Abstrict A thermal air flow meter for measuring the flow rate of intake
air supplied to an internal combustion engine comprises a thermal
sensor disposed in an intake air passage, a drive circuit including
a circuit pattern formed on a circuit board for driving the thermal
sensor, a shielding case having at least a bottom wall on which
the circuit board is mounted and side walls, a through capacitor
disposed on one of the side walls of the shielding case, a conductor
extending through the capacitor so that the conductor is electrically
connected in an AC mode through the capacitor to the shielding case
and having one end connected to a corresponding terminal of the
drive circuit and the other end connected to an external terminal,
a metal case in which the shielding case is mounted and a connecting
member electrically connecting the shielding case to the metal case
at an area on the bottom wall selected between an edge of the bottom
wall connected to the one side wall and an edge of the pattern formed
on the circuit board closest to the one side wall, whereby the endurance
electromagnetic compatability of the air flow meter is improved.
Claims We claim:
1. A thermal air flow meter comprising:
(a) drive circuit means for driving a thermal sensor to be disposed
in an intake air passage through which air to be measured flows
and including a circuit board and a drive circuit mounted on said
circuit board;
(b) an electrically conductive shielding case in which said drive
circuit means is disposed and having a bottom wall on which said
circuit board is fixed and side walls connected to respective edges
of said bottom wall;
(c) capacitor means including at least one through-capacitor fixed
to one of said side walls of said shielding case;
(d) a metal base on which said shielding case is mounted;
(e) at least one conductor having one end extending outside of
said metal case for connection to an external electric terminal
and another end connected to a terminal of said drive circuit in
said shielding case, said one conductor being electrically connected
through said through-capacitor means to said shielding case;
(f) connecting means for electrically connecting said shielding
case to said metal base in a selected area on said bottom wall of
said shielding case between an edge of said bottom wall connected
to said one side wall in which said capacitor means is fixed and
an edge of said circuit board closest to said one side wall.
2. A thermal air flow meter according to claim 1 wherein said
connecting means includes a metal member extending through said
shielding case and said metal base.
3. A thermal air flow meter according to claim 2 wherein said
metal member is positioned between said circuit board and said one
side wall of said shielding case.
4. A thermal air flow meter according to claim 1 wherein said
capacitor is formed with a through-hole through which said conductor
extends from outside to inside of said shielding case.
5. A thermal air flow meter according to claim 1 wherein said
selected area at which said connecting means connects said shielding
case to said metal base is substantially immediately beneath said
capacitor means and is spaced from said drive circuit mounted on
said circuit board.
6. A thermal air flow meter comprising:
a thermal sensor disposed in an intake air passage through which
an air to be measured flows;
a printed driving circuit formed on a circuit board for driving
said thermal sensor and having terminals to which electrical conductors
are connected for electrical connection of said driving circuit
to external electrical equipment;
a shielding case made of an electrical conductive material and
having a bottom wall to which said cirucit board is fixed and side
walls connected to respective edges of said bottom walls;
a flow meter case for mounting said shielding case therein and
having a base made of an electrical conductive material to which
the bottom wall of said shielding case is fixed;
capacitor means including a plurality of capacitors fixed to one
of the side walls of said shielding case and electrically connected
to that the conductors connected to the terminals of said driving
circuit are electrically connected through said capacitors, respectively,
to said shielding case; and
means for electrically connected said shielding case to the base
of said flow meter case at a portion selected in an area on said
base corresponding to an area on the bottom wall of the shielding
case disposed between an edge of the bottom wall connected to the
one side wall of the shielding case to which said capacitor means
is fixed and an edge of the circuit board closest to said one side
wall.
7. A thermal air flow meter according to claim 6 wherein said
connecting means includes a metal member extending at said selected
portion through said shielding case and said metal base.
8. A thermal air flow of meter according to claim 6 wherein each
of said capacitors is formed with a through-hole through which a
conductor extends from outside to inside of said one side wall of
said shielding case.
9. A thermal air flow meter according to claim 6 wherein said
selected area at which said connecting means connects said shielding
case to said metal base is substantially immediately beneath said
capacitor means and is spaced from said drive circuit mounted on
said circuit board.
10. A thermal air flow meter according to claim 7 wherein said
metal member is positioned between said circuit board and said one
side wall of said shielding case.
Description BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a hot wire type air flow meter, and more
particularly to an air flow meter which is suitable for measuring
the flow rate of air supplied to an internal combustion engine and
which operates with an excellent endurance electromagnetic compatibility.
2. Description of the Related Art
A hot wire type flow meter is disclosed in, for example, Japanese
patent unexamined publication JP-A-No. 58-6414. In the prior art
hot wire type flow meter, through-capacitors are fixed to conductor
leading openings respectively of a sheilding case of an electrical
conductive material accommodating a drive circuit which drives a
flow rate sensor, and power supply and signal output conductors,
to be connected to the drive circuit are extended into the shielding
case through the respective through-capacitors thereby preventing
the drive circuit from being disturbed by outside noises.
In the prior art flow meter described above, the power supply and
signal output conductors connected to the drive circuit driving
the flow rate sensor are electrically connected in an AC mode through
the capacitors to the shielding case. However, no consideration
is given in the prior art flow meter as to electrically connecting
the conductive shielding case to a flow meter case in which the
shielding case is mounted and the shielding case is merely fixed
to the flow meter case by an adhesive. Therefore, the shielding
case could not always be fixed to the flow meter case in a relation
accurately positioned relative to the latter, and, because the former
was not electrically connected to the latter, the endurance electromagnetic
compatibility of the flow meter has been quite low.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a thermal air
flow meter in which through capacitors, through which conductors
connecting a drive circuit to a positive terminal and a negative
terminal of a power supply and an output signal line extend, are
electrically connected to a flow meter case at a portion thereof
made of an electrically conductive material and disposed in a selected
area thereby to improve the endurance electromagnetic compatibility
of the air flow meter.
According to the present invention which attains the above object,
the conductors and the output signal line described above extend
through the through-capacitors which are electrically connected
in an AC mode through the through-capacitors to a shielding case
of an electrically conductive material accommodating the drive circuit,
and the shielding case is brought into electrical contact with the
flow meter case at a portion thereof made of an electrically conductive
material and disposed in a specific area which is determined as
follows. That is, a circuit board formed with a circuit pattern
for the drive circuit is attached to a bottom wall of the shield
case and the through-capacitors are fixed to one of the side walls
of the shield case connected to the bottom wall. The specific area
is selected within an area on a bottom wall or base of the flow
meter case corresponding to the area on the bottom wall of the shielding
case between an edge of the circuit board closest to the one side
wall on which the through-capacitators are fixed and an edge of
the bottom wall connected to the one side wall.
In an embodiment of the thermal air flow meter according to the
present invention, signal terminals connected to an external wiring
harness to receive power supplied to a drive circuit and to transmit
an output signal from the drive circuit are connected to the drive
circuit by electrical conductive members or wires extending through
associated through-capacitors fixed to a shielding case of an electrical
conductive material. The through capacitors are electrically connected
in an AC mode through the capacitors to the shielding case, to a
bottom wall of which a circuit board having a circuit pattern constituting
the drive circuit is fixed. The shielding case is brought into electrical
contact with an electrically conductive portion of a flow meter
case at a selected area as mentioned above. With this arrangement,
electrical noise generated from the external wiring harness under
influence of radio wave can be earthed through the through-capacitors
to the electrically conductive portion of the flow meter case or
to an air intake body defining an intake air passage, before the
electrical noise is transmitted to the drive circuit of the hot
wire type air flow meter. Therefore, the drive circuit can be shielded
against the adverse effect of the radio wave, and the endurance
electromagnetic compatibility of the air flow meter can be improved.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic sectional view of an embodiment of the hot
wire type air flow meter of the present invention when its sensor
is disposed in a bypass air passage.
FIG. 2 is a view similar to FIG. 1 to show its sensor disposed
in a main air passage.
FIGS. 3 and 4 are a schematic plan view and a schematic sectional
view respectively of the parts including the drive circuit of the
hot wire type air flow meter shown in FIG. 1.
FIGS. 5 to 9 are schematic sectional views showing various manners
of grounding the shielding case to the base of the flow meter case.
FIG. 10 is a graph showing the results of the endurance electromagnetic
compatibility test of the air flow rate sensor.
FIGS. 11 and 12 are a schematic plan view and a schematic sectional
view respectively of a trial-manufactured model of the air flow
meter of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Preferred embodiments of the present invention will now be described
with reference to the drawings.
FIG. 1 is a schematic sectional view of an embodiment of the hot
wire type air flow meter of the present invention designed for measuring
the flow rate of intake air of an internal combustion engine.
Referring to FIG. 1 the air flow meter includes a hot wire 1 made
by winding a filament of platinum around a bobbin of alumina and
coating thereon a surface covering of a glass material. This hot
wire 1 is welded to exposed portions of a pair of supporting pins
9 inserted into a supporting member 8 of an electrical insulator
supporting a base 4 of a flow meter case, which base is made of
an electrically conductive material. A cold wire 2 operating as
an electrical resistor similar to the hot wire 1 is provided for
the purpose of compensation of the temperature of intake air and
is similarly welded to exposed portions of supporting pins 9 inserted
into the supporting member 8. The supporting pins 9 are electrically
connected to a drive circuit schematically shown at 11A formed on
a circuit board 11 through a wiring connection 6 of aluminum. The
circuit board 11 is fixed by an adhesive to a shielding case 12
made of an electrically conductive material, and the shieldingn
case 12 is also fixed by an adhesive to the base 4 of the flow meter
case. The top and side walls of the flow meter case 3 are made of
a molded resin material and are mounted on the base 4 which carries
the shielding case 12 and associated parts.
The hot wire type air flow meter, including the drive circuit on
the circuit board 11 and the hot and cold wires 1 and 2 welded to
the supporting pins 9 is mounted on an air intake body 10 in which
a bypass air passage 21 and a main air passage 22 are formed. When
the air flow meter is mounted on the air intake body 10 the hot
and cold wires 1 and 2 are located in the bypass air passage 21.
The drive circuit 11A supplies current to the hot wire 1 to heat
the hot wire 1 so that the difference between the temperature of
the hot wire 1 and that of intake air flow 5 is maintained constant
regardless of the quantity of intake air flow 5 while the temperature
variation of intake air flow 5 is detected and compensated by using
a cold wire 2. Therefore, when the flow rate of air flowing through
the bypass air passage 21 is high, a large current is supplied to
the hot wire 1 while when the flow rate of air is low, a small
current is supplied to the hot wire 1 so that the temperature difference
can be maintained constant. The relation between the value of current
supplied to the hot wire 1 and the flow rate of intake air flow
5 is expressed by a monotone increasing function. Thus, by detecting
the value of current supplied to the hot wire 1 the flow rate of
intake air flow 5 can be detected.
FIG. 2 shows an arrangement in which the hot wire type air flow
meter described with reference to FIG. 1 is mounted on the air intake
body 10 having a single air passage.
FIGS. 3 and 4 are a schematic plan view and a schematic sectional
view respectively of the hot wire type air flow meter described
with reference to FIG. 1. Referring to FIGS. 3 and 4 signal terminals
13 13' and 13" connectable with an external wiring harness,
(not shown) are connected to the power line 26 grounding line 27
and signal line 28 of the drive circuit 11 by wires 14 of an electrically
conductive material, respectively, and the wires 14 extend through
respective through-capacitors 15 which are electrically connected
to the shielding case 12. Further, the shielding case 12 is electrically
connected to the base 4 at a specific area 23 which is determined
as follows. That is, a circuit board 11 formed with a circuit pattern
(not shown) of the drive circuit 11A is attached to a bottom wall
of the shielding case and the through-capacitors 15 are fixed to
one of the side walls of the shielding case 12 connected to the
bottom wall. The specific area 23 is selected within an area on
the bottom wall or the base 4 of the flow meter case corresponding
to the area on the bottom wall of the shielding case 12 between
an edge of the circuit board 11 closest to the one side wall having
the through-capacitors therein and an edge of the bottom wall of
the shielding case connected to the one side wall.
FIG. 10 is a graph showing the results of the endurance electromagnetic
compatibility test of a trial-manufactured model of the hot wire
type air flow meter of the present invention. FIGS. 11 and 12 are
a schematic plan view and a schematic sectional view respectively
of the trial-manufactured model of the air flow meter. The characteristic
curve a shown in FIG. 10 represents the results of the endurance
electromagnetic compatibility test when the shielding case 12 and
the base 4 are entirely electrically isolated from each other. The
characteristic curve b shown in FIG. 10 represents the endurance
electromagnetic compatibility when a cup 24 press-fitted into the
base 4 is welded to the shielding case 12 at a point 25 and the
shielding case 12 is partly press-fitted on a pin 17 erected on
the base 4 so as to provide two spaced electrical connection points
at the outside of the array of the through-capacitors 15 mounted
on the shielding case 12 while the shielding case 12 is locally
electrically isolated from the base 4 in an area 26 lying immediately
beneath the circuit board 11 as shown in FIGS. 11 and 12. Further,
the characteristic curve c shown in FIG. 10 represents the endurance
electromagnetic compatibility when the shielding case 12 and the
base 4 are entirely electrically brought into contact with each
other. It will be seen from FIG. 10 that the present invention can
provide the endurance electromagnetic compatibility of the level
equivalent to the case where the shielding case 12 and the base
4 are extirely electrically brought into contact with each other.
FIGS. 5 to 9 show various manners of grounding the shielding case
12 to the base 4. FIG. 5 shows that the shielding case 12 and the
base 4 are electrically connected to each other by a screw 16 of
an electrically conductive material. FIG. 6 shows that the pin 17
of an electrically conductive material is press-fitted into the
base 4 and the shielding case 12 is previously formed with a mating
hole to be press-fitted on the pin 17. FIG. 7 shows that, after
a pin 18 of an electrically conductive material is press-fitted
into the base 4 the shielding case 12 is electrically connected
to the base 4 by caulking the head of the pin 18. FIG. 8 shows that
a projection 19 is formed as part of the base 4 by means of, for
example, a press, and the shielding case 12 is press-fitted to this
projection 19. FIG. 9 shows that the shielding case 12 and the base
4 are electrically connected to each other by an electrically conductive
adhesive 20.
In the hot wire type air flow meter described above, the present
invention specifies the range where the shielding case 12 enclosing
the drive circuit formed on the circuit board 11 is electrically
connected to the base 4 of the flow meter case. More precisely,
the signal terminals 13 13' and 13" connected to the external
wiring harness are connected to the drive circuit 11A by the wires
14 extending through the through-capacitors 15 which are electrically
connected to the shielding case 12 accommodating the drive circuit
11A. Further, the shielding case 12 is brought into electrical contact
with the base 4 of the flow meter case in a portion of the area
23 which lies nearly immediately beneath the through-capacitors
15 and which is outside of the conductors printed on the circuit
board on which the drive circuit 11A is formed. Therefore, the air
flow meter can be shielded from external electrical noise, and the
endurance electromagnetic compatibility can be greatly improved
without regard to the factors including the thickness of the adhesive
layer fixing the shielding case 12 to the base 4 of the flow meter
case.
Further, when the manner of joining the shielding case 12 to the
base 4 as shown in any one of FIGS. 5 to 8 is applied to a plurality
of spaced points, the shielding case 12 can be easily accurately
positioned relative to the base 4 and the manufacturing accuracy
can be greatly improved.
It will be understood from the foregoing description that the present
invention provides a hot wire type air flow meter in which a shielding
case accommodating a drive circuit is electrically connected to
a base of a flow meter case in a portion of an area which lies nearly
immediately beneath through-capacitors mounted on the shielding
case and which is outside of conductors printed on a circuit board
on which the drive circuit is formed. Therefore, the endurance electromagnetic
compatibility of the air flow meter can be greatly improved without
regard to the thickness of an adhesive layer used for fixing the
shielding case to the base. |