Measuring and testing – Volume or rate of flow – Thermal type
Reexamination Certificate
1998-10-27
2001-06-05
Patel, Harshad (Department: 2855)
Measuring and testing
Volume or rate of flow
Thermal type
C073S204150, C073S202000, C073S118040
Reexamination Certificate
active
06240775
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a flow rate sensor which is usually employed to measure an intake air flow rate in an internal combustion engine, particularly relates to a flow rate sensor which is used to measure the flow rate of a fluid on the basis of a heat transfer phenomenon where a heat is transferred either from a heating element or from a portion heated by the heating element to the fluid.
2. Description of the Related Art
Japanese Unexamined Patent Publication No. 8-313318 has disclosed a thermo-sensitive type flow rate sensor which is used to measure the flow rate of a fluid flowing through a predetermined flowing passage, on the basis of a heat transfer phenomenon where a heat is transferred either from a heating element or from a portion heated by the heating element to the fluid.
FIG. 36
is a front view illustrating a conventional thermo-sensitive type flow rate sensor disclosed in Japanese Unexamined Patent Publication No. 8-313318.
FIG. 37
is a cross sectional view of the thermo-sensitive type flow rate sensor of FIG.
36
.
Referring to
FIGS. 36 and 37
, a detecting pipe conduit
19
, which is formed with a bell mouth portion, is located within a main fluid passage
16
through which a fluid (whose flow rate is to be measured) flows. The fluid flows from the left to the right in FIG.
37
through the main fluid passage
16
, and a flow rate detecting element
12
is disposed within the detecting pipe conduit
19
.
The flow rate detecting element
12
is comprised of a ceramic substrate and a platinum layer formed by depositing platinum served as a thermo-sensitive electrically resistant material on the surface of the ceramic substrate. The thermo-sensitive electrically resistant material has a property whereby the electric resistance will change with changes in temperature. Further, the platinum layer is formed into a tooth pattern (a meander pattern) so as to serve as a flow rate detecting resistance
11
. Moreover, a fluid temperature compensating resistance
13
, which is used to compensate a temperature change of the flowing fluid, is also made of a platinum which is the thermo-sensitive electrically resistant material, and is disposed upstream of the detecting pipe conduit
19
. A fluid rectifying grating means
17
is made of a resin and is formed into a honeycomb structure. Such fluid rectifying grating means
17
is positioned close to the inlet of the main fluid passage
16
.
An electronic circuit case
15
accommodating an electronic circuit board
14
is provided on the outside of the main fluid passage
16
. Mounted and fixed on the electric circuit board
14
is an electronic circuit for calculating the flow rate of a flowing fluid. In practice, the electronic circuit is electrically connected with both the flow rate detecting resistance
11
and the fluid temperature compensating resistance
13
.
Referring now to
FIG. 36
, there is provided a connector
18
which is used to supply an electric power from the outside of the main fluid passage
16
to the flow rate sensor, and to obtain a flow rate signal from the flow rate sensor so as to send the flow rate signal to a predetermined place outside the main fluid passage
16
.
In use of such conventional thermo-sensitive type flow rate sensor
1
, an electric current flowing into the flow rate detecting resistance
11
of the flow rate detecting element
12
, is controlled by the electronic circuit attached on the circuit board
14
, in a manner such that an average temperature of the flow rate detecting resistance
11
will rise to a predetermined value which is 200° C. higher than a fluid temperature detected by the fluid temperature compensating resistance
13
. In more detail, when a flowing fluid quantity is small, an amount of heat being transferred from the flow rate detecting resistance
11
to the flowing fluid will also be small, thus an electric current necessary for heating will decrease. On the other hand, when a flowing fluid quantity is large, an amount of heat being transferred from the flow rate detecting resistance
11
to the flowing fluid will also be large, thus an electric current necessary for heating will be increased. Thus, in a thermo-sensitive type flow rate sensor
1
, an electric current for heating the resistance
11
is detected and used as a fluid rate signal, thereby detecting an actual flow rate of a fluid flowing through the main fluid passage
16
having a predetermined cross section area.
The thermo-sensitive type flow rate sensor
1
, which is constructed in the above mentioned manner, is often used as an intake air flow rate sensor for an automobile engine, as shown in FIG.
38
. Referring to
FIG. 38
, the flow rate sensor
1
is positioned within an intake air pipe
4
which is located downstream of an air cleaner element
2
enclosed in an air cleaner case
3
. The air cleaner element
2
is a filter means made of a non-woven fabric or a filter paper, which is used to capture the dust entrained in the intake air so as to prevent it from entering the engine. However, after an automobile has been running for a certain long time, the air cleaner element
2
will get blocked due to the dust. Thus, an air flow having passed through the air cleaner element
2
, when compared with a fluid having passed through a non-dust-blocked air cleaner element
2
, will be more easily subjected to a change in the flow speed distribution of a fluid on the downstream side of the air cleaner element
2
before the fluid arrives at the flow rate sensor
1
.
In fact, the flow rate detecting element
12
of the flow rate sensor
1
can detect only a part of the fluid flowing through the entire cross section of the main fluid passage
16
. Accordingly, although the total quantity of a fluid flowing through the main fluid passage
16
does not change, a change in the flow speed distribution of a fluid on the upstream side of the flow rate sensor
1
, will bring about an error to a flow rate detecting result.
In order to solve the above problem, it has been suggested that a fluid rectifying grating means
17
be provided in the main fluid passage
16
upstream of the flow rate sensor
1
, as shown in
FIGS. 36 and 37
. Another conventional flow rate sensor has been disclosed in Japanese Unexamined Patent Publication No. 7-71985. In order to obtain a sufficient fluid rectifying effect, this conventional flow rate sensor employs a honeycomb structure, a net-like grating structure or a combination the honeycomb structure and the net-like grating structure.
Further, Japanese Unexamined Patent Publication Nos. 5-340778, 2-28520, 6-288805, have disclosed that a main fluid passage may be converged to have a Venturi shape as shown in
FIG. 39
, thereby obtaining a similar fluid rectifying effect.
Thus, a conventional flow rate sensor usually involves a fluid rectifying grating means
17
to rectify the fluid whose flow rate is to be measured. On the other hand, to obtain a sufficient rectifying effect, such kind of fluid rectifying means should be made so that the holes formed therethrough are quite small and that each unit area has a lot of such holes. However, since the fluid rectifying means has a honeycomb structure and since such rectifying means is required to have a sufficient rigidity, it is difficult to manufacture the fluid rectifying means with a lot of holes. As a result, a finally obtained fluid rectifying means has only a small aperture ratio (a small aperture area).
Further, since fluids flowing across many holes of a fluid rectifying means are unstable, a lot of small eddies will get together to form an irregularly large flowing of the fluid. As a result, there will occur a not uniform phenomenon in both the boundary layer thickness and the frictional stress around the detecting section of the flow rate sensor, hence causing fluctuations and errors in a flow rate detecting signal and thus making it impossible to perform a correct flow rate detection.
Moreover, it is understood that the ventilation resistance on the flow
Hamada Shingo
Kotoh Satoru
Oshima Takeharu
Uramachi Hiroyuki
Yamakawa Tomoya
Mitsubishi Denki & Kabushiki Kaisha
Patel Harshad
Sughrue Mion Zinn Macpeak & Seas, PLLC
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