JPH0670560A - Method of driving ultrasonic motor - Google Patents

Abstract

PURPOSE:To provide an ultrasonic motor which operates stably even at low- speed operation. CONSTITUTION:The output signal of a voltage control oscillating circuit 7 is halved, and two signals 90 deg. different in phase are made by a 90 deg. phase shifting circuit 8, and those are applied to an ultrasonic motor through power amplifying circuits A9 and A10. The output pulse of the encoder 12 installed in a mover is converted into DC voltage by a DC converting circuit 13. An error amplifying circuit 14 amplifies the error between the DC voltage and set voltage Vd1, and at normal speed, a frequency control circuit 15 controls the speed of the mover by an error signal. The voltage generated by the sensor electrode E of a piezoelectric substance is inputted into a judging circuit 18 after the amplification of the error between the DC voltage and the set voltage Vd2 by a rectifying circuit 16. The judging circuit 17 inputs the output of the error amplifying circuit 17 into the frequency control circuit 15 if the speed of the mover becomes smaller than the predetermined value. The frequency control circuit 15 controls the speed by the output of the DC converting circuit 13 and the rectifying circuit 16.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of driving an ultrasonic motor which uses a piezoelectric body such as a piezoelectric ceramic to generate a driving force by exciting an elastic wave in a vibrating body.

[0002]

2. Description of the Related Art An ultrasonic motor and a conventional driving method thereof will be described below with reference to the drawings.

FIG. 4 is a cutaway perspective view of an elastic traveling wave type ring type ultrasonic motor.
A ring-shaped piezoelectric ceramic 2 as a driving piezoelectric body is bonded to one of the ring surfaces of 1 by an adhesive to form a ring-shaped 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 installed in pressure contact with the vibrating body 3 via the friction material 4 by a pressing means such as a spring (not shown here).

FIG. 5 is a structural diagram of a drive electrode formed on the piezoelectric ceramic 2. Two sets of drive electrode A and drive electrode B, which are positionally displaced by a quarter wavelength of an elastic traveling wave, are formed. ing. Drive electrode A and drive electrode B are each 1/2
It consists of a group of small electrodes with a length corresponding to the wavelength. Electrode C
And the electrode D have lengths corresponding to 3/4 wavelength and 1/4 wavelength, respectively, and are formed to create a positional shift corresponding to a quarter wavelength between the drive electrode A and the drive electrode B.

The drive electrode A and the drive electrode B each have 90
Two alternating voltages with different degrees of phase (eg sin wave and co
When the s wave) is applied, a progressive wave of flexural vibration as shown in FIG. 6 is excited in the vibrating body 3. Here, FIG. 6A shows the vibration mode of flexural vibration, and FIG. 6B shows the displacement distribution in the radial direction. In the ring-type ultrasonic motor, a progressive wave of flexural vibration in the radial first order and the circumferential third order or higher is excited in the vibrating body 3. The moving body 6 is frictionally driven by the lateral component of the wave front of the traveling wave, and rotates at a speed proportional to the amplitude of the traveling wave.

[0006]

In the conventional ultrasonic motor, the electrode C or the electrode D of the electrode structure of the piezoelectric body shown in FIG.
It is used as a sensor electrode for detecting the vibration of the vibrating body as it is or by dividing it, and the speed of the moving body is controlled by the output of this sensor electrode, or the moving body is provided with a speed detecting means typified by an encoder. The speed of the moving body is controlled by the output of the speed detecting means.

However, in the above speed control method, the ultrasonic motor operates stably at normal speed, but at low speed the amplitude of the vibrating body becomes small and the output of the sensor electrode decreases.
Further, the speed detecting means has a problem that the number of output pulses decreases and the operation of the ultrasonic motor becomes unstable. Further, in order to improve the controllability at low speed, it is conceivable to increase the resolution of the speed detecting means, but there is a problem that the speed detecting means has a limited capacity and a high cost.

The present invention is to solve the above-mentioned conventional problems. A velocity detecting means is provided on a moving body and a sensor electrode is provided on a piezoelectric body, and two outputs are used for controlling the moving body at low speed. Accordingly, it is an object of the present invention to provide an ultrasonic motor whose operation is always stable.

[0009]

A moving body of an ultrasonic motor is provided with speed detecting means such as an encoder, a piezoelectric body forming a vibrating body is provided with a drive electrode and a sensor electrode, and the output of the speed detecting means is usually provided. The speed of the moving body is controlled only by using the voltage generated at the sensor electrode as the control signal together with the output of the speed detecting means when the speed is low.

[0010]

By providing the moving body of the ultrasonic motor with the speed detecting means, the number of pulses according to the rotating speed of the moving body can be obtained from the speed detecting means, and the piezoelectric body forming the vibrating body is provided with the drive electrode and the sensor electrode. With this configuration, since the amplitude of the traveling wave of the vibrating body is proportional to the rotation speed of the moving body, it is possible to obtain the output voltage of the sensor electrode that is proportional to the rotating speed of the moving body. Then, normally, the speed of the moving body is controlled only by the output of the speed detecting means, and the number of output pulses of the speed detecting means decreases at low speed, so by using the voltage generated in the sensor electrode together with the output pulse as the control signal, By controlling the speed of the moving body, an ultrasonic motor whose operation is always stable is realized.

[0011]

Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a block diagram of an ultrasonic motor drive control circuit which is an embodiment of the ultrasonic motor drive control method of the present invention. In the figure, a voltage controlled oscillator circuit 7 has a control terminal T capable of varying an oscillation frequency by a voltage.
And outputs an oscillation signal of a frequency corresponding to the voltage applied to the control terminal T. The output signal of the voltage controlled oscillation circuit 7 is divided into two, and one signal is
The phase difference is 90 degrees, and the other signal is left unchanged, and two signals with a phase difference of 90 degrees are generated. These two signals are respectively the power amplification circuit A9 and the power amplification circuit B10.
Thus, the power is amplified to a voltage value sufficient to drive the ultrasonic motor and applied to the drive electrodes A and B of the ultrasonic motor, respectively.

As a result, a traveling wave of elastic vibration is excited in the vibrating body, and the moving body is frictionally driven by the lateral component of the wave front of the traveling wave and starts moving at a speed proportional to the amplitude of the traveling wave.

A rotary encoder 12 for detecting a rotation speed is installed on the moving body of the ultrasonic motor, and outputs a pulse voltage according to the rotating speed of the moving body. The output pulse of the encoder 12 is converted into a DC voltage proportional to the pulse frequency by the DC conversion circuit 13.

For example, the DC converter circuit 13 is composed of a frequency counter and a D / A converter, the pulse frequency is counted by the frequency counter, and the count value is converted into a DC voltage by the D / A converter. The set voltage Vd ₁ corresponding to the target moving speed of the moving body is input to one input terminal of the error amplification circuit 14, and the output of the DC conversion circuit 13 is input to the other input terminal. The error amplifying circuit 14 amplifies the error between the voltage corresponding to the moving speed of the moving body and the set voltage Vd ₁ and outputs the amplified error. At the normal speed, the frequency controlling circuit 1
Reference numeral ₅ controls the drive frequency of the vibrating body according to the error signal so that the DC output of the DC conversion circuit 13 becomes equal to the set voltage Vd ₁ .

FIG. 2 shows an example of the electrode structure of the piezoelectric body forming the vibrating body used in the embodiment shown in FIG.
Together with B, a sensor electrode E corresponding to 1/2 wavelength is formed. When the elastic traveling wave is excited in the vibrating body, a voltage proportional to the piezoelectric constant and the magnitude of vibration of the vibrating body is generated in the sensor electrode E by the piezoelectric effect.

The generated voltage is converted into a DC signal by the rectifier circuit 16. The set voltage Vd ₂ corresponding to the target moving speed of the moving body is input to one input terminal of the error amplification circuit 17, and the rectifier circuit 1 is input to the other input terminal.
Input the output of 6. The error amplifying circuit 17 amplifies the error between the voltage corresponding to the moving speed of the moving body and the set voltage Vd _2, and then inputs the error to the discriminating circuit 18. Discrimination circuit 17
Is to determine the magnitude of the DC output of the DC conversion circuit 13, and if the DC voltage becomes smaller than a predetermined value, that is, if the speed (rotation speed) of the moving body becomes smaller than a predetermined value. , The output of the error amplification circuit 17 to the frequency control circuit 1
Enter in 5. The frequency control circuit 15 includes the error amplification circuit 1
The drive frequency of the vibrating body is controlled so that the output of 7 becomes equal to the set voltage Vd ₂ , and the control accuracy is increased with reference to the output of the DC conversion circuit 13.

Since the output pulse interval of the encoder 12 becomes long at low speed, the DC voltage output of the DC conversion circuit 13 is also output in synchronization with the output pulse, so that the change of the moving body within the time of the pulse interval cannot be detected. Therefore, if the speed of the moving body is controlled only by the output of the DC conversion circuit 13, the speed change of the moving body within the time of the pulse interval cannot be eliminated.

If the voltage generated by the sensor electrode is converted into a DC signal by the rectifier circuit 16 and the speed of the DC signal is controlled, the speed of the moving body can be controlled at all times. However, when the speed is low, the amplitude of the vibrating body becomes small and the output of the sensor electrode also becomes small, so the S / N of the output of the rectifying circuit 16 deteriorates. Therefore, the frequency control circuit 15 causes the rectifier circuit 16 to
If the output of the control signal is used as a control signal, and the speed control of the moving body is corrected by the output of the DC conversion circuit 13 having a good S / N in synchronization with the output pulse of the encoder 12, an ultrasonic motor that always operates stably can be obtained. realizable.

FIG. 3 shows the frequency characteristics of the moving speed of the moving body of the ultrasonic motor. As shown in FIG. 3, the vibrating body of the ultrasonic motor has a hysteresis with respect to the driving frequency as in the case of the piezoelectric body alone. The characteristics are shown, and the operation becomes unstable in this hysteresis region. Therefore, driving the ultrasonic motor avoids the hysteresis region, and in the higher frequency region,
Moreover, the detection is performed in the frequency range within the stable operation frequency range f ₁ to f ₂ where the detection voltage of the sensor electrode is monotonic with respect to the drive frequency. If the set voltage is set so that the speed corresponds to the drive frequency f _d within this region, the moving speed (rotation speed) of the ultrasonic motor can be stably set by the above control even at low speed.

In the above embodiment, the ultrasonic motor is driven by an alternating voltage of a constant voltage. However, when the set speed of the moving body is low, the drive voltage is lowered, and when the set speed of the moving body is high, the drive voltage is raised. As described above, if the driving voltage is changed according to the speed of the moving body, efficient driving can always be performed.

In the above embodiment, the speed of the moving body is controlled by the output pulse of the speed detecting means at the normal speed. However, since the vibration of the vibrating body becomes large, the speed of the moving body can be controlled by the output voltage of the sensor electrode. The same effect can be obtained. Further, in the above embodiments, the speed control of the moving body is performed by frequency by limiting the driving frequency range, but the same effect can be obtained by limiting the driving frequency range and performing the speed control of the moving body by the driving voltage. can get.

[0023]

As described above, in the present invention, the normal speed controls the speed of the moving body by the output of speed detecting means such as an encoder or the voltage generated at the sensor electrode, and the voltage generated at the sensor electrode at low speed. By using the output pulse of the speed detecting means as a control signal together with the control signal, it is possible to realize an ultrasonic motor whose operation is always stable without increasing the resolution of the speed detecting means.

[Brief description of drawings]

FIG. 1 is a block diagram of a drive control circuit of an ultrasonic motor according to an embodiment of the present invention.

FIG. 2 is a plan view of drive electrodes and sensor electrodes formed on the piezoelectric body of the motor of the embodiment.

FIG. 3 is a frequency characteristic diagram of the rotation speed of the motor of the embodiment.

FIG. 4 is a cutaway perspective view of a conventional annular ultrasonic motor.

FIG. 5: Electrode structure of annular piezoelectric body of the conventional ultrasonic motor

FIG. 6A is a vibration mode diagram of bending vibration of the conventional ultrasonic motor, and FIG. 6B is a radial displacement distribution diagram of the motor.

[Explanation of symbols]

 7 Voltage Control Oscillation Circuit 8 90 Degree Phase Shift Circuit 9 Power Amplifier Circuit A 10 Power Amplifier Circuit B 11 Ultrasonic Motor 12 Encoder 13 DC Converter Circuit 14 Error Amplifier Circuit A 15 Frequency Control Circuit 16 Rectifier Circuit 17 Error Amplifier Circuit B 18 Discrimination circuit

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Katsushi Takeda 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (72) Takashi Nojima 1006 Kadoma, Kadoma City, Osaka Matsushita Electric Industrial Co., Ltd. 72) Inventor Katsumi Imada 1006 Kadoma, Kadoma-shi, Osaka Prefecture Matsushita Electric Industrial Co., Ltd.

Claims (1)

[Claims] 1. A vibrating body is formed by coupling a piezoelectric body to an elastic substrate, and a moving body is installed in pressure contact with the vibrating body, and a predetermined number is provided on each drive electrode formed on the piezoelectric body. In an ultrasonic motor that applies a drive voltage having a phase difference to excite a traveling wave in the vibrating body to move the moving body, a velocity detecting means is provided in the moving body, and the piezoelectric body and the driving electrode are provided with a sensor. The movement of the moving body is controlled by using only the output of the speed detecting means to control the speed of the moving body, and using the voltage generated at the sensor electrode together with the output of the speed detecting means as a control signal when the speed is low. A method for driving an ultrasonic motor, characterized by controlling the speed of a body.