JP3765534B2 - Thermal flow meter - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a thermal flow meter capable of suppressing power consumption without causing a decrease in fluid measurement accuracy.
[0002]
[Related background]
For example, as shown in FIG. 4, a micro flow sensor constituting a thermal flow meter is a resistance temperature measuring resistor in a fluid flow direction F with a heater element Rh made of a heating resistor provided on a silicon base B interposed therebetween. It has an element structure provided with a pair of temperature sensors Ru and Rd made of a body. The thermal flow meter utilizes the fact that the degree of diffusion (temperature distribution) of the heat generated from the heater element Rh changes due to the flow of the fluid, and the resistance value changes due to the heat of the temperature sensors Ru and Rd. It is configured to detect the flow rate Q of the fluid.
[0003]
Specifically, the heat generated from the heater element Rh is applied to the downstream temperature sensor Rd according to the flow rate Q of the fluid, so that the change in the resistance value due to the heat of the temperature sensor Rd is greater than that of the upstream temperature sensor Ru. The flow rate Q is measured by utilizing the fact that it is large. In the figure, Rr is a temperature sensor made of a resistance temperature detector provided at a position away from the heater element Rh, and is used for measuring the ambient temperature.
[0004]
FIG. 5 shows a schematic configuration of a thermal flow meter using the above-described microflow sensor. That is, the drive circuit of the heater element Rh forms a bridge circuit 1 using the heater element Rh, a temperature sensor Rr for measuring ambient temperature, and a pair of fixed resistors R1 and R2, and a voltage supplied from a predetermined power source. Vcc is applied to the bridge circuit 1 via the transistor Q, the bridge output voltage of the bridge circuit 1 is obtained by the differential amplifier 2, and the transistor Q is adjusted so that the bridge output voltage becomes zero (0). The heater driving voltage applied to the bridge circuit 1 by feedback control is adjusted. The heater driving circuit configured as described above controls the heat generation temperature of the heater element Rh so as to be always higher than the ambient temperature by a constant temperature difference ΔT.
[0005]
On the other hand, the flow rate detection circuit for detecting the flow rate Q of the fluid flowing along the micro flow sensor from the resistance value change due to the heat of the pair of temperature sensors Ru and Rd includes the pair of temperature sensors Ru and Rd and the pair of temperature sensors Ru and Rd. The bridge circuit 3 for measuring the flow rate is formed using the fixed resistors Rx and Ry, and the bridge output voltage (output corresponding to the temperature difference) corresponding to the change in the resistance value of the temperature sensors Ru and Rd is passed through the differential amplifier 4. Configured to detect. Then, the flow rate Q of the fluid flowing along the microflow sensor is obtained from the bridge output voltage detected via the differential amplifier 4 under the condition that the heating value of the heater element Rh is made constant by the heater driving circuit. It has become a thing.
[0006]
The flow rate Q is calculated, for example, by taking the bridge output voltage (sensor output) into an arithmetic processing unit (CPU).
[0007]
[Problems to be solved by the invention]
By the way, when this type of thermal flow meter is used as a household gas meter, it is important to save power. In particular, when a battery is used as the drive source, the power consumption becomes a big problem in ensuring stable flow rate measurement over a long period of time. However, in the conventional thermal type flow meter (gas meter), the heater element Rh is exclusively driven to generate heat at a temperature always higher than the ambient temperature by a constant temperature ΔT, so that the power consumption of the heater element Rh is large. I can't deny it.
[0008]
Thus, it is conceivable to reduce the drive power (power consumption) by reducing the current supplied to the heater element Rh. However, when the current supplied to the heater element Rh is reduced, the heat generation temperature of the heater element Rh is lowered accordingly. Therefore, a temperature distribution corresponding to the flow rate of the fluid may be formed between the temperature sensors Ru and Rd. It becomes impossible and the problem that the flow measurement accuracy deteriorates arises.
[0009]
The present invention has been made in consideration of such circumstances, and its purpose is to significantly reduce the power consumption of the heater element Rh without adversely affecting the accuracy of flow rate measurement, thereby reducing the power consumption. An object of the present invention is to provide a thermal flow meter that can be realized.
[0010]
[Means for Solving the Problems]
In order to achieve the above-described object, a thermal flow meter according to the present invention includes a heater element and first and second temperature sensors respectively provided in the direction of fluid flow with the heater element interposed therebetween. And
A heater driving circuit for driving the heater element to generate heat; and a flow rate calculating means for determining a flow rate of the fluid from a difference between temperatures detected by the first and second temperature sensors when the heater element generates heat.
Furthermore, the flow rate variation judging means for judging the presence or absence of flow rate variation from the output of the flow rate calculating means, and the heat generation temperature of the heater element driven to generate heat by the heater drive circuit is higher than the ambient temperature by a constant temperature during normal operation. Heat generation that sets the first temperature and switches to the second temperature set lower than the first temperature to set the power saving mode when the flow rate change determining unit determines that there is no flow rate change. A temperature switching means is provided.
[0011]
In a preferred aspect of the present invention, the heating temperature switching means is configured to variably set the heating temperature of the heater element by switching a resistance inserted in series with the heater element. That is, the present invention is characterized in that when there is no fluctuation in the flow rate of fluid, the current flowing through the heater element is reduced by switching the resistance inserted in series with the heater element to reduce the power consumption.
[0012]
Incidentally, the flow rate fluctuation determining means determines that there is no flow rate fluctuation when the flow rate obtained by the flow rate calculating means during normal operation is maintained within a predetermined fluctuation range for a certain time. It is realized as having. The heating temperature switching means includes temporary return means for temporarily canceling the power saving mode and returning to the normal operation mode every predetermined time when the power saving mode is set.
[0013]
In a preferred aspect of the present invention, when the power saving mode is temporarily canceled by the temporary return means, the flow rate calculation means recalculates the flow rate of the fluid, and the heating temperature switching means is , Detecting the presence or absence of the fluctuation from the flow rate of the fluid recalculated by the flow rate calculation means with the temporary release of the power saving mode and the flow rate measured immediately before setting the power saving mode, It is characterized by comprising a power saving mode canceling means for canceling the power saving mode when a change is detected .
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, a thermal flow meter according to an embodiment of the present invention will be described with reference to the drawings, taking as an example a case of using as a household gas meter.
FIG. 1 shows a schematic configuration of a main part of a thermal type flow meter according to this embodiment. This thermal flow meter is realized by using the microflow sensor having the element structure shown in FIG. 4 and basically configuring the heater drive circuit and the flow rate detection circuit as shown in FIG.
[0015]
In other words, the drive circuit for the heater element Rh forms the bridge circuit 1 selectively using the heater element Rh, the temperature sensor Rr for measuring the ambient temperature, and the four fixed resistors R1, R2, R3, R4. Is applied to the bridge circuit 1 via the transistor Q, the bridge output voltage of the bridge circuit 1 is obtained by the differential amplifier 2, and the transistor Q is adjusted so that the bridge output voltage becomes zero. Configured for feedback control.
[0016]
In particular, the fixed resistors R1 and R3 are connected in series to form one side of the bridge circuit 1, and are inserted in series with the heater element Rh. A transistor SW1 serving as a first switch element is connected in parallel between both ends of the fixed resistor R3 via a fixed resistor R4 in series. The transistor SW1 is turned on so that the fixed resistor R4 is connected in parallel to the fixed resistor R3. Further, a transistor SW2 as a second switch element is connected in parallel between both ends of the fixed resistor R3, and the fixed resistor R3 is selectively short-circuited by the conduction of the transistor SW2. .
[0017]
In other words, the fixed resistor R4 is provided so as to be selectively connected in parallel to the fixed resistor R3 by the conduction (on) / cutoff (off) of the first transistor SW1. , R4 are provided so as to be selectively inserted into the bridge circuit 1 by conduction (on) / cutoff (off) of the second transistor SW2. Note that these transistors SW1 and SW2 are selectively turned on (on) by heat generation temperature switching means described later.
[0018]
Incidentally, in the bridge circuit 1, when the transistors SW1 and SW2 are both in the cut-off (off) state, the fixed resistors R1 and R3 are inserted in series with the heater element Rh. R1 + R3) = Rr / R2
It operates to satisfy the bridge equilibrium condition. In this state, the heater element Rh is driven to generate heat at a first temperature T1 higher than the ambient temperature by a certain temperature [normal operation mode].
[0019]
On the other hand, when the second transistor SW2 is in the cut-off (off) state and only the first transistor SW1 is turned on (on), the fixed resistor R4 is connected in parallel to the fixed resistor R3. Accordingly, fixed resistors R3 and R4 and a fixed resistor R1 connected in parallel are inserted in series with the heater element Rh. Therefore, the heater drive circuit
Rh / {R1 + R3 · R4 / (R3 + R4)} = Rr / R2
It operates to satisfy the bridge equilibrium condition. In this case, since the resistance value inserted in series with the heater element Rh is reduced by the parallel connection of the fixed resistor R4 to the fixed resistor R3, the energization to the heater element Rh is performed by the operation satisfying the bridge equilibrium condition of the bridge drive circuit. Current can be suppressed. As a result, the heat generation temperature is suppressed to a heat generation temperature T2 lower than that in the normal operation mode described above [first power saving mode].
[0020]
On the other hand, when the second transistor SW2 is turned on, the fixed resistor R3 is short-circuited regardless of whether the first transistor SW1 is turned on or off, and only the fixed resistor R1 is inserted in series with the heater element Rh. Therefore, the heater drive circuit
Rh / R1 = Rr / R2
It operates to satisfy the bridge equilibrium condition. In this case, since the resistance value inserted in series with the heater element Rh is lower than that in the first power saving mode described above, the heater element Rh is energized by the operation satisfying the bridge equilibrium condition of the bridge drive circuit. The current is further suppressed. As a result, the heat generation temperature of the heater element Rh can be suppressed to a temperature T3 that is lower than that in the first power saving mode described above [second power saving mode].
[0021]
As will be described later, when the flow rate measurement is performed by causing the heater element Rh to generate heat at the temperature T1, the first transistor SW1 is driven (turned on) when there is no change in the flow rate of the fluid, and the heater SW The heat generation temperature of the element Rh is lowered to [T2] to save power. The second transistor SW is turned on when the flow rate is zero, and further reduces the heat generation temperature of the heater element Rh to [T3], thereby further reducing power consumption.
[0022]
By the way, the flow rate detection circuit for detecting the flow rate of the fluid flowing through the temperature sensors Ru and Rd from the difference between the temperatures detected by the pair of temperature sensors Ru and Rd is a pair of the temperature sensors Ru and Rd. The fixed resistances Rx and Ry are used to form a bridge circuit 3 for measuring the flow rate, and the bridge output voltages corresponding to changes in the resistance values of the temperature sensors Ru and Rd are detected by the temperature sensors Ru and Rd, respectively. It is comprised so that it may detect as sensor output Vout equivalent to the difference of these.
[0023]
A CPU (arithmetic processing circuit) 5 that inputs such a sensor output Vout generally includes a flow rate calculation means 6 for obtaining a flow rate Q corresponding to the sensor output Vout, and a flow rate obtained by the flow rate calculation means 6. The first and second transistors SW1 and SW2 are selectively turned on in response to the heat generation temperature switching means 7 for switching the heat generation temperature of the heater element Rh.
[0024]
The CPU 5 includes a memory 8 made of, for example, an EEPROM and a display device 9. The memory 8 stores or detects a flow rate conversion table (not shown) indicating the relationship between the flow rate Q of the fluid (gas) and the sensor output Vout for use in calculating the flow rate Q in the flow rate calculation means 6. It plays a role such as storing the integrated value (gas consumption) of the flow rate Q. The indicator 9 plays a role of displaying the flow rate Q obtained by the flow rate calculation means 6, the gas usage amount, and the like.
[0025]
Here, this thermal type flow meter is characterized in that the flow rate calculation means 6 is provided with a flow rate fluctuation determination means 6a for determining the presence or absence of fluctuations in the flow rate. The flow rate fluctuation determination unit 6a monitors the flow rate Q sequentially obtained by the flow rate calculation unit 6, and when the flow rate Q is maintained within a predetermined fluctuation range over a certain period of time, the fluctuation of the flow rate Q is changed. It has a function to judge that there is no.
[0026]
On the other hand, the exothermic temperature switching means 7 basically conducts (turns on) the first transistor SW1 when the flow rate fluctuation judging means 6a included in the flow rate calculating means 6 determines that there is no flow rate fluctuation. The heater element Rh has a function of setting the first power saving mode by switching the heat generation temperature of the heater element Rh from [T1] to [T2]. Further, when the flow rate Q detected by the flow rate calculation unit 6 is zero, the heat generation temperature switching unit 7 drives (turns on) the second transistor SW2 to set the heat generation temperature of the heater element Rh to [T1] or A function of switching from [T2] to [T3] to set the second power saving mode is provided. At the same time, the heat generation temperature switching means 7 cancels the power saving mode when a flow rate variation is detected by the flow rate variation determination means 6a in a state where the first or second power saving mode is set. Release means 7a is provided. The power saving mode canceling means 7a stops the conduction (on) driving of the transistors SW1 and SW2 described above, in other words, shuts off (off) both the transistors SW1 and SW2 to change the driving mode of the heater element Rh to the normal operation. Play the role of returning to the mode.
[0027]
That is, the heat generation temperature switching means 7 drives the heater element Rh at the predetermined first heat generation temperature T1 in the normal operation mode. When there is no fluctuation in the flow rate detected at this time, the first power saving mode is set and the heater element Rh is driven at a temperature T2 lower than the first heat generation temperature T1 to reduce the power consumption. Further, when the flow rate is zero, the second power saving mode is set, and the heater element Rh is driven at a lower temperature T3 to further save power. When the power saving mode is set and the flow rate fluctuates or the flow rate is not zero, the canceling means 7a quickly cancels the power saving mode and returns to the normal operation mode. It has become.
[0028]
In addition to such basic functions, the exothermic temperature switching means 7 includes a temporary return means 7b. When the first power saving mode described above is set, the temporary return means 7b temporarily returns to the normal operation mode at predetermined time intervals as shown in FIG. The heat generation temperature of Rh is returned to the first temperature T1, and the flow rate measurement by the flow rate calculation means 6 is performed with high accuracy. That is, since the heat generation temperature of the heater element Rh is low in the power saving mode, it is difficult to perform highly accurate flow rate measurement. Therefore, even if the power saving mode is set in the temporary return means 7b, it is set in the power saving mode that is set on the premise that there is no flow rate fluctuation by temporarily setting the normal operation mode every predetermined time. Even so, the flow rate is regularly measured with high accuracy.
[0029]
When it is determined that there is no flow rate variation and the power saving mode is set in this way, the flow rate calculation means 5a measures the flow rate Q obtained immediately before setting the power saving mode in the power saving mode. It is desirable that the flow rate Q is considered to be integrated and the flow rate Q is integrated. If the measured flow rate values are integrated in this way, the measurement flow rate value with low reliability detected in the state where the heater element Rh is driven at the low temperature T2 by the setting of the power saving mode is not used. Measurement reliability can be maintained sufficiently high.
[0030]
In addition, as described above, by returning the driving condition of the heater element Rh to the normal operation mode every predetermined time and measuring the flow rate again in this state, the flow rate fluctuates within a slight range that can be considered that there is no flow rate fluctuation. Even in this case, the flow rate can be accurately measured. Therefore, even if the heat generation temperature of the heater element Rh is set to a low temperature to save power, the measurement accuracy in the power saving mode can be sufficiently increased, and the measurement accuracy is not impaired as a whole.
[0031]
FIG. 3 shows an example of an overall processing procedure in the thermal type flow meter having the processing function described above. This processing procedure will be briefly described. First, it is determined whether or not the heater element Rh is operating in the power saving power mode in which the heater element Rh is driven to generate heat at temperatures T2 and T3 lower than those at the time of normal flow measurement (step S1). When not operating in the power saving mode, that is, when operating in the normal operation mode in which the heater element Rh is driven to generate heat at a predetermined temperature T1, the flow rate Q is determined according to the sensor output Vout by the detection circuit at that time. [Step S2]. Then, it is determined whether or not the measured flow rate Q is zero for a predetermined time [step S3]. If the flow rate Q is zero, after setting the second power saving mode described above [step S5], the process returns to the processing from step S1 described above.
[0032]
On the other hand, if the flow rate Q is not zero, it is next determined whether or not the flow rate Q is stable [step S6]. If the flow rate Q is not stable, that is, the flow rate is fluctuating. Return to the processing from step S1 without changing the operation mode. Whether or not the flow rate Q is stable is determined by determining whether or not the difference (variation amount) between the flow rate measured last time and the flow rate measured this time exceeds a predetermined determination threshold value.
[0033]
When the flow rate Q is stable, it is determined whether or not there is a flow rate variation by checking whether or not the variation in the flow rate Q is within a predetermined range for a predetermined time [step] S4] If the flow rate fluctuation is not zero, the process returns to the process from step S1 described above. On the other hand, when the flow rate fluctuation is zero, the first power saving mode described above is set [step S7], and the flow rate detected immediately before the setting of the power saving mode is set in the power saving mode. The detected flow rate is set [Step S8].
[0034]
On the other hand, when operating in the power saving mode [step S1], it is first determined whether or not to check the flow rate Q [step S9]. By the way, for the flow rate confirmation in the power saving mode, for example, when the flow rate measurement is sampled and executed at a predetermined cycle, it is sufficient to execute the flow rate once every 2 to 10 samplings. If the flow rate check is not executed, the previously detected flow rate Q is detected as the current detected flow rate [step S10]. However, when confirming the flow rate, the currently set power saving mode is temporarily canceled [step S11], and the flow rate measurement is performed by returning to the normal operation mode as described above [step S12].
[0035]
Thereafter, it is checked whether or not the flow rate is stable [step S13]. If the flow rate is stable even at this time, the power saving mode is continuously set again [step S14]. However, if the flow rate varies, the power saving mode described above is canceled, the normal operation mode is set again, and the flow rate is measured [steps S15 and S16].
[0036]
Thus, as described above, it is determined whether or not the flow rate fluctuates. When there is no fluctuation in the flow rate, the power saving mode is set and the heat generation temperature of the heater element Rh is set low, thereby suppressing the power consumption. According to the type flow meter, it is possible to significantly reduce the power consumed in the heater element Rh, and the overall power saving can be achieved. Moreover, even when the power saving mode is set, since the heater element Rh is driven to generate heat at a low temperature, when the flow of fluid starts, that is, when the use of gas starts or when the flow rate changes When there is, the heat generation temperature of the heater element Rh can be quickly returned to the normal heat generation temperature, so that there is no measurement delay and there is no inconvenience that the measurement error increases.
[0037]
In addition, when the power saving mode is set on condition that there is no fluctuation in the flow rate, the flow rate measured in the normal operation mode immediately before is set as the flow rate measured in the power saving mode. There is no risk of measurement errors associated with the settings. Moreover, when the power saving mode is set, the flow rate of the fluid is measured with high accuracy by temporarily returning to the normal operation mode every predetermined time, and the measured flow rate accumulated in the power saving mode is changed to the latest data. In this respect, the measurement accuracy can be maintained sufficiently high. As a result, even when the power saving mode is set and the power consumption in the heater element Rh is suppressed, the flow rate can be measured with high accuracy. In particular, in a gas meter for home use, the gas flow rate hardly fluctuates over a certain period of time, so that the power saving effect by setting the power saving mode described above is great. Furthermore, since the gas is generally not used for a long time at midnight or the like, a great effect in practical use such as a great power saving effect can be expected also in this respect.
[0038]
The present invention is not limited to the embodiment described above. For example, the above-described permissible width of the fluid flow rate fluctuation and the measurement time, which are determination conditions when setting the power saving mode, may be determined according to the specifications. Further, the heat generation drive of the heater element Rh is periodically executed at predetermined time intervals set as, for example, 1 second intervals, and the flow rate measurement is executed in a sampling manner in synchronization with the heat generation drive timing of the heater element Rh. It is also possible. In this way, it is possible to further reduce the power consumption by the amount that the heater element Rh is not continuously driven to generate heat. In addition, the present invention can be implemented with various modifications without departing from the scope of the invention.
[0039]
【The invention's effect】
As described above, according to the present invention, when the flow rate does not fluctuate, the heat generation temperature of the heater element is lowered, so that the power required for the heat generation drive of the heater element can be reduced, and the power saving is greatly reduced. Can be planned. In addition, when the power saving mode is set and the heater element is driven to generate heat at a low temperature, the flow rate measured in the normal operation mode immediately before that is obtained as a measured value in the power saving mode. Measurement accuracy is not impaired. When the power saving mode continues for a long time, the normal operation mode is temporarily set every predetermined time and the flow rate is measured again, so that the measurement accuracy can be maintained sufficiently high in this respect as well. Such a great effect in practice can be achieved.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of a main part of a thermal flow meter according to an embodiment of the present invention.
FIG. 2 is a diagram showing a concept of temporary normal operation mode setting and flow rate measurement at the time of setting a power saving mode in the thermal flow meter shown in FIG. 1;
FIG. 3 is a diagram showing an example of a schematic processing operation in the thermal type flow meter shown in FIG. 1;
FIG. 4 is a schematic configuration diagram of a microflow sensor.
FIG. 5 is a diagram showing a configuration example of a conventional general heater drive circuit and flow rate detection circuit.
[Explanation of symbols]
Rh Heater element Ru Temperature sensor (upstream side)
Rd Temperature sensor (downstream)
Rr temperature sensor (for ambient temperature measurement)
1 Bridge circuit (for heater drive)
2 Differential amplifier (heater drive circuit)
3 Bridge circuit (for flow measurement)
4 Differential amplifier 5 CPU
6 Flow rate calculation means 6a Flow rate fluctuation determination means 7 Heat generation temperature switching means 7a Power saving mode release means 7b Temporary return means 8 Memory 9 Display

Claims (6)

A thermal flow meter comprising a heater element and first and second temperature sensors provided in the direction of fluid flow with the heater element interposed therebetween,
A heater driving circuit for driving the heater element to generate heat;
A flow rate calculating means for obtaining a flow rate of the fluid from a difference between temperatures detected by the first and second temperature sensors when the heater element generates heat;
A flow rate fluctuation determining means for determining the presence or absence of a flow rate fluctuation from the output of the flow rate calculating means;
The heat generation temperature of the heater element driven to generate heat by the heater drive circuit is set to a first temperature that is higher than the ambient temperature by a constant temperature during normal operation, and it is determined that there is no flow rate variation by the flow rate variation determination means. when the heating temperature switching means for setting the power saving mode is switched to the second temperature is set lower than the first temperature,
Means for outputting the flow rate measured immediately before the power saving mode is set as the flow rate measured in the power saving mode during the power saving mode setting period. Type flow meter.   2. The thermal flow meter according to claim 1, wherein the heat generation temperature switching means switches a resistance inserted in series with the heater element to variably set the heat generation temperature of the heater element. The flow rate variability determination means, wherein when the flow rate demanded by said flow rate calculation means during normal operation is maintained within a predetermined variation width over a predetermined time, it is to determine that there is no variation in the flow rate Item 2. The thermal flow meter according to Item 1. The heating temperature switching means includes temporary return means for temporarily canceling the power saving mode and returning to the normal operation mode every predetermined time when the power saving mode is set. The thermal flow meter according to 1. The thermal flow meter according to claim 4, wherein when the power saving mode is temporarily canceled by the temporary return means, the flow rate calculation means recalculates the flow rate of the fluid . The flow rate variation determination unit is configured to detect the variation from the flow rate of the fluid recalculated by the flow rate calculation unit with the temporary release of the power saving mode and the flow rate measured immediately before setting the power saving mode. The thermal flow meter according to claim 4 , further comprising: a power saving mode canceling unit that detects presence / absence and cancels the power saving mode when a flow rate fluctuation is detected .

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