History of Distributed Engine Controls for Propulsion Systems

The control of jet propulsion engines has always required complex control systems. Digital Electronic Engine Controls (DEEC) were first developed in the 1970s for the Pratt and Whitney (PW) JT9D engine {1}. The transition to Full Authority Digital Engine Controls (FADEC) in the 1980s was a natural response to the demand for more capabilities for the engine and was made possible by the increasing capabilities of electronics. This trend continued into the 80's where the demand for increasing capabilities in terms of performance, reliability, maintainability and physical attributes has been answered with innovative engineered solutions propelled by the increasing performance and miniaturization of electronics. However, these improvements have had their limitations because of the harsh environmental challenges posed by the FADEC systems. In addition, the demand for high reliability has forced designers to use proven but older technologies. As a result today's FADEC systems contain electronics that lags behind technology that is available in the commercial world.

A FADEC system contains a central computer and interfacing electronics connected via dedicated cable harnesses to its associated control sensors and actuators. The FADEC receives the pilot throttle commands, power and fuel from the aircraft and provides information to the on-aircraft systems via communication data buses. A block diagram of a typical centralized FADEC system and its interfaces are shown in Figure 1. The FADEC systems are often bundled with other engine related systems such as Prognostics Health Monitoring (PHM) and their associated sensors.

FADEC systems are usually implemented as dual redundant channels with identical FADEC computers and dual redundant sensors and actuators. Each "channel" is fully capable of controlling the engine. Centralized FADEC consists of three major sections: The processor section which implements all control laws and strategies such as model following, and interfaces with the aircraft via serial data bus. The Input/ Output section consists of signal conditioning and drive electronics hardware for sensors and actuators along with Analog to Digital and Digital to Analog converters (ADC and DAC, respectively). The Input/ Output section is controlled by the processor, sometimes with the help of a separate processor, and provides sensor data to the processor and converts processor commands to the actuators. The redundant channels exchange sensor data, commands and health information over the Cross Channel Data Links (CCDL) for fail safe signal selection and redundancy management. The power supply section conditions aircraft power and provides power to all the sensors and actuators. All FADEC computers are normally designed to operate below 125 C.

Commercial engine FADEC systems are air cooled while military FADEC computers normally require fuel cooling due to their higher performance capabilities and heat loads. The FADEC systems must meet stringent requirements for performance, cost and physical size and weight constraints. As a result today's FADEC is a centralized, highly optimized controller. FADEC from different manufacturers are unique in their design and only suited for specific engines. The highly optimized nature of the FADEC system leads to large System Development and Demonstration (SDD) investments and consequently to high cost of maintenance Including continuous obsolescence management due to the use of older, commercial electronic components. In addition to hardware, obsolescence of the FADEC also requires software changes both of which are very costly. In order to address these issues, the AFRL introduce the idea of a "Universal FADEC" in 2003. The Universal FADEC was to be a distributed, modular and scalable control with plug-in cards capable of meeting the increasing needs of the next generation of propulsion systems. The Universal FADEC concept also considered the possible distribution of sensor and actuator electronics around the engine and the relocation of the FADEC computer off engine to a more environmentally friendly location. However, these ideas were not practical at the time, primarily due to the lack of reliable high temperature capable electronics.

Engine Controls Changed Aviation - 25 YEARS AGO!

Transformation from Analog to Digital Engine Controls

revolutionary change