JP2001183201A - Thermal type flowmeter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thermal type flowmeter capable of showing high accu racy and high control responsiveness without complicating a circuit configura tion. SOLUTION: The flow rate of fluid is measured based on an applied voltage to a heater 112, the applied voltage being controlled based on the output of a sensor circuit 104 containing a temperature sensing element affected by the heater 112. The applied voltage to the heater is the sum of a fixed base voltage in a predetermined period and added voltage during variable applying period. A two-level signal consisting of an L level representing insufficient heating of the temperature sensing element and an H level excluding the L level outputted from a comparator 108 is sampled at predetermined intervals. When the L level count is for every predetermined period within a prescribed range, the base voltage is not changed. When it is lager than an upper limit, the base voltage is increased by one step value, and when it is lower than a lower limit, the base voltage is decreased by one step value. The added voltage is applied to a heater control circuit applies during only a period when the two-level signal outputted from the comparator is the L level.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fluid flow sensing technology, and more particularly to an indirectly heated flow meter.

[0002]

2. Description of the Related Art
Various types of flow meters [flow rate sensors] (or flow rate sensors [flow rate sensors]) for measuring the flow rate (or flow rate) of various fluids, especially liquids, are used, but it is easy to reduce the cost. For this reason, a so-called thermal (particularly indirectly heated) flow meter is used.

[0003] In this indirectly heated flow meter, a sensor chip formed by laminating a thin-film heating element and a thin-film temperature sensing element on a substrate by using a thin-film technology via an insulating layer is interposed between the sensor chip and a fluid in a pipe. What is arrange | positioned so that heat transfer is possible is used.
By energizing the heating element, the temperature sensing element is heated to change the electrical characteristics of the temperature sensing element, for example, the value of electrical resistance. This change in the electric resistance value (based on the temperature rise of the temperature sensing element) changes according to the flow rate (flow velocity) of the fluid flowing in the pipe. This is because a part of the calorific value of the heating element is transmitted into the fluid, and the amount of heat diffused into the fluid changes according to the flow rate (flow velocity) of the fluid. This is because the amount of heat supplied changes and the electrical resistance value of the thermosensitive body changes. The change in the electric resistance value of the temperature sensing element also differs depending on the temperature of the fluid. Therefore, a temperature sensing element for temperature compensation is incorporated in an electric circuit for measuring the change in the electric resistance value of the temperature sensing element. It has also been practiced to minimize the change in the flow measurement value due to the temperature of the fluid.

[0004] Such an indirectly heated flow meter using a thin film element is described in, for example, JP-A-11-118566. In this flow meter, an electric circuit (detection circuit) including a bridge circuit is used to obtain an electric output corresponding to the flow rate of the fluid.

In the flow meter described in Japanese Patent Application Laid-Open No. 11-118566, the voltage applied to the heating element is changed in accordance with the change in the flow rate, thereby changing the heat generation state of the heating element to provide a sensor. The heating body is maintained at a predetermined temperature (heating state), and at that time, a flow value is obtained based on a voltage applied to the heating body.

The present invention is to improve the control of the voltage applied to the heating element in the indirectly heated flow meter as described above, and to realize high accuracy and high control response without complicating the circuit configuration. is there.

[0007]

According to the present invention, there is provided, in order to achieve the above object, a heat transfer between a heating element and a fluid arranged so as to be affected by the heat generated by the heating element. And a flow rate detection circuit including a flow rate detection temperature sensing element disposed so as to be capable of controlling the heat generation of the heat generation element by a voltage applied to the heat generation element, and an output of the flow rate detection circuit. Controlling the voltage applied to the heating element based on
A thermal flowmeter that measures the flow rate of the fluid based on the applied voltage, wherein a voltage applied to the heating element is set every predetermined period, and a base voltage whose value does not change within the predetermined period,
A comparator that is a constant value and is made up of the sum of the applied voltages that are variable in application time and that compares the output of the flow rate detection circuit with a reference value; and that the heating of the temperature sensing element is insufficient from the comparator. And a second level indicating the other level is output. The binary signal output from the comparator is sampled at a predetermined period, and the first level is changed every predetermined period. The obtained number is counted to obtain a count value within the predetermined period, and when the count value is within a predetermined range, the base voltage value is not changed in a subsequent predetermined period, and If the count value is larger than the upper limit of the predetermined range, the base voltage is increased by a predetermined step value in a subsequent predetermined period, and the count value becomes lower than the predetermined range. The base voltage so as to descend by the step value in a predetermined period following if smaller, the output binary signal is said first of said comparator
A thermal flow meter is provided, wherein the additional voltage is applied only during a period of a level.

In one embodiment of the present invention, the added voltage is 2 to 4 times a step value of the base voltage. Further, in one aspect of the present invention, the predetermined range of the count value is １ ／ of the number of times of sampling within the predetermined period.
A value that is smaller and larger than 0 is set as a lower limit, and a value that is larger than 1/2 of the number of times of sampling within the predetermined period and smaller than the number of times of sampling is set as an upper limit.

[0009]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a circuit diagram showing an embodiment of a flow meter according to the present invention, and FIGS. 2 and 3 are partial detailed views thereof.
FIG. 4 is a cross-sectional view of a flow detection part of the flow meter according to the present embodiment, and FIG. 5 is a cross-sectional view of a flow detection unit.

As shown in FIG. 4, a casing member 20 made of a material having good heat conductivity, such as aluminum, is used.
Is formed with a fluid flow passage 20a. The flow passage 20 has a lower portion connected to a fluid inflow opening (not shown) and an upper portion connected to a fluid outflow opening (not shown). It flows out of the opening (the flow direction is indicated by an arrow).

A flow rate detection unit 24 and a fluid temperature detection unit 26 are attached to the casing member 20 so as to face the flow passage 20a. As shown in FIG. 5, in the flow rate detection unit 24, the flow rate detection unit 42 is joined to the surface of the fin plate 44 serving as a heat transfer member by a bonding material 46 having good thermal conductivity.
Of the bonding pad 5 and the electrode terminal 48
0, a part of the flow rate detector 42, the bonding wire 50, a part of the fin plate 44, and a part of the electrode terminal 48 are accommodated in a synthetic resin housing 52. The flow rate detection unit 42 is a chip-like member formed by insulating a thin-film temperature sensing element and a thin-film heating element from each other on a rectangular substrate of about 0.4 mm thick and about 2 mm square made of, for example, silicon or alumina. .

The fluid temperature detecting unit 26 is equivalent to a unit using a fluid temperature detecting unit instead of the flow detecting unit 42 in the flow detecting unit 24. In the fluid temperature detecting unit 26, members corresponding to those of the flow rate detecting unit 24 are denoted by the same reference numerals with “′” added thereto. The fluid temperature detecting section has the same configuration as that in which the thin film heating element is removed from the flow rate detecting section 42.

The ends of the fin plates 44, 44 'projecting from the housings 52, 52' of the flow rate detecting unit 24 and the fluid temperature detecting unit 26 are connected to the casing member 20.
In the flow passage 20a. Fin plate 4
4, 44 'extend through the center in the cross-section in the passage section 8 having a substantially circular cross-section. The fin plates 44, 44 'are arranged along the flow direction of the fluid in the flow passage 20a, so that the flow rate detecting unit 42 and the fluid temperature detecting unit 42' can be connected to the fluid without greatly affecting the fluid flow. It is possible to transfer heat well between.

As shown in FIG. 1, a DC voltage is supplied from a reference power supply circuit 102 to a sensor circuit (detection circuit) 104. The sensor circuit 104 includes a bridge circuit as shown in FIG. The bridge circuit 104 includes a thin film temperature sensing element 104-1 for flow rate detection of the flow rate detection unit 24, a thin film temperature sensing element 104-2 for fluid temperature compensation of the fluid temperature detection unit 26, and a resistor 104-.
3, 104-4. The potentials Va and Vb at points a and b of the bridge circuit 104 are equal to the differential amplifier circuit (amplifier) 1.
06, and the output of the differential amplifier circuit 106 is input to the comparator 108. The output voltage signal of the amplifier 106 and the reference voltage (Vre
f) is output as a binary signal, and
When the output voltage signal 06 is lower than the reference voltage (Vref), a low (L) level [first level] is output, and when the output voltage signal is the same or higher, a high (H) level [second level] is output.

On the other hand, as shown in FIG. 1, the DC voltage from the reference power supply circuit 102 passes through a transistor 110 for controlling the current supplied to the thin film heating element 112 of the flow rate detection unit 24. Is supplied to the thin film heating element 112. That is, in the flow rate detecting unit 24, based on the heat generated by the thin film heating element 112, the thin film thermosensitive element 1 is affected by the heat absorption by the fluid to be detected via the fin plate 44.
The temperature sensing by 04-1 is executed. As a result of the temperature sensing, a difference between the potentials Va and Vb at points a and b of the bridge circuit 104 shown in FIG. 3 is obtained.

The value of (Va-Vb) changes when the temperature of the flow sensing temperature sensing element 104-1 changes according to the flow rate of the fluid. By appropriately setting the characteristics of the bridge circuit 104 in advance and appropriately setting the reference (Vref) of the comparator 108, the heating state of the thin-film thermosensitive body 104-1 is predetermined (that is, the temperature of the thin-film thermosensitive body 104-1). Is a predetermined value), the output voltage signal of the amplifier 106 can be set to the comparator reference voltage (Vref). In other words, the comparator reference voltage (Vref) is used when the thin-film temperature sensing element 104-1 is in a predetermined heating state.
6 is set to be the same as the value of the output voltage obtained from.

When the fluid flow rate increases or decreases, the comparator 108
Output changes. Using the output of the comparator 108, the heat generation of the thin film heating element (sensor heater) 112 is controlled. The CPU 120 is used to control the heat generation of the thin film heating element 112 and to perform a flow rate calculation operation. As shown in FIG. 1, the output of the comparator 108 is input to the heater control circuit 124 of the CPU 120 via the PLD 122. The output of the heater control circuit 124 is converted into an analog signal by the D / A converter 128, input to the amplifier 130, and the output voltage signal of the amplifier 130 is input to the base of the transistor 110. On the other hand, from the heater control circuit 124, the CPU 12
The signal is transmitted to the flow rate integration calculation circuit 132 within the range of 0, and the calculation result and the like are displayed from the flow rate integration calculation circuit 132 on the display unit 13.
4 and the necessary display is made on the display unit 134.

As shown in FIG.
Are the synchronization circuit 122a and the edge detection circuits 122b and 125
And a counter 122c. Also, the heater control circuit 1
Reference numeral 24 denotes an "L" level counter 124a and the comparison circuit 12
4b and a heater voltage circuit 124c.

The CPU 120 has a 4 MHz clock circuit 1
36, a clock signal is converted to a 1 MHz clock by a frequency dividing circuit 138 in the CPU 120, and a 125 MHz counter 122 in the PLD 122
c and an “L” level counter 124 a in the heater control circuit 124.

The output of the comparator 108 is a PLD
After passing through 122, the signal is input to the “L” level counter 124a, where it is sampled every 1 μsec cycle (predetermined cycle), and what the “L” level is within the 125 μsec period (predetermined period) set by the 125 counter 122c. The number of occurrences is counted. Data of the count value obtained by this count (count data CD)
Is input to the comparison circuit 124b, where it is compared with a predetermined range. This predetermined range is 12
1/2 of the number of times of sampling within a predetermined period of 5 μsec
(62.5) a value smaller than 0 and larger than 0 (for example, 4
3) may be set as the lower limit, and a value (for example, 82) larger than 1/2 of the number of times of sampling and smaller than the number of times of sampling (125) within the predetermined period of 125 μsec may be set as the upper limit.

Incidentally, in the heater voltage circuit 124c,
The control voltage input to the transistor 130 for controlling the applied voltage to the sensor heater 112
2, which may be used synonymously with the heater applied voltage in the present specification] comprises the sum of the base voltage (Eb) and the added voltage (Ec). The base voltage is selected from discrete values set in advance for each predetermined step value, and the value does not change within each predetermined period, whereby coarse control of heat generation of the heater is performed. The addition voltage is a constant value, and the time or timing of application is variable, thereby finely controlling the heat generation of the heater. It is appropriate that the added voltage be 2 to 4 times the base voltage step value.

In the comparison circuit 124b, the count data C
When D is equal to or more than the lower limit value Nd and equal to or less than the upper limit value Nu, it instructs the heater voltage circuit 124c to hold the previous base voltage as it is in the next predetermined period. When the count data CD is less than the lower limit value Nd,
Instruct the heater voltage circuit 124c to lower the base voltage by one step value from the previous base voltage value in the next predetermined period. Also, the count data C
If D exceeds the upper limit Nu, the heater voltage circuit 12
4c is instructed to increase the base voltage by one step value from the previous base voltage value in the next predetermined period.

On the other hand, the output of the comparator 108 is PL
After passing through D122, it is input to the heater voltage circuit 124c. Based on this input signal, the heater voltage circuit 124c
In this case, the application of the addition voltage (Ec) is performed during a period in which the input signal is maintained at the “L” level, and the application of the addition voltage (Ec) is not performed during other periods.

The above-described heater voltage control will be described more specifically with reference to the time chart of FIG.

In FIG. 6, the output signal (input signal to the comparator 108) of the amplifier 106 connected to the bridge circuit 104 and the reference voltage (Vre
The time change of the relationship with f) is shown, and the corresponding change of the output signal of the comparator 108 is shown. Corresponding to this, the comparison circuit 124 obtains the "L" level counter 124a every 125 .mu.sec.
The change of the count data CD input to b is shown. In response to this, the heater applied voltage (Eh)
Is shown over time. Correspondingly, a time change of the actual flow rate is schematically shown.

It is assumed that the lower limit Nd and the upper limit Nu set by the comparison circuit 124b are Nd = 43 and Nu = 82. When 43 ≦ CD ≦ 82, the comparison circuit 124
b, the heater voltage circuit 124c sets the next 125 μs following the predetermined period in which the count data CD was obtained.
During the predetermined period of ec, the base voltage Eb is maintained at the value of the immediately preceding predetermined period and is not changed. If CD <43, the heater voltage circuit 124
In c, the base voltage Eb is increased by one step voltage value (here, 10 mV) from the value of the immediately preceding predetermined period during the next 125 μsec predetermined period following the predetermined period in which the count data CD is obtained.
Just lower. If CD> 82, the comparison circuit 12
4b, the heater voltage circuit 124c
The next 125 μ following the predetermined period in which the count data CD was obtained
During a predetermined period of sec, the base voltage Eb is increased by one step voltage value (10 mV) from the value of the immediately preceding predetermined period.

On the other hand, in the heater voltage circuit 124c, while the output signal of the comparator 108 is at the "L" level, a predetermined additional voltage (Ec: 30 mV in this case) is applied, and the output signal of the comparator 108 becomes "H". No additional voltage is applied during the level period.

As described above, in the present embodiment, the base voltage within the subsequent predetermined period is appropriately set based on the count data CD obtained within the predetermined period, and the addition voltage application period is set in accordance with the output of the comparator. By combining the two types of control of appropriately setting, the response of the control is increased, the accuracy of the flow rate measurement is increased, and the thermal hysteresis is reduced with a simple device configuration.

The above-mentioned predetermined period, predetermined period, base voltage step value, added voltage value and the like can be appropriately set in consideration of the expected maximum flow rate change so as to cope therewith.

As described above, regardless of the change in the fluid flow rate, the temperature of the temperature sensing element 104-1 always becomes a predetermined value (that is, the heating state of the temperature sensing element 104-1 is predetermined. Is generated, the heat generation of the thin-film heating element 112 is controlled. Since the voltage (heater applied voltage) applied to the thin-film heating element 112 at this time corresponds to the fluid flow rate, the voltage is used as a flow rate output in the flow rate integration operation circuit 132 shown in FIGS. Take out. For example, an instantaneous flow is output every 0.5 sec, and the integrated flow is obtained by integrating the instantaneous flow.

That is, as shown in FIG.
Based on the value of the count data CD for each predetermined period obtained from the level counter 124a, an integrated value (） Ec) of the added voltage Ec applied for 0.5 seconds is obtained, and a base voltage value Eb obtained from the heater voltage circuit 124c is obtained. Based on 0.
Integrated value of base voltage Eb applied for 5 seconds (ΣEb)
And the sum of these values (ΣEc + ΣEb) is obtained (FIG. 7).
reference). This value is converted into an instantaneous flow rate value using a calibration curve (instantaneous flow rate conversion table) measured and stored in advance. This instantaneous flow rate conversion table is a data table indicating the relationship between the flow rate and the integrated value of the heater applied voltage for 0.5 seconds, and specifically, the relationship between the integrated value of the heater applied voltage and the flow rate value. When the flow rate value is obtained from the actually obtained heater applied voltage integrated value, data is complemented. Further, the integrated flow rate value is obtained by integrating the instantaneous flow rate values.

This flow rate output is displayed on the display unit 134. In addition, according to a command from the CPU 120, the instantaneous flow rate and the integrated flow rate can be appropriately stored in a memory, and further, can be transmitted to the outside through a telephone line or other communication line including a network. it can.

[0034]

As described above, according to the present invention,
By appropriately setting the base voltage within the subsequent predetermined period based on the count value obtained within the predetermined period and further appropriately setting the addition voltage application period according to the output of the comparator, Without complicating the circuit configuration, the responsiveness of the heater control is increased, the accuracy of the flow rate measurement is increased, and the thermal hysteresis is reduced.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an embodiment of a flow meter according to the present invention.

FIG. 2 is a partial detailed view of the circuit diagram of FIG. 1;

FIG. 3 is a partial detailed view of the circuit diagram of FIG. 1;

FIG. 4 is a sectional view of a flow rate detecting portion of the flow meter.

FIG. 5 is a cross-sectional view of the flow detection unit.

FIG. 6 is a time chart for explaining heater voltage control.

FIG. 7 is a time chart showing changes in heater voltage and calculation of a flow rate value.

[Explanation of symbols]

 Reference Signs List 20 casing member 20a fluid flow passage 24 flow detection unit 26 fluid temperature detection unit 42 flow detection unit 44, 44 'fin plate 46 bonding material 48, 48' electrode terminal 50 bonding wire 52, 52 'housing 104 bridge circuit 104-1 flow rate Thin film temperature sensing element for detection 104-2 Thin film temperature sensing element for temperature compensation 104-3, 104-4 Resistor 106 Differential amplifier circuit (amplifier) 108 Comparator 110 Transistor 112 Thin film heating element (heater) 130 Amplifier

Continued on the front page (72) Inventor Kiyoshi Yamagishi 1333-2 Hara-shi, Ageo-shi, Saitama F-term in Mitsui Kinzoku Mining Co., Ltd. F-term (reference) 2F035 EA04 EA08

Claims (3)

[Claims] 1. A flow rate detection device comprising: a heating element; and a flow rate detection temperature sensing element disposed so as to be affected by heat generated by the heating element and arranged so as to be able to transfer heat between the heating element and a fluid. And a circuit for controlling heat generation of the heating element by a voltage applied to the heating element, controlling a voltage applied to the heating element based on an output of the flow rate detection circuit, and controlling the voltage based on the applied voltage. A thermal flow meter for measuring a flow rate of a fluid, wherein an applied voltage to the heating element is set every predetermined period, and a constant value and a base voltage whose value does not change within the predetermined period, and an application time is variable. And a comparator for comparing the output of the flow rate detection circuit with a reference value. From the comparator, a first level indicating that the heating of the thermosensitive element is insufficient and other levels And the second level that indicates The binary signal output from the comparator is sampled at a predetermined cycle, and the number of times the first level is obtained every predetermined period is counted to obtain a count value within the predetermined period. When the count value is within a predetermined range, the value of the base voltage is not changed in the subsequent predetermined period, and when the count value is larger than the upper limit of the predetermined range, the base voltage is not changed in the subsequent predetermined period. The base voltage is increased by a predetermined step value, and when the count value is smaller than the lower limit of the predetermined range, the base voltage is decreased by the step value in a subsequent predetermined period. The thermal flowmeter according to claim 1, wherein the additional voltage is applied only during a period when the signal is at the first level. 2. The thermal flow meter according to claim 1, wherein the additional voltage is 2 to 4 times a step value of the base voltage. 3. The predetermined range of the count value is smaller than の of the number of times of sampling within the predetermined period and 0
3. The method according to claim 1, wherein a larger value is set as a lower limit value, and a value larger than 1/2 of the number of times of sampling within the predetermined period and smaller than the number of times of sampling is set as an upper limit value. A thermal flow meter according to Crab.