Abstrict A thermal type air flow meter includes a device for measuring an
intake air flow rate by heating and controlling a heating resistor,
and an analog-to-digital converter for digitizing an air flow rate
signal of the air flow rate measurement device. The air flow rate
measurement device inputs a reference voltage to the analog-to-digital
converter so that the reference voltage can be converted from an
analog signal to a digital signal, and the air flow rate signal
can be corrected in accordance with the value of the reference voltage.
A/D conversion of the air flow rate signal can be executed accurately
even when the voltage of the reference voltage source of the A/D
converter in an electrical control unit is fluctuating.
Claims What is claimed is:
1. A thermal type air flow meter comprising:
(1) a means for outputting an air flow rate signal corresponding
to the air flow rate flowing through a flow path by heating and
controlling a resistor provided in the flow path,
(2) a first digital conversion means for converting the air flow
rate signal to a digital signal;
(3) a constant voltage source for applying a constant voltage to
said first digital conversion means;
(4) a correction means for correcting a signal C obtained by digitally
converting an output signal of said signal output means by said
first digital conversion means by using,
a signal A obtained by digitally converting a voltage signal of
the reference voltage source in a control means by said first digital
conversion means, the control means being comprised of a second
digital conversion means for digitally converting an input signal,
a reference voltage source for applying a voltage to said second
digital conversion means, and a calculation means for calculating
a control input of an actuator based on an output of said second
digital conversion means, and
a signal B obtained by digitally converting a voltage signal of
said constant voltage source by said first digital conversion signal;
(5) an output means for outputting a signal corrected by said correction
means to said control means.
Description TECHNICAL FIELD
The present invention relates to a thermal type air flow meter,
and particularly to an intake air flow meter for measuring an air
flow rate taken into an internal combustion engine.
BACKGROUND ART
An analog-to-digital (A/D) conversion method of detecting an air
flow rate taken into an internal combustion engine and then performing
analog-to-digital conversion is disclosed, for example, in Japanese
Patent Application Laid-Open No. 2-85724. This apparatus comprises
an air flow rate detecting unit having a bridge circuit and detection
circuit for detecting an air flow rate, and an electrical control
unit having a circuit for converting the air flow rate signal to
a digital signal. The feature of the prior art resides in that a
circuit for correcting the air flow rate signal according to a reference
voltage of an A/D conversion circuit is inserted between the A/D
conversion circuit and the air flow detection circuit. Thereby,
it is possible to accurately perform the analog-to-digital conversion
of the air flow rate signal, since the air flow rate signal can
be corrected according to the variation of the reference voltage
of the A/D conversion circuit.
However, the method in which a circuit for correcting the air flow
rate signal according to a reference voltage of an A/D conversion
circuit is inserted between the A/D conversion circuit and the air
flow detection circuit, is not a way in which a highly accurate
A/D conversion can be obtained, because there is an error in the
correction circuit itself.
Further, while a highly accurate constant voltage source is used
as a reference voltage of the A/D conversion circuit, the constant
voltage source slightly fluctuates. Further, when the drive circuit
and the detection circuit for detecting an air flow rate are over
the operating voltage, they do not depend upon the voltage of the
voltage source. Therefore, the value of the air flow rate converted
from an analog signal to a digital signal has an error due to the
fluctuation of the reference voltage of the A/D conversion circuit.
For example, assumed that the reference voltage of the A/D conversion
circuit is Vcc, the number of bits in the A/D conversion circuit
is 8 and the value of the digital conversion is Dafs, the conversion
equation is as follows.
Namely, owing to the reference voltage Vcc of the A/D conversion
circuit, the converted value Dafs has an error.
DISCLOSURE OF INVENTION
An object of the invention is to provide an thermal air flow meter
in which it is possible to perform with high accuracy the A/D conversion
of an air flow rate signal, irrespective of the fluctuation of a
reference voltage of an A/D conversion circuit.
To attain the above object, the present invention provides a thermal
type air flow meter comprising a means for measuring intake air
flow rate by controlling the heating of a heating resistor, and
a means for performing the analog-to-digital conversion of the air
flow rate signal of the air flow rate measuring means, wherein a
reference voltage is input to the A/D conversion circuit and converted
from an analog signal to a digital signal therein, and wherein the
air flow rate signal is corrected according to the value of the
reference voltage.
According to the present invention, it is possible to performing
the A/D conversion of an air flow rate signal with high accuracy,
even when the voltage of the reference voltage source of the A/D
converter in an electrical control unit is fluctuating.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a block diagram illustrating an embodiment of the present
invention.
FIG. 2 is a detailed diagram of a thermal type air flow meter according
to an embodiment of the present invention.
FIG. 3 is a detailed diagram of a thermal type air flow meter according
to another embodiment of the present invention.
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, modes for carrying out the present invention will
be explained with reference to the drawings.
FIG. 1 shows an intake air flow meter according to an embodiment
of the present invention, in which a thermal type air flow meter,
an electrical control unit and an actuator are shown in blocks.
Referring to FIG. 1 for example, a thermal type air flow meter
10 comprises a drive circuit 11 (a signal output means) for detecting
an air flow rate signal of the air flowing through such as an intake
pipe of an engine installed in a vehicle, an analog-to-digital (called
as A/D) converter 13 for performing an A/D conversion of the air
flow rate signal detected by the drive circuit 11 a central processing
unit (called as a CPU) 14 for calculating the air flow rate signal
converted from an analog signal to a digital signal, an interface
15 for converting the operated air flow rate signal into an analog
signal, and a constant voltage source 12 for supplying a reference
voltage to the A/D converter 13. The above conversion may be performed
to a signal indicative of temperature of the intake air, because
it is also possible to detect the temperature of the intake air
along with the intake air flow rate. Further, the thermal type air
flow meter 10 is connected to a battery power source 51 or power
source for driving the drive circuit 11. The air flow rate signal
or the air temperature signal from the thermal type air flow meter
is supplied to an A/D converter 33 installed in an electrical control
unit 30 (ex. an engine control unit ECU). Further, in the A/D converter
33 another analog signal 52 of the air flow rate signal is also
input. The electrical control unit 30 includes the A/D converter
33 the CPU 34 the interface 35 and the reference voltage source
32 and controls the air flow rate signal from the thermal type
air flow meter 10 or the operation of an actuator 50 (ex. a fuel
injection valve for supplying the fuel to an engine, an ignition
plug, and electronically controlled throttle) based on the air flow
temperature signal. The reference voltage source 32 installed in
the electrical control unit 30 is input to the A/D converter 13
installed in the thermal type air flow meter 10. Now, the voltage
of the reference voltage source 32 is referred to as a reference
voltage. By inputting the reference voltage to the A/D converter
13 and operating it with the A/D conversion value of the air flow
rate signal detected by the drive circuit 11 it becomes possible
to convert an air flow rate signal of the thermal type air flow
meter with high accuracy even when the voltage of the reference
voltage source 32 in the electrical control unit 30 fluctuates.
Referring to FIG. 2 an embodiment of the present invention will
be explained in detail.
The thermal type air flow meter 10 has a drive circuit for detecting
an air flow rate. The drive circuit comprises a heating resistor
20 a temperature-sensitive resistor 21 resistors 22 23 24 an
amplifier 25 and a transistor 30. The air flow rate signal is further
amplified by resistors 27 28 29 and an amplifier 26 and supplied
to the A/D converter 13. A digital signal indicative of the air
flow rate converted by the A/D converter 13 is digitally processed
in the CPU 14. The processed signal is output as an air flow rate
signal through the digital-to-analog (referred to as D/A) converter
33 to an air flow rate output terminal 61. The temperature of the
intake air can be detected by the temperature-sensitive resistor
21 and used for the correction of the air flow rate according to
the variation of the temperature of the intake air. The temperature
of the intake air is digitally processed as an intake air temperature
signal by applying the voltage across both ends of the temperature-sensitive
resistor 21 to the A/D converter 13 and converting from an analog
signal to a digital signal, and calculating a current flowing through
the temperature-sensitive resistor 21 in the CPU 14. The processed
signal is output through the D/A converter 33 to an air temperature
output terminal 62 as an intake air temperature signal. The CPU
14 corrects the detected air flow rate signal or the air temperature
signal based on the data stored in an EEPROM 32 and a ROM 31 in
which data and programs are stored. While the D/A converter 43 is
required when the A/D converter 33 of the electrical control unit
30 shown in FIG. 1 is used, it is not necessary to use when the
processed signal is supplied digitally to the electrical control
unit 30. It depends upon the form of the signal-transmission between
the thermal type air flow meter 10 and the electrical control unit
30.
The reference voltage of the A/D converter 33 installed in the
thermal type air flow meter 10 is supplied from the constant voltage
source 12 and performed in the analog-to-digital conversion using
the constant voltage Vcc as a reference voltage. Assumed that the
number of bits in the A/D converter 13 is 8 and the voltage of
the air flow rate signal detected by a heating resistor 20 etc.
and amplified is Vafs, the value Dafs of the digital conversion
is expressed as follows.
Further, the voltage of the reference voltage source 32 installed
in the electrical control unit 30 is applied through a reference
voltage terminal 63 to the A/D converter 13. Assumed that the reference
voltage is Vref, the air flow rate signal Vafs is further converted
digitally according to the following equation in the CPU 14.
Where, AD(Vref) is the digital value obtained by converting the
reference voltage of the reference voltage source 32 in the electrical
control unit 32 in the A/D converter 13 and AD(Vcc) is the digital
value obtained by converting the voltage of the constant voltage
source 12 in the A/D converter 13 in a similar manner. If the number
of bits of the A/D converter 13 is, for example, 8 it always indicates
256 which is 8.sup.th power of 2.
Namely, the above equation can be further converted to the following
equation.
This equation represents that even when the reference voltage of
the reference voltage source 32 in the electrical control unit 30
fluctuates, it is possible to correct or compensate the air flow
rate signal of the thermal type air flow meter in accordance with
the magnitude of the fluctuation, by performing the A/D conversion
to the voltage of the reference voltage source 32 and calculating.
Another embodiment of the present invention will be explained.
Even if the reference voltage of the reference voltage source 32
in the electrical control unit 30 is used as the reference voltage
of the D/A converter shown in FIG. 2 it is possible to convert
accurately the air flow rate signal detected by the drive circuit
11 (see FIG. 3). |