JPH06347496A - Electric charge detecting circuit - Google Patents

Abstract

PURPOSE:To obtain an electric charge detecting circuit which is of a low cost, and can measure the output charge of a piezoelectric element accurately by suppressing a drift. CONSTITUTION:A piezoelectric element 12 is connected to the nonreversing input end of an operational amplifier 14. The nonreversing input end of the operational amplifier 14 is earthed through a transistor 16. A resistance 18 is connected to the base of the transistor 16, and a signal to control the transistor 16 is input through the resistance 18. The base of the transistor 16 is earthed through a resistance 20. Resistances 22 and 24 are connected to the reversing input end of the operational amplifier 14. When the charge of the piezoelectric element 12 is measured, a voltage is not applied to the base of the transistor 16 so as to make the transistor 16 in a noncontinuety condition. When the voltage of the piezoelectric element 12 is not measured, the voltage is applied to the base of the transistor 16 so as to make the transistor 16 in the continuety condition.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a charge detection circuit,
In particular, the present invention relates to a charge detection circuit used in a tire wheel load measuring device or the like for measuring a vehicle tire wheel load using a piezoelectric element, for example.

[0002]

2. Description of the Related Art FIG. 3 is a circuit diagram showing an example of a conventional charge detection circuit which is the background of the present invention. The charge detection circuit 1 includes a piezoelectric element 2. One end of the piezoelectric element 2 is grounded,
The other end of the piezoelectric element 2 is connected to the non-inverting input end of the operational amplifier 3. A leak resistance 4 is connected between both ends of the piezoelectric element 2. A resistor 5 is connected between the inverting input terminal and the output terminal of the operational amplifier 3. Further, the inverting input terminal of the operational amplifier 3 is grounded via the resistor 6. In the charge detection circuit 1 shown in FIG. 3, when a load is applied to the piezoelectric element 2, electric charge is generated in the piezoelectric element 2. The voltage due to this charge is amplified by the operational amplifier 3, and a voltage proportional to the weight is output from the output terminal of the operational amplifier 3.

FIG. 4 is a circuit diagram showing another example of the background of the present invention. In this charge detection circuit 1, the operational amplifier 3
Is connected to the non-inverting input terminal of the
Are connected. Further, a parallel circuit of the capacitor 7 and the leak resistor 4 is connected between the inverting input terminal and the output terminal of the operational amplifier 3. In the charge detection circuit 1 shown in FIG. 4, the charge generated in the piezoelectric element 2 is charged in the capacitor 7. Therefore, by amplifying the voltage of the capacitor 7 by the operational amplifier 3, a voltage proportional to the weight is output from the output terminal of the operational amplifier 3. These charge detection circuits 1
The leak resistance 4 used for is for discharging the electric charge and the leak current of the operational amplifier 3 due to the pyroelectric effect generated in the piezoelectric element 2 due to the change of the external temperature. The presence of the leak resistance 4 can prevent drift of the operational amplifier output due to unnecessary charges and leak current.

[0004]

However, in the charge detection circuit shown in FIG. 3, the frequency characteristic is determined by the resistance value of the leak resistance and the capacitance value of the piezoelectric element. In the charge detection circuit shown in FIG. 4, the frequency characteristic is determined by the resistance value of the leak resistance and the capacitance value of the capacitor. Furthermore, the output charge of the piezoelectric element is very small,
Moreover, since the output impedance is very large, the range of selection of the leak resistance is narrowed, and the degree of freedom in design is reduced. Moreover, an operational amplifier with a small leak current must be used, and an expensive operational amplifier is required.

Therefore, a main object of the present invention is to provide a charge detection circuit which is inexpensive and which can suppress the drift and accurately measure the output charge of the piezoelectric element.

[0006]

SUMMARY OF THE INVENTION According to the present invention, a piezoelectric element, an amplifier connected to the piezoelectric element, and a piezoelectric element connected between the piezoelectric element and the amplifier are provided to discharge electric charge between the piezoelectric element and the amplifier. The switching element is a charge detection circuit including a switching element and a non-conductive state when the charge of the piezoelectric element is detected, and a conductive state when the charge of the piezoelectric element is not detected.

[0007]

When the electric charge of the piezoelectric element is measured, the switching element is in a non-conductive state, and when the electric charge of the piezoelectric element is not measured, the switching element is in a conductive state. When the switching element is in the conductive state, unnecessary electric charges in the circuit are discharged.

[0008]

According to the present invention, unnecessary charges in the circuit are discharged when the charge of the piezoelectric element is not measured.
It is possible to prevent drift of the operational amplifier output due to such unnecessary charges. Moreover, since the switching element is in a non-conducting state when measuring the charge of the piezoelectric element,
The necessary charge generated in the piezoelectric element is not released, and the operational amplifier output voltage that is proportional to the load can be obtained accurately.
Further, in the charge detection circuit of the present invention, since the leak resistance is not used, the frequency characteristic is not limited by the leak resistance and the capacitance of other elements, and the degree of freedom in circuit design is high.

The above-mentioned objects, other objects, features and advantages of the present invention will become more apparent from the following detailed description of the embodiments with reference to the drawings.

[0010]

1 is a circuit diagram showing an embodiment of the present invention. The charge detection circuit 10 includes a piezoelectric element 12. One end of the piezoelectric element 12 is grounded, and the other end is connected to a non-inverting input end of an operational amplifier 14 as an amplifier. Further, the collector of the transistor 16 is connected to the non-inverting input terminal of the operational amplifier 14, and the emitter of the transistor 16 is grounded. A resistor 18 is provided at the base of the transistor 16.
A measurement start signal is input via. Further, the pace of transistor 16 is grounded via another resistor 20. Between the inverting input terminal and the output terminal of the operational amplifier 14,
The resistor 22 is connected. Further, the inverting input terminal of the operational amplifier 14 is grounded via the resistor 24.

The charge detection circuit 10 is used, for example, in a tire wheel load measuring device. The tire wheel load measuring device is a device for reading an electric charge generated in a piezoelectric element when a vehicle tire passes over the piezoelectric element and measuring a load distribution thereof. In the charge detection circuit 10, when the charge of the piezoelectric element 12 is not measured, the transistor 16
A voltage is applied to the base of the. Therefore, the transistor 16 becomes conductive, and the piezoelectric element 1 due to the pyroelectric effect is generated.
A charge of 2 and a leak current of the operational amplifier 14 are discharged from the circuit.

When measuring the electric charge of the piezoelectric element 12, no voltage is applied to the base of the transistor 16 and the transistor 16 becomes non-conductive. Therefore, the piezoelectric element 1
The voltage across the piezoelectric element 12 due to the electric charge generated in 2 is
It is amplified by the operational amplifier 14, and a voltage output proportional to the load can be obtained from the output terminal of the operational amplifier 14. The signal input to the base of the transistor 16 uses a tape switch, a laser, or the like to output the signal from the piezoelectric element 1.
It should be created by detecting that the tire is approaching 2.

As described above, when the electric charge of the piezoelectric element 12 is not measured, the unnecessary electric charge is released through the transistor 16. Therefore, when the electric charge of the piezoelectric element 12 is measured, the drift of the output voltage due to the unnecessary electric charge is suppressed. . Moreover,
Since the transistor 16 is in a non-conducting state when the charge of the piezoelectric element 12 is measured, the voltage across the piezoelectric element 12 is accurately amplified without the charge of the piezoelectric element 12 leaking. Further, unlike the conventional circuit, a leak resistance is not used in the charge detection circuit 10, so that the frequency characteristic is not limited by the leak resistance and the piezoelectric element 12, and the degree of freedom in circuit design is high.

FIG. 2 is a circuit diagram showing another embodiment of the present invention. In this charge detection circuit 10, the non-inverting input terminal of the operational amplifier 14 is grounded. The piezoelectric element 12 is connected to the inverting input terminal of the operational amplifier 14. further,
A parallel circuit of the capacitor 26 and the FET 28 is connected between the inverting input terminal and the output terminal of the operational amplifier 14. In this case, the source and drain of the FET 28 are connected to the inverting input terminal and the output terminal of the operational amplifier 14, respectively, and F
A measurement start signal is input to the gate of ET28.

Also in the charge detection circuit 10, the FET 28 is made conductive when the charge of the piezoelectric element 12 is not detected, and the FET 28 is made non-conductive when the charge of the piezoelectric element 12 is detected. In the charge detection circuit 10, the capacitor 26 is charged with the charge of the piezoelectric element 12, and the voltage of the capacitor 26 is amplified by the operational amplifier 14. Like the charge detection circuit shown in FIG. 1, the charge detection circuit 10 also releases unnecessary charges when the charge of the piezoelectric element 12 is not measured. Then, when the charge of the piezoelectric element 12 is measured, the voltage of the capacitor 26 due to the charge generated in the piezoelectric element 12 can be accurately amplified. Therefore, there is no drift and accurate load measurement can be performed. Moreover, since the charge detection circuit 10 also does not use the leak resistance, the frequency characteristic is not limited, and the degree of freedom in circuit design is high.

Although the transistors and FETs are used as the switching elements in the above embodiments, the switching elements are not limited to these, and other switching elements can be used. Further, in the charge detection circuit of the present invention, since unnecessary charges are completely removed, it is not necessary to use an expensive operational amplifier, and the charge detection circuit can be manufactured at low cost.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention.

FIG. 2 is a circuit diagram showing another embodiment of the present invention.

FIG. 3 is a circuit diagram showing an example of a conventional charge detection circuit which is the background of the present invention.

FIG. 4 is a circuit diagram showing another example of a conventional charge detection circuit which is the background of the present invention.

[Explanation of symbols]

 10 Charge Detection Circuit 12 Piezoelectric Element 14 Operational Amplifier 16 Transistor 28 FET

Claims (1)

[Claims] 1. A piezoelectric element, an amplifier connected to the piezoelectric element, and a switching element connected between the piezoelectric element and the amplifier for discharging a charge between the piezoelectric element and the amplifier. A charge detection circuit comprising: the switching element being in a non-conducting state when the electric charge of the piezoelectric element is detected, and being in a conducting state when the electric charge of the piezoelectric element is not detected.