Piezoelectric sensor

A piezoelectric sensor, comprising:

• a piezoelectric plate, obtained from a rotated Y-plate having a rotation angle θ around an X-axis, wherein when at least a two-fold symmetry axis, or a symmetry axis of a piezoelectric crystalline material having a mirror symmetry plane is set as the X-axis, and the rotation angle θ is set according to a type of the piezoelectric crystalline material;
• a detection region located on a surface parallel to an X-Z plane of the piezoelectric plate;
• and
• a transmitting part and a receiving part, located along a direction of the X-axis of the piezoelectric plate and opposite to each other at positions that sandwich the detection region,
• wherein when a guided wave excited in the piezoelectric plate by applying a frequency signal from the transmitting part satisfies an equation (A) below, the rotation angle θ satisfies:
• (a1) the guided wave propagating in the direction of the X-axis inside the piezoelectric plate has a P wave component, an SV wave component and an SH wave component;
• (b1) when a displacement of the P wave component is u1, a displacement of the SV wave component is u2 and a displacement of the SH wave component is u3, the displacement u1 becomes the maximum, and the displacements u2 and the u3 become less than 10% of the displacement u1 on the surface according to a following equation (1),
• ξh=mπ/(2λ) (A)
• ui=Aiexpj(λξx2)expj(ξx1−ωt), i=1, 2 and 3 (1)
• where 2h is a plate thickness of the piezoelectric plate, ξ is a wave number in the X-axis direction, λ is a wave number in a plate thickness direction normalized by ξ, m is a positive even number to make a QL mode excited, x2 is a coordinate in Y-axis direction, x1 is a coordinate in the X-axis direction, and ω is an angular frequency, and Ai is an amplitude ratio of a displacement of a wave corresponding to λi.