Force sensor

Details Force sensor Force sensor

1999-12-13

2003-02-25

Noori, Max (Department: 2855)

Measuring and testing

Dynamometers

Responsive to force

C073S862410

Reexamination Certificate

active

06523423

ABSTRACT:

BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT
A present invention relates to an excellently reliable force sensor which can three-dimensionally detect the magnitude and the direction of a physical quantity by detecting displacement of an operating member or a strain caused in the flexible plate by the displacement of the operating member and aims to protect the flexible plate from being damaged / destroyed by the action of impulsive force applied suddenly or serious external force without deterioration of detection sensitivity.
There has been increasing a demand for sensors capable of accurately detecting a physical quantity such as acceleration, magnetism, or the like, in fields of an automobile and a mechanical industries. Particularly, there is a need for a small-sized sensor capable of detecting such a physical quantity for each of two- or three-dimensional components. For example, an acceleration sensor is used for a mechanism for automatically recovering a balance when an automobile lost its balance due to sudden wheeling, a side wind, or the like, a collision sensing mechanism, a mechanism for adjusting self-supporting stability of a crane, or the like, a mechanism for adjusting a flow rate of fluid or for opening and shutting a valve by sensing a change in a speed of fluid flowing through pipe, etc.
As such a sensor, Japanese Patent Laid-Open 5-26744 discloses a sensor
95
in which a plurality of piezoelectric element
98
are disposed on a flexible plate
97
suspending a weight
96
as shown in FIG.
15
. In addition, as shown in
FIG. 14
, Japanese Patent Laid-Open 8-94661 discloses a sensor
90
in which a supporting base
98
, a flexible plate
92
, and a weight
94
disposed inside the sensor are unitarily formed using a piezoelectric material, one end surface of a cylindrical supporting base
93
is blocked up by the flexible plate
92
, the columnar weight
94
is suspended by the flexible plate
92
at the center of a hollow portion of the supporting base
93
, a plurality of upper electrodes
91
A-
91
E are disposed on the surface of the flexible plate
92
, and a lower electrode
91
F is disposed on the lower surfaces of the supporting base
93
, the flexible plate
92
, and the weight
94
.
These sensors are constituted so that the flexible plates are bent by a force corresponding to a physical quantity such as a direct force acting on the weight from outside, a force of inertia due to an acceleration and a magnetic attraction. The sensors can detect the magnitude and the direction of a physical quantity by detecting an electric charge generated in a piezoelectric body in accordance with bending of the flexible plate. Such a sensor is hereinbelow referred to as “a force sensor ”.
In three-dimensional detection of a force, that is, in detection of a force in X, Y, and Z axial directions (hereinafter referred to as three axial directions) which shows rectangular coordinates, for example, in a sensor shown in
FIG. 14
, a displacement can be detected by a charge generated in the upper electrode
91
E in the case that the weight
94
is displaced in the z-axial direction which is a direction of suspension of the weight
94
. At this time, force in the X-axial direction and in the Y-axial direction is prevented from being detected by wiring so that the charges generated in the upper electrodes
91
A and
91
B offset each other and the charges generated in the upper electrodes
91
C and
91
D offset each other. In the same manner, a displacement in each of the directions can be measured by charges generated in the upper electrodes
91
A and
91
B when the weight
94
is displaced in the X-axial direction and charges generated in the upper electrodes
91
C and
91
D when the weight
94
is displaced in the Y-axial direction. Thus, when the weight
94
is displaced in any direction, the magnitude and the direction of an applied physical quantity can be known by synthesizes detected components in each axial direction.
The aforementioned force sensor uses a stress generated in the flexible plate by a displacement of the weight. When the stress exceeds the fracture strength of the flexible plate, a crack is caused in the flexible plate and results in breakage. For example, when a sensor is dropped and collides against the ground due to a mistake, or the like, an excessive acceleration is applied to the weight, which cause a crack in a boundary portion between the weight and the flexible plate. Thus, breakage of a sensor is sometimes caused.
In addition, when fatigue of the flexible plate advances due to a long-term use, breakage is sometimes caused even if the stress generated in the flexible plate does not reach the material fracture strength. This is because a kind, a shape, or the like, of a material for each member has conventionally been set up so that a desired detection sensitivity is obtained without breakage of the flexible plate when the maximum acceleration to be detected is given to the sensor.
By thickening the flexible plate to increase the strength, it is possible to avoid the breakage with a certain probability even if, for example, the aforementioned sudden serious external force acts. However, the flexible plate becomes difficult to bent in this case, and therefore sensitivity of the sensor is lowered.
For example, FIG.
16
(
a
) shows an example of a result that a stress generated in the flexible plate
71
when an operating member
72
is displaced with
1
G acceleration applied to the acceleration sensor
70
having a flexible plate
71
comprising a piezoelectric body, an operating member
72
, a supporting base
73
, and a structure of the same rank as that of the acceleration sensor
90
shown in
FIG. 14
in Z-axial direction is obtained by FEM simulation. Here, there were used, as a parameter, a supporting base
73
made of zirconia and having an inner diameter of 3.5 mm, an operating member
72
made of zirconia and having a diameter of 1.8 mm and a thickness of 0.635 mm, and a flexible plate
71
having a zirconia plate having a thickness of 0.015 mm and a PZT element having a thickness of 0.02 mm superposed on the zirconia plate to have a film form. Physical properties of the zirconia and the PZT element used for the simulation are shown in Table 1.
TABLE 1
Young's modulus
(10
6
kg/cm
2
)
Specific gravity
Zirconia
2
6.0
PZT
0.62
7.6
As shown by a curved line(stress line) showing a stress in FIG.
16
(
a
), when an acceleration is given in the Z-axial direction, a large stress force is generated in the X-axial and Y-axial directions, and the stress line has a peak. This shows that the stress is concentrated in a narrow range of length (a transversal axis). Therefore, an acceleration sensor has conventionally designed so as to avoid breakage by adjusting thickness of a flexible plate as a whole so as not to exceed limit of breakage of the flexible plate by; for example, supposing that the maximum stress is excessive stress upon collision with the ground. However, this method cause a problem of deterioration in sensitivity of the sensor because a mode of generating stress is not changed, and therefore, if a peak value of the stress is set to be the same as or lower than the limit of breakage, whole magnitude of stress to be generated becomes small.
This is hereinbelow explained from another view point. The whole magnitude of stress shows a detecting sensitivity of a sensor, and an area surrounded by a stress curve and a transverse axis showing a length of a flexible plate expresses a detection sensitivity. Therefore, as shown in FIG.
16
(
b
), a mode of generation of overall stress, that is, only a peak value becomes low without changing an outline having a peak shape, an area surrounded by the stress curve and the transverse axis becomes small, and a detecting sensitivity is lowered.
In a force sensor, it is important to improve productivity besides a problem of balance of the aforementioned sensitivity and reliability. The weight and the supporting base are required to have high rigidity to hardly bend to purely detect an a

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