Miniaturization of Thin Plate Type Bolt Loosening Detection Sensor

We have proposed a novel autonomous bolt loosening detection technique using thin plate type sensors to keep integrity of machines and structures. The thin plate type sensor consists of a thin plate and piezoelectric elements and bolt loosening is detected by the natural frequency change of the plate. The validity of the sensor is shown by experimental results in previous works. However, the edge length of the sonsor is six times of the diameter of the target bolt and too large for practical use, so that it is desired to miniaturize the sensor. We have also proposed to use an oscillator circuit whose oscillating frequency is determined by the natural frequency of the sensor to evaluate the natural frequency change induced by bolt loosening. However, the oscillating frequency of the circuit does not coincide with the natural frequency and difference between two frequencies is sensitive to circuit parameters. In this paper, we propose slit cut thin plate type sensor as a miniaturized sensor and modify the oscillator circuit for natural frequency evaluation. The slit cut thin plate type sensor consists of a plate that has slits and a ring type piezoelectric element. The slits of the sensor eliminate degenerate modes that have harmful effects on natural frequency tracking and improve the tolerance for noise. The modified oscillator circuit is based on positive velocity feedback and the oscillating frequency of the circuit coincides with the natural frequency perfectly.