Measuring and testing – Volume or rate of flow – Thermal type
Reexamination Certificate
2002-10-22
2004-06-08
Lefkowitz, Edward (Department: 2855)
Measuring and testing
Volume or rate of flow
Thermal type
Reexamination Certificate
active
06745625
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a fluid flow rate measuring apparatus used at a location requiring measurement of a flow rate of fluid such as air, for example, at an engine control device in a motor vehicle or an air conditioning appliance.
2. Description of the Prior Art
FIG. 13
is a fragmentary sectional view of a conventional fluid flow rate measuring apparatus disclosed in Japanese Patent Laid-Open Publication No. 11-326003(1999). The conventional fluid flow rate measuring apparatus includes a silicon substrate
101
, an air space
102
defined in the silicon substrate
101
by etching, thin film members, i.e., thin-wall portions
103
and
104
bridged over the air space
102
, first and second heating elements
105
and
106
and first and second temperature detecting elements
107
and
108
. The heating elements
105
and
106
and the temperature detecting elements
107
and
108
are made of a temperature sensitive resistance material whose resistance value varies according to temperature. For example, platinum is used as the temperature sensitive resistance material. The first and second heating elements
105
and
106
are manufactured so as to have substantially identical resistance values and temperature coefficients. The first and second temperature detecting elements
107
and
108
are also manufactured so as to have substantially identical resistance values and temperature coefficients.
In
FIG. 13
, the first heating element
105
and the first temperature detecting element
107
are spaced away from each other in order to facilitate understanding of their arrangements but are actually formed at substantially identical locations so as to be held in close contact with each other thermally. Likewise, the second heating element
106
and the second temperature detecting element
108
are spaced away from each other in order to facilitate understanding of their arrangements but are actually formed at substantially identical locations so as to be held in close contact with each other thermally.
FIG. 14
shows a circuit of the conventional fluid flow rate measuring apparatus of FIG.
13
. The circuit includes fixed resistances
109
and
110
which form a bridge circuit
117
with the first and second temperature detecting elements
107
and
108
, a comparator
111
for comparing intermediate potentials
118
and
119
of the bridge circuit
117
, an inverter
112
, electronic switches
113
and
114
, a power source
115
and a fluid flow path
116
. The conventional circuit is operated as follows. When a difference between the intermediate potentials
118
and
119
is produced in case there is no flow of fluid, the comparator
111
detects this difference between the intermediate potentials
118
and
119
so as to control the electronic switches
113
and
114
. If the fixed resistances
109
and
110
are set to have an identical resistance value, the first and second temperature detecting elements
107
and
108
also have an identical resistance value and thus, have an identical temperature. In case there is no flow of the fluid, on-state periods of the electronic switches
113
and
114
become identical with each other and thus the ratio of electric power supplied to the first heating element
105
and to the second heating element
106
is 50%:50%.
Subsequently, a case in which the fluid is flowing is described. When the fluid flows in the direction of the arrow in
FIG. 14
, heat is transferred from the first heating element
105
and the first temperature detecting element
107
to the fluid, so that a temperature of the first temperature detecting element
107
drops. The fluid which absorbed heat from the first heating element
105
and the first temperature detecting element
107
at an upstream side transfers the heat to the second temperature detecting element
108
and thus, a temperature of the second temperature detecting element
108
rises. Therefore, the intermediate potential
118
becomes lower than the intermediate potential
119
and thus, an output of the comparator
111
is at high level. Accordingly, the electronic switch
113
is turned on and thus, electric current flows through the first heating element
105
. As a result, the first heating element
105
is heated by Joule heat so as to raise the temperature of the first temperature detecting element
107
. Since the first heating element
105
and the first temperature detecting element
107
are cooled by the fluid flow, an on-state period of the electronic switch
113
, which should elapse before the intermediate potential
118
exceeds the intermediate potential
119
, becomes longer than that of a case in which there is no flow of the fluid. At the time the intermediate potential
118
has risen so as to exceed the intermediate potential
119
, the electronic switch
114
is turned on and thus, electric current flows through the second heating element
106
. Therefore, the second heating element
106
is heated by Joule heat so as to raise the temperature of the second temperature detecting element
108
and thus, the intermediate potential
119
rises. Since the second heating element
106
and the second temperature detecting element
108
are warmed by the fluid flow, an on-state period of the electronic switch
114
, which should elapse before the intermediate potential
119
exceeds the intermediate potential
118
, becomes shorter than that of the case in which there is no flow of the fluid. At the time the intermediate potential
119
has exceeded the intermediate potential
118
, the electronic switch
114
is turned off and the electronic switch
113
is turned on, so that electric current flows through the first heating element
105
again.
By repeating the above mentioned operations, the intermediate potentials
118
and
119
ire held equally again. Therefore, even if there is a flow of the fluid, the temperatures of the first and second temperature detecting elements
107
and
108
are controlled equally. At this time, quantity of electric power supplied to the first heating element
105
becomes larger than that supplied to the second heating element
106
. For example the ratio of the quantity of electric power supplied to the first heating element
105
and to the second heating element
106
is 60%:40%.
FIG. 15
shows an output waveform in the above mentioned operations of the conventional fluid flow rate measuring apparatus of FIG.
13
. An output voltage Vout in
FIG. 14
has a pulse waveform shown in FIG.
15
. As a flow rate of the fluid rises further, quantity of electric power supplied to the first heating element
105
is increased more. Hence, in
FIG. 15
, an interval t1 increases and an interval t2 decreases. Therefore, if a difference d of duty ratios is measured by using the following equation (1), an output dependent on the flow rate can be obtained.
d=
(
t
1
−t
2)/(
t
1
+t
2) (1)
FIG. 16
having an ordinate representing output and an abscissa representing flow rate shows such output characteristics. Furthermore, the difference d of duty ratios in the equation (1) can be expressed as follows by using a heat release value P1 of the first heating element
105
and a heat release value P2 of the second heating element
106
.
(
t
1
−t
2)/(
t
1
+t
2)=(
P
1
−P
2)/(
P
1
+P
2) (2)
In this technique, if a back flow occurs, the interval t1 decreases and the interval t2 increases, so that the output is inverted and thus, it is possible to detect the back flow.
FIG. 17
shows dependency of temperature distribution on flow rate in the conventional fluid flow rate measuring apparatus of FIG.
13
. In
FIG. 17
, flow rates v1, v2 and v3 have the relation of (0<v1<v2<v3). Temperature drop of the first temperature detecting element
107
caused by increase of the flow rate is larger than temperature rise of the second temperature detecting element
108
. Therefore, if the temperatures of the first and second temperature detect
Lefkowitz Edward
Leydig , Voit & Mayer, Ltd.
Mitsubishi Denki & Kabushiki Kaisha
Thompson Jewel V.
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