## Abstrict According to the invention, to correct the curvature characteristic
in output due to individual difference between the frequency output
type hot-wire flow meters, a frequency output type hot-wire flow
meter comprises a heating resistor disposed in the measurement target
fluid and a voltage frequency conversion circuit for converting
the air flow rate voltage signal that is proportional to the current
flowing through the heating resistor, wherein the voltage frequency
conversion circuit converts the air flow rate voltage signal non-linearly
to generate the output frequency.
## Claims What is claimed is:
1. A frequency output type hot-wire flow meter comprising a heating
resistor disposed in measurement target fluid and a voltage frequency
conversion circuit for converting an air flow rate voltage signal
obtained from said heating resistor to a frequency, wherein said
voltage frequency conversion circuit generates an output frequency
that is non-linear with respect to said flow rate voltage signal.
2. A frequency output type hot-wire flow meter comprising a heating
resistor disposed in measurement target fluid and a voltage frequency
conversion circuit for converting a flow rate voltage signal obtained
from said heating resistor to a frequency, said voltage frequency
conversion circuit comprising an integrating capacitor, two current
sources for providing a flow rate current signal that is proportional
to said flow rate voltage signal in the direction to increase and
in the direction to decrease a voltage of said integrating capacitor,
a switch for connecting said two current sources to said integrating
capacitor alternately, and a switch change-over circuit for generating
a change-over signal to change over said switch so that a frequency
of triangular wave obtained from said voltage of said integrating
capacitor is output as an output frequency, wherein said voltage
frequency conversion circuit generates said output frequency that
is non-linear with respect to said flow rate voltage signal by means
of a correction resistor connected to said integrating capacitor.
3. The frequency output type hot-wire flow meter according to claim
2 wherein said voltage frequency conversion circuit comprises a
correction resistor connected in parallel to said integrating capacitor.
4. The frequency output type hot-wire flow meter according to claim
2 wherein said voltage frequency conversion circuit comprises a
correction resistor connected in series to said integrating capacitor
and a third current source for supplying a constant current to a
connection point between said integrating capacitor and said correction
resistor.
5. The frequency output type hot-wire flow meter according to claim
2 wherein said voltage frequency conversion circuit comprises a
second integrating capacitor connected in parallel to said integrating
capacitor and a correction resistor connected between said two integrating
capacitors.
6. A frequency output type hot-wire flow meter comprising a heating
resistor disposed in measurement target fluid and a voltage frequency
conversion circuit for converting a flow rate voltage signal obtained
from said heating resistor to a frequency, said voltage frequency
conversion circuit comprising an integrating capacitor, two current
sources for providing a flow rate current signal that is proportional
to said flow rate voltage signal in the direction to increase and
in the direction to decrease a voltage of said integrating capacitor,
a switch for connecting said two current sources to said integrating
capacitor alternately, a switch change-over circuit for generating
a change-over signal to change over said switch when said voltage
of said integrating capacitor reaches an upper threshold voltage
or a lower threshold voltage so that a frequency of triangular wave
obtained from said voltage of said integrating capacitor is output
as an output frequency, wherein said switch change-over circuit
has a correction standard voltage, and a timing for generating said
change-over signal is changed based on said correction standard
voltage to thereby generate said output frequency that is non-linear
with respect to said flow rate voltage signal.
7. A frequency output type hot-wire flow meter comprising a heating
resistor disposed in measurement target fluid and a voltage frequency
conversion circuit for converting a flow rate voltage signal obtained
from said heating resistor to a frequency, said voltage frequency
conversion circuit comprising an integrating capacitor, two current
sources for providing a flow rate current signal that is proportional
to said flow rate voltage signal in the direction to increase and
in the direction to decrease a voltage of said integrating capacitor,
a switch for connecting said two current sources to said integrating
capacitor alternately, a switch change-over circuit for generating
a change-over signal to change over said switch when said voltage
of said integrating capacitor reaches an upper threshold voltage
or a lower threshold voltage so that a frequency of triangular wave
obtained from said voltage of said integrating capacitor is output
as an output frequency, wherein said switch change-over circuit
has a delaying circuit, and a timing for generating said change-over
signal is changed by means of said delaying circuit to thereby generate
said output frequency that is non-linear with respect to said flow
rate voltage signal.
8. The frequency output type hot-wire flow meter according to claim
7 wherein said delaying circuit has a correction resistor and a
correction capacitor, and said correction resistor and said correction
capacitor together control a delaying time of said change-over signal.
9. A frequency output type hot-wire flow meter comprising a heating
resistor disposed in measurement target fluid and a voltage frequency
conversion circuit for converting a flow rate voltage signal obtained
from said heating resistor to a frequency, said voltage frequency
conversion circuit comprising an integrating capacitor, two current
sources for providing a current signal in the direction to increase
and in the direction to decrease a voltage of said integrating capacitor,
a switch for connecting said two current sources to said integrating
capacitor alternately, and a switch change-over circuit for generating
a change-over signal to change over said switch when said voltage
of said integrating capacitor reaches an upper threshold voltage
or a lower threshold voltage so that a frequency of triangular wave
obtained from said voltage of said integrating capacitor is output
as an output frequency, wherein one of said two current sources
provides a flow rate current signal that is proportional to said
flow rate voltage signal in the direction to increase said voltage
of said integrating capacitor, and the other one of said two current
sources provides a predetermined constant current in the direction
to decrease said voltage of said integrating capacitor to thereby
generate said output frequency that is non-linear with respect to
said flow rate voltage signal.
10. A frequency output type hot-wire flow meter comprising a heating
resistor disposed in measurement target fluid and a voltage frequency
conversion circuit for converting a flow rate voltage signal obtained
from said heating resistor to a frequency, said voltage frequency
conversion circuit comprising an integrating capacitor, two current
sources for providing a flow rate current signal that is proportional
to said flow rate voltage signal in the direction to increase and
in the direction to decrease a voltage of said integrating capacitor,
a switch for connecting said two current sources to said integrating
capacitor alternately, and a switch change-over circuit for generating
a change-over signal to change over said switch when said voltage
of said integrating capacitor reaches an upper threshold voltage
or a lower threshold voltage so that a frequency of triangular wave
obtained from said voltage of said integrating capacitor is output
as an output frequency, wherein said flow meter further comprises
a third. current source for providing a predetermined constant current
signal in the direction to increase said voltage of said integrating
capacitor; and a fourth current source for providing a predetermined
constant current signal in the direction to decrease said voltage
of said integrating capacitor to thereby generate said output frequency
that is non-linear with respect to said flow rate voltage signal.
## Description FIELD OF THE INVENTION
[0001] This invention relates to a frequency output type hot-wire
flow meter that is used suitably for measurement of air flow rate.
BACKGROUND OF THE INVENTION
[0002] An exemplary conventional frequency output type hot-wire
flow meter is described with reference to FIG. 12. The frequency
output type hot-wire flow meter comprises a heating resistor 101
disposed in measurement target air flow, a fixed resistor 102 connected
to the heating resistor 101 in series, a buffer circuit 103 and
a voltage frequency conversion circuit 104. The target air mass
flow is measured based on the heating resistor current I101 that
flows through the heating resistor 101. The measurement principle
of flow rate for the hot-wire flow meter is well known and the detailed
description of the measurement principle is omitted herein.
[0003] The air flow rate voltage signal Vi proportional to the
heating resistor current I101 is obtained from the heating resistor
current I101. The air flow rate voltage signal Vi is supplied to
the voltage frequency conversion circuit 104 through the buffer
circuit 103. The voltage frequency conversion circuit 104 converts
the air flow rate voltage signal Vi linearly to generate the output
frequency Fo as described below.
[0004] An exemplary voltage frequency conversion circuit 104 is
described with reference to FIG. 13. The voltage frequency conversion
circuit 104 comprises two constant current sources 11 and 12 a
switch 15 a switch change-over circuit 19 for generating a switch
change-over signal, and an integrating capacitor 21 connected to
the output side of the switch 15 and a buffer circuit 27.
[0005] Two constant current sources 11 and 12 receive the air flow
rate voltage signal Vi and output the air flow rate current signal
Ii that is proportional to the air flow rate voltage signal Vi respectively.
Two constant current sources 11 and 12 may be, for example, a current
mirror circuit. One of the two constant current sources is connected
to the integrating capacitor 21 by means of the switch 15. When
the first constant current source 11 is connected to the integrating
capacitor 21 the air flow rate current signal Ii flows in such
a direction that charges are accumulated in the integrating capacitor
21. As the result, the voltage Vic of the integrating capacitor
21 rises. When the second constant current source 12 is connected
to the integrating capacitor 21 the air flow rate current signal
flows in such a direction that charges are released from the integrating
capacitor 21. As the result, the voltage Vic of the integrating
capacitor 21 falls.
[0006] The changing rate of the voltage Vic of the integrating
capacitor 21 is proportional to the air flow rate current signal
Ii. In other words, the changing rate of the voltage Vic of the
integrating capacitor 21 is proportional to the air flow rate voltage
signal Vi.
[0007] Operation of the voltage frequency conversion circuit 104
is described with reference to FIG. 14. The switch change-over circuit
19 has an upper limit threshold voltage Vthh and a lower limit voltage
Vthl(Vthh >Vthl) and compares the voltage Vic of the integrating
capacitor 21 with the threshold voltage to generate a change-over
signal. If the voltage Vic of the integrating capacitor 21 is lower
than the upper threshold voltage Vthh, the switch change-over circuit
19 generates a change-over signal so that the first constant current
source 11 is connected to the integrating capacitor 21. As the result,
the voltage Vic of the integrating capacitor 21 rises.
[0008] When the voltage Vic of the integrating capacitor 21 reaches
the upper threshold voltage Vthh, the switch change-over circuit
19 generates a change-over signal so that the second constant current
source 12 is connected to the integrating capacitor 21. As the result,
the voltage Vic of the integrating capacitor 21 falls. When the
voltage Vic of the integrating capacitor 21 reaches the lower threshold
voltage Vthl, the switch change-over circuit 19 generates a change-over
signal so that the first constant current source 11 is connected
to the integrating capacitor 21. As the result, the voltage Vic
of the integrating capacitor 21 rises.
[0009] The buffer circuit 27 generates a voltage signal that corresponds
to the voltage Vic of the integrating capacitor 21. Therefore triangular
waves are generated as shown in FIG. 14.
[0010] As described hereinabove, the changing rate of the voltage
Vic of the integrating capacitor 21 is proportional to the air flow
rate current signal Ii. Therefore the gradient of the triangular
wave is proportional to the air flow rate current signal Ii. The
larger gradient of the triangular wave results in the larger frequency
Fo of the triangular wave, and the smaller gradient of a triangular
wave results in the smaller frequency Fo of the triangular wave.
Therefore the frequency Fo of the triangular wave is proportional
to the air flow rate current signal Ii. In other words, the frequency
Fo of the triangular wave is proportional to the air flow rate voltage
signal Vi. The frequency Fo of the triangular wave output from the
voltage frequency conversion circuit 104 is represented according
to the following equation.
[0011] Equation 1
Fo=Vi/(R.multidot.C.multidot.dVth)=Ii/(C.multidot.dVth)
[0012] Wherein R denotes the resistance value of the fixed resistor
10 connected to the air flow rate voltage signal Vi, C denotes the
electrostatic capacity of the integrating capacitor 21 and dvth
denotes the rage of change of the voltage Vic of the integrating
capacitor 21 namely the difference between the maximum value and
the minimum value. R, C, and dvth are all constant values.
[0013] As represented according to the equation 1 the air flow
rate voltage signal Vi is converted linearly to obtain the output
frequency Fo in the voltage frequency conversion circuit 104 of
the conventional frequency output type hot-wire flow meter. The
period T of a triangular wave is represented according to the following
equation. 1 T = 1 / Fo = ( 1 / Vi ) ( R C dVth ) = ( 1 / Ii ) (
C dVth ) Equation 2
[0014] Curvature of an output of the conventional frequency output
type hot-wire flow meter that is caused by the difference between
individual hot-wire flow meters is described with reference to FIG.
15A, FIG. 15B, and FIG. 15C. FIG. 15A is a graph showing the relation
between the actual air flow rate Vair and the air flow rate voltage
signal Vi obtained from the heating resistor. The straight line
M shows an exemplary standard characteristic of the frequency output
type hot-wire flow meter. The curve X shows characteristic that
deviates from the standard characteristic due to individual difference.
In other words, the relation between the actual air flow rate Vair
and the air flow rate voltage signal has a curvature deviated from
the standard characteristic.
[0015] FIG. 15B shows the conversion relation between the air flow
rate voltage signal Vi and the output frequency Fo. The straight
line L shows the linear conversion relation between the air flow
rate voltage signal Vi and the output frequency Fo, and shows the
frequency conversion relation in the voltage frequency conversion
circuit 104 of the conventional frequency output type hot-wire flow
meter.
[0016] FIG. 15C shows the relation between the actual air flow
rate Vair and the output frequency Fo obtained by converting the
air flow rate voltage signal Vi according to the conversion relation
shown in FIG. 15B. A curve M.times.L shown in FIG. 15C shows a result
obtained by linearly converting the air flow rate voltage signal
Vi obtained by means of the frequency output type hot-wire flow
meter having the standard characteristic shown by the straight line
M in FIG. 15A according to the straight line L shown in FIG. 15B.
A curve X.times.L shown in FIG. 15C shows a result obtained by linearly
converting the air flow rate voltage signal Vi by means of the frequency
output type hot-wire flow meter having the curvature characteristic
shown by the curve X in FIG. 15A according to the straight line
L shown in FIG. 15B.
[0017] As it is obvious from the comparison between the curve M.times.L
and the curve X.times.L, the frequency output type hot-wire flow
meter having the standard characteristic shows a linear relation
between the actual air flow rate Vair and the output frequency Fo,
but on the other hand the frequency output type hot-wire flow meter
having the curvature characteristic shows a non-linear relation
between the actual air flow rate Vair and the output frequency Fo.
For example, a product that shows characteristic of the straight
line M.times.L may pass inspection, but a product that shows characteristic
of the curve X.times.L may not pass inspection.
[0018] A standard characteristic is shown by the straight line
M in FIG. 15A, but the standard characteristic may be a curve. The
curve X shown in FIG. 15A is an exemplary curvature characteristic,
and curvature characteristic can be various as shown by a curve
Y.
[0019] The voltage output type air flow meter has been known as
well as the frequency output type air flow meter. A technique for
correcting the non-linear characteristic relation between the mass
flow rate of the measurement target and the output voltage signal
has been known. JP-A No. 190647/1999 discloses a technique of correction
arithmetic using a microcomputer for the voltage output type hot-wire
flow meter.
[0020] JP-A No. 337382/1999 JP-A No. 94406/1996 and JP-A No.
62012/1996 disclose techniques for correcting non-linear characteristic
using a linearizing circuit for the voltage output type hot-wire
flow meter.
[0021] However, these techniques are used to correct non-linear
characteristic, but are not used to correct the curvature characteristic
of output due to individual difference between flow meters. A negative
feedback amplifier circuit using an operational amplifier corrects
the output of the above-mentioned linearizing circuit. The curvature
characteristic of output cannot be corrected by this technique because
the flow rate signal and the output signal are in a linear relation.
[0022] [Patent document 1]
[0023] JP-A No. 190647/1999
[0024] [Patent document 2]
[0025] JP-A No. 337382/1999
[0026] [Patent document 3]
[0027] JP-A No. 94406/1996
[0028] [Patent document 4]
[0029] JP-A No. 62012/1996
[0030] As described hereinabove, the frequency output type hot-wire
flow meter causes the curvature characteristic of output due to
individual difference between fluid passages and heating resistors,
and due to change of production line, production lot, and the likes.
SUMMARY OF THE INVENTION
[0031] It is an object of the present invention to provide a frequency
output type hot-wire flow meter that is capable of correcting the
curvature characteristic of output due to individual difference.
[0032] According to the present invention, a frequency output type
hot-wire flow meter comprises a heating resistor disposed in measurement
target fluid and a voltage frequency conversion circuit for converting
an air flow rate voltage signal obtained from the heating resistor
to a frequency, wherein the voltage frequency conversion circuit
generates an output frequency that is non-linear with respect to
the flow rate voltage signal.
[0033] In other words, according to the present invention, the
conversion relation between the air flow rate voltage signal Vi
and the output frequency Fo is non-linear as shown by the curve
NL in FIG. 15B. Therefore, the air flow rate voltage signal Vi obtained
by using a frequency output type hot-wire flow meter having the
curvature characteristic shown by the curve X in FIG. 15A is converted
to the frequency according to non-linear conversion shown by the
curve NL in FIG. 15B to obtain the approximately linear characteristic
X.times.NL that is similar to the standard characteristic M.times.L
as shown by the curve X.times.NL in FIG. 15C.
[0034] According to the present invention, the curvature characteristic
of the output of a frequency output type hot-wire flow meter is
detected in the production process and corrected in the adjustment
step, and as the result the production accuracy of the frequency
output type hot-wire flow meter is improved.
[0035] According to the present invention, the curvature characteristic
of the output of a frequency output type hot-wire flow meter can
be controlled without using a correction LSI and a microcomputer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] Preferred embodiments of the present invention will be described
in detail based on the followings, wherein:
[0037] FIG. 1 is a diagram showing an exemplary frequency output
type hot-wire flow meter in accordance with the present invention;
[0038] FIG. 2 is a diagram showing the first embodiment of the
voltage frequency conversion circuit of the frequency output type
hot-wire flow meter in accordance with the present invention;
[0039] FIG. 3 is a diagram showing the second embodiment of the
voltage frequency conversion circuit of the frequency output type
hot-wire flow meter in accordance with the present invention;
[0040] FIG. 4 is a diagram showing the third embodiment of the
voltage frequency conversion circuit of the frequency output type
hot-wire flow meter in accordance with the present invention;
[0041] FIG. 5 is a diagram showing the fourth embodiment of the
voltage frequency conversion circuit of the frequency output type
hot-wire flow meter in accordance with the present invention;
[0042] FIG. 6 is a diagram showing the fifth embodiment of the
voltage frequency conversion circuit of the frequency output type
hot-wire flow meter in accordance with the present invention;
[0043] FIG. 7 is a diagram showing a part of the fifth embodiment
of the voltage frequency conversion circuit shown in FIG. 6;
[0044] FIG. 8 is a diagram describing the operation of the fifth
embodiment of the voltage frequency conversion circuit shown in
FIG. 6;
[0045] FIG. 9 is a diagram showing the sixth embodiment of the
voltage frequency conversion circuit of the frequency output type
hot-wire flow meter in accordance with the present invention;
[0046] FIG. 10 is a diagram describing the operation of the sixth
embodiment of the voltage frequency conversion circuit shown in
FIG. 9;
[0047] FIG. 11 is a diagram showing the seventh embodiment of the
voltage frequency conversion circuit of the frequency output type
hot-wire flow meter in accordance with the present invention;
[0048] FIG. 12 is a diagram showing an exemplary conventional frequency
output type hot-wire flow meter;
[0049] FIG. 13 is a diagram showing an exemplary voltage frequency
conversion circuit of the conventional frequency output type hot-wire
flow meter;
[0050] FIG. 14 is a diagram for describing the operation of the
voltage frequency conversion circuit shown in FIG. 13; and
[0051] FIGS. 15A, 15B, and 15C are graphs for describing the curvature
characteristic of the frequency output type hot-wire flow meter.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0052] Embodiments of the frequency output type hot-wire flow meter
of the present invention will be described in detail hereinafter
with reference to the drawings. Embodiments in which the fluid is
air are described.
[0053] The concept of the frequency output type hot-wire flow meter
of the present invention is described with reference to FIG. 1.
A frequency output type hot-wire flow meter of this embodiment comprises
a heating resistor 101 disposed in measurement target air flow,
a fixed resistor 102 connected in series to the heating resistor
101 a buffer circuit 103 and a voltage frequency conversion circuit
104. The voltage frequency conversion circuit 104 has a correction
resistor 25.
[0054] The frequency output type hot-wire flow meter of this embodiment
is different from the conventional frequency output type hot-wire
flow meter shown in FIG. 12 in that the voltage frequency conversion
circuit 104 has the correction resistor 25. By providing the correction
resistor 25 the air flow rate voltage signal Vi is converted non-linearly
to the output frequency Fo as shown by the curve NL in FIG. 15B.
[0055] The air flow rate voltage signal Vi is obtained from the
heating resistor current I101 that flows through the heating resistor
101. The air flow rate voltage signal Vi is supplied to the voltage
frequency conversion circuit 104 through the buffer circuit 103.
The voltage frequency conversion circuit 104 converts the air flow
rate voltage signal Vi non-linearly to generate the output frequency
Fo. The relation between the air flow rate voltage signal Vi and
the output frequency Fo is represented by the following equation.
[0056] Equation 3
Fo=Vi/[(R+A).multidot.C.multidot.(dVth+B)+D.multidot.Vi]
[0057] Where A, B, and C denote variable values. The period T of
the triangular wave generated in the voltage frequency conversion
circuit 104 is represented by the following equation.
[0058] Equation 4
T=1/Fo=(1/Vi).multidot.(R+A).multidot.C.multidot.(dVth+B)+D
[0059] In comparison of Equation 4 with Equation 3 the fixed resistor
R is substituted with (R+A), the variable value dvth of the voltage
Vic of the integrating capacitor 21 is substituted with (dvth+B),
and a delaying constant D that is not dependent on the air flow
rate voltage signal Vi is added.
[0060] Accordingly the period T of the triangular wave changes
non-linearly with respect to the air flow rate voltage signal Vi
in this embodiment as shown in Equation 4. A desired non-linearity
can be obtained by selecting the values of A, B, and D properly.
The curvature characteristic of the output can be corrected to obtain
a desired form by selecting the values of A, B, and D properly.
An exemplary selection is described below.
[0061] The first embodiment of the voltage frequency conversion
circuit of the frequency output type hot-wire flow meter of the
present invention is described with reference to FIG. 2. The voltage
frequency conversion circuit of the present embodiment comprises
two constant current sources 11 and 12 for supplying the air flow
rate current signal Ii that is proportional to the air flow rate
voltage signal Vi, a switch 15 for changing over between the two
constant current sources 11 and 12 a switch change-over circuit
19 for generating a change-over signal to change over the switch
15 an integrating capacitor 21 a correction resistor 25 and a
buffer circuit 27.
[0062] The voltage frequency conversion circuit of this embodiment
is different from the exemplary conventional voltage frequency conversion
circuit shown in FIG. 13 in that the correction resistor 25 is added.
The correction resistor 25 is connected to the integrating capacitor
21 in parallel to realize non-linear conversion from the air flow
rate voltage signal Vi to the output frequency Fo. Non-linearity
can be controlled by changing the value of resistance of the correction
resistor 25. Relation between the air flow rate voltage signal Vi
and the output frequency Fo is represented by the following equation.
2 Fo = Vi / ( 2 RC ( Vthh - Vthl ) ) = Ii / ( 2 C ( Vthh - Vthl
) ) Equation 5
[0063] In the above-mentioned example, the changing rate of the
voltage Vic of the integrating capacitor 21 is not proportional
to the air flow current signal Ii because the correction resistor
25 is added. Accordingly the gradient of the triangular wave is
not proportional to the air flow rate current signal Ii.
[0064] When the first constant current source 11 is connected to
the integrating capacitor 21 the air flow rate current signal Ii
flows not only to the integrating capacitor 21 but also to the correction
resistor 25. The air flow rate current signal Ii flowing through
the correction resistor 25 causes the current flowing to the integrating
capacitor 21 to decrease, and influences the rising rate of the
voltage Vic of the integrating capacitor 21. If the air flow rate
current signal Ii is large, the influence of the air flow rate current
signal Ii flowing through the correction resistor 25 is not significant,
but on the other hand if the air flow rate current signal is small,
the influence of the air flow rate current signal Ii flowing through
the correction resistor 25 is significant because the current that
flows through the correction resistor 25 is constant.
[0065] If the air flow rate current signal Ii is large, the rising
rate of the voltage Vic of the integrating capacitor 21 is approximately
proportional to the air flow rate current signal Ii. In other words,
the gradient of the triangular wave is proportional to the air flow
rate current signal Ii. Accordingly the relation between the air
flow rate current signal Vi and the output frequency Fo is linear.
[0066] However, if the air flow rate current signal Ii is small,
the rising rate of the voltage Vic of the integrating capacitor
21 is not proportional to the air flow rate current signal Ii. In
other words, the gradient of the triangular wave is not proportional
to the air flow rate current signal Ii. Accordingly the relation
between the air flow rate current voltage signal Vi and the output
frequency Fo is non-linear as shown by the curve NL in FIG. 15B.
[0067] When the second constant current source 12 is connected
to the integrating capacitor 21 the air flow rate current signal
Ii flows out not only from the integrating capacitor 21 but also
from the correction resistor 25. Then the voltage Vic of the integrating
capacitor 21 falls. The air flow rate current signal Ii flows out
also from the correction resistor 25 to influence the falling rate
of the voltage Vic of the integrating capacitor 21. If the air flow
rate current signal Ii is large, the influence of the air flow rate
current signal Ii flowing through the correction resistor 25 is
not significant, and on the other hand if the air flow rate current
signal Ii is small, the influence of the air flow rate current signal
Ii flowing through the correction resistor 25 is significant.
[0068] The changing rate of the voltage Vic of the integrating
capacitor 21 not only in rising but also in falling, is non-linear
with respect to the air flow rate current signal Ii due to the influence
of the current that flows through the correction resistor 25. Accordingly
the gradient of the triangular wave is non-linear with respect to
the air flow rate current signal Ii, and the output frequency Fo
is non-linear with respect to the air flow rate voltage signal Vi
as shown by the curve NL in FIG. 15B.
[0069] The second embodiment of the voltage frequency conversion
circuit of the frequency output type hot-wire flow meter of the
present invention is described with reference to FIG. 3. The voltage
frequency conversion circuit of the present embodiment comprises
two constant current sources 11 and 12 for supplying the air flow
rate current signal Ii that is proportional to the air flow rate
voltage signal Vi, a third constant current source 13 a switch
15 for changing over between the two constant current sources 11
and 12 a second switch 16 for connecting or disconnecting the third
constant current source 13 a switch change-over circuit 19 for
generating a change-over signal for changing over the switches 15
and 16 an integrating capacitor 21 a correction resistor 25 and
a buffer circuit 27.
[0070] The voltage frequency conversion circuit of this embodiment
is different from the exemplary conventional voltage frequency conversion
circuit shown in FIG. 13 in that the third constant current source
13 and the correction resistor 25 are added. Providing of the third
constant current source 13 and the correction resistor 25 realizes
non-linear conversion from the air flow rate voltage signal Vi to
the output frequency Fo.
[0071] The integrating capacitor 21 and the correction resistor
25 are connected to each other in series, and the third constant
current source 13 is connected to the connection point between the
integrating capacitor 21 and the correction resistor 25 through
the second switch 16. The third constant current source 13 outputs
an auxiliary current Ii2 that is proportional to the air flow rate
current signal Ii flowing to the first constant current source 11
and the second constant current source 12.
[0072] Non-linearity can be controlled by changing the auxiliary
current Ii2 supplied from the third constant current source 13 and
the resistance value of the correction resistor 25.
[0073] The second switch 16 is linked to the first switch 15 and
both switches are operated by the same change-over signal. Accordingly
when the first constant current source 11 is connected to the integrating
capacitor 21 the third constant current source 13 is connected
to the connection point between the integrating capacitor 21 and
the correction resistor 25. The voltage V25 rises due to the auxiliary
current Ii2 supplied from the third constant current source 13
wherein V25 denotes the voltage of the connection point between
the integrating capacitor 21 and the correction resistor 25. The
higher connection point voltage V25 results in the higher rising
rate of the Vic of the integrating capacitor 21.
[0074] Hence, when the first switch 15 functions to connect the
first constant current source 11 to the integration capacitor 21
the auxiliary current Ii2 is supplied from the third constant current
source 13 to the connection point between the integrating capacitor
21 and the correction resistor 25. As the result, the time required
for the voltage Vic of the integration capacitor 21 to reach the
upper threshold voltage Vthh is shortened, and the gradient of the
triangular wave rises. In other words, the period T is shortened
and the output frequency Fo becomes high.
[0075] If the air flow rate current signal Ii is small, the auxiliary
current Ii2 supplied from the third constant current source 13 is
small and the voltage V25 of the connection point is low. Therefore
the effect of the integration capacitor 21 on the voltage Vic is
not significant. In other words, if the air flow rate current signal
Ii is small, the relation between the air flow rate voltage signal
Vi and the output frequency Fo is linear. However, if the air flow
rate current signal Ii is large, the auxiliary current Ii2 supplied
from the third constant current source 13 is large and the voltage
V25 of the connection point is high. Therefore the effect of the
integration capacitor 21 on the voltage Vic is significant. In other
words, if the air flow rate current signal Ii is large, the relation
between the air flow rate voltage signal Vi and the output frequency
Fo is non-linear.
[0076] When the second constant current source 12 is connected
to the integrating capacitor 21 the third constant current source
13 is disconnected from the connection point between the integrating
capacitor 21 and the correction resistor 25. The air flow rate current
signal Ii flows out from the integration capacitor 21 to the second
constant current source 12. Then the voltage Vic of the integrating
capacitor 21 falls. The falling rate of the voltage Vic of the integrating
capacitor 21 is not influenced by the third constant current source
13.
[0077] The third embodiment of the voltage frequency conversion
circuit of the frequency output type hot-wire flow meter of the
present invention is described with reference to FIG. 4. The voltage
frequency conversion circuit of this embodiment comprises two constant
current sources 11 and 12 for supplying the air flow rate current
signal Ii that is proportional to the air flow rate voltage signal
Vi, a switch 15 for changing over between the two constant current
sources 11 and 12 a switch change-over circuit 19 for generating
a change-over signal to change over the switch 15 a correction
resistor 25 two integrating capacitors 21 and 23 and a buffer
circuit 27.
[0078] The voltage frequency conversion circuit of this embodiment
is different from the exemplary conventional voltage frequency conversion
circuit shown in FIG. 13 in that the correction resistor 25 and
the second integrating capacitor 23 are provided.
[0079] In this embodiment, two integrating capacitors 21 and 23
are connected in parallel and the correction resistor 25 is connected
between the two integrating capacitors 21 and 23 to thereby realize
non-linear conversion from the air flow rate current signal Vi to
the output frequency Fo. Non-linearity can be controlled by changing
the resistance of correction resistor 25 and the electrostatic capacity
of second integrating capacitor 23.
[0080] When the first constant current source 11 is connected to
the integrating capacitor 21 the air flow rate current signal Ii
flows not only to the integrating capacitor 21 but also through
the correction resistor 25. The air flow rate current signal Ii
flowing through the correction resistor 25 causes the current flowing
through the integrating capacitor 21 to decrease and influences
the rising rate of the voltage Vic of the integrating capacitor
21.
[0081] The larger air flow rate current signal Ii that flows through
the first and second constant current sources 11 and 12 results
in the larger voltage difference V25 of the correction resistor
25 wherein V25 denotes the voltage difference between terminals
of the correction resistor 25. The larger voltage difference V25
of the correction resistor 25 results in less charges that are stored
in the second integrating capacitor 23 and then the time required
for the voltage Vic of the first integrating capacitor 21 to reach
the upper threshold voltage Vthh is shortened and the gradient of
the triangular wave becomes large. In other words, the period T
is shortened and the output frequency Fo becomes higher.
[0082] On the other hand, the smaller air flow current signal Ii
results in less voltage difference V25 of the correction resistor
25. The smaller voltage difference 25 of the correction resistor
25 has a less influence on the voltage Vic of the first integrating
capacitor 21. In other words, the smaller air flow rate current
signal Ii gives the linear relation between the air flow rate voltage
signal Vi and the output frequency Fo, but the larger air flow rate
current signal Ii gives the non-linear relation.
[0083] The fourth embodiment of the voltage frequency conversion
circuit of the frequency output type hot-wire flow meter of the
present invention is described with reference to FIG. 5. The voltage
frequency conversion circuit of this embodiment comprises two constant
current sources 11 and 12 for supplying the air flow rate current
signal Ii that is proportional to the air flow rate voltage signal
Vi, a switch 15 for changing over between the two constant current
sources 11 and 12 a switch change-over circuit 19 for generating
change-over signal to change over the switch 15 an integrating
capacitor 21 and a buffer circuit 27. A correction standard voltage
Vrc is used for the switch change-over circuit 19.
[0084] The voltage frequency conversion circuit of this embodiment
is different from the exemplary conventional voltage frequency conversion
circuit shown in FIG. 13 in that the correction standard voltage
Vrc is used for the switch change-over circuit 19. Using of the
correction standard voltage Vrc realizes the non-linear conversion
from the air flow rate voltage signal Vi to the output frequency
Fo. The non-linearity can be controlled by changing the correction
standard voltage Vrc.
[0085] The threshold voltages Vthh and Vthl of the switch change-over
circuit 19 are controlled so as to be proportional to the correction
standard voltage Vrc. The upper threshold voltage Vthh and the lower
threshold voltage Vthl are represented by the following equations
respectively.
[0086] Equation 6
Vthh=.alpha..multidot.Vrc
Vthl=.beta..multidot.Vrc
[0087] The output frequency Fo is obtained by substituting the
equation 6 into the equation 5. 3 Fo = Vi / ( 2 R C Vrc ( - ) )
= Ii / ( 2 C Vrc ( - ) ) Equation 7
[0088] It can be seen from the right side member of this equation
that if the correction standard voltage Vrc is changed to be depends
on the air flow rate current signal Ii, the output frequency Fo
changes non-linearly with respect to the air flow rate voltage signal
Vi. For example, if the correction standard voltage Vrc is controlled
to be proportional to the air flow rate current signal Ii, the larger
air flow rate current signal Ii results in the large correction
standard voltage Vrc. As the result, the output frequency Fo is
smaller than that obtained from the proportional relation between
the output frequency Fo and the air flow rate current signal Ii.
On the other hand, if the correction standard voltage Vrc is controlled
to be reversely proportional to the air flow rate current signal
Ii, the larger air flow rate current signal Ii result in the smaller
correction standard voltage Vrc. As the result, the output frequency
Fo is larger than that obtained from the proportional relation between
the output frequency Fo and the air flow rate current signal Ii.
[0089] The fifth embodiment of the voltage frequency conversion
circuit of the frequency output type hot-wire flow meter of the
present invention is described with reference to FIG. 6 FIG. 7
and FIG. 8. The voltage frequency conversion circuit of this embodiment
comprises two constant current sources 11 and 12 for supplying the
air flow rate current signal Ii that is proportional to the air
flow rate voltage signal Vi, a switch 15 for changing over between
the two constant current sources 11 and 12 a switch change-over
circuit 19 for generating a change-over signal to change over the
switch 15 a delaying circuit 31 disposed on the output side of
the switch change-over circuit 19 an integrating capacitor 21
and a buffer circuit 27. For example, mono-stable multi-vibrator
may be used for the delaying circuit 31 as a constituent.
[0090] The voltage frequency conversion circuit of this embodiment
is different from the exemplary conventional voltage frequency conversion
circuit shown in FIG. 13 in that the delaying circuit 31 is connected
to the switch change-over circuit 19. Connection of the delaying
circuit 21 to the switch change-over circuit 19 realizes the non-linear
conversion from the air flow rate voltage signal Vi to the output
frequency Fo.
[0091] As shown in FIG. 7 the delaying circuit 31 has a correction
resistor 25 and a correction capacitor 32 to change the time constant,
namely the delay time. The non-linearity can be controlled by changing
the delay time of the delaying circuit 31.
[0092] The triangular wave generated by means of the voltage frequency
conversion circuit of this embodiment is described with reference
to FIG. 8. D denotes the delay time of the delaying circuit 31.
The changing rate of the voltage Vic of the integrating capacitor
21 namely the gradient of the triangular wave, is proportional
to the air flow rate current signal Ii and is independent of the
delaying circuit 31. The switch change-over circuit 19 does not
generate a change-over signal immediately when the voltage Vic of
the integrating capacitor 21 reaches the upper threshold voltage
Vthh, but generates a change-over signal for switching from the
first constant current source 11 to the second constant current
source 12 after delay time D. Next, the switch change-over circuit
19 does not generate a change-over signal immediately when the voltage
Vic of the integrating capacitor 21 reaches the lower threshold
voltage Vthl, but generates a change-over signal for switching from
the second constant current source 12 to the first constant current
source 11 after delay time D. Therefore the period of the triangular
wave is T+4D for the voltage frequency conversion circuit having
the delaying circuit, wherein the period of the triangular wave
is T for the voltage frequency conversion circuit having no delaying
circuit. The output frequency Fo is represented by the following
equation for the voltage frequency conversion circuit having the
delaying circuit 31.
[0093] Equation 8
Fo=1/(T+4D)
[0094] Wherein T denotes sum of the time required for the voltage
Vic of the integrating capacitor 21 to reach the lower threshold
voltage Vthl from the upper threshold voltage Vthh and the time
required for the voltage Vic of the integrating capacitor 21 to
reach the upper threshold voltage Vthh from the lower threshold
voltage Vthl. T changes with the gradient of the triangular wave,
in other words, the air flow rate voltage signal Vi. T is obtained
according to the equation 2. On the other hand, 4D is constant always.
Therefore the output frequency Fo changes non-linearly with respect
to the air flow rate voltage signal Vi as shown by the curve NL
in FIG. 15B.
[0095] The sixth embodiment of the voltage frequency conversion
circuit of the frequency output type hot-wire flow meter of the
present invention is described with reference to FIG. 9 and FIG.
10. The voltage frequency conversion circuit of this embodiment
comprises two constant current sources 11 and 14 disposed in series,
a switch 15 for changing over between the two constant current sources
11 and 14 a switch 19 for generating a change-over signal to change
over the switch 15 an integrating capacitor 21 and a buffer circuit
27.
[0096] The first constant current source 11 supplies the air flow
rate current signal Ii that is proportional to the air flow rate
voltage signal Vi, and the second constant current source 14 supplies
a predetermined constant current I.sub.14 that is independent of
the air flow rate voltage signal Vi.
[0097] The voltage frequency conversion circuit of this embodiment
is different from the conventionally exemplary voltage frequency
conversion circuit shown in FIG. 13 in that the second constant
current source 14 supplies a constant current I14 that is independent
of the air flow rate voltage signal Vi. In this embodiment, the
second constant current source 14 supplying a predetermined constant
current that is independent of the air flow rate voltage signal
Vi realizes the non-linear conversion from the air flow rate voltage
signal Vi to the output frequency Fo. The non-linearity can be controlled
by changing the value of a predetermined constant current I.sub.14
supplied from the second constant current source 14.
[0098] Operation of the voltage frequency conversion circuit shown
in FIG. 9 is described with reference to FIG. 10. When the first
constant current source 11 is connected to the integrating capacitor
21 the voltage Vic of the integrating capacitor 21 rises. At that
time the rising rate of the voltage Vic of the integrating capacitor
21 is proportional to the air flow rate current signal Ii supplied
from the first constant current source 11. When the voltage Vic
of the integrating capacitor 21 reaches the upper threshold voltage
Vthh, the switch change-over circuit 19 generates a change-over
signal for switching from the first constant current source 11 to
the second constant current source 14. Then the second constant
current source 14 is connected to the integrating capacitor 21
and the voltage Vic of the integrating capacitor 21 falls. At that
time the falling rate of the voltage Vic of the integrating capacitor
21 is proportional to the constant current I.sub.14 supplied from
the second constant current source 14. The constant current I.sub.14
supplied from the second constant current source 14 is independent
of the air flow rate voltage signal Vi and is constant.
[0099] Therefore, by selecting the predetermined value of the constant
current I.sub.14 supplied from the second constant current source
14 the falling rate of the voltage Vic of the integrating capacitor
21 can be controlled. In other words, the predetermined value of
the constant current I.sub.14 is selected to thereby control the
time D required for the voltage Vic of the integrating capacitor
21 to reach the lower threshold voltage Vthl from the upper threshold
voltage Vthh.
[0100] The time T required for the voltage Vic of the integrating
capacitor 21 to reach the upper threshold voltage Vthh from the
lower threshold voltage Vthl is proportional to the air flow rate
voltage signal Vi. However, the time D required for the voltage
Vic of the integrating capacitor 21 to reach the lower threshold
voltage Vthl from the upper threshold voltage Vthh is determined
depending on the constant current I.sub.14 supplied from the constant
current source 14. Therefore the period T of the triangular wave
is non-linear with respect to the air flow rate voltage signal Vi.
The output frequency Fo is represented by the following equation.
[0101] Equation 9
Fo=1/(T+D)
[0102] Where T is determined according to the equation 2. The time
D is a constant to be determined depending on the constant current
I.sub.14 supplied from the second constant current source 14.
[0103] The seventh embodiment of the voltage frequency conversion
circuit of the frequency output type hot-wire flow meter of the
present invention is described with reference to FIG. 11. The voltage
frequency conversion circuit of this embodiment comprises two pairs
of constant current sources 11 13 and 12 14 a switch 15 for changing
over between two pairs of constant current sources 11 13 and 12
14 a switch change-over circuit 19 for generating a change-over
signal to change over the switch 15 an integrating capacitor 21
and a buffer circuit 27.
[0104] Out of the two pair of constant current sources, the first
pair of constant current sources 11 and 12 supplies the air flow
rate current signal Ii that is proportional to the air flow rate
voltage signal Vi, but the second pair of constant current sources
13 and 14 supplies the predetermined constant currents I.sub.13
and I.sub.14 that are independent of the air flow rate voltage signal
Vi.
[0105] The voltage frequency conversion circuit of this embodiment
is different from the exemplary conventional voltage frequency conversion
circuit shown in FIG. 13 in that the second constant current sources
13 and 14 for supplying the predetermined constant current I.sub.13
and I.sub.14 that are independent of the air flow rate voltage signal
Vi are provided. The provided second constant current sources 13
and 14 realize the non-linear conversion from the air flow rate
voltage signal Vi to the output frequency Fo. The non-linearity
can be controlled by changing the value of the constant currents
I.sub.13 and I.sub.14 supplied from the second constant current
sources 13 and 14.
[0106] If the voltage Vic of the integrating capacitor 21 is lower
than the upper threshold voltage Vthh, the first pair of constant
current sources 11 and 13 is connected to the integrating capacitor
21. The sum Ii+I.sub.13 of the air flow rate current signal Ii that
is proportional to the air flow rate voltage signal Vi and the constant
current I.sub.13 that is independent of the air flow rate voltage
signal Vi flows into the integrating capacitor 21.
[0107] As the result, the voltage Vic of the integrating capacitor
21 rises. At that time the rising rate of the voltage Vic of the
integrating capacitor 21 is dependent of the sum of the two currents
Ii+I.sub.13 but not proportional to the air flow rate voltage signal
Vi.
[0108] When the voltage Vic of the integrating capacitor 21 reaches
the upper threshold voltage Vthh, the second pair of constant current
sources 13 and 14 are connected to the integrating capacitor 21.
The sum current Ii+I.sub.14 of the air flow rate current signal
Ii that is proportional to the air flow rate voltage signal Vi and
the constant current I.sub.13 that is independent of the air flow
rate voltage signal Vi flows out from the integrating capacitor
21.
[0109] As the result the voltage Vic of the integrating capacitor
21 falls. At that time the falling rate of the voltage Vic of the
integrating capacitor 21 is dependent of the sum of the two currents
Ii+I.sub.14 and not proportional to the air flow rate voltage signal
Vi.
[0110] Therefore, the gradient of the triangular wave is dependent
of the sum of the air flow rate current signal Ii that is proportional
to the air flow rate voltage signal Vi and the constant current
that is independent of the air flow rate voltage signal Vi, and
not proportional to the air flow rate voltage signal Vi. In other
words, the output frequency Fo changes non-linearly with respect
to the air flow rate voltage signal Vi as shown by the curve NL
in FIG. 15B.
[0111] Embodiments of the present invention are described hereinabove,
however, the present invention is by no means limited to the above-mentioned
embodiments, and it is understood by those skilled in the art that
various changes and modifications may be made in the invention without
departing from the spirit and scope thereof.
[0112] According to the present invention, the curvature characteristic
of the output of a frequency output type hot-wire flow meter is
detected in the production process and corrected in the adjustment
step, and as the result the production accuracy of the frequency
output type hot-wire flow meter is improved.
[0113] According to the present invention, the curvature characteristic
of the output of a frequency output type hot-wire flow meter can
be controlled without using a correction LSI and a microcomputer. |