BACKGROUND  
       [0001]     The present invention relates generally to a redundant control system and method. More specifically, the present invention relates to a modular redundant control system and method that avoids abrupt changes in control signals, which are commonly encountered with a degradation of a transmitter (or sensor) signal or failure of a transmitter (or sensor) in present redundant systems.  
         [0002]     Typically, redundant control systems include at least two transmitters (or sensors) in communication with a controller to protect against transmitter (or sensor) signal degradation or transmitter (or sensor) failure that may cause a disruption of the turbine control system. In such system a control signal is derived by determining a median signal of the transmitters&#39; (or sensors&#39; ) signals. However, this approach results in abrupt changes in the control signal, which often results in undesirable disruptions in the control system operation, sometimes referred to as “bumps” in turbine control systems.  
         [0003]     Another approach is described in U.S. Pat. No. 5,715,178 to Scarola et al., which discloses an algorithm that averages outputs of sensors measuring the same parameter to provide an averaged (or mean) sensor output, which is compared to the original sensor outputs. When the deviation between the averaged sensor output and any one of the original sensor outputs is too great, the sensor output with the greatest deviation is removed from the calculation. This process is repeated until all the remaining sensor outputs in the calculation are within an acceptable deviation of the averaged sensor output; thereafter the averaged sensor output is utilized in the system. This approach also results in abrupt changes in the signal utilized in the system, which often results in undesirable disruptions in the system operation.  
       SUMMARY  
       [0004]     In one exemplary embodiment of the invention a redundant control system including at least three redundant signals is presented. The redundant control system includes a failure circuit associated with each of the redundant signals. The failure circuit generates a failure signal in response to an occurrence of a failure of a corresponding on or the redundant signals. The redundant control system further includes a select circuit associated with each of the redundant signals. The select circuit is receptive to a default signal associated with the corresponding one of the redundant signals, to the failure signal, and to the corresponding one of the redundant signals. The select circuit selects the default signal when the failure signal indicates a failure of the corresponding one of the redundant signals and selects the corresponding one of the redundant signals when the failure signal does not indicate a failure for the corresponding one of the redundant signals. The redundant control system still further includes a median determination circuit that is receptive to the default signals selected and the redundant signals selected to determine a median signal indicative of a median of the default signals selected and the redundant signals selected.  
         [0005]     In another exemplary embodiment of the invention a redundant control method including at least three redundant signals is also presented. The redundant control method includes determining an occurrence of a failure of any one of the redundant signals. The redundant control method further includes for each of the redundant signals, selecting a default signal associated with the corresponding one of the redundant signals when a failure of the corresponding one of the redundant signals has been determined and selecting the corresponding one of the redundant signals when a failure of the corresponding one of the redundant signals has not been determined. The redundant control method also includes determining a median signal of the default signals selected and the redundant signals selected  
         [0006]     In yet another exemplary embodiment of the invention a redundant control system including at least three redundant signals is presented. The redundant control system includes means for determining an occurrence of a failure of any one of the redundant signals. The redundant control system further includes means for selecting, associated with each of the redundant signals, a default signal associated with the corresponding one of the redundant signals when a failure of the corresponding one of the redundant signals has been determined and the corresponding one of the redundant signals when a failure of the corresponding one of the redundant signals has not been determined. The redundant control system still further includes means for determining a median signal of the default signals selected and the redundant signals selected. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying figures, wherein:  
         [0008]      FIG. 1  is a block diagram of an exemplary embodiment of a continuous median failure control system in accordance with the present invention;  
         [0009]      FIG. 2  is a block diagram of a Failure Detection function of the system of  FIG. 1 ;  
         [0010]      FIG. 3  is a block diagram of an Update Setpoint function of the system of  FIG. 1 ;  
         [0011]      FIG. 4  is a block diagram of a Failure Alarm Status Determination function of the system of  FIG. 1 ;  
         [0012]      FIG. 5  is a block diagram of a Calculate Bias function of the system of  FIG. 1 ; and  
         [0013]      FIG. 6  is a block diagram of a Median Drop function of the system of  FIG. 1 . 
     
    
     DETAILED DESCRIPTION  
       [0014]     Referring to  FIG. 1 , a block diagram of an exemplary embodiment of a continuous median failure control system for use in a redundant turbine control system is generally shown at  10 . Signals from redundant transmitters (the transmitters are not shown, but include pressure transducers, flow transducers, and other devices that transmit signals where redundancy would be advantageous in a system) are presented at signal buss or lines  12 , referred to herein as “present transmitter signals”. While only a single line is used herein to illustrate a bus or a plurality of signal lines, it is intended that each signal be associated with an individual conductor of the bus or an individual signal line. Nevertheless, it is also within the scope of the present invention that know multiplexing techniques could be employed to carry multiple signals over a single conductor. These transmitter signals are presented to a Failure Detection function (circuit)  14  and a Get Previous Value function (circuit)  16 . Get Previous Value function  16  stores a record of previous transmitter signal values of the transmitter signals that it receives. These previous transmitter signal values are then presented to the Failure Detection function  12  by buss or signal lines  18 . Failure Detection function  14  compares each of the present transmitter signals with the corresponding (i.e., corresponding to each of the transmitters) previous transmitter signals, whereby a failure is detected when a difference is outside respective limits. Failure Detection function  14  is discussed in more detail hereinafter with reference to  FIG. 2 . Failure Detection function  14  provides corresponding failure detection signals (“Failure High” and “Failure Low” signals) in response to these comparisons, which are presented to an Update Setpoint Status function (circuit)  20  and a Failure Alarm Status Determination function (circuit)  22  by a buss or signal lines  24 . Failure Alarm Status Determination function  22  receives Acknowledged Fixed signals, indicative of corresponding transmitter signals that are provided for a safe mode of operation for the system, presented at bus or signal lines  26 . Failure Alarm Status Determination function  22  employs a logic circuit to provide corresponding alarm signals indicating that a failure has occurred. “High Failure” and “Low Failure” signals are generated by the logic circuit of the Failure Alarm Status Determination function  22  and are presented at bus or signal lines  28  to Update Setpoint Status function  20 , a Median Drop function (circuit  30 , and a Median Drop function (circuit)  32 . Failure Alarm Status Determination function  22  is discussed in more detail hereinafter with reference to  FIG. 4 . Update Setpoint Status function  20  employs a logic circuit to provide corresponding “Failure” signals (regardless if it was a high failure or a low failure) at buss or signal lines  34 , indicating that a setpoint (e.g., an operational parameter) requires updating. These Failure signals are presented to a Calculate Bias and Update Setpoint function (circuit)  36  by buss or signal lines  34 . The Update Setpoint Status function  20  is discussed in more detail hereinafter with reference to  FIG. 3 . The Median Drop functions  30  and  32  determine Present (presented at buss or signal lines  38 ) and Previous (presented at buss or signal lines  40 ) Median signal values for the transmitter signals that have not failed. The Present and Previous Median signal values are presented to Calculate Bias and Update Setpoint function  36  by buss or signal lines  38  and  40 , respectively. The median Drop functions  30  and  32  are discussed in more detail (although only one is discussed as the same arrangement is applicable to both) hereinafter with reference to  FIG. 6 .  
         [0015]     Calculate Bias and Update Setpoint functions  36  receives a Fixed (setpoint) signal as an input at signal line  42 . The Present and Previous median signal values and the Failure signals are utilized to update a setpoint, resulting in the Biased Setpoint presented at signal line  44 . Calculate Bias and Update Setpoint function  36  is discussed in more detail hereinafter with reference to  FIG. 5 . The Bias Signal and the Present Median signal are combined to determine a difference at a Summer function (circuit)  46 , resulting in an Output signal presented at signal line  48 .  
         [0016]     Referring now to  Fig. 2 , Failure Detection function  14  is generally shown. While  FIG. 2  only shows the configuration for one of the transmitter signals, it will be appreciated that the configuration is applicable for each of the other redundant transmitter signals. The present transmitter signal presented at signal line  12  and the previous transmitter signal presented at signal line  18  are compared at a Summer function (circuit)  50 , resulting in a difference at a signal line  52 . This difference is presented to a Compare function (circuit)  54 , which compares the difference to determine if it is greater than a Failure High Constant limit or less than a Failure High Constant limit. Compare function  54  may simply comprise a comparator as is well know. When this difference is greater than the Failure High Constant limit the Failure High signal is set (at signal line  24   a  of signal lines  24 ) and when this difference is less than the Failure High Constant limit the Failure Low signal is set (at signal line  24   b  of signal lines  24 ).  
         [0017]     Referring to  FIG. 3 , Update Setpoint Status function  20  is generally shown. While  FIG. 3  only shows the configuration for one of the transmitter signals, it will be appreciated that the configuration is applicable for each of the other redundant transmitter signals. The Failure High signal (at signal line  24   a ) and the Failure Low signal (at signal line  24   b ) are inputs to a logic OR gate  56 , with the output thereof (at a signal line  58 ) input to an logic OR gate  60 . The Acknowledged Fixed signal (at signal line  26 ) is an input to logic AND gates  62  and  64 . The Low Failure signal at signal line  28   a  is also an input to logic AND gate  62 , with the output thereof (at a signal line  68 ) input to logic OR gate  60 . The High Failure signal at a signal line  28   b  is also an input to logic AND gate  64 , with the output thereof (at a signal line  72 ) input to logic OR gate  60 . The output of logic OR gate  60  is the Failure signal at signal line  34 .  
         [0018]     Referring now to  FIG. 4 , Failure Alarm Status Determination function  22  is generally shown. While  FIG. 4  only shows the configuration for one of the transmitter signals, it will be appreciated that the configuration is applicable for each of the other redundant transmitter signals. The Failure High signal (at signal line  24   a  )is an input to a logic AND gate  74 . The High Failure signal (at signal line  28   b ) is also an input (at an inverted input) to logic AND gate  74 . The Low Failure signal (at signal line  28   a ) is also an input (at an inverted input) to logic AND gate  74 . The output of logic AND gate  74  (at a signal line  76 ) is input to a logic OR gate  78 . The Acknowledged Fixed signal (at signal line  26 ) is an input (at an inverted input) to logic AND gates  80  and  82 . The High Failure signal is also an input to logic AND gate  80 , with the output thereof (at a signal line  84  input to logic OR gate  78 . The output of logic OR gate  78  is the High Failure signal at line  28   b.  The Failure Low signal (at signal line  24   b )is an input to a logic AND gate  86 . The High Failure signal is also an input (at an inverted input) to a logic AND gate  86 . The Low Failure signal is also an input (at an inverted input) to logic AND gate  86 . The output of logic AND gate  86  (at a signal line  88 ) is input to a logic OR gate  90 . The Low Failure signal is also an input to logic AND gate  82 , with the output thereof (at a signal line  92 ) input to logic OR gate  90 . The output of logic OR gate  90  is the Low Failure signal at line  28   a.  The High Failure and Low Failure signals are inputs to a logic OR gate  94 , with the output thereof being the alarm signal (at a signal line  96 ). It will be appreciated that since the foregoing is a cascaded arrangement of logic gates, it may require one or more clock cycles for the alarm signal to be updated.  
         [0019]     Referring now to  FIG. 5 , Calculate Bias and Update Setpoint function  36  is generally shown. The Failure signals from update Set Point Status function  20  are input to a logic OR gate (not shown) to generate a single Failure signal, whenever there is a failure. This single Failure signal (at signal line  34 ) is an input at the select or enable input of a Select function (circuit)  98 . Select function  98  may comprise a select or gate integrated circuit that simply pass an input to the output when enabled, as is known. The Present Median signal (at signal line  38 ) and the Previous median signal (at signal line  40 ) are combined at a Summer function (circuit)  100  to provide a difference at a signal line  102 . This difference signal is passed through to the output (at a signal line  104 ) of Select function  98  when selected, i.e., the Failure signal indicates a failure. Otherwise the output is set to zero (i.e., the Fixed signal), at signal line  42 . This difference signal (which can be positive or negative) when selected becomes a Bias signal (at a signal line  104 ), which is added to the current Setpoint (at a signal line  106 ) at a Summer function (circuit)  108 . This combined signal, i.e., the Biased Setpoint signal from Summer function  108  is presented at signal line  44 . The Biased Setpoint becomes the Setpoint. This is an important feature as use of this Biased Signal in conjunction with the changing Present Median avoids abrupt changes in the Output Signal. This assures a smooth transmission during a failure, as compared to the abrupt (or bump) changes encountered in the prior art systems.  
         [0020]     Referring now to  FIG. 6 , Median Drop function  30  is generally show, with Median Drop function  32  having essentially the same configuration. The High Failure and Low Failure signals generated by the logic circuit of the Failure Alarm Status Determination function  22  are presented at signal lines  28   a - f  to for each of the transmitter signals (in this example there is three transmitters). These High Failure and Low Failure signals are input to select inputs of Select functions (circuits)  110 ,  112 , and  114 . A “Drop High” signal and a “Drop Low” signal are provided for each of the transmitter signals at signal lines  116   a - f.  The Drop High and Drop Low signals are default safe mode signals for each type of failure, i.e., high or low. These default signals are set to assure safe mode operation of the system, in other words they are selected to be outside of the normal range of the transmitter signal levels. Setting the Drop signals outside the normal range of the transmitter signal levels ensures that the Drop signals will not be included in the calculation of the median. These Drop High and Drop Low signals are input to inputs of the Select functions. For each of the Select Functions, when a High Failure signal is set, the select input of the Select function in enabled, thereby passing the Drop High signal to an output of the Select function. Further, when a Low Failure signal is set, the select input of the Select function in enabled, thereby passing the Drop Low signal to an output of the Select function. Otherwise, the present transmitter signal (at respective signal line  1   a - c ) is passed to the output of the Select function. These output signals are present at signal lines  116 ,  188 , and  120  for Select function  110 ,  112 , and  114 , respectively, to inputs of a Median function (circuit)  122 . The Median function  122  calculates the median of the inputs and presents a Median signal at an output thereof at a signal line  124 .  
         [0021]     Referring also to  FIG. 3 , the Failure signal at line  34  is generated for each of the transmitter signals, corresponding Update Set Point Status functions  20 . The Failure signals indicated a failure (high or low) for each corresponding transmitter signals. For purposes of this discussion, the three transmitter signals will be referred to A, B, and C. The Failure signals for A and B are combined by, e.g., an AND gate (not shown) to provide a FailAB signal at a signal line  126 , whereby FailAB is set when both transmitter signals associated with transmitter signals A and B fail. The Failure signals for B and C are combined by, e.g., an AND gate (not shown) to provide a FailBC signal at a signal line  128 , whereby FailBC is set when both transmitter signals associated with transmitter signals B and C fail. The Failure signals for A and C are combined by, e.g., an AND gate (not shown) to provide a FailAC signal at a signal line  130 , whereby FailAC is set when both transmitter signals associated with transmitter signals A and C fails.  
         [0022]     The FailBC signal is input to a select input of a Select function (circuit)  132 . The present transmitter signal for transmitter signal A is input to an input of the Select function  132 . When the FailBC signal is set, the select input of the Select function  132  in enabled, thereby passing the present transmitter signal A to an output of the Select function  132 , at a signal line  134 . Otherwise, the Median signal (at signal line  124 ) is passed to the output (at signal line  134 ) of the Select function  132 . The FailAC signal is input to a select input of a Select function  126 . The present transmitter signal for transmitter signal B is input to an input of the Select function  136 . When the FailAC signal is set, the select input of the Select function  136  in enabled, thereby passing the present transmitter signal B to an output of the Select function  136 , at a signal line  138 . Otherwise the output signal from the Select function  132  (at signal line  134 ) is passed to the output (at signal line  138 ) of the Select function  136 . The FailAB signal is input to a select input of a Select function (circuit)  140 . The present transmitter signal for transmitter signal C is input to an input of the Select function  140 . When the FailAB signal is set, the select input of the Select function  140  in enables, thereby passing the present transmitter signal C to an output of the Select function  140 , at signal line  38 . Otherwise, the output signal from the Select function  136  (at signal line  138 ) is passed to the output (at signal line  28 ) of the Select function  140 .  
         [0023]     While the invention has been described with reference to an exemplary embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.
A redundant control system including at least three redundant signals is presented. The redundant control system includes a failure circuit associated with each of the redundant signals. The failure circuit generates a failure signal in response to an occurrence of a failure of a corresponding one of the redundant signals. A select circuit is associated with each of the redundant signals. The select circuit is receptive to a default signal associated with the corresponding one of the redundant signals, to the failure signal, and to the corresponding one of the redundant signals. The select circuit selects the default signal when the failure signal indicates a failure of the corresponding one of the redundant signals and selects the corresponding one of the redundant signals when the failure signal does not indicate a failure for the corresponding one of the redundant signals. A median determination circuit is receptive to the default signals selected and the redundant signals selected to determine a median signal indicative of a median of the default signals selected and the redundant signals selected.