U.S. patent number 3,895,639 [Application Number 05/399,067] was granted by the patent office on 1975-07-22 for apparatus for producing an interference signal at a selected location.
Invention is credited to Hans Rodler.
United States Patent |
3,895,639 |
Rodler |
July 22, 1975 |
Apparatus for producing an interference signal at a selected
location
Abstract
An electrotherapeutic apparatus for producing a beat or
interference frequency at a selected body location comprises two
pairs of electrodes connected to the body. Alternating current is
supplied to each pair of electrodes from the two outputs of an
oscillator, with the current paths between the electrodes of each
pair crossing each other at the selected body location. A phase
shifter rhythmically changes the phase of the current in one of the
current paths.
Inventors: |
Rodler; Hans (A-8053 Graz,
OE) |
Family
ID: |
26874033 |
Appl.
No.: |
05/399,067 |
Filed: |
September 20, 1973 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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178159 |
Sep 7, 1971 |
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Current U.S.
Class: |
607/67 |
Current CPC
Class: |
A61N
1/323 (20130101) |
Current International
Class: |
A61N
1/32 (20060101); A61n 001/36 () |
Field of
Search: |
;128/419R,420,421,422,423 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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871,672 |
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Jun 1961 |
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GB |
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467,502 |
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Jun 1937 |
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GB |
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Primary Examiner: Kamm; William E.
Attorney, Agent or Firm: Kelman; Kurt
Parent Case Text
This is a continuation-in-part application of my copending
application Ser. No. 178,159, filed Sept. 7, 1971, now
abandoned.
The present invention relates to improvements in apparatus for
producing a beat or heterodyne frequency at a selected location of
a body, and is particularly useful in electrotherapy for the human
body.
In known apparatus of this type, two pairs of electrodes are
connected or applied to the body. Independent oscillating means
supplies each pair of electrodes with alternating current, and the
current paths between the electrodes of each pair cross each other
at the selected body location.
This type of electrotherapy has the particular advantage that a
weak current of relatively high frequency is transmitted between
the skin and the applied electrodes, which does not irritate the
skin, while a relatively strong interference current of low
frequency is produced at the intersection of the two current paths,
due to the superposition of one current on the other and the
corresponding frequency difference between the two currents.
Furthermore, the selected location may be accurately determined by
a suitable arrangement of the electrodes, and this location may be
in regions deep in the body and remote from the skin. Accordingly,
this type of electrotherapy has been successfully used to relieve
pain, to exercise muscles, to treat joint diseases or neuralgia, to
induce sleep, to improve blood circulation, and to alleviate
inflammations.
In known apparatus of this general type, two separate and
independent oscillators have been used to supply the oscillations
to the electrodes, the difference between the frequencies of the
two oscillators being very small, i.e. within the range of about
0.5 to 20 cycles per second (cps). In the usual oscillator
frequency of 5000 cps, one cycle per second corresponds to a
tolerance of 0.02 percent, which is an almost unattainable accuracy
since the two oscillators influence each other and their
frequencies tend to become equal when the difference becomes too
small. Since the frequency of at least one of the oscillators must
be controllable, it is impossible to attain such accuracy even with
the use of quartz oscillators. Furthermore, these conventional
electrotherapeutic machines produce only interference currents of
sine waves.
It is the primary object of this invention to avoid the indicated
disadvantages of such apparatus and to provide a beat or
interference frequency generator of simple construction and capable
of producing interference currents of any desired wave shape and
frequency.
The above and other objects are accomplished in accordance with the
invention by providing a single oscillator having two outputs
respectively connected to a respective one of the pairs of
electrodes, and a phase shifter for rhythmically changing the phase
of the current in one of the current paths. The oscillator outputs
supply an alternating current to each pair of electrodes, and the
electrodes are arranged about a selected location so that the
current paths of the pairs of electrodes intersect at this
location. The phase shifter is arranged between one of the
oscillator outputs and the pair of electrodes connected thereto for
rhythmically changing the phase of the current in one current
path.
In one embodiment, the oscillator is a rectangular wave oscillator.
One of the oscillator outputs is connected to a first pair of the
electrodes, and an output amplifier and if desired a wave converter
are arranged between the one output and the first pair of
electrodes. A monostable multivibrator, whose pulse width is
controlled by a low-frequency oscillator, is connected to the other
output, a first differentiating circuit being arranged between the
other output and the multivibrator. A further differentiating
circuit is connected to the multivibrator, and the other pair of
electrodes is connected to another oscillator controlled by the
differentiated edge of the oscillations produced by the
multivibrator. An output amplifier and if desired a wave converter
are arranged between the other oscillator and the other pair of
electrodes.
Since a low-frequency oscillator controls the pulse width of the
multivibrator, the generated pulse becomes narrower and wider in
correspondence to the rhythm of this oscillator. Therefore, the
trailing edge of the oscillations produced with the multivibrator
changes its phase position in respect to the basic oscillation
rhythmically in the frequency of the controlling low-frequency
oscillator. By differentiating the trailing edge and controlling a
further oscillator, which may also be a monostable multivibrator
but may be a sine wave oscillator, too, the further oscillator may
be made to generate oscillations rhythmically phaseshifted in
respect to the first oscillations generated in the rectangular wave
oscillator. The phase shift may be changed between 5.degree. and
355.degree.. The frequencies of the two output currents are the
same, the phase of the second output current being variable in
respect of the phase of the first output current. When the two
frequencies are brought into interference at the point of
intersection of the two current paths, an interference oscillation
is produced. With an alternating current source, the enveloping
curve of the interference oscillation may take any shape or form,
the wave shape being controlled by the low-frequency oscillator. If
this oscillator changes the pulse width of the first monostable
multivibrator rectangularly, a rectangular wave interference
oscillation is generated. If the low-frequency oscillator generates
a sine wave oscillation, the interference oscillations is
sineshaped, too.
According to one preferred feature of the present invention, the
output amplifiers have an output for alternating current and an
output for direct current. Also, the transformers may preferably be
switched out of the operating circuit of the apparatus. This has
the advantage that a rectangular direct current pulse is received
from the D.C. output. When brought into interference, this makes
two variations possible, i.e. the D.C. pulses may be brought into
interference in opposite polarity, in which case the pulses cancel
each other at the same phase and produce alternating current at the
opposite phase, or they may be brought into interference at the
same polarity, in which case a D.C. pulse of the same frequency as
the basic frequency is produced at the same phase and a D.C. pulse
is produced at opposite phase as long as the phase shift is
180.degree.. Thus, it is possible to produce a direct current deep
in the tissues of the body although pulses of relatively high
frequency are generated.
In another embodiment of the invention, two or more monostable
multivibrators are connected to the rectangular wave oscillator via
a differentiating circuit, the pulse width of the first monostable
multivibrators being controlled by a low-frequency oscillator and
each monostable multivibrator having an end stage with a patient
output, an output amplifier with a patient output being
additionally directly controlled by the rectangular oscillator.
This has the advantage that three or more output amplifiers and
thus three or more current streams for patients may be operated
simultaneously. The first circuit receives directly the basic
frequency, the second circuit receives the frequency from one
monostable multivibrator, and the third one receives it from the
other monostable multivibrator in the indicated operating circuit.
By suitably adjusting the basic pulse width and thus the phase
position of the first monostable multivibrator, current
amplification may be attained at the interference point, i.e. the
interference point may be located more accurately. The considerable
advantage of such an arrangement resides in the fact that the
operating circuit comprises only one frequency-determining
oscillator, the frequency constant of this oscillator not being
critical. Therefore, the interference frequency may be made as
small as desired. Furthermore, by using D.C. pulses of the same
amplitude and equidistantly paced, D.C. voltage may be produced at
the interference point. A multiphased arrangement makes it possible
to project the interference point more accurately and to increase
the energy at the interference point in respect of the electrodes.
Claims
What is claimed is:
1. Apparatus for producing an interference signal at a selected
location comprising, in combination, oscillator means for
furnishing an oscillator output signal having a determined
frequency and a reference phase; phase shift means connected to
said oscillator means cyclically varying the phase of said
oscillator output signal, thereby furnishing a phase-shifted
oscillator output signal; first electrode means connected to said
oscillator means for creating a first current having said
determined frequency at said selected location in response to said
oscillator output signal; and second electrode means connected to
said phase shift means for creating a second current having said
determined frequency and a phase varying cyclically with respect to
the phase of said first current at said selected location in
response to said phase-shifted oscillator output signal, whereby
interference between said first and second currents creates said
interference signal at said selected location.
2. Apparatus as set forth in claim 1, wherein said oscillator means
comprise a sine wave oscillator having a first and second output
each for furnishing said oscillator output signal; and wherein said
phase shift means comprise a first phase shift circuit including a
capacitor and a variable resistor connected to said second output,
and means for cyclically varying the resistance of said variable
resistor.
3. Apparatus as set forth in claim 2, wherein said first and second
electrode means respectively comprise a first and second amplifier
each having an output, and a first and second pair of electrodes
respectively connected to said output of said first and second
amplifier.
4. Apparatus as set forth in claim 3, wherein said phase shift
means further comprise an additional phase shift circuit having a
capacitor and a variable resistor interconnected between said
oscillator output and said first electrode means, and means for
cyclically varying the resistance of said variable resistor in said
additional phase shift circuit in the direction opposite to the
variation of resistance of said variable resistor in said first
phase shift circuit.
5. Apparatus as set forth in claim 1, wherein said oscillator means
comprise rectangular wave generator means for furnishing a
rectangular wave having leading edges each indicative of the start
of a cycle; and wherein said phase-shift means comprise delay means
connected to said rectangular wave generator means for furnishing a
trigger signal after a variable time delay following each of said
leading edges, and second wave generator means connected to said
time delay means for furnishing a cycle of a second wave in
response to each of said trigger signals, whereby said second wave
has the same frequency but a varying phase shift relative to said
rectangular wave.
6. Apparatus as set forth in claim 5, wherein said second wave
generator means comprise pulse furnishing means for furnishing a
pulse in response to each of said trigger signals.
7. Apparatus as set forth in claim 6, wherein said time delay means
comprise first differentiating circuit means connected to said
rectangular wave generator means for differentiating said
rectangular wave and furnishing first trigger signals, each
indicative of one of said leading edges; first monostable
multivibrator means having a trigger input connected to said first
differentiating circuit means and a control input, for furnishing a
delay pulse having a pulse width varying as a function of the
amplitude of a control signal applied at said control input in
response to each of said first trigger signals; and low frequency
oscillator means for furnishing a low frequency control signal to
said control input of said first monostable multivibrator means,
whereby each of said delay pulses has a trailing edge occuring at a
varying time delay with respect to said leading edges of said first
rectangular wave; second differentiating circuit means connected to
said first multivibrator means for differentiating said delay
pulses and furnishing second trigger signals in response to said
trailing edges of said delay pulses; and wherein said pulse
furnishing means comprise second monostable multivibrator means
having a trigger input connected to said second differentiating
circuit means, for furnishing a pulse having a determined pulse
width in response to each of said second trigger signals.
8. Apparatus as set forth in claim 1, wherein said phase shift
means comprise bridge circuit means having input terminals
connected to said oscillator means and output terminals connected
to said second electrode means, and magnetic amplifier means having
output windings connected in an arm of said bridge circuit and
having input windings, and means coupled to said input winding for
applying a cyclically varying current thereto, thereby cyclically
varying the inductance of said output windings and the phase of the
signal at said output terminals of said bridge circuit.
9. Apparatus as set forth in claim 1, wherein said phase shift
means comprise a three phase stator connected to said oscillator
means, a rotor connected to said second electrode means, and means
for continuously rotating said rotor relative to said stator.
Description
The above and other objects, advantages and features of this
invention will be better understood by reference to the following
detailed description of one preferred embodiment, taken in
conjunction with the accompanying drawing wherein
FIG. 1 is a schematic view of a portion of a human body to which
two pairs of electrodes of an apparatus according to the invention
are applied;
FIG. 2 is a circuit diagram illustrating a very simple circuit for
operating the apparatus;
FIG. 3 is a circuit diagram illustrating another operating
circuit;
FIG. 4 is a detailed diagram of the operating circuit of FIG.
3;
FIG. 5 diagrammatically illustrates a detail of the circuit of FIG.
2;
FIG. 6 shows yet another operating circuit;
FIGS. 7 and 9 diagrammatically illustrate a detail of the circuit
of FIG. 6;
FIG. 8 shows still another operating circuit;
FIG. 10 is a circuit diagram of a further embodiment of the
operating circuit;
FIG. 11 shows a control circuit for the phase shifting means of
FIG. 10;
FIG. 12 shows the arrangement of the electrode pairs in the
operating circuit of FIG. 10; and
FIGS. 13 and 15 are circuit diagrams of three additional operating
circuit embodiments.
Referring now to the drawing and first to FIG. 1, there is shown
the oscillator 1 having two outputs constituted by pairs of
terminals 2, 3 and 4, 5. The terminals 2, 3 of one output are
connected to electrodes 6, 7 of a first pair of electrodes, and
terminals 4, 5 are connected to electrodes 8, 9 of a second pair of
electrodes. The two pairs of electrodes are connected or applied to
a portion of the human body 10 to be subjected to electrotherapy.
When alternating currents whose phases are shifted in respect of
each other are supplied to the respective pairs of electrodes from
the output terminals, an interference current is produced at the
location of intersection of the current paths 11 and 12 between the
electrodes of the respective pairs. The desired location 13 and
depth of the location of intersection of the current paths is
determined by the positions of the electrodes on the body, the
electrodes being quadrangularly arranged, as is well understood by
those skilled in this art.
In the very simple operating circuit shown in FIG. 2, terminals 2,
3 of one of the outputs of oscillator 1a is connected to the pair
of electrodes 6, 7, see FIG. 1, by means of secondary winding 34 of
transformer 31 receiving the sine wave output of the oscillator,
the signal being amplified by amplifier 35 connected between the
transformer winding and electrodes 6, 7. The oscillator produces a
sine wave of about 5 KHz (thousand cycles per second).
Phase shift circuit 32, part of phase shift means 32, 33, is
connected between the other oscillator output terminals 4, 5 and
electrodes 8, 9 of the other pair in accordance with the present
invention. Phase shift circuit 32, part of phase shift means 32,
33, comprises secondary winding 39 of transformer 31 from whose
center tapping point a phase-shifted signal is transmitted to
amplifier 35' connected between the tapping point and electrodes 8,
9 so that this pair of electrodes receives an amplified
phase-shifted signal. It further comprises function generator 33
connected to one end of winding 39 and controlling motor 42 driving
the adjustable element of potentiometer 38 for adjustment of the
same, and condenser 40 connected to the other end of winding 39. In
this manner, the generator 33 electromechanically controls
potentiometer 38 and thus the current phase supplied to electrodes
8, 9 rhythmically.
The diagram of FIG. 5 shows the two parts of the voltage of
transformer winding 39 as vectors 36, 36, the part voltage of
potentiometer 38 as vector 37 and the part voltage of condenser 40
as vector 43. The two vectors 36, 36 form the base of a right
triangle whose two sides are constituted by vectors 37 and 43. The
output voltage vector is tapped from the center of the base and the
point of intersection between vectors 37 and 43, which point lies
in a circle about the center point of the base. As vector 37
decreases, the output voltage becomes closer and closer to the
voltage of vector 36. If the resistance of potentiometer 38
increases to decrease vector 43 and proportionally increase vector
37, the phase of the current is shifted in the opposite
direction.
If desired, the potentiometer and the condenser could be adjusted
together, thus increasing the region of the phase variation.
The circuit diagram of FIG. 3 shows rectangular wave oscillator 14
having a differentiating circuit 15 connected to one pair of its
output terminals, which generates an impulse from the leading edge
of the rectangular pulse of the oscillator. This pulse controls a
monostable multivibrator 16 whose pulse width is controlled by
low-frequency oscillator 17. The differentiating circuit 18
connected to the multivibrator generates a new pulse from the
trailing edge of the oscillations generated in the multivibrator,
and this new pulse controls a second monostable multivibrator 19.
The pulse width of the multivibrator 19 is so adjusted that the
pulse durations and interruptions are of equal duration. The output
amplifier and wave converter 21 delivers the current from
multivibrator 19 to electrodes 9, 8 which are applied to the
patient. Another output amplifier and converter 20 is connected to
the other pair of output terminals of oscillator 14 to deliver
current to electrodes 6, 7 applied to the patient. The two currents
are phase-shifted in relation to each other by the amount of the
pulse width of the multivibrator 16.
FIG. 4 is a circuit diagram showing the operating circuit of the
circuit components of FIG. 3 in more detail. The circuit elements
are well known and, as readily available articles of commerce,
require no further description.
The rectangular wave generator 1b is an astable multivibrator which
is connected to the differentiating circuit 15 by means of coupling
transformer 33. The differentiating circuit 15 is connected to
monostable multivibrator 16 by means of a coupling diode 22 to
suppress the pulses of the second portion of the pulses delivered
by differentiating circuit 15.
The pulse width of the monostable multivibrator 16 is controlled by
the low-frequency oscillator 17 which is connected to multivibrator
16 via amplifier 23, the oscillator 17 being a Wien bridge
generator. Potentiometers 31 control the frequency of the Wien
bridge generator 17. If desired, the wave shape of the oscillations
generated by the Wien bridge may be adjusted in a known manner by
potentiometers (not shown).
The rectangular pulses generated by multivibrator 16 are
differentiated in differentiating circuit 18 and are delivered to
the monostable multivibrator 19 via diode coupling 24 which
suppresses the ascending pulse portion. The latter multivibrator is
so adjusted that the lengths of the pulses and interruptions are
equal. Therefore, the phase position of the rectangular pulses
varies rhythmically in respect of the rectangular pulses generated
directly by the astable multivibrator. Since the monostable
multivibrator 19 is always controlled by the astable multivibrator,
proper operation is assured even at stationary phase shifting.
The rectangular oscillation is supplied from the multivibrator 19
to a driving stage 25 and amplified at output amplifier 21. By
suitably dimensioning the switching elements of driving stage 25
and amplifier 21 the rectangular pulses may be converted into sine
wave pulses. The output amplifier 21 is connected by means of a
transformer coupling to the first pair of terminals 4, 5.
The rectangular pulses of astable multivibrator 16 are delivered
via condenser 26 and a driving stage 27 to output amplifier 20. By
suitable dimensioning the switching elements of the driving stage
and the amplifier the rectangular pulses may be converted into sine
wave pulses. The output amplifier 20 is connected by means of a
transformer to the second pair of terminals 6, 7.
The two phase-shifted rectangular pulses are superimposed in a
transformer coil 29, the generated interference current is
amplified and supplied to an indicator lamp 30 which shows the
interference voltage.
The circuit is supplied by a current source 23 which includes a
Wheatstone bridge, condensers and a Zener diode 24 to maintain the
voltage constant.
The operating circuit of FIG. 6 is a modified version of that of
FIG. 2, differing therefrom in that phase shifting circuits 32',
32' are connected between each output of sine wave oscillator 1a
and each electrode pair, the phase shifted signals being amplified
by amplifiers 35, 35'. Function generator 33' controls the phase
shifting potentiometers in the phase shifting circuits in the same
manner as described in connection with FIG. 2. In this circuit
arrangement, each phase shifter needs to effectuate only a
90.degree. shift since this will encompass a phase shift region
between 0.degree. and 180.degree. for the two phase shifters.
FIGS. 7 and 9 show the phases of a respective one of the phase
shifters 32', 32' in the same manner as described in connection
with FIG. 5.
FIG. 8 illustrates an operating circuit with electronic circuit
elements. Since wave generator 1c is constituted by transistor 49,
resonance circuit 44 and feed-back winding 45. Phase shifting means
32a, 32a are connected to the secondary 39', 39' of transformer 31'
which is connected to one output of the sine wave generator
(compare FIG. 2). Each phase shifting circuit 32a again comprises a
condenser 40' and an adjustable resistance constituted by field
effect transistor 46. Function generator 33a rhythmically controls
the transistors 46, 46 to change the resistance thereof
rhythmically. Resistances 47, 47 transmit the biasing potential
from terminal 18 to the transistors.
FIGS. 10 and 11 show an embodiment of the apparatus wherein four
pairs of electrodes are arranged for application to a patient so as
to provide a multi-phase treatment for the patient's body. As
shown, sine waves from oscillator 1d are transmitted to two pairs
of electrodes 6, 7 and 8, 9 as well as two additional pairs of
electrodes 50, 50 and 51, 51, the output signals from the
oscillator being phase shifted by respective phase shifting
circuits 32b connected between the oscillator and each of the four
pairs of electrodes. Function generator 33b controls the phase
shifting circuits so that each circuit produces a different phase
shift, as in the embodiments of FIGS. 6 and 8, the phase shifted
signals again being amplified by amplifiers 35, 35', each of the
four amplifier feeding an amplified phase-shifted signal to a
respective one of the four pairs of electrodes. The phase shifting
circuits may be those illustrated in FIG. 8, for example.
FIG. 11 illustrates the phase shift control for the four circuits
to obtain different phases in each circuit. The two integrated
analog amplifiers 52, 53 form a triangular function generator,
different direct current voltages being added to the control
voltage of this generator at connection 31 receiving these voltages
from resistors 55, 56 and resistance controls 54. This produces
triangular voltages at control signal output points 57, 58, 59, 60
which have added thereto different direct current voltages.
If desired, the phase shifting circuits may be differently
dimensioned whereby the initial output signal phases are different
so that the function generator 33b may be in parallel circuit with
the phase shifting circuits 32b.
FIG. 12 shows the arrangement of the four pairs of electrodes, the
electrodes of each pair being substantially diametrically opposite
each other in respect of a common point of intersection where the
current density is multiplied so that an intensive electrical
treatment is obtained in depth at a desired point within a
patient's body, the amplitudes of the current at the respective
electrodes being uniform.
In the operating circuit of FIG. 13, the phase shifting of the
output signal from oscillator 1e to the pair of electrodes 8, 9,
via amplifier 35', comprises a conventional bucket brigade delay
line device 63 and an impulse generator whose frequency is
controlled by function generator 33c. The impulse generator
consists essentially of a multivibrator constituted by transistors
64 and 66, the frequency-controlling resistances being formed by
transistors 65 and 67 which, in turn, are connected to generator
33a at 70, the generator controlling the resistances and thus
rhythmically changing the frequency of the impulse generator 64,
66. As is known, bucket storage devices store analog signals, the
storage time depending on the frequency of the impulse generator.
In this manner, a phase change is produced between the input signal
at input 71 of the phase shifting circuit and the output signal at
output 62 thereof, this change being linearly proportional to the
frequency of the impulse generator. Thus, a rhythmic change in the
frequency of the impulse generator produces a rhythmic phase
change.
In the operating circuit of FIG. 14, the phase shifting is effected
by a transductor or magnetic amplifier arrangement. Thus, the sine
wave signal coming from oscillator 1f is transmitted to a bridge
circuit consisting of three resistors 75 and the transductor or
magnetic amplifier means 73, 74. A second winding 76, 76 controlled
by function generator 33d pre-magnetizes the inductors 73 and 74
differently so that the inductance of the inductors is rhythmically
changed by generator 33d. This produces a phase-shifted output
signal which is transmitted to amplifier 35' for electrodes 8, 9
while the original signal is transmitted directly from oscillator
1f to amplifier 35 for electrodes 6, 7. The vector diagram of this
circuit is similar to that of FIG. 5.
Finally, FIG. 15 shows a purely electromechanical phase shifting
means. In this embodiment, the phase of the output signal from
oscillator 1g to electrodes 8, 9 is shifted by an arrangement
equivalent to a three-phase motor, the stator having three windings
77, 78, 79 which receive the output signal from sine wave generator
1g, the third phase winding 78 receiving the signal from the
generator via condenser 81. A fourth winding 80 is rotatably
mounted in the rotor space and produces a phase-shifted output
signal which is transmitted to amplifier 35' of electrodes 8, 9.
The phase depends on the angular position of coil 80 and this may
be rhythmically changed by motor 33e driving the coil. Of course,
the coil may also be rotated by an electronic 3-phase sine wave
generator, the principle of operation being the provision of a
rotary coil within the field provided by surrounding three
surrounding coils. Thus, the same voltage is induced in the fourth,
rotary coil 80 in each angular position thereof. Only the phase of
the voltage is changed in dependence on this angular position.
It will thus be appreciated that a variety of phase shifting means
may be devised by those skilled in the art and, depending thereon,
the oscillator providing alternating current to the pairs of
electrodes may generate rectangular or sine waves. What is
essential is that the phase of the current receiving from the
osillator by one pair of electrodes is shifted in respect to that
of the other pair of electrodes.
While the invention is particularly useful in electrotherapy, it
may be applied whenever it is desired to produce a beat or
heterodyne frequency. For instance, the apparatus may be used to
heat or melt metallic work pieces at selected locations,
particularly in their interior. It may also be useful in signal
transmissions, in which case the stable and phase-modulated
oscillations may be transmitted over two independent transmission
paths and then brought into interference in the receiver. In this
manner, the modulation value is available in the receiver as
amplitudemodulated value so that the modulation value may be
reconstituted by simple demodulation and disturbances in the
transmission path may be eliminated at the receiver by limiting the
amplitude.
* * * * *