Measuring and testing – Volume or rate of flow – Thermal type
Reexamination Certificate
2000-01-12
2001-11-13
Patel, Harshad (Department: 2855)
Measuring and testing
Volume or rate of flow
Thermal type
Reexamination Certificate
active
06314807
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a thermal-type flow sensor for measuring a flow rate, for example, of intake air in an internal combustion engine. More particularly, the present invention is concerned with a thermal-type flow sensor for measuring a flow rate (or flow velocity) of a fluid by taking advantage of a phenomenon of heat transfer from a heat generating element (or a part heated by the heat generating element) to the fluid, which sensor can assure an enhanced detection sensitivity and an improved reliability.
2. Description of Related Art
For better understanding of the concept underlying the present invention, description will first be made of conventional thermal-type flow sensors known heretofore by reference to figures.
FIG. 18
shows in a top plan view a bridge type flow rate detecting device
18
employed in a conventional thermal-type flow sensor disclosed, for example, in Japanese Patent Publication No. 7659/1993, wherein the flow rate detecting device
18
is shown in a state where a protection film
3
is removed, and
FIG. 19
shows a side-elevational sectional view of the same taken along a line X—X in FIG.
18
.
Referring to
FIGS. 18 and 19
, a plate-like substrate
1
is made of a silicon semiconductor material.
The thermal-type flow sensor is further composed of a supporting or base film
2
and the protection film
3
each formed of an insulative silicon nitride material deposited over a whole top surface of the substrate
1
. A heat generating resistor pattern
4
deposited on the base film
2
is formed of a heat-sensitive resistance film such as of permalloy, platinum or the like. In this conjunction, the term “heat-sensitive resistance film” means a resistant film formed of a material whose resistance value exhibits a temperature dependency.
Temperature measuring resistor patterns
5
and
6
also deposited on the base film
2
are each formed of a heat-sensitive resistance film similarly to the heat generating resistor pattern
4
. The temperature measuring resistor patterns
5
and
6
are disposed, respectively, at both sides of the heat generating resistor pattern
4
on a same plane as the latter. More specifically, the temperature measuring resistor patterns
5
and
6
are juxtaposed in a planar array in a fluid flow direction (indicated by an arrow G in
FIG. 18
) with the heat generating resistor pattern
4
being interposed therebetween.
A reference resistor pattern
7
also deposited on the base film
2
is formed of a heat-sensitive resistance film similarly to the patterns mentioned above and deposited or disposed on a same plane as the heat generating resistor pattern
4
and the temperature measuring resistor patterns
5
and
6
.
The heat generating resistor pattern
4
, the temperature measuring resistor patterns
5
and
6
and the reference resistor pattern
7
are incorporated in a control circuit of a thermal-type flow sensor in the manner well known in the art, although illustration thereof is omitted.
More specifically, the reference resistor pattern
7
constitutes a bridge circuit through cooperation with the temperature measuring resistor patterns
5
and
6
, wherein a constant voltage is applied across the bridge circuit from the control circuit. On the other hand, a heating current is fed to the heat generating resistor pattern
4
from the control circuit, whereby a voltage making across the heat generating resistor pattern
4
and corresponding to the heating current is outputted as a flow-rate measurement signal.
A pair of openings
8
are formed in the vicinity of the array or region of the heat generating resistor pattern
4
and the temperature measuring resistor patterns
5
and
6
at upstream and downstream sides thereof, wherein the pair of openings
8
are communicated to each other through an air space
9
.
The air space
9
is formed by removing partially the silicon semiconductor material through the openings
8
by using a liquid-phase etchant which does not exert any adverse influence to the silicon nitride film.
In this manner, the array composed of the heat generating resistor pattern
4
and the temperature measuring resistor patterns
5
and
6
forms a bridge portion
11
(low heat capacity portion).
Next, description will be directed to operation of the conventional thermal-type flow sensor in which the flow rate detecting device
18
shown in
FIGS. 18 and 19
is employed.
The heating current supplied to the heat generating resistor pattern
4
from the control circuit (not shown) is so controlled that the heat generating resistor pattern
4
can be heated to a predetermined temperature which is higher, for example, by 200° C. than the temperature of the plate-like substrate
1
which is detected by the reference resistor pattern
7
.
Heat generated by the heat generating resistor pattern
4
is transferred to the temperature measuring resistor patterns
5
and
6
by way of the base film
2
and the protection film
3
and/or other heat-sensitive resistance film(s), if present.
In this conjunction, it is to be noted that the temperature measuring resistor patterns
5
and
6
are disposed at respective positions symmetrically to each other with reference to the heat generating resistor pattern
4
. Accordingly, so long as no fluid flow exists, there will arise no difference in the resistance value between the temperature measuring resistor patterns
5
and
6
.
By contrast, when fluid flow such as air flow exists on and along the temperature measuring resistor patterns
5
and
6
, the temperature measuring resistor pattern located at the upstream side as viewed in the fluid flow direction is cooled by the air, while the temperature measuring resistor pattern located at the downstream side is not cooled to a same extent as the temperature measuring resistor pattern positioned at the upstream side, because the downstream temperature measuring resistor pattern is less susceptible to the influence of heat transferred from the heat generating resistor pattern
4
to the air when compared with the upstream temperature measuring resistor pattern.
By way of example, it is assumed that the air flow takes place in the direction indicated by the arrow G in
FIGS. 18 and 19
. Then, the temperature of the upstream temperature measuring resistor pattern becomes lower than that of the downstream temperature measuring resistor pattern
6
. In general, difference in the resistance value between the temperature measuring resistor patterns
5
and
6
increases as the flowing velocity or flow rate of the fluid (air) becomes high.
Thus, by detecting the resistance values of the temperature measuring resistor patterns
5
and
6
, respectively, it is possible to measure the flowing velocity or the flow rate of the air.
Such measurement of the flow rate can equally be performed even in the case where the air flows in the direction opposite to that indicated by the arrow G because then the temperature of the temperature measuring resistor pattern
6
becomes lower than that of the temperature measuring resistor pattern
5
. Besides, with the arrangement of the heat generating resistor pattern
4
and the temperature measuring resistor patterns
5
and
6
, the fluid flow direction can also be detected.
The foregoing description has been made of the flow rate detecting device
18
which includes the bridge portion
11
as the low heat capacity portion. It is however to be mentioned that a wide variety of flow rate detecting devices have also been proposed in which a diaphragm, for example, is employed as the low heat capacity portion.
FIG. 20
is a top plan view showing a diaphragm type flow rate detecting device
18
a
employed in a conventional thermal-type flow sensor, wherein the flow rate detecting device
18
a
is shown in a state where a protection film is removed, and
FIG. 21
is a side-elevational sectional view of the same taken along a line Y—Y in FIG.
20
. In
FIGS. 20 and 21
, components same as or equivalent to those mentioned hereinbef
Kawai Masahiro
Yamakawa Tomoya
Mitsubishi Denki & Kabushiki Kaisha
Patel Harshad
Sughrue Mion Zinn Macpeak & Seas, PLLC
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