1. Field of the Invention
The present invention relates to a method of oscillating an ultrasonic vibrator for use in ultrasonically cleaning (including deburring) workpieces immersed in a cleaning solution.
2. Description of the Prior Art
For ultrasonically cleaning workpieces immersed in a cleaning solution in a cleaning tank, it has been customary to apply a periodic voltage signal to an ultrasonic vibrator having a piezoelectric element, the periodic voltage signal having a frequency equal to the natural frequency of the ultrasonic vibrator, to oscillate the ultrasonic vibrator at its natural frequency for thereby radiating an ultrasonic energy into the cleaning solution. The radiated ultrasonic energy produces a cavitation in the cleaning solution, which generates shock waves to clean and deburr the workpieces immersed in the cleaning solution.
It is generally known that the cavitation in the cleaning solution appears at a depth depending on the frequency of the radiated ultrasonic energy, i.e., the natural frequency (resonant frequency) of the piezoelectric element of the ultrasonic vibrator. More specifically, when the ultrasonic energy is radiated from the bottom of the cleaning tank toward the surface level of the cleaning solution in the cleaning tank, the cavitation is produced intensively at a depth equal to a quarter wavelength, and also at depths positioned successively at quarter wavelength intervals from that depth toward the bottom of the cleaning tank.
For uniformly cleaning and deburring the workpieces immersed in the cleaning solution, it is preferable to generate the cavitation uniformly in the cleaning solution without being dispersed in the cleaning solution. To generate the cavitation uniformly in the cleaning solution, it is desirable to radiate the ultrasonic energy at a higher frequency. It is also generally known that the higher the frequency of the radiated ultrasonic energy, the more the ultrasonic energy is attenuated in the cleaning solution, resulting in a lowered cavitation effect. For effective cleaning or deburring of the workpieces, therefore, it is preferable to radiate the ultrasonic energy at a lower frequency. Since the generation and effect of the cavitation vary depending on the frequency of the ultrasonic energy, the frequency of the ultrasonic energy should be selected in view of the purpose for which the workpieces are to be cleaned and the degree to which the workpieces are to be cleaned. For example, if a stronger cleaning capability is desirable, then the ultrasonic energy should be applied at a lower frequency. If the workpieces to be cleaned are fragile, then the ultrasonic energy should be applied at a higher frequency in order to prevent the workpieces from being damaged by the cavitation.
However, where an ultrasonic vibrator having a single natural frequency is oscillated at the natural frequency, the above requirements cannot be satisfied under various conditions.
One solution has been to employ an ultrasonic vibrator having a plurality of piezoelectric elements having respective different natural frequencies, and repeatedly apply a plurality of signals having frequencies equal to the natural frequencies to the respective piezoelectric elements for respective periods of time. Therefore, ultrasonic energies are radiated at different frequencies from the single ultrasonic vibrator into the ultrasonic solution.
When the ultrasonic energies are radiated into the ultrasonic solution, cavitations are produced at relatively close depths, respectively, in the cleaning solution. As a result, the cavitations are distributed comparatively uniformly in the cleaning solution, and it is possible to obtain an effective cavitation effect primarily based on those ultrasonic energies which have lower frequencies. A suitable choice of periods of time for which the ultrasonic energies having different frequencies are radiated is effective to serve different purposes for which workpieces are to be cleaned.
The ultrasonic vibrator with plural piezoelectric elements having respective different natural frequencies, however, is difficult and expensive to manufacture. Another problem is that the cavitation distribution becomes unstable because the natural frequencies of the piezoelectric elements tend to vary due to the heat produced thereby when the ultrasonic vibrator is oscillated. Consequently, it has been difficult to clean and deburr the workpieces uniformly with the cavitations.