Measuring and testing – Fluid pressure gauge – Electrical
Reexamination Certificate
2000-09-21
2002-07-23
Oen, William (Department: 2855)
Measuring and testing
Fluid pressure gauge
Electrical
Reexamination Certificate
active
06422088
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a physical or dynamic quantity detecting apparatus and a sensor failure or abnormality detecting system incorporated in this apparatus.
A conventional pressure sensor comprises a semiconductor substrate with a thin diaphragm portion. Two pressure detecting elements (i. e., gauge resistors) are formed in a central region of this diaphragm portion, and another two pressure detecting elements are formed in a peripheral portion of this diaphragm portion. These four pressure detecting elements are arranged to form a Wheatstone's bridge circuit. When a pressure is applied on the diaphragm portion, the resistance of each pressure detecting element varies due to a piezoresistance effect. A significant difference (i.e., an output voltage) appears between a midpoint of the pressure detecting elements in the central region and a midpoint of the pressure detecting elements in the peripheral region. The pressure sensor amplifies and adjusts this output voltage appropriately to produce an electric signal representing the pressure applied on the diaphragm portion.
As described above, the pressure sensor amplifies and adjusts a voltage difference produced from the bridge circuit. However, the bridge circuit produces an erroneous voltage difference when spoiled or wounded. In such a case, the pressure sensor produces an erroneous electric signal.
The PCT application WO97/05464 discloses a pressure sensor having a failure detecting capability. According to this prior art, the pressure sensor detects an abnormal distribution of the stress in the diaphragm portion. To this end, the diaphragm portion is separated into two regions at the center. A bridge circuit is formed in each region. A sensor failure is detectable based on a deviation in the output voltage of respective bridge circuits. However, this pressure sensor is not desirable in that the area of pressure sensing portion is doubled due to separation of the diaphragm portion along the center line. Thus, downsizing of the pressure sensor is difficult.
Another conventional pressure sensor includes a sensor chip adhered on a pedestal by an anode bonding so as to form a reference pressure chamber. The pressure detection is performed based on a pressure difference between an external pressure and the reference pressure. In general, adhering of the sensor chip onto the pedestal is performed in a vacuum to avoid oxidation of aluminum wiring formed on the sensor chip. Thus, the reference pressure chamber is maintained in a vacuum condition (i.e., 0 atm absolute).
When this pressure sensor is subjected to an extremely high pressure (for example, in the detection of a hydraulic braking pressure), it is necessary to match the voltage range of a sensor output with the measuring pressure varying in a wide zone. Thus, the sensitivity of the pressure sensor must be suppressed to a low level.
This makes it difficult to detect a sensor failure by solely monitoring the sensor output.
Furthermore, when the sensor chip peeled off the pedestal, the reference pressure chamber communicates with an external atmosphere. Thus, the reference pressure becomes equal to the external pressure. However, when the same pressure is detected as two series of signals like the above-described failure detecting method, a similar situation possibly appear when these signals are compared mutually. Hence, it is impossible to accurately detect the sensor failure.
SUMMARY OF THE MVENTION
In view of the above-described problems, the present invention has an object to provide a pressure sensor which is surely capable of detecting a sensor failure when the resistance value in a bridge circuit has varied due to trouble or damage.
Furthermore, the present invention has another object to provide a pressure sensor which is preferable in downsizing of the pressure sensor.
Moreover, the present invention has another object to provide a pressure sensor which is capable of detecting a sensor failure caused by a defective airtightness of the reference pressure chamber formed by adhering a sensor chip on a pedestal.
To accomplish the above and other related objects, the present invention provides a first pressure sensor comprising a semiconductor substrate having a diaphragm portion, a pressure detecting bridge circuit comprising gauge resistors having resistance values varying in response to a pressure applied on the diaphragm portion of the semiconductor substrate, and a reference voltage generating circuit connected between one end and the other end of the pressure detecting bridge circuit for generating a reference voltage, the reference voltage generating circuit comprising non-sensitive resistors having resistance values not varying in response to the pressure applied on the diaphragm portion. A failure judgement of the pressure detecting bridge circuit is performed based on a voltage difference between two midpoints of the pressure detecting bridge circuit as well as a voltage difference between a voltage level of either of two midpoints of the pressure detecting bridge circuit and the reference voltage level of the reference voltage generating circuit.
With this arrangement, it becomes possible to surely detect a sensor failure when the resistance value of the bridge circuit is changed due to the failure.
Furthermore, it is preferable that the gauge resistors constituting the pressure detecting bridge circuit are constituted by separate gauge resistors, and a failure judgement of the pressure detecting bridge circuit is performed based on a voltage difference between two midpoints of the pressure detecting bridge circuit as well as a voltage difference between a pair of intermediate terminals selected from intermediate terminals of the separate gauge resisters, the selected pair of intermediate terminals having the same voltage level in a condition where no pressure is applied on the diaphragm portion.
With this arrangement, it becomes possible to surely detect a sensor failure when the resistance value of the bridge circuit is changed due to the failure.
Furthermore, it is preferable that a failure judgement of the pressure detecting bridge circuit is performed based on voltage differences between one and the other terminals of the bridge circuit and first and second intermediate terminals of the bridge circuit.
With this arrangement, it becomes possible to surely detect a sensor failure when the resistance value of the bridge circuit is changed due to the failure.
Furthermore, it is desirable that an amplification circuit is provided to amplify each voltage difference, and the failure judgement of the pressure detecting bridge circuit is performed based on an output signal of the amplification circuit.
Furthermore, the present invention provides a second pressure sensor comprising a semiconductor substrate having a diaphragm portion, a pressure detecting bridge circuit for outputting an electric signal representing a pressure applied on the diaphragm portion of the semiconductor substrate, the pressure detecting bridge circuit comprising two gauge resistors provided in a first direction and two other gauge resistors provided in a second direction normal to the first direction. A failure detecting circuit, provided at a predetermined region of the diaphragm portion where the pressure detecting bridge circuit is not formed, outputs an electric signal in response to the pressure applied on the diaphragm portion. The failure detecting circuit has a sensitivity different from that of the pressure detecting bridge circuit. A failure judging means is provided for performing a failurejudgement of the pressure detecting bridge circuit based on the output signals of the pressure detecting bridge circuit and the failure detecting circuit.
In this manner, by using the output signals of the pressure detecting bridge circuit and the failure detecting circuit which have mutually different sensitivities, it becomes possible to surely detect a sensor failure when an output of the pressure detecting bridge circuit is changed due to the f
Ishio Seiichiro
Murakami Yoshifumi
Oba Nobukazu
Suzuki Yasutoshi
Tanaka Hiroaki
Denso Corporation
Law Offices of David G. Posz
Oen William
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