JPH0684898B2 - Heat-generating resistance element of thermal air flow meter - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a so-called heat-sensitive air flow meter using a heating resistor by electric heating, and particularly to a heat-sensitive air flowmeter suitable for measuring an intake air amount of an internal combustion engine. The present invention relates to a heating resistance element of a flow meter.

[Background of the Invention]

As a heating resistance element (hereinafter referred to as HWE) of a heat-sensitive air flow meter (hereinafter referred to as AFM), for example, Japanese Patent Laid-Open No. 58-210520
As disclosed in Japanese Unexamined Patent Application Publication No. JP-A No. 2004-187, a platinum wire is stretched in an air passage, and as disclosed in JP-A-59-104513, a platinum wire is wound around a small bobbin. Things are known.

By the way, the former of these has high sensitivity, but the response is so sensitive that it detects even minute turbulence of the air flow, so it is difficult to capture the signal into the control device mainly composed of the microcomputer. In addition, since the characteristic change due to the adhesion of dust is remarkable, a special circuit for incinerating dust is required.

On the other hand, the latter, as shown in FIG.
Attach the leads 52 to both ends of the
It is a structure in which a platinum fine wire 53 is wound around the periphery of 51 with equal pitches, and a glass coat 54 is applied, and a heat insulating adhesive 55 is adopted,
The lead 52 is thinned to improve the response characteristics.

Therefore, the HWE50 with this structure is slightly inferior in response characteristics to the former platinum element wire, but because of that, signal processing is easy and the influence of dust is relatively small, so it has been widely put to practical use. There is.

Figure 8 shows the installation of such a HWE50.
It is attached together with the temperature compensation resistor 60 of the bypass passage B.

By the way, the conventional HWE50 shown in FIG. 7 and FIG.
As shown in Fig. 9, the response characteristics of AFM using
The rise response was 0.15 seconds or less, and the 100% response was 1.8 to 2.5 seconds.

However, in recent years, when an AFM is mounted on an engine and engine control is performed by a microcomputer,
The subtle response delay of the AFM adversely affects the responsiveness of the engine, and it is often the case that a poor driving condition is complained especially in the low speed range. Then, the high speed response of HWE of AFM came to be demanded.

Therefore, when comparing the degree of influence on drivability for the test items with different HWE response speeds, 100% rise and fall responses were obtained.
We have found that it is necessary to have a response time of 0.5 seconds or less and a 95% response of 0.1 seconds or less.

[Object of the Invention]

The present invention has been made based on the above technical background.
AEM (heat-sensitive air flow meter) with sufficient responsiveness while using a bobbin type resistor as HWE (heating resistance element)
The purpose is to obtain

[Outline of Invention]

In order to achieve this object, the present invention extends the lead wire of the bobbin type resistor into the air passage and bridges the resistor to the lead wire to attach the lead wire to the lead wire. The feature is that the heat shrinkage amount of is sufficiently reduced.

Example of Invention

Hereinafter, the present invention will be described in detail with reference to illustrated embodiments.

In the embodiments described below, various innovations have been made in addition to the features of the present invention described above. Therefore, first, the outline of the embodiments including these points will be described below.

First, a factor for reducing the amount of heat sink to the lead and a method for solving the factor will be described, which is related to the above-mentioned feature of the present invention.

As described above, the conventional heating resistor mounting structure is as shown in FIG. 8 and the heating resistor structure is as shown in FIG. 7. As is clear from these figures, the conventional heating resistor is The body structure has two problems with heat conduction to the leads, which adversely affects the response characteristics. Firstly, the distance between the terminal and the heat-generating resistor and the heat-generating portion, that is, the lead is short, and secondly, the heat insulating glass is used between the alumina pipe and the lead. Adhesive glass is 3τ (95%)
Although it was effective for speeding up the following responses, there is almost no effect of improving the response time from 4τ to 100% response.

Therefore, as an improvement measure, the leads should be made as thin and long as possible to increase the heat conduction resistance and at the same time be well adapted to the temperature of the air flow. On the other hand, the joint between the bobbin and the lead was made through a small amount of noble metal so that there was no stagnation of heat and the conductivity was improved.

As a specific structure, in the following examples, the outer diameter φ0.35 to φ
A platinum thin film is deposited on the surface of a 0.30 alumina pipe or rod by a spatula, and laser trimming grooves are formed from end to end so that heat can be generated over the entire length.
This heating resistor is connected to the wire lead bridged to the passage by solid-phase joining with a noble metal foil at the end of the heating resistor or with a small amount of braze containing the precious metal as a main component, and the wire lead is bridged. Alternatively, the structure is such that it is sandwiched.
The wire leads are attached in different steps to reduce dust adhesion, and the heating resistor is attached at an angle of 30 ° to 60 ° to the flow. The length between the heating resistor joint of the wire lead outer diameter φ0.1 to φ0.05 and the support metal fitting is 5 nm or more to prevent heat sink to the lead, and to prevent air flow temperature changes and wall temperature changes. Made it less susceptible. In addition, the heating resistor was attached to the small bench lily in advance and placed in the main passage for the purpose of reducing the cost, eliminating the directionality when mounted on the engine, or reducing noise.

Next, regarding the shape of the resistor, in general, the thermal time constant τ when the flow velocity of an infinitely long heat generating cylinder is changed in the flow of air is the diameter d of the cylinder, the heat capacity of the cylinder.
C _p ρ, expressed as a function of the heat transfer coefficient h from the cylinder to the air flow, as shown in equation (1).

Therefore, as a means for improving the response characteristics, firstly, the outer diameter d of the cylinder is reduced. Secondly, the heat transfer coefficient h is increased.
Thirdly, since the actual object is not an infinitely long cylinder but the leads are fixed to the cylinder, the heat conduction amount of the leads is reduced. And so on.

Therefore, in the following examples, the conventional outer diameter d is φ0.5, but it is reduced to φ0.35 to φ0.3, and the winding of a thin wire is not mass-produced. By applying the production technology, we decided to adopt a structure in which a thin metal film such as platinum is deposited on the outer circumference of the bobbin with a barrel spatter and a trimming groove is formed in a spiral shape. Next, the heat transfer coefficient h
As a means for increasing the heating temperature, the resistance value R of the constant temperature difference type heating resistor is halved to increase the supply current to the resistor while increasing the total length of the heating resistor to increase the unit heat capacity of the resistor. The ratio of surface area to 1.5 times. Therefore, the heat transfer coefficient with respect to the flow of air could be improved by 4 to 5 times as compared with the conventional one as shown in FIG.

Hereinafter, specific examples of the present invention will be described.

FIG. 3 and FIG. 4 show an embodiment of the present invention, showing a state in which it is installed in a duct that serves as an air passage, and FIG. 10 shows an AFM circuit. The resistor (HF) 1 constitutes the other resistors 11 and 12, and the temperature compensation resistor (CF) 13 constitutes a bridge. The feedback voltage circuit that drives the transistor 15 by differentially amplifying the differential voltage of the bridge resistor via the amplifier 14 is configured. HF1 and CF13 are two lead wires 17,1 bridged to small bench lily 16, respectively.
It is supported and fixed to 7 ', 18, 18' via a bonding material 19. The temperature of HF1 is controlled so that the difference from the temperature of the air flow detected by CF13 is constant at, for example, 200 ° C.

The drive circuit is housed in a housing 31 assembled to the duct 30 and connected to the lead wires by relay leads 20, 21, 22, 23. The small bellows lily 16 is arranged at the center of the passage of the duct 30, HF is arranged upstream and CF is arranged downstream. A rectifying wire mesh 32 is incorporated upstream of the duct 30.

Figure 1 shows the details of HF1, which is φ0.3, length 6m.
A platinum thin film 3 having a thickness of 4 .mu.m is deposited on the outer periphery of the m-free alumina blank 2 by a barrel sputtering, and after heat treatment, resistance trimming is performed by laser. The trimming groove 4 is a uniform pitch from end to end of the alumina bobbin.

By obtaining the relationship between the initial resistance value and the number of trimming grooves in advance, the variation in the resistance value R can be within ± 3%. This element and the lead wire are fixed by pressure welding at high temperature via gold foil, for example, or by brazing in an inert gas via a noble metal braze. Further, a glass coating 5 is applied to the outer periphery of the element and the joint between the lead wire and the element in a thickness of 5 μm to 10 μm. Here, the alumina-free rod 2 may be a pipe having an outer diameter of φ0.35 and an inner diameter of φ0,2. In the case of a pipe, it is difficult to join the lead wire at the side end as shown in FIG. Therefore, as shown in FIG. 2, the outer periphery of the end portion is joined.

In addition, lead wires 17, 17 ', 18, 1 supporting HF1, CF13
8'is required to withstand a vibration load of 60 G, and uses a wire with an outer diameter of φ0.1. The length is shortened to 5 mm and relayed to a thick lead pin on the way. The length of the lead wire is determined considering that the influence of wall temperature can be reduced.

Next, as one means for increasing the heat transfer coefficient h and improving the response characteristics, the resistance value R of the HF1 and the resistance value R11 of the resistor 11 of the bridge are decreased so that the current flowing in the HF1 is applied to the transistor.
We will try to raise it as much as possible within the tolerance limit of 15.

Next, the HFM according to this embodiment is attached to a test stand for flow rate measurement, and the relationship between the flow rate Q and the current I _h flowing through HF1 is measured. The measurement results are shown in FIG. When expressed as I _h ² with respect to, Q and I _h are in a straight line because they have a fourth root relation. Compared with the AFM by the conventional element, the HF element according to the embodiment of the present invention has a large ratio of the outer diameter d to the length l, and it is difficult to match the resistance value with the conventional winding method. It can not be wound completely from the end to the end, and there is no heat generation at the end
About 30% remained, but in this embodiment, it was possible to perform trimming by arbitrarily changing the trimming pitch by the laser, and it became possible to spread the heat generating portion from one end to the other.
It is possible to increase the heat transfer coefficient h (h corresponds to B in equation (2)) by decreasing the resistance value of 1 and increasing the current flowing to HF1, and as a result, as shown in FIG. That is the improvement. In addition, here Is.

Next, let's compare the performance by instantaneously switching the air flow with a solenoid valve and blowing it onto the AFM, measuring the instantaneous response characteristics. An example of the measurement result is shown. According to graph 6
% (3τ) response time is 0.066 seconds, 98% (4τ) response time
0.099 seconds, 100% response time is 0.159 seconds. On the other hand, the 100% response time of the conventional product shown in Fig. 9 is 1.9 seconds.
The following improvements could be made. Therefore, according to this embodiment, it is possible to eliminate a driving operation disorder due to a response delay during low-speed to high-speed acceleration when the AFM is mounted on an automobile engine.

Further, by disposing the element in the central portion of the main passage of the AFM as in this embodiment, the influence of the wall surface flow is reduced and the disturbance of the output signal of the AFM can be reduced. As shown in Fig. 3 and Fig. 4, in order to reduce the influence of dust, the element is mounted at an angle of 30 to 60 degrees to the flow direction, so that dust is attached more than horizontal mounting. The amount was reduced.

As described above, according to one embodiment of the present invention, the ratio of the outer diameter d to the length 1 of the element is increased to extend the heat generating portion from one end of the bobbin to the other end and a small amount at the end. Since the bridge was bridged to two thin wire leads via the bonding material, the response characteristics could be significantly improved. Moreover, the temperature characteristics could be improved by appropriately lengthening the wire leads. It was possible to reduce the influence of wall flow and reduce noise compared to the case where the element was attached to a small pliers lily in the center of the main passage. Also, the effect of wall temperature was reduced. It was possible to make it even cheaper. It was possible to reduce the influence of dust by mounting the lead wire in different steps and tilting the bridge element in the flow direction.

〔The invention's effect〕

As described above, according to the present invention, it is possible to provide sufficient response characteristics despite the use of the bobbin type resistor, and therefore, it is possible to control the internal combustion engine for automobiles while fully utilizing the advantages of the prior art. It is possible to easily provide the heat generating resistance element of the heat-sensitive air flow meter capable of reliably performing the above.

[Brief description of drawings]

FIG. 1 shows an embodiment of the present invention. (A) is a front view, (b) is a side view of (a) as seen from the direction of arrow P, and FIG. 2 is another embodiment of the present invention. An example is shown, (a) is a front view, (b) is a side view of (a) as seen from the direction of arrow P,
3 is a side sectional view of a heat-sensitive air flowmeter according to an embodiment of the present invention, FIG. 4 is a front view seen from the direction of arrow P in FIG. 3,
FIG. 5 is a characteristic diagram according to one embodiment of the present invention shown in comparison with a conventional example, FIG. 6 is a response characteristic diagram in one embodiment of the present invention, FIG. 7 is a front view of the conventional example, and FIG. Is an explanatory view showing the mounting state of the conventional example, FIG. 9 is a response characteristic diagram of the conventional example,
Figure 10 is a circuit diagram of a thermal air flow meter. 1 ... Heating resistor (HF), 2 ... Alumina bobbin, 3 ...
… Platinum thin film, 4 …… Trimming groove, 5 …… Glass coating,
17,17 ′ …… Lead wire, 19 …… Joining material.

Claims (8)

[Claims] 1. A bobbin type resistor having two lead wires parallel to each other which are stretched at right angles to the air flow direction in an air passage whose flow rate is to be measured, and which are bridged between these two lead wires. A heating resistance element characterized by being configured by attaching a body. 2. The bobbin-shaped resistor according to claim 1, wherein the bobbin-shaped resistor comprises a metal thin film adhered to the outer periphery of a ceramic pipe or a round bar as an electrical resistor. A heating resistor element. 3. A heating resistance element according to claim 2, wherein the metal thin film is a platinum thin film, and the platinum thin film has a spiral groove for resistance value trimming. 4. The bobbin type resistor according to claim 3, wherein d is the outer diameter and l is the length.
= 0.3 to 0.35 mm, l / d> 11.5. 5. The bobbin type resistor according to claim 2, wherein the bobbin type resistor is attached to the lead wire by either solid phase joining using a precious metal foil or brazing joining containing a precious metal as a main component. A heating resistance element characterized by being performed. 6. A heating resistor element according to claim 1, wherein at least one of the outer peripheral surfaces of the lead wire and the bobbin-shaped resistor and the mounting portions thereof are covered with glass. 7. The bobbin-shaped resistor according to claim 1, wherein the bobbin-shaped resistor is held so as to be inclined in the range of 30 to 60 degrees from the position perpendicular to the air flow direction toward the air flow direction. A heating resistance element, wherein the two lead wires are stretched in an air passage. 8. A heat-sensitive air flow meter comprising a heat-generating resistance element arranged in an air passage whose flow rate is to be measured, wherein the heat-generating resistance element is stretched in the air passage at a right angle to the air flow direction. A heat-sensitive air flow meter, comprising: a plurality of parallel lead wires; and a bobbin type resistor bridged between the lead wires.