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BACKGROUND OF THE INVENTION
Field of the Invention and Related Art Statement
The present invention relates to an apparatus for generating an ultrasonic
oscillation, and more particularly to an apparatus for generating an
ultrasonic oscillation comprising an ultrasonic transducer having an
ultrasonic vibrating element for producing an ultrasonic oscillation and a
probe for transmitting the oscillation produced by the ultrasonic
vibrating element, and a driving circuit for supplying a driving signal to
the ultrasonic vibrating element.
Heretofore, there have been proposed various kinds of apparatuses using the
ultrasonic transducer. For instance, ultrasonic surgical knives and
ultrasonic working machines have been developed. In these ultrasonic
apparatuses, it is advantageous to effect the impedance matching between
the ultrasonic transducer and the driving circuit in order to improve the
driving efficiency of the ultrasonic vibrating element.
FIGS. 1A and 1B show the known ultrasonic probe in which vibrating rods 2
and 3 having different lengths are detachably secured to an ultrasonic
transducer 1. The inventor of the instant application has experimentally
confirmed that the impedance of the probe illustrated in FIG. 1B is
smaller than that of the probe depicted in FIG. 1A by about five times.
Therefore, when these probes are driven by the same driving circuit, if
the output impedance of the driving circuit is fixedly matched to either
one of the vibrating rods 2 and 3, the impedance matching could not be
attained for the other of the vibrating rods 3 or 2 and the driving
efficiency of the ultrasonic vibrating element 1 would be decreased to a
great extent.
In order to mitigate the above mentioned drawback, in a Japanese Patent
Laid-open Publication Kokai Sho 63-162086, there has been proposed an
ultrasonic transducer in which taps on a secondary side of a coupling
transformer for electromagnetically coupling the ultrasonic vibrating
element and the driving circuit are switched in accordance with the
amplitude of the oscillation or vibration of the ultrasonic vibrating
element.
However, in the ultrasonic oscillation generating apparatus disclosed in
said Japanese Patent Laid-open Publication Kokai Sho 63-162086, there is a
problem that the impedance matching could be no more attained due to the
fact that the adjustment of the impedance matching is carried out on the
basis of the amplitude of the oscillation. That is to say, in the
ultrasonic transducer the amplitude of the oscillation is proportional to
an amplitude of a current passing through the ultrasonic vibrating
element, so that if the impedance of the transducer circuit is increased
twice, the taps on the secondary side of the coupling transformer are
changed such that the voltage applied to the ultrasonic vibrating element
is increased also twice in order to keep the amplitude of the current
unchanged. Then, the impedance of the transducer circuit viewed from the
primary side of the transformer is decreased by four times, because the
ratio of the primary winding to the secondary winding becomes 1:2. This
results in that the impedance of the load for the driving circuit is
decreased by two times, although the impedance of the transducer circuit
is increased by two times. Therefore, the impedance matching could not be
attained and the ultrasonic vibrating element could not be driven
efficiently.
Further, in the known ultrasonic generating apparatus, the taps are
provided on the secondary side of the coupling transformer, i.e. on the
vibrating element side of the transformer. When the apparatus is applied
to the ultrasonic surgical knife, a switching circuit for switching the
secondary winding portions is arranged in the circuitry on the patient
side, so that the electrical insulation should be effected to a very high
degree in order to achieve protection against the electric leakage and
discharge. This apparently increases the cost of the apparatus.
In order to attain a proper impedance matching, it would be also considered
that the impedance of ultrasonic transducers to be used are previously
measured and when an ultrasonic transducer is used, the impedance matching
is attained manually in accordance with the measured impedance of the
relevant transducer. In such a solution, there might be produced another
problem of the misoperation of the user and the driving circuit might be
broken under the overload condition.
In Japanese Patent Laid-open Publication Kokai Sho 63-212341 and 63-212342,
there are described further known ultrasonic apparatuses in which objects
such as hematoma and tumor produced within a patient body are broken into
pieces by irradiating the ultrasonic beam thereupon by inserting the
ultrasonic endoscope and pieces of the objects are sucked out of the body
via a tube arranged in the endoscope. In such ultrasonic surgical
operating apparatus, it is desired that the amplitude of the ultrasonic
probe driven by the ultrasonic vibrating element is kept constant
regardless of the acoustic impedance of the objects. As explained above,
since the amplitude of the ultrasonic vibrating element is proportional to
the amplitude of the current passing through the element, the output of
the oscillator in the driving circuit is supplied to the vibrating element
via a voltage controlled amplifier (VCA) whose amplification factor can be
changed by a control voltage, and the amplification factor of the VCA is
adjusted in accordance with the driving current such that the driving
current can be kept constant.
In the above mentioned ultrasonic apparatus in which the ultrasonic
vibrating element is driven by the constant current circuit, the
construction of the apparatus can be made simple and the amplitude of the
ultrasonic oscillation can be maintained substantially constant. However,
the electric stability of the known apparatus sometimes becomes
deteriorated. For example, in the above explained ultrasonic surgical
apparatus, when the tip of the ultrasonic probe is urged against the
object, the electric property of the probe is changed to a large extent in
accordance with the objects and the amplitude of the driving current
becomes extremely small. Then, the constant current circuit operates such
that the control voltage for the VCA is abnormally increased and the
voltage applied to the ultrasonic vibrating element becomes larger than
threshold voltages of the element and driving circuit, so that they might
be broken. This is quite dangerous for the patient.
SUMMARY OF THE INVENTION
The present invention has for its object to provide a novel and useful
apparatus for generating the ultrasonic oscillation in which the above
mentioned drawbacks of the known apparatuses can be removed and the
impedance matching between the ultrasonic vibrating element and the
driving circuit can be always attained correctly in an automatic manner
and the ultrasonic vibrating element can be driven always efficiently.
It is another object of the present invention to provide an apparatus for
generating the ultrasonic oscillation in which the ultrasonic vibrating
element can be always driven stably without damaging the driving circuit
and element as well as without damaging or injuring the patient.
According to the invention, an apparatus for generating an ultrasonic
oscillation comprises:
an ultrasonic transducer having an ultrasonic vibrating element for
producing an ultrasonic oscillation and a probe for transmitting the
ultrasonic oscillation produced by the ultrasonic vibrating element;
a driving circuit for supplying a driving signal to said ultrasonic
vibrating element:
an impedance matching means connected between said ultrasonic transducer
and said driving circuit for matching the output impedance of the driving
circuit to the impedance of said ultrasonic transducer;
an impedance detecting means for detecting the impedance of said ultrasonic
transducer to generate an impedance detecting signal; and
controlling means for automatically controlling said impedance matching
means in accordance with said impedance detection signal supplied from
said impedance detecting means such that the output impedance of said
driving circuit is matched to the impedance of said ultrasonic transducer.
In a preferred embodiment of the apparatus according to the invention, said
impedance matching means comprises a matching transformer having a
plurality of primary windings and a secondary winding connected to the
ultrasonic transducer, a switching circuit for selectively connecting one
of said primary windings to the driving circuit, and a control circuit for
controlling said switching circuit in accordance with said impedance
detection signal supplied from said impedance detecting means such that
the output impedance of said driving circuit is matched with the impedance
of the ultrasonic transducer.
According to further aspect of the invention, an apparatus for generating
an ultrasonic oscillation comprises:
an ultrasonic transducer having an ultrasonic vibrating element for
producing an ultrasonic oscillation and a probe for transmitting the
ultrasonic oscillation produced by the ultrasonic vibrating element;
a driving circuit for supplying a driving signal to said ultrasonic
vibrating element;
an impedance matching means connected between said ultrasonic transducer
and said driving circuit for matching the output impedance of the driving
circuit to the impedance of said ultrasonic transducer;
a probe identifying means for identifying a kind of said ultrasonic
transducer to generate a probe identification signal; and
controlling means for automatically controlling said impedance matching
means in accordance with said probe identification signal supplied from
said probe identifying means such that the output impedance of said
driving circuit is matched to the impedance of said ultrasonic transducer.
According to another aspect of the invention, an apparatus for generating
an ultrasonic oscillation comprises:
an ultrasonic transducer having an ultrasonic vibrating element for
producing an ultrasonic oscillation and a probe for transmitting the
oscillation produced by the ultrasonic vibrating element:
a driving circuit having a voltage controlled amplifier for supplying a
driving power to said ultrasonic vibrating element;
a feedback loop connected between said ultrasonic transducer and said
driving circuit for detecting a current of said driving power supplied to
said ultrasonic transducer and applying a control voltage corresponding to
said current of the driving power to said voltage controlled amplifier to
control an amplification factor of the voltage controlled amplifier; and
a voltage limiting means connected in said feedback loop for limiting an
amplitude of said control voltage.
According to still another aspect of the invention, an apparatus for
generating an ultrasonic oscillation comprises:
an ultrasonic transducer having an ultrasonic vibrating element for
producing an ultrasonic oscillation and a probe for transmitting the
ultrasonic oscillation produced by the ultrasonic vibrating element;
a driving circuit having a voltage controlled amplifier for supplying a
driving power to said ultrasonic vibrating element;
a feedback loop connected between said ultrasonic transducer and said
driving circuit for detecting a current of said driving power supplied to
said ultrasonic transducer and applying a control voltage corresponding to
said current of the driving power to said voltage controlled amplifier to
control an amplification factor of the voltage controlled amplifier;
voltage limiting means having a plurality of voltage limiting elements
connected in said feedback loop for limiting an amplitude of said control
voltage;
probe identifying means for identifying a kind of said ultrasonic
transducer to generate a probe identification signal; and
controlling means for automatically switching said plurality of voltage
limiting elements of the voltage limiting means in accordance with the
probe identification signal produced by the probe identifying means.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A and 1B are schematic views showing the vibration mode of the known
ultrasonic oscillation generating circuit;
FIG. 2 is a block diagram illustrating the principal construction of the
ultrasonic oscillation generating apparatus according to the invention;
FIG. 3 is a block diagram depicting a first embodiment of the ultrasonic
oscillation generating apparatus according to the invention;
FIG. 4 is a flow chart for explaining the generation of the apparatus shown
in FIG. 3;
FIGS. 5A and 5B are graphs showing the frequency characteristic of the
driving voltage;
FIGS. 6A and 6B are is a block diagram illustrating a second embodiment of
the apparatus according to the invention;
FIG. 7 is a graph representing the relationship between the frequency
characteristic of the driving voltage and the output of the voltage
comparator;
FIG. 8 is a block diagram showing the construction of the phase comparator;
FIG. 9 is a graph showing the frequency range of the ultrasonic vibrating
element and the oscillator;
FIG. 10 is a block diagram illustrating a third embodiment of the apparatus
according to the invention;
FIG. 11 is a block diagram depicting a fourth embodiment of the apparatus
according to the invention;
FIG. 12 is a block diagram showing the basic construction of the ultrasonic
oscillation generating apparatus according to the invention, in which the
amplitude of the driving signal is limited;
FIG. 13 is a block diagram showing an embodiment of the apparatus according
to the invention;
FIG. 14 is a block diagram illustrating another embodiment of the apparatus
according to the invention;
FIG. 15 is circuit diagram showing the detailed construction of a major
part of the apparatus; and
FIG. 16 is a side view depicting the whole construction.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Embodiments
FIG. 2 shows the principal construction of the apparatus for generating the
ultrasonic oscillation according to the invention. An output signal
generated from a driving circuit 11 is supplied via a matching transformer
12 which serves as a variable matching means for an ultrasonic transducer
or probe 13 including ultrasonic vibrating element 13a and vibrating rod
13b. The matching transformer 12 comprises taps 14-1 and 14-2 provided on
the primary side of the transformer for changing the number of winding
turns. To the primary side of the transformer 12 is connected an impedance
detection circuit 15 for detecting the impedance of the ultrasonic
transducer 13. In accordance with the impedance of the ultrasonic
transducer 13 detected by the impedance detection circuit 15, a
controlling means comprising a preset circuit 16 and a relay 17 is driven
such that the output impedance of the driving circuit 11 is changed by
changing the taps 14-1 and 14-2 to attain the optimum condition for the
ultrasonic transducer 13. In this manner, the matching between the driving
circuit 11 and the ultrasonic transducer 13 can be attained in an
automatic manner, so that the ultrasonic vibrating element 13a can be
always driven efficiently.
By constructing the apparatus in the manner explained above, the impedance
of the ultrasonic transducer 13 can be always correctly matched to the
impedance of the driving circuit 11 in an automatic manner, so that the
ultrasonic transducer can be driven in a very efficient manner.
FIG. 3 is a block diagram showing a first embodiment of the ultrasonic
oscillation generating apparatus according to the invention. In the
present embodiment, voltage phase and current phase of a driving signal
for the ultrasonic transducer comprising an ultrasonic vibrating element
21 are detected, and the frequency of the driving signal is automatically
controlled to be equal to a resonance frequency of the ultrasonic
vibrating element by a resonance point tracking circuit 22 in accordance
with the detected voltage and current phases. That is to say, the
so-called phase lock loop control is effected.
An output signal from the resonance point tracking circuit 22 is supplied
to a filter 23 and is converted into a driving signal having a sinusoidal
waveform. Then, the driving signal is supplied via a voltage controlled
amplifier (VCA) 24 whose amplification factor can be controlled and a
power amplifier 25 to a primary side of a matching transformer 26. The
matching transformer 26 includes two primary windings 27-1 and 27-2 and a
secondary winding 28. The two primary windings 27-1 and 27-2 may be
connected in series or in parallel with each other by means of a relay 29.
To the secondary winding 28 are connected the ultrasonic vibrating element
21 and a compensating inductor 30 for canceling the damping capacitance of
the element 21. It should be noted that in the present embodiment the
ratio of turns of the windings 27-1, 27-2 and 28 is set to 1:1:2 so that
the ratio of the primary and secondary windings may be changed between 1:1
and 1:2.
The voltage applied to the ultrasonic vibrating element 21 via the power
amplifier 25 is detected by a potentiometer 31 connected in parallel with
the primary side of the matching transformer 26, and an output of the
potentiometer 31 is applied to a differential amplifier 32 to remove
in-phase noise contained therein. The current passing through the
ultrasonic vibrating element 21 is detected by a current sensor 33
connected in series with the primary side of the matching transformer 26
and the detected signal is supplied to a differential amplifier 34 to
remove in-phase noise contained therein.
The voltage detection signal generated by the differential amplifier 32 is
supplied to a comparator 35 to detect a voltage phase signal .theta..sub.V
and is also supplied to an absolute value detecting circuit 36 to derive
an absolute value of the detected voltage .vertline.V.vertline..
Similarly, the current detection signal generated from the differential
amplifier 34 is supplied to a comparator 37 to produce a current phase
signal .theta..sub.1 and is supplied to an absolute value detecting
circuit 38 to derive an absolute value of the detected current
.vertline.I.vertline..
The voltage phase signal .theta..sub.V produced from the comparator 35 is
supplied to a resonance point tracking circuit 22 as well as to a
resonance point detecting circuit 39 for detecting the resonance point by
sweeping the frequency of the driving signal, which will be explained
later. The current phase signal .theta..sub.1 produced by the comparator
37 is supplied to the resonance point detecting circuit 39 as well as to
the resonance point tracking circuit 22 via a switch 40. The absolute
value .vertline.V.vertline. of the voltage detection signal generated by
the absolute value detecting circuit 36 is supplied to one input of a
voltage comparator 41 and the absolute value .vertline.I.vertline. of the
current detection signal generated by the absolute value detecting circuit
38 is supplied to one input of a differential amplifier 42. To the other
input of the differential amplifier 41 is applied a predetermined preset
voltage V.sub.Z and a difference between the absolute value
.vertline.V.vertline. and the preset value V.sub.Z is latched by a relay
control circuit 43 to control the relay 29. To the other input of the
differential amplifier 42 is applied a preset current signal from a
current preset circuit 44 to detect a difference between the absolute
value .vertline.I.vertline. of the current detection signal and the preset
current value. The amplification factor of the VCA 24 is controlled by the
output signal of the differential amplifier 42 such that the difference
becomes zero, so that the ultrasonic vibrating element 21 can be driven
with the constant current corresponding to the preset current value. It
should be noted that the output of the resonance point detecting circuit
39 is supplied to a control circuit 45.
The current preset circuit 44 comprises means for generating a reference
voltage V.sub.o for presetting the low constant current during the start
time period, a variable resistor 46 for presetting a constant current for
driving the ultrasonic vibrating element 21 at a predetermined amplitude
in the resonance point tracking mode, a switch 47 for connecting or
disconnecting the variable resistor 46, and a switch 48 for forcedly
stopping the vibration of the ultrasonic vibrating element 21 by making
the control voltage applied to VCA 24 zero.
In order to sweep the frequency of the driving signal for the ultrasonic
vibrating element 21, there is provided a generator 49 for generating the
sawtooth signal and the sawtooth signal is supplied to a voltage
controlled oscillator (VCO) 50 to generate a reference sweep signal having
a linearly varying frequency. The reference sweep signal thus produced is
supplied to the resonance point tracking circuit 22 via the switch 40. It
should be noted that the switch 40, relay control circuit 43, switches 47,
48, generator 49 and other circuits are controlled by the control circuit
45. Further, to the control circuit 45 is connected a switch 51 for
actuating and stopping the apparatus.
Now the operation of the apparatus of this embodiment will be explained
also with reference to a flow chart shown in FIG. 4.
While the switch 51 is made off, the switch 40 is connected to VCO 50, and
switches 47 and 48 are made on and off, respectively. Moreover the winding
ratio of the primary and secondary sides of the matching transformer 26 is
set to 1:1. When the switch 51 is made on, the ultrasonic vibrating
element 21 is driven at the low constant current set by the reference
voltage V.sub.o of the current preset circuit 44, and further the
generator 49 is actuated to control VCO 50 and the frequency of the
driving signal is swept in accordance with the reference sweep signal
generated from VCO 50. That is to say, the resonance point tracking
circuit 22 is locked to the output from VCO 50 and the oscillation
frequency of the PLL is scanned by scanning the output frequency of VCO 50
in accordance with the output of the generator 49.
During the above mentioned frequency scan, the ultrasonic vibrating element
21 is driven at the constant current mode owing to the operation of the
current preset circuit 44, differential amplitude 42 and VCA 24.
Therefore, if the constant current is set to .vertline.I.sub.o .vertline.,
the impedance .vertline.Z.vertline. can be detected by monitoring the
voltage .vertline.V.vertline. from the following equation:
##EQU1##
The impedance .vertline.Z.vertline. becomes minimum at the resonance point
f.sub.r during the frequency sweep, so that the voltage
.vertline.V.vertline. also becomes minimum at the resonance point f.sub.r
as shown in FIGS. 5A and 5B which correspond to the .vertline.V.vertline.
property of the ultrasonic transducer illustrated in FIGS. 1A and 1B.
Therefore, by comparing .vertline.V.vertline. with the preset voltage
V.sub.Z in the comparator 41, it can be detected that the ultrasonic
transducer having the small impedance is connected when the comparator 41
generates the output signal during the scan, and the ultrasonic transducer
having the large impedance is connected when the comparator 41 does not
generate the output signal.
When it is detected that the impedance .vertline.Z.vertline. of the
ultrasonic transducer is large, the switch 48 is made on to set the
control voltage for VCA 24 to zero to stop the output of the power
amplifier 25. In this condition, the relay 29 is actuated in accordance
with the impedance detection result latched in the relay control circuit
43 and the ratio of the windings of the matching transformer 26 is changed
to 1:2. In this manner, the output impedance of the driving circuit is
matched to the impedance of the ultrasonic transducer connected to the
driving circuit. After that, the switch 48 is made off and the generator
49 is actuated again to scan the driving frequency for the ultrasonic
vibrating element 21 in accordance with the reference sweep signal, and
the resonance point is detected by the resonance point detection circuit
39 in accordance with the voltage phase signal .theta..sub.V and current
phase signal .theta..sub.1. At the resonance point, the phase difference
between these phase signals becomes zero.
After the resonance point has been detected, the switch 40 is connected to
the comparator 37 to lock the resonance point tracking operation. When the
tracking is locked, the switch 47 is made off and the ultrasonic vibrating
element 21 is driven at the current value set by the variable resistor 46.
When it is detected that the ultrasonic transducer has the small impedance
.vertline.Z.vertline. and the winding ratio of the matching transformer 26
may be remained 1:1, the resonance point is detected during this frequency
scan, so that the rescan is not effected and the switch 40 is connected to
the comparator 37 after the resonance point has been detected to enter
into the lock in mode.
As explained above, in the present embodiment, the dynamic impedance of the
ultrasonic vibrating element 21 is detected to automatically correct the
impedance matching, and thus the error in the manual matching can be
effectively avoided and the ultrasonic transducer can be driven always
efficiently even if the impedance of the probe connected to the ultrasonic
vibrating element is changed to a great extent. In this manner, the
ultrasonic apparatus such as the ultrasonic surgical knife and ultrasonic
working machine can be driven efficiently. Further the impedance matching
is effected by changing the taps on the primary side of the matching
transformer, so that when the apparatus is applied to the medical devices
such as the ultrasonic surgical knife, it is not necessary to include the
switching circuits and control circuits in the circuit on the patient side
and the patient can be protected against the danger such as the breakage
of insulation and leakage.
FIGS. 6A and 6B are is a block diagram showing an embodiment of the
ultrasonic surgical knife to which the ultrasonic oscillation generating
apparatus according to the invention is applied. In the present
embodiment, a hand piece 55 comprises an ultrasonic vibrating element 56
of Langevin type to which a short probe 57 and a long probe 58 having
different impedance may be detachably secured. The ultrasonic vibrating
element 56 is connected to a secondary side of a matching transformer 59
and is driven by the output of a phase lock loop (PLL) 60. The matching
transformer 59 comprises two primary windings 61-1 and 61-2 and a
secondary winding 62. In a similar manner to the embodiment shown in FIG.
2, the ratio of turns between the primary and secondary windings can be
changed by a select circuit 63 including a relay and control circuit.
Across the vibrating element 56 is connected an inductor 64 for canceling
the damping capacitance of the element.
The phase lock loop 60 comprises a phase comparator (PC) 65, a charge pump
66 for converting the digital output of the phase comparator into the
analog signal, a loop filter 67 and a voltage controlled oscillator (VCO)
68. The output of the charge pump 66 is applied to VCO 68 as the control
voltage via the loop filter 67. The output of VCO 68 is supplied to the
filter 69 as well as to a frequency divider 70, and the output of the
frequency divider 70 is supplied to the filter 69, so that the rectangular
output signal from VCO 69 is converted into a sinusoidal driving signal
which contains only the resonance component of the ultrasonic vibrating
element 56 to avoid unnecessary heat radiation from the element. In the
present embodiment, the filter 69 is formed by a switched capacitor filter
(SCF) whose cut-off frequency can be changed by an external clock input.
When the filter is composed of such SCF, the amplitude variation and the
phase rotation of the output signal of the filter can be removed, and
therefore the constant current control and PLL are hardly affected by
these variations so that the rectangular-sinusoidal conversion can be
carried out in an ideal manner. Moreover, the impedance of the ultrasonic
vibrating element 56 can be detected precisely during the frequency sweep
of the driving signal, because only the fundamental wave is used.
The output of the filter 69 is supplied via voltage controlled amplifier
(VCA) 71, buffer amplifier 72, switch circuit 73 and power amplifier 74 to
the primary side of a matching transformer 59. By means of the matching
transformer 59, it is possible to electrically isolate the circuit of the
ultrasonic vibrating element 56 from the driving circuit and to attain the
impedance matching between the power amplifier 74 and the ultrasonic
vibrating element 56.
The voltage applied to the ultrasonic vibrating element 56 and the current
flowing through the element are detected by a voltage and current
detecting circuit 75 which includes voltage detecting potentiometer and
current sensor which are similar to those shown in FIG. 2. The obtained
voltage and current detection signals are supplied to differential
amplifiers 76 and 77, respectively. In this manner the problem of in-phase
noise which is inherent to the detection of the high voltage and large
current can be effectively removed by means of the differential amplifiers
76 and 77. Furthermore, although the positive and negative terminals of
the power amplifier 74 are connected inversely or the power amplifier is
not a type whose one output terminal is not connected to the ground, the
voltage and current detection signals can be obtained stably.
The voltage detection signal V generated from the differential amplifier 76
is supplied to a comparator 7i to detect a voltage phase detection signal
.theta..sub.V as well as to an absolute value detecting circuit 79 to
detect an absolute value of the amplitude of the detected voltage
.vertline.V.vertline.. Similarly the current detection signal T produced
by the differential amplifier 77 is supplied to as comparator 80 to derive
a current phase detection signal .theta..sub.1 as well as to the solute
value detecting circuit 81 to detect an absolute value of the detected
current .vertline.I.vertline..
The voltage phase signal .theta..sub.V derived from the comparator 78 is
supplied to a phase comparator 82 as well as to variable input terminal V
of the phase comparator 65 of PLL 60. The current phase signal
.theta..sub.I derived from the comparator 80 is supplied to the phase
comparator 82 as well as to a contact F of a switching circuit 83. The
absolute voltage signal .vertline.V.vertline. derived from the absolute
value detector 79 is supplied to a voltage comparator 84. The frequency
characteristic of the absolute value of the voltage detection signal is
shown in FIG. 7. In the voltage comparator 84, the absolute voltage value
.vertline.V.vertline. is compared with a predetermined threshold value L,
and the voltage comparator produces an output signal S when the absolute
value is smaller than the threshold value L. Since the driving circuit of
the present embodiment operates in the constant current mode, the absolute
value of the driving voltage represents the impedance of the ultrasonic
vibrating element 56. When the voltage comparator 84 generates the signal
S, the phase comparator 82 is enabled to detect the phase difference
.DELTA..theta. between the voltage phase signal .theta..sub.V and the
current phase signal .theta..sub.I. When the frequency of the driving
signal becomes equal to the desired resonance frequency f.sub.r of the
ultrasonic vibrating element 56, the phase difference .DELTA..theta.
becomes zero. Then, the phase comparator 82 generates a resonance
detection signal R. This resonance detection signal R is supplied to a
latch circuit 109 to change the state of the latch circuit 109. Then, the
switching arm of the switching circuit 83 is changed from the contact S to
the contact F and the current phase signal .theta..sub.I is supplied to
the reference input terminal R of the phase comparator 65 and PLL 60 is
operated in the feedback control mode in which the driving signal
frequency is automatically controlled to follow the resonance frequency of
the ultrasonic vibrating element 56. At the same time, a light emitting
diode 85 is lit to denote that PLL 60 is driven into the feedback control
mode. It should be noted that the output signal derived from the latch
circuit 109 is also supplied to a control circuit 93. The function of this
control circuit 93 will be explained in detail hereinafter.
FIG. 8 is a circuit diagram illustrating a detailed construction of the
phase comparator 82 and voltage comparator 84. The phase comparator 82
includes three D-flip-flops (D-FF) 89, 90, 91 and an OR gate 92. The
voltage phase signal .theta..sub.V generated from the phase comparator 78
is applied to D-input of the first D-FF 89 and the current phase signal
.theta..sub.I generated from the phase comparator 80 is applied to clock
input CK of D-FF 89. Q and Q outputs of this D-FF 89 are applied to clock
input CK of D-FF 90 and clock input CK of D-FF 91, and Q outputs of these
D-FFs 90 and 91 are applied to the OR gate 92. An output signal from the
OR gate 92 is supplied to the latch circuit 109 as the resonance point
detection signal R. To D inputs of D-FFs 90 and 91 are applied a supply
source voltage V.sub.CC. The voltage comparator 84 comprises a comparator
IC 84a and the absolute voltage signal .vertline.V.vertline. derived from
the absolute value detector 79 is applied to an inverted input of the
operational amplifier and a variable voltage source 84b is connected to
the non-inverted input. A voltage set by the variable voltage source 84b
represents the threshold level L shown in FIG. 7. An output signal from
the operational amplifier 84a is applied to clear terminals CLR of D-FFs
90 and 91.
As illustrated in FIGS. 6A and 6B, the absolute current signal
.vertline.I.vertline. generated from the absolute value detector 81 is
supplied to an inverted input of a differential amplifier 94. To a
non-inverted input of the differential amplifier 94 is applied a preset
signal generated fro ma current setting circuit 95. An output signal of
the differential amplifier 94 is applied, via a limiter 96, to a control
input terminal of the voltage controlled amplifier 71 to control the
amplification factor thereof such that the ultrasonic vibr | | |
