JP4805091B2 - Thermal mass flow sensor and mass flow controller - Google Patents

Description

  The present invention relates to a thermal mass flow sensor used for flow control in a mass flow controller, for example.

  Conventionally, a thermal mass provided with a sensor unit including an upstream sensor unit and a downstream sensor unit formed by winding two thermal resistors in a coil shape outside a metal hollow thin tube through which a sample gas flows. A flow sensor is known.

  Specifically, the hollow thin tube is heated by the thermal resistor, and when the sample gas is not flowing, the temperature distribution is symmetrical with respect to the centers of the two coils. On the other hand, when the sample gas flows in the hollow thin tube, the sample gas heated by the upstream sensor unit flows into the downstream sensor unit, and therefore the temperature is higher than that of the upstream sensor unit. A temperature difference is formed between the upstream sensor unit and the downstream sensor unit. As a result, the temperature distribution becomes asymmetric.

Since a certain relationship is established between the temperature difference (ΔT) and the mass flow rate of the sample gas at this time, the mass flow rate can be measured by detecting the temperature difference using a bridge circuit provided for both sensor units. (For example, refer patent document 1).
Patent Publication 7-271447

  However, in such a conventional configuration, for example, when the average temperature of the entire sensor unit (coil) is increased in order to increase the sensor sensitivity or expand the full scale (FS) range, a chemical reaction occurs depending on the gas type. It has the problem of being easy.

  Further, in the case where the number of turns per unit length is the same throughout the entire sensor unit as in the prior art, when the average temperature of the hollow thin tubes is increased by the sensor unit, the temperature distribution of the hollow thin tubes after the average temperature is increased ( Compared with FIG. 13 (Jb)) and the temperature distribution of the hollow thin tube before raising the average temperature (FIG. 13 (Ja)), the sharpness of the peak appearing at the center of the temperature distribution is hardly changed, and a minute change is preferable. Therefore, it cannot be said that the sensor sensitivity can be improved.

  In particular, in order to accurately detect the flow rate of low vapor pressure gas, the linearity of the flow rate change with respect to the temperature difference generated between the upstream side thermal resistor and the downstream side thermal resistor (hereinafter abbreviated as “linearity”). It is necessary to flow the flow rate that can be secured to the sensor unit (referring to the full-scale flow rate that flows to the sensor unit as the flow rate that can ensure linearity is called the first stage restriction), compared to the case where other sample gases are detected. However, there is a problem that the sensor sensitivity is reduced.

  This is not only the case of the constant current type control described above, but also in the case of the constant temperature type control, the linearity of the flow rate change with respect to the power difference and the detection of a minute change are also a problem. Yes.

  The present invention has been made paying attention to such problems, and the main purpose is to expand the area where the linearity can be ensured, expand the full scale, and detect the flow rate with high sensitivity. To provide a mass flow controller that can provide a mass flow sensor and, when employed in a mass flow controller, expands the full scale and realizes highly accurate flow control.

  That is, the thermal mass flow sensor according to the present invention is composed of a hollow thin tube through which a sample gas flows and a thermal resistor whose electric resistance value increases or decreases as the temperature changes, and is provided upstream and downstream of the hollow thin tube, respectively. A thermal mass comprising an upstream sensor section and a downstream sensor section wound in a coil shape, and detecting the flow rate of the sample gas flowing through the hollow thin tube based on a change in resistance value of both sensor sections. The flow sensor, wherein both of the sensor parts have a larger number of turns per unit length of the thermal resistor than the outer area in a predetermined area inside each sensor part of each of both ends. A winding portion is provided.

  Here, “more windings per unit length” means, for example, that only a predetermined area inside each sensor unit is wound in a plurality of stages (for example, two stages) while keeping the winding interval of the thermal resistor constant. Thus, the number of windings in the predetermined region on the inside is larger than that in the outer region, and the winding interval in the predetermined region inside each sensor unit while the number of winding steps is one, the winding interval in the outer region For example, an aspect of increasing the number of turns in a predetermined region inside the narrower region by narrowing the width of the wire is included.

  According to such a configuration, by providing the thermal resistor of each sensor part with a predetermined width, linearity (between the upstream sensor part and the downstream sensor part) can be obtained with respect to the molecules in the sample gas flowing in the hollow thin tube. The amount of heat necessary to ensure the linearity of the flow rate change with respect to the temperature difference (power difference) generated in

  On the other hand, a multiple peak provided in a predetermined region inside each sensor unit can sharpen the central peak appearing in the temperature distribution of the entire sensor unit, as indicated by Ha in FIG. Therefore, the upstream / downstream difference can be increased at the time of detection, and the change can be captured with high sensitivity, so that the sensor sensitivity is improved.

  In particular, even for sample gases that require a first-stage restriction, such as low vapor pressure gas, and where full scale is severely restricted, the area where linearity can be secured is expanded to expand full scale, and the sensor is highly sensitive. Therefore, the flow rate can be detected well.

  In order to sharpen the peak of the temperature distribution that appears at the center of the hollow thin tube, it is preferable that each of the multiple winding portions is provided from the inner end of each sensor portion.

  As a desirable aspect of the multi-winding portion of the present invention, among the one thermal resistor, the first-stage winding portion wound around the outer periphery of the hollow thin tube with a predetermined width, and the one thermal resistor A multi-stage winding section formed by overlapping all or a part of the remaining heat-sensitive resistor wound around the first-stage winding section on the first-stage winding section so as to have a width smaller than the predetermined width. And the multi-winding part is constituted by the multi-stage winding part.

  As a desirable mode of the present invention, one in which the winding width of the multi-stage winding portion is set to ½ or less of the winding width of the first-stage winding portion can be mentioned.

  By the way, in order to provide a mass flow controller that can expand the full scale and realize high-accuracy flow control, the mass flow controller includes the above-described thermal mass flow sensor, and a flow control valve provided in the flow path through which the sample gas flows. It is desirable that the apparatus further comprises a measured flow rate value output from the thermal mass flow sensor and a control unit that controls the valve opening degree of the flow rate control valve based on a set flow rate value that is a target flow rate.

  As described above, the thermal mass flow sensor according to the present invention is necessary for ensuring the linearity with respect to the molecules in the sample gas flowing in the hollow thin tube by providing the thermal resistor of each sensor portion with a predetermined width. The amount of heat can be given.

  On the other hand, the central peak appearing in the temperature distribution of the entire sensor portion can be sharpened by the multiple winding portions provided in a predetermined region inside each sensor portion. Therefore, the upstream / downstream difference can be increased at the time of detection, and the change can be captured with high sensitivity, so that the sensor sensitivity is improved.

  In particular, even for sample gases that require a first-stage restriction, such as low vapor pressure gas, and where full scale is severely restricted, the area where linearity can be secured is expanded to expand full scale, and the sensor is highly sensitive. Therefore, the flow rate can be detected well.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

  As shown in the schematic diagram of FIG. 1, the thermal mass flow sensor A of the present embodiment includes a hollow thin tube 1 and an upstream side as an upstream side sensor unit provided on the upstream side and the downstream side of the hollow thin tube 1 respectively. A coil 2a and a downstream coil 2b as a downstream sensor section (hereinafter collectively referred to as "coil 2"), and after the instantaneous flow rate of the sample gas G is detected by the coil 2 as an electrical signal (resistance value) The electric signal is amplified by the internal electric circuit 3 (bridge circuit 31, amplifier circuit 32, correction circuit 33) and output as a sensor output signal having a value corresponding to the detected flow rate. Hereinafter, each part is demonstrated concretely.

  The hollow thin tube 1 is made of stainless steel having substantially the same cross-sectional shape in the longitudinal direction. In this embodiment, the hollow thin tube 1 is erected on the base 4 so as to form an inverted U shape. Note that, depending on the embodiment, the inner diameter and the outer diameter of the hollow thin tube 1 can be changed, and other hollow materials can be used. Further, the base 4 that supports the hollow thin tube 1 may be omitted.

  The upstream coil 2a is formed by winding a thermal resistor K whose electrical resistance value increases or decreases with a change in temperature. In this embodiment, as shown in FIG. As a multi-winding portion in which the number of turns per unit length of the thermal resistor K is larger than that in the outer region Y in the predetermined region X on the coil 2b side (in other words, the inner predetermined region X where each coil 2 approaches). A multi-winding portion T is provided.

  Specifically, in this embodiment, the multiple winding portion T is set to the second winding portion m2 that is double-wound with respect to the first winding portion m1.

  Here, the first winding portion m1 is formed by winding the outer circumference of the hollow thin tube 1 with a predetermined width in one thermal resistor K.

  Further, the second-stage winding part m2 includes all of the remaining thermal resistor K wound around the first-stage winding part m1 among the one thermal resistor K. The wire portion m1 is wound around the outer periphery of the first-stage winding portion m1 from the inner end m11 to the outer end m12 so as to be narrower (about 1/10) than the predetermined width.

  The downstream coil 2b is formed by winding the same thermal resistor K as the thermal resistor K used for the upstream coil 2a symmetrically with respect to the center line CL. Multiple winding portions in which the number of turns per unit length of the thermal resistor K is larger than that of the outer region Y in the predetermined region X on the upstream coil 2a side (in other words, the predetermined region X on the inner side where each coil 2 approaches) T is comprised. The configuration of the multiple winding portion T of the downstream coil 2b is the same as the configuration of the multiple winding portion T of the upstream coil 2a.

  Hereinafter, in order to clarify the role of the first winding part m1 and the second winding part m2 constituting the coil 2 (influence on sensor sensitivity characteristics, influence on linearity), a simulation by CAE analysis is performed. It was. The results of the results will be described with reference to FIGS.

In the simulation, the average temperature of the coil 2 is constant, and C ₄ F ₈ is used as the sample gas. Moreover, conditions, such as winding specification, were made into the following conditions except when clearly shown in FIGS.

<C4F8>
Molecular weight: 200g
Density (760 Torr (21.1 degrees)): 8.66 kg / m3
Constant pressure specific heat (760 Torr (21.1 degrees)): 159 J / (mol · K)
Heat conduction specific heat (760 Torr (21.1 degrees)): 0.0102 W / (m · K)
Viscosity (760 Torr (21.1 degrees)): 0.012 mPa · s

  (1) FIG. 3 shows a simulation result when the first stage winding width is “changed” and the second stage winding width is “fixed”.

Here, in FIG. 3, the horizontal axis represents the flow rate of the sample gas C ₄ F ₈ , and the vertical axis represents the average temperature difference created by the upstream coil 2a and the center of the downstream coil 2b. Note that this temperature difference is a normalized temperature difference in order to uniformly prepare conditions other than the test parameters.

  As can be seen from FIG. 3, even if the first stage winding width is increased while the second stage winding width is fixed, no significant change appears in the sensitivity change. That is, with respect to the sensor sensitivity characteristic, it is considered that the winding width of the second winding part m2 is dominant.

  (2) FIG. 4 shows a simulation result when the first stage winding width is “fixed” and the second stage winding width is “changed”.

  Here, in FIG. 4, the horizontal axis and the vertical axis are the same as those in FIG.

  As can be seen from FIG. 4, even if the winding width of the second stage is increased while the winding width of the first stage is fixed, the change in linearity is small. That is, with regard to linearity, it is considered that the first-stage winding portion m1 is dominant.

  (3) When the first and second winding widths are substantially equal (winding specification I) and when the first winding width is about 8.3 times the second winding width (winding) The simulation result comparing with the line specification II) is shown in FIG.

  Here, in FIG. 5, the horizontal axis and the vertical axis are the same as those in FIG.

  As can be seen from FIG. 5, the winding specification II is superior to the winding specification I in terms of linearity.

  (4) Next, in order to clarify the influence when the arrangement position of the second-stage winding part m2 with respect to the first-stage winding part m1 is changed (FIGS. 6 to 8), simulation by CAE analysis is performed. Went. The role of each winding part will be described with reference to the result shown in FIG.

  The simulation conditions are the same as those described above.

  FIG. 6 is an arrangement according to the present embodiment, in which the second winding portion m2 is connected to the inner end portion (in other words, each coil 2 of the first winding portion m1). It is a figure which shows the aspect arrange | positioned in "both inside").

  FIG. 7 is an arrangement according to a comparative example with the present embodiment, and is a diagram showing an aspect in which the second-stage winding part m2 is arranged at the center of the first-stage winding part m1.

  FIG. 8 shows an arrangement according to a comparative example with the present embodiment, in which the second winding portion m2 is placed on the outer end portion (in other words, the side where the coil 2 in the first winding portion m1 does not come together). Then, it is a figure which shows the aspect arrange | positioned in "both outside") of each coil 2. FIG.

  In FIG. 9, the horizontal axis and the vertical axis are the same as those in FIG.

  As can be seen from FIG. 9, the case where the second winding part m <b> 2 is arranged in “both sides” on the side where the coil 2 in the first winding part m <b> 1 approaches is the other two cases. In comparison, the temperature difference between the upstream coil 2a and the downstream coil 2b can be increased, so that it is understood that the sensor sensitivity can be improved.

  Therefore, according to the thermal mass flow sensor A according to the present embodiment, the thermal resistor K wound around the outer region Y of each coil 2 is efficient for the molecules in the sample gas flowing in the hollow thin tube 1. The amount of heat necessary to ensure linearity can be given to the.

  On the other hand, the multiple peaks T provided in the predetermined region X inside each coil 2 can sharpen the central peak appearing in the temperature distribution of the entire coil 2 as indicated by Ha in FIG. Therefore, the upstream / downstream difference can be increased at the time of detection, and the change can be captured with high sensitivity, so that the sensor sensitivity is improved.

  Thus, the sensor sensitivity can be improved by sharpening the peak of the temperature distribution appearing at the center of the hollow thin tube 1 without unnecessarily increasing the average temperature of the entire sensor unit (coil 2). Even for a sample gas that requires a first stage restriction and the full scale is severely restricted, the flow rate can be detected satisfactorily because the area where the linearity can be secured can be expanded to expand the full scale and the sensor can be highly sensitive.

  Moreover, since the multiple winding part T as each multiple winding part is provided from the inner end of each coil 2, the peak of the temperature distribution which appears in the center of the hollow thin tube 1 can be sharpened.

  In addition, the first-stage winding portion m1 wound around the outer periphery of the hollow thin tube 1 with a predetermined width in the single thermal resistor K, and the first-stage winding in the single thermal resistor K. A second winding portion m2 formed by overlapping all or a part of the remaining thermal resistor K wound around the wire portion m1 on the first winding portion m1 and winding it to be narrower than the predetermined width; The multi-winding portion T is constituted by the second-stage winding portion m2, so that it can be easily manufactured, and the first-stage winding portion m1 wound with a predetermined width is included in the hollow thin tube 1 A second winding portion wound on the first winding portion m1 so as to be narrower than the predetermined width while giving heat necessary for ensuring linearity to molecules in the flowing sample gas. The central peak appearing in the temperature distribution of the entire coil 2 can be preferably sharpened by m2. In this way, it is possible to increase the sensitivity of the sensor while ensuring linearity and expanding the full scale.

  If the thermal mass flow sensor A is incorporated in the mass flow controller, the thermal mass flow sensor A can expand the full scale by increasing the area where linearity can be secured, and can detect the flow rate with high sensitivity. The full scale as a mass flow controller can be expanded, and the mass flow controller which can implement | achieve very highly accurate flow control can be provided.

  As a specific mode of the mass flow controller B in which the thermal mass flow sensor A is incorporated, for example, as shown in FIG. 11, a hollow thin tube R1 (a coil of the thermal mass flow sensor A in the hollow thin tube R1 is disposed in the middle of the flow path. 2) and a flow path control valve V provided on the gas flow path R on the downstream side of the merge point Rx; The control flow rate value output from the thermal mass flow sensor A and the control unit C that controls the valve opening degree of the flow rate control valve V based on the set flow rate value that is the target flow rate are included. However, it is not limited to this.

  The present invention is not limited to the above embodiment.

  For example, the second-stage winding part m2 is provided from the inner end m11 on the side where the coil 2 in the first-stage winding part m1 comes into contact, but as shown in FIG. You may make it provide from the position Xoff offset outside.

  In addition, the winding width of the second winding portion m2 is not limited to that of the present embodiment, and can be, for example, about half the winding width of the first winding portion m1.

  Alternatively, only the predetermined region X inside each coil 2 may be three or more stages while keeping the winding interval of the thermal resistor K constant.

  Moreover, in this embodiment, only the predetermined area | region X inside each coil 2 is wound in two steps, making the winding space | interval of the thermal resistor K constant, and the number of turns in the inner predetermined area | region X is set to the outer area | region Y. Although the number of windings is one, the winding interval in the predetermined region X inside each coil 2 is narrower than the winding interval in the outer region Y, for example. The number of turns in the predetermined region X may be increased.

  Moreover, although the clearance gap is provided between each coil 2, it can also be set as the structure without a clearance gap.

  Further, the thermal mass flow sensor of the present invention is not limited to the constant current type described above, but can also be applied to a constant temperature type.

  In addition, the specific configuration of each part is not limited to the above embodiment, and various modifications can be made without departing from the spirit of the present invention.

The schematic diagram which shows the thermal mass flow sensor which concerns on one Embodiment of this invention. The figure which shows the coil in the same embodiment typically. The figure which shows the simulation result in the same embodiment. The figure which shows the simulation result in the same embodiment. The figure which shows the simulation result in the same embodiment. The figure which shows typically the arrangement | positioning aspect of the 2nd winding part in the embodiment. The figure which shows typically the different arrangement | positioning aspect for comparing with the arrangement | positioning aspect of FIG. The figure which shows typically the different arrangement | positioning aspect for comparing with the arrangement | positioning aspect of FIG. The figure which shows the simulation result about the arrangement | positioning aspect of the 2nd winding part shown in FIGS. The figure explaining the sensor sensitivity in the same embodiment. The schematic diagram which shows the mass flow controller which employ | adopted the thermal mass flow sensor in the same embodiment. The figure which shows typically the arrangement | positioning aspect of the 2nd winding part in other embodiment of this invention. The figure explaining the sensor sensitivity of the conventional structure.

Explanation of symbols

1 ... Hollow thin tube 2 ... Sensor (coil)
2a ... Upstream sensor section (upstream coil)
2b ··· Downstream sensor section (downstream coil)
A ... Thermal mass flow sensor B ... Mass flow controller G ... Sample gas K ... Thermal resistor m1 ... Winding part m2 in the first stage ... 2nd winding part m11 ... Inner end T ... Multiple winding parts (multiple winding parts)
X: Inside area Y ... Outer area

Claims (5)

A hollow thin tube through which the sample gas flows;
An upstream sensor unit and a downstream sensor unit, each of which is formed of a heat-sensitive resistor whose electrical resistance value increases or decreases with a change in temperature and is wound around each of the upstream side and the downstream side of the hollow thin tube in a coil shape;
A thermal mass flow sensor configured to detect the flow rate of the sample gas flowing through the hollow thin tube based on the resistance value change of the two sensor parts,
The two sensor parts have a plurality of winding parts in which the number of turns per unit length of the thermal resistor is larger than that of the outer area in a predetermined area inside each sensor part of both end parts. A thermal mass flow sensor characterized by comprising:   The thermal mass flow sensor according to claim 1, wherein each of the multiple winding portions is provided from an inner end of each sensor portion. A first-stage winding portion formed by winding a predetermined width around the outer periphery of the hollow thin tube of one thermal resistor;
Of the one thermal resistor, all or a part of the remaining thermal resistor wound around the first-stage winding portion is overlapped with the first-stage winding portion so that the width is narrower than the predetermined width. A multi-stage winding portion formed by winding,
The thermal mass flow sensor according to claim 1 or 2, wherein the multiple winding portion is constituted by the multi-stage winding portion.   4. The thermal mass flow sensor according to claim 3, wherein the winding width of the multi-stage winding portion is set to ½ or less of the winding width of the first-stage winding portion. The thermal mass flow sensor according to any one of claims 1 to 4,
A flow control valve provided in the flow path through which the sample gas flows;
A mass flow comprising: a measured flow value output from the thermal mass flow sensor; and a control unit that controls a valve opening degree of the flow control valve based on a set flow value that is a target flow rate. controller.