sheet 2 is judged.