JP3238984B2 - Heat-sensitive microbridge flowmeter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat-sensitive microbridge-type current meter for measuring a gas flow velocity.

[0002]

2. Description of the Related Art Conventional velocimeters are disclosed in
Japanese Patent Application Laid-Open No. 1-235726 discloses one disclosed under the name of "mass flow meter and flow control device". In this basic structure, a moat is formed on a substrate by anisotropic etching, a beam is passed over the moat, and a heating element made of an electric resistor and a heat-sensitive portion for sensing the heating element are arranged on the beam. This makes it possible to reduce the heat capacity of the heat-sensitive portion, reduce the heat loss to the substrate and the beam, and reduce the heat generation and the input power required for temperature measurement.

[0003]

As described above, in the conventional structure, the beam is heated by the heating element, and the heating element input electric power supplied to stabilize the temperature difference between the beam and the fluid is output from the heat-sensitive section. A method of measuring the flow velocity by detecting the flow rate has been adopted.

However, such a measuring method has the following problems. That is, beams and room temperature (fluid)
The power supply to the heating element on the beam is adjusted so as to keep the temperature difference between the temperature and the temperature constant constant. However, when the room temperature (Tr) rises, the rise (ΔTr) and the temperature coefficient of the heating element resistance material (α) ), The change in the resistance value (the product of α and ΔTr) is included in the heating element output. For this reason, there is a problem that the output value changes every time the room temperature changes, and accurate detection of the flow velocity cannot be performed.

[0005] Further, as the flowmeter is used for a long period of time, the heating element exposed to a high temperature and a large amount of electricity is thermally degraded, resulting in a change in the temperature coefficient of resistance with time.
Due to such a change with time, there is a problem that accurate measurement of the flow velocity over a long period of time becomes impossible.

[0006]

According to the first aspect of the present invention, a moat is formed by etching a substrate, a beam is formed so as to straddle over the moat, and a heating element and the heat generating member are formed on the beam. A heat-generating microbridge flowmeter provided with a heating element for measuring the temperature of the body, and a fluid temperature sensor for measuring the temperature of the fluid on the substrate; A room temperature calibration resistor connected in series with the heating element for correcting a signal is provided, and the heating element for measuring the amount of current outside the substrate and a current measurement resistor connected in series to the room temperature calibration resistor. An electric body in which the heating element, the fluid temperature measuring resistor, and the balance adjusting resistor are connected in order to keep the temperature difference between the heating element and the fluid temperature measuring resistor constant. A bridge circuit is provided, and the voltage across the heating element is The respective voltage drop values of the lower value, the voltage drop value at both ends of the heating element and the room temperature calibration resistor, the voltage drop value at both ends of the room temperature calibration resistor, and the voltage drop value at both ends of the current measurement resistor are changed as needed. An electric measurement circuit for measuring and outputting as a digital signal is provided; a digital memory for storing and holding the digital signal is provided;

According to the second aspect of the present invention, a moat is formed by etching a substrate, a beam is formed so as to straddle over the moat, and a heating element and a temperature of the heating element are measured on the beam. A heat-sensitive microbridge flowmeter provided with a heating element and a fluid temperature measuring resistor for measuring the temperature of a fluid on the substrate, wherein the flow rate output signal is corrected on the substrate. A room temperature calibration resistor connected in series with a heating element is provided, and the heating element for measuring the amount of current and a current measurement resistor connected in series to the room temperature calibration resistor are provided outside the substrate. An electric bridge circuit in which the heating element temperature measuring resistor, the fluid temperature measuring resistor, and the balance adjusting resistor are connected to make a temperature difference between the body and the fluid temperature measuring resistor constant; The voltage drop between both ends of the heating element and the room temperature An electric measurement circuit is provided which measures each voltage drop value between the voltage drop value across the positive resistor and the voltage drop value across the current measurement resistor as needed and outputs it as a digital signal, and stores and holds the digital signal. A memory is provided, and an arithmetic processing circuit is provided for performing arithmetic processing for removing errors caused by changes in room temperature and errors caused by changes in the temperature coefficient of the resistor due to thermal fatigue caused by long-term use.

[0008]

According to the first aspect of the invention, it is possible to automatically remove an error component due to room temperature fluctuation from the flow velocity output as needed.

According to the second aspect of the present invention, it is possible to remove an error component due to room temperature fluctuation from the flow velocity output, and to automatically change a fluctuation component due to a temporal change in the temperature coefficient of resistance of the heating element during long-term use. Can be removed.

[0010]

An embodiment of the present invention will be described with reference to FIG. The substrate 1 is made of Si, and an SiO ₂ film (not shown), which is a heat insulating layer, is formed on the surface of the substrate 1 by using an oxidation method or a sputtering vacuum film forming method. The heat insulating layer is made of Si ₃ N ₄ or Ta ₂ which is a metal oxide.
O ₅ , Al ₂ O ₃ , or a multilayer film combining a SiO ₂ film and a Si ₃ N ₄ film may be used. The film thickness in this case is 0.5 to 2 μm. Next, the moat 2 is formed by performing anisotropic etching with KOH on the surface of the substrate 1. The depth of the moat 2 needs to be equal to or greater than the thermal boundary layer that can be formed at the minimum flow velocity to be measured. Next, by performing such anisotropic etching, so as to straddle the sky above the moat 2,
The beam 3 is formed. On the surface of the beam 3, a Ta ₂ O ₅ film is formed as an adhesion strength reinforcing layer in order to improve the adhesion with the film formed thereon. Note that Ti, Cr, Ta, NiCr, TiN
May be used. Next, a heating element (Rh) is placed on the beam 3.
And a heating element temperature measuring resistor (Rs) 5 adjacent to the heating element 4 for measuring the temperature of the heating element 4. As a material for the heating element 4 and the heating element temperature measuring resistor 5, Pt which is a metal having a high specific resistance is used.
Ta may be used. Note that the material is not limited to these materials, and any material may be used as long as the specific resistance characteristics have temperature dependency. Next, the substrate 1 located upstream of the beam 3 and the moat 2
Above, a fluid temperature sensor (Rf) for measuring the temperature of fluid A
6 is provided. The fluid temperature measuring resistor 6 is provided on the surface of the substrate 1 which is thermally isolated from the heating element 4 and uses Pt or the like having a high temperature coefficient of resistance. The Pt layer preferably has a thickness of 500 to 5000 ° depending on the condition for sufficiently lowering the critical density and the condition for setting the resistance value.

Next, a room temperature calibrating resistor (Rc) 7 is wired on the downstream edge of the large-volume substrate 1 to be a heat sink which is thermally isolated from the heating element 4. This room temperature calibration resistor 7 is connected in series with the heating element 4 for correcting the flow velocity output signal on the substrate 1. The location of the room temperature calibration resistor 7 with respect to the flow of the fluid A is not particularly limited, but is preferably formed on the substrate 1 having a sufficient volume to allow a large heat capacity. Next, a Ta ₂ O ₅ layer is formed over the entire surface including the heating element 4, the heating element temperature measuring resistor 5, the fluid temperature measuring resistor 6, and the room temperature calibrating resistor 7 formed as described above. Through a protective layer (both not shown)
Are laminated. This Ta ₂ O ₅ layer may also serve as a protective layer.

In the above structure, the Ta ₂ O ₅ layer as the adhesion strength reinforcing layer is preferably as thin as possible.
_In the case of forming a protective layer made of ₂ (may be Si ₃ N ₄ ), 100 to 700 °
Good degree. Further, the protective layer is preferably thinner for the purpose of reducing the heat capacity and for the purpose of improving mass productivity in order to improve the sensitivity of the resistance temperature detector, but the protective layer is preferably thicker for the purpose. It is preferable that the film thickness be kept in the range of 800 to 5000 °. Further, the adhesion strength reinforcing layer and each resistor layer can be formed by a vacuum film forming method such as an evaporation method, an EB evaporation method, and a sputtering method. The shape can be cut out by a lift-off method, an Ar sputter etching method, or the like.

Further, the thermosensitive microbridge type current meter having the above-mentioned substrate structure is provided with the following various measuring circuits. That is, FIG. 2 shows only the wiring circuit of each resistor on the substrate 1 of FIG. A current measurement resistor (Ri) 8 for measuring the amount of current is provided outside the substrate.
Is connected in series with the heating element 4 and the room temperature calibration resistor 7. Further, a Wheatstone bridge circuit 8 as an electric bridge circuit is provided to make the temperature difference between the heating element 4 and the fluid temperature measuring resistor 6 constant. The Wheatstone bridge 8 includes a heating element temperature measuring resistor 5 and a fluid temperature measuring resistor 6.
And the resistors 10a and 10b for balance adjustment.

Further, as shown in FIG. 1, an electric measurement circuit 13 comprising a drive correction circuit 11 and an A / D conversion circuit 12
Are connected to the circuit of the substrate 1 of the present flowmeter. The electric measuring circuit 13 includes a voltage drop value Vh across the heating element 4 and a voltage drop value Vou across the heating element 4 and the room temperature calibration resistor 7.
t, a voltage drop value Vc across the room temperature calibration resistor 7 and a voltage drop value Vi across the current measurement resistor 8 are measured as needed, converted into digital signals and output. Further, such an electric measurement circuit 13 is connected to an arithmetic processing circuit 14 which performs arithmetic processing for removing an error due to a change in room temperature. The arithmetic processing circuit 14 includes a nonvolatile memory 15 as a digital memory for storing and holding digital signals.
Is attached.

In such a configuration, a calculation process for removing the room temperature fluctuation component from the flow velocity output of the flow velocity A will be described below. However, here, the heating element 4 will be described as Rh, the heating element temperature measuring resistor 5 will be Rs, the fluid temperature measuring resistor 6 will be Rf, the room temperature calibration resistor 7 will be Rc, and the current measuring resistor 8 will be Ri. .

In the circuit of FIG. 2, a calibration temperature point is set in advance and is set to To (for example, 20 ° C.). The calibration resistance value of Rc and Rh at To is measured in advance, and the value is calculated as Rco
And Rho. Since Rc and Rh are made of the same resistance material, they have the same temperature coefficient of resistance (α). When a drive voltage Vd is applied to the electric drive circuit of the sensor element, Rh, Rc and R
A current i flows through i. Then, at the time of driving, the room temperature Tr
Has changed from the correction point To by ΔTr. Assuming that the difference between the target room temperature and the temperature of the beam 3 is ΔT, the temperature rise ΔTh of Rh is as follows: ΔTh = ΔT + ΔTr (1) In this case, R is determined only by ΔT in equation (1).
It is an object of this embodiment to measure the output of h regardless of the magnitude of the room temperature fluctuation ΔTr. Then, R
The voltage drop value Vout at both ends of h and Rc and the voltage drop value Vc of only Rc are measured. Furthermore, in order to monitor the current values flowing through Rh and Rc, Ri having a very small temperature coefficient is used.
Are connected in series, the voltage drop value Vi is measured, and the current value i
Is calculated in advance. When the power is supplied, Rh increases the temperature by ΔTh as shown in the equation (1), and the value of Rh becomes Rh = Rho (1 + α · ΔTh) (2). Further, since Rc is thermally isolated from the heat generating portion, the temperature rises only in ΔTr, which is a change in room temperature from the calibration point, and the value of Rc is: ). Here, Rho and Rco are resistance values at the calibration temperature Tc, which are measured in advance and are constants. α is R
Temperature coefficients of h and Rc. In this case, since Rc is connected in series with Rh, heat generation is likely to occur in the substrate 1 on which Rc is disposed, but this is likely to be a problem. Acting as a heat sink against heat generation, the substrate 1 actually has only a very small temperature rise. Furthermore, since the measurement is performed at the time when the thermal steady state is reached, there is no problem since the rise in the substrate temperature due to the Rc self-heating is included in the so-called ΔTr.

On the other hand, the voltage drop value of each resistor is as follows: Vh = i · Rh (4) Vc = i · Rc (5) The temperature rise obtained by removing the fluctuation of the room temperature from the equation (1) is as follows: ΔT = ΔTh−ΔTr (6) True voltage drop V of Rh from which the fluctuation component has been removed
An output signal that is true is as follows: Vtrue = i · {Rho (1 + α · ΔT)} (7) From the equations (2) to (5), the temperature change in each resistor is as follows: ΔTh = (1 / α) ｛Vh / (i · Rho) −1｝ (8) ΔTr = (1 / α) ｛Vc / (I · Rco) −1｝ (9) By substituting the equations (6), (8), and (9) into the equation (7), the output signal from which the room temperature fluctuation component has been removed is Vtrue = Vh + i · Rho−Vc · Rho / Rco (10)

In this embodiment, Rho and Rco in the equation (10) are measured in advance, and the values are stored in the nonvolatile memory 15. And Vh (or Vou
t), Vc, Vi, the three voltage drop values,
In step 1, detection is performed as needed. Next, the analog voltage drop value thus detected is converted into a digital value by A / D conversion using the A / D conversion circuit 12, and then, based on the equation (10) in the arithmetic processing circuit 14. Perform digital arithmetic processing. At the time of this arithmetic processing, the calibrated signal intensity is converted into a flow velocity value using an interpolation function or an interpolation reference data table stored in the nonvolatile memory 15 in advance.

Therefore, by performing such a series of processes, it is possible to detect an accurate flow rate without the influence of room temperature fluctuation. As described above, in the present embodiment, the error due to the fluctuation in the room temperature can be automatically removed at any time, so that the flow velocity can be detected with high accuracy. Also,
In the substrate structure as shown in FIG. 1, two resistors Rc and Ri increase as compared with the conventional substrate structure. However, Rc can be formed on the substrate 1 on which the elements are provided by the same resistor pattern.
Ri only increases by one as an external component,
Since the configuration of the entire electric circuit is not complicated, there is no influence on productivity.

Next, an embodiment of the present invention will be described. The description of the same parts as those of the first aspect is omitted, and the same reference numerals are used for the same parts.

In the present embodiment, the internal configurations of the electric measurement circuit 13 and the arithmetic processing circuit 14 in FIG. 1 are changed. That is, the electrical measurement circuit 13 includes a voltage drop value Vh across the heating element (Rh) 4, a voltage drop value Vc across the room temperature calibration resistor (Rc) 7, and a current measurement resistor (Ri) 8. , And each voltage drop value with respect to the voltage drop value Vi at both ends is measured at any time and output as a digital signal. Further, the arithmetic processing circuit 14 performs arithmetic processing not only for the removal of the error due to the room temperature change ΔTr, but also for the removal of the error due to the change in the temperature coefficient of the resistor due to thermal fatigue caused by long-term use.

In such a configuration, an arithmetic process for removing both the room temperature fluctuation component and the fluctuation component due to the temporal change of the heating element resistance temperature coefficient from the flow velocity output will be described below. When the current meter is used for a long period of time, Rh causes thermal fatigue deterioration, and causes a gradual change in the temperature coefficient of resistance α. Therefore, the element is now subjected to a sufficient aging process before being used as a current meter, and the temperature coefficient of resistance when stabilized by this is set to α ₀ and stored in the nonvolatile memory 15. In actual use, since the temperature coefficient of resistance fluctuates almost linearly, it approximates α = β · α ₀ １ (1 + x) α ₀ (11).

Here, it should be noted that Rh heated to a high temperature is susceptible to fatigue, and Rc formed on the heat sink is less susceptible to fatigue. From this, Rh in equation (2)
Is as follows: Rh = Rho (1 + β · α ₀ · ΔTh) (12), but Rc in the equation (3) is not affected. Similarly, equation (7) also, Vtrue = i · Rho (1 + β · α 0 · ΔTh) ... (13) , and the flow rate output to remove room temperature fluctuation component to be thereby determined is, Vtrue = Vh + β · i · Rho−β · Vc · Rho / Rco (14)

Next, the value of β is determined. Vh, Vc is, because it is a Vh = i · Rho (1 + β · α 0 · ΔTh) ... (15) Vc = i · Rco (1 + β · α 0 · ΔTh) ... (16), and re-organize here, the _{β · α 0 · ΔTh = Vh} / (i · Rho) -1 ... (17) α 0 · ΔTr = Vc / (i · Rco) -1 ... (18). Then, when the difference between (17) and (18) is taken, α ₀ (β · Th−ΔTr) = Vh / (i · Rho) −Vc / (i · Rco) (19)

Here, since β ≒ (1 + x), equation (14) of the flow velocity output can be solved by obtaining x. Then, when Expression (19) is expanded for x,

[0026]

(Equation 1)

The following equation can be obtained. In this case, since the value of the measured value Vc is known, the value of ΔTr can be calculated from equation (9). Further, since the coefficient in the equation (20) is a constant measured in advance, a value measured as needed, or a value obtained from an arbitrary calculation process, x which is a fluctuation component of the resistance temperature coefficient should be obtained as needed. Can be.
By substituting the value of x into β in equation (14),
In addition to eliminating errors due to changes in the temperature coefficient of resistance over time,
Flow velocity measurement from which the room temperature fluctuation component ΔTr has been removed can be performed.

Therefore, in this embodiment, the error component ΔTr due to room temperature fluctuation can be removed from the flow velocity output, and the fluctuation component x due to the temporal change of the resistance temperature coefficient of Rh during long-term use is automatically removed as needed. It is possible,
Thereby, the flow velocity can be detected with high accuracy for a long period of time.

[0029]

According to the first aspect of the present invention, a moat is formed by etching a substrate, a beam is formed so as to straddle over the moat, and a heating element and a temperature of the heating element are formed on the beam. In a heat-sensitive micro-bridge flowmeter provided with a heating element and a resistance temperature sensor for measuring the temperature of a fluid on the substrate, the correction of the flow velocity output signal is performed on the substrate. Providing a resistor for room temperature calibration connected in series with the heating element to be performed, and providing a resistor for current measurement serially connected to the heating element and the resistor for room temperature calibration to measure the amount of current outside the substrate, In order to keep the temperature difference between the heating element and the fluid temperature measuring resistor constant, an electric bridge circuit is provided in which the heating element temperature measuring resistor, the fluid temperature measuring resistor and the balance adjusting resistor are connected. The voltage drop across the heating element and the heat generation The voltage drop values at both ends of the room temperature calibration resistor, the voltage drop values at both ends of the room temperature calibration resistor, and the voltage drop values at both ends of the current measurement resistor are measured as needed and are digital signals. An electric measurement circuit for outputting the digital signal is provided, a digital memory for storing and holding the digital signal is provided, and an arithmetic processing circuit for performing an arithmetic processing for removing an error due to a change in room temperature is provided. Thus, the flow velocity can be detected with high accuracy.

According to the second aspect of the present invention, a moat is formed by etching a substrate, a beam is formed so as to straddle over the moat, and a heating element and a temperature of the heating element are measured on the beam. A heat-sensitive microbridge flowmeter provided with a heating element and a fluid temperature measuring resistor for measuring the temperature of a fluid on the substrate, wherein the flow rate output signal is corrected on the substrate. A room temperature calibration resistor connected in series with a heating element is provided, and the heating element for measuring the amount of current and a current measurement resistor connected in series to the room temperature calibration resistor are provided outside the substrate. An electric bridge circuit in which the heating element temperature measuring resistor, the fluid temperature measuring resistor, and the balance adjusting resistor are connected to make a temperature difference between the body and the fluid temperature measuring resistor constant; The voltage drop across the heating element and the room temperature A digital memory for storing an electric measurement circuit for measuring each voltage drop value of a voltage drop value between both ends of the resistor for use and a voltage drop value between both ends of the resistor for current measurement and outputting the digital signal as a digital signal, and storing and holding the digital signal And an arithmetic processing circuit that removes errors caused by changes in room temperature and errors caused by changes in the temperature coefficient of the resistor due to thermal fatigue caused by long-term use. Be able to
A fluctuation component due to a temporal change in the resistance temperature coefficient of the heating element during long-term use can be automatically removed as needed, and thus, the flow velocity can be detected with higher accuracy for a long time.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing a heat-sensitive microbridge type flow meter according to an embodiment of the present invention.

FIG. 2 is a circuit diagram showing an electric circuit configuration formed by connecting various resistors.

[Explanation of symbols]

 Reference Signs List 1 board 2 moat 3 beam 4 heating element 5 heating element temperature measuring resistor 6 fluid temperature measuring resistor 7 room temperature calibration resistor 8 current measurement resistor 9 electric bridge circuit 10a, 10b balance adjustment resistor 13 electric measurement circuit 14 arithmetic processing circuit 15 digital memory

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-61-235726 (JP, A) JP-A-63-201529 (JP, A) JP-A-2-281108 (JP, A) JP-A-4- 332867 (JP, A) JP-A-1-296111 (JP, A) JP-A-3-500687 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01F 1/68-1 / 699

Claims (2)

(57) [Claims] A moat is formed by etching a substrate, a beam is formed so as to straddle over the moat, and a heating element and a heating element temperature measuring resistor for measuring the temperature of the heating element on the beam. In a heat-sensitive micro-bridge type flow meter provided with a fluid temperature measuring resistor for measuring the temperature of a fluid on the substrate, connected in series with the heating element for correcting a flow velocity output signal on the substrate. Provide a room temperature calibration resistor, provided a heating element and a current measurement resistor connected in series to the room temperature calibration resistor to measure the amount of current outside the substrate,
In order to keep the temperature difference between the heating element and the fluid temperature measuring resistor constant, an electric bridge circuit is provided in which the heating element temperature measuring resistor, the fluid temperature measuring resistor and the balance adjusting resistor are connected. A voltage drop value at both ends of the heating element, a voltage drop value at both ends of the heating element and the room temperature calibration resistor, a voltage drop value at both ends of the room temperature calibration resistor, and a voltage drop value at both ends of the current measurement resistor. An electrical measurement circuit that measures each voltage drop value at any time and outputs it as a digital signal, a digital memory that stores and holds the digital signal is provided, and an arithmetic processing circuit that performs arithmetic processing for removing an error due to a change in room temperature is provided. A heat-sensitive micro-bridge type flow meter characterized by the following. 2. A moat is formed by etching a substrate, a beam is formed so as to straddle over the moat, and a heating element and a heating element temperature measuring resistor for measuring the temperature of the heating element on the beam. In a heat-sensitive micro-bridge type flow meter provided with a fluid temperature measuring resistor for measuring the temperature of a fluid on the substrate, connected in series with the heating element for correcting a flow velocity output signal on the substrate. Provide a room temperature calibration resistor, provided a heating element and a current measurement resistor connected in series to the room temperature calibration resistor to measure the amount of current outside the substrate,
In order to keep the temperature difference between the heating element and the fluid temperature measuring resistor constant, an electric bridge circuit is provided in which the heating element temperature measuring resistor, the fluid temperature measuring resistor and the balance adjusting resistor are connected. Electrical measurement for measuring the voltage drop values at both ends of the heating element, the voltage drop values at both ends of the room temperature calibration resistor, and the voltage drop values at both ends of the current measurement resistor at any time, and outputting them as digital signals. A digital memory for storing and holding the digital signal; and an arithmetic processing circuit for performing an arithmetic processing for removing an error due to a change in room temperature and an error due to a change in a temperature coefficient of the resistor due to thermal fatigue resulting from long-term use. A heat-sensitive micro-bridge type flow meter characterized by the following.