JPH08146032A - Filter circuit of piezoelectric acceleration sensor - Google Patents

Abstract

PURPOSE: To set a low band-shutting frequency low and reduce leak changes by equivalently connecting a capacitor in parallel to a static capacity of a piezoelectric element to increase the static capacity. CONSTITUTION: A capacitor 5 is equivalently connected, in parallel, to a static capacitance 1 of a piezoelectric element, so that the capacitance 1 is increased by the capacitor 5. In this case, a low band-shutting frequency determined by a load resistance 3 connected, in parallel, to the capacitor 5 and the capacitance 1 is smaller than a conventional low band-shutting frequency determined by a load resistance and a static capacitance. Since the low band-shutting frequency can be made small, the load resistance can be decreased and leak changes can be reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a filter circuit of a piezoelectric acceleration sensor as a vibrator or a transducer used for controlling an engine of a vehicle.

[0002]

2. Description of the Related Art FIG. 2 is a block diagram showing a filter circuit configuration of a conventional piezoelectric acceleration sensor. In FIG. 2, reference numeral 1 is the capacitance of the piezoelectric element that constitutes the piezoelectric acceleration sensor. 2 is a buffer and 3 is a load resistance. Reference numeral 4 is an output terminal to a circuit in the subsequent stage which controls the engine of the vehicle.

When the engine control of the vehicle is performed using the piezoelectric type acceleration sensor, the low frequency component of the signal from the piezoelectric type acceleration sensor is removed and only the high frequency component is output to the circuit at the latter stage for performing the engine control. There is a need. In the conventional filter circuit shown in FIG. 2, the low cutoff frequency (F _CL ) determined by the load impedance (R) and the electrostatic capacitance (C ₀ ) of the piezoelectric element is represented by the following formula.

F _CL = 1 / 2πC ₀ R However, in the filter circuit of the conventional piezoelectric acceleration sensor, the lower limit of the low cutoff frequency (F _CL ) limited by the capacitance (C ₀ ) of the piezoelectric element is set. It is limited.

[0005]

However, the piezoelectric type
The output of the acceleration sensor is required to be closer to DC.
Therefore, the low cutoff frequency (F_CL) As low as possible
There is a need. However, the conventional piezoelectric accelerometer
In the filter circuit of the sensor, the low cutoff frequency
(F_CL) To set a low
Required for this reason, semiconductors, circuit elements and printed circuit boards
Low cutoff frequency (F _CL) Special
There is a problem that variations and fluctuations in sex occur. Well
Moreover, since the piezoelectric element has temperature characteristics,
Lower cutoff frequency (F_CL) Changes the characteristics of
There is a problem. In addition, the piezoelectric acceleration sensor has high sensitivity.
Therefore, when using it for high acceleration,
There is a problem of exceeding the Mick Range.

The present invention solves the above-mentioned conventional problems, and provides an excellent piezoelectric acceleration sensor filter circuit in which the low cutoff frequency (F _CL ) can be set to a low value and which is little changed by temperature. The purpose is.

[0007]

In order to achieve the above object, the present invention provides a piezoelectric element for detecting acceleration, a capacitor equivalently connected in parallel with the capacitance of the piezoelectric element, and this capacitor. And a load resistor connected in parallel with the buffer circuit and a buffer circuit for amplifying the output of the piezoelectric element.

[0008]

The present invention is configured as described above, and by setting the low cutoff frequency to be low, the load impedance can be lowered, leakage fluctuation can be reduced, and the temperature characteristic of capacitance can be improved. Moreover, since the capacitor 5 causes a loss of sensitivity, the dynamic range when a large acceleration is applied to the piezoelectric element can be widened.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIG. FIG. 1 is a block diagram showing a filter circuit configuration of a piezoelectric acceleration sensor. In FIG. 1, reference numeral 1 is the electrostatic capacity of the piezoelectric element that constitutes the piezoelectric acceleration sensor. Reference numeral 2 is a buffer circuit, and 3 is a load resistance. Four
Is an output terminal to a circuit in the subsequent stage that controls the engine of the vehicle. Reference numeral 5 denotes a capacitor, which is equivalent to a piezoelectric element and operates in parallel as a capacitor for temperature compensation.

Next, the operation of the above embodiment will be described with reference to the drawings. In FIG. 1, a capacitor 5 is connected in parallel with the electrostatic capacitance 1 of the piezoelectric element constituting the piezoelectric acceleration sensor in parallel to increase the electrostatic capacitance. In this case, the low cutoff frequency (F _CL ) determined by the load impedance R3 and the electrostatic capacitance is expressed by the following formula.

F _CL = 1 / 2π (C ₀ + C ₁ ) R From the above formula (C ₀ + C ₁ ), the conventional load impedance (R) and the capacitance of the piezoelectric element (C ₀ ) are determined. This can be done by making the low cutoff frequency (F _CL ) smaller than the low cutoff frequency.

As described above, according to the above-mentioned embodiment, since the low cutoff frequency can be reduced, the load impedance can be lowered and the leakage fluctuation can be reduced. Further, by connecting the capacitor 5 in parallel with the electrostatic capacitance 1 of the piezoelectric element in an equivalent manner, the temperature characteristic of the electrostatic capacitance of the entire circuit can be improved, and further, the capacitor 5 causes sensitivity loss. Therefore, it is possible to widen the dynamic range when a large acceleration is applied to the piezoelectric element.

Incidentally, the variation of C ₀ is usually about 20.
%, But it is desirable to select a material having less variation in C ₁ and temperature characteristics.

Although the low cutoff frequency is set low in the above embodiment, the low cutoff frequency is fixed and the capacitor 5 may be connected so as to reduce the resistance value of the load impedance. good.

Further, a capacitor 5 having a temperature characteristic opposite to that of the piezoelectric element forming the piezoelectric acceleration sensor may be equivalently connected in parallel.

[0016]

As is apparent from the above embodiment, the present invention reduces the low cutoff frequency by connecting a capacitor in parallel with the capacitance of the piezoelectric element constituting the piezoelectric acceleration sensor. It is possible to set the load impedance, reduce the load impedance, reduce the leakage fluctuation, and improve the temperature characteristic of the capacitance. Further, by selecting a capacitor that is equivalently connected in parallel with the capacitance of the piezoelectric element in parallel with the variation in temperature, it is possible to reduce variation in the entire circuit.

[Brief description of drawings]

FIG. 1 is a block diagram of a filter circuit of a piezoelectric acceleration sensor according to an embodiment of the present invention.

FIG. 2 is a block diagram of a filter circuit of a conventional piezoelectric acceleration sensor.

[Explanation of symbols]

 1 Capacitance of piezoelectric element 2 Buffer circuit 3 Load resistance 4 Output terminal 5 Capacitor

Claims (1)

[Claims] 1. A piezoelectric element for detecting an acceleration, a capacitor equivalently connected in parallel with the capacitance of the piezoelectric element, a load resistance connected in parallel with the capacitor, and an output of the piezoelectric element. A filter circuit for a piezoelectric acceleration sensor having a buffer circuit for amplification.