JP4435351B2 - Thermal flow meter - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention belongs to a fluid flow rate detection technique, and particularly relates to an indirectly heated flow meter.
[0002]
[Prior art and problems to be solved by the invention]
Conventionally, various types of flowmeters [flow sensors] (or flowmeters [flow velocity sensors]) that measure the flow rate (or flow velocity) of various fluids, particularly liquids, have been used. For this reason, so-called thermal type (especially side heat type) flow meters are used.
[0003]
This indirectly heated flow meter uses a thin film technology on a substrate to transfer heat between a thin film heating element and a thin film temperature sensor through an insulating layer between the fluid in the pipe. The one arranged as possible is used. By energizing the heating element, the temperature sensing element is heated, and the electrical characteristics of the temperature sensing element, for example, the value of electric resistance are changed. This change in electrical 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 part of the calorific value of the heating element is transmitted into the fluid, and the amount of heat that diffuses into the fluid changes according to the flow rate (flow velocity) of the fluid, and in response to this, the heat sensing element This is because the amount of heat supplied changes and the electric resistance value of the temperature sensing element changes. The change in the electric resistance value of the temperature sensing element also varies depending on the temperature of the fluid. Therefore, a temperature sensing element for temperature compensation should be incorporated in the electric circuit for measuring the change in the electric resistance value of the temperature sensing element. The change of the flow rate measurement value due to the temperature of the fluid is also minimized.
[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 electrical output corresponding to the flow rate of the fluid.
[0005]
Further, in the flow meter described in Japanese Patent Application Laid-Open No. 11-118566, by changing the voltage applied to the heating element in response to the flow rate change, the heating state of the heating element is changed, so that the temperature sensing element A predetermined temperature (heating state) is maintained, and a flow rate value is obtained based on a voltage applied to the heating element at that time.
[0006]
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 responsiveness without complicating the circuit configuration.
[0007]
[Means for Solving the Problems]
According to the present invention, the object as described above is achieved.
A heating element, and a flow rate detection circuit including a flow rate detection temperature sensor arranged so as to be affected by the heat generated by the heating element and capable of transferring heat between the fluid and the fluid. And
Heat generated by the heating element is controlled by the voltage applied to the heating element, the voltage applied to the heating element is controlled based on the output of the flow rate detection circuit, and the flow rate of the fluid is measured based on the applied voltage. Type flow meter,
The applied voltage to the heating element is set for each predetermined period, and is composed of a total of a base voltage whose value does not change within the predetermined period and an addition voltage that is a constant value and variable in application time,
A comparator for comparing the output of the flow rate detection circuit with a reference value; from the comparator, a first level indicating that heating of the temperature sensing element is insufficient; A binary signal consisting of
The output binary signal of the comparator is sampled at a predetermined cycle, the number of times the first level is obtained for each predetermined period is counted to obtain a count value within the predetermined period, and the count value is determined in advance. If the predetermined value is within the predetermined range, the base voltage value is not changed in the subsequent predetermined period. If the count value is larger than the upper limit of the predetermined range, the base voltage is predetermined in the subsequent predetermined period. 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,
A thermal flow meter, wherein the addition voltage is applied only during a period when the output binary signal of the comparator is at the first level;
Is provided.
[0008]
In one aspect of the present invention, the addition voltage is 2 to 4 times the step value of the base voltage. In one embodiment of the present invention, the predetermined range of the count value has a lower limit value that is smaller than 1/2 of the number of samplings within the predetermined period and greater than 0, and the number of samplings within the predetermined period The upper limit value is a value larger than ½ and smaller than the number of samplings.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[0010]
FIG. 1 is a circuit diagram showing an embodiment of the flowmeter of the present invention, and FIGS. 2 and 3 are partial detailed views thereof. FIG. 4 is a cross-sectional view of the flow rate detection portion of the flowmeter of the present embodiment, and FIG. 5 is a cross-sectional view of the flow rate detection unit.
[0011]
As shown in FIG. 4, a fluid flow passage 20a is formed in the casing member 20 made of a material having good thermal conductivity such as aluminum. 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), and the fluid flowing from the fluid inflow opening flows upward through the flow passage 20a and flows out of the fluid. It flows out of the opening (the direction of flow is indicated by an arrow).
[0012]
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 bonded to the surface of the fin plate 44, which is a heat transfer member, by a bonding material 46 with good thermal conductivity, and the electrode pad of the flow rate detection unit 42. And the electrode terminal 48 are connected by a bonding wire 50, and the flow rate detection unit 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 made of 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 thickness and about 2 mm square made of, for example, silicon or alumina. .
[0013]
The fluid temperature detection unit 26 corresponds to a unit using a fluid temperature detection unit instead of the flow rate detection unit 42 in the flow rate detection unit 24. In the fluid temperature detection unit 26, members corresponding to those of the flow rate detection unit 24 are denoted by the same reference numerals with “′”. The fluid temperature detection unit has the same configuration as that obtained by removing the thin film heating element from the flow rate detection unit 42.
[0014]
End portions of the fin plates 44 and 44 ′ protruding from the housings 52 and 52 ′ of the flow rate detection unit 24 and the fluid temperature detection unit 26 extend into the flow passage 20 a of the casing member 20. The fin plates 44, 44 ′ extend through the center in the cross section of the flow passage portion 8 having a substantially circular cross section. Since the fin plates 44 and 44 ′ are arranged along the flow direction of the fluid in the flow passage 20 a, the flow rate detection unit 42, the fluid temperature detection unit 42 ′, and the fluid are not affected without greatly affecting the fluid flow. It is possible to transfer heat between the two.
[0015]
As shown in FIG. 1, a DC voltage is supplied from the reference power supply circuit 102 to the sensor circuit (detection circuit) 104. As shown in FIG. 3, the sensor circuit 104 includes a bridge circuit. The bridge circuit 104 includes a flow rate detection thin film temperature sensor 104-1 of the flow rate detection unit 24, a fluid temperature compensation thin film temperature sensor 104-2 of the fluid temperature detection unit 26, and resistors 104-3 and 104-4. Comprising. The potentials Va and Vb at the points a and b of the bridge circuit 104 are input to the differential amplifier circuit (amplifier) 106, and the output of the differential amplifier circuit 106 is input to the comparator 108. The comparator 108 outputs a comparison result between the output voltage signal of the amplifier 106 and the reference voltage (Vref) as a binary signal. When the output voltage signal of the amplifier 106 is lower than the reference voltage (Vref), the comparator 108 outputs low (L). When the level [first level] is output and is the same or higher, the high (H) level [second level] is output.
[0016]
On the other hand, as shown in FIG. 1, the DC voltage from the reference power supply circuit 102 generates thin film heat via a transistor 110 for controlling the current supplied to the thin film heating element 112 of the flow rate detection unit 24. To the body 112. That is, in the flow rate detection unit 24, the temperature sensing by the thin film temperature sensing element 104-1 is performed under the influence of heat absorption by the fluid to be sensed via the fin plate 44 based on the heat generated by the thin film heating element 112. As a result of the temperature sensing, the difference between the potentials Va and Vb at points a and b of the bridge circuit 104 shown in FIG. 3 is obtained.
[0017]
The value of (Va−Vb) changes as the temperature of the flow rate detection temperature sensor 104-1 changes according to the flow rate of the fluid. When the characteristics of the bridge circuit 104 are set in advance and the reference (Vref) of the comparator 108 is set as appropriate, the heating state of the thin film temperature sensor 104-1 is predetermined (that is, the temperature of the thin film temperature sensor 104-1). 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 set to be the same as the value of the output voltage obtained from the amplifier 106 when the thin film temperature sensor 104-1 is in a predetermined heating state.
[0018]
When the fluid flow rate increases or decreases, the output of the comparator 108 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 further calculate the flow rate. 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 and 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, a signal is transmitted from the heater control circuit 124 to the flow rate integration calculation circuit 132 in the CPU 120, and the calculation result and the like are output from the flow rate integration calculation circuit 132 to the display unit 134. Made.
[0019]
As shown in FIG. 2, the PLD 122 includes a synchronization circuit 122a, an edge detection circuit 122b, and a 125 counter 122c. The heater control circuit 124 includes an “L” level counter 124a, a comparison circuit 124b, and a heater voltage circuit 124c.
[0020]
The CPU 120 receives a clock signal from the 4 MHz clock circuit 136, and this clock signal is converted into a 1 MHz clock by the frequency dividing circuit 138 in the CPU 120, and the 125 counter 122 c in the PLD 122 and the “L” level counter in the heater control circuit 124. It is input to 124a.
[0021]
The output of the comparator 108 is input to the “L” level counter 124a after passing through the PLD 122, where it is sampled every 1 μsec period (predetermined period), and within the 125 μsec period (predetermined period) set by the 125 counter 122c. It is counted how many times the “L” level appears. The count value data (count data CD) obtained by this count is input to the comparison circuit 124b, where it is compared with a predetermined range. This predetermined range has a value lower than 1/2 (62.5) of the number of samplings within a predetermined period of 125 μsec and greater than 0 (for example, 43) as a lower limit, and 1 of the number of samplings within the predetermined period of 125 μsec. A value (for example, 82) that is larger than / 2 and smaller than the number of samplings (125) can be set as the upper limit value.
[0022]
By the way, in the heater voltage circuit 124c, the control voltage input to the transistor 130 for controlling the applied voltage to the sensor heater 112 [this corresponds to the applied voltage to the heater 112, and in this specification, heater application is performed. [Sometimes used synonymously with voltage] consists of 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 the heater heat generation is roughly controlled. The addition voltage is a constant value, and the application time or timing is variable, whereby the heater heat generation is finely controlled. The added voltage is suitably 2 to 4 times the base voltage step value.
[0023]
When the count data CD is not less than the lower limit value Nd and not more than the upper limit value Nu, the comparison circuit 124b instructs the heater voltage circuit 124c to hold the previous base voltage as it is for the next predetermined period. When the count data CD is less than the lower limit value Nd, the heater voltage circuit 124c is instructed to lower the base voltage by one step value from the previous base voltage value in the next predetermined period. When the count data CD exceeds the upper limit value Nu, the heater voltage circuit 124c is instructed to increase the base voltage by one step value from the previous base voltage value in the next predetermined period.
[0024]
On the other hand, the output of the comparator 108 is input to the heater voltage circuit 124 c after passing through the PLD 122. Based on this input signal, the heater voltage circuit 124c applies the addition voltage (Ec) while the input signal is maintained at the “L” level, and applies the addition voltage (Ec) during other periods. Do not do.
[0025]
The heater voltage control as described above will be described more specifically with reference to the time chart of FIG.
[0026]
In FIG. 6, the time change of the relationship between the output signal (input signal to the comparator 108) of the amplifier 106 connected to the bridge circuit 104 and the reference voltage (Vref) of the comparator 108 is shown. A change in the output signal of 108 is shown. Correspondingly, a change in the count data CD obtained by the “L” level counter 124a and inputted to the comparison circuit 124b every 125 μsec is shown. Correspondingly, the time change of the heater applied voltage (Eh) is shown. Correspondingly, the time change of the actual flow rate is schematically shown.
[0027]
It is assumed that the lower limit value Nd and the upper limit value Nu set in the comparison circuit 124b are Nd = 43 and Nu = 82. In the case of 43 ≦ CD ≦ 82, in response to an instruction from the comparison circuit 124b, the heater voltage circuit 124c applies the base voltage Eb for the predetermined period of 125 μsec following the predetermined period from which the count data CD is obtained for the previous predetermined period. Keep the value and do not change it. In the case of CD <43, in response to an instruction from the comparison circuit 124b, the heater voltage circuit 124c sets the base voltage Eb from the value of the immediately preceding predetermined period for the predetermined period of 125 μsec following the predetermined period of obtaining the count data CD. The voltage is decreased by one step voltage value (here, 10 mV). In the case of CD> 82, the heater voltage circuit 124c determines the base voltage Eb from the value of the immediately preceding predetermined period during the predetermined period of 125 μsec following the predetermined period in which the count data CD is obtained in accordance with an instruction from the comparison circuit 124b. Increase by one step voltage value (10 mV).
[0028]
On the other hand, in the heater voltage circuit 124c, a predetermined addition voltage (Ec: 30 mV in this case) is applied while the output signal of the comparator 108 is at "L" level, and the output signal of the comparator 108 is at "H" level. No additional voltage is applied during the period.
[0029]
As described above, in the present embodiment, the base voltage in the subsequent predetermined period is appropriately set based on the count data CD obtained in the predetermined period, and the addition voltage application period is appropriately set according to the output of the comparator. By combining these two types of control, the control responsiveness is improved with a simple device configuration, the accuracy of flow rate measurement is increased, and the thermal hysteresis is reduced.
[0030]
Note that the predetermined period, the predetermined period, the base voltage step value, the added voltage value, and the like can be appropriately set so as to cope with the expected maximum flow rate change.
[0031]
As described above, regardless of the change in the fluid flow rate, the temperature of the flow sensing temperature detector 104-1 is always a predetermined value (that is, the heating state of the flow detection temperature detector 104-1 is a predetermined value). As shown, 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, it is used as a flow rate output in the flow rate integration arithmetic circuit 132 shown in FIGS. Take out. For example, an instantaneous flow rate is output every 0.5 sec, and the integrated flow rate is obtained by integrating the instantaneous flow rate.
[0032]
That is, as shown in FIG. 2, the integrated value (ΣEc) of the addition voltage Ec applied for 0.5 seconds based on the value of the count data CD for each predetermined period obtained from the “L” level counter 124a. Further, based on the base voltage value Eb obtained from the heater voltage circuit 124c, an integrated value (ΣEb) of the base voltage Eb applied for 0.5 seconds is obtained, and a total value (ΣEc + ΣEb) is obtained (see FIG. 7). . 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 showing the relationship between the integrated value of the heater applied voltage for 0.5 seconds and the flow rate. Specifically, it shows the relationship between the integrated value of the heater applied voltage and the flow rate repeatedly. Therefore, when the flow rate value is obtained from the heater applied voltage integrated value actually obtained, the data is supplemented. Further, the integrated flow value is obtained by integrating the instantaneous flow value.
[0033]
This flow rate output is displayed by the display unit 134. The instantaneous flow rate and the integrated flow rate can be appropriately stored in the memory by a command from the CPU 120, and further transmitted to the outside via a communication line including a telephone line and other networks. it can.
[0034]
【The invention's effect】
As described above, according to the present invention, the base voltage in the subsequent predetermined period is appropriately set based on the count value obtained in the predetermined period, and the addition voltage application period is appropriately set according to the output of the comparator. By combining the two types of controls, the responsiveness of the heater control is increased without complicating the circuit configuration, the accuracy of 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 of the present invention.
FIG. 2 is a partial detail view of the circuit diagram of FIG. 1;
FIG. 3 is a partial detail view of the circuit diagram of FIG. 1;
FIG. 4 is a cross-sectional view of a flow rate detection portion of the flow meter.
FIG. 5 is a cross-sectional view of a flow rate 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 flow rate values.
[Explanation of symbols]
20 Casing member 20a Fluid flow passage 24 Flow rate detection unit 26 Fluid temperature detection unit 42 Flow rate 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 104-2 for detection Temperature compensation thin film temperature sensing element 104-3, 104-4 Resistor 106 Differential amplifier circuit (amplifier)
108 Comparator 110 Transistor 112 Thin-film heating element (heater)
130 amplifiers

Claims (3)

A heating element, and a flow rate detection circuit including a flow rate detection temperature sensor arranged so as to be affected by the heat generated by the heating element and capable of transferring heat between the fluid and the fluid. And
Heat generated by the heating element is controlled by the voltage applied to the heating element, the voltage applied to the heating element is controlled based on the output of the flow rate detection circuit, and the flow rate of the fluid is measured based on the applied voltage. Type flow meter,
The applied voltage to the heating element is set for each predetermined period, and is composed of a total of a base voltage whose value does not change within the predetermined period and an addition voltage that is a constant value and variable in application time,
A comparator for comparing the output of the flow rate detection circuit with a reference value; from the comparator, a first level indicating that heating of the temperature sensing element is insufficient; A binary signal consisting of
The output binary signal of the comparator is sampled at a predetermined cycle, the number of times the first level is obtained for each predetermined period is counted to obtain a count value within the predetermined period, and the count value is determined in advance. If the predetermined value is within the predetermined range, the base voltage value is not changed in the subsequent predetermined period. If the count value is larger than the upper limit of the predetermined range, the base voltage is predetermined in the subsequent predetermined period. 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 type flow meter, wherein the added voltage is applied only during a period when the output binary signal of the comparator is at the first level. The thermal flow meter according to claim 1, wherein the addition voltage is 2 to 4 times the step value of the base voltage. The predetermined range of the count value is smaller than 1/2 of the number of samplings within the predetermined period and greater than 0 as a lower limit value, and is greater than 1/2 of the number of samplings within the predetermined period and the number of samplings The thermal flow meter according to claim 1, wherein a smaller value is an upper limit value.

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