Abstrict A heat type air flow meter with improved resistance to radio wave
interference by restraining a high frequency noise from a resistor
and the supporting materials thereof provided in air passage. At
least one high frequency band cut-off filter having a capacitor
and a inductor is provided between, on one hand, a heating resistor
and a heat sensitive resistor and, on the other hand, a temperature
control circuit, and an intermediate connecting point of one end
of the capacitor connected with one end of the inductor is also
connected to both resistors. Thereby, as the resistivity to the
radio wave interference is effectively improved cost reduction and
compactness of the flow meter can be achieved.
Claims We claim:
1. A heat type air flow meter having an air passage and a metal
body acting as base ground, comprising a heating resistor and a
heat sensitive resistor operatively arranged in the air passage,
and a temperature control circuit for controlling the heating resistor,
said circuit having at least one high frequency band cut-off filter
comprising a capacitor and an inductor between, on one hand, the
heating resistor and the heat sensitive resistor and, on the other
hand, the temperature control circuit, wherein
an intermediate connection point where one end of the capacitor
is connected to one end of the inductor is connected to both resistors
and another end of the inductor is connected to the temperature
control circuit and another end of the capacitor is connected to
the metal body separate from a signal ground.
2. A heat type air flow meter as defined in claim 1 wherein said
high frequency band cut-off filter is provided in a portion of said
circuit which supplies an electric current to the resistors.
3. A heat type air flow meter as defined in claim 1 wherein said
high frequency band cut-off filter is provided between the heat
sensitive resistor and the temperature control circuit.
4. A heat type air flow meter as defined in claim 1 wherein said
high frequency band cut-off filter is provided in a portion fed
back to the temperature control circuit from the heat sensitive
resistor.
5. A heat type air flow meter as defined in claim 1 wherein said
high frequency band cut-off filter is connected between the heating
resistor and the temperature control circuit and between the heat
sensitive resistor and the temperature control circuit.
6. A heat type air flow meter as defined in claim 1 wherein said
high frequency band cut-off filter is connected to between the heat
sensitive resistor and the temperature control circuit.
7. A heat type air flow meter as defined in claim 1 wherein said
high frequency band cut-off filter comprises the inductor and the
capacitor and is connected to a metal body as a base ground so as
to comprise a sensor module installed in a body forming a main air
passage.
Description BACKGROUND OF THE INVENTION
The present invention relates to a heat type air flow meter, especially
relates to a heat type air flow meter which is improved so as to
suppress radio wave interference.
As an improved method for suppressing radio wave interference in
a heat type air flow meter, a high frequency band cut-off filter
is provided between both bodies of a heating resistance body and
a heat sensitive resistance body and the temperature control circuit
as described in Japanese Patent Application Laid-open 2-124429(1990).
The cause of a malfunction of the heat type air flow meter under
high frequency and strong electric field is the high frequency electric
current induced in the both resistances of the heat generation resistance
and heat sensitive resistance, and as both resistance elements are
covered on metal body having shield effect generally, even the method
mentioned in the Japanese Patent Laid-Open issue 2-124429 was not
able to fully suppress the radio wave interference.
The air passage body is today made of resin, however, and therefore
the above-mentioned method cannot be used. Then, as high frequency
band cut-off filter is generally used, the high frequency noise
which is induced in both resistance bodies invades the control circuit
with high frequency band cut-off filter mentioned above is taken
in.
Furthermore, the high frequency band cut-off filter mentioned above
is provided on a signal ground. Consequently, there is a danger
that the high frequency noise acts on a common mode of the control
circuit and causes false movements.
SUMMARY OF THE INVENTION
In the present invention, an LC circuit constituting a high frequency
band cut-off filter is connected to a base ground separated from
the signal ground to prevent the high frequency noise invading the
control circuit.
An object of the present invention is to remove and restrain the
high frequency noise transmitted from the resistance bodies and
a support members provided in an air passage and thereby to provide
a heat type air flow meter having improved resistivity to the radio
wave interference.
In the heat type air flow meter having the heat resistance body
and a heat sensitive body provided in the air passage, and the temperature
control circuit for controlling the temperature generated in the
heat resistance body mentioned, at least a high frequency band cut-off
filter constructed with a capacitor and a inductor between the temperature
control circuit and both resistance bodies of the heat sensitive
resistor and the heat generation resistance is provided. In the
high frequency band cut-off filter, the intermediate connection
point at which one end of the capacitor is connected to one end
of the inductor is connected to a side of both resistance bodies,
and the other end of the inductor is connected to the temperature
control circuit side.
When an air passage consists of an electrically non-conductive
material according to the present invention, a high frequency noise
is induced in the resistance bodies and the supporting materials
provided in the air passage under a condition of high frequency
and strong electric field.
However, the high frequency band cut-off filter (herein after called
the LC filter) raises the impedance in the control circuit side,
and the high frequency noise escapes to the base ground. At this
time, if the base ground is separated from the signal ground, the
high frequency noise does not affect the signal ground in the common
mode. Further, as a T-type filter is formed with the inductor having
the resistance body and the LC filter, the influence given to the
temperature control circuit by the noise can be restrained, with
the noise otherwise invading a power supply, the signal ground and
an output terminal of the temperature control circuit through an
input and output harness is effectively restrained.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a front view of a heat type air flow meter showing an
embodiment of the present invention.
FIG. 2 is cross section along line AA of FIG. 1.
FIG. 3 is a schematic diagram of an embodiment of the temperature
control circuit in the sensor module shown in FIG. 1.
FIG. 4 is a schematic diagram showing another embodiment of the
temperature control circuit.
FIG. 5 is a graph illustrating an evaluation between the resistivity
to the radio wave interference.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In FIG. 1 a heating resistance body 7 as a sensor for measuring
air flow quantity and a heat sensitive resistor 8 are provided in
a bypass passage 3 in a main air passage 2. A sensor module 4 containing
a temperature control circuit 9 (FIG. 3 or FIG. 4) and a metal body
5 acting as base ground separate from a signal ground are installed
on an outside wall of body 1 forming the main air passage 2.
After a portion of air flow which flows in the direction shown
by the broad arrow in FIG. 2 is flowed into the bypass passage 3
that portion joins in a main air flow from a opening portion 6 on
the side of the passage 3 to reduce backfire.
One embodiment of the temperature control circuit 9 prepared for
the sensor module 4 of FIG. 1 is shown in FIG. 3. Heating resistor
7 and heat sensitive resistor 8 are provided in bypass passage 3
in the main air passage 2 as described above. Resistances R1 R2
R3 and the resistance bodies 78 mentioned above form a bridge circuit,
and a temperature difference between a temperature of the heating
resistor 7 and the heat sensitive resistor 8 is controlled so as
to keep within a constant value by an operational amplifier OP.
Thereby, the electric current which flows through the heating resistor
7 is controlled as a function of the air flow quantity which is
thereby accurately measured.
Capacitors C1 C2 are provided as a general noise protection technique
for the control circuit. Filter F is inserted in a portion of the
circuit supplying an electric current controlled by the operational
amplifier OP in both resistance bodies 78.
The intermediate connection point of one side of the capacitor
Cf and one side of the inductor Lf is also connected to both resistance
bodies 78 and other side of the inductor Lf is connected to the
temperature control circuit 9. The other side of the capacitor Cf
connects to a metal body 5 acts as the base ground (and herein after
called the base ground) which is separated from the signal ground.
If the above-described heat type air flow meter were placed in
a strong, high frequency electrical field, a harmful high frequency
noise would be induced in the control movements to the supporting
materials connecting both resistors 78 with the temperature control
circuit 9.
However, the capacitor Cf of the filter F bypasses the high frequency
noise to the base ground and the inductor Lf acts as a function
for restraining the high frequency noise.
In the circuit configuration shown in FIG. 3 in particular, the
resistivity to the radio wave interference is improved to a level
where there is no practical problem prevent insertion of the LC
filter in only the one portion of the circuit.
Moreover, the filter F forms a T-type filter with the inductance
included in the both resistors 78 in a circuit thereof.
As a result, the influence of the noise invading through a input
and output harness of the heat type air flow meter into the power
supply, the signal ground, the output terminal of the temperature
control circuit 9 gives to temperature control circuit 9 can be
restrained.
In the operational amplifier OP2 of the embodiment shown in FIG.
4 the heat sensitive resistor 8 and resistors R7 R8 form a non-reverse
amplifier, with the use of a lowered resistance of the heat sensitive
resistor 8 the same as the heating resistor 7. The heating resistor
7 and the resistors R1 R2 R3 and the non-reverse amplifier have
a movement condition the same as that of the bridge circuit. On
this account, the heating resistor 7 is controlled so as to keep
the temperature difference with the heat sensitive resistor 8 within
a predetermined value by the operational amplifier OP1. Therefore,
the electric current which flows through the heating resistor 7
is controlled as a function of the air flow quantity which is thus
accurately detected. The capacitors C1 C2 C3 provide general noise
prevention in the control circuit. Filter F1 is inserted in the
portion where a signal returns from the heat resistor 7 to the temperature
control circuit 9. Filters F2 F3 are inserted at both ends of the
heat sensitive resistor 8.
Furthermore, the construction of each filter is similar to the
filter in FIG. 3. The intermediate connection point of one end of
each capacitor and one end of each inductor is connected to both
resistors, and other side of each inductor is connected to the temperature
control circuit 9. The other side of each capacitor is contacted
the base ground separate from the signal ground.
According to the above described construction, the heat type air
flow meter placed in the strong, high frequency electric field has
high frequency noise induced in the heating resistor 7 the heat
sensitive resistor 8 and the supporting materials.
The capacitor in the each filter bypasses the induced high frequency
noise into the base ground, and the inductor restrains the high
frequency noise.
Particularly according to this embodiment, resistance to radio
wave interference is improved to a level where there is not any
problem in practical use by inserting the LC filter in the three
positions.
Further, depending upon the environment of use only the filters
F1 F2 or F1 F3 or in some cases only filter F3 may be necessary
to restrain radio wave interference. |