JPH02179282A - Drive circuit for ultrasonic motor - Google Patents

Abstract

PURPOSE:To simplify designing and regulating by determining parallel resonance frequency newly generated in a booster transformer and an ultrasonic motor according to predetermined conditions. CONSTITUTION:An ultrasonic motor has a driver, and its piezoelectric element forms an equivalent circuit of a resistance Ra, a capacitance Ca, and an inductance La. When the ultrasonic motor is driven, a parallel resonance of an inductance L2 of the secondary coil of a booster transformer 19 and a capacitance C1 of the driver is generated. This parallel resonance frequency is so selected as to be higher than the resonance frequency of the ultrasonic motor and lower than the frequency at which amplitude characteristic initially becomes a lowest level in this high range.

Description

DETAILED DESCRIPTION OF THE INVENTION "Field of Industrial Application" The present invention relates to an active circuit for stably driving an ultrasonic motor.

"Prior Art" Ultrasonic motors are known in which a piezoelectric element such as a piezoelectric ceramic is attached to a stator, and the rotor is rotated by exciting elastic vibrations in the stator.

FIG. 9 schematically shows the driving body 11 and rotating body 12 of this type of motor. A thin plate-like piezoelectric element 14 is attached to the rear surface of the circular stator 13, and the surrounding Radial slits are formed along the .

The stator 13 and the piezoelectric element 14 serve as the driving body 11 and generate a vibration traveling wave λ on the front surface thereof.

The rotating body 12 includes a circular rotor 15 and a slider 16 attached thereto, and the slider 16 is pressed against the stator 13. This rotating body 12 is driven by the vibration of the driving body 11 and rotates in the opposite direction to the direction of the vibration traveling wave.

FIG. 10 is an enlarged rear view of the driver 11. As shown in the figure, the piezoelectric element 14 is polarized, and three electrodes 1 are connected to the polarization.
4a, 14b, and 14c are provided. These electrodes 1
4a and 14b are two systems of high frequency voltage Va with a phase difference of 90°.

It is designed to give vb. To give a specific example, V
It is applied as a=Vsin (ωt) and Vb=Vcos (ωt). Note that the stator 13 is connected to the power supply circuit as the other common electrode (for example, a ground electrode) for each of the electrodes 14a to 14c.

In addition, the power supply voltage (high frequency voltage) V applied as described above
Since a and Vb require 20 to 300 volts, when used as a drive source for small devices such as cameras and video cameras, a drive circuit such as an inverter that boosts the battery voltage and outputs AC voltage is used. ing.

FIG. 11 shows an example of the above-mentioned drive circuit.

In this drive circuit, the switches 17a and 17b are alternately
Further, the switches 18a and 18b are alternately opened and closed to cause one step-up transformer 19 to output a power supply voltage Va, and the other step-up transformer 20 to output a power supply voltage Vb. In this drive circuit, the DC voltage of the battery power supply 21 (for example, 3 to 1
2 volts) is converted into an alternating voltage of 20-300 volts and applied to the electrodes 14a, 14b of the piezoelectric element 14. In addition, in the mounted circuit, switches 17a, 17b, 18a, 1
A semiconductor switch 8b is configured to be repeatedly turned on and off according to predetermined operating conditions by a control circuit, and the power supply voltages Va and vb are outputted at a set driving frequency.

"Problem to be Solved by the Invention" The above-mentioned ultrasonic motor has a resonance frequency f unique to the driving body 11.
m and an anti-resonant frequency fn, and in many cases, the frequency is slightly higher than the resonant frequency fm (anti-resonant frequency fn
The configuration is such that the power supply voltages Va and Vb (lower) are used for driving.

FIG. 12 is an impedance characteristic curve A of the driving body 11 expressed with the frequency f as a parameter, and FIG. 13 is an amplitude characteristic curve B of the driving body 11 similarly expressed with the frequency f as a parameter.

As can be seen from these characteristic curves A and B, the resonant frequency f
Since the amplitude level becomes higher as it approaches m, a frequency slightly higher than this resonance frequency fm is set as the drive frequency fs.

Further, the frequency fd is the frequency of the amplitude characteristic that first becomes the lowest level in a region higher than the resonance frequency fm.

Note that it is widely known that when the drive frequency fs becomes lower than the resonance frequency fm, the lli dynamic performance becomes extremely poor.

By the way, ultrasonic motors depend on the size of the load.

Furthermore, the resonant frequency fm fluctuates due to external conditions such as changes in ambient temperature and assembly errors.

For example, when the resonance frequency fm changes in the lower direction, the driving frequency fs changes as Δfs in terms of amplitude characteristics, and as a result, an amplitude change Do occurs, which changes the driving performance (torque, rotation speed, etc.) of the ultrasonic motor. decreases.

In order to solve this problem, various means configurations have been proposed, such as circuits that suppress fluctuations in the resonant frequency fm. There are very few things.

In view of the above-mentioned circumstances, it is an object of the present invention to develop a drive circuit capable of stably driving an ultrasonic motor regardless of fluctuations in the resonance frequency fm.

"Means for Solving the Problems" In order to achieve the above object, the present invention provides an active circuit for driving an ultrasonic motor by supplying power with the output voltage of a step-up transformer. The parallel resonance frequency generated by the inductance of the output coil of the step-up transformer is between the resonance frequency of the motor and the frequency at which the amplitude characteristic of the motor first reaches its lowest level in a region higher than this resonance frequency. We propose an ultrasonic motor drive circuit characterized by having a circuit configuration as follows.

Further, in the present invention, the step-up transformer described above supplies power with an output voltage at a drive frequency that is higher than the resonant frequency of the ultrasonic motor and lower than the frequency at which the amplitude characteristic first reaches its lowest level in the region higher than the resonant frequency. We propose a drive circuit for an ultrasonic motor characterized by the following configuration.

"Function" As mentioned above, by determining the parallel resonance frequency (anti-resonance frequency) newly generated by the step-up transformer and the ultrasonic motor according to predetermined conditions, the amplitude in the frequency band immediately after exceeding the resonance frequency can be increased. The characteristics can be improved to have less level fluctuations.

As a result, changes in the torque, rotational speed, etc. of the ultrasonic motor due to fluctuations in the resonance frequency are extremely reduced.

In addition, the frequency of the output voltage of the step-up transformer can be set between the resonant frequency and the frequency of the amplitude characteristic point where the level is first lowest in the region higher than this resonant frequency, and the drive frequency can be set with a margin. This is advantageous for single-circuit design and adjustment work.

"Example" Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a simplified diagram showing a driving body 11 of an ultrasonic motor, and when one electrode 14a of this driving body 11 is electrically expressed, the equivalent circuit shown in FIG. 2 is obtained. The same applies to the other electrode 14b.

In this equivalent circuit, Ra, Ca, and La are the resistance, capacitance, and inductance of the piezoelectric element 14, and Go is the piezoelectric element 14.
This is the capacitance generated between the stator 13 and the stator 13. Note that the relationship is Co>Ca.

From this, when the ultrasonic motor is driven, the circuit configuration is as shown in FIG. Parallel resonance will occur. However, capacitance C = Co + Ca.

Regarding this point, the same relationship holds true for the step-up transformer 20 and the driver 11.

The frequency of this parallel resonance (anti-resonance frequency) fnl.

higher than the resonant frequency fm of the ultrasonic motor;

If the frequency is selected to be lower than the frequency fd at which the amplitude characteristic first reaches its lowest level in a region higher than the resonance frequency fm, the impedance characteristic will be as shown in FIG. 4, and the amplitude characteristic will appear as shown in FIG. 5.

That is, as can be seen from FIG. 5, the amplitude characteristics between the frequencies fm and fd have relatively few level changes.

Therefore, the supply voltage Va output from the step-up transformer
, Vb, that is, the drive frequency fs, is set as shown in the figure, the resonance frequency fm changes as Δ
fs, and the change in amplitude characteristics can be suppressed to about D4.

In this way, even if the resonant frequency fm fluctuates due to factors such as load size, changes in ambient temperature, or assembly errors of the ultrasonic motor, stable operation of the ultrasonic motor is possible by improving the amplitude characteristics. If the 6-parallel resonance frequency fn is selected higher than the frequency fd, the impedance characteristic will be as shown in FIG. 6, and as a result, the amplitude characteristic will have a large level change as shown in FIG.

In this case, due to the fluctuation of the resonance frequency fm, the driving frequency changes as Δfs, and the level change of the amplitude characteristic becomes large as Δfs. Therefore, the parallel resonance frequency fn□ needs to be set according to the condition fm<fn and <fd (7).

The parallel resonant frequency fn is It can be determined by the formula.

Note that the capacitance C□ of the driving body 11 is calculated in advance, and the inductance L2 can be determined by the number of turns of the secondary coil.

When setting the parallel resonance frequency fn1 using the existing step-up transformers 22, 23, as shown in FIG. You can also use it as

"Effects of the Invention" As described above, according to the drive circuit according to the present invention, the frequency band immediately after the resonance frequency is improved to have an amplitude characteristic with little level change. Alternatively, even if the resonant frequency fluctuates due to external conditions such as errors during motor assembly, stable operation of the ultrasonic motor is possible without deteriorating drive performance such as torque and rotational speed.

Furthermore, since the reactive current of the ultrasonic motor can be suppressed to a low level during operation, there is an advantage that power consumption by the motor can be reduced.

Further, since the drive frequency can be arbitrarily set within a frequency band in which the level change of the amplitude characteristic is small, the design and adjustment of the drive circuit are simplified.

[Brief explanation of the drawing]

Figure 1 is a simplified diagram showing the driving body of the ultrasonic motor, Figure 2 is a diagram showing an equivalent circuit of the driving body, Figure 3 is a circuit diagram showing the connection between the above equivalent circuit and a step-up transformer, and Figure 4. and fifth
The figure shows an impedance characteristic and an amplitude characteristic showing an embodiment of the present invention, FIG. 6 shows an impedance characteristic when the parallel resonance frequency fn is set higher than the frequency fd, and FIG. Figure 6 is a diagram showing amplitude characteristics cursed by impedance characteristics, Figure 8 is a simple drive circuit diagram showing another embodiment of the present invention, Figure 9 is a simplified diagram of an ultrasonic motor, and Figure 10 is a drive circuit diagram. FIG. 11 is a circuit diagram showing a conventional example of a drive circuit, and FIGS. 12 and 13 are diagrams showing impedance characteristics and amplitude characteristics of the ultrasonic motor itself. 11...Driver 12...Rotating body 13...Stator 14...Piezoelectric elements 14a, 14b, 14c="Electrode 15...Rotor 16...Slider 19.20 .22.23...Step-up transformer 24.2
5...Coil m1 diagram j12 diagram 1jQ W 3! Figure 5 Chestnut diagram Figure 5

Claims (2)

[Claims] (1) In a drive circuit that drives an ultrasonic motor by supplying power with the output voltage of a step-up transformer, a parallel resonance frequency generated by the capacitance of the motor driver and the inductance of the output coil of the step-up transformer is
An ultrasonic motor drive circuit characterized in that the circuit is configured to be between the resonant frequency of the motor and a frequency at which the amplitude characteristic of the motor first reaches its lowest level in a region higher than the resonant frequency. (2) The frequency of the output voltage is set within the range between the resonant frequency of the motor and the frequency at which the amplitude characteristic of the motor first reaches its lowest level in a region higher than the resonant frequency. An ultrasonic motor drive circuit according to claim (1).