Abstrict In the thermal process air flow meter according to the invention
first, second and third electrode terminals are arranged on the
same surface at one end of a substrate. A first exothermic resistor
is connected between the first and second terminals, and a second
exothermic resistor is connected between the second and third terminals,
with the first exothermic resistor on a front surface of a substrate
and the second exothermic resistor on the opposite (back) surface.
The substrate may also carry the semiconductor elements of an electronic
circuit used to analyze electric signals from the terminals. In
a preferred embodiment of the invention, the substrate is mounted
at an incline relative to the direction of unobstructed air flow
in the air passage.
Claims We claim:
1. Bidirectional air flow detector comprising:
a substrate arranged in an air flow path and inclined at an angle
relative to air flow in said air flow path;
a first exothermic resistor arranged on a first surface of said
substrate, said first exothermic resistor outputting a signal indicative
of heat dissipated thereby;
a second exothermic resistor arranged on a second surface of said
substrate opposite said first surface, said second exothermic resistor
outputting a signal indicative of heat dissipated thereby, said
first and second exothermic resistors being electrically connected
to each other in series;
an air flow detection circuit for detecting air flow, said air
flow detection circuit comprising a bridge circuit having said electrically
connected first and second exothermic resistors forming a first
arm of said bridge circuit between adjacent bridge points, and at
least one additional resistor forming a second arm of said bridge
circuit;
a flow detection circuit for comparing said output signals of said
first and second exothermic resistors and for outputting a direction
signal based on relative magnitude thereof.
2. Bidirectional air flow detector according to claim 1 wherein
said flow direction circuit comprises:
a first sensor for sensing a voltage across said first exothermic
resistor;
a second sensor for sensing a voltage across said second exothermic
resistor; and
a comparator coupled to receive output signals from said first
and second sensors.
3. Bidirectional air flow detector according to claim 2 wherein
said first and second sensors comprise first and second differential
amplifiers connected in parallel to said first and second exothermic
resistors, respectively.
4. Bidirectional air flow detector according to claim 3 wherein
said flow detection circuit comprises:
a feedback amplifier connected diagonally in said bridge circuit;
and
a switch element with controls current flowing in said bridge circuit
in response to an output of said feedback amplifier.
5. An air flow detector arrangement comprising:
a substrate;
an air flow detector having a first heat sensitive detector element,
a second heat sensitive detector element and a circuit for determining
an amount and direction of air flow based on outputs from both of
said first and second heat sensitive detector elements, said first
and second heat sensitive detector elements being mounted on opposite
sides of said substrate;
a plurality of electrodes arranged on one of said first and second
surfaces at an end of said substrate, whereby one of said first
and second heat sensitive detector elements is arranged on the surface
of said substrate on which said plurality of electrodes is arranged,
and the other of said first and second heat sensitive detector elements
is arranged on the surface of said substrate opposite that on which
the plurality of electrodes is arranged;
wherein said first heat sensitive detector element is connected
between a first pair of said electrodes, and said second heat sensitive
detector element is connected between a second pair of said electrodes,
the one of said first and second heat sensitive detector elements
which is arranged on the surface opposite the surface on which the
plurality of electrodes is arranged being electrically connected
to at least one of the electrodes through a plated hole in said
substrate between said first and second surfaces, and one electrode
of said second pair being the same as an electrode of said first
pair.
6. Apparatus according to claim 5 wherein said first and second
heat sensitive detector elements are aligned in registration on
opposite surfaces of said substrate.
7. Apparatus according to claim 5 when said first and second heat
sensitive detector elements are film type exothermic resistors.
8. Apparatus according to claim 5 further comprising:
a switch arrangement for controlling a flow of current through
said first and second heat sensitive detector elements;
means for measuring electric current flow through said first and
second heat sensitive detector elements;
means for detecting a first voltage across said first heat sensitive
detector element;
means for detecting a second voltage across said second heat sensitive
detector element; and
a comparator coupled to receive and compare said first and second
voltages.
9. Air flow detector arrangement according to claim 5 wherein said
first and second heat sensitive detector elements comprise first
and second exothermic resistors, respectively.
10. Apparatus according to claim 9 wherein said first and second
exothermic resistors are arranged on opposite surfaces of a substrate
which is disposed in said air flow path and inclined at an angle
relative to air flow therein.
11. Apparatus according to claim 9 wherein said first and second
exothermic resistors are aligned in registration with each other
on said opposite surfaces.
Description BACKGROUND OF THE INVENTION
This invention relates to exothermic resistor elements, and to
a thermal process air flow meter of the type used to measure forward
and backward air flows in internal combustion engines, and the like.
Thermal process flow meters are frequently used for electronically
controlled fuel injection systems for internal combustion engines,
because of their advantages such as compact size, quick response,
etc. Such air flow meters are disclosed, for example, in Japanese
Patent Applications Laid-Open No. 265118/1988 and No. 185416/1989
as well as in U.S. Pat. No. 5086650.
In conventional exothermic resistor elements, electrode terminals
are provided on both the right and left ends, with the number of
terminals being four in some cases and three in others. When three
electrode terminals are provided, they are arranged on opposite
walls of a suction passageway, which complicates the connection
of the exothermic resistor element to the electric circuit. Those
elements also occupy excessive space for mounting.
In addition, in conventional thermal process air flow meters, if
both the first and second exothermic resistors are arranged parallel
to the direction of air flow, on the same surface of a substrate,
the difference in their respective heat transmission is small. When
they are inclined relative to each other, the difference in the
heat transmission is less affected by turbulence, but it is impossible
to generate a significant difference between the amount of radiated
heat. As a result, the heat response is slowed when the air flow
reverses, which makes it difficult to detect the direction of air
flow accurately. This problem has remained unsolved heretofore.
U.S. Pat. No. 5086650 discloses an air flow detector in which
the substrate is inclined relative to the air flow direction, and
bi-directional measurement is achieved by placing a first exothermic
air flow detector to measure air flow in a forward direction on
one side of the substrate, and a second (identical) exothermic air
flow detector to measure reverse air flow on the other side. Each
of the respective detectors has a heater element situated adjacent
to heat detectors, and air flow is measured by pulsing the heater
and measuring the phase shift in the output signal of a downstream
detector. This arrangement achieves bidirectional air flow detection
but requires a complex configuration of heater and sensors as well
as complex circuitry.
SUMMARY OF THE INVENTION
One object of the present invention, is to provide a thermal process
air flow meter that generates a significant difference between the
radiated heat from the exothermic resistance elements,.is simplified
for easier connection to the electric circuit, and can accurately
detect and measure the direction of air flow.
This and other objects and advantages are achieved by the exothermic
resistor element according to the invention in which first, second
and third electrode terminals are arranged on the same surface at
one end of a substrate, which may be made for example from aluminum
oxide. A first exothermic resistor is connected between the first
and second terminals, and a second exothermic resistor is connected
between the said second and third terminals, with the first exothermic
resistor on a front surface of the substrate and the second exothermic
resistor on the opposite (back) surface. Holes through the substrate
are provided between the second and third terminals and the respective
ends of the second exothermic resistor, for the purpose of electrical
connection.
The substrate may also carry the semiconductor elements of an electronic
circuit used to analyze electric signals from the terminals. The
semiconductor elements may be in the form of an integrated circuit
incorporated into a semiconductor chip.
In a preferred embodiment of the air flow meter according to the
invention, a substrate with exothermic resistors on opposite sides
thereof is mounted at an incline relative to the direction of unobstructed
air flow in the air passage. In this manner, an air flow is channeled
over the resistor on one side, and the resistor on the other side
is effectively shielded from the air flow, depending on the direction
of air movement. (The first and second resistors may also be mounted
on separate substrates which are inclined in this manner.) Thus,
when the air flow direction changes, a significant difference exists
between the heat transmission of the first and second resistors
due to differences in the thickness of the temperature boundary
layers at their respective surfaces. As a result, a difference can
be detected between the heat radiated by the first and second resistors,
and it is possible to improve the response time when the air flow
is reversed, so that the air flow direction can be detected and
measured accurately.
With the configuration mentioned above, both the number of connections
on the terminals and size of the substrate can be reduced. Also,
the substrate can be connected directly to the electric circuit
at one end, which simplifies the overall structure and provides
for easier connection of the exothermic resistors.
Other objects, advantages and novel features of the present invention
will become apparent from the following detailed description of
the invention when considered in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a first embodiment of the exothermic resistor element
according to the invention;
FIG. 2(a) is a plan view of an exothermic resistor element of a
second embodiment of the invention;
FIG. 2(b) is a side view of the exothermic resistor element of
FIG. 2(a);
FIG. 3(a) is a plan view of the exothermic resistor element in
a third embodiment of the invention;
FIG. 3(b) is a cross sectional side view of the exothermic resistor
of FIG. 3 (a), taken along line III(b)-III (b) of FIG. 3 (a);
FIG. 4 is a plan view of the mounting structure according to the
invention;
FIG. 5 is a partial horizontal cross sectional view of the mounting
structure of FIG. 4 taken along line V--V of FIG. 4;
FIG. 6 shows part of the electric circuit used in the invention;
FIGS. 7a and 7b are partial enlargements of additional embodiments
of the exothermic resistor element according to the invention;
FIGS. 8a and b contain a graphic depiction of the calculation results
for heat transmission on a flat plate in a uniform air flow; and
FIG. 9 is a plan view of the exothermic resistor element in another
embodiment of this invention.
DETAILED DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a first embodiment of the exothermic resistor element
according to the invention, which has first, second and third terminals
14 15 and 16. A first exothermic resistor 6a is connected between
the first and second terminals 14 and 15 and a second exothermic
resistor 6b is connected between the second and third terminals
15 and 16. The terminals 14 15 16 and the exothermic resistors
6a and 6b are formed of platinum or nickel film on the substrate
5 which is made of an electrically insulating material. The terminal
15 is common to both of the exothermic resistors 6a and 6b. The
first, second and third terminals 14 15 and 16 are arranged at
one end of the substrate on the same surface.
FIG. 2(a) shows a plan view of another embodiment of the exothermic
resistor element according to the invention, while FIG. 2(b) shows
a side view. In this embodiment, each of the terminals 15 16 has
first and second parts arranged on front and back surfaces of the
substrate. The first exothermic resistor 6a is provided on the front
surface of the substrate 5 and the second exothermic resistor 6b
is provided on the back surface. The first terminal 14 and a first
part 15a of the second electrode terminal are provided on the front
surface, and the first exothermic resistor 6a is connected between
them. Second parts of the second and third terminals 15b and 16b
are provided on the back surface of the substrate, and the second
exothermic resistor 6b is connected between them. In addition, a
plated passageway or hole 18 is provided through the substrate 5
as a means of electrically connecting the first and second parts
15a and 15b of the second terminal, and the first and second parts
16a and 16b of the third terminal.
FIG. 3 shows another embodiment of the exothermic resistor element
according to the invention. FIG. 3(a) shows a plan view and FIG.
3(b) shows a cross-section taken along the line III(b)-III(b) in
FIG. 3(a). The first and second exothermic resistors 6a and 6b are
formed on the surface of a semiconductor element 20 and are made,
for example, in the form of film resistors. They are coupled to
terminals 14 15 and 16 with the terminal 15 being common to both.
FIGS. 4 and 5 show an embodiment of a thermal process air flow
meter in which the exothermic resistor element according to the
invention is mounted. FIG. 4 shows a plan view, and FIG. 5 shows
a horizontal cross-section taken along line V--V of FIG. 4.
An electric circuit 2 is provided on the outer periphery of the
external passageway 1. An inner passageway 4 is mounted inside the
external passageway 1 by means of the fixture member 3. The substrate
5 the first and second exothermic resistors 6a and 6b provided
thereon, the exothermic resistor circuit 8 having the first, second,
and third terminals, and a temperature compensation resistor 7 (which
detects the temperature of intake air; see FIG. 6) are all arranged
in the inner passageway 4. The exothermic resistor element 8 is
connected directly to the electric circuit through the fixture member
3.
FIG. 6 shows a detection circuit, which is part of the electric
circuit 2 in FIG. 5. This circuit comprises a bridge circuit having
a feedback amplifier 11 a transistor 9 first and second exothermic
resistors 6a and 6b, a temperature compensation resistor 7 and
fixed resistors 12 and 13. The bridge circuit itself is configured
as a conventional hot wire detector, which measures the quantity
of air flow in a known manner. A flow direction detection circuit
comprising differential amplifiers 10a, b and comparator 10c detects
the difference between the output signals of terminals 15 and 16
on the one hand, and terminals 15 and 14 on the other, (representing
the amount of radiated heat from the first and second exothermic
resistors 6a and 6b), to judge the direction of air flow. That is,
in the case of forward air flow, the amount of heat dissipated by
the resistor 6a is increased while that of resistor 6b is not. Hence,
the voltage drop V.sub.R6a is smaller than V.sub.R6b, and the comparator
10c outputs a logical 1 signal. For reverse air flow, the situation
is the opposite, and comparator 10c outputs a logical 0 signal.
The quantity of air flow is measured by the voltage at node 16 in
a conventional manner, as noted previously. (Flow signal should
be zero flow and not be used.)
As explained above, the three terminals 14 through 16 are disposed
at one end of the substrate. This arrangement is effective to achieve
the following improvements for transmitting the electric signals
from the exothermic resistor element 8 to the electric circuit 2.
(i) The substrate size can be reduced, with the number of electrode
terminals reduced to three;
(ii) The electrode terminals can be connected to the electric circuit
with only three wires;
(iii) The wiring space required for such connection is reduced;
and
(iv) The electrode terminals are connected to the electric circuit
in the shortest distance.
These advantages collectively provide a simpler structure for the
substrate, and facilitate easier mounting of the components thereon.
FIG. 7a illustrates the flow of air around an exothermic resistor
circuit 8 according to the invention, which is mounted in the inner
passageway 4 of the thermal process air flow meter. In this embodiment,
which incorporates the exothermic resistor element 8 shown in FIG.
2 (with resistors 6a, 6b on opposite sides of the substrate), the
substrate 5 is inclined at an angle relative to a horizontal flow
of air inside the passageway 4. With this arrangement, a forward
air flow (left to right in the figure) is channeled over the exothermic
resistor 6a by the substrate itself, which effectively blocks the
flow of air in the immediate vicinity of the surface of the exothermic
resistor 6b. When the flow reverses on the other hand, air is channeled
over the resistor 6b, while resistor 6a is effectively shielded
from the reverse air flow. In other words, the air flows on the
surface of the first exothermic resistor 6a but is diverted from
the surface of the second exothermic resistor 6b, in the forward
flow, and vice versa.
Usually, the air flow in the inner passageway 4 is laminar, and
it is well known that the heat transmission of a flat plate placed
in such a uniform laminar air flow is closely related to the thickness
of the temperature boundary layer on the plate surface. Thus, the
temperature boundary layer 19a on the surface of the first exothermic
resistor 6a becomes thinner than the temperature boundary layer
19b of the second exothermic resistor 6b because of the difference
in the amount of local air turbulence. Therefore, heat transmission
differs significantly between the first and second exothermic resistors
6a and 6b.
As noted previously, the respective exothermic resistors 6a, 6b
may also be mounted on separate substrates. See FIG. 7b.
FIG. 8 is a graphic illustration of the heat transmission of the
flat plate in a uniform flow of air. The surface length along the
air flow direction of the flat plate is displayed on the horizontal
axis, while the heat transmission at a given point on the surface
is shown on the vertical axis. The solid lines represent the heat
transmission rate on the front and rear sides of the slope when
the flat plate is inclined by 30.degree. against the air flow. The
broken line indicates the curve of the heat transmission rate when
the flat plate is mounted horizontally. In the latter case, there
is only a single curve, since there is no difference in the heat
transmission between the front and rear sides of the plate. When
the plate is inclined as shown in FIG. 7a, however, two curves appear,
indicating a significant difference in the heat transmission rate
between the front and rear sides of the inclined plate.
In this embodiment, therefore, the amount of radiated heat output
by the first and second exothermic resistors mounted on the front
and back surfaces can be easily detected, so quick heat responses
can be expected when the air flow is reversed. Thus, the backward
flow signal can be determined earlier. In addition, forward and
backward flows can be distinguished clearly and the distinction
is less affected by turbulence and disturbances, even when they
occur. As a result, an erroneous backward air flow signal is prevented,
so that the reverse air flow can be detected and measured accurately.
In the embodiment shown in FIG. 9 the exothermic resistors 6a
and 6b are arranged directly opposite each other on the front and
back surfaces of the substrate. The difference in heat transmission
rate between the front and rear sides of the flat plate in this
case can also be detected in the manner described above. Thus, the
purpose of this invention can be achieved. The width of the substrate
can also be reduced by approximately a half, compared to that in
the embodiment shown in FIG. 7a.
Since this invention allows the number of connections in the electrode
terminals (and the substrate size) to be reduced, the exothermic
resistor element structure can be simplified for easier connection
of the exothermic resistor element to the electric circuit. On the
other hand, the amount of heat radiated from the first and second
exothermic resistors can be distinguished clearly, providing a thermal
process air flow meter having a high precision backward flow detecting
function.
Although the invention has been described and illustrated in detail,
it is to be clearly understood that the same is by way of illustration
and example, and is not to be taken by way of limitation. The spirit
and scope of the present invention are to be limited only by the
terms of the appended claims. |