JPH0584867B2 - - Google Patents

Description

[Detailed description of the invention]

<Industrial Application Field> The present invention relates to a flow sensor that can detect the two-dimensional direction and flow velocity of fluid flow. <Prior Art and its Problems> As a conventionally proposed thermal type flow sensor capable of detecting direction, there is a semiconductor type flow velocity detector described in Japanese Patent Application Laid-Open No. 60-247169. This flow sensor uses silicon for the substrate, so
Heat conduction is very good, and the temperature difference within the chip caused by the gas flow is about 1/100 to 1/10 degrees Celsius (Electronic Materials, December 1983 issue, p. 38-43). Therefore, the only way to accurately determine this temperature difference is to assemble pairs of highly sensitive temperature measuring transistors in a bridge and detect deviations from the equilibrium state. An example of a detection circuit for a silicon flow sensor is shown in FIG. This flow sensor uses silicon process technology and is therefore excellent in mass production. However, it has drawbacks such as large variations in temperature characteristics between elements and the inability to set a high heat generation temperature. <Object of the Invention> The present invention has been made in order to eliminate the above-mentioned drawbacks, and provides a flow sensor that utilizes a semiconductor process, has excellent mass productivity and homogeneity, and is capable of stable operation even at high temperatures. The purpose is to <Summary of the Invention> The present invention provides a flow sensor in which a heat-generating resistor is placed in the center of a thermally insulated substrate and temperature-measuring resistors are installed at symmetrical positions on both sides of the heat-generating resistor. The flow rate of the fluid is controlled so that it is maintained at a constant temperature higher than the temperature of the fluid, and the flow rate of the fluid is detected by the current flowing through the heating resistor, which changes according to the flow rate of the fluid, or the electric potential or voltage, which changes in response to the current. At the same time, the temperature difference generated between the two temperature measuring resistors is detected as a current difference or a voltage difference, and the flow direction is detected. The present invention makes it possible to sufficiently increase the temperature difference within the chip by using a thermally insulating substrate, such as a glass substrate, with a thermal conductivity of 2w/mk or less. It has the feature of being able to increase the temperature difference between symmetrical positions that sandwich the resistor. Since glass has a sufficiently low thermal conductivity of 1/130 compared to silicon, it is possible to make the temperature gradient sufficiently large within a small chip, but this will be explained using a model. Consider the case where heat is constantly transmitted along a thin round rod or fin as shown in Figure 3. Let T ₁ be the temperature at the base of the fin, T be the temperature at a point x apart, and T ₀ be the ambient temperature. Assume that heat moves from the surface of the fins by convection, and that the heat flux lost per unit area is given by the following equation: q=h(T-T ₀ )...(1) For further simplification, the local heat transfer coefficient h
is assumed to be constant over the entire surface of the fin. Let the cross-sectional area of the fin be a and the circumference length be p. Temperature changes in the cross section are ignored. That is, T=
Assume that f(x). In FIG. 3, consider an element of length dx. In this case, the amount of heat lost by conduction = (dQ/dx) dx = -ak (d ² T/dx ² ) dx The amount of heat lost from the surface by convection = h (T-
T ₀ ) pdx From the heat balance, akd ² T/dx ² dx−h(T−T ₀ )pdx=0 or d ² T/dx ² −hp/ak(T−T ₀ )=0 ……(2) Therefore, by substituting θ=T−T ₀ , equation (2) becomes d ² θ/dx ² −λ ² θ=0 (3). Here λ=√. The solution to equation (3) is θ=Acosh〓x+Bsinh〓x...(4). The boundary conditions are x=0 and θ=θ ₁ ∴A=θ ₁ x=L and d〓/dx=0 (assuming that heat does not escape from the edge of the fin), so〓Asinh〓L+〓Bcosh〓L= 0 Therefore, since B=-θ ₁ sinh〓L/cosh〓L, the temperature distribution is given by the following equation.

[C] The specific shape of Figure 3 is shown in Figure 4.
1.10wm ^-1 k ^-1 ), silicon (thermal conductivity: 148wm ^-1
Figure 5 shows the results of calculations using the case (k ^-1 ) as an example.
In Figure 5, the temperature at the base of the fin is T ₁ = 100℃,
Ambient temperature T ₀ = 25℃, heat transfer coefficient h = 10kcal/
The calculation is performed as (hr・m ²・deg). As can be seen from Figure 5, by using a thermal insulator such as glass for the substrate, a sufficiently large temperature difference can be created within a small chip, and the heat generated within the chip due to the flow of fluid can be reduced. It is possible to set a large temperature difference between symmetrical positions across the resistor. Furthermore, among the materials used for heat generating resistors, heat generating resistor temperature monitors, and temperature measuring resistors, noble metals such as platinum have somewhat weak adhesion to glass, which poses problems in terms of reliability. If you use a glass substrate coated with alumina,
Sufficiently strong adhesion can be provided between the substrate and the heat generating resistor, the heat generating resistor temperature monitor, and the temperature measuring resistor. As described above, an object of the present invention is to provide a highly reliable flow sensor in which a heat generating resistor, a heat generating resistor temperature monitor, and a temperature measuring resistor are integrated into the same element. . <Example> FIGS. 1A and 1B are a schematic plan view and a cross-sectional view of a flow sensor showing an example of the present invention. An alumina thin film 2 is deposited on a glass substrate 1 by appropriately using a thin film forming technique such as a vacuum evaporation method, a sputtering method, or a plasma CVD method. A metal thin film having a large temperature coefficient of resistance, such as platinum, is similarly deposited on the alumina thin film 2 by vacuum evaporation, sputtering, plasma CVD, etc., and then patterned by etching technology to form a heat generating resistor 3a, a heat generating resistor 3a, etc. A resistor temperature monitor 3b and a pair of temperature measuring resistors 4a, 4b are arranged at an appropriate distance apart, as shown in FIG. 1A. Temperature measurement resistor 4
a and 4b are placed in symmetrical positions with respect to the heating resistor 3a. Next, the glass substrate 1 is cut into a set of the heating resistor 3a, the heating resistor temperature monitor 3b, and the temperature measuring resistors 4a and 4b to form individual sensor elements. The obtained sensor element has a glass substrate 1, a heat generating resistor 3a, a heat generating resistor temperature monitor 3b, a temperature measuring resistor 4a,
4b is arranged. Moreover, the size of the sensor elements is minute, on the order of several millimeters, and sensor elements with uniform characteristics can be mass-produced by performing so-called wafer processing, in which a large number of sensor elements are simultaneously fabricated on one substrate. The obtained sensor element is adhered to a support stand (not shown), and the leads of the heat generating resistor 3a, the heat generating resistor temperature monitor 3b, and the temperature measuring resistors 4a and 4b are connected to form the flow sensor of this embodiment. shall be. As the material for the metal thin film, nickel or a nickel alloy having a large resistance temperature coefficient other than platinum, or a temperature-sensitive resistor material such as a thermistor may be used instead of the metal thin film. FIG. 6 shows a schematic circuit diagram of a flow sensor using the heat generating resistor 3a, the heat generating resistor temperature monitor 3b, and the temperature measuring resistors 4a and 4b manufactured in this way. A heat generating resistor 3a, a heat generating resistor temperature monitor 3b, temperature measuring resistors 4a and 4b, and a fluid temperature compensating resistor 5 manufactured by the above manufacturing method are placed in a flow path (not shown) through which the fluid passes. It will be installed. The fluid temperature compensating resistor 5 and the heat generating resistor temperature monitor 3b are connected to other electrical resistors 6 and 7, respectively, to form a bridge A. An intermediate connection point between the fluid temperature compensating resistor 5 and the heat generating resistor temperature monitor 3b is grounded. These bridges A are connected to a feedback circuit that differentially amplifies the voltage difference between the bridge resistors with an amplifier 8, controls the base potential of the switching transistor 9, and drives the transistor 9, and controls the voltage of the heating resistor 3a. are doing.
The temperature measuring resistors 4a and 4b are connected to other constant current sources 10 and 11, respectively, to form a bridge B. An intermediate connection point between the temperature measuring resistors 4a and 4b is grounded. FIG. 7 shows the flow velocity-output characteristic of the voltage V _h of the heating resistor 3a which changes depending on the flow velocity v _f . In the bridge A, the heating resistor 3a is maintained at a constant temperature higher than the temperature of the fluid by the electric resistor 12. If the fluid velocity is high, heat generating resistor 3
A large amount of heat is removed from a. Conversely, when the velocity of the fluid is slow, the amount of heat taken away from the heat generating resistor 3a is also small. Therefore, the temperature of the fluid is measured by the fluid temperature compensating resistor 5, and the current value flowing through the heat generating resistor 3a is controlled via the feedback circuit so as to keep the temperature difference between the fluid temperature and the heat generating resistor 3a constant. Then, find the current (voltage) value corresponding to the fluid flow rate (flow rate). FIG. 8 shows the relationship between the fluid flow direction θ and the temperature difference between the temperature measuring resistors 4a and 4b in the bridge circuit B.
It is calculated as the voltage difference due to . In the bridge circuit B, the temperature measuring resistors 4a and 4b are connected to constant current sources 10 and 11, respectively, and convert changes in the temperature distribution within the substrate caused by changes in the flow direction of the fluid into voltage changes, Output as voltage difference
Find V _D. In this embodiment, the flow velocity of the fluid is determined by a bridge circuit using a heat generating resistor temperature monitor 3b and a fluid temperature compensating resistor 5, but the heat generating resistor temperature monitor 3b is omitted and the heat generating resistor 3a is omitted. The fluid flow velocity may be determined by a bridge circuit using a fluid temperature compensating resistor 5 and a fluid temperature compensating resistor 5. <Effects of the Invention> As described above, the flow sensor of the present invention has a pattern near the center of the surface in contact with the fluid to be measured of the substrate made of a thermally insulating material with a thermal conductivity of 2 w/m·k or less. A heating resistor made of a formed metal thin film, and a patterned metal thin film formed on the surface of the substrate in contact with the fluid to be measured at substantially symmetrical positions on both sides of the heating resistor. and a temperature measuring resistor, and detects the flow velocity of the fluid to be measured by detecting the energy necessary to maintain the temperature of the heat generating resistor higher than the temperature of the fluid to be measured by a predetermined temperature. At the same time, the flow direction of the fluid to be measured is detected by detecting the temperature difference that occurs between the temperature measurement resistors at this time, so a sufficiently large temperature difference can be detected within the small chip. As a result, the direction of fluid flow can be detected with high precision within a smaller sensor chip than in the past.

[Brief explanation of the drawing]

FIG. 1 is a schematic plan view and a sectional view of a flow sensor showing one embodiment of the present invention. FIG. 2 is a detection circuit diagram of a conventional silicon flow sensor. Third
4 and 4 are explanatory diagrams for explaining the principle of the thermal flow sensor. FIG. 5 is an explanatory diagram showing the results of calculating the temperature of each part in FIG. 4. FIG. 6 is a schematic circuit diagram of the flow sensor shown in FIG. 1. FIG. 7 is a flow rate vs. output characteristic diagram of the flow sensor shown in FIG. 1. FIG. 8 is a characteristic diagram obtained by determining the fluid flow direction as a voltage difference. 1...Glass substrate, 2...Alumina thin film, 3a
...Heating resistor, 3b...Heating resistor temperature monitor, 4a, 4b...Resistor for temperature measurement.

Claims (1)

[Claims] 1. A substrate made of a thermally insulating material with a thermal conductivity of 2 w/m·k or less, and a metal thin film formed in a pattern near the center of the surface of the substrate that is in contact with the fluid to be measured. a temperature measuring resistor comprising a metal thin film formed in a pattern on the surface of the substrate in contact with the fluid to be measured at substantially symmetrical positions on both sides of the heating resistor; Detecting the flow velocity of the fluid to be measured by detecting the energy necessary to maintain the temperature of the heating resistor higher than the temperature of the fluid to be measured by a predetermined temperature, and at this time, A flow sensor characterized in that a flow direction of a fluid to be measured is detected by detecting a temperature difference occurring between the temperature measuring resistors.