Measuring and testing – Volume or rate of flow – Thermal type
Reexamination Certificate
2002-01-03
2003-02-18
Williams, Hezron (Department: 2855)
Measuring and testing
Volume or rate of flow
Thermal type
C073S118040, C073S204270, C073S118040, C073S204180, C073S204190, C073S861010, C123S478000
Reexamination Certificate
active
06520009
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus for measuring an air flow rate by using a heat generating resistor, and more particularly to an air flow rate measuring apparatus advantageously utilized for controlling the air-fuel ratio of an internal combustion engine of an vehicle.
2. Description of the Related Art
It has been well known that one of apparatus for measuring a flow rate of a fluid such as air is a flow rate measuring apparatus of a heat generating resistor type.
The flow rate measuring apparatus of the heat generating resistor type is utilized in such a manner that the heat generating resistor supplied with energy electically heated is provided in a fluid of which flow rate is under measurement. Then, the flowing velocity is detected by using that the quantity of heat taken away from the heat generating resistor by the fluid becomes a function of the flowing velocity, whereby the flow rate is measured.
Recently, because of the advantages that will be described later on, the flow rate measuring apparatus of the heat generating resistor type has been widely utilized for measuring an intake air flow rate for the air-fuel ratio control of an engine (internal combustion engine) of a vehicle.
According to the flow rate measuring apparatus of the heat generating resistor type, it is possible to measure the mass flow rate directly. For this reason, there can be obtained advantages that data obtained therefrom need not be corrected based on the atmospheric pressure or temperature.
Further, the flow rate measuring apparatus of the heat generating resistor type has a nonlinear characteristic such that the relationship between the flow rate and the detected signal exhibits a steep slope in the low flow rate region and the slope becomes gentler as the flow rate is increased. For this reason, it is possible to obtain a wide measurement range in which an error rate need not be differently set. Moreover, even if the flow rate is small with the result that the flow rate exhibits only a small change, it is possible to obtain an output voltage change large enough to be detected by an analog-to-digital converter with a reasonable resolution. Accordingly, the flow rate measuring apparatus of the heat generating resistor type is extremely advantageous in application to a control of idling engine speed for stabilizing the same.
Conversely, due to the nonlinear characteristic, the flow rate measuring apparatus requires a correction processing for linearizing the output. For this reason, in a prior art, as for example shown in
FIG. 11
, a flow rate signal V generated from a flow rate detecting unit
3
is supplied to an engine control unit
2
in which the flow rate signal V is subjected to a linearizing processing and also subjected to an averaging processing by a filter upon necessity. Thus, data indicative of an air flow rate Q is obtained.
The engine control unit
2
calculates a fuel injection amount for an engine under consideration of other parameters such as a throttle valve opening degree, an engine speed as shown in the figure. Thus, the air-fuel ratio for the engine is controlled for the engine not shown.
The flow rate detecting unit
3
is composed of a heat generating resistor
3
a
as a detecting element. The heat generating resistor
3
a
is provided within an air passage A such as an intake manifold of the engine or the like so that the heat generating resistor
3
a
is exposed to an intake air flow AF.
On the other hand, there has been proposed a prior art arrangement of the flow rate measuring apparatus of the heat generating resistor type in which, as shown in
FIG. 12
, influence of backward flow caused from an intake air ripple of the engine within the intake manifold is corrected.
According to the prior art arrangement, as shown in
FIG. 12
, a flow rate detecting unit
30
is provided with, in addition to the original heat generating resistor
3
a
, another heat generating resistor
3
b
serving for detecting the backward flow of intake air with this arrangement, data indicative of an air flow rate Q having been subjected to the backward flow correction can be obtained.
A prior art relating to this kind of correction can be found in Japanese Patent Laid-Open No. Hei 8-94406, for example.
The above-described prior art, however, does not take into account deterioration in detection precision due to the averaging processing which is carried out by a filter in addition to the linearizing processing on the flow rate signal. Thus, the prior art will encounter the following difficulties.
As described above, the flow rate measuring apparatus of the heat generating resistor type has some advantages and disadvantages.
One of advantages is that the mass flow amount can be directly measured, the data obtained therefrom need not be corrected based on the change in atmospheric pressure or temperature. Another advantage is that the flow rate measuring apparatus of the heat generating resistor type has a nonlinear characteristic that the relationship between the flow rate and the output voltage exhibits a steep slope in a low flow rate region while a gentle slope in a high flow rate region. Therefore, even when the flow rate is low and thus it exhibits small flow rate fluctuation such as when the engine is placed in an idling drive mode, it is possible to obtain an output voltage fluctuation amount large enough to be detected by an analog-to-digital converter with a reasonable resolution, which fact is useful for stabilizing the idling speed of the engine.
Conversely, the flow rate measuring apparatus of the heat generating resistor type has a drawback that the nonlinear characteristic causes a measurement error.
This phenomenon is caused because the relationship between the flow rate and the output voltage is not linear, with the result that the mean value of the output voltage is decreased with respect to the mean value of the flow rate due to the engine speed ripple or the like.
This phenomenon particularly acts on increase in the air-fuel ratio (the ratio of fuel to air is decreased) in view of the engine control standpoint, leading to decrease in output of the engine.
Now, the drawback of the prior art will hereinafter be described.
FIGS. 13 and 14
show the relationship between the ripple amplitude and the detected voltage of the heat generating resistor of the flow rate measuring apparatus of the heat generating resistor type according to the prior art arrangement shown in FIG.
11
.
These diagrams are characteristic diagrams in which the air flow rate Q is plotted in abscissa while the voltage value V of the detected signal detected by the heat generating resistor
3
a
is plotted in ordinate. In this case,
FIG. 13
is a characteristic diagram in which the engine control unit does not carry out the averaging processing with a hard filter while
FIG. 14
is a characteristic diagram in which the engine control unit carries out the averaging processing with a hard filter or the like.
Since air flowing the intake manifold of the engine ripples with the opening and closing motion of the intake valve, the detected air flow rate is also rippled as shown in the figures.
Since the temperature of the heat generating resistor
3
a
substantially faithfully responds to the fluctuation of the air flow rate, the detected signal also ripples as shown in the figures.
At this time, since the voltage value characteristic of the detected signal detected by the heat generating resistor
3
a
relative to the air flow rate exhibits nonlinearity as shown in the figure, the mean value of the detected signal is decreased relative to the mean value of the original air flow rate Q, which fact leads to a measurement error derived from the nonlinearity of the heat generating resistor and the rippled amplitude.
However, if the heat generating resistor could detect the intake air ripple without any response delay, the detected ripple could be sequentially converted into the air flow rate, and the mean value thereof could be calculated, then the d
Igarashi Shinya
Kobayashi Chihiro
Crowell & Moring LLP
Thompson Jewel
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