This paper describes Pratt &Whitney's pioneering work in “constrained dynamic inversion,” a control algorithm architecture for multivariable systems that must operate tight to limits. A hallmark of gas turbine control is the prevalence and fundamental importance of tightly holding limits. When constrained dynamic inversion is applied to gas turbine systems control, this algorithm enables operation closer to physical and operational limits, while also providing faster and more precise responses. In addition to more fully exploiting systems physical capabilities, this architecture provides for the independent and finely coordinated control of system variables-of-interest even when each effector affects all variables. A distinctive feature of this algorithm is that it can be implemented on state of the art controllers at update rates consistent with vehicle control.
The control architecture and its capabilities are described in terms of its application to the F-135 propulsion system for the F-35 Lightning II aircraft. Its dynamic control requirements provided a strong impetus to the development of constrained dynamic inversion. In order to support vertical flight, this propulsion system is more profoundly multivariable, requires more independent control of goals, and runs tighter to more numerous limits, than prior gas turbines. This control algorithm enabled the F-35 Lightning II propulsion system to operate successfully in short takeoff and vertical landing (STVOL) flight mode. In this mode, the propulsion system produces all or most of the forces and moments required for flight control. Constrained dynamic inversion is the most technically challenging control architecture ever attempted in aerospace due to the unprecedented precision and dynamic performance requirements of the STOVL aircraft.