Note: Descriptions are shown in the official language in which they were submitted.
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TITLE: ELECTRIC FIELD SENSOR
FIELD OF THE INVENTION
This invention relates to electric field sensors in
the medical field for the detection of alternating~electrical
fields originating from within the body to produce electro
cardiograms (ECGs) and electro-encephalograms (EEGs) and the
like, as well as heart rate monitoring. It also relates to
other applications for sensing external electric fields.
BACKGROUND TO THE INVENTION
The detection of electrical potentials occurring on
the human body is the basis for ECG/EEG diagnostic procedures
used to assess heart conditions and brain functions (hereafter
"ECG"). An extensive science has been established on the
basis of coupling conductive electrodes to the human body to
sense the low-level electrical signals that the body is able
to generate.
A feature of this technology in the past has been
to focus on reducing electrical resistance at the
skin/electrode interface. For this purpose ECG electrodes are
often used in conjunction with conductive gels and suction cup
attachment mechanisms. These arrangements are uncomfortable
for the user, restrict mobility, and have limited useful life.
Dry Electrodes - Prior Art Approach
Investigations have been made into using capacitive
pickups to detect electrostatic potentials on the skin of a
patient. Examples in the literature include the text
"Introduction to Bio-Electrodes" by Clifford D. Ferris,
published by Plenum Press in 1974. In this text the author
discusses experiments with insulated, capacitive electrodes
based upon the configuration (page 184):
"Body surface (skin)/Dielectric/metal/FET".
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A shielded single electrode and a two-electrode
circuit based on a capacitive electrode are depicted on page
185. Electrode capacitance is reported as 14 uF/cmz at page
187.
The text "Electrodes and Measurement of Bio-Electric
Events" by L.A. Geddes, published in 1972 by Wiley-
Interscience discusses "dry electrodes" at pages 98 - 103. A
single electrode circuit based on a insulated anodized
electrode and FET transistor is depicted at page 100. A value
for electrode capacitance is reported at page 102 as being
3200 picoFarads. Capacitance ranges of 5000-20000
picoFarads/cm2 are referenced at page 102. In particular,
this reference reports (page 102):
"At present there are attempts to provide ultra
thin films of insulating materials having high
dielectric constants and strengths so that a high
electrode-to-subject capacitance will be
attained...".
This statement recites that obtaining a high level of
capacitive coupling is an objective and necessarily presumes
that such electrodes will be placed in intimate contact with
the body of the subject being measured.
In the text "Principles of Applied Biomedical
Instrumentation" 2nd edition, L.A. Geddes, L.E. Bater
published by Wiley Interscience, 1975, the author observes (at
page 217):
"To obtain an electrode-subject impedance that is
as low as possible, every effort is made to obtain
a high capacitance by using a very thin dielectric
having a high dielectric constant."
Capacitance values from 5,000 pF/cm2 to 20,000 pF/cm2 are
cited.
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A Technical Note entitled "New Technologies for In-
Flight, Pasteless Bioelectrodes" by D. Prutchiand A.M. Sagi-
Dolev, published in Aviation, Space and Environmental
Medicine, June 1993 (page 552) describes a capacitive, dry
bioelectrode for obtaining EEG and ECG signals obtained
through a plate anodized with aluminum oxide. Coating
thicknesses of 50um and 170 um are referenced. Allowing for
a dielectric value of 10 (for aluminum oxide) this thickness
would provide an electrode with the ability to develop a
capacitance of about 50 pF/cm2 to 180 pF/cm2, if intimately
presented to a conducting surface.
Accordingly, the prior art has addressed the problem
of capacitive dry electrodes in terms of developing high
capacitive values for insulated electrodes placed in intimate
contact with the surface being monitored. These prior
investigative efforts have been focused on maximizing the
coupling between the electrode and the skin surface carrying
the potential to be detected. This has led to electrodes that
employ thin dielectric surfaces that are capable of providing
capacitive values from about 50-1000 picoFarads/cm2 and
higher. It is a necessary adjunct to establishing high
capacitive coupling to a body that the electrodes be pressed
intimately against the surface being sensed, and that the
surface be smooth and free of defects.
True Effective Capacitance
It is believed that all of the capacitive values
cited in the prior art references are based on the premise
that cited capacitance values are for the maximum capacitance
that an insulated electrode can develop when pressed against
a conductive surface.
A capacitive pickup electrode for an ECG system may be
designed to have a capacitive value of several hundred
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picoFarads per square centimeters when its insulated plate
surface is laid over a smooth, highly conductive counter-
electrode surface, such as a sheet of copper. This is the
condition for maximum capacitance. However, when placed
proximate to the human skin, the dead layer of the skin acts
effectively as an insulating spacer, removing the plate of the
pickup electrode further from the source of the electric field
being sensed. In such a configuration, the effective value of
the capacitive coupling between a typical, high capacitance
pickup electrode e.g. 100+pF/cmz and the field source within
the human body may be on the order of 1-100 picoFarads/cm2
depending on the intimacy of contact with the body and the
presence of sweat or hair on the skin. The prior art has
endeavoured to maximize this capacitance value.
Difficulties of Intimate Coupling
The results of prior art endeavours have been only
moderately successful. One problem that has arisen is the
extensive sensitivity of these capacitive electrodes of prior
design to variations in the gap or intimacy of contact between
the electrode and the skin. When intimate contact is the
objective, even the presence of hair or sweat can cause
variations in the value of capacitive coupling being
established. The procedure of pressing dry electrodes against
the body has presented similar inconveniences to those arising
in the use of conductive electrodes, e.g., discomfort and
limited mobility due to intimate contact protocols. In
particular, prior art systems have never been reported as
operating through clothing fabric. No proposal has been made
to obtain alternating electrical signals of the ECG, EEG type,
etc. through use of dry capacitive electrodes that are not
positioned at fixed locations on the skin surface of a
subject.
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Further difficulties associated with the use of dry
electrodes pressed into intimate contact with the skin of a
person are tribo-electric effects - electrical charges created
by sliding friction and pressure. Tribo-electric effects
deliver large, essentially static charges, to the pickup
electrode.
Such charges impose a near DC or very low frequency
drift in the background level over which the more relevant,
higher frequency signals are imposed. To discharge the
amplifier input and pickup electrode of such capacitively
acquired charge, the input resistive impedance of the high
impedance first stage amplifier should be carefully selected.
Thus a particular concern when sensing alternating
signals is the band-pass capabilities of the sensing system.
Ideally, the pickup electrode should drive an amplifier with
a complementary input impedance which, in the case of ECGs is
able to process low level, e.g. milli-volt, signals in the
range 0.05 HZ to 150 HZ. The lower cut-off frequency should
be stable in order to restore the bias value of the driven
amplifiers to its normal value in cases where the circuit is
over-driven by a very low frequency or DC offset signal.
To minimize the disruptions caused by very low
frequency or DC over-driven off-sets, the capacitive coupling
to the body (C) should be matched to the input impedance of
the amplifier sensor (R) via a preferred, tuned RC-relation.
This allows the sensor to have a stable band pass. U.S.
patent 3,744,482 addresses this issue with a tuned feed-back
loop. However, for the tuning of the sensor input to be
consistent, both the resistive -R and capacitive -C values
should be stable.
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Variance in Capacitance
A pickup electrode may be of such a design as to
permit it to achieve high value capacitive coupling, as for
example maximum values of 50-100+ picoFarads/cm2 when placed
on a conductive plate. This can be effected through use of
thin or high dielectric value insulative layers. A difficulty
arises, however, in ensuring that the frequency cut-off of the
RC network at the input stage is appropriately tuned when the
pickup electrode is capable of high capacitance coupling.
This difficulty arises from the fact that a pickup electrode
with potentially high capacitance will exhibit varying actual
capacitive coupling values when placed adjacent to the body
generating the electric field, particularly when an attempt is
made to place such an electrode in intimate contact with the
skin of the human body being sensed.
By example, the actual capacitive coupling value may
range over several hundred percent if the electrode is pressed
very tightly against skin wetted with body sweat. In this
situation, since capacitance varies inversely with the gap
separating with the capacitor electrodes, the system is
operating in the separation-sensitive region of a graphic plot
of Capacitance vs Separation Distance (cf Figure 5).
When the effective capacitance of the pickup
electrode varies substantially, the cut-off value of the RC
filter arrangement will vary correspondingly. This will
reduce the performance of the RC combination as a well-tuned,
high-pass, low frequency cut-off filter. Settling times for
low frequency signal artifacts will be lengthened as the
capacitive value of C is doubled or tripled.
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Background Noise Refection
A major source of noise for electronic systems is
ambient 60 Hz signals (in North America) arising from the
power system. It is known that sixty hertz background noise
can be eliminated or greatly reduced through the use of a
differential amplifier arrangement. However, for maximum
rejection of common mode noise to be achieved, the inputs to
both branches of the differential amplifier should be fully
balanced. If the inputs are not balanced improper signal
differencing will occur and the output will be disturbed by
the imbalance. In the case of ECG systems, balance would
ideally be achieved by having two separate ECG pickup
electrodes couple to the source body originating the
electrical field with the same degree of capacitive coupling.
Where intimate-contact, high capacitance electrodes
are employed, this balancing is hard to maintain. A need
exists for a more stable system to be employed for these types
of applications. The invention herein addresses this need.
In summary, a need exists in the medical field to
provide an electrical field sensor for detecting alternating
signals that is less demanding in terms of electrode/body
coupling. In non-medical fields, useful applications may also
arise where the measurement of an oscillating surface charge
is to be effected without contact arising between the charged
surface and the electrical sensor. The invention herein
addresses such needs.
The invention in its general form will first be
described, and then its implementation in terms of specific
embodiments will be detailed with reference to the drawings
following hereafter. These embodiments are intended to
demonstrate the principle of the invention, and the manner of
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its implementation. The invention in its broadest and more
specific forms will then be further described, and defined, in
each of the individual claims which conclude this
Specification.
SUMMARY OF THE INVENTION
According to one aspect of the invention the signal
pickup procedure for obtaining an electric field or ECG signal
and the like is carried-out under a configuration wherein the
effective capacitance coupling the electrical field source to
a high impedance sensing amplifier is relatively insensitive
to variations in the separation between the body that serves
as a field source and the pickup electrode. Small
displacements of the pick-up electrode lead to little change
in the degree of capacitance coupling between the electrical
field source and the sensing amplifier.
According to the invention in one aspect, an
electric field sensor is provided that includes a first pick-
up electrode for placement next to a surface whose electrical
field is to be sensed through capacitive coupling. This pick-
up electrode is not operated, as in the past, to achieve high
capacitive coupling values for such electrodes, i.e. operating
in the separation-sensitive region of a Capacitance vs.
Separation Distance graph (as per Figure 5). Rather, by the
arrangements of the present invention, the value of the
capacitive couplings between the source field and the sensing
amplifier is kept small i.e. under 40 picoFarads/cm2,
preferably 20 picoFarads/cm2, more preferably, 1-10
picoFarads/cm2. This may be achieved by avoiding intimate
contact with the body e.g. by positioning the plate of the
pickup electrode at a "stand-off" location that reduces the
sensitivity of the measured output to motion effects i.e.
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variations in the separation of the pick-up electrode from the
surface of the body being sensed. And it may be achieved by
placing a limiting capacitor in series with the input to the
sensing amplifier.
To ensure the "stand-off" effect of the invention
first arrangement is achieved, an insulating layer may be
provided over the electrode to separate it from a body by a
gap that ensures that capacitive coupling does not vary
sensitivity with separation. In some cases, useful signals can
be obtained by placing sensors of the invention over
protective layers already present on the body.
The objective in designing the sensor. in accordance
with this criterion is to ensure that the overall, effective
capacitance formed between the pick-up electrode and any
surface that may be presented to the outer face of the pick-up
electrode will always have a value in the region of a plot of
capacitance value versus separation distance wherein, upon
displacement of the electrode by a standard amount, the
capacitance is varied by a limited percentage value.
Equivalently, changes in the surface condition of
the field-emitting object, e.g. the appearance of sweat on
skin, does not significantly change the degree of capacitive
coupling that is present when the sensor is operating under
the conditions of the invention.
In particular, and preferably, when the separation
of the electrode from the surface varies by 0.1 mm or less,
the capacitance value of the coupling between the body and the
pick-up electrode varies by less than 500. More preferably
the capacitive value varies by less than 20%.
By providing an ECG pickup with an insulative layer
that precludes the pickup electrode from achieving capacitance
values of higher than a specific value, e.g. 40, 20 or less,
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preferably 10 picoFarads/cm2, an ECG system so equipped will
be inherently suited for operation in the preferred, second,
separation-insensitive region (as per Figure 5?. The presence
of such a capacitance-limiting insulative layer will preclude
an electrode from operating in the first, separation sensitive
zone.
It is preferable for the insulating layer to have a
thickness which is equal to, or greater than, the size of
surface irregularities of the body being measured, and equal
to or greater than the variations in the sensor-to-body
separation gap.
This is completely counter-intuitive to the
methodologies applied by the prior art experiments with
capacitive, "dry" electrodes which employ extremely thin
dielectric layers and then proceed to place the sensor in
intimate contact with the surface of the body being sensed.
Thus, the present invention, in one aspect, employs
a dielectric layer for the pick-up electrode that ensures that
sensing is occurring at a stand-off location which is
insensitive to minor motion and/or surface irregularities as
well as temporal changes in surface characteeristics.
The instability arising from the variations in the
coupling capacitance of the pickup electrode can be addressed
in a further manner, namely by inserting into the input of the
high impedance sensing amplifier that receives signals from
the pickup electrode a series capacitor of fixed and limited
value. This limiting capacitor should preferably have a
minimum value that is greater than the input capacitance of
the amplifier stage that is driven by the signal received from
the body through both the pickup electrode and the limiting
capacitor. As a preferred upper limit, the limiting capacitor
may have a value that is less than the effective coupling
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capacitance between the pickup electrode and the body. Values
for this limiting capacitor outside this preferred range may
also be adopted. The inclusion of such a series capacitor has
the same effect in constraining variations in the effective,
overall capacitance value of the coupling between the
electrical field source and the input amplifier as the "stand-
off" variant of the invention referenced above.
When this alternate procedure for rendering the
input amplifier relatively insensitive to the electrode/body
separation distance, e.g. placing a limiting capacitor in
series at the input to the first stage amplifier, is employed,
use of a series limiting capacitor of appropriate value, e.9.
40 picoFarads, will set an upper limit on the capacitance
coupling between the field source and the input amplifier. As
the pickup electrode is in series with the limiting capacitor,
the combined capacitance of the two cannot exceed the value of
the limiting capacitance. Because the value of the limiting
capacitor is fixed, the RC value for the high pass filter at
the input stage is stabilized. Even if the pickup electrode
has a relatively high maximum possible capacitance, e.g. over
1000 picoFarads, because it is in series with the limiting
capacitor, it cannot absorb a substantial static charge.
Viewed alternately, if the pickup electrode were to achieve in
fact, a very high level of capacitance coupling to the body,
at a value greatly exceeding the capacitance value of the
limiting capacitor e.g. 10:1, then we may treat it as having
a minimal, or transparent impedance contribution to the
combined series capacitance of the amplifier's input. This
will still leave the limiting capacitor, e.g. 40 pf as
dominating the capacitive coupling between the field source
and the input amplifier.
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Voltage Divider Network
In detecting electric field signals through the
capacitive pickup arrangement of the invention, the signal
being sensed by the input amplifier is essentially being taken
from across a voltage divider network defined by the pickup
electrode, the limiting capacitor (if present), the input
capacitance of the amplifier and the remaining electrical
coupling (either resistive or capacitive or both) at the other
end of the voltage divider network which is connected to the
body which is the source of the electric field. Assuming this
last connection is of relatively low impedance, the signal
strength seen at the input to the amplifier depends on the
ratio of the input capacitance of the amplifier to the other
capacitors in the series chain. If the input capacitance of
the amplifier is small, then most of the signal strength will
appear across this capacitance, and be sensed by the
amplifier.
In actual use, the effective capacitive value of the
pickup electrode may be on the order of the value of the
limiting capacitor. In this case, its impedance contribution
will become significant. For example, the pickup electrode
effective coupling capacitance being equal in value to that of
the limiting capacitance --e.g. 40 picoFarads-- then the
combined, net capacitance of these two elements in series
would drop to half of their individual capacitance values e.g.
20 picoFarads. This will not, however, have a serious
deleterious effect on the signal detection performance of the
overall system so long as the input capacitance to the high
impedance amplifier is small e.g. 2-5 picoFarads.
Differential Amplifier/Dual Inputs
As is done in the case of conductive electrode ECG
systems, two pick-up sensors may be applied at two distinct
locations on the skin. By taking the difference in the output
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signals from two locations on the body the benefits of common
mode noise rejection may be obtained. The objective of
minimizing variations in such capacitance values is also
important for this special case arrangement in ECG-measuring
systems: the use of dual input differential amplifiers to
obtain rejection of common mode noise.
Differential amplifiers used to reject common mode
noise, e.g. ambient 60 Hz, fail to achieve full rejection when
the bias levels of the amplifiers are imbalanced or if the
amplifiers have unequal RC characteristics. To maximize the
prospects that these levels and characteristics are balanced,
both branches of the pickup elements should have similar
settling times when disrupted by an off-setting, very low
frequency signal. This requires that the effective
capacitance of the couplings within both branches between the
sensed body and amplifier inputs be similar. The invention
addresses means for achieving this last criterion.
Clothin~pported Arrays
On the foregoing basis, this invention provides a
means for detecting electrical fields present on the surface
of a body without the use of conductive gels and suction-based
appliances. Useful signals may be obtained based on the
combination of multiple electrodes assembled in a fixed,
preformated array. Thus, multiple electrodes, e.g. 4 or more,
may be carried by a clothing-type of support as an array that
can be readily donned or removed with minimal inconvenience.
This provides considerable freedom for the tele-monitoring of
patients while they engage in daily routines. Freedom from
the limitations of conventional tele-monitoring arrangements
represents a valuable advance in this field.
The foregoing summarizes the principal features of
the invention and some of its optional aspects. The invention
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may be further understood by the description of the preferred
embodiments, in conjunction with the drawings, which now
follow.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1A is a combined pictorial/electrical
schematic depiction of a single pick-up of the invention in
position adjacent to a body whose electrical field is to be
sensed. The voltage divider network is capacitively coupled
to the body at both ends and drives an operational amplifier.
Figure 1B is a conventional electrical schematic
corresponding to the input portion driving the amplifier of
Figure 1A.
Figures 1C and 1D are the schematics of Figure 1A
and 1B with the added presence of a series capacitor in the
amplifier input.
Figure 2A is Figure 1A with the substitution of a
resistive, conductive coupling to the body at one end of the
voltage divider network. A smaller parallel capacitive
coupling remains present as well.
Figure 2B is a conventional electrical schematic
corresponding to Figure 2A.
Figure 3 is an electrical schematic for a dual pick-
up electrode configuration, based on the pick-up of Figure 1A,
with signals being fed to a differential amplifier, but with
dual, parallel Schotkey diodes as input leakage resistors.
Figure 4 is an expanded electrical schematic of the
circuit of Figure 3 with the additional presence of an
amplifier and optical coupler to provide electrical isolation.
Figure 5 is a graph showing the change of
capacitance of pick-up electrodes with various surface areas
as a function of separation distance for the electrodes.
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Figure 6 is a graph showing the percentage change in
capacitance for a 0.1 mm change in electrode-to-body gap
distance as a function of nominal electrode-to-body gap
distance over a range of 0.0 to 1.0 mm, assuming the body acts
as a perfect electrode.
Figure 7 is a plan view of an electrical circuit
corresponding to Figure 4 laid-out in a belt to be worn over
the chest of a patient.
Figure 8 is a pictorial depiction of the belt of
Figure 7 in place over the chest of a patient.
Figure 9 is a pictorial version of a garment worn by
a patient that carries four pick-up electrodes.
Figure 10 is a graph of total effective coupling
capacitance between the sensed body and the input to the
amplifier of the sensor, plotted as a function of the
separation distance of the electrode from the surface being
sensed. Three curves are shown, two with a limiting series
capacitor present and one with no limiting capacitor present.
Figure 11 is similar to figure 10 but with the
vertical scale for the input capacitance increased by a factor
of ten and showing one curve with and one curve without a
limiting capacitor present.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In Figure 1A a pictorial schematic is shown of an
electrical sensor system incorporating a pick-up electrode 1
in the form of a flat conductive surface placed adjacent a
first location 2 on a body 3 where an electrical signal is to
be sensed originating from an electrical signal generator 4
within the body 3 that provides a source voltage VS. The
pick-up electrode 1 develops a capacitive coupling to the body
3 through an intervening dielectric layer separating it from
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the body 3. This capacitive coupling for the pick-up
electrode 1 is represented schematically by the capacitor Cp.
The electrode 1 is connected to the input of an
operational amplifier - IC1A, or its equivalent such as a
field effect transistor. Input resistance RI connected
between the amplifier input and circuit ground has a
resistance value of on the order of 1012 ohms and serves to
discharge the input of DC offsets and restore proper voltage
input levels while accepting signals of the desired frequency.
The output Vo from the voltage divider network which
drives the operational amplifier IC1A, shown in Figure 1B, is
measured across input resistor RI that extends between the
input of the operational amplifier IC1A through circuit ground
to a reference capacitor CR that is coupled to the body 3 at
a second, separate location 5. This location 5 may be
separated from the first location 2 in obtaining conventional
ECG signals. The locations 2,5 may also be proximate, e.g.
adjacent, at certain body locations and still provide useful
signals.
Capacitive coupling through reference capacitor CR
is effected by means of an electrode (not shown in Figure 1A)
that is separated from the body 3 by a non-conducting material
that acts as a dielectric. Conveniently, the case for an on
board battery holder can serve as this electrode, as shown
further below.
The "standoff" or low-capacitance feature of the
invention enables signal pickup without skin shaving and over
some clothing layers. As a natural outcome of standoff
operation, electrodes of the invention are less sensitive to
the electrode dielectric characteristics than those of the
prior art. Satisfactory values of electrode dielectric
constants have been found in the range 1 to 10 and the signal
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characteristics on unprepared skin, hairy skin, and over
clothing are essentially unchanged over this range. Flexible
and compressible materials with advantageous mechanical
properties can now be used.
These advantages arise because the nominal electrode
capacitance in "standoff" operation is less than the typical
"parasitic" capacitance which is created between the body and
the electrode (see section "True Effective Capacitance"). The
parasitic capacitance is an unavoidable consequence of hair,
air, dead skin layers, skin inhomogeneities, and clothing
fabrics. The standoff dielectric restricts the electrode
capacitance to values which are smaller, and thus dominant,
over the parasitic capacitance. This is essentially the
reverse of the condition found in prior art electrodes where
parasitic effects could dominate the coupling on hairy skin
and where over-clothing pickup was not feasible for similar
reasons.
This aspect of the invention enables the use of
electrode materials not suitable for prior art capacitive
electrodes and possessing highly desirable mechanical
properties. Materials such as rubbers, plastics, foams, and
fabrics can be used as electrode substrates in order to
provide flexibility, elasticity, softness and conformability
to the body. These features provide advantages of user
comfort and mechanical stability of the electrode when placed
against the body. Furthermore a wide range of materials can
be used for the internal construction of electrodes providing
flexibility, compressibility etc to the whole electrode
structure. This is in contrast to prior art which required
stiff constructions to provide mechanical support for brittle,
fragile, or moderately flexible, thin substrates possessing
carefully contrived dielectric and mechanical properties.
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Inside the body 3, the signal generator 4 is seen as
being subject to internal resistance RH within the body 3.
The input portion of circuit of Figure 1A is redrawn
as Figure 1B in more conventional form. In Figure 1B, the
capacitance Co arises from the combined input capacitance of
the operational amplifier IC1A and the input resistor RI. The
total apparent input resistance of this amplifier is
represented by Ro, including the resistive value of the input
resistor RI. Collectively, the capacitances Cp, Co, CR act as
a voltage divider network whereby the output voltage Vo is
proportional to the source voltage VS.
In Figures 2A and 2B, the coupling to the body 3 at
the end of the voltage divider network opposite to the pick-up
electrode 1, is effected principally by a direct, conductive
contact. The resistance of the interface is indicated by RR.
Necessarily, some slight capacitance coupling is also still
present, indicated by C'R.
The output signal of the sensor is extracted by
measuring the voltage difference across an electrical
component in the voltage divider network that is connected to
the subject electrical source. This should be done through a
high impedance, low capacitance sensing circuit or sensing
means to minimize signal loss. A field effect transistor or
operational amplifier having an input impedance of on the
order of 1012 ohms and an input capacitance of about 3
picoFarads has been found to be satisfactory when the other
capacitors) in the voltage divider network have values of on
the order of 10 picoFarads. Used in conjunction with a pick-
up electrode having an area of on the order of one to ten
square centimetres, dielectric media having a total effective
dielectric constant of 1-10 and a body-to-surface gap distance
of on the order of 0.1 to 4 millimetres, signal values of the
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order of 1 millivolt or less may be detected from the skin
surface of the human body.
With this type of sensor configuration useful
signals may be obtained with the plate of the pick-up
electrode separated from the skin or sensed body by a gap that
allows the pick-up to qualify as a "stand-off" electrode. As
the gap varies, the strength of the output signal will vary.
But by operating the sensor in the capacitance/gap separation
region specified by the criterion of the invention, such
variations will not detract inordinately from the value of the
signals being obtained.
A pickup electrode that is removed (i.e. placed at
a distance) somewhat from the electrical field source is able
to supply a satisfactory signal by reason of the mathematical
relationship that exists between the value of capacitance and
the separation distance existing between capacitor plates or
electrodes, cf Figure 5. Since capacitance varies inversely
with separation, the mathematical form of a curve for
capacitance value plotted against separation distance is in
the shape of a hyperbola. This means that the capacitance
performance of a pickup electrode can operate in two distinct
regions:
1) a first region wherein the separation distance is
small and the curve is steep, corresponding to the
situation where the capacitance value will vary
highly, with great sensitivity, in response to
small changes in the separation distance; and
2) a second region wherein the separation is greater,
the curve is relatively flat, and the capacitance
value varies relatively insensitively with similar
changes in the separation distance.
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For the purposes of the present invention, the preferred
region of operation according to one variant of the invention
is in the second, separation-insensitive zone.
In Figure 5 a graphic plot is depicted of the
variation of capacitance C with a variation in the separation
distance d at various separation distances d, based upon the
theoretical formula: C =k.A/d
where: C is the effective capacitance of, for example Cp,
d is the separation distance of the electrode plate
from the body giving rise to the capacitance,
A is the area, or effective area, of the pick-up
electrode 1; and
k is a proportionality constant affected by the
dielectric material in the separation gap.
In Figure 5 the value of the dielectric constant is assumed to
be that of air, i.e. 1.0 and the plates forming the
capacitance are assumed to be fully conductive. This is
therefore an idealized variant on the case of coupling to the
human body.
Four curves are shown in Figure 5 for pick-up
electrodes 1 having surface areas as follows:
a = 1 cm2 c = 5 0 cmz
b = 10 cm2 d = 10 0 cm2
Each capacitance curve can be separated into two
important regions: region 6, in which the capacitance changes
relatively rapidly with a given change in separation distance;
and region 8 of the invention in which the capacitance changes
relatively slowly with a similar given change in separation
distance. These regions are generally separated on Figure 5
by boundary line 7.
For a capacitor with an electrode area of 1 cm2, the
line 7 passes approximately through a capacitive value of
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about 40 picoFarads. For capacitors with an electrode area of
around 25 cm2 the line 7 passes approximately through a
capacitive value (of about 200 picofarads. For capacitive
values above 200 picoFarads, region 6 approximately
corresponds to the zone with d = 0.1 mm or less; while for
capacitive values below 200 picofarads region 8 approximately
corresponds to the values above d = 0.1 mm.
An important implication of Figure 5 is that sensors
with capacitance values within region 6 are very sensitive to
small additional changes in the separation distance (delta-d).
In contrast, sensors with capacitance values corresponding to
region 8 are relatively insensitive to such changes. This is
illustrated more succinctly in Figure 6.
In Figure 6, the percentage change in capacitance
corresponding to a delta-d = 0.1 mm is graphed as a function
of the nominal separation distance d.
Figure 6 is dimensionless along the Y axis and
applies to all capacitive sensors which obey or approximately
obey the relation C=kA/d. According to the invention the
capacitive value of the pick-up electrode, and other
capacitive sensors when employed, are designed to operate in
region 8' of Figure 6, as opposed to region 6' from which it
is separated by boundary line 7'.
In the region 8' this latter regime the
capacitance, and hence the output signal is sufficiently
insensitive to spatial and temporal body surface variations so
as to provide the advantages of signal stability inherent in
the invention.
Figures 5 and 6 premise that operation in regions 8
and 8' can be effected by achieving low capacitance coupling
between the body and the pickup electrode. Figures 10 and 11
apply to an alternate case wherein the capacitive coupling
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between the pickup electrode and the body is high, but the
results of achieving system operation in preferred regions
8,8' is still obtained. This is achieved by insertion of a
series limiting capacitor CL in the input to the first stage
amplifier of the sensor.
This series limiting capacitor may have a preferred
value that is greater than the input capacitance of the first
stage amplifier, and less than the effective value of the
capacitance coupling between the pickup electrode and the body
whose electrical field is being sensed, e.g. between 5 and 40
picoFarads.
In Figures 1A and 1B the pickup capacitor Cp is
shown as being directly coupled to the operational amplifier
1C1A. In Figures 1C and 1D a series capacitor CL is shown
added between the pickup capacitor Cp and the amplifier input
(at which Vo is detected). The effect of this limiting
capacitor CL is to place a maximum value on the capacitance
extending between the body 3 and the signal sensing means
1C1A. The pickup electrode's capacitance CP is in series with
the limiting capacitor CL. Collectively, they behave as a
single capacitor having a total net value CT = 1/ (1/CL + 1/CP) .
Figures 10 and 11 plot the behaviour of CT as a
function of the separation distance present for the pickup
capacitor CP.
This net value capacitor CT provides a more stable,
separation-insensitive circuit performance that occurs in its
absence. This is particularly true when CL is smaller than CP.
A convenient formula for establishing a value for CL
is that CL should be less than 5 (picoFarads/cmz) times the
area of the pickup electrode (in cm2).
The consequence is that a similar regions 8c,8d of
insensitivity to displacement of the pickup electrode exists
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in Figures 10 and 11, parallelling regions 8 and 8' in Figures
and 6 and delimited by line 7c,7d. A similar preferred
criterion for performance of the invention can also be
established for the circuit arrangement of Figure 1C, 1D,
5 namely, a 0.1 mm displacement of the pickup electrode causes
a 50% or small change in the net capacitance Cn. Preferably
the change is less than 20% more preferably, less than 10%.
Thus, the same effect of desensitizing the signal
pickup and coupling capacitance from DC offsets can be
achieved through the presence of a limiting capacitor CL in
the input link between the pickup capacitor Cp and the signal
sensing means 1C1A.
Over-Driven Amplifier
For the present invention, the input resistance
present at the input to the high impedance amplifier can be
provided from two sources:
1) the inherent input resistance of the amplifier,
typically 1013-1014 ohms;
2) the input resistance of an added, external, input
resistor, RI.
A preferred value for this resistance RI may be determined by
considering the pickup electrode and input resistance as an RC
high frequency passing filter.
Assuming an effective pickup electrode capacitive
value of 60 picoFarads and a low frequency cut-off of 0.05 Hz
established by the RC input value of the first stage
amplifier, a preferred value of 4 x lOlz ohms may be provided
for the input stage input resistance RI.
Occasionally, the near-DC signals delivered to the
pickup electrode will be so substantial as to drive the signal
at the input amplifier to the limit of its range of response.
When overdriven; the recovery period (before a normal input
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level can be re-established by the input resistor) is
increased. To shorten the recovery period in such cases it is
convenient to provide the input stage with a non-linear input
resistance. This can be achieved by grounding the input
through pairs of Schotkey diodes, D1, DZ in Figure 3, connected
in parallel.
The forward resistance of Schotkey diodes before
breakdown occurs can be on the order of 1013 ohms . By choosing
diodes with a forward breakdown voltage that is above the
level of the signal of interest, the "reset" function of the
input resistance of the high impedance amplifier can be
improved. If the breakdown voltage of the Schotkey diode is
chosen to be at the voltage level for saturation of the input
amplifier, then the "shorting" effect occurring after
breakdown will not distort the signal of interest as long as
the amplifier is operating within or inside its saturation
cut-off limits.
As the forward resistance of the Schotkey diodes
prior to breakdown may be higher than the appropriate value to
provide an input resistance suited to the given low frequency
cut-off for the RC filter, such diodes D1, D2 may have to be
accompanied by a parallel input resistor RI that establishes
the appropriate net value for input resistance for small level
signals.
In Figure 3 two pick-ups similar to that of
Figure 1A (except for the substitution of diodes D1, Dz for
the input resistor RI) are used to drive a differential
amplifier IC3A through input operational amplifiers IC1A and
IC2A. The second additional pickup electrode 1A is placed at
a location 10, separated from the first and second locations
2 and 5. Within the body the signal source VS may be treated
as distributing its potential over the resistors RB/R'B/RB.
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By use of this differential signal detection
circuit, common mode noise present in the two pick-up circuits
will be minimized.
Figure 4 shows the circuit of Figure 3 extended by
an optical isolator ISO1 driven by an operational amplifier
IC4A which is, in turn, driven by the output from the
differential amplifier 1C3A. By mounting these circuits as
close as possible to the pick-up electrode 1 1A, interference
from ambient 60 Hz electromagnetic signals can be minimized.
In Figure 4, a shielding conductive layer 11 is
depicted as overlying the externally-directed side of the
circuitry. This layer/structure 11 is preferably connected to
the circuit common point but need not necessarily be so
connected. In some configurations this shield may be
"floating". Its role is to exclude effects arising from
intruding electro-magnetic signals, e.g. 60Hz, originating in
the environment. In non-earthed applications the shield
distributes ambient, intruding signals equally to both
pickups, contributing to common mode noise rejection. It is
highly desirable that a shield of some type be employed in one
or other of such configurations.
In Figure 7 a belt 12 is depicted that carries the
circuit of Figure 4. The hatched areas are decorative. The
pick-up electrodes 1, 1A are mounted on a substrate 13
comprising a MYLAR~T"" or KAPTON~T"" film that serves both as a
spacer and as an insulating dielectric of approximately 0.13
mm thickness. The pick-up electrodes 1, 1A have been measured
against a copper plate as providing a capacitance value of 20
picoFarads respectively.
The pickup electrodes are completed by the addition
of a "standoff" dielectric which is bonded to the undersurface
of the Mylar or Kapton film directly beneath the electrodes.
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Had the original film been chosen of sufficient thickness to
realize the full benefits of "standoff" operation, the extra
dielectric would be optional.
The belt 12 of Figure 7 has its own on-board power
supply in the form of batteries 14. The case 15 of the
batteries 14 is connected to circuit common point and serves
as an electrode to provide the reference capacitor CR. A
measured value for its capacitance, when placed against a
copper plate, of 160 picoFarads has been observed with the
case 15 coupled to the entire circuit. The substrate 13 for
the belt 12, is made of KAPTON~T"" has a thickness of 5
thousandths of an inch. This forms the principal dielectric
element for CR. The nature of the dielectric material has
little effect on the invention for reasons discussed above.
The shield 11 (not shown but present) in the belt 12
of Figure 7 is in the form of a flexible conductive layer,
with an insulated undersurface that overlies the circuitry on
the outer side portion of the belt 12. This shielding layer
should be close enough to the pickup electrodes 1 to evenly
distribute ambient noise signals, and sufficiently spaced from
the pickup electrode/body interface so as to not detract from
signal pickup by the pickup electrodes.
The pick-up electrodes l, 1A in Figure 4 are held by
the substrate 13 of the belt 12, at a fixed interval. This
interval is dimensioned to permit the electrodes 1 to
respectively overlie electrical nodes (not shown) on the body
3 of a wearer 16 as shown in Figure 8. The belt 12 is held in
place by tension developed by connectors, e.g. hook-and-loop
fastening means, once positioned on the body 3. While a
narrow belt 12 is depicted in Figure 8, a wider belt or vest
15 could carry three, four or more electrodes 1 as shown in
Figure 9.
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An advantage of the invention is that multiple pick-
up electrodes can be assembled in a preformated, fixed array
that can be fitted to the body collectively, as a unitary
assembly, much as in the manner of donning an article of
clothing. This permits a wearer to be "fitted-up" for
electrical field measurement in a very short period of time.
Data acquisition can readily be suspended and resumed by the
simple act of removing and then re-donning the pre-assembled
array. No components are consumed in this process.
The electrodes 1 of such a piece of apparel as shown
in Figure 9 may feed signals to a radio transmitter 19 carried
by the wearer 16. In this manner an especially convenient
form of tele-monitoring can be achieved.
Apart from the option of providing the pickup
electrode with an insulative layer that inherently suits its
operation in the preferred, separation-insensitive zone, the
actual freedom from having to place the pickup electrode in
intimate contact with the body whose field is to be sensed,
has considerable advantages. These include:
1) the pickup electrode need not be tightly fixed at a
specific location on the skin. Small lateral
displacements are permissible. Adhesives are
avoided;
2) the pickup electrode need not be applied under
excessive pressure against the skin. Discomfort is
avoided;
3) the skin need not be prepared to receive the
electrode, as by shaving or rubbing;
4) an insulative layer, such as a pad or layer of
clothing may be present between the electrode and
the skin. This can be useful to increase comfort
and absorb sweat. With the electrode at a removed
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"stand-off", location, the presence of sweat on
the skin does not substantially affect the degree
of capacitive coupling between the body and the
amplifier; and
5) Conformable or compressible electrode substrates
such as foams and fabrics can be used for comfort
and mechanical stability. This is unlike the prior
art which utilized hard dielectric surfaces or thin
films of limited flexibility that required
mechanically stiff constructions and were incapable
of fitting around all body curvatures.
These are substantial conveniences for patients who must
submit to ECG examinations. This is particularly true in
respect to extended-period ECG monitoring procedures.
CONCLUSION
The foregoing has constituted a description of
specific embodiments showing how the invention may be applied
and put into use. These embodiments are only exemplary. The
invention in its broadest, and more specific aspects, is
further described and defined in the claims which now follow.
These claims, and the language used therein, are to
be understood in terms of the variants of the invention which
have been described. They are not to be restricted to such
variants, but are to be read as covering the full scope of the
invention as is implicit within the invention and the
disclosure that has been provided herein.
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