1. Technical Field of the Invention
This invention relates to an ultrasonic motor to be driven by a self-excited oscillation circuit and, more particularly, to an ultrasonic motor to be driven on two signals different in phase and to an electronic apparatus using the same.
2. Description of the Related Art
Recently, the ultrasonic motors using piezoelectric elements have been actively developed. Various kinds of ultrasonic motors have already been placed in the market. Generally, the driving schemes of the ultrasonic motors, in many cases, use a scheme two phase-different signals are supplied to the piezoelectric element thereby causing traveling waves onto the vibrating body, and a scheme of utilizing two standing waves to turn the displacement at a vibrating-body end surface into elliptic motion.
However, these driving schemes are of the so-called separately-excited oscillation scheme requiring an external oscillation circuit for obtaining a particular frequency. Thus complicates the driving circuits and increases the size thereof.
In such a circumstance, there is a recent devise and reduction into practice of a scheme utilizing an ultrasonic motor having a piezoelectric element as a vibrator to combine this with an amplifier circuit, thereby constituting a self-excited oscillation circuit for causing drive due to the oscillation of the ultrasonic motor itself. Such a scheme of an ultrasonic motor is disclosed, for example, in Japanese Patent Laid-open No. 251952/1996.
However, the ultrasonic motor as shown in Japanese Patent Laid-open No. 251952/1996 uses only one of a standing wave. This makes vibrating-body displacement almost in linear motion, thereby leading to reduced conversion efficiency, increased wear amount on the moving body and shortened life. Furthermore, the part of the piezoelectric element used for drive is small in ratio relative to the entire piezoelectric element, making impossible to obtain high output.
Therefore, the present invention has been made in order to solve the above problem and it is an object of the present invention to increase the output and efficiency of an ultrasonic motor, reducing the wear on the moving body and increase the life of the ultrasonic motor.
It is another object to mount such an ultrasonic motor on an electronic apparatus thereby improving the performances of an electronic apparatus including the size and thickness reduction, power saving, etc.
An ultrasonic motor according to the present invention is an ultrasonic-motor having a moving body to be driven by vibration of a vibrating body having a piezoelectric element, comprising: the piezoelectric element having first, second and third electrodes; an output signal from the third electrode being amplified by an amplifier circuit and fed back to the first electrode thereby constituting a self-excited oscillation circuit; phase selection means provided to select whether to invert or not invert a signal based on an oscillation signal of the self-excited oscillation circuit according to an external instruction signal; and an output signal of the phase selection means being applied to the second electrode to obtain a drive force.
Furthermore, an ultrasonic motor according to the invention is an ultrasonic motor having a moving body to be driven by vibration of a vibrating body having a piezoelectric element, comprising: the piezoelectric element having first, second and third electrodes; an amplifier circuit for amplifying an output signal from the third electrode; first signal switching means provided to select whether to output an output signal of the amplifier circuit to the first electrode or to the second electrode, according to an external instruction signal; and a self-excited oscillation circuit formed by the third electrode, the amplifier circuit and an electrode selected by the first signal switching means; and second signal switching means provided to select whether to output a signal based on an oscillation signal of the self-excited oscillation circuit to the first electrode or to the second electrode, according to an external instruction signal.
The foregoing ultrasonic motor is configured to obtain positive self-excited oscillation between the two electrodes of the piezoelectric element, i.e. electrically in a single phase. The oscillation signal obtained here in is passed through a buffer circuit and then delivered to the second electrode through a phase-shift circuit, thereby making it possible to supply two signals different in phase to the ultrasonic motor without having an effect upon oscillation stability of self-excited oscillation.
Furthermore, the provision of either phase selection means for selecting a presence or absence of phase inversion of a signal obtained from the phase-shift circuit or signal switching means for switching between an electrode to generate a self-excited oscillation signal and an electrode to be supplied by a signal different in phase, thereby easily realizing the switching in moving direction of the moving body.