Note: Descriptions are shown in the official language in which they were submitted.
CA 02688945 2009-12-21
SYSTEM FOR ASSET TRACKING
Cross Reference to Related' Application
This application claims priority from United States Provisional Patent
Application No. 611193,730 filed December 19, 2008 entitled System for Asset
Tracking.
Field of the Invention
This invention relates to the field of radio frequency identification systems
and
in particular to a system for asset tracking using radio frequency
identification and improved
antenna adapted for use in safety systems.
Background of the Invention
The system for asset tracking according to the present invention, including
the
improved antenna forming a part thereof, aims for example to assist in
improving workers'
safety in hazardous workplaces, when incorporated as part of for example a
radio frequency
identification (RFID) based personal safety system (PSS) such as described in
US patent
application 11/822,911 and published under publication number US02008-0018472-
Al on
January 24, 2008, which provides one example of how aspects of the present
invention may be
employed. The PSS, in summary, prevents an accident from happening between a
worker and
a mobile machine such as a forklift. In a typical workplace, there are a
number of forklifts that
circulate in close proximity to a group of workers, naturally increasing the
chances for an
accident to happen. The PSS is intended to improve the workers' safety in all-
time and real-
time radio-frequency (RF) wireless ranging system without any intervention
from the worker
or the machine operator by providing an independent monitoring wireless sensor
that logs the
distance between the forklift and the worker, and then controls the machine
for example slows
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down or stops the machine when this distance becomes less then a pre-defined
danger zone.
Machine operator and worker warnings are incorporated into the machine and the
workers vest
or harness. The PSS is intended to include indoor usage, where the use of a
conventional
narrowband technique is excluded because of it vulnerability to the multipath
and fading
signals, and unsuitable due to low accuracy in short distance ranging
applications. In Contra-
distinction spread spectrum systems use techniques that are specifically
suitable for
communication in severe multipath environments. The distance measurement
accuracy in
such systems is highly improved due to the wideband nature of the signal. The
Chirp
Modulation Spread Spectrum (CSS) is one kind of these techniques and presents
further
advantages when it comes to short distance ranging, such as removing of the
"near-far"
problem faced in short distance ranging with other systems.
The PSS uses directional antennas directionally detecting RFID tags within the
danger zones around the machine, and, cooperating with the tags worn by the
workmen,
antennas cooperating with the tags worn by the workmen, antennas cooperating
with the tags
where the antennas are distributed about the article worn by the workmen so as
to surround the
workmen. Where the article worn by the workman is a safety vest, the safety
vests may
include shoulder antenna and side antenna which each wrap around the vest so
as to be
exposed to both the front and back of the vest.
Summary of the Invention
In summary, the asset tracking system according to the present invention
includes one or more RFID readers and one or more RFID tags cooperating with
the readers,
and at least one garment, wherein the readers each include an RF wideband
transceiver and a
linearly polarized antenna. Each reader polls periodically a corresponding one
or more RFID
tag to determine a distance of a particular tag from each reader. Each reader
computes the
tag's distance in real-time and updates a corresponding database in real-time
for an on-demand
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reaction as determined by a processor in each reader. Each tag includes a
wideband
transceiver and an antenna array. The antenna array is implanted on each
garment and ensures
a spherical coverage around each tag. The antenna array includes two or more
patch antenna
distributed around each garment so as to provide full 360 degree and spherical
coverage
around each garment, and wherein each patch antenna is provided on a
dielectric. Coverage is
thereby provided by the patch antenna array and removes the need of having a
single line of
sight. Each tag may include an on-board processor adapted to adaptively switch
between the
antennas in the antenna array so as to select an appropriate antenna of the
array for a signal
from a corresponding reader.
Each patch antenna may be a linearly-polarized antenna with a high cross-polar
component so as to receive signals with random polarization, whereby the
reader's linearly-
polarized signals are received without losing 3dB of signal power that
otherwise would be lost
with a circularly polarized patch.
The readers may be a single reader adapted to track one or more of tags
without
the position of the single reader being known. The reader may be mounted on
either a mobile
or stationary platform. Where two or more readers are at known physical
positions with
respect to each other, the tags may be tracked and their coordinates
determined.
Brief Description of the Drawings
Figure 1 is, in plan view, a diagrammatic illustration of the reaction and
warning zones for forward and backward covering directional antennas mounted
on a piece of
mobile machinery, and the superposition of the coverage of a monopole antenna
also mounted
on the mobile machine.
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Figure 2 is a front view of a safety vest according to one embodiment of the
present invention.
Figure 3a is a plan view of an enlargement of one of the patch antenna of
Figure 2.
Figure 3b is an exploded view of the patch antenna of Figure 3a.
Figure 4a is a test worker standing during base line reading using dual front
and rear patch antennas on the workers vest.
Figure 4b is the test worker of Figure 4a facing the reader and stooping to
pick
up a box during detection testing.
Figure 4c is the test worker of Figure 4b right side onto the reader during
testing to detect the worker.
Figure 4d is the test worker of Figure 4c lying down, with right side on the
reader during detection testing.
Figure 4e is the test worker of Figure 4d stooping to pick up a box while
facing
away from the reader during detection testing.
Figure 4f is the test worker of Figure 4e holding a tin-foil covered box in
front
of his vest so as to cover the fi-ont patch antenna.
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Figure 5 is a magnitude versus radial degrees plot of the dual-antenna
arrangement, co-polar, when measured in the horizontal H-plane and the dual-
antenna is worn
by a test worker.
Figure 6 is a diagrammatic view of an RFID transponder tag and the
corresponding RFID transponder detection system.
Figure 7a and 7b are, respectively, diagrammatic representations of a multiple
sensor network, and a multiple sensor network mounted, in plan view, on a
vehicle.
Figure 8 is in plan view wheel loader bucket type, depicting a full coverage
zone.
Figure 9 is, in perspective view, a mobile machine having an appendage
moving in proximity to a worker.
Detailed Description of Embodiments of the Invention
In the PSS example, the worker's RF sensor includes the antenna connected to
a transceiver and a processor that communicates with the machine-mounted
sensor via the
transceiver and antenna. The machine mounted sensor monitors the paths that
the machine, for
example a forklift would take in the forward and backward directions, and
therefore includes
two directional antennas, namely a forward-looking and a rearward looking
antenna. Only one
of these antennas is activated at a time depending on the direction of
movement. Each of the
workers' sensors is given a unique identification (ID), so that the system is
a full RFID
wireless system. Moreover, many sensors with another set of IDs may be mounted
on objects
like walls or posts so that the machine may be programmed to react differently
according to
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the set of IDs detected. Some machine types only require a single monopole
antennae as per
Figure 18.
The PSS described by way of example herein inhibits accidents from happening
if workers are located in the front or the rear of a forklift. However, the
sides of the forklift are
not covered, as the front and rear directive antennas can not see the sides
due to their high
gain. Protecting the sides of a forklift could be advantageous, for example in
the instance of
another forklift driving towards the unprotected sides of the first forklift,
or if a workman is
present at the side of the first forklift and appears to be in a potential
danger. There are
different ways to cover the forklift sides. Installing another directional
antenna on each side is
one of the options. This technique requires the use of two extra RFID sensors
and their
antennas. This increases the total cost of the safety system.
Another solution includes using one monopole antenna connected to a RF
sensor that monitors the sides. The monopole's donut-shaped radiation pattern
allows for
coverage of both sides of the forklift, in addition to the front and the rear.
The monopole
coverage zone need not be larger than 10% of the directional ones.
This further embodiment of the PSS will thus include two directional antennas
8 for the forward and backward directions, A and B respectively in Figure 1,
and one
monopole antenna providing monopole coverage C for primarily monitoring the
sides of the
forklift F, as illustrated diagrammatically in Figure 1. The directional
antennas may be
connected either to two different RF sensors or to a single sensor by a single-
pole double-
throw (SPDT) switch. The direction of movement of the forklift selects which
sensor is ON in
the first case (using 2 sensors) or controls the switch through- way in the
second case (using a
SDPT).
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When available, if the monopole's and directional antennae sensors use
different radio channels, then they may be used simultaneously without
interfering with each
other. However, if they share the same communication channel, they would
employ a
switching mode to avoid jamming each other.
To give an example of operation such as illustrated in Figure 1, when the
directional antenna providing the warning and reaction zones G and H
respectively are
switched ON, a detected tag signal, for example with ID1, will be considered
as a worker
24present in that antenna's field-of-view (FOV). Next, when the monopole's
radio is ON (and
the other radios for zones G and H are OFF) then one the following scenarios
would be
possible:
1- If ID I is detected, then the worker 24 (ID I holder) is located in the
front
(or rear) of the forklift. The forklift was made aware of this presence
during the directional antenna ON step in the preceding cycle.
2- If ID2 is detected, then this ID holder must be located at one of the
forklift sides as ID2 was not detected in zones G or H during the
directional antenna ON step in the preceding cycle. The machine
operator is then notified accordingly.
3- If no ID is detected by the monopole antenna, then no one is present
near the sides, and the ID I located by the directional antennas is located
at a further distance from the front (or the rear) than the distance
covered by the coverage zone, for example partially that of zone H, of
the monopole antenna. However, the directional antenna may still keep
tracking the position of ID1.
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The monopole radio may use different types of modulation at different
frequency bands other than the directional antennae's. Using a narrowband
signal could be
sufficient and hence the switching mode would not be required.
The data available at the machine-mounted sensor may be forwarded to a
logging gateway or central computer through a Wi-Fi connection. This adds
another
dimension to the system. For instance, a designated supervisor can monitor any
deficiency in
the machines sensors or even send instructions to those sensors without
interruption to the
work pace.
In one embodiment of the Personnel Safety System, interrogators (for example
a reader) are installed on mobile machines and tags are worn by workers 24 (or
visitors) who
are going to be in proximity to the mobile machines for example forklift F.
System reliability
is important, so the communication between a reader and a tag must be
established without fail
every time the worker (tag) comes close to the machine (reader). To do so the
antennas on the
tag and the reader must be configured in such a way that a line of sight (LOS)
is always
guaranteed between them. A monopole, or any other antenna with an
omnidirectional
radiation pattern, mounted on the worker's hard hat would be one. This may
work for some
applications. However, in actual workplaces workers stack the hats on top of
each other or
treat them very roughly, and the possibility of damage to the antenna and
electronics which are
built into the hat is very high.
A safety vest 12 is another part of the safety equipment that workers are
typically required to wear at their workplace. The vest typically presents a
large profile or area
where electronics and an antenna may be embedded. Using a monopole-like
antenna
embedded in the safety vest is not practical because the worker's body
adversely affects the
performance of the antenna in two ways. First, the worker's body will
profoundly change the
antenna radiation pattern, leading to "dead spots" and more frequent non-line-
of-sight
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CA 02688945 2009-12-21
situations. Second, antenna mismatching and radiation absorption by the body
will strongly
decrease the antenna radiation efficiency. Furthermore, exposing the body to
excessive
radiowave radiation is not acceptable by safety guidelines for wireless
design, and useless
dissipation of power sharply reduces the life of the system battery.
Therefore, the antenna system 10 proposed according to the present invention
includes a shielding layer that prevents the body of worker 24, when wearing
vest 12, from
affecting the performance of antenna mounted on or in the vest. Microstrip
patch antennas
with ground plane are well-suited for such applications. In an embodiment
operatively at
2.4GHz, the maximum size of a single antenna is not large enough to wrap it
around the
typical vest and thus two or more antennas may be necessary to cover the whole
circumference
of the body. Covering the entire circumference is important so that the vest
may be detected
by the machine reader no matter which way the worker is oriented or turned
relative to the
machine.
Space diversity techniques are used, with two microstrip antennas 10a
integrated into the safety vest 12, shown simplified as a harness in Figure 2.
The topology
chosen for this application has one antenna system 10 in the front 12a and the
other in the back
12b of the upper parts of the opposite shoulders 14 of the vest 12. Other
configurations are
also possible, such as placing the antenna on the sides 16 of the vest over
the shoulders, or
even integrating antennas under the reflective strips 18 of the vest.
In a preferred embodiment such as seen in Figures 2, 3a and 3b, each antenna
system 10 integrated into or onto a safety vest 12 includes a microstrip patch
antenna 10a.
Each patch antenna 10a includes an antennae plate 20, such as supplied by AP
Circuits of
Calgary, Alberta, Canada, which is mounted or built on a low cost rigid
substrate 22.. The use
of a hard substrate 22 is mainly to eliminate any bending and warping effect
that would affect
the antenna performance. The total antenna size may advantageously be about
60x60 mm (2
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CA 02688945 2009-12-21
6/16 x 2 61116 inches), and is fed through an inset feed 20a. The patch
antennae dimensions are
optimized to cover the entire ISM frequency band 2.4-2.485GHz. The antenna
back plane 22a
may be mounted on a rubber backing material 22h, which may have a reflective
coating, and
which may advantageously elevate the antennae from the front shoulder 12a and
rear shoulder
23 of worker 24. Plate 20 may also be in one embodiment convex rather than
planar, which
may improve the wave pattern of the antennae. Whether elevated for improved
performance
due to increased LOS visibility, the antenna system 10 may be fitted into
pockets built into the
shoulders of the vest, wherein, again, reference to vests herein is intended
to include other
garments including harnesses.
The size of the patch antenna plate 20 can be further reduced if high
permittivity dielectric such as ceramic is used. For example, the size may be
reduced to 25x25
mm by using dielectric with dielectric constant of 12.
In one embodiment, not intended to be limiting, inset feed 20a is provided
with
a coax connector 20b. It may also be useful to empty so-called leaky coax
which when
electrically connected to the antenna and a length distributed over or around
the workman's
shoulders may decrease areas of reduced coverage.
The patch antenna 10a has proven to have great immunity against the human
body effect. In fact, its input impedance seems to see almost no effect
whether the antenna is
in free space or placed anywhere against the body of for example worker 24.
The matching
level generated by the inset feed is good enough to keep the antenna impedance
tuned
regardless of how the antenna is used.
In experiments, the antenna fabricated on standard epoxy measured a gain of
3.5 dB and 3.2 dB in free space and on body, respectively, which is sufficient
for this
application. The E- and H-planes were measured in free space and on the body.
In free space,
CA 02688945 2009-12-21
the 3-dB beam aperture at 2.45 GHz was 76 degree and 97 degree in the E- and
H-planes,
respectively. When worn, the 3-dB beam aperture at 2.45 GHz became 61 degree
and 127 in
the E- and H-planes, respectively. These angles helped to determine the number
of antennas
required and the angle of orientation of the antenna on the body that provide
the best coverage
in the azimuthal plane and that ensure a full coverage of the worker's body
boundary.
Several processing schemes may be used to transmit and receive by either of
the two antennas. A selecting scheme selects the antenna that presents the
highest Signal-to-
Noise (SNR) ratio. A combining scheme maintains the connection on both
antennas and
weights the received signals to deliver the desired signal. The Switching
scheme is the
simplest method. It switches the front-end input between the receiving
antennas and selects
the received signal with a level higher than a certain threshold. An improved
switching
scheme was tailored for this application in which the RF front-end compares
the signal level
received by the two antennas and, in addition, ranges the Reader and then
selects the shorter
distance to filter out the reflected path. The connection between the front-
end and the antennas
is made by an RF switch integrated on the Tag PCB. Using more than one antenna
is also
possible by using a Single Pole Multiple Throw RF switch.
The radiation pattern of two antennas placed on the front and back,
respectively, is shown in Figure 5. The radiation pattern diagrams show that
the superposition
of the front and back antennas' radiation patterns overlap quite nicely to
cover the full 360
degrees circumference around the body. The mild dip of about 5dB at the 90
elevation is
expected due to the nature of the flat antenna radiation, and also some
masking by the
shoulders of the person wearing the vest.
In fact, the radiation patterns 28a and 28b in Figure 5 of, respectively, the
front
and back antenna show that the front antenna assures high intensity signal on
the low elevation
fore left side and high intensity signal on the low elevation back right side
for the back
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CA 02688945 2009-12-21
antenna. However the signal gets weaker on the frontal right hand side for the
front antenna
and the rear left side for the back antenna. Therefore, one possible way to
improve the side
coverage can be achieved by adding a second antenna on each side of the
shoulders, that is
front left, front right, rear left and rear right antenna, to have a full and
uniform coverage. This
could work well; it would increase, however, the switching time between the
antenna array
elements.
In a set of tests, detection of a worker 24 was measured when the worker was
at
various angles and in various postures such as seen in Figures 4a - 4f. Again
a single worker
was employed with a single reader. The worker and rear separation was 17
meters. The
workers tag and reader output power was minus 20dBm. The reader had a monopole
antenna.
The workers vest had front and rear patch antennas on opposite shoulders.
Figure 4a depicts
the worker in the baseline pose. Figure 4b depicts the worker bending to pick
up the box while
standing side-on to the reader. Figure 4d depicts the worker lying or prone
simulating the
worker performing work while lying down or sleeping. Figure 4e depicts the
worker lifting
the box while facing away from the RFID reader. Figure 4f depicts the worker
holding the
box in front of the vest shoulder mounted antenna. The box, as tested, was
covered with tin
foil.
In the testing where the worker was in the stance of Figure 4b, the front
antenna
20 on the vest 12 was detected and the rear antenna 20 gave reflected path
data. In the stance
of Figure 4c, the front antenna, that is on the shoulder 14 of the vest
closest to the RFID
reader, was detected. In the stance of Figure 4d, the front antenna of the
workers vest was
detected. In the stance of Figure 4e, the rear antenna of the workers vest was
detected and the
front antenna gave reflected data. In the stance of Figure 4f visual
evaluation of the data
showed no significant problems. In this set of tests the worker was detected
in all stance
positions. Reflected path measurements were present. At least one sensor
reported correct
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ranging distance in all instances. The error rates were similar to the
baseline testing although
the broadcast miss-rate was slightly higher than that of the baseline testing.
The conclusions were thus drawn that the carrying of box 26 in front of a
workers antenna 20 on the workers vest 12 had little effect on detection of
the worker by the
RFID reader and that the various box carrying and lifting scenarios showed
substantially no
difference based on the box positions. It was further concluded that the dual
antenna provided
full 360 degrees of coverage for the worker wearing the vest, not withstanding
that results
from individual rotation tests suggested a 5 degree angle on each side where
measurements
may not have been reliable. Thus the dual antenna was an improvement and not
merely the
sum of the individual antennas as the dual antenna provided very good results
during the 360
degree turn tests in the 5 degree angle positions where measurements were not
as reliable
when testing the individual antennas. It was determined that current antenna
linear vertical
polarization was sufficient to provide detection when the test worker was bent
over.
It should be noted that a fully integrated solution is possible by using
fabric
antennas directly sewn on the safety fabric. A simple fabric antenna can be
made of a sheet of
conductive fabric laid on an unwoven fabric material such as fleece or
polyester. This will
remove the need to use coaxial cables to connect the PCB to antennas 20
mounted for example
by epoxy on rigid substrates 22
In the preferred embodiment antennae, plate 20 is planar. The top layer 20a of
the patch Antenna plate 20 may be mounted onto an etched upper surface on
planar dielectric
substrate 22. This kind of antenna radiates with a quite a uniform field
intensity in the upper
hemisphere. Typical the radiation pattern of a patch antenna shows strong
radiation intensity
in the boresight direction, and then slow decrease of the intensity as the
observer or RFID
reader is off the boresight. This is expected for this non high directional
antenna. At
elevations close to 90 degrees from the boresight, the signal intensity is
about 10dB below the
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peak signal. This may lead to losing signal tracking when the patch antenna is
ranged from the
side of the shoulders. Using two or more antennas proved to improve signal
coverage without
losing signal tracking. However, it has been noticed that, it would be
advantageous to slightly
redesign the antennae to allow a larger side lobe that would extend past the
wearers shoulder
creating a more uniform wave pattern around the worker. This would allow lower
overall
system power levels increasing battery life on the vests.
Human body style is a contributing factor for spherical wave pattern coverage
around the worker. Antennae alignment and proper fit of the garment are both
important
considerations and perhaps as many as six different varieties or sizes of vest
will be needed to
ensure proper fit. For example on a barrel chested worker the antennae will
not be aligned
properly on a vertical plane and so the addition of a shaped piece of material
(perhaps rubber)
under the antennae will work to properly align the antennae within the vest.
Every worker will
have to be test fit for their vest or harness to ensure adequate coverage.
These dips in wave pattern coverage only come into play at the lower power
levels and with an ill fitting vest slightly higher base power level would
have to be employed.
The current patch antenna may be made more sensitive to receive any reflected
wave by making it a circularly polarized patch. This may further increase the
patch antenna
gain for the non-collinear signals, and hence making the patch more
susceptible to signals
from the Reader's linearly polarized antenna received with any random
orientation.
An RFID system such as shown by way of example in Figure 6 incorporating a
garment having a transponder (and corresponding antenna mounted and arranged
according to
the present invention) offers the unique feature of being able to accurately
range an object, or
person, or a machine equipped with the same RFID system, from any angle by the
RFID
reader. The ranging accuracy may be as accurate as 0.5 meters indoors or
outdoors regardless
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CA 02688945 2009-12-21
of the multipath level. This unique combination of ranging by the RFID reader
and garment
(such as vest 12) coverage makes this system very competitive to track using
an RFID reader
and locate any assets such as those wearing a RFID tag or a garment in any
environment
without worrying about keeping a single line of sight between the RFID reader
and the asset.
At slow speed we use a very lower power level to keep the machine's wave
pattern spread to a minimum. As the machine speeds up the system automatically
shifts to our
operating power level (we don't keep increasing the power levels). But for a
very long range
detection over 100 meters the power levels may be amplified depending upon the
work
environment.
In one embodiment of the present invention, the power level is automatically
adjusted with the speed of the mobile machine. The faster the machine is
moving, the higher
the power level. This leads to the problem, however that the most dangerous
scenario for a
worker is when the mobile machine is close to the worker and moving slowly,
which may be
counter-intuitive to what would be expected. That is, it may conventionally be
assumed that a
mobile machine would be most dangerous while moving quickly. In the instance
of a piece of
mobile equipment that has a moving appendage or tool it may be desirable to
have two
different non-interfering detection systems, for example an excavator such as
the one in Figure
9 has the ability to swing the digging boom in one direction while the whole
machine moves in
a different direction. In a case like this both of the sensing systems would
send inputs to the
main processor and would not cancel out each others signals.
System design can incorporate totally enclosed combined antennae/processor
units - each one capable of individual operation or they can interact with one
another for use
where triangulation is desirable for example on large equipment such as haul
trucks or earth
moving equipment.
CA 02688945 2009-12-21
By using complete units the system can have real-time asset location and
tracking and if coupled with a central processor the system accuracy and
distance locating is
speeded up considerably. (Wireless communication or hard wired).
With this application of the present invention and as diagrammatically
illustrated in Figures 7a and 7b, we can determine which one of eight or more
zones an asset is
in and in the way an appropriate response to the situation can be determined.
The vest is an integral part of the whole system that can be tailored to suit
any
jobsite requirements. Some machines such as skidsteer loaders only need a
small reaction zone
and possibly no warning zone - depending entirely upon the jobsite situation.
Our customers
and regulatory bodies for workplace safety can decide the exact requirements
and our system
can be programmed to meet those requirements. The system can be a basic-
worker
detection/warning apparatus, or be integrated into the mobile equipment
control system in
order to automatically activate certain pre-programmed responses.
Until anti-lock braking systems become standard on mobile equipment, this
safety system will not reach its full potential. -Right now it is possible to
slow these machines
by adjusting engine RPM and or throttle settings, but bringing a machine to a
safe stop
automatically would likely require anti-lock braking technology.
In situations where, instead of the machine being mobile, the machine is
stationary and the worker is the only mobile player in the scenario; in such
circumstances,
instead of or in conjunction with, the system of RFID tracking and the use of
the antennae
described herein may be supplemented by a magnetic-field-based warning zone
detection
system for example using magnetic field sensor's according to the RuBee IEEE
standard. In
the prior art applicant is aware of United States Patent No. 5,939,986, which
issued on August
17, 1999 to Schiffbauer et al.
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As will be apparent to those skilled in the an in the light of the foregoing
disclosure, many alterations and modifications are possible in the practice of
this invention
without departing from the spirit or scope thereof. Accordingly, the scope of
the invention is
to be construed in accordance with the substance defined by the following
claims.
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