Measuring and testing – Volume or rate of flow – Thermal type
Reexamination Certificate
2000-11-29
2004-02-03
Lefkowitz, Edward (Department: 2855)
Measuring and testing
Volume or rate of flow
Thermal type
Reexamination Certificate
active
06684693
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a heat generation type flow sensor destined for applications where measurement of a flow rate (also referred to simply as flow) of a fluid medium such as intake air in an internal combustion engine of a motor vehicle is required for performing, for example, an air/fuel ratio control for the internal combustion engine. More particularly, the present invention is concerned with a heat generation type flow sensor which can ensure an enhanced detection sensitivity and a high reliability.
2. Description of Related Art
For better understanding of the concept underlying the present invention, description will first be made of conventional heat generation type flow sensors known heretofore by reference to the drawings.
FIG. 12
shows in a top plan view a flow measuring element employed in a conventional heat generation type flow sensor, as described, for example, in Japanese PCT Application Publication No. 500490/1998, and
FIG. 13
is a circuit diagram showing an equivalent circuit of a flow-rate measuring bridge circuit in which the heat generation type flow sensor is employed.
Referring to
FIG. 12
, the measuring element is comprised of a substrate
120
, and a diaphragm
110
formed on the substrate
120
. Provided on the diaphragm
110
are a heat generating resistor
112
, a pair of first and second temperature detecting resistors
113
and
114
, another pair of third and fourth temperature detecting resistors
115
and
116
, and a connecting resistor
117
for interconnecting the third and fourth temperature detecting resistors
115
and
116
. The diaphragm
110
is heated to a predetermined temperature by means of the heat generating resistor
112
. Assuming that a fluid medium such as the air flows in the direction indicated by an arrow in
FIG. 12
, the diaphragm
110
is subjected to cooling by the flow of the fluid medium. In this conjunction, it is noted that the temperature detecting resistors
113
and
115
located upstream of the heat generating resistor
112
are cooled more intensively than the temperature detecting resistors
114
and
116
disposed downstream of the heat generating resistor
112
. Thus, by detecting the difference in temperature between the upstream and downstream temperature detecting resistors, the flow rate of the fluid medium can be measured.
Next, referring to
FIG. 13
, description will be directed to the basic operation of the flow-rate measuring circuit in which the conventional heat generation type flow sensor is employed. As can be seen in
FIG. 13
, the first and second temperature detecting resistors
113
and
114
cooperate to form a first measuring bridge arm having an intermediate tap
133
. On the other hand, the third and fourth temperature detecting resistors
115
and
116
and the connecting resistor
117
cooperate to form a second measuring bridge arm having two taps
134
and
135
. The taps
134
and
135
are connected in series by means of adjusting resistors
145
and
146
, wherein the serial circuitry is connected in parallel to the connecting resistor
117
with a tap
147
being led out from a junction between the adjusting resistors
145
and
146
.
A tap
131
led out from a junction between the first temperature detecting resistor
113
and the fourth temperature detecting resistor
116
is connected to a power source (voltage source) while a tap
132
led out from a junction between the second temperature detecting resistor
114
and the third temperature detecting resistor
115
is connected to the ground. Parenthetically, the taps
131
,
132
,
133
,
134
and
135
correspond to bonding pads of the measuring element shown in a top plan view of
FIG. 14
, as described later on. By adjusting resistance values of the adjusting resistors
145
and
146
, the zero point of the flow-rate measuring bridge circuit can be adjusted.
In the flow sensor of the so-called temperature difference detection type structured as described above, temperature lowering at the upstream side of the heat generating resistor
112
is significant when the flow rate of the fluid medium is in a low range, presenting thus a high flow sensitivity. However, as the flow rate of the fluid medium increases, the temperature difference between the upstream side and the downstream side of the heat generating resistor
112
decreases with the flow sensitivity being correspondingly lowered. Ordinarily, no remarkable dependency is observed in the relations between the flow sensitivity on one hand and the sizes of the heat generating resistor and the diaphragm on the other hand. In general, the heat generation type flow sensor is practically so designed that the width of the strip-like heat generating resistor does not exceed one third (⅓) of the width of the diaphragm with a view to reducing the power consumption.
Furthermore, since such feedback control is ordinarily adopted that the temperature of the heat generating resistor
112
remains constant regardless of variation of the flow rate of the fluid medium, the temperature detecting resistors
113
,
114
,
115
and
116
tend to incur error in the detected flow rate due to a thermal lag in the response to the change or variation of the flow rate even though high responsivity of the heating current can be assured.
FIG. 14
shows in a top plan view a fluid flow measuring element
201
employed in another conventional heat generation type flow sensor described in Japanese Patent Application Laid-Open Publication No. 311750/1998 (JP-A-H10-311750). Referring to
FIG. 14
, the measuring element
201
is comprised of a substrate
220
and a diaphragm
210
formed on the substrate
220
. Formed on the diaphragm
210
are heating conductors
202
a
and
202
b
and a temperature detecting resistor
204
. Additionally, a fluid temperature detecting resistor
207
is deposited on the substrate
220
. These resistance elements are connected to an external circuit
214
(see
FIG. 15
) by way of bonding pads
330
a
,
330
b
,
330
c
,
330
d
,
330
e
,
330
f
and
330
g.
As is shown in
FIG. 15
, the measuring element
201
includes a supporting member
213
b
on which the fluid temperature detecting resistor
207
is fixedly supported so that both surfaces of the fluid temperature detecting resistor
207
are exposed directly to the air flow. Further, mounted fixedly on the supporting member
213
b
is the external circuit
214
which is electrically connected to the measuring element
201
by means of bonding wires
216
. Besides, the external circuit
214
and the wire-bonded portion (i.e., interconnected portion of the bonding wires
216
, the measuring element
201
and the external circuit
214
) are covered hermetically by a cap member
213
a
for the purpose of protection of the wire-bonded portion.
Turning back to
FIG. 14
, the heating current is fed to the heat generating resistors
202
a
and
202
b
so as to keep the temperature of the temperature detecting resistor
204
higher than that of the fluid temperature detecting resistor
207
by a predetermined temperature. Thus, the flow rate of the fluid medium such as the air or the like can be detected on the basis of the heating current flowing through the heat generating resistors
202
a
and
202
b
. The heat generating resistors
202
a
and
202
b
are connected in series to each other so that the same heating current flows through both the heat generating resistors
202
a
and
202
b
. Accordingly, by comparing difference in voltage between the upstream heat generating resistor
202
a
and the downstream heat generating resistor
202
b
, the direction of the fluid or air flow can be determined.
The flow sensor of heating current detection type structured as described above can certainly exhibit an enhanced responsivity to the change of the flow rate. However, this type sensor suffers a problem that the sensitivity is low in a low range of flow rate because of nonnegligible heat losses due to heat conduction to the substrate
220
a
Ariyoshi Yuji
Tanimoto Kouji
Yamakawa Tomoya
Lefkowitz Edward
Mitsubishi Denki & Kabushiki Kaisha
Sughrue & Mion, PLLC
Thompson Jewel V.
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