Abstrict In a ratiometric output type air flow meter with a heat generating
resistor, a ratiometric circuit is provided with a divider circuit
which is constituted by invariable resistors and divides an external
reference voltage Vref and a multiplier circuit. The invariable
resistor is grounded, via the invariable resistor. To one of input
terminals of the multiplier circuit an output signal V0 of an operational
amplifier is inputted, to the other input terminal of the multiplier
circuit voltage Vex divided by the invariable resistors is inputted,
and the output signal Vout of the multiplier circuit is outputted
to an engine control unit. A correction use resistor is connected
at the junction of the invariable resistor and a current detection
resistor, and is grounded via a ground wiring resistance. Thus the
voltage drop Vg caused by the ground wiring is compensated by the
correction use resistor, and the voltage (Vex-Vg) representing the
input to the multiplier circuit is kept constant regardless to the
magnitude of a heating current flowing through the heat generating
resistor. Thereby, a ratiometric output and heat generating resistor
type air flow meter which permits a highly accurate air flow rate
measurement is realized without being affected by variation due
to the heating current flowing through the heat generating resistor.
Claims What is claimed is:
1. An air flow meter, comprising a heat generating resistor disposed
in an air flow passage and supplied with a heating current to heat
the same up to a predetermined temperature, a ratiometric circuit
converting an air flow rate signal V0 detected based on the heating
current to a signal proportional to an external reference voltage
Vref and outputting the same, and a correction circuit configured
to correct a conversion error of said ratiometric circuit which
is caused by the heating current flowing through a ground wiring.
2. An air flow meter according to claim 1 wherein said ratiometric
circuit comprises a divider circuit which divides the external reference
voltage Vref to obtain an input voltage Vex and a multiplier which
receives the input voltage Vex and the air flow rate signal V0 as
the input signals thereof and outputs an output signal Vout, and
said correction circuit includes a correction use resistor which
is connected in series to said divider circuit at a side of the
reference potential in the ground wiring of the air flow meter and
corrects a voltage drop due to the heating current flowing through
the ground wiring.
3. An air flow meter according to claim 1 wherein said ratiometric
circuit comprises a divider circuit which divides the external reference
voltage Vref to obtain an input voltage Vex and a multiplier which
receives the input voltage Vex and the air flow rate signal V0 as
the input signals thereof and outputs an output signal Vout as the
output of the ratiometric circuit, and the correction circuit includes
a correction use resistor which is connected between the division
point in the divider circuit generating the voltage Vex and an input
terminal of the multiplier where the air flow rate signal V0 is
inputted and corrects a voltage drop due to the heating current
flowing through the ground wiring.
4. An air f low meter with a heat generating resistor according
to claim 1 wherein said ratiometric circuit comprises a divider
circuit which divides the external reference voltage Vref to obtain
an input voltage Vex and a multiplier which receives the input voltage
Vex and the air flow rate signal V0 as the input signals thereof,
and said correction circuit includes a correction use variable resistor
which is connected in series to the divider circuit at a side of
the reference potential in the ground wiring of the air flow meter
for permitting a linearity adjustment of the output signals of the
air flow meter by varying the resistance value thereof and an output
characteristic adjusting circuit disposed at the output terminal
of said ratiometric circuit and outputting an output after adjusting
the output characteristics of the air flow meter.
5. An air flow meter according to claim 1 wherein said ratiometric
circuit comprises a divider circuit which divides the reference
voltage Vref to obtain an input voltage Vex and a multiplier which
receives the input voltage Vex and the air flow rate signal V0 as
the input signals thereof, and said correction circuit includes
a correction use variable resistor which is connected between a
division point in said divider circuit generating the voltage Vex
and an input terminal of said multiplier where air flow the rate
signal V0 is inputted for permitting a linearity adjustment of the
output signals of the air flow meter by varying the resistance value
thereof and an output characteristic adjusting circuit disposed
at the output terminal of said ratiometric circuit and outputting
an output after adjusting the output characteristics of the air
flow meter.
6. An air flow meter comprising:
a heat generating resistor disposed in an air flow passage and
arranged to be supplied with a heating current to maintain a predetermined
temperature;
a current detection resistor for detecting the heating current
flowing through said heat generating resistor, said current detection
resistor being connected in series with said heat generating resistor
downstream thereof;
a first operational amplifier of which a non-inverting input terminal
is connected to the junction between said heat generating resistor
and said current detection resistor and of which an inverting input
terminal is connected to the output terminal thereof, whereby said
first operational amplifier outputs an amplified output signal voltage
V0 corresponding to the heating current and representing a flow
rate of air flowing through the air flow passage;
a ratiometric circuit including a multiplier circuit and a series
circuit of a first invariable resistor disposed upstream and a second
invariable resistor disposed downstream, said series circuit of
the first and second invariable resistors being connected between
an external reference voltage Vref and a ground wiring, the junction
between first and second invariable resistors of said series circuit
being connected to one of two terminals of said multiplier circuit
and the other terminal thereof being connected to the output terminal
of said first operational amplifier outputting the amplified output
signal voltage V0; and
a correction circuit being coupled to said ratiometric circuit,
said correction circuit compensates conversion errors introduced
in said ratiometric circuit due to variation in voltage drop across
the ground wiring caused by the heating current flowing therethrough.
7. An air flow meter according to claim 6 wherein said correction
circuit includes a third resistor connected to the ground wiring
downstream of the second invariable resistor in said series circuit.
8. An air flow meter according to claim 6 wherein said correction
circuit includes a third resistor connected between the two input
terminals of said multiplier circuit.
9. An air flow meter according to claim 7 further comprising an
output characteristic adjusting circuit including a second operational
amplifier of which non-inverting input terminal is connected to
the output terminal of said multiplier circuit, and inverting input
terminal is connected to the ground wiring through a first variable
adjusting resistor and is connected to the output terminal thereof
through a second variable adjusting resistor, wherein said third
resistor is a variable resistor.
10. An air flow meter according to claim 8 further comprising
an output characteristic adjusting circuit including a second operational
amplifier of which non-inverting input terminal is connected to
the output terminal of said multiplier circuits and of which inverting
input terminal is connected to the ground wiring through a first
variable adjusting resistor and is connected to the output terminal
thereof through a second variable adjusting resistor, wherein said
third resistor is a variable resistor.
11. An air flow meter comprising:
a heat generating resistor;
an operational amplifier;
a first resistor;
a second resistor;
a third resistor; and
a multiplier circuit, the air flow meter further comprising:
said second resistor being connected to a reference voltage (Vref)
through said first resistor, said heat generating resistor being
connected to a non-inverting input terminal of said operational
amplifier and the output terminal thereof being connected to an
inverting input terminal, the output terminal of said operational
amplifier being connected to an input terminal of said multiplier
circuit and a junction of said first resistor and said second resistor
being connected to another input terminal of said operational amplifier,
said third resistor being provided between a junction point of said
second resistor and said heat generating resistor and one of terminals
of a grounding wire of which other terminal being grounded, and
the resistance value of said third resistor being determined to
be substantially equal to the resistance value of said second resistor
multiplied by the resistance value of said grounding wire and divided
by the resistance value of said first resistor.
12. An air flow meter according to claim 11 further comprising
an amplification rate adjusting circuit disposed at the output terminal
of said multiplier circuit, wherein said third resistor is a variable
resistor.
13. An air flow meter comprising:
a heat generating resistor;
an operational amplifier;
a first resistor;
a second resistor;
a third resistor; and
a multiplier circuit, the air flow meter further comprising:
a non-inverting input terminal of said operational amplifier being
connected to said heat generating resistor and an inverting input
terminal of said operational amplifier being connected to the output
terminal thereof, to one of input terminals of said multiplier circuit
the output terminal of said operational amplifier is connected and
to the other input terminal of which is connected to a reference
voltage (Vref) through said first resistor and further is grounded
by a grounding wire, said heat generating resistor being connected
to said grounding wire through said second resistor, the output
terminal of said operational amplifier being connected to the other
input terminal of said multiplier circuit through said third resistor,
and the resistance value of said third resistor being determined
to be substantially equal to the resistance value of said second
resistor multiplied by the resistance value of said first resistor
and divided by the resistance value of said grounding wire.
14. An air flow meter according to claim 13 further comprising
an amplification rate adjusting circuit disposed at the output terminal
of said multiplier circuit, wherein said third resistor is a variable
resistor.
Description BACKGROUND OF THE INVENTION
This application claims the priority of Japanese application 10-184533
filed Jun. 30 1998 in Japan, the disclosure of which is expressly
incorporated by reference herein.
The present invention relates to an air flow rate measurement device
used for control of an automobile engine.
An air flow meter with a heat generating resistor is one of the
devices which detect intake air flow rate of an internal combustion
engine. The air flow meter with a heat generating resistor is designed
in such a manner that the heat generating resistor disposed in an
air flow passage is heated up to a predetermined temperature and
the intake air flow rate is detected based on a heating current
supplied to the heat generating resistor.
An example of such air flow meters with a heat generating resistor
is an intake air flow rate detecting device disclosed in JP-A-2-85724(1990).
The known intake air flow rate detecting device is provided with
a correction circuit, in other words a ratiometric circuit which
corrects an output signal from an intake air flow rate detecting
circuit depending on variation of a reference voltage supplied to
an A/D converter circuit and outputs the same to the A/D converter
circuit, and with the provision of the ratiometric circuit, an intake
air flow rate detecting device is realized which permits an accurate
intake air flow rate detection regardless of a possible variation
of the reference voltage supplied to the A/D converter circuit.
However, in the known intake air flow rate detecting device when
current flowing through the heat generating resistor increases,
the voltage drop at the reference potential (GND) increases due
to the wiring resistance, a ground potential Vg serving as a reference
potential for the ratiometric circuit becomes higher than a ground
potential Vgnd serving as a reference potential for an engine control
unit (ECU), and the amplification rate of the ratiometric circuit
decreases, thereby a problem is posed which causes errors and reduces
accuracy in the output signal of the intake air flow rate detecting
device.
FIG. 5 shows a schematic circuit diagram of the above explained
conventional ratiometric output type air flow meter 50 with a heat
generating resistor.
In FIG. 5 the collector of a transistor 60 is connected to a power
source and the emitter thereof is grounded via a heat generating
resistor 2 and a current detecting resistor 1. Numeral 5 denotes
a wiring resistance between the current detecting resistor 1 and
the ground. Further, the junction point between the heat generating
resistor 2 and the current detecting resistor 1 is connected to
a non-inverted input terminal of an operational amplifier 61 and
an inverted input terminal of the operational amplifier 61 is connected
to an output terminal thereof.
Still further, the output terminal of the operational amplifier
61 is connected to one of two input terminals of a multiplier circuit
11 in a ratiometric unit 10. The other input terminal of the multiplier
circuit 11 is connected to the junction point between invariable
resistors 3 and 4 connected in series with each other. The invariable
resistor 3 is grounded via the invariable resistor 4 and the wiring
resistance 5 and the invariable resistor 3 is supplied with an
external reference voltage Vref for an engine control unit (ECU)
20. An output Vout of the multiplier circuit 11 is supplied to the
ECU 20.
Although not illustrated in FIG. 5 a set of resistors is connected
in parallel with the heat generating resistor 2 and the current
detecting resistor 1 so as to constitute a bridge circuit. Therefore,
when the bridge circuit imbalances due to increase in air flow rate,
an output signal from an operational amplifier (not shown) for the
bridge circuit, which is designed to sense such increase, is supplied
to the base of the transistor 60 to supply a current to the heat
generating resistor 2. In this instance, the larger the flow rate
increase is, the supply current has to be increased accordingly;
therefore, through the detection of output voltage V0 at the operation
amplifier 61 which corresponds to the supply current, the air flow
rate can be measured.
Now, when the grounding potential Vgnd for the ECU 20 in the device
shown in FIG. 5 is used as the reference, because of the existence
of the wiring resistance 5 between the ratiometric output type air
flow meter 50 with a heat generating resistor and the ECU 20 when
current Ia flowing through the heat generating resistor 2 increases,
the reference grounding potential Vg of the ratiometric circuit
10 becomes higher than the reference grounding potential Vgnd.
The output voltage Vout of the ratiometric circuit 10 is expressed
as follows, wherein it is assumed that the voltage proportional
to the current flowing through the heat generating resistor 2 is
V0 the proportion constant thereof is A and the external reference
voltage for the circuit is Vref;
Because of the differences in the reference grounding potentials,
an error Err is caused in the output voltage Vout as expressed by
the following equation (2);
Since the reference ground potential Vg corresponds to a voltage
drop Ia.times.R5 which is caused when the heating current Ia for
the heating resistor 2 flows through the wiring resistance 5 (wherein
the resistance value thereof is assumed as R5), and the heating
current Ia is proportional to a voltage V2 detected by the current
detection resistor 1 the reference ground potential Vg is proportional
to V2.times.R5.
Further, since the air flow rate detection voltage V0 is also proportional
to the heating current Ia and the heating current Ia is proportional
to the voltage V2 and wherein it is assumed the proportion constant
thereof is B, the equation (2) can be expressed by the following
equation (3), wherein the error Err is expressed by a quadratic
function of the voltage V2;
Even if the error Err expressed by the quadratic function of the
voltage V2 is attempted to be adjusted by a characteristic adjusting
means making use of a DC amplifier circuit, the correction of the
error is difficult and the problem as referred to above is posed
that the accuracy in the output signal of the air flow meter is
reduced.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a ratiometric
output type air flow meter with a heat generating resistor which
permits a highly accurate air flow rate measurement regardless of
the variation in the ground potential caused by the current flowing
through the heat generating resistor.
In order to achieve the above object, the ratiometric output type
air flow meter with a heat generating resistor in which the heat
generating resistor is disposed in an air flow passage and is supplied
with a heating current to heat the same up to a predetermined temperature
and which includes a ratiometric circuit converting an air flow
rate signal V0 detected based on the heating current to a signal
proportionated to an external reference voltage Vref and outputting
the same, comprises a correction circuit which is designed to correct
the conversion error through the ratiometric circuit which is caused
by the heating current flowing through the wiring for grounding.
When the current flowing through the heat generating resistor increases,
the reference potential drop due to the wiring resistance increases
accordingly, thereby, the ground potential Vg serving as the reference
potential for the ratiometric circuit becomes higher than the ground
potential Vgnd serving as the reference potential for an engine
control unit (ECU). Therefore, the amplification rate of the ratiometric
circuit decreases, causing errors therein and reduced accuracy in
output signals of the concerned air flow meter reduces.
Accordingly, when the conversion errors in the ratiometric circuit
are corrected with the correction circuit, the accuracy reduction
in the output signals of the air flow meter is prevented.
Preferably, in the above described ratiometric output type air
flow meter with a heat generating resistor, the ratiometric circuit
comprises a divider circuit which divides the external reference
voltage Vref to obtain an input voltage Vex and a multiplier which
receives the input voltage Vex and the air flow rate signal V0 as
the input signals thereof and outputs an output signal Vout as the
output of the ratiometric circuit, and the correction circuit is
connected in series to the divider circuit at the side of the reference
potential in the ground wiring of the air flow meter and includes
a correction use resistor for correcting a voltage drop due to the
heating current flowing through the ground wiring.
Further preferably, in the above described ratiometric output type
air flow meter with a heat generating resistor the ratiometric circuit
comprises a divider circuit which divides the external reference
voltage Vref to obtain an input voltage Vex and a multiplier which
receives the input voltage Vex and the air flow rate signal V0 as
the input signals thereof and outputs an output signal Vout as the
output of the ratiometric circuit. The correction circuit is connected
between the division point in the divider circuit generating the
voltage Vex and an input terminal of the multiplier where the air
flow rate signal V0 is inputted and includes a correction use resistor
for correcting a voltage drop due to the heating current flowing
through the ground wiring.
Further preferably, in the above explained ratiometric output type
air flow meter with a heat generating resistor, the ratiometric
circuit comprises a divider circuit which divides the external reference
voltage Vref to obtain an input voltage Vex and a multiplier which
receives the input voltage Vex and the air flow rate signal V0 as
the input signals thereof. The correction circuit is connected in
series to the divider circuit at the side of the reference potential
in the ground wiring of the air flow meter and includes a correction
use variable resistor which can adjust a linearity of the output
signals of the air flow meter by varying the resistance value thereof
and an output characteristic adjusting circuit disposed at the output
terminal of the ratiometric circuit and for adjusting the output
characteristics of the air flow meter.
Further preferably, in the above described ratiometric output type
air flow meter with a heat generating resistor, the ratiometric
circuit comprises a divider circuit which divides the external reference
voltage Vref to obtain an input voltage Vex and a multiplier which
receives the input voltage Vex and the air flow rate signal V0 as
the input signals thereof. The correction circuit is connected between
the division point in the divider circuit generating the voltage
Vex and an input terminal of the multiplier where the air flow rate
signal V0 is inputted and includes a correction use variable resistor
which can adjust a linearity of the output signals of the air flow
meter by varying the resistance value thereof and an output characteristic
adjusting circuit disposed at the output terminal of the ratiometric
circuit and for adjusting the output characteristics of the air
flow meter.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects, advantages and novel features of the present invention
will become apparent from the following detailed description of
the invention when considered in conjunction with the accompanying
drawings.
FIG. 1 is a schematic circuit diagram of a ratiometric output type
air flow meter with a heat generating resistor representing a first
embodiment according to the present invention;
FIG. 2 is a schematic circuit diagram of a ratiometric output type
air flow meter with a heat generating resistor representing a second
embodiment according to the present invention;
FIG. 3 is a schematic circuit diagram of a ratiometric output type
air flow meter with a heat generating resistor representing a third
embodiment according to the present invention,
FIG. 4 is a schematic circuit diagram of a ratiometric output type
air flow meter with a heat generating resistor representing a fourth
embodiment according to the present invention; and
FIG. 5 is a schematic circuit diagram of the already described
conventional ratiometric output type air flow meter with a heat
generating resistor.
DETAILED DESCRIPTION OF THE DRAWINGS
The embodiments according to the present invention are explained
with reference to the drawings.
FIG. 1 shows a ratiometric output and heat generating resistor
type air flow meter 40. A basic difference of this air flow meter
40 from the conventional air flow meter 50 as shown in FIG. 5 is
the provision of a correction circuit including a correction use
resistor 6 disposed between the junction of a current detection
resistor 1 and an invariable resistor 4 and a wiring resistance
5 but the other circuit structure of FIG. 1 embodiment is substantially
the same as that of FIG. 5.
Namely, the ratiometric circuit 10 in FIGS. 1 and 5 is provided
with the divider circuit which is constituted by the invariable
resistors 3 and 4 connected in series each other and divides the
external reference voltage Vref and the multiplier circuit 11. The
invariable resistor 3 is grounded via the invariable resistor 4
and through the ground wiring.
To one of the input terminals of the multiplier circuit 11 is applied
voltage V0 representing the voltage at the junction of the heat
generating resistor 2 and the current detection use resistor 1 and
appearing at the output signal of the operational amplifier 61 and
to the other input terminal of the multiplier circuit 11 is applied
voltage Vex divided by the invariable resistors 3 and 4 namely
the voltage appearing at the junction of the invariable resistors
3 and 4. The output signal of the multiplier circuit 11 is outputted
to the ECU 20 as the output signal Vout of the ratiometric circuit
10.
The correction use resistor 6 is connected at the side of the reference
potential of the ground wiring, namely at the junction of the invariable
resistor 4 and the current detection resistor 1 and is grounded
with the ground wiring resistance 5. Thus, the voltage drop caused
by the ground wiring is compensated by the correction use resistor
6.
Namely, in the first embodiment according to the present invention
as shown in FIG. 1 with the provision of the correction circuit
including the correction use resistor 6 the voltage (Vex-Vg) representing
the input to the multiplier circuit 11 is kept constant regardless
to the magnitude of the heating current Ia flowing through the heat
generating resistor 2 which will be explained below.
When assuming that the potential between the current detection
use resistor 1 and the correction use resistor 6 is Va, the current
flowing through the invariable resistors 3 and 4 is Ib and the resistance
values of the resistors 3 4 5 and 6 are respectively R3 R4 R5
and R6 the current Ib and the potential Va are respectively expressed
by the following equations (4) and (5);
Ib=(Vref-Va)/(R3+R4) (4)
From the above equations (4) and (5) the following equations (6)
and (7) are derived.
When substituting Va expressed by the equation (7) for the equation
(4), the following equation (8) is obtained
When calculating Vex-Vg by making use of Ib determined by the equation
(8), the following equation (9) is obtained.
Accordingly, in order to keep Vex-Vg constant regardless of the
magnitude of the heating current Ia, it is necessary to satisfy
the following equation (10);
Namely, if the resistance value R6 of the resistor 6 is selected
to satisfy the following equation (11), the above condition expressed
by the equation (10) is satisfied
As has been explained above, according to the first embodiment
of the present invention, the correction use resistor 6 is connected
between the junction of the invariable resistor 4 which is connected
to the other input terminal of the multiplier circuit 11 in the
ratiometric circuit 10 with the current detection use resistor 1
connected to the heat generating resistor 2 and the wiring resistance
5 and the resistance value R6 of the correction use resistor 6
is selected to be equal to (R4.multidot.R5/R3), consequently, the
voltage value (Vex-Vg) representing one of the inputs for the multiplier
circuit 11 is kept constant regardless to the magnitude of the heating
current Ia. Thereby, a ratiometric output and heat generating resistor
type air flow meter which permits a highly accurate air flow rate
measurement is realized without being affected by variation of the
ground potential due to the current flowing through the heat generating
resistor 2.
In FIG. 1 the discrete correction use resistor 6 is connected
to the ground wiring. However, alternatively the ground wiring can
be elongated to increase the resistance of the ground wiring and
to add the corresponding resistance value R6 to the original wiring
resistance value.
FIG. 2 shows a ratiometric output and heat generating resistor
type air flow meter 40 what is different from the conventional air
flow meter 50 as shown in FIG. 5 by the provision of a correction
circuit including a correction use resistor 7 disposed between the
output terminal of the operational amplifier 61 and the junction
of the invariable resistors 3 and 4. Otherwise, the structure of
FIG. 2 is substantially the same as that of FIG. 5.
With the provision of the above correction use resistor 7 the
voltage value (Vex-Vg) representing the input to the multiplier
circuit 11 is kept constant regardless to the magnitude of the current
flowing through the heat generating resistor 2 which will be explained
hereinbelow.
When assuming that the resistance value of the correction use resistor
7 is R7 the currents flowing through the invariable resistors 3
and 4 and the resistor 7 are respectively as IR3 IR4 and IR7 the
current IR4 is expressed by the following equation (12);
From the above equation (12) the following equation (13) is obtained;
From the above equation (13) the following equation (14) is obtained;
From the above equation (14) and the relationship Vg=R5.multidot.Ia,
the following equation (15) is obtained;
Wherein, since V0=(R1+R5).multidot.Ia, the following equation (16)
is obtained;
Accordingly, in order to keep the voltage value Vex-Vg expressed
by the above equation (16) constant regardless to the magnitude
of the heating current Ia, it is necessary to satisfy the following
equation (17);
Namely, if the resistance value R7 of the resistor 7 is selected
to satisfy the following equation (18), the above condition expressed
by the equation (17) is satisfied;
As has been already explained, according to the second embodiment
of the present invention, the correction use resistor 7 is connected
between the junction of the invariable resistors 3 and 4 in the
ratiometric circuit 10 namely at the voltage division point of
the reference voltage Vref and the output of the operational amplifier
61 and the resistance value R7 of the correction use resistor 7
is selected to be equal to (R1.multidot.R3/R5). The voltage value
(Vex-Vg) representing one of the inputs for the multiplier circuit
11 is kept constant regardless of the magnitude of the heating current
Ia, and thereby, a ratiometric output and heat generating resistor
type air flow meter which permits a highly accurate air flow rate
measurement is realized without being affected by variation of the
ground potential due to the current flowing through the heat generating
resistor 2.
Further, in FIG. 2 since the resistance value of the correction
use resistor 7 is large in comparison with that of the correction
use resistor 6 in FIG. 1 the adjustment thereof is easy.
FIG. 3 shows a ratiometric output and heat generating resistor
type air flow meter 40 which is different from the conventional
air flow meter 50 as shown in FIG. 5 by the provision of a correction
circuit including a correction use variable resistor 66 disposed
between the junction of a current detection resistor 1 and the invariable
resistor 4 and the wiring resistance 5 as well as the provision
of an output characteristic adjusting circuit disposed at the output
terminal of the ratiometric circuit 10 structure of FIG. 3 is substantially
the same as that of FIG. 5.
The output characteristic adjusting circuit 30 is constituted by
an operational amplifier 12 and adjusting resistors 8 and 9 and
to the non-inverting input terminal of the operational amplifier
12 is applied an output signal of the multiplier circuit 11. Further,
the inverting input terminal of the operational amplifier 12 is
connected to the junction between the variable resistor 66 and the
wiring resistance 5 via the adjusting resistor 8. Still further,
the junction between the inverting input terminal of the operational
amplifier 12 and the adjusting resistor 8 is connected to the output
terminal of the operational amplifier 12 via the adjusting resistor
9.
In this third embodiment, the dispersion in linearity of the air
flow meter 40 due to characteristics of the heat generating resistor
2 and the wiring resistance 5 is corrected by making use of the
variable resistor 66 which serves to adjust the output component
in the multiplier circuit 11 determined by the term of second degree
of the heating current Ia, however, through the adjustment of the
variable resistor 66 the constant amplification rate of the air
flow meter as a whole is lost, the output characteristic adjusting
circuit 30 is introduced to ensure the amplification rate of the
air flow meter constant.
Namely, with the variable resistor 66 the output linearity is adjusted
so that (Vex-Vg) representing the input to the multiplier circuit
11 is kept constant without being affected by the magnitude of the
heating current Ia, and with the adjusting resistors 8 and 9 the
output slope is adjusted.
As has been previously explained, according to the third embodiment
of the present invention, the correction use variable resistor 66
is connected between the junction of the invariable resistors 4
which is connected to the other input terminal of the multiplier
circuit 11 in the ratiometric circuit 10 with the current detection
use resistor 1 connected to the heat generating resistor 2 and the
wiring resistance 5 as well as the output characteristic adjusting
circuit 30 is disposed at the output terminal of the ratiometric
circuit 10. The resistance value of the correction use variable
resistor 66 is selected so that, the voltage value (Vex-Vg) representing
one of the inputs for the multiplier circuit 11 is kept constant
regardless of the magnitude of the heating current Ia, and the amplification
rate is adjusted by the output characteristic adjusting circuit
30. Thereby, a ratiometric output and heat generating resistor type
air flow meter which permits a highly accurate air flow rate measurement
is realized without being affected by variation of the ground potential
due to the current flowing through the heat generating resistor
2 and further without damaging the total output characteristic of
the air flow meter.
Further, according to the third embodiment, since the output characteristics
of the air flow meter can be adjusted by adjusting the output component
thereof determined by the term of second degree of the heating current
Ia, a further advantage can be obtained that even when such as passage
configuration of the air flow meter and the combination thereof
with the ECU are modified, the conversion characteristic for converting
the output voltage from the air flow meter 40 to air flow rate which
is stored in the ECU can be used without changes.
FIG. 4 shows a ratiometric output and heat generating resistor
type air flow meter which is different from the conventional air
flow meter 50 shown in FIG. 5 by the provision of a correction circuit
including a correction use variable resistor 77 disposed between
the output terminal of the operational amplifier 61 and the junction
of the invariable resistors 3 and 4 as well as the provision of
the output characteristic adjusting circuit 30 disposed at the output
terminal of the ratiometric circuit 10. Otherwise, the circuit structure
of FIG. 4 embodiment is substantially the same as that of FIG. 5.
The output characteristic adjusting circuit 30 is constituted in
the same manner as that shown in FIG. 3 and the adjusting resistor
8 is grounded via the wiring resistance 5.
In this fourth embodiment, with the variable resistor 77 the output
curve characteristics is adjusted so that the voltage value (Vex-Vg)
representing the input to the multiplier circuit 11 is kept constant
without being affected by the magnitude of the heating current Ia.
The output slope thereof is adjusted by the adjusting resistors
8 and 9 in the output characteristic adjusting circuit 30 while
setting the outputs at the upper and lower control points constant
by making use of the variable resistor 77 and the output characteristic
adjusting circuit 30.
As has been explained above, according to the fourth embodiment
of the present invention, since the correction use variable resistor
77 is connected between the output terminal of the operational amplifier
61 and the junction of the invariable resistors 3 and 4 as well
as the output characteristic adjusting circuit 30 is disposed at
the output terminal of the ratiometric circuit 10. The resistance
value of the correction use variable resistor 77 is selected so
that, the voltage value (Vex-Vg) representing one of the inputs
for the multiplier circuit 11 is kept constant regardless of the
magnitude of the heating current Ia, and the amplification rate
is adjusted by the output characteristic adjusting circuit 30. Thereby,
a ratiometric output and heat generating resistor type air flow
meter which permits a highly accurate air flow rate measurement
is realized without being affected by variation of the ground potential
due to the current flowing through the heat generating resistor
2 and further without damaging the total output characteristic of
the air flow meter.
As has been explained herein, with the present invention the following
advantages are obtained.
Since the input signals to the multiplier circuit in the ratiometric
circuit can be kept constant without being affected by the magnitude
of the heating current, a ratiometric output and heat generating
resistor type air flow meter which permits a highly accurate air
flow rate measurement is thereby realized without being affected
by variation of the ground potential due to the current flowing
through the heat generating resistor.
Further, since the input signals to the multiplier circuit in the
ratiometric circuit can be kept constant without being affected
by the magnitude of the heating current, as well as the output characteristic
adjusting circuit is disposed at the output terminal of the ratiometric
circuit, a ratiometric output and heat generating resistor type
air flow meter which permits a highly accurate air flow rate measurement
is thereby realized without being affected by variation of the ground
potential due to the current flowing through the heat generating
resistor and further without damaging the total output characteristic
of the air flow meter.
Still further, with the provision of the correction use variable
resistor, the output characteristics of the air flow meter can be
adjusted by adjusting the output component in the multiplier circuit
determined by the term of second degree of the heating current.
A further advantage can be obtained that even when such as passage
configuration of the air flow meter and the combination thereof
with the ECU are modified, the conversion characteristic for converting
the output voltage from the air flow meter to air flow rate which
is stored in the ECU can be used without changes.
The foregoing disclosure has been set forth merely to illustrate
the invention and is not intended to be limiting. Since modifications
of the disclosed embodiments incorporating the spirit and substance
of the invention may occur to persons skilled in the art, the invention
should be construed to include everything within the scope of the
appended claims and equivalents thereof. |