JPH109919A - Mass flowmeter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To correct the temperature drift of flow rate perfectly including the aging by correcting the drift of output due to variation in the characteristic of each component caused by aging through the use of a microcomputer. SOLUTION: When a fluid flows through a duct 10, a current IU flows through the resistance RSU of an upstream sensor while a current ID flows through the resistance RSD of a downstream sensor. At that time, the sensor output voltage VO (VO=RSUIURSDID) is proportional to the mass flow rate of the fluid. The output voltage VO is amplified at an amplifying circuit section 7 and converted at an A/D converting section 12 into a digital signal before being inputted to a microcomputer 11. Terminal voltages of ambient temperature detecting resistances Rtu , Rtd and reference resistances R30u, R30d are also converted into digital signals which are then inputted to the microcomputer 11. The microcomputer 11 automatically calculates a mass flow rate signal for which drift due to variation in the characteristic of each component caused by aging is corrected through processing based on these input signals.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mass flow meter for accurately measuring the mass of a fluid having a relatively small flow rate such as gas.

[0002]

2. Description of the Related Art In a film forming process and an etching process for manufacturing a semiconductor product, a gas flow control device for controlling a constant amount of various gases is mainly used. A mass flow meter for accurately measuring the gas flow rate is incorporated. As a method in which the flow meter detects the gas flow rate precisely, for example, US Pat.
No. 759 and Japanese Patent Application Laid-Open No. 1-158175 are known and are now in practical use.

This constant temperature difference type sensor has a structure as shown in FIG. 4, and a pair of sensor resistors Rs having a certain temperature coefficient of resistance are provided upstream and downstream of a fluid line 6 through which a gas fluid flows.
u and Rsd are wound in a coil shape, for example, platinum resistances Rtu, Rtd for detecting the ambient temperature in series with the sensor resistance.
And reference resistances R30u and R30d whose resistance temperature coefficients are substantially zero are connected. These and resistors R1u, R2u, R1d, R2d form a bridge circuit on the upstream side and on the downstream side, respectively, and a control circuit 4, which makes the sum of the sensor resistance, the ambient temperature detection resistance and the reference resistance equal, 5 is controlled so that this relationship always holds at any flow rate value on both the upstream and downstream sides. Since the mass flow rate of the fluid flowing at this time is proportional to the difference between the respective voltages of the upstream and downstream sensor resistances, the flow rate can be detected by measuring the potential difference.

When a fluid flows through the fluid line 6, Iu is supplied to the upstream sensor resistance Rsu and Id is supplied to the downstream sensor resistance Rsd.
Current flows at this time, and at this time, the upstream side control circuit unit 4 outputs Rsu = (R
30u + Rtu) .R2u / R1u, and the downstream control circuit 5 is Rs
It is constantly controlled so that d = (R30d + Rtd) .R1d / R2d. Usually, R2u / R1u = 1, R1d / R2d = 1,
And R30u + Rtu = R30d + Rtd, so that generally Rsu = (R30u + Rtu) and Rsd = (R30d
+ Rtd). Here, the sensor output voltage V _O is V _O
= Rsu · Iu−Rsd · Id, and when the electrical and thermal characteristics of the corresponding electronic components of both the upstream and downstream bridge circuits are completely identical to each other, the above equations are used. The mass flow rate of the fluid is proportional to the value of Iu-Id. In particular, when the flow rate is zero, Iu = Id, and the sensor output voltage V _O also becomes zero.

However, in practice, there are always variations in the initial resistance values of the sensor resistor, the ambient temperature detection resistor, and the zero point adjustment resistor, and variations in the temperature coefficient of resistance. Cannot be exactly the same as each other. Therefore, the output value of the sensor does not indicate the true flow rate value, and a deviation from the true value occurs,
This causes temperature drift. In order to correct this drift, a method of zero point adjustment has conventionally been adopted in which any of the resistors R1u, R2u, R1d, and R2d is made variable to cancel the drift amount. However, in this method, only the drift at the ambient temperature at that time, that is, the so-called zero-point drift is corrected, and when the ambient temperature changes, a new drift occurs.

As another technique for solving the problem of such temperature drift, the following method is currently being practiced. That is, the amount of zero point drift at that time at each ambient temperature is measured in advance as a digital amount using an A / D converter or the like, and the initial data is stored in an electronic element having a memory function incorporated in the flow meter. It is stored as a value. Similarly, the initial value is subtracted from the current sensor output value converted into a digital quantity by an A / D converter or the like by using an arithmetic function of a microcomputer or the like incorporated in the flow meter to obtain a digital value. There is also a method of compensating for the drift component. However, even in such a method, it is not possible to compensate for the drift with time caused when the characteristic value of each component of the flow meter changes with time. Once occurs, the flow rate detection accuracy is deteriorated because there is no longer any means for correction, and the current situation is that the accuracy of the entire flow meter is greatly reduced.

[0007]

As described above, the conventional mass flow meter has no means for correcting drift caused by the change over time of the electrical or thermal characteristics of the constituent parts. Because of the absence, once it occurred, the accuracy and reliability had to be reduced.

[0008]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems by simply adjusting the zero point using a variable resistor or digitally subtracting the initial drift as in the conventional drift correction method. Instead of a temporary flow rate correction method, the microcomputer incorporated in the flow meter captures the characteristic value information that has changed over time of each component constituting the flow meter as digital signals, and from these values the current drift is calculated. By calculating the amount by the correction calculation program built in the microcomputer, not only the temporary flow rate correction but also the time-dependent change of the sensor output, which is the main cause of the decrease in the accuracy of the flow meter, is completely corrected. , And can be output as a true flow signal.

That is, according to the present invention, an upstream sensor resistor is provided upstream of a fluid pipe through which a fluid flows, and an upstream ambient temperature detecting resistor and an upstream reference resistor having a resistance temperature coefficient of substantially zero are connected in series. Together with the value of the upstream sensor resistance,
An upstream constant temperature difference circuit controlled so that the sum of the upstream ambient temperature detection resistance and the upstream reference resistance is always equal, and a downstream sensor resistance is provided downstream of the fluid pipeline. A downstream-side ambient temperature detection resistor and a downstream-side reference resistor having a resistance temperature coefficient of substantially zero are connected in series to this, and the value of the downstream-side sensor resistance, and the downstream-side ambient temperature detection resistance and the downstream-side reference resistance A downstream constant temperature difference circuit controlled so that the sum value is always equal, based on a difference in energy given to the upstream sensor resistance and the downstream sensor resistance, the mass flow rate of the fluid The mass flow meter according to the present invention is characterized in that, in the mass flow meter to be obtained, a drift amount of an output caused by a change in a characteristic value of each component over time is corrected by using an element having an arithmetic function.

[0010] By adopting the configuration as in the present invention, it is possible to completely correct even the temperature drift caused by the temporal change of the flow meter, which was considered impossible in the prior art, and to achieve high reliability. It can be a mass flow meter with high performance and accuracy.

The mass flow meter according to the present invention is built in the microcomputer based on initial characteristic value data of each component stored in advance and various characteristic value data of each component after aging. The correction calculation is performed by the set of flow value correction calculation programs, and the current temperature drift amount of the flow meter is quantitatively obtained. By calculating the difference between the drift amount and the current flow output value, a true flow signal is output that self-corrects the temperature drift over time of the mass flow meter automatically, greatly improving the flow output accuracy of the flow meter. Can be improved.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below in detail with reference to FIG. The configuration of the mass flow meter according to the present invention includes a microcomputer unit 11, an upstream control circuit unit 8, a downstream control circuit unit 9, a fluid pipeline 10, an upstream sensor resistance Rsu, and an upstream ambient temperature detection resistance Rtu shown in FIG. , Downstream sensor resistance Rsd, downstream ambient temperature detection resistance Rtd, upstream reference resistance R30u, downstream reference resistance R30d, upstream bridge resistance R1u, R2u, downstream bridge resistance R1d, R2d, A / D
It comprises a converter 12, a D / A converter 13, a microcomputer 11, and a group of flow rate correction calculation programs built in the microcomputer.

When the fluid flows through the fluid line 10, a current of Iu flows through the upstream sensor resistor Rsu and a current of Id flows through the downstream sensor resistor Rsd. At this time, the sensor output voltage VO becomes VO
= RsuIu-RsdId, and when the electrical and thermal characteristics of the corresponding electronic components between the upstream and downstream bridge circuits are completely the same, the mass flow rate of the fluid is proportional to the value of Iu-Id. I do. Since this output voltage is generally very small, it is amplified by the sensor output amplifier circuit 7 to a predetermined voltage, converted into a digital signal by the A / D converter 12, and then input to the microcomputer 11. You. Similarly, the terminal voltages of the upstream and downstream ambient temperature detection resistors and the terminal voltages of the upstream and downstream reference resistors are also converted into digital signals by the A / D converter and then input to the microcomputer. The microcomputer automatically calculates the present mass flow rate signal VO in which the drift with time of each component is corrected by the following arithmetic processing based on the input signals. However, the meanings of the symbols used in this operation are as follows.

Rsu is an upstream sensor resistance, Rtu is an upstream ambient temperature detection resistor, Rsd is a downstream sensor resistance, Rtd is a downstream ambient temperature detection resistor, Tsu is an upstream sensor resistance temperature, T
tu is the upstream ambient temperature detection resistance temperature, Tsd is the downstream sensor resistance temperature, Ttd is the downstream ambient temperature detection resistance temperature, Iu is the upstream current, Id is the downstream current, Psu is the input to the upstream sensor resistance, Ptu is the input to the upstream ambient temperature detection resistor,
Psd is the input to the downstream sensor resistance, Ptd is the input to the downstream ambient temperature detection resistor, Csu is the heat dissipation coefficient of the upstream sensor resistance, Ctu is the heat dissipation coefficient of the upstream ambient temperature detection resistor, C
sd is the heat dissipation coefficient of the downstream sensor resistance, Ctd is the heat dissipation coefficient of the downstream ambient temperature detection resistor, αsu is the primary resistance temperature coefficient of the upstream sensor resistance, and αtu is the primary resistance of the upstream ambient temperature detection resistor. The resistance temperature coefficient, αsd is the primary resistance temperature coefficient of the downstream sensor resistance, αtd is the primary resistance temperature coefficient of the downstream ambient temperature detection resistance, βsu is the secondary resistance temperature coefficient of the upstream sensor resistance, βtu is The secondary resistance temperature coefficient of the upstream ambient temperature detection resistor, βsd is the secondary resistance temperature coefficient of the downstream sensor resistance, βtd is the secondary resistance temperature coefficient of the downstream ambient temperature detection resistor, and Rsou is Tsou (° C.) Upstream sensor resistance at R
tou is the upstream ambient temperature detection resistance at Ttou (° C), R
sod is the downstream sensor resistance at Tsod (° C), Rtod is the downstream ambient temperature detection resistance at Ttod (° C), Rs25u
Is the upstream sensor resistance at Ts25u (° C), and Rt25u is T
The upstream ambient temperature detection resistance at t25u (° C), Rs25d is the downstream sensor resistance at Ts25d (° C), and Rt25d is Tt2
The downstream side ambient temperature detection resistance at 5d (° C), Rs50u is T
The upstream sensor resistance at s50u (° C), Rt50u is Tt50u
The upstream side ambient temperature detection resistance at (° C), Rs50d is Ts5
The downstream sensor resistance at 0d (° C), Rt50d is Tt50d
R30u is the upstream reference resistance, R30d is the downstream reference resistance, Ta is the ambient temperature,
N is the upstream zero-point adjustment resistance value ratio, M is the downstream zero-point adjustment resistance ratio R1u, R2u is the upstream bridge resistance, and R1d and R2d.
Is the downstream bridge resistance, Vsu is the terminal voltage of the upstream sensor resistance, Vtu is the terminal voltage of the upstream ambient temperature detection resistor, Vsd
Is the terminal voltage of the downstream sensor resistor, Vtd is the terminal voltage of the downstream ambient temperature detection resistor, Vu is the upstream sensor output, Vd is the downstream sensor output, and VO is the sensor output voltage.

The electrical and thermal equation for calculating the sensor output voltage VO is as follows. Psu = Csu (Tsu−Ta), Psu = RsuIu ² , Rsu = Rsou (1 + αsu (Tsu−Tsou) + βsu (Tsu−
^{Tsou) 2) Ptu = Ctu (} Ttu-Ta), Ptu = RtuIu 2, Rtu = Rtou (1 + αtu (Ttu-Ttou) + βtu (Ttu-
Ttou) ² ) Psd = Csd (Tsd−Ta), Psd = RsdId ² , Rsd = Rsod (1 + αsd (Tsd−Tsod) + βsd (Tsd−
Tsod) ² ) Ptd = Ctd (Ttd−Ta), Ptd = RtdId ² , Rtd = Rtod (1 + αtd (Ttd−Ttod) + βtd (Ttd−
Ttod) ² ) Rsu = N (R30u + Rtu), Rsd = M (R30d + Rtd), N = R2u / R1u, M = R2d / R1d, Vu = RsuIu, Vd = RsdId, VO = Vu−Vd,

Also, αsu, βsu, αtu, βtu, αsd, βs
The values of d, αtd, βtd, Csu, Ctu, Csd, and Ctd are as follows. αsu = ((Rs50u−Rs25u) Ts25uTs50u−Rsou (Ts
50u-Ts25u) (Ts50u + Ts25u)-(Rs50uTs25u-
Rs25uTs50u) (Ts25u + Ts50u)) ÷ (RsouTs25u
Ts50u (Ts50u−Ts25u)) αtu = ((Rt50u−Rt25u) Tt25uTt50u−Rtou (Tt)
50u-Tt25u) (Tt50u + Tt25u)-(Rt50uTt25u-
Rt25uTt50u) (Tt25u + Tt50u)) ÷ (RtouTt25u
Tt50u (Tt50u−Tt25u)) βsu = (Rsou (Ts50u−Ts25u) + Rs50uTs25u−Rs)
25uTs50u) ÷ (RsouTs25uTs50u (Ts50u-Ts25
u)) βtu = (Rtou (Tt50u−Tt25u) + Rt50uTt25u−Rt)
25uTt50u) ÷ (RtouTt25uTt50u (Tt50u-Tt25
u))

Αsd = ((Rs50d−Rs25d) Ts25dTs50d
−Rsod (Ts50d−Ts25d) (Ts50d + Ts25d) − (Rs
50dTs25d-Rs25dTs50d) (Ts25d + Ts50d)) ÷
(RsodTs25dTs50d (Ts50d-Ts25d)) αtd = ((Rt50d-Rt25d) Tt25dTt50d-Rtod (Tt
50d-Tt25d) (Tt50d + Tt25d)-(Rt50dTt25d-
Rt25dTt50d) (Tt25d + Tt50d)) ÷ (RtodTt25d
Tt50d (Tt50d-Tt25d)) βsd = (Rsod (Ts50d-Ts25d) + Rs50d Ts25d-Rs
25dTs50d) ÷ (RsodTs25dTs50d (Ts50d-Ts25
d)) βtd = (Rtod (Tt50d−Tt25d) + Rt50dTt25d−Rt)
25dTt50d) ÷ (RtodTt25dTt50d (Tt50d-Tt25
d))

Csu = RsuIu ² / (Tsu = (− αsu / 2β)
su) ± (αsu ² / 4βsu ² + (Rsu−Rsou) / Rsouβs
u) ^1/2 −Ta) Csd = RsdId ² / (Tsd = (− αsd / 2βsd) ± (αsd
² / 4βsd ² + (Rsd−Rsod) / Rsouβsd) ^1/2 −Ta) Ctu = RtuIu ² / (Ttu = (− αtu / 2βtu) ± (αtu
² / 4βtu ² + (Rtu−Rtou) / Rtouβtu) ^1/2 −Ta) Ctd = RtdId ² / (Ttd = (− αtd / 2βtd) ± (αtd)
² / 4βtd ² + (Rtd−Rtod) / Rsouβtd) ^1/2 −Ta)

By solving the above equation using these values, the output VO is represented by the following equation. VO = (CsuRsu ((- αsu / 2βsu) ± ((αsu 2/4
βsu ² ) + (Rsu−Rsou) / (Rsouβsu)) ^1/2 −T
a)) ¹ /2-(CsdRsd ((-αsd / 2βsd) ± ((αsd
² / 4βsd ² ) + (Rsd−Rsod) / (Rsodβsd)) ^1/2
−Ta)) ^1/2 where the values of Rsu and Rsd are the solutions of the following equation:
The ± signs are αsu, βsu, αtu, βtu, αsd, βsd, α
It can be determined by examining the values of td and βtd. Rsu / (Rsu / NR30u) = (Csu / Ctu) (-αsu
^{/ 2βsu) * ((αsu 2} / 4βsu 2) + (Rsu-Rsou)
/ Rsouβsu) ^1/2 -Ta) / ((-αtu / 2βtu)
((Αtu ² / 4βtu ² ) + (Rtu−Rtou) / Rtouβtu)
¹ / ^2- Ta) Rsd / (Rsd / N-R30d) = (Csd / Ctd) (-αsd
^{/ 2βsd) ((αsd 2 /} 4βsd 2) + (Rsd-Rsod) /
Rsodβsd) ^1/2 −Ta) / ((− αtd / 2βtd) ((αt
d ² / 4βtd ² ) + (Rtd−Rtod) / Rtodβtd) ^1/2 −T
a)

Since the above is a higher-order equation, it is practically impossible to find a root directly. Therefore, in the present invention, the numerical value is solved by the microcomputer 11 having the built-in flow meter. The microcomputer calculates the current ambient temperature from the value of the temperature detection resistor, and calculates the numerical solution of the above equation at that temperature using a group of built-in arithmetic programs.

Now, suppose that the output value is V2, and similarly, an output value based on the initial characteristic value before the components change over time is calculated by the arithmetic processing, and the value is V1. Can be obtained as D = V2-V1. FIGS. 3 and 4 show comparative examples of the drift amount obtained by the above-described arithmetic processing by the microcomputer and the actually measured drift amount of the sensor at each ambient temperature. The flow rate output value VR obtained here is obtained by subtracting the value of the drift amount D from the actual flow rate output value VC, and this output VR represents the total temperature drift amount due to the temporal change of each component as described above. It becomes the true flow output value automatically corrected. However, finding the final solution by sequentially solving all of the complicated equations as described above is a very difficult problem in practice because there is naturally a limitation in the operation speed of the current microcomputer.

However, considering the change of the characteristic value over time in each element of the sensor of the present system, the characteristic value of each element hardly changes at most in such a short time span. Since it does not occur, there is no need to perform the calculation at high speed sequentially here. Therefore, the actual calculation is performed using the idle time of the flow meter when it is not operating, and the calculated drift amount D is stored in the microcomputer, and the true flow value is output. It is also possible to use the device successively.

[0023]

As described above, in the flow meter of the present invention, it is possible to completely correct the temperature drift of the flow rate value caused by the change of the ambient temperature, including the time-dependent change. Can be output.

[Brief description of the drawings]

FIG. 1 is a diagram showing components of a mass flow sensor according to the present invention.

FIG. 2 is a comparative example 1 of an actual drift amount and a drift amount obtained by calculation.

FIG. 3 is a comparative example 2 of an actual drift amount and a drift amount obtained by calculation.

FIG. 4 is a diagram showing components of a conventional mass flow sensor.

[Explanation of symbols]

Rsu upstream sensor resistance, Rtu upstream ambient temperature detection resistance, Rsd downstream sensor resistance, Rtd downstream ambient temperature detection resistance, R30u upstream reference resistance, R30d downstream reference resistance, R1u, R2u upstream bridge resistance, R1d,
R2d Downstream bridge resistor, 3,7 Sensor output amplifier circuit, 4,8 Upstream current control circuit, 5,9 Downstream current control circuit, 6,10 Fluid line, 11 microcomputer, 12 A / D Converter, 13D
/ A converter,

Claims (1)

[Claims] An upstream sensor resistor is provided upstream of a fluid line through which a fluid flows, and an upstream ambient temperature detection resistor and an upstream reference resistor having a resistance temperature coefficient of substantially zero are connected in series to the upstream sensor resistor. The value of the side sensor resistance, the upstream constant temperature difference circuit controlled so that the sum of the upstream ambient temperature detection resistance and the upstream reference resistance is always equal, and on the downstream side of the fluid line A downstream sensor resistor is provided, and a downstream ambient temperature detection resistor and a downstream reference resistor having a resistance temperature coefficient of substantially zero are connected in series thereto, and the value of the downstream sensor resistance, the downstream ambient temperature detection resistor, A downstream constant temperature difference circuit controlled so that the sum value of the downstream reference resistance is always equal to the downstream reference resistance, based on a difference in energy given to the upstream sensor resistance and the downstream sensor resistance. , The mass flow of the fluid In a mass flow meter for obtaining an amount, a drift amount of an output caused by a change in a characteristic value of each constituent member over time is corrected using an element having an arithmetic function.