Abstrict A hot wire type air flow meter employing a hot wire temperature
dependent resistor having a heat generating portion and a non-heat
generating portion comprising a feedback control circuit controlling
the hot wire temperature dependent resistor temperature constant
which includes a phase shift circuit for inducing a sufficient phase
delay in feedback signals thereby compensating a second stage delay
in output signals of the air flow meter caused by the thermal inertia
of the non-heat generating portion and enhancing response characteristic
of the air flow meter.
Claims We claim:
1. A hot wire type air flow meter comprising:
a heat generating temperature dependent resistor formed of a heat
generating member and a non-heat generating member supporting the
heat generating member;
a current detecting resistor connected in series with said heat
generating temperature dependent resistor;
a switching transistor for controlling current flow to said heat
generating temperature dependent resistor; and
a feedback control circuit for said heat generating temperature
dependent resistor for maintaining the same at a predetermined temperature,
said feedback control circuit including a first operational amplifier
for amplifying a feedback signal derived from said current detecting
resistor, phase shift means for providing said feedback control
circuit with a transfer characteristic sufficient to induce a phase
delay in said feedback signal caused by a thermal inertia of the
non-heat generating member of said heat generating temperature dependent
resistor, said phase shift means including a resistor and a capacitor
connected in series, and a second operational amplifier which control
conductivity of said switching transistor with reference to the
phase delayed feedback signal, wherein said shift phase shift means
is disposed between said first and second operational amplifiers.
2. An air flow meter according to claim 1 further comprising a
voltage dividing circuit connected in parallel with said heat generating
temperature dependent resistor, the divided voltage from said voltage
dividing circuit being applied to one input terminal of said first
operational amplifier and a temperature detecting temperature dependent
resistor being arranged in a negative feedback circuit of said second
operational amplifier.
3. An air flow meter comprising:
a heat generating temperature dependent resistor formed of a heat
generating member and a non-heat generating member supporting the
heat generating member and disposed in an air flow passage;
a current detecting resistor connected in series with said heat
generating temperature dependent resistor;
a switching transistor for controlling current flow to said heat
generating temperature dependent resistor;
a feedback control circuit for said heat generating temperature
dependent resistor, said feedback control circuit including an operational
amplifier connected to the series circuit of said heat generating
temperature dependent resistor and said current detecting resistor
and to said switching transistor, and a phase shift circuit providing
said feedback control circuit with a transfer characteristic sufficient
to induce a phase delay in feedback signals to compensate a second
stage delay in output signals of the air flow meter caused by the
a thermal inertia of the non-heat generating member of said heat
generating temperature dependent resistor;
a series circuit including a temperature detecting resistor disposed
in the air passage and a reference resistor, said series circuit
being connected in parallel with the series circuit including said
heat generating temperature dependent resistor and said current
detecting resistor to constitute a bridge circuit, and said phase
shift circuit being disposed between said bridge circuit and said
operational amplifier.
4. An air flow meter comprising:
a heat generating temperature dependent resistor formed of a heat
generating member and a non-heat generating member supporting the
heat generating member, the non-heat generating member of said heat
generating temperature dependent resistor having a thermal inertia
causing a second stage delay in output signals of the air flow meter;
a current detecting resistor connected in series with said heat
generating temperature dependent resistor;
a switching transistor for controlling current flow to said heat
generating temperature dependent resistor; and
a feedback control circuit for said heat generating temperature
dependent resistor for maintaining the same at a predetermined temperature,
said feedback control circuit including phase shift means for providing
said feedback control circuit with a transfer characteristic sufficient
to induce a phase delay in feedback signals to compensate said second
stage delay in output signals of the air flow meter caused by the
thermal inertia of the non-heat generating member of said heat generating
temperature dependent resistor, wherein said phase shift means is
formed of a resistor and a capacitor connected in series.
5. An air flow meter according to claim 4 wherein said feedback
control circuit further includes a first operational amplifier which
controls conductivity of said switching transistor with reference
to the phase delayed feedback signals.
6. An air flow meter according to claim 5 further comprising a
second operational amplifier for amplifying the feedback signals.
7. An air flow meter according to claim 6 wherein said phase shift
means is disposed in a negative feedback circuit of said second
operational amplifier.
8. An air flow meter comprising:
a heat generating temperature dependent resistor formed of a heat
generating member and a non-heat generating member supporting the
heat generating member;
a current detecting resistor connected in series with said heat
generating temperature dependent resistor;
a switching transistor for controlling current flow to said heat
generating temperature dependent resistor; and
a feedback control circuit for said heat generating temperature
dependent resistor for maintaining the same at a predetermined temperature,
said feedback control circuit including phase shift means for providing
said feedback control circuit with a transfer characteristic sufficient
to induce a phase delay in feedback signals to compensate a delay
in output signals of the air flow meter caused by a thermal inertia
of the non-heat generating member of said heat generating temperature
dependent resistor, wherein said phase shift means is formed of
first and second resistors and a capacitor connected in series,
of which input terminals are provided at the ends of the entire
series circuit and of which output terminals are provided at the
ends of a series circuit formed of the second resistor and the capacitor.
9. An air flow meter comprising:
a heat generating temperature dependent resistor disposed in an
air flow passage and having portions which exhibit primary and secondary
kinds of thermal inertia, respectively;
a current detecting resistor connected in series with said heat
generating temperature dependent resistor; and
a feedback control circuit for performing feedback control with
respect to the current flowing through said heat generating temperature
dependent resistor, including:
(a) a switching transistor for controlling current flow to said
heat generating temperature dependent resistor;
(b) a phase shift circuit for providing said feedback control circuit
with a transfer characteristic sufficient to induce a phase delay
in feedback signals to compensate a second stage delay in output
signals of the air flow meter caused by a secondary thermal inertia
of said heat generating temperature dependent resistor, said phase
shift circuit including a resistor and a capacitor connected in
series; and
(c) a first operational amplifier which controls conductivity of
said switching transistor with reference to the phase delayed feedback
signals.
10. An air flow meter comprising:
a heat generating temperature dependent resistor disposed in an
air flow passage;
a current detecting resistor connected in series with said heat
generating temperature dependent resistor; and
a feedback control circuit for performing feedback control with
respect to current flowing through said heat generating temperature
dependent resistor, including:
(a) a switching transistor for controlling current flow to said
heat generating temperature dependent resistor;
(b) a phase shift circuit for providing said feedback control circuit
with a transfer characteristic sufficient to induce a phase delay
in feedback signals to compensate a second stage delay in output
signals of the air flow meter caused by a thermal inertia of said
heat generating temperature dependent resistor;
(c) an operational amplifier which controls conductivity of said
switching transistor with reference to the phase delayed feedback
signals; and
(d) a series circuit including a temperature detecting resistor
disposed in the air passage and a reference resistor, said series
circuit being connected in parallel with the series circuit including
said heat generating temperature dependent resistor and said current
detecting resistor to constitute a bridge circuit, and said phase
shift means being disposed between said bridge circuit and said
operational amplifier.
11. An air flow meter comprising:
a heat generating temperature dependent resistor disposed in an
air flow passage;
a current detecting resistor connected in series with said heat
generating temperature dependent resistor;
a switching transistor for controlling current flow to said heat
generating temperature dependent resistor;
a first operational amplifier which controls conductivity of said
switching transistor with reference to a phase delayed feedback
signal;
a phase shift circuit for providing a transfer characteristic sufficient
to induce the phase delay in said feedback signal caused by a thermal
inertia of said heat generating temperature dependent resistor,
said phase shift circuit including a resistor and a capacitor connected
in series; and
a second operational amplifier for amplifying the feedback signal
derived from said current detecting resistor, wherein said phase
shift circuit is disposed between said first and second operational
amplifiers.
12. An air flow meter comprising:
a heat generating temperature dependent resistor having a non-heat
generating portion and being disposed in an air flow passage, said
resistor having a thermal inertia causing a second stage delay in
output signals of the air flow meter resulting from said non-heat
generating portion thereof;
a current detecting resistor connected in series with said heat
generating temperature dependent resistor;
a switching transistor for controlling current flow to said heat
generating temperature dependent resistor; and
a feedback control circuit for controlling said switching transistor
with respect to the current flowing through said heat generating
temperature dependent resistor, said feedback control circuit including
a phase shift circuit providing said feedback control circuit with
a transfer characteristic sufficient to induce a phase delay in
feedback signals to compensate said second stage delay in output
signals of the air flow meter caused by the thermal inertia of the
non-heat generating portion of said heat generating temperature
dependent resistor, wherein said phase shift circuit is formed of
a resistor and a capacitor connected in series.
13. An air flow meter comprising:
a heat generating temperature dependent resistor disposed in an
air flow passage and having a thermal inertia causing a second stage
delay in output signals of the air flow meter;
a current detecting resistor connected in series with said heat
generating temperature dependent resistor;
a second series circuit, including a temperature detecting resistor
disposed in the air passage and a reference resistor, connected
in parallel with said first series circuit to constitute a bridge
circuit;
a switching transistor for controlling current flow to said heat
generating temperature dependent resistor;
a feedback control circuit for controlling said switching transistor
to maintain said heat generating temperature dependent resistor
at a predetermined temperature; and
a phase shift circuit connected between said bridge circuit and
said feedback control circuit and being provided with a transfer
characteristic sufficient to induce a phase delay in feedback of
signals to said switching transistor to compensate said second stage
delay.
14. An air flow meter according to claim 13 wherein said phase
shift circuit includes a capacitor and at least one resistor connected
in series across said temperature detecting resistor.
Description The present invention relates to a hot wire type air flow meter
for measuring an air flow rate by utilizing the cooling action of
an air flow on a heat generating temperature dependent resistor
disposed in an air flow passage, and more particularly the invention
relates to a hot wire type air flow meter including a heat generating
temperature dependent resistor formed of a heat generating portion
and a non-heat generating portion supporting the heat generating
portion, such as wound type, thin film type and thick film type
temperature dependent resistors. The hot wire type air flow meter
of the present invention is suitably applicable to the measurement
of an intake air flow rate of an automotive internal combustion
engine.
BACKGROUND OF THE INVENTION
The heat generating temperature dependent resistor formed of a
heat generating portion and a non-heat generating portion supporting
the heat generating portion such as wound type, thin film type and
thick film type temperature dependent resistors used in the hot
wire type air flow meter has two kinds of thermal inertias; one
kind of a primary thermal inertia defined by the heat generating
portion and causing a first stage delay in air flow rate signals,
and the other kind is a secondary thermal inertia defined by the
non-heat generating portion and further causing second stage delay
in the air flow rate signals in response to flow rate changes during
an operation of the hot wire type air flow meter.
The fuel amount of an automotive internal combustion engine is
controlled in accordance with detected air flow rate signals from
the air flow meter such that delayed or incorrect air flow rate
signals during the air flow rate changes cause an improper fuel
control for the automotive internal combustion engine.
Since the second stage delay in detected air flow rate signals
caused by the thermal inertia of the non-heat generating portion
of the hot wire resistor is substantial, and hitherto, many attempts
have been made to reduce the thermal inertia of the non-heat generating
portion in order to minimize the second stage delay in the air flow
rate signals caused during air flow rate changes, for example, as
disclosed in U.S. Pat. No. 4715224 issued on Dec. 29 1987. However
such attempts caused structural and manufacturing limitations of
the hot wire resistors having a non-heat generating portion.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a hot wire type
air flow meter using a heat generating temperature dependent resistor
including a non-heat generating support member with a sufficient
response characteristic during air flow rate changes, by compensating,
eliminating or cancelling out a substantial portion of the second
stage delay in the detected response signals during air flow rate
changes.
The object of the present invention is carried out by incorporating
a phase shift circuit into a feedback control circuit which performs
a constant temperature control for the heat generating temperature
dependent resistor whereby a transfer characteristic of the feedback
control circuit is modified to have a predetermined phase delay
characteristic sufficient to compensate the second stage delay in
the air flow rate signals during air flow rate changes which are
detected at the output terminal of the heat generating temperature
dependent resistor.
A hot wire type air flow meter of the present invention comprises
a heat generating temperature dependent resistor formed of a heat
generating member and a non-heat generating member supporting the
heat generating member; a switching transistor for controlling current
flow to the heat generating temperature dependent resistor; a feedback
control circuit for the heat generating temperature dependent resistor,
the feedback control circuit including a first operational amplifier
which controls the conductivity of the switching transistor for
maintaining the heat generating temperature dependent resistor at
a predetermined temperature. The feedback control circuit further
includes a phase shift circuit which provides the feedback control
circuit with a transfer characteristic to phase-delay feedback signals
enough to compensate the second stage delay in air flow rate signals
caused by the non-heat generating member of the heat generating
temperature dependent resistor.
Since a predetermined phase delay is provided for the transfer
characteristic of the feedback control circuit in compliance with
the second stage delay caused by the secondary thermal inertia of
the heat generating temperature dependent resistor, the second stage
delay is compensated for, thereby improving the response characteristic
as a whole of the hot wire type air flow meter of the present invention
when air the flow rate changes.
More particularly, since the phase of the output feedback signal
is delayed from that of the input feedback signal applied to the
feedback control circuit, when the flow rate changes in an increasing
direction, the output feedback signal provides an indication as
if the heat generating temperature dependent resistor were over-cooled,
so that the heating current is rapidly increased, and when the flow
rate changes in a decreasing direction, the output feedback signal
provides an indication as if the heat generating temperature dependent
resistor were over-heated, so that the heating current is rapidly
decreased. In other words, the present invention takes advantage
of an over-shooting phenomenon to reduce the second stage delay
included in the detected response air flow rate signals caused by
the thermal inertia of the non-heat generating support member of
the heat generating temperature dependent resistor.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a circuit diagram of a first embodiment of a hot wire
type air flow meter according to the present invention.
FIG. 2 shows a signal characteristic representing air flow rate
during step-wise air flow rate change according to a conventional
hot wire type air flow meter.
FIG. 3 shows a signal characteristic representing air flow rate
during step-wise air flow rate change according to the embodiment
of the present invention shown in FIG. 1.
FIG. 4 shows a part of a circuit diagram of a second embodiment
of a hot wire type air flow meter according to the present invention.
FIG. 5 shows a circuit diagram of a third embodiment of a hot wire
type air flow meter according to the present invention.
FIG. 6 shows a circuit diagram of a fourth embodiment of a hot
wire type air flow meter according to the present invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
A hot wire type air flow meter according to the present invention
will be explained in detail with reference to the embodiments shown
in the accompanying drawings.
Throughout the drawings, the same numeral denotes an equivalent
or similar element.
FIG. 1 shows an embodiment of the present invention. The reference
numeral 1 represents a heat generating hot wire temperature dependent
resistor, 2 a temperature detecting cold wire temperature dependent
resistor, 3 a current detecting resistor, 4 an amplifier, 5 and
6 operational amplifiers, 7 a switching transistor for controlling
a current flow to the heat generating resistor 1 8 to 15 resistors,
16 a phase shifter, 17 and 18 resistors, 19 a capacitor, 27 a base
resistor, V.sub.B power source voltage and Vcc reference voltage.
The heat generating resistor 1 for example, may be a wound type
resistor comprising a resistance wire are wound around a bobbin,
or a thin film type resistor wherein a thin film resistor is formed
on a substrate. The resistance of the heat generating resistor 1
changes in accordance with the temperature of the resistor. The
heat generating resistor 1 is disposed in a passage of air flow
whose rate is to be measured.
The temperature detecting resistor 2 generally has the same structure
as the heat generating resistor 1 and is disposed in the air flow
passage so as to detect the temperature of the air.
The current detecting resistor 3 detects a current i representing
air flow rate to be measured which is supplied to the heat generating
resistor 1 as a voltage V.sub.2.
The amplifier 4 includes an operational amplifier 5 and amplifies
the voltage V.sub.2 and outputs the amplified voltage to an operational
amplifier 6. The resistors 8 and 9 compensate for the intake air
temperature change in combination with the temperature detecting
resistor 2 and the resistors 10 to 13 adjust the offset voltage
of the operational amplifier 5. The resistors 14 and 15 in combination
determine a DC amplification rate of the amplifier circuit 4.
The operational amplifier 6 receives a feedback control signal
V.sub.0 from the amplifier 4 thereby controlling the transistor
7 so that the voltage V.sub.1 produced by the current i which flows
through the heat generating resistor 1 is equal to the control signal
V.sub.0 of the amplifier 4.
The amplifier 4 the operational amplifier 6 and the transistor
7 therefore constitute a feedback control system with respect to
the current i of the heat generating resistor 1. That is, the current
i is so controlled as to maintain the resistance of the heat generating
resistor 1 at a constant value. Thus, the heat generating resistor
1 is so controlled as to be maintained at a constant temperature,
so that the voltage V.sub.2 represents the air flow rate. More specifically,
the heat generating resistor 1 which is heated by the current i,
is also cooled by the air which passes through the air flow passage
where the heat generating resistor 1 is disposed. As a result, the
temperature of the heat generating resistor 1 is in a condition
of equilibrium between the amount of heating by the current i and
the amount of cooling by the air flow. Consequently, if the current
i is so controlled that the temperature of the heat generating resistor
1 namely, the resistance value thereof is at every moment constant,
the current i changes in accordance with the air flow rate. Thus,
the air flow rate is detected as a function of the current i.
It is now assumed that no phase shifter 16 is provided, and the
output V.sub.0 of the amplifier 4 is input to the operational amplifier
6 as it is, as in the prior art.
The output of the air flow meter is obtained as the voltage V.sub.2
as described above, but a second stage delay is produced in the
output due to the influence of the secondary thermal inertia which
exists in the heat generating resistor 1. The response characteristic
with respect to the stepped change of the air flow rate is shown
in FIG. 2.
The reason for this second stage delay is that when the heat generating
resistor 1 has a non-heat generating portion such as a coil bobbin
or a substrate, the change in the temperature of the non-heat generating
portion is delayed in accordance with the change in the air flow
rate with respect to the change in the temperature of the heat generating
portion.
In this embodiment, however, the phase shifter 16 is provided in
the feedback system leading from the output of the amplifier 4 to
the operational amplifier 6 as is seen from FIG. 1. The phase shifter
16 is composed of a first resistor 17 a second resistor 18 and
a capacitor 19 which are sequentially connected to each other in
series, thereby constituting a series circuit. The voltage V.sub.0
is applied between both ends a and c of the entire series circuit
as an input, and an output voltage V.sub.0 * is obtained from both
ends b and c of the series circuit consisting of the second resistor
18 and the capacitor 19 thereby modifying the transfer characteristics
of the feedback control system to cause a sufficient phase delay
in feedback signals to compensate for the second stage delay in
air flow rate signals.
The phase shifter 16 maintains its output V.sub.0 * at a constant
value when the output V.sub.0 of the amplifier 4 is constant, but
when the output V.sub.0 of the amplifier 4 has changed, the phase
shifter 16 induces a phase delay for the output V.sub.0 with the
following time constant T and amount of adjustment l defined by
the phase shifter 16 and modifies it as the output V.sub.0 *.
wherein R.sub.17 and R.sub.18 represent resistances of the resistors
17 and 18 respectively, and C represents the capacity of the capacitor
19.
In other words, the gain of the phase shifter 16 is reduced only
for high frequency components corresponding to changing signal components
other than non-changing DC signal components.
As a result, a predetermined phase delay is provided for the output
feedback signal with respect to the current i flowing through the
heat generating resistor 1 and the optimum compensation for the
secondary thermal inertia of the heat generating resistor 1 is obtained,
thereby compensating for the second stage delay in the detecting
voltage V.sub.2 with respect to a change in the air flow rate.
FIG. 3 shows the characteristic of the output voltage V.sub.2 exhibited
when the stepped change is provided for the air flow rate in the
same way as in FIG. 2. As is obvious from FIG. 3 according to the
embodiment shown in FIG. 1 it is possible to suppress the second
stage delay of the output voltage V.sub.2 and to obtain a sufficient
responsiveness.
FIG. 4 shows another embodiment of the present invention, wherein
only a portion corresponding to the amplifier 4 shown in FIG. 1
is shown. The phase shifter 16 is applied to the negative feedback
circuit of the operational amplifier 5 itself in the amplifier 4.
In this embodiment, instead of disposing the phase shifter 16 between
the operational amplifiers 5 and 6 as illustrated in FIG. 1.
FIG. 5 shows still another embodiment of the present invention,
wherein the heat generating resistor 1 constitutes a bridge circuit
together with the temperature detecting resistor 2 the current
detecting resistor 3 and a newly added reference resistor 20 and
the phase shifter 16 is disposed between the bridge circuit and
the operational amplifier 5 so that the equilibrium bridge circuit
so controls the heat generating resistor 1 as to be maintained at
a constant temperature.
FIG. 6 shows still further embodiment of the present invention
similar to that shown in FIG. 1 except that a voltage dividing circuit
composed of resistors 21 and 22 is connected in parallel with the
heat generating resistor 1 applying the divided voltage to the negative
terminal of the operational amplifier 6 and that the temperature
detecting resistor 2 is disposed in the negative feedback circuit
of the operational amplifier 5 wherein the numeral 23 is a negative
feedback resistor for the operational amplifier 5 24 and 28 resistors
for adjusting offset of the operational amplifier 5 and 25 and
26 input resistors for the operational amplifier 6.
In the embodiments shown in FIGS. 4 5 and 6 the phase shifter
16 is also inserted in the feedback control system of the heat generating
resistor 1 and the time constant T and the amount of adjustment
l of the phase shifter 16 are properly set, thereby eliminating
the second stage delay in the output voltage V.sub.2 and providing
a sufficient responsiveness.
According to the present invention, since it is possible to compensate
for the second stage delay in the output of the hot wire type air
flow meter which uses a heat generating resistor having a secondary
thermal inertia with respect to the change in the air flow rate,
the responsiveness of the output of the air flow meter with respect
to the change in the air flow rate is advantageously sufficiently
improved.
In addition, the present invention is advantageous in that since
the phase compensation is incorporated in the feedback loop for
the constant temperature control, the S/N ratio of the output is
improved, since the gain of the feedback control circuit with regard
to high frequency components corresponding to external disturbances
such as electric noise is reduced. |