JP2551412B2 - Ultrasonic motor driving method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a driving method of an ultrasonic motor that uses a piezoelectric body to generate a driving force.

(Prior Art) In recent years, attention has been paid to ultrasonic motors that use elastic vibration as a driving force by exciting elastic vibration in a driving body that uses a piezoelectric body such as a piezoelectric ceramic.

As shown in the perspective view of FIG. 2, a conventional ultrasonic motor includes a piezoelectric driving body 3 in which a ring-shaped piezoelectric ceramic 2 serving as a piezoelectric body is attached to one surface of a ring-shaped elastic body 1 and a ring-shaped elastic body 1. A moving body 6 in which a slider 4 made of a wear resistant material is attached to one surface of the elastic body 5 of the moving body 6 and the moving body 6 contacts the elastic body 1 of the driving body 3 via the slider 4. ing.

As shown in FIG. 3, the electrode structure of the piezoelectric ceramic 2 used in the ultrasonic motor shown in FIG. 2 is configured so that elastic waves of 9 wavelengths are laid in the circumferential direction, and each of them has a half length. 1
Electrode A and electrode B consisting of small regions corresponding to wavelengths, electrode C having a length of 3/4 wavelength, and electrode D having a length of ¼ wavelength and 1 wavelength, with electrode A being electrode B On the other hand, the positions are shifted by a quarter wavelength (= 90 degrees). Then, adjacent small electrode portions in the electrodes A and B of the piezoelectric ceramic 2 are polarized in the thickness direction with polarities opposite to each other, and are bonded to the elastic body 1 of the piezoelectric ceramic 2 (shown in FIG. 3). A common electrode (solid electrode) is formed on the entire surface (the surface opposite to the surface), and on the surface that does not adhere to the elastic body 1, as shown by the diagonal lines in FIG. Common electrode is formed.

When an AC electric field is applied to the piezoelectric ceramic 2, the driver 3
The traveling wave of the bending vibration is excited in the circumferential direction to drive the moving body 6. The arrow in FIG. 2 indicates the direction of rotation of the moving body 6 and indicates that the moving body 6 can rotate in both clockwise and counterclockwise directions.

Next, the operation of the conventional ultrasonic motor thus configured will be described.

When a voltage represented by V = V ₁ × sin (ωt) (1) (where V ₁ is the instantaneous value of the voltage, ω is the angular frequency, and t is the time) is applied to the electrode A of the piezoelectric ceramic 2, The driver 3 vibrates flexurally in the circumferential direction.

FIG. 4 is a perspective view in which the driving body of the ultrasonic motor of FIG. 2 is linearly approximated, and FIG. 4 (a) shows a state in which no voltage is applied to the piezoelectric body 2, and FIG. ) Indicates a state in which a voltage is applied to the piezoelectric body 2.

FIG. 5 is an enlarged view of the contact state between the moving body 6 and the driving body 3, in which one electrode A of the piezoelectric body 2 has V ₁ ×
When sin (ωt) and an AC voltage of V ₁ × cos (ωt) whose phases are shifted from each other by π / 2 are applied to the other electrode B, the driving body 3
A traveling wave of bending vibration can be generated in the circumferential direction. In general, if the amplitude of a traveling wave is ξ, then ξ = ξ ₁ × cos (ωt−kx) (2) where ξ _{1 is} the instantaneous value of the wave magnitude k is the wave number (2π / λ) λ is the wavelength x: Can be expressed as a position. This equation 2 can be rewritten as ξ = ξ ₁ × {cos (ωt) × cos (kx) + sin (ωt) × sin (kx)} (3), and in the equation 3, the traveling wave is temporally π Obtained as the sum of the products of waves cos (ωt) and sin (ωt) that are out of phase by / 2, and cos (kx) and sin (kx) that are out of phase by π / 2 It is shown that.

Since the piezoelectric body 2 is provided with the electrode group A and the electrode group B that are mutually phase-shifted by π / 2 (= λ / 4),
When an oscillator that generates an output with a frequency equal to the resonance frequency of the driving body 3 generates alternating voltages that are temporally out of phase with each other by π / 2 and is applied to the electrode group A and the electrode group B, A traveling wave of bending vibration can be generated.

FIG. 5 shows that point A of the driving body 3 is excited by the traveling wave,
It shows a state in which the major axis 2w and the minor axis 2u are in an elliptical motion, and when the moving body 6 placed on the driving body 3 contacts at the apex of the ellipse, V = in the direction opposite to the wave traveling direction. It shows a state of moving at a speed of ω × u. That is, the moving body 6 is pressed against the driving body 3 by an arbitrary static pressure and comes into contact with the surface of the driving body 3, and the frictional force between the moving body 6 and the driving body 3 causes the moving body 6 to move at a speed v in the direction opposite to the traveling direction of the wave. Driven.

Since the minor axis 2u (traveling direction) of the elliptic motion is proportional to the amplitude of vibration, the amplitude of vibration must be increased in order to increase the speed. Further, in order to excite the amplitude of vibration largely at a low voltage, it is necessary to drive in the vicinity of the resonance frequency where the impedance of the driver becomes small.

Therefore, in the conventional ultrasonic motor, two AC voltages whose phases are temporally different by π / 2 are used as drive signals,
Driving is performed at or near the resonance frequency of the driver.

(Problems to be Solved by the Invention) However, when driving in the vicinity of the resonance frequency, the admittance of the driving body 3 changes unstablely, and the amplitude of the driving body 3 changes unstablely even if it is driven at a constant voltage. Therefore, the speed of the moving body 6 was not stable.

That is, in the present invention, the instability in the ultrasonic motor drive is caused by the hysteresis characteristic in the ultrasonic motor, and when the drive frequency falls within this hysteresis loop, the operating point causes a jump phenomenon between the hysteresis curves. For the first time, it was clarified that this jump phenomenon caused an unstable operation, and the unstable operation was resolved.

Also, an ultrasonic motor that is driven at a frequency intermediate between the nearby frequency and the anti-resonance frequency, instead of near the resonance frequency,
Although it is described in JP-A-59-204482, even if the ultrasonic motor is driven at this driving frequency, the hysteresis characteristic of the ultrasonic motor fluctuates, so that this driving frequency enters the hysteresis loop to prevent the jump phenomenon. Occurs and makes the operation of the ultrasonic motor unstable.

Therefore, the ultrasonic motor driving method of the present invention was conceived in order to provide a driving method that can always realize stable operation.

(Means for Solving the Problems) An AC voltage is applied to a driving body composed of a piezoelectric body and an elastic body to excite elastic vibration, and a moving body brought into pressure contact with the driving body is subjected to the elastic vibration. In the ultrasonic motor driving method, the frequency of the AC voltage is always set higher than the highest frequency that exhibits a hysteresis loop in the characteristics of the admittance and frequency of the driving body.

In addition, the frequency of the AC voltage applied to the driver is always higher than the highest frequency that exhibits a hysteresis loop in the characteristics of the admittance and frequency of the driver, and within the frequency range lower than the antiresonance frequency of the driver. Set.

(Operation) The admittance of the driving body is stable and does not change abruptly due to changes in load, temperature, driving conditions, etc., and the moving body is always stable not only during initial operation and normal operation. When the drive body is driven at a frequency lower than the anti-resonance frequency of the drive body, the drive body can be efficiently driven at a low voltage.

(Embodiment) Next, a method of driving an ultrasonic motor of the present invention will be described in detail based on an embodiment shown in the drawings.

In the driving method of the ultrasonic motor, when the relationship between the frequency and the admittance in the vicinity of the resonance frequency was examined by an experiment, as shown in FIG. 6, the driving body has a nonlinearity because the driving frequency has a sweeping direction. f ₁ ~
exhibits a hysteresis in the region of f _6, the hysteresis loop area, not determined by the uniquely varies with temperature and load or driving conditions, and by also sweep direction resonance frequency in the vicinity of the frequency f ₁ and frequency f ₂ It became clear that it would change. The arrow in FIG. 6 indicates the direction of sweeping the drive frequency.

In addition, the frequency in the hysteresis loop region (f _{1 to} f ₆ )
In, the admittance of the driving body changes unstablely, and the amplitude of the driving body also changes unstablely even if it is driven by a constant voltage, so it is clear that the speed of the moving body changes unstablely. .

The driving frequency in the conventional ultrasonic motor driving method may be a frequency within the hysteresis loop region exhibited by the driving body due to the non-linear effect, so that the speed of the moving body changes instability and the speed of the motor changes. It wasn't stable.

Therefore, the ultrasonic motor driving method of the present invention is made based on such knowledge.

FIG. 1 is a characteristic curve diagram showing the relationship between the admittance of a driving body and the number of rotations of a moving body, which is the principle of the ultrasonic motor driving method of the present invention.

Arrows S ₁ and S ₂ in FIG. 1 indicate the sweep direction of the driving frequency, and frequencies f ₁ and f ₂ are resonances of the driving body when the driving frequency is swept from the lower side and swept from the higher side, respectively. Frequency.

The arrow J indicates that the driving body is driven at a frequency within the hysteresis loop region of the frequency characteristic shown in FIG.
It is shown that the admittance of the driving body causes a jump phenomenon due to the non-linearity due to changes in the environment such as load and temperature and the driving conditions, and the speed of the moving body drastically decreases accordingly. This jump phenomenon occurs at a frequency of the hysteresis loop in the region of the frequency f ₁ ~f _6. Also,
Even if this jump phenomenon does not occur, the characteristics are unstable in the hysteresis loop region of frequencies f _{1 to} f ₆ , and the rotational speed of the moving body changes due to a slight change in the drive frequency.

FIG. 1 shows an example of the operation when the driving frequency changes and enters the hysteresis loop region while the frequency characteristic of the driving body remains constant. Or the frequency characteristic of the driver changes due to the change of the load, the drive frequency frequency is constant, the frequency characteristic of the driver changes due to the change of the temperature or the load, and the drive frequency falls within the hysteresis loop region. Or even when it reaches the vicinity, the same phenomenon is exhibited.

As is clear from the above description, in order to realize an ultrasonic motor that does not only perform initial operation or normal operation, but also stable operation in any case, the drive frequency must be within or near the hysteresis loop region. It is necessary to drive the driver at a frequency that does not reach it.

Further, when the drive frequency is set to the anti-resonance frequency f ₅ or higher shown in FIG. 1, it operates stably because it is outside the hysteresis loop, but the impedance of the driver increases,
Due to the low voltage driving, the driving current becomes difficult to flow, and as a result, the amplitude of the vibration of the driving body becomes small and the speed of the moving body decreases.

Based on such knowledge, the ultrasonic motor driving method of the present invention avoids the hysteresis loop region (f _{1 to} f ₆ ) shown in FIG. Higher than ₁ , for example, set to a frequency within the range of frequencies (f _{3 to} f ₄ ),
At the set frequency, the admittance of the driving body is high and does not change drastically, and as a result, it is possible to realize an ultrasonic motor in which the rotational speed of the moving body is stable in any case. .

Further, if the driving frequency is set lower than the anti-resonance frequency of the driving body, a large vibration can be generated in the driving body with a low voltage, so that the ultrasonic motor can be driven with high efficiency.

(Effects of the Invention) As is apparent from the description based on the above embodiments, according to the present invention, an ultrasonic motor driving method having a hysteresis characteristic in the relationship between the admittance of the driving body and the rotation speed and frequency of the moving body. In the above, it is possible to provide a highly efficient ultrasonic motor driving method that can always perform stable operation even if the characteristics of the driving body change due to changes in the environment such as temperature and load and driving conditions.

[Brief description of drawings]

FIG. 1 is a characteristic curve diagram showing the admittance of a driving body of an ultrasonic motor for determining the driving frequency for realizing the ultrasonic motor driving method of the present invention, and the relationship between the rotational speed of the moving body and the frequency. FIG. 1 is a perspective view showing a conventional ultrasonic motor, FIG. 3 is a plan view showing the shape of a piezoelectric body and an electrode structure used in the conventional ultrasonic motor, and FIG. 4 is a drive of the ultrasonic motor. FIG. 5 is a schematic diagram showing a vibration state of a body portion, FIG. 5 is an explanatory diagram showing a driving principle of an ultrasonic motor, and FIG. 6 is a characteristic curve diagram showing a relationship between admittance of a driving body and frequency. Explanation of symbols f ₁ , f ₂ …… Resonance frequency f _{3 to} f ₄ …… Example of setting range of drive frequency f ₅ …… Anti-resonance frequency S ₁ , S ₂ …… Sweep direction J …… Indicates jump phenomenon Arrow

Front page continuation (72) Inventor Ritsuo Inaba 1006 Kadoma, Kadoma-shi, Matsushita Electric Industrial Co., Ltd. 59-204482 (JP, A) JP 61-144901 (JP, A) JP 61-157276 (JP, A) JP 62-203575 (JP, A) OHM Bunko (204) "Mechanical vibration and Prevention ", issued by Ohmsha, Ltd. on August 25, 1952, P. 186-188 REVIEW OF SCIENTI FIC INSTRUMENTS Vol. 57, No. 11 November 1986 P.I. 2886 ~ 2890

Claims (2)

(57) [Claims] 1. A superposition system for applying an AC voltage to a driving body composed of a piezoelectric body and an elastic body to excite elastic vibration and to move a moving body which is brought into pressure contact with the driving body by the elastic vibration. A method of driving an ultrasonic motor, wherein the frequency of the alternating voltage is always set higher than the highest frequency exhibiting a hysteresis loop in the characteristics of the admittance and frequency of the driving body. 2. The ultrasonic motor drive according to claim 1, wherein the frequency of the AC voltage applied to the driving body is set within a frequency range lower than the anti-resonance frequency of the driving body. Method.