Physical quantity detector

Abstract

A physical quantity detector includes a pressure-responsive bridge circuit that outputs a detection voltage including a voltage component corresponding to atmospheric pressure and a voltage component corresponding to a change quantity from the atmospheric pressure, a filter that eliminates the voltage component corresponding to the change quantity from the atmospheric pressure, and a variable gain unit that amplifies a voltage difference between the detection voltage outputted from the bridge circuit and the filter output voltage that is free form the change quantity from the atmospheric pressure.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings:

FIG. 1 is a circuit diagram of a physical quantity detector according to a first embodiment of the present invention;

FIG. 2 is a graph showing an example of a relation between a detected pressure and an output voltage of an amplifier in the first embodiment;

FIG. 3 is a circuit diagram of a filter in the first embodiment;

FIG. 4 is a circuit diagram of a level filter in the first embodiment;

FIG. 5 is a circuit diagram of a variable gain unit in the first embodiment;

FIG. 6 is a circuit diagram of a level shifter and a constant current generating circuit of a physical quantity detector according to a second embodiment of the present invention;

FIGS. 7A and 7B are circuit diagrams of variations of other embodiments of the present invention; and

FIG. 8 is a graph showing pressure changes within a vehicle door caused by collision against the door.

Claims

1. A physical quantity detector comprising: a detecting means that outputs a detection output voltage including a voltage component corresponding to an average physical quantity of a detection target and a voltage component corresponding to a change quantity from the average physical quantity;a filter means that eliminates the voltage component corresponding to the change quantity from the detection output voltage and outputs a filter output voltage that is free from the change quantity; anda ratio output means that outputs a ratio output signal indicating a ratio of the change quantity from the average physical quantity to the average physical quantity of the detection target, based on the filter output voltage and the detection output voltage.

2. A physical quantity detector comprising: a detecting means that outputs a detection output voltage including a voltage component corresponding to an average physical quantity of a detection target and a voltage component corresponding to a change quantity from the average physical quantity;a filter means that eliminates the voltage component corresponding to the change quantity from the detection output voltage and outputs a filter output voltage including the voltage component corresponding to the average physical quantity; anda variable gain unit that amplifies a voltage difference between the detection output voltage and the filter output voltage, with a gain that is inversely proportional to the filter output voltage.

3. The physical quantity detector according to claim 2, wherein the variable gain unit includes: a first transistor that has a control terminal applied with the detection output voltage;a second transistor that has a control terminal applied with the filter output voltage;a constant current generating circuit that generates two constant currents proportional to the filter output voltage, and feeds the constant currents to the first transistor and the second transistor, respectively;an input resistor connected between a first terminal of the first transistor connected with the constant current generating circuit and a first terminal of the second transistor connected with the constant current generating circuit;wherein the first transistor has a second terminal that outputs a voltage corresponding to a sum of the constant current generated by the constant generating circuit and a current flowing through the input resistor, and the second transistor has a second terminal that outputs a voltage corresponding to a difference between the constant current generated by the constant current generating circuit and the current flowing through the input resistor, anda differential amplifying circuit that outputs a voltage corresponding to a voltage difference between the voltage outputted from the second terminal of the first transistor and the voltage outputted from the second terminal of the second transistor.

4. The physical quantity detector according to claim 2, wherein: the detecting means includes an amplifier that amplifies the detection output voltage, and outputs an amplifier output voltage by adding a reference voltage to an amplified detection output voltage; andthe constant current generating circuit includes a potential dropping means that drops the filter output voltage by the reference voltage of the amplifier, and generates the constant currents proportional to the average physical quantity of the detection target.

5. The physical quantity detector according to claim 1, wherein the detecting means detects air pressure within a vehicle door as the detection target and outputs the detection output voltage indicating atmospheric pressure and a pressure change from the atmospheric pressure caused by collision against the vehicle door.

6. A physical quantity detector comprising: a first means that produces a detection output including both an average component corresponding to an average physical quantity of a detection target and a change component corresponding to a change quantity from the average physical quantity;a second means that produces an average output corresponding to the average component from the detection output;a third means that produces a change output corresponding to the change component from the detection output; anda fourth means that produces a ratio output indicating a ratio of the change quantity to the average physical quantity of the detection target based on the average output and the change output.

7. The physical quantity detector according to claim 6, wherein the first means detects atmospheric pressure within a vehicle door.