Seminar Report on Fadec

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SVKM’s NMIMS Mukesh Patel School of Technology Management and Engineering Department of Mechanical Engineering “Zero human interference on engine control” 0R “Full authority digital engine control” -Mithil Pandey Roll no.- 627 Abstract This report describes steps and procedures necessary to achieve a successful project on Zero Human Interference on Engine Control or Full Authority Digital Engine Control (FADEC). Topics discussed include an overview of full digital engine control, what is the role of FADEC in aircraft engine performance? How it functions and controls the engine through computer, its safety features. Its application or uses, advantages, disadvantages, technical challenges and approach are also discussed. The report includes industry partnership with companies or government and also the ongoing research on the project. This report also presents a short history of fuel controls, some of the objectives required for a successful FADEC system, a description of some of the hardware changes to meet these objectives, and an overview of some of the operational characteristics and system protection provided Introduction Turbine engines provide the propulsive force for a significant percentage of modern transportation systems and are especially important as the engine for a wide variety of aircraft. Although often viewed as a mature

Transcript of Seminar Report on Fadec

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SVKM’s NMIMS

Mukesh Patel School of Technology

Management and Engineering

Department of Mechanical

Engineering

“Zero human interference on engine control”

0R

“Full authority digital engine control”

-Mithil Pandey

Roll no.- 627

Abstract

This report describes steps and procedures necessary to achieve a successful project on Zero Human Interference on Engine Control or Full Authority Digital Engine Control (FADEC). Topics discussed include an overview of full digital engine control, what is the role of FADEC in aircraft engine performance? How it functions and controls the engine through computer, its safety features. Its application or uses, advantages, disadvantages, technical challenges and approach are also discussed. The report includes industry partnership with companies or government and also the ongoing research on the project. This report also presents a short history of fuel controls, some of the objectives required for a successful FADEC system, a description of some of the hardware changes to meet these

objectives, and an overview of some of the operational characteristics and system protection provided

Introduction

Turbine engines provide the propulsive force for a significant percentage of modern transportation systems and are especially important as the engine for a wide variety of aircraft. Although often viewed as a mature technology, a substantial amount of resources are expended to improve these Systems because of the large impact they have on society. The NASA program in Fundamental Aeronautics [1] describes one such research effort and is aimed at reducing emissions, fuel burn, and noise. Separately, the Department of Defense’s Versatile Affordable Advanced Turbine Engines (VAATE) Program [2] describes similar goals regarding fuel burn reduction with perhaps more emphasis on overall performance and reducing cost. A multitude of fundamental technologies are involved in realizing these improvements, however, many of them are only enabled or reach full potential through the use of supporting controls technology.

The control system is not generally considered to be the limiting factor in the performance of an engine. Controls do have a direct impact on performance by how well they enable the engine system to operate within its design envelope. Yet the control system negatively affects performance indirectly because it has physical mass and volume. It also uses electric power and dissipates heat which ultimately impacts

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weight and volume. More control capability through sensing and actuation could feasibly enable better engine performance; however this is constrained because the impact outweighs the gain. The control architecture can be a tool to reduce the negative impact of an existing control capability or provide additional performance capability with the same impact.

Changing the engine control system architecture towards a more “distributed” format has the potential to reduce overall life cycle costs, reduce control system weight, and provide an enabling path for new technologies which do not currently fit in the existing cost structure of small air vehicle systems.

FADEC – Full Authority Digital Engine Control

The newest version of a jet engine fuel control is called a FADEC - Full Authority Digital Electronic Control. The original fuel controls on early jet engines of the late 1940's and early 1950's were simply constructed and resembled a common gate valve connected to a throttle lever, Engines and controls became more sophisticated through the 1950's and 1960's, Better performance, more reliability, and increased safety became driving forces, the new

electronic fuel controls will be even smarter, more precise, more accurate and more reliable than present day fuel controls and they will be with us for many years.

Full Authority Digital Engine Control (FADEC) is a system consisting of a digital computer, called an electronic engine controller (EEC) or engine control unit (EEU), and its related accessories that control all aspects of aircraft engine performance. FADECs have been produced for both piston engines and jet engines.

FADEC is a system consisting of a digital computer and ancillary components that control an aircraft’s engine and propeller. First used in turbine-powered aircraft, and referred to as full authority digital electronic control, these sophisticated control systems are increasingly being used in piston powered aircraft.

In a spark ignition reciprocating engine the FADEC uses speed, temperature, and pressure sensors to monitor the status of each cylinder. A digital computer calculates the ideal pulse for each injector and adjusts ignition timing as necessary to achieve optimal performance. In a compression ignition engine the FADEC operates similarly and performs all of the same functions, excluding those specifically related to the spark ignition process.

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FADEC systems eliminate the need for magnetos, carburetor heat, mixture controls, and engine priming. A single throttle lever is characteristic of an aircraft equipped with a FADEC system. The pilot simply positions the throttle lever to a desired detent such as start, idle, cruise power, or max power, and the FADEC system adjusts the engine and propeller automatically for the mode selected. There is no need for the pilot to monitor or control the air/fuel mixture.

During aircraft starting, the FADEC primes the cylinders, adjusts the mixture, and positions the throttle based on engine temperature and ambient pressure. During cruise flight, the FADEC constantly monitors the engine and adjusts fuel flow, and ignition timing individually in each

cylinder. This precise control of the combustion process often results in decreased fuel consumption and increased horsepower.

FADEC systems are considered an essential part of the engine and propeller control, and may be powered by the aircraft’s main electrical system. In many aircraft FADEC uses power from a separate generator connected to the engine. In either case, there must be a backup electrical source available because failure of a FADEC system could result in a complete loss of engine thrust. To prevent loss of thrust, two separate and identical digital channels are incorporated for redundancy, each channel capable of providing all engine and propeller functions without limitations.

HISTORY/ANALYSIS

Developed in the early 1970s for military aircraft, electronic flight and engine-control system have found increasing application in

commercial fleets of the world. The goal of any engine control system is to allow the engine to perform at maximum efficiency for a given condition. The complexity of this task is proportional to the complexity of the

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engine. Originally, engine control systems consisted of simple mechanical linkages controlled by the pilot, but then evolved and became the responsibility of the third pilot-certified crew member, the flight engineer. By moving throttle levers directly connected to the engine, the pilot or the flight engineer could control fuel flow, power output, and many other engine parameters.

Following mechanical means of engine control came the introduction of analog electronic engine control. Analog electronic control varies an electrical signal to communicate the desired engine settings. The system was an evident improvement over mechanical control but had its drawbacks, including common electronic noise interference and reliability issues. Full authority analogue control was used in the 1960s and introduced as a component of the Rolls Royce Olympus 593 engine of the supersonic transport aircraft Concorde. However, the more critical inlet control was digital on the production aircraft.

Following analog electronic control, the logical progression was to digital electronic control systems. Later in the 1970s, NASA and Pratt and Whitney experimented with the first experimental FADEC, first flown on an F-111 fitted with a highly modified Pratt & Whitney TF30 left engine. The experiments led to Pratt & Whitney F100 and Pratt & Whitney PW2000 being the first military and civil engines, respectively, fitted with FADEC, and later the Pratt & Whitney PW4000 as the first commercial "dual FADEC" engine. The first FADEC in

service was developed for the Harrier II Pegasus engine by Dowty & Smiths Industries Controls.

The Rolls-Royce AE1107C is a 6250 shaft horsepower turboshaft engine manufactured by Allison Engine Co. (AEC). Two Full Authority Digital Engine Control (FADEC) fly-by-wire technology AE1107C engines are installed on each V-22 aircraft. The AE1107C has been certified as a Commercial Item under FAR 2.101 and Section 10 U.S.C. 2464.

WORKING:-

Full Authority Digital Engine (or Electronics) Control (FADEC) is a system consisting of a digital computer, called an electronic engine controller (EEC) or engine control unit (ECU), and its related accessories that control all aspects of aircraft engine performance. FADECs have been produced for both piston engines and jet engines.

ENGINE CONTROL UNIT

An engine control unit (ECU) is a type of electronic control unit that controls a series of actuators on an internal combustion engine to ensure the optimum running. It does this by reading values from a multitude of sensors within the engine bay, interpreting the data using multidimensional performance maps (called Look-up tables),

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and adjusting the engine actuators accordingly.

Before ECU's, air/fuel mixture, ignition timing, and idle speed were mechanically set and dynamically controlled by mechanical and pneumatic means. One of

the earliest attempts to use such a unitized and automated device to manage multiple engine control functions simultaneously was the "Kommandogerät" created by BMW in 1939, for their 801 14-cylinder aviation radial engine.

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Working Of ECU

Control of ignition timingA spark ignition enigine requires a spark to initiate combustion in the combustion chamber. An ECU can adjust the exact timing of the spark (called ignition timing) to provide better power and economy. If the ECU detects knock, a condition which is potentially destructive to engines, and "judges" it to be the result of the ignition timing being too early in the compression stroke, it will delay (retard) the timing of the spark to prevent this. Since knock tends to occur more easily at lower rpm, the ECU controlling an automatic transmission will often downshift into a lower gear as a first attempt to alleviate knock.

Control of Air/Fuel ratioFor an engine with fuel injection, an engine control unit (ECU) will determine the quantity of fuel to inject based on a number of parameters. If the Throttle position sensor  is showing the throttle peddle is pressed further down, the Mass flow sensor will measure the amount of additional air being sucked into the engine and the ECU will inject more fuel into the engine. If the Engine coolant temperature sensor is showing the engine has not warmed up yet, more fuel will be injected (causing the engine to run slightly 'rich' until the engine warms up). Mixture control on computer controlled carburetors works similarly but with a mixture control solenoid or stepper motor incorporated in the float bowl of the carburetor.

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Control of idle speedMost engine systems have idle speed control built into the ECU. The engine RPM is monitored by the crankshaft position sensor which plays a primary role in the engine timing functions for fuel injection, spark events, and valve timing. Idle speed is controlled by a programmable throttle stop or an idle air bypass control stepper motor. Early carburetor-based systems used a programmable throttle stop using a bidirectional DC motor. Early TBI systems used an idle air control stepper motor . Effective idle speed control must anticipate the engine load at idle. Changes in this idle load may come from HVAC systems, power steering systems, power brake systems, and electrical charging and supply systems. Engine temperature and transmission status, and lift and duration of camshaft also may change the engine load and/or the idle speed value desired.

A full authority throttle control system may be used to control idle speed, provide cruise control functions and top speed limitation.

Control of variable valve timingSome engines have Variable Valve timing . In such an engine, the ECU controls the time in the engine cycle at which the valves open. The valves are usually opened sooner at higher speed than at lower speed. This can optimize the flow of air into the cylinder, increasing power and economy.

Electronic valve controlExperimental engines have been made and tested that have no camshaft, but have full

electronic control of the intake and exhaust valve opening, valve closing and area of the valve opening. Such engines can be started and run without a starter motor for certain multi-cylinder engines equipped with precision timed electronic ignition and fuel injection. Such a static-start engine would provide the efficiency and pollution-reduction improvements of a mild hybrid electric drive, but without the expense and complexity of an oversized starter motor.

The first production engine of this type was invented ( in 2002) and introduced (in 2009) by Italian automaker Fiat in the Alfa Romeo MiTo. Their multi air engines use electronic valve control which drastically improve torque and horsepower, while reducing fuel consumption as much as 15%. Basically, the valves are opened by hydraulic pumps, which are operated by the ECU. The valves can open several times per intake stroke, based on engine load. The ECU then decides how much fuel should be injected to optimize combustion.

For instance, when driving at a steady speed, the valve will open and a bit of fuel will be injected, the valve then closes. But, when you suddenly stamp on the throttle, the valve will open again in that same intake stroke and much more fuel will be injected so that you start to accelerate immediately. The ECU then calculates engine load at that exact RPM and decides how to open the valve: early, or late, wide open, or just half open. The optimal opening and timing are always reached and combustion is as precise as possible. This, of course, is impossible with a normal camshaft, which opens the valve for the whole intake period, and always to full lift.

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Programmable ECUs

A special category of ECUs are those which are programmable. These units do not have a fixed behaviour and can be reprogrammed by the user.

Programmable ECUs are required where significant aftermarket modifications have been made to a vehicle's engine. Examples include adding or changing of a turbocharger, adding or changing of an intercooler, changing of the exhaust system, and conversion to run on alternative fuel. As a consequence of these changes, the old ECU may not provide appropriate control for the new configuration. In these situations, a programmable ECU can be wired in. These can be programmed/mapped

with a laptop connected using a serial or USB cable, while the engine is running.

The programmable ECU may control the amount of fuel to be injected into each cylinder. This varies depending on the engine's RPM and the position of the accelerator pedal (or the manifold air pressure). The engine tuner can adjust this by bringing up a spreadsheet like page on the laptop where each cell represents an intersection between a specific RPM value and an accelerator pedal position (or the throttle position, as it is called). In this cell a number corresponding to the amount of fuel to be injected is entered. This spreadsheet is often referred to as a fuel table or fuel map.

By modifying these values while monitoring the exhausts using a wide band lambda probe to see if the engine runs rich or lean,

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the tuner can find the optimal amount of fuel to inject to the engine at every different combination of RPM and throttle position. This process is often carried out at a dynamometer, giving the tuner a controlled environment to work in. An engine dynamometer gives a more precise calibration for racing applications. Tuners often utilize a chassis dynamometer for street and other high performance applications.

Other parameters that are often mappable are:

Ignition: Defines when the spark plug should fire for a cylinder.

Rev. limit: Defines the maximum RPM that the engine is allowed to reach. After this fuel and/or ignition is cut. Some vehicles have a "soft" cut-off before the "hard" cut-off.

Water temperature correction: Allows for additional fuel to be added when the engine is cold (choke) or dangerously hot.

Transient fueling: Tells the ECU to add a specific amount of fuel when throttle is applied. The term is "acceleration enrichment"

Low fuel pressure modifier: Tells the ECU to increase the injector fire time to compensate for a loss of fuel pressure.

Closed loop lambda: Lets the ECU monitor a permanently installed lambda probe and modify the fueling to achieve stoichiometric (ideal) combustion. On traditional petrol powered vehicles this air:fuel ratio is 14.7:1.

Some of the more advanced race ECUs include functionality such as launch control, limiting the power of the engine in first gear

to avoid burnouts. Other examples of advanced functions are:

Wastegate control: Sets up the behavior of a turbocharger’s wastegate, controlling boot.

Staged injection: Sets up the behavior of double injectors per cylinder, used to get a finer fuel injection control and atomization over a wide RPM range.

Variable cam timing:Tells the ECU how to control variable intake and exhaust cams.

Gear control: Tells the ECU to cut ignition during (sequential gearbox) upshifts or blip the throttle during downshifts.

A race ECU is often equipped with a data logger recording all sensors for later analysis using special software in a PC. This can be useful to track down engine stalls, misfires or other undesired behaviors during a race by downloading the log data and looking for anomalies after the event. The data logger usually has a capacity between 0.5 and 16 megabytes.

In order to communicate with the driver, a race ECU can often be connected to a "data stack", which is a simple dash board presenting the driver with the current RPM, speed and other basic engine data. These race stacks, which are almost always digital, talk to the ECU using one of several proprietary protocols running over RS232 or Canbus, connecting to the DLC connector (Data Link Connector) usually located on the underside of the dash, inline with the steering wheel

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Engine Control Architectures

Conventional turbo-shaft control systems are designed to concentrate active electronic components in a common enclosure to protect them from the environment. This is a legacy issue derived from the survivability of silicon-based electronics and the availability of integrated electronic functional components provided by the commercial markets. These constraints drive what is known as the “centralized” architecture; where the controller function is protectively housed in the Engine Control Unit (ECU), or Full Authority Digital Engine Control (FADEC). The remaining control elements, sensors and actuators, are devoid of sensitive semiconductors and are located throughout the engine in harsher environments. Typically, control systems are

designed to a set of requirements and for functionality specified at the onset of a program, accommodating for the sensor suite, actuators, power distribution, cockpit interfaces, and data bus drive connector pin count and harnessing. This renders each system unique. Future upgrades or unscheduled modifications to the control system require extensive and costly redesign and re-certification efforts potentially involving the controller, software, harness, connectors, and Fuel Delivery Unit (FDU) interface. These obstacles may ultimately be sufficient cause to postpone the insertion of functionality and technology on legacy platforms, or severely limit the reuse of hardware and software altogether. The complexity of these system interfaces are described in detail in the section on legacy centralized architecture.

Notionally this architecture is shown in Figure 1. Distributed control architecture is intended to eliminate the design constraints imposed by legacy systems and take

advantage of advances in new technology. These advances have extended the operational temperature range of many integrated circuits and made available

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powerful new components for the partitioning of complex systems. The objective is to place control system functions in locations which provide system optimization while enabling a more modular approach to system design thereby achieving cost efficiencies over the system life-cycle. Using digital communication between control system components is a main feature of this architecture. Digital communication standards eliminate the system complexities of a myriad of individual interfaces that constrain design flexibility. The flow of data between sensor, controller, and actuator is unaffected. In a fully distributed architecture, each control system function is decomposed into very simple tasks and then reconstructed into more complex functional blocks. Control system functionality is added in a modular fashion with fewer constraints and less impact on hardware. Modular, flexible, scalable engine control systems are the building blocks of future capability and cost effectiveness. This system architecture is notionally shown in Figure 2. Distributed turbine engine control has been an area of significant interest among propulsion control engineers for some time but has rarely been implemented in practice. Supporting technologies, like

high temperature electronics, are not mature and industry is only beginning to constructively collaborate on common approaches to minimize risk and share development costs. Limited distribution of control functions outside of a central FADEC have made sense in some larger aircraft engines where significant reductions in wiring harness weight are readily achieved. The small turbo-shaft engine, because of its compact topology and limited zones with a suitable environment, offer a greater challenge. In this effort, a partial distribution of control system functionality for the turbo-shaft engine has been identified as a reasonable compromise between the technology-driven fully distributed architecture and the desire to improve upon the present limitations of a legacy centralized control system. Simply described, in a partially distributed control system, several main components of the vehicle control system are interconnected using digital communications; these are the FADEC, the Fuel Delivery Unit (FDU), vehicle health monitoring, and flight controls. The remaining system sensors and actuators are legacy control system devices which are connected to one of the four major control components to minimize weight.

Hybrid digital designs

Hybrid digital/analog designs were popular in the mid 1980s. This used analog techniques to measure and process input parameters from the engine, then used a look

up table stored in a digital ROM chip to yield precomputed output values. Later systems compute these outputs dynamically. The ROM type of system is amenable to tuning if one knows the system well. The disadvantage of such systems is that the precomputed values are only optimal for an

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idealised, new engine. As the engine wears, the system is less able to compensate than a CPU based system.

Modern ECUs

Modern ECUs use a microprocessor  which can process the inputs from the engine sensors in real time. An electronic control unit contains the hardware and software (firmware). The hardware consists of electronic components on a printed circuit board (PCB), ceramic substrate or a thin laminate substrate. The main component on this circuit board is a microcontroller chip (CPU). The software is stored in the microcontroller or other chips on the PCB, typically in EPROMs or flah memory so the CPU can be re-programmed by uploading updated code or replacing chips. This is also referred to as an (electronic) Engine Management System (EMS).

Sophisticated engine management systems receive inputs from other sources, and control other parts of the engine; for instance, some variable valve timing systems are electronically controlled, and turbocharger wastegates can also be managed. They also may communicate with transmission control unit or directly interface electronically-controlled automatic transmission ,traction control system and the like. The Controller area network or CAN bus automotive network is often used to achieve communication between these devices.

Modern ECUs sometimes include features such a cruise control, transmission control, anti-skid brake control, and anti-theft control, etc.

General Motors' first ECUs had a small application of hybrid digital ECUs as a pilot program in 1979, but by 1980, all active programs were using microprocessor based systems. Due to the large ramp up of volume of ECUs that were produced to meet the US Clean Air Act requirements for 1981, only one ECU model could be built for the 1981 model year. The high volume ECU that was installed in GM vehicles from the first high volume year, 1981, onward was a modern microprocessor based system. GM moved rapidly to replace carburetor based systems to fuel injection type systems starting in 1980/1981 Cadillac engines, following in 1982 with the Pontiac 2.5L "GM iron duke engine" and the Corvette Chevorlet L83 "Cross-Fire" engine. In just a few years all GM carburetor based engines had been replaced by throttle body injection or intake manifold injection systems of various types. In 1988 Delco Electronics, Subsidiary of GM Hughes Electronics, produced more than 28,000 ECUs per day, the world's largest producer of on-board digital control computers at the time.

Applications

A typical civilian transport aircraft flight may illustrate the function of a FADEC. The flight crew first enters flight data such as wind condition runaway length, or cruise altitude, into the Flight management system(FMS). The FMS uses this data to calculate power settings for

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different phases of the flight. At takeoff, the flight crew advances the throttle to a predetermined setting, or opts for an auto-throttle takeoff if available. The FADECs now apply the calculated takeoff thrust setting by sending an electronic signal to the engines; there is no direct linkage to open fuel flow. This procedure can be repeated for any other phase of flight.

In flight, small changes in operation are constantly made to maintain efficiency. Maximum thrust is available for emergency situations if the throttle is advanced to full, but limitations can’t be exceeded; the flight crew has no means of manually overriding the Flight.

Other application

Such systems are used for many internal combustion engines in other applications. In aeronautical applications, the systems are known as “FADECs” (Full Authority Digital Engine Controls). This kind of electronic control is less common in piston-engined aeroplanes than in automobiles, because of the large costs of certifying parts for aviation use, relatively small demand, and the consequent stagnation of technological innovation in this market. Also a carbuerted engine with magneto ignition and a gravity feed fuel system does not require electrical power generated by an alternator to run, which is considered a safety advantage.

Electronic Concepts & Engineering has developed Full Authority Digital Engine Controls or FADECs for turbine engines, flight qualified engine monitoring electronics, and turbine engine control simulation systems.

Our extensive experience in FADEC turbine engine control system design and development includes:

Electronic Circuitry Design FADEC Enclosure Design (Ruggedized) Controller EMI/Environmental Testing and Qualification Software-Hardware Integration and Bench Testing System Integration and Engine Test Support Generating Control System Specifications Control Algorithm Development Real-time FADEC Software Development

In addition to FADEC development, we have also developed flight qualified turbine engine monitoring systems and Real-Time Hardware In-the-Loop (HIL) engine simulators.  The HIL systems were developed for DARPA, Lockheed Martin, Raytheon, and the US Air Force and have been used on the JASSM and TACTOM programs and are deployed at Wright Patterson Labs. 

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JASSM Missile Turbine Engine FADEC

They simulate the operational characteristics of the propulsion system over the flight envelope, thus allowing validation of the engine controller and accessories without performing expensive engine (or flight) testing.

We have the ability to provide embedded real-time software, documented to IEC 12207, MIL-STD-498, or customer standards.  In addition to the real-time embedded software, we routinely develop P.C. based operator interface software that allows the operator to control, test, status, reprogram, and log data from the embedded product.

Our product developments include features such as built-in-test (BIT), diagnostics, and prognostics.

Our designs typically integrate modern communication busses such as CAN (ISO 11898), Ethernet, USB, 802.11, MIL-1553, as well as others.  We have implemented CAN over Fiber Optics for high voltage/high current applications for EMI immunity.

In addition to FADEC development, we have the ability to provide design analysis and documentation such as reliability analysis (MIL-HDBK-217), finite element analysis (subcontracted), failure modes and effects analysis, sneak circuit analysis, interface control documents, qualification test plans and reports, to name a few.

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Digital Engine Control for Military Application

We typically take responsibility for product validation and qualification testing per the customer’s specification.  Tests normally are performed to FAA or Military standards including MIL-STD-810 and MIL-STD-461.  Depending on the type of testing, and the levels required, we may perform the testing in-house or use an established network of testing labs. 

Typical qualification testing for a FADEC includes: Corrosive Atmosphere (Salt Fog), Salt Water Immersion, High/Low Temperature, Temperature Shock, Humidity, Vibration, Shock, Altitude, Explosive Atmosphere, Fungus, Acceleration, EMI Suite, and Electro Static Discharge (ESD) to name several.  In addition to the qualification testing, we have the ability to perform Highly Accelerated Life Testing (HALT), Highly Accelerated Stress Screening (HASS), and Environmental Stress Screening (ESS) in house.  ESS (shake and bake) is usually required on each deliverable unit to reduce infant mortality.

Proactively Addressing FADEC Issues

To proactively addressing the FADEC issue, the FADEC design process must be examined carefully. Compare the present FADEC with what it can be in the future. The software component of FADEC is expanding rapidly. There are advancements in the hardware capability as well. It is much better to, simplify, standardize, reuse, and spend resources on advancement rather than by reinventing the wheel with each new application. To improve the FADEC, one must envision the present and ideal FADEC. Table 1 is an illustration of a typical current FADEC compared to what could be realized in the future. [3]

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Current FADEC Future (Ideal) FADEC

Not Upgradeable Upgradable

Single application mindset Multi-application

Unexpected Shutdown Measured proaction, reaction

Best Practices Essential Practices

Shorter Life Span Longer Life Span

Not Adaptable Adaptable

High Cost Lower Cost

Non-standard I/O Standard I/O / Open Interface Standards

Non-standard Power Supply Custom Electronics

Standard Power Supply COTS Electronics

Non-distributed Distributed-Modular Custom designs

Prognosis capability

Integrated with Control / Flight / Thermal /Power/ Human

factors

Maximum interoperability of diverse components Ease of customization and extension

One of the main requirements for modern FADEC systems is to implement great computing power with many interfaces and to keep the FADEC hardware effort to a minimum. On the other side the criticality potential of computer failures is considered as `hazardous'. The trend in FADEC development is to implement even more complex functions into the control software which consequently increases the authority and therefore the criticality potential of computer failures. In the mid 80's a double computer system was used to perform a parallel execution of

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the control software with identical input parameters to output identical results. A difference in any one of these computer results causes the comparator hardware to output a failure indication. This was considered to have a 100% coverage of computer failures. The problem with this system was certainly the relatively large hardware overhead and the limited intelligence of the comparator logic. Some other FADEC systems have implemented only a Watch Dog Timer and Bus Access Supervisory hardware to detect computer malfunctions. With this method the proof

for the achievements of the safety requirements have become almost impossible since adequate fault models of the computer components are difficult to establish due to their increasing functional complexity.