JP2548248B2 - Ultrasonic motor controller - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a control device for an ultrasonic motor that generates a driving force by using a piezoelectric body.

2. Description of the Related Art In recent years, for example, a driving voltage of several tens of KHz is applied to a vibrating body using a piezoelectric body such as a piezoelectric ceramic to excite elastic vibration, and the vibrating body is caused to expand and contract or to vibrate in thickness. Attention has been paid to an ultrasonic motor in which a driven body (moving body) is pressed and driven to rotate or linearly move the moving body.

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

FIG. 4 is a perspective view of a ring-shaped ultrasonic motor, in which a ring-shaped elastic body 15 and a ring-shaped piezoelectric body 14 are bonded together to form a vibrating body 16. Reference numeral 17 is a friction material made of a wear resistant material, and 18 is an elastic body, which are bonded to each other to form a moving body 19. The moving body 19 is in contact with the vibrating body 16 via the friction material 17. When a voltage is applied to the piezoelectric body 14, bending vibration is excited in the circumferential direction of the vibrating body 16, and this becomes a traveling wave, which drives the moving body 19. In addition, a protrusion 20 for extracting a mechanical output is installed on the vibrating body 16 in FIG.

FIG. 5 shows an example of the electrode structure of the piezoelectric body 14 used in the ultrasonic motor of FIG. In the figure, the configuration is such that nine elastic waves are laid in the circumferential direction. In the figure,
A and B are electrode groups each composed of a small area corresponding to a half wavelength, C is an electrode corresponding to three quarter wavelengths, and D is an electrode corresponding to a quarter wavelength. The electrodes C and D positionally make a phase difference of a quarter wavelength (= 90 °) between the electrode groups A and B. Adjacent small electrode portions in the electrodes A and B are electrodes used for polarizing the piezoelectric body 14, and the bonding surface of the piezoelectric body 14 with the elastic body 15 is a surface opposite to the surface shown in FIG. Yes, the electrodes on that surface are all-plane electrodes. 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 14 of the ultrasonic motor configured as above. However, if V ₀ : instantaneous value of voltage ω: angular frequency t: voltage V ₁ and V ₂ represented by time are applied respectively, ξ = ξ ₀ · (cos (ωt) · cos (kX ) + Sin (ωt) ・ s
in (kX)) = ξ ₀ · cos (ωt−kX) (3) where ξ: amplitude value of bending vibration ξ ₀ : instantaneous value of bending vibration k: wave number (2π / λ) λ: wavelength X: A bending vibration, which can be represented by a position, and which advances in the circumferential direction is excited.

FIG. 6 shows that the point A on the surface of the vibrating body 16 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 19 installed by pressing on the vibrating body 16 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.
= ΩxU. Also,
This speed becomes smaller than v when there is slip between the vibrating body 16 and the moving body 19. The arrow B in the figure indicates a moving body.
The traveling direction of 19 is shown, and the arrow C shows the traveling direction of this traveling wave. The velocity v of the moving body 19 described above is proportional to the instantaneous value ξ ₀ of this bending vibration.

By the way, it is known that the ultrasonic motor 6 is represented by an equivalent circuit as shown in FIG. In the figure, C ₀
Is the electric capacitance of the vibrating body 16, C ₁ is the compliance of the vibrating body 16, L ₁ is the mass, and R ₁ is the mechanical loss. The circuit composed of C ₁ , L ₁ and R ₁ is related to mechanical operation such as vibration and is called a mechanical bowl. Of the current i supplied to the electrode of the piezoelectric body 14, the current flowing through this mechanical bowl.
i _m is called the mechanical bowl current. When the level of the frequency voltage applied to the ultrasonic motor 6 is fixed and the frequency is changed,
The magnitude of the mechanical arm current i _m exhibits the characteristics shown in FIG. In other words, this figure shows that the admittance of the mechanical arm of the ultrasonic motor 6 changes with the same characteristic curve depending on the frequency of the applied frequency voltage. The size of the machine arm current i _m is proportional to the vibration amplitude of the vibrating body 19 as previously described, therefore proportional to the moving speed of the moving body 19.

When the ultrasonic motor 6 configured as described above is driven to perform a relatively rough control such that the moving speed of the moving body 19 is approximately a predetermined value, the moving body 19 is well known.
Instead of installing a speed sensor such as FG (frequency generator) to control the speed of the ultrasonic motor based on the speed information obtained from it, the level (amplitude) of the mechanical bowl current of the moving body 19 is as described above. Since it is proportional to the moving speed, the amplitude of this mechanical bowl current is controlled so as to have a constant value corresponding to a predetermined moving speed.

Problems to be Solved by the Invention By the way, when the ultrasonic motor configured as described above is started, a frequency voltage having a certain frequency is first applied, but it is very difficult to determine this frequency. This is because the frequency characteristic of the mechanical bowl current i _m (admittance of the mechanical bowl) relating to rotation changes as shown in FIG. 8 due to changes in temperature and humidity and the state of load from the outside. That is, the characteristic like (e) in a certain state shows the characteristic like (g) or the characteristic like (f) due to the change of the environment as described above.

When the frequency at start-up is set to f ₁ shown in FIG. 8, for example, when the characteristic of the mechanical bowl current i _m is as shown in (f), that is, when the admittance of the mechanical bowl is extremely small, no matter how high the voltage is, Even if (the piezoelectric body 14 is not destroyed) is applied, almost no current flows, and the ultrasonic motor 6 does not start at all. As a measure against this start-up, the start-up frequency was experimentally set by assuming the temperature and humidity change range and the load change so that the frequency does not become lower than the conventional resonance point (at the peak of the characteristic curve in FIG. 6). It is set.

However, in such a frequency setting, it is necessary to set the level of the applied voltage to a certain level in consideration of the characteristic (g), that is, the admittance becomes small at that frequency, and therefore the power supply becomes large. Or, the problem that the polarization state of the piezoelectric body 14 deteriorates occurs,
Also, when the set frequency becomes opposite to the frequency near the resonance point, the admittance increases.Therefore, if a large voltage is applied considering the case of small admittance as described above, the instantaneous There is also a problem that a large mechanical bowl current flows and the speed of the moving body 19 becomes abnormally high, which damages the equipment incorporating the ultrasonic motor 6.

In view of such a point, the present invention provides a control device for an ultrasonic motor that is stable even when the characteristics of a mechanical bowl at the time of starting the ultrasonic motor are changed, and that can lower the voltage applied to the piezoelectric body as compared with the related art. To aim.

Means for Solving the Problems According to the present invention, a piezoelectric body is driven by an alternating voltage to excite an elastic wave in a vibrating body composed of the piezoelectric body and an elastic body, thereby contacting the vibrating body. An ultrasonic motor for moving the installed moving body, an amplitude detecting means for detecting an amount corresponding to the vibration amplitude of the vibrating body, and an amplitude for controlling the vibration amplitude of the vibrating body based on the output of the amplitude detecting means. Control means, a predetermined amplitude detection means for detecting that the vibration amplitude of the vibrating body has reached a predetermined value, and outputting a predetermined amplitude detection signal,
And a frequency sweeping means for sweeping the frequency of the frequency voltage from a higher frequency to a lower frequency.

Operation According to the present invention, when the ultrasonic motor is activated, the frequency sweeping means is operated, the predetermined amplitude detecting means outputs the predetermined amplitude detection signal, and then the frequency sweeping means stops the sweeping operation. Operate the control means.

First Embodiment FIG. 1 is a block diagram of an ultrasonic motor control device according to a first embodiment of the present invention. In FIG. 1, 1 is a variable oscillator control circuit for controlling the oscillation frequency of the variable oscillator 2, 2 is an output frequency whose output frequency is determined by an input voltage value, and 3 is an output from the variable oscillator 2.
The 90 ° phase shift circuits 4 and 5 for generating two signals having different phases are amplified by amplifying the signals having different 90 ° phases to the voltage level necessary for driving the ultrasonic motor 6. power amplifier circuit which is controlled by a DC voltage value that amplification degree and applied to each electrode body 14 is applied to the control input, 7 mechanical arm current detector for detecting the mechanical arm current i _m, 8 is frequency signal Is a detection circuit for converting the output of the mechanical arm current detector 7 into a DC voltage, and 9 is an output of the detection circuit 8 and a mechanical arm current reference value V corresponding to the set mechanical arm current value.
A comparison circuit that compares _ir and outputs an error signal according to the difference, 10 is a compensation filter for stabilizing the control loop,
11 the amplitude level of the mechanical arm current i _m flowing to the ultrasonic motor by monitoring the output of the comparator circuit 9 (hereinafter, simply referred to as level) level detection circuit detects whether or not reached a predetermined value, 12 The level detection circuit 11 selects the contact.
The switch (S which is controlled by the output of the power amplifier circuit 4 and 5 and applied to the control inputs of the power amplification circuits 4 and 5 is the output of the compensation filter 10 or the constant voltage V _r1).
W) and 13 are control terminals for starting and stopping the ultrasonic motor 6. In this block diagram, the mechanical arm current detector 7, the detection circuit 8, the comparison circuit 9, the compensation filter 9,
A control loop (hereinafter, referred to as a machine arm current control loop) that sets the level of the machine arm current to a predetermined set value is formed by the power amplification circuits 4 and 5 and the ultrasonic motor 6. In view for FIG. 2 for explaining the method of detecting the mechanical arm current i _m, in (figure at least one applied to the piezoelectric element 14 through the transformer 21 of the phase of different two frequency voltage Is the output of the power amplifier 4. The mechanical arm current detector 7 is composed of a resistor R ₀ connected to the secondary side of the transformer 21 and an amplifier for amplifying the voltage applied to the capacitors C ₀ ′ and R ₀ as shown in the same figure. Wherein C ₀ of the equivalent circuit of Figure 7 the capacitor C ₀ '
When the winding ratio of the transformer 21 is set to 1: 1 and the current i _R0 flowing through the resistor R ₀ is i _R0 = (1 / (2 · R ₀ · C ₀ ′ · S + 1)) · i _m , The break frequency of the first term on the right side of this equation is several MHz
Since the driving frequency is several tens KHz, the above equation can be approximated as i _R0 = i _m . Therefore, the voltage applied to the resistor R ₀ is the mechanical arm current.
i becomes frequency voltage proportional to _m, it is possible to detect the mechanical arm current i _m by detecting the voltage as in FIG.

FIG. 3 is an operation waveform diagram at the time of starting the drive control device for the ultrasonic motor 6 shown in FIG. 1. (a) is a motor start / stop command signal input to the control terminal 13 and started at the'H 'level. , 'L' level gives a command to stop, (b) is the output of the variable oscillator control circuit 1, the variable oscillator control signal,
(C) an error signal corresponding to the level of the mechanical arm current i _m which is detected by the mechanical arm current detector 7 at the output of the comparator circuit 9, (d)
Is a level detection signal level of the mechanical arm current i _m at the output of the level detection circuit 11 becomes 'H' level reaches the predetermined value.

The operation of the control device for the ultrasonic motor of the present embodiment configured as described above will be described below.

Control terminal to start ultrasonic motor 6
When the motor start / stop signal (a) input to 13 changes from the “L” level to the “H” level, the variable oscillator control circuit 1 causes the variable oscillator 2 to apply the frequency voltage to the ultrasonic motor 6.
And at the same time on a time to the level of the mechanical arm current i _m flowing to the ultrasonic motor 6 is detected to have reached a predetermined value by the level detecting circuit 11 to be described later (FIG. 2 t), variable oscillator 2 Output a variable oscillator control signal (b) that changes from a high voltage to a low voltage with time as shown in FIG. 3 in order to sweep the output frequency from a high frequency (f _H ) to a low frequency (f _L ). Where the frequencies f _H and f _L
Is set in a range that sufficiently covers the change in the characteristics of the mechanical bowl due to the change in the environment described above. The output of the variable oscillator 2 is applied to the ultrasonic motor 6 (piezoelectric body 14) through the 90 ° phase shift circuit 3 and the power amplification circuits 4 and 5. When this frequency voltage is applied, the moving body 19 starts rotating motion. At this time, a constant voltage V _r1 is applied to each control input so that the amplification degree of the power amplification circuits 4 and 5 is at the center of the variable range (SW12 is connected to the 1 side). Therefore, a constant frequency voltage is applied to the ultrasonic motor 6.

Under this constant voltage, when the oscillating frequency of the variable oscillator changes to the lower side by the variable oscillator control circuit 1,
Long as the characteristics of the machine bowl is long as (f) (g), increased admittance machine bowl, it as go also increases mechanical arm current i _m. This increasing mechanical arm current
The level of i _m is converted into a direct current voltage proportional to the level by the mechanical arm current detector 7 and the detection circuit 8 and is input to the comparison circuit 9, where the direct current which is a reference value corresponding to the set mechanical arm current level is applied. The voltage is compared with the voltage V _ir and extracted as an error signal (error signal in FIG. 3C) corresponding to the difference. Configured level detecting circuit 11 in a known voltage comparator or the like, this the error voltage monitor mechanical arm current i _m flowing to the ultrasonic motor 6, a given mechanical arm current or error signal (C) is a predetermined level When it reaches (V _r2 ), the output is changed from the “L” level to the “H” level as shown in FIG. 3 and is output to SW12 and the variable oscillator control circuit 1 as a level detection signal (d). In the present embodiment, the predetermined mechanical arm current level is substantially matched with the set mechanical arm current level (the mechanical arm current level desired to flow to the ultrasonic motor 6 in the steady state).

When the'H 'level detection signal (d) is input, the variable oscillator control circuit 1 stops changing the variable oscillator control signal (b) from a high voltage to a low voltage, as shown in FIG. Hold that state. Therefore, the frequency sweep by the variable oscillator 2 is stopped, and the frequency voltage of the frequency at that time is continuously applied to the ultrasonic motor 6. SW12 is connected to the m side when the level detection signal (d) is input. Therefore, the mechanical arm current control loop is closed,
Thereafter controlled to always set value machine bowl current i _m changing the voltage to be applied by varying the amplification degree of the power amplifier circuit 4, 5 on the basis of the information on the mechanical arm current detector 7 to the ultrasonic motor 6 Then, the speed of the moving body 19 is made substantially constant.

As described above, according to the present embodiment, at the time of startup, the frequency is not fixed as in the conventional case, but the frequency is swept from the higher side to the lower side to change (increase) the admittance of the mechanical bowl, Since the set mechanical bowl current i _m is obtained, it is not necessary to apply a high voltage in consideration of changes in the characteristics, and the level of the frequency voltage to be applied at startup is always constant even if the characteristics of the mechanical bowl change. It may be a level that is lower than the conventional starting voltage and becomes the center of operation during the mechanical arm current control. Therefore, the polarization of the piezoelectric body 14 is less deteriorated than in the conventional case.
Since the sweep frequency is performed from the smaller always admittance as described above, startup and speed of the conventional way instantaneously machine bowl current i _m flowing mobile 19 becomes too abnormally fast damage to the machine Not even give.

Further, in this embodiment, since the amplification degree of the power amplification circuits 4 and 5 is set to be at the center of the variable range at the time of start-up, when the machine arm current is controlled, the machine arm characteristics against the fluctuation as shown in FIG. And a sufficient dynamic range can be obtained.

In the above-described embodiment, the method of controlling the mechanical bowl current by changing the voltage level applied to the ultrasonic motor 6 using the power amplification circuit whose amplification degree changes according to the control signal is adopted. Even if a method of controlling the mechanical bowl current by changing the average value of the voltage applied to the ultrasonic motor 6 by applying a so-called PWM voltage for changing the width, the same effect can be obtained without any problem. In this case, if the PWM duty is set to 50:50 until the startup level detection signal is output, the dynamic range can be made sufficiently wide when the machine arm current is controlled, as in the case described above.

Further, in the above-mentioned embodiment, the vibration amplification of the vibrating body is detected by detecting the mechanical arm current.
There is no problem even if the monitor electrode for detecting the driving state of the ultrasonic motor as provided in Japanese Patent No. 85684 is provided and the vibration amplitude of the vibrating body is detected by the monitor electrode.

Effects of the Invention As described above, according to the present invention, even if the temperature and humidity and the load change and the characteristics of the ultrasonic motor change, stable starting is performed at a low voltage without applying an unnecessary voltage. Further, it is possible to provide a control device for an ultrasonic motor capable of controlling the mechanical arm current almost at the center of operation, and its practical effect is great.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of an ultrasonic motor controller according to an embodiment of the present invention, FIG. 2 is a block diagram of a mechanical arm current detector according to the same embodiment, and FIG. FIG. 4 is a cutaway perspective view of an annular ultrasonic motor, FIG. 5 is a plan view showing the shape of a piezoelectric body used in the ultrasonic motor of FIG. 6 and an electrode structure. 6 is an explanatory diagram of the operating principle of the ultrasonic motor, FIG. 7 is an equivalent circuit diagram of the ultrasonic motor, and FIG. 8 is a frequency characteristic diagram of the mechanical arm current. 1. Variable oscillator control circuit 2. Variable oscillator 3, 3.
90 ° phase shift circuit, 4.5 ... power amplification circuit, 6 ... ultrasonic motor, 7 ... mechanical arm current detector, 8 ... detection circuit, 10
…… Compensation filter, 11 …… Level detection circuit, 21 …… Transformer.

Claims (5)

(57) [Claims] 1. A moving body placed in contact with the vibrating body by driving the piezoelectric body with an alternating voltage to excite elastic waves in the vibrating body composed of the piezoelectric body and the elastic body. An ultrasonic motor to be moved, an amplitude detecting means for detecting an amount corresponding to the vibration amplitude of the vibrating body, and an amplitude control means for controlling the vibration amplitude of the vibrating body to a predetermined value based on the output of the amplitude detecting means. A predetermined amplitude detecting means for detecting that the vibration amplitude of the vibrating body has reached a predetermined value and outputting a predetermined amplitude detection signal; and a frequency sweeping means for sweeping the frequency of the AC voltage from a higher side to a lower side. Having the ultrasonic motor, the frequency sweeping means is operated to start sweeping the frequency of the AC voltage from a frequency higher than a resonance frequency of the vibrating body to a lower frequency, and the output of the amplitude detecting means is The predetermined When the predetermined amplitude detection means detects that the predetermined amplitude has been reached, and the predetermined amplitude detection means outputs the predetermined amplitude detection signal, the frequency sweeping operation of the frequency sweeping means is stopped, and the resonance frequency of the vibrating body is exceeded. A control device for an ultrasonic motor, characterized in that the amplitude control means is operated at a frequency at which the sweeping is stopped to be high, and the vibration amplitude of the vibrating body is suppressed to a predetermined value. 2. An amplitude detecting means for detecting a current level (amplitude) of a mechanical arm current flowing into a mechanical arm of an ultrasonic motor as an amount corresponding to a vibration amplitude of a vibrating body. The control device for the ultrasonic motor according to claim 1. 3. A monitor electrode for detecting a driving state of an ultrasonic motor is provided, and an amplitude detecting means for detecting an output amplitude value of the monitor electrode as an amount corresponding to a vibration amplitude of a vibrating body is provided. The control device for the ultrasonic motor according to claim 1. 4. An amplitude control loop of the amplitude control means has a voltage control amplification means for amplifying a frequency voltage applied to the ultrasonic motor, the amplitude of which is changed by a control signal, and has a predetermined amplitude when the ultrasonic motor is started. The control of the ultrasonic motor according to claim 1, wherein a predetermined value is given as the control signal until the detection means detects that the amplitude value of the vibrating body has reached a predetermined value. apparatus. 5. An amplitude control loop of the amplitude control means has pulse width modulation means for applying a voltage whose pulse width is changed by a control signal to the ultrasonic motor, and has a predetermined amplitude when the ultrasonic motor is started. The duty of the pulse width is set to a predetermined value until the detection means detects that the amplitude value of the vibrating body has reached a predetermined value. Sonic motor control device.