JPH07177765A - Electrode structure of piezoelectric element in ultrasonic motor and polarizing method of piezoelectric element - Google Patents

Abstract

PURPOSE:To prevent as much as possible concentration of stress at the boundaries of respective polarized portions of a piezoelectric element and improve reliability, driving efficiency and output at the time of driving a motor. CONSTITUTION:An area of both end portions 20b in the circumferencial direction of a plurality of independent electrodes 20 arranged respectively to a couple of regions 3a, 3b at a single side of a circular piezoelectric element for rotating a rotor by generating therein a travelling wave is set smaller than the area of the central region 20a.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrode structure of a piezoelectric element in an ultrasonic motor and a method of polarizing the piezoelectric element. More specifically, the piezoelectric element made of ceramic or the like is polarized in different thickness directions and a driving voltage is applied. The present invention relates to a structure of a plurality of independent electrodes formed for applying a voltage and a polarization method of a piezoelectric element using the independent electrodes.

[0002]

2. Description of the Related Art In recent years, ultrasonic motors that convert electric energy into ultrasonic vibrations of mechanical energy to obtain rotational force have
It is used in the field of functional parts for automobiles and various robots because it has the advantages of quietness and high torque performance at low speed rotation.

As a method of the ultrasonic motor, a traveling wave type ultrasonic motor, a standing type ultrasonic motor, a composite vibrator type ultrasonic motor, etc. have been proposed, and particularly stable motor performance can be obtained for a long time. Technical studies have been made on a traveling wave type ultrasonic motor that can be used.

In a traveling wave type ultrasonic motor, a high frequency drive voltage is applied between an electrode formed on a piezoelectric element and a stator to cause the stator to resonate and vibrate to rotate the rotor.

As shown in FIG. 14, the piezoelectric element 1 is made of ceramic and is formed in an annular shape.
A plurality of independent electrodes 2 divided into a fan shape according to the wavelength are divided into two electrode groups 3a and 3b and are arranged side by side on the circumference. Between the two electrode groups 3a and 3b, an electrode region 4 partitioned by 3/4 wavelength and not contributing to driving and an electrode region 5 partitioned by ¼ wavelength not contributing to driving are provided. The electrode groups 3a and 3b are shifted from each other by ¼ wavelength. In addition, as shown in FIG. 15, two common electrodes 6a and 6b are provided on the opposite surface of each electrode group 3a and 3b.
Are formed so as to face each other. The independent electrode 2 and the common electrodes 6a and 6b are formed of a silver thin film suitable as an electrode when the piezoelectric element 1 is polarized. Then, as shown in FIG.
As shown in FIG. 6, a positive high voltage (several KV) is applied from the power source 7 to every other one of the common electrodes 6a and 6b and the independent electrode 2 of each electrode group 3a and 3b to perform polarization processing in the thickness direction. Then, a negative high voltage is applied from the power source 7 to the remaining independent electrodes 2 to perform polarization processing in the opposite thickness direction.

In this way, as shown in FIGS. 17 and 18, the polarized portions corresponding to the independent electrodes 2 of the piezoelectric element 1 in the two common electrodes 6a and 6b (hereinafter referred to as segments) 9 are formed. Polarization treatments in different thickness directions are alternately performed. The polarization direction and the polarization strength by this polarization treatment are indicated by an arrow Y. Further, the plus signs are common electrodes 6a and 6b.
Shows the polarization in the direction of the independent electrode 2 and the minus sign shows the polarization in the opposite direction.

Then, as shown in FIG. 17, the piezoelectric element 1 subjected to the polarization treatment is attached so that the independent electrode 2 comes into contact with an annular stator 8 (only part of which is shown). An ultrasonic motor is constructed by bringing a rotor (not shown) into pressure contact with. With such a configuration, when a high frequency voltage with a 90 ° phase shift is applied between the stator 8 and the common electrodes 6a and 6b, a progressive wave of bending vibration is generated in the circumferential direction of the piezoelectric element 1 and the stator 8. At this time, the waveform of the traveling wave is a sine wave. Then, the rotor moves in a direction opposite to the traveling direction of the traveling wave and is driven as a motor. This bending vibration is generated, for example, when a positive voltage is applied to the common electrodes 6a and 6b.
And a portion of the stator 8 corresponding to the plus sign extends in the circumferential direction, and each portion corresponding to the minus sign contracts in the circumferential direction. Conversely, when a negative voltage is applied, each part corresponding to the plus sign contracts and each part corresponding to the minus sign expands.

[0008]

However, in the piezoelectric element 1 of the ultrasonic motor described above, a portion corresponding to the plus sign of the piezoelectric element 1 is generated when a traveling wave of bending vibration is generated by the application of a high frequency voltage. The stress is likely to be concentrated on the boundary with the portion corresponding to the minus sign, that is, the boundary between the extended portion and the contracted portion. This stress concentration is such that in each segment 9 of the piezoelectric element 1, the distribution of the polarization intensity shown by the arrow Y in FIG. 17 is uniform in the circumferential direction, and when a voltage is applied, the boundary portion of each segment 9 (indicated by a two-dot chain line) It is caused by the fact that the bending energy generated in FIG. 10 has the same magnitude as that of the central portion 9a. Therefore, there is a problem that the boundary portion 10 of each segment 9 may be damaged due to this stress concentration, and the reliability at the time of driving the motor may be reduced.

The movement of the rotor via the stator 8 is mainly due to the action of bending energy generated in the central portion 9a of each segment 9. Therefore, the boundary 1
The bending energy generated at 0 is not used effectively for moving the rotor, and is wasted energy. Therefore, there is a problem that the energy loss increases and the driving efficiency deteriorates.

Further, when the applied voltage is increased to increase the amplitude of the stator 8, the bending energy in the vicinity of the boundary portion 10 is also increased, so that the amplitude causes internal strain in the boundary portion 10 of the piezoelectric element 1 and is unnecessary. There is a problem in that vibration is induced, which makes it impossible to improve the output of the ultrasonic motor.

The present invention has been made to solve the above-mentioned problems, and an object thereof is to prevent stress from concentrating at the boundaries between the polarized portions of the piezoelectric element as much as possible. An object of the present invention is to provide an electrode structure of a piezoelectric element in an ultrasonic motor and a polarization method of the piezoelectric element, which can improve reliability, driving efficiency, and output when driving the motor.

[0012]

In order to solve the above-mentioned problems, the present invention as set forth in claim 1 provides at least two areas on one surface of a circular piezoelectric element for rotating a rotor by generating a traveling wave in a stator. In the structure of a plurality of electrodes arranged, the area of both ends of the electrode in the circumferential direction is made smaller than the area of the central part.

Further, the invention according to claim 2 is the structure of a plurality of electrodes arranged in at least two regions of at least one surface of a circular piezoelectric element for generating a traveling wave in the stator to rotate the rotor. The electric conductivity at both ends in the circumferential direction of is smaller than that at the center.

According to a third aspect of the present invention, there is provided a method of performing a polarization process in a thickness direction on a plurality of polarization regions of a circular piezoelectric element which causes a stator to generate a traveling wave to rotate a rotor. A high voltage was applied to the central portion in the circumferential direction in to polarize each polarization region.

[0015]

According to the invention of claim 1 having such a structure, the area of both ends of the electrode in the circumferential direction is made smaller than the area of the central part, so that when the piezoelectric element is polarized through the electrode, The polarization intensity of the polarized portion corresponding to the electrode changes in the circumferential direction. That is, the polarization distribution in the vicinity of the boundary between the polarized parts corresponding to the respective electrodes is smaller than that in the central part. Therefore, the bending energy generated when the piezoelectric element is driven by applying a high-frequency voltage to each polarized portion changes according to the polarization strength, and the vicinity of the boundary is weak, the center is strong, and the boundary is strong. Stress is prevented from being concentrated in the area as much as possible, and it is less likely to be damaged and reliability is improved.

Further, as compared with the central portion of each polarized portion, the power consumption in the vicinity of the boundary portion which is not directly used as the driving energy of the motor can be small, so that the driving efficiency is improved. Further, even if the applied voltage is increased to increase the amplitude of the piezoelectric element, the bending energy in the vicinity of the boundary is small, so that internal strain is less likely to occur at the boundary. As a result, unnecessary vibration is not induced and the output of the ultrasonic motor is improved.

According to the second aspect of the present invention, the electric conductivity at both ends in the circumferential direction of the electrode is made smaller than the electric conductivity at the central portion, so that the polarization strength of the polarized portion corresponding to the electrode is increased. Since the distribution is strong in the central part and weak in the vicinity of the boundary part, the same operation as described above is achieved.

According to the third aspect of the invention, a high voltage is applied to the central portion in the circumferential direction of the plurality of polarization regions of the piezoelectric element, and the polarization process is performed on each polarization region. By such a polarization process, the polarization intensity of each polarization portion changes in the circumferential direction of the electrode, and the distribution is weak in the central portion and weak in the boundary portion. Further, the polarization intensity in the direction orthogonal to the circumferential direction becomes uniform. Therefore, the same effect as that of claims 1 and 2 can be obtained.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS (First Embodiment) A first embodiment of the present invention will be described below with reference to FIGS.

The same components as those of the prior art are designated by the same reference numerals and the description thereof will be omitted. As shown in FIG. 1, on one surface of the piezoelectric element 1, a plurality of independent electrodes 20 partitioned by ½ wavelength are formed in two electrode groups 3a and 3b, respectively. The width of the central portion 20a in the circumferential direction of each independent electrode 20 is the same as the width of the piezoelectric element 1, and both end portions 20b
Is formed into a flat barrel shape whose width is smaller than that of the piezoelectric element 1. The electrode area ratio (electrode area / piezoelectric element area) in the circumferential direction of each of the independent electrodes 20 is shown in the graph of FIG. In this graph, the electrode area ratio is 100% at the central portion 20a of the independent electrode 20, decreases toward both end portions 20b, and the boundary portion (illustrated by a chain double-dashed line) 10 of each segment 9 corresponding to the independent electrode 20 is shown. Is 0%.

As shown in FIG. 3, a high voltage is applied between the independent electrode 20 and the common electrodes 6a and 6b in the same manner as in the prior art, and as shown by an arrow Y, the piezoelectric element 1 Polarization treatments in different thickness directions are alternately applied. The polarization intensity in each segment 9 of the piezoelectric element 1 changes in the circumferential direction according to the shape of the independent electrode 20, and when the polarization intensity is indicated by the arrow Y, the central portion 9a of each segment 9 is the most. It is large and decreases toward the boundary 10.
This polarization strength is proportional to the electrode area ratio shown in FIG.

In the ultrasonic motor using the piezoelectric element 1 which is polarized as described above, the common electrodes 6a and 6b are used.
When a high-frequency voltage having a 90 ° phase shift is applied between the stator and a stator (not shown), each polarized segment 9 expands and contracts to generate bending energy. And
Due to this bending energy, a progressive wave of sinusoidal bending vibration is generated in the circumferential direction of the piezoelectric element 1. At this time, the bending energy is generated in a magnitude proportional to the polarization strength indicated by the arrow Y, is largest in the central portion 9a, and becomes smaller toward the boundary portion 10. Further, the distribution of the bending energy at this time is close to the waveform of the traveling wave (sine wave).

Therefore, the bending energy generated in the vicinity of the boundary portion 10 becomes small according to the polarization strength, so that the concentration of stress can be prevented, and as a result, the boundary portion 10 of each segment 9 is damaged. It is possible to improve reliability in driving the motor, as well as to reduce the possibility of the occurrence of damage.

Further, since the bending energy of the central portion 9a of each segment 9, which is a point for moving the rotor through the stator, is the largest and the boundary portion 10 is smaller, the distribution of the bending energy is improved and it is wasted. Energy consumption is gone. Therefore, the energy loss in the vicinity of the boundary 10 of each segment 9 is reduced, and the driving efficiency of the motor can be improved. Further, since the polarization strength in the vicinity of the boundary 10 of each segment 9 is small, the input current becomes small. Therefore, it is possible to reduce power consumption when driving the motor.

Further, even if the applied voltage is increased to increase the amplitude of the stator, the bending energy in the vicinity of the boundary portion 10 is small, so that internal strain is less likely to occur in the boundary portion 10. As a result, unnecessary vibration is not induced, and the output of the ultrasonic motor can be improved.

(Second Embodiment) Next, a second embodiment will be described. In this embodiment, the shape of the independent electrode 20 is different from that of the first embodiment.

As shown in FIG. 4, each of the independent electrodes 20 has a substantially center-shaped portion 20a in the circumferential direction having the same width as the width of the piezoelectric element 1, and has a substantially oval shape in plan view which gradually narrows toward both end portions 20b. Is formed in. Although not shown, the electrode area ratio of each independent electrode 20 in the circumferential direction is 100% at the central portion 20a of the independent electrode 20, decreases toward both end portions 20b, and the boundary portion 10 of each segment 9 is 0. %.

Therefore, in the structure of the independent electrode 20 described above, each segment 9 when the piezoelectric element 1 is polarized is processed.
The polarization intensity at changes in the circumferential direction according to the shape of the independent electrode 20, is largest in the central portion 9a, and becomes smaller toward the boundary portion 10. As a result, the bending energy generated in the vicinity of the boundary portion 10 when a high frequency voltage is applied to the piezoelectric element 1 is reduced, and it is possible to prevent stress from being concentrated on the boundary portion 10 as much as possible. Further, since the electrode area ratio is close to the same sine wave as the traveling wave, the polarization strength, that is, the distribution of the bending energy is also close to the traveling wave, so that the motor driving efficiency and the output can be further improved.

(Third Embodiment) Next, a third embodiment will be described. In this embodiment, the external shape of the independent electrode 20 is the same as the conventional one, but the internal shape is different.

As shown in FIG. 5, in each of the independent electrodes 20, the width of the central portion 20a and both end portions 20b in the circumferential direction is the piezoelectric element 1.
Is formed in the same plane fan shape as the width. Further, a plurality of comb-shaped notches 21 are formed along the circumferential direction of the piezoelectric element 1 from both end portions 20b of the independent electrode 20 toward the vicinity of the central portion 20a. Due to this notch 21, the electrode area ratio of the independent electrode 20 in the circumferential direction has the same area distribution as in the above embodiment.

In the structure of the independent electrode 20 described above, since the independent electrode 20 is formed over the entire surface of each segment 9 of the piezoelectric element 1, the polarization strength is changed over the entire segment 9 except the notch 21. Then, the polarization treatment can be performed. As a result, when a high frequency voltage is applied to the piezoelectric element 1, a smooth extension and contraction operation can be obtained from the central portion 9a of each segment 9 to the boundary portion 10.

(Fourth Embodiment) Next, a fourth embodiment will be described. In this embodiment, the internal shape of the independent electrode 20 is different from that of the third embodiment.

As shown in FIG. 6, four small holes 22a to 22d having different inner diameters are arranged in a plurality of rows (in this case, 7 in this case) from both end portions 20b of each fan-shaped independent electrode 20 toward the vicinity of the central portion 20a. Columns) respectively.
The small holes 22a to 22d are formed to have a larger diameter as they are closer to both ends 20b. The electrode area ratio is the same as that of the third embodiment due to the small holes 22a to 22d.

Also in the structure of the independent electrode 20 described above,
Similarly to the third embodiment, the polarization strength can be changed and the polarization treatment can be applied to the whole of each segment 9.
A smooth extension and contraction operation can be obtained when a high frequency voltage is applied to.

By changing the number and size of the small holes 22a to 22d, the electrode area ratio of the independent electrode 20 can be slightly changed to adjust the polarization strength. (Fifth Embodiment) Next, a fifth embodiment will be described. In this embodiment, the internal shape of the independent electrode 20 is the third and fourth shapes.
Different from the embodiment.

As shown in FIG. 7, mesh-like meshes 23 are formed from both end portions 20b of each fan-shaped independent electrode 20 toward the central portion 20a. The mesh 23 has an electrode area ratio in the circumferential direction of the independent electrode 20 similar to that of the third embodiment.

Also in the structure of the independent electrode 20 described above,
The polarization strength can be changed by changing the polarization intensity of the entire segment 9, and smooth extension and contraction operations can be obtained when a high frequency voltage is applied to the piezoelectric element 1.

When the independent electrode 20 is formed by printing silver paste on the piezoelectric element 1, for example, the pattern of the mesh 23 can be formed relatively easily. (Sixth Embodiment) Next, a sixth embodiment will be described. In this embodiment, the material forming the independent electrode 20 is different.

As shown in FIG. 8, a semiconductor or carbon thin film having a conductivity lower than that of the silver thin film is formed in the vicinity of the central portion 20a of the independent electrode 20 in the vicinity of the central portion 20a, and in the vicinity of both end portions 20b (shown by a two-dot chain line) Consists of. Each thin film is formed by printing a paste-like silver powder and a semiconductor or carbon powder on each region of the surface of the piezoelectric element 1 and then performing a firing process.

When the piezoelectric element 1 is polarized by using the independent electrode 20 having the above-described structure, the semiconductor or carbon thin film is stronger than the polarization strength of the segment 9 corresponding to the silver thin film due to the difference in conductivity. The polarization intensity of the segment 9 corresponding to is decreased. Therefore, it is possible to obtain the distribution of the polarization strength in the circumferential direction of the segment 9. As a result,
When a high frequency voltage is applied to the piezoelectric element 1, the bending energy generated in the vicinity of the boundary portion 10 of the segment 9 can be made smaller than that in the central portion 9a.

(Seventh Embodiment) Next, a sixth embodiment will be described. In this embodiment, the external shape of the independent electrode 20 is the same as the conventional one, and the polarization method is different from the first to sixth embodiments.

As shown in FIG. 9, the independent electrode 2 is formed on one surface of the piezoelectric element 1, and the resistor 25 as an electrode is formed on the surface corresponding to each independent electrode 2 on the opposite side. ing. The resistor 25 is made of the same thin silver film as the independent electrode 2.

Next, the polarization method of the piezoelectric element 1 will be described. A positive high voltage (several K) between every other independent electrode 2 and every other resistor 25 corresponding to the independent electrode 2.
V) is applied from a power source (not shown). At the same time, a negative high voltage is applied between the remaining independent electrode 2 and the resistor 25. At this time, as shown in FIGS. 10 to 12, the conductive plate 26 that is brought into contact with the independent electrode 2 and the resistor 25 when a high voltage is applied has a plate shape having the same width as the radial width of the independent electrode 2. What is formed in is used. Further, the conductive plate 26 is arranged so as to contact the central portion 2a in the circumferential direction of the independent electrode 2 and the central portion 25a in the circumferential direction of the resistor 25.

When a positive high voltage is applied,
As shown in FIG. 11, the polarization intensity in the central portion 9a of each segment (polarization region) 9 is maximum, and the polarization intensity in the vicinity of the boundary portion 10 is from the resistor 25 having the minimum polarization intensity toward the independent electrode 2. Polarization processing is performed. Also, when a negative high voltage is applied,
As shown in FIG. 12, the polarization process is performed in the direction from the independent electrode 2 having the maximum polarization strength in the central portion 9a of each segment 9 and the minimum polarization strength in the vicinity of the boundary portion 10 to the resistor 25. It In this way, as shown in FIG. 13, the polarization treatments of the respective segments 9 in the different thickness directions are alternately performed, and the polarization strength is greatest in the central portion 9a in the circumferential direction, and the polarization strength is increased toward the boundary portion 10. The distribution becomes smaller. In addition, a uniform distribution of polarization strength is obtained in the radial direction of each segment 9.

After the polarization treatment, two common electrodes (not shown) made of the same silver thin film are formed on the surface of the piezoelectric element 1 on which the resistor 25 is formed so as to cover the resistor 25. As a result, the piezoelectric element 1 having the independent electrode 2 and the common electrode is obtained.

Piezoelectric element 1 polarized as described above
Since the polarization intensity is distributed in the circumferential direction in each segment 9, the bending energy generated when a high-frequency voltage is applied is large in the central portion 9a in the circumferential direction and smaller toward the boundary portion 10. To be distributed. Therefore, it is possible to prevent stress from concentrating on the boundary portion 10 as much as possible, and it is possible to improve reliability, drive efficiency, and output when the motor is driven.

The present invention is not limited to the above-mentioned embodiment, and may be carried out as follows, for example, without departing from the spirit of the present invention. (1) In each of the above embodiments, each segment 9 of the piezoelectric element 1
Although the polarization treatments in different thickness directions are alternately performed, the polarization treatments may be performed in the same thickness direction. In this case, the common electrodes 6a and 6b are separately formed in a shape corresponding to the independent electrode 2, each segment 9 is polarized, and a voltage of opposite polarity is alternately applied between the divided electrodes and the independent electrode. You can do it like this.

(2) Independent electrode 2 and common electrodes 6a, 6b
May be formed of copper, nickel or the like other than silver. In this case, the thin film formed in the vicinity of both ends 20b of the independent electrode 20 in the sixth embodiment may have a conductivity lower than that of each of the above materials.

(3) It is preferable that the piezoelectric element 1 has a circular shape such as a perfect circle or an ellipse, which forms a loop. (4) The piezoelectric element 1 may be a polymer such as PVDF (polyvinylidene fluoride) other than ceramics.

[0050]

As described above in detail, according to the present invention, it is possible to prevent the stress from being concentrated on the boundary portion of each polarized portion of the piezoelectric element as much as possible, and to improve the reliability in driving the motor. It has an excellent effect that the driving efficiency and the output can be improved.

[Brief description of drawings]

FIG. 1 is a plan view showing a piezoelectric element having an independent electrode according to a first embodiment of the present invention.

FIG. 2 is a graph showing an area ratio of each segment of a piezoelectric element of an independent electrode.

FIG. 3 is a partial schematic cross-sectional view showing a piezoelectric element that has been polarized.

FIG. 4 is a partial plan view showing a piezoelectric element having an independent electrode according to a second embodiment.

FIG. 5 is a partial plan view showing a piezoelectric element having an independent electrode according to a third embodiment.

FIG. 6 is a partial plan view showing a piezoelectric element having an independent electrode according to a fourth embodiment.

FIG. 7 is a partial plan view showing a piezoelectric element having an independent electrode according to a fifth embodiment.

FIG. 8 is a partial plan view showing a piezoelectric element having an independent electrode according to a sixth embodiment.

FIG. 9 is a schematic cross-sectional view showing a polarization treatment method for a piezoelectric element in which independent electrodes and resistors are formed on each surface of a seventh embodiment.

FIG. 10 is also a partial plan view showing a polarization treatment method for the piezoelectric element.

FIG. 11 is a partial schematic cross-sectional view showing a piezoelectric element that has been polarized by applying a positive high voltage.

FIG. 12 is a partial schematic cross-sectional view showing a piezoelectric element polarized by applying a negative high voltage.

FIG. 13 is a graph showing the polarization strength in the circumferential direction of one segment.

FIG. 14 is a plan view showing a piezoelectric element in which a conventional independent electrode is formed.

FIG. 15 is a bottom view showing a piezoelectric element on which a common electrode is formed.

FIG. 16 is also a schematic cross-sectional view showing a polarization treatment method for the piezoelectric element.

FIG. 17 is a partial schematic cross-sectional view showing a state in which a polarized piezoelectric element is attached to a stator.

FIG. 18 is a bottom view showing the polarized piezoelectric element.

[Explanation of symbols]

1 ... Piezoelectric element, 3a, 3b ... Electrode group, 4 ... Electrode region not contributing to driving, 5 ... Electrode region not contributing to driving,
9 ... Segment (polarized region), 20 ... Independent electrode, 20a
... central part, 20b ... end part.

Claims (3)

[Claims] 1. A structure of a plurality of electrodes arranged in two regions of at least one surface of a circular piezoelectric element for rotating a rotor by generating a traveling wave in a stator, wherein an area of both ends in a circumferential direction of the electrode The electrode structure of the piezoelectric element in the ultrasonic motor is characterized in that the area is smaller than the area in the central portion. 2. In a structure of a plurality of electrodes arranged in at least two regions of at least one surface of a circular piezoelectric element for rotating a rotor by generating a traveling wave in a stator, the conductivity of both ends in the circumferential direction of the electrode. The electrode structure of the piezoelectric element in the ultrasonic motor is characterized in that the conductivity is smaller than the conductivity in the central portion. 3. A method of performing polarization processing in a thickness direction on a plurality of polarization regions of a circular piezoelectric element for rotating a rotor by generating a traveling wave in a stator, wherein a central portion of the polarization region in a circumferential direction. A method of polarizing a piezoelectric element in an ultrasonic motor, characterized in that a high voltage is applied to the electrodes to perform polarization processing in each polarization region.