Stability Analysis of Distributed Engine Control Systems

Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio

Tuesday, December 01 2009

This design is inherently more powerful, flexible, and scalable than a centralized control approach.

Full Authority Digital Engine Control (FADEC), based on a centralized architecture framework, is being widely used for gas turbine engine control. However, current FADEC is not able to meet the increased burden imposed by the advanced intelligent propulsion system concepts. This has necessitated development of the Distributed Engine Control (DEC) system. FADEC based on Distributed Control Systems (DCS) offers modularity, improved control sys-tem prognostics, and fault tolerance, along with reducing the impact of hardware obsolescence.

In the traditional Centralized Control System (CCS) configuration, the centralized control processor handles all processing functions, including the operating system, task scheduling, I/O, protection, communication, and control algorithms. All computations are performed by a single controller and the control signals are transmitted to each individual actuator. The transmission of the control signal is through individual analog connections with each sensor and actuator. FADEC houses the electronics required for data acquisition and signal conditioning. In order to provide required safety and reliability, dualchannel communication links are used.

In order to reduce the wire harness length, the optimal location for the FADEC is near the combustion chamber; however, this is not practically possible, as extra structural rigidity has to be added to protect it against high temperature and vibration. Hence, the FADEC is placed on the aircraft engine fan case, which increases the wire harness length. As data communication is analog, the wire harness and connectors have to be shielded from noise and signal attenuations. This shielding increases the control system weight. The operational life of FADEC is one-third of the engine life. As the FADEC uses nonstandard input/output interfaces, it is not easily upgradable. This increases obsolescence cost of the engine.

In Distributed Engine Control (DEC), the functions of FADEC are distributed at the component level (see figure). Each sensor/actuator can be replaced by a smart sensor/actuator. These smart modules can include local processing capability to allow modular signal acquisition and conditioning, digital data bus communications, and diagnostics and health management functionality. A serial communication network can be used to connect these smart modules with FADEC. There are no restrictions on the location of the DEC unit; therefore, it can be placed at a location where it is subjected to less vibration and a hostile environment.