JPH04147016A - Thermal-type flow sensor - Google Patents

Abstract

PURPOSE:To enable sure removal of dust sticking to a resistor by a method wherein impurities sticking on a resistance layer of a substrate are removed by a thermal expansion change of the substrate itself which is caused by heating of a heater for removing the impurities. CONSTITUTION:A sensing element 7 is held by a holding member 21 so that it extends in the direction perpendicular to an air current. An electroconductive part 72a is provided on the downstream side of the air current of the element 7, while a heating part 72b is connected in series electrically to the electroconductive part 72, and a flow rate is detected from a difference between the heating temperature of the heating part 72b and the temperature of a resistor of a temperature compensation element. Besides, a heater 72c for removing dust is connected in series electrically to the heating part 72b and provided on the upstream side of the air current of the element 7. By forming the heater 72c only in the end part on the upstream side of the air current of the element 7 in this way, a rapid thermal expansion accompanying quick heating can be facilitated only in the end part on the upstream side of the air current of the element 7, and thereby the dust sticking to the vicinity of the heater 72c can be shaken off efficiently.

Description

[Detailed Description of the Invention] [Industrial Field of Application] [Prior Art] Thermal flow rate sensors have conventionally been employed in internal combustion engines for automobiles, and the flow rate sensors detect temperature changes in the intake system of the engine. Provide a resistor (for example, a platinum wire) that shows a change in resistance value,
By controlling the current supply to this resistor according to the temperature state of the resistor, a signal corresponding to the flow rate of air taken into the engine is obtained, and the air is remeasured.

However, when the resistor is exposed to the air for a long period of time, floating dust contained in the airflow adheres to the resistor, causing a problem in that the output characteristics change.

In order to deal with such problems, JP-A-54
In Publication No. 76182, after the end of a normal measurement cycle, an excessive current is temporarily passed through the resistor to heat the resistor to a temperature higher than the normal operating temperature, thereby incinerating the deposits (burn-off). ing.

[Problem to be solved by the invention]

By the way, when a resistor is placed in the intake system as described above, the floating dust that adheres to the resistor consists of inorganic dust and carbon particles, and in order to burn off the adhered substances, the resistor must be heated to 800°C or higher. Needs to be heated.

However, even if a heat-resistant platinum wire is used as the resistor, repeated burn-offs will cause changes in the resistance-temperature characteristics of the platinum wire, making accurate flow measurement impossible.

Furthermore, if an element is used in which a platinum resistive film or the like is provided on a substrate made of insulating heat-resistant resin, burn-off cannot be performed because the burn-off temperature is higher than the heat-resistant temperature of the substrate itself. Even if a semiconductor material is used, burn-off causes distortion at the junction between the substrate and the resistive film, significantly changing the characteristics of the resistive film.

The present invention has been made in view of the above problems, and its purpose is to provide a thermal flow sensor that reliably removes dust that has repeatedly adhered to a resistor without changing the characteristics of the resistor layer. It is something to do.

[Means for Solving the Problems] Accordingly, the present invention has a resistance layer formed on a substrate, which has a temperature-dependent resistance characteristic, and includes a heat-generating portion that generates heat when energized, and the substrate is arranged in a passageway through which a fluid flows. This is a thermal flow sensor that detects the flow rate based on the heat generation state of the resistance layer, which is held parallel to the flow of fluid at the substrate. The present invention provides a thermal flow sensor having an impurity removal heating element that removes impurities adhering to the substrate from the substrate due to the thermal expansion of the substrate caused by the thermal expansion of the substrate.

[Function] By employing the present invention, when removing impurities attached to the resistance layer of the flow sensor, the impurities are not burned out, but are generated by the heat generated by the impurity removal heating element.
Since impurities attached to the resistance layer on the substrate are removed by thermal expansion changes of the substrate itself, impurity dust attached to the substrate can be removed even at a temperature lower than the heat generation of the resistance layer.

〔Effect of the invention〕

According to the present invention, it is possible to obtain a thermal flow sensor that reliably removes dust repeatedly attached to a resistor without changing the characteristics of the resistor.

〔Example〕

Hereinafter, the present invention will be explained in detail with reference to the drawings.

FIG. 1 is an overall schematic diagram showing an internal combustion engine to which a thermal flow sensor according to the present invention is applied. In FIG. 1, an air cleaner 3 and a rectifying grid 4 are provided in an intake passage 2 of an internal combustion engine 1.
Air is inhaled through.

A measurement pipe (duct) 5 is fixed in this intake passage 2 by a stay 6, and the inside thereof is made of an electrical resistance material that shows a resistance value change according to temperature changes in order to measure the air flow rate. The sensing element 7 is a resistance layer that is provided with a heat-generating portion that generates heat when the sensor is exposed to the outside air temperature. A temperature compensation element 8 is provided for compensation. These sensing element 7 and temperature compensation element 8 are connected to a sensor circuit 9 formed on the hybrid substrate.

The sensor circuit 9 is composed of a feed bank circuit and a heater control circuit. Feedhack control of calorific value and its sensor output■. The heater control circuit is a circuit that controls the dust removal heater so as to heat the sensing element 7 at regular intervals. The control circuit IO is configured by a microcomputer, for example, and controls the fuel injection valve 11.

2 to 5 show a first embodiment of the present invention,
As shown in FIG. 2, holding members 21 and 22 are fixed in the duct 5, which is fixed approximately at the center of the intake passage 2 by the stay 6, and which are arranged parallel to the air flow direction. Sensing element 7 at 21 and 22, respectively.
A temperature compensation element 8 is fixed, and the sensing element 7 and the temperature compensation element 8, like the holding elements 21, 22, are arranged parallel to the air flow direction. That is, the sensing element 7 and the temperature compensating element 8 are arranged so that only their minimum dimension portions, that is, their thickness portions oppose the air flow.

FIG. 3 shows a sectional view of the sensing element 7. The sensing element 7 has a resistance pattern 72 made of platinum (Pt), which is an electrical resistance material whose resistance value changes according to temperature changes, on a thin plate-shaped substrate 71 made of silicon single crystal, via an insulating film (for example, 5iO) 71a. is formed into a thin film. Furthermore, this resistance pattern 72 is made of Si
It is covered with an electrically insulating passivation film 74 made of O, SiN, or the like.

FIG. 4 is a front view of the sensing element 7. Here, the resistor pattern 72 is formed by depositing Pt.

The conductive portion 72a2, the heat generating portion 72b, and the dust removal heat generating portion 72c are formed by forming a thin film by sputtering or the like, and by chemical or dry etching.

As shown in FIG. 4, the conductive part 72a is located downstream of the sensing element 7 in the airflow direction, extends in a direction perpendicular to the airflow direction, and allows electricity to flow to the lower end of the heat generating part 72a. It is for. Further, the heat generating part 72b is electrically connected in series with the conductive part 72a, and in the approximate center of the sensing element 7, the conductive part 72a
As mentioned above, this heat generating part 7
The flow rate is detected based on the difference between the heat generation temperature of the temperature compensation element 2b and the temperature of a resistor (not shown) of the temperature compensation element 8. Furthermore, the dust removal heat generating part 72c is electrically connected in series with the heat generating part 72a, is located upstream of the sensing element 7 in the air flow, and is formed substantially parallel to the heat generating part 72b. Furthermore, this dust removal heat generating part 72c is a heat generating part 72c.
70 within 2.0 seconds by making the width smaller than b.
The temperature is adjusted to 0°C or higher.

Further, in order to ensure conduction of electricity to these resistor patterns 72, butt portions 76a, 76b, and 76c are formed above each resistor pattern without intervening the passivation film 74.

As shown in FIG. 1, the sensing element 7 formed in this manner is held by the holding member 21 so as to extend in a direction perpendicular to the air flow.

By the way, the holding member 21 that holds the sensing element 7 is made of a material with high thermal conductivity, such as aluminum, copper, or denim, and desirably, a material with even lower specific heat is selected.

The sensing element 7 is attached to the holding member 21 with a heat insulating material 23 and an adhesive 25 as shown in FIG.
a, 25b.

As the heat insulating material 23, materials having low thermal conductivity such as mullite, zirconia, zircon, SiO□ glass, etc. are used.

Further, the holding member 21 has a concave concave portion 21a formed on the upstream side with respect to the air flow. And this recess 21
The sensing element 7 is attached to the holding member 21 so that the heat generating part 72b of the sensing element 7 is located within the range of a.
is set for. By doing this, the holding member 21 prevents the flow of air near the sensing element 7 from being disturbed.

Furthermore, a lead member 30 made of alumina or the like is fixed to the holding member 21 with an adhesive (not shown), and a lead wire made of Au or the like is electrically connected to the sensor circuit 9. 31a, 31b
, 31c are formed by printing and baking. and,
The lead wires 31a, 31b, 31c are wires 24a, 31c formed by wire bonding with pad portions 76a, 76b, 76c of the sensing element 70, respectively.
They are electrically connected via 24b and 24c.

In order to make the transient temperature characteristics of the system of the sensing element 7 and the system of the temperature compensation element 8 the same, the sensing element 7 and the temperature compensation element 8 are substantially made of the same substrate material, the same amount of heat, and the same amount of heat. It is constructed in accordance with the dimensions and fixed symmetrically to a retaining member 21, 22 of identical construction.

In the first embodiment, a dust removal heater 72c that quickly heats up
By forming only the airflow upstream end of the sensing element 7, rapid heating of the sensing element 7 is made possible. Therefore, rapid thermal expansion due to rapid heating can be promoted only at the upstream end of the airflow of the sensing element 7, and dust adhering to the vicinity of the dust removal heater 72C of the sensing element 7 can be efficiently shaken off. I was able to make it possible.

Therefore, in the thermal flowmeter of the first embodiment, when removing dust attached to the sensing element 7, not only the conductive part 71a of the sensing element 7 itself is not heated, but also the heating temperature of the sensing element 7 is reduced. can be made to be lower than the heat-resistant temperature of the conductive part 71a, which is lower than the regulation temperature of dust, so it is possible to obtain an excellent thermal flowmeter in which the characteristics do not change even when the sensing element 7 is repeatedly regenerated. can.

Next, the flow rate detection method of the thermal flow rate sensor of the present invention will be used in detail using a circuit diagram.

FIG. 6 shows a circuit diagram of the sensor circuit 9. This sensor circuit 9 includes a measurement circuit section 80 and a dust removal heater control circuit section 8.
Consists of 1.

The measurement circuit section 82 includes a measurement bridge 82. 1st Oheamp 83. Second operational amplifier 84. Constant voltage generation circuit 85. It consists of a flip-flop 86. Further, the calculation circuit section 82 applies a constant voltage (2) to the measurement bridge 82 in synchronization with the measurement start signal TiN inputted from the control circuit 10, and starts heating the heat generating section 72b of the sensing element 7 in the bridge.

As the temperature of the heat generating portion 72b of the sensing element 7 rises, the resistance value of the resistor forming the bridge 82 increases, the balance of the bridge 82 is established, and the voltage application to the bridge 82 is stopped. After that, when the air flow rate in the intake passage 2 increases, the temperature of the heat generating part 72b of the sensing element 7 decreases, causing the bridge 82 to become unbalanced.
Application of voltage to is resumed.

In the end, the air flow rate in the duct 5 is measured by the time of application to the bridge 80.

Next, dust removal is performed based on the dust removal heater control circuit, and this dust removal heater control circuit 81 is connected to the control bridge 86. 1st operational amplifier 87. 2nd operational amplifier 8B, NAND circuit 89° monostable multivibrator 90. It is formed from a flip-flop 91. and,
The operation of this control circuit 81 is such that voltage application is started by inputting a start signal outputted from the control circuit 10 at predetermined time intervals, and a rise and fall 86 occur as the temperature of the dust removal heater 72c increases. The application of voltage is stopped when the bridge 8'6 is balanced due to an increase in the resistance of the resistor, or when the start signal is turned off. In addition, a dust removal heater 72c
The temperature rising time is adjusted by the value of the resistor 92.

(Second Example) Using the thermal flow sensor shown in the first example, the removal effect of the dust removal heater 71c was investigated. Table 1 shows the error in measuring the flow rate when the temperature and heating time of the dust removal heater 71c are used as parameters, and Table 2 shows the change in resistance value of the sensing element (72a+72b) at that time. The test conditions were 200 g/s and 300 hr, and the dust removal heater 71c was energized and heated at an interval of 2 times/hr.

Here, the flow rate measurement error is a combination of the error due to dust adhesion to the sensing element 7 and the error due to deterioration such as cracks in the sensing element 7 due to the cooling/heating cycle during dust removal from the sensing element.

Further, the change in resistance value of the sensing element 7 indicates an error due to deterioration of the sensing element 7.

As shown in Tables 1 and 2, the set temperature is 300.
At 900°C, a sufficient dust removal effect cannot be obtained regardless of the heating time, and at 900°C, the resistance of the conductive part 71a made of platinum film changes as shown in Table 2, making it difficult to use. becomes.

For example, if you compare the heating time of 1 second and 2 seconds when set to 500°C, it is clear that as the heating rate becomes slower, the adhering dust is removed due to the impact thermal stress caused by rapid thermal expansion. It can be seen that the effect is decreasing. Furthermore, the seventh
As shown in the figure, as the temperature increase rate eases, the effect of selectively heating only the edges of the board is lost, and the temperature of the entire board rises.As a result, as is clear from Table 2, the sensing The resistance of the element also begins to change.

From the above results, the sensing element resistance (72a+
In order to obtain a good effect of removing attached dust without changing 72c), it is found that it is appropriate to raise the temperature to 400''C-800C within 2 seconds.

In the above embodiment, the material of the substrate 71 is single crystal silicon, but the same effect can be obtained by using a heat-resistant material such as glass, ceramic, or laminated material.

Further, in the above embodiment, the substrate was held on one side, but even if the substrate was held on both ends as shown in FIG. 8, the same effect as in the above embodiment can be obtained. That is clear.

(Left below) Table Flow rate measurement error (%) (200g/s Removal of dust - 2 times/hr) (Left below) Table 2 Sensing element section (72a+72b) resistance change (
%) (conditions same as above)

[Brief explanation of drawings]

FIG. 1 is an overall schematic diagram of an internal combustion engine to which a thermal flow sensor according to the present invention is applied, FIG. 2 is a partially cutaway configuration diagram of a duct, and FIG. 3 is a diagram of a substrate having a resistance layer. 4 is a block diagram of the board, FIG. 5 is an enlarged block diagram showing the mounting part of the board, FIG. 6 is a circuit diagram showing one embodiment of the sensor circuit, and FIG. It is a characteristic diagram which shows the heat distribution of the board|substrate when the heat generating part for dust removal of this invention generates heat. 7... Sensing element (M board), 72b...
Heat generating part (resistance layer), 72c... Heating element for removing impurities.

Claims (2)

[Claims] (1) A substrate formed in a flat plate shape and a resistance layer formed on the substrate and having a temperature-dependent resistance characteristic and including a heat generating part that generates heat when energized are provided, and the substrate is placed in a passage through which a fluid flows. The resistance layer is held parallel to the flow of fluid, and due to the heating state of the resistance layer,
A thermal flow rate sensor for detecting flow rate, which is formed on the substrate and removes impurities attached to the substrate by thermal expansion of the substrate caused by heat generation at a temperature lower than that of the resistance layer. A thermal flow rate sensor characterized by having a heating element for removing impurities. (2) The heating element for removing impurities is formed on the upstream side of the fluid on the substrate, extending in a direction perpendicular to the flow of the fluid. Thermal flow sensor.