U.S. patent application number 10/428540 was filed with the patent office on 2003-10-30 for method and apparatus for electromagnetic position and orientation tracking with distortion compensation employing a modulated signal.
Invention is credited to Khalfin, Igor.
Application Number | 20030201767 10/428540 |
Document ID | / |
Family ID | 29710128 |
Filed Date | 2003-10-30 |
United States Patent
Application |
20030201767 |
Kind Code |
A1 |
Khalfin, Igor |
October 30, 2003 |
METHOD AND APPARATUS FOR ELECTROMAGNETIC POSITION AND ORIENTATION
TRACKING WITH DISTORTION COMPENSATION EMPLOYING A MODULATED
SIGNAL
Abstract
A bounding box or volume of interest is flooded with a modulated
AC electromagnetic signal from a source. Different types of
modulated signals may be used, including single-tone AM and FM. One
or more sensors disposed on an object or body within the volume are
then used to detect the signal, and a digital and/or analog
spectral and phase analysis is performed on the received signal in
hardware or software. The processing distinguishes between the
direct source to sensor response and the response due to eddy
currents. By removing the response due to the distorters, the
effects of the electromagnetic distortion can be removed through a
more conventional "free-space" solution. The invention finds
applicability in a wide variety of environments, including head
tracking systems and helmet-mounted displays for fighter aircraft;
head trackers for armored vehicles; medical-guided surgery and
biopsy; remote sensing, among other potential uses.
Inventors: |
Khalfin, Igor; (Pleasanton,
CA) |
Correspondence
Address: |
John G. Posa
Gifford, Krass, Groh, Sprinkle,
Anderson & Citkowski, P.C.
280 N. Old Woodward Ave., Suite 400
Birmingham
MI
48009-5394
US
|
Family ID: |
29710128 |
Appl. No.: |
10/428540 |
Filed: |
May 2, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10428540 |
May 2, 2003 |
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10164081 |
Jun 4, 2002 |
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10428540 |
May 2, 2003 |
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09430978 |
Nov 1, 1999 |
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6400139 |
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60377918 |
May 2, 2002 |
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Current U.S.
Class: |
324/207.17 ;
324/248; 324/252; 324/253; 600/407 |
Current CPC
Class: |
G01V 3/08 20130101; G01S
5/02 20130101; G01S 1/024 20130101; A61B 5/062 20130101; G02B
27/017 20130101; A61B 5/06 20130101 |
Class at
Publication: |
324/207.17 ;
324/248; 324/252; 324/253; 600/407 |
International
Class: |
G01B 007/14; G01B
007/30 |
Claims
I claim:
1. A method of determining the position and orientation of an
object or body within a bounded volume containing an AC
electromagnetic field distorter, comprising the steps of:
modulating an AC magnetic field carrier frequency with a modulation
waveform to generate a source signal having induction-vector
components corresponding to the carrier and modulation
(satellites); stabilizing magnitude and phase of the modulated
source signal with respect to the internal reference providing a
sensor to measure the induction-vector components at the location
of the object or body; analyzing the induction-vector components of
the carrier and satellites to distinguish between source-sensor
coupling and source-distorter-sensor coupling; and using the
source-sensor coupling to compute the position and orientation of
the sensor, and hence, the object or body.
2. The method of claim 1, wherein the AC magnetic field carrier is
amplitude modulated.
3. The method of claim 1, wherein the AC magnetic field carrier is
frequency modulated.
4. The method of claim 1, wherein the AC magnetic field carrier is
amplitude or frequency modulated with a single tone.
5. The method of claim 1, wherein the frequency of the modulation
is somehow lower than the carrier.
6. The method of claim 1, wherein the steps (generation and
measurements) are performed within narrow frequency bands.
7. The method of claim 1, wherein the symmetry between carrier and
satellites is known.
8. The method of claim 1, further including the steps of:
positioning at least one stationary witness sensor near or within
the volume of interest; measuring the induction-vector components
at the witness sensor using a known fixed position and orientation;
and using the induction-vector components from each witness sensor
to more accurately compute the position and orientation.
9. The method of claim 1, wherein the signal received by the sensor
is a time derivative of the source signal multiplied by a coupling
constant.
10. The method of claim 1, wherein the object or body includes a
person's head or other body part.
11. The method of claim 1, wherein the object or body includes a
medical instrument.
12. The method of claim 1, wherein the object or body is associated
with remote sensing.
13. The method of claim 1, wherein the steps are performed in real
time or near real time.
14. The method of claim 1, wherein the sensor operates along two or
more independent axes to detect multiple degrees of freedom.
15. The method of claim 1, wherein the sensor is a magnetic-field
search coil.
16. The method of claim 15, wherein the three non-parallel sensor
search coils and three non-parallel source coils for six degrees of
freedom.
17. The method of claim 1, wherein the sensor is a solid-state (GMR
or PSS), quantum (SQID), or flux gage magnetic flux sensor.
18. A system for determining the position and orientation of an
object or body within a bounded volume containing an AC
electromagnetic field distorter, comprising: a source of a
modulated AC magnetic field having induction-vector components; an
electronic circuit stabilizing magnitude and phase of the modulated
source signal with respect to the internal reference; a sensor to
measure the induction-vector components at the location of the
object or body; and one or more processors to perform the following
functions: analyze the induction-vector components of the carrier
and satellites to distinguish between source-sensor coupling and
source-distorter-sensor coupling, and compute the position and
orientation of the sensor, and hence, the object or body based on
the source-sensor coupling.
19. The system of claim 18, wherein the AC magnetic field is
amplitude modulated.
20. The system of claim 18, wherein the AC magnetic field is
frequency modulated.
21. The system of claim 18, wherein the AC magnetic field carrier
is amplitude or frequency modulated with a single tone.
22. The system of claim 18, wherein the frequency of the modulation
is somehow lower than the carrier.
23. The system of claim 18, wherein the AC magnetic field
components are generated and measured within narrow frequency
bands.
24. The system of claim 18, wherein the symmetry between carrier
and satellite frequencies is known.
25. The system of claim 18, wherein the sensor operates along two
or more independent axes to detect multiple degrees of freedom.
26. The system of claim 18, further including a stationary witness
sensor positioned near or within the volume of interest to measure
the induction-vector components using a known fixed position and
orientation.
27. The system of claim 18, wherein the signal received by the
sensor is a time derivative of the source signal multiplied by a
coupling constant.
28. The system of claim 18, wherein the object or body includes a
person's head or other body part.
29. The system of claim 18, wherein the object or body includes a
medical instrument.
30. The system of claim 18, wherein the steps are performed in real
time or near real time.
31. The system of claim 18, wherein the sensor operates along two
or more independent axes to detect multiple degrees of freedom.
32. The system of claim 18, wherein the sensor is a magnetic-field
search coil.
33. The system of claim 18, wherein the three non-parallel sensor
search coils and three non-parallel source coils for six degrees of
freedom.
34. The system of claim 18, wherein the sensor is a solid-state
(GMR or PSS), quantum (SQID), or flux gage magnetic flux
sensor.
35. In a tracking system of the type wherein source of an AC
electromagnetic field having induction-vector components is
received by a sensor on an object or body within a bounded volume
to determining the position and orientation of the object or body
in the presence of a field distorter, the improvement comprising:
modulating the AC electromagnetic field; stabilizing magnitude and
phase of the modulated source field components analyzing the
induction-vector components corresponding to the carrier and
modulation frequencies and detected by the sensor to distinguish
between source-sensor coupling and source-distorter-sensor
coupling; and computing the position and orientation of the sensor,
and hence, the object or body based on the source-sensor coupling.
Description
REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority from U.S. Provisional
Patent Application Serial No. 60/377,918, filed May 2, 2002; and is
a continuation of U.S. patent application Ser. No. 10/164,081,
filed Jun. 4, 2002, which is a continuation-in-part of U.S. patent
application Ser. No. 09/430,978, filed Nov. 1, 1999, now U.S. Pat.
No. 6,400,139. The entire content of each application is
incorporated herein by reference.
FIELD OF THE INVENTION
[0002] This invention relates generally to position/orientation
tracking and, in particular, to methods and apparatus for
accurately tracking position, orientation and movement within a
volume in the presence of electromagnetic distortion and/or
noise.
BACKGROUND OF THE INVENTION
[0003] Position and orientation tracking systems ("trackers") are
well known in the art. For example, U.S. Pat. Nos. 4,287,809 and
4,394,831 to Egli et al.; U.S. Pat. No. 4,737,794 to Jones; U.S.
Pat. No. 4,314,251 to Raab; and U.S. Pat. No. 5,453,686 to
Anderson, are directed to AC electromagnetic trackers. U.S. Pat.
No. 5,645,077 to Foxlin discloses an inertial system, and
combination systems, consisting or two different trackers, such as
optical and magnetic, are described in U.S. Pat. No. 5,831,260 to
Hansen and U.S. Pat. No. 6,288,785 B1 to Frantz et al. Other
pertinent references include U.S. Pat. No. 5,752,513 to Acker et
al. and U.S. Pat. No. 5,640,170 to Anderson.
[0004] AC electromagnetic trackers have definite advantages over
other types of systems. For one, AC trackers provide the highest
solution/update rate with the greatest accuracy, not affected by
obstructed field of view, in contrast to optical solutions. AC
trackers do not require reference sensor/unit and drift stable
apparatus of the type required by inertial units, and they are not
affected by the Earth's magnetic field and ferrous materials, in
contrast to DC magnetic systems.
[0005] The main disadvantage of AC trackers is that they are quite
susceptible to distortion due to eddy currents in conductive
materials in or near the motion box. To overcome this phenomenon,
magnetic trackers often require costly and time-consuming
calibration/mapping procedures to function correctly in the
distorted environment. With mapping, the magnetic field profile is
measured at multiple points associated with the volume of interest
(motion box) prior to the actual tracking, as discussed in commonly
assigned U.S. Pat. No. 6,377,041 to Jones et al., and some of the
references cited therein. While mapping may be done quickly and
accurately, any changes in the motion box will require repeating of
the mapping procedure.
[0006] Another approach, described in U.S. Pat. No. 6,147,480 to
Osadchy et al., allows the AC tracker to trace moving metal
(distortion) by measuring the signal without distortion (acquiring
baseline signals). This signal is then compared with a signal in
the presence of distorting object(s) by measuring the phase error
of the received signal. While such a system does work, it is not
always practical to acquire baseline signal without distortion; in
many cases, in an aircraft cockpit, for example, the distortion is
always present.
[0007] The approach described in U.S. Pat. No. 6,172,499 B1 to
Westley introduces at least two frequencies per source channel, and
uses the difference in responses to compensate for the eddy current
distortion. This approach requires a guess about the eddy currents
loop geometry, and the efficiency of the distortion compensation
and operational frequencies depends on assumptions regarding the
distorted environment, including the physical characteristics of
the distorting materials where the system will be working. In
addition, this approach requires a comparatively wide-band receiver
(sensor and ADC processing), thus reducing noise stability.
[0008] The methods and apparatus for distortion compensated AC
tracking described in our commonly assigned U.S. Pat. Nos.
6,400,139 and 6,369,564, both to Khalfin et al., take advantage of
wired "witness" sensors to obtain real-time information concerning
the distortion (this is done by analyzing field profile that is
superposition of the source field and distortion fields at the
locations of "witness" sensors given sufficient "witness" sensors
data). In addition, the '564 patent describes the signal processing
from a resonantly tuned wireless passive sensor, 90.degree. phase
shifted with respect to the source to enable separation of the
distortion signal.
[0009] Despite these advances, the need remains for apparatus and
methods of compensation for spurious, eddy-current-induced fields
in AC electromagnetic tracking systems. Such a solution could take
advantage of the fact that the electromagnetic coupling which
creates these eddy currents is strongly dependent on the frequency
of the transmitted AC magnetic field. In addition, eddy currents
are phase shifted with respect to the magnetic tracker source drive
current that generates the magnetic field.
SUMMARY OF THE INVENTION
[0010] According to the system and method described herein, a
bounding box or volume of interest is flooded with a modulated AC
electromagnetic signal from a source. Different types of modulated
signals may be used, including single-tone AM and FM. One or more
sensors disposed on an object or body within the volume are then
used to detect the signal, and a digital and/or analog spectral and
phase analysis is performed on the received signal in hardware or
software. The processing distinguishes between the direct source to
sensor response and the response due to eddy currents. After
removing the response due to the distorters, the electromagnetic
position/orientation problem can be treated through a conventional
"free-space" solution.
[0011] The disclosed system and methods do not require witness
sensors, though the approach may be used in a combination with
them. The invention operates in a narrow frequency band to ensure
noise stability, and preferably uses high operating frequencies
(e.g., of about 20 kHz-50 kHz) to ensure high signal quality and
increased operation range.
[0012] A rapid solution update rate may be used to achieve real
time (per frame) distortion compensation without any prior
knowledge about physical properties of the distorters. At the same
time, the system preserves all known advantages of AC trackers, but
without the need for a calibration/mapping procedure, which has
proven to be the main obstacle to more widespread applications of
AC electromagnetic tracking technology.
[0013] The invention finds applicability in a wide variety of
environments, including head tracking systems and helmet-mounted
displays for fighter aircraft; head trackers for armored vehicles;
medical-guided surgery and biopsy; remote sensing, among other
potential uses.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a block diagram of a preferred embodiment of the
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0015] According to the invention, a source processor generates a
modulated waveform, for example, a single-tone FM or single-tone AM
signal, preferably having a modulation frequency close to the
carrier frequency (for example, 30 kHz carrier and 20 kHz
modulation). In the preferred embodiment, the waveform is detected
by a sensor containing several non-coplanar coils, and a
demodulation and spectral analysis is performed on the received
signal.
[0016] The symmetry pattern of the carrier/satellites allows the
system to distinguish between source-sensor coupling and
source-distorter-sensor coupling, since each coupling modality is
frequency-dependent and has unique phase characteristics. The
restored source-sensor response then is plugged into a "free-space"
solution which is well known in the art.
[0017] Thus, in contrast to prior-art systems, the inventive
approach monitors and stabilizes the phase and the magnitude of the
source (transmitter) current with respect to the internal
reference. Without such stabilization, source drive current may
change due to the electromagnetic coupling between the source and
conducting materials in the volume of interest, as well as because
of drift in electronics hardware.
[0018] A functional diagram of a system according to the preferred
embodiment of the invention is presented in FIG. 1. A source
processor 111 generates the signal components representative of the
modulated waveform. The modulated signal feeds source drivers 112
and is transmitted by a B-field source 113.
[0019] The modulated signal is received by a sensor or sensors 121,
and the induction-vector components are demodulated and analyzed by
analog and/or digital signal processors 122 and 123. At this stage,
the signal is cleansed from the effects of the electromagnetic
distortion. The resulting data is then transmitted to the
"free-space" position and orientation processor 131, and the result
is output to a utilization device via I/O controller 132.
[0020] It should be noted that the system with additional sensors
working as "witnesses" might be arranged thus providing additional
distortion and error correction. The use of witness sensors is
disclosed in U.S. Pat. No. 6,400,139 to Khalfin et al., the entire
content of which is incorporated herein by reference.
[0021] The number of coils per sensor and per source may vary,
depending on the number of DOF (degrees of freedom) to be measured.
It is preferred to have three nonparallel sensor coils and three
non-parallel source coils for six degrees of freedom. In addition
to search coils, the sensors may use any appropriate magnetic flux
sensing device including, but not limited to, solid-state (GMR or
PSS), quantum (SQID), or flux gage sensors.
EXAMPLE
[0022] The method of distortion compensation in the case of
single-tone FM modulation will now be described with the
understanding that various alternative modulation schemes may be
used.
[0023] The waveform generated by the source drive may be given
as:
cos(2.pi.f.sub.ct-.alpha.sin(2.pi.f.sub.mt)) (1)
[0024] This signal has well-pronounced components at
f.sub.c-f.sub.m, f.sub.c, and f.sub.c+f.sub.m,
f.sub.c>f.sub.m.
[0025] The sensor(s) receive a signal that is time derivative of
the source signal multiplied by a coupling constant that contains
all sufficient information about position and orientation of the
sensor with respect to the source. In a non-distorted environment,
the data is sampled at 90 degrees behind the source, i.e., -sin DFT
(discrete Fourier transform) and, after the DFT, the satellite and
carrier components are: 1 2 b = S1 ( 2 a ) b = S2 ( 2 b ) - 2 b =
S3 , ( 2 c )
[0026] where S1, S2, and S3 are sensor responses normalized to the
frequencies, b corresponds to the matrix element of the coupling
matrix: .vertline..vertline.b.vertline..vertline.=.mu..mu..sub.o
Att A.sub.eff D M. All further considerations have as their goal
the restoration of S2 in a distorted/scattering environment.
[0027] In the presence of distortion, an additional term quadratic
with respect to the frequency (linear, after normalization) appears
with magnitude .alpha. and with the phase shift of the carrier
frequency .phi., given that in this case the data are sampled at
the zero phase of the non-distorted response. In general, the
satellites exhibit different phase shifts from the carrier which
are proportional to the deviation of frequencies. Note, no
information or estimates about the values of .phi. and a are
necessary for further computations.
[0028] For the Im part of the acquired signal, e.g., 90 degrees
behind the non-distorted source phase, we have: 2 - 2 a 2 ( f c - f
m ) sin ( - ) + 2 b = SD1 ( 3 a ) -a2.pi.(f.sub.c)sin(.phi.)+b=SD2
(3b) 3 2 a 2 ( f c + f m ) sin ( + ) - 2 b = SD3 ( 3 c )
[0029] For the Re part we have: 4 - 2 a 2 ( f c - f m ) cos ( - ) =
CD1 ( 4 a ) -a2.pi.(f.sub.c)cos(.phi.)=CD2 (4b) 5 2 a 2 ( f c + f m
) cos ( + ) = CD3 ( 4 c )
[0030] Combining terms in equations (3) and (4), and using symmetry
of the satellites with respect to the carrier, we arrive at: 6 (
SD1 + SD3 ) 2 + ( CD1 - CD3 ) 2 - 2 CD2 2 = 2 ( SD2 - S2 ) 2 f m 2
f c 2 ( 5 )
[0031] Taking into account practical consideration that the
distortion contribution to the signal is not greater than the
direct response and removing the ambiguity of sign we obtain the
result: 7 S2 = b = sign ( SD2 ) [ - 1 f c f m ( SD1 + SD3 ) 2 + (
CD1 - CD3 ) 2 - 2 CD2 2 + sign ( SD2 ) SD2 ] ( 6 )
[0032] Equation (6) defines a restored, "non-distorted" signal in
the presence of distortion.
[0033] The values of b from different sensor coils (3 for 6DOF) are
sufficient to find position and attitude matrix as it was noted
after equation (2).
* * * * *