JP2003097989A - Thermal flowmeter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thermal flowmeter which accurately detects an abnormally heated heater element Rh, can prevent a micro flow sensor from being thermally destroyed by overheating the heater element, and further can forestall firing of combustible gas. SOLUTION: The thermal flowmeter has the heater element Rh, a first and a second temperature sensor Ru, Rd which are disposed on the straddling sides of the heater element in the passing direction of fluid, a monitoring means which monitors the heater driving voltage Vh applied on the heater element, and a means for stopping driving heater which stops energizing and driving the heater element when the heater driving voltage exceeds a reference voltage being set beforehand. Furthermore, in such a state that the heater element is not energized or heated, the heater element is forced to be energized and driven, and then the stop of driving the heater element is released in accordance with the heater driving voltage at the time, and thermal flowmeter is rapidly restored into the normal operation.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention comprises a heater element and first and second temperature sensors provided in the fluid flow direction with the heater element interposed therebetween. The present invention relates to a thermal type flow meter capable of preventing thermal destruction due to heating.

[0002]

2. Related Background Art A micro flow sensor constituting a thermal type flow meter has a silicon base B as shown in FIG.
It has an element structure in which a pair of temperature sensors Ru and Rd formed of temperature measuring resistors are provided in the fluid flow direction F with a heater element Rh formed of a heating resistor provided therebetween. The thermal flow meter utilizes the fact that the diffusion degree (temperature distribution) of the heat emitted from the heater element Rh changes due to the flow of the fluid, and based on the resistance change due to the heat of the temperature sensors Ru and Rd, It is configured to detect the flow rate Q of the fluid.

Specifically, the heat generated from the heater element Rh is added to the temperature sensor Rd on the downstream side according to the flow rate Q of the fluid, so that the resistance value change due to the heat of the temperature sensor Rd causes the temperature on the upstream side to change. The flow rate Q is measured by utilizing the fact that it is larger than the sensor Ru. Rr in the figure
Is a temperature sensor including a resistance temperature detector provided at a position distant from the heater element Rh, and is used for measuring the ambient temperature.

FIG. 6 shows a schematic structure of a thermal type flow meter using the above-mentioned micro flow sensor. That is, the drive circuit of the heater element Rh includes the heater element Rh, a temperature sensor Rr for measuring the ambient temperature, and a pair of fixed resistors R1, R.
2 is used to form a bridge circuit 1, a voltage Vcc supplied from a predetermined power source is applied to the bridge circuit 1 via a transistor Q, and the bridge output voltage of the bridge circuit 1 is applied to a differential amplifier 2. Then, the transistor Q is feedback-controlled so that the bridge output voltage becomes zero, and the heater driving voltage applied to the bridge circuit 1 is adjusted. The heater driving circuit configured as described above controls the heat generation temperature of the heater element Rh to be always higher than the ambient temperature by a constant temperature difference.

On the other hand, the flow rate detection circuit for detecting the flow rate Q of the fluid flowing along the microflow sensor from the change in resistance value of the pair of temperature sensors Ru and Rd due to heat is
The pair of temperature sensors Ru, Rd and the pair of fixed resistors Rx,
The Ry is used to form the bridge circuit 3 for flow rate measurement, and the bridge output voltage according to the change in the resistance value of the temperature sensors Ru and Rd is detected via the differential amplifier 4. Then, under the condition that the amount of heat generated by the heater element Rh is made constant by the heater drive circuit, 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. It has become a thing.

[0006]

By the way, in such a thermal type flow meter, when an electric component constituting the micro flow sensor or the heater drive circuit fails, an excessive heater drive voltage is applied to the heater element Rh. May occur.
If such an excessive heater driving voltage applied to the heater element Rh is left as it is, the amount of heat generated by the heater element Rh becomes excessive and the microflow sensor may be thermally damaged.

When this type of thermal flow meter is used to measure the flow rate of the flammable gas, the flammable gas may ignite due to overheating of the heater element Rh. Therefore, it is important to prevent overheating (abnormal temperature increase) of the heater element Rh. However, in the related art, since the abnormality of the microflow sensor is detected only from the sensor output detected using the pair of temperature sensors Ru and Rd, the abnormal heating of the heater element Rh cannot be accurately detected. There was a problem to say.

The present invention has been made in consideration of such circumstances, and an object thereof is to accurately detect abnormal heating of the heater element Rh to prevent thermal destruction of the microflow sensor due to overheating of the heater element Rh. It is an object of the present invention to provide a thermal type flow meter capable of preventing the ignition of combustible gas.

[0009]

In order to achieve the above-mentioned object, a thermal type flow meter according to the present invention comprises a heater element and first and second heater elements provided in the fluid flow direction with the heater element interposed therebetween. A second temperature sensor, particularly a monitoring means for monitoring a heater driving voltage applied to the heater element, and a heater driving for stopping energization driving of the heater element according to the heater driving voltage. And a stop means.

That is, when the heater driving voltage applied to the heater element becomes excessive due to a failure of electric parts,
The heater drive stopping means is operated to stop the energization drive of the heater element, thereby preventing abnormal heating of the heater element, preventing thermal destruction of the microflow sensor, and measuring the flow rate of combustible gas. It is characterized in that the danger of ignition of the above flammable gas is surely avoided in the case of being used for.

The thermal type flow meter according to the present invention further comprises a heater control means for controlling a heater driving voltage applied to the heater element to control a heating temperature of the heater element, and a heater controlling voltage for monitoring the heater driving voltage. And a heater drive stopping means for stopping the energization drive of the heater element when the heater drive voltage exceeds a predetermined reference voltage.

That is, even when the heating temperature of the heater element is controlled by the heater control means, the energization drive of the heater element is stopped when the heater driving voltage exceeds a predetermined reference voltage. It is possible to prevent the element from abnormally generating heat, so that the microflow sensor can be prevented from being thermally destroyed.

Further, the thermal type flow meter according to the present invention further comprises a trial means for forcibly energizing and energizing the heater element for a predetermined time when energization and heating of the heater element is stopped, and the forcible heater element And a release unit for releasing the stop of energization drive of the heater element by the heater drive stop unit when the heater drive voltage during energization drive is lower than a predetermined reference voltage.

As described above, when the heating of the heater element is stopped, the heater element is forcibly energized and driven for a predetermined time.
By examining the heater driving voltage at that time, it is possible to easily determine whether or not the factor at the time of stopping the energization heating of the heater element has been eliminated. Specifically, if the heater driving voltage is lower than a predetermined reference voltage, it can be confirmed that the cause of stopping the energization and heating of the heater element is eliminated. The restoration process can be executed smoothly.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION A thermal flow meter according to an embodiment of the present invention will be described below in detail with reference to the drawings. FIG. 1 shows a schematic configuration diagram of a main part of a thermal type flow meter according to this embodiment. This thermal flowmeter is realized by using the microflow sensor having the element structure shown in FIG. 5 and configuring the heater drive circuit and the flow rate detection circuit as shown in FIG.
That is, the drive circuit of the heater element Rh is
And a temperature sensor Rr for measuring ambient temperature, and a pair of fixed resistors R1 and R2 to form a bridge circuit 1, which applies a predetermined power supply voltage Vcc to the bridge circuit 1 via a transistor Q and The bridge output voltage of the circuit 1 is obtained by the differential amplifier 2, and the transistor Q is feedback-controlled so that the bridge output voltage becomes zero.

A detection circuit for detecting a change in resistance value of the pair of temperature sensors Ru and Rd due to heat uses a bridge circuit for flow rate measurement using the pair of temperature sensors Ru and Rd and a pair of fixed resistors Rx and Ry. 3 forming a temperature sensor R
Bridge output voltage Vout according to the change of resistance of u and Rd
Is detected via the differential amplifier 4. The bridge output voltage Vout detected by the detection circuit (differential amplifier 4) is given to the CPU 5 which is an arithmetic processing unit, and the flow output detecting means 5a which is a part of the processing function thereof outputs the bridge output voltage Vout. A corresponding flow rate Q is required.

The CPU 5 comprises a memory 6 composed of an EEPROM and a display 7. The memory 6 is a table showing the relationship between the bridge output voltage Vout and the flow rate Q,
A table showing the relationship between the applied voltage Vh of the heater element Rh and the flow rate Q is stored, for example, according to the type of fluid, and the allowable maximum value of the heater drive voltage applied to the heater element Rh is stored. Is. Further, the display 7 plays a role of displaying the flow rate Q of the fluid obtained by the flow rate detecting means 5a and displaying various messages described later.

The CPU 5 monitors not only the flow rate detecting means 5a described above but also the heater driving voltage Vh (bridge driving voltage) applied to the heater element Rh, and the heater driving voltage Vh is stored in the memory 6. When the maximum allowable value (reference voltage) is exceeded, the heater element Rh
The drive stop means 5b for stopping the energization drive is provided. The drive stopping means 5b is, for example, a transistor S as a switch element connected in parallel with the heater element Rh.
By making W conductive (ON), the switch element Rh is short-circuited, thereby playing a role of stopping the conduction drive of the switch element Rh, specifically, stopping the application of the heater drive voltage Vh to the heater element Rh. A switch element (not shown) connected in series to the bridge circuit 1 is cut off (OF
F) and then the heater driving voltage Vh to the heater element Rh
May be stopped. Then, by stopping the conduction drive of the switch element Rh, the switch element Rh
It is intended to prevent abnormal heat generation of h.

Further, the CPU 5 has the drive stopping means 5
When the energization drive of the heater element Rh is stopped by b, the transistor SW is forcibly cut off (OFF) for a predetermined time (short time), and the heater element Rh is trially driven to conduct. The means 5c is provided. Furthermore, this trial means 5c allows the heater element Rh to be used for a short time.
The heater driving voltage Vh applied to the heater element Rh when the heater is driven to conduct is detected, and the heater driving voltage Vh at that time is detected.
When h is below the above-mentioned maximum allowable value (reference voltage), the heater element Rh by the drive stopping means 5b is used.
The release means 5d is provided for releasing the stop of the energization drive.

FIG. 2 shows such processing functions 5a, 5b, 5
The flow of the basic processing operation in the CPU 5 provided with c and 5d is shown. The processing operation of the CPU 5 will be described. First, the CPU 5 selects the processing routine according to whether or not abnormality processing for the heater applied voltage Vh is being performed [step S1]. When the abnormality processing is not being performed, the heater driving voltage Vh applied to the heater element Rh is detected [step S2], and the heater driving voltage Vh is when hydrogen is flowing through the microflow sensor. [Step S
3].

That is, the heater element Rh is controlled by the above-mentioned drive circuit so as to be always higher than the ambient temperature by a constant temperature difference. However, the degree of heat diffusion varies depending on the type of fluid flowing through the microflow sensor. Therefore, when the heating temperature of the heater element Rh is always higher than the ambient temperature by a constant temperature difference, the heater driving voltage applied to the heater element Rh is increased. Vh also differs depending on the type of fluid. For example, in FIG. 3, the flow rate Q of the fluid and the drive voltage V of the heater element Rh are shown.
As shown in the relationship of h, when the fluid is hydrogen, the driving voltage Vh applied to the heater element Rh is high (characteristic a), whereas in the case of air containing much nitrogen, the heater element Rh is
The drive voltage Vh applied to h is relatively low (characteristic b).

The above-mentioned determination [step S3] determines whether or not the fluid passing through the microflow sensor is hydrogen based on the difference in the heater driving voltage Vh which varies depending on the type of fluid. . When a fluid (gas) other than hydrogen is flowing, a warning message is displayed by, for example, outputting a message indicating that the inside of the microflow sensor is being purged [step S4].

On the other hand, when the fluid (gas) flowing through the microsensor is hydrogen, whether or not the heater driving voltage Vh at that time is less than a predetermined allowable maximum value (reference voltage) or not. Is determined [step S5]. When the heater drive voltage Vh exceeds a predetermined allowable maximum value (reference voltage), the drive stopping means 5b described above is used.
To drive the transistor SW to drive the heater element Rh.
The energization drive is stopped [step S6]. At this time, it is determined whether or not the heater driving voltage Vh is a voltage of a failure level corresponding to the power supply voltage Vcc [step S7]. Then, according to the determination result, whether the cause that the heater driving voltage Vh exceeds the allowable maximum value (reference voltage) is because the flow rate of hydrogen is excessive or the failure of the microflow sensor is determined. Discrimination is performed and the cause is displayed as an error [steps S8 and S9].

On the other hand, as described above, the heater element Rh
Stopping the energization drive of is equivalent to executing the abnormal process for the heater drive voltage. Therefore, after the lapse of a predetermined time, when the above-mentioned abnormal processing is being executed when this processing is started again [step S1], whether or not the abnormal processing is a processing for a failure of the microflow sensor is next performed. Is determined [step S11]. If the sensor has a failure, the abnormality process for the failure is continuously executed.

However, if the microflow sensor is not in failure, the trial means 5b is activated to forcibly turn off the transistor SW to turn off the heater element R.
The current h is temporarily energized [step S12]. Then, the drive voltage Vh of the heater element Rh at that time is detected, and it is determined whether or not the heater drive voltage Vh has returned to the above-described allowable maximum value (reference voltage) or less [step S
13]. If it is confirmed by this determination that the heater drive voltage Vh has returned to the allowable maximum value (reference voltage) or less, the above-described drive stopping means 5b cancels the energization drive stop of the heater element Rh, and the heater The element Rh is energized and driven as it is. However, when the heater drive voltage Vh is still in the state of exceeding the allowable maximum value (reference voltage), the transistor SW is turned on again to turn on the heater element R.
The energization drive of h is stopped [step S14].

Thus, according to the thermal type flow meter having the processing functions 5b, 5c, 5d for controlling the energization drive of the heater element Rh in this way, the drive voltage Vh of the heater element Rh is equal to its allowable maximum value (reference voltage). ), The energization drive of the heater element Rh is stopped, so that the heater element Rh can be prevented from abnormally generating heat and its overheating can be effectively prevented. Therefore, it is possible to effectively avoid the risk that the microflow sensor is thermally damaged due to abnormal heat generation of the heater element Rh. Further, even when the flow rate Q of combustible gas such as hydrogen is measured, the heater element Rh does not overheat, so that hydrogen (combustible gas) does not ignite. That is, it is possible to prevent the heater element Rh from reaching an abnormally heated (heated) state in advance, so that it is possible to effectively avoid occurrence of a defect due to abnormal heating of the heater element Rh. Moreover, there is an advantage that abnormal heating of the heater element Rh can be surely prevented with a simple configuration.

When the energization drive of the heater element Rh is stopped, the heater element Rh is forcibly energized as described above, and the heater drive voltage Vh at that time is checked to determine whether or not the failure is recovered. Since the determination is made, when the cause of the failure is removed, it is possible to quickly return to the normal flow rate measuring operation. That is, when the flow rate Q of the fluid temporarily increases as shown in FIG. 4A, the driving voltage of the heater element Rh increases as shown in FIG. 4B as the flow rate Q increases excessively. When the maximum allowable value (reference voltage) is exceeded,
As shown in (c), the heater drive stop signal is output and the energization drive of the heater element Rh is stopped. When the energization of the heater element Rh is stopped, the heater element Rh is tentatively energized and the energization drive of the heater element Rh is restarted according to the heater driving voltage Vh at that time. To be judged. Therefore, if the flow rate Q of the fluid becomes temporarily excessive, the flow rate Q returns to the normal amount, whereby the normal flow rate measuring operation can be promptly returned.

Incidentally, it cannot be denied that the output (detected flow rate Q) of the microflow sensor is temporarily interrupted when the driving of the heater element Rh is stopped as shown in FIG. 4 (d). However, since abnormal heating of the heater element Rh can be prevented in advance, it is possible to expect an effect over temporary stop of the flow rate detection operation. In particular, since it is possible to reliably avoid the ignition of flammable gas itself, the practical advantage in ensuring explosion-proof safety is extremely high.

In particular, the drive voltage V applied to the heater element Rh
Since it is determined from h that the driving state is normal or abnormal, even if the heating temperature of the heater element Rh is suppressed to be low due to an excessive flow rate, the abnormal state can be reliably detected. can do. Therefore, even when the heating temperature of the heater element Rh is low, it is possible to expect a secondary effect such as determining whether or not the detected flow rate at that time is normal. .

The present invention is not limited to the above embodiment. Here, the bridge circuit 1 is configured to control the driving of the heater element Rh. However, for example, while reading the ambient temperature measured by the temperature sensor Rr in the CPU 5, the driving power of the heater element Rh is controlled and its heat generation temperature ( The same can be applied when controlling the heating temperature). It can also be applied to a thermal type flow meter configured to measure the flow rate while controlling the temperature of the heater element Rh to be constant, and further measure the flow rate of flammable gas such as oxygen in addition to hydrogen. It is also applicable to flow meters. In addition, the present invention can be variously modified and implemented without departing from the scope of the invention.

[0031]

As described above, according to the present invention, the energization drive of the heater element is stopped according to the drive voltage applied to the heater element, so that abnormal heat generation (heating) of the heater element is prevented. It can be prevented, thermal destruction of the microflow sensor can be prevented in advance, and further, problems such as ignition of combustible gas can be effectively avoided, which can bring about great practical effects.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of a main part of a thermal type flow meter according to an embodiment of the present invention.

FIG. 2 is a diagram showing an example of a processing procedure in a CPU in the thermal type flow meter shown in FIG.

FIG. 3 is a diagram showing a relationship between a flow rate Q and a heater driving voltage Vh which differ depending on the type of fluid.

FIG. 4 is a diagram schematically showing an operation form of a thermal type flow meter with respect to a change in fluid flow rate.

FIG. 5 is a schematic configuration diagram of a microflow sensor.

FIG. 6 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 side) Rr temperature sensor (for ambient temperature measurement) 1 bridge circuit (for driving the heater) 2 differential amplifier 3 bridge circuit (for flow rate measurement) 4 differential amplifier 5 CPU 5a Flow rate detection means 5b Drive stop means 5c Trial means 5d Release means 6 memory (EEPROM) 7 Display Q transistor (for temperature control) SW transistor (for energization control)

Claims (3)

[Claims] 1. A heater element and first and second heater elements provided in the fluid flow direction with the heater element interposed therebetween.
A thermal type flow meter comprising: a temperature sensor for monitoring the heater driving voltage applied to the heater element; and a heater for stopping energization driving of the heater element according to the heater driving voltage. A thermal type flow meter comprising: a drive stopping means. 2. A heater element and first and second heater elements respectively provided in the fluid flow direction with the heater element interposed therebetween.
And a heater control means for controlling a heater driving voltage applied to the heater element to control a heat generation temperature of the heater element, and monitoring the heater driving voltage. A thermal flow meter, comprising: heater driving stop means for stopping energization driving of the heater element when the heater driving voltage exceeds a predetermined reference voltage. 3. The thermal type flow meter according to claim 1, further comprising a trial means for forcibly energizing and driving the heater element for a predetermined time when energization heating of the heater element is stopped. When the heater driving voltage at the time of energizing the conventional heater element is lower than a predetermined reference voltage, the heater driving stopping means cancels the energization driving stop of the heater element. A thermal type flow meter.