JPH10213469A - Method for compensating property of heat sensitive type flow sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a property compensating method capable of maintaining the accuracy required for a heat sensitive type flow sensor. SOLUTION: In a heat sensitive type flow sensor for measuring the flow rate of fluid on the basis of voltage Vu of an upstream side heat generator located in the upstream side of fluid flow, voltage Vd of a downstream side heat generator located in the downstream and a difference between these voltages Vud=Vu-Vd, when a value of the voltage difference Vud is smaller than a threshold value, the threshold value ±Vdu(0) is previously stored to consider the flow rate as zero. When the flow rate is judged to be zero or near zero on the basis of the value of voltage difference Vud outputted from the heat sensitive type flow sensor, the value of the voltage difference Vud is recorded at any time. When the hourly change of these recorded values of the voltage difference is considered to be the systematic one, a new threshold value to consider the flow rate as zero on the basis of these values of the voltage difference Vud is determined to be renewed and recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for compensating characteristics of a heat-sensitive flow sensor for measuring a flow rate using a voltage change of a heating element cooled by a flow of a fluid.

[0002]

2. Description of the Related Art In recent years, a flow meter using a fluidic fluid element has been actively studied as a next-generation gas meter that replaces a conventional integral type membrane meter. However,
The fluidic fluid element is 150 [L / H] (“L”).
Represents a liter; the same applies to the following. Since a low flow rate cannot be measured, a heat-sensitive flow sensor having a heating element exposed to a fluid on a microbridge is used as a flow meter for measuring a low flow rate.

[0003] Such a heat-sensitive flow sensor generally has a flow rate of approximately 0 to 150 L / H, ± 1.
It is required that measurement can be performed with an accuracy of ± 5 [L / H]. Further, since the gas meter needs to be driven outdoors by a battery for a long period of ten years, it is necessary to maintain the required accuracy for ten years under an environmental condition of −20 to + 70 ° C. for the thermal flow sensor. There is. For this purpose, the drift of the output value (zero drift) or the fluctuation of the sensitivity when the flow rate of the flow sensor is extremely small cannot be ignored, and it is necessary to compensate for it by some method.

Several countermeasures have been proposed for compensating for the zero drift of such a thermal flow sensor. For example, according to Japanese Utility Model Laid-Open No. 1-87209, an error signal with respect to a measurement signal is obtained by periodically stopping power supply to a heating element to create a reference state.

[0005]

However, according to such measures, the circuit when the heating element is at room temperature and the circuit when the heating element is in the measurement state and the flow rate is zero are as follows.
Since the heating elements have different resistance values, they are not in the same circuit.The signal output when power supply to the heating elements is stopped is only an approximation of the zero flow signal during measurement, and zero drift in a strict sense No compensation can be made. By the way, the heat-sensitive flow sensor element and its driving circuit are used for measuring flow rate accuracy,
For example, although it is possible to create a drift that suppresses a signal output equivalent to 1 [L / H] in flow rate until a short period of time is suppressed, the drift is equivalent to 1 [L / H] over a long period of 10 years. It is very difficult to keep the signal output below this, and it is necessary to compensate systematically.

In this regard, according to Japanese Patent Application Laid-Open No. 3-53128, the difference between the offset voltage when the heating element is turned on and the offset voltage when the heating element is turned off is corrected by another power supply. However, even in this method, since the resistance value changes with the aging of the heating element, as a result, the temperature of the heating element also changes, and it is possible to compensate for the phenomenon of sensitivity change and sensitivity change due to environmental temperature change. Can not.

Therefore, the present invention relates to a heat-sensitive flow sensor for applying the heat-sensitive flow sensor to a gas meter or the like which needs to maintain the required accuracy over a long period of, for example, 10 years. An object of the present invention is to provide a method for compensating sensor characteristics.

[0008]

According to the first aspect of the present invention,
Voltage Vu of upstream heating element located on the upstream side of fluid flow
And a thermosensitive flow sensor that measures the flow rate of the fluid based on the voltage Vd of the downstream heating element located on the downstream side and the voltage difference Vud = Vu−Vd, the value of the voltage difference Vud is smaller than a certain threshold voltage. Is smaller, a threshold voltage for considering the flow rate to be zero is recorded in advance, and the voltage difference Vud is recorded.
When it is determined that the flow rate is zero or near zero based on the value of Vd, the value of the voltage difference Vud is recorded at any time, and the time change of these recorded values of the voltage difference Vud is determined to be a systematic change. In such a case, a new threshold voltage for determining that the flow rate is zero is determined based on the values of these voltage values Vud and updated and recorded.

A thermal flow sensor measures an instantaneous flow rate, that is, a flow velocity, and the flow rate is determined by multiplying a measured value (flow velocity) by a certain time. Also, when the thermal flow sensor is applied to a battery-operated gas meter or the like, if the thermal flow sensor is constantly driven, the battery will be drastically consumed. For example, it is impossible to replace the battery for a long period of 10 years without battery replacement. Therefore, in reality, an intermittent drive method is required in which the thermal flow sensor is driven once every several seconds to measure the flow velocity. Therefore, the flow rate is estimated from the instantaneous value. In the case of such an intermittent method,
It has an error compared to the conventional integral type membrane meter,
As a practical problem, the error is small in the measured flow rate range (low flow rate range) of the thermal flow sensor, and the billing system for gas and the like does not fluctuate and does not become a social problem. In addition, when a measurement error is considered, it is very difficult to precisely measure the flow rate zero with a thermal flow sensor. Therefore, for example, when the flow rate is 1 [L / H] or less, even if it is considered that the flow rate is zero, the error is smaller than the error of the intermittent method, and is accepted socially. Therefore, in the present invention, first, it is assumed that the flow rate is regarded as zero for a sensor output which is equal to or less than a predetermined output of the thermal flow sensor corresponding to the flow rate such as 1 [L / H]. And For this reason, in each thermal type flow sensor, the value of zero flow rate and the sensitivity are measured, and when the value of the voltage difference Vud is smaller than a certain threshold voltage, a threshold voltage for considering it as zero flow rate is recorded in advance. Keep it. On the other hand, in the measurement of the low flow rate region to be measured by the thermal type flow sensor, the drift of the output value becomes a problem, but in order to compensate for such zero drift,
The fact that the output of the thermal flow sensor does not fluctuate in a short term at zero flow rate is used. That is, the fact that the voltage is equal to or lower than the previously recorded threshold voltage is used. Therefore, the sensor output (voltage difference Vud) included in a range several times the threshold voltage is recorded over time. By recording as needed over time, it is possible to judge from the situation before and after that whether the voltage is a fluctuation caused by the actual flow of the fluid or a drift of the output when the flow rate is zero (zero drift). Therefore, when it is determined that the drift is zero, a new threshold voltage for determining that the flow rate is zero is determined and updated and recorded, whereby the zero drift is compensated for a long period of time.

According to a second aspect of the present invention, the voltage Vu of the upstream heating element located on the upstream side of the fluid flow, the voltage Vd of the downstream heating element located on the downstream side, and the voltage difference Vud = V
In a heat-sensitive flow sensor that measures the flow rate of a fluid based on u-Vd, a relational expression between the voltage difference Vud and the flow rate is calculated and recorded in advance, and the flow rate calculated based on the relational expression is zero or a predetermined value. When it is determined that the flow rate is a set constant flow rate, the value of the voltage difference Vud at that time is recorded as needed, and the time change of the recorded value of the voltage difference Vud is equal to or greater than a preset value. In some cases, the relational expression corrected based on the value of the voltage difference Vud is calculated and updated and recorded. In a long-term use such as 10 years, the sensitivity of the thermal type flow sensor may change. However, according to the present invention, the sensor output (voltage difference Vud) is obtained even for a predetermined fixed flow rate.
Is recorded over time, and if the time change is greater than a set value, it is determined that the sensitivity has changed, and the voltage difference Vu is determined.
By correcting and updating the relational expression between d and the flow rate, fluctuations in the sensitivity of the thermal flow sensor are compensated for a long period of time.

According to a third aspect of the present invention, the voltage Vu of the upstream heating element located on the upstream side of the fluid flow, the voltage Vd of the downstream heating element located on the downstream side, and the voltage difference Vud = V
In a heat-sensitive flow sensor that measures the flow rate of a fluid based on u-Vd, the relationship between the voltage difference Vud and the flow rate is approximated by a plurality of broken lines of a primary straight line, and a relational expression related to the primary straight line is recorded in advance. When it is determined that the flow rate calculated based on the relational expression is zero or the flow rate at the intersection of the primary straight line or the maximum measured flow rate value, the voltage difference Vu at that time is determined.
The value of d is recorded from time to time, and these recorded voltage differences Vud
Is greater than a preset value, the coefficient of the relational expression of the primary straight line is corrected based on the value of the voltage difference Vud and updated and recorded. Therefore, the fluctuation of the sensitivity of the thermal flow sensor is compensated for a long period of time as in the case of the second aspect of the present invention. You don't have to keep track of a lot of changes over time,
Approximating the relationship between the voltage difference Vud and the flow rate by a plurality of linear lines of the primary straight line makes it simple, and can reduce the load on the CPU, the memory, and the like.

According to a fourth aspect of the present invention, there is provided a method for compensating a characteristic of a thermal flow sensor according to the first, second or third aspect.
A temperature measuring resistor disposed at a position not thermally affected by the upstream heating element and the downstream heating element to measure an environmental temperature;
The relationship between the voltage difference Vud and the flow rate is approximated by broken lines of a plurality of primary straight lines, and the temperature dependence of the coefficient of the relational expression of the primary straight line is recorded in advance, and based on the output value of the resistance temperature detector, The coefficient of the relational expression of the primary straight line is corrected. Therefore, the relationship between the sensor output (voltage difference Vud) and the flow rate is approximated by a plurality of broken lines of a primary straight line, and the temperature dependence of the coefficient of the relational expression of the primary straight line is recorded in advance, and the relationship is determined according to the environmental temperature. Since the coefficients of the relational expressions of the respective primary straight lines are corrected, the compensation of the flow rate with respect to the environmental temperature is also easily performed.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described with reference to the drawings. First, FIG. 1 shows an example of a heat-sensitive flow sensor used in the present embodiment. In the thermal flow sensor 1, for example, a bridge 4 is formed by a moat 3 on a silicon substrate 2, and two heating elements 5 are formed on the bridge 4 by thin film resistors. Therefore, these heating elements 5
Is exposed in the fluid flow. One of these heating elements 5 is an upstream heating element 5u located on the upstream side of the fluid flow indicated by the arrow, and the other is a downstream heating element 5d located on the downstream side. Further, a thin film temperature measuring resistor 6 for measuring the environmental temperature is formed at a position where the upstream heating element 5u and the downstream heating element 5d are not thermally affected, for example, at a corner on the upstream side. This heat-sensitive flow sensor 1 is applied to a gas meter.

Such a heat-sensitive flow sensor 1 has a voltage Vu of the upstream heating element 5u and a voltage Vd of the downstream heating element 5d.
It is based on measuring the flow rate of the fluid based on the voltage difference Vud = Vu−Vd. Here, the thermosensitive flow sensor 1 measures an instantaneous flow rate, that is, a flow velocity, and the flow rate is determined by multiplying a measured value (flow velocity) by a certain time. In addition, when the thermal flow sensor 1 is applied to a battery-operated gas meter, if the thermal flow sensor 1 is constantly driven, the battery is greatly consumed. For example, it is impossible to replace the battery for a long period of 10 years. In practice, the intermittent driving method in which the thermosensitive flow sensor 1 is driven once every several seconds to measure the flow velocity is inevitable. Therefore, the flow rate is estimated from the instantaneous value. In the case of such an intermittent method, there is an error as compared with the conventional integral type film meter, but as a practical problem, the error is slight in the measurement flow rate range (low flow rate range) of the thermosensitive flow sensor 1. However, the gas billing system will not fluctuate and become a social problem. In addition, when a measurement error is considered, it is very difficult to exactly measure zero flow with the thermal type flow sensor 1. Therefore, for example, when the gas flow rate is 1 [L / H] or less, even if it is considered that the flow rate is zero, the error is smaller than the error of the intermittent method, and is accepted socially.

Therefore, in this embodiment, first,
It is assumed that a measured flow rate is assumed to be zero for an output of a thermosensitive flow sensor which is equal to or less than an output (voltage difference Vud (0)) corresponding to a flow rate such as a gas flow rate 1 [L / H]. .
For this reason, in each of the thermosensitive flow sensors 1, the value and the sensitivity of the zero flow rate are measured, and when the value of the voltage difference Vud is smaller than a certain threshold voltage, the threshold voltage for determining that the flow rate is zero is stored in advance in the memory. To record. In this case, it is expected that the sensor output (voltage difference Vud) will take a negative value near zero flow rate due to noise or the like. Therefore, it is practical to set a certain threshold voltage value to ± Vud (0). . Incidentally, here, an example will be described in which the flow rate of 1 [L / H] or less is regarded as zero flow rate. However, in determining a certain threshold voltage, the range in which the flow rate is regarded as zero is limited to 1 [L / H]. For example, 0.5 [L / H]
Or 1.5 [L / H] or the like, and the point is that it may be finally determined according to the accuracy required for the gas meter.

On the other hand, in the measurement in the low flow rate range to be measured by the thermosensitive flow sensor 1, drift of the output value becomes a problem. However, in order to compensate for such zero drift systematically, in the present embodiment, The fact that the output of the thermal flow sensor does not fluctuate in the short term at zero flow rate is used. That is, a predetermined threshold voltage ± Vud (0)
Take advantage of the following: Therefore, a sensor output (voltage difference Vud) included in a range several times as large as a certain threshold voltage ± Vud (0) is recorded over time. Usually, the range (width) to be several times is desirably about 0.5 to 1 times the precision required near zero flow rate.

The output (voltage difference Vud) of the heat-sensitive flow sensor 1 when the flow rate is zero is shown in FIG.
Assuming that Vud (0) and the measurement accuracy at a flow rate of 1 to 10 [L / H] is ± 2 [L / H], about ± 2 Vud (0) is appropriate as a recording width. .

As shown in FIG. 2, if the output value Vud where the flow rate is zero or near zero (within the range of ± Vud (0)) is recorded as an output over time,
From the situation before and after that, for example, the output fluctuation (output increase) due to the actual flow of the gas from time T1 to time T2, but the output fluctuation (output increase) from time T3 to time T4 is a heat-sensitive type. It can be determined that this is due to an increase in the zero drift of the flow sensor 1. When it is determined that the drift is caused by the rise of the zero drift as described above, the variation over time is regarded as the drift D (0) of the zero flow, and a new threshold voltage for assuming the zero flow is ± Vud (0). + D (0) is determined and updated and recorded in the memory.

Here, the determination as to whether it is voltage fluctuation caused as a result of actually flowing gas or drift of output at zero flow rate (zero drift) is made, for example, at time TX shown in FIG. The determination can be made based on whether or not the difference between the average value of several points before and the average value of several points after time TX is larger than Vud (0). For example, at time T1, the difference between the average value of several points before that and the average value of several points after it is larger than Vud (0), but at time T3, the average value of several points before that And the average value of several points after that is V
Since it is smaller than ud (0), the difference between the two situations can be determined. In this way, when it is determined that the drift is zero, a new threshold voltage ± Vu
By determining and updating d (0) + D (0), it is possible to compensate for a long-term zero drift that is difficult to achieve with a single sensor chip or a single circuit.

In the thermal type flow sensor 1 used as a gas meter according to the present embodiment, 10
In long-term use, as long as years, its sensitivity can also change. In the present embodiment, in order to compensate for such fluctuations in sensor sensitivity, the sensor output values are recorded as needed over time for some predetermined fixed flow rates other than zero flow rate. . For example, assume that the output voltage value of the thermal flow sensor 1 when the initial flow rate value is determined to be a certain constant value X [L / H] is Vdu (X) (the relationship between this flow rate and the output voltage value is , Equal to the relationship pre-written in the gas meter memory at the factory)
It is assumed that the output voltage value Vdu (X) determined to be X [L / H] shows a temporal change as shown in FIG. 3, for example.
Then, the variation ΔVdu (X) is due to the zero drift D (0) of the sensor and the variation ΔK in sensitivity. Therefore,
The output voltage value Vdu (X) is set to a preset value (a certain ±
(The threshold voltage, which may be about twice as large as the error output allowed at each steady flow rate) or more. The voltage value Vdu (X) is recorded as a compensation value. When the number of registrations of these compensation values reaches a certain number or more, a relational expression between the gas flow rate and the output voltage value Vdu (X) of the thermal flow sensor 1 is newly calculated using the compensation values, The new relational expression is updated and recorded in the memory. Accordingly, fluctuations in the sensitivity of the thermal flow sensor 1 can be compensated for a long period of time, such as 10 years.

By the way, when a new relational expression (calibration curve) corrected by using some compensation values as described above is obtained again, in order to improve the accuracy, it is necessary to change over time for a considerably large number of measurement points. It needs to be recorded. further,
A process of recalculating the calibration curve is required, and the load on the CPU and the memory increases. By reducing such burden,
In order to simplify the operation, the calibration curve is approximated by a polygonal line of a plurality of primary straight lines, and the output of each point of the zero flow rate, the flow rate at the intersection of the primary straight lines (broken line), and the maximum measured flow rate is changed as needed. If the change (temporal change) of those values is greater than a preset value, the two coefficients of the relational expression relating to the primary straight line are corrected based on the output value of each point and updated and recorded. You may make it. According to this method, the load on the CPU and the memory can be reduced, and the sensitivity fluctuation can be compensated at low cost.

The compensation of the measured flow rate with respect to the environmental temperature will be described. In this compensation, the relationship between the voltage difference Vud, which is the output of the thermal flow sensor 1, and the flow rate is determined by a plurality of 1s.
The temperature dependence of the coefficient of the relational expression of the primary straight line is recorded in advance by approximating the broken line of the secondary straight line, and the coefficient of the relational expression of the primary straight line is corrected based on the output value of the resistance temperature detector. Done.

For example, FIG. 4 shows the relationship between the propane flow rate and the sensor output Vud for three types of ambient temperature of -20.degree. C., 25.degree. C. and 70.degree. C. when the gas to be measured is propane gas. FIG. 5A and FIG. 5B show the relationship between these coefficients a and b in the case where these relations are approximated by a polygonal line using a relational expression of a primary straight line, flow rate = a · Vud + b (a and b are coefficients). )become that way.
Indicates each linear region divided by polygonal line approximation. As is clear from FIGS. 5A and 5B, if the environmental temperature T is determined from the output of the resistance temperature detector, the coefficients a and b are uniquely determined. Therefore, the values of a (T) and b (T) are recorded in the memory for each of the areas divided by the polygonal line approximation, and by knowing the environmental temperature T, the coefficients of the relational expression of the linear line are corrected. By performing the update recording, the measured flow rate with respect to the environmental temperature can be easily compensated.

In the present embodiment, the example of the heat-sensitive flow sensor 1 applied to a gas meter such as propane gas has been described. However, the present invention is not limited to the gas meter, but may be, for example, an air conditioner for controlling the air flow rate of an air conditioner. For flow sensor,
The present invention can be similarly applied to a flow sensor for controlling a supply air amount of a combustion device.

[0025]

According to the first aspect of the present invention, a zero-drift can be compensated for a long-term thermal flow sensor used for a gas meter or the like for a long period of time, and the required measurement accuracy is obtained. Can be maintained.

According to the second aspect of the present invention, even when the sensitivity of the thermal flow sensor changes, the fluctuation of the sensitivity can be compensated for a long period, and the required measurement accuracy can be maintained. it can.

According to the third aspect of the present invention, in order to obtain the effect of the second aspect of the present invention, it is not necessary to record a considerable amount of change over time, and the burden on the CPU and the memory is reduced. And can be easily realized.

According to the fourth aspect of the present invention, since the flow rate is compensated for the environmental temperature, the required measurement accuracy can be maintained more reliably.

[Brief description of the drawings]

FIG. 1 is a schematic plan view showing an example of a thermal flow sensor applied to an embodiment of the present invention.

FIG. 2 is a characteristic diagram showing a change over time of a sensor output when a flow rate is zero for explaining an embodiment of the present invention.

FIG. 3 is a characteristic diagram showing a temporal change of a sensor output at a certain flow rate X [L / H].

FIG. 4 is a characteristic diagram showing a relationship between several types of flow rates and sensor outputs when environmental temperatures are different.

FIG. 5A is a characteristic diagram showing a temperature dependence of a coefficient a,
(B) is a characteristic diagram showing the temperature dependence of the coefficient b.

[Explanation of symbols]

 Reference Signs List 1 heat-sensitive flow sensor 5u upstream-side heating element 5d downstream-side heating element 6 RTD

Claims (4)

[Claims] 1. A flow rate of a fluid based on a voltage Vu of an upstream heating element located on an upstream side of a flow of a fluid, a voltage Vd of a downstream heating element located on a downstream side, and a voltage difference Vud = Vu−Vd. When the value of the voltage difference Vud is smaller than a certain threshold voltage, a threshold voltage for considering the flow rate to be zero is recorded in advance, and the flow rate is determined based on the value of the voltage difference Vud. Is determined to be zero or near zero, the value of the voltage difference Vud is recorded at any time. If the time change of the recorded value of the voltage difference Vud is determined to be a systematic change, these values are determined. A method for compensating characteristics of a heat-sensitive flow sensor, characterized in that a new threshold voltage for determining that the flow rate is zero is determined based on the value of the voltage value Vud and updated and recorded. 2. The flow rate of a fluid based on a voltage Vu of an upstream heating element located on the upstream side of the flow of the fluid, a voltage Vd of a downstream heating element located on the downstream side, and a voltage difference Vud = Vu−Vd. In a heat-sensitive flow sensor for measuring the flow rate, a relational expression between the voltage difference Vud and the flow rate is calculated and recorded in advance, and the flow rate calculated based on the relational expression is zero or a predetermined fixed flow rate. When it is determined that there is, the value of the voltage difference Vud at that time is recorded as needed, and when the time change of the recorded value of the voltage difference Vud is equal to or larger than a preset value, the voltage difference Vud is used.
A method for compensating a characteristic of a thermal flow sensor, wherein the relational expression corrected based on the value of ud is calculated and updated and recorded. 3. The flow rate of the fluid based on the voltage Vu of the upstream heating element located on the upstream side of the flow of the fluid, the voltage Vd of the downstream heating element located on the downstream side, and the voltage difference Vud = Vu−Vd. The relationship between the voltage difference Vud and the flow rate is approximated by broken lines of a plurality of primary straight lines, and a relational expression relating to the primary straight line is recorded in advance, and the relationship is calculated based on the relational expression. When it is determined that the flow rate is zero or the flow rate at the intersection of the primary straight line or the maximum measured flow rate value, the value of the voltage difference Vud at that time is recorded as needed, and the time change of the recorded value of the voltage difference Vud is performed. Is greater than or equal to a preset value, these voltage differences V
A characteristic compensation method for a thermal flow sensor, wherein the coefficient of the relational expression of the primary straight line is corrected based on the value of ud and updated and recorded. 4. A temperature measuring resistor which is disposed at a position where the upstream heating element and the downstream heating element are not thermally affected to measure an environmental temperature, and has a plurality of relations between a voltage difference Vud and a flow rate. Of 1
The temperature dependency of the coefficient of the relational expression of the primary straight line is recorded in advance by approximating the broken line of the next straight line, and the coefficient of the relational expression of the first straight line is corrected based on the output value of the resistance temperature detector. 4. The method for compensating for the characteristics of a thermosensitive flow sensor according to claim 1, wherein said method is performed.