In modern high-performance aircraft, the Fuel Thermal Management System (FTMS) plays a critical role in the overall thermal energy management of the aircraft. Actuator and state constraints in the FTMS limit the thermal endurance and capabilities of the aircraft. Thus, an effective control strategy must plan and execute optimized transient fuel mass and temperature trajectories subject to these constraints over the entire course of operation. For the control of linear systems, hierarchical Model Predictive Control (MPC) has shown to be an effective approach to coordinating both short- and long-term system operation with reduced computational complexity. However, for controlling nonlinear systems, common approaches to system linearization may no longer be effective due to the long prediction horizons of upper-level controllers. This paper explores the limitations of using linear models for hierarchical MPC of the nonlinear FTMS found in aircraft. Numerical simulation results show that linearized models work well for lower-level controllers with short prediction horizons but lead to significant reductions in aircraft thermal endurance when used for upper-level controllers with long prediction horizons. Therefore, a mixed-linearity hierarchical MPC formulation is presented with a nonlinear upper-level controller and a linear lower-level controller to achieve both high performance and high computational efficiency.
A nonlinear hierarchical model predictive control (MPC) framework is proposed and applied to maximize the thermal endurance of aircraft. Effectively controlling the fuel temperatures in a nonlinear multitimescale aircraft fuel thermal management system (FTMS) requires controllers capable of long-term planning and fast update rates. In this article, a twolevel hierarchical MPC controller is formulated using successive linearization (SL) that directly accounts for the multitimescale and nonlinear system dynamics to achieve accurate predictive capabilities and computational efficiency. Detailed simulation results show that the proposed hierarchical structure can increase aircraft thermal endurance by at least 21% compared to a centralized approach while significantly reducing the computational cost. The results also show that SL provides a valuable framework for efficiently accounting for nonlinear system dynamics within both levels of the hierarchical MPC formulation.
Dynamic compressors are known to suffer from surge, which can severely damage compressor components and disturb production. Surge may arise by the occurrence of disturbances (e.g. compressor discharge valve closure) that would bring its operating point to a region at low flows delimited by the so called surge line (SL). Therefore, dynamic compressors are always equipped with anti-surge mechanisms: typically a fast actuating recycle valve controlled by a PI anti-surge controller. Since surge develops extremely fast, the compressor is usually not allowed to operate too close to the surge line. A surge safety margin is considered, which is the region between the SL and a surge control line (SCL), which may be defined as a line parallel and to the right of the SL. Once the compressor crosses the SCL towards the SL, PI controller action starts. Depending on a number of factors the recommended surge margin adopted may vary. This control objective (keep the compressor away from the SL) is conflicting with energy efficiency requirements, since higher efficiency operating points are located close to the SL. Therefore, it is desirable to operate the compressor using the smallest possible surge margin that still guarantees anti-surge action is effective. In this paper we propose a method for triggering the compressor anti-surge action, aiming at a faster action than traditional PI control, which could enable the adoption of reduced surge margins and operation with higher efficiency. Given a typical single compressor system topology, the possibility of a compressor reaching the surge region coming from a stable operating point can be retrieved through the state of the system actuators. Considering a certain combination of values of the system actuators (compressor suction valve opening, discharge valve opening and motor drive torque), if the steady state operating point (calculated based on a nonlinear variable speed compressor system model) lies to the right of the SCL, then no anti-surge action is necessary. If the resulting steady state lies to the left of the SCL, anti-surge action is deemed necessary and therefore triggered. The proposed anti-surge control method relies on an offline computation of necessary recycle valve openings for each possible combination of the system actuators positions mentioned above, considering a predefined discrete set of values from the actuators positioning ranges. This generates a look-up table for online use. The values from the look-up table are used to trigger a “steady state control action” for the recycle valve that would keep the system stable after transients vanish. They are also used to identify the necessary compressor flow set-point for a feedback controller, which is responsible for ensuring that the system trajectory goes from the state upon anti-surge activation to the desired steady state. The control action from this feedback controller is added to the steady state control action so that its contribution should vanish after transients vanish. A PI and a sliding mode controller are used in feedback control action and results are compared to the traditional anti-surge control approach.
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