Acceleration detector

An acceleration detector comprising a housing adapted to be rigidly secured to an object whose acceleration is to be detected and a transducer assembly disposed within the housing for detecting the acceleration of the housing. The transducer assembly comprises a disc-shaped diaphragm having a circular central region flexible in response to the acceleration of the housing. A disc-shaped piezoelectric element is attached to the central region of the diaphragm for generating a signal representative of the acceleration of the housing. Positioning means are disposed on the diaphragm and the piezoelectric element for positioning the piezoelectric element relative to the central region of the diaphragm. The positioning means may comprise a projection disposed on the diaphragm and engageable with the piezoelectric element. The projection may be disposed at the center of the diaphragm and the piezoelectric element may have a central hole engageable with the positioning projection. The projection may be disposed on the diaphragm and engageable with an outer peripheral edge of the piezoelectric element.

BACKGROUND OF THE INVENTION 
This invention relates to an acceleration detector and, more particularly, 
to an acceleration detector for detecting knocking of an internal 
combustion engine. 
A conventional acceleration detector for detecting knocking of an internal 
combustion engine comprises a housing adapted to be rigidly secured to an 
engine and an acceleration transducer assembly disposed within the housing 
for detecting the acceleration on the housing which represents knocking of 
the engine. The housing comprises a base and a cover securely connected 
together. The transducer assembly comprises a disc-shaped diaphragm and a 
piezoelectric element concentrically attached to the center of the 
diaphragm for sensing flexure thereof for generating a signal 
representative of the acceleration of the housing. The diaphragm is 
rigidly supported by the housing at its outer continuous circumference 
edge portion. Typically, the circumference edge portion is firmly clamped 
between edges of the base and the cover of the housing which are 
mechanically connected together by means of caulking. 
The sensitivity of the acceleration transducer assembly comprising the 
diaphragm and the piezoelectric element is at its best at the resonance 
frequency f.sub.0 which is determined by the diameter and the thickness of 
the diaphragm and the piezoelectric element, whereupon the output from the 
transducer assembly is at its maximum. The resonance frequency f.sub.0 is 
an important characteristic of the acceleration detector and it is 
required that the deviation of the resonance frequency f.sub.0 from one 
detector to another is minimized. 
Another important factor of changing the resonance frequency f.sub.0 of the 
acceleration transducer assembly is the accuracy of the concentric 
positioning of the piezoelectric element relative to the diaphragm. If the 
piezoelectric element is bonded to the diaphragm with their centers 
misaligned, the resonance frequency f.sub.0 deviates from one detector to 
another. 
With the conventional acceleration detector as described above, the 
disc-shaped piezoelectric element is simply bonded to the circular 
diaphragm, so that they are quite easily attached at an eccentric 
position, making the deviation of the resonance frequency f.sub.0 large 
and frequent. 
SUMMARY OF THE INVENTION 
Accordingly, one object of the present invention is to provide an 
acceleration detector free from the above discussed problem of the 
conventional acceleration detector. 
Another object of the present invention is to provide an acceleration 
detector in which accurate alignment of the diaphragm and the 
piezoelectric element can be easily achieved. 
Another object of the present invention is to provide an acceleration 
detector in which accurate alignment of the diaphragm and the 
piezoelectric element can be easily achieved with a simple structure. 
Still another object of the present invention is to provide a simple 
process for accurately positioning the piezoelectric element relative to 
the diaphragm. 
With the above objects in view, the acceleration detector comprises a 
housing adapted to be rigidly secured to an object whose acceleration is 
to be detected such as an internal combustion engine and a transducer 
assembly disposed within the housing for detecting the acceleration of the 
housing. The transducer assembly comprises a substantially disc-shaped 
diaphragm having a substantially circular central region flexible in 
response to the acceleration of the housing. A substantially disc-shaped 
piezoelectric element is attached to the central region of the diaphragm 
for sensing flexure of the central region and generating a signal 
representative of the acceleration of the housing. The acceleration 
detector also comprises positioning means disposed on the diaphragm and 
the piezoelectric element for positioning the piezoelectric element 
relative to the central region of the diaphragm. 
The positioning means may comprise a positioning projection disposed on the 
diaphragm and engageable with the piezoelectric element. The projection 
may be disposed at the center of the diaphragm and engageable with a 
central hole on the piezoelectric element, or may be disposed in a ring so 
as to be engageable with an outer peripheral edge of the piezoelectric 
element. 
Alternatively, the positioning means may comprise inner surfaces of central 
holes in the diaphragm and the piezoelectric element, or an outer 
circumferential edge of the central region of the diaphragm and an outer 
edge of the piezoelectric element. 
A process for positioning a piezoelectric element relative to a diaphragm 
of an acceleration detector also comprises the steps of inserting a 
positioning pin into the central hole of the diaphragm so that at least 
one end of the pin projects from the diaphragm, and placing the 
piezoelectric element on the diaphragm with the projected end of the pin 
inserted into the central hole of the piezoelectric element. 
Alternatively, positioning pins may be placed around the outer edge of the 
central region of the diaphragm so that at least one end of the pins 
projects from the diaphragm, and the piezoelectric element is placed on 
the diaphragm with the outer edge of the piezoelectric element brought 
into contact with the projected ends of the positioning pins.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrates an acceleration detector constructed in accordance with 
the present invention, which comprises a housing 1 having a metallic base 
2 with a screw 3 and a flange 4 and a plastic cap 5 with a flange 6. The 
base 2 is adapted to be rigidly secured by the screw 3 to an object whose 
acceleration is to be detected, such as an internal combustion engine (not 
shown). The acceleration detector also comprises an acceleration 
transducer assembly 7 disposed within the housing 1 for detecting the 
acceleration of the internal combustion engine and therefore the housing 
1. The transducer assembly 7 is placed on an annular planar surface 
defined by a step 8 on the base 2 inside of the flange 4. The cap 5 is 
firmly attached to the base 2 by caulking the flange 4 of the base 2 over 
the flange 6 of the cap 5 with the edge portion of the transducer assembly 
7 firmly sandwiched between the cap flange 6 through a spring washer 6a 
and the step 8 of the base 2. 
The transducer assembly 7 comprises a substantially disc-shaped metallic 
diaphragm 11 and a substantially disc-shaped piezoelectric element 12 
attached to the diaphragm 11 for sensing the flexure of the diaphragm 11 
and generating an electrical signal representative of the acceleration of 
the housing 1 attached to the engine. The signal from the piezoelectric 
element 12 is supplied through a lead 13 connected to the piezoelectric 
element 12 and an output terminal 14 insert-molded into the plastic cap 5. 
As best seen from FIGS. 2 and 3, the disc-shaped diaphragm 11 of the 
acceleration transducer assembly 7 comprises a substantially circular 
central region 15 flexible in response to the acceleration of the housing 
1 and having the piezoelectric element 12 attached thereon. The diaphragm 
11 also comprises an annular outer region 16 which is integrally disposed 
around the central region 15 and which is not attached to the 
piezoelectric element 12. As illustrated in FIG. 1, the outer region 16 of 
the diaphragm 11 is rigidly connected to the housing 1 by being clamped 
between the cap flange 4 through the spring washer 6a and the base step 8. 
According to the present invention, the diaphragm 11 and the piezoelectric 
element 12 have positioning surfaces 21 and 22 for concentrically 
positioning the piezoelectric element 12 relative to the central region 15 
of the diaphragm 11. In the embodiment illustrated in FIGS. 2 and 3, the 
positioning surface 21 on the diaphragm 11 is defined by an outer surface 
of a projection 23 formed at the center of the diaphragm 11. The 
positioning surface 22 on the piezoelectric element 12 is defined by an 
inner surface of a circular through hole 24 formed at the center of the 
element 12. The through hole 24 is sized so as to receive the projection 
23 therein without any substantial play between the positioning surfaces 
21 and 22. 
During assembly of the acceleration transducer assembly 7, a bonding agent 
(not shown) is applied on at least one of the diaphragm 11 or the 
piezoelectric element 12, and the diaphragm 11 and the element 12 are put 
together with the projection 23 on the diaphragm 11 guided by and received 
in the through hole 24 of the piezoelectric element 12, whereby the 
element 12 is precisely concentrically positioned with respect to the 
diaphragm 11. 
FIGS. 4 and 5 illustrate another embodiment of a transducer assembly 25 of 
the acceleration detector of the present invention, in which a diaphragm 
26 has a central through hole 27 having an inner surface 28 defining a 
positioning surface, and a piezoelectric element 29 has a central through 
hole 30 having an inner surface 31 defining another positioning surface. 
These central holes 27 and 30 have equal diameters. During assembly, a jig 
32 having a positioning pin 33 having a diameter slightly smaller than the 
diameter of the through holes 27 and 30 is used. The diaphragm 26 is 
placed on the jig 32 so that the positioning pin 33 is inserted into the 
central hole 27 of the diaphragm 26 and at least one end of the 
positioning pin 33 projects from the diaphragm 26. Then, the piezoelectric 
element 29 is also placed on the diaphragm 26 with the projected end of 
the positioning pin 33 inserted into the central through hole 30 of the 
piezoelectric element 29. Since the diaphragm 26 and the piezoelectric 
element 29 are guided by the common positioning pin 33 extending through 
the central through holes 27 and 30, they are precisely concentrically 
positioned relative to each other. The positioning pin 33 is withdrawn 
from the through holes 27 and 30 after the bonding agent (not shown) 
between the diaphragm 26 and the element 29 is cured. 
FIGS. 6 to 8 illustrate still another embodiment of a transducer assembly 
34 for use in the acceleration detector of the present invention. The 
illustrated transducer assembly 34 comprises a gear-shaped diaphragm 35 
having a substantially circular central region 36 on which the 
piezoelectric element 37 is attached and an outer region in the form of a 
plurality of radial arms 38 each having an outer end rigidly connected to 
the housing 1 by being clamped between the cap flange 4 through the spring 
washer 6a and the base step 8. The central region 36 has a diameter 
substantially equal to the diameter of the piezoelectric element 37 as 
illustrated in FIGS. 7 and 8. Therefore, the accurate concentric 
positioning of the piezoelectric element 37 on the central region 36 of 
the diaphragm 35 during bonding can be relatively easily achieved by 
employing positioning pins 39 (FIGS. 7 and 8) planted on a jig 40. In this 
embodiment, the positioning surfaces are an outer circumference of the 
circular central region 36 and the outer circumference of the 
piezoelectric element 37. 
FIGS. 9 to 11 illustrate another acceleration transducer assembly 42 which 
can be used in the acceleration detector of the present invention. In this 
embodiment, the transducer assembly 42 comprises a diaphragm 43 having 
four projections 44 having radially inner surfaces 45 disposed along an 
outer circumferential edge of a circular central region 46 of the 
diaphragm 43. When a piezoelectric element 47 is bonded to the diaphragm 
43, an outer circumferential edge of the piezoelectric element 47 which 
serves as a positioning surface is guided and positioned by the inner 
surfaces 45 which serve as the positioning surfaces on the diaphragm 43. 
While this embodiment is effective for positioning the piezoelectric 
element 47 relative to the diaphragm 43, the frequency characteristics of 
the output from the transducer assembly 42 is illustrated by a curve a 
with a disturbance a.sub.1 in FIG. 12, which depicts how the output from 
the transducer assembly 42 varies as the frequency changes. 
FIGS. 13 to 16 illustrate another embodiment in which a diaphragm 50 
similar to the gear-shaped diaphragm 35 illustrated in FIGS. 6 to 8 has 
four positioning projections 51 at the outer circumference of the central 
region 52 of the diaphragm 50 or around the piezoelectric element 53. The 
positioning projections 51 are in the form of hooks extending from the 
outer edge of the central region 52 of the diaphragm 50 and substantially 
perpendicularly bent toward the side on which the piezoelectric element 53 
is attached. The transducer of this embodiment exhibits a frequency 
characteristic of an output as illustrated by a curve b of FIG. 17 which 
has disturbances b.sub.1 and b.sub.2. 
FIGS. 18 and 19 illustrate another acceleration transducer 55 in which the 
positioning projection comprises a single annular projection 56 disposed 
along an outer peripheral edge of a piezoelectric element 57. The 
output-frequency curve of this transducer 55 also has a disturbance. 
As has been described, according to the present invention, the acceleration 
detector comprises positioning means disposed on the diaphragm and the 
piezoelectric element for positioning of the piezoelectric element 
relative to the central region of the diaphragm. The positioning means may 
comprise a positioning projection disposed on the diaphragm and engageable 
with the piezoelectric element. The projection may be disposed at the 
center of the diaphragm and engageable with a central hole on the 
piezoelectric element, or may be disposed in a ring so as to be engageable 
with an outer peripheral edge of the piezoelectric element. Alternatively, 
the positioning means may comprise inner surfaces of central holes in the 
diaphragm and the piezoelectric element, or an outer circumferential edge 
of the central region of the diaphragm and an outer edge of the 
piezoelectric element. Accordingly, with the acceleration detector of the 
present invention it is easy to accurately align the diaphragm and the 
piezoelectric element, and it has a simple structure.