Technical Report on Vibration and Its Sensors 2

The gap (displacement) is measured from the capacitance of the capacitor formed by the sensor and the target measurement object. Therefore, the measurement target is limited to conductors. The capacitance C is a function of the opposing surface area S of the conductor and the gap D, and the above relationship holds when the sensor and the opposing conductor (measurement target) are parallel plates. Here, if the area S is constant, the gap D is inversely proportional to the capacitance C. Thus, if the capacitance C can be measured, the gap D can be determined.

Advantages and Points of Caution

advantage point	non-contact detection All conductors can be used without calibration. High precision and high stability Can be measured from zero gap proximity. Measurable up to DC
Points to note	A connection with the measurement target is necessary. Affected by oil, water, and other substances on the measured surface. The necessary measurement area is relatively large.

Single crystal of quartz and barium titanate generate charges on their surface when subjected to force. This phenomenon is called the piezoelectric effect. Materials that exhibit the piezoelectric effect are referred to as piezoelectric materials (piezoelectric elements). Piezoelectric accelerometers use piezoelectric elements as springs in a seismic system and simultaneously as mechanical-electrical conversion elements. They output electrical signals proportional to vibrational acceleration. Piezoelectric accelerometers are fundamentally classified into two types: compression type and shear type, based on how the force is applied to the piezoelectric element. Structural diagrams of each type are shown on the right. The compression type has a structure where the piezoelectric element is sandwiched between the sensor base and a weight. The shear type has a structure where the piezoelectric element is fixed between a post standing vertically on the base and a weight. Traditionally, the compression type was used, but recently, the shear type, which is less affected by base strain and sudden temperature changes, has become more common. Our piezoelectric accelerometers are mostly of the shear type, with some exceptions.

Advantages and Points of Caution

advantage point	Measurement is relatively easy. The system can be realized at a low cost. High stability and little change over time.
Points to note	Due to the contact method, be aware of mass effect and contact resonance. Measurement of uniformly accelerated motion is not possible.

Change in Frequency Characteristics Due to Sensor Fixation Method

When fixing a sensor to the measurement target, the frequency characteristics of the sensor change depending on the method used. Please refer to the diagram below. It is necessary to adopt the optimal fixing method according to the measurement frequency range.