U.S. patent application number 13/488072 was filed with the patent office on 2012-09-27 for signal demodulation.
This patent application is currently assigned to Nellcor Puritan Bennett LLC. Invention is credited to Ethan Petersen.
Application Number | 20120245441 13/488072 |
Document ID | / |
Family ID | 40753232 |
Filed Date | 2012-09-27 |
United States Patent
Application |
20120245441 |
Kind Code |
A1 |
Petersen; Ethan |
September 27, 2012 |
SIGNAL DEMODULATION
Abstract
A method for processing an analog composite signal in a system
has the steps of receiving a composite signal with at least one
first signal component and at least one interfering signal
component; filtering the composite signal with a filter having a
transfer function H(s); sampling the filtered composite signal in
periodic intervals wherein each periodic interval has n samples;
forming a matrix equation representing the composite signal wherein
the matrix equation has a signal vector with the at least first one
signal component and the at least one interfering signal component
and a matrix comprising weighted coefficients; solving the matrix
equation to determine the at least one signal component; outputting
the at least one signal component.
Inventors: |
Petersen; Ethan; (Castro
Valley, CA) |
Assignee: |
Nellcor Puritan Bennett LLC
Boulder
CO
|
Family ID: |
40753232 |
Appl. No.: |
13/488072 |
Filed: |
June 4, 2012 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
11955824 |
Dec 13, 2007 |
8204567 |
|
|
13488072 |
|
|
|
|
Current U.S.
Class: |
600/323 |
Current CPC
Class: |
A61B 5/7203 20130101;
A61B 5/14551 20130101; A61B 5/02028 20130101; A61B 5/725
20130101 |
Class at
Publication: |
600/323 |
International
Class: |
A61B 5/1455 20060101
A61B005/1455 |
Claims
1. A system, comprising: a filter configured to receive a signal
from a physiological sensor, wherein the signal comprises a first
signal component and a second signal component comprising an
interference component, and wherein the filter is configured to
output a filtered signal; an analog-to-digital converter configured
to receive the filtered signal and sample the filtered signal in
periodic intervals to generate a sampled filtered signal, wherein
each periodic interval comprises n samples; and a processor
configured to: receive the sampled filtered signal; solve a matrix
equation representing the sampled filtered signal for the first
signal component, wherein the matrix equation comprises a signal
vector comprising the first and second components and a matrix
comprising weighted coefficients; and determine a physiological
parameter based on the first signal component and not the second
signal component.
2. The system of claim 1, wherein the processor is configured to
determine the interference component by a linear approximation
between a first and last sample of each periodic interval.
3. The system of claim 1, wherein the filter comprises a variable
filter and wherein the processor is configured to determine an
inverse of the matrix of coefficients during a start-up phase of
the system
4. The system of claim 1, wherein the filter comprises a constant
filter and wherein the processor is configured to determine an
inverse of matrix of coefficients.
5. The system of claim 1, wherein the filter comprises an adaptive
filter and wherein the processor is configured to determine an
inverse of the matrix of coefficients after each adaptation of the
adaptive filter.
6. The system of claim 1, wherein the physiological sensor
comprises a pulse oximetry sensor.
7. The system of claim 1, wherein the second signal component
comprises an ambient light signal component.
8. The system of claim 7, wherein the ambient light signal
component is approximated by a linear approximation between a first
and last sample of each periodic interval.
9. The system of claim 1, wherein the second signal component
comprises cable transient components.
10. The system of claim 9, wherein the weighted coefficients of the
matrix equation for the cable transients are determined by a
transfer function of the filter and a sample position within a
periodic interval
11. The system of claim 1, wherein the first signal component
comprises a red signal component.
12. The system of claim 1, wherein the first signal component
comprises an infrared signal component and a red signal
component.
13. The system of claim 1, wherein the red signal component and the
IR signal component are timely separated within each periodic
interval and the red signal component and the IR signal component
each comprise a predetermined signal length having an on and off
transient.
14. A method, comprising: receiving a composite signal comprising a
red component, an infrared component, and at least one interfering
signal component; filtering the composite signal with a filter
having a transfer function H(s); sampling the filtered composite
signal with an analog to digital converter in periodic intervals
wherein each periodic interval comprises n samples; and using a
processor: solving a matrix equation representing the composite
signal for the red component and the infrared component, wherein
the matrix equation comprises a signal vector comprising the red
component, the infrared component, and the at least one interfering
signal component and a matrix comprising weighted coefficients,
wherein weighted coefficients for the at least one interfering
signal component are based on an impulse response and a magnitude
of an impulse; generating an output based on the red component and
the infrared component.
15. The method of claim 14, wherein the red component and the
infrared component are represented by an impulse response at a
periodic interval multiplied by a measured current.
16. The method of claim 14, wherein the interfering signal
component comprises an ambient light signal component and cable
transients.
17. The method of claim 16, wherein the weighted coefficients for
the cable transients are determined by the transfer function and a
sample position within a periodic interval.
18. A system, comprising: a sensor comprising one or more light
emitters and a detector configured to detect light emitted by the
one or more light emitter and generate a signal, wherein the signal
comprises a primary signal component and at least one of an ambient
light component or a cable cross-talk component; a patient monitor
comprising: a filter configured to receive the signal and
outputting a filtered signal; an analog-to-digital converter
configured to receive the filtered signal and sample the filtered
composite signal in periodic intervals wherein each periodic
interval comprises n samples; a signal processor configured to:
receive the sampled filtered signal; solve a matrix equation for
the primary signal component, wherein the matrix equation comprises
a signal vector comprising the primary signal component and at
least one of the ambient signal component or the cable cross-talk
component and a matrix comprising weighted coefficients; and
determine a physiological parameter based at least in part on the
primary signal component.
19. The system of claim 18, wherein the at least one of an ambient
light component or a cable cross-talk component is approximated by
a linear approximation between a first and last sample of each
periodic interval.
20. The system of claim 18, wherein n is determined based on an
accuracy of the physiological parameter.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of application Ser. No.
11/955,824, filed Dec. 13, 2007, entitled "Signal Demodulation" in
the name of Ethan Petersen and assigned to Nellcor Puritan Bennett
LLC, which is incorporated by reference herein in its entirety.
TECHNICAL FIELD
[0002] The technical field of the present application relates to
oximeter signal processing.
BACKGROUND
[0003] Pulse oximeters are used to indirectly measures the amount
of oxygen in a patient's blood and for measuring the pulse of a
patient. Furthermore, they can be used to measure changes in blood
volume in the skin, producing a photoplethysmograph. Pulse
oximeters are usually attached to a medical monitor so staff can
see a patient's oxygenation at all times. Most monitors also
display in addition the heart rate.
[0004] A pulse oximeter is a particularly convenient non-invasive
measurement instrument. Typically it has a pair of small
light-emitting diodes (LEDs) facing a photodiode through a
translucent part of the patient's body, usually a fingertip or an
earlobe. One LED is red, with wavelength of approximately 660 nm,
and the other is infrared, using a wavelength of approximately 905,
910, or 940 nm. Absorption at these wavelengths differs
significantly between oxyhemoglobin and its deoxygenated form,
therefore from the ratio of the absorption of the red and infrared
light the oxy/deoxyhemoglobin ratio can be calculated.
[0005] The monitored signal is modulated by the heart beat because
the arterial blood vessels expand and contract with each heartbeat.
Oximeters are furthermore subject to various interferences. For
example, ambient light, in particular light emitted from
fluorescent lighting, can introduce a significant interfering
signal. Capacitive coupling in the patient cable between the LED
wires and the detector wires is also a large source of additional
errors. Generally on the rising and falling edges of the LED
voltage an impulse current appears in the detector lines due to
this capacitive coupling.
SUMMARY
[0006] According to an embodiment, a method for processing an
analog composite signal in a system, may comprise the steps of;
receiving a composite signal comprising at least one first signal
component and at least one interfering signal component; filtering
the composite signal with a filter having a transfer function H(s);
sampling the filtered composite signal in periodic intervals
wherein each periodic interval comprises n samples; forming a
matrix equation representing the composite signal wherein the
matrix equation comprises a signal vector comprising the at least
first one signal component and the at least one interfering signal
component and a matrix comprising weighted coefficients; solving
the matrix equation to determine the at least one signal component;
and outputting the at least one signal component.
[0007] According to a further embodiment, an interfering signal
component can be approximated by a linear approximation between a
first and last sample of each periodic interval. According to a
further embodiment, the weighted coefficients for a transient
interfering signal component can be determined by the transfer
function and a sample position within a periodic interval.
According to a further embodiment, the system can be an oximeter
system comprising an oximeter sensor generating a Red signal
component and an Infrared (IR) signal component as signal
components, wherein an ambient light signal component and cable
transients may be interfering signal components. According to a
further embodiment, the ambient light signal component may be
approximated by a linear approximation between a first and last
sample of each periodic interval. According to a further
embodiment, the weighted coefficients for the cable transients may
be determined by the transfer function and a sample position within
a periodic interval. According to a further embodiment, the Red
signal component and the IR signal component may be timely
separated within each periodic interval and the Red signal
component and the IR signal component each may comprise a
predetermined signal length having an on and off transient.
According to a further embodiment, the ambient light signal
component may be approximated by a linear approximation between a
first and last sample of each periodic interval and the weighted
coefficients for the cable transients are determined by the
transfer function and a sample position within a periodic interval,
and wherein a coefficient matrix may comprise first and second
coefficients for the linear approximation, switch on and switch off
coefficients for the cable transients, a Red coefficient, and an IR
coefficient.
[0008] According to another embodiment, a system for processing an
analog composite signal comprising at least one first signal
component and at least one interfering signal component, may
comprise: a filter having a transfer function H(s) receiving the
composite signal and outputting a filtered composite signal; an
analog-to-digital converter receiving the filtered composite signal
and sampling the filtered composite signal in periodic intervals
wherein each periodic interval comprises n samples; and a signal
processor receiving the sampled filtered composite signal, wherein
the signal processor forms a matrix equation representing the
composite signal wherein the matrix equation comprises a signal
vector comprising the at least first one signal component and the
at least one interfering signal component and a matrix comprising
weighted coefficients, wherein the signal processor is furthermore
operable to solve the matrix equation to calculate the at least one
signal component and to output the at least one signal
component.
[0009] According to a further embodiment, an interfering signal may
be approximated by a linear approximation between a first and last
sample of each periodic interval. According to a further
embodiment, the weighted coefficients for a transient interfering
signal component may be determined by the transfer function and a
sample position within a periodic interval. According to a further
embodiment, the system can be an oximeter system comprising an
oximeter sensor generating a Red signal component and an Infrared
(IR) signal component as signal components, wherein an ambient
light signal component and cable transients are interfering signal
components. According to a further embodiment, the ambient light
signal component may be approximated by a linear approximation
between a first and last sample of each periodic interval.
According to a further embodiment, the weighted coefficients for
the cable transients may be determined by the transfer function and
a sample position within a periodic interval. According to a
further embodiment, the Red signal component and the IR signal
component may be timely separated within each periodic interval and
the Red signal component and the IR signal component each may
comprise a predetermined signal length having an on and off
transient. According to a further embodiment, the ambient light
signal component may be approximated by a linear approximation
between a first and last sample of each periodic interval and the
weighted coefficients for the cable transients may be determined by
the transfer function and a sample position within a periodic
interval, wherein a coefficient matrix may comprise first and
second coefficients for the linear approximation, switch on and
switch off coefficients for the cable transients, a Red
coefficient, and an IR coefficient.
[0010] According to yet another embodiment, an oximeter system may
comprise an oximeter sensor generating an output signal with a Red
signal component and an Infrared (IR) signal component which are
timely separated within a periodic interval wherein the Red signal
component and the IR signal component each comprise a predetermined
signal length having an on and off transient, a filter having a
transfer function H(s) receiving a composite signal consisting of
the oximeter sensor output signal and at least one interfering
signal component, wherein the filter outputs a filtered composite
signal; an analog-to-digital converter receiving the filtered
composite signal and sampling the filtered composite signal in
periodic intervals wherein each periodic interval comprises n
samples; and a signal processor receiving the sampled filtered
composite signal, wherein the signal processor forms a matrix
equation representing the composite signal wherein the matrix
equation comprises a signal vector comprising the Red and IR signal
components and the at least one interfering signal component and a
matrix comprising weighted coefficients, wherein the signal
processor is furthermore operable to solve the matrix equation to
calculate the Red and IR signal components and to output the Red
and IR signal components.
[0011] According to a further embodiment, an ambient light signal
component and cable transients may be interfering signal
components. According to a further embodiment, the ambient light
signal component may be approximated by a linear approximation
between a first and last sample of each periodic interval.
According to a further embodiment, the weighted coefficients for
the cable transients may be determined by the transfer function and
a sample position within a periodic interval. According to a
further embodiment, the ambient light component may be approximated
by a linear approximation between a first and last sample of each
periodic interval and the weighted coefficients for the cable
transients are determined by the transfer function and a sample
position within a periodic interval, wherein a coefficient matrix
may comprise first and second coefficients for the linear
approximation, switch on and switch off coefficients for the cable
transients, a Red coefficient, and an IR coefficient.
[0012] According to yet another embodiment, a method for processing
an analog composite signal in an oximeter system, may comprise the
steps of: receiving a composite signal comprising at Red signal
component and an infrared (IR) signal component from an oximeter
sensor and at least one interfering signal component; filtering the
composite signal with a filter having a transfer function H(s);
sampling the filtered composite signal in periodic intervals
wherein each periodic interval comprises n samples; forming a
matrix equation representing the system wherein the matrix equation
comprises a signal vector comprising the Red and IR signal
component and the at least one interfering signal component and a
matrix comprising weighted coefficients; solving the matrix
equation to calculate the Red and IR signal components; and
outputting the Red and IR signal components.
[0013] According to a further embodiment, an ambient light signal
component and cable transients may be interfering signal
components, wherein the ambient light can be approximated by a
linear approximation between a first and last sample of each
periodic interval and wherein the weighted coefficients for the
cable transients may be determined by the transfer function and a
sample position within a periodic interval. According to a further
embodiment, the Red signal component and the IR signal component
can be timely separated within each periodic interval and the Red
signal component and the IR signal component each may comprise a
predetermined signal length having an on and off transient.
According to a further embodiment, the ambient light can be
approximated by a linear approximation between a first and last
sample of each periodic interval and the weighted coefficients for
the cable transients are determined by the transfer function and a
sample position within a periodic interval, and wherein a
coefficient matrix comprises first and second coefficients for the
linear approximation, switch on and switch off coefficients for the
cable transients, a Red coefficient, and an IR coefficient.
[0014] Other technical advantages of the present disclosure will be
readily apparent to one skilled in the art from the following
figures, descriptions, and claims. Various embodiments of the
present application obtain only a subset of the advantages set
forth. No one advantage is critical to the embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] A more complete understanding of the present disclosure and
advantages thereof may be acquired by referring to the following
description taken in conjunction with the accompanying drawings, in
which like reference numbers indicate like features, and
wherein:
[0016] FIG. 1 is a block diagram of a typical oximeter arrangement,
and the sources of interfering signals;
[0017] FIG. 2 depicts the various components of an oximeter input
signal;
[0018] FIG. 3 an example of the impulse response of a filter to a
chain of impulses.
[0019] FIG. 4 shows how a piecewise linear approximation is applied
to a signal representing ambient light;
[0020] FIG. 5 shows how the signal is processed after it is
digitized.
DETAILED DESCRIPTION
[0021] As stated above, oximeter detectors are subject to a variety
of interfering signals. Currently the largest source of error in
the electronics of a pulse oximeter arises from capacitive coupling
in the patient cable between the LED wires and the detector wires.
On the rising and falling edges of the LED voltage an impulse
current appears in the detector lines due to this capacitive
coupling. For example, it has been determined that the error in
measured photo current due to such impulse currents can be up to
around 71 pA, for example, out of a batch of 17 new cables. Older
cables that have been worn will have compromised shields that could
result in a much larger error.
[0022] FIG. 1 shows a block diagram explaining the influences of
the main interfering sources in oximeter systems. Generally, an
oximeter sensor comprises a red LED and IR LED whose emitted light
is passed through a patient's tissue. A detector receives these
signals but also receives some ambient light as shown on the left
side of FIG. 1. Node 110 simulates a summing point within the photo
detector or detectors of an oximeter system. Thus, the photo
detector produces a signal 120 which comprises the RED component,
the IR component, and an ambient light component. Node 130
simulates the summing point of capacitive cable transient signals
introduced into the detector signal. Thus, output signal 140 now
comprises in addition to the signals mentioned above, the cable
transient signals. Signal 140 is then fed into filter 150
comprising a transfer function H(s). The output signal of filter
150 is then fed to an analog-to-digital-converter 160.
[0023] FIG. 2 shows exemplary signal curves for each signal
component as shown in FIG. 1 as well as the composite signal. For
each Red and IR signal pulse, according to an embodiment, 8 samples
P1 . . . 8 are taken as indicated on the bottom x-axis. During the
time frame P1 . . . 8, the ambient light, shown as the dotted line
which can be dominated by components of the 50 Hz/60 Hz power line
signals, is approximated by a linear line as shown in the top curve
between points X1 and X2. The transient pulses caused by the rising
and falling edges of the Red and IR signals are shown as signals
W1, W2, W3, and W4. Next follows the Red signal and then the IR
signal. The bottom curve represents the composite signal as it is
fed to the filter 150. This composite signal represents a sum of
the above signals.
[0024] This signal is then sampled by an analog-to-digital
converter 160 as indicated at the bottom line of FIG. 2. As shown
in FIG. 2, 8 samples are produced for each Red and IR pulse.
However, according to other embodiments, more than eight samples
can be generated which will improve performance. The composite
signal which is filtered by filter 150 and sampled by
analog-to-digital-converter 160 comprises the component signals as
discussed with respect to FIG. 3. Thus, each component signal is
first filtered before it is sampled by analog-to-digital converter
160. The filter is used for anti-aliasing and to help eliminate out
of band noise. Thus, the filter 150 has a transfer function of H(s)
that spreads out the composite signal in the time domain. Since the
filter 150 is a linear system, each of the components can be
analyzed by assuming they have all gone through the filter
independently. The result is that an impulse will have energy
spread across all the sample periods.
[0025] FIG. 3 shows an exemplary output signal from signal filter
150 to which a series of periodic pulses W1 is fed. The respective
sample points P1 . . . 8 produced by the
analog-to-digital-converter 160 resulting from the pulses W1 fed to
filter 150 are shown in FIG. 3 by the vertical lines ending with a
crossbar. The magnitude of the sample at each sampling point is,
thus, a function of the magnitude of the impulse W1 and the impulse
response of the system. Since the time between the impulse W1 and
the sample time is constant, the size of the sample at P1 is a
constant times the magnitude of the impulse. This results in:
P 1 = k 1 W 1 P 2 = k 2 W 1 P 3 = k 3 W 1 and so on .
##EQU00001##
[0026] The results for the Red and IR components of the composite
signal can be represented in a similar way, as a constant
representing the impulse response at that time multiplied by the
current. This results in:
TABLE-US-00001 Red component IR component P1 = c1 R P1 = b5 I P2 =
c2 R P2 = b6 I P3 = c3 R P3 = b7 I ##EQU00002## ##EQU00003## and so
on.
[0027] The component of the signal representing the ambient light
can be approximated for a sample period (P1 . . . P8) by a linear
approximation A.sub.n between points X1 and X2 as shown in FIG. 4,
wherein point X1 is associated with sample time P1 and X2 is
associated with sample time P8. A new approximation A.sub.n+1
follows for the next eight samples as indicated in FIG. 4. The
terms for ambient light only can, thus, be represented as:
P 1 = X 1 P 2 = 6 7 X 1 + 1 7 X 2 P 3 = 5 7 X 1 + 2 7 X 2 P 4 = 4 7
X 1 + 2 7 X 2 and so on . ##EQU00004##
[0028] The magnitude of the sample for the composite signal is the
sum of all components. For instance:
P1=1X1+0X2+k1W1+k7W2+k5W3+k3W4+c1R+b5I
[0029] The whole system can, thus, be represented in matrix form
as:
[ P 1 P 2 P 3 P 4 P 5 P 6 P 7 P 8 ] = [ 7 7 0 7 k 1 k 7 k 5 k 3 c 1
b 5 6 7 1 7 k 2 k 8 k 6 k 4 c 2 b 6 5 7 2 7 k 3 k 1 k 7 k 5 c 3 b 7
4 7 3 7 k 4 k 2 k 8 k 6 c 4 b 8 3 7 4 7 k 5 k 3 k 1 k 7 c 5 b 1 2 7
5 7 k 6 k 4 k 2 k 8 c 6 b 2 1 7 6 7 k 7 k 5 k 3 k 1 c 7 b 3 0 7 7 7
k 8 k 6 k 4 k 2 c 8 b 4 ] [ X 1 X 2 W 1 W 2 W 3 W 4 R I ]
##EQU00005##
[0030] or as a matrix equation as:
{circumflex over (P)}={circumflex over (K)}{circumflex over
(L)}
[0031] After measuring samples P1, P2, P3 . . . P8, the individual
components of the composite signal can be isolated by solving the
system of equations.
{circumflex over (L)}={circumflex over (K)}.sup.-1{circumflex over
(P)}
[0032] In practice only the Red and IR components need to be solved
as the other components are usually of no interest. This can be
done by only computing the results for the bottom two rows of the
system. The matrix of coefficients is a constant determined by the
impulse response of the system. To solve the matrix for the Red and
IR components, the inverse of the matrix only needs to be computed
once for a particular front end filter 150, which can be done at
start-up if a variable filter design is used or during the design
of the system if the system uses a constant filter. Also an
adaptive filter might be used. Then, the computation has to be
performed after each adaptation.
[0033] As a result, the cable transients W can be eliminated from
the signal on a real time basis. Stray capacitances in the cable
will no longer be an issue. This also allows a front end to be
designed with a much tighter anti-aliasing filter which will reduce
noise and interference.
[0034] As mentioned above, a better performance can be achieved by
increasing the number of samples per Red and IR measuring period.
This oversampling will result in an over determined system that can
be solved by using a pseudo-inverse to the constant matrix which
gives a result that is a least squares fit to the sampled data. In
general more over sampling will result in a more accurate
measurement.
[0035] According to a further embodiment, the same technique can be
used for more than two wavelength signals. This may also result in
an over determined system that can be solved with a
pseudo-inverse.
[0036] The above described concept is not limited to the error
signals discussed, i.e., the ambient light signal and the cable
transients. Other known error sources can be included in the matrix
as discussed above.
[0037] FIG. 5 shows an example of a system for solving the matrix
equations. The data stream generated by the analog-to-digital
converter 160 is fed to a matrix 410. Separate equations 420 and
430 for the Red signal and for the IR signal are computed to solve
the matrix and generate the respective component signals for the
Red and IR signals without the external error signals introduced to
the signal fed to the analog-to-digital converter 160. The system
shown can be easily implemented in a digital signal processor,
microcontroller, or application specific integrated circuit
(ASIC).
[0038] The invention, therefore, is well adapted to carry out the
objects and attain the ends and advantages mentioned, as well as
others inherent therein. While the invention has been depicted,
described, and is defined by reference to particular preferred
embodiments of the invention, such references do not imply a
limitation on the invention, and no such limitation is to be
inferred. The invention is capable of considerable modification,
alteration, and equivalents in form and function, as will occur to
those ordinarily skilled in the pertinent arts. The depicted and
described preferred embodiments of the invention are exemplary
only, and are not exhaustive of the scope of the invention.
Consequently, the invention is intended to be limited only by the
spirit and scope of the appended claims, giving full cognizance to
equivalents in all respects.
* * * * *