Abstrict To perform with high accuracy the analog-to-digital conversion
of signals of an air flow meter irrespective of fluctuation of a
reference voltage of an analog-to-digital conversion circuit. A
thermal type air flow meter including means for measuring an intake
air flow rate by heating and controlling a heating resistor and
means for performing analog-to-digital conversion of an air flow
rate signal of the air flow rate measurement means, which inputs
a reference voltage to the analog-to-digital conversion circuit
so that the reference voltage can be converted from the analog signal
to the digital signal by the analog-to-digital conversion circuit,
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 1. A thermal type air flow meter comprising 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, and a means for converting the air flow rate signal
to a digital signal, wherein the air flow rate signal output from
said digital conversion means is corrected by inputting a reference
voltage to said digital conversion means.
2. A thermal type air flow meter according to claim 1 wherein
said reference voltage is a reference voltage of a second digital
conversion means forming a control means for controlling an actuator
by inputting a signal from the thermal type air flow meter.
3. A thermal type air flow meter according to claim 1 wherein
said control means is an engine control unit (ECU) for controlling
an engine for a vehicle, and said actuator is a fuel injection valve
for supplying fuel to the engine.
4. 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.
5. A thermal type air flow meter according to claim 4 further
comprising an analog conversion means for converting an input signal
to an analog signal and outputting provided between said correction
means and said output means.
6. A thermal type air flow meter according to claim 4 wherein
said control means is an engine control unit (ECU) installed in
a vehicle, and said actuator is a fuel injection valve for supplying
fuel to the engine.
7. A thermal type air flow meter comprising 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, a means for converting the air flow rate signal
to a digital signal, and an analog conversion means for converting
the air flow rate signal digitally converted by said digital conversion
means into an analog signal, wherein the air flow rate signal output
from said analog conversion means is corrected by inputting a reference
voltage to said analog conversion means.
8. A thermal type air flow meter according to claim 7 wherein
said reference voltage is a reference voltage of a second digital
conversion means forming a control means for controlling an actuator
by inputting a signal from the thermal type air flow meter.
9. A thermal type air flow meter according to claim 8 wherein
said control means is an engine control unit (ECU) for controlling
an engine for a vehicle, and said actuator is a fuel injection valve
for supplying fuel to the engine.
10. 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) an analog conversion
means for converting an input signal into an analog signal; (4)
a constant voltage source for applying a constant voltage to said
analog conversion means; (5) a correction means for correcting a
signal C obtained by converting an output signal of said signal
output means into an analog signal by said analog conversion means
by using, a signal A obtained by converting a voltage signal of
the reference voltage source in a control means into an analog signal
by said analog 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 converting a voltage signal of said constant voltage source into
an analog signal by said analog conversion means; (6) an output
means for outputting a signal corrected by said correction means
to said control means.
11. A thermal type air flow meter according to claim 10 wherein
said control means is an engine control unit (ECU) installed in
a vehicle, and said actuator is a fuel injection valve for supplying
fuel to the engine.
Description TECHNICAL FIELD
[0001] 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
[0002] 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.
[0003] 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.
[0004] 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.
Dafs=Vafs/Vcc*256
[0005] Namely, owing to the reference voltage Vcc of the A/D conversion
circuit, the converted value Dafs has an error.
DISCLOSURE OF INVENTION
[0006] 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.
[0007] 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.
[0008] 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
[0009] FIG. 1 is a block diagram illustrating an embodiment of
the present invention.
[0010] FIG. 2 is a detailed diagram of a thermal type air flow
meter according to an embodiment of the present invention.
[0011] 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
[0012] Hereinafter, modes for carrying out the present invention
will be explained with reference to the drawings.
[0013] 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.
[0014] 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 10 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.
[0015] Referring to FIG. 2 an embodiment of the present invention
will be explained in detail.
[0016] 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.
[0017] 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.
Dafs=Vafs/Vcc*256
[0018] 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.
Dout=(Vafs/Vcc*256)*(AD(Vcc)/AD(Vref))
[0019] 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.
[0020] Namely, the above equation can be further converted to the
following equation.
Dout=AD(Vafs)*256)/AD(Vref))
[0021] 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.
[0022] 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). |