JPH0746910B2 - 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 which use elastic vibration as a driving force by exciting elastic vibration in a vibrating body using 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. 5 is a perspective view of a ring-shaped ultrasonic motor, and a ring-shaped piezoelectric ceramic 2 is bonded to one of the ring-shaped surfaces of a ring-shaped elastic body 1 as a piezoelectric body to form a vibrating body 3. Reference numeral 4 is a friction material 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 vibrating body 3 via the friction material 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 vibrating body 3,
The moving body 6 is driven. The arrow in the figure indicates the rotation direction of the moving body 6.

FIG. 6 shows an example of the electrode structure of the piezoelectric ceramic 2 used in the ultrasonic motor of FIG. In the figure, nine elastic waves are arranged in the circumferential direction. In the figure,
Each of A and B is an electrode group including a small region corresponding to a half wavelength, C is a quarter wavelength, and D is a quarter wavelength electrode. The electrodes C and D have a phase difference of a quarter wavelength (= 90 degrees) between the electrode groups A and B. Adjacent small electrode portions in the electrodes A and B are polarized in the thickness direction opposite to each other. The bonding surface of the piezoelectric body 2 with the elastic body 1 is
It is a surface opposite to the surface shown in FIG. 6, and the electrode is a solid electrode. In use, the electrode groups A and B are short-circuited and used as indicated by the hatched lines in FIG.

V ₁ = V ₀ × sin (ωt) (1) V ₂ = V ₀ × cos (ωt) (2) on the electrodes A and B of the piezoelectric body 2 of the ultrasonic motor configured as described above, V ₀ : Instantaneous value of voltage ω: Angular frequency t: By applying voltages V ₁ and V ₂ represented by time, respectively, the vibrating body 3 has ξ = ξ ₀ × (cos (ωt) × cos (kx) + Sin (ωt) × sin (kx)) = ξ ₀ × cos (ωt−kx) (2) where ξ: amplitude value of bending vibration ξ ₀ : instantaneous value of bending vibration k: wave number (2π / λ) λ: Wavelength x: Excitation of the bending vibration, which can be expressed by the position, is performed in the circumferential direction.

FIG. 7 shows that the point A on the surface of the vibrating body 3 makes an elliptic motion of the long axis 2w and the short axis 2u by the excitation of the traveling wave, and the moving body 6 installed by pressing on the vibrating body 3 has an elliptical shape. By contacting in the vicinity of the apex, frictional force causes v in the direction opposite to the traveling direction of the wave.
It shows that the object moves at a speed of ω × u.

Problems to be Solved by the Invention If the shape of the vibrating body 3 is determined, the minor axis of the above elliptical locus is
Since it is proportional to the bending vibration amplitude, the wave amplitude must be increased in order to increase the velocity. Also,
In order to obtain a large amplitude with low voltage driving, it is necessary to drive near the resonance frequency of the vibrating body. However, the resonance characteristics of the vibrating body change due to changes in temperature and load, so
If the driving is performed at a constant frequency as in the conventional case, the relative relationship between the driving frequency and the resonance frequency changes and the characteristics of the ultrasonic motor change. Further, the resonance characteristics of the vibrating body of the ultrasonic motor are different at the time of startup and at the time of normal operation even if the load is the same. Then, particularly when the load is large at the time of starting, there is a problem that the frequency characteristic of the impedance seen from the electric input terminal of the vibrating body changes greatly and the starting cannot be performed.

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 performs stable start-up and normal operation even when temperature or load changes.

Means for Solving Problems The first and second phase differences between the drive voltage of the vibrator and the inflow current are set, and the drive voltage of the drive voltage is adjusted so that the operation phase difference is close to the first set phase difference during normal operation. When the phase difference between the drive voltage and the inflow current exceeds the second set value by controlling the frequency, that is, the drive frequency of the vibrating body is swept when the phase difference between the drive voltage and the inflow current exceeds the second set value, that is, when the temperature or the load largely changes. The sweep is stopped at a drive frequency near the set value of, and the frequency control of the drive voltage is restarted so that the operating phase difference is near the first set phase difference.

Even if the resonance characteristics of the vibrating body change due to changes in operating temperature and load, and as a result, the relative relationship between the resonance frequency of the vibrating body and the drive frequency changes, the operating phase difference is close to the first set phase difference By controlling the frequency of the drive voltage so that the above, the relative relationship is kept constant by controlling the drive frequency. When the phase difference between the drive voltage and the inflow current becomes equal to or larger than the second set value at the time of start-up or when the temperature or the load largely changes, the frequency of the drive voltage of the vibrating body is swept from the higher side to the lower side, After stopping the sweep at a drive frequency near the first set value and finding a range in which the above-mentioned control can be reliably executed, frequency control of the drive voltage is performed so that the operating phase difference is near the first set phase difference. A stable operation can always be performed by restarting.

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 this circuit starts operating, that is, when the ultrasonic motor is activated, the voltage controlled oscillator 7 oscillates according to the control voltage input to the control terminal C. The control voltage at this time is set by the adder 8 to the initial set value O
Is output as is. The output of the voltage controlled oscillator 7 is divided into two, one of which is passed through the 90-degree phase shifter 9 to the power amplifier 10,
The other is directly input to the power amplifier 11 and amplified to a value required to drive the vibrating body 3. The outputs of the power amplifiers 10 and 11 are applied to the piezoelectric body 2 respectively,
Drive the vibrating body.

A resistor R is connected to one input terminal of the piezoelectric body,
The current flowing in the piezoelectric body is detected by the current detector 13 by the voltage across the resistor R. Further, the voltage detector 12 detects the drive voltage applied to the piezoelectric body 2. The phase difference detector 14 generates a voltage proportional to the phase difference between the current and the voltage from the outputs of the current detector 13 and the voltage detector 12. The phase comparator 15 compares the output of the phase difference detector 14 with the operation set value P ₁ and outputs a voltage proportional to the difference. However, when the difference is 0, the output voltage is 0
I am trying to become. Therefore, the set voltage O and a voltage proportional to the phase difference are input to the adder 8, and the sum of the two voltages is output. The output voltage of the adder 8 is input to the control terminal C of the voltage controlled oscillator 7, and the voltage controlled oscillator 7 changes the driving frequency so as to reduce the difference between the set value P ₁ and the phase difference during driving.

FIG. 2 is a frequency characteristic of the current flowing into the piezoelectric body and the phase difference between the drive voltage and the current for the purpose of explaining the specific circuit of FIG. In the figure, the solid line represents the characteristics of the ultrasonic motor during one operation, and the dotted line represents the characteristics during another operation. f ₀ is a drive frequency at the time of starting the ultrasonic motor corresponding to the set voltage O, P ₁ is an operation set value, and the phase comparator 15 outputs a voltage O ₁ proportional to P ₀ −P ₁ . The voltage O ₁ is output as it is by the controller 17, is added to the set voltage O by the adder 8, and is input to the control terminal C of the voltage controlled oscillator 7. In this case, lowering the driving frequency if P ₀ -P ₁ is positive, P ₀
If −P ₁ is negative, the phase difference can be controlled to P ₁ by increasing the drive frequency. Therefore, the vibrating body is driven with the current value i ₁ at the frequency f ₁ with respect to P ₁ . If the drive frequency is constant,
If the characteristics of the vibrating body change as indicated by the dotted line in the figure due to changes in temperature and load, the inflow current value becomes i ₂ , and the speed of the ultrasonic motor changes greatly. However, according to the drive circuit shown in FIG. 1, the control works so that the phase difference at the time of driving becomes close to P ₁ , so the drive frequency changes to f ₂ and the current value at that time becomes i ₃ . The change in current value can be reduced. Therefore,
The characteristics of the ultrasonic motor are stable against temperature and load.

When the resonance frequency of the ultrasonic motor is greatly reduced as shown by the dotted line in FIG. 3, the phase at the frequency f ₀ at the start is P _0.
Changes to P ₃ . However, in the vicinity of the phase difference P ₃ , the change in the phase difference due to the drive frequency is extremely small, that is, P ₃ −P ₁
Is extremely small, and it is impossible to control the phase difference at the time of driving to be near P ₁ as described above.

When the resonance frequency of the ultrasonic motor greatly increases as shown by the dotted line in FIG. 4, the phase of the drive voltage set at start-up at the frequency f ₀ changes from P ₀ to P ₄ . However, the phase difference between the driving voltage and the current flowing into the piezoelectric body that constitutes the oscillator of the ultrasonic motor changes the sign of the slope as shown in the same figure at the vicinity of the resonance frequency as shown in FIG. The phase difference P ₁ is taken at two frequencies, and when the drive frequency set at startup becomes lower than the lowest frequency of the two frequencies corresponding to the operation set point P ₁ , the difference P ₄ −P ₁ becomes positive. The drive frequency is always controlled to be lower, and the phase difference cannot settle to the set value P ₁ .

Therefore, if the phase comparator 16 detects that the phase difference between the drive voltage and the current is equal to or greater than the second set value P ₂ , the phase comparator 16
Upon receiving the output of 16, the controller 17 disconnects the output of the phase comparator 15 and outputs a signal for sweeping the drive frequency. Second
The sweep start frequency f _{3 in the} figure is set to the frequency obtained by adding the largest expected frequency change to the above-mentioned start frequency f ₀ .
In response to the output of the controller 17, as shown in FIGS. 2, 3, and 4, of the two frequencies corresponding to the operation set point P ₁ , the operation set point on the frequency side higher than the resonance frequency. As set to P ₁ , the voltage controlled oscillator 7 sweeps the driving frequency from the higher side. The phase difference is output by the output of the phase comparator 15.
When it becomes equal to P ₁ , the controller 17 stops the output of the sweep signal of the driving frequency and returns to the control for setting the phase difference to P ₁ .

According to the present invention, it is possible to provide an ultrasonic motor that always performs stable start-up and normal operation even if temperature or load changes.

EFFECTS OF THE INVENTION According to the present invention, it is possible to provide an ultrasonic motor that always performs stable start-up and normal operation even if temperature or load changes. Further, in the present invention, as shown in FIG. 2 to FIG. 4, a frequency region in which the phase difference change with respect to the frequency change is higher than the resonance frequency is selected so that the predetermined phase difference is obtained in the frequency region. Since frequency control is performed, stable control can be performed and the ultrasonic motor can be operated stably.

[Brief description of drawings]

FIG. 1 is a block diagram of a specific circuit for realizing the ultrasonic motor driving method of the present invention, and FIG. 2 is a driving current during normal operation.
Phase difference frequency characteristic diagram of voltage and current, FIG. 3 is a drive current of the vibrating body, frequency characteristic diagram when the phase difference frequency characteristic of the voltage and current is greatly reduced, and FIG. FIG. 5 is a cutaway perspective view of the annular ultrasonic motor, and FIG. 6 is the shape of the piezoelectric body used in the ultrasonic motor of FIG. FIG. 7 is a plan view showing the electrode structure, and FIG. 7 is an explanatory diagram of the operating principle of the ultrasonic motor. 7 ... Voltage controlled oscillator, 8 ... Adder, 9 ... 90 degree phase shifter, 10, 11 ... Power amplifier, 12 ... Voltage detector, 13 ...
Current detector, 14 ... phase difference detector, 15, 16 ... phase comparator, 17 ... controller.

Claims (1)

[Claims] 1. A moving body placed in contact with the vibrating body by driving the piezoelectric body with an alternating voltage to excite an elastic traveling wave into the vibrating body composed of the piezoelectric body and the elastic body. Is a method of driving an ultrasonic motor to move a first and a second phase difference between the drive voltage and the inflow current of the piezoelectric body, and the phase difference between the drive voltage and the inflow current is set during normal operation. The frequency of the drive voltage is controlled in a frequency range higher than the resonance frequency of the vibrating body so as to be close to the first set phase difference, and the phase difference between the drive voltage and the inflow current is set to the second set. When the frequency becomes equal to or higher than the value, the frequency of the driving voltage is swept from the high frequency side to the low frequency side, and the phase difference becomes close to the first set value. The frequency higher than the resonance frequency of the vibrating body. Stop the sweep at The control during the normal operation for controlling the frequency of the driving voltage in a frequency region higher than the resonance frequency of the vibrating body so that the phase difference is close to the first set phase difference is restarted. Ultrasonic motor driving method.