Abstract:
A method for sensing and reacting to jet engine fan speed in excess of a fan speed computed from an aircraft throttle input is disclosed. Engine fan speeds are detected by adding test conditions to a main central processing unit (CPU) and adding test conditions to an independent overspeed module to provide additional protection from main CPU computational errors. Interpretation of sensor data relating to engine speed may initiate a modeling routine for sensor data. Comparison of measured/computed sensor data to desired conditions using logic and data tables is a used. If an anomaly is detected, engine fuel cutback devices are engaged.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to electronic control and monitoring of certain jet engine performance parameters and more specifically to electronic control of jet engine overspeed and overthrust. 
     Under some operating conditions, multiple engine aircraft may be susceptible to lateral stability problems when one engine erroneously produces more than the commanded level of thrust. For example, one such operating condition is that of the aborted takeoff, and more specifically, runway procedures following an aborted takeoff. 
     To minimize the effects of an engine failing during such flight conditions, specific techniques are used to increase the aircrafts tolerance to uncommanded asymmetric thrust. Aircraft engines typically include control features that detect and respond with control action when engine rotor speeds reach the maximum level to which the engines are certified. Other aircraft include systems that sense a difference in side-to-side engine thrust and then automatically adjust rudder position to compensate for the fan speed differences. 
     Such aircraft stability control techniques are directed to lateral stability control in flight, but are limited during landing or aborted take-off. It would be desirable to address uncommanded engine overthrust detection and control by adding test conditions to known overspeed detection schemes, thereby providing improved reliability in the detection and control of uncommanded overthrust conditions. 
     BRIEF SUMMARY OF THE INVENTION 
     In an exemplary embodiment, a control system senses jet engine fan speed in excess of a fan speed computed from an aircraft throttle input and responds by adding test conditions to a main central processing unit (CPU) and adding test conditions to an independent overspeed module that provide additional protection from main CPU computational errors. The test conditions include interpretation of sensor data relating to the speed of an engine and further include modeling capability for sensor data when actual sensor data is not within a predetermined tolerance. Logic and data tables are used to compare measured/computed conditions to desired conditions and if an anomaly is detected, an engine fuel cutback device is engaged reducing fuel flow to the engine which is not functioning as desired. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified block diagram of a typical engine control system with independent engine overspeed protection; 
     FIG. 2 is a schematic diagram of one known engine control system with independent engine overspeed protection; and 
     FIG. 3 (is a schematic diagram of an adaptation of the overspeed protection system shown in FIG. 2 to provide overthrust protection. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a simplified block diagram of an engine control system  10  with independent overspeed detection. System  10  is dual redundant in that system  10  includes two identical engine control and engine overspeed modules, channel A  12  and channel B  14 . Channel A  12  includes a main central processing unit  16  and an overspeed module  18 . Channel B  14  also includes a main central processing unit  20  and an overspeed module  22 . Overspeed modules  18  and  22  include logic and other circuitry for reducing engine power level when an overspeed condition has been detected and a signal is transmitted to a fuel cutback device  24 . 
     The redundancy of system  10  is enhanced because channel A  12  and channel B  14  each receive inputs from sensors (not shown) which are also redundant. Channel A  12  receives sensor input directly from inputs  26  and channel B  14  receives sensor input directly from inputs  28 . Inputs  26  and  28  cross feed the same information to the other, respectively, channel B  14  and channel A  12 . To provide protection from spurious indications or non-intended engine fuel cutbacks, specific inputs from inputs  26  are routed into overspeed module  22  of channel B  14  and specific inputs from inputs  28  are routed to overspeed module  18  in channel A  12 . 
     FIG. 2 is a schematic diagram of an engine speed detection and control system  50  in accordance with the block diagram of system  10  shown in FIG.  1 . System  50  is redundant and includes two identical engine control channels, control channel A  52  and control channel B  54 . Channel A  52  includes a main central processing unit  56  and an overspeed module  58 . Channel B  54  also includes a main central processing unit  60  and an overspeed module  62 . Overspeed modules  58  and  62  are equipped with logic circuitry or other means of protecting an engine when an overspeed condition has been detected by routing a signal to a fuel cutback device  64 . 
     Control channel A  52  and control channel B  54  receive inputs from sensors (not shown) which are also dual redundant. Control channel A  52  receives sensor input from sensor inputs  66  while control channel B  54  receives sensor input from sensor inputs  68 . Inputs from sensor inputs  66  are cross-connected from channel A  52  to channel B  54 . Respectively, inputs from sensor inputs  68  are cross-connected from channel B  54  to channel A  52 . Certain inputs from inputs  66  are routed into overspeed module  62  of control channel B  54  and certain of inputs from inputs  68  are routed to overspeed module  58  in control channel A  52 . 
     In the exemplary embodiment shown, inputs  66  are an N 1  probe  70  which represents an engine speed reading from a sensor sensing a phonic wheel on a fan shaft (not shown) of an engine, and is referred to as a fan speed probe. N 2  Probe  72  represents an engine speed reading from a sensor sensing a gear from a gear box (not shown) which is driven by the engine core (not shown). N 2  PMA  74  represents an engine speed reading from a sensor sensing the sinusoidal alternator voltage. The same data from a second set of identical sensors is available at input  68 . N 1  command parameters  76  include other inputs to system  50 , which after validation and selection in module  78  are used in module  82  to compute a fan speed (N 1 ) command. 
     N 1  probe  70 , N 2  Probe  72  and N 1  command parameters  76  are inputs into a validation and selection module  78  within main central processing units  56  and  60 . Validation and selection module  78  is configured to select the best sensor data available using a logical algorithm. Best sensor data is defined as data from the sensor that has readings with the least variations. In one embodiment, the algorithm will choose to model N 1  and N 2  if all of the sensor readings are above a particular threshold with respect to the variations in sensor readings, otherwise an average of the sensor readings are used. As shown by test box  80 , if the selected N 1  and N 2  readings, either modeled or averaged from sensor readings, are greater than a predetermined limit, an error signal is generated and transmitted to overspeed module  58 , which in turn connects a source of power to fuel cutback device  64 . 
     In overspeed module A  58 , sensor readings from both inputs  66  and  68  are compared to pre-determined limits to determine if an error condition exists. If any of N 1  probe  70 , N 2  Probe  72  or N 2  PMA  74  from inputs  66 , as shown in test box  84 , shows a condition that is out of tolerance with the pre-determined limits, a return line from fuel cutback device  64  is connected to a power source return. As an alternate to the signal from test box  80 , if the equivalent sensor readings from input  68  and as shown in test box  86 , show an error condition, a source of power will be connected to fuel cutback device  64 . Fuel to the engine is cut back only when the signal from test box  86  connects a power source to fuel cutback device  64  and the signal from test box  84  connects the return line from fuel cutback device  64 , thus preventing spurious energizing of fuel cutback device  64 . While only one channel  52  of system  50  is fully described, the same description applies to channel B  54 , which also can cause fuel cutback device  64  to engage. 
     FIG. 3 is a schematic diagram of one embodiment of an engine overspeed and overthrust detection and control system  100 . In addition to systems in place for detection and control of engine overspeed similar to those described in FIG. 2, included in FIG. 3 are systems and logic to detect and control engine overthrust. As shown in FIG. 3, system  100  is dual redundant and includes two identical engine control channels, control channel A  102  and control channel B  104 . Channel A  102  includes a main central processing unit  106  and an overspeed module  108 . Channel B  104  also includes a main central processing unit  110  and an overspeed module  112 . Overspeed modules  108  and  112  are equipped with logic circuitry and other means of limiting fuel flow to an engine when an overspeed/overthrust condition has been detected by routing a signal to a fuel cutback device  64 . 
     Inputs  66  and  68  are identical to those described in FIG.  2 . Inputs  66  and  68  include the same two redundant sets of sensors N 1  probe  70 , N 2  Probe  72  and N 2  PMA  74  as described above. N 1  command parameters  76  are also included. All of N 1  probe  70 , N 2  Probe  72 , N 2  PMA  74  and N 1  command parameters  76  are inputs into a validation and selection module  114  within main central processing unit  106 . The same type of sensor data from the second set of sensors from input  68  and N 1  command parameters  76  are input into a validation and selection module  114  within processing unit  110 . Validation and selection module  114  selects or models the best engine control parameters as previously described. Another function of validation and selection module  114  is to output signals to module  116  which generates a fan speed (N 1 ) command. One error condition detected within main central processing unit  106  is resultant from a comparison of the selected fan speed probe signal to the calculated fan speed command  1170 . In the embodiment shown in FIG. 3, comparison  118  is modified. If the selected fan speed probe signal is greater than calculated fan speed command  116  by a predetermined amount and for a predetermined length of time, an error exists and logic activates a switch connecting one side of the power supply to fuel cutback device  64 . 
     In the embodiment shown in FIG. 3, validation and selection module  114  further selects and outputs a throttle lever angle (TLA-Sel) signal to a table  120  which determines the expected throttle position based on a state of switches  122  and  124  on the throttle. The TLA-sel signal is compared to an independent determination of the positions of a near-idle switch  122  and a near maximum climb switch  124 , which in combination are referred to as sensed aircraft throttle position. Sensed aircraft throttle position is input to overspeed module  108  from two switches, namely  122 , detecting near-idle throttle position and  124  detecting near max climb throttle position. These switch positions are compared with the expected throttle positions determined, as described previously, in table  120  and any disagreement communicated to the Main CPU  106  and to the Overspeed Module  108 . 
     Sensed aircraft throttle position further determines a maximum fan speed limit as shown in table  126 . The maximum fan speed limit is compared to a sensor fan speed from N 1  probe  70 . If sensor fan speed from N 1  probe  70  is greater than maximum fan speed limit as determined from aircraft throttle position switches for a predetermined length of time as shown in test box  128 , an error condition has occurred and a switch is closed connecting a power source to fuel cutback device  64 . Overspeed module  108  is further configured to perform the same comparison with B-channel sensor data as shown in test box  130 , resulting in a switch closure which connects the power return line from fuel cutback device  64 . 
     Control channel B  104  is redundant to control channel A  102 , and includes the same error detection schemes and switch position determinations and signal applications to fuel cutback device  64  as control channel A  102 . It should be understood that while the embodiments described in FIG. 3 describe aircraft throttle switch inputs as the device inputting aircraft throttle position to overspeed modules  108  and  112  other devices may be employed, for example, proximity switches, photoelectric sensors, Rotary Variable Differential Transformers (RVDT), Linear Variable Differential Transformers (LVDT) and other transducer devices. 
     The control techniques above described adapt the best of known overspeed logic by adding test conditions in the main CPUs and the independent overspeed modules that sense and react to engine fan speed in excess of that computed from the aircraft throttle input. To provide additional protection from CPU computational errors, independent determination of aircraft throttle position is made by using sensors on the throttle itself. Comparison of the throttle position computed by the CPUs and the throttle position detected in the overspeed modules indicate difference faults and provide an independent test for overthrust conditions. While the overspeed modules described above are referred to as being independent, and shown as such in FIGS. 1,  2  and  3 , independence refers to functionality only. It is understood that the overspeed modules may be embodied as either hardware separate from the main CPU modules or, in the alternative, may be embodied as a separate function within the main CPU modules. 
     The control technique above described is adaptable for alternate jet engine thrust parameters such as engine pressure ratio and core speed. While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.