JPH0227221A - Mass flow controller - Google Patents

Abstract

PURPOSE:To achieve a higher temperature characteristic by connecting a resistor with a value of below a specified percentage of a resistance value of a thermal resistance wire in series to the thermal resistance wire. CONSTITUTION:In a mass flow controller which uses a thermal flow rate sensor provided with a downstream thermal resistance wire 1 and an upstream thermal resistance wire 2 on the circumference of a fine flow tube 3, a resistor R4 with a value of below 2% of a resistance value thereof is connected in series to the resistance wires 1 and 2 on the side of R1 (or R2) of the resistor R1 and R2 thereof 1 and 2. Therefore, a resistance value increases by 2% while a temperature coefficient decreases by almost 2%. This enables external correction of the temperature coefficients of the thermal resistance wires along with a limited drift for a change in the resistance value itself and the drift is corrected by addition of an external resistor with due consideration given to this fact. In this manner, under a system of winding a thermal resistance wire, a temperature characteristic can be upgraded simply by connecting an external resistor.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field 1] The present invention relates to a mass flow controller used for precisely controlling gas flow rate t/, and particularly relates to an improvement in a heat-sensitive flow rate sensor.

[Prior Art] Conventionally, as shown in the schematic diagram of FIG. 10, as a heat-sensitive flow sensor of a mass flow controller, 2 fl heat-sensitive resistance wires 1.2 are wound around the outer periphery of a trickle pipe 2 on the downstream side and the upstream side. Linear sensors are widely used. The trickle pipe 3 is used to divide the gas and determine the mass flow rate. The heat-sensitive resistance wire 1.2 provides the gas with an almost constant σ thermal envelope through the thin wall of the trickle tube 3 at all times. Further, the resistance value of the heat-sensitive resistance wire changes depending on the temperature. When gas flows from the inlet 4 to the outlet 5 as shown by the arrow in the figure, heat is carried from the upstream heat-sensitive resistance wire 2 to the downstream heat-sensitive resistance wire 1, so the temperature on the downstream side is higher than on the upstream side. It gets expensive.

The two heat-sensitive resistance wires are integrated into a bridge circuit with a constant input current. The output voltage is proportional to the difference in resistance of the heat-sensitive resistance wire (R, -R2) caused by the temperature difference (ΔT). The other two parameters are heat input (P) and specific heat (
C9) are both -・definite. Unlike gas viscosity and thermal conductivity, specific heat is preferable as a substitution parameter. This is because the specific heat is 5, which is inherent to gases, so it remains almost constant over a wide range even if the temperature or pressure changes.

Although there is essentially no linear relationship between output and mass flow rate, there is a nearly linear relationship within the range of normal use.

FIG. 9 is a circuit diagram showing the configuration of the eleventh bridge.

In FIG. 9, R and R2 are respectively downstream heat-sensitive resistance wires 1. This is the resistance of the upstream heat-sensitive resistor [12].

R3 and R4 and v, R1 are the opposite side resistances of the bridge connected to R1 and R2. The collector of transistor Q1 that drives this bridge is connected.

A detection resistor R5 for detecting the current flowing through the bridge is connected in series with the bridge. There is a Zener diode that generates a reference voltage ■7, and it is connected to VC, (for example +1
5■) is connected. The output voltage of the detection resistor R5 and the reference voltage 7 are connected to the input of the operational amplifier A1 via resistors R and R, respectively. The output of the operational amplifier A1 is connected to the base of a bridge driving transistor Q1.

A bridge output can be obtained from the midpoint of the bridge balance resistor and the connection point between the heat-sensitive resistance wires.

The bridge output is connected to the input of an operational amplifier Δ2 whose gain is determined by resistors R11, R12, R13, and R14.

Therefore, if the temperature changes due to the gas flow rate, the resistance values of R and R will change, resulting in an output V from the operational amplifier A2. is obtained.

If the gain of A1 is large enough, the input voltage of operational amplifier A1, v+, will increase due to feedback.

■ are equal. Here, V = V2 V = IXR5 + (However, ■ is bridge drive N current) Therefore, V = IXR5 and f = V /R5 is obtained, and the bridge drive current I is constant by the operational amplifier A1 and the bridge drive transistor Q1. Electrified.

As a mass flow controller, for example, a heat-sensitive resistance wire 1 with a large temperature coefficient is installed on the downstream side and the first flow side of the trickle hanging pipe 2.
．． 2, maintains the nR value supplied to each heat-sensitive resistance wire constant, and detects the temperature distribution of the heat-sensitive part that changes as gas flows (
For example, the passing fluid is conditioned by changing the fluid temperature to W411!5 (Japanese Patent Publication No. 56-23094), and the temperature of the fluid is changed to a different temperature value in the heat exchange action when the fluid passes, and the temperature is adjusted. For example, Japanese Patent Laid-Open Publication No. 59/1983 discloses a method of measuring flowing mountains by displaying the energy expended in at least one of the steps of changing the temperature and changing the temperature.
-18423).

However, the former has the disadvantage of lacking responsiveness because the speed at which the temperature distribution changes is influenced by the heat capacity of the tricklestone tube and its coating. In addition, although the latter has a better response speed than the former, the operating principle is the same as that of a hot wire anemometer, so the zero point is likely to fluctuate due to changes in ambient temperature, differences in the heat capacity n of the fluid, etc. There are drawbacks. In order to solve this drawback, it has been proposed to provide a constant temperature circuit (JP-A-62-13120 and JP-A-61-1281).
23 Publication), the actual situation is that it has not been widely put into practical use because the manufacturing cost of the circuit is high.

[Problems to be Solved by the Invention] The present invention has been made to address the above-mentioned problems, and therefore aims to improve the temperature characteristics while maintaining the conventional method of winding a temperature-sensitive resistance wire.

It is generally said that the temperature characteristics of a sensor made of a wire wound with a temperature-sensitive resistance wire are poor. This is said to be caused by the principle of the heat-sensitive flow M sensor. That is, since the temperature difference caused by the gas flowing in the trickle loss tube is small, the temperature characteristics of the sensor are determined by the temperature characteristics of a DC amplifier including a bridge circuit that extracts the temperature difference as a voltage.

Therefore, as a way to improve the temperature characteristics, it is possible to use an amplifier with small temperature drift, or to correct the output by setting a temperature slope, but these methods increase the cost. The effect is small.

After conducting a detailed analysis of the characteristics of the sensor, the inventor discovered that sensor drift is caused by unevenness in the resistance values of the two heat-sensitive resistance wires, heat dissipation, and imbalance in the temperature coefficient of resistance that occur during sensor manufacturing. That became clear.

In other words, the conventional sensor is 21Il! 1 heat-sensitive resistance wire cannot be created under exactly the same conditions, so it has the disadvantage of having drift.

[Means for Solving the Problems] In order to solve the above problems, the present invention utilizes the change in electrical resistance due to the temperature difference caused by the gas flowing in the trickle pipe to generate a voltage corresponding to the quality 1 ffl 5t. In a mass flow controller using a heat-sensitive flow sensor in which at least two output heat-sensitive resistance wires are provided on the outer periphery of a trickle A tube, a wire with a resistance value of 2% or less of the resistance value of the heat-sensitive resistance wire is connected in series with the heat-sensitive resistance wire. This is a mass 70-controller characterized by having 11 heat-sensitive flow rate sensors connected to resistors having a value.

A heat-sensitive flow sensor is used in which a resistor having a resistance value 50 times or more the resistance value of the heat-sensitive resistance wire is connected in parallel with the heat-sensitive resistance wire.

Furthermore, a resistor having a resistance value of 2% or less of the resistance value of the heat-sensitive resistance wire is connected in series with the heat-sensitive resistance wire, and a resistor having a resistance value of 50 times or more the resistance value of the heat-sensitive resistance wire is connected in parallel with the heat-sensitive resistance wire. This uses a heat-sensitive flow m sensor connected to a resistor with .

The present invention attempts to actively change the temperature coefficient from the outside in order to correct the drift.

Letting α be the temperature coefficient of the resistance R1 of the downstream heat-sensitive resistance wire 1, the resistance value is expressed by equation (1).

R-Ro (1+αT) ・(1)Ro
=Resistance value at reference temperature T: Temperature Connect a resistor of 1 te in series to this resistor.

Here, R=Ro/N (2), J3<, and the combined resistance R is as shown in equation (3).

■Self N R, = R (1 ton N core a ton) - (3) Similarly R
Consider the case where resistors 1 are connected in parallel.

R=nR0 ... (4) Then, the combined resistance R becomes as shown in equation (5).

n n Old Shiro□Ro (1-1-7α-cho), -(5) Now,
If it is N-50, then N2L'-N = 1.02°N+1 curvature 0.98, so equation (3) is the thermal resistance wire 1
It is shown that when a resistor R having a resistance value of 2% is connected in series with , the resistance value increases by 2% and the temperature coefficient decreases by approximately 2%.

On the other hand, Equation (5) shows that when n-50 is added, both the resistance value and the temperature coefficient decrease by approximately 2% by adding a resistor in parallel.

It has been found that the temperature coefficient of the heat-sensitive resistance wire can be corrected externally in this way. Furthermore, the drift caused by a change in the resistance value itself is small, and even if this is taken into account, it is possible to correct the drift by adding an external resistor.

FIG. 11 shows the analysis results of the zero point trit of the sensor due to the difference in the resistance value of the heat-sensitive resistance wires, where the downstream heat-sensitive resistance wire 1 is the upstream heat-sensitive resistance l! 4%, 8%, 12% compared to 2.
This figure shows the bridge output when the flow l is zero when the flow is 16% or 20% larger. If the resistance value of the heat-sensitive resistance wire differs by 20%, an output drift of 1.5 mV will occur.

In FIG. 1, R1=R1o(1+α1T) R2−R2o(1+α2T) Here, R10′ and R20 are the resistance values at R1° sub-temperature, respectively.

R2 group a1=a2=a 4700ppm/'CR1o=R
20(1+D) Rlo: O≦ at 50o25℃
D≦0.2.

Figure 12 shows the analysis results of the zero point drift of the sensor due to the difference in the temperature coefficient of the heat-sensitive resistance wires, showing that the downstream heat-sensitive resistance wire 1 is 0.2%, 0.4%, 0
．． The figure shows the output when the flow rate is zero when the flow rate is 6%, 0.8%, and 1% larger.

In Figure 12, R1=R1o (1 to α1 T) R2=R2o(1+α2T) α1≠α2 R10"" R20 α=α(1+H) 0≦H≦0.01.

If the temperature coefficient differs by 1%, the drift i・
occurs.

In this way, there are drifts in heat dissipation of resistance values, differences in resistance temperature coefficients, etc.! As a result of I calculation, it was found that the difference in temperature coefficient was the biggest cause of drift.

In FIG. 13, assuming that the downstream heat-sensitive resistance wire 1 has a resistance value 20% higher than that of the downstream heat-sensitive resistance wire 2, the parallel resistance R1 to the upstream heat-sensitive resistance wire 2 is set to 10 Ω. 20Ω
, 30Ω, 40Ω.

Example of correcting zero point drift by adding Ω to 50 (analysis result)
shows. It can be seen that a resistance of 20Ω minimizes the drift.

In FIG. 13, R1o=R2o(1+0.2> α 1 = α 2 (1+ t1 )R2(Il
This means that H is moved by the parallel resistor R9 attached to l.

Normally, manufacturing variations in sensors are within 2%, so in the present invention, the range of change in temperature coefficient due to added resistance is suppressed to 2%.

[Example] Hereinafter, the present invention will be explained in more detail based on examples.

A heat-sensitive flow sensor was fabricated using a stainless steel vibrator having an outer diameter Q of 5 mm and a wall thickness of 30 μm as the I8 flow N tube and having the configuration shown in FIG. 1. A heat-sensitive resistance wire was wound over a width of 10 mm. The resistance value of one resistance wire was 50Ω. 11 currents at 0.05A2 in the resistance wire using constant current control method
1iEt, sen υ was heated. The resistance value during heating was 75Ω when the ambient temperature was 25°C.

Figure 14 shows experimental results showing an example of zero point drift correction.
4.7Ω each as parallel resistance R1 to R21111
. Ω to 10. The case where Ω is connected to 22 and Ω is connected to 47 is shown.

From this figure, it can be seen that before the improvement, that is, when the resistor Q and the parallel resistor R0 were not attached, the drift that was 0.4 mV was reduced to 0.1 mV by simply adding a resistor of Ω to 47 in parallel. .

Although FIG. 1 is a bridge circuit diagram in the case where a parallel resistor RI) is attached to the R2 side, the parallel resistor R may be attached to the R1 side as shown in FIG. Also, as shown in Figure 3, the parallel resistance R,1,
R and 2 may be attached to the R1 side and R2 side, respectively,
In this case, it is necessary to insert a resistor VR2 for adjustment.

Furthermore, similar effects can be obtained by adding a series resistor R8 to the R1 or R2 side as shown in FIGS. 4 and 5. Also, as shown in Fig. 6, the series resistors R31°Rs2 are connected to R1
It is also possible to attach it to the R2 side and the R2 side, but in this case, it is necessary to insert a resistor VR2 for adjustment.

In addition, when the parallel resistor R is attached to the R1 side and the series resistor R is attached to the R2 side as shown in Figure 7, or when the series resistor R is attached to the R1 side and the parallel resistor R1 is attached to the R2 side as shown in Figure 8. It is also possible, but it would be too complicated and would not be of much practical use.

[Effects of the Invention] As detailed above, the present invention makes it possible to dramatically improve temperature characteristics by simply connecting an external resistor while retaining the conventional method of winding a temperature-sensitive resistance wire. Extremely effective.

[Brief explanation of the drawing]

Figures 1 to 8 are bridge circuit diagrams for extracting a sensor signal of a heat-sensitive flow sensor used in the mass flow controller of the present invention, and Figure 9 is a bridge circuit for extracting a sensor signal of a heat-sensitive flow rate sensor used in a conventional mass flow controller. Figure 10 is a schematic diagram of a heat-sensitive flow sensor, Figure 11 is a diagram showing the analysis results of the zero point drift of the sensor due to the difference in resistance of the heat-sensitive paper Wc wire, and Figure 12 is a diagram showing the temperature coefficient of the heat-sensitive resistance wire. Figures 13 and 14 show the analysis results of the zero point drift of the sensor due to the difference, and Figures 13 and 14 C show the analytical and experimental results of an example in which a parallel resistance is added to the upstream heat-sensitive resistance wire to correct the zero point drift, respectively. be. 1... Downstream side heat-sensitive resistance wire, 2... Upstream side heat-sensitive resistance wire, 3... Ribbon a tube, R, R81, R82... Series additional resistance, R, Rpl, Rp2... Parallel addition resistance. Figure (N c-c''''C) C100C) c-c- (N book) ♂ Separate g ← Stomach 5 ← → Ambient temperature C) → hIr Rino 8J outlet) Tokiberan I'll leave it out to R.

Claims (3)

[Claims] (1) At least two heat-sensitive resistance wires are provided on the outer periphery of the narrow flow tube that output a voltage corresponding to the mass flow rate by utilizing the change in electrical resistance caused by the temperature difference caused by the gas flowing inside the narrow flow tube. A mass flow controller using a heat-sensitive flow rate sensor, characterized in that the heat-sensitive flow rate sensor is connected in series with the heat-sensitive resistance wire to a resistor having a resistance value of 2% or less of the resistance value of the heat-sensitive resistance wire. . (2) A mass flow controller characterized in that a heat-sensitive flow rate sensor is used, in which a resistor having a resistance value 50 times or more the resistance value of the heat-sensitive resistance wire is connected in parallel with the heat-sensitive resistance wire. (3) A resistor with a resistance value of 2% or less of the resistance value of the heat-sensitive resistance wire is connected in series with the heat-sensitive resistance wire, and a resistor with a resistance value of 50 times or more of the resistance value of the heat-sensitive resistance wire is connected in parallel with the heat-sensitive resistance wire. A mass flow controller characterized by using a heat-sensitive flow sensor connected to a resistor having a certain value.