JP2879220B2 - Ultrasonic motor drive circuit - Google Patents

Description

The present invention relates to a drive circuit for stably driving an ultrasonic motor.

"Conventional technology" A piezoelectric element such as piezoelectric ceramic is attached to the stator,
An ultrasonic motor that excites the stator with elastic vibration to rotate the rotor is known.

FIG. 9 schematically shows a driving body 11 and a rotating body 12 of this type of motor. A thin plate-shaped piezoelectric element 14 is adhered to a rear surface of a circular stator 13 and a peripheral surface is attached to a front surface thereof. Are formed along the slit.

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

The rotating body 12 is formed by attaching a slider 16 to a circular rotor 15, and presses the slider 16 against the stator 13. The rotating body 12 is driven by the vibration of the driving body 11 and rotates in the direction opposite to the traveling wave direction.

FIG. 10 is an enlarged rear view of the driving body 11, in which the piezoelectric element 14 is polarized as shown in FIG.
4b and 14c are provided. To these electrodes 14a and 14b, two high-frequency voltages Va and Vb having a phase difference of 90 ° are applied. To give a specific example, Va = Vsin
(Ωt), Vb = Vcos (ωt). 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 feed pressure (high-frequency voltage) applied as described above
Since Va 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 an AC voltage is used. ing.

 FIG. 11 shows an example of the drive circuit described above.

In this drive circuit, the switches 17a and 17b are alternately opened and closed, and the switches 18a and 18b are alternately opened and closed to supply the supply voltage Va from one boost transformer 19 and the supply voltage Vb from the other transformer 20 respectively. Output. In this drive circuit, the DC voltage (for example, 3 to 12 volts) of the battery power supply 21 is 20 to 300 volts.
The voltage is converted to an AC voltage of volts, and the electrodes 14a, 14
Added to 4b. The switches 17a, 17
The semiconductor switches b, 18a, and 18b are configured as semiconductor switches, and are repeatedly turned ON and OFF in accordance with operating conditions predetermined by a control circuit. The power supply pressures Va and Vb are output at a set driving frequency fs.

"Problems to be Solved by the Invention" The above-described ultrasonic motor has a resonance frequency fm unique to the driving body 11.
And an anti-resonance frequency fn, and in many cases, are driven by power supply pressures Va and Vb at a frequency fs slightly higher than the resonance frequency fm (lower than the anti-resonance frequency fn).

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

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

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

When the driving frequency fs becomes lower than the resonance frequency fm,
It is widely known that the driving performance becomes extremely poor.

By the way, the resonance frequency fm of the ultrasonic motor fluctuates depending on the size of the load and external conditions such as a change in ambient temperature and an error in assembly. For example, when the resonance frequency fm fluctuates in a lower direction, the drive frequency fs shifts like Δfs on the amplitude characteristic, and as a result, an amplitude change Do occurs, and the drive performance (torque, rotation speed, etc.) of the ultrasonic motor Decrease.

In order to solve such a problem, various means configurations such as a circuit that suppresses the fluctuation of the resonance frequency fm have been proposed. However, it is suitable for implementation such as a drive circuit having many circuit components and a complicated configuration. There are few things.

The present invention has been made in view of the above circumstances, and has as its object to develop a drive circuit capable of stably driving an ultrasonic motor irrespective of a change in a resonance frequency fm.

"Means for Solving the Problems" In order to achieve the above object, according to the present invention, in a driving circuit for driving an ultrasonic motor by supplying power from an output voltage of a step-up transformer, the capacitance of a driving body of the motor and The parallel resonance frequency fn ₁ generated by the inductance of the output coil of the step-up transformer is the resonance frequency fm of the motor, and the frequency fd at which the amplitude characteristic of the motor becomes the lowest level first in a region higher than the resonance frequency fm. The present invention proposes a drive circuit for an ultrasonic motor, wherein the circuit is configured so as to fall within the range and the frequency of the output voltage is set so as to be set within the range of the frequencies fm and fd.

"Effect" as described above, by determining the parallel resonance frequency fn ₁ caused by the step-up transformer and the ultrasonic motor according to a predetermined condition, the amplitude characteristic level variation in the frequency band immediately beyond the resonance frequency fm Can be improved to less.

As a result, by setting the drive frequency fs within the range between the above-mentioned frequencies fm and fd, changes in the torque and the number of revolutions of the ultrasonic motor due to the fluctuation of the resonance frequency fm are extremely reduced.

Also, the frequency of the output voltage of the step-up transformer (drive frequency
fs) can be set between the resonance frequency fm and the frequency fd of the amplitude characteristic point which becomes the lowest level first in a region higher than the resonance frequency fm, and the drive frequency fs can be set with a margin. This is advantageous for circuit design and adjustment work.

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

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

In this equivalent circuit, Ra, Ca, and La are the resistance, capacitance, and inductance of the piezoelectric element 14, and Co is the piezoelectric element 14 and the stator.
It is the capacitance that occurs between 13 and so on. Note that the relationship is Co≫Ca. Therefore, when the ultrasonic motor is driven, the third becomes the circuit configuration as shown in FIG., The inductance L ₂ of the secondary coil 19s of the step-up transformer 19 has,
Parallel resonance to the capacitance C ₁ of driver 11 has will occur. Here, the capacitance C ₁ = Co + Ca.

In this regard, the same relationship holds for the step-up transformer 20 (see FIG. 11) and the drive circuit 11.

FIG. 4 shows that the frequency fn _{1 of the} parallel resonance is selected to be higher than the resonance frequency fm of the ultrasonic motor and lower than the frequency fd at which the amplitude characteristic first becomes the lowest level in a region higher than the resonance frequency fm. It becomes impedance characteristic,
The amplitude characteristic appears as shown in FIG.

That is, the amplitude characteristic between the frequencies fm and fd is the fifth
As can be seen from the figure, the characteristics have relatively little level change.

Therefore, the feed voltage V output from the step-up transformer
a, the frequency of Vb, that is, by setting as illustrated driving frequency fs, moves to the resonance frequency fm is Δfs accordance variation, it is possible to suppress a change in the amplitude characteristic in the order D _1.

In this way, even if the resonance frequency fm fluctuates due to the size of the load, changes in the ambient temperature, or errors in assembling the ultrasonic motor, stable operation of the ultrasonic motor can be achieved by improving the amplitude characteristics. Become.

If the selected higher than the frequency fd of the parallel resonance frequency fn ₁ becomes the impedance characteristic shown in FIG. 6, as a result, becomes amplitude characteristics is large in level change as shown in Figure 7.

In this case, the driving frequency by variation of the resonance frequency fm is shifted as? Fs, the level change in the amplitude characteristic increases as D _2. Therefore, for the parallel resonance frequency fn _1, it is necessary to set according to the conditions of fm <fn ₁ <fd.

The parallel resonance frequency fn ₁ is Can be obtained by the following equation.

Incidentally, the capacitance C ₁ of the driver 11 is calculated in advance,
Inductance L ₂ can be determined by the number of turns of the secondary coil.

Using the existing step-up transformers 22 and 23, the parallel resonance frequency fn
_{When 1} is set, as shown in FIG. 8, coils 24 and 25 whose inductance can be varied may be connected to the output side of each of the step-up transformers 22 and 23.

[Effects of the Invention] As described above, in the drive circuit according to the present invention, the parallel resonance frequency fn ₁ is equal to the resonance frequency fm of the ultrasonic motor, and the amplitude characteristic of the motor has the lowest level in a region higher than the resonance frequency fm. The frequency band immediately after the resonance frequency fm is exceeded by improving the frequency characteristic of the motor with little level fluctuation by configuring so as to be between the frequency fd that becomes
Since the drive frequency fs was set between the above-mentioned frequencies fm and fd, the resonance frequency fm fluctuated due to external conditions such as changes in load conditions and ambient temperature, or errors in assembling the motor. However, stable operation can be performed without lowering the driving performance such as the torque and the number of revolutions of the ultrasonic motor.

In addition, since the reactive current of the ultrasonic motor can be kept low in the operating state, there is an advantage that power consumption by the motor is reduced.

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

[Brief description of the drawings]

1 is a simplified diagram showing a driving body of an ultrasonic motor, FIG. 2 is a diagram showing an equivalent circuit of the driving body, FIG. 3 is a circuit diagram showing a connection between the equivalent circuit and a step-up transformer, and FIG. And the fifth
Figure figure showing the impedance characteristic and the amplitude characteristic of an embodiment of the present invention, FIG. 6 is a parallel resonance frequency fn ₁ frequency fd
FIG. 7 is a diagram showing impedance characteristics when set to a higher position, FIG. 7 is a diagram showing amplitude characteristics appearing by the impedance characteristics of FIG. 6, and FIG. 8 is a simplified drive circuit showing another embodiment of the present invention. Fig. 9, Fig. 9 is a simplified diagram of an ultrasonic motor, Fig. 10 is an enlarged rear view of a driving body, Fig. 11 is a circuit diagram showing a conventional example of a driving circuit, Figs. 12 and 13 are ultrasonic motors themselves. FIG. 3 is a diagram showing impedance characteristics and amplitude characteristics of the present invention. 11 ... Driver 12 ... Rotator 13 ... Stator 14 ... Piezoelectric elements 14a, 14b, 14c ... Electrode 15 ... Rotor 16 ... Slider 19, 20, 22, 23 ... Step-up transformer 24, 25 ... …coil

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-63-178775 (JP, A) JP-A-63-202278 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) H02N 2/00

Claims (1)

(57) [Claims] 1. A drive circuit for driving an ultrasonic motor by supplying power with an output voltage of a step-up transformer, wherein a parallel resonance frequency generated by a capacitance of a driver of the motor and an inductance of an output coil of the step-up transformer.
fn ₁ is the resonance frequency fm of the motor and the resonance frequency f
The circuit is configured so that the amplitude characteristic of this motor is initially at the lowest frequency fd in the region higher than m,
And a driving circuit for the ultrasonic motor, wherein the frequency of the output voltage is set so as to be set within the range between the frequencies fm and fd.