United States Patent [19]
`Polson
`
`I 1111111111111111 11111 IIIII IIIII IIIII IIIII IIIII IIIII IIIII IIIIII Ill lllll llll
`US005766 l 24A
`[lll Patent Number:
`[45] Date of Patent:
`
`5,766,124
`Jun. 16, 1998
`
`(54] MAGNETIC STIMULATOR FOR NEURO(cid:173)
`MUSCULAR TISSUE
`
`Primary Examiner-John P. Lacyk
`Attorney, Agent, or Firm-Walter C. Farley
`
`(75]
`
`Inventor: Michael John Ross Polson, Narberth,
`Great Britain
`
`[73] Assignee: The Magstim Company Limited,
`Whitland. Great Britain
`
`(21) Appl. No.: 608,680
`Feb. 29, 1996
`
`[22) Filed:
`Int. Cl.6
`....................................................... A61N 1/00
`(51)
`(52) U.S. Cl. ................................................................ 600/13
`(58) Field of Search ............................................. 600/9-15
`
`(56]
`
`References Cited
`
`U.S. PATENf DOCUMENfS
`
`[57)
`
`ABSTRACT
`
`A magnetic stimulator of neuro-muscular tissue has a dis(cid:173)
`charge capacitor, at least one discharge control switch for
`allowing discharge of the discharge capacitor into a stimu(cid:173)
`lating coil and a circuit for recovering energy from the
`stimulating coil when current flow from the discharge
`capacitor to the stimulating coil is interrupted, the energy
`being recovered either by the discharge capacitor or a
`capacitor additional to the discharge capacitor. The addi(cid:173)
`tional capacitor may be a reservoir capacitor connected for
`charging by a power supply and there may be a circuit for
`pumping charge from the reservoir capacitor to a transfer
`capacitor and for pumping charge from the transfer capacitor
`to the discharge capacitor.
`
`5,181,902
`5,314,401
`
`1/1993 Erickson et al ........................... 600/13
`5/1994 Tepper ...................................... 600/14
`
`17 Claims, 7 Drawing Sheets
`
`HIGH
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`Allergan EX1064
`Page 1
`
`

`

`U.S. Patent
`
`Jun. 16, 1998
`
`Sheet 1 of 7
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`5,766,124
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`Allergan EX1064
`Page 2
`
`

`

`U.S. Patent
`
`Jun. 16, 1998
`
`Sheet 2 of 7
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`Allergan EX1064
`Page 3
`
`

`

`U.S. Patent
`
`Jun. 16, 1998
`
`Sheet 3 of 7
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`Allergan EX1064
`Page 4
`
`

`

`U.S. Patent
`
`Jun. 16, 1998
`
`Sheet 4 of 7
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`Allergan EX1064
`Page 5
`
`

`

`U.S. Patent
`
`Jun. 16, 1998
`
`Sheet 5 of 7
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`Allergan EX1064
`Page 6
`
`

`

`U.S. Patent
`
`Jun. 16, 1998
`
`Sheet 6 of 7
`
`5,766,124
`
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`Allergan EX1064
`Page 7
`
`

`

`U.S. Patent
`
`Jun. 16, 1998
`
`Sheet 7 of 7
`
`5,766,124
`
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`
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`Allergan EX1064
`Page 8
`
`

`

`1
`MAGNETIC STIMULATOR FOR NEURO(cid:173)
`MUSCULAR TISSUE
`
`5,766,124
`
`2
`FIGS. 4A to 4C show waveforms relating to a final or
`discharge capacitor in the embodiment shown in FIG. 1;
`FIGS. SA to SC show waveforms relating to a discharge
`capacitor in the embodiment shown in FIG. 2;
`FIG. 6 illustrates another embodiment of the invention;
`FIG. 7 illustrates a modification of the embodiment
`shown in FIG. 6;
`FIG. 8 illustrates another embodiment of the invention.
`generally resembling the embodiment shown in FIG. 2;
`FIG. 9 illustrates an embodiment which provides for
`recovery of energy back to the discharge capacitor;
`FIG. 10 illustrates an embodiment employing a stimulator
`coil forming part of a transformer; and
`FIG. 11 illustrates an embodiment which employs induc(cid:173)
`tors for controlling the discharge of energy into the stimu(cid:173)
`lating coil.
`DETAILED DESCRJPl1ON OF PREFERRED
`EMBODThlENTS
`The stimulator shown in FIG. 1 is based on a charge
`storage capacitor which stores an electric charge which is
`transferred. preferably by way of an intermediate or 'trans(cid:173)
`fer' capacitor. to another capacitor which is discharged into
`25 a stimulating coil. The coil provides in response to the
`discharge of current through it a high value time-varying
`magnetic field which induces electric current in neuro(cid:173)
`muscular tissue. The design of the stimulating coil is not
`generally relevant to the present invention and it need not be
`30 permanently connected to the remainder of the circuit.
`In the embodiment shown. a high voltage power supply 1.
`which may be of any suitable construction and may be
`variable. is provided for charging a first capacitor 2. here(cid:173)
`inafter called the 'reservoir capacitor'. Discharge of the
`35 reservoir capacitor is controlled by a controllable series
`switch 3. which, like all the other controllable switches in
`the embodiment. may be a thyristor but could be any of a
`large variety of suitable switches. The thyristor 3 is con(cid:173)
`nected to a series inductor 4. which is connected to the upper
`40 plate of a transfer capacitor 5 of which the lower plate is
`connected to the lower plate of capacitor 2. A reverse diode
`6 is connected across the capacitor 5. the diode blocking
`current in the direction of normal current flow through the
`switch 3 and the inductor 4.
`The upper plate of the transfer capacitor 5 is connected by
`way of an inductor 7 to the anodes of two thyristor switches
`8a and 8d of which the cathodes are connected to opposite
`plates of a third or discharge capacitor 9. The inductor 7, like
`the inductor 4. is a current limiter which is capable of
`50 transient energy storage. The lower plate of the transfer
`capacitor 5 is connected to the cathodes of thyristors Sb and
`Sc. of which the anodes are connected to the upper and lower
`plates respectively of the discharge capacitor 9. The upper
`plate of the capacitor 9 is connected to the anode of a
`55 thyristor 10a and the cathode of a thyristor 10d. the cathode
`of thyristor 10a and the anode of thyristor 10d being
`connected to upper and lower terminals respectively of the
`stimulating coil 11. Similarly. the lower plate of the capaci(cid:173)
`tor 9 is connected to the anode of thyristor 10b and to the
`60 cathode of thyristor 10c. the cathode of thyristor 10b and the
`anode of thyristor 10b being connected to the upper and
`lower terminals of the stimulating coil 11. Thus the thyris(cid:173)
`tors 10a-10d constitute a bridge 10 which determines a
`unidirectional flow of current through the coil 11 irrespec-
`65 tive of the polarity of the voltage across the capacitor 5. In
`this embodiment the lower terminal of the stimulating coil
`11 is provided with a ground connection 12.
`
`FIELD OF THE INVENTION
`The present invention relates to the magnetic stimulation 5
`of neuro-muscular tissue. The stimulation is achieved by
`creating a rapidly changing magnetic field. typically of the
`order of 20 k'.fesla/sec. in the vicinity of the tissue. An
`electric current is thereby induced in the tissue and causes
`stimulation of the tissue.
`
`lO
`
`BACKGROUND TO THE INVENTION
`
`15
`
`Various forms of magnetic stimulators are known. for
`example from US-A-4940453.
`Known magnetic stimulators comprise generally a charg(cid:173)
`ing circuit. a capacitor. a discharge control such as a con(cid:173)
`trolled rectifier for allowing discharge of the capacitor
`through the stimulating coil and some circuit elements for
`limiting the effect of undesirable electrical transients. The 20
`coil itself may be adapted to fit over a human cranium but
`may take any of a variety of forms currently known in the
`art.
`Known systems exhibit a variety of disadvantages. par(cid:173)
`ticularly the difficulty of achieving any modulation of the
`magnetic pulse output. a continuously variable high voltage
`power supply. a large reservoir capacitor and a large instan(cid:173)
`taneous power output of the high voltage power supply.
`The present invention provides a generally more versatile
`and improved stimulator which reduces at least some of
`these disadvantages.
`
`45
`
`SUMMARY OF THE INVENITON
`One aspect of the invention is the use of at least one
`additional capacitor to which charge is supplied or trans(cid:173)
`ferred and from which charge is transferred to a discharge
`capacitor which discharges into the stimulating coil. In one
`preferred form of the invention charge is pumped under the
`control of switches from a reservoir capacitor to an inter(cid:173)
`mediate transfer capacitor which is employed to replenish
`the charge on a capacitor which is controlled to discharge
`into the stimulating coil. The transfer of charge between
`successive capacitors may be controlled to occur by way of
`transient energy storage which may be provided by an
`inductor or inductors.
`The use of at least one additional capacitor enables a
`substantial increase in the rate of discharge pulses and also
`a substantial variation in the amplitude of them.
`Another aspect of the invention is the controlled recovery
`of energy from the stimulating coil and in particular means
`for recovery of energy from the stimulating coil to the
`discharge capacitor or another capacitor when the current
`flow from the discharge capacitor to the stimulating coil is
`interrupted. The other capacitor may be the transfer
`capacitor. if there is one. or the reservoir capacitor. The
`recovery of energy from the stimulating coil may be
`obtained by way of appropriately connected rectifiers or a
`transformer coupling.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1 illustrates one embodiment of a magnetic stimu(cid:173)
`lator according to the invention;
`FIG. 2 illustrates a second embodiment of a magnetic
`stimulator according to the invention;
`FIGS. 3A to 3D show waveforms relating to a transfer
`capacitor;
`
`Allergan EX1064
`Page 9
`
`

`

`3
`It may be seen that the thyristor switch 3 and the capacitor
`2 are a means for charging the transfer capacitor 5. the
`switches are a means for controlling the transfer of charge
`from the transfer capacitor 5 to the discharge capacitor 9 and
`the switches 10 are a means for controlling the discharge of
`the capacitor 9 through the stimulating coil 11. A control
`circuit for the thyristors is not shown because its operation
`will be obvious from the description that follows.
`The arrangement shown in FIG. 1 preferably operates as
`follows.
`FIG. 3A illustrates the voltage across the transfer
`capacitor. the voltage waveform of FIG. 3A being shown to
`an expanded timescale in FIG. 3B and to an even larger scale
`in FIG. 3C. FIG. 3D illustrates the current flow through the
`transfer capacitor and, in dashed lines. the current flow
`through the diode 6.
`FIG. 4A illustrates the voltage across the discharge
`capacitor 9. the voltage across the capacitor 9 being shown
`to a larger timescale in FIG. 4B. and the current flow through
`the discharge capacitor in FIG. 4C.
`Initially, the reservoir capacitor 2 would be charged to a
`high voltage such as 3 kV and the discharge capacitor 9
`would be charged to 1.8 kV, the upper plate being positively
`charged.
`A first pulse stimulus in a train of pulses for the stimu(cid:173)
`lating coil is caused by rendering the switches 10a and 10c
`conductive. by applying trigger signals to the gates of these
`thyristors. so that capacitor 9 discharges by way of the loop
`comprising the thyristor 10a, the stimulating coil 11 and the
`thyristor 10c. The discharge current reaches its peak value
`(41 in FIG. 4C) after a quarter of the resonant period of the
`loop. typically, for example, 40 microseconds, and then
`begins to diminish. After. for example, 80 microseconds,
`half the resonant period. the discharge current has dimio- 35
`ished to zero. Most of the · energy in the loop has been
`returned to the capacitor 9. At this point. thyristors 10a and
`10c stop conducting. The voltage 42 (FIG. 4B) on the
`discharge capacitor is about 80% of its initial magnitude but
`the polarity is reversed, the lower plate being positive.
`The stimulating pulses are intended to be a minimum of
`10 milliseconds apart. During the rest time, the capacitor 9
`is replenished so that it is recharged to a selected voltage
`which may be the same as or different from the initial
`voltage (1.8 kV). lo particular. the thyristor 3 in series with 45
`the reservoir capacitor 2 is made conductive, whereby
`charge is transferred from the reservoir capacitor 2 to the
`transfer capacitor 5. The series inductor 4 is chosen so that
`this transfer is complete in a time of the order of 75
`microseconds. the peak current being about 130 amps in a so
`typical system. After half of a resonant cycle, the current
`from the reservoir capacitor has fallen to zero and the
`thyristor 3 is turned off. The voltage on the transfer capacitor
`5 will now be greater than the voltage on the reservoir
`capacitor owing to the pumping action of the inductor 4.
`Next. either the thyristors 8a and Sc are rendered con(cid:173)
`ductive or the thyristors 8b and 8d are rendered conductive.
`depending on the polarity of the capacitor 9. As mentioned
`above, after the first pulse, the lower plate is positive so that
`thyristors 8a and 8c will be rendered conductive. Charge is 60
`thereby transferred from the transfer capacitor 5 to the
`capacitor 9 because the transfer capacitor S has been charged
`to a higher voltage than the discharge capacitor 9. At the
`point when the voltage at the transfer capacitor S has fallen
`to zero. substantially all the energy previously stored in it is 65
`transiently stored in the inductor 7. Current continues to flow
`through the inductor, the capacitor 9 and the clamping diode
`
`4
`6 until all the energy from the transfer capacitor has been
`transferred to the capacitor 9. At this point, current in the
`circuit comprising the transfer capacitor. the inductor 7. the
`capacitor 9 and the relevant conductive thyristors 8a and Sc
`5 has fallen to zero. so that the thyristors 8a and 8c tum off.
`The recharge cycle then repeats by means of triggering the
`thyristor 3 in series with the reservoir capacitor. lo the next
`cycle, in order to transfer charge between the transfer
`capacitor S and the capacitor 9. the thyristors Sb and 8d will
`10 be rendered conductive.
`lo practice the replenishment cycle of operation may be
`repeated a multiplicity of times.
`Once the discharge capacitor 9 has been recharged, the
`next stimulus in the train of stimuli can be delivered by
`15 triggering the switches 10b and 10d so that the direction of
`current flow in the path of the second stimulus is the same
`as that for the first stimulus. for which the thyristors 10a and
`10c are made conductive.
`lo the second embodiment. shown in FIG. 2. some parts.
`shown by like reference numerals, are common to the
`embodiment already described with reference to FIG. 1.
`Thus the reservoir capacitor 2 is charged from the high
`voltage supply 1 and can be discharged by means of the
`controlled switch 3 by way of the inductor 4 into the transfer
`25 capacitor S. from which charge can be transferred to the
`discharge capacitor by way of the inductor 7 when the
`thyristor Se is rendered conductive. The discharge capacitor
`9 may be discharged through the stimulating coil 11 by way
`of thyristor toe. lo anti-parallel with the thyristor lOe is a
`30 diode rectifier 12.
`The preferred method of operation of the embodiment
`shown in FIG. 2 is as follows. FIG. SA shows the waveform
`of the voltage across the capacitor 9. FIG. SB shows this
`waveform to a larger timescale and FIG. SC shows the
`current through the capacitor 9.
`Initially, the reservoir capacitor 2 may be charged. for
`example to 3 kV, and the capacitor 9 may be charged to 1.8
`kV, the upper plate being positive.
`The first stimulus is delivered by rendering thyristor He
`conductive so that the discharge current Is starts to flow in
`the stimulating coil 11. The current reaches a peak value 51
`after about a quarter of the resonant period and then starts to
`fall. After half the resonant period, in this example 80
`microseconds, the current has reduced to zero (point 52,
`FIG. SC) and most of the energy has been returned to the
`capacitor 9.At this point. the diode becomes forward biased.
`allowing the second part of the resonant cycle to follow. The
`current flows in the opposite direction and at the end of the
`cycle the current to the capacitor 9 has again fallen to zero
`(point 53), and the diode 12 is reversed biased. No further
`current can flow because the thyristor switch lOe has been
`turned off. The voltage 54 (FIG. SB) on the capacitor 9 is
`typically about 65% of its original magnitude. and has the
`55 original polarity.
`The stimuli may be a minimum of. for example, 10
`milliseconds apart and during the inter-pulse period the
`capacitor 9 is replenished by means of rendering the thyris(cid:173)
`tor switch 3 conductive, so that charge is transferred from
`the reservoir capacitor 2 to the transfer capacitor 5. The
`series inductor is chosen such that the charge transfer lasts
`about 75 microseconds, the peak current being typically
`about 130 amps. After half a resonant cycle, this charge
`current from capacitor 2 has fallen to zero and the thyristor
`3 turns off. The voltage on the transfer capacitor 5 is now
`higher than the reservoir voltage owing to the pumping
`action of the inductor. Next. the thyristor 8 is triggered so as
`
`20
`
`40
`
`Allergan EX1064
`Page 10
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`

`

`5,766J24
`
`5
`to render this thyristor switch conductive. Thereby charge is
`transferred from the transfer capacitor S to the discharge
`capacitor 9. because the transfer capacitor is charged to a
`higher voltage than the capacitor. At the point when the
`voltage on the transfer capacitor S has fallen to zero virtually 5
`all the energy in the loop comprising the two capacitors S
`and 9. the inductor 7 and the thyristor Se is stored transiently
`in the inductor 7. The current continues to flow by way of the
`diode 6 until all the energy has been transferred into the
`capacitor 9. At this point the current has again fallen to zero
`and the thyristor Se is turned off. The recharge cycle is
`repeated by triggering the thyristor switch 3 until sufficient
`charge has been transferred to replenish the capacitor 9.
`Again, although only a few recharge cycles SS are shown in
`FIG. SB. in practice there may be a greater number of
`recharge cycles. for example twenty.
`When the discharge capacitor 9 has been recharged. the
`next stimulus in the train may be delivered by the triggering
`of the thyristor lOe.
`The embodiment shown in FIG. 2 is simpler than that
`shown in FIG. 1 but is less efficient. and requires two voltage
`reversals instead of one for each discharge of the capacitor
`9.
`
`The embodiments shown in FIGS. 1 and 2 are in practice
`capable of providing variable length trains of pulses within
`successive periods PL P2 etc (FIG. 3A) which have high
`repetition rates. such as 100 Hz. and can be amplitude
`modulated. typically about 20%, and/or frequency modu(cid:173)
`lated without requiring multiple parallel discharge systems
`or a variable high voltage power supply with high instan(cid:173)
`taneous power output. For example. recharging a 70µF
`capacitor to 1.8 kV in 10 microseconds requires about 11 kW
`whereas the preferred embodiment could utilize a 500 VA
`transformer in the supply unit 1.
`FIG. 6 illustrates an embodiment which generally
`resembles that shown in FIG. 1 because it has a reservoir
`capacitor 2 connected to a high voltage power supply. and a
`discharge capacitor 9 which is controllable to discharge into
`the stimulating coil 11 when the switch 10 is closed. 40
`However. in place of the transfer capacitor is a transfer
`inductor 1S which acts to store energy transiently during
`energy transfer between the reservoir capacitor 2 and the
`discharge capacitor 9.
`The transfer inductor 15 forms distinct loops with the 45
`capacitor 2 and the capacitor 9, these loops containing
`respective switches 3 and 8. A rectifier 13 is coupled to
`convey energy unidirectionally from the stimulating coil
`back to the reservoir capacitor 2 in response to the inter(cid:173)
`ruption of the flow of current between the capacitor 9 and the so
`coil 11.
`In the circuit shown in FIG. 6, the high voltage supply
`continually supplies a charging current to the reservoir
`capacitor 2. maintaining its voltage close to a selected
`maximum. The discharge capacitor 9 is charged to a required
`level as follows. The switch 3 is closed, allowing current la
`to flow in the transfer inductor 1S. When this current reaches
`some predetermined value, the switch 3 is opened and
`switch 8 closed simultaneously, the current lb in the transfer
`inductor 15 charging the capacitor 9. The switch 8 may be
`opened when the current in the respective loop has decayed
`to zero. A stimulating pulse can be delivered by the closure
`of the switch 10. When the current Is in the coil is near a
`peak value. the switch 10 may be opened. allowing a current
`Ir to flow by way of the back-coupling diode 13 to charge the
`reservoir capacitor 2. The arrangement enables the reservoir
`capacitor's voltage to be lower than the discharge capaci-
`
`30
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`25
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`6
`tor's voltage and the control of the energy transferred to the
`discharge capacitor 9 for each cycle. Furthermore. the dis(cid:173)
`charge capacitor does not experience significant voltage
`reversal.
`FIG. 7 illustrates an embodiment which uses both a
`transfer capacitor S and a transfer inductor between the
`reservoir capacitor 2 and the discharge capacitor 9. In this
`embodiment. the branch containing the transfer inductor 1S
`in FIG. 6 contains the transfer capacitor S in series with
`10 inductor 15. Ganged series switches 3a. 3b are disposed
`between the reservoir capacitor 2 and the transfer branch and
`ganged switches Sa. Sc are disposed between the transfer
`branch and the discharge capacitor. Opposite ends of the
`stimulating coil are connected by the diode rectifiers 13a and
`15 13b to respective plates of the reservoir capacitor 2 so that
`current flowing in the stimulating coil after closure of the
`discharge control switch 10 can flow to charge the reservoir
`capacitor 2 when the switch 10 is opened, as described
`above with reference to FIG. 6. The transfer inductor 15 in
`20 FIG. 7 may be much smaller than that required for the circuit
`of FIG. 6 because it acts substantially only as a current
`limiter. Otherwise the circuit operates similarly to that
`shown in FIG. 6. The switches 3 and 8 switch only when
`current flow is zero and may comprise thyristors.
`FIG. 8 illustrates an embodiment in which the transfer of
`charge from the reservoir capacitor 2 to the discharge
`capacitor 9 resembles that occurring in the embodiment
`shown in FIGS. 1 and 2. Charge may be pumped by the
`closing and opening of switch 2 to the transfer capacitor S
`and charge may be pumped from the transfer capacitor S to
`the discharge capacitor 9. The discharge loop resembles that
`described with reference to FIG. 2, the thyristor l0e allow(cid:173)
`ing discharge of the discharge capacitor 9 into the stimulat(cid:173)
`ing coil 11 and the diode 12 allowing reverse flow of charge
`35 back to the discharge capacitor in response to the cessation
`of current flow around the discharge loop. as previously
`described. This embodiment, like the embodiments in FIGS.
`1 and 2. may employ thyristors for all the controlled
`switches because all the switching may occur when the
`resepctive current is zero.
`FIG. 8 may be modified by the removal of capacitor S and
`switch 8 shown within the chain-line; the inductor 1S then
`needs to be in series between the switch 3 and the discharge
`capacitor 9.
`FIG. 9 illustrates a somewhat different embodiment which
`may include a reservoir capacitor 2 and a transfer capacitor
`S as previously described but which may be arranged. as
`shown. such that the discharge capacitor 9 is connected to
`the high voltage supply by way of ganged switches 16, 16a
`that enable reversal of the supply voltage applied to the
`discharge capacitor. The discharge capacitor is connected to
`the stimulating coil 11 by way of a rectifying bridge con(cid:173)
`taining two controlled rectifiers 17. 17a. and two diode
`rectifiers 18, 18a. Once the capacitor 9 has been charged to
`55 the required energy level. the controlled rectifier 17 may be
`made conductive. so that current 11 flows in the coil 11. The
`current reaches a maximum after a time determined by the
`resonant frequency of the discharge loop comprising the
`capacitor 9 and the coil 11. The current decreases to zero. at
`60 which point the controlled rectifier 17 can be switched off.
`The voltage on the capacitor 9 is now reversed and the next
`discharge requires the firing of controlled rectifier 17a.
`allowing current 12 to flow in the coil in the same direction
`as during the previous cycle. The discharge capacitor 9
`65 recovers a substantial portion of the energy held at the start
`of a cycle and needs only replenishment. rather than a full
`charging cycle, from the supply by means of the two-pole
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`20
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`25
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`7
`two-way switches 16, 16a. the operation of the switches
`being selected to connect the supply in the correct polarity
`to the capacitor 9.
`FIG. 10 illustrates a modification which may employ any
`of the energy transfer techniques described with reference to 5
`FIGS. 1 to 8. The relevant components. including the high
`voltage supply and the reservoir capacitor, have been omit(cid:173)
`ted from FIG. 10 for the sake of simplicity.
`In the modification shown in FIG. 10, charge is trans(cid:173)
`ferred to the discharge capacitor 9 by means of closure of the
`switch 8. Closure of the switch 10 allows discharge of the
`capacitor 9 into the stimulating coil 11. which comprises two
`interleaved windings Ila. lib acting as a transformer. When
`the current in winding lla is at or near a maximum. the
`switch 10 may be opened. The cessation of current flow in
`the primary winding lla induces a flow of current in the
`secondary winding lib. This current may be used to
`recharge by way of the diode 13 the discharge capacitor 9 or
`the reservoir capacitor (if there is one), so that energy is
`recovered from the stimulating coil.
`All the foregoing embodiments include means enabling
`the recovery of energy from the stimulating coil to a charge
`storage capacitor when a discharge control switch between
`the discharge capacitor and the coil is opened or made
`non-conductive so as to interrupt current flow from the
`discharge capacitor to the stimulating coil.
`FIG. 11 illustrates another embodiment of the invention.
`which is intended to be a modification of the circuit shown
`in either FIG. 8 or FIG. 9. In this embodiment, the discharge
`capacitor 9 may have charge coupled to it either directly
`from the power supply or indirectly by means of a reservoir
`capacitor and, if desired. a transfer capacitor as previously
`described with reference to the preceding Figures. Between
`the discharge capacitor 9 and the stimulating coil 11 is a
`network 20 which may be. for example. either a controlled
`rectifier switch and a by-pass diode, as shown in FIG. 8 or
`a thyristor and diode bridge as shown in FIG. 9.
`Additionally. the stimulating coil 11 is in parallel with a
`ladder network 21 comprising a plurality of parallel
`branches each containing an inductor 22. 22a ... 22n and a
`respective switch 23. 23a . . . 23n. Preferably the induc(cid:173)
`tances of the branches of the ladder network increase
`according to powers of two.
`Initially the discharge capacitor would be charged to a
`much higher energy level than is required for a first pulse in
`a series and all the inductors 22. 22a etc may be switched
`into parallel connection with the stimulating coil. During the
`first discharge, the energy that was stored in the capacitor 9
`is divided between the stimulating coil and the inductor
`network in proportion to the ratio of the respective induc(cid:173)
`tance values. The discharge cycle proceeds as described with
`reference to FIG. 8 or FIG. 9. terminating with the storage
`of recovered energy in the capacitor 9. Because the circuit
`components are imperfect. some of the energy will have
`been lost so there will be less energy in the capacitor at the
`end of the cycle than there was at the beginning. Before the
`initiation of the second pulse in the train. some of the
`inductors in the network are switched out of circuit, so that
`when the discharge takes place. a greater proportion of the
`capacitor energy is directed into the stimulating coil. This
`process is repeated for each pulse in the train. such that the
`value of inductance in parallel with the stimulating coil
`progressively decreases and the same absolute energy is
`delivered to the coil during every pulse in the chain. This 65
`circuit does not require the discharge capacitor to be 'topped
`up' between pulses and thereby reduces stresses on the
`
`8
`power supply. However. a control circuit for the switches 23.
`23a . . . 23n has to calculate the appropriate parallel
`inductance changes for different stimulating coils with a
`range of inductances and energy losses.
`I claim:
`1. A magnetic stimulator of neuro-muscular tissue com(cid:173)
`prising a stimulating coil. a discharge capacitor. means for
`controlling discharge of said discharge capacitor into said
`stimulating coil, a reservoir capacitor, and means for pump-
`10 ing ~arge from said reservoir capacitor to said discharge
`capacitor.
`2. The stimulator of claim 1 wherein said means for
`pumping comprises an energy storage device. switch means
`for discharging said reservoir capacitor into said energy
`storage device and means for transferring energy from said
`15 storage device to said discharge capacitor.
`3. The stimulator of claim 1 wherein said means for
`pumping comprises
`an energy storage device;
`a first electrical loop including said energy storage device.
`said reservoir capacitor and a first controllable switch;
`and
`a second electrical loop including said energy storage
`device. said discharge capacitor and a second control(cid:173)
`lable switch.
`4. The stimulator of claim 3 wherein said energy storage
`device comprises an inductor.
`5. The stimulator of claim 3 wherein said energy storage
`device comprises a transfer capacitor.
`6. The stimulator of claim 5 wherein each of said first and
`30 second electrical loops includes an inductor.
`7. The stimulator of claim 1 further comprising a ladder
`network in parallel with said stimulating coil. said ladder
`network having a plurality of branches each including an
`inductor in series with a controllable switch.
`8. The stimulator of claim 1 wherein said means for
`controlling comprises a switch operative to interrupt curren