Linear power control for ultrasonic probe with tuned reactance

An apparatus for driving an ultrasonic probe comprising:

• first means for generating an alternating current driving signal for said probe thereby causing load current to flow in said probe and having an input for receiving a first control signal to determine the frequency of said driving signal;
• second means coupled to said first means to determine the mechanical resonant frequency of said probe under then existing conditions at predetermined times by causing said first means to generate said driving signal at a first frequency and for recording the load current flowing through said probe which results, and for causing said first means to increment the frequency of said driving signal to a second frequency and for recording the resulting load current through said probe at said second frequency, and, if the load current at said second frequency is less than the load current at said first frequency, for calculating the slope of said load current versus frequency function for the load current samples adjacent said load current peak, and for comparing the calculated slope to a constant which is positive and greater than the slope at any false peak in the load current versus frequency function, and, if the slope is greater than said constant, for generating said first control signal to control said first means so as to generate said driving signal at said mechanical resonant frequency of said probe which caused said peak load current to flow;
• a tuning inductor means for coupling said driving signal to said probe and having an input for receiving a second control signal which controls the amount of inductance of said tuning inductor and for changing inductance in accordance with said second control signal;
• means coupled to said tuning inductor and to said first means and to said second means for determining the actual phase angle between the driving signal for said probe and the resulting load current and for determining the difference between said actual phase angle and a desired range of phase angles and for generating said second control signal to tune said tuning inductor to bring said phase angle within said desired range of phase angles after said second means has tuned said first means to generate said driving signal at said mechanical resonance frequency of said probe.

An apparatus for driving an ultrasonic probe comprising:

• first means for generating a driving signal for said probe thereby causing load current to flow in said probe and having an input for receiving a first control signal to determine the frequency of said driving signal;
• second means coupled to said first means to occasionally determine the mechanical resonant frequency of said probe under then existing conditions and to generate said first control signal to tune said first means for generating a driving signal at said mechanical resonant frequency of said probe;
• a tuning reactance means for coupling said drive signal to said probe and having an input for receiving a second control signal which controls the amount of reactance coupled to said load and for changing the amount of said reactance in accordance with the value of said second control signal;
• third means coupled to said tuning reactance means for determining the actual phase angle between the driving signal voltage for said probe and the resulting load current and for determining the difference between said actual phase angle and a predetermined minimum phase angle and for generating said second control signal to tune said tuning reactance means to change its reactance so as to minimize said phase angel;
• and wherein said first means also has an input for receiving a third control signal which controls the amplitude of the driving signal generated by said first means and wherein said apparatus further comprises means coupled to said first means and including a foot operated control with a range of deflection, for determining the desired amount of power to be dissipated in said probe by determining the percentage of full deflection of said foot operated control and for generating said third control signal so as to control the amplitude of said driving signal such that the power dissipated in said probe is a linear function of the deflection of said foot operated control;
• and wherein said second means includes means for generating said second control signal so as to cause said tuning reactance means to assume a minimum inductance and then for sweeping the frequency of said drive signal through a band of frequencies and for recording the resulting drive current drawn by said probe for each frequency and for comparing said drive signal current at each said frequency to the highest drive current previously recorded and for calculating the slope of a function relating each said load current value to the corresponding frequency of said drive signal at least at said peak load current and for comparing the value of said slope to a minimum slope value, and for generating said first control signal to set the drive frequency to the frequency which results in a peak load current having a slope greater than said minimum slope;
• and
• wherein said third means includes means to calculate one-half the difference between said actual phase angle and said predetermined minimum phase angle and for generating said second control signal so as to adjust the inductance of said tuning inductor means to alter the actual phase angle by one-half said phase angle difference and to continue this process of altering said inductance of said tuning inductor means until the difference between said actual phase angle and said predetermined minimum phase angle is within a specified tolerance.

An apparatus for driving an ultrasonic probe having a piezoelectric crystal excitation device for exciting a metal rod comprising:

• first means for finding and tracking the mechanical resonance frequency of said probe by monitoring the load current and the slope of the load current versus driving frequency functions and for generating a driving signal and applying it to said crystal, said driving signal having a frequency substantially equal to the mechanical resonance frequency of said probe;
• tunable reactance means coupled to said piezoelectric crystal for cancelling the reactive component of the load impedance represented by said piezoelectric crystal excitation device such that the output impedance of said first means is substantially the complex conjugate of the load impedance;
• and
• further comprising resonance tuning means in said first means for tuning said tunable reactance means so as to have minimum reactance during a resonance tuning phase and for sweeping the frequency of said driving signal during said resonance tuning phase until a peak load current occurs which has a slope of change of load current per unit change in frequency greater than a predetermined minimum value, and for fixing the frequency of said drive signal at the frequency which caused said peak load current to occur which satisfies said minimum slope value criteria, and then for allowing said tunable reactance means to tune out said reactive component at said resonant frequency of said driving signal.

A method of driving an ultrasonically driven probe comprising the steps of:

• (1) generating a driving signal to said probe having a frequency within a band of frequencies which encompasses the mechanical resonance frequency of said probe and applying said driving signal to said probe;
• (2) sampling and storing the amount of drive current drawn by said probe at said frequency;
• (3) comparing said drive current sample to the highest drive current sample previously recorded for other drive signal frequencies in said band of frequencies;
• (4) calculating the slope of a function relating each said drive current sample to the corresponding drive signal frequency which caused that drive current to flow for substantially all said drive current samples;
• (5) incrementing the frequency of said driving signal;
• (6) repeating steps 1 through 5 until a mechanical resonant frequency is found where the corresponding drive current sample is larger than all other drive current samples and where the slope of said function at said mechanical resonant frequency which caused said highest load current to flow is greater than a predetermined constant selected to eliminate spurious current peaks caused by phenomena other than mechanical resonance from being mistaken as the load current peak associated with the actual mechanical resonance.