Patent ID: 7476233

Claim:
An ultrasonic surgical system including a controllable ultrasonic energy generator, a hand piece with a blade that is vibrated at an ultrasonic resonance frequency rate by energy from the generator, and a switch for indicating to the generator the amplitude and frequency of the energy supplied to the hand piece, said ultrasonic generator comprising: an analog input drive signal generator which generates an input drive signal having an amplitude and frequency; an amplifier which receives the analog input drive signal and supplies the energy through a transformer and the transformer output is fed to the hand piece through a line in response thereto; a current sensor that senses the current at the transformer output and produces a current signal related thereto; a comparator which compares the current signal to a variable preset current value and produces a difference signal that is applied to the analog input drive signal generator so as to change the amplitude of the drive signal to cause the current signal to match the preset value; a voltage sensor which senses the voltage at the transformer output and produces a voltage signal related thereto; microprocessor means for producing one or more frequency signals that sweeps either from above or below a target resonance frequency of the handpiece; a digital phase detector which compares the current signal to the voltage signal at the one or more frequency signals and generates a digital phase code related to the phase difference between them; a digital impedance detector which compares the ratio of the voltage signal to the current signal at the one or more frequency signals and generates a digital impedance code related thereto; a digital controller which receives the digital phase code and the digital impedance code and produces a digital frequency code in response thereto for determining the target resonance frequency of the hand piece; and a direct digital synthesis circuit for converting the digital frequency code to an analog frequency signal that is applied to the analog input drive signal generator so as to maintain the frequency at the resonance frequency.