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.

FADEC based on Distributed Architecture. In Distributed Engine Control(DEC), the functions of FADEC are distributed at the component level. Eachsensor/actuator can be replaced by a smart sensor/actuator.
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.

In DEC, the appropriate selection of communication architecture is very important, as the performance of the DEC will be dependent on the performance of the communication network. The network must have sufficient bandwidth and latency to enable closed loop control. It must also be robust to accommodate the safety and critical functions.

Distributed Control Systems have multiple, independent processes and the output of any distributed process is based on the quality of input data, which must be transmitted accurately and without any delay. DEC systems can be viewed as a Networked Control System (NCS) with distributed sensors and actuators. Here, the control loops are closed through a real-time communication network. This addition of a communication network in the system introduces complexity in the design. There are various factors introduced as a result of a communication network. They include networkinduced delay, packet dropouts, and bandwidth constraints, which have to be considered for ensuring desired functionality of the NCS.

The distributed control approach will be inherently more powerful, flexible, and scalable than a centralized control approach. However, there are major technical challenges to the realization of DEC. High-temperature electronics, selection of appropriate communication architecture, and partitioning of the centralized controller are a few. Silicon-On- Insulator (SOI) is one of the promising high-temperature electronics technologies. The operational temperature for SOI is in the range of 225ºC to 250ºC. For a temperature higher than this, the technology is still in the development stage. It is projected that use of silicon carbide electronics will enable use of electronics at temperatures above 600ºC. In DEC, the appropriate selection of communication architecture is very important, as the performance of the DEC will be dependent on the performance of the communication network.

This work was done by Rama K. Yedavalli and Rohit K. Belapurkar of The Ohio State University, and Alireza R. Behbahani of the Air Force Research Laboratory. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Mechanics/Machinery category. AFRL-0123


This Brief includes a Technical Support Package (TSP).
Stability Analysis of Distributed Engine Control Systems

(reference AFRL-0123) is currently available for download from the TSP library.

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This article first appeared in the December, 2009 issue of Defense Tech Briefs Magazine.

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