JP2771949B2 - Thermal flow sensor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thermal flow sensor for measuring the presence / absence of a flow of a liquid, and more particularly, to a resistance value of a thermal flow detecting resistance element which is cooled by a flow of a fluid and changes. The present invention relates to a thermal type flow sensor which is operated based on an output voltage obtained from a bridge circuit and operates stably even when the liquid temperature changes.

[0002]

2. Description of the Related Art Conventionally, as a thermal type flow sensor for detecting a flow of a fluid, a flow meter for successively measuring a flow rate and a flow switch for simply detecting the presence or absence of a predetermined flow have been used. As a flow meter and a flowing water switch, a heater and a pair of thermocouples are installed in a pipe in which the fluid flows, and the temperature drop of the fluid heated by the heater is measured by the thermocouple, so that the flow rate of the liquid and the flow rate of the fluid are measured. Is used. However, the thermal flow meter using a thermocouple has a problem that the response speed is slow. Therefore, when a quick response is required, the flow velocity and the flow rate are measured by a thermal flow meter. On the other hand, a thermal anemometer is also used as a flowing water switch. In this case, a resistance element for flow rate detection is arranged in a fluid, heated, cooled by the flow of the fluid, and changed for the fluid detection. By detecting the resistance value of the resistance element as an output voltage in the bridge circuit, and comparing the output voltage with a certain threshold value to determine,
It is operated as an ON-OFF switch.

That is, in a conventional thermal current meter,
One of the resistance elements forming the bridge circuit is disposed as a flow rate detection resistance element in a pipe through which a fluid flows, and current is constantly passed through the resistance element to heat the resistance element. Here, when the flow velocity of the fluid changes, the amount of heat that the fluid takes away from the resistance element changes, so that the resistance value of the flow rate detection resistance element changes. The thermal anemometer measures a flow rate and a flow rate by detecting a change in a current flowing through a flow rate detecting resistance element having a changed resistance value as an output voltage from a bridge circuit.
According to the thermal anemometer, the flow rate can be accurately and responsively measured, so that it is widely used recently.

However, when the temperature of the fluid passing through the pipeline changes, the temperature of the flow detecting resistance element changes even if the flow rate is the same, and the amount of heat taken off by the fluid from the flow detecting resistance element differs. It will be. Accordingly, the output voltage detected by the bridge circuit as a change in the current flowing through the flow rate detecting resistance element having a changed resistance value differs depending on the temperature of the fluid even if the flow rate is the same. That is, there is a problem that an error occurs in the flow rate measurement due to a change in the temperature of the fluid.
That is, FIG. 4 shows the relationship between the output voltage and the flow rate measured by a conventional one-element type thermal flow sensor.
Here, the one-element type thermal flow sensor is one in which only one resistance element for flow rate detection is arranged in a pipe in which a fluid flows.

Using this one-element thermal flow sensor,
The liquid used for the measurement is water. Liquid temperature (water temperature)
Are experimented with six types of 0, 10, 20, 30, 40, and 50 (° C.). It can be seen that the change in the output voltage with respect to the change (increase) in the flow rate is small at all water temperatures. However, as can be seen from FIG. 4, the value of the output voltage for the same flow rate differs greatly depending on the water temperature, and the effect of the liquid temperature on the output voltage is large. Therefore, when measuring the flow rate of liquids with different temperatures, the output voltage changes with the temperature of the liquid, and the same output voltage is not applied to all the liquids with different temperatures. The threshold cannot be determined and it cannot operate as a flush switch.

In order to eliminate a measurement error due to a change in the temperature of a fluid, in a thermal type anemometer, in order to correct the temperature of a resistance element for flow rate detection forming a bridge circuit, a resistance element for flow rate detection and a temperature sensor for water temperature detection are used. A resistance element is mounted in the flow path to form a bridge circuit.

[0007]

However, the conventional thermal flow sensor has the following problems. That is, the change in the resistance value of the flow detection resistor itself caused by the self-heating of the flow detection resistor has not been considered. Then, the bridge circuit is in a state of equilibrium (a state in which no current flows in the galvanometer of the bridge circuit)
The bridge circuit was operating in a state deviated from the above. Therefore, there is a problem that a malfunction occurs in the operation as the water flow switch.

That is, FIG. 5 shows the relationship between the output voltage and the flow rate measured by a two-element flow switch using a conventional flow rate detecting element and a water temperature detecting element. Using this two-element thermal flow sensor, the liquid used for measurement is:
Water. The temperature of the liquid (water temperature) is 0, 10, 20, 3
Experiments are performed with six types of 0, 40, and 50 (° C.). As can be seen from FIG. 5, in the two-element thermal flow sensor, the output voltage value for the same flow rate is different at all the water temperatures, as compared with the one-element flow sensor of FIG. Not as large as in the case of a one-element flow sensor. That is, the effect of the temperature of the liquid on the output voltage is
It turns out that it is not so large.

However, when measuring the flow rates of liquids having different temperatures, the output voltage slightly changes depending on the temperature of the liquid, and the output voltage does not become the same constant for all the liquids having different temperatures. However, there has been a problem that a fixed threshold value cannot be determined, and the device cannot operate as a water flow switch. The reason is that the resistance value of the flow detection resistor itself changes due to self-heating of the flow detection resistor.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problem, and the same output voltage is detected for all liquids having different temperatures without being affected by the temperature of the liquid. Accordingly, an object of the present invention is to provide a thermal flow sensor capable of stably detecting the presence or absence of a flow.

[0011]

In order to achieve this object, a thermal type flow sensor according to the present invention is formed to include a resistance element for detecting a flow rate and a resistance element for detecting a water temperature disposed in a liquid passage. A thermal flow sensor having a bridge circuit provided therein, the thermal flow sensor having a temperature rise correction resistance element outside the liquid passage and connected in series with a water temperature detection resistance element constituting the bridge circuit. In the above thermal type flow sensor, when the resistance value of the temperature rise correction resistance element is energized to the bridge circuit and the resistance value of the flow rate detection resistance element changes due to temperature rise, the bridge circuit is electrically balanced. It is characterized by a resistance value.

[0012]

The thermal flow sensor of the present invention having the above-described structure is disposed in a liquid passage and detects the presence or absence of a liquid flow. Here, the flow rate detecting resistance element constituting the bridge circuit is constantly heated to a temperature about 20 to 30 degrees higher than the fluid temperature. The flow rate detecting resistance element heated in the flow path is deprived of heat by the fluid. Here, the amount of heat taken by the fluid is proportional to the flow velocity. Then, heat is deprived as the flow velocity increases, and the temperature of the flow rate detecting resistance element decreases. Then, the resistance value of the flow detection resistance element decreases, the voltage division ratio of the bridge in the flow detection resistance element changes, and a bridge voltage is generated. The flow velocity is measured by detecting the change in the voltage as an output voltage.

When the temperature of the fluid changes, the temperature of the water temperature detecting resistance element becomes the same as the temperature of the fluid, and the bridge circuit performs temperature correction for the temperature change of the fluid. In addition, a minute change in the resistance value of the flow rate detection resistance element caused by being heated to a constant temperature is connected in series to the water temperature detection resistance element at a position opposite to the flow rate detection resistance element of the bridge circuit. The temperature rise correction resistance element corrects the error caused by the heating of the flow rate detection resistance element in order to bring the bridge circuit into an equilibrium state when the flow rate detection resistance element is energized and heated to a predetermined temperature. Is done. As a result, even if the temperature of the fluid changes, the equilibrium state in the resistance element for flow rate detection is constant, so that it is possible to easily take a threshold value and obtain a thermal type flow sensor that operates stably. it can.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A thermal flow sensor 1 according to an embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows the configuration of the thermal flow sensor 1. The thermal type flow sensor 1 has a resistance element 2 for detecting a water temperature and a resistance element 3 for detecting a flow rate which are arranged in series in a flow direction of the fluid F such that a detection portion contacts the fluid F in a passage in which the fluid F flows. I have. Then, the water temperature detection resistance element 2 and the flow rate detection resistance element 3
Are covered with a protective cover 4 which is a protective member. The space between the water temperature detecting resistance element 2, the flow rate detection resistance element 3 and the protective cover 4 is filled with a molding material 5. The molding material 5 fixes the water temperature detecting resistance element 2 and the flow rate detection resistance element 3 and the like, transfers heat, and prevents water droplets and moisture from entering from outside the air side.

The protective cover 4 is provided with an O for preventing leakage.
It is attached to the body part 7 via the ring 6. The body portion 7 is a cylindrical tube, and the inlet through which the fluid F flows in and the outlet through which the fluid F flows out are formed slightly larger in diameter. The fluid F flows in from the left and flows out in the right. In this embodiment, the resistance element 2 for detecting the water temperature and the resistance element 3 for detecting the flow rate are arranged in series with respect to the flow. However, they may be arranged in parallel. You may.

In the body portion 7, the cross-sectional area of the flow path through which the fluid F flows in the pipe is formed to be constant. Further, the flow of the fluid F is configured to be in a laminar flow state inside the body portion 7. Since the water temperature detecting resistance element 2 and the flow rate detection resistance element 3 are provided substantially at the center of the flow path, they are in contact with the fluid F at the position where the velocity of the fluid F is highest. Therefore, the maximum velocity of the fluid F is detected as a thermal flow sensor.

FIG. 2 shows a bridge circuit for measuring a change in current of the resistance element 3 for flow rate detection. The bridge circuit includes a water temperature detecting resistance element 2 (resistance value R4) and a temperature rise correction resistance element 8 (resistance value R5) connected in series therewith.
, A flow detecting resistance element 3 (resistance value R3), and two fixed resistance elements (resistance value R1 and resistance value R
2). Here, the temperature rise correction resistance element 8 (resistance value R5) is connected to the water temperature detection resistance element 2 (resistance value R4) in series outside the fluid passage.
Two fixed resistance elements (resistance value R1 and resistance value R2)
Are for the flow rate detection resistance element 3 (resistance value R3) and the water temperature detection resistance element 2 (resistance value R4), respectively. Vs is a power supply voltage, and Vb is an output voltage.

In the bridge circuit, when the fluid F is not flowing, the water temperature detecting resistance element 2 is at the same temperature as the fluid F, and the flow rate detection resistance element 3 is heated. Here, since the water temperature detection resistance element 2 and the flow rate detection resistance element 3 have different temperatures, their resistance values are slightly different, and there is a small potential difference between V1 and V2. In the present invention, a temperature rise correction resistance element 8 is connected in series with the water temperature detection resistance element 2 in order to correct this minute potential difference.

Next, how to determine the resistance value of the temperature rise correction resistance element 8 will be described. Equation 1 is a calculation equation for calculating the output voltage Vb detected from the bridge circuit of FIG.

(Equation 1) In the bridge circuit of FIG. 2, the output voltage Vb is determined by the difference between the voltages V2 and V1 at the connection point of the bridge circuit. However, in practice, the temperature coefficient (T
CR), it is necessary to add the correction. Therefore, the actual output voltage is the corrected output voltage Vb '
Is calculated by Here, since R1 'and R2' are mounted outside the fluid passage, they are corrected by the ambient temperature TR. Also, since R3 'is in the fluid passage,
Since the temperature is corrected by the water temperature TW and further heated to a predetermined temperature, the temperature is corrected by the temperature rise TO heated by energization. Further, since R4 'is in the fluid passage, the water temperature T4'
It is corrected by W. Further, since R5 'is mounted outside the fluid passage, it is corrected by the ambient temperature TR.

Next, the thermal type flow sensor 1 having the above configuration
The operation of will be described. In this embodiment, the flow rate of water, which is one of the fluids F, is measured. The fluid F flows into the body 7 from the left. Inside the body 7, the flow of the fluid F is in a laminar flow state. Since the water temperature detecting resistance element 2 and the flow rate detection resistance element 3 are attached at the center position of the fluid passage, the maximum velocity of the fluid F is detected. Here, the flow rate detecting resistance element 3 is constantly heated to a predetermined temperature. This predetermined temperature is set to be about 20 to 30 degrees higher than the temperature of the fluid F.

Then, heat is deprived by the flow rate detecting resistance element 3 and the fluid heated in the flow path. Here, the amount of heat taken by the fluid is proportional to the flow velocity. Then, heat is deprived as the flow velocity increases, and the temperature of the flow rate detecting resistance element 3 decreases. Then, the resistance value R3 of the flow detection resistance element 3 decreases, the voltage division ratio based on the resistance values R3 and R1 of the flow detection resistance element 3 changes, and V1 decreases. Then, a bridge output voltage Vb is generated. The flow velocity is measured by detecting the change in the voltage as an output voltage. Here, the fluid F
When the temperature of the fluid F changes, the temperature of the water temperature detecting resistance element 2 changes as in the case of the flow rate detection resistance element 3, and the error due to the temperature change of the fluid F is corrected by the bridge circuit. In addition, when the resistance element 3 for flow rate detection is heated to a predetermined temperature, the resistance value of the resistance element 3 for flow rate detection slightly changes, but the error due to this change is corrected by the temperature rise correction resistance element 8.

Next, FIG. 3 shows experimental results of the thermal type flow sensor 1 which is an embodiment of the present invention. This FIG.
It shows the relationship between the flow rate and the output voltage. The horizontal axis shows the flow rate, and the vertical axis shows the output voltage. FIG. 3 shows an example in which water is used as the liquid. And the temperature of the water is 0, 10, 20, 30, 40, 50 (° C.),
Six types of water are used. As can be seen from FIG. 3, compared to the conventional FIGS. 4 and 5, at all water temperatures,
It can be seen that the output voltage values are almost the same. That is, it is understood that there is almost no influence of the liquid temperature on the output voltage.

This is also confirmed from the value of the corrected output voltage Vb 'in the above equation (1). That is, when calculating the corrected output voltage Vb ', the correction resistance values (R1', R2 ', R3', R
4 ', R5'), the temperature coefficient of resistance (TCR), the ambient temperature, and the like. For example, when calculating the corrected output voltage Vb ′ when the water temperature is 0 ° C., the resistance value of each resistance element (R1 = 100Ω, R2 = 1000Ω,
R3 = 100Ω, R4 = 1000Ω, R5 = 48Ω) and the temperature coefficient (TCR1 =
0.0001, TCR2 = 0.0001, TCR3 =
0.005, TCR4 = 0.005, TCR5 = 0.0
001) and the ambient temperature = 20 ° C. and the water temperature = 0 ° C. are substituted into the equations for calculating the correction resistance value of each resistance element, whereby the correction resistance value (R1 ′ =
100.2Ω, R2 ′ = 1002Ω, R3 ′ = 100
Ω, R4 ′ = 1000Ω, R5 ′ = 48.096Ω). Further, when these values are substituted into the equation for calculating the corrected output voltage Vb 'and calculated, Vb' = 0.1755
V is calculated.

When calculating the corrected output voltage Vb 'when the water temperature is 50 ° C., the resistance value (R
1 = 100Ω, R2 = 1000Ω, R3 = 100Ω, R
4 = 1000Ω, R5 = 48Ω) and the temperature coefficient of the resistance value of each resistance element (TCR1 = 0.0001, TCR
2 = 0.0001, TCR3 = 0.005, TCR4 =
0.005, TCR5 = 0.0001), ambient temperature = 20 ° C., water temperature = 50 ° C., etc., are substituted into the equations for determining the correction resistance value of each resistance element, so that the correction resistance value of each resistance element ( R1 '= 100.2Ω, R2'
= 11002Ω, R3 ′ = 125Ω, R4 ′ = 1250
Ω, R5 ′ = 48.096 Ω). Further, when these values are substituted into the equation for calculating the corrected output voltage Vb 'and calculated, Vb' = 0.1395 V is calculated.

When the water temperature is 0 ° C. and 50 ° C., the difference between the corrected output voltages Vb ′ is 0.036 V, which is extremely small. That is, it is understood that there is almost no influence of the liquid temperature on the output voltage. This also indicates that the output voltage values are almost the same for all water temperatures.

As described above in detail, according to the thermal type flow sensor 1 of the present embodiment, the thermal flow sensor 1 is formed to include the flow rate detecting resistance element 3 and the water temperature detection resistance element 2 disposed in the liquid passage. Since the thermal type flow sensor 1 having the bridge circuit provided has the temperature rise correction resistance element 8 outside the liquid passage and connected in series with the water temperature detection resistance element 2 constituting the bridge circuit, Not only the temperature change of F,
Since the change in the resistance value generated by heating the flow rate detecting resistance element 3 is also corrected, an output voltage less related to the temperature of the fluid F can be obtained, an appropriate threshold value can be easily selected, and the stable value can be obtained. The thermal type flow sensor 1 which performs the above operation can be obtained. Further, since a complicated temperature correction circuit is not required, a low-cost thermal flow sensor 1 can be provided.

Further, the thermal type flow sensor 1 of the present embodiment is configured such that when the resistance value of the temperature rise correction resistance element 8 is energized to the bridge circuit and the resistance value of the flow rate detection resistance element 3 changes due to temperature rise. Since the resistance value is such that the bridge circuit is electrically balanced, the voltage generated by heating the flow rate detecting resistance element 3 to a predetermined temperature is also corrected, so that an output voltage less related to the temperature of the fluid F is obtained. Thus, it is possible to obtain the thermal type flow sensor 1 which can easily select an appropriate threshold value and operate stably.

The embodiments of the present invention have been described above.
The present invention is not limited to the above embodiment, but can be applied in various ways. For example, in the present embodiment, the resistance value of the temperature rise correction resistance element 8 is obtained by calculation, but the resistance of the temperature rise correction resistance element 8 is set to a variable resistance, and the value of the temperature rise correction resistance element 8 is determined by experiment. May be. In this embodiment, the flow switch is described. However, the present invention can be applied to a thermal flow meter for measuring a flow rate.

[0029]

As is apparent from the above description, a thermal type having a bridge circuit formed by including a resistance element for detecting a flow rate and a resistance element for detecting a water temperature disposed in a liquid passage of the present invention. In the flow sensor, outside the liquid passage,
The temperature rise correction resistance element connected in series with the water temperature detection resistance element that constitutes the bridge circuit generates not only the temperature change of the fluid but also the heating of the flow rate detection resistance element to a predetermined temperature. Therefore, it is possible to obtain an output voltage less related to the temperature of the fluid, easily select an appropriate threshold value, and obtain a thermal type flow sensor that operates stably. Further, since a complicated temperature correction circuit is not required, a low-cost thermal flow sensor can be provided.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a configuration of a thermal flow sensor 1 according to an embodiment of the present invention.

FIG. 2 is a bridge circuit diagram of a flow measuring unit of the thermal type flow sensor 1.

FIG. 3 is a data diagram showing an experimental result of the thermal type flow sensor 1;

FIG. 4 is a data diagram showing experimental results of a conventional one-element type thermal flow meter.

FIG. 5 is a data diagram showing experimental results of a conventional two-element thermal flow meter.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Thermal type flow sensor 2 Water temperature detection resistance element 3 Flow rate detection resistance element 4 Protective cover 8 Temperature rise correction resistance element F Fluid

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) G01F 1/68

Claims (1)

(57) [Claims] [Claim 1] have a bridge circuit which is contained in form a liquid passage disposed flow rate sensing resistor element and the temperature sensing resistor element in the temperature coefficient of the flow rate detecting resistance element TC
The thermal type flow sensor is R3, the In the outside the liquid passage has a temperature rise correction resistor element connected to said temperature sensing resistor element in series constituting said bridge circuit, the temperature increase correction resistor element The resistance value R5
The resistance value of the resistance element for flow rate detection is
Is changed by the temperature rise TO and R3 '= R3 + R3 × T
When CR3 × TO, the bridge circuit is
Value R5 = R3 '× R2 / R1-R4 to be balanced
Thermal flow sensor, characterized in that it.