FLOW RATE SENSOR

Abstract: A flow rate sensor is provided. This sensor includes a flow rate detecting element (1). The flow rate detecting element (1) is provided within a flow passage (10) so as to expose the flow rate of a medium whether at the center of the passageway or at the inner surfaces of the passage wall. The flow rate detecting element has a plurality of openings (1a) for allowing the medium to pass therethrough without pressure loss. Thus, the flow rate sensor may precisely detect the flow rate of a medium flowing within the passage regardless of insecurity in the flow distribution of the medium.

Description: The present invention relates generally to a flow rate sensor, and more particularly to a flow rate sensor which is suitable for measuring the flow rate of intake air into an internal combustion engine for example, according to the preamble of claim 1 A hot-wire air flow meter as a flow rate sensor is well known in the art. This meter is used for measuring the flow rate of intake air into an internal combustion engine. The meter generally includes a small winding type hot-wire probe which has a relatively great mechanical strength and an air temperature probe for detecting the temperature of intake air. These probes are disposed within a bypass air passage. It will be appreciated that a hot-wire air flow meter may provide a high degree of mechanical strength against the common using conditions of automotive vehicles such as vibrations, thermal shock, and backfire. Miniaturization, ease of mass production, and rapid response time are also attributes of this type of sensor. However, in such a hot-wire air flow meter, the hot-wire probe as a flow rate detecting element is arranged within a bypass air passage independent of the main air passage. Therefore, flow rate measurement is subject to inaccuracy due to pulsations caused by the opening and closing of the intake valves of the engine which causes flow velocity and distribution to vary continually, creating an extremely insecure condition with asymmetrical distributions therein due to the many curved sections and necessary straight sections within an air passage. Due to spatial restrictions a hot wire type flow rate sensor cannot take such conditions into account. The US-A-4 304 129 discloses a flow measuring apparatus including an electric heater and temperature sensitive resistors upstream and downstream the heater. The flow rate is calculated from -the heat energy supplied by the heater and the temperature difference upstream and downstream the heater derived from the resistance of the electric resistors. The US-A-2 776 565 discloses a flow measuring apparatus having a heater and two temperature sensitive devices downstream the heater. The apparatus responds to a cyclically varying temperature signal with the phasing of the temperature signal being indicative of the flow rate. It is, accordingly, an object of the present invention to provide a flow rate sensor for detecting the flow rate of a medium precisely even when the flow distribution thereof is insecure within a flow passage. This object is complied with by the features of claim 1. According to one aspect of the present invention, there is provided an apparatus for measuring the flow rate of a medium (includes liquids and/or gases) within a passage, which comprises a first means for radiating heat energy to the medium flowing within the passage at a plurality of points transversly offset from each other with respect to the flow direction of the medium, a second means for determining the heat quantity transferred from the first means to the medium to provide a signal indicative thereof, and a third means for determining the flow rate of the medium flowing within the passage based on the signal output from the second means. In the preferred mode, the first means may be a flow rate element which is formed of a resistance film and has a plurality of openings for allowing the medium to flow therethrough without pressure loss. The flow rate element is arranged so as to cover the entire stream area of a diameter section of the passage. In addition, the flow rate element may take the form of a honeycomb which is disposed transversly to the flow direction within the passage. According to another aspect of the invention, there is provided an apparatus for measuring the flow rate of air within an intake passage of an internal combustion engine, which comprises a flow rate detecting element, provided within the intake passage, or generating a heat energy to be dissipated to the flowing air at a plurality of points defined on a plane transverse to the flow direction of the air, a first means for determining the amount of heat dissipated to the air by the flow rate detecting element to provide a signal indicative thereof, and a second means for determining the flow rate of the air flowing within the intake passage based on the signal from the first means. The flow rate detecting element has a plurality of openings for allowing the air to flow therethrough without pressure loss. In the preferred mode, the flow rate detecting element has a surface area (S) and a resistance value (Rw). A temperature sensor for detecting a temperature (Ta) of the air flowing within the intake passage to provide a signal indicative thereof is provided. The first and second means constitutes a control circuit for applying a current to the flow rate detecting element to generate heat having a temperature of (Tw). The control circuit is operable to control the current so as to maintain a value defined by the relation (Tw - Ta) X S / Rw at a constant level based on the signal output from the temperature sensor to determine the flow rate of the air based on the magnitude of the current applied to the flow rate detecting element. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1(a) is a sectional view which shows an arrangement of a flow rate sensor according to the present invention and a temperature sensor within a flow passageway. Fig. 1(b) is a front view taken along the line B in Fig. 1(a) which shows a temperature sensor. Fig. 1(c) is a front view taken along the line C in Fig. 1(a) which shows a flow rate sensor. Figs. 2(a) to 2(e) are perspective views which show a fabrication sequence for a flow rate detecting element of a flow rate sensor. Fig. 3(a) is a graphic depiction of the flow velocity profile of intake air into an engine. Fig. 3(b) is a graphic depiction of flow rate variations at various engine speeds. Fig. 4(a) is a sectional view which shows an arrangement of a second embodiment of a flow rate sensor according to the invention within a flow passageway. Fig. 4(b) is a front view taken along the line B in Fig. 4(a) which shows a flow rate sensor of a second embodiment. Figs. 5(a) to 5(d) is front view which shows a fabrication sequence for a flow rate detecting element of a flow rate sensor of a second embodiment. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, particularly to Fig. 1(a), there is illustrated an arrangement of a flow rate sensor 1 according to the present invention and a temperature sensor 2 in an flow passage 10. The flow rate sensor 1 is available as an air flow meter for an internal combustion engine for example. In the flow passage 10, the flow rate sensor 1 and the temperature sensor 2 are mounted perpendicularly to the flow direction R. Referring to Fig. 1(b), the temperature sensor 2 is shown. This sensor is comprised of a thermistor element 2a and a bar 2b and is operable to measure the temperature of a medium within a passage to provide a signal indicative thereof to a control circuit 10. The bar is made of a ceramic material such as an alumina. The thermistor element is disposed in the center of the bar. Referring to Fig. 1(c), the flow rate sensor 1 is shown. This sensor comprises honeycomb shaped cells formed of a flow rate detecting element 1a and a circular support member 1b. The support member supports the flow rate sensor 1 within a duct 3 so that the flow rate sensor occupies all flow areas of the flow passage 10. The medium to be measured passes through the spaces 1c in the cells. Referring to Fig. 2, a process for producing the flow rate sensor is shown. First, a metallic foil 5 is prepared, as shown in Fig. 2(a), which is made of nickel, aluminum, or iron alloys and has a thickness of 50 to 100 (µm). An insulating material such as silicon dioxide is, as shown in Fig. 2(b), then disposed on a surface of the metallic foil 5 by means of a spattering method to form a thin insulating film 6. A resistance material such as platinum, tungsten, or nickel is further evaporated on the surface of the insulating film 6 by means of the same method to form a metallic film 7 The thin insulating film 6 is made of a plurality of thin layers which are made of silicon dioxide and have a thickness of 1 to 3 (µm) respectively are laminated by a spattering method to form the insulating thin film having a thickness of 5 to 10 (µm). After disposing the films, a laminated member is formed. Subsequently, a conductive paste 8 is applied to the metallic film 7, at a thickness of 20 to 50 (µm), in strips spaced at predetermined intervals as shown in Fig. 2(c) to form a finished laminated member 50. A plurality of finished laminated members 50 are further laminated to provide a laminated assembly, as shown in Fig. 2(d). On the uppermost and lowermost finished laminated members, as shown in Fig. 2(c), metallic foil electrode strips 9 are connected to the respective thin metallic films 7 thereof as terminals for the control circuit 10. With this arrangement, weight loads are supplied against both sides of the laminated assembly from the directions P and the laminated assembly is compressed. It is also dried at a predetermined temperature. After a certain period of time, the compressed laminated assembly is stretched from the top and bottom thereof (against the direction of compression) to obtain the honeycomb shaped flow rate detecting element 1a as shown in Fig. 2(d and e). Finally, a support member 1b covers the circumference of the flow rate element 1a to form a flow rate sensor 1 (see Fig. 1(c)). The principle of operation of the flow rate sensor according to the invention will be described hereinbelow. An electric resistance Rw provided by the metallic film 7 of the flow rate detecting element 1a of the flow rate sensor 1 is expressed as follows: Rw = Rw₀ (1 + αTw) Where Tw is a temperature of the metallic film 7. Rw₀ is a resistance value of the metallic film 7 at a reference temperature, for example, at 0 degrees. The α is a temperature coefficient of the metallic film 7. When the application of a current i is provided to the metallic film 7, heat generated thereby, or a heat quantity Q is transferred to the medium at V flow velocity. The heat quantity Q is determined by the following formula; Q = i²Rw = {C₁ + C₂(σ V) 1/2 } X (Tw - Ta) S Thus, i² = {C₁ + C₂(σ V) 1/2 } X (Tw - Ta) S / Rw Where σ is the density of the medium flowing through the flow passage. S is the surface area of the metallic film 7. Ta is the temperature of the medium. C₁ and C₂ are constants. Thus, as is clear from the above formula (2), by electrically controlling the value of (Tw - Ta) S / Rw regardless of the temperature Ta of the medium, the current i may be expressed as a product of the density of the medium and the flow velocity, or a function dependent only upon mass flow rate. The current i may be found to obtain the mass flow rate. As is well known in the art, various control means which use a signal from a sensor for detecting a temperature Ta of the medium may be provided for maintaining the value of the formula (3) at a constant value. In this embodiment, the control circuit 10 controls a current to be applied to the flow rate sensor 1 based on a temperature signal output from the temperature sensor 2 so as to maintain the value of the formula (3) at a constant value. A U.S. Patent 4,311,042 to Hoshiya et al. discloses a hot-wire air flow meter for an internal combustion engine. In a system of this invention, a hot-wire and a resistor is disposed within an intake passageway of an intake manifold. A control circuit is provided which will increase the voltage across the hot-wire to compensate for the energy dissipated by the hot-wire so that the temperature of the hot-wire is maintained at a constant value. With this provision, such a hot-wire flow meter may accurately measure the flow rate of the medium. A U.S. patent 4,505,248 to Yuzawa et al. discloses a hot-wire air flow meter control system for engine. The contents of these disclosure are hereby incorporated by reference. The flow rate element 1a, as shown in Fig. 1(a), is provided perpendicularly to the stream direction R of the medium and covers the entire stream area of a diameter section of the passage. Further, the element 1a extends parallel to the flow direction R with a certain thickness. In a flow passage such as an intake manifold of an engine, a medium (air) tends to flow with an insecure flow profile with respect to space and time. In an engine, due to the restricted space within a flow passage, the provision of the necessary lengths of straight sections in the flow passage is difficult. The flow passage includes many curved sections. The intake stream of the engine is therefore subject to extreme insecurity. Fig. 3(a) illustrates the variations per hour of in the velocity profiles of a medium S at a cross section perpendicular to a stream direction within a flow passage. It will be appreciated that the stream of flow into the intake of the engine is spatially insecure. Further, fluctuation, or pulsation flow which is synchronized with valve timing of an engine cylinder tends to occur in the stream of flow into the intake manifold of an engine. The more the difference in medium pressure between the intake side of a flow passage, where a throttle valve is opened greatly, and the cylinder side is reduced, the greater the magnitude of pulsation flow becomes. Fig. 3(b) illustrates pulsation flow in a medium flowing into the intake when the throttle valve is opened greatly with respect to various engine speeds. It will be noted that the stream of flow into the intake of the engine is always variable in relation to space and time and is extremely insecure. In order to always measure such an insecure stream precisely as a mean flow rate, the provision of a flow rate element which is disposed perpendicularly to the stream direction so as to cover all areas of a diameter of the flow passage is preferable. Accordingly, in this embodiment according to the invention, the honeycomb shaped metallic film element is provided within a flow passage so as to be arranged perpendicularly to the flow direction of the medium and so as to cover a diameter section of the flow passage. The metallic film element therefore encounters the medium across the entire flowpath from the center of the passageway to the inner surface of the passage walls regardless of momentary variations in the flow stream. Thus, the flow rate detecting element according to the invention can determine the flow rate of a medium passing through the flow passage. The honeycomb structure of the flow rate element takes the form of cells having openings, so that no pressure loss induced by the flow rate element tends to occur. For installation of the flow rate element within the flow passage so as to cover all cross area thereof, a method for arranging a mesh of platinum or tungsten wires, or so forth may be proposed. However, with this method, the provision of a retaining means for the mesh within the passage while maintaining the mechanical strength of the wires is difficult. Additionally, the cost for provision of this retaining means is high. For these reasons, the above described retaining means is unsuitable for an automotive vehicle for example. Moreover, hot wire probes, as described in the prior art, may be arranged at some places within a flow passage. However, the arrangement of a plurality of flow rate elements induces a great pressure loss. The precise detection of a flow rate given average spatial flow variations would require a good many flow rate elements, causing the cost to become high. This method is therefore unsuitable for automotive vehicles. Referring to Figs. 4(a) to 5(d), wherein like numbers refer like parts in Fig. 1 and description thereof will be omitted, a second embodiment of a flow rate sensor according to the invention is shown. This flow rate sensor 1 includes a disk shaped of metallic foil 5. An insulating film 6 and a metallic film 7 are laminated on the metallic foil 5 as a resistance member and a plurality of openings 1c are provided therein. The total aperture area of the openings 1c are more than or equal to 30 percent of a surface area of the flow rate sensor 1. This sensor is adapted for providing no pressure loss when medium passes therethrough. Figs. 5(a) to 5(d) shows a process for producing the flow rate sensor 1 of the second embodiment. Materials to be used and the thicknesses thereof are similar to the above first embodiment. First, a disk shaped metallic foil 5 is prepared, see Fig. 5(a). A plurality of circular openings are provided in the metallic foil by etching. Next, an insulating film 6 and a metallic film 7 are laminated on the metallic foil 5 by a spattering method. A pair of metallic film electrodes 9A is also laminated on edge section of the metallic film 7. Openings are formed at areas which correspond to the openings 1C in the metallic foil formed by etching, see Fig. 5(b). Third, metallic strip electrodes 9 are bound to the metallic film electrodes 9A by solder. Finally, a support member 1b is installed on the outer surface. A flow rate sensor 1 according to the second embodiment is thereby made. Similarly to the first embodiment, a flow rate sensor 1 is disposed perpendicularly to the stream direction of a medium within a flow passage. In addition, the flow rate sensor has a plurality of openings to allow the medium to pass therethrough without resistance. Thus, regardless of momentary flow variations, the flow rate element according to the second embodiment can determine flow rate of a medium flowing within the flow passage with no pressure loss to the medium.

Claims: 1. Apparatus for measuring the flow rate of a medium within a passage, comprising a temperature sensor (2) for detecting the temperature of the medium flowing within the passage to provide a signal indicative thereof, a heater for radiating heat energy to be dissipated in the medium flowing within the passage, and a control unit (10) characterized by a single flow rate element (1) consisting of a thin laminated film made of an insulating material (6) and a conductive material (5) said element having a plurality of openings and being mounted within the passage downstream of the temperature sensor (2) so as to allow the medium to flow through the openings for radiating heat energy to be dissipated in the medium uniformly over the entire stream area of the diameter section of the passage, the control unit (10) controlling the amount of heat dissipated in the medium such that a difference in temperature between the medium detected by the temperature sensor (2) and the temperature of the medium generated by the heat energy supplied by the flow rate element (1) is maintained at a constant level, the control unit determining the flow rate of the medium within the passage based on the amount of heat dissipated by the flow rate element (1). 2. Apparatus as claimed in claim 1, characterized in that the control unit (10) determines the flow rate of the medium based on the magnitude of the current applied to the now rate element (1). 3. Apparatus as claimed in claim 2, characterized in that said control unit (10) controls the current (i) so as to maintain a value defined by the relation (Tw - Ta) x S / Rw at a constant level, where (Ta) is the temperature detected by the temperature sensor (2), (Tw) the temperature of the medium achieved by the energy supplied by the now rate element (1), (S) is the surface area of the flow rate element and (Rw) the resistance value thereof. 4. Apparatus as claimed in one of claims 1 to 3, characterized in that said flow rate element (1) is made of a metallic foil (5), an insulating film (6) formed on said metallic foil and a metallic film (7) formed on said insulating film. 5. Apparatus as claimed in one of the preceding claims, characterized in that said laminated film is in the form of a honeycomb.

Inventors: KOBAYASHI, HIROSHI
Applicants: NISSAN MOTOR CO., LTD.
Application year: 1989
Main classification: G01F1
IPC codes: G01F1