Abstract:
A health monitoring system for a fuel cell stack using current fuel cell architecture to enable the electronic control unit (ECU) to continue to monitor the health of the fuel cell stack despite a component failure. The system uses an embedded measurement module (EMM) connected to a group of fuel cells in the fuel cell stack to monitor the health of that group of fuel cells. The EMM produces a pulse width modulation signal that is sent to the ECU. A total voltage value for the group of fuel cells is embedded into the calibration signal or end of frame sequence. The ECU uses an algorithm to determine a missing voltage of at least one fuel cell in the event of the component failure of that fuel cell by adding up the cumulative value for each fuel cell reporting their voltage and subtracting that value from the total voltage value found in the end of frame sequence.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to a method and apparatus for monitoring fuel cell voltages in a fuel cell stack, and more particularly to making connector and component failures in the monitoring circuitry transparent to the user without losing the information that the failed connector or component was designed to provide. Furthermore, the method and apparatus provide for a redundant power supply and ground connection for the monitoring circuitry measurements. 
     Fuel cells produce energy by the electrochemical processing of reactants and the subsequent generation of electric current. A typical fuel cell configuration includes a polymer membrane (e.g., a proton exchange membrane (PEM)) with catalyst layers on both sides to promote the respective oxidation and reduction of hydrogen and oxygen. Additional components, including a pair of gas diffusion media layers disposed against the respective catalyst layers and cathode and anode bipolar plates placed outside the gas diffusion media layers; these bipolar plates define flow channels therein to facilitate the introduction of the externally-provided reactants to the catalyst-coated PEM. The various components are compressed together to form the fuel cell. To increase the power output, numerous fuel cells are arranged together to form a fuel cell stack. 
     Fuel cell stacks are monitored for their electrical viability through a set of diagnostic connectors which are attached to the bipolar plates of each fuel cell within the fuel cell stack. An embedded measurement module (EMM), via the diagnostic connectors, monitors the voltage of each fuel cell and reports that health to the vehicle electronic control unit (ECU). 
     A problem with this approach is that the EMM is unable to address any failure modes that may arise, making the system vulnerable to diagnostic connector and component failures. Furthermore, if the diagnostic connector that powers the EMM fails, the EMM will not provide any information on any of the fuel cells to the vehicle ECU. A way is needed to circumvent those failures to get the needed data to monitor the health of the fuel stack. 
     SUMMARY OF THE INVENTION 
     In view of the above and other problems, the present disclosure modifies the end of frame sequence of the pulse width modulation (PWM) signal to provide a new data set to be used in calculating the voltage of each fuel cell connected to the EMM in the event of a connection or component failure. The role of the end of frame sequence is to provide a calibration signal and to indicate the beginning of a data stream. In the preferred embodiment, the end of frame sequence is modified to add a total voltage value of all the fuel cells connected to the EMM. In the event of a connection or component failure, the sum of all the fuel cell voltages reported are subtracted from the total voltage value to find the missing fuel cell voltage due to the failure. Furthermore, a redundant or alternate power and ground connection are added to the current EMM architecture to overcome a failure in the primary power or ground connection. 
     According to a first aspect of the present invention, a redundant health monitoring system is disclosed. The system uses an EMM with a cell voltage monitoring circuit and a plurality of diagnostic connectors. The plurality of diagnostic connectors are coupled to a substrate and electrically connected with a plurality of fuel cells in the fuel cell stack. The cell voltage monitoring circuit is configured to send a plurality of pulses and an end of frame sequence to a receiver circuit. Each pulse in the plurality of pulses corresponds to an individual voltage for an individual fuel cell in a plurality of fuel cells in the fuel cell stack. The end of frame sequence indicates the beginning of the plurality of pulses as well as a total voltage for the plurality of fuel cells connected to the EMM. A central processing unit with an algorithm programmed therein sums up the individual voltages and the sum is subtracted from the total voltage to determine a missing voltage of at least one fuel cell. 
     According to a second aspect of the present invention, a method of redundant health monitoring of a fuel cell stack is disclosed. The method entails providing an EMM that is electrically connected to a plurality of fuel cells in the fuel cell stack. The EMM sends out a plurality of pulses wherein each pulse in the plurality of pulses indicates an individual voltage for an individual fuel cell in a plurality of fuel cells in the fuel cell stack. An end of frame sequence is defined that indicates the beginning of the plurality of pulses, and a total voltage for the plurality of fuel cells connected to the embedded measurement module. A missing voltage value is calculated corresponding to at least one of the fuel cells in the fuel cell stack using an algorithm. The individual voltages for each fuel cell are added up to a sum of the individual voltages and the sum is subtracted from the total voltage to determine the missing voltage value. 
     According to a third aspect of the present invention, a redundant health monitoring system for a fuel cell stack is disclosed. The system uses an EMM with a cell voltage monitoring circuit and a plurality of diagnostic connectors. The plurality of diagnostic connectors are coupled to a substrate and electrically connected with a plurality of fuel cells in the fuel cell stack. The cell voltage monitoring circuit is configured to optically send a plurality of pulses and an end of frame sequence to a receiver circuit. Each pulse in the plurality of pulses corresponds to an individual voltage for an individual fuel cell in a plurality of fuel cells in the fuel cell stack. The end of frame sequence indicates the beginning of the plurality of pulses as well as a total voltage for the plurality of fuel cells connected to the EMM. A central processing unit with an algorithm programmed therein sums up the individual voltages and the sum is subtracted from the total voltage to determine a missing voltage of at least one fuel cell. The EMM has a primary power source and a redundant or alternate power source. The redundant power source is configured to use at least one diode electrically connected to an individual diagnostic connector from the plurality of diagnostic connectors to provide the alternate power source for the EMM. The embedded measurement module also has a primary ground connection and a redundant or alternate ground connection. The redundant ground connection is electrically connected to at least one individual diagnostic connector from the plurality of diagnostic connectors to provide the alternate ground source for the EMM. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a schematic block diagram of a fuel cell system with health monitoring apparatus according to an aspect of the disclosure; 
         FIG. 2  depicts a PWM signal that includes an index synchronization sequence and cell voltage pulses; and 
         FIG. 3  depicts a comparison between a cell voltage measurement signal including an index synchronization sequence, a saw tooth comparison signal and an output signal. 
     
    
    
     DETAILED DESCRIPTION 
     Referring first to  FIG. 1 , a schematic block diagram of a fuel cell system  10  is shown. The fuel cell system  10  has a fuel cell stack  12 , an embedded measurement module (EMM)  26  and a receiver circuit  30 . In this non-limiting embodiment, fuel cell stack  12  has a plurality of stacked fuel cells  14 . The EMM  26  comprises a stack interconnect  16  having a plurality of diagnostic connectors  18  and a cell voltage monitoring circuit  28 . The diagnostic connectors  18  are mounted to a substrate and are in electrical contact with a plurality of bipolar plates  20  that separate the fuel cells  14  in the fuel cell stack  12 . In one non-limiting embodiment, the stack interconnect  16  includes twenty diagnostic connectors  18 . All twenty diagnostic connectors  18  are in contact with seventeen bipolar plates  20  which define sixteen fuel cells  14 . This enables the EMM  26  to monitor the voltage of sixteen fuel cells  14 . Those sixteen fuel cells are represented in  FIG. 1  as  14 A- 14 P and define a cell group  15 . Seventeen of the diagnostic connectors  18  are in contact with seventeen bipolar plates  20 . The remaining three diagnostic connectors  18  make redundant connections to three of the seventeen bipolar plates  20 . In one embodiment, the stack interconnect  16  is an embedded interconnect that is part of the fuel cell stack  12 , although other types of interconnects may be equally applicable. 
     The EMM  26  is created with a standard twenty diagnostic connectors  18 . The architecture of the fuel cell stack  12  encompasses three hundred and twenty fuel cells  14 . Using an even twenty EMMs  26  to monitor the fuel cell stack  12 , each EMM  26  will monitor sixteen fuel cells  14 , leaving three diagnostic connectors  18  on each EMM  26  free for other purposes. 
     The cell voltage monitoring circuit  28  comprises a communication module  44 , a power supply  48 , and a pulse generator (discussed in more detail below). The communication module  44  in this non-limited example uses a LED to communicate optically. The communication module  44  may also use electrical or radio frequency to communicate as well. 
     A power supply  48  provides the electricity needed for the EMM  26  to operate. The EMM  26  derives its power directly from the fuel cell stack  12  through two dedicated connections in the diagnostic connector  18 . One connection is for power and the second is for ground. In the preferred embodiment, an alternate power source  58  provides power directly from the fuel cell stack  12  in the event of a connection or component failure. The alternate power source  58  is provided from a redundant diagnostic connector via one of the plurality of leads  32 . At least one diode  59  is connected to an individual diagnostic connector from the plurality of diagnostic connectors  18  and at least one diode  59  is connected from one of the plurality of leads  32  to the power supply  48  to create the alternate power source  58 . As used throughout this application, the alternate power source  58  is also a redundant power source. 
     In the preferred embodiment, an alternate ground connection  53  is provided to ensure the EMM  26  has a connection to ground in the event of a connection or component failure. The alternate ground connection  53  is directly connected from the fuel cells  14  via one of the plurality of leads  32 . The alternate ground connection is provided from the redundant diagnostic connector and connects directly to the power supply  48 . As used throughout this application, the alternate ground connection  53  is also a redundant ground connection. 
     Another component of the cell voltage monitoring circuit  28  is the pulse generator. The pulse generator comprises a multiplexer  34 , an instrumentation amplifier  38 , a saw tooth wave generator  42 , a counter circuit  36 , a comparator  40 , and a reference circuit  50 . The components of the pulse generator are described in greater detail below. 
     In the cell voltage monitoring circuit  28 , the plurality of leads  32  is electrically coupled to each diagnostic connector  18  in the stack interconnect  16 . An opposite end of each lead  32  is electrically coupled to the multiplexer  34  that selectively provides two voltage potential signals from the diagnostic connectors  18  to the instrumentation amplifier  38  at any given point in time. The counter circuit  36  provides sequence signals to the multiplexer  34  to cause the multiplexer  34  to selectively and sequentially switch from one of the leads  32  to a next one of the leads  32 . The output of the multiplexer  34  is amplified in the instrumentation amplifier  38  such that the signal has a magnitude that identifies the voltage of the particular fuel cell  14  being measured. The amplified cell voltage signal is provided to the comparator  40  that compares the signal to an inverted saw tooth wave provided by the saw tooth wave generator  42 , where the output of the comparator  40  is a PWM signal. The PWM signal is shown in  FIG. 2 . The PWM signal has two parts; a series of end of frame synchronization pulses and a data stream. The width of the pulses in the data stream define a cell voltage as will be discussed in detail below. The PWM signal is provided to the communication module  44  that generates a communication signal  46  having an on/off time determined by the pulses. 
     The data stream is a sequence of pulses wherein each pulse corresponds to a voltage measurement of each fuel cell  14  in the cell group  15 . The end of frame sequence is introduced into the PWM signal after a last fuel cell  14 P voltage measurement pulse so that it provides an indication that the next pulse after the end of frame sequence is the voltage measurement pulse for a first fuel cell  14 A in the cell group  15 . The cell voltage monitoring circuit  28  of the type being discussed sequentially measures the voltage of the plurality of fuel cells  14  in order in a cell group  15 . When the voltage of the last fuel cell  14 P in the cell group  15  is measured, the sequence returns to the first fuel cell  14 A in the cell group  15  and begins to measure the voltage, sequencing through the cell group  15  in this manner at the rate set by the saw tooth wave generator  42 . 
       FIG. 2  is a graph with time on the horizontal axis and magnitude on the vertical axis showing the PWM signal  60  of the type that is output from the comparator  40 . The end of frame synchronization pulses  64  provide a reference pattern that when decoded provides an indication that the data stream  190  is next. The first cell in the cell group (refer to  FIG. 1 ) will be the first signal  194  in the data stream  190  after the end of frame synchronization pulses  64  and the last signal  196  corresponding to the last cell in the cell group with end of the data stream  190 . The format or pattern of the frame synchronization pulses  64  in this embodiment is a high pulse  66  followed by a low pulse  68 , followed by a high pulse  66  and then followed by a last pulse  198  (H-L-H-L). This pattern is specifically selected to provide a defined sequence of the end of frame synchronization pulses  64  that is very unlikely to occur in the actual voltage measurements of the fuel cells shown in  FIG. 1 , thus providing a good indication that the pulses are the end of frame synchronization pulses  64 . In the preferred embodiment, the pulse width of the first three end of frame synchronization pulses  64  (H-L-H) may always be the same for the high pulses  66  and the low pulses  68 . The pulse width of the last pulse  198  of the end of frame synchronization pulses  64  may be used to indicate the total voltage of the cell group. The last pulse  198  may not have the same pulse width as the first three pulses  66  and  68 . The pulse width of the pulses  62  of the data stream  190  is created by the actual voltage of the fuel cells in the cell group, as will be discussed in detail below. 
     The width of the end of frame synchronization pulses  64  may be chosen so that the magnitude of the pulse width is known, consistent and outside of any possible pulse width of the data stream  190 . In one non-limiting example, the width of the high pulses  66  represents 1.235V and the width of the low pulse  68  represents −1.235V. The modulation provided by the saw tooth wave ( FIG. 3 ) creates the PWM signal  60  so that the high voltage has a narrow pulse width and the low voltage has a wide pulse width. 
     In the present disclosure, the pulse width of the last pulse  198  of the end of frame synchronization pulses  64  is modified to indicate a total voltage measurement of all the fuel cells in the cell group in  FIG. 1 . The total voltage measurement is larger in magnitude than any single voltage measurement of the cell group in the data stream  190 . The magnitude of a total voltage measurement serves two functions. The first is the traditional function of indicating the end of the data stream  190 . The second is an actual, useable total voltage measurement that can be interpreted and used by the electronic control unit (ECU). 
     The end of frame synchronization pulses  64  can be injected into the PWM signal  60  in any suitable matter. In the fuel cell system  10 , the reference circuit  50  generates the sequenced values that become the end of frame synchronization pulses  64 . The counter circuit  36  sequences the signals from the reference circuit  50  into the PWM signal  60  after the last fuel cell  14 P in the cell group  15  is measured. The total voltage measurement is provided to a series of pins on line  52  to the multiplexer  34  which presents the voltage values in sequence to the instrumentation amplifier  38  and then to the comparator  40 . The differentiation between the high pulses  66 , the low pulse  68  and the last pulse  198  end of frame synchronization pulses is provided by the modulation using the saw tooth wave  70  shown in  FIG. 3  and discussed in more detail below. 
     The end of frame synchronization pulses  64  allow the voltage measurements to be calibrated, in this non-limiting example, 250 times per second. In other words, the amount of time that the pulse is high is compared to the high pulse  66  to give the voltage measurement that will be less than that value. Because the sequence of the end of frame synchronization pulses  64  represents a start for the data stream  190 , and those measurements are taken in the order of the fuel cells  14  in the fuel cell stack  12 , each pulse  62  specifically identifies which fuel cell  14  in the cell group  15  being monitored is associated with that pulse  62 . 
       FIG. 3  is a graph with time on the horizontal axis and magnitude on the vertical axis showing a relationship between the inputs to the comparator  40  to provide the modulation of the cell voltage measurement signals and the communication signal  46  that is output from the communication module  44 . At the top of  FIG. 3 , the saw tooth wave  70  from the saw tooth wave generator  42  is shown super-imposed over a voltage signal  72  from the instrumentation amplifier  38 . Section  74  of the voltage signal  72  includes four square wave pulses that are the end of frame synchronization pulses provided by the reference circuit  50 . When those four square wave pulses are modulated by the saw tooth wave  70 , the end of frame synchronization pulses are produced as shown in  FIG. 2 . The positive portion  76  of the voltage signal  72  will become the narrow width pulses  80  of the communication signal  46  and the negative portion  78  of the voltage signal  72  will become the wide width pulses  82  of the communication signal  46 . 
     In the preferred embodiment, if the saw tooth wave  70  is greater in magnitude than the voltage signal  72 , then the comparator outputs a pulse  62 , which causes a LED in the communication module to conduct and generate the optical signal. This is shown by the bottom of the graph on  FIG. 3  where “1” represents the LED being on and “0” represents the LED being off. Particularly, the angle provided by the saw tooth wave  70  creates the modulation for the width of the pulse being relative to the magnitude of the voltage measurement signal. Therefore, the pulses of the optical signal are narrower for the high voltage than they are for the low voltage. As the magnitude of the voltage pulse  84  goes up, it is covered by a narrower part of the saw tooth wave  70 , which creates a narrower pulse in the optical signal. Thus, the greater the magnitude of the voltage pulse  84  represented by a higher voltage of the particular fuel cell being measured, the narrower the pulse for that voltage measurement, which represents a higher voltage. 
     Referring back to  FIG. 1 , in the preferred embodiment using optical communication, the receiver circuit  30  includes a series of receiver channels where there is a single channel  90  for each of the EMMs  26 . Each channel  90  includes a photodiode  92  that receives the communication signal  46  and a trans-impedance amplifier  94  that converts the diode current to a representative voltage. The voltage signal from the trans-impedance amplifier  94  is then sent to a comparator  96  to make sure it is within a desired range, and if so, is then sent to a master central processing unit (CPU)  98 , which receives the signals from all of the channels  90 . The CPU  98  decodes the on/off sequence of the voltage signal to identify the end of frame synchronization pulses  64  so that each new group of actual voltage measurement signals are recalibrated to the startup calibration sequence at each measurement. The CPU  98  uses the width of the voltage pulses that have been decoded to identify a minimum cell voltage, a maximum cell voltage, an average cell voltage, and the actual voltage of each cell. This information is provided to a dual controller area network (CAN)  100  that provides the information to a vehicle bus through a serial interface circuit (SIU)  102  and then to a vehicle ECU (not shown) that controls the fuel cell system  10 , such as controlling reactant flow rates, stack relative humidity, etc. The edge-to-edge time of the PWM signal  60  does not exceed the capability of the timer capture unit in the CPU  98 . 
     While certain representative embodiments and details have been shown for purposes of illustrating the invention, it will be apparent to those skilled in the art that various changes may be made without departing from the scope of the invention, which is defined in the appended claims. For example the communication between the EMM  26  and the receiver circuit  30  could be done electrically via wires, radio waves or optically as illustrated. Furthermore the substrate can be a printed circuit board.