Measuring and testing – Fluid pressure gauge – Diaphragm
Patent
1991-06-10
1993-03-09
Woodiel, Donald O.
Measuring and testing
Fluid pressure gauge
Diaphragm
73708, 73721, 338 4, 338 42, G01L 708, G01L 906
Patent
active
051917980
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a pressure sensor using a strain gauge, and, more particularly, to a thin film pressure sensor constituted in such a manner that a diaphragm is made of a metal such as stainless steel and a strain gauge made of a semiconductor thin film such as silicone thin film or the like is formed on the diaphragm via an insulating film.
BACKGROUND OF THE INVENTION
There has been a variety of sensors in which a semiconductor strain gauge is employed, the semiconductor strain gauge utilizing a piezo electric effect characterized in that a large resistance change is displayed when strain is applied to a semiconductor. For example, there has been a thin film pressure sensor constituted in such a manner that a diaphragm is made of a metal such as stainless steel and a strain gauge made of a semiconductor thin film such as silicone thin film or the like is formed on the diaphragm via an insulating film.
As shown in FIG. 1A which is a cross sectional view, the thin film pressure sensor is structured in such a manner that a sensor portion 5 is, via a silicon oxide (SiO.sub.2) film 2 serving as an insulating film, formed on the surface of a diaphragm 1 made of stainless steel. The sensor portion 5 comprises: a strain gauge 3 constituted by polycrystalline silicone layer patterns formed on the SiO.sub.2 film 2; and an electrode 4 constituted by aluminum layer patterns and for supplying power to the strain gauge 3, the sensor portion 5 being covered with a passivation film 6 made of a silicone nitride layer. As shown in FIG. 1B, the sensor portion 5 is constituted by four patterns R1 to R4 forming the strain gauge 3 and six circuit patterns E1 to E6 for forming the electrode 4, the electrode 4 serving to supply power to the strain gauge 3. As shown in FIG. 1C, the sensor portion 5 forms a bridge circuit when it is expressed as an equivalent circuit so that the pressure can be measured by detecting the change in voltage between the electrode circuit patterns E2 and E5 due to the change in the level of the resistance of the strain gauge 3 when pressure is applied to the thin film pressure sensor.
However, the semiconductor has a disadvantage in that its characteristics excessively depend upon temperature. However, it has an excellent repetitive reproductionality against temperature. The characteristics of this type raise the reliability of the device after the compensation has been performed.
In the case of a semiconductor thin film pressure sensor, the resistance of the strain gauge changes in accordance with temperature as well as in accordance with the specific resistance of the strain gauge and the resistance change due to pressure. Therefore, both the pressure sensitivity and the zero point in a bridge circuit formed by combining strain gauges are inevitably changed. Since accuracy of the pressure sensor depends upon the way of performing the temperature compensation, a variety of methods have been attempted.
FIG. 2 illustrates an equivalent circuit acting in accordance with a method of compensating the temperature change of the strain gauge by using a temperature compensating circuit with a pressure sensor arranged to be operated by a rated voltage, the temperature compensating circuit being formed by combining a transistor and resistors.
The temperature compensating circuit 7 comprises an NPN-type transistor Tr and resistors R5 and R6 connected between the emitter of the transistor Tr and the base of the same and between the collector of the transistor Tr and the base of the same, the emitter of the transistor Tr being connected to the contact between electrode circuit patterns E4 and E6. The collector of the transistor Tr is connected to the positive side of a power supply source Vin via a circuit pattern E8 (omitted from FIG. 1).
The temperature compensating circuit 7 of the type described above is, as shown in FIG. 3, disposed outside the thin pressure sensor. That is, a thin film pressure sensor 100 is included in a case 101, and is included, together With the
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Inagaki Hiroshi
Kobayashi Yukio
Suzuki Noritake
Tabata Aki
Tajika Jun
Kabushiki Kaisha Komatsu Seisakusho
Woodiel Donald O.
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