JP2001099656A - Acceleration/angular velocity sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an acceleration/angular velocity sensor using a piezoelectric vibration gyro which is compact, simple in structure and simple to manufacture, highly accurate and utilizes an energy-confined vibration mode. SOLUTION: A strip-shaped first electrode is set in a direction at right angles to an excitation direction to one of two positions separated via a predetermined distance in the excitation direction at a nearly central part of a primary face of a part polarized in a thickness direction of a piezoelectric plate. Moreover, strip-shaped second and third electrodes are formed extending via a distance in the direction at right angles to the excitation direction to the other position. A driving voltage for excitation is impressed between the first electrode and, the second and third electrodes, and electromotive forces corresponding to vibrations by a Coriolis force generated between the second and third electrodes are detected. A sum of the electromotive forces is detected as an acceleration, and a difference of the electromotive forces is detected as an angular velocity. The acceleration/angular velocity sensor is thus obtained with the utilization of a vibrational energy-confined mode between the first electrode and, the second and third electrodes.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a sensor for detecting acceleration and angular velocity, and more particularly, to a piezoelectric vibration type acceleration sensor and an angular velocity sensor using an energy trapping vibration mode of a piezoelectric vibrator.

[0002]

2. Description of the Related Art Conventionally, a so-called piezoelectric vibration gyroscope using a piezoelectric vibrator (hereinafter referred to as a piezoelectric vibration gyroscope for simplicity) is known as an angular velocity sensor. When the piezoelectric vibrator rotates around an axis perpendicular to the excitation direction while the piezoelectric vibrator is excited in a certain direction, the piezoelectric vibrating gyroscope is perpendicular to the excitation direction and the rotation axis. It detects the rotational angular velocity by detecting the Coriolis force generated in the direction of. More recently, for example, car navigation systems, VT
It has come to be used for a camera shake correction mechanism of an R camera.

As a piezoelectric vibrating gyroscope, a piezoelectric vibrating gyroscope utilizing an energy trapping vibration mode using a piezoelectric vibrator vibrating in an energy trapping vibration mode in which vibration energy is concentrated in the vicinity of a driving electrode is disclosed in, for example, Japanese Patent Application Laid-Open Publication No. Sho. 62-162915 and JP-A-5-3225
No. 80.

In the piezoelectric vibrating gyroscope utilizing the energy trapping vibration mode, since the vibration energy is concentrated on a local portion of the piezoelectric vibrator, the support of the piezoelectric vibrator is easy and easy, and the need for a separated lead wire is eliminated. There are advantages.

Japanese Patent Application Laid-Open No. Sho 62-162915 discloses that in order to confine vibration energy locally, the thickness of the vibrator is locally increased, the portion is polarized in the thickness direction, and a driving electrode is provided on the opposite end face of the thick local portion. And a detection electrode provided on the opposite side surface. As another example, there is disclosed an example in which a drive electrode and a detection electrode are provided on one surface of a piezoelectric plate, and interdigital electrodes are provided between the drive electrodes as detection electrodes.

Japanese Patent Application Laid-Open No. 5-322580 discloses that a partial area of a piezoelectric plate is polarized in a thickness direction and a main surface of the polarized area has
A piezoelectric vibrating gyroscope is disclosed in which a pair of opposing electrodes are provided so that the opposing directions are at right angles, one opposing electrode is a drive electrode, and the other opposing electrode is a detection electrode.

[0007]

Problems to be Solved by the Invention
The piezoelectric vibrating gyroscope using the energy confinement vibration mode disclosed in Japanese Patent No. 15 has manufacturing difficulties that the thickness of the piezoelectric plate must be locally increased or the interdigital electrodes must be formed. In addition, since the pair of drive electrodes and the pair of detection electrodes are provided near each other, the drive electric field applied from the drive electrodes to the piezoelectric plate is affected by the detection electrodes, and the direction of the drive electric field changes, thereby obtaining accuracy. The disadvantage of not being seen.

The piezoelectric vibrating gyroscope utilizing the energy trapping vibration mode disclosed in Japanese Patent Application Laid-Open No. 5-322580 has a simple structure and is easy to manufacture. However, a pair of driving electrodes and a pair of detecting electrodes are provided near each other. So
The drive electric field applied from the drive electrode into the piezoelectric plate is affected by the detection electrode, and the direction of the drive electric field changes, making it difficult to obtain a highly accurate detection output.

On the other hand, in the application of the piezoelectric vibrating gyroscope, it is sometimes necessary to know not only the angular velocity of rotation applied to the piezoelectric vibrating gyroscope from the outside but also the acceleration. The conventional piezoelectric vibrating gyroscope detects only angular velocity and cannot detect acceleration at the same time.

Accordingly, an object of the present invention is to provide a piezoelectric vibration type acceleration / angular velocity sensor which is small in size, simple in structure and manufacture, and utilizes a high-precision energy trapping vibration mode.

According to the present invention, an output electrode is shared with a part of a drive electrode in a limited area of a main surface of a piezoelectric plate.
An object of the present invention is to provide an acceleration / angular velocity sensor using an energy trapping vibration mode in which an output electrode does not adversely affect a driving electric field.

[0012]

According to the present invention, there is provided a piezoelectric vibrator which is excited in one direction along the plane of a piezoelectric plate, detects vibration caused by Coriolis force generated by rotation of the piezoelectric plate, and detects a rotational angular velocity. In the gyroscope, one of two positions on the main surface of a portion having polarization in the thickness direction of the piezoelectric plate and separated from each other by a predetermined distance in the excitation direction is stripped in a direction perpendicular to the excitation direction. And a strip-shaped second and third electrode extending at a distance from each other in a direction substantially perpendicular to the excitation direction at the other position. A driving voltage for excitation is applied between the second and third electrodes, and an electromotive force corresponding to the vibration caused by the Coriolis force generated in the second and third electrodes is detected, respectively. Acceleration as the sum of The acceleration is detected as the difference between the two electromotive forces, and the energy trapping mode of the thickness shear vibration generated between the first electrode and the second and third electrodes is used. An acceleration / angular velocity sensor is obtained.

In the acceleration / angular velocity sensor, the second and third electrodes are respectively connected to first and second current detection circuits having a virtual grounding function.
A drive voltage for excitation is applied to the electrodes of the first and second electrodes.
, And the difference can be obtained as an angular velocity detection output.

In the above acceleration / angular velocity sensor,
An addition circuit connected between the outputs of the first and second current detection circuits for detecting the sum of the output voltages of the two current detection circuits, a first synchronous detection circuit connected to the output of the addition circuit,
An acceleration detection circuit comprising a first rectifier circuit connected to an output of the first synchronous detection circuit and obtaining an acceleration detection output from the first rectification circuit; and outputs of the first and second current detection circuits A differential circuit connected between the two current detection circuits for detecting a difference between output voltages of the two current detection circuits, a second synchronous detection circuit connected to the differential circuit output, and an output of the second synchronous detection circuit. An angular velocity detection circuit comprising a second rectifier circuit connected thereto and obtaining an angular velocity detection output from the second rectifier circuit; and a signal of a self-excitation drive frequency connected between outputs of the first and second current detection circuits. And a drive circuit connected to the output of the oscillation circuit and applying an AC voltage of the self-excitation drive frequency to the first electrode.

As the piezoelectric plate, a piezoelectric ceramic in which only a region between the first to third electrodes and a region in the vicinity thereof is polarized in the thickness direction can be used.
Alternatively, a piezoelectric crystal plate having a polarization axis in the thickness direction can be used.

According to the present invention, there is provided a piezoelectric vibration gyroscope which excites in one direction along a plane of a piezoelectric plate, detects vibration due to Coriolis force generated by rotation of the piezoelectric plate, and detects a rotational angular velocity. One of two positions on the main surface of a portion having polarization in the thickness direction of the piezoelectric plate, which is separated from each other by a predetermined distance in the excitation direction, has a strip-shaped first shape in a direction perpendicular to the excitation direction. And at the other position, strip-shaped second and third electrodes extending at a distance from each other in a direction substantially perpendicular to the excitation direction, and the first electrode and the second and third electrodes are formed. A drive voltage for excitation is applied between the second electrode and the third electrode.
And an electromotive force corresponding to the vibration caused by the Coriolis force generated in the third electrode is detected, and an acceleration is detected as a sum of the two electromotive forces. An acceleration sensor using the energy trapping mode of the thickness shear vibration generated between the second and third electrodes can be obtained.

In this acceleration sensor, the second and third electrodes are respectively connected to first and second current detecting circuits having a virtual grounding function, and a driving voltage for excitation is applied to the first electrode. To obtain the sum of the outputs of the first and second current detection circuits as an acceleration detection output.

In this acceleration sensor, an addition circuit is connected between the outputs of the first and second current detection circuits for detecting the sum of the output voltages of the two current detection circuits, and is connected to the output of the addition circuit. An acceleration detection circuit comprising a synchronous detection circuit, a rectifier circuit connected to an output of the synchronous detection circuit, for obtaining an acceleration detection output from the rectifier circuit, and an automatic detection circuit connected between outputs of the first and second current detection circuits. An oscillation circuit for oscillating a signal of the excitation drive frequency, and a drive circuit connected to an output of the oscillation circuit and applying an AC voltage of the self-excitation drive frequency to the first electrode can be provided.

[0019]

FIG. 1 is a perspective view showing a configuration of a vibrator of an energy-trap type vibrating gyroscope used in an embodiment of the present invention. As shown in FIG. 1, the piezoelectric vibrator uses a piezoelectric plate 10 made of a piezoelectric ceramic plate such as PZT or barium titanate and having a central portion having a polarization axis in a thickness direction. A strip-shaped first electrode 11 is provided on the main surface of the central portion of the piezoelectric plate 10. A second electrode 12 and a third electrode 13 are formed apart from each other at a position separated from the first electrode 11 by a predetermined distance in parallel with the first electrode. These electrodes 1
1, 12, 13 are terminal portions 1 for leading out to the outside.
4, 15, and 16 are connected. Note that these electrodes and terminal portions are preferably made of silver paste or gold sputtering. Of course, other conductive films can be employed. The terminal for leading out to the outside may be a lead wire without being provided on the vibrator.

As shown in the figure, three-dimensional coordinates are determined in which the thickness direction is the Z axis, the direction facing the electrodes is the X axis, and the direction orthogonal thereto is the Y axis.

When a driving voltage (alternating current) is applied between the first electrode 11 and the second and third electrodes, vibration in the X direction is excited. In this state, when the piezoelectric plate 10 rotates around the Z axis, a vibration due to the Coriolis force occurs in the Y direction, thereby generating an electromotive force between the second and third electrodes. By detecting this electromotive force, the magnitude of the vibration due to the Coriolis force can be detected.

Since the energy of the vibration is confined in the central portion of the piezoelectric plate and does not reach the periphery, it is easy to support the peripheral portion of the piezoelectric plate.

FIG. 2 is a block diagram showing a circuit configuration connected to the piezoelectric vibrator 1 of FIG. Referring to FIG.
Current detection circuits 18 and 19 are connected to the second and third electrodes 12 and 13 of the piezoelectric vibrator 10, respectively. The output side of the current detection circuits 18 and 19 has an addition circuit 2
2 is connected, and an acceleration detection output is obtained via the first synchronous detection circuit 23 and the first rectification circuit 24. On the other hand, the output side of the current detection circuits 18 and 19 further includes a differential circuit 2
5 is connected, and an angular velocity detection output is obtained via a second synchronous detection circuit 26 and a second rectification circuit 27. The current detection circuits 18 and 19 are connected to an oscillation circuit 28 for satisfying a self-excited oscillation condition, and a driving circuit 2 for applying X-direction vibration.
It is connected to the first electrode 11 through the line 9 and constitutes a self-excited oscillation circuit. An AC voltage having a frequency substantially equal to the resonance frequency of the thickness shear vibration of the piezoelectric vibrator is applied to the electrode 11 by the self-excited oscillation circuit.

FIG. 3 shows the current detection circuits 18 and 1 of FIG.
9 is a diagram showing a configuration example of No. 9 and has a function of virtually grounding the second and third electrodes 12 and 13.
In this circuit, the non-inverting input terminal (+) of the operational amplifier 31 is grounded to the reference voltage, and a resistor R is connected from the output terminal of the operational amplifier 31 to the inverting input terminal. The inverting input terminal (-) is always kept at the ground reference potential by the virtual ground function of the operational amplifier. When a current flows into this inverting terminal, it is converted into a voltage by the resistor R. That is, an output of Vout = −iR is obtained. That is, this current detection circuit is a circuit that has a functionally zero input impedance and can obtain an output voltage proportional to the input current.

This current detection circuit is the current detection circuit 1 shown in FIG.
Used for 8 and 19. At that time, the inverting input terminal (-) is connected to the second and third electrodes 12 and 13. As a result, the driving voltage is applied between the first electrode 11 and the second and third electrodes.
And between the electrodes 12 and 13 of the second
And the electromotive force between the third electrodes 12 and 13 can be detected as a potential difference between the outputs of the current detection circuits 18 and 19.

Next, the driving principle of the piezoelectric vibrator in the k-th embodiment will be specifically described with reference to the drawings.

FIGS. 4A and 4B show FIGS. 1 and 2 respectively.
FIG. 2 is a plan view showing a basic structure of the energy trap type vibrator shown in FIG.
Referring to FIGS. 4A and 4B, X is placed on the same plane at the center of the piezoelectric plate 10 polarized in the thickness direction (Z-axis direction).
Slit-shaped electrodes D1 and D2 facing each other with an interval in the axial direction are formed. T1 and T2 are terminal portions. When a voltage is applied between the terminals T1 and T2, an electric field is applied to a region (inter-electrode region) of the piezoelectric plate 10 between the opposing electrodes D1 and D2 in a direction substantially parallel to the plate surface (X direction). Therefore, due to the interaction between the electric field and the polarization in the thickness direction (Z-axis direction) orthogonal to the electric field, a strain is generated in the inter-electrode region in the X direction. Electrode D1,
If the dimension of D2 is designed according to the characteristics of the piezoelectric plate 10 and the applied voltage is an AC voltage having a frequency matching the resonance frequency of the inter-electrode region, thickness shear vibration can be excited in the inter-electrode region. The vibration is attenuated around the inter-electrode region and confined without propagating. That is, an energy trapping oscillator can be configured. Also, this vibration
Since the thickness shear vibration is generated by an electric field parallel to the surface of the piezoelectric plate, it is called a parallel electric field excitation type thickness shear vibration. Note that the thickness shear vibration is vibration in which the direction of displacement of the piezoelectric plate is parallel to the plate surface and the direction of wave propagation is the thickness direction of the plate. In order to illustrate the state of this vibration, FIG. 5 shows a displacement distribution in the thickness direction (Z-axis direction) when resonating at a half wavelength.

The vibrator shown in FIGS. 1 and 2 utilizes the above basic structure. That is, the first electrode 11
The second electrode 12 and the third electrode 13 which are one electrode and oppose this are D2 electrodes. The D2 electrode is divided into two to form a detection electrode and forms a second electrode 12 and a third electrode 13, which are respectively connected to current detection circuits 18 and 19 having a virtual ground function. Thus, the second electrode 12 and the third electrode 13 are virtually maintained at the reference potential, and thus can be regarded as a ground terminal in terms of potential. Therefore, when a drive voltage for excitation having a frequency substantially equal to the resonance frequency of the thickness-shear vibration mode of the piezoelectric plate is applied to the first electrode 11, the first, second, In the region (inter-electrode region) surrounded by the first and second electrodes 11, 12 and 13, the center of the first electrode 11 and the second and third electrodes 12 and 1 are located.
Direction (X direction) of the line connecting the midpoints of the lines connecting the centers of 3
The thickness shear vibration of the energy trapping vibration mode occurs.

In this state, when the piezoelectric plate 10 is rotated around an axis orthogonal to the main surface thereof, the direction (Y) perpendicular to the direction of the excited thickness shear vibration is generated by the action of Coriolis force. Direction), thickness shear vibration occurs. By the thickness shear vibration generated by this Coriolis force, the first
Between the first electrode 11 and the second electrode 12 and the first electrode 1
The impedance between the first and third electrodes 13 changes,
As a result, the value of the current flowing into the current detection circuits 18 and 19 changes. Second electrode 12 and third electrode 1
As described above, 3 is arranged symmetrically with respect to the direction of the excited thickness-shear vibration (X direction), so that the current that changes due to the vibration due to the Coriolis force flowing into the current detection circuits 18 and 19 is , Equal in amplitude and 180
The currents have different phases. Therefore, the current detection circuit 1
The output voltages of 8 and 19 are also voltages having phases different from each other by 180 degrees.
2 to obtain a DC output voltage proportional to the applied rotational angular velocity as a detection output by synchronously detecting the voltage at a predetermined timing by a synchronous detection circuit 23 and rectifying the voltage by a rectifier circuit 24. Can be done.

On the other hand, the current detection circuits 18 and 19 are provided with an oscillation circuit 25 for satisfying a self-excited oscillation condition and an oscillator driving circuit 2
6 and connected to the electrode 11 to form a self-excited oscillation loop. Thus, the vibrator can be efficiently driven by automatically resonating the resonance frequency of the vibrator, so that a gyro with high sensitivity can be obtained.

FIG. 6 shows another circuit configuration.
Electrode 11 is grounded, and a resistor R is provided between the second and third electrodes.
A voltage dividing circuit composed of a series circuit of R1 and R2 is connected, and the oscillation driving circuit 30 is controlled by the divided voltage. The oscillation drive circuit is grounded, so that a drive voltage is applied between the first electrode 11 and the second and third electrodes. Note that the oscillation drive circuit 30 includes the oscillation circuit 28 and the drive circuit 29 shown in FIG. Both ends of the voltage dividing circuit are connected to the differential amplifier circuit 25. The differential amplifier circuit 25, the detection circuit 23, and the rectifier circuit 24 are the same as those in FIG.

Here, the expression that the piezoelectric plate used in the present invention has polarization in the thickness direction is not limited to the one polarized only in the thickness direction, but also includes the one having a polarization component in the thickness direction. Shall be considered. Of course, the larger the polarization component in the thickness direction is, the better the polarization component is in the thickness direction.

When a piezoelectric ceramic is used as the piezoelectric plate 10, a polarization process is required as is well known. However, the polarization region may be over the entire piezoelectric plate, or may be limited to only the region where vibration is confined. May be.

As the piezoelectric plate 10, a piezoelectric crystal plate (for example, quartz, LiNbO3, LiTO3, etc.) can be used. In that case, in order to have a polarization axis in the thickness direction, Z
Although a cut plate is most preferred, a rotational Y-cut plate can also be used.

[0035]

According to the present invention, driving is performed using only a pair of slit-like electrodes arranged in parallel on the main surface of the piezoelectric plate, and one of the pair of electrodes is divided into two. Since the detection is performed as a detection electrode, the drive electric field is not adversely affected by the presence of the detection electrode, and the Y-direction vibration can be detected while maintaining the electric field for exciting the X-direction vibration. A vibrating gyroscope can be obtained. In addition, since the energy-confined vibration is used, it is easy to support and a highly reliable gyro can be obtained.

Further, it is not necessary to locally change the thickness of the piezoelectric plate or to form a complicated electrode structure. As a vibrator, a pair of parallel electrodes (one of which is provided in a predetermined region of an arbitrary-shaped piezoelectric plate) is used. (Although it is necessary to divide it into two parts), the structure and manufacturing are simple.

[Brief description of the drawings]

FIG. 1 is a perspective view showing a configuration of a piezoelectric vibrator according to an embodiment of the present invention.

FIG. 2 is a block diagram showing a circuit configuration of a gyro using a piezoelectric vibrator.

FIG. 3 is a circuit diagram illustrating an example of a current detection circuit used in the circuit of FIG. 2;

4A and 4B are diagrams showing a basic structure of a vibrator employed in the piezoelectric vibrator of FIGS. 1 and 2, wherein FIG. 4A is a plan view and FIG.

FIG. 5 is a diagram showing a displacement distribution in a thickness direction in thickness shear vibration of the oscillator having the basic structure of FIG. 4;

FIG. 6 is a block diagram showing another circuit configuration of a gyro using a piezoelectric vibrator.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Piezoelectric vibrator 10 Piezoelectric plate 11 1st electrode 12 2nd electrode 13 3rd electrode 14, 15, 16, terminal part 18 Current detection circuit 19 Current detection circuit 22 Addition circuit 23, 26 Synchronous detection circuit 24, 27 Rectifier circuit 25 Differential circuit 28 Oscillation circuit 29 Drive circuit 30 Oscillation drive circuit 31 Operational amplifier

Claims (8)

[Claims] 1. Exciting in one direction along a plane of a piezoelectric plate,
In a piezoelectric vibrating gyroscope for detecting a rotational angular velocity by detecting vibration due to Coriolis force generated by rotation of the piezoelectric plate, the piezoelectric plate has a main surface having a polarization in a thickness direction of the piezoelectric plate. Of the two positions separated by a predetermined distance, a strip-shaped first electrode is provided at one position in a direction perpendicular to the excitation direction, and the other electrode is extended at a distance substantially perpendicular to the excitation direction at the other position. Forming second and third electrodes in the form of strips,
A driving voltage for excitation is applied between the first electrode and the second and third electrodes, and electromotive forces corresponding to the Coriolis force generated in the second and third electrodes are detected. The acceleration is detected as the sum of the two electromotive forces, and the acceleration is detected as the difference between the two electromotive forces, between the first electrode and the second and third electrodes. An acceleration / angular velocity sensor using the energy trapping mode of the generated thickness shear vibration. 2. The acceleration / angular velocity sensor according to claim 1, wherein the second and third electrodes are respectively connected to first and second current detection circuits having a virtual grounding function.
A drive voltage for excitation is applied to the first electrode,
An acceleration / angular velocity sensor, wherein the sum of the outputs of the second current detection circuit and the second current detection circuit is obtained as an acceleration detection output, and the difference is obtained as an angular velocity detection output. 3. The acceleration / angular velocity sensor according to claim 2, wherein the addition circuit is connected between outputs of the first and second current detection circuits and detects a sum of output voltages of the two current detection circuits. An acceleration detection circuit comprising a first synchronous detection circuit connected to the output of the adder circuit, and a first rectifier circuit connected to the output of the first synchronous detection circuit, for obtaining an acceleration detection output from the first rectifier circuit A differential circuit connected between the outputs of the first and second current detection circuits for detecting a difference between output voltages of the two current detection circuits, and a second synchronization circuit connected to the differential circuit output An angular velocity detection circuit comprising a detection circuit, a second rectification circuit connected to the output of the second synchronous detection circuit, and obtaining an angular velocity detection output from the second rectification circuit; and the first and second current detection circuits Self-excited drive frequency connected between circuit outputs An oscillation circuit for oscillating a number of signals, and a drive circuit connected to an output of the oscillation circuit and applying an AC voltage of the self-excitation drive frequency to the first electrode. Acceleration and angular velocity sensors. 4. The acceleration / angular velocity sensor according to claim 1, wherein a piezoelectric ceramic is used as the piezoelectric plate, and only a region between the first to third electrodes of the piezoelectric ceramic and a region in the vicinity thereof. An acceleration / angular velocity sensor characterized in that is polarized in the thickness direction. 5. The acceleration / angular velocity sensor according to claim 1, wherein a piezoelectric crystal plate having a polarization axis in a thickness direction is used as the piezoelectric plate. 6. A piezoelectric vibrating gyroscope that excites in one direction along a plane of a piezoelectric plate, detects vibration due to Coriolis force generated by rotation of the piezoelectric plate, and detects a rotational angular velocity. A first electrode having a strip shape in one of two positions on a main surface of a portion having polarization in a direction and separated by a predetermined distance in the excitation direction in a direction perpendicular to the excitation direction; At the other position, strip-shaped second and third electrodes extending at a distance from each other in a direction substantially perpendicular to the excitation direction are formed, and the first electrode and the second and third electrodes are connected to each other. A drive voltage for excitation is applied during that time, and electromotive forces corresponding to the vibrations caused by the Coriolis force generated in the second and third electrodes are respectively detected, and acceleration is detected as the sum of the two electromotive forces. It is characterized by An acceleration sensor using an energy trapping mode of thickness shear vibration generated between the first electrode and the second and third electrodes. 7. The acceleration sensor according to claim 6, wherein the second and third electrodes are respectively connected to first and second current detection circuits having a virtual ground function, and the first and second electrodes are connected to each other. An acceleration sensor, wherein a driving voltage for excitation is applied to an electrode, and a sum of outputs of the first and second current detection circuits is obtained as an acceleration detection output. 8. The acceleration sensor according to claim 7, wherein the addition circuit is connected between outputs of the first and second current detection circuits and detects a sum of output voltages of the two current detection circuits. A synchronous detection circuit connected to the output, and an acceleration detection circuit comprising a rectifier circuit connected to the output of the synchronous detection circuit and obtaining an acceleration detection output from the rectifier circuit;
An oscillation circuit connected between outputs of the first and second current detection circuits for oscillating a signal of a self-excitation drive frequency;
A drive circuit connected to an output of the oscillation circuit and applying an AC voltage having the self-excited drive frequency to the first electrode.