Abstrict A thermal air flow meter is equipped with a flow rate detector
utilizing the phenomenon that heat is carried away in proportion
to flow rate. The flow rate detector comprises a support, a heat-sensitive
resistor formed on the support and leads attached to both ends of
the support. To make the temperature distribution uniform, the heat-sensitive
resistor is formed in such a manner that the resistance per unit
length of the heat-sensitive resistor at either end of the support
is greater than the resistance per unit length of the heat-sensitive
resistor at the center of the support. This arrangement can provide
a thermal air flow meter which has a rapid response to changes in
flow rate.
Claims What is claimed is:
1. In an internal combustion engine a thermal air flow meter including
a flow rate detector for detecting a proportion of a flow rate of
air in dependence upon heat carried away by the air the flow rate
detector comprising a support means, means disposed on said support
means for increasing a response time of the thermal air flow meter
and improving an accuracy of the measurement of flow rate including
a heat-sensitive resistor formed on said support means in such a
manner that the resistance per unit length of said heat-sensitive
resistor at either end of said support means is greater than the
resistance per unit length of said heat-sensitive resistor at a
center of said support means, and leads attached to both ends of
said support means and electrically connected to said heat sensitive
resistor.
2. The thermal air flow meter as defined in claim 1 wherein said
heat-sensitive resistor is linear and is wound around the outer
periphery of said support means in such a manner that said resistor
is wound densely at either end of said support means and coarsely
at the center area thereof.
3. The thermal air flow meter as defined in claim 2 wherein said
support means is cylindrical, said leads are inserted into and attached
to said cylindrical support, and said heat-sensitive resistor is
wound densely at least over the portions corresponding to the portions
at which said lead are inserted into said cylindrical support.
4. The thermal air flow meter as defined in claim 1 wherein said
heat-sensitive resistor has a film-like form and is constructed
by trimming a film formed on the surface of said support means,
and the pitch of said trimming is such that it is dense at either
end of said support means and coarse at the center of said support
means.
Description BACKGROUND OF THE INVENTION
This invention relates to a thermal air flow meter, and, more particularly,
to a thermal air flow meter suitable for measuring the quantity
of air taken in by an internal-combustion engine.
Various systems for measuring the quantity of air taken in by an
internal-combustion engine have been known in the past, such as
movable vane types, types utilizing Karman's vortex sheets, and
so forth. Thermal air flow meters disclosed in, for example, U.S.
Pat. Nos. 3747577 and 4304128 have recently gained wide application
because they usually have a rapid response and can measure the mass
flow rate of the air. A thermal air flow meter of the aforementioned
type includes a platinum wire of a diameter of between 70 .mu.m
and 100 .mu.m stretched within an intake pipe of an internal combustion
engine to act as a flow rate detector. A disadvantage of this construction
resides in the fact that the thermal air flow meter does not have
sufficient durability when the internal-combustion engine is running
badly, and the flow meter undergoes mechanical damage due to backfiring.
In, for example, U.S. Pat. No. 4264961 an improved thermal air
flow meter solving this problem is proposed wherein a part of the
air flowing through the intake pipe is led into a by-pass pipe,
and the platinum wire, acting as the flow rate detector, is mounted
in this by-pass pipe. Since the by-pass pipe has a maximum diameter
of 1 cm, the flow rate detector must also be compact. However, since
the flow meter measures flow rate by utilizing the phenomenon that
the resistance of platinum wire varies with temperature, a higher
sensitivity can be obtained by a higher-resistance platinum wire.
Accordingly, the flow rate detector is constructed by winding platinum
wire around the outer periphery of a piece of insulating material
to make the flow meter compact and increase its resistance. With
this construction, however, another problem occurs in that the response
is lower than that of the system described above, because of the
heat capacity of the bobbin used as the support. This problem is
not limited to the type of meter which utilizes a by-pass pipe,
but also to any meter which utilizes a compact flow rate detector.
The response problem is more critical when the thermal air flow
meter described above is, for example, used for a single-point fuel
injection system.
In single-point fuel injection, a single injection valve is provided
at a point at which the intake pipes of the engine join, and hence
the distances from the fuel injection position to the cylinder inlets
is longer than those in multi-point fuel injection, and the time
taken for the fuel to arrive at the cylinders is longer. Similarly,
since the distance from the fuel injection valve to each cylinder
differs from cylinder to cylinder, delicate matching must be carried
out. Although attempts have been made to compensate for the difference
by use of computer software, these have not been entirely successful.
After all, in single-point injection, only one injection valve distributes
fuel to each cylinder so that delicate matching must be made whenever
the model of the car, and hence the shape of the intake pipes, changes.
Particularly during acceleration or during high-speed operation,
the detection accuracy must be improved so that the pulsating flow
of intake air in the engine can be followed precisely, using a highly
accurate flow rate sensor.
This problem of response occurs not only in the control of an internal-combustion
engine, such as in the single-point fuel injection system, but also
in the measurement of flow rates in general if changes in such flow
rates are rapid.
It is therefore an object of the present invention to provide a
thermal air flow meter which is equipped with a compact flow rate
detector, but which still provides a rapid response.
In a thermal air flow meter equipped with a flow rate detector
utilizing the phenomenon that heat is carried away in proportion
to the flow rate, the thermal air flow meter in accordance with
the present invention is characterized in that the flow rate detector
comprises a support, a heat-sensitive resistor formed on the support,
and leads attached to both ends of the support. The heat-sensitive
resistor is formed on the support in such a manner that the resistance
per unit length of the heat-sensitive resistor at either end of
the support is greater than the resistance per unit length of the
heat-sensitive resistor at its center.
Various experiments and studies have been carried out using conventional
flow rate detectors of the wound type, and have clarified the following
points.
In a conventional flow rate detector, platinum leads which function
both as supports and conductors are attached to both ends of a 2
mm-long bobbin made of an insulating material, and platinum wire
is wound at constant pitch onto this bobbin. When a current flows
through the platinum wire and the heat thus generated is controlled
so that the wire is at a predetermined temperature, the temperature
distribution is such that it is highest at the center and drops
towards the leads. Accordingly, when the set temperature for the
flow rate detector is, for example, 170.degree. C., (this can be
effected by making the resistance of the flow rate detector a predetermined
value), the maximum temperature at the center is about 250.degree.
C.
The reason why the temperature distribution is so large can be
attributed to the following. In the initial stages when the current
starts to flow through the resistance wire, the quantity of heat
generated per unit length is the same; but as the heat is transferred,
temperature differences occur between the glass coating, the bobbin,
and the leads that are in contact with the winding. These temperature
differences change the resistance of each part of the resistance
wire. For instance, the resistance rises locally at the center,
further increasing the quantity of heat generated. When specific
structures are examined, it is first of all obvious that the bobbin
center is hollow while the two end portions hold the leads and adhesive
for the leads, so that they have different volumes which induce
differences in heat capacity. Secondly, heat sinks are generated
from both end portions because leads of a precious metal are fitted.
In accordance with the present invention, since the resistance
of the bobbin is smaller at the center thereof than that at either
end, heat generation is less at the center of the bobbin and is
more at either end, so that the present invention can provide a
thermal air flow meter in which the temperature distribution along
the flow rate detector can be made substantially uniform, and which
has a rapid response.
The resistance of the heat-sensitive resistor per unit length at
the center of the bobbin can be easily made different from that
at either end by winding the resistor onto a support coarsely at
the center and densely at either end, if the heat-sensitive resistor
is a wire, or by making the trimming pitch dense at either end and
coarse at the center if it is produced by trimming after the formation
of a film.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a section through a thermal air flow meter measuring
the quantity of air taken in by an internal-combustion engine, in
accordance with one embodiment of the present invention;
FIG. 2 is a plan view of a portion of the thermal air flow meter
of FIG. 1;
FIG. 3 is a bottom plan view of FIG. 1 with the skirt portion is
removed;
FIG. 4 is a circuit diagram of the thermal air flow meter constructed
in accordance with one embodiment of the present invention;
FIG. 5A is a photomicrograph side view of the flow rate detector
of the thermal air flow meter in accordance with one embodiment
of the present invention at a magnification of fifty;
FIG. 5B is a longitudinal cross-sectional view of the detector
of FIG. 5A at a magnification of fifty;
FIG. 5C is a graph of the temperature distribution along a flow
rate detector;
FIG. 6 is a graph of the response characteristics of one embodiment
of the present invention;
FIG. 7 is a graph of the response characteristics of a prior art
flow meter;
FIG. 8 is a graph comparing the characteristics of a prior art
flow meter and the flow meter of the present invention;
FIG. 9 is a side view of the flow rate detector of a thermal air
flow meter in accordance with another embodiment of the present
invention; and
FIG. 10 is a longitudinal cross-sectional view of the detector
of FIG. 9.
DETAILED DESCRIPTION
Referring now to the drawings wherein like reference numerals are
used throughout the various views to designate like parts and, more
particularly, to FIGS. 1-3 according to these figures, a thermal
air flow meter suitable for measuring the quantity of air taken
in by an internal-combustion engine includes a body portion 10
formed by aluminum die casting, a head portion 11 fitted onto the
body portion 10 with an air cleaner (not shown) being connected
upstream of the head portion. A skirt portion 12 formed by aluminum
die casting is connected at downstream end thereof connected to
an engine (not shown). Part of each of the inner walls of the portion
10 and skirt portion 12 forms form a venturi portion 14. A by-pass
passage 18 is formed in the body portion 10 as well as a main passage
16 with the by-pass passage 18 including a linear portion 18a which
is parallel to the main passage 16 and a curved portion 18b surrounding
the main passage 16.
As shown most clearly in FIG. 3 the curved portion 18b of the
by-pass passage 18 surrounds about 3/4 of the main passage 16. As
shown in FIG. 1 the outlet 18c of the by-pass passage 18 is shaped
as a laterally elongated slot. A heat-sensitive resistor 20 for
measuring the air flow rate, and a heat-sensitive resistor 22 for
compensating for the air temperature, are positioned as sensors
inside the linear portion 18a of the bypass passage 18. The two
heat sensitive resistors 20 22 are attached to support pins which
are connected to an electric circuit within a circuit case 26 through
a piece of heat-insulating material 24. Hot wire resistors or hot
film resistors may be used as the heat-sensitive resistors 20 22
these resistors will be described elsewhere in further detail.
In the electric circuit of FIG. 4 a voltage V.sub.B, from a power
source such as a car, is applied to the heat-sensitive resistor
20 for measuring the flow rate, through a transistor 30. A resistor
32 is connected in series with the heat-sensitive resistor 20 and
is used for measuring a current flowing through the heat-sensitve
resistor 20. The voltage across the ends of the heat-sensitive resistor
20 is divided by resistors 34 36. The divided voltage is applied
to one of the input terminals of a differential amplifier 40 through
a resistor 38. The negative input of an amplifier 42 is grounded
by a resistor 44. The heat-sensitive resistor 22 for temperature
compensation and a resistor 46 function as feedback resistors for
the amplifier 42. The output of the amplifier 42 is applied to the
positive input of the amplifier 40 through a resistor 48. Accordingly,
the amplifier 40 produces an output by amplifying the divided voltage
across the two ends of the heat-sensitive resistor 20 and the voltage
at a junction B between the heat-sensitive resistor 20 and the resistor
32 by an amplification factor in response to the ambient temperature,
so that it is equal to the voltage amplified by the amplifier 42.
The output of the amplifier 40 is applied to the base of the transistor
30 and the amplifier 40 produces an output such that the output
is equal to this input, thereby controlling the transistor 30. As
a result, the heat-sensitive resistor 20 is maintained at a predetermined
temperature of about 170.degree. C. higher than the ambient temperature
of the heat-sensitive resistors 20 22.
A series circuit of a resistor 52 and a variable resistor 54 is
connected to a constant voltage source 50 with a junction of the
resistors 52 54 connected to the negative input of the amplifier
42 through a resistor 56 and with the resistors 52 54 being used
for offset adjustment. The output of the constant voltage source
50 connected to the battery voltage V.sub.B, is divided by resistors
58 60 with a junction of the resistors being connected to the
positive input of the amplifier 40 by a diode 62. This circuit is
a starting circuit, when the key switch of a vehicle is turned on,
a predetermined voltage is applied through the diode 56 so that
the two inputs to the amplifier 40 become different, and an output
is forced from the amplifier 40. The junction of the heat-sensitive
resistor 20 and the resistor 32 is connected to an amplifier 64.
The amplifier 64 produces an output which is the voltage across
the two ends of the resistor 32. Since the resistance of the resistor
32 is constant, the output V.sub.S1 of the amplifier 68 indicates
the current flowing through the resistor 32 that is, the current
flowing through the heat-sensitive resistor 20 and is a signal
indicating the air flow rate. The output V.sub.S2 of the amplifier
40 also indicates the current flowing through the heat-sensitive
resistor 20 and is an air flow rate signal.
As shown in FIGS. 5A, 5B, in a heat-sensitive resistor acting as
a flow rate detector, platinum leads 102 104 are inserted into
either end of a hollow alumina bobbin 100 and are bonded by borosilicate
glass 106 108 respectively. A platinum wire 110 is wound around
the outer periphery of the bobbin 100 in such a manner that the
coil produced is coarser at the center and denser at either end.
In other words, this winding method is different from the conventional
uniform winding. Both ends 112 114 of the platinum wire 110 are
spot-welded to the leads 102 104 respectively. Lead glass is also
applied around the platinum wire 110 and is baked at 600.degree.
C. to form a 10.mu. to 20.mu.-thick protective film 116. Both ends
of the leads 102 104 are spot-welded to support pins 118 (the right-hand
support pin is omitted in the drawing).
The bobbin 110 may be made of any electrically insulating material,
such as, for example, magnesia and zirconia, besides alumina. Other
metallic wires can be used as the leads 102 104 so long as they
are electrically conductive. Platinum paste or metal alloys can
be used as the adhesive 106 108. The platinum wire 110 may be substituted
by other metal wires so long as they are electrically conductive
and have a large coefficient of thermal resistance, but from the
aspect of stability, platinum is preferred.
The dimensions of each part of the embodiment shown in FIGS. 5A
and 5B are as follows:
Total length (L) of bobbin 100: 2 mm
Length l.sub.1 at each end of bobbin 100 at which dense winding
of platinum wire is difficult: 0.1 mm
Length l.sub.2 at each end of bobbin 100 over which platinum wire
is wound densely: 0.45 mm
Length l.sub.3 of the center of bobbin 100 over which platinum
wire is wound coarsely: 0.9 mm
Length l.sub.4 of leads 102 104 inserted into ends of bobbin 100:
0.5 mm
Outer diameter d.sub.1 of bobbin 100: 0.5 mm
Inner diameter d.sub.2 of bobbin 100: 0.3 mm
Outer diameter d.sub.3 of leads 102 104: 0.15 mm
Outer diameter d.sub.4 of support pins 118: 0.8 mm
The method of winding the platimum wire 110 will now be described.
The platinum wire 110 is wound fourteen times around the densely-wound
end portions of the bobbin 100. The platinum wire 110 is 20.mu.-thick
and the spacing between one loop of platinum wire and the next is
12 .mu.m. On the other hand, the platinum wire 110 is wound seven
times around the coarsely-wound portions at either end of the bobbin
so that the spacing between one loop of platinum wire and the next
is 108 .mu.m. Incidentally, it is preferable to wind the platinum
wire completely up to both ends of the bobbin 100 but a sufficiently
tight coil can not always be obtained at the start of winding, because
winding is difficult there.
The length of the center of the bobbin 100 is about 50% of the
total length, and is substantially 50% when the two end portions
are added. Since the bobbin is hollow at the center, however, the
volumetric ratio of the center to that of the two end portions is
40:60 (%). Accordingly, the ratio of their heat capacities is also
substantially this ratio. Furthermore, since the leads 102 104
are connected to the two end portions, heat sinks must be considered.
For these reasons 20% of the total length of the platinum wire is
wound around the center, that is, that portion accounting for 20%
of the total resistance, 40% of the total length of the platinum
wire, or 40% of the total resistance, is wound around each end portion.
As a result, the temperature distribution of the flow rate detector
is such as shown in FIG. 5C, the temperature difference between
the center and either end portion is at most 10.degree. C., and
the maximum temperature has dropped to about 200.degree. C. The
response of this embodiment was measured, with the results shown
in FIG. 6. Similarly, the response of a prior-art detector produced
by winding platinum wire at equal pitch (thirty-five coils with
a spacing of 35 .mu.m due to the uniform winding) is shown in FIG.
7.
The graphs of FIGS. 6 and 7 show the response outputs of the flow
meters when the air flow rate was increased in a single step from
0 Kg/H to 200 Kg/H at time 0 and the response output of the flow
meters when the air flow rate was similarly decreased in a single
step from 200 Kg/H to 0 Kg/H. Numerals in the graphs show the response
time 3.tau. after the change in flow rate until 95% of the full
scale was reached. In the prior art flow meter, the rise response
time was 1700 ms and the fall response time 107 ms. In the present
embodiment, the rise and fall response times were improved to 105
ms and 60 ms, respectively. The rise response time, in particular,
was reduced by about 1/7.
Since the rise response time is reduced, the difference between
the rise response time and the fall response time is small, so that
when a pulsating flow in the engine is being detected, the flow
rate can be detected with an accuracy approximately that of the
mean value, and the detection accuracy of the flow rate can be improved.
Since the heat transferred to the air flow is increased, the sensitivity
of a heat-flow sensor can be improved. FIG. 8 shows two characteristics,
that is, the characteristics of a prior art sensor and those of
a sensor with coarse winding, in which the square of the current
I.sub.H applied to the resistor is plotted along the ordinate and
the square root of the flow rate Q along the abscissa. The power
consumption is reduced by 15% on the low flow-rate side for the
sensor with the coarse central winding, in comparison with that
of the prior art. The heat transferred to the air flow is improved
by about 20% in terms of gradient, and the sensitivity can be improved.
Experiments were carried out involving changes in the winding ratios.
The results are shown in Table 1.
In the prior art flow meter, the wire is wound thirty-five times
at equal pitch. Example 1 is the embodiment of the invention described
above in which the wire is wound densely fourteen times at either
end and coarsely seven times at the center. It was found to be difficult
to wind the wire more than fourteen times within a zone of the length
of 0.45 mm from either end (because neighboring platinum wires come
into contact with each other). Accordingly, the experiment was repeated
by winding the wire thirteen times on either side and nine times
at the center, without changing the overall number of windings in
Example 2. This experiment provided a rise response time of 180
ms and a fall response time of 83 ms, these values are superior
to those of the prior art but are inferior to those of Example 1.
In other words, with Example 2 a heat sink occurs from either end,
and the temperature is higher at the center than at either end so
that variations occur in the temperature distribution. In Example
3 the number of windings was the same as those of Example 1 but
there were two more coils at the center to increase slightly the
temperature at the center. With this example, the response characteristics
were substantially the same as those of Example 1.
In other words, the response can be improved by winding the wire
more densely at either end than at the center. The density of the
winding is preferably determined experimentally in accordance with
the overall heat capacity distribution and the magnitude of the
heat sink at each end.
TABLE 1 ______________________________________ Each end Center
Total Rise Fall ______________________________________ Prior art
11.3 11.3 35 1700 ms 107 ms Example 1 14 7 35 105 60 Example 2 13
9 35 180 83 Example 3 14 9 37 100 56 ______________________________________
The examples described above had hollow bobbins in order to restrict
any increase in the total heat capacity, and facilitate the fitting
of the leads. However, the bobbin can be a cylindrical bobbin, or
a support on a flat sheet. In such a case, the heat capacity is
substantially uniform so that heat sinks must be primarily considered.
In the embodiment of FIGS. 9 and 10 leads 202 and 204 are inserted
into both ends of a hollow bobbin 200 of an insulating material,
and the bobbin 200 and the leads 202 204 are bonded by adhesive
206 208. A platinum film 210 is provided around the outer periphery
of the bobbin 200. The platinum film 210 is spirally trimmed by
a laser so that the trimming pitch is dense at either end and is
coarse at the center thereof. In this manner, the resistance at
either end can be made large and the resistance at the center small,
even if the film is trimmed by the laser to a constant width. This
arrangement makes the temperature distribution in the axial direction
of the bobbin 200 constant, in the same way as in the embodiment
described above. In this embodiment, the connection of the platinum
film 210 to the leads 202 204 can be done easily by forming the
platinum film 210 so that it reaches the leads 202 204.
In a thermal air flow meter of the type in which a platinum film
is formed on a flat sheet, the platinum film 210 is trimmed into
a zigzag such that its pitch is dense at either end and coarse at
the center. Thus, substantially the same effects as those of the
embodiments described above can be obtained.
In accordance with the present invention, the temperature distribution
of the flow rate detector can be made substantially uniform to improve
the heat transfer quantity thereof with respect to the air flow.
Accordingly, the sensitivity as well as the accuracy of the measurement
of the flow rate can be improved.
The rise response time can be made faster by reducing the highest
temperature of the bobbin.
Since a value approximately the same as the true value of a pulsating
quantity can be detected by reducing the difference between the
rise and fall response time, the accuracy of measuring flow rate
can be improved. |