Abstract:
A method and apparatus for a high frequency resistance measurement instrument with a reasonable dynamic range, which militates against saturation in the presence of one or more large undesirable signals, the high frequency measurement instrument capable of employing multiple alternating currents of differing frequencies to maintain measurement capability despite signal chain saturation in the presence of a large undesirable signal.

Description:
FIELD OF THE INVENTION 
     This invention relates to a method of operating parameter measurement within a fuel cell system. More particularly, the invention is directed to a method and apparatus for measurement of fuel cell high frequency resistance. 
     BACKGROUND SUMMARY 
     High frequency resistance is a well-known and extensively documented property of fuel cells. By measuring the high frequency resistance of a fuel cell within a specific band of excitation current frequencies, the state-of-humidification of a proton exchange membrane can be measured. The high frequency resistance in a proton exchange membrane of the fuel cell is typically measured at a single frequency between 500 Hz and 1500 Hz. 
     Traditional methods for measuring high frequency resistance begin by first inducing an alternating current at the frequency of interest (typically 1000 Hz) through the fuel cell or stack. Next, the instrument measures the actual current ripple flowing through the fuel cell or stack and the voltage ripple induced on the cell or stack via the injected alternating current. These signals are filtered and amplified by the instrument. The instrument then determines the high frequency resistance by dividing the magnitude of the voltage ripple waveform by the magnitude of the current ripple waveform. In accordance with Ohm&#39;s law, the resulting value is a resistance, which may be further scaled by the number of cells in the stack or the active area of the fuel cell membrane to yield the unit-area-resistance of the membrane. 
     In noisy electrical environments, such as that of a high voltage distribution system in an electrically-propelled vehicle, other large alternating current ripples may be present in addition to the induced ripple current and resultant ripple voltage. These large undesirable signals may exist at a single frequency persistently, or they may have variable frequency content depending on vehicle speed, engine load, or other factors. 
     The presence of such large undesirable signals can be problematic for traditional single-frequency high frequency resistance measurement systems. For example, if the induced ripple current is approximately 0.3 A, and the large undesirable signal is 100 A at the same frequency, a circuit with a dynamic range greater than 70 db must be realized to prevent saturation of the signal chain. 
     It would be desirable to produce a high frequency resistance measurement instrument with a reasonable dynamic range, which militates against saturation in the presence of one or more large undesirable signals. It would be further desirable to produce a high frequency resistance instrument that can be employed at multiple induced AC currents of differing frequencies to maintain measurement capability despite signal chain saturation in the presence of a large undesirable signal. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a high frequency resistance measurement instrument with a reasonable dynamic range, capable of employing multiple alternating currents of differing frequencies to maintain measurement capability despite signal chain saturation in the presence of a large undesirable signal has surprisingly been discovered. 
     In one embodiment, the method for measurement of high frequency resistance in a fuel cell system includes the steps of providing a fuel cell stack, providing a microcontroller, inducing at least one excitation signal into the fuel cell stack, determining the presence of undesirable signals in the fuel cell stack using the microcontroller, and determining the high frequency resistance as a function of the excitation signal at frequencies with acceptable undesirable signal levels. 
     In another embodiment, the method for measurement of high frequency resistance in a fuel cell system includes the steps of providing a fuel cell stack, providing a microcontroller, providing a plurality of channels wherein the total number of channels is at least one greater than the total number of expected undesirable signals, tuning each channel at a separate fixed frequency, determining a number of states, designating at least one channel as an inactive channel and the remaining channels as active channels during each state, inducing at least one excitation signal into the fuel cell stack at the frequencies of the active channels during each state, sampling an output of the channels using the microcontroller, evaluating the validity of the output, computing a high frequency resistance value of the fuel cell system, and outputting the high frequency resistance value. 
     In another embodiment, the apparatus for measuring high frequency resistance in a fuel cell system includes a microcontroller, at least one channel electronically linked to the microcontroller, and a frequency tuning device electronically linked to each channel. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an apparatus for measurement of fuel cell high frequency resistance according to an embodiment of the invention; 
         FIG. 2  is a flow diagram illustrating a method of controlling the apparatus for measurement of fuel cell high frequency resistance of  FIG. 1 ; 
         FIG. 3  is a graph showing a plot of noise floor v. frequency during operation of a vehicle and illustrating ripple noise during operation of the vehicle; 
         FIG. 4  is a flow diagram illustrating a method of controlling an apparatus for measurement of fuel cell high frequency resistance according to another embodiment of the invention; 
         FIG. 5  is a flow diagram illustrating a subroutine for the method of controlling the apparatus for measurement of fuel cell high frequency resistance of  FIG. 4 ; 
         FIG. 6  is a graph showing a plot of noise floor v. frequency during operation of a vehicle and illustrating ripple noise during operation of the vehicle for a third embodiment of the invention; and 
         FIG. 7  is a flow diagram illustrating a method of controlling an apparatus for measurement of fuel cell high frequency resistance according to the third embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The following detailed description and appended drawings describe and illustrate various exemplary embodiments of the invention. The description and drawings serve to enable one skilled in the art to make and use the invention, and are not intended to limit the scope of the invention in any manner. In respect of the methods disclosed, the steps presented are exemplary in nature, and thus, the order of the steps is not necessary or critical. 
       FIG. 1  shows a fuel cell stack  10  consisting of a plurality of individual fuel cells which are connected electrically in series and/or in parallel. The operation of various types of fuel cell systems are commonly known in the art; one embodiment can be found in commonly owned U.S. Pat. No. 6,849,352; hereby incorporated herein by reference in its entirety. Therefore, only the operation of a fuel cell system as pertinent to this invention will be explained in the description. 
     In the embodiment shown herein, a high frequency resistance measurement system contains a total number of(N+1) high frequency measurement channels  55 . Where N is the number of separate undesirable signals the designer expects to encounter. In  FIG. 1 , N is assumed to equal two, although it is understood that more or fewer signals may be present. Each high frequency measurement channel  55  contains a separate current signal chain  42  and a voltage signal chain  64 . Additionally, the number of channels provided may differ without departing from the scope of the invention. 
     An AC current sensor  11  includes a current transducer  12  that is linked to the fuel cell stack  10 . An amplifier  16  is linked to the current transducer  12  via an electrical connection  14 . The electrical connections may be any conventional means of electrical connection such as insulated wire, for example. A carrier rejection filter  20  is linked to the amplifier  16  via an electrical connection  18 . 
     The carrier rejection filter  20  is linked to a peak detector  88  in a common saturation detection circuit  84  via an electrical connection  22 . 
     The carrier rejection filter  20  is linked to the first integrator filter  24 , a second integrator filter  26 , and a third integrator filter  28  via the electrical connection  22 . The first integrator filter  24  is linked to the first integrator circuit  30  via an electrical connection  25  in a first high frequency resistance measurement channel  57 . The second integrator filter  26  is linked to an integrator circuit  32  via an electrical connection  27  in a second high frequency resistance measurement channel  59 . The third integrator filter  28  is linked to an integrator circuit  34  via an electrical connection  29  in a third high frequency resistance measurement channel  53 . The integrator filters  24 ,  26 , and  28  have tuning frequencies matching those in the high frequency measurement channels  53 ,  59 , and  57 . 
     A filter  44  is linked to the integrator circuit  30  via an electrical connection  36  in the current signal chain  42  for the first high frequency resistance measurement channel  57 . An electrical connection  45  links the filter  44  to a detection circuit  50 . The integrator circuit  32  is linked to a filter  46  via an electrical connection  38  in the current signal chain  42  for the second high frequency resistance measurement channel  59 . A detection circuit  52  is linked to the filter  46  via an electrical connection  47 . The integrator circuit  34  is linked to a filter  48  via an electrical connection  40  in the current signal chain  42  for the third high frequency resistance measurement channel  53 . An electrical connection  49  links the filter  48  to a detection circuit  54 . 
     An analog to digital converter  106  is linked to the detection circuit  50  via an electrical connection  94 . Electrically linked to the analog to digital converter  106  is the detection circuit  52  via an electrical connection  96 . The detection circuit  54  is linked to the analog digital converter  106  via an electrical connection  98 . The analog to digital converter  106  is included in a microcontroller  108 . 
     The fuel cell stack  10  is electrically connected to a safety isolation barrier circuit  56 . The safety isolation barrier circuit  56  is electrically linked to a carrier rejection filter  60  via an electrical connection  58 . The carrier rejection filter  60  is linked to a peak detector  86  in the common saturation detection circuit  84  via an electrical connection  62 . The detectors  86  and  88  are linked to the converter  106  via electrical connections  90  and  92  respectively. 
     The carrier rejection filter  60  is linked to a filter  66  in the voltage signal chain  64  for the first high frequency resistance measurement channel  57 . The carrier rejection filter  60  is linked to a filter  68  in the voltage signal chain  64  for the second high frequency resistance measurement channel  59 . The carrier rejection filter  60  is linked to a filter  70  in the voltage signal chain  64  for the third high frequency resistance measurement channel  53 . 
     An electrical connection  72  links the filter  66  to a detection circuit  78 . The filter  68  is linked to a detection circuit  80  via an electrical connection  74 . An electrical connection  76  links the filter  70  to a detection circuit  82 . 
     The detection circuits  78 ,  80  and  82  are linked to the analog to digital converter  106  via electrical connections  104 ,  102  and  100  respectively. 
     In operation, the fuel cell stack  10  generates electrical current and voltage in a manner commonly known the art. An excitation signal, containing a ripple current and causing a ripple voltage, is induced in the fuel cell stack  10 . The excitation signal is generated using the fuel cell stack  10  end cell heaters (not shown). The output of the end cell heaters is regulated using pulse-width-modulation. The frequency of the ripple current is varied by controlling the carrier frequency of the pulse-width-modulation. Additionally, other sources of inducing the excitation signal may be used without departing from the scope of the invention. 
     The alternating current sensor  11  measures the alternating current flow through the fuel cell stack  10  via the current transducer  12 . The current transducer  12  converts the magnitude of the current into a voltage signal which is proportional to the flow of the current through the fuel cell stack  10 . The voltage signal is amplified by the amplifier  16 , and filtered by the carrier rejection filter  20 . 
     Some current transducers  12  require an integrator circuit  13 , which may saturate in the presence of large undesirable signals. Therefore, the alternating current sensor integrator  11  may include the separate integrator filters  24 ,  26 ,  28  for each high frequency measurement channel  55 . The integrator filters  24 ,  26 ,  28  are tuned to frequencies matching those of the individual high frequency measurement channels  55 . 
     The high frequency measurement channels  55  include the filters  44 ,  46 ,  48  on the current signal chain  42  and the filters  66 ,  68 ,  70  on the voltage signal chain  64  tuned to corresponding frequencies. The frequencies of interest of each high frequency measurement channel  55  are tuned to avoid known fixed-frequency undesirable signals. The filters  44 ,  46 ,  48 ,  66 ,  68 ,  70  limit the exposure of each individual voltage signal chain  64  and current signal chain  42  to nearby large undesirable signals, while allowing saturation of a high frequency measurement channel  55  whose tuning frequency coincides with the frequency of a large undesirable signal. When a large undesirable signal is present and causing one high frequency measurement channel  55  to saturate, the filters on the adjacent high frequency measurement channel  55  attenuate the undesirable signal sufficiently that the induced ripple current can be recovered without saturation. Additionally, it is understood that various types of frequency tuning devices known in the art can be used without departing from the scope of the invention. 
     The current signal chains  42  receive the voltage signal from the alternating current sensor  11  and are the current measurement and conditioning portion of the high frequency measurement channels  55 . The current signal chains  42  measure the current at a predetermined frequency tuned by the filters  44 ,  46 ,  48  via the detection circuits  50 ,  52 ,  54 . The current signal chains  42  also condition the measurement as an output signal to the microcontroller  108 . 
     The safety isolation barrier circuit  56  is used in the high frequency measurement apparatus if the voltage generated across the fuel cell stack  10  is sufficient to be dangerous to persons or connected equipment. The safety isolation barrier circuit  56  provides galvanic isolation between high voltage and low voltage portions of the system. The carrier rejection filter  60  filters the frequencies of interest from the voltage received from the fuel cell stack  10  via the safety isolation barrier circuit  56 . 
     The voltage signal chains  64  receive the voltage signal from the carrier rejection filter  60  and are the voltage measurement and conditioning portion of the high frequency measurement channels  55 . The voltage signal chains  64  measure the voltage at a predetermined frequency tuned by the filters  66 ,  68 ,  70  via the detection circuits  78 ,  80 ,  82 . The voltage signal chains also condition the measurement as an output signal. 
     Referring now to  FIG. 2 , the microcontroller  108  operates as a state machine to augment the selection of the most-accurate high frequency measurement value. The microcontroller  108  continuously samples the outputs of each high frequency measurement channel  55  of the voltage signal chain  64  and the current signal chain  42 . The number of states implemented depends on the total number of high frequency measurement channels  55  and the total number of high frequency measurement channels  55  which can be excited simultaneously. 
     In each state  120 ,  128 ,  134  excitation signals are injected in the fuel cell stack  10  at all but one of the high frequency measurement channels  55  frequencies. The high frequency measurement channels  55  in which current ripples are excited are designated as active channels, and the other channels are designated as inactive channels. 
     The microcontroller  108  evaluates the validity of each voltage and current ripple measurement on each active channel, to determine if a large undesirable signal is causing the high frequency measurement channel sensor to saturate in steps  122 ,  130 , and  138 . If the microcontroller  108  does not detect saturation, the high frequency measurement value for the high frequency measurement channel is computed in steps  124 ,  132 , and  140  and the microcontroller selects the best measurement to output as an electrical signal in steps  126 ,  136 , and  142 . The microcontroller also checks the unexcited channels for the presence of any voltage or current in the absence of intentional excitation. Measurements taken on channels with less unexcited ripple are considered more valid in measurements taken in subsequent states. High frequency measurement channels exhibiting saturation or high levels of unexcited ripple are considered less-valid in subsequent states. The microcontroller  108  may output a single or multiple high frequency measurements in each state depending on the validity algorithm. The microcontroller  108  may also output the frequency at which the high frequency output measurements were taken. The microcontroller  108  returns to the first state  120  after cycling through all the states  144 . 
     In another embodiment, the high frequency measurement apparatus is realized using at least one of the high frequency measurement channels  55 . The microcontroller  108  continuously scans through a predetermined measurement range  151  as shown in  FIG. 3  for a desired frequency  153 . As shown in  FIG. 5 , the microcontroller  108  scans from an Enter  166  with a first measurement channel at  168  until locating a frequency where the level of unexcited ripple is minimal and no saturating undesirable signals exist  170 . If the microcontroller  108  has scanned all measurements without finding a minimum value (Y at  172 ) then it branches at  174  and an invalid signal is returned  180 . If the microcontroller  108  has not scanned all measurements without finding a minimum value (N at  172 ) it branches at  176  and then the minimum value is returned  178 . In  FIG. 4 , from a Start  150  the subroutine of  FIG. 5  is run in step  152  and the high frequency resistance excitation is set to the desired frequency  153  in a step  154  determined by the minimum value returned  178 . The microcontroller  108  measures the high frequency resistance for one measurement channel at the desired frequency  153  at  156 . The microcontroller  108  can measure the high frequency resistance at the desired frequency  153  using the first high frequency measurement channel or a second high frequency measurement channel. The subroutine of  FIG. 5  is run in the step  158  and the current frequency is compared with the best frequency at  160 . The method branches at  162  to step  156  if the current frequency is the best frequency (Y at  160 ) and branches at  164  to step  154  if not (N at  160 ). 
     In another embodiment the high frequency measurement apparatus is realized using a single high frequency measurement channel  55  located at a center frequency  190  in a predetermined measurement range  192  as shown in  FIG. 6 . The microcontroller  108  measures the noise at the center frequency  190  at a step  194  in  FIG. 7 . The microcontroller  108  measures the high frequency resistance at the center frequency  190  in a step  198  if the noise level is below saturation (Y at  196 ). 
     If noise at the center frequency is not below saturation level (N at  196 ), the microcontroller  108  modulates the frequency of the high frequency measurement channel  55  an even modulation distance Δ above and below the center frequency  190 . The modulation distance is set at a minimum in a step  202 , and the microcontroller  108  measures the noise at the center frequency  190  plus and minus the modulation distance Δ at  204 . The microcontroller increases the modulation distance Δ in a step  206  when the modulation distance is not already at the maximum (N at  208 ). If the modulation distance is at the maximum the system reports an error in a step  216  and restarts in a step  218 . 
     The high frequency measurement apparatus induces the excitation signal and measures the high frequency resistance at the modulated frequency in a step  212  when the noise is below saturation (Y at  210 ), and then restarts at  214 . A step  200  activates the excitation during measurement only. 
     From the foregoing description, one ordinarily skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, make various changes and modifications to the invention to adapt it to various usages and conditions.