JP2020030081A - Piezoelectric acceleration sensor - Google Patents

Abstract

The present invention relates to a piezoelectric acceleration sensor, and more particularly to a piezoelectric acceleration sensor that compensates for temperature characteristics of a piezoelectric element.
A temperature correction capacitor is connected in series to a detection unit of a piezoelectric charge output acceleration sensor, and the capacitance of the temperature correction capacitor is changed to the temperature correction capacitor. The temperature coefficient of the capacitance synthesized by the detection unit 8 of the type charge output acceleration sensor is set to a value such that the product of the temperature coefficient of the detection unit 8 and the temperature coefficient of the voltage sensitivity becomes 1 to obtain a temperature characteristic of the charge sensitivity. Is provided by a piezoelectric charge output acceleration sensor in which is flattened.
[Selection diagram] FIG.

Description

  The present invention relates to a piezoelectric acceleration sensor, and more particularly, to a piezoelectric acceleration sensor that compensates for temperature characteristics of a piezoelectric element.

  Piezoelectric acceleration sensors have a structure in which a piezoelectric element, which is a detection element, holds a load mass. When acceleration is applied to the sensor, inertial force acts on the load mass, and the piezoelectric element holding the load mass The element exerts its inertial force. Since a piezoelectric element has a characteristic of generating an electric charge proportional to a force applied thereto, an electric charge is generated by an inertial force applied to the piezoelectric element. As a result, the charge output of the piezoelectric acceleration sensor is proportional to the applied acceleration and can be used as an acceleration signal. A general piezoelectric acceleration sensor has a structure as shown in FIG. Two layers of the piezoelectric elements 1 and 1 and the load mass 2 are fastened to the case 6 with the fastening screws 4 with the electrode plate 3 interposed therebetween. The electric charge generated by the piezoelectric element is output from the electrode plate 3 to a measuring device (an amplifier such as a charge amplifier) (not shown) via the connector 5 and the conductor 7.

  Normally, a signal from this charge generation type sensor converts charges into a voltage signal using a charge signal conversion amplifier generally called a charge amplifier. A charge signal conversion amplifier system in which a feedback capacitor (integration capacitor) is provided between an inverting input and an output of an amplifier having an infinite gain with an opposite phase is often used. As described above, the charge signal output from the piezoelectric sensor has been converted into a voltage signal by the charge amplifier, and then subjected to various signal processing as an acceleration signal and used.

Compared with other methods (servo type, semiconductor type, strain gauge type), such a piezoelectric acceleration sensor is more robust, has a wider frequency band, a wider use acceleration range, and a wider use temperature range. For this reason, it has been widely used especially for industrial purposes.
However, there has been a demand for a solution to the problem that the sensitivity characteristics of the piezoelectric acceleration sensor are affected by temperature. That is, in the piezoelectric acceleration sensor, the characteristics of the piezoelectric element serving as the detecting element have a temperature characteristic, and when the temperature changes, the amount of generated charge changes even when the same force is applied. In addition, the members that hold the detection element (mainly metals) expand and contract due to temperature, and the force that fixes the detection element changes, thereby avoiding defects that affect the characteristics of the detection element that generate electric charge. Absent.
Therefore, measures have been required to avoid the influence of temperature on the sensitivity characteristics of the piezoelectric acceleration sensor.

  As a countermeasure, research has been required for a method of electrically compensating the piezoelectric material itself and the members attached to the piezoelectric material and the detected acceleration data. In the former case, it is necessary to improve the material characteristics of the piezoelectric element, and a piezoelectric element made of a special material is required. However, the detection sensitivity of a piezoelectric material with little fluctuation due to temperature is often extremely low. There is also a method of using a material having a low coefficient of thermal expansion for a member attached to the sensor, but it has been difficult to sufficiently suppress the influence of the coefficient of thermal expansion.

In the latter case, there is a method of incorporating a temperature sensor separately and electrically correcting the detected acceleration data, but in this method, it is necessary to install the temperature sensor in an environment where the temperature of the detection element matches the temperature of the detection element. However, it is also difficult to implement, and therefore, the temperature of the acceleration detecting unit and the temperature of the temperature sensor do not always match, and it is difficult to perform accurate temperature correction.
Therefore, in our research, we decided to solve the problem by devising a circuit configuration that performs temperature compensation on the sensor output.

According to Patent Document 1, in a piezoelectric acceleration sensor, by using a capacitor having a positive temperature characteristic for an integration capacitor of a charge amplifier connected in parallel to a piezoelectric element, an output of a charge sensitivity having a positive temperature characteristic is obtained. An attempt is made to correct in the operation of converting to voltage sensitivity.
However, since the charge amplifier or integrating capacitor cannot exist in the same place as the piezoelectric element as the detection unit, temperature correction can be performed when the temperature is stable, but especially in the transient period when the temperature is changing. However, there is a problem that it is difficult to perform accurate temperature correction due to a large temperature difference.

According to Patent Document 2, the low-frequency cutoff frequency is determined by the capacitance of the piezoelectric element and the load resistance of the input of the buffer amplifier. Since the capacitance of the piezoelectric element has a temperature characteristic, if the capacitance of the piezoelectric element changes with temperature, the low cutoff frequency also changes. Therefore, an attempt is made to improve the temperature characteristics of the capacitance by connecting a capacitor in parallel with the load resistor.
However, the purpose of this patent is to reduce the fluctuation of the low-frequency cutoff frequency due to the temperature change, and does not aim to correct the sensitivity fluctuation due to the temperature.

Japanese Utility Model Publication No. 2-150567 JP-A-8-146032

An object of the present invention is to eliminate a measurement error caused by a temperature characteristic of charge sensitivity of a general piezoelectric acceleration sensor. To solve this problem, a trend analysis of the temperature characteristics of the charge sensitivity of the piezoelectric acceleration sensor and its capacitance is performed in advance, and as a countermeasure, a predetermined temperature correction capacitor is signaled at the output stage of the charge sensitivity of the piezoelectric acceleration sensor. The problem was solved by connecting the output in series.
More specifically, the electric charge output from the acceleration sensor is composed of a series combined capacitance of the temperature correction capacitor to be added and the capacitance of the piezoelectric element of the acceleration detection unit, and the electric charge generated by the piezoelectric element of the acceleration detection unit. And a voltage value obtained from the capacitance thereof, to solve the problem.

  The present invention has been made in view of such circumstances, and the temperature correction method does not require the use of a special component for temperature correction such as a thermistor, and a readily available capacitor having a small capacitance change due to temperature can be used. It can be realized at very low cost.

  In addition, this temperature correction capacitor does not need to be installed in principle near the piezoelectric element of the acceleration detector, and even if there is a temperature difference between the piezoelectric element and the temperature correction capacitor, it does not affect the temperature correction. The temperature correction capacitor can be installed on a printed circuit board separated from a detection unit such as a relay board or a built-in charge amplifier. In addition, accurate temperature correction is possible even in applications where there is a sudden temperature change.

The present invention measures the change coefficient of the charge sensitivity due to the temperature of the detection unit in the piezoelectric charge output acceleration sensor and the change coefficient of the capacitance of the detection unit due to the temperature at a plurality of locations of the temperature, and changes the charge sensitivity. A coefficient A and a change coefficient B of the capacitance were calculated, and the change coefficient A, the change coefficient B, and the capacitance D of the detection unit at a reference temperature were calculated by Expression (1). A temperature compensating capacitor 9 having an electrostatic capacitance E is connected in series to the detection unit 8 to perform temperature compensation. As a result, a flattened change based on 20 ° C. before and after the temperature compensation is compared. It is provided by a piezoelectric charge output acceleration sensor in which the degree of flattening is in the range of 0.9 to 1.05 at a coefficient rate.
E = D (BA) / (A-1) (1)
Here, the change coefficient A is a change coefficient of the charge sensitivity of the detection unit 8 due to the temperature, the change coefficient B is a change coefficient of the capacitance of the detection unit 8 due to the temperature, and D is a static coefficient of the detection unit 8 at the lower limit temperature of the temperature correction range. It is capacitance. Each of the change coefficients is a coefficient calculated from a value at the upper limit of the temperature correction range based on the lower limit of the temperature correction range.

The present invention is also provided by the piezoelectric charge output acceleration sensor, wherein the degree of flattening in the temperature range in which the temperature compensation is performed is in the range of 0.95 to 1.05.
The piezoelectric type according to claim 1, wherein the charge sensitivity can be used at −40 ° C. to + 120 ° C. within a range of 0.95 to 1.05 based on 20 ° C. comparing before and after the compensation of the charge sensitivity. Effectively provided by a charge output acceleration sensor.

According to the present invention, as the piezoelectric charge output acceleration sensor, the temperature correction capacitor is connected in series to the detection unit 8, so that the coefficient of change of the charge sensitivity of the piezoelectric charge output acceleration sensor with temperature is reduced. Further, by selecting the capacitance of the temperature compensating capacitor, the effect of obtaining a piezoelectric charge output acceleration sensor usable in a wide temperature range can be obtained.
More preferably, the piezoelectric charge output as described above, which can be used at -40 ° C to + 120 ° C, with a coefficient of change of 0.95 to 1.05 based on 20 ° C comparing before and after compensation of the charge sensitivity. The effect provided by the acceleration sensor is obtained.
As a result, an effect as a very effective piezoelectric charge output acceleration sensor can be obtained by selecting the simpler circuit configuration of the present invention.
As the temperature compensating capacitor, a ceramic capacitor having a very small temperature coefficient (for example, within ± 60 ppm) which can be easily obtained can be used, and the installation location of the temperature compensating capacitor is not limited. Therefore, there is no need to be in contact with the detection unit, and there is no need to be near the detection unit, and the degree of freedom is extremely high.

  Although the structure of the piezoelectric acceleration sensor is generally shown in FIG. 1, the present invention is not limited to this, and other structures in accordance with the principle of the present invention, for example, a connection cable to a charge amplifier can be applied by other methods.

FIG. 1 is a basic configuration diagram of a piezoelectric acceleration sensor. FIG. 2 is an equivalent circuit of the piezoelectric acceleration sensor according to the present invention. FIG. 3 shows an equivalent circuit of the acceleration sensor according to the present invention, in which the detecting unit 8 is represented by a voltage source and a capacitance, and a temperature correcting capacitor 9 is connected. FIG. 4 is a temperature characteristic diagram showing the charge sensitivity according to the temperature of the detection unit of the piezoelectric acceleration sensor and the change in capacitance according to the temperature of the detection unit. FIG. 5 is a temperature characteristic diagram showing FIG. 4 showing the change coefficient A of the charge sensitivity, the change coefficient B of the capacitance D of the detection unit with temperature, and the change coefficient C of the voltage sensitivity of the detection unit with temperature. The change coefficient A of the charge sensitivity is 1.3, and the temperature coefficient B of the capacitance is 1.59. FIG. 6 is a diagram showing the temperature characteristics of the charge sensitivity of the piezoelectric acceleration sensor before and after temperature correction in the embodiment of the present invention.

  Hereinafter, an embodiment for carrying out the present invention will be described with reference to the drawings. The same reference numerals are given to the configurations, members, and processes shown in each drawing, and the repeated description will be omitted as appropriate. The modes for carrying out the invention are not intended to limit the invention, but are exemplifications, and all features and combinations described in the embodiments are not necessarily essential to the invention. .

Hereinafter, an embodiment of the present invention will be specifically described with reference to FIGS.
FIG. 2 is an equivalent circuit in which a piezoelectric charge output acceleration sensor (hereinafter may be abbreviated as “acceleration sensor”) of the present invention is represented by a detection unit 8 and a temperature correction capacitor 9. FIG. 3 is an equivalent circuit diagram showing the detection unit 8 as a voltage source 11 and a detection unit capacitance 10 (capacitance D), and a temperature correction capacitor 9 added thereto according to the present invention.

By the way, in general, a stable temperature characteristic at least in a low temperature range (−40 ° C.) to a high temperature range (120 ° C., usually about 80 ° C.) is required as a working temperature environment of the piezoelectric charge output acceleration sensor.
Here, the temperature characteristics are shown in FIG. 4, where the horizontal axis represents temperature, and the range is from −20 ° C. to 120 ° C. The left side of the vertical axis represents the internal capacitance (unit pF) of the detection unit 8 of the acceleration sensor, and the right side of the vertical axis represents the charge sensitivity (unit pC / m / s ² ) of the detection unit 8 of the acceleration sensor. .

However, as shown in FIGS. 4 and 5, even if the same acceleration is applied, the charge sensitivity of the charge output of the detection unit 8 of the acceleration sensor changes with temperature. FIG. 5 shows a change coefficient A of the charge sensitivity depending on the temperature of the detection unit 8 and a change coefficient B of the capacitance of the detection unit 8 depending on the temperature. With reference to a temperature of 20 ° C. (the value at 20 ° C. in FIG. 4 was set to 1), a change coefficient A of the charge sensitivity of the detection unit at a temperature of 120 ° C. and a change coefficient B of the capacitance of the detection unit 8 due to the temperature are shown. Indicated.
When the change coefficient is compared with the temperature of 20 ° C., the slope of the change coefficient tends to be higher at around 120 ° C. than at around −20 ° C., which is not a straight line. Even if the temperature correction is performed, the temperature coefficient cannot be completely set to 1, indicating that the temperature coefficient has a certain width.

Also, let us focus on the change coefficient A of the charge sensitivity. As can be seen from FIG. 5, a change coefficient of about 0.9 near -20 ° C. and about 1.3 near 120 ° C. is observed at a temperature of 20 ° C. As can be seen from these measurements, it can be seen that they are of considerable magnitude in temperature measurement of acceleration, which cannot be ignored, compared to room temperature.
As described above, it was confirmed that the charge sensitivity of the piezoelectric charge output acceleration sensor changes depending on the temperature, and specifically, the charge sensitivity increases with a slope of about 0.3% / ° C. with respect to the temperature rise.

In this measurement, the acceleration sensor having the detection unit 8 was placed in a thermostat and vibration was applied to perform measurement. The figure shows the detection unit capacitance and charge sensitivity obtained at each temperature.
In addition, the acceleration sensor has a variation in the curve of FIG. 4 due to manufacturing variations of the piezoelectric material or the like to be used, and it is about ± 5% experimentally. Further, it has been confirmed that different piezoelectric materials produce different differences.

  For this reason, in the present invention, it is necessary to obtain data for FIG. 4 in advance. For the same piezoelectric material, it is necessary to take some sample data and obtain the curve of FIG. 4 as an average value.

Next, the present invention utilizes the fact that the charge sensitivity of the detection unit 8 of the acceleration sensor is the product of the voltage sensitivity and the detection unit capacitance, and calculates the temperature coefficient B of the capacitance of the detection unit 8 as the voltage sensitivity. If the product of the temperature coefficient and the temperature coefficient can be corrected to 1, the temperature coefficient of the charge sensitivity can be set to 1, that is, the value with the minimum change with respect to temperature. Specifically, a temperature correction capacitor is connected in series to the detection unit 8 of the acceleration sensor, thereby artificially adjusting the temperature coefficient of the capacitance of the detection unit.
Specifically, by using a temperature correcting capacitor 9 (capacitance E) and connecting the temperature correcting capacitor 9 and the detecting unit 8 in series, the result shown in FIG. 6 was obtained. .
According to this, the charge sensitivity characteristic of the piezoelectric charge output before temperature correction is 1.12 at 120 ° C. on the basis of 20 ° C., but after temperature compensation (addition of the temperature correction capacitor 9), the charge sensitivity characteristic is 0 ° on the basis of 20 ° C. A significant improvement to 0.98 (120 ° C.) was observed.

Hereinafter, a theoretical description of a calculation formula for calculating the temperature correction capacitor 9 (capacitance E) will be given.
Generally, charge is expressed as the product of the capacitance of a charge source and a voltage.
The capacitance at the lower limit temperature of the temperature correction range of the detection unit 8 is D, the capacitance at the upper limit temperature is C0H, the charge sensitivity SQL at the lower limit temperature of the temperature correction range, the charge sensitivity SQH at the upper limit temperature is connected in series. Let E be the capacitance of the temperature correction capacitor. If the combined capacitance of the detecting unit 8 and the temperature correcting capacitor 9 at the lower limit temperature and the upper limit temperature is CL and CH, respectively, equations (2) and (3) are obtained.

CL = DE / (D + E) (2)
CH = C0H.E / (C0H + E) (3)

Assuming that the voltage sensitivity at the lower limit temperature obtained from the charge sensitivity SQL, SQH and the capacitance D, C0H of the detection unit 8 is SVL, and the voltage sensitivity at the upper limit temperature is SVH.

SVL = SQL / D (4)
SVH = SQH / C0H (5)

Assuming that the charge sensitivity at the lower limit temperature when the capacitance E of the temperature correction capacitor 9 connected in series is connected is SQLL and the charge sensitivity at the upper limit temperature is SQHH, the charge sensitivity is the voltage sensitivity and the capacitance. And the equations (6) and (7) are obtained from the equations (4) and (5).

SQLL = CL · SVL = CL · SQL / D (6)
SQHH = CH.SVH = CH.SQH / C0H (7)

Correcting the charge sensitivity so that there is no change in the temperature characteristic means that both SQLL and SQHH are equalized, and therefore, Equation (8) is obtained.
SQLL = SQHH (8)

In Expression (8), Expression (9) is obtained from Expressions (2) and (3) and Expressions (6) and (7).

CL · (SQL / D) = CH · (SQH / C0H)
(D · E / (D + E)) · (SQL / D) = (C0H · E / C0H + E)) · (SQH / C0H)
(E / (D + E)) · SQL = (E / (C0H + E)) · SQH
SQL / (D + E) = SQH / (C0H + E) (9)

Here, the temperature coefficient A of the charge sensitivity of the detection unit 8 and the temperature coefficient B of the capacitance are used.

SQH = A · SQL (10)
C0H = BD (11)

Substituting equations (10) and (11) into equation (9)

SQL / (D + E) = A · SQL / (B · D + E)
1 / (D + E) = A / (B · D + E)
B · D + E = A · (D + E)
B • D + E = A • D + A • E
E−A · E = A · D−B · D = D · (A−B)
(1-A) · E = D · (AB)
E = D · (AB) / (1-A) (1)

Equation (1) is obtained.
Since E> 0, it is necessary that A> 1 and B> A are satisfied.

DESCRIPTION OF SYMBOLS 1 Piezoelectric element 2 Load mass 3 Electrode plate 4 Tightening screw 5 Connector 6 Case 7 Conductor 8 Detector 9 Temperature compensation capacitor 10 Detector capacitance 11 Voltage source

A A coefficient of change of the charge sensitivity of the detection unit 8 with temperature B A change coefficient of the capacitance D of the detection unit 8 with temperature C A change coefficient D of the voltage sensitivity of the detection unit 8 The capacitance of the detection unit 8 (the lower limit temperature of the temperature correction range) )
E Capacitance of capacitor 9 for temperature compensation

Claims (3)

A change coefficient A of the charge sensitivity of the detection unit in the piezoelectric charge output acceleration sensor 8 due to the temperature and a change coefficient B of the capacitance D of the detection unit due to the temperature are measured at a plurality of points of the temperature. The change coefficient B is calculated, and the change coefficient A and the temperature correction capacitor 9 having the capacitance E calculated by the equation (1) from the change coefficient B are serially connected to the detection unit 8. Connected, and the temperature compensation of the charge sensitivity is performed by the capacitance E calculated from the change coefficient A and the change coefficient B of the charge sensitivity. As a result, a comparison is made between 20 ° C. before and after the temperature compensation. A piezoelectric charge output acceleration sensor wherein a flattened degree is in a range of 0.9 to 1.05 at a flattened change coefficient rate.
E = D (BA) / (A-1) (1)
Here, the change coefficient A is a change coefficient of the charge sensitivity of the detection unit 8 due to the temperature, the change coefficient B is a change coefficient of the capacitance of the detection unit 8 due to the temperature, and D is a static coefficient of the detection unit 8 at the lower limit temperature of the temperature correction range. It is capacitance. Each of the change coefficients is a coefficient calculated from a value at the upper limit of the temperature correction range based on the lower limit of the temperature correction range.   2. The piezoelectric charge output acceleration sensor according to claim 1, wherein the degree of flattening is in a range of 0.95 to 1.05.   The piezoelectric type according to claim 1, wherein a change coefficient based on 20 ° C. comparing before and after the compensation of the charge sensitivity is in a range of 0.95 to 1.05 and can be used at −40 ° C. to + 120 ° C. 3. Charge output acceleration sensor.

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