Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR CALCULATING TRANSDUCER CAPACITANCE
TO DETERMINE TRANSDUCER TEMPERATURE
BACKGROUND OF THE INVENTION
1. FIELD OF THE INVENTION
The present invention generally relates to ultrasonic surgical, systems and,
more
particularly; to a method for determining the temperature of an ultrasonic
transducer.
2. DESCRIPTION OF THE RELATED ART
It is known that electric scalpels and lasers can be used as a surgical
instrument to
perform the dual function of simultaneously effecting the incision and
hemostatis of soft tissue by
cauterizing tissues and blood vessels. However, such instruments employ very
high temperatures
to achieve coagulation, causing vaporization and fumes as well as splattering.
Additionally, the
use of such instruments often results in relatively wide zones of thermal
tissue damage.
Cutting and cauterizing of tissue by means of surgical blades vibrated at high
speeds
by ultrasonic drive mechanisms is also well known. One of the problems
associated with such
ultrasonic cutting instruments is uncontrolled or undamped vibrations and the
heat, as well as
material fatigue resulting therefrom. In an operating room environment
attempts have been made
to control this heating problem by the inclusion of cooling systems with heat
exchangers to cool
the blade. In one known system, for example, the ultrasonic cutting and tissue
fragmentation
CA 02359742 2009-09-04
system requires a cooling system augmented with a water circulating jacket and
means for
irrigation and aspiration of the cutting site. Another known system requires
the delivery of
cryogenic fluids to the cutting blade.
It is known to limit the current delivered to the transducer as a means for
limiting
the heat generated therein. However, this could result in insufficient power
to the blade at a time
when it is needed for the most effective treatment of the patient. U.S. Patent
No. 5,026,387 to
Thomas, which is assigned to the assignee of the present application discloses
a system for
controlling the heat in an ultrasonic surgical cutting and hemostasis system
without the use
of a coolant, by controlling the heat in an ultrasonic surgical cutting and
hemostasis system
without the use of a coolant, by controlling the drive energy supplied to the
blade. In the
system according to this patent an ultrasonic generator is provided which
produces an
electrical signal of a particular voltage, current and frequency, e.g. 55,500
cycles per second.
The generator is connected by a cable to a hand piece which contains
piezoceramic elements
forming an ultrasonic transducer. In response to a switch on the hand piece or
a foot switch
connected to the generator by another cable, the generator signal is applied
to the transducer,
which causes a longitudinal vibration of its elements. A structure connects
the transducer to
a surgical blade, which is thus vibrated at ultrasonic frequencies when the
generator signal is
applied to the transducer. The structure is designed to resonate at the
selected frequency, thus
amplifying the motion initiated by the transducer.
The signal provided to the transducer is controlled so as to provide power on
demand to the transducer in response to the continuous or periodic sensing of
the loading condition
(tissue contact or withdrawal) of the blade. As a result, the device goes from
a low power, idle
state to a selectable high power, cutting state automatically depending on
whether the scalpel is
or is not in contact with tissue. A third, high power coagulation mode is
manually selectable with
automatic return to an idle power level when the blade is not in contact with
tissue. Since the
ultrasonic power is not continuously supplied to the blade, it generates less
ambient heat, but
imparts sufficient energy to the tissue for incisions and cauterization when
necessary. .
The control system in the Thomas patent is of the analog type. A phase lock
loop
(that includes a voltage controlled oscillator, a frequency divider, a power
switch, a matching
network and a phase detector), stabilizes the frequency applied to the hand
piece. A
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microprocessor controls the amount of power by sampling the frequency, current
and voltage
applied to the hand piece, because these parameters change with load on the
blade.
The power versus load curve in a generator in a typical ultrasonic surgical
system,
such as that described in the Thomas patent, has two segments. The first
segment has a positive
slope of increasing power as the load increases, which indicates constant
current delivery. The
second segment has a negative slope of decreasing power as the load increases,
which indicates
a constant or saturated output voltage. The regulated current for the first
segment is fixed by the
design of the electronic components and the second segment voltage is limited
by the maximum
output voltage of the design. This arrangement is inflexible since the power
versus load
characteristics of the output of such a system can not be optimized to various
types of hand piece
transducers and ultrasonic blades. The performance of traditional analog
ultrasonic power systems
for surgical instruments is affected by the component tolerances and their
variability in the
generator electronics due to changes in operating temperature. In particular,
temperature changes
can cause wide variations in key system parameters such as frequency lock
range, drive signal
level, and other system performance measures.
In order to operate an ultrasonic surgical system in an efficient manner,
during
startup the frequency of the signal supplied to the hand piece transducer is
swept over a range to
locate the resonance frequency. Once it is found, the generator phase lock
loop locks on to the
resonance frequency, continues to monitor the transducer current to voltage
phase angle, and
maintains the transducer resonating by driving it at the resonance frequency.
A key function of
such systems is to maintain the transducer resonating across load and
temperature changes that
vary the resonance frequency. However, these traditional ultrasonic drive
systems have little to
no flexibility with regards to adaptive frequency control. Such flexibility is
key to the system's
ability to discriminate undesired resonances. In particular, these systems can
only search for
resonance in one direction, i.e., with increasing or decreasing frequencies
and their search pattern
is fixed. The system cannot: (i) hop over other resonance modes or make any
heuristic decisions,
such as what resonance to skip or lock onto, and (ii) ensure delivery of power
only when
appropriate frequency lock is achieved.
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The prior art ultrasonic generator systems also have little flexibility with
regard to
amplitude control, which would allow the system to employ adaptive control
algorithms and
decision making. For example, these fixed systems lack the ability to make
heuristic decisions
with regards to the output drive, e.g., current or frequency, based on the
load on the blade and/or
the current to voltage phase angle. It also limits the system's ability to set
optimal transducer
drive signal levels for consistent efficient performance, which would increase
the useful life of the
transducer and ensure safe operating conditions for the blade. Further, the
lack of control over
amplitude and frequency control reduces the system's ability to perform
diagnostic tests on the
transducer/blade system and to support troubleshooting in general.
Some limited diagnostic tests performed in the past involve sending a signal
to the
transducer to cause the blade to move and the system to be brought into
resonance or some other
'vibration mode. The response of the blade is then determined by measuring the
electrical signal
supplied to the transducer when the system is in one of these modes. The
ultrasonic system
described in U.S. Patent No. 7,476,233 possesses the ability to sweep the
output drive
frequency, monitor the frequency response of the ultrasonic transducer and
blade, extract
parameters from this response, and use these parameters for system
diagnostics. This
frequency sweep and response measurement mode is achieved via a digital code
such that the
output drive frequency can be stepped with high resolution, accuracy, and
repeatability not
existent in prior art ultrasonic systems.
When using ultrasonic surgical generators, access to the transducer
temperature is
of particular importance. The temperature of the transducer can be used to
optimize the overall
performance of the ultrasonic surgical system, as well as to enhance the
overall safety of the
system during use, such as to determine whether it is safe to handle or grab
the hand piece. For
example, during use of the ultrasonic surgical system, such as while
performing surgery, the
impedance of the transducer can increase such that electrical losses within
the transducer increase
which can lead to excessive hand piece temperatures. It is therefore
advantageous to know the
temperature of the transducer to prevent undesired effects, such as injury to
an operator as a result
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of grabbing a hot hand piece, or to prevent injury to a patient as a result of
exposure to bare hand
piece surfaces.
Measuring the temperature of the transducer is relatively simple.
Traditionally,
thermocouples, thermistors and other classical temperature sensors are used to
measure the
transducer temperature for control and safety purposes. However, these methods
increase the cost
of the hand piece, and add additional wires and connections which could
potentially reduce the
reliability of the ultrasonic surgical system. Another way to determine the
transducer temperature
is to measure the shunt capacitance of the transducer (C0), and use it to
calculate the transducer
temperature.
A transducer with or without a blade will always possess non-resonant
frequencies
at which Co of the transducer can be measured. However, the particular non-
resonant frequencies
will vary depending on which blade is attached and the type of transducer in
use. Given a known
non-resonant frequency of a blade, the measurement of Co is relatively simple
and fast to perform.
However, if the resonant frequencies of the blade must first be identified and
Co then measured
at non-resonant frequencies, a considerable amount of time and effort will be
consumed. In such
a case, the determination of Co is difficult, because the frequency at which
Co is measured
preferably resides at a non-resonant frequency. Typically, the particular non-
resonant frequencies
used to measure C0 are almost always present in the blade. However, if the
design of the blade
is changed, the detection of these particular non-resonant frequencies is not
assured. Accordingly,
there is a need for a method for ensuring isolation of Co from resonances or a
nearby resonance
to determine the transducer/blade temperature.
SUMMARY OF THE INVENTION
The invention is a method for calculating the capacitance of a transducer (Co)
without knowing the exact resonance frequency of a transducer/blade
combination. The invention
also comprises a method for determining the temperature of the transducer
without the use of a
temperature sensor, or the like. The method of the invention is achieved by
sweeping across a
broad frequency range which contains resonant and non-resonant frequencies
where Co can be
measured. A pre-defined frequency range is set independently of the resonance
frequency of a
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specific transducer/blade combination. CO of the transducer/blade is measured
at several different
frequencies within the pre-defined frequency range to ensure that invalid CO
measurements are
disregarded, and the temperature of the transducer is calculated based on
valid CO measurements.
The method according to the invention is adaptive, in that CO is identified
independently of any variances of the resonance frequencies of the blade which
may occur. In this
manner, the method provides greater design freedom for future transducer and
blade designs, since
the location of a "quiet" non-resonant zone within a specific frequency range
is not required. By
using selective averaging of CO and measurements at different frequencies, the
present invention
achieves CO measurements which are more accurate than those obtained by a
single Co
measurement. Moreover, by eliminating Co measurements which appear disrupted
by resonances
at specific frequencies and by focusing only on distinct potentially valid CO
values, a rapid
calculation and an accurate identification of the shunt capacitance is
achieved. In accordance with
the invention, during manufacture of the hand piece, the measured capacitance
at an off-resonance
frequency (i.e., CO at a frequency other than resonance) is stored in non-
volatile memory located
in the hand piece (i.e., in an integrated circuit memory inside the connector,
cable or body of the
hand piece).
In an embodiment of the invention, the hand piece is measured to determine its
impedance ZHP. A comparison is made to determine whether the phase of the hand
piece is within
acceptable limits. If the absolute value of the phase of the hand piece is
less than a predetermined
value, then the drive frequency level is incremented by a fixed amount. If, on
the other hand, the
absolute value of the phase of the hand piece is greater than the
predetermined value, then ZHP of
the hand piece is measured a number of times.
An average CO value is computed at each generator drive frequency level for
each
non-resonant frequency. The drive frequency is incremented, and a check is
made to determined
whether the drive frequency is greater than a maximum frequency or whether the
total number of
ZHP measurements is greater than a predetermined number. If either of these
conditions are met,
then the average value of the C0 values measured at each drive frequency is
computed. If, on the
other hand, the drive frequency is less than the maximum frequency or the
total number of ZHP
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measurements is less than the predetermined number then additional CO values
are determined. In
the preferred embodiment, the maximum frequency is 44.5 kHz.
To determine whether the transducer temperature is within acceptable limits, a
calculation is performed to determine a calculated value for Co. The
calculated value is compared
to a CO value stored in non-volatile memory during manufacture of the hand
piece. If the
calculated value for C. is greater than a predetermined threshold above the C.
value stored in
non-volatile memory, then the transducer temperature is excessive and a
warning is provided to
the user. In the preferred embodiment, power to the hand piece is removed
until such time as the
shunt capacitance falls below the predetermined threshold.
In the preferred embodiment of the invention, the hand piece is measured at
fixed
frequency intervals to determine its impedance ZHP at each frequency interval.
Using the data
points obtained during the impedance measurement, a curve fit is then
performed to obtain a curve
fit equation.
This equation is solved at a number of equally spaced frequency values to
arrive
at a group of distinct impedance values. The shunt capacitance is calculated
for each of the
distinct impedance values. The maximum calculated capacitance value and the
minimum
calculated shunt capacitance value is discarded. An average of the remaining
values is then
calculated to thereby "smooth" the high and low values, and arrive at a final
shunt capacitance
value.
If the shunt capacitance is greater than a predetermined threshold based on a
CO/Temp relationship, then the transducer temperature is excessive and a
warning is provided to
the user. Alternatively, power to the hand piece is removed until such time as
the shunt
capacitance falls below the predetermined threshold. In the preferred
embodiment, the
predetermined threshold is a fixed amount above the capacitance of the hand
piece/blade at room
temperature, and the fixed amount is 462 pF.
In another embodiment of the invention, the rate of change of the measured
shunt
capacitance (CO) of the transducer is measured and compared to a predetermined
threshold. If the
rate of change is greater than the predetermined threshold, the
transducer/blade is on the verge of
over heating, or will do so in the near future. The CO of the transducer is
measured when a
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surgeon first activates the hand piece using the foot switch of the ultrasonic
generator or the switch
on the hand piece. A second measurement is performed upon release of either
switch by the
surgeon. The difference between the two measurements is calculated and divided
by a time
difference to arrive at a value which is representative of the rate of change
of the capacitance.
Here, the time difference is the time between the surgeon activating and
releasing the foot switch.
If the rate of change value for the shunt capacitance exceeds a predetermined
threshold stored in
memory, a warning is provided to the surgeon before the temperature of the
transducer becomes
excessive, and therefore presents a danger of injury to the surgeon or
patient.
In a further embodiment of the invention, the temperature of the transducer is
determined without the use of temperature sensors. This is accomplished by
using non-volatile
memory which is embedded in the hand piece to enhance the overall performance
and safety of
the system. The measured capacitance at an off-resonance frequency (i.e., the
shunt capacitance
(CO) at a frequency other than resonance) is stored in the non-volatile
memory. Linear regression
analysis of the values of the transducer capacitance, as it changes with
temperature and hand piece
use, is also stored in non-volatile memory in the generator.
Prior to and/or during hand piece activation, the generator performs a "read"
of
the room temperature capacitance data from the hand piece. The actual
capacitance of the hand
piece is then measured in accordance with the invention, and the actual
transducer temperature is
calculated using a polynomial curve stored in the non-volatile memory of the
generator. The
temperature data is then used to determined whether it is safe to activate the
hand piece, as well
as to determine what levels of parameters to expect during diagnostic
measurements. In this
manner, a means to indirectly measure the temperature of the transducer is
achieved. In addition,
the need for temperature sensors, wires and connector pins for performing
temperature
measurements are eliminated.
Using the method of the invention, greater design freedom for future
transducer and
blade designs is achieved, since the location of a "quiet" non-resonant zone
within a specific
frequency range is not required. By eliminating the need to measure resonance
frequencies, the
invention greatly increases and enhances the speed at which Co is determined.
By using selective
averaging of CO and measurements at different frequencies, the present
invention achieves Co
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measurement which are more accurate than those obtained by a single Co
measurement. By eliminating Co measurements which appear disrupted by
resonances at specific frequencies and by focusing only on distinct
potentially valid
Co values, a rapid calculation and an accurate identification of the shunt
capacitance
is achieved. Moreover, the "sampling process" is improved due to the avoidance
of
resonances and/or resonance zones which are located adjacent to frequencies at
which Co measurements are performed. In addition the method provides indirect
measurement of the temperature of the transducer, and the need for temperature
sensors, wires and connector pins for performing temperature measurements are
eliminated.
In a further aspect, there is provided a method for determining
temperature of a transducer of an ultrasonic hand piece; comprising the steps
of:
determining a shunt capacitance of the transducer;
calculating the temperature of the transducer based on the shunt
capacitance of the transducer; and
providing a warning to a user of the hand piece if one of the
temperature of
the transducer and a rate of change of the temperature is excessive;
wherein said determining step comprises the steps of:
applying an ultrasonic drive signal to the hand pieceiblade across a
pre-defined frequency range;
measuring a first hand piece shunt capacitance when a user first
activates the hand piece/blade;
measuring a second hand piece/blade shunt capacitance when the
surgeon deactivates the hand piece/blade;
calculating a time difference between when the hand piece/blade is
activated and deactivated using a time when the first measured hand
piece/blade
shunt capacitance is obtained and a time when the second measured hand
piece/blade shunt capacitance is obtained;
computing a rate of change value of the hand piece/blade shunt
capacitance using the calculated time difference;
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i
determining whether the rate of change value of the hand piece/blade
shunt capacitance is greater than a predetermined threshold above a value
stored in
memory; and
providing a warning to the user, if the rate of change value of the
hand piece/blade shunt capacitance is greater than the predetermined threshold
above the value stored in memory.
In a further aspect, there is provided a method for determining
temperature of a transducer of an ultrasonic hand piece, comprising the steps
of:
determining a shunt capacitance of the transducer comprising the
steps of;
applying an ultrasonic drive signal to the transducer across a pre-
defined frequency range;
measuring the hand piece impedance at fixed frequency intervals to
obtain a measured impedance at each frequency interval;
performing a curve fit based on each measured impedance at each
frequency interval to obtain a curve fit equation;
solving the curve fit equation at equally spaced frequency values to
obtain a group of distinct impedance values;
calculating a shunt capacitance based on each distinct impedance
value;
discarding a maximum and a minimum calculated shunt capacitance
value to obtain a residual group of shunt capacitances; and
averaging the residual group of shunt capacitances to obtain a final
shunt capacitance value of the hand piece;
calculating the temperature of the transducer based on the shunt
capacitance of the transducer; and
providing a warning to a user of the hand piece if one of the
temperature of the transducer and a rate of change of the temperature is
excessive.
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BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other advantages and features of the invention will become
more
apparent from the detailed description of the preferred embodiments of the
invention given below
with reference to the accompanying drawings in which:
FIG. 1 is an illustration of a console for an ultrasonic surgical cutting and
hemostasis system, as well as a hand piece and foot switch in which the method
of the present.
invention is implemented;
FIG. 2 is a schematic view of a cross section through the ultrasonic scalpel
hand
piece of the system of FIG. 1;
FIGS. 3(a) and 3(b) are block diagrams illustrating an ultrasonic generator
for
implementing the method of the invention;
FIG. 4 is a schematic illustration of transducer drive circuitry of a power
transformer of Fig. 3(b).
FIG. 5 is a flow chart illustrating an embodiment of the method of the
invention;
FIG. 6 is a flow chart illustrating another embodiment of the method of the
invention;
FIG. 7 is a flow chart illustrating another embodiment of the method of the
invention;
FIGS. 8(a) and 8(b) are flow charts illustrating a preferred embodiment of the
method of the invention; and
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FIG. 9 is a graph of capacitance vs. temperature for a hand piece fitted with
a test
tip.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 is an illustration of a system for implei'ienting the method in
accordance
with the invention. By means of a first set of wires in cable 26, electrical
energy, i.e., drive
current, is sent from the console 10 to a hand piece 30 where it imparts
ultrasonic longitudinal
movement to a surgical device, such as a sharp scalpel blade 32. This blade
can be used for
simultaneous dissection and cauterization of tissue. The supply of ultrasonic
current to the hand
piece 30 may be under the control of a switch 34 located on the hand piece,
which is connected
to the generator in console 10 via wires in cable 26. The generator may also
be controlled by a
foot switch 40, which is connected to the console 10 by another cable 50.
Thus, in use a surgeon
may apply an ultrasonic electrical signal to the hand piece, causing the blade
to vibrate
longitudinally at an ultrasonic frequency, by operating the switch 34 on the
hand piece with his
finger, or by operating the foot switch 40 with his foot.
The generator console 10 includes a liquid crystal display device 12, which
can be
used for indicating the selected cutting power level in various means such as
percentage of
maximum cutting power or numerical power levels associated with cutting power.
The liquid
crystal display device 12 can also be utilized to display other parameters of
the system. Power
switch 11 is used to turn on the unit. While it is warming up, the "standby"
light 13 is
illuminated. When it is ready for operation, the "ready" indicator 14 is
illuminated and the
standby light goes out. If the unit is supplying maximum power, the MAX
indicator is
illuminated. If a lesser power is selected, the MIN indicator is illuminated.
The level of power
when MIN is active is set by button 16.
If a diagnostic test is to be performed, it is initiated by the "test" button
19. For
safety reasons, e.g., to make sure a test is not started while the blade is
touching the surgeon or
other personnel, the button 19 may be depressed in combination with hand piece
switch 34 or foot
switch 40. Also, if the hand switch 34 is to be operative instead of foot
switch 40, "hand
activation" button 18 on the front panel must be selected or enabled using
button 18.
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When power is applied to the ultrasonic hand piece by operation of either
switch
34 or 40, the assembly will cause the surgical scalpel or blade to vibrate
longitudinally at
approximately 55.5 kHz, and the amount of longitudinal movement will vary
proportionately with
the amount of driving power (current) applied, as adjustably selected by the
user. When relatively
high cutting power is applied, the blade is designed to move longitudinally in
the range of about
40 to 100 microns at the ultrasonic vibrational rate. Such ultrasonic
vibration of the blade will
generate heat as the blade contacts tissue, i.e., the acceleration of the
blade through the tissue
converts the mechanical energy of the moving blade to thermal energy in a very
narrow and
localized area. This localized heat creates a narrow zone of coagulation,
which will reduce or
eliminate bleeding in small vessels, such as those less than one millimeter in
diameter. The cutting
efficiency of the blade, as well as the degree of hemostasis, will vary with
the level of driving
power applied, the cutting rate of the surgeon, the nature of the tissue type
and the vascularity of
the tissue.
As illustrated in more detail in FIG. 2, the ultrasonic hand piece 30 houses a
piezoelectric transducer 36 for converting electrical energy to mechanical
energy that results in
longitudinal vibrational motion of the ends of the transducer. The transducer
36 is in the form of
a stack of ceramic piezoelectric elements with a motion null point located at
some point along the
stack. The transducer stack is mounted between two cylinders 31 and 33. In
addition a cylinder
35 is attached to cylinder 33, which is mounted to the housing at another
motion null point 37.
A horn 38 is also attached to the null point on one side and to a coupler 39
on the other side.
Blade 32 is fixed to the coupler 39. As a result, the blade 32 will vibrate in
the longitudinal
direction at an ultrasonic frequency with the transducer 36. The ends of the
transducer achieve
maximum motion with a portion of the stack constituting a motionless node,
when the transducer
is driven with a maximum current at the transducers' resonant frequency.
However, the current
providing the maximum motion will vary with each hand piece and is a value
stored in the
non-volatile memory of the hand piece so the system can use it.
The parts of the hand piece are designed such that the combination will
oscillate at
the same resonant frequency. In particular, the elements are tuned such that
the resulting length
of each such element is one-half wavelength. Longitudinal back and forth
motion is amplified as
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the diameter closer to the blade 32 of the acoustical mounting horn 38
decreases. Thus, the horn
38 as well as the blade/coupler are shaped and dimensioned so as to amplify
blade motion and
provide harmonic vibration in resonance with the rest of the acoustic system,
which produces the
maximum back and forth motion of the end of the acoustical mounting horn 38
close to the blade
32. A motion at the transducer stack is amplified by the horn 38 into a
movement of about 20 to
25 microns. A motion at the coupler 39 is amplified by the blade 32 into a
blade movement of
about 40 to 100 microns.
The system which creates the ultrasonic electrical signal for driving the
transducer
in the hand piece is illustrated in FIGS. 3(a) and 3(b). This drive system is
flexible and can create
a drive signal at a desired frequency and power level setting. A DSP 60 or
microprocessor in the
system is used for monitoring the appropriate power parameters and vibratory
frequency as well
as causing the appropriate power level to be provided in either the minimum or
maximum
operating modes. The DSP 60 or microprocessor also stores computer programs
which are used
to perform diagnostic tests on components of the system, such as the
transducer/blade.
For example, under the control of a program stored in the DSP or
microprocessor
60, such as a phase correction algorithm, the frequency during startup can be
set to a particular
value, e.g., 50 kHz. It can than be caused to sweep up at a particular rate
until a change in
impedance, indicating the approach to resonance, is detected.. Then the sweep
rate can be reduced
so that the system does not overshoot the resonance frequency, e.g., 55 kHz.
The sweep rate can
be achieved by having the frequency change in increments, e.g., 50 cycles. If
a slower rate is
desired, the program can decrease the increment, e.g., to 25 cycles which both
can be based
adaptively on the measured transducer impedance magnitude and phase. Of
course, a faster rate
can be achieved by increasing the size of the increment. Further, the rate of
sweep can be changed
by changing the rate at which the frequency increment is updated.
If it is known that there is a undesired resonant mode, e.g., at say 51 kHz,
the
program can cause the frequency to sweep down, e.g., from 60 kHz, to find
resonance. Also, the
system can sweep up from 50 kHz and hop over 51 kHz where the undesired
resonance is located.
In any event, the system has a great degree of flexibility
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In operation, the user sets a particular power level to be used with the
surgical
instrument. This is done with power level selection switch 16 on the front
panel of the console.
The switch generates signals 150 that are applied to the DSP 60. The DSP 60
then displays the
selected power level by sending a signal on line 152 (FIG. 3(b)) to the
console front panel display
12. Further, the DSP or microprocessor 60 generates a digital current level
signal 148 that is
converted to an analog signal by digital-to-analog converter (DAC) 130. A
signal representing
the average output current from circuit 120 is applied to the negative input
of node 132. The
output of node 132 is a current error signal or amplitude control signal which
is applied to direct
digital synthesis (DDS) circuit 128 to adjust the amplitude of its output, as
opposed to the
frequency of its output, which is controlled by the signal on line 146 from
the DSP or
microprocessor 60. The arrangement of current level signal 148, DAC 130,
summing node 130,
and signal supplied by average output voltage 122 allows the DSP or
microprocessor 60 to adjust
the output current such that it can generate a desired power versus load curve
when not in constant
current mode.
To actually cause the surgical blade to vibrate, the user activates the foot
switch 40
or the hand piece switch 34. This activation puts a signal on line 154 in FIG.
3(a). This signal
is effective to cause power to be delivered from push-pull amplifier 78 to the
transducer 36. When
the DSP or microprocessor 60 has achieved lock on the hand piece transducer
resonance frequency
and power has been successfully applied to the hand piece transducer, an audio
drive signal is put
on line 156. This causes an audio indication in the system to sound, which
communicates to the
user that power is being delivered to the hand piece and that the scalpel is
active and operational.
FIG. 4 is a schematic illustration of transducer drive circuitry of a power
transformer of FIG. 3(b). The transducer is represented by an equivalent
electrical circuit with
The components CO, Ls, C, and Rs form a transducer equivalent circuit Tequ;v,
where CO is a shunt
capacitance and represents the electrical capacitance of the piezoelectric
elements of the
piezoelectric transducer 36 shown in FIG. 2.
L,, CS and Rs are an electrical equivalent of the overall mechanical system
and
collectively represent the mechanical branch. L, is the effective mass of the
system, C, is the
13
CA 02359742 2001-10-17
effective compliance and RS represents mechanical losses associated with
friction, internal material
dissipation and/or the power delivered to the tissue.
Inductor L, is matched to the shunt capacitance C. at the resonance of the
ultrasonic
system, such as approximately 55.5 kHz. Hence, L, and C." electrically cancel
each other at the
resonant frequency. As a result, all of the drive current will flow through
the mechanical branch.
This helps to ensure that the ultrasonic excursion of the transducer is
primarily proportional to the
drive current.
The two resistors RP/2 sum in series to a resistance of RP. This resistance
helps to
establish an upper limit of the overall impedance of the output circuit, and
also establishes an
upper limit for the drive voltage. In preferred embodiments, R. is a
relatively large resistance.
At resonance, the parallel combination of RP and R, is effectively R, because
Rs is much smaller
then RP, even when coagulating and cutting tissue.
The series combination of capacitors C,, and C,.2 forms a voltage divider.
Together
these capacitors reduce the high voltage that typically drives the transducer
to a level which is
appropriate for signal processing by integrated circuits (not shown). A
transformer V, couples the
reduced voltage to the feedback circuitry (voltage sense 92 of FIG. 3(b)) and
also provides
isolation between the drive circuitry and the other circuitry of the
generator.
A small voltage drop is provided across the series combination of resistors R3
and
R4. In the preferred embodiment, the series combination is a relatively low
resistance in the order
of ohms. The voltage drop across R3 and R4 is proportional to the drive
current. This voltage is
provided to the feedback circuitry (current sense 88 of FIG. 3(b)) through a
transformer IT, which
also isolates the drive circuitry from the rest of the circuitry of the
generator. This signal
represents current in the control algorithms implemented in the generator.
R, and R2 are used to establish a minimum impedance level to the control
circuitry
for use in the control algorithms. The resistance is divided between the two
output arms Von,,,
Vo,2 of the power transformer to help mitigate electromagnetic radiation and
leakage current.
FIG. 5 is a flow chart illustrating an embodiment of the method of the
invention.
Under control of the program stored in the DSP or microprocessor,60 shown in
FIGS. 3(a) and
3(b), the method of the invention is implemented by applying an ultrasonic
signal to the transducer
14
CA 02359742 2001-10-17
36 to drive the transducer/blade across a pre-defined frequency range, such as
from 34 kHz to 36
kHz, as indicated in step 500. The pre-defined frequency range is set such
that it will contain
non-resonant frequencies where Co can be measured, and is set independently of
a specific
transducer/blade combination. The determination of the resonance frequency is
not made during
the initial attempt to measure Co. Instead, Co is measured at several
different frequencies
(preferably at least 5 frequencies) within and spaced along the pre-defined
frequency range, as
indicated in step 510.
Next, the measured Co values are compared, as indicated in step 520. In step
530,
a determination is made whether any of the measured Co values vary from each
other by more than
percent. Co values which substantially vary from a majority of the measured Co
values are
10 deemed invalid and disregarded, and Co values which pass this test are
deemed valid, as indicated
in step 532. This "filtering" helps to eliminate invalid C0 values, such as CO
values which have
been adversely influenced by a resonance or nearby resonance.
Next, a determination is made whether there are enough remaining valid Co
values
to ensure statistical validity, as indicated in step 534. In the preferred
embodiment, the number
of remaining values is at least 3. If an insufficient number of remaining
valid Co values exists,
a return to step 510 occurs. The method of the invention continues looping and
measuring
additional Co values until enough remaining valid C. are measured to ensure
statistical validity.
Once a statistically valid set of Co values is obtained, the valid Co values
are averaged to.obtain
a derived Co value for the transducer which is used to determine whether the
actual temperature
of the transducer is excessive, as indicated in step 540.
To determine whether the transducer temperature is within acceptable limits, a
calculation is performed in accordance with the relationship:
ACo= CS - Co, Eq. 1
where CS is the capacitance at an off-resonance frequency which is stored in
non-volatile memory
located in the hand piece at room temperature.
If A Co is greater than a predetermined threshold based on the Co/Temp
relationship shown in FIG. 8, then the transducer temperature is excessive and
a warning is
CA 02359742 2001-10-17
provided to the user. Alternatively, power to the hand piece is removed until
such time as the
shunt capacitance falls below the predetermined threshold. In the preferred
embodiment, the
predetermined threshold is a fixed amount above the capacitance of the hand
piece/blade at room
temperature, and the fixed amount is 462 pF.
FIG. 6 is a flow chart illustrating another embodiment of the method of the
invention. During manufacture of the hand piece, the measured capacitance at
an off-resonance
frequency at room temperature (i.e., CO at a frequency other than resonance)
is stored in
non-volatile memory located in the hand piece (i.e., in an integrated circuit
memory inside the
connector, cable or body of the hand piece). Under control of the program
stored in the DSP or
microprocessor 60 shown in FIGS. 3(a) and 3(b), the method is implemented by
applying an
ultrasonic signal to the transducer 36 to drive the transducer/blade across a
pre-defined frequency
range, such as from 34 kHz to 44 kHz, as indicated in step 600.
The hand piece is measured to determine its impedance ZHP, as indicated in
step
610. A comparison is made to determine whether the absolute value of the phase
difference
between the voltage and current of the hand piece drive signal is greater than
89.5 , as indicated
in step 620. If the absolute value of the phase difference of the hand piece
drive signal is less than
89.5 , then the drive frequency is incremented by 25 Hz, as indicated in step
625. If, on the other
hand, the absolute value of the phase difference of the hand piece drive
signal is less than 89.5 ,
then ZHP of the hand piece is measured a number of times, as indicated in step
630. In the
preferred embodiment; the impedance is measured 10 times.
An average C. is computed at the drive frequency in accordance with the
relationship:
1
CO 2JrfI ZHPI , Eq. 2
where f is the drive frequency of the generator.
The drive frequency is incremented by 25 Hz, as indicated in step 650. A check
is made to determined whether the drive frequency is greater than 44.5 kHz or
whether the number
of ZHP measurements is greater than 100, as indicated in step 660. If the
answer to either test is
16
CA 02359742 2001-10-17
yes, then the average value of the Co values measured at each drive frequency
is computed, as
indicated in step 670. If the drive frequency is less than 44.5 kHz and the
number of ZHP
measurements is less than 100, a return to step 610 occurs.
To determine whether the transducer temperature is within acceptable limits, a
calculation is performed in accordance with the relationship:
A C0= CS - Co, Eq. 3
where C. is the capacitance at an off-resonance frequency at room temperature
which is stored in
non-volatile memory located in the hand piece.
If A Co is greater than a predetermined threshold based on the C0/Temp
relationship shown in FIG. 8, then the transducer temperature is excessive and
a warning is
provided to the user. Alternatively, power to the hand piece is removed until
such time as the
shunt capacitance falls below the predetermined threshold. In the preferred
embodiment, the
predetermined threshold is a fixed amount above the capacitance of the hand
piece/blade at room
temperature, and the fixed amount is 462 pF.
FIG. 7 is a flow chart illustrating an alternative embodiment of the method of
the
invention. Here, the measured rate of change of the shunt capacitance (Co) of
the transducer is
compared to a predetermined threshold above a Co rate of change value stored
in non-volatile
memory. During manufacture of the hand piece, the measured capacitance at an
off-resonance
frequency at room temperature is stored in non-volatile memory located in the
hand piece. Under
control of the program stored in the DSP or microprocessor 60 shown in FIGS.
3(a) and 3(b), the
method is implemented by applying an ultrasonic signal to the transducer 36 to
drive the
transducer/blade across a pre-defined frequency range, such as from 34 kHz to
36 kHz, as
indicated in step 700.
The Co of the transducer is first measured when a surgeon first activates the
hand
piece using the foot switch of the ultrasonic generator or the switch on the
hand piece, as indicated
in step 710. A second measurement is performed upon release of either switch
by the surgeon,
as indicated in step 720. Of note, no measurements are performed during actual
use of the
ultrasonic surgical system due to the time required to process each
capacitance measurement.
17
CA 02359742 2001-10-17
Next, the difference between the first and second measurements is calculated
and
divided by the difference in time between when the first and second
measurements were obtained
to arrive at a value which is representative of the rate of change of the
capacitance, as indicated
in step 730.
A check is made to determine whether the rate of change value for the shunt
capacitance exceeds the predetermined threshold above the Co rate of change
value stored in the
non-volatile memory, as indicated in 740. If the rate of change value for the
shunt capacitance
exceeds the predetermined threshold, a warning is provided to the surgeon
before the temperature
of the transducer becomes excessive, and therefore presents a danger of injury
to the surgeon or
patient, as indicated in step 745. On the other hand, if the rate of change is
less than the
predetermined value, the test is ended, as indicated in step 750. Of note, the
rate of change value
for the shunt capacitance is directly related to the rate of temperature rise
of the transducer (see
FIG. 8). In the preferred embodiment, the predetermined threshold is 120
pF/min.
FIGS. 8(a) and 8(b) are flow charts illustrating a preferred embodiment of the
method of the invention. Under control of the program stored in the DSP or
microprocessor 60
shown in FIGS. 3(a) and 3(b), the method is implemented by applying an
ultrasonic signal to the
transducer 36 to drive the transducer/blade across a pre-defined frequency
range, such as from
34.5 kHz to 44.5 kHz, as indicated in step 800.
The hand piece is measured at fixed frequency intervals to determine its
impedance
ZHP at each frequency interval, as indicated in step 810. In the preferred
embodiment, the fixed
frequency interval is 50 Hz. Typically, resonances for known blades are not
found in the
pre-defined frequency range. However, Co can be influenced by resonances which
are located
slightly above or below the sweep range. Depending on whether the measurement
is performed
above or below the resonance frequency where ZHP is measured, resonances in
the vicinity of the
frequency tend to change the measured impedance value such that Co is shifted
above or below the
true Co value. The effect of these resonances is to cause errors in the
measurement of the shunt
capacitance (Co) when discrete measurements are performed.
In accordance with the invention, this effect is mitigated by using data
points
obtained in step 810 to perform a curve fit, as indicated in step 820. In
preferred embodiments,
18
CA 02359742 2001-10-17
the curve fit is a least squares curve fit which is performed in accordance
with the following
relationship:
ZNP = afo 2 + bfo + c , Eq. 4
where a, b and c are constants which are calculated via the curve fit and fo
is a fixed frequency at
which the hand piece impedance is measured.
The relationship in Eq. 4 is solved at a number of equally spaced frequency
values
to arrive at a group of distinct impedance values, as indicated in step 830.
In the preferred
embodiment, a total of eleven equally spaced frequencies across the sweep
range (i.e., 34.5 kHz,
35.5 kHz ... 44.5 kHz) are evaluated and the fixed frequency interval is 1000
Hz.
CO is calculated for each of the distinct impedance values, as indicated in
step 840.
In preferred embodiments, the calculation of CO is performed in accordance
with the relationship:
CO = -(1/f0) *(ZHP2 -1/RP2)' - (Cv1 * Cv2)/(Cv1 + Cv2) 1/(f02 * L,) - C, -
CPcb , Eq. 5
where Co is the shunt capacitance, fo is a fixed frequency at which the hand
piece impedance is
measured, ZHP is the calculated impedance at the fixed frequency fo, RP is a
value of a limiting
resistor, Cv1 and Cv2 are values of voltage dividing capacitors, L, is a value
stored in memory of
the generator which represents a transducer tuning inductor, CPcb is the
contribution of capacitance
from a printed circuit board in the generator and Cc is the capacitance of the
hand piece cable.
The maximum calculated shunt capacitance value and the minimum calculated
shunt
capacitance value is discarded, as indicated in step 850. An average of the
remaining values is
then calculated to thereby "smooth" the high and low values, and arrive at a
final shunt
capacitance value, as indicated in step 860.
If Co is greater than a predetermined threshold based on the C0/Temp
relationship
shown in FIG. 9, then the transducer temperature is excessive and a warning is
provided to the
user, as indicated in step 870. Alternatively, power to the hand piece is
removed until such time
as the shunt capacitance falls below the predetermined threshold. In the
preferred embodiment,
the predetermined threshold is a fixed amount above the capacitance of the
hand piece/blade at
room temperature, and the fixed amount is 462 pF.
19
CA 02359742 2001-10-17
By performing the curve fit, upward and downward fluctuations of the impedance
measurements created by the resonances are "smoothed out" such that their
effect (independent
of their location relative to the sweep range) is significantly reduced.
Calculating, discarding the
high and low shunt capacitance values, and subsequently averaging the
remaining shunt
capacitance values further aids to "smooth" the data. As a result, measurement
errors are also
reduced.
Of note, if a resonance occurs in the middle of the sweep range, the curve
fitting
significantly reduces the influence of the resonances upon the measured shunt
capacitance. In
contemplated embodiments, linear (i.e., a first order equation) and quadratic
curve (i.e., a second
order equation) fits are used. However, any curve fit may be used provided
that the equation
smooths the data, as opposed to following it precisely. For instance, a curve
fit which follows the
measured data exactly is not beneficial, since no data smoothing would occur.
In another embodiment of the invention, during manufacture of the hand piece,
the
measured capacitance at an off-resonance frequency (i.e., the shunt
capacitance (CO) at a frequency
other than resonance) is stored in non-volatile memory located in the hand
piece (i.e., in an
integrated circuit memory inside the connector, cable or body of the hand
piece). Linear
regression analysis of the values of the transducer capacitance, as it changes
with temperature and
hand piece use, is also stored in non-volatile memory located in the
generator.
Prior to and/or during hand piece activation, the generator performs a "read"
of
the room temperature capacitance data from the hand piece. The actual
capacitance of the hand
piece is then measured in accordance with the invention, and the actual
transducer temperature is
calculated using a polynomial curve (see FIG. 8, for example) stored in the
non-volatile memory
of the generator.
The temperature data is then used to determined whether it is safe to activate
the
hand piece, as well as to determine what levels of parameters to expect during
diagnostic
measurements. It will be appreciated that the actual temperature measurement
can be utilized for
other purposes, such as to determine whether he hand piece operating at
optimal conditions and
to predict changes in the hand piece resonance frequency.
CA 02359742 2001-10-17
In alternative embodiments, the curve fitting is performed as a supplement to
instances where the magnitude of the phased difference between the voltage and
current applied
to the hand piece/blade is used to filter the data prior to calculation of the
shunt capacitance.
Using the method of the invention, the need to obtain prior knowledge of the
transducer resonance is eliminated, and thus the speed at which Co is
determined is greatly
enhanced. By selectively averaging CO measurement obtained at different
frequencies, a highly
accurate CO measurement is obtained. Moreover, by eliminating CO measurements
which appear
disrupted by resonances and by focusing only on distinct potentially valid CO
values, a faster
calculation and identification of highly accurate CO values is achieved. As a
result, an indicator
of problems before the temperature of the hand piece becomes excessive is
achieved.
Although the invention has been described and. illustrated in detail, it is to
be clearly
understood that the same is by way of illustration and example, and is not to
be taken by way of
limitation. The spirit and scope of the present invention are to be limited
only by the terms of the
appended claims.
21
