Measuring and testing – Volume or rate of flow – Thermal type
Reexamination Certificate
1999-06-30
2002-03-19
Noori, Max (Department: 2855)
Measuring and testing
Volume or rate of flow
Thermal type
Reexamination Certificate
active
06357293
ABSTRACT:
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 V
0
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 V
0
, the proportion constant thereof is A and the external reference voltage for the circuit is Vref;
Vout=A×V
0
×(Vref−Vg) (1)
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);
Err=−A×V
0
×Vg (2)
Since the reference ground potential Vg corresponds to a voltage drop Ia×R
5
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 R
5
), and the heating current Ia is proportional to a voltage V
2
detected by the current detection resistor
1
, the reference ground potential Vg is proportional to V
2
×R
5
.
Further, since the air flow rate detection voltage V
0
is also proportional to the heating current Ia and the heating current Ia is proportional to the voltage V
2
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 V
2
;
Err=−B×(V
2
)
2
×R
5
(3)
Even if the error Err expressed by the quadratic function of the voltage V
2
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 V
0
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 ratiom
Akamatsu Masuo
Ando Ryo
Crowell & Moring LLP
Hitachi , Ltd.
Noori Max
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