U.S. patent application number 09/887410 was filed with the patent office on 2001-11-29 for physiological sensor array.
This patent application is currently assigned to Nexan Telemed Limited. Invention is credited to Harry, Andrea J., Johnson, Paul, Kumar, Harpal S., Mullarkey, William J., New, William JR., Nicolson, Laurence J., Place, John D., Wilson, Adrian J..
Application Number | 20010047127 09/887410 |
Document ID | / |
Family ID | 23123482 |
Filed Date | 2001-11-29 |
United States Patent
Application |
20010047127 |
Kind Code |
A1 |
New, William JR. ; et
al. |
November 29, 2001 |
Physiological sensor array
Abstract
A physiological sensor device for attachment to a mammalian
subject comprising an output transmitter, at least two
physiological sensors each for sensing one of the subject's
physiological parameters, and a controller operably in
communication with the physiological sensors which controller
communicates a signal comprising data representative of both the
sensed physiological parameters to the output transmitter which
operably transmits the signal to a remote location, wherein the
controller comprises a multiplexer which operably switches the data
from both the physiological sensors into a serial output signal. In
a preferred embodiment, respiration is detected by a bend sensor
including an elongate member and an electrical component mounted
thereon which electrical component has an electrical property which
varies in dependence on the extent of bending of the elongate
member. Other parameters such as temperature and full waveform ECG
may also be measured.
Inventors: |
New, William JR.; (Woodside,
CA) ; Harry, Andrea J.; (Cambridge, GB) ;
Johnson, Paul; (Oxford, GB) ; Kumar, Harpal S.;
(Cambridge, GB) ; Mullarkey, William J.; (Wigan,
GB) ; Nicolson, Laurence J.; (Liverpool, GB) ;
Place, John D.; (Suffolk, GB) ; Wilson, Adrian
J.; (Sheffield, GB) |
Correspondence
Address: |
Woodcock Washburn Kurtz
MacKiewicz & Norris LLP
One Liberty Place, 46th Floor
Philadelphia
PA
19103
US
|
Assignee: |
Nexan Telemed Limited
|
Family ID: |
23123482 |
Appl. No.: |
09/887410 |
Filed: |
June 22, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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09887410 |
Jun 22, 2001 |
|
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09292159 |
Apr 15, 1999 |
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Current U.S.
Class: |
600/300 ;
600/301; 600/508; 600/529; 600/549 |
Current CPC
Class: |
A61B 5/1135 20130101;
H04M 11/002 20130101; A61B 5/0205 20130101; A61B 5/282 20210101;
Y10S 128/903 20130101; A61B 5/0002 20130101 |
Class at
Publication: |
600/300 ;
600/301; 600/549; 600/529; 600/508 |
International
Class: |
A61B 005/00 |
Claims
What we claim is:
1. A physiological sensor device for attachment to a mammalian
subject comprising an output transmitter, at least two
physiological sensors each for sensing one of the subject's
physiological parameters, and a controller operably in
communication with the physiological sensors which controller
communicates a signal comprising data representative of both the
sensed physiological parameters to said output transmitter, which
operably transmits the signal to a remote location, wherein the
controller comprises a multiplexer which operably switches the data
from the physiological sensors into a serial output signal.
2. A physiological sensor device according to claim 1 wherein the
multiplexer is implemented in an application specific integrated
circuit (ASIC).
3. A physiological sensor device according to claim 2 wherein the
output transmitter enables wireless transmission of the signal to
the remote location.
4. A physiological sensor device according to claim 1 wherein the
controller samples an analog signal from the physiological sensors
and converts the sampled analog signal into a digital signal for
presentation to the output transmitter.
5. A physiological sensor device according to claim 1 wherein said
at least two physiological sensors comprises at least two from an
ECG sensor, a respiration sensor, a motion sensor, and a
temperature sensor.
6. A physiological sensor device according to claim 5 wherein said
at least two physiological sensors comprises a first respiration
sensor and a second respiration sensor.
7. A physiological sensor device according to claim 6 wherein at
least one of the first respiration sensor and second respiration
sensor comprises a bend sensor.
8. A physiological sensor device according to claim 1 wherein a
first physiological sensor operably detects ECG data and the
controller operably communicates a signal representative of the ECG
data to the output transmitter at a first sampling frequency, and a
second physiological sensor operably detects at least one of
respiration, motion, and temperature data and operably communicates
a signal representative of said at least one data at a second
sampling frequency to the output transmitter.
9. A physiological sensor device according to claim 8 wherein the
first sampling frequency is greater than the second sampling
frequency.
10. A physiological sensor device according to claim 9 wherein the
first sampling frequency is approximately ten times greater than
the second sampling frequency.
11. A physiological sensor device according to claim 10 wherein the
first sampling frequency is on the order of 250 Hz.
12. A physiological sensor device for attachment to a mammalian
subject comprising a bend sensor which comprises an elongate member
and an electrical component mounted thereon which electrical
component has an electrical property which varies in dependence on
the extent of bending of the elongate member, and an electrical
monitoring device for detecting variation in the electrical
property of the electrical component thereby to determine a
physiological parameter of a subject in use.
13. A physiological sensor device according to claim 12 wherein the
electrical component comprises an elongate resistor superimposed on
the elongate member.
14. A physiological sensor device according to claim 13 wherein the
resistor comprises a track of conductive ink and a series of at
least two areas of highly conductive material over the conductive
ink thereby to effect a series of individual conductive sensors
having a combined resistance less than the track of conductive ink
without the areas of highly conductive material.
15. A physiological sensor device according to claim 14 wherein the
electrically conductive ink forms a substantially U-shaped
track.
16. A physiological sensor device according to claim 12 wherein the
elongate member comprises a flexible substrate.
17. A physiological sensor device according to claim 12 which is
adapted to attach to a human chest.
18. A physiological sensor device according to claim 17 wherein the
bend sensor is adapted for application, at least in part, over the
pectoral muscle of a subject in use.
19. A physiological sensor device according to claim 17 wherein the
bend sensor extends between a precordial position to the axilla of
a human subject when attached to the subject in use.
20. A physiological sensor device comprising two electrode sensors
operably locatable on a patient, a current generator for driving a
current to each of the electrode sensors, and an impedance
measuring device which determines variation in the impedance of the
electrode sensors when attached to the subject in use, wherein a
variation in the motion of the subject is measured as a variation
in the impedance at the electrode sensors caused by such
motion.
21. A physiological sensor device according to claim 20 wherein the
current generator comprises a sine wave generator which operably
alternately drives each of the two electrode sensors.
22. A physiological sensor device according to claim 20 wherein the
impedance measuring device comprises a differential amplifier
having an input from each of the two electrode sensors.
23. A physiological sensor device according to claim 22 wherein a
signal output by the differential amplifier passes through a filter
and demodulator before being AC coupled to a further stage of
amplification.
24. A physiological sensor device according to claim 23 wherein the
current generator generates an alternating current and the
impedance measuring device comprises an anti-aliasing filter.
25. A physiological sensor device according to claim 24 wherein the
two electrode sensors are drive electrodes in a four electrode
sensor arrangement for monitoring subject respiration.
26. A physiological sensor device according to claim 20 comprising
an output transmitter which enables wireless transmission of an
output signal representative of the subject's motion to a remote
location.
27. A portable physiological sensor device comprising a plurality
of electrode sensors for use in measuring electrocardiographic data
and respiratory data of a subject, wherein at least one of the
electrode sensors is used in both the electrocardiographic and
respiratory measurements, and an output transmitter responsive to
outputs of said electrodes sensors so as to enable wireless
transmission of the electrocardiographic data and respiratory data
to a remote location.
28. A physiological sensor device according to claim 27 wherein a
signal from at least one of said plurality of electrode sensors is
sampled periodically at a first sampling frequency by an ECG
measuring device and periodically at a second sampling frequency by
a respiration measuring device.
29. A physiological sensor device according to claim 28 wherein the
first sampling frequency is greater than the second sampling
frequency.
30. A physiological sensor device according to claim 29 wherein the
first sampling frequency is approximately ten times greater than
the second sampling frequency.
31. A physiological sensor device according to claim 30 wherein the
first sampling frequency is on the order of 250 Hz and the second
sampling frequency is on the order of 25 Hz.
32. A portable physiological sensor device attachable to a
mammalian subject in use and comprising two electrode sensors
adapted to assist in monitoring at least one of
electrocardiographic data, and respiratory data of the subject and
further comprising a motion detector which operably monitors the
variation in impedance between the two electrode sensors thereby to
determine the extent of motion of the patient in use.
33. A physiological sensor device according to claim 32 further
comprising a first respiration sensor comprising the two electrode
sensors and two further electrode sensors, wherein one pair of the
two electrode sensors and the two further electrode sensors forms a
pair of drive electrodes to which a drive current is operably
applied, and the other pair of the two electrode sensors and the
further electrode sensors forms a pair of input electrodes to the
respiration sensor.
34. A physiological sensor device according to claim 33 wherein the
first respiration sensor comprises a differential amplifier having
an input from each of the input electrodes.
35. A physiological sensor device according to claim 32 further
comprising a bend sensor which measures respiration.
36. A physiological sensor device according to claim 32 further
comprising a temperature sensor.
37. A physiological sensor device according to claim 32 further
comprising a controller which samples an output from
electrocardiographic electrode sensors at a first sampling
frequency, and another physiological sensor at a second sampling
frequency which is less than the first sampling frequency.
38. A physiological sensor device according to claim 37 wherein the
controller samples an output from at least two of the physiological
sensors, other than the electrocardiographic sensor, at
substantially the same second sampling frequency.
39. A physiological sensor device according to claim 37 wherein the
first sampling frequency is approximately ten times the second
sampling frequency.
40. A physiological sensor device for attachment to a mammalian
subject comprising a sensor for acquiring physiological data about
the subject in use, and an output transmitter which receives a
signal representative of the physiological data from the sensor and
transmits the signal to a remote location, wherein the output
transmitter comprises an inductive element for inductively coupling
the output transmitter to a remote receiver at the remote
location.
41. A physiological sensor device according to claim 40 wherein the
output transmitter comprises a reservoir capacitor in parallel with
the inductive element.
42. A physiological sensor device according to claim 40 wherein the
inductive element has first and second ends each of which ends is
connected via a pair of switches to signal supply lines within the
output transmitter to enable a reversing of the polarity across the
inductive element between the supply lines.
43. A physiological sensor device according to claim 40 wherein the
inductive element comprises a coil which forms part of an H-bridge
circuit in the output transmitter.
44. A physiological sensor device according to claim 40 wherein the
inductive element comprises a rectangular substantially flat
coil.
45. A physiological sensor device for attachment to a mammalian
subject, having one or more physiological sensors, an output
transmitter for transmitting a signal from the one or more
physiological sensors to a remote location, and a memory for
storing a serial number for identifying the physiological sensor
device which serial number is transmittable by the output
transmitter with the signal.
46. A physiological sensor device according to claim 45 wherein the
serial number is randomly generated.
47. A physiological sensor device according to claim 45 comprising
a random number generator and a controller for selecting a randomly
generated number from the random number generator and storing the
selected randomly generated number as the device serial number in
the memory.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a physiological sensor
device or sensor array for attachment to a mammalian subject in
order to obtain data about one or more physiological parameters of
the subject. In particular, the device relates to a physiological
sensor device in the form of a patch attachable to the chest of a
human subject to enable sensing of physiological data such as
electro-cardiographic data and/or respiration data.
[0003] 2. Description of the Prior Art
[0004] The prior art includes U.S. Pat. No. 3,943,918 to Lewis
which discloses an ECG signal sensing and transmitting device for
use in the care of medical patients requiring monitoring of cardiac
functions. The device disclosed is a single use, disposable unit
consisting principally of a foam pad having a pair of circular
electrodes in one face of the rectangular foam block. The block
comprises electrical circuitry which transmits an RF signal to a
receiver which is required to be within 100 feet of the patient.
Subsequent filtering and amplification of the signal takes place at
a monitoring station comprising a receiver and the like. The device
is disposable after one use but, as a result, is somewhat crude and
only comprises two electrodes for very basic ECG measurements.
[0005] U.S. Pat. No. 4,121,573 discloses a chest sensor for
monitoring cardiac rhythms of a patient using a pair of spaced
circular electrodes mounted on a foam pad. Electrical connectors
between the electrodes and electronic circuitry for acquiring and
transmitting cardiac rhythm signals is provided by independent
electrical leads or wires. The circuitry and wires are located on
the rear surface of a first layer of foam and held in position by a
second layer of foam. Accordingly, a fairly deep configuration of
layers of foam, electronic circuitry and electrodes is provided in
this rather crude two electrode device.
[0006] U.S. Pat. No. 4,957,109 discloses an electrode array for use
in generating electrocardiographic signals for a patient. The array
comprises ten different electrode regions (comprising pairs of
semicircular electrodes) for attachment to different parts of the
human body. The electrodes are interconnected to an output
connector for attachment to signal processing apparatus. The
electrode sensors and electrical conductors between the electrodes
and the output conductor are formed on a large flexible circuit
board having a large dentritic or tree-like configuration to enable
location of the electrodes at appropriate positions on the human
body for standard twelve lead diagnostic electrocardiogram studies.
A digital infra red signal having multiplexed data from each of the
ECG electrodes is transmitted to a remote location in use. While
fairly sophisticated, this arrangement only contemplates
point-in-time 12 lead ECG studies and is not disposable.
[0007] U.S. Pat. No. 5,634,468 discloses a sensor for physiological
monitoring of a patient, consisting of a rectangular patch having a
central structural member formed of MYLAR.TM. encased in an
adhesive hydrogel. One side of the sensor has four circular
electrodes for contacting the patient. The electrodes are wired to
an electronic package on the opposite side of the structural
member. The electronics package is adapted to receive ECG data and
transmits the data to a monitoring unit. However, this small sensor
is limited to measuring ECG signals.
[0008] U.S. Pat. No. 5,353,793 discloses sensor apparatus for
making ECG measurements comprising a band which passes entirely
round a patient's chest. The chest band can have optional shoulder
straps and an optional abdominal band. Electrodes are positioned
around the inner circumferential surface of the band for monitoring
respiration, pulse and ECG signals. The ECG electrodes are simple
conductive sensors in electrical contact with the skin. The pulse
and respiration sensor comprises a tension sensor consisting of a
piezoelectric element. A minimum of 7 ECG sensors is provided but
up to 18 can be spaced around the band. Two or more of the
piezoelectric sensors can be provided in a single chest band. The
various sensors are connected by cabling and accordingly the
apparatus as a whole is quite bulky. Also, the data from the
sensors is transferred to a remote location by wire via a
connector. While the possibility of a radio link is mentioned,
there is no detail as to how this would be achieved cost
effectively to allow for disposability and yet ensure accurate and
efficient data transfer from the various sensors.
[0009] International patent specification WO 94/01039 discloses
physiological monitoring apparatus having a strip assembly for
attachment to a patient's chest. The strip comprises a series of
nine electrically conductive electrode sensors for attachment to
the precordial region of a patient's chest for obtaining ECG data.
The strip only measures ECG data which is wirelessly transmitted on
a multiplexed analog signal which modulates an RF carrier signal
for transmission to a remote data analysis station which can be up
to 50 to 100 meters from the strip. The emphasis here is to provide
a complete ECG study of a patient using a portable system, and
accordingly, there is no discussion of disposability and efficient
communication of data from different types of sensors other than
ECG.
SUMMARY OF THE INVENTION
[0010] An object of the invention is to avoid or at least mitigate
the problems of the prior art. In particular, the invention seeks
to provide an improved physiological sensor device which enables
accurate and or continuous collection of various types of
physiological data using a relatively inexpensive electrical system
which can viably be disposed of after a single use over say a 24
hour period. A further object of the invention is to provide a
device which is able to collect a variety of types of physiological
data, such as ECG, respiration, motion and/or temperature for
example, while still being relatively inexpensive to manufacture. A
yet further object is to use a single sensor for acquiring more
than one type of physiological data.
[0011] Accordingly, a first aspect of the invention provides a
portable and disposable physiological sensor device for attachment
to a mammalian subject comprising physiological sensors for sensing
the subject's physiological parameters, such as ECG or respiration,
and a controller operably in communication with the physiological
sensor for communicating a signal representative of the sensed
physiological parameter to an output which operably transmits the
signal to a remote location.
[0012] Preferably, at least two physiological sensors are provided,
each for sensing different ones of the subject's physiological
parameters. The controller is operably in communication with the
physiological sensors so as to communicate a signal comprising data
representative of both the sensed physiological parameters to an
output transmitter which operably transmits the signal to a remote
location. Preferably, the controller comprises a multiplexer which
operably switches the data from both the physiological sensors into
a serial output signal.
[0013] In a preferred embodiment, the device of the invention
comprises an application specific integrated circuit, or ASIC,
which is designed to have components which form part of a
physiological sensor and also part of the controller, such as the
multiplexer, for communicating the signals between the sensor and
output. Preferably, the output enables wireless transmission of the
signal to a remote location, for example, using a digitally
modulated electromagnetic carrier frequency such as a low frequency
RF carrier for inductive coupling with a receiver. Also, the
controller samples an analog signal from the physiological sensor
and converts the sampled signal into a digital signal using an
analog to digital converter.
[0014] Alternatively, a first and second respiration sensor may be
provided, one of which preferably comprises a bend sensor
locatable, for example, on the subject's chest and preferably over
or adjacent the subject's pectoral muscle.
[0015] In accordance with the invention, the output preferably
transmits a transmission signal comprising a data signal from two
or more physiological sensors. Beneficially, the rate of
transmission of the different signals from the two physiological
sensors can be varied. Preferably, a first physiological sensor
operably detects ECG data and the controller operably communicates
a signal representative of the ECG data to the output transmitter
at a first sampling frequency, and a second physiological sensor
operably detects at least one of respiration, motion, and
temperature data and operably communicates a signal representative
of that data at a second sampling frequency to the output
transmitter. Preferably, the first sampling frequency is as large
as or greater than the second sampling frequency and, more
preferably, approximately ten times greater than the second
sampling frequency. For example, the first sampling frequency might
be 250 Hz while the second sampling frequency might be 25 Hz.
[0016] According to another aspect of the invention, a disposable
physiological sensor device for attachment to a mammalian subject
is provided which is adapted for continuous use over a 24, or
indeed longer, say 48, hour period, comprising a physiological
sensor, a controller operably in communication with the
physiological sensor which controller generates a signal
representative of the subject's physiological parameters such as
ECG or respiration, and an output for transmitting the signal to a
remote location. Accordingly, the device is generally disposable
after a single continuous use. Beneficially, two or more
physiological sensors are provided on the device.
[0017] According to a further aspect of the invention, a
physiological sensor device for attachment to a mammalian subject
is provided comprising a bend sensor which comprises an elongate
member and an electrical component mounted thereon which electrical
component has an electrical property which varies in dependence on
the extent of bending of the elongate member, and an electrical
monitoring device for detecting variation in the electrical
property of the electrical component thereby to determine a
physiological parameter such as respiration, of a subject in use.
Preferably, the electrical component comprises an elongate resistor
superimposed on the elongate member. The resistor can comprise a
track of conductive ink and a series of two or more areas of highly
conductive material such as metallic material over the conductive
ink, thereby to effect a series of individual conductive sensors
having a combined resistance less than the track of conductive ink
without the areas of highly conductive material. Preferably, the
elongate resistor or track of conductive ink, is substantially
U-shaped. The elongate member can comprise a flexible substrate
such as MYLAR.TM.. The device of the invention is preferably
adapted to attach to a human chest for example over or adjacent the
pectoral muscle of a subject and more preferably between a
precordial position and the axilla of the subject in use, thereby
to enable monitoring of the subject's respiration, for example.
[0018] A further aspect of the invention provides a physiological
sensor device comprising two electrode sensors operably locatable
on a patient, a current generator for driving a current to each of
the electrode sensors, and an impedance measuring device for
determining variation in the impedance of the electrode sensors
when attached to the subject in use thereby to determine a
variation in the motion of the subject in use due to variation in
the impedance at the electrical sensors caused by such motion.
Preferably, the current generator comprises a sine wave generator
which operably independently drives each of the two electrode
sensors. The impedance measuring device can comprise a differential
amplifier having an input from each of the two electrode sensors.
The output signal from the differential amplifier can pass through
a filter and demodulator before being AC coupled to a further stage
of amplification. Preferably, the current generator generates an
alternating current and the impedance measuring device comprises an
anti-aliasing device after the further stage of amplification to
ensure proper detection of the impedance signal. Preferably, the
two electrode sensors are drive electrodes in a four electrode
sensor arrangement for monitoring subject respiration.
[0019] A yet further aspect of the invention provides a portable
physiological sensor device comprising a plurality of electrode
sensors for use in measuring electrocardiographic data and
respiratory data of a subject, wherein at least one of the
electrode sensors is used in both the electrocardiographic and
respiratory measurements, and an output transmitter responsive to
outputs of the electrodes sensors so as to enable wireless
transmission of the electrocardiographic data and respiratory data
to a remote location. Preferably, the signal from at least one
electrode sensor is sampled periodically by an ECG measuring device
at a first sampling frequency and periodically at a second sampling
frequency by a respiration measuring device. Preferably, the first
sampling frequency is greater than the second sampling frequency.
In a preferred embodiment, the first sampling frequency is
approximately ten times greater than the second sampling frequency
and, for example, can be 250 Hz compared to a second sampling
frequency of 25 Hz.
[0020] A further aspect of the invention provides a physiological
sensor device attachable to a mammalian subject in use and
comprising two electrode sensors adapted to assist in monitoring
one of electrocardiographic data, and respiratory data of the
subject and further comprising a motion detector which operably
monitors the variation in impedance between the two electrode
sensors thereby to determine the extent of motion of the patient in
use. Preferably, the device comprises a first respiration sensor
having the two electrode sensors and two further electrode sensors,
wherein one pair of the two electrode sensors and the two further
electrode sensors forms a pair of drive electrodes to which drive
current is operably applied, and the other pair of the two
electrode sensors and the further electrode sensors forms a pair of
input electrodes to the respiration sensor. Preferably, the first
respiration sensor comprises a differential amplifier having an
input from each of the input electrodes. Also, the device
preferably comprises a second respiration sensor having, for
example, a bend sensor. The device can further comprise a
temperature sensor.
[0021] Another aspect of the invention provides a physiological
sensor device for attachment to a mammalian subject comprising a
sensor for acquiring physiological data about the subject in use,
and an output transmitter which receives a signal representative of
the physiological data from the sensor and transmits the signal to
a remote location, wherein the output transmitter comprises an
inductive element for inductively coupling the output transmitter
to a remote receiver at the remote location. Beneficially the
inductive coupling enables efficient low powered transfer of data
from the device.
[0022] Preferably, the output transmitter comprises a reservoir
capacitor in parallel with the inductive element, thereby to
mitigate power loss during transmission. Also, the inductive
element preferably has first and second ends each of which ends is
connected via a pair of switches to signal supply lines within the
output transmitter to enable a reversing of the polarity across the
inductive element between the supply lines. Preferably, the
inductive element comprises a coil which forms part of an H-bridge
circuit in the output transmitter. In a preferred embodiment the
inductive element comprises a rectangular, substantially flat
coil.
[0023] A yet further aspect of the invention provides a
physiological sensor device for attachment to a mammalian subject,
having one or more physiological sensors, an output for
transmitting a signal from the one or more physiological sensors to
a remote location, and a memory for storing a serial number for
identifying the physiological sensor device which serial number is
transmittable by the output with the signal. Preferably, the serial
number is randomly generated. The device preferably comprises a
random number generator and a controller for selecting a randomly
generated number from the random number generator and storing the
selected randomly generated number as the device serial number in
the memory.
[0024] Beneficially, each of the physiological sensors according to
any aspect of the invention can comprise any one or more of the
following: a first stage of amplification, possibly including a
variable amplifier stage; a filter, for example, a band pass
filter; and a demodulation stage to remove any carrier frequency. A
further stage of amplification, again possibly including a variable
amplifier stage, can be provided followed by an anti-aliasing stage
and subsequent switched capacitor low pass filter stage before
passing the signal onto a multiplexer operably controlled to drive
signals from pre-selected sensors to an analog to digital converter
which in turn can be fed to the output where the multiplexed
digital signal can be used to modulate a pre-selected carrier
frequency.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] The invention will now be described, by way of example only,
with reference to the accompanying drawings, in which:
[0026] FIG. 1 is a schematic diagram of a physiological monitoring
system in which a sensor device according to the invention can be
suitably used;
[0027] FIG. 2 is a schematic block diagram of the system shown in
FIG. 1;
[0028] FIG. 3 is a schematic front elevation view of a device
according to the invention;
[0029] FIG. 4 is a schematic diagram of the connections of various
sensors and other external devices to the ASIC of the
invention;
[0030] FIG. 5 is a schematic block diagram of the electronic system
forming the device or array of the invention;
[0031] FIG. 6 is a pulse diagram of the timing sequences for
communication between the ASIC and PIC micro-controller via a
serial link;
[0032] FIG. 7 is a pulse diagram showing the preferred 250 Hz ECG
sample frequency, and 25 Hz sample frequency for other
physiological parameters over a 40 millisecond time frame;
[0033] FIG. 8 is a schematic diagram of the drive arrangement to
the transmission coil; and
[0034] FIG. 9 is a schematic waveform diagram for operating the
drive circuit shown in FIG. 8.
DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS
[0035] FIG. 1 illustrates a physiological sensor device or array 10
according to the invention as attached to the chest of a human
subject or patient S. Device 10 comprises an array of sensors 10a
(to be described later), which generate data about the
physiological condition of the subject. This data is transmitted to
a portable signal transfer unit 12. In turn, signal transfer unit
12 transfers a signal representative of the measured physiological
parameters to a base station 14 which operably communicates with a
remote monitoring station 16, which can comprise a suitably
programmed computer 16b. This communication is, for example, via a
telemetry or telephonic link T, such as a land based telephone
system, using, for example, modems 14c and 16a.
[0036] The basic structure of the different components in the
system is shown in the schematic block diagram of FIG. 2. As can be
seen, device 10 comprises an array of sensors 10a in communication
with suitable electronics forming a controller for processing and
communicating physiological data to the signal transfer unit 12. In
this example, device 10 comprises an application specific
integrated circuit (ASIC) 59, and a communications programmable
integrated circuit, COMMS PIC or micro-controller 61. Further
details of the preferred form of electronics and details of the
sensors are described below, while further details of the structure
and method of manufacture of the device 10 are given in our
co-pending patent application entitled "Physiological Sensor
Device", U.S. patent application Ser. No. (Attorney Reference No.
NEXT-0005), which is incorporated herein by reference.
[0037] The system further comprises a portable signal transfer unit
12 having a receiver 12a in communication with a processor 12b
which, in turn, enables two-way transfer of data and commands to
base station 14 via a radio module 12c. Further details of a
preferred form of unit 12 are given in a co-pending patent
application entitled "Portable Signal Transfer Unit", U.S. patent
application Ser. No. (Attorney Reference No. NEXT-0014), which is
incorporated herein by reference.
[0038] Base station unit 14 comprises a radio module 14a for
receiving data signals from signal transfer unit 12, a processor
14b suitably programmed to manipulate the physiological data and to
enable transfer from base station 14 to remote monitoring station
16 via a modem 14c and a link T. Remote monitoring station 16 can
comprise a modem 16a and programmable computer 16b, for example.
Further details of the base station 14 and remote monitoring
station 16, as well as the system as a whole, including details of
the format of transmitted data and transmission protocols between
the device 10 and signal transfer unit 12, are given in a
co-pending application entitled "Portable Remote Patient
Telemonitoring System", U.S. patent application Ser. No. (Attorney
Reference No. NEXT-0002), which is incorporated herein by
reference.
[0039] FIG. 3 illustrates a front elevation view of a preferred
embodiment of physiological sensor device 10 according to the
invention showing the face of the device which is attached to a
subject in use. Device 10 comprises a first sensor region 20, for
positioning in a precordial position, and a second sensor region
22, connected thereto by a yoke or web 24. Device 10 further
comprises a third sensor region 26, for positioning just below the
subject's axilla under the subject's left arm. Third region 26 is
attached to first region 20 by a web 28. First region 20 comprises
three electrode sensors 30, 32 and 34, while second region 22
comprises two electrode sensors 36 and 38. A sixth electrode sensor
is provided at the bottom of third sensor region 26 in the form of
electrode 40. The electrode sensors can be used to monitor such
physiological parameters as heart rate, respiration and/or motion
as described below, and are operably in electrical contact with a
patient's skin through the use of a conductive gel.
[0040] A further physiological sensor 42 is provided on first
sensor region 20 proximal the web 28 and hence adjacent or at least
proximal a subject's chest in use. Sensor 42 can be used to
determine the extent of movement of a subject's chest and hence to
monitor respiration, for example. The sensor 42 can comprise a bend
sensor having a flexible nonconductive substrate onto which is
mounted a strip of conductive material such as ink which in turn
has a series of highly conductive areas, for example of metal,
mounted on top of the conductive strip. The resistance of the
conductive material varies according to the extent of bending of
the flexible substrate. Such sensors are available from
Abrams/Gentille Entertainment Inc of New York, N.Y., for example.
The bend sensor 42 can be positioned anywhere on the chest but
preferably is located between a precordial position and the
axilla--such as over or adjacent the pectoral muscle. A further
physiological sensor is provided in the form of a temperature
sensor 44 which can for example comprise a thermistor.
[0041] A printed circuit board 46 is provided for the on-board
electrical system having suitable electronics such as the ASIC 59
and communications PIC which are operably in communication with the
various sensors via conductive tracks 48. The system is able to
communicate to a signal transfer unit 12 via an aerial or coil
antenna 50 which beneficially can be substantially flat and
rectangular to fit comfortably around PCB 46. Also shown in FIG. 3
is a series of slots 52, 54 and 56 which pass through sensor
regions 20 and 22. These slots provide an air gap separation, and
hence non-conductive divide, between adjacent electrodes and also
provide additional flexibility to sensor regions 20 and 22. Also
shown schematically in FIG. 3 is an aperture 58 which passes
through the layers of device 10 to accommodate suitable batteries
attached to printed circuit board 46.
[0042] In use, device 10 is attached to a patient such that first
sensor region 20 is positioned in a precordial position on the
chest, web 24 lies across the sternum and second sensor region 22
is located in substantial horizontal alignment with region 20, on
the right hand side of the chest. Web 28 passes over the pectoral
muscle of a subject, and third region 26 is preferably located just
below the left axilla or left armpit of the patient.
[0043] The connection of the various sensors to ASIC 59 mounted on
printed circuit board 46 is shown in FIG. 4. In a preferred
embodiment, electrode 30 acts as an ECG reference electrode and
electrodes 40 and 36 act as positive and negative sense ECG
electrodes, respectively. Preferably, the three electrode sensors
act to approximate the standard ECG lead II signal. Of course,
other combinations of electrodes 30, 32, 34, 36, 38 and 40 could be
used as the reference, positive and negative sense ECG electrodes
where preferably the reference electrode is as far away as possible
from the positive and negative sense electrodes as possible. The
present arrangement is particularly adapted for common use of one
of the electrodes as part of the respiration sensor as well as the
ECG sensor.
[0044] Electrode 36 also acts as a respiration sensor as do
electrode sensors 32, 34 and 38, which utilization is described in
greater detail below. Additionally, bend sensor 42 is connected to
ASIC 59 through a voltage divider and filter configuration
comprising resistor R4 and capacitor C8. Temperature sensor 44
comprises a thermistor TH1 connected to ASIC 59 through a voltage
divider arrangement comprising resistor R5.
[0045] Power to the electronic system is provided by a battery
which in this embodiment comprises three 1.4 volt zinc-air cells
B1-B3 such as DA675 cells, which in series provide a maximum of
approximately 4.2 volts between power rails on PCB 46. In fact, the
majority of components require a 2.5 volt source and the use of
three 1.4 volt cells has been found to provide a suitable power
source for the required operational duration of about 24 hours for
the system.
[0046] Further description of the various pins identified in FIG. 4
is given in Table 1, and a further block diagram of the functional
components of ASIC 59 and external components is provided in FIG.
5.
[0047] The ECG sensor or circuit, illustrated in FIGS. 4 and 5, is
required to amplify a small differential voltage from two chest
electrodes 36, 40 (sensed on the ECGInP/RespInP and ECGInN Pins of
ASIC 59), while rejecting a much larger common mode signal. A third
electrode 30 connected to the ECGRef pin acts as a ground
reference. The signals are AC coupled into the ASIC 59 using an
off-chip RC network (FIG. 4). The ASIC 59 amplifies and filters the
input signal, and has provision for adjustable gain and offset. The
filtered ECG signal is preferably sampled at 250 Hz and converted
to a digital signal by the A/D converter 120.
[0048] In greater detail the ECG circuit comprises a differential
or operational amplifier 60 connected to electrode sensors 36 and
40 as shown in FIG. 5. Amplifier 60 can for example provide 54 dB
amplification. The signal is then fed to a variable amplifier 62 of
between 0 to 10.5 dB. The ECG signal passes through an
anti-aliasing device 64 to ensure proper sampling at the 250 Hz
sampling frequency. The output signal then passes to a switching
capacitor low pass circuit 66 which in turn is fed to multiplexer
70.
[0049] A high frequency cut off of 85 Hz might be provided by
switching capacitor low pass circuit 66, but this can be altered,
for example, by changing the crystal frequency of the crystal
oscillator 116 thereby to increase the high frequency cut off from
85 Hz to 100 Hz, for example.
[0050] Temperature sensing is done using an off-chip
thermistor/resistor potential divider. A single pin Temp on the
ASIC 59 is used to interface to the temperature sensor 44. No
signal conditioning need be performed; instead the voltage is
multiplexed directly into the A/D converter 120. Preferably A/D
converter 120 uses VDD as a reference voltage, hence variations due
to changing battery voltage are canceled. Conversion of this
digital value into a temperature value is done externally to the
ASIC 59. Skin temperature data can be collected at 25 Hz.
Preferably, the range of temperatures monitored by thermistor 44 is
between 25 and 45.degree. Centigrade with an accuracy of
about+/-0.5.degree. C.
[0051] One method of respiration measurement uses tetrapolar
electrodes to detect changes in the impedance of the body's chest
cavity. One pair of electrodes 38, 32 is driven with a
reciprocating current from the RespDrvP and RespDrvN pins, which
causes a voltage to be developed across the body in proportion to
its impedance. For example, a 50 kHz sine wave generator 72 can be
used. Electrodes 34 and 36 are used to sense the voltages. One
electrode 36 is shared with an ECG input (pin EcgInP/RespInP), and
the other electrode 34 connects via pin RespInN to ASIC 59. These
inputs are fed to a first stage amplifier 90 for example providing
34 dB amplification from which the signal is passed to a variable
amplifier 92 enabling further amplification from between say 0 to
10.5 dB. The signal passes through a band pass filter 94 and on to
demodulation circuitry 96. The demodulation circuitry 96 preferably
consists of a precision rectifier followed by a low pass filter,
which gives better performance than a peak detector type circuit in
the presence of noise.
[0052] The demodulated respiration signal is capacitively coupled
via capacitor C7 to pin RespACIn and passes through a second stage
amplification comprising, for example, a 34 dB amplifier 98 and
variable amplifier 100 (again between 0 and 10.5 dB preferably) and
on to an anti-aliasing device 102. Finally, the signal passes
through switching capacitor and low pass filter circuit 104 before
being passed onto multiplexer 70. The demodulator 96 and second
gain stage and filter (98 to 104) can be independently powered down
if not required (e.g. if the alternative method of respiration
measurement is used). As can be seen, two pins RespACOut and
RespACIn on ASIC 59 are used with an off-chip capacitor C7 to AC
couple the demodulated respiration signal to the second gain
stage.
[0053] Respiration can also be measured using bend sensor 42. This
method uses a resistance bend sensor 42 for respiration measurement
which sensor has an impedance which in a preferred embodiment
typically varies between 12 kW when flat and 40 kW when bent
through 90 degrees. The change in impedance when used as a
respiratory monitor is generally about 500-1000.OMEGA.. In another
example, a bend sensor having a variable impedance of between 26
k.OMEGA. and 200 k.OMEGA. (between 0 and 90.degree. of bend) can
provide a variation in impedance of between 500.OMEGA. and
2000.OMEGA. during normal respiration. Beneficially, deep breaths
can be detected using such bend sensors by a large variation in
impedance of around 5 k.OMEGA.. Bend sensor 42 can comprise a
sensor having a flexible non-conductive substrate onto which is
mounted a strip of conductive material such as ink which in turn
has a series of highly conductive areas for example of metal
mounted on top of the conductive strip. Such sensors are available
from Abrams/Gentille Entertainment Inc of New York, for example. In
one embodiment, the conductive track is U-shaped having an
electrical contact at the end of each of the arms of the track.
[0054] The bend sensor 42 is connected in series with an external
resistor R4. This combination is connected across the supply rails,
thus acting as a potential divider. The midpoint of the divider is
AC coupled into the ASIC 59 via the BendSense pin. The signal is
then amplified via amplifiers 106 (e.g., 26 dB) and 108 (e.g.
0-10.5 dB), passed through anti-aliasing device 110 and filtered at
switching capacitor 112 before being driven into the multiplexer
70.
[0055] An impedance measurement for motion detection is preferably
also obtained from the monitored voltage between the respiration
(impedance) drive electrodes 38 and 32, i.e., at the RespDrvP and
RespDrvN pins. The demodulator and gain/filter stage are similar in
principle to that of the Respiration (Impedance) channel, and can
be independently disabled. As shown in FIG. 5, it can be seen that
a differential amplifier 74, of say 16.9 dB, is connected to the
outputs from sine wave generator 72 to electrodes 38 and 32. The
differential output signal is passed through a variable amplifier
76 (of say between 0 and 10.5 dB) and subsequently through band
pass filter 78 and demodulation circuitry 80. The output from the
demodulation circuitry 80 is AC coupled via capacitor C3 to
amplifier 82 (of say 20 dB) and then on to variable amplifier 84
(of say 0 to 10.5 dB). The signal then passes through anti-aliasing
circuitry 86 and a switched capacitor low pass filter circuit 88.
The output signal is finally fed to multiplexer 70.
[0056] The movement detection is accordingly an effective
measurement of variations in the impedance between electrode
sensors 32 and 38. This impedance variation is most likely to be
caused due to variations in the electrical conductivity of the
conductive gel between the electrode and the user's skin, whereby
movement of the subject causes some movement of device 10 and hence
minor variations at the conductive interface between device 10 and
the patient can be detected. The size of signal detected can be
used to determine the extent of movement. Alternatively, a
threshold value for the detected movement signal can be
predetermined such that signals below the threshold value indicate
that the patient is substantially stationary whereas signals above
the threshold value indicate that the patient is undergoing some
form of movement.
[0057] Accordingly, in the illustrated embodiment, five different
sets of physiological data are gathered by the various sensors and
suitably demodulated, filtered and passed through to multiplexer
70. In turn, multiplexer 70 sequentially feeds the appropriately
sampled signals in a predetermined sequence through to analog to
digital converter 120. Also, ASIC 59 comprises a control and
configuration logic unit 121 to provide control and proper timing
of the predetermined operation within the ASIC 59.
[0058] The analog signals from the various sensor channels are
converted into digital values using an on-chip Analog to Digital
(A/D) converter 120, where the input of the A/D converter is
multiplexed between the various sensor channels. The A/D converter
120 preferably uses an on-chip resistor chain connected across the
power supply as a reference. Beneficially, the A/D converter 120
can be powered down when inactive (i.e., between samples) and can
be a successive approximation type converter with 10 bit
resolution, for example.
[0059] The ASIC 59 also has two pins (Xtal/Clk and XtaIN/ClkN)
dedicated to a crystal oscillator circuit 116. If the on-chip
crystal oscillator is used, then an external crystal and two
capacitors should be connected. A clock signal can then be taken
from the XtaIN/ClkN pin. If an external clock signal is used then
the external crystal and capacitors should preferably be removed
and the external clock driven into the Xtal/Clk input.
[0060] Commands and data are passed between the ASIC 59 and an
external micro-controller device 61 via a 3-wire link (pins SD, SQ,
SClk) serial link interface 114. The micro-controller 61 can write
into various configuration registers within the ASIC 59, and can
read data from any of the sensor channels using the Analog to
Digital converter 120 on the ASIC 59. The serial port forming
interface 114 consists of three wires:
[0061] 1. SClk (serial clock): This is an input to the ASIC 59.
[0062] 2. SQ (serial data output): This output carries the
measurement data and other information from ASIC 59 to
micro-controller 61.
[0063] 3. SD (serial data input): This input will carry the
commands and data into the ASIC 59 from micro-controller 61.
[0064] The serial link interface 114 supports three types of
operation: commands, read operations, and write operations.
Commands are performed by clocking an eight bit operation code into
the ASIC 59. For register write operations, a further byte of data
is clocked into the ASIC 59. For register read operations, up to
ten bits of data are clocked out on the SQ pin after the operation
code has been received.
[0065] The rising edge of SClk is active, and data on the SD pin is
read on the rising edge. Data changes on the SQ pin happen
subsequent to a rising edge. The signal SClk need not be clocked
continuously. The ASIC 59 recognizes the beginning of an operation
code when a logic 1 is clocked in. When SClk is toggling but an
operation code is not being clocked into the ASIC 59 (e.g., when
read data is being clocked out), then SD should be held at logic
0.
[0066] An example of Command, Read, and Write operations is shown
in FIG. 6.
[0067] The ASIC 59 has a collection of registers which may be read
or written to via serial port 114. These hold various data such as
the values on the sensor channels, configuration settings, and
status information. Write commands are followed by eight bits of
data, with the most significant bit being clocked in first. For
read commands, the SQ pin goes to the value of the MSB of the
output word on the first clock edge after the operation code has
been received. Each active clock edge clocks the next least
significant bit out of the ASIC 59. After the LSB has been output,
SQ goes to logic 0 and remains in that state until another register
read operation code is received by the ASIC 59.
[0068] The crystal oscillator 116 (on chip or from an external
source) is divided down to provide the clock for the switched
capacitor filters and the A/D converter 120. The outputs of the
switched capacitor SC filters (66, 88, 104, 112) naturally have
periods between clock pulses where their outputs are stable, which
would be the most desirable place for the A/D converter 120 to do a
conversion. However, because the external micro-controller 61
cannot be guaranteed to be running in exact lock-step with the ASIC
59, there is a danger that samples could be dropped occasionally.
Therefore the ASIC 59 preferably only does a conversion on one of
the sensor channels when explicitly instructed to do so by the
micro-controller 61 (via the serial link 114). Requests for ECG
conversions are issued by the micro-controller 61 at, in this
example, a precise 250 Hz rate, with requests for motion,
respiration and temperature measurements being interleaved between
the ECG measurements at a 25 Hz rate, as shown in FIG. 7. FIG. 7
shows the position of measurements in a 40 ms repeating time
frame.
[0069] Since the SC filter clocks and the micro-controller clock
are not necessarily synchronized, it may be that the A/D converter
120 will be instructed to do a conversion while the filter outputs
are in transition between two adjacent samples. However, as the A/D
converter 120 gives an output that is somewhere between the two
adjacent levels, there are no large spikes or spurious values.
Also, clock noise does not cause a problem since the SC filters
have smoothing filters on their outputs to remove this. However,
the A/D converter 120 can be configured (using a register bit) to
delay its conversion until the SC filter outputs have settled.
[0070] If the power supply voltage drops below a certain threshold,
the SC filters will no longer operate correctly. In order that the
external micro-controller 61 can determine the integrity of the
sensor measurements, a battery monitor circuit 118 (FIG. 5)
periodically measures whether the supply voltage is adequate, and
updates a bit in the status register accordingly.
[0071] The ASIC 59 preferably has a built in power-up reset
function which puts the device into a defined state when power is
first applied. It then waits for a predetermined time for the power
supply and clock to settle. The power-up delay function preferably
comprises a length of power-up sequence of 100 ms maximum (after
power supply settles), and a power supply settling time of 100 ms
maximum to within 5% of the final value, which preferably increases
monotonically. Power-up can occur on removing an air impermeable
membrane from the batteries B1, B2 and B3.
[0072] A wireless communication link is used to communicate between
the sensor array, that is device 10, and the signal transfer unit
12. As a backup option, the same drive circuit can be used to drive
via a wire link (not shown). The ASIC 59 can contain an H-bridge
driver 122 for this purpose. An external micro-controller 61 drives
data for the H-bridge onto the Hin pin, the external antenna 50
being connected to the HOutP and HOutN pins. The ASIC 59 has
protection circuitry to prevent the battery from being removed: at
power up the H-bridge driver is reset to an inactive state, and
during normal operation the H-bridge driver will be deactivated if
there have been no data transitions for a certain amount of
time.
[0073] The driver 122 comprises transistors which carry high peak
currents, and can carry bi-directional current to allow energy to
be recirculated into the supply. In order to minimize coupling or
interference in the sensor amplifier circuits, the H-bridge driver
preferably has its own dedicated power supply pins HV.sub.DD and
HV.sub.SS. These pins preferably have good external decoupling.
[0074] In a preferred embodiment, the H-bridge driver specification
is as follows:
1 Output resistance 1 .OMEGA. max. Output capacitance (per pin) 500
pF max. Drive transistor turn on time 100 ns max. Timeout delay 32
.mu.s.
[0075] The purpose of the coil-driving circuitry 122 in the
transmitter is to deliver sufficient signal current into the
primary coil to yield the required magnetic field strength at the
receiving coil (on signal transfer unit 12) while incurring minimal
energy loss, and thus minimizing supply current.
[0076] In order to save power in its circuitry, it is necessary to
avoid resistive losses in the output stages of the transmitter.
This may be achieved by avoiding the use of a linear output stage
and instead using switching circuitry. Switching devices may be
used to apply the power supply directly to the transmitter coil 50
and will in principle result in no power loss, since perfect
switches dissipate no power in either their on or off states.
Practical switches may be implemented using transistors, such as
those present in the output stages of digital integrated circuits.
A switching output stage is more easily applicable to drive signals
having a constant amplitude, such as frequency or phase modulation,
including direct spread-spectrum digital modulation. Amplitude
modulation may be achieved, but at the expense of the complication
of pulse-width modulation, where the duty cycle of the output may
be varied.
[0077] Two ways are available to recycle the energy used in
building up the magnetic field during each voltage cycle applied to
the coil. In the case of a narrow-band transmission system, the
coil 50 may be resonated at the transmitted frequency using a
capacitor connected across it. Each time the magnetic field in the
coil 50 collapses, the energy released passes into the capacitor,
where it is stored as charge. When the field has completely
collapsed, the energy now stored in the capacitor starts to return
to the coil 50 to build up the field in the opposite direction to
give, in principle, a lossless system. In practice, of course,
there will be residual losses from the series resistance of the
coil 50, so a drive circuit is necessary. Either a voltage drive
may be used to drive a series resonant circuit, or a current drive
may be used with a parallel resonant circuit. At typical circuit
impedance levels, the power supply in electronic equipment is
usually better considered as a voltage source, so a switching
circuit driving a voltage square wave into a series resonant
circuit will be appropriate.
[0078] Apart from the limited frequency band over which it is
effective, a drawback of using resonance is that the impedance
level of the coil 50 is altered. In the case of a series resonant
circuit, for example, the impedance of the circuit at resonance is
simply that of the self resistance of the coil, which will
intentionally be low to reduce losses. For a given power supply
voltage, this may result in an inappropriately high current in the
coil. A coil resistance of 5.OMEGA., for example, would lead to a
current of 1 A when fed from a 5V supply.
[0079] An alternative technique, which overcomes the drawbacks of
the resonant circuit, is to drive the coil 50 using four switches
in a bridge configuration as shown in FIG. 8. In this
configuration, the energy from the collapsing magnetic field is
returned to the power supply, ready for the next cycle. For this
technique to succeed, the power supply must be able to store the
returned energy, which is easily arranged by the use of a reservoir
capacitor 124. The resulting circuit may be used at any frequency
(in principle), while the self-inductance of the coil 50 acts as a
convenient way of defining the output current. The switches may be
electronic devices, such as discrete transistors or components in
the output stages of digital integrated circuits in ASIC 59.
[0080] The operation of this circuit is illustrated by the
waveforms given in FIG. 9, which show the voltage v(t) applied to
the coil and the resulting current I(t) for the case of a
continuous square wave drive signal and a perfect circuit
exhibiting no resistive losses. During intervals A and B, the
applied voltage is positive, so the current linearly increases,
while for intervals C and D, the applied voltage is negative, so
the current linearly decreases. During intervals B and D, the
current has the same polarity as the applied voltage, indicating
the energy is being supplied to the coil and a magnetic field is
being built up. During intervals A and C, the current has the
opposite polarity to the applied voltage, indicating that energy is
being transferred from the coil 50 to the supply as the magnetic
field collapses.
[0081] A transmitter constructed using the bridge technique
described above uses Advanced CMOS driver ICs to drive the ends of
the transmitter coil 50 with opposite polarities from a
frequency-modulated test signal. A 74AC540 (inverting) and 74AC541
(non inverting) buffer are used with all eight sections of each IC
connected in parallel to give an output resistance of approximately
1.OMEGA.. The transmitting coil 50 is preferably rectangular with
dimensions of approximately 175.times.58 mm and has 40 turns such
that it is substantially flat; its self inductance may be
approximately 600 .mu.H. In a preferred embodiment the value of the
reservoir capacitor 124 is about 10 micro Farads, and for example
two parallel capacitors are used such as a ceramic capacitor of
about 47 nanoFarads and an electrolytic capacitor of about 10 micro
Farads. For comparative purposes, this transmitter of the invention
was tested with the receiver used in an earlier test with a
different method of driving the primary coil, where a 20-turn
receiving coil of approximately 55 mm square was used. The result
of the test was that a similar range was obtained with the circuit
of the invention to that with the earlier one. The major advance
was that the supply current to the transmitter output stage was now
1.3 mA at 5 V, which may be contrasted with the previously achieved
figure of 40 mA.
[0082] In the bridge-drive circuit described above, the coil 50 is
always applied a voltage of the same magnitude as the supply
voltage, but of alternating polarity. The circuit is thus only
suitable for two-valued drive signals, though these may form a
pulse-modulated signal representing a lower frequency waveform. By
separating the drive signals to the two push-pull bridge output
stages, the coil 50 may have to be fed with a third drive level of
zero voltage. In this state, the magnetic field in the coil 50 is
maintained, ideally without loss. The result is that a third signal
value may be used, which may allow a better representation of
signals by pulse modulation.
[0083] The circuit may be further extended by the use of additional
supply lines at different voltage levels, each fitted with energy
storage capacitors and connected to both ends of the primary coil
50 by means of switching devices. Care must be taken in the
selection of switching signals to ensure that the energy into and
out of each capacitor is balanced over the long term. Using this
arrangement the digital multiplexed signals from ADC 120 are
transmitted on a pre-selected carrier frequency via inductive
coupling of coil 50 to a remote receiver coil in a signal transfer
unit 12.
[0084] In this regard a variety of protocols for the modulation
technique of a fundamental carrier frequency such as between say 50
kHz and 150 kHz and preferably between 54 kHz and 144 kHz is
described in our co-pending patent application entitled "Portable
Remote Patient Telemonitoring System", U.S. patent application Ser.
No. (Attorney Reference No. NEXT-0002). As described therein, in a
preferred embodiment, a quadrature phase shift keying (QPSK)
technique is used whereby the binary data is transmitted in bit
pairs and each consecutive bit pair corresponds to a different
phase offset of 0, 90, 180 or 270.degree. of the transmitted signal
relative to the fixed carrier. In a preferred embodiment, digital
frequency modulation in a 4 kHz band on channels with 6 kHz
separation in the above frequency range is preferred enabling
transmission up to several meters.
[0085] Preferably, a randomly generated unique serial number is
generated for each device 10 and inserted in the data for
transmission and hence easy tracking of the individual device 10.
Of course, ordered serial numbers can be used for each of devices
10 manufactured but in the preferred form a random number generator
such as a pre-programmed microchip is used to assign the serial
number which is stored on micro-controller 21. In a preferred
embodiment, micro-controller 61 comprises its own random number
generator. Accordingly, a randomly generated number is selected by
micro-controller 61 to represent the serial number for a given
device 10.
[0086] Although an exemplary embodiment of the invention has been
described in detail above, those skilled in the art will readily
appreciate that many additional modifications are possible in the
exemplary embodiment without materially departing from the novel
teachings and advantages of the invention. All such modifications
are intended to be included within the scope of this invention as
defined in the following claims.
2 Pin Name Direction Description V.sub.DO I/O Power connection
V.sub.SS I/O Ground connection Xtal/Clk Input Crystal/Clock input
XtalN/ClkN Output Crystal/Clock output SD Input Serial link data
input SQ Output Serial link data output SClk Input Serial link
clock ECGRef Output ECG reference connection ECGInP/ Input ECG
sense (+) RespInP Respiration (impedance) sense (+) ECGInN Input
ECG sense (-) Temp Input Temperature sensor input RespDrvP Output
Respiration (impedance) current drive (+)/ Motion sense (+)
RespDrvN Output Respiration (impedance) current drive (-)/ Motion
sense (-) RespInN Input Respiration (impedance) sense (-) RespACOut
Output Respiration AC coupling output RespACIn Input Respiration AC
coupling input BendSense Input Respiration (bend sensor) input
MotACOut Output Motion AC coupling output MotACIn Input Motion AC
coupling input HV.sub.DD I/O H-Bridge Power connection HV.sub.SS
I/O H-Bridge Ground connection HIn Input H-Bridge input signal
HOutP Output H-Bridge output drive (+) HOutN Output H-Bridge output
drive (-)
* * * * *