Abstract:
A control system trims a gas turbine engine in real-time to provide a desired amount of thrust. The control system includes a controller coupled to the engine for receiving inputs from the engine regarding a status of the engine. The controller includes a processor and a non-volatile memory coupled to the processor. The processor is programmed to execute control logic. An engine power schedule representing values for a controlled variable is stored in the memory.

Description:
BACKGROUND OF THE INVENTION 
     This application relates generally to gas turbine engines and, more particularly, to control systems for gas turbine engine. 
     Because an amount of thrust produced by an aircraft engine can not be measured in flight, gas turbine engines typically use control systems that indirectly control thrust by controlling engine fan speed or engine pressure ratio. Specifically, such control systems infer engine thrust from parameters that can be measured, such as a rotational speed of a fan or a ratio of nozzle inlet pressure to fan inlet pressure. The measured parameters are compared to power management schedules preloaded into the control systems. 
     To account for engine-to-engine manufacturing quality variations, deterioration of engine components over time, control sensor measurement errors, and changes in operating conditions, such as humidity, the control systems typically preset each control parameter within the power management schedule at a higher value than is actually needed. As a result, actual thrust produced is at least equal to, but usually higher than, an amount of engine thrust desired. 
     Because the aircraft engines are not trimmed in real-time, the control systems are pre-programmed to produce a minimum amount of thrust from even a deteriorated engine. Accordingly, engines that have not deteriorated produce more thrust than necessary for a given set of operating parameters. The additional thrust causes the engines to operate with increased operating temperatures. Furthermore, because the schedules do not change with time or in response to specific engine characteristics, such engines may never be trimmmed to produce an optimal desired thrust. Over time, continued operation of the engine at increased temperatures may shorten engine life, increase operating costs, and limit user flexibility in selecting operating ranges for the engine. 
     BRIEF SUMMARY OF THE INVENTION 
     In an exemplary embodiment, a control system trims a gas turbine engine in real-time to provide a desired amount of thrust. The control system includes a controller coupled to the engine for receiving inputs from the engine regarding a status of the engine. The controller includes a processor and a memory coupled to the processor. The processor is programmed to execute control logic. An engine power schedule representing values for a controlled variable is stored in the memory. 
     During operation, the processor uses the engine inputs to determine a commanded fuel flow that corresponds to an amount of thrust desired. Because the control system trims the engine in real-time and does not control the engine using fixed schedules that do not change in response to changing operating characteristics of the engine, on-wing engine life for the engine is increased. Furthermore, because the engine is trimmed in real-time and is not trimmed using schedules that result in producing more thrust than necessary, excess thrust of the engine is reduced and the engine operates with lower operating temperatures, lower operating costs, and more reliability. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a logic diagram of a control system for use with an aircraft engine; and 
     FIG. 2 is a logic diagram of an alternative embodiment of a control system for use with an aircraft engine. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a logic diagram of a control system  10  for use with an aircraft engine  12 . Control system  10  includes a fan speed trim estimation unit  20 , a plurality of summing junctions  22 , a power management schedule memory  24 , and a regulator unit  26 . Power management schedule memory  24  and regulator unit  26  are known in the art. Trim estimation unit  20  receives inputs  30  from engine  12  and applies inputs  30  to a thrust-vs.-fan speed schedule (not shown) to produce a steady-state power management parameter trim value  34 . In one embodiment, inputs  30  are measured values of exhaust gas temperature, EGT and a power management feedback parameter. 
     Summing junctions  22  include a first summing junction  36  and a second summing junction  38 . Second summing junction  38  is well known in the art. First summing junction  36  sums a power management parameter reference value  40  from power management schedule memory  24 . Power management schedule  24  includes a table of values of power management parameter references  40  as a function of thrust demand. Thrust demand is determined from throttle lever angle (TLA)  44  and values  42  received from engine sensors that are indicative of flight conditions. In one embodiment, values  42  include fan inlet temperature (T 2 ). Summing junction  36  sums reference parameter value  40  and reference parameter trim value  34  to produce a modified reference parameter value  46 . 
     Second summing junction  38  subtracts feedback parameter value  48  provided by engine  12  from modified reference parameter value  46  to produce control error  50 . In one embodiment, power management parameter reference  40  is a reference fan speed, power management parameter trim value  34  is fan speed trim, feedback parameter value  48  is sensed fan speed, modified reference parameter value  46  is modified reference fan speed, and control error  50  is fan speed error. In another embodiment, power management parameter reference  40  is a reference engine pressure ratio, power management parameter trim value  34  is engine pressure ratio trim, feedback parameter value  48  is sensed engine pressure ratio, modified reference parameter value  46  is modified reference engine pressure ratio, and control error  50  is engine pressure ratio error. 
     Control error  50  generated by second summing junction  38  is supplied to regulator unit  26 . Regulator unit  26  produces a commanded fuel flow output  52  based on a combination of information pre-programmed into regulator unit  26  and error  50 . Commanded fuel flow output  52  is provided to engine  12  to produce a desired thrust  56  based on throttle lever angle  44  and values  42  received from engine sensors, feedback parameter value  48 , and engine value  30 . In one embodiment, value  42  is fan inlet temperature, feedback parameter value  48  is sensed fan speed, and engine value  30  is sensed exhaust gas temperature. 
     Trim estimation unit  20  uses input values  30  indicative of engine condition and engine power levels to compute power management trim value  34 . In one embodiment, engine values  30  include exhaust gas temperature and fan speed. Trim estimation unit  20  also includes logic to account for engine dynamics and to ensure that modifications to trim value  34  are made at steady-state conditions. In one embodiment, trim estimation unit  20  consists of a lookup table and steady-state detection logic. In another embodiment, trim estimation unit  20  uses curve-fits or physics models to obtain trim value  34  as a function of engine condition and power level. In a further embodiment, trim estimation unit  20  and first summing junction  36  are implemented in a non-volatile memory unit coupled to a processor that implements power management schedule memory  24 , regulator unit  26 , and summing junction  38 . 
     As engine  12  deteriorates over time, engine values  30  indicating sensed exhaust gas temperature  30  changes in response. Because control system  10  trims engine using fan speed trim value  34  and is not controlled based on fixed power management schedules  24  that do not change with time or in response to operating conditions of engine  12 , on-wing engine life for engine  12  is increased. Furthermore, because engine  12  is trimmed in real-time and is not trimmed based on schedules that are designed to produce more thrust than necessary, excess thrust from engine  12  is reduced and engine  12  operates with lower operating temperatures. 
     FIG. 2 is a logic diagram of an alternative embodiment of a control system  100  for use with aircraft engine  12 . Components in control system  100  that are identical to components of control system  10  (shown in FIG. 1) are identified in FIG. 2 using the same reference numerals used in FIG.  1 . Accordingly, control system  100  includes regulator unit  26  and summing junction  36 . Control system  100  also includes an engine quality estimation unit  102 , a thrust estimation unit  104 , a fan speed trim estimation unit  108  that is substantially similar to fan speed trim estimation unit  20  (shown in FIG.  1 ), and a power management schedule  113  that is substantially similar to power management schedule  24  (shown in FIG.  1 ). In an alternative embodiment, control system  100  does not include engine quality estimation unit  102 . 
     Thrust estimation unit  104  uses sensor values  114  from engine  12  to determine an estimated thrust  116 . In one embodiment, sensor values  114  are measured values for pressures, temperatures, and/or rotor speeds. In another embodiment, thrust estimation unit  104  uses a table-lookup scheme to determine estimated thrust  116 . In yet another embodiment, thrust estimation unit  104  uses a regressor to determine estimated thrust  116 . In a further embodiment, thrust estimation unit  104  uses a neural network model to determine estimated thrust  116 . In still a further embodiment, thrust estimation unit  104  uses a physics-based model to determine estimated thrust  116 . In yet another embodiment, thrust estimation unit  104  uses engine quality estimates  118  computed by estimation unit  102 . 
     Estimation unit  102  uses sensor values  120  from engine  12  to produce engine quality estimates  118  indicative of engine component health. In one embodiment, sensor values  110  are measured values for temperatures, pressures, and rotor speeds. In another embodiment, estimation unit  102  uses a regression matrix to generate engine quality estimates  118 . In yet another embodiment, estimation unit  102  uses a Kalman filter to generate engine quality estimates  118 . In a further embodiment, estimation unit  102  uses a neural network to generate engine quality estimates. 
     Power management schedule  113  is substantially similar power management schedule  24  and includes the functionality included in power management schedule  24 . Power management schedule  113  also provides a value for desired thrust  120 . In one embodiment, desired thrust  120  is computed from throttle lever angle  44  and values  42  received from engine sensors that are indicative of flight conditions. 
     Trim estimation unit  108  receives estimated thrust  116  provided by thrust estimation unit  104  and desired thrust  120  provided by power management schedules  113  to produce a power management parameter trim value  126 . Similarly to trim estimation unit  20 , power management trim value  126  is updated each time engine thrust from engine  12  reaches a steady-state value. In one embodiment, power management parameter value  40  is a reference fan speed, parameter trim value  126  is a fan speed trim value, feedback input parameter value  48  is a sensed fan speed, modified reference parameter  46  is a modified reference fan speed, and control error  50  is a fan speed error. 
     Control error  50  generated by second summing junction  38  is supplied to regulator unit  26 . Regulator unit  26  produces commanded fuel flow  52  based on a combination of information pre-programmed into regulator unit  26  and provided by control error  50 . Commanded fuel flow  52  is provided to engine  12  to produce desired thrust  56  based on throttle lever angle  44 , a fan inlet temperature value  42 , a sensed fan speed value  48 , and sensed values of pressures, temperatures, and/or rotor speeds provided by sensor values  114  and  120 . 
     In one embodiment, estimation units  102 ,  104 , and  108 , and summing junction  36  are implemented in a non-volatile memory unit coupled to a processor that implements power management schedule memory  130 , regulator unit  26 , and summing junction  38 . 
     As engine  12  deteriorates over time, sensor values  114  and  120  change in response. Because control system  100  trims engine using fan speed trim value  126  and is not controlled based on fixed power management schedules  130  that do not change with time or in response to operating conditions of engine  12 , on-wing engine life for engine  12  is increased. Furthermore, because engine  12  is trimmed in realtime and is not trimmed based on schedules that are designed to produce more thrust than necessary, excess thrust from engine  12  is reduced and engine  12  operates with lower operating temperatures. 
     The above-described control system for a gas turbine engine is cost-effective and reliable. The control system includes a processor coupled to the engine to receive real-time inputs from the engine. Based on the real-time inputs, the control system is capable of trimming the engine to produce a desired amount of engine thrust. As a result of the control system trimming the engine in real-time, on-wing life for the engine is increased, operating costs for the engine are lowered, and the engine operates with lower operating temperatures. 
     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.