JP3424974B2 - Flow sensor - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a flow sensor used for a mass flow meter for measuring a gas flow rate or a mass flow controller for measuring a gas flow rate and controlling the gas flow rate. 2. Description of the Related Art For example, a conventional flow sensor used in a mass flow controller has a capillary having an inner diameter of 0.3 to 1 mm.
A wire of about 0 μm is wound as a heater on the upstream side and the downstream side. When the gas flows inside the capillary, the upstream heater is deprived of heat by the gas flow, and the downstream heater is heated by the heat carried by the gas flow. The flow rate can be detected by this heat balance. In this case, in the flow rate sensor having the above-described structure, the winding has a function as a temperature sensor in addition to a function as a heater. In order to investigate the change in temperature balance, upstream and downstream temperature sensors were incorporated in a bridge circuit, and a delicate resistance change was output. [0004] However, in the above-mentioned winding, the resistance value is as small as about several hundreds Ω, and
As shown in the graph II of FIG. 4, the rate that varies with temperature (TCR) is too small as 0.00 per 1 ° C. 4. Therefore, the amplification factor of the amplifier of the bridge circuit is large, and is greatly affected by the temperature influence of the amplifier itself. In addition, there is a disadvantage that the sensor output greatly changes due to a minute change in contact resistance at a welding portion of the winding due to a change with time. Further, in order for the bridge circuit to be realized even when the temperature of the gas or the environmental temperature is changed, the resistance value and the resistance temperature coefficient of the two windings must exactly match. When manufacturing windings, the temperature coefficient of resistance is not measured due to the difficulty of measurement, but the resistance value must be measured and within the strict standard range of ± 0.5Ω. However, this requires a great deal of care in the production, which inevitably increases the production cost. SUMMARY OF THE INVENTION The present invention has been made in consideration of the above circumstances, and has as its object to provide a flow sensor which is easy to manufacture and can perform stable measurement over a wide flow range which requires almost no adjustment. And In order to achieve the above object, a flow sensor according to the present invention is provided with a heater on a surface of a pipe through which gas flows, and an upstream and downstream side of the heater. A first temperature sensor and a second temperature sensor are provided on the surface of the pipe, respectively, and a third temperature sensor is provided on the upstream side of the first temperature sensor. The heater is heated so that the average value is always higher than the temperature of the third temperature sensor by a constant temperature, and the resistance values of the first temperature sensor and the second temperature sensor when the gas flow rate is zero are Ru ₀ and Rd ₀ , respectively. ,
When the resistance values of the first temperature sensor and the second temperature sensor when the gas is flowing are Ru and Rd, respectively, {(Ru−Rd) − (Ru ₀ −Rd ₀ )} × f (Ru−Rd) The mass flow rate of the gas in the pipeline is determined based on the following equation. In the flow sensor, first, the heater is energized with the gas flow rate being zero at the time of adjustment, and the outputs of the first and second thermistors at this time are stored in a microcomputer. Obtain a calibration curve with the temperature. Next, in a state where the gas is flowing, the average value of the temperatures of the first and second thermistors provided on both the upper and lower sides of the heater is higher by a certain temperature than the gas temperature detected by the third thermistor. to supply power to the heating data. The gas flow rate can be obtained by using the output difference between the first thermistor and the second thermistor at this time and the calibration curve. 1 to 3 show an example of a flow sensor according to the present invention. First, FIG. 1 and FIG. 2 show the configuration of a sensor unit 10 of a flow rate sensor. Reference numeral 11 denotes a pipeline through which a gas G flows. A ceramic heater 12 is attached. Reference numeral 13 denotes a device which holds the capillary 11 horizontally and which is supplied from the heater 12 to the capillary 11.
Is a substantially U-shaped radiator plate for improving heat radiation to the space, and the capillary 11 is provided with a vertical portion 1 erected at appropriate intervals.
3a and 13b are inserted, and the heater 12 is held so as to be positioned between the vertical portions 13a and 13b. Reference numerals 14 and 15 denote temperature sensors attached to the surface of the capillary 11 on the upstream and downstream sides of the heater 12 and between the vertical portions 13a and 13b, for example, a chip-type thermistor (hereinafter referred to as a thermistor). 1 thermistor 14 and second thermistor 15).
It is arranged at the part where the temperature changes most by the gas flow. 16 is a capillary 1 on the upstream side of the first thermistor 14
1, more specifically, a temperature sensor attached to the surface of the capillary 11 slightly upstream of the vertical portion 13a on the upstream side, for example, a chip type thermistor (hereinafter referred to as a third thermistor). This is for measuring the temperature of the gas G inside. Hereinafter, the resistance values of the thermistors 14 to 16 are represented by Ru, Rd, and Rg, respectively.
It expresses. The heater 12, thermistor 14
16 are wire-bonded to a ceramic substrate 17 arranged near the capillary 11 via gold wires 18. Reference numeral 19 denotes a plurality of lead extraction pins provided on the ceramic board 17. By the way, a commercially available thermistor is oxidized at a high temperature of 80.degree. C. or more, thereby preventing the resistance value from changing. Therefore, in this state, the capillary 1
1 cannot be stuck on the surface. Therefore, in this embodiment, as the thermistors 14 to 16, a bare chip thermistor is hermetically sealed by applying a hermetic seal, and an inert gas such as nitrogen gas is sealed in the inside, or the inside is evacuated. Is used. FIG. 4 is a graph showing a change in resistance of a metal resistor used in a conventional winding and a thermistor used in the present invention with temperature. A curve I shows a metal resistor (TCR400).
0 ppm) and the curve II shows the resistance change of the thermistor (B constant 3800), and each resistance value at 80 ° C. is represented as 1. The function of the resistance value of each resistor and the temperature is as follows. Metal resistor: R = R ₀ {1 + α (T−T ₀ )} Thermistor: R = R ₀ exp {B (1 / T−1 / T ₀ )} where R: resistance value, R ₀ : reference temperature , T:
Temperature (K), T ₀ : reference temperature, α: temperature coefficient of resistance, B:
B constant As shown in FIG. 4, the resistance value of the metal resistor hardly changes (about 0.004 / ° C.) when the temperature changes by several degrees. In contrast, thermistors vary by a few percent per degree Celsius. In addition, it changes almost linearly in a narrow temperature range, and its linearity is as excellent as about 2 to 3% (B constant is usually about 3800). Even if the linearity is poor, there is no problem because the calibration curve is created in the microcomputer 21 as described later. FIG. 3 shows an example of a sensor drive section 20 for driving the heater 12 in the sensor section 10 and processing sensor signals. In this figure, reference numeral 21 denotes a microcomputer, for example, an EEPROM 22 connected thereto. I have. The microcomputer 21 receives signals a, b, and c representing resistance changes of the thermistors 14 to 16, and outputs a control signal d to the heater 12 to the amplifying circuit 2.
3 is output. In order to convert the resistance change of the thermistors 14 to 16 into electric signals a, b and c, a constant current may be applied to the microcomputer 21 to obtain a voltage signal, and a capacitor having a small temperature coefficient may be used.
A CR oscillation circuit may be configured and used as a frequency signal. The EEPROM 22 stores calibration curve data, data obtained in the adjustment stage, and the like, and these data are stored even when the power is turned off. First, the calibration curve data will be described.
Is the sum of the resistance value Rg of the third thermistor 16 and the resistance values of the first thermistor 14 and the second thermistor 15 (Ru +
Rd), the data of the resistance value Rg of the third thermistor 16 is taken in order to measure the gas temperature, and then (Ru + Rd) becomes a value according to this calibration curve. Power is supplied to the heater 12. Thereby, the heating of the heater 12 can be controlled to (gas temperature + constant temperature difference). In the adjustment stage, when a voltage is supplied to the heater 12 in a state where the gas flow rate is zero, the first thermistor 14 and the second thermistor 15 are warmed, and the respective resistance values change. The first thermistor 1 at this time
4. The resistances Ru ₀ and Rd ₀ of the second thermistor 15 are measured. FIG. 6 shows the change in the heater voltage and the resistance value of the first thermistor 14 and the second thermistor 15 at this time, and the curve I in the figure shows the resistance value R of the first thermistor 14.
u ₀ and the curve II represents the resistance Rd of the second thermistor 15.
₀ , and the curve III shows (Ru ₀ + Rd ₀ ).
The relationship between (Ru ₀ + Rd ₀₎ and (Ru ₀ -Rd ₀₎ is expressed as shown by the solid line in FIG. After the adjustment as described above, the gas flow rate is measured as follows. That is, the first and second thermistors 14 and 1 provided on the upper and lower sides of the heater 12, respectively.
Power is supplied to the heater 12 so that the average value of the temperature at 5 is higher than the gas temperature detected by the third thermistor 16 by a certain temperature.
The outputs Ru and Rd of the thermistors 14 and 15 are input to the microcomputer 21 to obtain the sum (Ru + Rd). Then, the value obtained by subtracting the value of (Ru ₀ −Rd ₀ ) corresponding to (Ru + Rd) at this time is (Ru−Rd−Rd ₀ ).
By multiplying by the function of Rd), i.e., f (Ru-Rd), the gas flow rate X is obtained. This can be expressed as follows. X = {(Ru−Rd) − (Ru ₀ −Rd ₀ )} × f (Ru−Rd) (1) Then, the zero point in the flow sensor of the present invention is (Ru−Ru) in the above equation (1). -Rd)-(Ru
₀ -Rd ₀ ). Therefore, unlike the related art, the zero point hardly fluctuates due to the resistance value balance. Also, for the span, the above f (Ru
-Rd) is corrected so that there is no temperature effect. In the flow sensor according to the present invention, the thermistors 14 to 16 used for temperature measurement have a large change in the detection output as compared with the windings, so that the gain of the amplifier may be small. As a result, it is not necessary to consider the temperature influence of the amplifier. Since the resistance value is large and the resistance change is large, a stable output can be obtained. Further, since the outputs of the thermistors 14 and 15 at a gas flow rate of zero are input to the microcomputer 21, the thermistors 14, 1
It is not necessary to consider variations in the resistance value and the B constant of No. 5.
Therefore, the sensor can be easily manufactured, and the yield is greatly improved. Since the balance between the resistance values of the thermistors 14 and 15 is stored in the microcomputer 21, even if the environmental temperature or the gas temperature changes, the state of zero gas flow is determined from the stored contents. Can be. Therefore, even if the environmental temperature changes, there is almost no change in the flow rate instruction, and there is no need to accommodate the sensor in a thermostat and adjust the temperature. The present invention is not limited to the above-described embodiment.
mm may be used. And heater 1
The winding type 2 may be used. The thermistor is easily oxidized by oxygen in the atmosphere, and its characteristics are easily changed. Since the leakage current when a reverse bias is applied to the PN junction of the silicon semiconductor has the same characteristics as the thermistor, a PN diode may be used as the temperature sensors 14 to 16 instead of the thermistor. As described above, according to the present invention, a heater is provided on the surface of a pipe through which gas flows,
Temperature sensors are provided at the upstream position and the downstream position of the heater, and the gas flow rate is measured based on the change in the output of the temperature sensor when the heater is energized. Thus, a flow sensor can be obtained at low cost. Also in the adjustment stage,
Adjustment for temperature adjustment becomes unnecessary. Therefore, there is no temperature effect caused by the mutual variation of the temperature sensors, and a flow sensor which operates stably over a wide temperature range can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view showing an example of a configuration of a sensor unit of a flow sensor. FIG. 2 is a sectional view schematically showing a main part of the sensor unit. FIG. 3 is a diagram schematically showing an example of a configuration of a sensor driving section of the flow sensor. FIG. 4 is a diagram showing a change in resistance of a metal resistor used in a conventional winding and a thermistor used in the present invention with temperature. FIG. 5 is a diagram showing an example of a calibration curve. FIG. 6 shows the heater voltage when the gas flow rate is zero,
FIG. 5 is a diagram illustrating changes in resistance values of a first thermistor and a second thermistor. FIG. 7 is a diagram illustrating a relationship between a sum and a difference between two thermistor outputs. [Description of Signs] 11 ... conduit, 12 ... heater, 14 ... first temperature sensor, 1
5: second temperature sensor, 16: third temperature sensor, G: gas.

Claims (1)

(57) [Claim 1] A heater is provided on the surface of a pipe through which gas flows, and a first temperature sensor and a second temperature sensor are provided on the surfaces of the pipes upstream and downstream of the heater, respectively. A second temperature sensor is provided, and a third temperature sensor is further provided upstream of the first temperature sensor, and an average value of the temperature of the first temperature sensor and the temperature of the second temperature sensor is always constant than the temperature of the third temperature sensor. The heater is heated so as to increase by the temperature, and the resistance values of the first temperature sensor and the second temperature sensor when the gas flow rate is zero are set to Ru ₀ and Rd ₀ , respectively, and the first temperature sensor when the gas is flowing When the resistance values of the second temperature sensor are Ru and Rd, respectively, based on the following formula, {(Ru−Rd) − (Ru ₀ −Rd ₀ )} × f (Ru−Rd) It is characterized by finding the mass flow rate of gas The flow rate sensor that.