A stacked body constituted by a first insulating sheet, a first terminal plate, a piezoelectric element, a second terminal plate, a second insulating sheet, a weight, and a disk spring is fitted over a cylindrical portion of a base, clamped and held between a nut and a flange portion of the base, and resin-molded. Annular partial electrodes having a width narrower than a radial width of the piezoelectric element are formed on front and rear surfaces of the piezoelectric element. The first and second terminal plates have annular portions formed so as to have annular shapes having an outside diameter and an inside diameter substantially equal to those of the partial electrodes, and are stacked such that the annular portions are placed in close contact with entire surfaces of the partial electrodes.
1. A knock sensor comprising:
a base having:
an annular flange portion; and
a cylindrical portion disposed so as to extend axially from said flange portion, a penetrating aperture being formed so as to pass through said flange portion and said cylindrical portion in an axial direction;
an annular piezoelectric element having electrode portions formed on front and rear surfaces thereof so as to face each other, said piezoelectric element being fitted over said cylindrical portion;
first and second terminal plates fitted over said cylindrical portion so as to hold said piezoelectric element from opposite sides, said first and second terminal plates closely contacting said electrode portions to extract an output signal from said piezoelectric element;
an annular weight fitted over said cylindrical portion to impart a vibrational force to said piezoelectric element;
a first insulating sheet interposed between said first terminal plate and said flange portion to insulate said first terminal plate and said flange portion from each other electrically;
a second insulating sheet interposed between said second terminal plate and said weight to insulate said second terminal plate and said weight from each other electrically; and
a holding means for clamping and holding a stacked body against said flange portion, said stacked body being constituted by said first insulating sheet, said first terminal plate, said piezoelectric element, said second terminal plate, said second insulating sheet, and said weight,
wherein:
said electrode portions are constituted by annular partial electrodes having a width narrower than a radial width of said piezoelectric element and formed on said front and rear surfaces of said piezoelectric element, and
said first and second terminal plates have annular portions formed so as to have annular shapes having an outside diameter and an inside diameter substantially equal to those of said partial electrodes, and are stacked such that said annular portions are placed in close contact with entire surfaces of said partial electrodes.
2. The knock sensor according to claim 1 , wherein:
said partial electrodes are formed at a radially-central portion of said front and rear surfaces of said piezoelectric element so as to be concentric with said piezoelectric element.
3. The knock sensor according to claim 1 , wherein:
said partial electrodes are formed at a radially-outer peripheral edge portion of said front and rear surfaces of said piezoelectric element so as to be concentric with said piezoelectric element.
4. The knock sensor according to claim 1 , wherein:
said partial electrodes are formed at a radially-inner peripheral edge portion of said front and rear surfaces of said piezoelectric element so as to be concentric with said piezoelectric element.
5. The knock sensor according to claim 1 , wherein:
a gap between regions of said front and rear surfaces of said piezoelectric element where said partial electrodes are not formed and said first and second insulating sheets is filled with an electrically-insulating resin.
6. The knock sensor according to claim 5 , wherein:
said gap between said regions of said front and rear surfaces of said piezoelectric element where said partial electrodes are not formed and said first and second insulating sheets is greater than or equal to 0.3 mm.