JPH0766009B2 - Acceleration sensor - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic suspension control system for automobiles, an acceleration sensor used for detecting acceleration such as collision, acceleration / deceleration feeling amount, and the like.

2. Description of the Related Art Conventionally, this type of acceleration sensor uses a piezoelectric ceramic center fixed piezoelectric ceramic as a transducer. The frequency characteristic of this piezoelectric ceramic has a very large impedance at 0.5 Hz to 200 Hz used as an acceleration sensor, so impedance conversion is performed using a field effect transistor (FET) with a large input impedance. Hereinafter, the above conventional example will be described with reference to the drawings.

In FIG. 4, 31 is a transducer made of a piezoelectric ceramic center fixed type piezoelectric ceramic, 32 is an FET for impedance conversion, and 33 is an input resistance (Ri) for preventing destruction of the FET 32 due to latch-up. Only the drain of FET 32 is sent from the sensor side to the circuit body side, the load resistance (R _L ) 34 is connected to the circuit body side, the power supply (V _CC ) 35 is connected to this R _L 34, and the capacitor ( c) At 36, the direct current component is cut and the output is taken out.

Then, when acceleration is applied to the sensor, the transducer
The piezoelectric ceramic of 31 bends and the amount of charge changes. This change in the amount of charge, that is, the change in voltage can be applied to the gate of the FET 32, impedance conversion can be performed by the FET 32, and the change in voltage can be extracted. At this time, the Ri 33 acts as a high-pass filter with the piezoelectric ceramic of the transducer 31, so that it is necessary to increase the value in order to sufficiently transmit the signal to the low frequency band to the FET 32. Here, the low-cut frequency f _L is expressed by the following equation.

And the value of Ri33 is usually Co≈7000 _P F, so f _L = 0.
In case of 5Hz, Ri = 50MΩ and very high resistance is required.

The one shown in FIG. 5 uses a bias resistor (Rs) 37, and other configurations are the same as those in FIG.

In FIG. 6, Ri 33 is connected between the gate and the source of the FET 32 in order to increase the input resistance of the FET 32 by using Rs37, and other configurations are the same as those in FIG.

Problems to be Solved by the Invention However, of the above-mentioned conventional examples, in the configuration shown in FIG.
Since the source of T32 is directly grounded, the temperature characteristic of FET32 itself appears in the output as it is.

On the other hand, in the configuration shown in FIG. 5, the resistance of Rs37 makes it resistant to temperature changes to some extent. However, when Rs37 is inserted, the amplification degree by FET32 decreases,
You will not be able to get a large output.

In the configuration shown in FIG. 6 as well, although it becomes more resistant to temperature changes as in the configuration shown in FIG. 5, the amplification degree by the FET 32 decreases and it becomes impossible to obtain a large output.

The present invention solves the problems of the conventional example as described above.
It is an object of the present invention to provide an acceleration sensor that is resistant to temperature changes and can be set to an arbitrary output level.

Means for Solving the Problems In order to solve the above problems, the present invention provides a transducer using a piezoelectric ceramic and a first impedance conversion type in which an output of the transducer is applied and a field effect transistor is used in an input stage. No. 1 operational amplifier, the output of this first operational amplifier is applied, and the low-frequency cut circuit composed of a capacitor and a resistor is connected to the input terminal of the first operational amplifier to reduce the low-frequency cut frequency of the low-frequency cut circuit. A resistance to determine and a second operational amplifier, to which the output of the low-frequency cut circuit is applied, has a low-pass and amplification function, and uses a field effect transistor in the input stage.

Action The present invention has the following actions due to the above configuration. That is, since impedance conversion, filtering, and amplification are performed by an operational amplifier using a field effect transistor in the input stage, it is resistant to temperature changes, and the degree of amplification can be increased and the output level can be increased.

EXAMPLES Examples of the present invention will be described below with reference to the drawings.

First, a first embodiment of the present invention will be described. FIG. 1 is a circuit diagram showing a first embodiment of the present invention.

In FIG. 1, reference numeral 1 is a transducer composed of a piezoelectric disk center fixed type piezoelectric ceramic.
As shown in the figure, of the electrodes 22 on both sides of the piezoelectric ceramic 21, one electrode 22 is attached to the metal plate 23, the central portion of the metal plate 23 is attached to the support 24, the metal plate 23, the piezoelectric ceramic 21 Covered by case 25. And support 24
Is fixed to the engine 26 when detecting knocking of an object to be detected, for example, the vibration of the engine 26 is transmitted to the piezoelectric ceramic 21 via the support 24 and the metal plate 23, and the piezoelectric ceramic 21 responds to the vibration. It is designed to be able to take out electrical signals. Reference numeral 2 is an operational amplifier for impedance conversion whose input stage is composed of a field effect transistor (FET), and the output is directly fed back to the input terminal. 3 is an input resistor (Ri), 4 is a capacitor, 5 is a resistor, and these capacitors 4 and 5 constitute a low-frequency cut circuit (temperature dlit compensation circuit).
Is applied. Reference numeral 6 denotes an operational amplifier to which a signal from the low-frequency cut circuit is applied, the input stage of which is composed of a FET, and the operational amplifier 6 has a feedback circuit including a resistor (Rf) 7 and a capacitor (Cf) 8. 9 is a bias resistor, 10 is a DC cut capacitor, 11 and 12 are bias resistors, 13 and
14 is a resistor and a capacitor for removing the power supply ripple.

Next, the operation of the above embodiment will be described.

When acceleration is applied to the sensor, the piezoelectric ceramic 1 bends,
The amount of charge changes in proportion to this. This change in the amount of charge,
That is, the change in voltage is applied to the operational amplifier 2. Since this operational amplifier 2 uses a field effect transistor (FET) in the input stage, it is possible to set the bias current to 40 pA or less even at high temperature, and at this time, the input resistance Ri3 is Ri = 50 MΩ as described above. Therefore, the DC output of the operational amplifier 2 is as small as 0.2V, and it does not saturate and almost no loss occurs. Since the operational amplifier 2 directly feeds back the output to the input terminal, the input impedance of the operational amplifier 2 becomes very large as a whole, and the input loss becomes very small. That is, the output of the piezoelectric ceramic in the transducer 1 is efficiently applied to the operational amplifier 2 and impedance conversion is performed. The output of the operational amplifier 2 is applied to a low cut circuit composed of a capacitor 4 and a resistor 5. The low-frequency cut circuit cuts the output level of the operational amplifier 2 when the output level changes with temperature. That is, since the operational amplifier 2 alone does not take measures against the temperature change, a level change corresponding to the temperature change occurs on the output side, and this is cut by the low frequency cut circuit. The signal temperature-compensated by the low frequency cut circuit is applied to the operational amplifier 6. This operational amplifier 6
Is provided with a feedback circuit composed of R _f7 and C _f8 , and since the input stage is composed of a FET, the value of R _f7 can be made relatively large and the gain can be kept large. That is, R _f7 is set to a large value with this operational amplifier 6 to act as an amplifier having a large gain. Therefore, the output finally obtained is relatively large. And C
_f8 acts as a low pass filter. That is, the combined impedance of C _f8 and R _f7 that constitutes the feedback circuit changes to a large impedance when the frequency is low, and changes to a small impedance when the frequency is high, so the feedback amount is small when the frequency is low and the frequency is high. As the gain increases, the gain of the operational amplifier 6 increases in the low range, and the gain of the operational amplifier 6 decreases in the high range.

Next, a second embodiment of the present invention will be described. FIG. 3 is a circuit diagram showing a second embodiment of the present invention.

In this embodiment, a quad FET input operational amplifier is used. The same parts as those in the first embodiment are designated by the same reference numerals, and the description thereof will be omitted. Only different configurations will be described. In FIG. 3, 15 is an operational amplifier, 16,
Reference numeral 17 is a resistor, and a comparison circuit (a hysteresis can be added) is configured by these operational amplifiers 15, 16 and 17, and the operational amplifier 15 compares the output of the operational amplifier 6 with a constant voltage divided by the resistors 16 and 17. Then, when the output of the operational amplifier 6 becomes larger than the constant voltage, a predetermined signal is output. 18 is an operational amplifier for stabilizing the bias, and 19 and 20 are capacitors for removing power supply ripple.

According to the present embodiment, the output can be increased when the acceleration larger than the preset acceleration is applied to the sensor.

The operational amplifiers 2 and 6 of the first embodiment and the operational amplifiers 2, 6, 15, and 18 of the second embodiment may be individually provided, and two or four operational amplifier ICs are used. As a result, the circuit can be easily obtained.

As described above, according to the present invention, the voltage change of the transducer using the piezoelectric ceramic is applied to the first operational amplifier for impedance conversion using the field effect transistor at the input stage, and the low frequency cut circuit is used. The output level fluctuation of the first operational amplifier is cut at a cut frequency defined by a resistor connected to the input terminal of the first operational amplifier, and applied to the second operational amplifier using a field effect transistor in the input stage. Since impedance conversion, filtering, and amplification are performed, it is resistant to temperature changes, and the degree of amplification can be increased and the output level can be increased.

[Brief description of drawings]

1 and 2 show an acceleration sensor according to a first embodiment of the present invention. FIG. 1 is a circuit diagram, FIG. 2 is a partially cutaway front view of a transducer, and FIG. 3 is a second embodiment of the present invention. An example circuit diagram and FIGS. 4 to 6 are circuit diagrams showing conventional acceleration sensors, respectively. 1 ... Transducer, 2 ... Operational amplifier, 3 ... Input resistance (Ri), 4 ... Capacitor, 5 ... Resistor, 6 ... Operational amplifier, 7 ... Resistance ( _Rf ), 8 ... Capacitor ( _Cf ) , 15 …… operational amplifier, 18 …… operational amplifier.

Claims (1)

[Claims] 1. A transducer using a piezoelectric ceramic, an output of this transducer is applied, a first operational amplifier for impedance conversion using a field effect transistor in an input stage, and an output of this first operational amplifier is applied. A reduction cut circuit consisting of a capacitor and a resistor, a resistor connected to the input terminal of the first operational amplifier to determine the low cut frequency of the low cut circuit, and an output of the low cut circuit applied, An acceleration sensor having a low-pass and amplification function and a second operational amplifier using a field effect transistor in an input stage.