Capacitance type load sensor

Details Capacitance type load sensor Capacitance type load sensor

2001-03-28

2003-04-29

Oda, Christine (Department: 2858)

Electricity: measuring and testing

Impedance, admittance or other quantities representative of...

Lumped type parameters

C073S862626

Reexamination Certificate

active

06556024

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to a capacitance type load sensor for detecting load based on a change in capacitance caused by the load applied thereto.
2. Related Art
Conventional capacitance type load sensors comprise a pair of electrodes that are spaced and electrically insulated from each other to constitute a capacitor, and detect load on the basis of that change in the capacitance of the capacitor which is caused when one electrode applied with the load is flexed to change the distance between the electrodes.
FIG. 11
shows, by way of example, a conventional capacitance type load sensor which comprises an insulating board
1
formed with an electrode pattern
2
and an electrode plate
3
made of stainless steel, for instance. The insulating board
1
and the electrode plate
3
are spaced from each other, with spacers
4
,
5
interposed therebetween, and are connected with each other by means of rivets
6
and
7
, to thereby constitute a capacitor. Lead wires
8
,
9
are connected to the electrode pattern
2
and the electrode plate
3
, respectively. When load is applied to the electrode plate
3
in the direction shown by arrow P to cause the electrode plate to be flexed so that the gap G between the electrode pattern
2
and the electrode plate
3
changes in magnitude and hence the capacitance of the capacitor changes, the load sensor detects such change in capacitance, as the load applied to the electrode plate through the medium of the lead wires
8
,
9
.
In the load sensor having the aforementioned construction, the measurement accuracy becomes higher with the increase in an amount of change in capacitance, i.e., an amount of flexure of the electrode plate
3
caused when load is applied. On the other hand, stress generated in the electrode plate
3
becomes greater with the increase in amount of flexure of the electrode plate
3
. If the generated stress exceeds the elastic limit, fatigue limit or impact breakdown limit, the durability of the electrode plate
3
is lowered, so that the durability and reliability of the load sensor, especially long-term reliability thereof, may be worsened. As apparent from the foregoing explanation, when load is applied, the electrode plate
3
is desirable to be flexed by a large amount from the viewpoint of improving the measurement accuracy, but to be flexed by a small amount from the viewpoint of improving the reliability.
That is, in a load sensor for detecting load based on a change in capacitance of a capacitor comprised of an electrode pattern
2
formed on an insulating board
1
and an electrode plate
3
that is adapted to be flexed when the load is applied, a conflict arises between a solution for improvement of measurement accuracy and that for improvement of reliability, and hence it is difficult to improve both the measurement accuracy and the reliability at the same time.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a capacitance type load sensor which is high in measurement accuracy and long-term reliability.
A capacitance type load sensor of this invention comprises a first electrode, a second electrode disposed to face the first electrode and to be spaced therefrom and cooperating with the first electrode to form a capacitor, first and second holding members made of an insulating material, and an elastic member having opposite end portions thereof associated with the first and second holding members, respectively. The first and second holding members support the first and second electrodes for relative motion in the direction along which a facing area between the electrodes changes, with the distance between the electrodes kept constant. The elastic member is elastically deformable in the direction of the relative motion between the first and second electrodes. Load to be measured by the load sensor is applied to the first and second holding members or to the elastic member.
In the load sensor of this invention, load is applied to the first and second holding members or to the elastic member, and hence no load is applied to the first and second electrodes. Thus, there is a low possibility of causing deformation or damage of the electrodes upon application of load, and accordingly the load sensor is excellent in long-term reliability. When the elastic member is elastically deformed upon application of load, a relative motion occurs between the first and second electrodes, respectively supported by the first and second holding members with which the opposite end portions of the elastic member are associated, with the electrode distance kept constant. Thus, a change occurs in the facing area between the electrodes and therefore the capacitance therebetween changes. The amount of change in the capacitance accurately indicates the magnitude of the load applied to the load sensor, so that the measurement accuracy of the load sensor is excellent. In this manner, the present invention provides a capacitance type load sensor which is high in measurement accuracy and long-term reliability.
In the present invention, preferably, the first and second electrodes are each formed into a hollow cylinder shape which is open at its at least one end face. The second electrode is disposed coaxially with the first electrode. More preferably, the first and second electrodes are each formed into a hollow circular cylinder shape.
With this preferred embodiment, the facing area between the first and second electrodes and accordingly an amount of change in the facing area per unit load become sufficiently large, without the need of making the transverse sectional dimensions of the electrodes large, thereby providing a load sensor which is compact in size and high in measurement accuracy.
In the present invention, preferably, the first and second electrodes are each formed into a hollow cylinder shape which is open at its opposite end faces. The first holding member is comprised of a hollow outer tube having a peripheral wall and an end wall formed integrally therewith at one end of the peripheral wall. The second holding member is comprised of a hollow inner tube disposed in the hollow outer tube and having a peripheral wall and an end wall formed integrally therewith. The outer and inner tubes are arranged for relative motion along the longitudinal axis of the load sensor. The first electrode is attached to the inner peripheral face of the peripheral wall of the outer tube, and the second electrode is attached to the outer peripheral face of the peripheral wall of the inner tube.
With this preferred embodiment, the first and second holding members positively hold the first and second electrodes for relative motion, with the electrode distance maintained constant. Thus, an amount of relative motion between the first and second electrodes caused by the application of load and accordingly an amount of change in the facing area between the electrodes accurately correspond to the magnitude of the applied load, whereby the measurement accuracy, in particular, of the load sensor is improved.
In the preferred embodiment, preferably, the elastic member is comprised of a coil spring having a coil portion and two extension portions extending from the coil portion outward along the longitudinal axis of the load sensor. The coil portion of the coil spring is disposed within the interior space of the second electrode, and the two extension portions of the coil spring are coupled to the end walls of the outer and inner tubes, respectively. The load to be measured by the load sensor is applied to at least one of the two extension portions of the coil spring.
With this preferred embodiment, when load is applied to one or both of the two extension portions of the coil spring, the coil portion of the coil spring is elastically deformed by an amount corresponding to the magnitude of the load. The amount of elastic deformation of the coil portion accurately indicates the magnitude of the applied load, thereby contributing to improved measurement accuracy of the load sensor. Fur

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