Flow sensor

Details Flow sensor Flow sensor

1999-07-26

2002-10-29

Fuller, Benjamin R. (Department: 2855)

Measuring and testing

Volume or rate of flow

Thermal type

Reexamination Certificate

active

06470742

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a flow sensor for measuring the flow velocity or flow rate of a fluid such as intake air for an internal combustion engine. More specifically, it relates to a flow sensor, equipped with a heating element, for measuring the flow rate of a fluid based on a heat transfer phenomenon from the heating element or a part heated by the heating element to the fluid.
2. Description of the Prior Art
FIGS.
13
(
a
) and
13
(
b
) are diagrams showing the constitution of a flow detection element (diaphragm sensor)
51
used in a conventional flow sensor disclosed by Japanese Laid-open Patent Application No. 4-230808, for example. FIG.
13
(
a
) is a plan view and FIG.
13
(
b
) is a sectional view cut on line D—D of FIG.
13
(
a
). In FIGS.
13
(
a
) and
13
(
b
), reference numeral
1
denotes a plate substrate made from a silicon semiconductor. A cavity
12
which has a trapezoidal section and does not communicate with the front side of the plate substrate
1
is formed in a center portion of the rear side of the plate substrate
1
by anisotropic etching to fabricate a thin diaphragm
13
in the plate substrate
1
on the bottom side of the cavity
12
, that is, the front side of the plate substrate
1
.
A thin film heating element
3
is formed at a center portion of the surface of the diaphragm
13
and thin film resistance thermometers
52
and
53
are formed symmetrical on both sides of the heating element
3
at a predetermined interval therebetween. Slit portions
54
a
and
54
b
which are belt-like holes and extend through the diaphragm
13
are formed between the heating element
3
and the resistance thermometers
52
and
53
in a longitudinal direction, and slit portions
55
a
and
55
b
which consist of a plurality of square holes extending through the diaphragm
13
and aligned with one another are formed outside the resistance thermometers
52
and
53
in a longitudinal direction. Slit portions
56
c
,
56
d
,
57
c
and
57
d
which are holes extending through the diaphragm
13
are formed at both ends in a longitudinal direction of the heating element
3
and the resistance thermometers
52
and
53
, respectively. These slit portions
54
a
to
57
d
are formed by general photolithography or wet or dry etching.
In the above FIGS.
13
(
a
) and
13
(
b
), the electrodes of the heating element
3
and the resistance thermometers
52
and
53
and thin-film conductor patterns forming the power lines of the heating element
3
and the resistance thermometers
52
and
53
formed on the plate substrate
1
are omitted.
A description is subsequently given of the operation of the above flow detection element
51
of the prior art.
The front side (heating element
3
side) of the flow detection element
51
is made parallel to the flow passage of a fluid to be measured, the longitudinal directions of the heating element
3
and the resistance thermometers
52
and
53
are made perpendicular to the flow of the fluid, and a current to be applied to the heating element
3
is controlled such that the temperature of the heating element
3
should be higher than the temperature of the fluid by a predetermined value. Since the resistance thermometers
52
and
53
are arranged symmetrical about the heating element
3
, when the fluid does not flow (flow velocity is zero), the temperatures of the above resistance thermometers
52
and
53
are equal to each other.
When the fluid flows in a direction shown by an arrow V, the resistance thermometer
52
on an upperstream side is cooled and the temperature thereof becomes lower than that when the flow velocity is zero. A reduction in the temperature of the above resistance thermometer
52
becomes greater as the flow velocity increases. Meanwhile, since the resistance thermometer
53
on a downstream side is located on the downstream side of the heating element
3
, when the flow velocity is the same, the temperature of the resistance thermometer
53
does not become as low as that of the resistance thermometer
52
on the upperstream side. Therefore, there is a temperature difference between the resistance thermometer
52
on the upperstream side and the resistance thermometer
53
on the downstream side according to the flow velocity of the fluid. Then, by detecting a resistance difference between the resistance thermometer
52
and the resistance thermometer
53
, which corresponds to the above temperature difference, by means of detection means such as an unshown Wheatstone bridge circuit incorporating the resistance thermometers
52
and
53
, the flow velocity of the fluid can be measured.
Thus, in the above prior art, changes in output caused by the adhesion of dust are reduced by forming the cavity
12
in the rear side of the plate substrate
1
to fabricate the thin diaphragm
13
. Further, the slit portions
54
a
to
57
d
are formed in the diaphragm
13
to reduce a heat flow from the heating element
3
to the resistance thermometers
52
and
53
, thereby suppressing a rise in the temperatures of the resistance thermometers
52
and
53
and reducing a heat flow from the heating element
3
to the plate substrate
1
to improve sensitivity.
To obtain high sensitivity and responsibility for a flow detection element having such a diaphragm structure, the heat responsibility of the diaphragm
13
must be improved by reducing the thickness of the diaphragm
13
regardless of the existence of the slit portions. However, when the thickness of the diaphragm
13
is reduced, the ratio of the thickness of the diaphragm
13
to the thickness of a heat sensitive resistor film forming the heating element
3
and the resistance thermometers
52
and
53
becomes large. Therefore, as the thickness of the diaphragm
13
decreases, the difference of a material structure in a thickness direction between a portion with the heat sensitive resistor film and a portion without the heat sensitive resistor film becomes larger, whereby the diaphragm
13
deforms (initial deformation) when the heat sensitive resistor film and the cavity are formed. This deformation is caused by the difference of internal stress between the materials of the films. When the initial deformation of the diaphragm
13
occurs and electricity is applied to the heating element
3
to generate heat, the deformation of the diaphragm
13
becomes larger due to the differences of thermal or mechanical properties between the material of the diaphragm
13
(silicon which is the material of the substrate) and the material of the heat sensitive resistor film such as the heating element
3
formed thereon (for example, a metal material such as platinum). When the deformation is large, large stress is generated between the diaphragm
13
and the heat sensitive resistor film, thereby causing the heat sensitive resistor film forming the heating element
3
and the resistance thermometers
52
and
53
to be separated from the surface of the diaphragm
13
. This exerts an adverse effect on the detection characteristics of the flow sensor.
Further, when the large deformation of the diaphragm
13
occurs, there are differences in the amount of deformation of the film when it serves as a flow sensor due to differences in the thermal or mechanical properties of the film, which may influence the detection characteristics of the flow meter and make accurate flow detection impossible.
If the above deformation is asymmetrical within the plane of the diaphragm
13
at the time of forming thin-film patterns or applying electricity for heating, the separation of the film and the difference of deformation become more marked, thereby deteriorating the detection characteristics of the flow sensor.
In view of the above problems of the prior art, it is an object of the present invention to provide a flow sensor which has excellent responsibility, sensitivity and reliability and high flow detection accuracy by suppressing the deformation of a diaphragm.
SUMMARY OF THE INVENTION
According to a first aspect of the

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