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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates generally to biological tissue stimulators
and more particularly to biological tissue stimulators utilizing a
constant current output stage.
Biological tissue stimulators are known to be medically useful. In one
example, a transcutaneous electrical nerve stimulator (TENS) is utilized
to mask pain signals in a human body before they reach the brain, giving
the subject apparent relief from the pain. In such a TENS device,
electrical stimulation pulses, usually current pulses of a selected rate,
amplitude, pulse width and duty cycle, are delivered to the skin of the
subject by one or more electrodes, each containing a pair of electrode
elements. The timing characteristics of the delivered electrical
stimulation pulses may be predetermined, as for example, by the
prescribing physician and/or may be individually selected or controlled by
switches available to be operated by the subject. Additionally, individual
parameters, or even entire pulse programs, can be varied in a
predetermined or random basis by the TENS device itself.
Another example of a useful biological tissue stimulator is a neuromuscular
stimulator (NMS) which can be utilized to electrically stimulate muscle
activity of a patient. Electrical stimulation pulses, again probably
current pulses of a carefully controlled rate, amplitude, pulse width and
sequence are delivered by one or more pairs of electrodes to a site or
sites near the muscle to be stimulated in order to activate or contract
the muscle. The initiation and control of such sequence of pulses may be
patient-controlled.
Biological tissue stimulators typically provide a series of electrical
stimulation pulses to the biological tissue according to a preset program.
It is generally desirable to provide an electrical stimulation pulse of a
known current value. Output circuits are commonly arranged to produce
so-called "constant current" outputs. The output stage of a biological
tissue stimulator sees a load impedance which consists primarily of the
electrode-tissue interface impedance and the impedance of the biological
tissue between the electrode elements. This impedance can vary
significantly, for example, from about 200 ohms to 2500 ohms. The
"constant current" output stage supplies the same current to the
worst-case expected load impedance, i.e., the highest expected load
impedance, as to a more normal (lower) load impedance. The high voltage
power supply driving the output stage must be able to deliver a high
enough voltage to maintain the desired current level through this
"worst-case" load impedance. If the high voltage power supply output level
is fixed at this maximum value, however, then a much higher voltage is
supplied to the output stage than is otherwise required when the load
impedance is in a lower, more normally anticipated, range. This excess
high voltage power is then dissipated and "lost", resulting in an
energy-inefficient use of such a high voltage supply.
It is highly desirable that biological tissue stimulators be portable
devices, that is, be compact in size and low in weight Both attributes
make for a more comfortable and easily used biological tissue stimulator.
The biological tissue stimulator may be required to be connected to the
body for extended periods of time and, thus, ease of portability and
unobtrusivness are of critical importance.
For portability, biological tissue stimulators must be battery-powered
since connection to a household power supply would greatly limit the
geographical range of operation. It is advantageous that the battery life
of the biological tissue stimulator be as long as possible. Insufficient
battery life limits the freedom of extensive use without resupply, creates
the bothersome duty of physically changing the batteries and substantially
increases the cost of operation. Also, the deterioration of battery
voltage may result in less than optimum performance characteristics. If
the battery discharges more often, a greater percentage of time may be
spent during nonoptimum operating conditions and, thus, the biological
tissue stimulator may not achieve the advantageous results that it was
intended to produce.
Battery life may be increased by increasing the size and weight of the
batteries. This, however, is not desirable since increasing of the size
and weight of the biological tissue stimulator limits the places,
locations and environments for the biological tissue stimulator is likely
to be used. Thus, a heavy, bulky biological stimulator is not truly
portable.
SUMMARY OF THE INVENTION
The present invention provides a biological tissue stimulator with a
substantially improved battery life over a similar stimulator without the
high voltage adjustment feature of the present invention.
Since the actual load impedance is not known and may vary over the course
of time, the biological tissue stimulator of the present invention
implicitly determines that impedance by comparing the output signal of the
high voltage power supply against the voltage actually dissipated across
the load impedance. This is accomplished by measuring the "left over"
voltage from the high voltage power supply at the end of an electrical
stimulation pulse. If sufficient excess voltage is available, the output
voltage of the adjustable high voltage power supply can be decreased.
Thus, the next electrical stimulation pulse will be associated with a
lower amount of excess high voltage. This process is then repeated over
successive electrical stimulation pulses. Through this iterative process,
eventually a level of high voltage is reached which does not significantly
waste high voltage power.
Since each step of this iterative process typically occurs in a time of
much less than a second, the biological tissue stimulator will quickly,
compared to the lifetime of the batteries, reach the desired high voltage
level. Since the load impedance is usually known to change only gradually,
the iterative process typically will reach a relative steady state without
significant load impedance changes. If the required current amplitude
output should change, as e.g., by a manual change in the amplitude
controlled by the patient or automatically under stimulation program
control, the high voltage signal again will seek the minimum required
level to maintain constant current.
Should the amount of high voltage signal available ever be insufficient to
supply the required current output of the electrical stimulation pulse,
then the high voltage power supply adjusts the high voltage signal by
increasing its output voltage level until the required current level or
amplitude is reached. Again, this can be done iteratively over a
successive number of electrical stimulation pulses. Alternatively, instead
of iteratively increasing the high voltage stepwise by known or relatively
small calculated steps, it may be wise to immediately step the high
voltage to the maximum possible level. This would ensure that as much of
the required electrical stimulation program is run as is possible. Current
would not be lost while the high voltage power supply incrementally
attained the higher required level. Since the measurement of the
insufficient high voltage signal occurs with an actual load, the high
voltage signal will be insufficient until it can be properly readjusted
over at least one electrical stimulation pulse. After the high voltage
signal is deemed insufficient and raised to its maximum level, the
automatic decreasing steps previously described can occur again ensuring
that no major amount of battery life is lost.
Thus, the biological tissue stimulator of the present invention is adapted
to supply an electrical stimulation signal to a biological load. The
stimulator has a battery and a high voltage power supply operatively
coupled to the battery producing a high voltage signal. An output circuit
is coupled to the high voltage signal for supplying the electrical
stimulation signal of a predetermined current amplitude. Electrodes are
coupled to the electrical stimulation pulse and adapted to be coupled to
biological tissue and deliver the electrical stimulation signal so that
the electrical stimulation signal may be applied to the biological tissue.
An adjustment circuit is coupled to the output circuit and to the high
voltage power supply for comparing the amplitude of the high voltage
signal to the amplitude of the voltage drop across the biological load and
then adjusting the amplitude of the high voltage signal dependent upon the
result of the comparing. It is preferred that the adjustment circuit
adjust the amplitude of the high voltage signal to minimize the difference
in amplitude between the high voltage signal and the voltage drop across
the biological load thereby maximizing the energy efficiency of the
biological stimulator. Further, it is preferred that the high voltage
power supply be a switching type power supply with the amplitude of the
high voltage signal being dependent upon control signals generated by an
adjustment circuit.
It is preferred that the adjustment circuit include a voltage gating
circuit coupled to the output circuit for gating a slack voltage
representing the voltage from the high voltage signal not dissipated
across the biological load. A comparison circuit coupled to the slack
voltage then compares the slack voltage against a predetermined reference
voltage producing a comparison output indicative of such comparison
relative to a predetermined threshold. The adjustment circuit then further
has a switching circuit coupled to the comparison circuit and to the high
voltage power supply for supplying control signals so that the amplitude
of the high voltage signal is properly adjusted relative to the
predetermined threshold. In another embodiment, the slack voltage is
compared against first and second predetermined reference voltages
providing a "window" between which the slack voltage would be allowed to
range.
In one embodiment of the present invention, the adjustment circuit of the
biological tissue stimulator determines the impedance of the biological
load and adjusts the amplitude of a high voltage signal to minimize the
amplitude of the high voltage signal and still have sufficient voltage
amplitude to allow the output circuit to produce the electrical
stimulation signal at a predetermined current amplitude.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing advantages, construction and operation of the present
invention will become more readily apparent from the following description
and accompanying drawings in which:
FIG. 1 is block diagram of one embodiment of the present invention;
FIG. 2 is a block diagram of an alternative embodiment of the present
invention;
FIG. 3 is a block diagram of another alternative embodiment of the present
invention;
FIG. 4 is a schematic diagram of a portion of one embodiment of the present
invention;
FIG. 5 is a schematic diagram of another portion of one embodiment of the
present invention;
FIG. 6 is a timing diagram of electrical signals of the present invention;
and
FIG. 7 is flow chart of increase/decrease decisions of the control portion
of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A block diagram of the biological tissue stimulator 10 of the present
invention is illustrated in FIG. 1. A battery 12, of conventional design,
provides a portable compact energy source for the biological tissue
stimulator 10. The low voltage power provided by the battery 12 is
supplied to a high voltage power supply 14. High voltage power supply 14
steps up the low voltage supply of the battery 12 to a high voltage signal
which is delivered to the output stage 16. Output stage 16 supplies an
electrical stimulation pulse to an electrode system 18, consisting of
electrode elements 20 and 22. Electrode system 18 is designed to be
attached to biological tissue so that the output stage 16 can deliver an
electrical stimulation pulse to electrode elements 20 and 22 by passing a
current of known value between electrode elements 20 and 22, thereby
stimulating the biological tissue Since it is generally the current
through the biological tissue which determines the therapeutic advantage
obtained by the biological tissue stimulator, output stage 16 delivers an
electrical stimulation pulse which is of a known and predetermined current
amplitude. The particular electrical stimulation pulse to be delivered is
determined by control section 24. Control section 24 determines the
particular electrical stimulation pulse to be delivered, either directly
from the amplitude control operated by the user of the biological tissue
stimulator, from a fixed set of operating parameters (e.g., pulse rate,
width and amplitude) stored in the biological tissue stimulator 10, or as
selected from one of a number of preprogrammed stimulation programs which
may vary the operating parameters of the electrical stimulation pulses.
Control section 24 is of conventional design and is well known in the art.
Similarly, electrode system 18 is of conventional design and well known
with respect to biological tissue stimulators 10.
The load for the output stage 16 is determined primarily from the
electrode-biological tissue interface impedance, which is essentially the
impedance at the point of contact between electrode elements 20 and 22
with the biological tissue, and the impedance of the biological tissue
between electrode elements 20 and 22. When electrode system 18 is coupled
to the biological tissue (not shown), the output stage 16 will adjust the
voltage of the electrical stimulation pulse to ensure that the proper
predetermined current amplitude of the electrical stimulation pulse is
maintained, as determined by control section 24. To be able to supply the
proper current amplitude for the electrical stimulation pulse delivered to
electrode system 18, output stage 16 must have a sufficient operating
voltage from high voltage power supply 14 to enable output stage 16 to
deliver the proper current amplitude under worst-case load impedance
conditions. In these worst-case load impedance conditions, the combined
impedance of the electrode system 18's biological tissue interface, the
electrode systems and the biological tissue impedance is high, requiring a
large voltage to supply the proper electrical stimulation pulse of proper
current amplitude. However, when the output load impedance levels are at
more " normal" levels, a lower high voltage signal is required by output
stage 16 and, hence, required from high voltage power supply 14. If the
worst-case high voltage signal level is maintained, then a substantial
amount of power and, hence, energy from battery 12 is lost or dissipated
in the circuitry not across the biological tissue when the load impedance
is not at its maximum value. Hence, the biological tissue stimulator 10 of
the present invention has an adjustment circuit 26 coupled to the output
stage 16. The adjustment circuit 26 compares the amplitude of the high
voltage signal delivered to the output stage 16 by the high voltage power
supply 14 with the actual voltage drop across the load impedance and,
hence, can determine whether there is excess high voltage signal available
and, hence, whether energy from the battery 12 is being wasted. Adjustment
circuit 26 then supplies control signals to high voltage power supply 14
in order to adjust the amplitude of the high voltage signal which high
voltage power supply 14 delivers to the output stage 16. Usually this is
done to minimize the difference between the voltage supplied by the high
voltage power supply to the output stage 16 and the actual voltage drop
across the load impedance. Alternatively, adjustment circuit 26 may
determine the actual impedance of the biological load, or load impedance,
directly and then send control signals to the high voltage power supply 14
to minimize the high voltage signals supplied by the high voltage power
supply 14 to minimize the high voltage signal supplied by the high voltage
power supply 14 to the output stage 16 and still allow the output stage 16
to produce the electrical stimulation pulse of the proper current
amplitude.
More detail of the adjustment circuit 26 can be seen in the biological
tissue stimulator 10 illustrated in FIG. 2. The biological tissue
stimulator 10 again has battery 12, the same adjustable high voltage power
supply 14, the same output stage 16, the same electrode system 18 and the
same control section 24. Voltage gating circuit 28 is coupled directly to
output stage 16 which gates a "slack" voltage 30 representing the amount
of the output voltage from the adjustable high voltage power supply 14
delivered to the output stage 16 which is not dissipated across the
biological load impedance through electrode system 18. This amount of
slack voltage 30 represents the excess in amplitude of the high voltage
signal delivered by the high voltage power supply 14 to the output stage
16 over and above what is required to maintain the proper current
amplitude of the electrical stimulation pulse supplied by output stage 16
to electrode system 18. A comparator 32 then compares slack voltage 30
against a known reference voltage 34 and sends a comparison output
indicative of the result of such comparison to control signal generator
36. Reference voltage 34 is selected to be larger than zero in order to
minimize the amount of hunting that otherwise would result in constant
readjustment of the high voltage signal amplitude level but yet
sufficiently small so as to substantially minimize the waste of the energy
of battery 12. Generally, with a high voltage signal which varies in the
range of 10 to 75 volts, a reference voltage of 6 volts is tolerable. In
this embodiment, high voltage power supply 14 is a switching type power
supply. That is, high voltage power supply 14 operates to step up the low
voltage level of battery 12 to the high voltage signal level through a
series of intermittent connections of the battery 12 to an inductor,
successively "stepping up" the voltage level across the inductor. The
successive connections of battery 12 to the inductor in the high voltage
power supply 14 is controlled by control signals 38 supplied from the
control signal generator 36. Thus, the control signal generator, through
the application of control signals 38, can directly vary the amplitude of
the high voltage signal delivered from the high voltage power supply 14 to
the output stage 16. Should the result of the comparison made by
comparator 32 with the reference of voltage 34 indicate that there is an
excess of slack voltage 30, then control signal generator 36 will vary the
application of control signals 38 to the high voltage power supply 14 so
as to lower the amplitude of the high voltage signal delivered from the
high voltage power supply 14 to the output stage 16. Should the result of
comparator 32 in comparing a slack voltage 30 to the reference voltage 34
indicate that the slack voltage 30 is insufficiently high, then an
increased output level from high voltage power supply 14 is called for.
Thus, the control signal generator 36 would modify the control signals 38
sent to the high voltage power supply 14 to increase the amplitude of the
high voltage signal delivered from the high voltage power supply 14 to the
output stage 16.
FIG. 3 illustrates a block diagram for an alternative embodiment of the
biological stimulator 10 of the present invention. Battery 12, high
voltage power supply 14, output stage 16, electrode 18, control section
24, voltage gate 28 are identical to corresponding elements described in
FIG. 2. In this embodiment, however, a comparator 40 compares slack
voltage 30 against a first reference voltage 42 and against a second
reference voltage 44. The value of the second reference voltage 44 is
chosen to be greater than the value of the first reference voltage 42 so
as to create a "window" or range between which slack voltage 30 is allowed
to fluctuate without resulting in changes in the control signals 38
delivered by control signal generator 36. Should comparator 40 indicate
that slack voltage 30 exceeds the second reference voltage 44 then control
signal generator 36 will modify the control signals 38 to decrease the
amplitude of the high voltage signal delivered from the high voltage power
supply 14 to the output stage 16 since the excess amount of slack voltage
30 indicates that energy from the battery 12 is being wasted. Should,
however, comparator 40 indicate that slack voltage 30 is less than the
first reference voltage 42, then the level of high voltage signal from
high voltage power supply 14 to output stage 16 is nearly or is
insufficient. Thus, control signal generator 36 will modify control
signals 38 to the high voltage power supply 14 in order to increase the
amplitude of the high voltage signal delivered to the output stage 16.
A detailed schematic diagram of the biological tissue stimulator 10 of the
present invention is illustrated in FIGS. 4 and 5. Battery 12 is shown
supplying a battery voltage 46 and is shown connected to a conventional
four volt power supply 48 for supplying conventional logic voltage 50 to
run the integrated circuits in the accompanying schematic. Battery 12 is a
nine volt (nominal) battery. Battery voltage 46 is connected through a
filter capacitor 52 to an inductor 54 as part of the high voltage power
supply 14. This circuit uses a power oscillator circuit incorporating
transistors 56 and 58 as well as accompanying components resistors 60, 62,
64, 66 and capacitors 68 and 70. This power oscillator circuit generators
transients across inductor 54 which are rectified by diode 71 and filtered
by capacitor 72 to achieve a positive direct current output voltage which
is the high voltage signal 74 delivered by the high voltage power supply
14 to the output stage 16. Zener diode 76 and accompanying resistor 78
provide a safety mechanism preventing the high voltage signal 74 from
exceeding a known preset value. The power oscillator circuit of the high
voltage power supply 14 is gated by comparator 80 through resistor 82 and
diode 84 according to the voltage monitored from the programmable divider
circuit of resistors 86, 88, 90, 92, 94 and 96 and associated diodes 98,
100, 102, 104 and 106. The other side of the input to comparator 80 is
obtained from reference voltage 108 which is obtained via voltage
reference signal circuit 110 and associated resistor 112 coupled to the
logic power supply 50 providing a known stable reference voltage 108.
Comparator 114 and dual flip-flop 116 along with associated resistors 118,
120 and 122 permit disabling of the high voltage power supply from control
circuit 24.
Control circuit 24 also supplies a true level enable signal 124 and its
complement 126 to output stage 16. This supplies both positive and
negative power for the differential amplifier circuit consisting of
transistors 128 and 130, associated resistors 132, 134, 136, 138,
capacitors 140, 142, and diodes 144, 146 and 148. The current level
provided by this output stage 16 is determined by the wiper at variable
resistor 132, as for example, being an amplitude control potiometer
controlled by the user. The remainder of the current controlled output
stage 16 is formed from transistors 150, 152, 154, 156, resistors 158,
160, 162, 164, 166, 168, diodes 170, 172, 174 and capacitors 176 and 178.
Transistor 154 is driven from the high voltage signal 74 as obtained from
the high voltage power supply 14. The signal across capacitor 178 is
supplied to electrode system 18 for delivery to the biological tissue
load.
Slack voltage 30 is obtained from the collector of transistor 152 and
routed to the voltage gate circuit 28. Here it is applied to series pass
transistor 180. Transistors 182 and 184 provide base drive to transistor
180 whenever the enable signal 186 is obtained from control circuit 24.
Resistors 188, 190 and diode 192 complete the voltage gating circuit.
Voltage gating circuit 28 operates to supply the slack voltage 30 which
exists at the end of the electrical stimulation pulse which most closely
approximates the excess voltage available from high voltage signal 74. In
this embodiment, the output from voltage gating circuit 28 supplied
optionally to voltage attenuation circuit 194 which through resistors 196
and 198 serve to reduce the voltage to the proper level for subsequent
processing. Also, resistors 200 and 202 serve to represent other possible
inputs from other channels of the biological tissue stimulator to show
that a single comparator 40 can serve a plurality of output stages 16
representing different channels of output of the biological tissue
stimulator 10. The output of voltage attenuation 194 is supplied to
comparator 40. Comparator 204 provides a low comparison threshold of four
volts and comparator 206 provides a high comparison threshold of ten volts
for the window comparator as described in the block diagram of FIG. 3. The
lower threshold voltage of four volts is chosen to permit electrical
stimulation pulses having a current amplitude of 80 milliamperes through
large load impedances and resistor 160 and transistor 152. The upper
voltage threshold of ten volts was chosen to accomodate the "quantal"
magnitude of adjustments for the high voltage power supply 14 provided by
the programmable voltage divider of FIG. 5. Resistors 208, 210, 212, 214,
216 and capacitors 218 and 220 complete the comparator circuit 40 is
supplied to control circuit 24, a portion of which serves as the control
signal generator 36 of the block diagram of FIG. 3. This portion of
control section 24, representing the control signal generator 36, supplies
the control signals across the programmable voltage divider of FIG. 5 to
supply the control signals to the power oscillator circuit of the high
voltage power supply of FIG. 5.
The component values illustrated in the schematic diagram of FIGS. 4 and 5
are shown in the following table:
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Reference Numeral
Value or Component Number
______________________________________
52 6.8 microfarads, 16
volts
54 1.1 millihenries
56 BC848C
58 BST52
60 2.2 kilohms
62 100 kilohms
64 100 kilohms
66 3.3 kilohms
68 0.01 microfarads
70 33 picofarads
71 BAS19
72 6.8 microfarads, 80
volts
76 BZX84-C75
78 S10 kilohms
80 IR-9022N
82 100 kilohms
84 BAV 70
86 8.2 megohms
88 1 megohm
90 249 kilohms
92 499 kilohms
94 1 megohm
96 2 megohms
98 BAW56
100 BAW56
102 BAW56
104 BAW56
106 BAW56
110 LM385BZ-1.2
114 IC-5B
116 HEF4013BT
118 1 megohm
120 1 megohm
122 100 kilohms
128 BC848C
130 BC848C
132 47 kilohms, adjustable
134 47 kilohms
136 13.7 kilohms
138 15 kilohms
140 1 microfarad
142 100 picofarads
144 BAV99
146 BAV99
148 BAV70
150 BC846B
152 BSR43
154 MMBT5401
156 BSR43
158 1 kilohm
160 26.7 ohms
162 12 kilohms
164 33 kilohms
166 100 kilohms
168 100 kilohms
170 BAV70
172 BAV70
174 BAV70
176 6.8 microfarads, 50
vo1ts
178 470 picofarads
180 MMBT5401
182 MMBT5401
184 BC846B
188 5.6 megohms
190 5.6 megohms
192 BAV70
196 1 megohm
198 100 kilohms
200 1 megohm
202 1 megohm
204 LM239D
206 LM239D
208 10 kilohms
210 10 kilohms
212 1 megohm
214 100 kilohms
215 1 megohm
216 499 kilohms
218 0.047 microfarads
220 0.047 microfarads
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The timing diagram of FIG. 6 shows the output circuit 16 enable signals 124
and its complement 126 representing the enabling of this particular output
stage 16 and the accompanying output electrical stimulation pulse 214
delivered to electrode system 18. The slack voltage signal 30 shows the
high voltage signal 74 being dissipated across a highly capacitive
biological load, through the electrode system 18, from a maximum value
near the start of the output stage enable and exponentially decreasing as
the power from the high voltage signal 74 is dissipated across the
biological load. As can be seen in the exemplary timing diagram of FIG. 6,
not all of the high voltage signal 74 is dissipated across the biological
load. Thus, at the end of the output stage enable a discrete amount | | |
