U.S. patent application number 09/738944 was filed with the patent office on 2001-10-11 for method and apparatus for controlling a temperature-controlled probe.
This patent application is currently assigned to ORATEC Interventions, Inc.. Invention is credited to Kannenberg, Donald P., Lisiecki, Andrew I..
Application Number | 20010029369 09/738944 |
Document ID | / |
Family ID | 23143126 |
Filed Date | 2001-10-11 |
United States Patent
Application |
20010029369 |
Kind Code |
A1 |
Kannenberg, Donald P. ; et
al. |
October 11, 2001 |
Method and apparatus for controlling a temperature-controlled
probe
Abstract
A method and apparatus to control a power output of a probe
connected to a controller in a thermal energy controller system to
maintain a target temperature. The system includes a probe, a
controller/generator and a means for connecting the probe to the
controller. The probe has a thermal element and a temperature
sensor. The temperature sensor provides a sensed temperature. The
method and apparatus allow the controller to more effectively
accommodate different types of probes by providing selectable probe
settings for the probes. The controller modifies its operation in
response to the selected probe setting. In this way, the power
output of each type of probe can be more effectively controlled to
better maintain the selected target temperature.
Inventors: |
Kannenberg, Donald P.; (San
Jose, CA) ; Lisiecki, Andrew I.; (Irvine,
CA) |
Correspondence
Address: |
Edward N. Bachand
Flehr Hohbach Test Albritton & Herbert LLP
Suite 3400
Four Embarcadero Center
San Francisco
CA
94111-4187
US
|
Assignee: |
ORATEC Interventions, Inc.
|
Family ID: |
23143126 |
Appl. No.: |
09/738944 |
Filed: |
February 26, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09738944 |
Feb 26, 2001 |
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09296690 |
Apr 21, 1999 |
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6162217 |
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Current U.S.
Class: |
606/34 |
Current CPC
Class: |
A61B 2018/00642
20130101; A61B 2018/0237 20130101; A61B 2018/00702 20130101; A61B
2018/00178 20130101; A61B 2017/00482 20130101; A61B 18/14 20130101;
A61B 2017/00084 20130101; A61B 18/1206 20130101; A61B 18/08
20130101; A61B 2018/00988 20130101; A61B 2018/0262 20130101; A61B
2018/00684 20130101; A61B 2018/00791 20130101 |
Class at
Publication: |
606/34 |
International
Class: |
A61B 018/04 |
Claims
What is claimed is:
1. A method for controlling a power output of a probe connected to
a system including controller circuitry to maintain a target
temperature, the probe having a thermal element and a temperature
sensor, the temperature sensor providing a sensed temperature,
comprising the steps of: providing, in a memory, at least one set
of probe settings including at least one gain parameter and
corresponding to predetermined operating characteristics for a
particular probe; receiving the target temperature; receiving a
first probe setting corresponding to a desired set of operating
characteristics for a probe; selecting a set from the at least one
set of probe settings in response to the first probe setting;
generating an error signal by comparing the sensed temperature to
the target temperature; determining an output control signal by
applying a control function to the error signal, the control
function using the at least one gain parameter from the selected
set; and controlling an amount of power output to the thermal
element in response to the output control signal to maintain the
target temperature.
2. The method of claim 1 wherein the at least one gain parameter of
the at least one set of probe settings includes a set specific
proportional gain factor and a set specific integral gain factor,
and wherein: said step of determining an output control signal
includes the steps of: generating a proportional signal by
multiplying the error signal by the selected set specific
proportional gain factor; generating an integral signal by
integrating the error signal and multiplying the integrated error
signal by the selected set specific integral gain factor; and
summing the proportional signal and the integral signal to generate
the output control signal.
3. The method of claim 2 wherein the at least one gain parameter of
the at least one set of probe settings includes a set specific
derivative gain factor, and wherein: said step of determining the
output control signal includes the step of: generating a derivative
signal by applying a derivative function to the sensed temperature
to generate an intermediate signal, and multiplying the
intermediate signal by the selected set specific derivative gain
factor, wherein said step of summing also sums the derivative
signal to generate the output control signal.
4. The method of claim 1 wherein the at least one gain parameter of
the at least one set of probe settings includes a set specific
proportional gain factor, a set specific integral gain factor and a
set specific derivative gain factor, and wherein: said step of
determining an output control signal includes the steps of:
generating a proportional signal by multiplying the error signal by
the selected set specific proportional gain factor; generating an
integral signal by integrating the error signal and multiplying the
integrated error signal by the selected set specific integral gain
factor; generating a derivative signal by comparing the sensed
temperature to a previous sensed temperature and applying a
derivative function to generate an intermediate signal and
multiplying the intermediate signal by the selected set specific
derivative gain factor; and summing the proportional signal, the
integral signal and the derivative signal to generate the output
control signal.
5. The method of claim 1 further comprising the step of: limiting
the output control signal to a predetermined output value when the
output control signal exceeds a predetermined threshold.
6. The method of claim 2 further comprising the step of: when the
predetermined output value exceeds a predetermined threshold value,
disabling said step of generating the integral signal.
7. The method of claim 2 further comprising the steps of:
determining an antiwindup difference by subtracting a maximum
predetermined output control value from the output control signal;
generating an antiwindup adjustment signal by multiplying the
antiwindup difference by an antiwindup gain factor; and generating
a modified error signal by subtracting the antiwindup adjustment
signal from the error signal, wherein said step of generating an
integral signal integrates the modified error signal by the
integral gain factor.
8. The method of claim 1 further comprising the steps of: receiving
a ramp parameter corresponding to a particular profile at which to
ramp up the output power; and changing the target temperature in
response to the ramp parameter.
9. The method of claim 8 further comprising the steps of: receiving
a switch setting corresponding to a particular profile at which to
ramp up the output power; and changing the target temperature in
response to the switch setting.
10. The method of claim 9 wherein said step of selecting the set
from the at least one set of probe settings selects a first set of
probe settings in response to the switch setting, and further
comprising the steps of: determining an intermediate target
temperature in response to the switch setting; and selecting a
second set of probe settings in response to the switch setting when
the intermediate target temperature is reached.
11. The method of claim 1 wherein the at least one gain parameter
of the at least one set of probe settings includes a set specific
derivative gain factor, and further comprising the step of:
generating a derivative signal by multiplying the sensed
temperature by a first predetermined signal, subtracting a
temporary integral signal generated from a previous sensed
temperature to generate an intermediate signal, and multiplying the
intermediate signal by the selected set specific derivative gain
factor, wherein said step of summing also sums the derivative
signal to generate the output control signal.
12. The method of claim 1 wherein the at least one set of probe
settings includes a set specific default target temperature and a
set specific default maximum output power, and wherein said step of
receiving the target temperature sets the target temperature to the
selected set specific default target temperature, and wherein said
step of controlling the amount of power output to the thermal
element clamps the power output to the selected set specific
default maximum output power.
13. A method for controlling a power output of a probe connected to
a system including controller circuitry to maintain a target
temperature, the probe having a thermal element and a temperature
sensor, the temperature sensor providing a sensed temperature,
comprising the steps of: providing, in a memory, at least one set
of probe settings including at least one gain parameter and
corresponding to predetermined operating characteristics for a
particular probe, the at least one gain parameter including a set
specific proportional gain factor, a set specific integral gain
factor and a set specific derivative gain factor; receiving the
target temperature; receiving a first probe setting corresponding
to a desired set of operating characteristics for a probe;
selecting a set from the at least one set of probe settings in
response to the first probe setting; receiving a sensed
temperature; generating an error signal by comparing the sensed
temperature to the target temperature; generating a proportional
signal by multiplying the error signal by the selected set specific
proportional gain factor; generating an integral signal by
integrating the error signal and multiplying the integrated error
signal by the selected set specific integral gain factor;
generating a derivative signal by comparing the sensed temperature
to a previous sensed temperature and applying a derivative function
to generate an intermediate signal and multiplying the intermediate
signal by the selected set specific derivative gain factor; summing
the proportional signal, the integral signal and the derivative
signal to generate the output control signal; controlling an amount
of power output to the thermal element in response to the output
control signal to maintain the target temperature; determining an
antiwindup difference by subtracting a maximum predetermined output
control value from the output control signal; generating an
antiwindup adjustment signal by multiplying the antiwindup
difference by an antiwindup gain factor; and generating a modified
error signal by subtracting the antiwindup adjustment signal from
the error signal, wherein said step of generating the integral
signal integrates the modified error signal by the integral gain
factor.
14. A system for controlling the power output of a probe connected
to the system to maintain a temperature, the probe having a thermal
element, and a temperature sensor providing a sensed temperature,
comprising: a processor; and a memory storing at least one set of
probe settings, the at least one set including at least one gain
parameter and corresponding to a predetermined operating
characteristics for a particular probe, the memory also storing
instructions that cause the processor to: receive a target
temperature value; receive a first probe setting corresponding to a
desired set of operating characteristics for a probe; select a set
of the probe settings in response to the first probe setting;
generate an error value by comparing the sensed temperature value
to the target temperature value; and determine an output control
value by applying a control function to the error value, the
control function using the at least one gain parameter from the
selected set whereby an amount of power is output to the thermal
element in response to the output control value to maintain the
temperature.
15. The system of claim 14 wherein the at least one gain parameter
of the sets of probe settings includes a set specific proportional
gain factor and a set specific integral gain factor, and wherein:
the instructions that determine the output control value includes
instructions to: generate a proportional value by multiplying the
error value by the selected set specific proportional gain factor;
generate an integral signal by integrating the error value and
multiplying the integrated error value by the selected set specific
integral gain factor; generate a derivative value by comparing the
sensed temperature value to a previous sensed temperature value and
applying a derivative function to generate an intermediate value
and multiplying the intermediate value by the selected set specific
derivative gain factor; and sum the proportional value, the
integral value and the derivative value to generate the output
control value.
16. The system of claim 14 wherein the memory further includes
instructions to limit the output control value to a predetermined
output value when the output control value exceeds a predetermined
threshold.
17. The system of claim 14 wherein the memory further includes
instructions to: receive a switch setting corresponding to a
particular profile at which to ramp up the output power; and change
the target temperature in response to the switch setting.
18. A computer program product for controlling a power level of a
probe connected to a medical device computer system, the computer
program product for use in conjunction with the computer system,
the computer program product comprising a computer readable storage
medium and a computer program mechanism embedded therein, the
computer program mechanism comprising: at least one set of probe
settings including at least one gain parameter and corresponding to
a predetermined operating characteristics for a particular probe; a
first set of instructions that receive a target temperature value;
a second set of instructions that receive a first probe setting
corresponding to a desired set of operating characteristics for a
probe; a third set of instructions that select a set from the at
least one set of probe settings in response to the first probe
setting; a fourth set of instructions that receive a sensed
temperature value; a fifth set of instructions that generate an
error value by comparing the sensed temperature value to the target
temperature value; a sixth set of instructions that determine an
output control value by applying a control function to the error
value, the control function using the at least one gain parameter
from the selected set; and a seventh set of instructions that cause
an amount of power to be output to a thermal element in response to
the output control value.
19. The computer program product of claim 18 wherein the at least
one gain parameter of the at least one set of probe settings
includes a set specific proportional gain factor and a set specific
integral gain factor, and wherein: the instructions that determine
the output control value include instructions to: generate a
proportional value by multiplying the error value by the selected
set specific proportional gain factor; generate an integral signal
by integrating the error value and multiplying the integrated error
value by the selected set specific integral gain factor; generate a
derivative value by comparing the sensed temperature value to a
previous sensed temperature value and applying a derivative
function to generate an intermediate value and multiplying the
intermediate value by a selected set specific derivative gain
factor; and sum the proportional value, the integral value and the
derivative value to generate the output control value.
20. The computer program product of claim 18 wherein the computer
program mechanism further includes instructions to limit the output
control value to a predetermined output value when the output
control value exceeds a predetermined threshold.
21. The computer program product of claim 18 wherein the computer
program mechanism further includes instructions to: receive a
switch setting corresponding to a particular profile at which to
ramp up the output power; and change the target temperature value
in response to the switch setting.
Description
BRIEF DESCRIPTION OF THE INVENTION
[0001] This invention relates generally to medical devices. More
particularly, this invention relates to a method and apparatus for
controlling the temperature of a probe that is used to vary the
thermal energy delivered to tissue during a surgical procedure.
BACKGROUND OF THE INVENTION
[0002] Soft tissue is the most abundant tissue in the human body.
Most soft tissue is collagen--over 90% of the organic matter in
tendons and ligaments is collagen. The connective tissue in joints
is comprised of soft tissue, generally collagen tissue. When soft
tissue in a joint is damaged, the healing process is often long and
painful.
[0003] Well-known methods for addressing the treatment of soft
tissue in injured joints include strengthening exercises, open
surgery, and arthroscopic techniques. Using current treatments,
many people with injured joints suffer from prolonged pain, loss of
motion, nerve injury, and some develop osteoarthritis. The soft
tissue in many injured joints never heals enough to return the
damaged joint to its full range of function.
[0004] It is known in the art that thermal energy applied to soft
tissue, such as collagen tissue, in joints may alter or manipulate
the tissue to provide a therapeutic response during thermal
therapy. In particular, applying controlled thermal energy to soft
tissue in an injured joint can cause the collagenous tissue to
shrink, thereby tightening unstable joints.
[0005] Medical probes for the rehabilitative thermal treatment of
soft tissues are known in the art. Examples of these probes include
laser probes and RF heated probes. While these tools meet the basic
need for rehabilitative thermal treatment of soft tissues, such as
collagen tissues, many suffer from temperature over-shoot and
under-shoot fluctuation causing unpredictable results in the
thermal alteration.
[0006] One medical probe in U.S. Pat. No. 5,458,596 to Lax, et al.,
discloses examples of a probe with a proximal and distal end that
employs heat for the controlled contraction of soft tissue.
However, a potential drawback of many prior art probes is that the
probe's temperature can become unstable when heat from the probe is
dissipated into the mass of the treated tissue. This situation can
be a particular problem when treating dense tissue; dense tissue
acts as a heat sink thereby requiring additional energy input to
maintain the desired temperature. The application of additional
energy in an attempt to compensate for the heat sink effect can
cause an underdamped effect before settling out at the correct
temperature.
[0007] In general, a system is overdamped when its damping factor
is greater than one and has a slow response time. A system is
critically damped when its damping factor is exactly one. A system
is underdamped when its damping factor is less than one. In an
underdamped system, "ringing" is a problem because it can cause the
momentary application of temperatures that are too high for the
safe heating of soft tissue. When this occurs, damage to the soft
tissue may result from charring, ablation or the introduction of
unwanted and harmful effects on the soft tissue causing injury.
[0008] Typically, the medical probes are attached to a controller
to control the power output of the probe based on an actual
temperature measurement from a temperature sensor such as a
thermocouple in the probe and a set predetermined target
temperature. The controller is part of a system that includes
circuitry to monitor sensed temperature from the temperature
sensor. Temperature-controlled probes are designed to provide
precise coagulation to eliminate damage, charring, and bubbles.
Different size probes with various configurations are available to
treat various joint sizes including the shoulder, knee, ankle,
wrist and the elbow.
[0009] Precise temperature control of the system in which the
probes are used is required during various types of thermal therapy
of soft tissue. For example, during hyperthermia which is defined
as the treatment of diseased soft tissue by raising the bodily
temperature by physical means, some prior art probes have
difficulty in providing smooth and consistent heating because the
preferred materials for the energy delivery electrodes are highly
thermally responsive materials. Such materials generally do not
retain large amounts of heat energy. At initiation, the controller
rapidly heats the probe to achieve the target temperature which can
result in an overshoot problem. During application, contact with
large tissue masses tends to cause underdamped fluctuations in the
probe temperature due to vast differences in the temperature of the
surrounding tissue mass. Likewise, one skilled in the art will
appreciate that similar problems may occur during a desired
reduction in the soft tissue temperature.
[0010] In addition, different probes have different operating
characteristics. Applications using larger probes typically need
relatively large amounts of power to reach and maintain the desired
temperature. Applications using smaller probes, such as a spine
probe, need a well-controlled and precise stable temperature.
However, the typical controller uses the same method to control the
power output of all the different probes and does not change its
control process in response to different types of probes further
contributing to overshoot and undershoot problems.
[0011] Therefore, a method and apparatus are needed that allows the
controller to change operation in response to the type of probe
attached. This method and apparatus should also reduce temperature
overshoot and oscillation while initiating and applying
treatment.
SUMMARY OF THE INVENTION
[0012] A method and apparatus control the power output to a probe
to maintain a target temperature. The probe is part of a system
including a means for connecting a probe to a controller. The probe
has a thermal element and a temperature sensor. The temperature
sensor provides a sensed temperature. The method and apparatus
allows the system and the controller to more effectively
accommodate different types of probes by providing at least one
selectable probe setting for the probes. The controller modifies
its operation in response to the selected probe setting. In this
way, the power output of the probe is more effectively controlled
to maintain the target temperature.
[0013] A memory provides at least one set of probe settings. The
set includes at least one gain parameter and corresponds to
predetermined operating characteristics for a probe. A target
temperature is received. A first probe setting corresponding to a
desired set of operating characteristics for a probe is also
received. A set of probe settings is selected in response to the
first probe setting. An error signal is generated by comparing the
sensed temperature to the target temperature. An output control
signal is determined by applying a control function to the error
signal. The control function uses the gain parameter from the
selected set of probe settings. An amount of power is supplied to
the thermal element in response to the output control signal.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] For a better understanding of the invention, reference
should be made to the following detailed description taken in
conjunction with the accompanying drawings, in which:
[0015] FIG. 1 illustrates a controller and probe in accordance with
an embodiment of the present invention;
[0016] FIG. 2 illustrates the controller of FIG. 1 in accordance
with an embodiment of the present invention;
[0017] FIG. 3 illustrates an exemplary table, stored in the memory
of FIG. 2, associating a particular probe setting with a particular
switch position;
[0018] FIG. 4 illustrates one embodiment of a
proportional-integral-deriva- tive (PID) control function of the
present invention;
[0019] FIG. 5 illustrates an embodiment of the derivative operation
of FIG. 4;
[0020] FIG. 6 illustrates a second embodiment of a PID control
function of the present invention;
[0021] FIG. 7 illustrates a third embodiment of a PID control
function of the present invention;
[0022] FIG. 8 is a flowchart of the PID control function of FIG.
4;
[0023] FIG. 9 is a flowchart of the derivative operation of FIG. 5
that is used in step 128 of FIG. 8;
[0024] FIG. 10 is a flowchart of an antiwindup function;
[0025] FIG. 11 is a flowchart of an alternate embodiment of an
antiwindup function;
[0026] FIG. 12 is a flowchart of an embodiment which varies the
target temperature to reach the final target temperature;
[0027] FIG. 13 is an exemplary temperature profile stored in the
memory of FIG. 2; and
[0028] FIG. 14 is a detailed flowchart of step 188 of FIG. 13.
[0029] Like reference numerals refer to corresponding parts
throughout the drawings.
DETAILED DESCRIPTION OF THE INVENTION
[0030] In FIG. 1, a probe 16 is attached to a temperature
controller 20 of the present invention. The temperature controller
20 may also be defined as a generator. The probe 16 has a thermal
element 22 attached to a probe tip 24. The thermal element 22
provides a means of altering the temperature of tissue by heating
or cooling. The thermal element 22 includes any of the following: a
transducer that delivers RF energy to the tissue, a resistive
heating element that delivers thermal energy to the tissue, or a
cooling element. Examples of probes and energy delivery are set
forth in greater detail in U.S. Pat. No. 5,458,596 to Lax et al.
which is incorporated herein by reference. The cooling element
includes a means for cooling with liquid nitrogen, or a Peltier
cell. A temperature sensor 26, such as a thermocouple, senses the
surrounding temperature. The controller 20 receives the sensed
temperature from the temperature sensor 26 and controls the amount
of power that is supplied to the thermal element 22 to change the
temperature of the probe tip 24 or to change the temperature of the
tissue such as during the delivery of RF energy to the tissue.
[0031] In a preferred embodiment, the temperature controller 20 is
part of a medical system used by physicians to adjust thermal
energy to soft tissue. To set a target temperature, a physician
activates a control 28, such as a knob or a digital switch, on the
controller 20. The target temperature is displayed on a display 30
in degrees Celsius. To select the operating characteristics of the
controller, the physician adjusts a multiposition switch 32, such
as a thumbwheel switch. The operating characteristics are
determined by the type of probe 16 and the type of tissue subject
to thermal therapy. In other words, each switch position is
associated with a probe and tissue combination. The physician may
obtain the desired operating characteristics, and therefore switch
position, from the manufacturer of the controller 20. Such
information may be included in the instructions for use (IFU). In
this way the physician can set both the temperature and operating
characteristics for different probes.
[0032] FIG. 2 illustrates the controller 20 in more detail. A
processor 34 communicates with a memory 36 the control 28, the
display 30, the multiposition switch 32, and a power control
circuit 38 which controls a power source 40 which is attached to
the probe. The processor 34 includes a microprocessor and
peripheral ports that attach to the control 28, display 30, the
multiposition switch 32 and the power control circuit 38. The
memory 36 includes semiconductor memory. In an alternate
embodiment, the memory 36 includes disk memory.
[0033] The memory 36 stores a PID_Temperature_Control procedure 42
and a PID_generation procedure 43, which will be described below,
sets of probe settings, Probe_Settings_1 to Probe_Settings_N, 44 to
46, respectively, a Temperature Profile 47, and a Switch Setting
Table 48. An exemplary probe setting 46 includes a proportional
gain factor Kp, an integral gain factor Ki and a derivative gain
factor Kd. In addition, the exemplary probe setting 46 may also
include a default target temperature and a default maximum power
value. The processor 34 executes the PID_Temperature_Control
procedure 42 to control the probe temperature using a PID control
methodology that is implemented in the PID_generation procedure
43.
[0034] Table 1 below shows a preferred set of gain settings.
1TABLE 1 Gain Settings Proportional Gain Integral Gain Derivative
Gain Gain Set Kp Ki Kd A 0.031 0.008 0.008 B 0.063 0.016 0.016 C
0.125 0.031 0.031 D 0.250 0.125 0.063 E 0.500 0.250 0.125
[0035] The higher gain settings (D and E) are beneficial in an
application in which the physician is heating a large area of
tissue and must move the probe across the tissue. A greater degree
of temperature oscillation may be tolerated due to the larger mass
of tissue available to absorb the variations.
[0036] The lower gain settings (A, B and C) are beneficial in an
application where the probe is stationary for long periods of time
and the temperature is varied slowly, over minutes. The lower gain
settings provide more precise temperature control.
[0037] The memory of FIG. 2 also stores a Task_scheduler 49a, a
Set_target_temperature procedure 49b, a PID_control procedure 49c
and a target_temperature 49d which will be explained below with
reference to FIGS. 12, 13 and 14.
[0038] In FIG. 3, the switch setting table 48 associates each
multiposition switch setting with a set of probe settings. Table 2,
below, shows the exemplary switch settings of table 48 of FIG. 3.
Table 2 summarizes the relationship between various switch
positions, the default temperature, the default maximum output
power, gain settings and probe type.
2TABLE 2 Switch Settings Default Default Switch Temperature Maximum
Gain Set Position (.degree. C.) Power (W) (See Table 1) Probe Type
0 55 50 C small 1 55 40 C small 2 55 30 C small 3 55 20 C small 4
67 30 C large 5 67 40 C large 6 67 50 C large 7 60 30 C large 8 60
40 C large 9 60 50 C large 10 55 20 D small 11 55 30 D small 12 67
40 D large 13 67 50 D large 14 80 40 D large 15 80 50 D large
[0039] In FIG. 4, a hardware implementation of one embodiment of a
proportional-integral-derivative (PID) temperature control is
illustrated, in which block 50 identifies the components of a
hardware implementation which accomplishes the method of the
present invention. For ease of illustration, the invention will be
described with respect to a hardware implementation. A person
skilled in the art will appreciate that a software implementation
may also be used based on the disclosure herein. In a preferred
embodiment, the temperature control block 50 is implemented in
software in the PID_Temperature_Control procedure 42. The hardware
implementation and various embodiments will first be discussed,
followed by a discussion of the software using flowcharts.
[0040] On the controller, the physician sets the temperature using
the control 28 with associated circuitry which outputs a digital
target temperature signal. The digital target temperature signal is
multiplied by a constant gain value, Ks, by amplifier 52. The
constant gain value is typically ten.
[0041] During operation, the probe tip 24 alters the temperature of
the tissue 56. The thermocouple 26 senses the surrounding change in
temperature and outputs an analog sensed temperature signal
corresponding to the sensed temperature. An analog-to-digital (A/D)
converter 58 converts the analog sensed temperature signal to a
digital sensed temperature value. The A/D converter 58 is also
calibrated to multiply the sensed temperature signal by a
predetermined value, such as ten to match the temperature
signal.
[0042] A first summer 60 subtracts the digital sensed temperature
value from the digital target temperature value to generate an
error value or error signal, e(t).
[0043] A PID generation block 61 generates three signals or
values--a proportional value, an integral value and a derivative
value. In the software implementation, the PID generation block 61
is implemented using the PID_generation procedure 43 of FIG. 2.
[0044] To generate a proportional signal or value, a first
amplifier 62 multiplies the error value by the proportional gain
factor Kp.
[0045] To generate the integral value or signal, a second summer 64
subtracts an anti-integral windup signal from the error signal and
supplies its output via a switch 66 to an integrator 68 which
integrates the adjusted error value, as represented by the 1/s
Laplace transform, to generate an intermediate value or signal. In
a digital implementation, the integrator 68 can use any of the
following well-known algorithms including the trapezoidal, Euler,
rectangular and Runge-Kutta algorithms. A second amplifier 70
multiplies the intermediate value by the integral gain factor Ki to
generate the integral value.
[0046] To generate the derivative value, the derivative unit 72
applies a transfer function to the sensed temperature value to
generate an intermediate derivative signal or value. A third
amplifier 74 multiplies the intermediate derivative signal or value
by the derivative gain factor Kd. The transfer function will be
discussed in detail below and is represented as a Laplace transform
as follows: 1 - s 0.25 s + 1
[0047] A third summer 76 adds the proportional value, the integral
value and the derivative value to generate a PID control value or
signal.
[0048] According to a preferred embodiment of the present
invention, the proportional gain factor, the integral gain factor,
and the derivative gain factor are determined from the
multiposition switch setting, the table and the sets of settings in
the memory before starting the PID control operation. In this way,
the PID control function and gains of the proportional, integral
and derivative values can be customized to different types of
probes.
[0049] A clamping circuit 78 determines if the PID control value
exceeds a predetermined threshold to output an adjusted PID control
value. If so, the clamping circuit 78 outputs a maximum allowed
power value to the power control circuit 38 to limit or clamp the
amount of power supplied to the probe to prevent overheating.
Otherwise, the clamping circuit 78 outputs the PID control value.
In one embodiment, the PID_Temperature_Control procedure determines
the default maximum allowed power from the default maximum power
value of table 48 of FIG. 3. In an alternate embodiment, the
physician sets the maximum allowed power.
[0050] An antiwindup circuit also helps to limit the amount of
power by preventing the integrator from including large power
surges, thereby allowing the integrator to more effectively output
a stable steady state value and therefore a more stable operating
temperature of the probe. A fourth summer 82 subtracts the adjusted
PID control value from the PID control value to generate an
antiwindup difference. A fourth amplifier 84 multiplies the
antiwindup difference by an antiwindup gain factor Kw, typically
four, to generate an antiwindup error. The second summer 64
subtracts the antiwindup error from the error value.
[0051] Since the adjusted PID control value is typically equal to
the PID control value, the antiwindup difference is typically zero
and the error value supplied to the integrator 68 is not affected.
When the PID control value is large, for example when power is
first turned on, the PID control value may exceed the maximum
allowable power and the PID control value will be clamped. In this
case the antiwindup difference will be greater than zero and a
positive value will be supplied to the positive input of the second
summer 64 to reduce the magnitude of the error value supplied to
the integrator, thereby reducing the effect of large surges.
[0052] The physician uses a foot switch 86 to control the amount of
power that is supplied to the probe. The foot switch power control
86 controls the switch position of switches 66 and 38. When the
foot switch power control 86 is not engaged, a zero value is
supplied to the integrator 68 via the zero block 92 at a first
switch position. Similarly, another zero block 94 is used by the
power control circuit 38 such that no power is output to the probe.
When the foot switch power control 86 is engaged, switch 66 changes
to a second switch position and allows the output of the second
summer 64 to flow to the integrator 68. In addition, switch 38
changes to a second switch position and allows the output control
value to flow from the clamping circuit 78 to the probe.
[0053] In FIG. 5, the derivative unit 72 implementing the transfer
function described above is shown. The derivative unit 72 receives
an input signal X and outputs a value Y. A fifth amplifier 96
multiplies the input signal X by a value A0. The derivative unit 72
includes an integrator 98 that dampens the effect of the derivative
function thereby reducing the sensitivity of the derivative unit 72
to large changes in the input signal and to noise. In a digital
implementation, the integrator 98 can use any of the following
well-known algorithms including the trapezoidal, Euler, rectangular
and Runge-Kutta algorithms. At power on, the integrator 98 output
is initialized to zero. A sixth amplifier 100 multiplies the
integrator output by A0 to generate a modified integrated signal. A
fifth summer 102 subtracts the modified integrated signal from the
multiplied input signal. A seventh amplifier 104 multiplies the
output of the fifth summer 102 by B1 to generate the intermediate
integrated value. In a preferred embodiment, A0 is equal to four
and B1 is equal to one.
[0054] FIG. 6 is similar to FIG. 4, except that the antiwindup
function is implemented differently. This implementation uses the
antiwindup difference as a switch to stop the integrator from
integrating, thereby resulting in an improved steady state
operation. When the antiwindup difference is equal to zero the
integrator 68 integrates. When the antiwindup difference is not
equal to zero, the integrator 68 stops integrating.
[0055] As described above, the fourth summer 82 generates the
antiwindup difference. A comparator 106 compares the antiwindup
difference to a zero value 107. An inverter 108 inverts the output
by the comparator 106. In response to the output of the inverter
108 and a signal from the foot switch, the AND gate 110 generates a
position control signal that controls switch 64.
[0056] In particular, when the foot switch is not engaged by the
physician, the foot switch power control signal is a zero value,
the AND gate 110 outputs a digital zero value, and the switch 64
moves to the first switch position to output a zero value, thereby
preventing the integrator 68 from integrating.
[0057] When the foot switch is engaged, the foot switch power
control signal is a digital one value and the AND gate 110 will
respond to the antiwindup circuit. When the antiwindup difference
is equal to zero, the comparator 106 outputs a digital zero value
which is inverted to a digital one value by inverter 108. Since the
inverter 108 is outputting a digital one value, the AND gate 110
outputs a digital one value and switch 64 is positioned at the
second switch position, as shown in FIG. 6, and the integrator 68
integrates the error signal e(t).
[0058] When the antiwindup difference is not equal to zero, the
antiwindup difference is a positive value, the comparator 106
outputs a digital one value and the inverter 108 outputs a zero
value. In response to the zero value from the inverter 108, the AND
gate 110 outputs a digital zero value and switch 64 is positioned
at the first switch position to output the zero value to the
integrator 68, thereby preventing the integrator 68 from
integrating.
[0059] FIG. 7 is similar to FIG. 6 except that the error signal
e(t) is supplied to the derivative block 72.
[0060] FIG. 8 is a flowchart of the PID_Temperature_Control
procedure 42 of FIG. 2 that implements the PID control method of
FIG. 4. In step 112, sets of probe settings and a table associating
the probe settings with switch settings are provided in the memory,
as described above. Each set corresponds to predetermined operating
characteristics for a particular probe. In step 114, the
PID_Temperature_Control procedure 42 receives a target temperature.
The target temperature can be set by the physician in degrees
Celsius. The target temperature value used by the PID temperature
controller is the temperature set by the physician in degrees
Celsius multiplied by a factor, such as ten. In step 116, the
PID_Temperature_Control procedure 42 receives a first setting
corresponding to a desired set of operating characteristics from
the multiposition switch. In step 118, the PID_Temperature_Control
procedure 42 selects a particular set of the sets of probe settings
in response to the multiposition switch setting. The particular set
has the proportional, integral and derivative gain factors, Kp, Ki
and Kd, respectively, as described above, that will be used by the
PID_generation procedure. If the physician has not set a target
temperature, the default target temperature that is stored in
memory for the selected switch setting is used. In step 119, the
PID_Temperature_Control procedure waits a predetermined amount of
time before the next sample period. In one embodiment the
predetermined amount of time is equal to twenty milliseconds. In
other words, the PID_Temperature_Control procedure samples the
sensed temperature value output by the probe every twenty
milliseconds. In one implementation, the PID_Temperature_Control
procedure uses interrupts to trigger the sample periods. In step
120, a sensed temperature value is received. Similar to the target
temperature, the sensed temperature value represents the actual
temperature in degrees Celsius and multiplied by a factor of ten.
In step 122, an error value is generated by subtracting the sensed
temperature from the target temperature.
[0061] As shown by the dashed lines, steps 124 to 130 are
implemented in the PID_generation procedure 43 of FIG. 2 which is
invoked by the PID_Temperature_Control procedure. The
PID_generation procedure also corresponds to the PID generation
block 61 of FIG. 4. In step 124, a proportional value is generated
by multiplying the error value by the particular proportional gain
parameter, Kp. In step 126, an integral value is generated by
subtracting the anti-integral windup value from the error value,
integrating the resulting value of the subtraction and multiplying
the integrated adjusted error value by the particular integral gain
parameter, Ki. The integrator 68 can be implemented with any of the
following well-known algorithms including the trapezoidal, Euler,
rectangular and Runge-Kutta algorithms. In step 128, a derivative
value is generated by applying a derivative transfer function to
the sensed temperature value, as described above, and multiplying
the result of the transfer function by the particular derivative
gain parameter. In step 130, an output control signal is generated
by summing the proportional value, the integral value and the
derivative value.
[0062] In step 132, the output control signal is clamped to a
predetermined output value when the output control signal exceeds a
predetermined threshold value. The predetermined threshold value is
the default set power from table 2. The predetermined threshold
value can be set by the physician. Alternately, based on the
multiposition switch setting, the default maximum power value
stored in one of the tables, described above, is used. In step 134,
an amount of power is output to the thermal element of the probe in
response to the output control signal, and the process repeats at
step 120.
[0063] FIG. 9 is a detailed flowchart of step 128 of FIG. 8 which
generates the derivative value. In step 136, the current sensed
temperature value is multiplied by a first constant, A0. In step
138, subtract an integrated output value from the multiplied
current sensed temperature to generate a temporary value.
Initially, the integrated output value is zero and is modified with
each current sensed temperature reading. In step 140, the temporary
value is multiplied by a second constant, B1, to generate the
derivative value. In step 142, a new integrated value is generated
based on a previous sensed temperature value and the current sensed
temperature value. The integration can be performed using any of
the following well-known algorithms including the trapezoidal,
Euler, rectangular and Runge-Kutta algorithms. The new integrated
value is multiplied by the first constant, A0, to generate another
integrated output value which is used in subsequent calculations.
As described above, preferably, the first constant, A0, is equal to
four and the second constant, B1, is equal to one.
[0064] In an alternate embodiment of FIGS. 8 and 9, the error
values are input to the derivative operation instead of the sensed
temperature values.
[0065] FIG. 10 is a flowchart of the PID_Temperature_Control
procedure 42 of FIG. 2 that implements the antiwindup function of
FIG. 4. In step 152, an antiwindup difference is determined by
subtracting a maximum predetermined clamping value from the output
control signal. In step 154, an antiwindup adjustment value is
generated by multiplying the antiwindup difference by an antiwindup
gain factor. In step 156, the antiwindup adjustment value is
subtracted from the error value to generate a modified error value.
In step 158, the modified error value is integrated.
[0066] FIG. 11 is a flowchart of the PID_Temperature_Control
procedure 42 of FIG. 2 that implements the alternate embodiment of
the antiwindup function of FIG. 6. In step 162, an antiwindup
difference is determined by subtracting a maximum predetermined
clamping value from the output control signal. In step 164, when
the antiwindup difference is not zero, the procedure stops
integrating the error values.
[0067] FIG. 12 is a flowchart of the PID_Temperature_Control
procedure 43 of FIG. 2 that implements variable temperature
setting. Physicians may want to change the temperature profile
depending on the application. When operating on large joints, the
physician may want to use the probe in a high power mode to heat
the probe quickly and maintain the target temperature. However,
when operating on the spine, the physician may want to use a low
power mode with a very controlled temperature and no overshoot.
[0068] In this embodiment of the invention, the physician via the
multiposition switch can select a particular temperature profile
(47, FIG. 2). The physician also may set a final target
temperature. In FIG. 12, in step 172, in the
PID_Temperature_Control procedure, the selected switch position
corresponds to a particular temperature profile with a ramp
parameter at which to ramp up the output temperature. Referring
also to FIG. 13, additional exemplary temperature profiles are
shown. Each profile 176, 178 stores a ramp parameter (Ramp 1, Ramp
N), gain settings, and a final target temperature. Referring back
to FIG. 12, in step 180, in response to the switch position, the
target temperature is initialized to a starting temperature based
on the ramp parameter. The set of gain factors associated with the
ramp parameter are retrieved and loaded into a PID control block
for use by the PID_control procedure. In step 182, the
PID_Temperature_Control procedure configures the microprocessor to
generate an interrupt at predetermined intervals, preferably every
twenty milliseconds.
[0069] The steps in block 184 are executed in response to the
interrupt. In step 186, the target temperature is set using the
Set_target_temperature procedure (49b, FIG. 2). If step 186 is
being executed in response to a first interrupt, the target
temperature is already set to the starting temperature. Otherwise,
the target temperature is changed by adding the ramp parameter to
the target temperature if a predetermined amount of time has
elasped between successive target temperature changes. Preferably,
the target temperature is changed every thirty seconds. If the sum
of the ramp parameter and the target temperature exceeds the final
target temperature, then the target temperature is set to the final
target temperature.
[0070] In step 188, the PID_control procedure (49c, FIG. 2) is
executed to control the temperature of the probe. The PID_control
procedure is executed at each interrupt, every twenty milliseconds.
The PID_control procedure will be shown in further detail in FIG.
14.
[0071] In step 190, the PID_Temperature_Control procedure waits for
the next interrupt to occur.
[0072] Preferably, the microprocessor executes a task scheduler
(49a, FIG. 2), such as a round-robin task scheduler, to generate
the interrupts and to execute the Set_target_temperature procedure
and the PID_control procedure as tasks. The target temperature is
stored in the memory (49d, FIG. 2) for access by both the
Set_target_temperature procedure and the PID_control procedure.
[0073] In an alternate embodiment, the Set_target_temperature
procedure changes the gain factors in addition to changing the
target temperature.
[0074] For example, for a particular switch position setting, a low
power application with a very controlled temperature is desired.
Based on the switch position, the PID_Temperature_Control procedure
sets an initial target temperature that is much lower than the
final target temperature. The PID_Temperature_Control procedure
also uses the predetermined set of gain values associated with the
particular switch position setting and the interrupts are
configured. In response to the interrupts, the
Set_target_temperature procedure and the PID_control procedure are
executed every twenty milliseconds.
[0075] After thirty seconds have passed, the Set_target_temperature
procedure increments the initial target temperature by a
predetermined amount, such as one degree, to generate the next
target temperature. In this way, the Set_target_temperature
procedure increments the intermediate target temperature until the
final desired target temperature is reached. As a result, the
temperature of the probe is very well-controlled and overshoot is
substantially avoided.
[0076] In FIG. 14, a flowchart of the PID_Control procedure of step
188 of FIG. 12 is shown. The PID_Control procedure uses steps
120-134 of FIG. 8, which were described above. The antiwindup
adjustment of FIG. 10 or FIG. 11 can be used with the PID_Control
procedure of FIG. 14.
[0077] In this way, a method and apparatus are provided that
control the sensed temperature of a probe in a strictly controlled
manner to avoid overdamping and underdamping. In addition,
depending on the type of probe, the target temperature can be set
to increase or decrease the tissue temperature. Therefore, the
method and apparatus can control both high temperature and low
temperature probes to heat or cool tissue.
[0078] The foregoing description, for purposes of explanation, used
specific nomenclature to provide a thorough understanding of the
invention. However, it will be apparent to one skilled in the art
that the specific details are not required in order to practice the
invention. In other instances, well known circuits and devices are
shown in block diagram form in order to avoid unnecessary
distraction from the underlying invention. Thus, the foregoing
descriptions of specific embodiments of the present invention are
presented for purposes of illustration and description. They are
not intended to be exhaustive or to limit the invention to the
precise forms disclosed, obviously many modifications and
variations are possible in view of the above teachings. The
embodiments were chosen and described in order to best explain the
principles of the invention and its practical applications, to
thereby enable others skilled in the art to best utilize the
invention and various embodiments with various modifications as are
suited to the particular use contemplated. It is intended that the
scope of the invention be defined by the following claims and their
equivalents.
* * * * *