Circuit for driving ultrasonic transducer

Claims

What is claimed is:

1. A circuit for driving an ultrasonic transducer comprising

an oscillating means for generating a driving signal whose frequency is controlled in accordance with a frequency control signal;

a first phase detecting means for detecting a phase of said driving signal to generate a first phase detection signal;

a second phase detecting means for detecting a phase of the vibration of the ultrasonic transducer to generate a second phase detection signal;

a reference signal generating means for generating a reference signal whose frequency is continuously changed; and

a frequency control means for selectively changing the operation of the driving circuit between a sweep control mode in which the frequency of the driving signal generated from said oscillating means is controlled to follow the frequency of said reference signal by comparing one of said first and second phase detection signals with said reference signal and a feedback control mode in which the frequency of the driving signal generated from said oscillating means is controlled to follow a resonance frequency of the ultrasonic transducer by comparing said first and second phase detection signals with each other.

2. A driving circuit according to claim 1, wherein said frequency control means comprises a resonance point detecting circuit for detecting such a condition that the driving signal is phase-locked with the vibration of the ultrasonic transducer which is vibrated at its resonance frequency and producing a resonance point detection signal, and a switching circuit for changing the operation mode of the driving circuit between the sweep control mode and the feedback control mode in response to said resonance point detection signal.

3. A driving circuit according to claim 2, wherein said frequency controlling means further comprises a phase comparator having a reference input terminal, a variable input terminal and an output terminal, said reference input terminal being connected to an output terminal of said said switching circuit, and said variable input terminal being connected to an output terminal of the other of said first and second phase detecting means, and said oscillating means comprises a voltage controlled oscillator having a control input terminal coupled with said output terminal of said phase comparator, whereby said phase comparator and voltage controlled oscillator constitute a phase lock loop in which the phase difference between said first and second phase detection signals is detected by said phase comparator and an oscillation frequency of the voltage controlled oscillator is controlled in accordance with said phase difference so that the frequency of the driving signal is automatically adjusted to follow the resonance frequency of said ultrasonic transducer.

4. A driving circuit according to claim 3, wherein said frequency controlling means further comprises a loop filter connected between said output terminal of the phase comparator and the control input terminal of the voltage controlled oscillator and having the integrating function for integrating a phase difference derived from said phase comparator.

5. A driving circuit according to claim 3, wherein said first phase detecting means comprises a voltage phase detector for detecting the phase of a voltage of the driving signal, and said second phase detecting means comprises a current phase detector for detecting the phase of a current of the driving signal.

6. A driving circuit according to claim 3, wherein said first phase detecting means comprises a voltage phase detector for detecting the phase of a voltage of the driving signal, and said second phase detecting means comprises a vibration sensor applied on the ultrasonic transducer for detecting the vibration of the ultrasonic transducer.

7. A driving circuit according to claim 2, wherein said resonance point detecting circuit comprises an impedance detector for detecting an impedance of the ultrasonic transducer and generating an enabling signal when the impedance of the ultrasonic transducer is reduced below a predetermined threshold level, and a phase comparator which is enabled in response to said enabling signal to initiate to compare the phases of said first and second phase detection signals with each other and produce said resonance point detection signal when a phase difference between said phases becomes zero.

8. A driving circuit according to claim 7, wherein the driving circuit further comprises a means for keeping constant an amplitude of a current of the driving signal, and said impedance detector comprises an absolute value detecting circuit for detecting an absolute value of a voltage of said driving signal and a voltage comparator for comparing the absolute value of the voltage of the driving signal with a predetermined threshold value which corresponds to said predetermined threshold level, said enabling signal being generated when the absolute value of the voltage of the driving signal becomes smaller than said predetermined threshold value.

9. A driving circuit according to claim 8, wherein said means for keeping constant the amplitude of the current of the driving signal comprises a voltage controlled amplifier for amplifying the driving signal and having a control input terminal, an absolute value detector for detecting an absolute value of the current of the driving signal, a presetting means for generating a control voltage corresponding to said predetermined threshold value, and a differential amplifier for deriving a difference between said absolute value of the current and said control voltage, whereby said difference is applied to said control input terminal of the voltage controlled amplifier to change an amplification factor thereof such that the amplitude of the current of the driving signal is kept to said predetermined threshold level.

10. A driving circuit according to claim 3, wherein said frequency controlling means further comprises an out-of range detector for producing a reset signal by detecting a condition in which the frequency of the driving signal is changed beyond a predetermined frequency range, and a control circuit for driving said switching circuit in response to said reset signal such that the frequency controlling means is operated in the sweep control mode when said reset signal is generated.

11. A driving circuit according to claim 10, wherein said out-of range detector comprises a window comparator for comparing said phase difference generated by the phase comparator with upper and lower threshold levels and producing said reset signal when the phase difference is changed beyond said upper and lower threshold levels.

12. A driving circuit according to claim 10, wherein said out-of range detector comprises an absolute value detector for detecting an absolute value of the voltage of the driving signal, a differentiating circuit for differentiating the absolute value of the voltage of the driving signal, and a gate circuit for generating said reset signal when said differentiating circuit generates a large output signal.

13. A driving circuit according to claim 10, wherein said frequency controlling means further comprises a counter for counting the reset signals and generating an abnormality indication signal when the number of the reset signals exceeds a predetermined number.

14. A driving circuit according to claim 1, wherein said reference signal generating means is constructed such that the frequency of the reference signal is varied monotonously.

15. A circuit for driving an ultrasonic transducer and including a driving signal generating means which has an open loop control mode in which the frequency of the driving signal is increased or decreased continuously and a feedback control mode in which the frequency of the driving signal is controlled in accordance with a phase difference between voltage and current of the driving signal such that the frequency of the driving signal follows a varying resonance frequency of the ultrasonic transducer, and a switching means for selectively driving said driving signal generating means between said open loop control mode and the feedback control mode by comparing the phases of the voltage and current of the driving signal, the improvement being characterized in that said switching means comprises an impedance detector for detecting an impedance of the ultrasonic transducer, a comparator for comparing the impedance of the ultrasonic transducer with a predetermined threshold value and generating a resonance point detecting signal when the impedance is decreased below said threshold value, and a switch for changing the open loop control mode into the feedback control mode in response to said resonance point detection signal.

16. A driving circuit according to claim 15, wherein the driving signal generating means comprises a means for keeping constant an amplitude of a current of the driving signal, and said impedance detector comprises an absolute value detecting circuit for detecting an absolute value of a voltage of said driving signal and a voltage comparator for comparing the absolute value of the voltage of the driving signal with a predetermined threshold value which corresponds to said predetermined threshold level, said enabling signal being generated when the absolute value of the voltage of the driving signal becomes smaller than said predetermined threshold value.

17. A driving circuit according to claim 16, wherein said means for keeping constant the amplitude of the current of the driving signal comprises a voltage controlled amplifier for amplifying the driving signal and having a control input terminal, an absolute value detector for detecting an absolute value of the current of the driving signal, a presetting means for generating a control voltage corresponding to said predetermined threshold value, and a differential amplifier for deriving a difference between said absolute value of the current and said control voltage, whereby said difference is applied to said control input terminal of the voltage controlled amplifier to change an amplification factor thereof such that the amplitude of the current of the driving signal is kept to said predetermined threshold level.

18. A driving circuit according to claim 17, wherein said impedance detector comprises an absolute value detecting circuit for detecting an absolute value of a voltage of said driving signal and a voltage comparator for comparing the absolute value of the voltage of the driving signal with a predetermined threshold value which corresponds to said predetermined threshold level, said open loop control mode is changed into said feedback control mode when the absolute value of the voltage of the driving signal becomes smaller than said predetermined threshold value.

19. A driving circuit according to claim 15, wherein said driving signal generating means further comprises an out-of range detector for producing a reset signal by detecting a condition in which the frequency of the driving signal is shifted from the resonance frequency of the ultrasonic transducer to such an extent that the frequency of the driving signal could be no more adjusted to follow the resonance frequency, and a control circuit for driving said switching circuit in response to said reset signal such that the driving signal generating means is operated in the open loop control mode when said reset signal is generated.

20. A driving circuit according to claim 19, wherein said out-of range detector is constructed such that said reset signal is generated when the frequency of the driving signal is changed beyond a predetermined frequency range.

21. A driving circuit for generating a driving signal for an ultrasonic transducer including

a frequency sweep means for effecting the frequency sweep over a predetermined frequency range;

a means for operating the driving circuit under a sweep control mode in which the frequency of the driving signal is increased or decreased continuously within said predetermined frequency range with the aid of said frequency sweep means;

a means for operating the driving circuit under a feedback control mode in which the frequency of the driving signal is controlled to follow a varying resonance frequency of the ultrasonic transducer; and

a switching means for selectively driving said driving circuit between said sweep control mode and the feedback control mode by comparing the phases of the voltage and current of the driving signal, the improvement being characterized in that said switching means comprises an impedance detector for detecting an impedance of the ultrasonic transducer, a comparator for comparing the impedance of the ultrasonic transducer with a predetermined threshold value and generating a resonance point detection signal when the impedance is decreased below said threshold value, and a switching circuit for changing the sweep control mode into the feedback control mode in response to said resonance point detection signal.

22. A driving circuit according to claim 21, wherein said means for operating the driving circuit under the sweep control mode and feedback control mode comprises a phase lock loop including a voltage controlled oscillator having a frequency control terminal and a phase comparator for comparing a phase of the driving signal and a vibration phase of the ultrasonic transducer, said frequency sweep means comprises a reference signal generating circuit for generating a reference signal whose frequency is varied continuously, and said switching means is so constructed that in the sweep control mode, the reference signal is supplied to said phase comparator and in the feedback control mode, a signal for representing the phase of the vibration of the ultrasonic transducer is supplied to said phase comparator.

23. A driving circuit according to claim 22, wherein said signal for representing the phase of the vibration of the ultrasonic transducer is generated from a circuit for detecting a phase of a current of the driving signal passing through the ultrasonic transducer.

24. A driving circuit according to claim 22, wherein said signal for representing the phase of the vibration of the ultrasonic transducer is generated from a vibration sensor arranged on the ultrasonic transducer.

25. A driving circuit according to claim 21, wherein the driving circuit further comprises a means for keeping constant an amplitude of a current of the driving signal, and said impedance detector comprises an absolute value detecting circuit for detecting an absolute value of a voltage of said driving signal and a voltage comparator for comparing the absolute value of the voltage of the driving signal with a predetermined threshold value which corresponds to said predetermined threshold value.

26. A driving circuit according to claim 25, wherein said means for keeping constant the amplitude of the current of the driving signal comprises a voltage controlled amplifier for amplifying the driving signal and having a control input terminal, an absolute value detector for detecting an absolute value of the current of the driving signal, a presetting means for generating a control voltage corresponding to said predetermined threshold value, and a differential amplifier for deriving a difference between said absolute value of the current and said control voltage, whereby said difference is applied to said control input terminal of the voltage controlled amplifier to change an amplification factor thereof such that the amplitude of the current of the driving signal is kept to said predetermined threshold level.

27. A driving circuit according to claim 26, wherein said impedance detector comprises an absolute value detecting circuit for detecting an absolute value of a voltage of said driving signal and a voltage comparator for comparing the absolute value of the voltage of the driving signal with a predetermined threshold value which corresponds to said predetermined threshold value, said open loop control mode is changed into said feedback control mode when the absolute value of the voltage of the driving signal becomes smaller than said predetermined threshold value.

28. A driving circuit for generating a driving signal for an ultrasonic transducer comprising

a phase lock loop for adjusting a frequency of the driving signal to follow a resonance frequency of the ultrasonic transducer in accordance with a voltage phase detection signal representing a phase of a voltage of the driving signal and a current phase detection signal;

a reference signal generating circuit for generating a reference signal having a frequency which is varied continuously; and

a control means for selectively driving said phase lock loop between a first control mode in which during a start period, the frequency of the driving signal is controlled in accordance with a phase difference between the reference signal and the voltage phase detection signal until the driving signal is phase-locked with a resonance frequency of the ultrasonic transducer, and a second control mode in which after the driving signal has been phase-locked with the resonance frequency of the ultrasonic transducer, the frequency of the driving signal is controlled in accordance with a phase difference between the voltage phase detection signal and the current phase detection signal.

29. A driving circuit according to claim 28, wherein said frequency of the reference signal is changed monotonously.

30. A driving circuit according to claim 29, wherein said control means comprises a resonance point detecting means for generating a resonance point detection signal when the driving signal is phase-locked with the resonance frequency of the ultrasonic transducer, and a switching circuit for responding to said resonance point detection signal to change the operation of the driving circuit from the first control mode to the second control model.

31. A driving circuit according to claim 30, further comprising a current control means for keeping constant an amplitude of the current of the driving signal.

32. A driving circuit according to claim 31, wherein said current control means is constructed such that during the first control mode, the amplitude of the level, and during the second control mode the amplitude of the current of the driving signal is set to a higher current level.

33. A driving circuit according to claim 32, wherein said current control means further comprises a means for adjusting said lower current level and higher current lever.

34. A driving circuit according to claim 32, wherein said current control means comprises a plurality of lower current level presetting circuits, a plurality of higher current level presetting circuits, a detector for detecting a kind of a probe installed in the ultrasonic transducer to produce a probe identification signal, and a switching means for selectively connecting one of said plurality of the lower current presetting circuits and one of said plurality of the higher current level presetting circuits in accordance with said probe identification signal.

35. A driving circuit according to claim 34, wherein said probe identification signal is constructed such that the kind of the probe is identified by detecting an impedance of the ultrasonic transducer.

36. A driving circuit according to claim 32, wherein said current control means further comprises a limiting means for limiting the amplitude of the current of the driving signal.

37. A driving circuit for driving an ultrasonic transducer comprising

a frequency sweep means for effecting the frequency sweep over a predetermined frequency range;

a means for operating the driving circuit under a sweep control mode in which the frequency of the driving signal is increased or decreased continuously within said predetermined frequency range with the aid of said frequency sweep means;

a means for operating the driving circuit under a feedback control mode in which the frequency of the driving signal is controlled to follow a varying resonance frequency of the ultrasonic transducer;

a switching means for selectively driving said driving circuit between said sweep control mode and the feedback control mode;

a means for amplifying the driving signal with a variable amplification factor; and

an amplification control means for controlling the amplification factor of said amplifying means such that the amplification factor is gradually increased from a lower value during the sweep control mode to a higher value during the feedback control mode.

38. A driving circuit according to claim 37, wherein said amplifying means comprises a voltage controlled amplifier whose amplification factor is controlled by a control voltage applied to a control input terminal thereof, and said amplification control means comprises a first voltage source means for generating a lower control voltage corresponding to said lower value of the amplification factor, a second voltage source means for generating a higher control voltage corresponding to said higher value of the amplification factor, and a time constant circuit connectable to an output of said second voltage source means to increase gradually the control voltage from the lower value to the higher value in accordance with a time constant of the time constant circuit.

39. A driving circuit according to claim 37, wherein said amplifying means comprises a voltage controlled amplifier whose amplification factor is controlled by a control voltage applied to a control input terminal thereof, and said amplification control means comprises a microprocessor for generating a gradually increasing amplification control signal and an analog-digital converter for converting the amplification control signal into an analog control voltage whose amplitude is gradually increased.

40. A driving circuit according to claim 38, wherein said first voltage source means includes a plurality of voltage sources for generating different lower control voltages, said second voltage source means includes a plurality of voltage sources for generating different higher control voltages, a means for detecting a kind of a probe installed in the ultrasonic transducer to generate a probe identification signal, and a switching means for selectively applying one of said plurality of the lower control voltages and one of said plurality of the higher control voltages to the control input terminal of the voltage controlled amplifier in accordance with said probe identification signal.

41. A driving circuit according to claim 40, wherein said probe identification signal is constructed such that the kind of the probe is identified by detecting an impedance of the ultrasonic transducer.

BACKGROUND OF THE INVENTION

1. Field of the Invention and Related Art Statement

The present invention relates to a circuit for driving an ultrasonic transducer, and more particularly relates to a circuit for driving an ultrasonic transducer for use in surgical operations.

There have been developed various kinds of devices using ultrasonic transducers such as ultrasonic surgical knives, ultrasonic working tools, ultrasonic atomizes, ultrasonic bonding machines and ultrasonic welding machines. In these ultrasonic devices, in order to improve the efficiency it is desired to drive the ultrasonic transducer at a resonance frequency. However, since the resonance frequency is changed in accordance with temperature variations, it is rather difficult to vibrate the ultrasonic transducer always at the resonance frequency, when there is not provided any means for compensating for the variation of the resonance frequency. Furthermore, the resonance frequency of the ultrasonic transducer is changed in accordance with the condition of the load applied thereto. Therefore, it is required to adjust or change the frequency of the driving signal in accordance with the variation of the resonance frequency of the ultrasonic transducer.

Several solutions for satisfying the above mentioned requirement have been proposed in, for instance U.S. Patent Nos. 4,275,363, 4,587,958, 4,724,401 and 4,754,186. In these known ultrasonic transducer driving circuits, there is provided a phase lock loop (PLL) and the frequency of the signal for driving the ultrasonic transducer is automatically controlled to follow the varying resonance frequency of the ultrasonic transducer. However, the inventor present has confirmed that the known ultrasonic transducer driving circuits including a PLL have a serious drawback which will be explained hereinbelow.

FIG. 1 shows an equivalent circuit of the piezoelectric type ultrasonic transducer. The ultrasonic transducer 1 is expressed by a parallel circuit of series-connected resistor R, inductor L and capacitance C and a damping capacitance C.sub.d. In a practical circuit, in order to cancel the effect of the damping capacitance C.sub.d, a compensating inductor L.sub.d is connected in parallel with said parallel circuit of the ultrasonic transducer 1. In such a circuit, the frequency characteristic of a phase difference .DELTA..theta. between the driving voltage and the driving current can be represented by a curve shown in FIG. 2A, and the frequency characteristic of an impedance .vertline.Z.vertline. viewed in the direction shown by an arrow A in FIG. 1 is illustrated in FIG. 2B. As illustrated in FIGS. 2A and 2B, the phase difference .DELTA..theta. becomes zero at a resonance frequency f.sub.r as well as antiresonance frequencies f.sub.1 and f.sub.2, these antiresonance frequencies being positioned on respective sides of the resonance frequency f.sub.r, and the impedance .vertline.Z.vertline. becomes minimum at the resonance frequency f.sub.r and becomes maximum at the antiresonance frequencies f.sub.1 and f.sub.2. In the known driving circuit, the phase difference .DELTA..theta. between the driving voltage and the driving current is detected to adjust the driving frequency into the resonance frequency f.sub.r through the feedback control of the PLL. As can be understood from the curves shown in FIGS. 2A and 2B, the phase difference .DELTA..theta. becomes zero not only at the desired resonance frequency f.sub.r, but also at the antiresonance frequencies f.sub.1 and f.sub.2, so that the feedback control of the PLL in which the frequency of the driving signal is adjusted to follow the resonance frequency of the ultrasonic transducer is effective only within the frequency range between the two antiresonance frequencies f.sub.1 and f.sub.2, and if the vibrating frequency of the ultrasonic transducer decreases lower than the antiresonance frequency f.sub.1 or increases higher than the antiresonance frequency f.sub.2, the feedback control of the PLL could not be performed correctly and the driving frequency would further decrease or increase continuously. Particularly, in the time of starting the vibration, the driving signal could not be easily locked with the vibration of the ultrasonic transducer at the desired resonance frequency f.sub.r.

In order to mitigate the above mentioned drawback, it has been known to restrict the frequency control range of the PLL with the aid of a limiter. In this case, since the limiter has to be set or designed very precisely, the circuit construction is liable to be complicated, so that such a driving circuit has not been actually realized.

In the above mentioned U.S. Pat. No. 4,275,363, there has been proposed to provide a sweep circuit by means of which a control voltage applied to a control terminal of a voltage controlled oscillator (VCO) in the PLL is changed in a monotonous manner so that the oscillation frequency of the VCO is changed within a predetermined range, and when the resonance point is detected, the sweep operation is stopped and the PLL is set into the feedback control mode. However, in this known driving circuit, the output voltage from the sweep circuit is applied to the VCO at the transient between the sweep control mode and the feedback control mode and a finite voltage is applied to the VCO as an offset voltage. This offset voltage affects the loop characteristics of the PLL, and the PLL could not operate correctly.

In the U.S. Pat. No. 4,754,186, there is proposed another method of locking the oscillation frequency of the VCO to the resonance frequency of the transducer. In this known method, when the VCO oscillates at a frequency lower than the lower antiresonance frequency f.sub.1, a pulse signal is added to the feedback signal in the PLL so that the oscillation frequency of the VCO is increased and is locked into the desired resonance frequency f.sub.r. However, in this known method, since the locking operation is dependent upon the loop characteristics of the PLL, it is quite difficult to positively lock the oscillation frequency of the VCO into the resonance frequency f.sub.r of the ultrasonic transducer.

It should be noted that the above described problem occurs not only during the starting period but also during the usual operation. That is to say, in the case that the ultrasonic transducer is used in a surgical knife, when a very large load is applied to the ultrasonic transducer, the ultrasonic transducer could not be vibrated at the resonance frequency f.sub.r and the driving frequency might be out of the automatic resonance point following range. Then, it is necessary to effect the lock-in operation again.

SUMMARY OF THE INVENTION

The present invention has for its object to provide an ultrasonic transducer driving circuit in which the driving frequency can be positively and accurately locked into a desired resonance frequency of the ultrasonic transducer without producing any undesired offset in the phase lock loop.

According to the invention, a circuit for driving an ultrasonic transducer comprises

an oscillating means for generating a driving signal whose frequency is controlled in accordance with a frequency control signal;

a first phase detecting means for detecting a phase of said driving signal to generate a first phase detection signal;

a second phase detecting means for detecting a phase of the vibration of the ultrasonic transducer to generate a second phase detection signal;

a reference signal generating means for generating a reference signal whose frequency is continuously changed; and

a frequency control means for selectively changing the operation of the driving circuit between a sweep control mode in which the frequency of the driving signal generated from said oscillating means is controlled to follow the frequency of said reference signal by comparing one of said first and second phase detection signals with said reference signal and a feedback control mode in which the frequency of the driving signal generated from said oscillating means is controlled to follow a resonance frequency of the ultrasonic transducer by comparing said first and second phase detection signals with each other.

In a preferred embodiment of the driving circuit according to the invention, said first phase detecting means detects the phase of a voltage of the driving signal and said second phase detecting means detects the phase of a current of the driving signal. As is well known in the art, the ultrasonic transducer vibrates in synchronism with the driving current, so that the phase of the driving current represents the phase of the vibration of the ultrasonic transducer.

As explained above with reference to the curves shown in FIGS. 2A and 2B, the phase difference .DELTA..theta. between the driving voltage and the driving current becomes zero and the impedance .vertline.Z.vertline. of the ultrasonic transducer becomes minimum when the ultrasonic transducer vibrates at the resonance frequency f.sub.r.

Therefore, in a preferred embodiment of the driving circuit according to the present invention, said frequency control means comprises a means for detecting the impedance of the ultrasonic transducer, a means for comparing the thus detected impedance with a predetermined threshold level, and a switching means for changing the operation of the driving circuit between the sweep control mode and the feedback control mode when the impedance of the ultrasonic transducer exceeds said threshold level.

As explained above, the vibration amplitude of the ultrasonic transducer is proportional to the amplitude of the driving current, so that it is preferable to energize the ultrasonic transducer under the constant current driving mode. In this case, the impedance of the ultrasonic transducer becomes proportional to the driving voltage. Therefore, in another preferred embodiment of the driving circuit according to the invention, said frequency control means comprises a means for detecting the amplitude of the voltage of the driving signal, a means for comparing the thus detected amplitude with a predetermined threshold level, and a switching means for changing the operation of the driving circuit between the sweep control mode and the feedback control mode when the amplitude of the voltage of the driving signal exceeds said threshold level.

The inventor has conducted various experiments and has found that when the driving circuit is operated under the sweep control mode, it is not necessary to keep the amplitude of the driving current at a normal value, but it is advantageous to reduce the driving current to a low safety level. According to another aspect of the invention, the amplitude of the driving current is limited to a level lower than the nominal value during the sweep control mode, and after the frequency of the driving signal is locked into the resonance frequency of the ultrasonic transducer, the amplitude of the current of the driving signal is increased to the nominal value.

The inventor has further confirmed that when the amplitude of the current of the driving signal is abruptly increased from the low value in the sweep control mode into the nominal value in the feedback control mode, the vibration frequency of the ultrasonic transducer is sometimes changed and the resonance condition might be lost. According to still another aspect of the invention, the amplitude of the current of the driving signal is increased gradually from the lower value to the nominal value for a relatively long time period when the frequency of the driving signal is locked into the resonance frequency of the ultrasonic transducer. By this measure, the frequency of the driving signal is kept at the desired resonance frequency of the ultrasonic transducer even if the amplitude of the driving current is changed from the lower value to the higher nominal value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an equivalent circuit of the ultrasonic vibrating element;

FIGS. 2A and 2B show frequency characteristics of the phase difference between the driving voltage and current and the impedance of the ultrasonic vibrating element;

FIG. 3 is a block diagram illustrating a first embodiment of the driving circuit according to the invention;

FIGS. 4, 5, 6 and 7 are circuit diagrams depicting several embodiments of the switching circuit shown in FIG. 3;

FIGS. 8A to 8D are signal waveforms for explaining the operation of the driving circuit of FIG. 3;

FIG. 9 is a block diagram illustrating a second embodiment of the driving circuit according to the invention;

FIG. 10 is a circuit diagram showing the detailed construction of the voltage-current detector shown in FIG. 9;

FIG. 11 shows the frequency characteristic of the driving voltage;

FIG. 12 is a circuit diagram showing the phase comparator shown in FIG. 9;

FIG. 13 is a diagram representing the frequency ranges of the voltage controlled oscillator and ultrasonic transducers;

FIG. 14 is a flow chart showing the operation of the driving circuit illustrated in FIG. 9;

FIG. 15 is a side view depicting the ultrasonic transducer and the probe connected thereto;

FIG. 16 is a block diagram illustrating a modification of the first embodiment of the driving circuit according to the invention shown in FIG. 3;

FIG. 17A is a block diagram showing a modification of the second embodiment of the driving circuit according to the invention illustrated in FIG. 9, and FIG. 17B is a circuit diagram showing the detailed construction of the in-phase detector shown in FIG. 17A;

FIG. 18 is a block diagram depicting a third embodiment of the driving circuit according to the invention;

FIG. 19 is a graph showing the variation of the vibration amplitude control voltage;

FIG. 20 is a block diagram illustrating a fourth embodiment of the driving circuit according to the invention;

FIGS. 21A and 21B are graphs representing the variation of the control voltage;

FIG. 22 is a block diagram showing a fifth embodiment of the driving circuit according to the invention;

FIG. 23 is a circuit diagram of the timer circuit shown in FIG. 22;

FIGS. 24A to 24F are signal waveforms for explaining the operation of the driving circuit illustrated in FIG. 22;

FIG. 25 is a circuit diagram of the electronic switch usable is the driving circuit shown in FIG. 22;

FIG. 26 is a block diagram depicting a sixth embodiment of the driving circuit according to the invention;

FIGS. 27A to 27F are signal waveforms for explaining the operation of the circuit of FIG. 26;

FIG. 28 is a schematic view showing a part of a modification of the driving circuit of FIG. 26;

FIG. 29 is a block diagram illustrating a seventh embodiment of the driving circuit according to the invention;

FIG. 30 is a graph showing the variation of the voltage for controlling the amplification of the voltage controlled amplifier shown in FIG. 29; and

FIG. 31 is a block diagram showing an eighth embodiment of the driving circuit according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3 is a block diagram showing a first embodiment of the ultrasonic transducer driving circuit according to the invention. In the present embodiment, the ultrasonic transducer is applied to an ultrasonic surgical knife of Langevine type. A piezoelectric ultrasonic vibrating element 11 of Langevine type is installed in a hand piece 10 and is driven by the driving circuit including a phase lock loop, i.e. PLL 12 and a power amplifier 13. Between the power amplifier 13 and the ultrasonic vibrating element 11 there is arranged a voltage-current detecting circuit 14 to detect voltage and current phases of a driving signal applied to the ultrasonic vibrating element to generate voltage phase signal .theta..sub.v and current phase signal .theta..sub.I. The voltage-current detecting circuit 14 also detects an impedance of the ultrasonic vibrating element to derive an impedance signal .vertline.Z.vertline.. As explained above, the voltage phase signal .theta..sub.v represents the phase of the driving signal and the current phase signal .theta..sub.I expresses the vibration phase of the ultrasonic vibrating element 11. Further, the impedance .vertline.Z.vertline. of the ultrasonic vibrating element 11 becomes minimum when the ultrasonic element vibrates at the desired resonance frequency f.sub.r as shown in FIG. 2A.

The phase lock loop 12 includes a phase comparator (PC) 15, a loop filter 16 and a voltage controlled oscillator (VCO) 17. An output signal from VCO 17 is supplied to the power amplifier 13 and is amplified to a level which is sufficient for driving the ultrasonic vibrating element 11. The phase comparator 15 has a variable input terminal V and a reference input terminal R, and to the variable input terminal V is applied the voltage phase signal .theta..sub.v and to the reference input terminal R is selectively supplied the current phase signal .theta..sub.I via a switching circuit (SW) 18 whose switching arm is connected to a contact F coupled with the voltage and current detecting circuit 14.

There is further provided a reference signal generating circuit 19 comprising a reference signal generator 20 and an oscillator 21 which is controlled by an output signal from the reference signal generator 20. It should be noted that the reference signal generator 20 may be formed by a voltage generator and the oscillator 21 may be constructed by the voltage controlled oscillator. A reference signal .theta..sub.ref generated from the reference signal generating circuit 19 is also selectively supplied to the reference input terminal R of the phase comparator 15 via the switching circuit 18. To this end, a contact S of the switching circuit 18 is connected to the reference signal generating circuit 19. The reference signal generator 20 is formed to generate a ramp voltage whose amplitude is increased monotonously, and this continuously increasing voltage is applied to the control input of the voltage controlled oscillator 21, so that the frequency of the reference signal .theta..sub.ref generated from the reference signal generating circuit 19 is increased also continuously.

The voltage and current phase signals .theta..sub.v and .theta..sub.I and the impedance signal .vertline.Z.vertline. generated from the voltage and current detecting circuit 14 are also supplied to a resonance point detecting circuit 22. In the resonance point detecting circuit 22, whether or not the frequency of the driving signal is equal to the resonance frequency f.sub.r of the ultrasonic vibrating element 11. When the frequency of the driving signal is not identical with the resonance frequency f.sub.r, the switching arm of the switching circuit 18 is connected to the contact S, and when the driving frequency becomes equal to the resonance frequency f.sub.r, the switching arm is changed from the contact S to the contact F. Therefore, when the driving frequency is not equal to the resonance frequency f.sub.r, the phase comparator 15 detects the phase difference between the voltage phase signal .theta..sub.v and the reference signal .theta..sub.ref so that the frequency of the driving signal is increased in accordance with the increasing frequency of the reference signal. When the resonance point detecting circuit 22 detects the in-phase condition, the switching arm of the switching circuit 18 is changed from the contact S to the contact F. Then, the phase lock loop 12 begins to operate normally and the frequency of the driving signal is automatically controlled to follow the resonance frequency of the ultrasonic vibrating element 11. The former operation mode is termed the sweep control mode, while the latter operation mode is called the feedback control mode.

FIGS. 4 to 7 illustrate some embodiments of the switching circuit 18 of the driving circuit according to the invention. In the switching circuit 18 shown in FIG. 4, the current phase signal .theta..sub.I and reference signal .theta..sub.ref are applied to three-state buffers 25-1 and 25-2, respectively. To a control terminal of the first three-state buffer 25-1 is directly