JPH05149767A - Mass flowmeter - Google Patents

Abstract

PURPOSE:To measure the mass flow rate of a small amount of fluid with high accuracy. CONSTITUTION:In this mass flowmeter adopting a constant-temperature drive system, a correction circuit 10 which performs zero-drift adjustment on output voltages of heat sensitive coils R1 and R2 is connected in series with a temperature correction circuit 11 which performs gain adjustment. In addition, zero drift correction is calculated by using the sum of the output voltages of the coils R1 and R2 as a gas temperature index.

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 used for industrial fluid conveyance equipment, and more particularly, to a mass flow rate of a small amount of fluid flowing in a conduit with high accuracy and fast response. The present invention relates to a mass flow meter for measuring.

[0002]

2. Description of the Related Art As a conventional mass flow meter, a constant current method and a constant temperature driving method are known. What is the constant current method?
A pair of self-heating type temperature measuring elements each having a large temperature coefficient are wound around the upstream side and the downstream side of the conduit through which the fluid flows to form a heat sensitive coil, and the current value supplied to each heat sensitive is held constant,
The flow rate is measured by detecting the temperature change of the heat-sensitive coil which changes as the fluid flows. However, the constant current method has a problem of poor responsiveness because it utilizes the temperature change of the heat sensitive coil, and is not satisfactory as a mass flow meter used in a system that supplies a small amount of fluid with high accuracy. there were.

As a means for improving the poor response in the constant current method, the constant temperature driving method has been put into practical use. The constant temperature drive method is to form a thermosensitive coil by wrapping a pair of self-heating type temperature measuring elements each having a large temperature coefficient on the upstream side and the downstream side of the conduit through which the fluid flows, to form a bridge circuit by each thermosensitive coil. The temperature of the heat sensitive coil is controlled to a constant value, and the mass flow rate of the fluid is calculated from the potential difference between the bridge circuits.

However, since the operating principle of the constant temperature drive system is the same as that of the hot-wire anemometer, the voltage between the bridge circuits in the state where no fluid is flowing due to the change in ambient temperature and the difference in heat capacity of the fluid is There is a drawback that the zero point shown tends to fluctuate. Further, even when the fluid is flowing, the potential difference between the bridge circuits is likely to fluctuate due to changes in the fluid temperature and the like.

One of means for solving these problems is proposed by Japanese Patent Laid-Open No. 61-128123 as shown in FIG. Here, the fluid F is flowing in the conduit 1 in the direction indicated by the arrow. The two heat sensitive coils R1 and R2 are bonded to the upstream side and the downstream side of the conduit 1 with UV curable resin or the like to form the sensor unit 2. Heat sensitive coil R1, R2
Are connected to constant temperature control circuits 3 and 4, respectively, and control them so that the temperatures of the heat sensitive coils R1 and R2 are always equal and constant. Therefore, the voltages P1 and P2 output from the constant temperature control circuits 3 and 4 are proportional to the amount of energy required to maintain the heat sensitive coils R1 and R2 at the constant temperature in each constant temperature control circuit.

Here, the voltage difference P1-P2 is proportional to the mass flow rate of the fluid F as shown by the solid line in FIG. 10, and the mass flow rate can be measured by measuring P1-P2. .. However, when the gas temperature of the fluid F rises, the values of P1 and P2 change as shown by the dotted line in FIG.
P1 'and P2'. Therefore, an accurate flow rate cannot be measured unless the values of P1 and P2 are corrected. On the other hand, the sum of the voltages P1
Focusing on the fact that + P2 fluctuates in proportion to the temperature of the fluid F, the value of P1-P2 is corrected by using the value obtained by dividing P1-P2 by P1 + P2.

[0007]

However, in the prior art disclosed in Japanese Patent Application Laid-Open No. Sho 62-132120, the temperature correction is not sufficient, and the flow rate cannot be measured with high accuracy and fast responsiveness. It was That is, since the zero drift compensation that corrects the zero point when the fluid is not flowing and the temperature correction when the fluid is flowing are performed by one arithmetic circuit, it is possible to reduce the ambient temperature change and the gas temperature change. It was not possible to measure the mass flow rate with high accuracy and high responsiveness.

In the prior art, attention is paid to the fact that the value of P1 + P2 changes in accordance with the change in the gas temperature of the fluid F, P1-P
By dividing 2 by P1 + P2, the temperature of the value of P1-P2 is corrected. However, in the actual mass flowmeter, the nonuniformity of the characteristics of the upstream and downstream heat-sensitive coils R1 and R2 (variation in resistance of the heat-sensitive coils R1 and R2,
Due to the heat transfer coefficient variation, etc., the output (P1-P2) is not completely zero even when the fluid F is not flowing at all, so zero adjustment is necessary. Further, it is necessary to correct the zero drift in which the zero point changes in temperature. Therefore, as in the prior art, P1 + P is used as an index of gas temperature.
If the two kinds of temperature corrections are simultaneously performed by using No. 2 as described above, accurate correction cannot be performed, and an error or a time delay occurs in the measured mass flow rate.

According to the prior art, P1-P2 is replaced by P1 + P2
The data corrected by dividing by is shown in FIG. The output at a flow rate of 0 cc / min is 0 V when the temperature is 25 degrees or lower, but is slightly negative when the temperature is 25 degrees or higher. This is problematic as a mass flow meter used in a supply system for the purpose of accurately conveying a small amount of fluid.

Further, in order to perform zero drift compensation,
When the fluid is not flowing, the voltage between the bridge circuits is made equal, but if the zero drift compensation is performed by this method, a difference occurs in the temperatures of the inter-heating coils R1 and R2, and the responsiveness deteriorates. There was a flaw. On the other hand, in recent years, for example, in the manufacturing process of semiconductors, it has been required to supply a small amount of corrosive gas or the like at a flow rate with an accuracy of 1% or less, and the accuracy requirement is becoming more severe. Therefore, the prior art could not meet the demand.

Further, although the variation value of P1-P2 when the gas temperature of the fluid varies due to the property of the fluid, in the prior art, it is simply (P1-P2) /
Since the flow rate is measured by the value of (P1 + P2), the temperature correction is constant even for mass flowmeters used for different fluids, and it is possible to perform appropriate temperature correction according to the nature of the fluid. could not.

[0012]

In order to achieve this object, the mass flowmeter of the present invention has the following construction. (1) A conduit through which a fluid flows, and the resistance value changes depending on the temperature of the fluid, which is wound around the conduit independently of each other.
One resistor and a control means for calculating the mass flow rate of the fluid flowing through the conduit from the difference in energy applied to the resistor while keeping the temperature of the resistor constant, wherein the control means is When the mass flow rate is zero, zero drift calculation is performed so that the energy difference becomes constant even if the ambient temperature changes.

(2) In the above-mentioned (1), the control means corrects the energy difference when the fluid temperature is changed when the fluid is further flown. Calculate. (3) According to the above (1) or (2), based on a value obtained by adding or subtracting an energy difference to a value obtained by multiplying a sum of energy given to the resistor by a predetermined coefficient by the control means. , Performing zero drift calculation of the fluid.

(4) In the above-mentioned (2), the temperature correction calculation is performed by multiplying the energy difference by a linear function whose variable is the sum of energy given to the resistor by the control means. To do.

[0015]

The conduit of the mass flowmeter according to the present invention having the above-mentioned structure causes a fluid to flow inside and conveys the fluid. Further, the two resistors are wound around the conduit independently of each other, and the resistance value changes according to the temperature of the fluid. The control means calculates the mass flow rate of the fluid flowing through the conduit from the difference in energy applied to the resistor while keeping the temperature of the resistor constant. further,
When the mass flow rate of the fluid is zero, the control means performs the zero drift calculation so that the energy difference becomes constant even if the ambient temperature changes.

Further, the control means further performs a temperature correction calculation for correcting the energy difference when the fluid is further flowed and the ambient temperature changes. Further, the control means performs the zero drift calculation of the fluid based on the value obtained by adding or subtracting the value obtained by multiplying the sum of the energy given to the resistor by a predetermined coefficient to the energy difference.

Further, the user inputs an arbitrary value by the input means. Further, the control means calculates the input value from the input means as the predetermined coefficient. Further, the control means is
The temperature correction calculation is performed by multiplying the difference in energy by a linear function having the sum of energy given to the resistor as a variable.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A mass flowmeter which is an embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows a control circuit of a mass flowmeter which is an embodiment of the present invention. Here, the numbers used in the conventional description are used for the components having the same functions as those of the conventional control circuit.

The fluid F flows in the conduit 1 in the direction indicated by the arrow. The conduit 1 is a SUS316 tube having an inner diameter of 0.5 mm and a length of 20 mm. Two heat sensitive coils R1 and R2 are formed on the upstream side and the downstream side of the conduit 1 by winding a heat sensitive resistance wire having a diameter of 25 μm for 70 turns.
The heat-sensitive resistance wire is made of a material having a large temperature coefficient such as iron or nickel alloy. The heat sensitive coils R1 and R2 are bonded to the conduit 1 with UV curable resin or the like to form the sensor unit 2.

The heat sensitive coils R1 and R2 are connected to constant temperature control circuits 3 and 4, respectively. The constant temperature control circuits 3 and 4 control the heat sensitive coils R1 and R2 so that they are always equal and constant. The voltages P1 and P2 output from the constant temperature control circuits 3 and 4 indicate the amount of energy required to maintain the heat sensitive coils R1 and R2 at constant temperature in each constant temperature control circuit. The voltages P1 and P2 are amplified as current values by the transistors and are subtracted by the subtraction circuit 5 and the addition circuit 6.
Is supplied to.

The adder circuit 6 is connected to resistors R3 and R4 for multiplying a value obtained by adding P1 and P2 by a predetermined coefficient value m. In the case of this embodiment, R3 = 30 kΩ, R
4 = 1 kΩ and m = 0.033. Further, the subtractor circuit 5 is provided with three resistors of R4 = 10 kΩ for outputting the difference between P1 and P2 as it is.

The output of the subtraction circuit 5, the output of the addition circuit 6 and the output of the constant voltage circuit 8 are connected to the correction circuit 9.
The subtraction circuit 5, the addition circuit 6, the constant voltage circuit 8 and the correction circuit 9 constitute a zero drift compensation circuit 10.
The zero drift compensation circuit 10 outputs Vs. The output of the correction circuit 9 is connected to a temperature compensation circuit 11 that performs temperature compensation by gain compensation. The temperature compensation circuit 11 has a thermistor 12, which is a sensor for measuring the ambient temperature, as a constituent element. The temperature compensation circuit 11 outputs Vo.

Next, the operation of this embodiment will be described.
An electric current is applied to the heat sensitive coils R1 and R2, and the heat sensitive coil R
1 and R2 are controlled to be maintained at a constant temperature.
When the fluid F is flown in the direction of the arrow in the conduit 1, the temperature of the heat sensitive coil R1 wound on the upstream side of the conduit 1 becomes low because the heat is taken away by the fluid. In order to raise it, the output voltage P1 becomes larger than the output voltage Po when the fluid F is not flowing. Further, since the heat-sensitive coil R2 wound on the downstream side of the conduit 1 is given heat by the fluid F heated by the heat-sensitive coil R1, the temperature thereof becomes high.
In order to lower it, the output voltage P2 becomes smaller than the output voltage Po when the fluid F is not flowing.

As shown in FIG. 5, the heat-sensitive coil R on the upstream side
The output voltage P1 of 1 increases in direct proportion to the flow rate of the fluid F. On the other hand, the output voltage P2 of the thermal coil R2 on the downstream side is
The flow rate of the fluid F decreases inversely with the flow rate up to 10 cc / min, and rises from around 10 cc / min. In the region where the flow rate is small, the fluid F is heated to a high temperature by the heat sensitive coil R1, and the heat sensitive coil R2 is
Contrary to 1, the heat is applied from the fluid F, so that the output voltage P2 of the heat sensitive coil R2 decreases. However, when the flow rate increases, the fluid F reaches the heat sensitive coil R2 at a low temperature, and the heat is absorbed in the heat sensitive coil R2, so that the output voltage P2 starts to increase.

By the way, the voltage difference P1-P2 calculated by the subtraction circuit 5 is proportional to the mass flow rate of the fluid F as shown by the solid line in FIG. 2, and the mass flow rate is measured by measuring P1-P2. Can be measured. However, if the gas temperature of the fluid F rises, as shown by the dotted line in FIG.
The values of 1 and P2 vary and the accurate flow rate cannot be measured unless the values of P1 and P2 are corrected.

On the other hand, as shown in FIG. 2, the value of P1 + P2 changes to the position shown by the dotted line when the gas temperature of the fluid F changes. In this embodiment, as shown in FIG. 6, the potential difference P1-P2 between the bridge circuits, which is measured as the mass flow rate, changes at 0 cc / min in a one-dimensional manner with respect to the ambient temperature change. Paying attention to the fact that P1 + P2 is used as an index of temperature change, zero drift compensation is performed.

Next, the zero drift compensation will be described in detail. The constant temperature control circuits 3 and 4 are heat sensitive coils R1 and R.
2 is controlled so that they are always equal and constant.
Here, the voltage P output from the constant temperature control circuits 3 and 4
Reference numerals 1 and P2 indicate the amounts of energy required to maintain the thermosensitive coils R1 and R2 at constant temperatures in the respective constant temperature control circuits. These are the same as in the conventional device, and detailed description thereof will be omitted. The voltages P1 and P2 are amplified as current values by the respective transistors and supplied to the subtraction circuit 5 and the addition circuit 6.

The addition circuit 6 multiplies the value obtained by adding P1 and P2 by a predetermined coefficient value m. In the case of this embodiment,
R3 = 30 kΩ, R4 = 1 kΩ, and m = 0.033
Has become. Therefore, -0.033 * (P1 + P2)
Is output from the adder circuit 6. Further, the subtraction circuit 5 has a P
The difference between 1 and P2- (P1-P2) is output.

The correction circuit 9 adds the output of the constant voltage circuit 8 to the output of the subtraction circuit 5 and the output of the addition circuit 6. In this embodiment, the constant voltage circuit outputs -0.275V. Therefore, the Vs output by the zero drift compensation circuit 10 is Vs = (P1-P2) + m * (P1 + P
2) -n. Here, in this embodiment, m = 0.03
3, n = 0.275.

Next, a method of calculating m and n will be described. That is, in the above device, P without flowing the fluid F at all
1 + P2 is measured, the data shown in FIG. 3 is created, approximated by a straight line, P1-P2 = 0 is set, and the resulting quadratic equation is solved for m and n. Data in which Vs is zero-drift corrected by the above calculation formula is shown in FIG. As a matter of course, the data of the mass flow rate of the fluid F of 0 cc / min is 0 for all temperatures. As a result, even if the flow rate F is a value very close to 0 cc / min, the potential difference between the bridge circuits can be accurately temperature-corrected, so that even if a small amount of the fluid F is transported in a state where the temperature changes, the fluid sent is sent. The mass flow rate of F can be accurately grasped and adjusted.

The zero-drift corrected output Vs is connected to a temperature compensating circuit 11 which performs temperature compensation by gain compensation. The temperature compensation circuit 11 has a thermistor 12, which is a sensor for measuring the ambient temperature, as a constituent element. The temperature compensation circuit 11 outputs Vo. Vo is the thermistor 1
The gain is corrected by the output R7 of 2 and the fluctuation of the inclination due to the temperature change of the potential difference between the bridge circuits is corrected.

In order to correct the variation of the inclination due to the temperature change of the potential difference between the bridge circuits by the gain correction, P1 + P2 is used as an index of the ambient temperature without using the thermistor,
As shown in FIG. 7, by multiplying the zero-drift-corrected energy difference by a linear function having P1 + P2 as a variable, correction may be performed by the following formula Vo = Vs * {P- (P1 + P2) * Q}. .. According to this, the temperature can be corrected without using the thermistor, so that the cost can be reduced.

In this embodiment, the zero drift correction is carried out by using P1 + P2 as an index of the ambient temperature.
Zero drift correction may be performed using the ambient temperature measured by a thermistor. The control circuit diagram at that time is shown in FIG. The output of the thermistor is P1 + P in the above embodiment.
It is used instead of 2 for calculation, and detailed description thereof will be omitted.

[0034]

As is apparent from the above description, the mass flowmeter of the present invention has the temperature of two resistors which are wound around the conduit independently of each other and whose resistance value changes according to the temperature of the fluid. When the mass flow rate of the fluid is zero, the control means for calculating the mass flow rate of the fluid flowing through the conduit from the difference in the energy applied to the resistor while keeping the constant value becomes constant even if the ambient temperature changes. Since the zero drift calculation is performed like this, even when measuring a small amount of fluid, it is possible to accurately correct the change in mass flow rate due to temperature changes, so it is possible to measure a small amount of fluid accurately and with high responsiveness. You can

Further, since the control means performs the temperature correction calculation for correcting the energy difference when the fluid temperature is changed when the fluid is further flown, a small amount of fluid is measured. Even in this case, since the fluctuation of the mass flow rate due to the temperature change can be accurately corrected, a small amount of fluid can be accurately measured.

Further, since the control means performs the temperature correction calculation by multiplying the energy difference by a linear function having the sum of energy given to the resistors as a variable, it is not necessary to use an extra thermistor or the like. Therefore, the cost can be reduced.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a control configuration of a mass flowmeter that is an embodiment of the present invention.

FIG. 2 is a characteristic diagram showing a relationship between a mass flow rate, an output voltage, and a temperature change when a fluid flows through a conduit.

FIG. 3 is a characteristic diagram showing a relationship between gas temperature and output voltage.

FIG. 4 is a characteristic diagram showing a characteristic of zero-drift-corrected voltage.

FIG. 5 is a characteristic diagram showing a relationship between a mass flow rate and an output voltage when a fluid flows through a conduit.

FIG. 6 is a characteristic diagram showing a relationship between output voltage and temperature before zero drift correction and temperature correction.

FIG. 7 is a circuit diagram showing a control configuration of a mass flowmeter according to a second embodiment of the present invention.

FIG. 8 is a circuit diagram showing a control configuration of a mass flowmeter that is a third embodiment of the present invention.

FIG. 9 is a circuit diagram showing a control configuration of a mass flowmeter which is a conventional technique.

FIG. 10 is a characteristic diagram showing an output voltage of a conventional mass flowmeter.

FIG. 11 is a characteristic diagram showing a temperature-corrected output voltage of a conventional mass flowmeter.

[Explanation of symbols]

 1 conduit 2 sensor part 3 constant temperature control circuit 4 constant temperature control circuit 5 subtraction circuit 6 addition circuit 8 constant voltage circuit 9 correction circuit 10 zero drift compensation circuit 11 temperature compensation circuit 12 thermistor F fluid R1 heat sensitive coil R2 heat sensitive coil

Claims (4)

[Claims] 1. A conduit through which a fluid flows, two resistors which are independently wound around the conduit and whose resistance value changes according to the temperature of the fluid, and the temperature of the resistor is kept constant. While controlling the mass flow rate of the fluid flowing through the conduit from the difference in energy applied to the resistor, the control means controls the ambient temperature to change when the mass flow rate of the fluid is zero. Even if the energy difference is constant, the zero drift calculation is performed. 2. The device according to claim 1, wherein the control means performs a temperature correction calculation for correcting the energy difference when the fluid is further flowed and the fluid temperature changes. A mass flowmeter characterized by the above. 3. The method according to claim 1 or 2, wherein a value obtained by multiplying a sum of energy given to the resistor by a predetermined coefficient is added to or subtracted from the energy difference,
A mass flowmeter for performing zero drift calculation of the fluid. 4. The temperature correction calculation according to claim 2, wherein the control means multiplies a linear function having a variable of a sum of energy given to the resistor by the energy difference. A mass flowmeter characterized by the above.