Measuring and testing – Volume or rate of flow – Thermal type
Reexamination Certificate
1999-12-15
2002-02-26
Patel, Harshad (Department: 2855)
Measuring and testing
Volume or rate of flow
Thermal type
C073S204150
Reexamination Certificate
active
06349596
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a thermal type air flow sensor for measuring an air flow rate employing a heating resistor. More particularly, the invention relates to a thermal type air flow sensor suitable for measuring an intake air flow rate of an internal combustion engine or so forth.
Conventionally, a thermal type air flow sensor has been used as a sensor for measuring an intake air flow rate flowing through an air intake passage of an internal combustion engine of an automotive vehicle or so forth. Such thermal type air flow sensor has been evaluated for capability of directly detecting a mass flow rate.
In the recent year, a thermal type air flow sensor fabricated by a semiconductor fine patterning technology on a semiconductor substrate, such as silicon (Si) or the like, has been attracting attention for capability of fabrication in relatively easy and by a mass production system, and of driving at low power.
As a basic principle of the thermal type air flow sensor employing such conventional semiconductor technology, there is one illustrate in
FIGS. 12A and 12B
, for example.
FIG. 12A
is a circuit diagram of the thermal type air flow sensor and
FIG. 12B
is a plan view showing a layout of a heating resistor Rh and an air temperature measuring resistor Rc for measuring air flow rate.
The heating resistor Rh of shown example operates as both of an air flow rate measuring element and a heater. On the other hand, an air temperature measuring resistor Rc is used for control to maintain a temperature difference of the heating resistor and an air temperature constant even when a temperature of an intake air is varied. These resistors Rh and Rc are formed with temperature sensitive resistors having common directionality of variation of resistance values relative to a temperature. Resistance values of the heating resistor Rh and the air temperature measuring resistor Rc are set so that a large current flows through the heating resistor Rh for causing heat generation, and, in contrast, a little current not causing little heat generation flows through the air temperature measuring resistor Rc. These heating resistor Rh and the air temperature measuring resistor Rc form a bridge circuit together with fixed resistors R
1
and R
2
. A voltage between the resistors Rh and R
1
and a voltage between resistors Rc and R
2
are input to an operational amplifier Op for controlling a heating current flowing through the heating resistor Rh via the operational amplifier Op and a transistor Tr so that a temperature difference between the heating resistor Rh and an air temperature (air temperature measuring resistor Rc) becomes a predetermined temperature &Dgr;Th. The heating current becomes a value corresponding to an air flow rate. Then, by converting this current into a voltage by the resistor R
1
, the air flow rate is detected.
As shown in
FIG. 12B
, upon fabricating the heating resistor Rh and the air temperature measuring resistor Rc by semiconductor fine patterning on a semiconductor substrate
300
, the heating resistor Rh and the air temperature measuring resistor Rc are formed via an electrically insulative film (electrically insulative layer) on the semiconductor substrate
300
, such as a silicon (Si) substrate or the like. However, concerning the heating resistor Rh, a part of the semiconductor substrate
300
is removed to certainly define a space (cavity portion)
301
to arrange the overall heating resistor Rh via the electrically insulative layer on the space
301
formed by removal of part of the semiconductor substrate. Thus, escape of heat of the heating resistor Rh by heat transmission through the semiconductor substrate
300
can be avoided (prevention of heat radiation other than air flow rate). On the other hand, the air temperature measuring resistor Rc is required to restrict heat generation as small as possible so as to enhance accuracy of measurement of air temperature. Therefore, the air temperature measuring resistor Rc is arranged on the semiconductor substrate
300
outside of the space
301
.
FIGS. 13A and 13B
are illustration showing a principle of another example of the conventional thermal type air flow sensor.
In the shown example, a temperature measuring resistor Rs heated by the heating resistor Rh (which temperature measuring resistor Rs is as it were, a temperature sensing resistor detecting a heat of the heating resistor Rh), a air temperature measuring resistor Rc and fixed resistors R
1
and R
2
form a bridge circuit. A voltage between the resistors Rs and R
1
and a voltage between the resistors Rc and R
2
are input to an operational amplifier Op
1
to control a heating current flowing through the heating resistor Rh via the bridge circuit, the operational amplifier Op
1
and the transistor Tr so that a temperature difference between the temperature measuring resistor Rs, thus the heating resistor Rh and the air temperature (air temperature measuring resistor Rc) is maintained at a predetermined temperature. The heating resistor Rh this managed the temperature heats a temperature measuring resistor Ru arranged upstream side of the heating resistor Rh and also a temperature measuring resistor Rd arranged downstream side of the heating resistor Rh. The temperature measuring resistors Ru and Rd form a bridge circuit together with fixed resistors R
1
′ and R
2
′. When air flow is generated, a difference of calorific values to be removed from the upstream side and downstream side temperature measuring resistors Ru and Rd depending upon air flow rate due to positional relationship thereof. By detecting the difference by an operational amplifier Op
2
, air flow rate can be detected.
Even in such type, the air temperature measuring resistor Rc to be used for maintaining the temperature difference between the heating resistor Rh and the air temperature at a predetermined value, is arranged outside of the cavity portion
301
formed by removing a part of substrate
300
. On the other hand, all of the heating resistor Rh and the temperature measuring resistors Rs, Ru and Rd intended to be heated by the heating resistor are arranged on the cavity portion
301
via the electrically insulative layer (electrically insulative film).
As the thermal type air flow sensor utilizing the principle set forth above, there are sensors disclosed in JP-A-2-259527, JP-A-4-320927, JP-A-6-273208, JP-A-6-50783, JP-A-8-14976, JP-A-10-160538, and Tokuhyo Hei No. 10-500490.
In the prior art set forth above, sufficient consideration has not been given for an error in detection of air flow rate in the case where fouling substance, such as dust or so forth contained in the intake air, adheres or deposits on a surface of the thermal type air flow sensor. If such thermal type air flow sensor is continuously used for a relatively long period, it is expected that the initial accuracy cannot be maintained for the reason set forth above.
As fouling substances for the thermal type air flow sensor possibly contained in the intake air of the internal combustion engine may be Si, Fe, Ca, Mg, Na contained in solid particle, typically sand, NaCl, MgCl
2
, CaCl
2
contained in snow melting agent, engine lubricant oil contained in blow-by gas, H
2
O, C, an impregnating oil of an air filter in a wet type air cleaner, and so forth, for example. The substances set forth above may adhere on the surface of the thermal type air flow sensor due to intermolecular attraction, liquid bridging force, electrostatic force, and composite force thereof.
Once the fouling substance adhere on the surface of the thermal type air flow sensor, thermal transmission from the heating resistor to ambient air or aspect of thermal transmission can be varied due to the adhered or deposited substance to degrade accuracy of measurement to be insufficient. Such problem can be caused even for different types of thermal type air flow sensors as illustrated in
FIGS. 12A
,
12
B and
13
.
SUMMARY OF THE INVENTION
The present invention has been worked out in view of the problems se
Nakada Keiichi
Watanabe Izumi
Yoneda Hiroshi
Crowell & Moring LLP
Hitachi , Ltd.
Patel Harshad
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