GB2177212A

GB2177212A - Flow sensor

Info

Publication number: GB2177212A
Application number: GB08613053A
Authority: GB
Inventors: Junich Tanaka
Original assignee: Sharp Corp
Current assignee: Sharp Corp
Priority date: 1985-05-30
Filing date: 1986-05-29
Publication date: 1987-01-14
Also published as: DE3617770A1; DE3617770C2; JPS61274222A; GB8613053D0

Abstract

A thermal type flow sensor comprises a heating resistor (4) for detecting changes in the amount of heat transferred from the heating resistor placed within a fluid to the fluid. The heating resistor is composed of a patterned thin metal film on an insulating plate (1). The metal is preferably platinum and the substrate (1) may be alumina. A fluid temperature sensing resistor 3 also in film form may be provided to copensate for fluctuation of the sensor output due to changes in the temperature of the fluid. <IMAGE>

Description

SPECIFICATION Flow sensor This invention relates to a thermal type flow sensor comprising a heating resistor and, as desired, a fluid temperature sensing resistor, both of which are composed of a fine patterned platinum thin-film on an insulating plate.

A flow meter in which a main flow tube is provided with a by-pass tube through which a portion of the main flow is directed and to which a heater is connected is widely used as a thermal type flow sensor. This flow meter detects the flow rate from the distribution of heat that arises when fluid flows through the by-pass tube while the by-pass tube is heated. Such a flow meter has such excellent accuracy that it is widely used for the control of the flow rate of semiconductor gases, etc., but it is not suited for miniaturiztion and/or mass production. Moreover, its production cost is so high that it can only be applied to a limited field.

Another thermal type flow sensor comprises a heating resistor and a fluid temperature sensing resistor, by which the flow rate is calculated based on changes in the amount of heat transferred from the heating resistor to the fluid surrounding this heating resistor. Taking into account the temperature of the fluid detected by the fluid temperature sensing resistor, the difference in temperature between the fluid and the heating resistor is maintained at a fixed value so that compensation for changes in the fluid temperature can be made and a quick response can be obtained regardless of the heat capacity of the heating resistor. Moreover, when the difference in temperature mentioned above is set at a large value, it is possible to enhance the output of the flow sensor. A wire of platinum, tungsten, or the like is used as the heating resistor and as the fluid temperature sensing resistor.However, the resistance of the wire is small and the resistance from one flow sensor to another varies greatly, so that adjustability of the heating temperature and accuracy of the temperature measurement are poor. Moreover, thin wire is used, so that manufacture is difficult and mass production is not possible.

There is also a flow sensor using a heating transistor and a fluid temperature sensing transistor both of which are buiit on a silicon chip. This flow sensor detects the flow rate in the same manner as that mentioned above.

This flow sensor is manufactured by the use of a silicon processing technique, so that it is readily mass-produced, but the temperature characteristics of the sensor are various, resulting in difficulties of the establishment of a high heating temperature.

Notwithstanding that thermal type flow sensors have an enlarged application in such fields as the control of the flow rate of semiconductor gases, the control of the mixture of a number of gases, the control of the flow rate in internal-combustion engines, and measuring wind velocity in air-conditioning systems, they have not yet been put into practical use due to the above-mentioned problems.

A thermal type flow sensor according to this invention comprises a heating resistor for detecting changes in the amount of heat transferred from the heating resistor placed within a fluid to the fluid, said heating resistor being composed of a patterned thin metal film on an insulating plate.

In a preferred embodiment, the thermal type flow sensor further comprises a fluid temperature sensing resistor composed of a patterned thin metal film on an insulating plate, whereby compensation for fluctuation in the output of said sensor due to changes in the temperature of said fluid can be attained.

The thin metal film is, in a preferred embodiment, made of platinum.

Thus, the invention described herein makes possible one or more of the objects of (1) providing a thermal type flow sensor which can be mass-produced with uniform sensor characteristics using a process for the production of semiconductor devices; (2) providing a thermal type flow sensor which attains stabilized operation even at a high temperature; (3) providing a thermal type flow sensor which can be produced at low cost; (4) providing a thermal type flow sensor which can be miniaturized; (5) providing a thermal type flow sensor which consumes less electrical power; (6) providing a thermal type flow sensor which can be used for the measurement of various fluids, in a case wherein platinum is used for the patterned thin metal film on an insulating plate; and (7) providing a thermal type flow sensor in which, in the case wherein platinum is used for a patterned thin metal film on an insulating plate, the difference in temperature between the fluid and the heating resistor can be set at a large value, resulting in the enhancement of the output voltage of the sensor.

The invention is further described, by way of example, with reference to the accompanying drawings, in which: Figs. 1(A) and 1(B) are respectively, a plan view and a sectional view showing the heating resistor and/or the fluid temperature sensing resistor of a thermal type flow sensor of this invention, and Fig.2 is a schematic diagram showing use of a thermal type flow sensor of this invention.

Figs. 1(A) and 1(B) show a heater and/or a temperature sensor of this invention, which are produced as follows: A thin platinum film is formed on an alumina substrate 1 by the sputtering method, followed by annealing to stabilize the resistability thereof. Then, on the thin platinum film, a resist is formed into a given pattern as a mask in a succeeding etch ing process by a photolithography technique.

The thin platinum film is then subjected to an etching treatment using the sputtering etching method. After etching the resist is removed by the oxygen plasma etching method, resulting in a meandering patterned resistor path 2 of the thin platinum film having a fixed resistance. In this manner, a desired heating resistor and a desired fluid temperature sensing resistor can be obtained.

A glass substrate, a semiconductor substrate prepared by the formation of an insulating film, such as an aluminium oxide film or a silicon oxide film on a silicon wafer, etc., can be used for the substrate of the sensor of this invention. The thin platinum film can be also produced by deposition techniques other than the sputtering method, a printing method, etc.

The etching and the resist removal can be effected by a wet process.

Fig.2 shows a flow sensor using a heating resistor 4 and a fluid temperature sensing resistor 3, each made as described above with reference to Figs. 1A and 1B. The fluid temperature sensing resistor 3 and the heating resistor 4 as a set of resistors are placed within a flow path 7 through which fluid flows in the direction of the arrow. The fluid temperature sensing resistor 3 is positioned upstream of the heating resistor 4. The fluid temperature sensing resistor 3 and the heating resistor 4 are connected to electrical resistor elements 5 and 6, respectively, resulting in a bridge circuit. The connection point between the electrical resistor elements 5 and 6 is earthed.The bridge circuit is connected to a feedback circuit in which the difference in potential between one resistance bridge (composed of the fluid temperature sensing resistor 3 and the electrical resistor element 5) and the other resistance bridge (composed of the heating resistor 4 and the electrical resistor element 6) is amplified by a differential amplifier 8 to control the base potential of a transistor 9, in which the emitter terminal is commonly connected to the fluid temperature sensing resistor 3 and the heating resistor 4, allowing the transistor 9 to be driven.Both the fluid temperature sensing resistor 3 and the heating resistor 4 are controlled by the feedback circuit such that the difference in temperature between the resistor 3 and resistor 4 can be maintained at a fixed level regardless of changes in the temperature of the fluid such as an oil, a chemical reagent, a gas, etc., which flows through the path 7.

When the transistor 9 is switched on, current is supplied from an input terminal 10 to the heating resistor 4, resulting in heat generation in the heating resistor 4. When the flow rate of the fluid flowing through the path 7 is high, a large amount of heat is transferred from the heating resistor 4 to the fluid. On the contrary, when the flow rate of the fluid is low, a small amount of heat is transferred from the heating resistor 4 to the fluid.

Thus, the flow rate of the fluid can be determined by detecting changes in the amount of heat transferred from the heating resistor to the fluid in the following manner: Current that is applied to the heating resistor 4 is maintained at a fixed level and the temperature of the heating resistor 4 is measured while the fluid flows. Alternatively, the difference in temperature between the fluid and the heating resistor 4 is maintained at a fixed level by the control of current that is applied to the heating resistor 4, and the flow rate of the fluid can be calculated from changes in the current.

This example is of the latter mode in which the difference in temperature between the fluid and the heating resistor 4 is maintained at a fixed level. Although this example utilises a bridge circuit incorporating both the fluid temperature sensing resistor 3 and the heating resistor 4 therein in order to maintain the temperature difference between the fluid and the heating resistor 4 at a fixed level, the fluid temperature sensing resistor 3 is not essential. In the case wherein the fluid temperature sensing resistor is not incorporated or the temperature of the fluid hardly varies, a temperature detector such as a thermocoupler, etc., is connected to the heating resistor and current can be controlled by the control of the switching operation of the transistor 9 such that the temperature of the heating resistor 4 is maintained at a fixed level.

In the example shown in Fig.2, in order to make it possible to determine the flow rate even when the temperature of the fluid varies, the fluid temperature sensing resistor 3 is disposed upstream of the heating resistor 4 resulting in a bridge circuit, whereby the temperature of the fluid is measured, and current supplied to the heating resistor 4 is controlled by the feedback circuit such that the difference in temperature between the fluid and the heating resistor 4 can be maintained at a fixed level. The said temperature difference therebetween is maintained at a fixed level in the above-mentioned manner, so that a quick response to changes in the flow rate can be obtained regardless of the heat capacity of the heating resistor 4. When the temperature difference is established at a large value, the output of the flow sensor can be increased.

In a preferred embodiment the temperature sensing resistor 3 has a substantially greater resistance than the flow sensing resistor 4.

For example, the resistor 3 may have a resistance of about 1 KOhm and the resistor 4 a resistance of about 300 Ohms. The resistors 5 and 6 may then have resistances of about 50 KOhm and 15 KOhm, respectively. However, the temperature and flow sensing resistors do not have to have different resistances.

It is understood that various other modifications can be readily made without departing from the scope of this invention. Accordingly, the scope of the claims is not limited to the description as set forth herein, but rather that the claims be construed as encompassing all the features of patentable novelty that reside in the present invention, including all features that would be treated as equivalents thereof by those skilled in the art to which this invention pertains.

Claims

1. A thermal type flow sensor comprising a heating resistor for detecting changes in the amount of heat transferred from the heating resistor placed within a fluid to the fluid, said heating resistor being composed of a patterned thin metal film on an insulating plate.

2. A thermal type flow sensor according to claim 1, which further comprises a fluid temperature sensing resistor composed of a patterned thin metal film on an insulating plate, whereby compensation for fluctuation in the output of said sensor due to changes in the temperature of said fluid can be attained.

3. A thermal type flow sensor according to claim 1 or 2, wherein said thin metal film is made of platinum.

4. A thermal type flow sensor, constructed and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.