Abstrict A heat sensitive flow meter comprising a power source for changing
an offset voltage .DELTA.E at the time of application of a power
voltage. The power source is connected to the non-inversion input
terminal of an operation amplifier for amplifying a potential difference
between a first heat sensitive element for measuring the temperature
of a fluid and a second heat sensitive element which is cooled by
the fluid. The heat sensitive flow meter can suppress an output
error right after the application of a power voltage and shorten
a target signal reach time.
Claims What is claimed is:
1. A heat sensitive flow meter comprising:
a bridge circuit having a first heat sensitive element for measuring
a temperature of a fluid in a first bridge branch and a second heat
sensitive element which generates heat with a heating current and
is cooled by a flow of the fluid in a second bridge branch;
a differential amplifier for amplifying a potential difference
between the first and second heat sensitive elements;
a fixed temperature difference control circuit for controlling
the heating current so that a temperature difference between the
first and second heat sensitive elements becomes fixed;
a power source for supplying an offset voltage to an input circuit
of the differential amplifier; and
an offset voltage changing means for temporarily changing the offset
voltage at the time of application of a power voltage.
2. The heat sensitive flow meter of claim 1 wherein the offset
voltage changing means adds the offset voltage at the time of application
of a power voltage and gradually reduces the offset voltage.
3. The heat sensitive flow meter of claim 1 wherein the offset
voltage changing means subtracts the offset voltage at the time
of application of a power voltage and gradually reduces the offset
voltage.
4. The heat sensitive flow meter of claim 1 wherein the offset
voltage changing means is composed of a primary filter circuit and
a time constant of the primary filter circuit is set to almost a
same value as a heat time constant of the second heat sensitive
element.
5. The heat sensitive flow meter of claim 1 wherein the offset
voltage changing means is composed of a secondary or higher-order
filter circuit and a time constant of the filter circuit is set
to almost a same value as a heat time constant of the second heat
sensitive element.
6. The heat sensitive flow meter of claim 1 wherein an input to
the offset voltage changing means is independent from an output
of the bridge circuit.
Description BACKGROUND OF THE INVENTION
1. [Field of the Invention]
The present invention relates to a heat sensitive flow meter for
detecting the flow rate of a fluid using heat sensitive elements.
2. [Description of the Prior Art]
In an electronically controlled fuel injector for a car engine,
it is important to measure the quantity of intake air for an engine
for the control of air/fuel ratio with high accuracy. A heat sensitive
flow meter is used in an air flow detector. Two heat sensitive elements
for detecting the flow rate of a fluid such as air are formed on
a ceramic substrate by winding a temperature sensitive resistor
film made from platinum or a platinum wire. The temperature of the
fluid is detected by a first heat sensitive element making use of
changes in the resistance of this heat sensitive element caused
by temperature variations and a current is supplied to a second
heat sensitive element so that the temperature of the second heat
sensitive element becomes higher than the temperature of the first
heat sensitive element by a predetermined temperature. When the
second heat sensitive element is cooled by the flow of the fluid,
a current supplied to maintain the second heat sensitive element
at a predetermined temperature increases, and the flow rate of the
fluid is measured from this change in current value. This control
system is called "fixed temperature difference control system"
or "fixed temperature control system".
This heat sensitive flow meter in which the first and second heat
sensitive elements and a plurality of fixed resistors constitute
a bridge circuit and this bridge circuit is controlled by an operation
amplifier is already known. There is also known a technology for
controlling the responsibility and safety of this bridge circuit
by adjusting the off-set voltage of this operation amplifier to
a predetermined value.
For example, FIG. 10 is a circuit diagram showing an example of
a conventional heat sensitive flow meter, and FIG. 3(C) is a graph
showing the waveform of output B when a power voltage is applied
to this conventional heat sensitive flow meter. In FIG. 10 a terminal
T to which a power voltage is applied is connected to the connector
of a transistor 8 the emitter of the transistor 8 is connected
to one end of a first heat sensitive element 1 and to one end of
a second heat sensitive element 2 the other end of the first heat
sensitive element 1 is connected to one end of a fixed resistor
3 the other end of the fixed resistor 3 is connected to one end
of a fixed resistor 4 and to the inversion input terminal of an
operation amplifier 7 and the other end of the fixed resistor 4
is grounded. The other end of the second heat sensitive element
2 is the output (V5) of the bridge circuit and connected to one
end of a fixed resistor 5 and to the non-inversion input terminal
of the operation amplifier 7 through a DC offset voltage 33. The
output of the operation amplifier 7 is connected to the base of
the transistor 8 and a current is supplied from a power source
to the bridge circuit through the transistor 8 to maintain the balance
of the bridge circuit. The above first heat sensitive element 1
is connected to a bridge branch SA and the second heat sensitive
element 2 is connected to a bridge branch SB. The first and second
heat sensitive elements 1 and 2 are placed at predetermined locations
on the above ceramic substrate.
Describing the operation of this heat sensitive flow meter, when
the flow rate of air increases, the second heat sensitive element
2 placed in the flow of air is cooled and the resistance value thereof
decreases, thereby increasing the potential of a connection point
between the second heat sensitive element 2 and the fixed resistor
5. This voltage change raises the non-inversion input voltage of
the operation amplifier 7 an output voltage thereby rises, a current
is supplied to the bridge circuit through the transistor 8 the
second heat sensitive element 2 generates heat with this current,
and the temperature of the second heat sensitive element 2 is thereby
increased to maintain a fixed temperature difference between it
and the first heat sensitive element 1. Generally speaking, as the
operation amplifier 7 has primary delay characteristics and the
second heat sensitive element 2 has a thermal delay, a fixed temperature
difference control circuit shows secondary delay characteristics.
Since the DC offset voltage 33 is provided for the stable operation
of the secondary delay system, the circuit can operate stably at
the entire range of flow rate.
Heat generated from the second heat sensitive element 2 is transmitted
to the air and to a support section for supporting the second heat
sensitive element 2 and consumed as a loss. When a power voltage
is applied, a heat transmission loss to this support section cannot
be ignored and this heat transmission gradually changes over a long
time. For example, when a power voltage is applied, as shown in
the output B of FIG. 3(A), a flow signal shows a tendency to gradually
reach a final flow rate from a flow rate a little higher than the
final flow rate.
It is known that, when the flow rate sharply changes, responsibility
is reduced by the influence of a heat transmission loss to the support
section. As the prior art for improving responsibility, FIG. 11
shows a fixed temperature difference control circuit for a heat
sensitive flow meter disclosed by Japanese Laid-open Patent Application
No. 7-63588. When this circuit is compared with the circuit of FIG.
10 a differential circuit 34 is connected to the output of the
bridge circuit comprising the second heat sensitive element 2 and
the fixed resistor 5 and the output of the differential circuit
34 is divided into two and connected to comparators 35 and 36 the
outputs of the comparators 35 and 36 are integrated and connected
to a constant current circuit 37 and the output of the constant
current circuit 37 is connected to the non-inversion input terminal
of the operation amplifier 7. One end of a fixed resistor 6 is connected
to the non-inversion input terminal of the operation amplifier 7
and the other end of the fixed resistor 6 is connected to the outputs
of the second heat sensitive element 2 and the fixed resistor 5
to form a loop, and the output of a constant current circuit 38
is connected to the non-inversion input terminal of the operation
amplifier 7. This constant current circuit 38 is driven by a voltage
Vcc obtained by dividing a power voltage by unshown resistors and
controlled by the feedback control of the loop to supply an offset
voltage .DELTA.E.
When the flow rate sharply changes, the feed-back control is carried
out according to a flow change signal to temporarily change this
offset voltage .DELTA.E, thereby improving resposibility.
In the prior art, when the heat sensitive elements 1 and 2 generate
heat by the application of a power voltage, they transmit heat to
the support section and then become stable at a predetermined temperature.
Therefore, it takes time to reach a target signal, thereby deteriorating
stability at the time of application of a power voltage and causing
an output error until the heat sensitive elements 1 and 2 become
stable.
In a heat sensitive flow meter disclosed by Japanese Laid-open
Patent Application No. 7-63588 responsibility is improved by temporarily
changing the offset voltage .DELTA.E according to a flow change
signal. However, since the heat sensitive elements become stable
after they transmit heat to the support section, the influence of
an output error at the time of application of a power voltage is
not cancelled. In addition, when the heat sensitive elements 1 and
2 having low responsibility are used and the offset voltage .DELTA.E
is temporarily changed according to a flow signal, the output of
the bridge circuit readily oscillates due to feedback control, especially
at the time of application of a power voltage.
SUMMARY OF THE INVENTION
The present invention has been made to eliminate the above problems
of the prior art and is aimed to suppress an output error right
after the application of a power voltage without carrying out feedback
control and shorten a target signal reach time.
According to a first aspect of the present invention, there is
provided a heat sensitive flow meter in which a power source for
supplying an offset voltage to the input circuit of a differential
amplifier is provided and an offset voltage changing means for temporarily
changing the offset voltage at the time of application of a power
voltage is provided.
According to a second aspect of the present invention, there is
provided a heat sensitive flow meter in which the offset voltage
changing means adds the offset voltage at the time of application
of a power voltage and gradually reduces the offset voltage.
According to a third aspect of the present invention, there is
provided a heat sensitive flow meter in which the offset voltage
changing means subtracts the offset voltage at the time of application
of a power voltage and gradually reduces the offset voltage.
According to a fourth aspect of the present invention, there is
provided a heat sensitive flow meter in which the offset voltage
changing means is composed of a primary filter circuit and the time
constant of the filter circuit is set to almost the same value as
the heat time constant of the second heat sensitive element.
According to a fifth aspect of the present invention, there is
provided a heat sensitive flow meter in which the offset voltage
changing means is composed of a secondary or higher-order filter
circuit and the time constant of this filter circuit is set to almost
the same value as the heat time constant of the second heat sensitive
element.
The above and other objects, features and advantages of the invention
will become more apparent from the following description when taken
in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
FIG. 1 is a circuit diagram showing the constitution of the fixed
temperature difference control circuit of a heat sensitive flow
meter according to Embodiment 1 of the present invention;
FIG. 2 is a circuit diagram showing the constitution of a power
source according to Embodiment 1;
FIGS. 3(A), 3(B) and 3(C) are graphs showing power source, offset
voltage and output waveforms according to Embodiment 1;
FIG. 4 is a circuit diagram showing the constitution of a power
source according to Embodiment 2 of the present invention;
FIGS. 5(A), 5(B) and 5(C) are graphs showing power source, offset
voltage and output waveforms according to Embodiment 2;
FIGS. 6(A) and 6(B) are circuit diagrams showing the constitutions
of power sources according to Embodiment 3 of the present invention;
FIGS. 7(A) and 7(B) are circuit diagrams showing the constitutions
of power sources according to Embodiment 4 of the present invention;
FIG. 8 is a circuit diagram showing the constitution of a fixed
temperature difference control circuit according to Embodiment 5
of the present invention;
FIGS. 9(A), 9(B) and 9(C) are graph showing a power source, offset
voltage and output waveforms according to Embodiment 5;
FIG. 10 is a circuit diagram showing the constitution of a heat
sensitive flow meter of the prior art; and
FIG. 11 is a circuit diagram showing the constitution of a heat
sensitive flow meter of the prior art.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Preferred Embodiments of the present invention will be described
hereinafter with reference to the accompanying drawings.
Embodiment 1
FIG. 1 is a circuit diagram showing an example of the fixed temperature
difference control circuit of a heat sensitive flow meter according
to Embodiment 1 of the present invention, FIG. 2 is a circuit diagram
of a power source, and FIGS. 3(A), 3(B) and 3(C) are graphs showing
power voltage, offset voltage and output waveforms, respectively.
In these figures, the same or corresponding elements as those of
FIG. 10 and FIG. 11 are given the same reference numerals.
In this Embodiment 1 reference numeral 1 denotes a first heat
sensitive element for detecting the temperature of a fluid, 2 a
second heat sensitive element which generates heat when a heating
current is supplied, and 3 4 5 and 6 fixed resistors. The heat
sensitive elements 1 and 2 and the fixed resistors 3 4 5 and
6 constitute a bridge circuit. Denoted by 7 is an operation amplifier
as a differential amplifier for amplifying the imbalanced voltage
of the bridge circuit, 8 a transistor as a power amplifier for supplying
a current to the bridge circuit, and 9 a power source whose output
is connected to the non-inversion input terminal of the operation
amplifier 7 and which incorporates a filter circuit as an offset
voltage changing means and is driven with a voltage Vcc which is
turned on simultaneously with the application of a power voltage.
A source current I9 which is controlled by the filter circuit flows
into the fixed resistor 6 (resistance value R6) connected to the
non-inversion input terminal of the operation amplifier 7 to supply
an offset voltage .DELTA.E (I9.times.R6) to the non-inversion input
terminal of the operation amplifier 7.
Since it takes time (heat time constant) for the second heat sensitive
element 2 to become stable at a predetermined resistance value right
after the application of a power voltage, the voltage of the second
heat sensitive element 2 reaches a predetermined value with a thermal
delay. During this time, the output V5 of the bridge circuit changes
and an error is generated. To cancel this voltage change of the
output V5 of the bridge circuit, the time constant .tau. of the
filter circuit is adjusted to the same value as the above heat time
constant in advance, the current I9 is supplied to the fixed resistor
6 from the power source 9 to form an offset voltage .DELTA.E, this
offset voltage .DELTA.E is added to the voltage V5 of the second
heat sensitive element 2 and the obtained total voltage is supplied
to the operation amplifier 7. Thus, an output error at the time
of application of a power voltage is eliminated without the feedback
control of the operation amplifier 7. Since the output V5 of the
bridge circuit reaches a target signal without generating an output
error, the target signal reach time can be shortened.
FIG. 2 shows the internal configuration of the power source 9 which
comprises an operation amplifier 19 transistors 11 and 12 fixed
resistors 13 14 15 16 and 17 and a capacitor 18. The fixed resistor
17 and the capacitor 18 constitute a primary filter circuit. One
end of the fixed resistor 13 is connected to a terminal to which
a voltage Vcc is supplied, the other end of the fixed resistor 13
is connected to one end of the fixed resistor 14 the other end
of the fixed resistor 14 is grounded, and the fixed resistors 13
and 14 are connected to the non-inversion input terminal of the
operation amplifier 10. To the terminal to which the voltage Vcc
is supplied is connected one end of the fixed resistor 16 the other
end of the fixed resistor 16 is connected to one end of the fixed
resistor 15 and the other end of the fixed resistor 15 is connected
to the inversion input terminal of the operation amplifier 10. The
other end of the fixed resistor 16 is connected to one end of the
fixed resistor 17 the other end of the fixed resistor 17 is connected
to one end of the capacitor 18 and the other end of the capacitor
18 is grounded. The other end of the fixed resistor 16 is connected
to the emitter of the transistor 12 the base of the transistor
12 is connected to the emitter of the transistor 11 and the base
of the transistor 11 is connected to the output of the operation
amplifier 10. The collector terminals of the transistors 11 and
12 are connected to each other, and the source current I9 supplied
from these collector terminals flows into the fixed resistor 6 to
supply the offset voltage .DELTA.E to the operation amplifier 7.
A voltage is gradually charged into the filter circuit on the inversion
input terminal side of the operation amplifier 10 so that a current
gradually rises and becomes stable at a predetermined value. The
resistance values of the fixed resistors 13 14 15 16 and 17 are
represented by R13 R14 R15 R16 and R17 respectively, and the
capacity of the capacitor 18 is represented by C18.
The time constant .tau. (R17.times.C18) of the filter circuit composed
of the fixed resistor 17 and the capacitor 18 is adjusted to the
same value as the heat time constant of the second heat sensitive
element 2 in advance. This heat time constant is directly measured
by applying a constant current or constant voltage to the second
heat sensitive element 2 and the time constants .tau. of the filter
circuit is adjusted to the same value as this heat time constant.
This time constant .tau., which differs according to the heat capacity
of the second heat sensitive element 2 is several tens of seconds
at the most.
Since the first heat sensitive element 1 rarely generates heat
compared with the second heat sensitive element 2 the heat time
constant of the first heat sensitive element 1 rarely has an influence
upon the heat time constant of the second heat sensitive element
2. However, when the first heat sensitive element 1 generates heat
of itself or the temperature of the first heat sensitive element
1 is changed by heat generated by the second heat sensitive element
2 the heat time constant of the first heat sensitive element 1
has a slight influence upon the heat time constant of the second
heat sensitive element 2.
However, even when this influence exists, the time constant .tau.
of the filter circuit may be set to cancel a total heat time constant
as it is considered that only the second heat sensitive element
2 has the total heat time constant on the basis of the heat time
constant of the first heat sensitive element 1 because the bridge
circuit is a differential circuit for comparing the heat sensitive
elements 1 and 2. To measure this total heat time constant, the
total time constant of the bridge circuit may be measured while
the final output waveform of a flow meter is monitored with an oscilloscope
or the like. That is, the time required for the output V5 to reach
a target signal without operating the power source 9 that is, a
voltage change delay time may be measured.
When the heat time constant of the first heat sensitive element
1 has an influence, it may be considered that only the first heat
sensitive element 1 has the total heat time constant on the basis
of the heat time constant of the second heat sensitive element 2.
To cancel this total heat time constant, the power source 9 and
the fixed resistor 6 are connected to the inversion terminal of
the operation amplifier 7 to supply an offset voltage so as to eliminate
an output error. This is the same as described above in that the
total heat time constant is corrected though the basis is different.
A description is subsequently given of the operation of this fixed
temperature difference control circuit with reference to FIGS. 3(A),
3(B) and 3(C). The output A of the circuit of this Embodiment when
a power voltage is applied to the terminal T at the timing shown
in FIG. 3(A) is compared with the output B of the prior art circuit.
FIG. 3(A) shows a power voltage and Vcc, FIG. 3(B) shows the waveform
of the offset voltage .DELTA.E, and FIG. 3(C) shows the waveform
of the output V5 of the bridge circuit. When a power voltage is
applied at the timing shown in FIG. 3(A), a voltage equal to a voltage
at both ends of the fixed resistor 13 is generated at both ends
of the fixed resistor 16 in FIG. 2 and a current I16 represented
by the following equation flows into the fixed resistor 16.
Meanwhile, a current is charged into the capacitor 18 with a time
constant .tau. (C18.times.R17) and the charged current I18 is represented
by the following equation when an initial value thereof is represented
by Is and a final value thereof is represented by Ie.
That is, the current I18 flowing into the capacitor 18 gradually
decreases from its initial value and becomes null, and is charged
into the capacitor 18 so that the emitter current of the transistor
12 rises from its initial value to a predetermined value. Therefore,
a current It flowing into the transistors 11 and 12 changes from
its initial value according to the current I18 that is, it decreases
from its initial value and becomes stable at a predetermined value.
Since the source current I9 is supplied from the transistors 11
and 12 according to the change of this current It, the source current
I9 gradually decreases from its initial value and becomes stable
at a predetermined value. Since the product of the source current
I9 and the resistance of the fixed resistor 6 is the offset voltage
.DELTA.E, the offset voltage .DELTA.E right after the application
of a power voltage is added to the voltage of the non-inversion
input terminal of the operation amplifier 7 as an initial value
.DELTA.Es. Thereafter, the offset voltage .DELTA.E decreases and
becomes stable at a final value .DELTA.Ee. The initial value .DELTA.Es
is represented by the following equation.
The final value .DELTA.Ee changes as shown by the following equation.
To cancel the output B of the prior art as the output V5 of the
bridge circuit, the offset voltage .DELTA.E gradually changes from
its initial value .DELTA.Es during a time Th to a predetermined
value as shown by the output A of the present invention.
In FIG. 3(C), the time Th is required to initially heat the second
heat sensitive element 2. Since the second heat sensitive element
2 does not generate heat until the resistance value thereof reaches
a predetermined value during this time Th, the bridge circuit supplies
the maximum current from the transistor 8 to the second heat sensitive
element 2 to generate heat. That is, as the bridge circuit does
not start regular control, the operation amplifier 7 operates to
supply the maximum current and outputs a voltage as high as the
power voltage of the operation amplifier 7. Therefore, the output
V5 becomes high during the time Th. Meanwhile, in the power source
9 the source current I9 is controlled by the filter circuit to
cancel the error of the output V5. Since the offset voltage .DELTA.E
changes gradually, when the bridge circuit starts regular control
after the time Th, the output V5 of the present invention becomes
stable at a target signal right after the time Th unlike the output
B of the prior art which becomes stable gradually at a target signal.
Therefore, it is possible to shorten the time required for the output
V5 of the bridge circuit to become stable at the target signal.
The output V5 of the bridge circuit increases linearly together
with the offset voltage .DELTA.E when the flow rate is fixed. Therefore,
when the time constant .tau. is set to substantially the same value
as the heat time constant, an error caused by the influence of a
heat transmission loss of the heat sensitive elements 1 and 2 to
the support section can be apparently cancelled, thereby making
it possible to shorten the target signal reach time at the time
of application of a power voltage.
According to the above constitution, since the offset voltage .DELTA.E
of the operation amplifier 7 for controlling the bridge circuit
is temporarily changed only at the time of application of a power
voltage, an error at the time of the heat generation of the heat
sensitive elements 1 and 2 can be corrected and a more accurate
output V5 can be obtained.
Embodiment 2
In the above Embodiment 1 the power source 9 in which the other
end of the capacitor 18 of the filter circuit is grounded to the
earth has been described. In this Embodiment 2 as shown in FIG.
4 one end of a capacitor 20 is connected to a terminal to which
Vcc is supplied, the other end of the capacitor 20 is connected
to one end of a fixed resistor 19 the other end of the fixed resistor
19 is connected to the fixed resistors 15 and 16 and to the emitter
of the transistor 12 and a filter circuit composed of the fixed
resistor 19 (resistance value R19) and the capacitor 20 (capacitance
value C18) constitutes the power source 9. In this Embodiment 2
as shown in FIG. 5(C), the circuit operation of the prior art is
assumed to be such that the output V5 of the bridge circuit gradually
rises right after the application of a power voltage as shown by
output D. The reason why the output changes like this is that the
heat of the support section transmitted from the second heat sensitive
element 2 has an influence upon the temperature measurement of the
first heat sensitive element 1. To cancel this influence, the time
constant of the filter circuit is adjusted in advance to cancel
the total heat time constant of the second heat sensitive element
2. Therefore, even if the heat of the support section transmitted
from the second heat sensitive element 2 exists, an output error
can be eliminated without influencing the temperature measurement
of the first heat sensitive element 1. In this case, the feedback
control of the operation amplifier is not carried out as well.
A description is subsequently given of the operation of this fixed
temperature difference control circuit with reference to FIGS. 5(A),
5(B) and 5(C). The output C of this Embodiment is compared with
the output D of the prior art circuit when a power voltage is applied
at the timing shown in FIG. 5(A). FIG. 5(A) shows the waveforms
of a power voltage and Vcc, FIG. 5(B) shows the waveform of an offset
voltage .DELTA.E and FIG. 5(C) shows the waveform of the output
V5 of the bridge circuit. When a power voltage is applied at the
timing shown in FIG. 5(A), a voltage equal to a voltage at both
ends of the fixed resistor 13 is generated at both ends of the fixed
resistor 16 in FIG. 4 and a current I16 represented by the following
equation flows into the fixed resistor 16.
Meanwhile, a current is charged into the capacitor 20 with a time
constant .tau. (C20.times.R19) and the charged current I20 is represented
by the following equation when its initial value is represented
by Is and its final value is represented by Ie.
That is, the current I20 flowing into the capacitor 20 gradually
decreases from its initial value Is and becomes null, and is charged
into the capacitor 20 so that the potential of the emitter terminal
of the transistor 12 falls from its initial value to a predetermined
value. Therefore, as a current It flowing into the transistors 11
and 12 changes from its initial value according to the current I20
it gradually decreases from its initial value and becomes stable
at a predetermined value. Since the source current I9 is supplied
from the transistors 11 and 12 along with the change of this current
It, it gradually decreases from its initial value and becomes stable
at a predetermined value. Since the product of the source current
I9 and the resistance value of the fixed resistor 6 becomes the
offset voltage .DELTA.E, the offset voltage .DELTA.E right after
the application of a power voltage is subtracted from the voltage
of the non-inversion input terminal of the operator amplifier 7
as an initial value .DELTA.Es. Thereafter, the offset voltage .DELTA.E
decreases and becomes stable at a final value .DELTA.Ee. The initial
value .DELTA.Es is represented by the following equation.
The final value .DELTA.Ee changes as shown by the following equation.
Meanwhile, since the output V5 of the bridge circuit increases
linearly together with .DELTA.E when the flow rate is fixed, when
the above time constant is set to substantially the same value as
the heat time constant as in Embodiment 1 an error can be apparently
cancelled, thereby making it possible to shorten the target signal
reach time at the time of application of a power voltage.
In FIG. 5(C), the time Th is a time required to initially heat
the second heat sensitive element 2 as in Embodiment 1.
Embodiment 3
In the above Embodiment 1 the filter circuit is provided on the
non-inversion input terminal side of the operation amplifier 10.
In this Embodiment 3 as shown in FIG. 6(A), one end of a capacitor
22 is connected to a terminal T and to one end of the fixed resistor
13 the other end of the capacitor 22 is connected to one end of
a fixed resistor 21 the other end of the fixed resistor 21 is connected
to the other end of the fixed resistor 13 and to the non-inversion
input terminal of the operation amplifier 10 and a filter circuit
composed of the fixed resistor 21 and the capacitor 22 is provided
to control the potential of the non-inversion input terminal of
the operation amplifier 10. Alternatively, as shown in FIG. 6(B),
there may be provided a filter circuit composed of (1) a fixed resistor
23 one end of which is connected to the other end of the fixed resistor
13 and to the non-inversion input terminal of the operation amplifier
10 and (2) a capacitor 24 one end of which is connected to the other
end of the fixed resistor 23 and the other end of which is grounded
to the earth. Thus, the source current I9 for canceling an output
error can be supplied like Embodiments 1 and 2 even when the potential
of the non-inversion input terminal which is a reference potential
for the operation of the operation amplifier 10 is controlled.
Embodiment 4
In the above Embodiment 1 the primary filter circuit is provided.
In this Embodiment 4 a secondary filter circuit composed of fixed
resistors 25 and 26 and capacitors 27 and 28 is provided. As shown
in FIG. 7(A), one end of the fixed resistor 25 is connected to the
other end of the fixed resistor 16 and to the emitter terminal of
the transistor 12 the other end of the fixed resistor 25 is connected
to one end of the fixed resistor 26 and to one end of the capacitor
27 the other ends of the fixed resistor 26 and the capacitor 27
are connected to one end of the capacitor 28 and the other end
of the capacitor 28 is grounded. Alternatively, a secondary filter
circuit composed of fixed resistors 29 and 30 and capacitors 31
and 32 may be provided. As shown in FIG. 7(B), one end of the fixed
resistor 29 is connected to the other end of the fixed resistor
16 and to the emitter terminal of the transistor 12 the other end
of the fixed resistor 29 is connected to one end of the fixed resistor
30 and to one end of the capacitor 31 the other ends of the fixed
resistor 30 and the capacitor 31 are connected to one end of the
capacitor 32 and the other end of the capacitor 32 is connected
to the terminal Vcc to which a power voltage is supplied.
When an output error is canceled with the secondary filter circuit,
the output (charge) of the secondary filter circuit changes more
sharply than the primary filter circuit, thereby making it possible
to further shorten the target signal reach time at the time of application
of a power voltage. An active filter using an operation amplifier
may be provided. A secondary or higher-order filter circuit may
also be provided.
Embodiment 5
In the above Embodiment 1 the offset voltage .DELTA.E is controlled
by the filter circuit incorporated in the power source 9. In this
Embodiment 5 as shown in FIG. 8 a primary or secondary filter
circuit 50 like one described above is provided between the power
source 9 and a terminal to which a voltage Vcc is supplied so that
the rise of the voltage Vcc is delayed as shown by the waveform
E of FIG. 9(A) to delay the supply of the offset voltage .DELTA.E
so as to provide the same performance as in Embodiments 1 2 3
and 4. E of FIG. 9 shows the waveform of the circuit of the present
invention and F shows the waveform of the circuit of the prior art.
Embodiment 6
In the above Embodiment 1 the filter circuit is grounded to the
earth. In the Embodiment 2 the filter circuit is connected to the
terminal to which a voltage Vcc is supplied. In this Embodiment
6 a switching means for selecting between the grounding of the
filter circuit to the earth or the connection of the filter circuit
to the terminal to which the voltage Vcc is supplied is provided.
Since the internal configuration of the filter circuit is the same,
the switching means makes it easy to select a method for changing
the offset voltage .DELTA.E. This can be effected by connecting
a jumper wire (not shown) to the switching means. This switching
means may be provided in the heat sensitive flow meters of FIG.
6(A), FIG. 6(B), FIG. 7(A) and FIG. 7(B).
As having been described above, according to the first aspect of
the present invention, the power source for supplying an offset
voltage to the input circuit of the differential amplifier is provided
and offset voltage changing means for temporarily changing the offset
voltage at the time of application of a power voltage is provided.
Therefore, an output error right after the application of a power
voltage can be suppressed and the target signal reach time can be
shortened.
According to the second aspect of the present invention, since
the offset voltage changing means adds the offset voltage at the
time of application of a power voltage and gradually reduces the
offset voltage, an output error right after the application of a
power voltage can be suppressed and the target signal reach time
can be shortened.
According to the third aspect of the present invention, since the
offset voltage changing means subtracts the offset voltage at the
time of application of a power voltage and gradually reduces the
offset voltage, even if heat transmitted to the support section
from the second sensitive heat element exists, the time constant
of the filter circuit is adjusted to cancel this heat time constant
and an output error can be canceled.
According to the fourth aspect of the present invention, since
the offset voltage changing means is composed of a primary filter
circuit and the time constant of the filter circuit is set to almost
the same value as the heat time constant of the second heat sensitive
element, an output error right after the application of a power
voltage can be suppressed and the target signal reach time can be
shortened.
According to the fifth aspect of the present invention, since the
offset voltage changing means is composed of a secondary or higher-order
filter circuit and the time constant of the filter circuit is set
to almost the same value as the heat time constant of the second
heat sensitive element, the target signal reach time can be shortened. |