Abstract:
A method for validating a pressure sensor between a source of a gas and a pressure regulator that regulates a gas flow. The method includes providing a valve command signal and selecting a bias signal from a bias table. The method also includes comparing the selected bias signal to the valve command signal and determining there is a pressure sensor error if the difference between the selected bias signal and the valve command signal is above a predetermined threshold.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a system and method for validating a pressure sensor signal from a high pressure gas storage system and, more particularly, to a system and method for validating a pressure sensor signal from a high pressure gas storage system, where an algorithm uses information from electrical pressure controls to validate the high pressure sensor signal. 
     2. Discussion of the Related Art 
     Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cell systems as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today&#39;s vehicles employing internal combustion engines. Fuel cell vehicles are expected to rapidly increase in popularity in the near future in the automotive marketplace. 
     Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically, but not always, include finely divided catalytic particles, usually a highly active catalyst such as platinum (Pt) that is typically supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation. 
     Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack. In one known type of fuel cell system, the hydrogen gas fuel is injected into the anode side of the fuel cell stack by one or more injectors. The injector controls the amount of injected fuel for a particular stack current density based on a pulse width modulation (PWM) control signal that controls the opening and closing of the injector. 
     A fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow fields are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow fields are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows. 
     Typically, hydrogen gas for the fuel cell system is stored at high pressure in a tank system including one or more interconnected pressure vessels on the vehicle to provide the hydrogen gas necessary for the fuel cell stack. The pressure within the vessels can be 700 bar or more. In one known design, the pressure vessels include an inner plastic liner that provides a gas tight seal for the hydrogen gas, and an outer carbon fiber composite layer that provides the structural integrity of the vessel. 
     A hydrogen storage system typically includes at least one pressure regulator as part of the various and numerous valves, gauges, and fittings necessary for operation of the hydrogen storage system that reduces the pressure of the hydrogen gas from the high pressure in the vessels to a constant pressure suitable for the fuel cell stack. Various pressure regulators are known in the art to provide this function, including mechanical pressure regulators and electronic pressure regulators. 
     High pressure gas tank systems typically require pressure regulators to control the outlet flow and to reduce the pressure of the outlet flow. In some applications this function is done by one or more mechanical pressure regulators. In other applications this function is done by an electrical pressure regulator. For pressure controls as well as for driver information and for communication with refueling stations, a high pressure sensor is utilized. The high pressure sensor signal is also used for the observation of the lower pressure limit of the high pressure gas tank/vessel to prevent liner damage in the tank that is caused by low tank pressure. In current systems, if the high pressure sensor signal fails, the low pressure threshold for the vessel has to be increased for safety reasons and to prevent negative driver impact. Thus, there is a need in the art for an algorithm that uses information from electrical pressure controls to validate the high pressure sensor signal in a manner that reduces the amount the low pressure threshold is increased. 
     SUMMARY OF THE INVENTION 
     In accordance with the teachings of the present invention, a method is disclosed for validating a pressure sensor between a source of a gas and a pressure regulator that regulates a gas flow. The method includes providing a valve command signal and selecting a bias signal from a bias table. The method also includes comparing the selected bias signal to the valve command signal and determining there is a pressure sensor error if the difference between the selected bias signal and the valve command signal is above a predetermined threshold. 
     Additional features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic block diagram of a high pressure gas tank system for a fuel cell stack; and 
         FIG. 2  is a block diagram showing the operation of an algorithm for validating a pressure sensor signal of the system shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The following discussion of the embodiments of the invention directed to a system and method for validating a pressure sensor signal from electrical controlled high pressure gas storage systems is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. For example, as mentioned, the present invention has application in a system for providing hydrogen gas to a fuel cell stack. However, as will be appreciated by those skilled in the art, the system and method of the invention may have application for controlling a pressure regulator in association with an injector that injects a gas for other applications. 
       FIG. 1  is a simplified schematic block diagram of a high pressure gas tank system  10  for providing hydrogen gas to a fuel cell stack  12 . The tank system  10  includes a first high pressure tank  14  and a second high pressure tank  16 , where it will be understood that the tank system  10  could include any suitable number of high pressure tanks. The tanks  14  and  16  may contain the hydrogen gas at a pressure upwards of 700 bar, and can be any high pressure tank suitable for the purposes discussed herein, such as the high pressure vessel discussed above having an inner plastic liner and an outer structural composite layer. The high pressure tank  14  includes a tank shut-off valve  20  provided in an output line  18  from the tank  14  and the high pressure tank  16  includes a tank shut-off valve  24  provided in an output line  22  from the tank  16 . The shut-off valves  20  and  24  generally provide a safety control for the high pressure in the tanks  14  and  16 , respectively. The tank output lines  18  and  22  are coupled to an anode side input line  26  that provides the hydrogen gas stored in the tanks  14  and  16  to the fuel cell stack  12 . A pressure sensor  36  is provided in the line  26  to provide a high pressure reading of the pressure within the tanks  14  and  16  when the valves  20  and  24  are open for system control purposes. In known systems, validation of the pressure reading of high pressure tanks such as the tanks  14  and  16  requires a second pressure sensor (not shown). Without a second pressure sensor, known systems must utilize safety margins that have a lower pressure range, and that provide a lower reliability of a tank system in a vehicle when compared to the tank system  10  that includes a validation algorithm, discussed in detail below. 
     An electronic pressure regulator  28  is provided in the input line  26  downstream from the pressure sensor  36  that selectively reduces and provides a constant pressure of the gas from the high pressure of the tanks  14  and  16  to a pressure suitable for the fuel cell stack  12 , in a manner that is well understood by those skilled in the art. In one embodiment, the pressure regulator  28  is a proportional valve having an adjustable orifice. As is well understood by those skilled in the art, the size of the orifice in the regulator  28  and the pressure upstream in the input line  26  controls the flow rate and the amount of gas that is provided downstream of the pressure regulator  28 . 
     The reduced pressure hydrogen gas in the input line  26  downstream of the pressure regulator  28  is injected into the anode side of the fuel cell stack  12  by an injector  30 . The position of the orifice in the pressure regulator  28  is selectively controlled by a controller  34  to control the pressure of the gas in the input line  26 , as measured by two redundant pressure sensors  38  and  40  that are in the line  26  downstream of the pressure regulator  28  and that measure the pressure in a gas volume that is between the pressure regulator  28  and the injector  30 . The pressure of the gas volume between the pressure regulator  28  and the injector  30  is controlled to be equal to or less than a target pressure. The injector  30  is controlled by a PWM signal to provide the proper amount of hydrogen gas to the fuel cell stack  12  for a particular stack current density. A duty cycle and frequency of the PWM signal may be defined as described in U.S. patent application Ser. No. 13/217,888 entitled, “Advanced Controls Algorithm for an Electronic Pressure Regulator System with Pulsed Disturbances,” filed Aug. 25, 2011, assigned to the assignee of the present application and incorporated herein by reference. 
     Although a single injector is shown in this non-limiting embodiment to inject the hydrogen gas into the stack  12 , those skilled in the art will understand that a fuel cell system may include a bank of several injectors that inject the hydrogen gas into the stack  12 . The controller  34  receives the pressure measurements from the pressure sensors  36 ,  38  and  40  and the PWM signal that controls the opening and closing of the injector  30 , and the controller  34  controls the position of the pressure regulator  28  so that the pressure in the volume of gas between the pressure regulator  28  and the injector  30  remains substantially constant during normal system operation. A pressure relief valve  42  is coupled to an exhaust line  44  and may be used to release gas from the input line  26  into a cathode exhaust or ambient if the pressure in the input line  26  exceeds a predetermined threshold. 
     As stated above, the pressure regulator  28  works as a proportional orifice. A valve piston is admitted with a pressure difference between the tanks  14  and  16  as measured by the pressure sensor  36  and the input line  26  downstream of the pressure regulator  28  as measured by the pressure sensor  38  or  40 . This pressure difference determines the force at a given flow, where force is current at a solenoid valve. The relation between tank pressure and a duty cycle for a feed-forward controller may be used, as described in U.S. patent application Ser. No. 13/217,888 entitled, “Advanced Controls Algorithm for an Electronic Pressure Regulator System with Pulsed Disturbances” discussed above and incorporated by reference. Thus, at a given flow, the interpolated value from a bias look-up table can be compared with the actual DC command, as described in more detail below. If this absolute difference is higher than a threshold, a sensor failure of the pressure sensor  36  may be detected. This validation approach allows for a failure of the pressure sensor  36  to be detected without requiring a second sensor. The validation algorithm discussed in detail below also provides the additional benefits of less wiring, lower sensor costs and higher tightness due to a saved sensor port, i.e., less space is required. Further benefits include no additional electronic controls unit (ECU) input needed, less EMC (define) sensitivity, low controls effort, usage of existing tables, higher reliability compared to using one sensor and a higher pressure range in the case of sensor failure. 
       FIG. 2  is a schematic block diagram of a system  50  that shows the operation of an algorithm operating in the controller  34  for controlling the position of the pressure regulator  28 . The system  50  includes a proportional-integral (PI) controller  52  that provides a duty cycle (DC) pressure error control signal that defines a difference between a desired pressure of a gas volume in the input line  26  and the actual pressure of the gas volume in the input line  26 , as measured by one or both of the pressure sensors  38  and  40 . Particularly, a pressure request signal is provided on line  54  and an actual pressure measurement signal is provided on line  56  from the pressure sensor  38  or  40  that are sent to a subtractor  58  that generates a pressure error signal between the two signals, where the error signal, i.e., the controls deviation of the pressure set-point, is provided to the PI controller  52 . The pressure request signal is the calibrated pressure for the input line  26  that will provide the desired amount of hydrogen gas through the control of the injector  30 . The controller  52  operates as a standard PI controller that attempts to reduce the error signal by controlling the deviation between the pressure set-point and the actual pressure in the input line  26 . 
     The system  50  provides a feed-forward bias signal that determines when the pressure regulator  28  is opened, where the bias signal is determined by the requested mass flow of hydrogen gas to the fuel cell stack  12  and the pressure at the upstream location of the pressure regulator  28  as measured by the pressure sensor  36 . Particularly, an average hydrogen gas mass flow request signal, such as determined by the position of the throttle in the vehicle, is provided on line  78  and the measured high pressure in the tanks  14  and  16 , as provided by the sensor  36 , is provided on line  80 , where the pressure measurement signal is filtered by a filter  82 . The mass flow signal and the pressure signal are set to a two-dimensional look-up table  84  that selects the proper bias signal for opening the pressure regulator  28  the proper amount for the desired hydrogen gas flow to the injector  30 . The pressure measurement signal from the pressure sensor  36  needs to be filtered because disturbances caused by the opening and closing of the pressure regulator  28  could influence the bias signal provided by the look-up table  84 , causing unwanted pressure oscillations. The bias signal from the look-up table  84  is a DC signal whose magnitude changes in step increments if the average hydrogen gas mass flow request and/or the high pressure measurement change enough to select a different value in the look-up table  84 . At a constant flow there is a monotonically increasing of the duty cycle in relation to the vessel pressure The main idea behind a feed-forward controller is that the bias value is as exact as possible so that the PI-controller  52  only has to correct a deviation between the bias value and system behavior. Thus, the PI-controller  52  has less work and the controls are more stable. 
     The output from the look-up table  84  is added to the output from the PI controller  52  by an adder  86  to provide a valve command  88  that determines the position of the orifice of the pressure regulator  28  that is synchronized to the opening and closing of the injector  30 . Therefore, the feed-forward control provided by the bias signal from the table  84  provides most of the control for the position of the pressure regulator  28  and the error control signal from the PI controller of the controller  52  slightly modifies that bias signal in the adder  86  to correct the deviation between the desired pressure and the actual pressure in the input line  26  at box  88 . The pressure sensor signal validation algorithm compares the output of the bias look-up table  84  to the valve command from the adder  86 . Due to the fact that there is a high correlation between the bias DC and the vessel pressure at steady state operation, the difference between the bias DC and the pressure of the tanks  14  and  16  as measured by the pressure sensor  36  is evaluated by the validation algorithm at box  90 . To avoid the evaluation of dynamic effects, a low-pass-filter  92  is implemented. An absolute difference box  94  is compared with a calibrateable threshold box  96  at relational operator box  98 . The relational operator box  98  is fed into a double-debounce box  100 , where the impact from the relational operator box  98  has to be a constant true for a predetermined threshold amount of time before the output from the double-debounce box  100  is set to true at box  102 . In this way, peak disturbances do not effect the validation algorithm. A similar function may be performed using a x-out-of-y function, known to those skilled in the art. Thus, the validation algorithm discussed above may be used anytime the fuel cell stack  12  is producing energy. However, for improved accuracy, the validation algorithm may be disabled when the flow of gas is idle and the pressure of the input line  26  is high. 
     As stated above, the validation algorithm ensures that the pressure sensor  36  is functioning properly, thereby preventing the pressure in the tanks  14  and  16  from dropping below a predetermined value such that damage to the tanks  14  and  16  is prevented. The predetermined value of the low pressure threshold for the tanks  14  and  16  that is used to prevent damage to the tanks  14  and  16  depends on the size and the construction of the tanks. 
     As will be well understood by those skilled in the art, the several and various steps and processes discussed herein to describe the invention may be referring to operations performed by a computer, a processor, or other electronic calculating device that manipulates and/or transforms data using electrical phenomenon. Those computers and electronic devices may employ various volatile and/or non-volatile memories including non-transitory computer-readable medium with an executable program stored thereon including various code or executable instructions able to be performed by the computer or processor, where the memory and/or computer-readable medium may include all forms and types of memory and other computer-readable media. 
     The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.