JPH0710187B2 - Ultrasonic motor driving method - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of driving an ultrasonic motor that uses piezoelectric material to generate a driving force.

2. Description of the Related 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.

Hereinafter, a conventional technique of an ultrasonic motor will be described with reference to the drawings.

FIG. 4 is a perspective view of a conventional ultrasonic motor, in which a ring-shaped piezoelectric ceramic 2 is bonded as a piezoelectric body to one of the ring-shaped surfaces of a ring-shaped elastic body 1 to form a piezoelectric driver 3. Reference numeral 4 is a slider made of a wear resistant material, and 5 is an elastic body, which are bonded to each other to form a moving body 6. The moving body 6 is in contact with the driving body 3 via the slider 4. When an electric field is applied to the piezoelectric body 2, a traveling wave of bending vibration is excited in the circumferential direction of the driving body 3 to drive the moving body 6. The arrow in the figure indicates the rotation direction of the moving body 6.

FIG. 5 shows an example of the electrode structure of the piezoelectric ceramic 2 used in the ultrasonic motor of FIG. In the figure, elastic waves of 9 wavelengths are arranged in the circumferential direction. In the figure, A and B are electrodes each consisting of a small region corresponding to a half wavelength, C is a three quarter wavelength, and D is a quarter wavelength electrode. Therefore, the A electrode and the B electrode have a positional phase shift of ¼ wavelength (= 90 degrees). Electrode A,
Adjacent small electrode portions in B are polarized in the thickness direction opposite to each other. The surface of the piezoelectric ceramic 2 bonded to the elastic body 1 is the surface opposite to the surface shown in FIG. 5, and the electrode is a solid electrode. When used, the electrode groups A and B are short-circuited and used as indicated by the hatched lines in FIG.

The operation of the ultrasonic motor configured as described above will be described below. When a voltage represented by V = V ₁ × sin (ωt) (1) is applied to the electrode A of the piezoelectric body 2 (where V ₁ is an instantaneous value of voltage, ω is an angular frequency, and t is time), driving The body 3 vibrates flexurally in the circumferential direction.

FIG. 6 is a perspective view when the driving body of the ultrasonic motor of FIG. 4 is linearly approximated. FIG. 6A shows the piezoelectric body 2 when no voltage is applied, and FIG. 6B shows the piezoelectric body. A state when a voltage is applied to the body 2 is shown.

FIG. 7 is an enlarged view of the contact state between the moving body 6 and the driving body 3. V ₁ × sin (ω
t), the phase of V ₁ × cos (ωt) relative to the other electrode B is π /
If a voltage deviated by 2 is applied, a traveling wave of bending vibration can be generated in the circumferential direction of the driving body 3. Generally, when the amplitude of a traveling wave is ξ, ξ = ξ ₁ × cos (ωt−kx) (2) where ξ _{1 is} the instantaneous value of the wave size k: wave number (2π / λ) λ: wavelength x: position Can be expressed as Equation (2) can be rewritten as ξ = ξ ₁ × (cos (ωt) × cos (kx) + sin (ωt) × sin (kx)) (3), and in Equation (3), the traveling wave is temporally π / Obtained by the sum of the products of the waves cos (ωt) and sin (ωt) that are phase-shifted by 2 and cos (kx) and sin (kx) that are phase-shifted by π / 2. Is shown. According to the above description, the piezoelectric body 2 has the electrode groups A and B that are phase-shifted from each other by π / 2 (= λ / 4).
An alternating voltage with a phase shift of π / 2 is generated from the output of the oscillator having a frequency output equal to the resonance frequency of the above, and applied to the electrode group to generate a traveling wave of bending vibration in the driving body 3. .

Fig. 7 shows that the point A of the driver is excited by the traveling wave and the long axis 2
w, showing the elliptical motion of the short axis 2u, the driver 3
It is shown that the moving body 6 placed on top of the ellipse moves at a velocity of v = ω × u in the direction opposite to the traveling direction of the wave when the moving body 6 contacts at the apex of the ellipse. That is, the moving body 6 is pressed against the driving body 3 with an arbitrary static pressure, 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 velocity v in the direction opposite to the traveling direction of the wave. Driven.

Since the minor axis (traveling direction) of the ellipse is proportional to the amplitude of the wave, the amplitude of the wave must be increased in order to increase the velocity. Also, in order to increase the amplitude of the wave at a low voltage, the drive must be driven near the resonance frequency of the drive. However, since the resonance frequency of the driver changes due to temperature and load fluctuations, driving with a constant frequency as in the past changes the relative relationship between the drive frequency and the resonance frequency and changes the characteristics of the ultrasonic motor. Will end up.

Problems to be Solved by the Invention As described above, the conventional ultrasonic motor uses two AC voltages having a constant frequency whose phases are temporally different by π / 2 as drive signals. Therefore, when the resonance frequency of the driving body changes due to changes in temperature and load, the relationship between the resonance frequency and the driving frequency changes, and the characteristics of the motor change abruptly.

The present invention has been made in view of the above points, and an object of the present invention is to provide an ultrasonic motor that always operates stably even if the temperature or load changes.

Means for Solving the Problems A resonant frequency of the driver is swept within a frequency variable range set so as to fall within the range, and the frequency of the AC drive voltage is swept from the lower side to the higher side, and the driver is driven. The resonance frequency is detected, and the driving frequency of the AC driving voltage applied to the piezoelectric body forming the driving body is set to be lower than the anti-resonance frequency of the driving body and higher than the resonance frequency.

Operation Sweep the frequency of the AC drive voltage from low to high and detect the resonance frequency of the driving body to obtain the maximum value of the resonance frequency hysteresis due to the nonlinear body of the driving body, and configure the driving body. By setting the drive frequency of the AC drive voltage applied to the piezoelectric body so that it is lower than the antiresonance frequency of the drive body and higher than the resonance frequency, even if the resonance frequency of the drive body changes due to temperature or load fluctuations. Since the drive frequency can be changed to an appropriate frequency according to the change, it is possible to provide an ultrasonic motor that always operates stably.

Embodiment Hereinafter, one embodiment of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a block diagram of a specific circuit for realizing the ultrasonic motor driving method of the present invention. When the circuit starts operating, the sweep controller 12 causes the control voltage generator 13 to sweep the voltage. This sweep voltage is input to the control terminal T of the voltage controlled oscillator 7. Then, the output frequency of the voltage controlled oscillator 7 is swept according to the sweep voltage. Here, the sweep voltage is set so that the resonance frequency of the driver always falls within the sweep range.
The output of the voltage controlled oscillator 7 is divided into two, one of which is input to the power amplifier 9 through the 90-degree phase shifter 8 and the other of which is directly input to the power amplifier 10 to have a value necessary to drive the driver 3. Is amplified up to. The outputs of the power amplifiers 9 and 10 are respectively applied to the piezoelectric bodies 2 forming the driving body 3 of the ultrasonic motor to drive the driving body 3.

The resistor 11 is connected to the input terminal of the driving body 3, and the current flowing through the driving body 3 is detected by the current detector 14 by the voltage across the resistor 11. Further, the voltage detector 15 detects the drive voltage applied to the driving body 3. The phase difference detector 16 generates a voltage proportional to the phase difference between current and voltage from the outputs of the current detector 14 and the voltage detector 15. The output of the phase difference detector 16 is input to the 1 input of the phase range comparator 17 and the memory 18, respectively. A minimum value discriminator 19 is connected to the storage device 18, discriminates the minimum value of the phase difference between the current and voltage, and the drive frequency controller 20 stores the drive frequency at that time. When the detection of the resonance frequency is finished, the drive frequency controller 20 sets the drive frequency from the already stored resonance frequency so that the drive frequency becomes a value between the resonance frequency and the anti-resonance frequency, and the voltage corresponding to the frequency is set. It is generated by the control voltage generator 13.

The set voltage P corresponding to the allowable range of the phase difference is input to the remaining one input terminal of the phase range comparator 17. If the resonance frequency of the driving body changes due to changes in temperature and load, and the voltage corresponding to the phase difference at the driving frequency set above deviates from the set voltage P, that is, the phase difference of the current voltage is set. If the phase difference deviates from the determined phase difference and is out of the allowable range of the phase difference, logical H is output. This output is input to the control terminal C of the sweep controller 12, the drive frequency sweep is started again, and the drive frequency is set as described above.

FIG. 2 is a frequency characteristic diagram of a current value, a voltage-current phase difference, and a rotational speed of a moving body when the driving body is driven at a constant voltage. Further, FIG. 3 is a hysteresis curve of admittance caused by the non-linearity of the driving body when the driving frequency is swept from the bottom and when the driving frequency is swept from the top.

As shown in FIG. 2, since the rotational speed of the moving body increases near the resonance frequency f ₁ of the driving body, it is preferable to drive the ultrasonic motor near this resonance frequency. However, in the frequency region between the resonance frequency f ₁ when the drive frequency is swept from the bottom in FIG. 3 and the resonance frequency f ₂ when the drive frequency is swept from the top, and in the very vicinity of those frequencies, the admittance of the driver is The driving frequency must be out of this range because the operation becomes very unstable due to the jump phenomenon that changes rapidly. Since the rotation speed sharply decreases at a frequency lower than this range, the driving frequency of the driving body is a frequency higher than this range and a frequency range lower than the anti-resonance frequency f ₅ is used.

Sweep the drive frequency from above in the frequency range from f ₃ to f _{4 in} Fig. 2 and set the voltage / current phase difference setting value to P ₁ and the allowable phase difference range setting value to P ₂ . For example, the drive frequency is set to the frequency when the rotation speed of the moving body reaches the rotation speed of N _{1 in} the figure. Wherein setting the phase difference between the voltage and current values P ₁ and the set value P ₂ of the allowable phase difference range driving frequency after setting is determined to be higher than the resonance frequency f ₁ of the driver. Also,
The temperature or load fluctuates, the resonant frequency of the driver changes, and as a result, the phase difference deviates from the set value P ₁
When it jumps out of P ₂ , the drive frequency sweep is started again.
Therefore, the rotation speed of the moving body is controlled within the range of N ₂ centered on N ₁ in the figure.

Although the control is performed by detecting the phase difference between the voltage and the current in the present embodiment, the same applies when the current value corresponding to the rotation speeds N ₁ and N ₂ or the rotation speed itself is adopted. However, when the rotation speed is adopted, a sensor for detecting the rotation speed is required.

According to the drive circuit of the present embodiment, the ultrasonic motor can always be stably driven at the rotation speed within the preset range even if the temperature or the load fluctuates.

Effects of the Invention As described above, according to the present invention, it is possible to provide an ultrasonic motor that operates stably even if the temperature or load changes.

[Brief description of drawings]

FIG. 1 is a block diagram of a specific circuit for realizing the driving method of the ultrasonic motor of the present invention, and FIG. 2 is a driving current, a phase difference between voltage and current, and rotation of a moving body when the driving body is driven with a constant voltage. Number characteristic diagram, FIG. 3 is a non-linear characteristic diagram of the admittance of the driving body, FIG. 4 is a perspective view of a conventional ultrasonic motor, and FIG. 5 is the shape and electrodes of the piezoelectric body used in FIG. FIG. 6 is a plan view showing the structure, FIG. 6 is a model diagram showing a vibration state of a driving body portion of the ultrasonic motor, and FIG. 7 is an explanatory view of the principle of the ultrasonic motor. 7 ... Voltage controlled oscillator, 8 ... 90 degree phase shifter, 9,10 ...
Power amplifier, 11 ... resistance, 12 ... sweep controller, 13 ... control voltage generator, 14 ... current detector, 15 ... voltage detector,
16 …… Phase difference detector, 17 …… Phase range comparator, 18 …… Memory device, 19 …… Minimum value discriminator, 20 …… Drive frequency controller.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP 62-85684 (JP, A) JP 59-185178 (JP, A) JP 63-202278 (JP, A) JP 59- 87078 (JP, A) JP 56-10792 (JP, A) JP 63-154076 (JP, A)

Claims (1)

[Claims] 1. A movement installed in contact with a piezoelectric body by driving the piezoelectric body with an alternating voltage to excite an elastic traveling wave in a drive body composed of the piezoelectric body and an elastic body. In an ultrasonic motor for moving a body, at least the resonance frequency of the driving body is swept within a frequency variable range set so as to be included in the range by sweeping the frequency of the AC drive voltage from the lower side to the higher side. A resonance frequency of the driving body is detected, and the driving frequency of the AC driving voltage applied to the piezoelectric body is set to be lower than the anti-resonance frequency of the driving body and higher than the resonance frequency. Sound wave motor driving method.