Abstrict An air flow meter circuit with a temperature compensation circuit
comprises an air flow rate detection circuit for generating a output
signal in accordance with an air flow rate, an output circuit for
amplifying the output signal of the air flow rate detection circuit
to produce an output signal whose value is so adjusted as to be
related to the value of the input signal in a predetermined relationship,
and a constant voltage circuit for supplying a predetermined reference
voltage to the air flow rate detection and output circuits. In order
to compensate the output of the air flow meter circuit for its changes
due to the influence of temperature coefficients of component parts
of both the air flow rate detection and output circuits, the temperature
coefficient of the output of the constant voltage circuit is so
adjusted as to cancel out the temperature coefficients of the parts
constituting the air flow meter circuit.
Claims I claim:
1. An air flow meter circuit comprising:
an air flow rate detection circuit for generating an output signal
in accordance with an air flow rate;
an output circuit for receiving the output signal of said air flow
rate detection circuit as an input signal and amplifying the input
signal to produce an output signal whose value is so adjusted as
to be related to the value of the input signal in a predetermined
relationship;
a constant voltage circuit for supplying .Iadd.as an output signal
.Iaddend. a predetermined constant voltage to said detection and
output circuits; and
temperature compensation means for adjusting the temperature coefficient
of the output signal of said constant voltage circuit such that
temperature coefficients of said detection and output circuits are
cancelled out to substantially zero a change in the output signal
.Iadd.of said output circuit .Iaddend. due to a change in the temperature
of the entirety of said air flow meter circuit.
2. An air flow meter circuit according to claim 1 wherein said
constant voltage circuit includes a Zener diode whose Zener voltage
is used as a reference voltage, and said temperature compensation
means includes a series connection of a constant voltage source
and current control means, said series connection being connected
to said Zener diode, said current control means being so adjusted
as to vary a Zener current to thereby set the temperature coefficient
of the output signal of said constant voltage circuit to a predetermined
value.
3. An air flow meter circuit according to claim 2 wherein said
constant voltage source is provided by the output voltage of said
constant voltage circuit, and said current control means comprises
a variable resistor.
4. An air flow meter circuit according to claim 3 wherein said
air flow rate detection circuit comprises a hot wire air flow rate
detection circuit including a hot wire heated to a predetermined
temperature and undergoing impingement of an air flow to change
current flowing in said hot wire, said current change being detected
for measurement of an air flow rate.
5. An air flow meter circuit according to claim 1 wherein said
output circuit comprises a non-inverting amplifier circuit having
an operational amplifier, and means for varying feedback rate and
input reference voltage of said non-inverting amplifier circuit
such that the input value versus the output value is so adjusted
as to be in the predetermined relationship.
6. An air flow meter circuit according to claim 2 wherein said
output circuit comprises a non inverting amplifier circuit having
an operational amplifier, and means for varying feedback rate and
input reference voltage of said non-inverting amplifier circuit
such that the input value versus the output value is so adjusted
as to be in the predetermined relationship.
7. An air flow meter circuit according to claim 3 wherein said
output circuit comprises a non-inverting amplifier circuit having
an operational amplifier, and means for varying feedback rate and
input reference voltage of said non-inverting amplifier circuit
such that the input value versus the output value is so adjusted
as to be in the predetermined relationship.
8. An air flow meter circuit according to claim 4 wherein said
output circuit comprises a non-inverting amplifier circuit having
an operational amplifier, and means for varying feedback rate and
input reference voltage of said non-inverting amplifier circuit
such that the input value versus the output value is so adjusted
as to be in the predetermined relationship.
9. An air flow meter circuit according to claim 5 wherein the output
voltage of said constant voltage circuit is used as the reference
voltage of said operational amplifier.
10. An air flow meter circuit according to claim 6 wherein the
output voltage of said constant voltage circuit is used as the reference
voltage of said operational amplifier.
11. An air flow meter circuit according to claim 7 wherein the
output voltage of said constant voltage circuit is used as the reference
voltage of said operational amplifier.
12. An air flow meter circuit according to claim 8 wherein the
output voltage of said constant voltage circuit is used as the reference
voltage of said operational amplifier.
Description The present invention generally relates to an air flow meter circuit
with a temperature compensation circuit and more particularly to
a temperature compensation circuit for compensating changes in output
die to changes in temperature of the entirety of the air flow meter
circuit.
In an internal combustion engine, for example, the intake air flow
rate of the internal combustion engine is detected and used as a
parameter for controlling the operation of the internal combustion
engine. For detection of the intake air flow rate, a hot wire air
flow meter, for example, is available wherein a hot wire heated
to a predetermined temperature is placed in an intake air path,
and a current flowing through the heated hot wire is measured to
detect an air flow rate. This type of air flow meter is disclosed
in, for example, U.S. Pat. No. 4297881 to Sasayama et al issued
on Nov. 3 1981. Since, in an air flow meter, the value of air flow
rate to be detected changes with the temperature of intake air,
it has hitherto been practice to compensate the detected output
for intake air temperature. Generally, in a hot wire air flow meter,
for example, a cold wire is provided in addition to the hot wire,
which cold wire is placed in the same intake air path to effect
the direction of the air temperature simultaneous with the compensation
therefor. Generally, this type of temperature compensation is also
employed in a variety of intake air flow meters of the other types.
Even with the compensation for the intake air temperature, there
still remains, in practice, a problem that parts constituting the
air flow meter circuit, for example, resistors change in their resistance
values as the ambient temperature changes. Thus, because of a temperature
characteristic of each of the component parts, the relation between
air flow rate and output value also has, in practice, a temperature
characteristic, i.e., temperature dependence. Especially, in the
case of the air flow meter for use in internal combustion engines,
a flow meter module is placed in an engine room and hence exposed
to large changes in temperature. Therefore, the problem of the temperature
dependence is serious.
The inventors have found that such temperature dependence has a
great effect on accuracies of the air flow meter and recognized
the necessity of compensation for the temperature characteristic.
In addition, higher accuracy is required for a temperature compensation
circuit of that air flow meter than for general temperature compensation
circuits, because the air flow rate is related to the output value
of the air flow detection circuit by a fourth-power exponential
function as will be described later with reference to a formula
and therefore, in order to measure a flow rate with 4% accuracy,
for example, the accuracy of the detection must be held to be 1%.
Furthermore, since the temperature coefficient of one air flow meter
usually differs from that of another, desirability is such that
desired adjustment of the temperature coefficient can be done with
ease and the temperature compensation never disturbs the predetermined
relation between air flow rate and output value. However, a highly
accurate temperature compensation circuit has not been materialized
heretofore which can meet the above particular conditions imposed
on the temperature compensation circuit of the air flow meter.
The present invention has been achieved with a view of solving
the novel subject matter found by the inventors and of meeting the
necessity of temperature compensation for the entirety of the air
flow meter circuit as well as the necessity of provision of a highly
accurate temperature compensation circuit.
Accordingly, an object of this invention is to provide an air flow
meter circuit subject to the temperature compensation which meets
the aforementioned subject matter.
Another object of this invention is to provide a highly accurate
temperature compensation circuit which is simplified in construction
and easy to adjust.
To accomplish the above objects, according to the present invention,
an air flow meter circuit incorporates a temperature compensation
circuit which is simplified in construction and adjustable in order
for its temperature coefficient to be a desired value, whereby the
temperature coefficients of other circuits than the temperature
compensation circuit are compensated so as to zero the temperature
coefficient of the entirety of the air flow meter circuit. To obtain
the temperature compensation circuit of simplified construction
according to the invention, the inventors take advantage of such
a characteristic of a Zener diode that the temperature coefficient
of the Zener voltage (Zener voltage change/temperature change) varies
with Zener current value, to realize a circuit whose temperature
coefficient can be set desirably by adjusting the Zener current.
The present invention will be apparent from the following detailed
description taken in conjunction with the accompanying drawings,
in which:
FIG. 1 is a block diagram showing the construction of an air flow
meter circuit;
FIG. 2 is a circuit diagram showing an air flow meter circuit with
a temperature compensation circuit according to an embodiment of
the invention;
FIG. 3 is a circuit diagram useful in explaining the temperature
compensation circuit of the invention; and
FIG. 4 is a graph showing an actual example of temperature coefficient
characteristic of a Zener diode.
Referring to FIG. 1 an air flow detection circuit 20 has its output
connected to the input of a zero-span circuit 30. The detection
circuit 20 and zero-span circuit 30 are supplied with a predetermined
reference voltage from a regulated voltage source 10.
An air flow to be measured, shown at wavy arrows in FIG. 1 impinges
upon an air flow rate detection device, not shown, of the detection
circuit 20 and a flow rate is detected as an electrical signal.
The detected signal is inputted to the zero-span circuit 30 where
the input value versus the output value is so adjusted as to be
in a predetermined relationship. Specifically, zero point and inclination
of a characteristic curve representative of the input value versus
the output value are determined. Such predetermined relationship
of the zero-span circuit 30 is determined by a request from a separate
control circuit, not shown, connected to the output of the zero-span
circuit 30.
FIG. 2 shows a preferred embodiment of an air flow meter with a
temperature compensation circuit according to the invention. The
hot wire air flow meter described previously is used as an air flow
meter in this embodiment. Referring to FIG. 2 power supply voltage
V.sub.+ is supplied to the collector of a transistor Tr 1 having
its emitter connected to a hot wire RH placed in an air flow path
(not shown). The other end of this hot wire RH is grounded via a
resistor R1. Connected across the collector and base of the transistor
Tr 1 is a resistor R12. Resistors R2 and R10 are connected, at one
end, in common to the emitter of the transistor Tr 1. The other
end of the resistor R2 is connected to the inverting input terminal
of an operational amplifier OP1 via a resistor R9. The other end
of the resistor 2 is also connected to one end of a variable resistor
R3 the other end of which is connected to the non-inverting input
terminal of an operational amplifier OP4 via a resistor R21. The
junction between the variable resistor R3 and resistor R21 is connected
with the junction between the hot wire RH and resistor R1. A voltage
drop V.sub.2 due to a current flowing through the hot wire RH and
resistor R1 is applied to the non-inverting input terminal of the
operational amplifier OP4 via the resistor R21. The other end of
the resistor R10 is connected to the inverting input terminal of
the operational amplifier OP1. Both the input terminals of this
operational amplifier OP1 are bridged via a capacitor C5.
The non-inverting input terminal of the operational amplifier OP1
is connected to a resistor R11 having the other end connected to
a resistor R14. A resistor R4 has one end connected to the output
of a constant voltage circuit 100 blocked by chained line and the
other end connected to a resistor R6 and a variable resistor Rr.
The other end of the resistor R6 is connected with the inverting
input terminal of an operational amplifier OP2. The non-inverting
input terminal of this operational amplifier OP2 is connected to
the junction between the hot wire RH and resistor R1. Connected
across the output terminal and inverting terminal of the operational
amplifier OP2 is a series connection of a cold wire RC and a resistor
R8. The cold wire RC is placed at a position where the temperature
of air flow to be measured can be detected. The output terminal
of the operational amplifier OP2 is also connected to the noninverting
input terminal of the operational amplifier OP1 via the resistor
R11 and ground via the resistor R14. A resistor R7 and a capacitor
C1 are connected, at one end, in common to the inverting input terminal
of the operational amplifier, with the other end of each of the
resistor R7 and capacitor C1 grounded.
The transistor Tr1 hot wire RH, cold wire RC, resistors R1 to
R14 capacitors SC1 and C5 and operational amplifiers OP1 and OP2
constitute a feedback circuit 200 which controls the current I.sub.H
such that the temperature of the hot wire RH is kept constant. Thus,
the voltage drop V.sub.2 due to the current flowing through the
resistor R1 stands for the flow rate detection signal. To be specific,
when the air flow impinges on the hot wire, this hot wire is deprived
of heat by aerial molecules and decreased in temperature. An amount
of current of the hot wire to compensate for a decreased temperature
thereof corresponds to an air flow rate. This feedback circuit 200
corresponds to the air flow detection circuit 20 shown in FIG. 1.
Connected to the resistor R4 of this feedback circuit 200 are one
end of a resistor R18 and the output terminal of an operational
amplifier OP3 i.e., the output of the constant voltage circuit
100. A variable resistor R19 and a resistor R20 are connected, at
one end, to the other end of the resistor R18. The variable resistor
R19 has the other end grounded and the resistor R20 has the other
end connected with the inverting input terminal of the operational
amplifier OP4.
The output terminal and the inverting input terminal of the operational
amplifier OP3 are connected together via a resistor R15 with that
inverting input terminal grounded via a series connection of a resistor
R15 and a diode D1. The non-inverting input terminal of the operational
amplifier OP3 is supplied with the power supply voltage V.sub.+
via a resistor R27 and grounded via a Zener diode ZD1 in backward
connection. A variable resistor R17 is connected between the output
terminal of the operational amplifier OP3 and the cathode of the
Zener diode ZD1 and the junction between the variable resistor
R17 and the cathode of the Zener diode ZD1 is connected with one
end of the capacitor C2 the other end of which is grounded. The
operational amplifier OP3 is also fed with the power supply voltage
V.sub.+ via a resistor R28. The resistor R28 is connected with one
end of a capacitor C3 and the cathode of a Zener diode ZD2. The
other end of the capacitor C3 and the anode of the Zener diode ZD2
are grounded. The capacitor C3 and Zener diode ZD2 are adapted to
protect the operational amplifier OP3 from surge voltage from the
power supply. In this manner, the constant voltage circuit 100 is
constituted.
Meanwhile, the inverting input terminal of the operational amplifier
OP4 is also connected with a series connection of a variable resistor
R22 and a resistor R23. The resistor R23 is grounded via a Zener
diode ZD3 in backward connection. The cathode of the Zener diode
ZD3 is connected via a resistor R24 to the output terminal of operational
amplifier OP4 which is grounded via a resistor R25. The resistor
R24 is connected with one end of a resistor R26 having the other
end connected to an output terminal V.sub.0. The resistors R18 to
R26 Zener diode ZD3 and operational amplifier OP4 constitute a
zero-span circuit 300. Like the operational amplifier OP3 each
of the operational amplifier OP1 OP2 and OP4 is also fed by the
power supply voltage V.sub.+ but for clarity of illustration, the
feed line is not depicted.
The operation of the FIG. 2 circuit will be described briefly.
To describe the operation of the feedback circuit 200 in the first
place, it should be understood that each of the hot wire RH and
cold wire RC has a platinum wire wound on an aluminum bobbin and
is placed in the intake air flow path so as to be sufficiently exposed
to the flowing air. Both the hot and cold wires have their own resistances
exhibiting a positive characteristic with respect to temperatures.
In other words, their resistances increase as the temperature rises.
A predetermined amount of current I.sub.H is fed from the transistor
Tr1 into the hot wire RH which in turn is heated to a temperature
which is higher than the temperature of the flowing air by a predetermined
temperature .DELTA.T.sub.H. Since the cold wire RC is on the other
hand connected to act as a feedback resistor for the operational
amplifier OP2 and only an extremely small amount of current is passed
through the cold wire RC, the temperature of the cold wire is hardly
affected by the current and maintained at the same value as that
of a temperature of the flowing air.
The voltage V.sub.2 caused across the resistor R1 by the current
flowing from the hot wire RH into the resistor R1 is amplified by
the operational amplifier OP2 and fed to the non-inverting input
terminal of the operational amplifier OP1. Since the sum of resistances
of the resistors R2 and R3 is set to be sufficiently greater than
a resistance of the hot wire RH, the current flowing in the resistor
R1 has substantially the same value as that of the current I.sub.H
flowing in the hot wire RH.
The operational amplifier OP2 effects a feedback by an amount which
is determined by the resistance of the cold wire RC, thereby compensating
the intake air for its temperature.
The operational amplifier OP1 compares a voltage divided a voltage
drop across the hot wire RH by the resistors R2 and R3 with the
output voltage of the operational amplifier OP2 to produce a output
voltage commensurate to the difference which in turn is feedback
to the hot wire RH via the transistor Tr1 so that the current I.sub.H
flowing in the hot wire RH is so controlled as to constantly keep
the temperature of the hot wire RH higher than the temperature of
the intake air by .DELTA.T.sub.H.
Consequently, as the intake air flow rate changes, the quantity
of heat deprived from the hot wire RH by the intake air changes
and then the current I.sub.H changes in a sense for cancelling a
temperature change of the hot wire RH being deprived of heat. Eventually,
the current I.sub.H changes as a function of the intake air flow
rate. Due to the fact that the current flowing in the resistor R1
substantially equals 1.sub.H, the voltage drop V.sub.2 across the
resistor R1 represents the intake air flow rate. Specifically, the
amount of intake air flow Q is related to the voltage V.sub.2 by
V.sub.2.alpha. Q1/4.
Thus, the voltage V.sub.2 fed to the operational amplifier OP4
is amplified thereby to produce a flow rate signal V.sub.0 at the
output terminal. This signal V.sub.0 may be inputted to a microcomputer
for engine control, for example, and used for air/fuel ratio control.
Next, the operation of the zero-span circuit 300 will be described.
This circuit is a non-inverting amplifier using an operational amplifier.
By adjusting the variable resistor R19 connected to the inverting
input terminal, the bias voltage of the operational amplifier OP4
can be varied to adjust the output signal V.sub.0 to a desired level.
Further, by adjusting the variable resistor R22 inserted in the
feedback loop of the operational amplifier OP4 the gain of the
non-inverting amplifier can be varied to desirably set the rate
of change of the output signal V.sub.0 relative to the input signal
V.sub.2 i.e., the input/output characteristic. Thank to the function
of adjustments, the output characteristic of the air flow meter
can be matched with the specification of a control circuit, not
shown, fed with the output signal V.sub.0. The Zener diode ZD3 provided
for the output of the operational amplifier OP4 is adapted to absorb
external high voltage noises which intrude into the zero-span circuit
300.
The operation of the constant voltage circuit 100 will now be described.
This circuit is adapted not only to supply the reference voltage
to the operational amplifiers included in the feedback circuit 200
and zero-span circuit 300 but also to achieve the temperature compensation
for the entirety of the air flow meter circuit which is the subject
matter of the present invention. In essentiality, the temperature
coefficient of that reference voltage is adjusted to compensate
the entirety of the air flow meter circuit for temperature. For
details, reference should be made to FIGS. 3 and 4.
In FIG. 3 the power supply voltage feed line inclusive of the
resistor R28 is not illustrated for simplicity. Further, there is
no illustration, in FIG. 3 of the parallel connection of the Zener
diode ZD2 and capacitor C3 for protecting the operational amplifier
OP3 from surge voltage, the capacitor C2 for noise protection and
the resistor R27 for passage of starting current of the constant
voltage circuit upon turn-on of the power supply source, all of
which are unessential to the present invention.
Taking a Zener diode of HZ 2B-LL type manufactured by Hitachi Ltd.,
for instance, a characteristic of temperature coefficient .gamma.Z
(mV/.degree.C.) relative to Zener current I.sub.Z (mA) of the Zener
diode playing the part of basis in the principle of the present
invention is plotted in FIG. 4 where abscissa represents Zener
current in logarithmic scale and ordinate temperature coefficient.
As will be seen from FIG. 4 the temperatures coefficient of Zener
voltage changes with the Zener current.
From the characteristic curve, the temperature coefficient .gamma.Z
of the Zener diode can be indicated by equation (1):
where .alpha.=4.78.times.10.sup.-4 and .beta.=2.54.times.10.sup.-4.
The embodiment of FIG. 3 comprises an operational amplifier OP,
resistors R.sub.A, R.sub.B and R.sub.C, a diode D1 and a Zener
diode ZD1. Then, denoting a forward voltage of the diode D1 by V.sub.F,
a Zener voltage of the Zener diode ZD1 by V.sub.Z, and a set output
voltage by V.sub.S, there results ##EQU1## By neglecting temperature
coefficients of the resistors R.sub.A and R.sub.B, the temperature
coefficient, .gamma.S, of the output voltage of the constant voltage
circuit is given by ##EQU2## where .gamma.F represents a temperature
coefficient of the diode D1.
It will be seen from equation(3) that the temperature coefficient
.gamma.S of the constant voltage circuit can be adjusted desirably
by varying the temperature coefficient .gamma.Z of the Zener diode
ZD1. Considering that the temperature coefficient .gamma.F of the
diode is generally of the order of -2 mV/.degree.C., .gamma.Z=-2
mV/.degree.C. may be set by adjusting the Zener current I.sub.Z
when the temperature coefficient .gamma.S of the output voltage
V.sub.S is desired to be about 0 mV/.degree.C., for example. The
value of .gamma.Z=-2 mV/.degree.C. is, however, outside of the controlling
range as will be noted from FIG. 4. Then, in order to obtain the
value of .gamma.S of about 0 mV/.degree.C., the diode D1 may be
short-circuited (placed out of use) to zero the term of .gamma.F
in equation (3) and the variable resistor RC may be adjusted so
as to set the Zener current I.sub.Z to about 5 mA which makes the
.gamma.Z substantially zero. The circuit of FIGS. 3 or 2 employs
the diode D1 because without the diode D1 it is necessary to make
the Zener current larger than 5 mA, followed by an increase in power
consumption in the power supply circuit, in order to provide the
constant voltage circuit with the positive temperature coefficient.
By the use of the diode D1 as in FIG. 2 the adjustment for the
positive temperature coefficient can be done with ease for a Zener
current I.sub.Z which is less than 5 mA. For example, to obtain
the positive temperature coefficient .gamma.S under the condition
that R.sub.B /R.sub.A =1.0 V.sub.F =0.7 V and V.sub.Z =2 V, there
stands .[..gamma.S=1.times.(.gamma.Z+2) (mV/.degree.C.) (4).]. .Iadd..gamma..sub.S
=1.times.(.gamma..sub.Z +2)+.gamma..sub.Z (mV/.degree.C) (4) .Iaddend.
From the characteristic curve of FIG. 4 .gamma.Z is 0.77 mV/.degree.C.
for I.sub.Z =1 mA and hence .[..gamma.S=1.23 mV/.degree.C. (5).].
.Iadd..gamma..sub.S =1.23-0.77=0.46 (mV/.degree.C) (5) .Iaddend.
is obtained which is positive.
Accordingly, the temperature coefficient .gamma.S of the constant
voltage circuit can be desirably set to be positive, zero or negative
by varying the value of the Zener current I.sub.Z through the adjustment
of the variable resistor R.sub.c.
Since the adjustment of the variable resistor R.sub.c has no appreciable
effect on the absolute value of the set voltage V.sub.S (actually,
the internal resistance and Zener voltage of the Zener diode are
negligibly slightly increased), the temperature compensation circuit
of FIG. 3 can be incorporated into a circuit requiring a suppressed
temperature dependence characteristic, whereby the variable resistor
R.sub.c is adjusted for temperature compensation to suppress the
temperature dependence. The FIG. 2 air flow meter circuit of the
present invention implements the basic principle of the temperature
compensation described thus far.
In the circuit of FIG. 2 the output signal V.sub.0 of the zero-span
circuit 300 is, ##EQU3## where I.sub.H is current flowing in the
hot wire RH and V.sub.Z is a Zener voltage of the Zener diode ZD1
as described previously, .Iadd.and resistor R.sub.23 is assumed
to be zero, .Iaddend.
and symbol ".vertline..vertline." denotes a parallel
resultant resistance of the resistors R18 and R19 to mean that R18.vertline..vertline.R19=R18.multidot.R19/R18+R19.
Assuming that, ##EQU4## the equation (6) is reduced to .[.V.sub.0
=C.multidot.V.sub.2 -D.multidot.V.sub.Z (7).]. .Iadd.V.sub.0 =C.multidot.V.sub.2
-D.multidot.V.sub.S (7) .Iaddend.
where V.sub.2 is a voltage drop across the resistor R1 as described
previously.
Pursuant to the King's formula, the relation between the flow rate
detection output V.sub.2 and the air flow rate Q is expressed by
where Q is in terms of Kg/h, A and B are coefficients, and V.sub.2
is given by
Consequently, the current I.sub.H in the hot wire RH is related
to the air flow rate Q by a fourth-power root function, and the
coefficients A, B, C and D are determined by the resistors constituting
the air flow meter circuit.
When the temperature of the air flow meter module changes, the
parameters, especially, C, D, I.sub.H and R1 change under the influence
of the temperature coefficients of the component elements constituting
the circuit and as a result, the output signal V.sub.0 of the air
flow meter circuit changes. The change in the output signal V.sub.0
can be cancelled by adjusting the temperature coefficient .gamma.S
of the output voltage V.sub.S of the constant voltage circuit 100
described with reference to the equations (2) and (3).
Denoting a change in the output signal V.sub.0 due to a change
in temperature by .DELTA.V.sub.0 from the equations (7) and (9)
there is obtained .[..DELTA.V.sub.0 =CR.sub.1 .DELTA.I.sub.H +I.sub.H
(C.DELTA.R.sub.1 +R.sub.1 .DELTA.A)-.DELTA.D.multidot.V.sub.S -D.DELTA.V.sub.S
(10).]. .Iadd..DELTA.V.sub.0 =CR.sub.1 .DELTA.I.sub.H +I.sub.H (C.DELTA.R.sub.1
+R.sub.1 .DELTA.C)-.DELTA.D.multidot.V.sub.S -D.multidot..DELTA.V.sub.S
( 10) .Iaddend.
The output change .DELTA.V.sub.0 of the air flow meter circuit
given in equation (10) can be zeroed by adjusting the temperature
dependent change .DELTA.V.sub.S of the output voltage V.sub.S of
the constant voltage circuit 100 through the adjustment of the temperature
coefficient .gamma..sub.S.
Practically, the output voltage of the air flow meter circuit placed
in a predetermined ambient temperature is first confirmed and thereafter,
the air flow meter circuit is placed in a different ambient temperature
and the variable resistor R.sub.c is adjusted so that the value
of the output voltage at condition of the latter ambient temperature
equals the output voltage at condition of the former ambient temperature.
As has been described, according to the present invention, the
air flow meter free from temperature dependence which is compensated
such that its output signal remains unchanged with variations in
the ambient temperature can be provided, and the highly accurate
temperature compensation circuit can also be provided. The application
of the temperature compensation circuit is not limited to the air
flow meter but may be extended to various control circuits which
similarly disagree with changes in the output signals due to temperature
changes. In addition, the temperature coefficient of the temperature
compensation circuit can be desirably set and this characteristic
may be applied to a circuit which is required to have a specified
temperature coefficient.
Further, the present invention is in no way limited to the embodiments
described thus for but may include many modifications without departing
from the spirit thereof and scope of claims. For example, the invention
may be applied to other types of air flow meter than the hot wire
air flow meter as exemplified in the foregoing embodiments. Furthermore,
the Zener current is adjusted by means of the variable resistor
connecting the output of the operational amplifier and the Zener
diode in the foregoing embodiments but for adjustment of the Zener
current, the variable resistor may be replaced with a fixed resistor
and the fixed resistor may be trimmed. Alternatively, the Zener
current may be adjusted by using a separate combination of constant
voltage source and current control device. |