Note: Descriptions are shown in the official language in which they were submitted.
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MONITORING FUEL CELLS USING RFID DEVICES
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates generally to uses of radio frequency
identification (RFID) devices in fuel cells, and, more particularly, to
monitoring cell
voltages and other operating parameters in solid polymer electrolyte fuel cell
stacks.
Description of the Related Art
Electrochemical fuel cells convert fuel and oxidant to generate electrical
power and reaction products. A representative type of fuel cell is the solid
polymer
electrolyte fuel cell which employs a solid polymer, ion exchange membrane
electrolyte. The membrane electrolyte is generally disposed between two
electrode
layers (a cathode and an anode layer) to form a membrane electrode assembly
(MEA).
In a typical solid polymer electrolyte fuel cell, the MEA is disposed between
two
electrically conductive separator or fluid flow field plates. Fluid flow field
plates have
at least one flow passage formed therein to direct a fluid reactant (either
fuel or oxidant)
to the appropriate electrode layer, namely, the anode on the fuel side and the
cathode on
the oxidant side. The flow field or separator plates also act as current
collectors and
provide mechanical support for the MEAs.
Since the output voltage of a single fuel cell is relatively low (e.g., less
than one volt under load), fuel cell power supplies typically contain many
cells that are
connected together, in series or in parallel, in order to increase the overall
output
voltage and power of the supply. In a series configuration, the fuel cells are
typically
arranged in a stack such that one side of a given flow field plate serves as
an anode side
plate for one cell while the other side of the plate serves as the cathode
side plate for the
adjacent cell. Such a flow field plate is referred to as a bipolar plate. A
stack of
multiple fuel cells is referred to as a fuel cell stack. The fuel cell stack
is typically held
together in its assembled state by tie rods and end plates. A compression
mechanism is
generally required to ensure sealing around internal stack manifolds and flow
fields,
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and also to ensure adequate electrical contact between the surfaces of the
plates and
MEAs.
Depending on the application, significant subsystems and controls rnay
be required to turn a fuel cell stack into a practical power supply. For
instance,
subsystems generally must provide reactants to the stack at proper pressures
and rates in
accordance with the applied electrical load. The practical operation of a
complete fuel
cell system can thus be quite complex and various process or operating
parameters may
need to be monitored to provide feedback for satisfactory control and/or to
provide a
warning in the event of an impending problem condition.
An example of an important potential problem condition in series stacks
is voltage reversal in a cell (or cells). (Voltage reversal can occur in a
weaker cell in a
series stack when that cell is incapable of providing current at the same
level as other
cells in the stack. In such a situation, a sufficiently high current generated
by the other
cells in the stack is forced through the weaker cell and drives it into
voltage reversal.)
Aside from being associated with a reduction in output power, voltage reversal
also can
result in internal damage to the reversed cells and the stack. It can
therefore be useful
to monitor individual cell voltages and to detect for any abnormally low
voltage during
operation in order to provide advance warning of a voltage reversal condition.
In, turn,
corrective action can then be taken to prevent cells from undergoing voltage
reversal,
and thus prevent any reversal-related damage from occurring.
However, it has proven difficult to develop a suitable cell voltage
monitor (CVM) for this purpose. A typical CVM collects voltage data via
suitable
electrical connections to the individual cells. Signals representative of the
cell voltages
are then generated and supplied to a processor which then determines whether a
problem condition exists and initiates appropriate action. Since the typical
processor
cannot handle high common mode voltages (i.e., voltages with respect to a
common
voltage or common ground) and since the voltages encountered in the typical
series
stack can be quite high (e.g., up to hundreds of volts between cells), the
generated
signals are usually electrically isolated from the cells themselves via
appropriate
isolation circuitry. Problems have been encountered though with the electrical
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connections made to the cells and with the circuitry that generates the
electrically
isolated signals representative of the cell voltages.
With regards to making electrical connections to the cells, the assembly
required is very labour intensive and it is becoming more difficult to align
and install
contacts as the designs of fuel cells advance and as the separator plates
become
progressively thinner and more closely spaced. Further, variations in the cell-
to-cell
spacing (due to manufacturing tolerances and to expansion and contraction
during
operation of the stack) must be accommodated. Further still, the fuel cell
stack may be
subject to vibration and thus reliable connections must be able to maintain
contact even
when subjected to vibration.
The signal generationlelectrical isolation circuitry in a CVM is desirably
located close to the electrical connections to the cells and hence close to
the stack.
(This minimizes the high voltage hardware required and the size of the
hazardous
voltage region in the system. Also the possibility of inadvertently shorting
out cells in
the stack through the CVM may be reduced.) However, in the immediate vicinity
of the
stack, the environment may be humid, hot, and either acidic or alkaline. For
instance,
in solid polymer electrolyte fuel cells, carbon separator plates may be
somewhat porous
and thus the environment in the immediate vicinity of the plates can be
somewhat
similar to that inside the cells. Consequently, any metallic hardware' in the
immediate
vicinity of the stack may be subject to corrosion and failure. In particular,
conductive
traces that separate large voltages (e.g., in printed circuit board based
isolation
circuitry) are subject to corrosion and bridging via dendrite formation. To
prevent this
type of failure, such hardware can be appropriately encapsulated or potted to
isolate it
from the corrosive environment. Still, it is not trivial to provide a
satisfactory
comprehensive, durable protective coating in this way.
Accordingly, although there have been advances in the field, there
remains a need for simple, reliable cell voltage monitors for fuel cell
stacks. The
present invention addresses these needs and provides further related
advantages.
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BRIEF SUMMARY OF THE INVENTION
Radio frequency identification (RFID) devices may be used to monitor
various operating parameters in fuel cells, including, for instance, the
voltage of
individual cells in a fuel cell stack. Thus, an RFID system may serve as an
improved
cell voltage monitor to check for voltage reversal conditions in individual
cells during
stack operation.
In order to monitor an operating parameter, an RFID transponder is'
provided in the fuel cell and the transponder is configured to sense and
transmit
information about that operating parameter.
In one embodiment, the transponder may be configured to transmit its
identification only when the operating parameter reaches a certain threshold
value (e.g.,
when the parameter falls below or alternatively when it exceeds the threshold
value). In
a different embodiment, the transponder may instead be configured to transmit
the
actual value of the operating parameter.
As mentioned above, the monitored operating parameter can be the cell
voltage. However, it is also possible to monitor other operating parameters
such as cell
impedance. Both cell voltage and impedance may be sensed by incorporating a
sensor
in the transponder which has a cathode contact and an anode contact
electrically
connected to the cathode and the anode in the fuel cell, respectively.
Half cell voltages (i.e., the voltage between a suitable reference electrode
and one of the cathode or anode voltages) may be monitored if a suitable
reference
electrode is employed in the fuel cell. The transponder would then comprise a
voltage
sensor that includes the reference electrode.
Other parameters that may also be monitored include the cell
temperature, a reactant pressure and/or flow rate, stack compression, and an
impurity
concentration. With appropriate sensors incorporated in the transponders, more
than
one parameter can be sensed and hence monitored at the same time using the
inventive
apparatus.
Along with appropriate sensors to sense the desired operating
parameters, the transponder may comprise an AlD converter to convert the
sensed
parameters into digital form for transmission. The transponder may be active
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(internally powered) or passive (externally powered, typically via interaction
with an
RFID reader).
An RFID monitored fuel cell system would typically comprise a series
stack of a plurality of the above transponder equipped fuel cells along with a
reader for
reading information transmitted from the transponders.
In an exemplary embodiment, the system is a solid polymer electrolyte
fuel cell system in which the invention serves as a cell voltage monitor to
protect
against voltage reversal. In the fuel cell stack, each cell comprises a
membrane
electrode assembly and each membrane electrode assembly comprises a cathode,
an
anode, an electrolyte and an electrochemically inactive manifold section. The
stack
further comprises flow field plates adjacent the anode and cathode of each
fuel cell.
Each cell is equipped with a transponder located in the manifold section of
the
membrane electrode assembly. The transponder comprises a voltage sensor with a
cathode pressure contact pad and an anode pressure contact pad mounted on
opposing
faces of the manifold section such that they electrically contact the flow
field plates
adjacent the cathode and anode, respectively. The manifold section is a
thermoplastic
and the transponder may be molded therein at the time of manufacture.
In the foregoing embodiment, the transponders sense and transmit
information regarding the cell voltage to the reader. However, to avoid any
"collision"
issues in this application (where signals from many transponders may interfere
with
each other), it is possible to have the transponders in each fuel cell remain
dormant
(silent) unless the cell voltage falls below some threshold value indicative
of an
impending voltage reversal. Thus, the transponders are configured to transmit
their
identification to the reader only when the cell voltage falls below this
threshold value.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAW1NG(S)
Figure 1 shows a schematic diagram of a solid polymer electrolyte fuel
cell system that includes a cell voltage monitor based on RFID devices.
Figure 2a shows a schematic diagram of a transponder configured to
transmit its identification in response to a sensed operating parameter.
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Figure 2b shows a schematic diagram of a transponder configured to
transmit data representative of a sensed operating parameter.
Figures 3a and 3b show an assembled view and an exploded view,
respectively, of a possible mounting arrangement for a voltage monitoring
transponder
in a fuel cell unit of a solid polymer electrolyte fuel cell stack.
Figure 4 shows the model described in Example 1 of a cell voltage
monitoring transponder.
Figure 5 shows the model described in Example 2 of a cell voltage
monitoring transponder.
DETAILED DESCRIPTION OF THE INVENTION
Radio frequency identification (RFID) devices are used in various
industries to identify and track goods. In a typical tracking application,
each item to be
tracked contains an RFID transponder and an item is identified using an RFID
reader
which communicates with the transponder at radio frequencies and determines
its
identification. RFID devices are slowly replacing barcodes as the technology
continues
to advance and the size and price of the devices drop. RFID devices offer
several
advantages over barcodes in that they do not need to be visible (i.e., can be
embedded
in an object) and they can provide a memory function.
An RFID system has at least one RFID transponder (which is often
called a tag and typically comprises an integrated circuit and an appropriate
coil/antenna), and at least one reader (which comprises a transceiver and an
appropriate
coil/antenna). Communication takes place between transponder(s) and readers)
via
magnetic coupling between their coils (that is, together the coils act like an
air core
transformer). The typical frequency band for operation is in the range of
about 30 KHz
to 2.5 GHz.
In the fuel cell industry, RFID technology can be valuable not only for
identifying and tracking components and/or products but also for monitoring
various
parameters in the fuel cell stack itself while it is operating. Although there
can be
significant electromagnetic noise in the vicinity of powerful fuel cell stacks
when in
use, it is possible in general for RFID devices to communicate successfully in
this
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environment. (Although, the noise level in certain locations may be
unacceptable, as
noted in the Examples below.)
While a number of parameters might desirably be monitored while
operating a stack, it is particularly useful to be able to monitor individual
cell voltages
in order to provide advance warning of an impending voltage reversal
condition. Figure
1 shows a schematic diagram of an exemplary solid polymer electrolyte fuel
cell system
which includes a cell voltage monitor in which RFID devices are used to
monitor each
cell in the stack.
In Figure 1, stack 1 comprises a plurality of fuel cell units 2 in a series
stack (for simplicity, only three units are shown in detail in Figure 1). Each
unit 2 in
the stack comprises a membrane electrode assembly (MEA) 3. MEA 3 comprises an
electrochemically active portion 4 and an inactive portion 5. Active portion 4
comprises a cathode, an anode, and an electrolyte (not specifically shown). In
the
illustrated embodiment, inactive portion 5 may serve to form internal
manifolds for
reactants and/or coolant. Each fuel cell unit 2 also comprises bipolar
separator plate 6
with cathode and anode flow fields 7 formed therein adjacent the cathode and
anode,
respectively, of adjacent MEAs 3. In this way, flow fields 7 in bipolar plate
6 serve to
distribute oxidant and fuel reactants to the cathode and anode, respectively.
In accordance with the invention, an RFID transponder 10 comprising
integrated circuit 8 and coil 9 is incorporated into each fuel cell unit 2. To
sense the
cell voltage, transponder 10 also includes cathode contact pad 11 and anode
contact pad
12 on opposite sides of inactive portion 5 but near active portion 4. Pads 1 l
and 12
physically contact the cathode and anode sides of adjacent bipolar plates 6,
respectively,
and are electrically connected to voltage inputs on integrated circuit 8 via
sense lines
13.
The cell voltage monitor in Figure 1 comprises transponders 10 and
reader 14 which is located within communication range of transponders 10.
Reader 14
comprises transceiver 15 and coil 16. Information representative of the
individual cell
voltages is communicated by transponders 10 to reader 14. In turn, reader 14
forwards
cell voltage information via line 17 to a processor (not shown) which analyzes
the
information, determines if a problem condition exists, and initiates
appropriate action.
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The system of Figure 1 provides many advantages over prior cell voltage
monitoring systems. Communication between transponders 10 and reader 14 is
wireless and no external electrical connections are required, thereby reducing
complexity and the possibility of electrical shorting between cells.
Transponders 10 are
electrically isolated from reader 14 and thus there are no high voltage
isolation issues
associated with the latter. Further, line of sight is not required between
reader 14 and
the fuel cell stack, so reader 14 can be isolated more from the corrosive
environment
around the stack. Another advantage is that the transponders 10 function
independently
and thus failure of a single transponder need not affect the functioning of
the rest of the
cell voltage monitor. Yet another advantage is that the components required
for the cell
voltage monitor shown in Figure 1 are relatively inexpensive, small, and, to a
great
extent, generic components that are readily available industrially. Finally,
the RFID
system may be used to perform additional functions. During manufacture of the
depicted stack, the component MEAs can be identified and tracked in a
conventional
manner using the embedded transponders. In addition, operating parameters
other than
cell voltage might also be monitored at the same time, simply by incorporating
appropriate additional sensors in the transponders and by sharing the other
existing
hardware (e.g., reader 14).
Depending on what information is desired, the transponders can be
configured to transmit information only when a problem condition exists or
alternatively can be configured to continuously transmit information about the
measured parameter. Where the application allows, the former may be preferred
since
reducing the transmission volume and/or the number of transmitting
transponders
reduces concerns about "tag collision" (i. e., where transponders sending
signals at the
same time confuse the reader). However, in the latter case, standard industry
practices
may be adopted to address any "tag collision" issues (e.g., with anti-
collision software).
Figures 2a and 2b show schematics of general transponder configurations
suitable for
each case. In Figure 2a for instance, the transponder is configured to
transmit its
identification only when the sensed parameter crosses a threshold value. In
Figure 2b,
the transponder is configured to transmit data representative of the parameter
value
itself.
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In Figure 2a, transponder 20 employs a conventional RFID integrated
circuit 21 which transmits the transponder identification when queried by the
reader.
Also shown in the schematic is a conventional transponder coil 22 and tuning
capacitor
23. (Tuning capacitor 23 may optionally be included within integrated circuit
21.) To
enable transponder 20 to sense and transmit information regarding an operating
parameter, transponder 20 also comprises sensor 24 and silencing circuit 25.
Sensor 24
senses the parameter to be monitored and supplies a representative signal to
silencing
circuit 25. When the measured parameter is in a normal range, silencing
circuit 25
electrically de-energizes integrated circuit 21 and prevents transponder 20
from
transmitting its identification when queried by the reader (thereby silencing
transponder
20). However, when the measured parameter crosses a predetermined threshold
value,
silencing circuit 25 is disabled, thereby allowing transponder 20 to respond
when
queried. Thus transponder 20 is dormant until a problem condition is sensed,
at which
point transponder 20 transmits its identification when queried by the reader.
A configuration like that depicted in Figure 2a is suitable for a cell
voltage monitor if all that is needed is a warning of an impending voltage
reversal
situation. In such a case, silencing circuit 25 may be disabled when the cell
voltage
drops below a threshold value. Thus, only that cell or cells with voltages
below this
threshold will respond when queried by the reader, thereby avoiding "tag
collision". In
the embodiment of Figure 1, cathode and anode contact pads 11, 12 serve as
sensor 24.
Silencing circuit 25 may simply comprise an appropriately configured
transistor and
capacitor (as illustrated in the Examples below). In a different embodiment,
transponder 20 can be configured to transmit at two different thresholds, the
first to
transmit an advance warning and the second to initiate a shutdown of the fuel
cell stack.
To do so, two levels of identification are required along with similax
silencing circuitry.
In Figure 2a, conventional RFID integrated circuit 21 is typically
passive, i.e., the modest power required to power it and thus to transmit the
transponder
identification is obtained from interaction with the reader. Sensor 24 and
silencing
circuit 25 may obtain power from the cell (in the case when cell voltage is
monitored)
or other target when it is possible to do so. Alternatively, power may be
obtained from
interaction with the reader (either from the loop comprising coil 22,
capacitor 23, and
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integrated circuit 21 or from a secondary coil circuit, as in Fig. 2b - not
shown in Fig.
2a). However, transponder 20 may also incorporate an alternate energy storage,
such as
a capacitor or battery, and thus be active instead.
Figure 2b shows an alternate transponder 30 in which the actual value of
the monitored operating parameter is provided to a reader. Here, a custom
integrated
circuit 31 is employed which transmits the cell (i.e., transponder)
identification along
with encoded information about the value of the operating parameter. Here,
sensor 34
senses the operating parameter and supplies a signal to A/D converter 36. A/D
converter 36 then converts this analog data into a digital signal which is
supplied to
custom integrated circuit 31. Finally, integrated circuit 31 encodes the
operating
parameter data for transmission.
Figure 2b shows an optional dual capacitor tuned coil arrangement
comprising coil 32, first capacitor 33, and second capacitor 35. This
arrangement
allows for transmissions at two different frequencies. (Alternatively, a
single capacitor
tuned coil arrangement similar to that shown in Figure 2a may be used
instead.)
As shown in Figure 2b, power for the various components in the
transponder may be obtained via a secondary coil arrangement comprising
secondary
coil 37, tuning capacitor 38, and full or half wave rectifier 39. Power can
thus be
obtained via interaction with the reader at another frequency. ~ther options
for
powering the components may be employed however instead of the depicted
secondary
coil arrangement (e.g., battery).
Whatever transponder configuration is selected, both the transponders
and the readers) should be located where electromagnetic noise cannot
interfere with
their operation. As illustrated in the Examples below, RFID systems of the
invention
are quite robust and can operate properly in all but perhaps the noisiest
locations (e.g.,
adjacent a high power inverter) in a typical high power fuel cell system.
For cell voltage monitoring purposes, a possible mounting arrangement
for a transponder in a solid polymer electrolyte fuel cell 40 is shown in
Figures 3a and
3b. (Figures 3a and 3b show an assembled view and an exploded view,
respectively.)
In these figures, integrated circuit 41 and coil 42 are embedded in a plastic,
electrochemically inactive, manifold section 49b at the end of MEA 49.
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manifolds for carrying reactants and coolant in the fuel cell stack are
created by
aligning the manifold openings in MEA 49 along with other components in the
cell
stack.) Voltage sensing cathode pressure contact pad 50 and anode pressure
contact pad
(not shown) axe mounted on opposite faces of inactive section 49b and press
against the
cathode flow field side of bipolar plate 51 and the anode flow field side of
an adjacent
bipolar plate 51 in the assembled cell. The cathode and anode pads are located
close to
electrochemically active section 49a in an area where the local voltage is
sufficiently
representative of the cell voltage (under high electrical load, there can be a
significant
variation in voltage along the active section of the fuel cell). The pads
should be made
of a material that is suitable for use within the cell environment, such as
that used in
bipolar plates 51 (e.g., carbon). The conductors connecting cathode and anode
pads to
integrated circuit 41 may also be embedded in plastic manifold section 49b to
protect
them from the cell environment. The transponder components depicted here can
readily
be molded into manifold section 49b during manufacture. The pressure contact
pads
can be covered during the molding operation such that the contact surfaces
remain
exposed prior to assembling the fuel cell stack (e.g., with removable tape).
While a suitable application for the invention is for use as a cell voltage
monitor, it may be desirable to monitor other operating parameters as well.
Typically,
the modifications required would be to the sensor type and its mounting
arrangement
and/or to the internals of the integrated circuit. For instance, to monitor
cell impedance
(primarily electrolyte impedance) in order to check electrolyte hydration in-
situ, similar
hardware to that described above might be employed. An appropriate current
signal
would typically be superimposed across the entire fuel cell stack and cell
impedance
would be determined from the voltage difference that results in the measured
cell. Half
cell voltages (i.e., the voltage between a reference electrode and one of the
cathode or
anode) might be made in a similar manner by incorporating a reference
electrode in a
suitable location within the cell (e.g., within the membrane electrolyte in a
sensitive
area, such as in the vicinity of a reactant port).
Cell temperature may be monitored using various temperature measuring
devices as a sensor (e.g., thermocouple, thermistor). To monitor reactant
pressures (for
instance, to detect for blockages or "flooding") or stack compression (to
watch for a
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sudden loss of stack compression), pressure sensors comprising strain gauge
bridges
may be used. In the former, a suitable location for the sensor could be in a
manifold or
flow field passage for that reactant. In the latter, the sensor could act as a
load cell and
be located in a region under significant compression in one or both end cells
in the
stack. Other parameters that might be monitored include a reactant flow rate
(thus
requiring a flow rate sensor) or perhaps the concentration of an impurity in a
reactant
stream (such as CO in the fuel stream or hydrogen in oxidant stream, and thus
requiring
a concentration sensor for the impurity species).
If RFID devices are used elsewhere in a fuel cell powered system, the
invention offers possible integration advantages. For instance, the same
readers might
be used to monitor subsystem parameters (e.g., in the oxidant or fuel supply
subsystems) as well as to monitor the fuel cell operating parameters.
Furthermore,
incorporating RFID devices in the fuel cell components allows for conventional
inventory and tracking of the components and/or assemblies.
While the preceding discussion has been directed primarily at solid
polymer electrolyte fuel cell types, the invention may be used in other
suitably low
temperature fuel cell types. A limitation of course is the maximum temperature
that the
transponders can handle (at present, commercially available devices are rated
up to
about 125° C).
EXAMPLE 1
A cell voltage monitoring system was designed for use in a solid
polymer electrolyte fuel cell stack. The transponder design was similar to
that generally
shown in Figure 2a and used commercially available RFID components, including
a
microID MCRF200 tag comprising a 125 kHz chip programmed in-house and a
microID Reader. (These components are part of a microID Developer's kit that
can be
obtained from Microchip.) Figure 4 shows a model of the operating transponder.
In the
model circuit of Figure 4, chip 60 represents the aforementioned 125 kHz chip
and coil
61 is a conventional coil provided with the kit. Resistor 62 and capacitor 63
represent
the resistance and capacitance found in the modeled circuit and signal 64
represents the
voltage induced in the circuit via interaction with the reader. The
conventional circuit
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was modified by providing cathode and anode voltage inputs from simulated cell
65 at .
points 71 and 72, respectively, and transistor 68 and capacitor 69 were added
to act as a
silencing circuit. When the voltage of cell 65 is greater than -0.3 V, the
transistor
68/capacitor 69 silencing circuit should de-tune the circuit, thereby
preventing the
transponder from transmitting its identification when queried by the reader.
However,
when the voltage of cell 65 falls below -0.3 V (a significant reversal
condition),
transistor 68 should open and the transponder should respond with its
identification
when queried.
Operation of the transponder of Figure 4 was simulated using SPICE
(open source modeling software from Berkeley) and the circuit operated as
planned. A
working unit was then assembled and tested using a reader located 1 cm away.
With
simulated test cell voltages above the threshold of -0.3 V, the transponder
was silent
when queried. At voltages just below -0.3 V, the transponder properly
transmitted its
unique identification when queried. Operation of the transponder was found to
be very
repeatable. (In additional testing with the reader at different distances from
the
transponder, it was observed that the threshold voltage decreased somewhat
with
distance from the reader. Again however, transponder operation was very
repeatable.)
Operation of the transponder was then checked in various locations immediately
adjacent an operating 150 kW heavy duty (for passenger buses) solid polymer
electrolyte fuel cell stack running between 20 to 275 amps to see if
electromagnetic
noise affected operation.- Except in the immediate vicinity of the system
inverter
cables, the digital response from the transponder was found to be consistent
and
repeatable. The circuit therefore performed as the model predicted.
This example demonstrates that an RFID based cell voltage monitor can
operate successfully in a fuel cell environment. Further, minimal
modifications to
conventional apparatus are required to make a working transponder.
EXAMPLE 2
A second cell voltage monitoring system was constructed that gave
improved performance over that of Example 1. In this case, a commercially
available
RFID tag and reader were employed that operated at the higher 13.56 MHz
frequency.
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This higher frequency system provided faster data transmission and a reduction
in
response time.
Figure 5 shows a model of this operating transponder. (In Figure 5, like
components to those in Figure 4 are labeled with the same numerals.) Here, a
surface
mount version of a MCRF450 tag 60 from Microchip was mounted on a thin, but
rigid
circuit board. The coil for powering tag 60 was comprised of circuit board
traces on
both sides of the board. Surface mount capacitors were used to tune the
circuit for
resonance. As in Example 1, a transistor based circuit and de-tuning capacitor
were
used to enable or disable the transponder from transmitting. However, in this
example,
the silencing circuit was comprised of two junction field effect transistors
(JFETs) 68,
73, instead of one, in order to improve the switching performance. (Note the
differences
in the cathode and anode voltage inputs from simulated cell 65, which again
appear at
points denoted 71 and 72, respectively. Also note that series resistor 74 has
been
provided in the silencing circuit to protect transistors 68, 73. Such a
resistor might also
be employed in the circuit of Figure 4 if desired.) The complete transponder
assembly
measured 1.2 cm in width, 5.1 cm in height and 0.8 mm in depth. MCRF450 tag 60
employs a built-in anti-collision mechanism that allows multiple tags to
operate in close
proximity to each other.
Three transponders were constructed as above and mounted side by side
with a spacing of 1.8 mm to represent a typical fuel cell configuration.
Inputs 71 and
72 of each transponder were then individually connected to 1.5 volt do
sources, whose
polarity could be reversed by pressing a button. This transponder array was
then
brought into proximity to a Microchip Anti-Collision Interrogator (reader).
A positive voltage was applied to the electrode voltage inputs of each
transponder and this simulated cell voltage monitoring system was then
operated. With
positive voltages across each set of inputs, the transponders remained silent
and did not
transmit their individual tag information (i.e., serial number). When the
polarity of the
voltage across a set of inputs was reversed however, the associated
transponder
properly transmitted its individual tag information, which was then
interpreted and
displayed on a computer screen. This test verified that the transponders could
operate
14
CA 02548203 2006-06-02
WO 2005/064720 PCT/US2004/042854
either individually, or all three at the same time, without interfering with
each other in a
typical fuel cell configuration.
The environment immediately surrounding the typical solid polymer
electrolyte fuel cell is harsh for most electronics, with temperatures near
80°C and a
relatively humidity that can approach 100%. To demonstrate the ability to
withstand
this environment, two transponders similar to the above, were given a spray
coating of
conventional circuit board conformal coating, then dipped in silicon, and heat
cured. A
large beaker was then filled half way with water, covered, and kept at a
constant
temperature of 80°C. One transponder was placed in the vapour space
above the water
and another transponder was placed under water. A reader was then positioned
near the
beaker so it could continuously read both transponders at the same time. This
test was
run for greater than 4 hours each day, for 5 days, without any failures.
While particular elements, embodiments and applications of the present
invention have been shown and described herein for purposes of illustration,
it will be
understood, of course, that the invention is not limited thereto since
modifications may
be made by those skilled in the art without departing from the spirit and
scope of the
present disclosure, particularly in light of the foregoing teachings.
