1. Field of the Invention
The present invention relates generally to an ultrasonic motor and method of driving the same for use in various types of driving apparatuses for optical devices, and more particularly to a standing wave type ultrasonic motor.
2. Description of the Related Art
Recently, ultrasonic motors have been used in the field of precision mechanical equipment and optical devices. As compared to a conventional electromagnetic motor, the ultrasonic motor has a smaller size and a higher torque. In general, there are two types of ultrasonic motors: rotary type ultrasonic motors and linear type ultrasonic motors.
FIG. 10 is a perspective view showing the structure of an ultrasonic oscillator in an ultrasonic linear motor disclosed in Jap. Pat. Appln. KOKAI Publication No. 6-105571. This ultrasonic linear motor can cause translational motion by using, as driving sources, piezoelectric elements which are electric-to-mechanic energy conversion elements. In the ultrasonic oscillator shown in FIG. 10, a pair of piezoelectric elements 2a and 2b are disposed on an upper surface of a rectangular-parallelepipedic basic elastic body 1. A pair of sliding members 3 are disposed on a lower surface of the basic elastic body 1. A holding elastic body 4 holds the piezoelectric elements 2a and 2b on the basic elastic body 1.
FIG. 11 schematically illustrates an extending/contracting vibration operation of the ultrasonic oscillator, and FIG. 12 schematically illustrates a bending vibration operation of the ultrasonic oscillator. When a sine-wave voltage is applied to the piezoelectric elements 2a and 2b in the ultrasonic oscillator, the basic elastic body 1 extends and contracts in the longitudinal direction, as shown in FIG. 11. At the same time, the basic elastic body 1 vibrates in a bending manner in accordance with transverse waves consisting of secondary standing waves propagating in the longitudinal direction, as shown in FIG. 12.
In the ultrasonic oscillator, the length and width of the basic elastic body 1 are set so that the primary resonance frequency of the extending/contracting vibration coincides with the frequency of the secondary bending vibration due to transverse waves. Thus, at the maximum bending point (the position of the loop of vibration) of the secondary standing waves, the displacement of the extending/contracting vibration and that of the bending vibration are compounded, and the material point on the basic elastic body 1 moves along an elliptic locus. Accordingly, by disposing the sliding members 3 at the maximum bending points, a driven object (not shown) to be pushed by the sliding members 3 can be translated.
FIGS. 13A and 13B show examples of signal waveforms for describing a method of driving the ultrasonic motor. FIG. 13A shows driving signals and FIG. 13B shows signal waveforms of extending/contracting vibration and bending vibration. When the ultrasonic motor is slightly driven, driving signals AS and BS consisting of burst waves are applied to the piezoelectric elements 2a and 2b. At this time, as shown in FIG. 13A, a phase difference between the driving signals AS and BS is set at .pi./2 or -.pi./2, thereby determining the direction of driving the driven object.
FIG. 14 illustrates the compounding of vibrational displacements due to the extending/contracting vibration and bending vibration in the ultrasonic oscillator. In FIG. 14, horizontal arrows denote displacement due to the extending/contracting vibration, and vertical arrows denote displacement due to the bending vibration. As is shown in FIG. 14, the phase of the direction of the bending vibration, as viewed in the direction of the extending/contracting vibration, generally varies from 0 to .pi., as seen from FIG. 14 in which shown are: (t=0, .pi./4, .pi./2, 3.pi./4, .pi.). In this manner, while the phase of the direction of the bending vibration, as viewed from the standpoint of the direction of the extending/contracting vibration, varies on the basis of ".pi.", the displacement due to the extending/contracting vibration and that due to the bending vibration are compounded. Thereby, the elliptic vibration of the ultrasonic oscillator rotates in one direction, as shown in FIG. 15 (in the counterclockwise direction in FIG. 15).
However, the following problem arises when the standing wave type ultrasonic motor is slightly moved by burst wave driving signals in the manner described above.
As is shown in FIG. 13B, each of the extending/contracting vibration and the bending vibration occurs at the same and constant frequency during the application of burst wave driving signals, i.e. in the vibration excitation period. However, in a free attenuation period of vibration after the vibration excitation period, the frequency of each of the extending/contracting vibration and the bending vibration tends to vary, as shown in FIG. 13B. The frequency of each of the extending/contracting vibration and the bending vibration in the free attenuation period is not necessarily kept constant. This tendency of variation in frequency is generally conspicuous in the bending vibration. If each of the frequency of the extending/contracting vibration and the bending vibration is not constant, the driving phase varies. As a result, the amplitude of the elliptic vibration as shown in FIG. 15 varies undesirably.
FIG. 16 shows an example of the variation in amplitude of the aforementioned elliptic vibration. In FIG. 16, the rotational direction of the elliptic vibration is reversed while the phase difference .DELTA..phi. between the extending/contracting vibration and bending vibration varies from 0 to .pi.. Furthermore, the rotational direction of the elliptic vibration is reversed once again while the phase difference .DELTA..phi. between the extending/contracting vibration and bending vibration varies from .pi. to 2.pi.. Thus, the rotational direction of the elliptic vibration restores to that at the time the phase difference .DELTA..phi. is 0. In this manner, in the free attenuation period of vibration, the elliptic vibration of the ultrasonic oscillator gradually attenuates while the rotational direction of phase is alternately changed.
When the rotational direction of elliptic vibration is thus changed, however, a reverse propelling force occurs when the rotational direction is reverse to the direction in which the driven object should be moved. Consequently, the driven object is moved in the reverse direction. As a result, fine positioning of the driven object becomes difficult and the precision of positioning deteriorates.