Measuring and testing – Volume or rate of flow – Thermal type
Reexamination Certificate
1998-10-30
2002-01-01
Patel, Harshad (Department: 2855)
Measuring and testing
Volume or rate of flow
Thermal type
Reexamination Certificate
active
06334359
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a thermal-type flowmeter for detecting a flow rate of a fluid by using heat-sensitive resistors. More particularly, the present invention is concerned with a signal conditioning interface circuit for a thermal-type flowmeter adapted for detecting a flow rate of intake air in an internal combustion engine, which circuit is designed for processing an output signal of a heat-sensitive flow sensor of the thermal-type flowmeter to thereby derive a detection voltage signal which indicates the flow rate and which is to undergo analogue-to-digital conversion for generating a digital signal to be supplied to an electronic control unit for the purpose of controlling fuel injection or other operation in the internal combustion engine.
2. Description of Related Art
In the thermal-type flowmeter designed for outputting a current indicating a flow rate of a fluid such as intake air in an internal combustion engine, it is known that the intake air flow signal outputted from the sensor is converted into a voltage signal for analogue-to-digital conversion by transmitting the intake air flow signal in the form of a current signal to a circuit stage preceding to an analogue-to-digital converter so that the flow rate information can be transmitted with high fidelity or reliability even if potential variation takes place in the thermal-type flowmeter and/or the electronic control unit such as the electronic fuel injection control unit.
For having better understanding of the invention, description will first be made of a conventional thermal-type flowmeter known heretofore by reference to
FIG. 8
to
FIG. 10
, in which
FIG. 8
is a circuit diagram showing a circuit configuration of a conventional thermal-type flowmeter disclosed, for example, in a Japanese Unexamined Patent Application Publication No. 216420/1990 (JP-A-2-216420),
FIG. 9
is a circuit diagram showing another conventional thermal-type flowmeter, and
FIG. 10
is a view for illustrating graphically input-versus-output characteristics of the thermal-type flowmeter shown in FIG.
9
.
Now referring to
FIG. 8
, a conventional thermal-type flowmeter denoted generally by reference numeral
1
includes an operational amplifier
1
b
having a non-inverting input terminal (+) to which applied is a flow-rate indicating voltage signal V
afs
outputted from an amplifier
1
a
constituting an output part of an air flow sensor (not shown) installed, for example, in an intake pipe of an internal combustion engine (not shown either). On the other hand, the output terminal of the operational amplifier
1
b
is connected to a base electrode of a transistor
1
c
while an inverting input terminal (−) of the operational amplifier
1
b
is connected an emitter electrode of the NPN-transistor
1
c
and one end of a reference resistor R
e
having the other end connected to a potential source of negative polarity. The collector of the transistor
1
c
is connected to a terminal of a reference voltage V
ref
in a fuel injection control unit
2
by way of a current detecting resistor R
c
. Further, an analogue-to-digital converter (hereinafter also referred to as the A/D converter in short) incorporated in the fuel injection control unit
2
has an analogue input terminal to which the reference voltage V
ref
is applied by way of a current detecting resistor R
c
.
In operation, when the flow-rate indicating voltage signal V
afs
is inputted to the non-inverting input terminal (+) of the operational amplifier
1
b
from the amplifier
1
a
, a base current I
b
flows to the base of the NPN-transistor
1
c
from the output terminal of the operational amplifier
1
b
. In that case, an emitter current I
e
flows through the reference resistor R
e
, generating an emitter voltage V
e
which is fed back to the inverting input terminal of the operational amplifier
1
b
. As a result of this, the emitter voltage V
e
becomes equal to the voltage level of the flow-rate voltage signal V
afs
.
In this conjunction, it should be mentioned that the emitter current I
e
may be regarded as being equal to the collector current I
c
so far as the current amplification factor of the NPN-transistor
1
c
is selected at a sufficiently large value. Accordingly, the input voltage V
c
which is applied to the analogue input terminal of the analogue-to-digital converter and which is given by
V
c
=V
ref
−R
c
. I
c
can be regarded as bearing a proportional relation to the flow-rate indicating voltage signal V
afs
.
Because the flow-rate indicating voltage signal V
afs
is outputted after having been converted into the collector current I
c
, the intake air flow signal indicating a flow rate of the intake air can be converted into an electric signal to be transmitted to the A/D converter with high fidelity without being affected by variations of potentials which may occur in the thermal-type flowmeter
1
and/or the fuel injection control unit
2
.
Next referring to
FIG. 9
which shows a circuit configuration of another conventional thermal-type flowmeter
1
, the flow-rate indicating voltage signal V
afs
outputted from the amplifier
1
a
constituting a part of the sensor circuit is applied to the non-inverting input terminal (+) of the operational amplifier
1
b
, as in the case of the thermal-type flowmeter
1
shown in FIG.
8
. The inverting input Terminal C-) of the operational amplifier
1
b
is connected to an emitter terminal of an NPN-transistor
1
c
and additionally to one end of a first reference resistor
1
e
which has the other end connected to the ground potential. The collector terminal of the NPN-transistor
1
c
is connected to a reference voltage V
cc
of a power supply circuit
1
d
by way of a first current detecting resistor
1
f.
A voltage V
2
making appearance across the first current detecting resistor
1
f
as a voltage drop is applied to a non-inverting input terminal of an operational amplifier
1
g
which has a non-inverting input terminal connected to a junction between the collector terminal of the NPN-transistor
1
c
and a first current detecting resistor
1
f
, and an output terminal connected to the base of a PNP-transistor
1
h
. Further, the inverting input terminal of the operational amplifier
1
g
is connected to an emitter of the PNP-transistor
1
h.
The emitter electrode of the PNP-transistor
1
h
is connected to the reference voltage V
cc
of the power supply circuit
1
d
by way of a second reference resistor R
e
while the collector electrode of the PNP-transistor
1
h
is connected to an analogue input terminal of an A/D converter incorporated in a fuel injection control unit
2
, wherein the analogue input terminal is connected to a ground potential terminal of the A/D converter by way of a second current detecting resistor R
c
. Owing to the circuit arrangement described above, the flow-rate indicating voltage signal V
afs
can be generated ultimately as a voltage drop V
c
making appearance across the second current detecting resistor R
c
. The voltage signal V
c
is inputted to the A/D converter.
In operation, the current which flows through the NPN-transistor
1
c
and which bears a proportional relation to the output voltage of the operational amplifier
1
b
undergoes a current-to-voltage conversion (also referred to as the I/V conversion) through the first current detecting resistor
1
f
connected to the collector terminal of the NPN-transistor
1
c
. Thus, inputted to the non-inverting input terminal of the operational amplifier
1
g
is a detection voltage V
2
resulting from subtraction of a voltage derived from the reference voltage V
cc
through the I/V conversion, i.e.,
V
2
=V
cc
−I
.1
f.
Further, a voltage generated across the second reference resistor R
e
is fed back to the inverting input terminal of the operational amplifier
1
g
. Accordingly, the detection voltage V
2
can assume a value or level which is in proportion to the level of the flow-rate indicating voltage s
Mitsubishi Denki & Kabushiki Kaisha
Patel Harshad
Sughrue & Mion, PLLC
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