Gas turbine engine controllers are designed to satisfy both performance and safety requirements. To achieve this, engine control logic includes three primary functions: steady-state control, transient control, and limit protection. The steady-state control logic is designed to maintain output power at a demanded level in the presence of disturbances and uncertainties. The transient control logic is designed to safely transition the engine from one power level to another within performance requirements. The limit protection logic is designed to ensure that critical parameters never violate constraints. These functions are all integrated via loop selection logic.
In a typical power management loop, the power request is converted to a setpoint. The difference between the setpoint and the corresponding feedback signal is used to drive power management compensation, which ultimately produces a rate command that is output to the loop selection logic. The loop selection logic either passes the rate command through to a common integrator or selects a rate command that is appropriate for the current engine operating mode.
In order to develop control logic for steady-state control, transient control, and limit protection, linear point models may be used to approximate the gas turbine engine system. Specifically, control laws are developed and tuned in a computational environment in which linear point models are used to represent the engine dynamics. Development and tuning occurs for one linear point model at a time, and the resulting individual control laws are integrated together to form control logic satisfying both performance and operability requirements across the complete operating envelope of the engine. The controller is then tested on a non-linear engine system model where additional tuning is accomplished. Once modeling and simulation development is completed, bench and engine testing is conducted to fine tune and verify the control logic. All of the models used for control design generally represent a single engine condition.
If the control logic is to perform acceptably, it must safely accommodate nonlinearities and uncertainties not present in the design models. In addition, control logic must accommodate the effects of disturbances, manufacturing variations, and degradations that will be present in all of the components of the system. Conventional control logic accommodates these factors by implementing margins within the control logic design. However, this approach may reduce engine performance by not allowing the control logic to access engine states in these margins.