## Abstrict A heating resistor type air flow-meter has a ratio-meter circuit
comprising a proportional circuit for outputting a signal V.sub.out
proportional to an input voltage V.sub.in a multiplying circuit
for outputting a multiplied signal V.sub.m proportional to a product
of a voltage difference VD between an internal reference voltage
VINT and an external reference voltage VREF and the voltage signal
V.sub.in and an adding circuit for adding the output signal V.sub.p
of the proportional circuit and the output signal V.sub.m of the
multiplying circuit.
## Claims What is claimed is:
1. A heating resistor type air flow-meter having a radio-meter
circuit for outputting an output signal V.sub.out representing an
air flow rate, by processing a voltage signal V.sub.in detected
from a current which heats a heating resistor, using an externally
generated second reference signal, wherein the ratio-meter circuit
comprises:
an internal reference electric power source for generating a first
reference signal;
a proportional circuit for receiving the voltage signal V.sub.in
and outputting a signal V.sub.p proportional thereto;
a multiplying circuit for outputting a signal V.sub.m proportional
to a product of (i) a difference signal between said first reference
signal and said second reference signal, and (ii) said voltage signal
V.sub.in ; and
an adding circuit for producing said output signal V.sub.out by
adding said signal V.sub.p and said signal V.sub.m.
2. A heating resistor type air flow-meter according to claim 1
wherein a ratio (V.sub.p /V.sub.m) of the output voltage V.sub.p
of said proportional circuit to the output voltage V.sub.m of said
multiplying circuit is larger than 4.
3. A heating resistor type air flow-meter according to claim 1
wherein said multiplying circuit is an analogue multiplying circuit.
4. A heating resistor type air flow-meter according to claim 3
wherein:
said analogue multiplying circuit comprises a transistor LOG amplifier
and a differential amplifying circuit coupled to receive an output
voltage of said LOG amplifier; and
said LOG amplifier is set so that an output voltage thereof becomes
0 (zero) when said second reference voltage signal has a median
value within a variation range of said reference voltage signal.
5. A heating resistor type air flow-meter comprising:
a detecting circuit for outputting a signal V.sub.in detected from
a current which heats a heating resistor provided in an air flow
passage;
an air temperature detector for generating a temperature signal
representing a temperature of air to be measured flowing in said
air flow passage;
an adding-subtracting unit for outputting a correction signal by
additively combining a voltage signal proportional to said temperature
signal with a second reference voltage signal input from an external
source; and
a ratio-meter circuit having an internal reference electric power
source for generating a first reference signal; a proportional circuit
for receiving the signal V.sub.in and outputting a signal V.sub.p
proportional thereto; a multiplying circuit for outputting a signal
V.sub.m proportional to a product of (i) a difference signal between
said first reference signal and said corrected signal and (ii) said
signal V.sub.in ; and an adding circuit for outputting an output
signal V.sub.out representing an air flow rate produced by adding
said proportional signal V.sub.p and said multiplied signal V.sub.m.
6. A ratio-meter circuit for a heating resistor type air flow meter
which generates a signal V.sub.in representing a heating current
flowing in an air flow detection resister, said ratio-meter comprising:
an internal source which provides a first reference signal;
an input for receiving an externally generated second reference
signal;
a different circuit which outputs a difference signal representing
a difference between the first and second reference signals;
a proportional circuit for receiving the signal V.sub.in and outputting
a signal V.sub.p proportional thereto;
a multiplying circuit for generating a signal V.sub.m proportional
to a product of said difference signal and said signal V.sub.in
; and
an adding circuit for adding the signal V.sub.p and the signal
V.sub.m, and outputting a signal representing a sum thereof.
## Description BACKGROUND OF THE INVENTION
The present invention relates to a heating resistor type air flow-meter
and an engine control system using said heating resistor type air
flow-meter, and more particularly relates to a heating resistor
type air flow-meter of ratio-metric output type and the engine control
system using said heating resistor type air flow-meter.
An output signal from a heating resistor type air flow-meter is
transmitted to an engine control unit (hereinafter referred to as
"ECU") and converted to a digital signal by an AD converter
(hereinafter referred to as "ADC") in the ECU, and then
an air flow rate is calculated. If voltage of an electric power
source of the ADC at that time, an error is caused in the digital
signal after converted. There is a method of reducing the error
where a reference voltage in the ECU is input to the heating resistor
air flow-meter as an external reference voltage, and the air flow
rate signal is proportionally adjusted corresponding to the external
reference voltage using a ratio-metric circuit inside the flow meter.
The technology is disclosed, for example, in Japanese Patent Application
Laid-Open No. 2-85724 (1990).
However, in the above conventional technology, the ratio-metric
function is simply added to the heating resistor air flow-meter,
and improvement of accuracy in the ratio-metric function is not
considered.
SUMMARY OF THE INVENTION
An object of the present invention is to improve an accuracy of
ratio-metric function and to provide a highly accurate heating resistor
type air flow-meter of ratio-metric output type and a highly accurate
heating resistor type air flow-meter.
Another object of the present invention is to provide a highly
accurate engine control apparatus.
The above object can be attained by providing a heating resistor
type air flow-meter of ratio-metric output type, wherein a ratio-metric
circuit for outputting an output signal V.sub.out expressing an
air flow rate by processing a voltage signal V.sub.in detected from
a current heating a heating resistor using a second reference voltage
signal input from an external means, the ratio-metric circuit comprising
an internal reference electric power source circuit for generating
a first reference signal; a proportional circuit for receiving the
voltage signal V.sub.in and outputting a proportional signal V.sub.p
proportional to the voltage signal V.sub.in ; a multiplying circuit
for outputting a multiplied signal V.sub.m proportional to a product
of "a difference signal between the first reference voltage
signal and the second reference voltage signal" and "the
voltage signal V.sub.in "; and an adding circuit for producing
the output signal V.sub.out by adding the proportional signal V.sub.p
and the multiplied signal V.sub.m.
Further, the above object can be attained by providing a heating
resistor type air flow-meter comprising a detecting circuit for
outputting a signal V.sub.in detected from a current heating a heating
resistor provided in an air flow passage; an air temperature detecting
means for detecting a temperature signal of air to be measured flowing
in the air flow passage; an adding-subtracting means for outputting
a correction signal by adding or subtracting a voltage signal proportional
to the temperature signal to or from a second reference voltage
signal input from an external means; and a ratio-metric circuit
having an internal reference electric power source circuit for generating
a first reference signal, a proportional circuit for receiving the
signal V.sub.in and outputting a proportional signal V.sub.p proportional
to the signal V.sub.in, a multiplying circuit for outputting a multiplied
signal V.sub.m proportional to a product of "a difference signal
between the first reference voltage signal and the second reference
voltage signal" and "the signal V.sub.in ", and an
adding circuit for outputting an output signal V.sub.out expressing
an air flow rate produced by adding the proportional signal V.sub.p
and the multiplied signal V.sub.m.
Furthermore, the above object can be attained by providing an engine
control apparatus comprising a heating resistor type air flow-meter
for outputting a signal V.sub.in detected from a current heating
a heating resistor; an internal reference electric power source
circuit for generating a first reference voltage signal; an engine
control unit having a reference electric power source circuit for
generating a second reference signal, a proportional circuit for
outputting a signal V.sub.p proportional to the signal V.sub.in,
a multiplying circuit for outputting a signal V.sub.m proportional
to a product of "a difference between the first reference voltage
signal and the second reference voltage signal" and "the
signal V.sub.in ", and an adding circuit for adding the signal
V.sub.p and the signal V.sub.in, the added signal being processed
to obtain an output signal V.sub.out expressing an air flow rate.
According to the present invention, since the ratio-metric output
signal is a signal formed by adding the output signal of the proportional
circuit and the output signal of the multiplying circuit, an error
of the ratio-metric circuit can be suppressed less than one-tenth
when change in the external reference voltage is, for example, .+-.5%
to the median value, and therefore the accuracy of the ratio-metric
function can be improved.
Further, since the measurement accuracy of the heating resistor
type air flow-meter is improved, it is possible to provide a highly
accurate engine control apparatus.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram showing a first embodiment of a heating resistor
type air flow-meter in accordance with the present invention.
FIG. 2 is a diagram showing another embodiment of a ratio-metric
circuit in accordance with the present invention.
FIG. 3 is a diagram showing a second embodiment of a heating resistor
type air flow-meter in accordance with the present invention.
FIG. 4 is a diagram showing a third embodiment of a heating resistor
type air flow-meter in accordance with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Embodiments of the present invention will be described below, referring
to the accompanying drawings.
Initially, an embodiment of a heating resistor type air flow-meter
of ratio-metric output type in accordance with the present invention
will be described.
FIG. 1 is a diagram showing a first embodiment of a heating resistor
type air flow-meter in accordance with the present invention. In
FIG. 1 a heating resistor type air flow-meter 80 (hereinafter,
referred to as "air flow-meter 80") is composed of a detecting
circuit 10 for detecting a voltage signal V.sub.in of an air flow
rate signal input to a ratio-metric circuit, and the ratio-metric
circuit 4 for outputting an output signal V.sub.out expressing an
air flow rate by processing the voltage signal V.sub.in using a
second reference voltage signal received from an external means.
That is, temperature of a heating resistor 2 placed in an air flow
is controlled by a constant temperature control circuit 1 so as
to maintain constant, and a heating current required to heat the
heating resistor 2 of a heated wire is detected through a current
detecting resistor to obtained a voltage signal V2. The detected
signal V2 is adjusted by an output characteristic adjusting circuit
3 and the adjusted output of a voltage signal V.sub.in is processed
to be converted into a ratio-metric output signal varied corresponding
to the voltage signal V.sub.in in the ratio-metric circuit 4 using
an external reference voltage VREF input from an external means
to obtain an output signal V.sub.out of the ratio-metric circuit
4 (or, an air flow meter 80 to be described later).
In the embodiment of FIG. 1 the ratio-metric circuit 4 comprises
an internal reference electric power source circuit 5 for generating
an internal reference voltage VINT of a first reference voltage
signal; a multiplying circuit 6 composed of a differential amplifier
6a for detecting a voltage VD of a difference between the external
reference voltage VREF of a second reference voltage signal input
from the external means and the internal reference voltage VINT,
and a multiplier 6b for outputting a voltage V.sub.m of a product
of the voltage VD of the differential amplifier 6a and the output
voltage V.sub.in of the output characteristic adjusting circuit
3; a proportional circuit 8 for outputting a voltage V.sub.p of
a constant (K2) times of the output voltage V.sub.in of the output
characteristic adjusting circuit 3; and an adding circuit 9 for
adding the voltage V.sub.m of the multiplier 6b and the output voltage
V.sub.p of the proportional circuit 8 to obtain the output voltage
V.sub.out expressing an air flow rate of the flow meter.
The above-mentioned relationships are expressed by the following
equations.
Therein the value K2 among the constants K1 and K2 is set as follows.
Then the output voltage V.sub.out proportional to the external
reference voltage VREF can be obtained as follows.
Letting an error ratio of the multiplying circuit 7 be .epsilon..sub.m,
then
and letting an error ratio of the output voltage V.sub.out be .epsilon..sub.v,
then
Therefore, the error ratio .epsilon..sub.v of the ratio-metric
circuit 4 to the error ratio .epsilon..sub.m of the multiplying
circuit 6 can be reduced as follows.
In the above equation, assuming that the median value of the external
reference voltage VREF is 5(V) and the variation range (variation
rate range) is .+-.5%, VREF=4.75.about.5.25 (V). Therefore, the
necessary condition to operate within the range of VREF>VINT
is as follows, assuming VINT=5.75
and it can be understood that the error ratio .epsilon..sub.v of
the ratio-metric circuit 4 is less than one-tenth of the error ratio
.epsilon..sub.m of the multiplying circuit 6. That is, according
to the present invention, the error ratio .epsilon..sub.v of the
ratio-metric circuit 4 can be suppressed less than one-tenth of
the error ratio of conventional one.
The ratio of the output voltage V.sub.p of the proportional circuit
8 to the output voltage V.sub.m of the multiplying circuit 6 is,
from (Equation 6) and (Equation 9) as follows.
Therefore, when VREF=5 and VINT=4.75 the ratio becomes as follows.
Further, when the variation range (variation rate range) is .+-.20%
in taking an operation margin into consideration, V=4 V, and thus
and the reducing rate of the error ratio is as follows.
From the above, it can be understood that by setting the ratio
(V.sub.p /V.sub.m) of the output voltage V.sub.p of the proportional
circuit 8 to the output voltage V.sub.m of the multiplying circuit
6 to a value larger than 4 (4 and 19 in the above cases), it is
possible to obtain a practical ratio-metric circuit in which the
operation margin is ensured and the error ratio .epsilon..sub.m
of the multiplying circuit 7 is reduced. In other words, it can
be said that the variation rate range of the second reference voltage
signal (generally, the external reference voltage VREF) is necessary
to be suppressed within 20 (%) of the median value of the variation
rate range of the second reference voltage signal (that is, the
external reference voltage VREF).
In a case of performing digital calculation to obtain a ratio-metric
output, sufficient accuracy can be obtained only by multiplying
calculation since there is no difference in calculation accuracy
between multiplying and adding. However, in order to perform digital
calculation, it is required to employ functional elements inferior
in temperature environment resistant characteristics (environment
resistance) such as an AD converter, a microprocessor and the like,
which causes a problem in that size of the ratio-metric circuit
becomes large.
It can be considered that the ratio-metric circuit is constructed
by an analogue multiplying circuit which is of a small circuit size,
does not require any functional elements inferior in environment
resistance, and is suitable in an environmental condition of vehicle
air flow meter. However, it has been difficult to provide the analogue
multiplying circuit with an accuracy equivalent to that of the analogue
differential amplifying circuit. However, since an appropriate accuracy
can be ensured by employing the idea of the present invention even
when an analogue multiplying circuit is employed, it becomes possible
to provide a heating resistor type air flow-meter having a highly
accurate ratio-metric function by making use of an analogue multiplying
circuit which is superior in environment resistance and simple in
circuit construction.
Description will be made below on an embodiment of a ratio-metric
circuit using the analogue multiplying circuit as a second embodiment
of a ratio-metric circuit. The ratio-metric circuit typically shown
in the first embodiment of the heating resistor type air flow-meter
shown in FIG. 1 is a first embodiment of a ratio-metric circuit.
FIG. 2 is a diagram showing another embodiment of a ratio-metric
circuit in accordance with the present invention. The figure shows
a detailed electronic circuit if the ratio-metric circuit.
The second embodiment of the ratio-metric circuit 4 comprises an
internal reference electric power source circuit 5; a multiplying
circuit 6 composed of a LOG amplifier 6c for outputting an output
voltage .DELTA.VBE proportional to a logarithm of the ratio of a
difference between the external reference voltage VREF and the internal
reference voltage VINT to the internal reference voltage VINT, and
a differential amplifying circuit 6d for differential-amplifying
the .DELTA.VBE by giving a bias current proportional to the input
voltage V.sub.in ; a proportional circuit 8; and an adding circuit
9. That is, currents of bipolar transistors 11 12 are supplied
to transistors 15 16 17 18 controlled by operational amplifiers
30 31 and by copying a difference .DELTA.VBE of base-emitter voltages
VBE between the transistors 11 12 to emitter coupling differential
type transistors 13 14 a ratio of current of a resistor 43 controlled
by an operational amplifier 32 to current of the transistor 13 is
made equal to a ratio of voltage V.sub.ex to voltage V.sub.ei input
to the operational amplifiers 30 31. Letting the current of the
transistors 11 12 in that condition be I.sub.c 1 I.sub.c 2 respectively,
the following relations can be obtained.
The base-emitter voltages VBE of the transistor can be expressed
as follows.
where k: Boltzmann constant, T: temperature [K], q: charge of electron,
I.sub.c : collector current of transistor, I.sub.s : collector saturation
current. Therefore, the difference .DELTA.VBE of base-emitter voltages
VBE between the transistors 11 12 can be expressed as follows.
Similarly, currents of the transistors 13 14 can be expressed
as follows.
From (Equation 23) and (Equation 25), the following relations can
be obtained.
Current IEE flowing in the resistor 43 is as follows.
From (Equation 18), (Equation 19), (Equation 27), (Equation 28)
and (Equation 29), the following relation can be obtained.
Since a current mirror circuit formed by the transistors 21 22
conduct current to a resistor 44 by reversing the polarity of the
current I.sub.c 3 of the transistor 13 voltage V.sub.m -V.sub.p
of a resistor 44 becomes as follows.
Since the voltage V.sub.p is a divided voltage of V.sub.in by resistors
45 46 the voltage V.sub.p can be expressed by the following equation.
Thus,
Therefore, by setting values of the resistors as the following
equation, the voltage V.sub.m becomes a ratio-metric output signal
proportional to V.sub.ex and V.sub.in.
This voltage V.sub.m is input to the amplifying circuit composed
of the operational amplifier 34 and the resistors 53 54 to obtain
an output signal V.sub.out of the flow-meter.
Here, (Equation 20) can be rewritten as follows.
Therefore,
In that condition, currents of the transistors 11 12 are equal
to each other, and from (Equation 23) the output voltage .DELTA.VBE
of the LOG amplifier 6c becomes 0 V (zero Volt). Since the "current
ratio of the differential transistor" becomes nearest to the
theoretical value when the currents are equal to each other, the
"current ratio of the differential transistor" is set
so that the above equation is satisfied, that is, the output voltage
.DELTA.VBE of the LOG amplifier 6c becomes 0 V (zero Volt) when
the external reference voltage VREF of the second reference voltage
signal is at the median value of the variation range. By doing so,
error in the ratio-metric transforming under a practical use condition
can be further reduced.
V.sub.ex and V.sub.ei can be expressed by the following (Equation
41) and (Equation 42) from FIG. 2 and by substituting these equations
into (Equation 38) the following (Equation 43) can be obtained.
The voltage VBE of the transistor is a function of temperature
T, as described by (Equation 21). Therefore, when temperatures of
respective transistors composing the LOG amplifier 6c and the differential
amplifier 6d are different, the value of (Equation 21) is varied
depending on the temperatures of the respective transistors. In
order to stabilize the output V.sub.m of multiplying calculation
by solving this problem, that is, to eliminate obstacles against
heat dissipation, it is preferable that the multiplying circuit
6 is formed on a single silicon substrate to thermally couple well.
In other words, it is preferable that the LOG amplifier 6c and the
differential amplifier 6d are formed on a single silicon substrate,
or the multiplying circuit 6 and adding circuit 8 are formed on
a single silicon substrate.
Another embodiment of a heating resistor type air flow-meter of
ratio-metric output type in accordance with the present invention
will be described.
FIG. 3 is a diagram showing a second embodiment of a heating resistor
type air flow-meter in accordance with the present invention. The
second embodiment of FIG. 3 is formed by adding an air temperature
detecting means 62 and an adding-subtracting means 63 to the first
embodiment of the air flow-meter shown in FIG. 1. That is, the heating
resistor type air flow-meter comprises the air temperature detecting
means 62 for detecting temperature of air to be measured, composed
of an air temperature sensor 60 and an air temperature detecting
circuit 61 and the adding-subtracting means 63 for processing (correcting)
a temperature signal VTMP output from the air temperature detecting
means 62 and the processed (corrected) signal is input to the ratio-metric
circuit 4 as a second reference voltage signal.
In other words, the heating resistor type air flow-meter in accordance
with the present invention comprises a heating resistor 2 provided
in an air flow passage; a constant temperature control circuit 1
for supplying current so as to maintain temperature of the heating
resistor 2 at a constant temperature; a detecting circuit 10 including
a current detecting resistor 12 for detecting the controlled current
supplied to the heating resistor and an output characteristic adjusting
circuit 3 for outputting a voltage signal V.sub.in ; an air temperature
detecting means 62 for detecting a temperature signal VTMP of air
to be measured flowing in said air flow passage; an adding-subtracting
means 63 for outputting a correction signal by adding or subtracting
a voltage signal proportional to the detected voltage VTMP of the
temperature signal to or from an external reference voltage VREF
of a second reference voltage signal input from an external means
(for example, an ECU) provided in the external; and a ratio-metric
circuit including an internal reference electric power source circuit
5 for generating an internal reference voltage VINT of a first reference
signal, a proportional circuit 8 for receiving the signal V.sub.in
and outputting a proportional signal V.sub.p proportional to the
signal V.sub.in, a multiplying circuit 6 for outputting a multiplied
signal V.sub.m proportional to a product of "a difference signal
between the first reference voltage signal and the corrected signal"
and "said signal V.sub.in ", and an adding circuit 9 for
outputting an output signal V.sub.out expressing an air flow rate
produced by adding the proportional signal V.sub.p and the multiplied
signal V.sub.m.
The second embodiment has an advantage in that the output characteristic
of the flow-meter can be performed temperature correction in a case
where the flow-rate characteristic of the flow-meter shows temperature
dependence. There is no need to say that the same effect can be
attained if the air temperature detecting means 62 is arranged outside
the flow-meter and the temperature signal VTMP is input to the flow-meter,
in the embodiment of FIG. 3.
In the first embodiment, "the second reference voltage signal
of the ratio-metric circuit is equal to the external reference voltage
VREF", but in the second embodiment, "the second reference
voltage signal of the ratio-metric circuit is equal to the corrected
signal of the external reference voltage VREF". In other words,
the second reference voltage signal is a voltage signal for inputting
the ratio-metric circuit and processing the input signal V.sub.in,
and the external reference voltage is a voltage signal to be used
by an AD converter 71 of an ECU 70 to be described later.
A further embodiment of a heating resistor type air flow-meter
of ratio-metric output type in accordance with the present invention
will be described.
FIG. 4 is a diagram showing a third embodiment of a heating resistor
type air flow-meter in accordance with the present invention. The
third embodiment of a heating resistor type air flow-meter of FIG.
4 is a part of an embodiment of an engine control apparatus. That
is, this embodiment of the heating resistor type air flow-meter
is shown as a part of the engine control apparatus 90 which is constructed
by that, in the ratio-metric circuit 5 in the first embodiment,
the internal reference electric power source circuit 5 of one component
of the ratio-metric circuit 4 is left inside the air flow-meter
80 side and the other components of the ratio-metric circuit 4 except
for the internal reference electric power source circuit 5 are arranged
inside the ECU 70 and the internal reference voltage VREF and the
air flow rate signal V.sub.in are transmitted from the air flow-meter
80 to the ECU 70. In other words, the components composing the ratio-metric
circuit characterizing the heating resistor type air flow-meter
of ratio-metric output type in accordance with the present invention
are separately arranged in the other units (the ECU 70 the engine
control apparatus 90 and so on).
In this embodiment, the engine control apparatus 90 is composed
of the air flow-meter 80 for outputting a signal V.sub.in detected
from current heating the heating resistor, includes the internal
reference electric power source circuit 5 for generating the first
reference voltage signal; and the ECU 70 including the ratio-metric
circuit 4a for processing an added signal as an output signal V.sub.out
expressing an air flow rate. In the ECU 70 there are arranged the
ratio-metric circuit 4a except for the internal reference electric
power source circuit 5 the AD converter 71 the reference electric
power source circuit 72 for supplying the reference voltage VREF
of the second reference voltage signal to the AD converter 71 and
the MPU (microprocessor) 73 for processing digital signals output
from the AD converter 71. By performing calculation of (Equation
9) using the ratio-metric circuit 4a in the ECU 70 even if the
reference voltage VREF of the ECU 70 is fluctuated, the digital
signal output from the AD converter 71 can be made insensitive to
the effect of the fluctuation.
According to the embodiment of FIG. 4 there is an advantage in
that the circuit construction of the ECU 80 can be made small in
size. Further, in a vehicle, since environmental disturbance such
as temperature, vibration, electromagnetic wave and so on is less
in a place to install the ECU 70 than a place to install the air
flow-meter 80 requirement of the environmental condition to the
ratio-metric circuit 4a is moderated and accordingly there is an
effect in that the correction system using the ratio-metric function
can be made economical.
The engine control apparatus 90 using the heating resistor type
air flow-meter like this embodiment can perform highly accurate
engine control since measuring accuracy of air flow rate is improved.
The internal reference electric power source circuit 5 may be installed
in the ECU 70.
According to the present invention, since an output of the ratio-metric
circuit is formed by adding an output signal of the proportional
circuit and an output signal of the multiplying circuit, the ratio
of the error of the multiplying circuit to the total error of the
ratio-metric circuit can be reduced and accordingly the ratio-metric
output characteristic of the heating resistor type air flow-meter
can be made more accurate. Particularly, the present invention is
effective in minimizing an output error near the median value of
the external reference voltage which is most frequently used.
By employing the present invention, there is an effect in that
accuracy of an engine control apparatus can be improved.
Further, by constructing the ratio-metric circuit by an analogue
multiplying circuit, the heating resistor type air flow-meter of
ratio-metric output type can be made low in cost. |