JPH09218066A - Flow rate sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a flow rate sensor which is adapted to burn away foreign matters adhering to a resistor for heating by heating the resistor for heating up to a high temperature cyclically or at any time as desired. SOLUTION: A resistor RH for heating and a resistor RT for temperature compensation are arranged as sensor elements in a tube 20 serving as a passage of a fluid to form a resistance bridge circuit together with resistances R1 and R2 . In a signal processing circuit for detecting flow rates comprising the bridge circuit, a differential amplifier OPA and a transistor Tr1, a power source voltage Vcc is applied directly to the resistor RH for heating through a transistor Tr2 for switching aside from a power source voltage Vcc of the signal processing circuit and when a pulse signal Ps is inputted into the base of the transistor Tr2, the resistor RH for heating is heated up to a high temperature by a time equivalent to the width of the pulse signal Ps.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a flow rate sensor used for a water heater, an air conditioner, etc., and more particularly to a flow rate sensor for removing foreign matter adhering to a heating resistor by incineration.

[0002]

2. Description of the Related Art Flow rate (flow velocity) of air from a water heater, air conditioner, etc.
As a flow rate sensor for measuring, there is a thermal type flow rate sensor which detects a flow rate of the fluid by exposing a heat-generating resistor to the flow of the fluid and detecting the heat radiation effect.

FIG. 4 is a diagram showing a basic circuit configuration of such a flow sensor, which is a so-called constant temperature difference method. In this flow rate sensor, a heat generating resistor RH and a temperature compensating resistor RT which are sensor elements are arranged in a tube 20 which is a fluid flow path, and a resistor bridge circuit is formed together with the resistors R1 and R2 to generate heat. The connection point between the resistor RH and the resistor R1 and the connection point between the temperature compensation resistor RT and the resistor R2 are respectively connected to the input ends of the differential amplifier (op amp) OPA, and the output end of the differential amplifier OPA is It is connected to the base of the transistor Tr1 and the emitter of the transistor Tr1 has a resistor R
It is connected to the connection point of 1 and R2, and the power supply voltage Vcc is applied to the collector of the transistor Tr1.

The heat-generating resistor RH has a constant temperature higher than the temperature of the fluid, for example, 200 in an air flow sensor of an automobile.
℃ to 250 ℃, 20 ℃ to 3 for applications such as water heaters and air conditioners
The temperature is controlled to be increased by 0 ° C., and the temperature compensating resistor RT detects the temperature of the fluid itself and compensates for the influence of the fluid temperature.

The supply current to the bridge circuit changes so as to maintain the equilibrium state of the bridge circuit with respect to the change of the resistance value according to the heat radiation amount of the heating resistor RH due to the change of the flow rate,
At the output terminal of the differential amplifier OPA, an output voltage V according to the flow rate
o is output.

Heating resistor RH and temperature compensating resistor RT
As the temperature sensitive resistor, a resistance pattern made of a metal film such as platinum is formed on an insulating substrate such as alumina, and a signal processing circuit including the bridge circuit and the differential amplifier is formed. It is attached to the circuit board by soldering.

[0007]

In such a flow rate sensor, both resistors RH and RT, which are sensor elements, are constantly exposed to the flow of air, and their surfaces are in use during use. There has been a problem that foreign matter such as oil, dust, and dust adheres to the surface, which lowers the measurement accuracy. Particularly, when a foreign substance adheres to the heating resistor RH, the heat transfer coefficient to the air of the heating resistor RH, the thermal emissivity, and the heat capacity of the heating resistor RH change, and the flow rate measurement (detection) accuracy and response There was a problem that the property deteriorates significantly. When used in applications such as water heaters, air conditioners, and ventilation fans used in the kitchen, oily foreign matter tends to adhere, and measures against such foreign matter adhesion are strongly required.

As a countermeasure against the adhesion of foreign matter, the heating resistor R
If H is constantly set to a high temperature of 200 ° C to 250 ° C,
Although foreign matter is less likely to adhere, there are problems such as consideration for safety and increased power consumption. Furthermore, the temperature of the mounting part of the heating resistor RH to the circuit board also becomes high. There is a problem in that it is difficult to reduce the size and cost of the entire sensor because the method for attaching (holding) the resistor RH can be restricted.

Therefore, an object of the present invention is to provide a flow rate sensor for incinerating and removing foreign matter adhering to the heating resistor by heating the heating resistor to a high temperature periodically or at a desired time.

[0010]

In order to achieve the above object, the invention according to claim 1 provides a heating resistor and a temperature compensating resistor, each having a thin film resistance pattern formed on an insulating substrate, in a fluid. And a signal processing circuit for processing a signal from each of the resistors, which detects the flow rate of the fluid by utilizing the heat dissipation of the heat-generating resistors. For a period of time, the heating resistor is provided with a switching means for directly applying a power supply voltage, and the heating resistor is heated to 300 ° C. to 500 ° C. to incinerate foreign matter adhering to the heating resistor. It is characterized by being removed.

According to a second aspect of the present invention, in the flow sensor according to the first aspect, the switching means is composed of a transistor, and the heating resistor is connected to a power source through the transistor. To do.

According to a third aspect of the present invention, in the flow sensor according to the first or second aspect, the length of the resistance pattern forming portion of the heating resistor is 1 / the length of the insulating substrate.
It is characterized by being 3 or less.

The flow rate is not detected during heating of the heating resistor for burning foreign matter.

According to the above construction, the heating resistor is 30
Since it is heated to a high temperature of 0 ° C to 500 ° C, it is possible to incinerate and remove foreign matter such as oil attached to the heat-generating resistor. This heating is carried out periodically or only for a short time at a desired time, so that the increase in power consumption can be suppressed, the heat has little influence on other parts and members, and the safety is not impaired. . Further, it is possible to suppress the temperature rise of the mounting portion of the heating resistor.

Further, by setting the length of the resistance pattern forming portion of the heating resistor to be 1/3 or less of the length of the insulating substrate, it is possible to suppress the temperature of the mounting portion to a temperature below which the solder does not melt. Become.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described with reference to the drawings showing embodiments thereof. In the following figures, the same reference numerals are given to those having the same or the same functions as those of the conventional example,
The description is omitted.

FIG. 1 is a circuit diagram of a flow sensor according to an embodiment of the present invention. As shown in FIG. 1, the flow rate sensor of the present embodiment is configured by adding a switching transistor Tr2 to the basic circuit described in FIG. 4 of the conventional example.
Specifically, the collector of the transistor Tr2 is connected to the connection point between the heating resistor RH and the resistor R1 of the resistance bridge circuit, the emitter of the transistor Tr2 is connected to the power supply Vcc, and the base of the transistor Tr2 is connected to the signal input end a. The transistor Tr2 is configured to be turned on / off by the pulse signal Ps which is connected and applied to the signal input end a.

That is, in the flow rate sensor of the present embodiment, the power supply voltage Vcc is directly applied to the heating resistor RH via the transistor Tr2 separately from the power supply voltage Vcc of the signal processing circuit for detecting the flow rate. Transistor Tr2
When the pulse signal Ps is input to the base of the above, the heating resistor RH is heated to a high temperature for the duration of the pulse width of the pulse signal Ps.

By this heating, the heating resistor R
Foreign matters such as oil, dust, and dust attached to the H resistance pattern portion are incinerated and removed. The pulse width of the pulse signal Ps, that is, the heating time, should be incinerated and scattered by the temperature of the resistance pattern forming portion of the heating resistor RH in consideration of the place of use and the type of foreign matter that adheres. The temperature is set in the range of 300 ° C to 500 ° C.

Application of the pulse signal Ps is performed automatically once a day to once every several days, or manually at a desired time. That is, the cycle of incineration of foreign matter is set according to the use environment, and it is possible to avoid a situation in which a large amount of foreign matter becomes inorganic dust and cannot be removed unless the incineration temperature is raised to 800 ° C.

The waveform of the pulse signal Ps is not limited to a rectangular shape, and a waveform of 0.6 to 0.7 V or more for turning on the transistor Tr2 is applied to the base of the transistor Tr2 for a predetermined time. If

Next, FIGS. 2 and 3 show the structures of the flow rate sensor and the heating resistor RH embodying the above circuit configuration.

As shown in FIG. 2, this flow sensor includes a circuit board 1 having a vertically long window portion 2, a heating resistor RH and a temperature compensation resistor R mounted so as to straddle the window portion 2.
T, a differential amplifier 4 arranged on the circuit board 1, a transistor 5 for current amplification, a resistor (chip resistor) 6 which constitutes a bridge circuit, etc., a switching transistor 7 and an input / output terminal 8 ... It is configured with. Two of the plurality of resistors 6 correspond to R1 and R2 of the circuit shown in FIG. 1. In an actual circuit, resistors for temperature adjustment and output offset adjustment are added. The heat-generating resistor RH and the temperature-compensating resistor RT are soldered so as to straddle the window portion 2 and at a predetermined distance in order to obtain a desired heat separation, and a soldering portion is provided to enhance mounting strength. A reinforcing material 9 such as an epoxy resin is applied so as to cover the.

Heating resistor RH and temperature compensating resistor RT
As shown in FIG. 3, a resistance pattern 14 made of a metal film of platinum or the like is formed on a rectangular insulating substrate 11 made of alumina or the like, and on both sides of the insulating substrate 11 to be a mounting portion,
Terminal electrodes 15 are formed on both ends of the resistance pattern 14 on the circuit,
15 is formed and configured as a chip type temperature sensitive resistor. The resistance pattern 14 is formed by etching the metal film 12 in the central portion of the insulating substrate 11 in the longitudinal direction and removing the metal film 12 with a laser trimmer to form a groove 13.

The length A of the resistance pattern forming portion of the heating resistor RH is formed to be ⅓ or less of the length B of the insulating substrate 11. Insulating substrate 11 has a thickness of 0.15 mm
0.3 mm, length 2.0 mm to 3.0 mm, resistance about 10
As a result of an experiment using the heating resistor RH having the above configuration of Ω, the power supply voltage VccDC5.0V was applied for about 1 minute to 2 minutes, and the temperature of the resistance pattern forming portion was 300 ° C. to 500 ° C.
It was confirmed that the temperature of the formation portion of the terminal electrode 15, that is, the attachment portion, can be suppressed to about 180 ° C. or lower, which is the melting point of the solder, when it is heated to ℃.

In the above construction, the heating resistor RH
The heating for incineration and removal of foreign matter adhering to is only performed for a short time (tens of seconds to several minutes) periodically or at a desired time, and it is possible to suppress an increase in power consumption, and to heat other parts and members. There is little impact on and safety is not compromised.

Further, by setting the length A of the resistance pattern forming portion of the heating resistor RH to be 1/3 or less of the length B of the insulating substrate 11, it is possible to suppress the temperature rise of the mounting portion and to melt the solder. The deterioration of the reinforcing agent 9 is prevented.

In the heating resistor RH of the above embodiment, the terminal electrodes 15 are arranged at both ends of the insulating substrate, but the invention is not limited to this, and the ones are arranged on one side of the insulating substrate 11. But it's okay. In this case, the temperature of the mounting portion can be further suppressed.

The signal processing circuit composed of the bridge circuit and the differential amplifier is not limited to the above embodiment.

[0030]

As described above, according to the flow rate sensor of the present invention, the heating resistor is heated to a high temperature of 300 ° C. to 500 ° C., and foreign matter such as oil adhered to the heating resistor is incinerated. Since it can be removed, it is possible to prevent the deterioration of the flow rate detection accuracy due to the adhesion of foreign matter to the heating resistor.

This heating is carried out periodically or only for a short period of time at a desired time, so that an increase in power consumption can be suppressed, the heat has little effect on other parts and members, and the safety is impaired. Nor. Further, it is possible to suppress the temperature rise of the mounting portion of the heating resistor.

Further, by setting the length of the resistance pattern forming portion of the heating resistor to be ⅓ or less of the length of the insulating substrate, the temperature of the mounting portion can be suppressed to a temperature at which the solder does not melt.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a flow sensor according to an embodiment of the present invention.

FIG. 2 is a plan view of a flow sensor of the circuit shown in FIG.

FIG. 3 is a perspective view showing an embodiment of a heating resistor used in the present invention.

FIG. 4 is a circuit diagram of a conventional flow sensor.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Circuit board 2 Window part RH Heating resistor RT Temperature compensation resistor 4, OPA Differential amplifier 5, Tr1 transistor 6, R1, R2 Resistor 7, Tr2 Switching transistor 8 Input / output terminal 9 Reinforcing agent 11 Insulating substrate 12 Metal film 13 Groove 14 Resistance pattern 15 Terminal electrode Vcc Power supply (voltage) Ps Pulse signal

Claims (3)

[Claims] 1. A signal processing circuit for arranging a heating resistor and a temperature compensating resistor, each having a thin film resistance pattern formed on an insulating substrate, in a fluid and processing a signal from each of the resistors. A flow rate sensor for detecting the flow rate of a fluid by utilizing heat dissipation of the heat generating resistor, provided with a switching means for directly applying a power supply voltage to the heat generating resistor for a predetermined time period periodically or at a desired time. And heating the heating resistor to 300 ° C. to 500 ° C. to incinerate and remove foreign matter attached to the heating resistor. 2. The flow rate sensor according to claim 1, wherein the switching means is composed of a transistor, and the heating resistor is connected to a power source through the transistor. 3. The flow sensor according to claim 1, wherein the length of the resistance pattern forming portion of the heating resistor is 1/3 or less of the length of the insulating substrate.