Piezoelectric acceleration sensor

1. A piezoelectric acceleration sensor comprising:

a piezoelectric element;

a metallic sheet; and

a circuit board, wherein:

the piezoelectric element is polarized in an upper-to-lower direction;

the circuit board includes a circuit portion and a roughly flat shaped base portion, the base portion protruding from an end portion of the circuit portion; and

one of surfaces of the metallic sheet is fixed to and supported by a surface of the base portion, the piezoelectric element being fixed to and supported by a remaining one of the surfaces of the metallic sheet such that the piezoelectric element and the base portion do not overlap each other in the upper-to-lower direction;

the metallic sheet is partially located on the base portion in the upper-to-lower direction;

the piezoelectric element is located on the metallic sheet in the upper-to-lower direction; and

the piezoelectric element is apart from the base portion in a left-to-right direction perpendicular to the upper-to-lower direction.

2. The piezoelectric acceleration sensor according to claim 1 , wherein as seen along the upper-to-lower direction, the metallic sheet is located to be apart from the circuit portion so as not to be brought into contact with the circuit portion in a case where the metallic sheet is bent by acceleration.

3. The piezoelectric acceleration sensor according to claim 1 , wherein as seen along the upper-to-lower direction, the piezoelectric element is located to be apart from the base portion so as to be bent entirely to one side by acceleration.

4. The piezoelectric acceleration sensor according to claim 3 , wherein a distance between the piezoelectric element and the base portion as seen along the upper-to-lower direction is designed so that the metallic sheet does not practically generate an undesirable vibration by acceleration.

5. The piezoelectric acceleration sensor according to claim 1 , wherein the metallic sheet is located near the circuit portion as compared with a head of the base portion.

6. The piezoelectric acceleration sensor according to claim 1 , wherein:

the base portion includes two base portions which project in parallel to each other in a common plane from the end portion of the circuit portion; and

opposite end portions in a lengthwise direction of the one of the surfaces of the metallic sheet are fixed to and supported by the surfaces of the base portions, respectively.

7. The piezoelectric acceleration sensor according to claim 6 , wherein as seen along the upper-to-lower direction, distances between the piezoelectric element and the respective base portions are substantially the same.

8. The piezoelectric acceleration sensor according to claim 1 , wherein the piezoelectric element and the metallic sheet constitute a unimorph piezoelectric vibrator.

9. The piezoelectric acceleration sensor according to claim 1 , wherein the piezoelectric element constitutes a bimorph piezoelectric vibrator.

10. The piezoelectric acceleration sensor according to claim 1 , further comprising a case, wherein:

the case has an accommodating space formed therewithin, the accommodating space having a support surface; and

the base portion has an opposite surface which is opposite, in the upper-to-lower direction, to the surface which supports the metallic sheet, the opposite surface being fixed to and supported by the support surface.