Heat-sensitive type flow sensor

Details Heat-sensitive type flow sensor Heat-sensitive type flow sensor

1998-11-10

2001-03-27

Patel, Harshad (Department: 2855)

Measuring and testing

Volume or rate of flow

Thermal type

C073S204180

Reexamination Certificate

active

06205854

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a heat-sensitive type flow sensor for detecting a flow rate of a fluid by using heat-sensitive resistors.
2. Description of Related Art
In a conventional heat-sensitive type flow sensor known heretofore, a bridge circuit is employed which is constituted by a plurality of resistance elements inclusive of a first heat-sensitive resistor for detecting an atmospheric or ambient temperature and a second heat-sensitive resistor which is disposed within a passage through which a fluid flows such as, for example, an intake pipe of an internal combustion engine and heated electrically. To this end, a heating current supplied to the second heat-sensitive resistor for electrically heating it is so controlled that the temperature thereof is held higher than the ambient temperature by a predetermined value, wherein the quantity of heat which is deprived of the heat-sensitive resistor by the fluid whose flow rate is to be measured is detected in terms of change of the heating current flowing through the second heat-sensitive resistor. Thus, the flow rate of the fluid such as the intake air can be detected on the basis of the change of the heating current as detected.
For having better understanding of the present invention, description will first be made in some detail of a conventional heat-sensitive type flow sensor.
FIG. 5
is a circuit diagram showing a circuit configuration of a hitherto known heat-sensitive type flow sensor such as disclosed, for example, in Japanese Unexamined Patent Application Publication No. 117436/1995 (JP-A-7-117436). Referring to the figure, the conventional heat-sensitive type flow sensor is comprised of a temperature control circuit
10
, an operational amplifier circuit
15
, a first constant current circuit
16
, a second constant current circuit
17
, a first constant current control circuit
18
, a third constant current circuit
19
, a fourth constant current circuit
20
and a second constant current control circuit
37
A, details of which will be described below.
At first, description will be directed to the temperature control circuit
10
. As can be seen in
FIG. 5
, the temperature control circuit
10
includes a bridge circuit constituted by resistors R
1
and R
2
, a flow rate detecting heat-sensitive resistor R
h
and an ambient temperature detecting heat-sensitive resistor R
c
, wherein a junction f between the resistor R
1
and the ambient temperature detecting heat-sensitive resistor R
c
and a junction b between the resistor R
2
and the flow rate detecting heat-sensitive resistor R
h
of the bridge circuit are connected, respectively, to input terminals of a differential amplifier
101
which has an output terminal connected to the base of a transistor
102
, the emitter of which is connected to a junction a between the flow rate detecting heat-sensitive resistor R
h
and the ambient temperature detecting heat-sensitive resistor R
c
while the collector of the transistor
102
is connected to a positive or plus electrode of a DC power source
103
having the other electrode connected to the ground potential.
Next, description will turn to operation of the temperature control circuit
10
. When the voltages at the junctions b and f become equal to each other, the bridge circuit assumes an equilibrium or balanced state. In this state, an electric current I
h
corresponding to the flow rate of a fluid concerned flows through the flow rate detecting heat-sensitive resistor R
h
. The output voltage V
H
at the junction b can be given by a product of the current I
h
and the resistance value of the resistor R
2
. This voltage V
H
is made use of as a flow rate signal.
With the view to compensating for dispersions in the flow-rate detection due to dispersions of resistance values of the heat-sensitive resistors R
h
and R
c
and the resistors R
1
and R
2
as well as temperature coefficients thereof, the detection output value at a predetermined flow rate (ordinarily a relatively low flow rate) is set as a target or desired value by adjusting the resistance value of the resistor R
1
for thereby changing or translating the detection characteristic of the heat-sensitive type flow sensor correspondingly.
Description will now turn to the operational amplifier circuit
15
which is designed to process the flow rate signal outputted from the temperature control circuit
10
. The operational amplifier circuit
15
includes an operational amplifier
106
having an inverting input terminal and an output terminal interconnected by way of a feedback resistor R
13
and an input resistor R
11
having one end connected to the junction b of the bridge circuit mentioned above. The other end of the input resistor R
11
is connected to the non-inverting input terminal of the operational amplifier
106
.
The first constant current circuit
16
includes a transistor
110
having an emitter coupled to a line of a reference source voltage V
ref
by way of a resistor R
25
and a collector connected to the non-inverting input terminal of the operational amplifier
106
. On the other hand, the second constant current circuit
17
includes a transistor
111
having an emitter electrode connected to the line of the reference source voltage V
ref
by way of a resistor R
26
and a collector connected to the inverting input terminal of the operational amplifier
106
. The base electrodes of both the transistors
110
and
111
are connected in cascade and connected in common to an output terminal of an operational amplifier
108
which constitutes a part of the first constant current control circuit
18
which will be described below.
The first constant current control circuit
18
mentioned above is so designed as to control the output current values I
16
and I
17
of the first and second constant current circuits
16
and
17
, respectively, on the basis of the preset reference source voltage V
ref
. To this end, the first constant current control circuit
18
is constituted by resistors R
20
, R
21
, R
22
, R
23
and R
24
and an operational amplifier
108
connected in such a manner as can be seen in FIG.
5
.
Further, the heat-sensitive type flow sensor includes the third constant current circuit
19
of a structure similar to that of the first constant current circuit
16
, the fourth constant current circuit
20
implemented in an essentially same structure as that of the second constant current circuit
17
and the second constant current control circuit
37
A implemented similarly to the first constant current control circuit
18
.
Next, description will turn to operation of the operational amplifier circuit
15
. The value or voltage level of the input voltage V
p
applied to the non-inverting input terminal of the operational amplifier
106
can be determined by subtracting from the output voltage V
H
of the temperature control circuit
10
a voltage drop making appearance across the resistor R
11
due to the current I
16
flowing through the resistor R
11
by way of the first constant current circuit
16
. Namely, the input voltage V
p
mentioned above can be given by the following expression (1):
V
p
=V
H
−(
R
11
×I
16
)  (1)
On the other hand, the value or voltage level of the input voltage V
n
applied to the inverting input terminal of the operational amplifier
106
can be determined by subtracting from the output voltage V
o
of the operational amplifier circuit
15
a voltage drop making appearance across the resistor R
13
due to the current I
17
flowing through the resistor R
13
by way of the second constant current circuit
17
. Namely, the input voltage V
p
mentioned above can be given by the following expression (2):
V
n
=V
o
−(
R
13
×I
17
)  (2)
The operational amplifier
106
controls the output voltage V
o
of the operational amplifier circuit
15
such that the condition given by V
p
=V
n
is satisfied. Thus, the output voltage V
o
of the heat-sensitive type flow sensor can be given by the

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