Flight controls, a brandnew technology - da lezioni di Tecnologie aeronautiche avanzate, digital Fly By Wire; docente Virgilio Conti

28/ott/2008 11.32.47 Virgilio E. Conti Contatta l'autore

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Flight controls, a brandnew technology

THE COMPUTERS FLY

Alitalia - Engineering and Maintenance Division

Technical training - Stage led by chief instructor Virgilio Conti                                                                                                              Fonts: Alitalia, Airbus Industrie, Boeing, NASA

Digital Fly By Wire

One hundred years after the Wright brothers, first powered flight, airplane designers are unshackled from the constraints that they lived with for the first seven decades of flight because of the emergence of digital fly-by-wire (DFBW) technology. Many technologies with significant advantages fail to catch on due to economic constraints, or sometimes simply because their time has not come. Fly-by-wire demonstrated that its time had come.


  • At its most basic level, fly-by-wire technology reduced the weight and maintenance costs of aircraft by replacing heavy mechanical systems with lightweight wires. But its real significance was its impact on aircraft design and performances capability.

  • Today, digital fly-by-wire (DFBW) systems are integral to the operation of a great many aircraft.

  • These systems provide numerous advantages over older mechanical arrangements. By replacing cables, linkages, push rods, pull rods, pulleys, and the like with electronic systems, digital fly-by-wire reduces weight, volume, the number of failure modes, friction, and maintenance.

  • It also enables designers to develop and pilots to fly radical new configurations that would be impossible without the digital technology.

 

Digital fly-by-wire aircraft can exhibit more precise and better maneuver control, greater combat survivability, and, for commercial airliners, a smoother ride.

New designers seek incredible maneuverability, survivability, efficiency, or special performance through configurations which rely on a DFBW system for stability and controllability (see appendix Airplane axis and stability). DFBW systems have contributed to major advances in human space flight, advanced fighters and bombers, and safe, modern civil transportation.

The story of digital fly-by-wire is a story of people, of successes, and of overcoming enormous obstacles and problems. The fundamental concept is relatively simple, but the realization of the concept in hardware and software safe enough for human use confronted the research-industry team with enormous challenges.

DFBW technology has demonstrated that was possible to build and integrate a combined hardware and software flight-control system, and that it had all the advantages expected of fly-by-wire with additional flexibility provided by embedding the flight-control laws in computer code. The world was apparently ready, and now the skies are filled with digital computers flying "in very tight formation."

The Airbus A-320 has a high level of success with its control system, which features a unique architecture to enhance reliability. The Airbus flight control system has migrated to all new models of that firm's aircraft. Even the conservative industry, Boeing, has launched the B-777 with a digital control system of more conventional design.

 

 

History of Flight-Control Technology

Flight controls philosophies had progressed through several stages of emphasis on inherent and dynamic stability to instability. Flight control technology initially had limited mechanical means and a great dependence on the pilot. By 1960, control of both unpiloted missiles and automatic control of piloted aircraft greatly reduced this reliance. A technology called "fly-by-wire" - due to its electrical, rather than mechanical, nature - made these accomplishments possible.

 

 

Active Control in History

In an active control system centered on a computer, sensors provide input and the actuators execute the output commands. This is a feedback system in which control-surface deflections caused by the actuators change the state of the sensors, which affects the output from the computer, and so on. Such systems had already been used in automatic flight guidance systems or autopilots.

The key difference between mechanical control systems and fly-by-wire systems is that the former are distance dependent and the latter are force dependent. A pilot pulling back on the control wheel of a light plane deflects the elevators upward. The distance the elevators move is proportional to the distance the control wheel is pulled away from the instrument panel. The actual proportions are a function of the cable length connecting the control wheel and control surface.
In a fly-by-wire system, the control device is usually a stick, either in the normal between-the-legs position or on the side armrest. Depending on the type of sensor used, either the distance of deflection of the control stick or the force applied by the pilot's hand is measured. This measurement is what is communicated to the computer system as the pilot's desires. This makes it possible to control the motion of an aircraft, rather than the surface positions of the elevators, rudders, and ailerons. The result is a new aircraft generation, more cost effective, safer and nicer to fly than a conventional aircraft.

 

 

Sensors

Sensors are carried on all aircraft. Depending on the sophistication of the autopilot and navigation system, there may be many different types of sensors. As an example, one of the simplest and most prevalent is the pitot-static system, which supplies information to the pilot and flight-control system about airspeed, vertical speed, and altitude. A relatively small hollow tube, the pitot, projects from the wing or fuselage of an aircraft in such a way that there is an unobstructed flow of air into it. The pressure of this ram air is compared with the pressure of stable outside air gathered through a port mounted away from turbulence.

Through comparison of the two pressures, indicated airspeed can be calculated.
In the case of the pitot-static system, the "sensors" are completely passive: the pitot tube and static port essentially sample the air directly. Also, since the static air sample is taken without any correction or correlation to the movement of the air outside the aircraft, the airspeed indicated on a gauge is not the ground speed. This is because it has no way of allowing for the effects of wind. An aircraft with a direct 20 knot-per-hour tailwind would actually be moving relative to the ground at the indicated airspeed plus 20 knots. This means that the pilot is responsible for calculating the effects of wind using weather data and computers that contain

information about the overall impact of wind from all directions.
Gyroscopic instruments are more complex. A gyroscope tends to resist forces applied to it once it is spinning. Thus, an instrument such as an attitude indicator can use the position of a gyroscope to correctly show changes in the angle of the wings and nose of an aircraft relative to the horizon. In this case, the sensor is the gyroscope itself coupled with some reference point. In simple aircraft the pilot is responsible for monitoring the attitude indicator to keep the aircraft straight and level or to use it as a reference in turns. In visual flight conditions, the attitude indicator is largely unnecessary since the pilot can use the actual horizon for reference. However, in instrument-based flying, the indicator is crucial to the pilot,s ability to maintain orientation.
Gyroscopic sensors can also be used to measure angular velocities. These "rate gyros" are the basis for stability augmentation systems and are important components in fly-by-wire controllers.

Another type of sensor especially used for the fly-by-wire project is the inertial measurement unit. Such devices had been developed in the 1940s. They measure accelerations in each axis of motion (see appendix Airplane axis and stability). This acceleration data is used in an inertial navigation system to calculate velocity and position without any other sensor input. This is handy in a vacuum. Since good inertial measurement units were common in the space program, they could readily provide data for the computer, the next device in the control chain.

 

The Role of the Computer

The central component of all fly-by-wire systems is the flight computer. The computer uses control laws specific to an aircraft to calculate the commands necessary to maintain stability and implement pilot desires. Control laws are the equations of motion that have to be solved to actively control an unstable aircraft. The values for these equations are specific to each aircraft design. That is why control laws embodied in electronic analog circuits make those circuits unusable in any other aircraft. There are two types of computers used in fly-by-wire systems: analog and digital.

Each type has advantages and disadvantages, and there was considerable debate among the engineers over which to use.

Analog computers exist in a wide variety of forms. In fact, long after the advent of digital computers, there were still many more analog computers in use than the digital ones so familiar today. The log-scale slide rule, once the dominant personal computing device, is an analog computer. It works by creating a mechanical analogy between the positions of numbers on its various scales and the products, quotients, squares, square roots, cube roots, etc., that it is used to calculate. Another type of mechanical analog computer was the differential analyzer, which was in scientific use from the early 1930s through the early 1950s: the "DA,"

as it was called, had cams of various shapes to model the terms of equations.

The analyzer filled a good-sized room and had to be operated by hand.

Such mechanical analog computers are not as practical flight-control devices as their electronic brethren. Such an electronic analog computer modeled the differential equations of the control laws and conveniently accepted voltage values as input and generated them as output. These voltages could then be amplified as commands to the actuators of the control system. Thus, by the early 1940s it was possible to use an analog computer in flight control. For nearly forty years thereafter, such devices formed a core enabling technology for fly-by-wire.
The fact that the control laws are hard-wired into an analog computer is both an advantage and a disadvantage. The advantage is that it is difficult or impossible to corrupt an analog computer through power transients, software viruses, Electro Static Discharge phenomena or other weaknesses experienced by digital computers. The disadvantage is that to "re-program" an analog computer, one must physically rearrange the circuits into a new structure that models the modified control laws. Furthermore, analog circuits are subject to signal drift in their responses, and this must be compensated, usually by voting of output from multiple circuits. Higher temperatures also affect analog computers because information is in the form of amplitudes, and temperature effects modulate the amplitude.

Nevertheless, analog computers were used in fly-by-wire tests.


Substituting circuits forced the engineers to face the key difference between analog and digital computation. Analog devices depend on a continuous stream of data signals. Digital circuits, by their very nature, need data to be transformed into a stream of bits. The problem is that the signal streams in a complex real-time system might be too dense and rapid for the analog-to-digital converters to deliver all the sensor data to the computer. This means that the data must in effect be sampled, rather than used in totality. The difficulty is in the accurate processing of sampled data in order to make it as useful as a complete data set. It was not until 1963 that the mathematical basis of digital control became widely available due to published work on sampling theory. Note that aircraft systems were not the only beneficiaries of this foundation.

Digital control in manufacturing, automobiles, and medical instrumentation has similar problems and has benefited from this information.

 


Another aspect of digital computers that needed to be improved before they could be used in aircraft was their size.The first general-purpose electronic computer, the ENIAC, at the University of Pennsylvania during World War II. It filled a very large room and required significant power and air conditioning to operate, primarily since it used vacuum tube technology.

 

The transistor improved the situation tremendously, and a discrete-circuit, transistorized computer built by IBM flew in the Gemini piloted spacecraft in the mid-1960s. The improvements in digital computer hardware made possible equally important improvements in the capability of the software that embodies the control laws of the aircraft. Whereas with an analog computer the "software" is essentially hardwired into the machine, a digital computer can be adapted to many different uses by changing its programming. A limitation on software for real-time systems in aerospacecraft is the size of a computer word. It not only affects the scale at which the computer can do computations; it affects the flexibility of its instruction set and the application software built for it. Engineers programmed early digital systems exclusively in low-level machine languages that are very difficult to inspect and understand and thus prone to human error. Early recognition of the inherently complex nature of these machine-based languages inspired the development of machine-independent languages such as FORTRAN, which express mathematical formulae in terms more recognizable by the average engineer. However, the use of such high-level languages requires special translation software such as interpreters and compilers that recast the language statements into machine code.

Even though these languages reflected a significant engineering improvement, they were not readily adaptable to the embedded computer systems demanded by fly-by-wire. They lacked statements to support functions such as scheduling of processes. Also, real-time systems have strict performance constraints, and engineering managers thought compiler-generated machine code was too inefficient to meet these requirements.

Effectors and Actuators

The last enabling technology for fly-by-wire flight control consists of the actuators that move the control surfaces. In the mechanically based flight-control systems, the control surfaces move under direct-cable positioning. This is replaced by electrical connections to actuators in fly-by-wire systems. In fact, the original meaning of "fly-by-wire" is limited to this technology alone.

  • Actuators are usually hydraulically powered jacks or oleoservocontrols; they are sometimes electrically (PBW) or pneumatically powered.

Gavin Jenney, one of the pioneers of the technology, says that, "When we were developing fly-by-wire, the purpose was to provide safe and reliable electrical control between the pilot and the flight control surfaces as a replacement for the mechanical connection." Such connections did not need either computers or sensors, but rather simple physical force to electrical force converters at one end, and electrically operated hydraulics at the other. Such systems could be made triply or quadruply redundant and still obtain weight savings along with reliability increases over even dual hydromechanical systems. It would have been nearly impossible to achieve practical fly-by-wire without the electrical actuators and their associated equipment.
This is what had been achieved in sensor, computer, and actuator technology when researchers was considering fly-by-wire for airplanes. The engineers felt that these technologies had reached a point where they would be practical to use. However, it would be necessary to make hard choices, the most difficult one being whether to use a digital or an

analog computer. Most other decisions depended on that one.

 

Analog versus Digital

T

Numbers are transmitted in electronic analog circuits as continuous current at varying voltage levels proportional to the values being transmitted. Volts are a measure of pressure, so the bigger the value, the larger the voltage

 

here are two ways to send numbers on electrical wires: continuously, or in ones and zeroes.

Digital signals are sent as streams of bits - binary digits - which can be either ones or zeroes. A specific bit length represents a word of information

 

Once in the computer, the data is manipulated with the control laws for the airplane. In an analog computer, the equations are represented by circuits that implement the mathematics. In a digital computer, the control laws are in software. This means that analog computers are effectively a single-airplane system; they cannot be moved from one type of aircraft to another without extensive physical changes. In a flight-test program, characterized by continuous tweaking of components, this could potentially be a problem. In a two-phase program like the fly-by-wire project, where an aircraft change is possible between phases,

analog computers are even more awkward.

Digital computers are more flexible due to software. The phrase "general purpose computer," which is only applied to digital machines, implies their ability to adapt through different software programs. However, digital computers have advantages in addition to their programmability. By proper scaling of the data represented in digital words, such computers can be made to be more accurate than their analog counterparts. They also can compensate for drift in analog subcomponents. An attraction for the engineers was that with a digital system, they could

include some logic in the control laws, making them more robust.

Analysis of the choice between analog and digital computers shows that at the time any comparison made based on considerations of pure size and complexity does not show much difference. For simple systems like short-lived missiles and non-combat aircraft, analog computers are best in most instances. Conversely, most complex systems have long-living applications that benefit from software changes. However, as one flight-control engineer said, "Just where this crossover point lies is difficult to judge." Therefore, the final decision had

some political aspects.



It is not clear that the engineers knew what they were getting into by starting to deal with software. Software’s flexibility is a bane as well as an advantage. It is too easy to change and very difficult to change correctly: fifty percent of all software modifications, including defect repairs, result in new defects. By 1972, researches came to think of a digital computer and its software as a patchboard in which any two points could be inadvertently, and invisibly, connected. In an analog system an incorrect connection was more easily visible. Nevertheless, they pressed on with digital technology. The problem then became getting a digital computer suitable for flight control. There were no widely available computers at the time with the size, power requirements, weight, reliability, and performance needed for flight. There were the computers used in piloted spacecraft, however.


A new project was encouraged to have industry interest in the results. But commercial manufacturers were essentially ignorant of digital fly-by-wire. Moreover, even if knowledgeable, they had to consider the three factors that were essential to any control system choice for commercial aircraft: safety, performance, and cost of ownership. So the selling point for the project surfaced in the demonstration that fly-by-wire, especially digital fly-by-wire, would have sufficient impact to get manufacturers on board.

 

It seems unlikely that anyone in the 1940s imagined that future counterparts of their room-sized computers would someday, a quarter-century later, fit into one-cubic-foot boxes and be many times more powerful. The challenge then, as now, was to create confidence that such systems could work as well as mechanical, hydraulic systems over long periods of time. Reliability in such systems is in many ways the sum of the reliability of the parts, but there is no doubt that some parts are considered more worrisome than others. This is particularly true of software.

 

 

Reliability and Its Impact on Later Designs

While electronic analog computers were small and powerful enough to work in flight-control systems as early as 1940, no one seriously considered using digital computers for that purpose until the late 1950s. The reason was simple: digital computers were still giants. They used vacuum tubes, they had large power and refrigeration support systems, and their circuitry was neither densely packed nor reliable. As far as aircraft designers were concerned, they were cargo. At that time, the chief cause of computer failure was not software defects, as it is today, but hardware faults.

 

At first glance, this seemed to ensure that digital computers would never find their way into flight-control systems. Objections to redundant digital computers centered on size, power, and weight. Triplicating the logic circuitry and adding majority organs meant a penalty in all three areas. However, within a few years, transistors matured enough to replace vacuum tubes, core memories became more reliable and rugged (though still not very dense), and physical miniaturization together with lower power requirements all became common. Therefore, interest in using digital computers in control systems increased, and the tecnology became

interesting to designers.

 

Flights with the Side-stick

There were experiments with side-sticks in other fly-by-wire programs. The major advantages of a side-stick were eliminating an obstruction to seeing the instrument panel and making it easier to design a reclined seat for better g-tolerance. There is never an abundance of real estate in a fighter cockpit, where instruments and switches are jammed together and mounted at least down to the pilot's knees. As the amount of avionics increased, this could only get worse. Hence, the invention of multifunction display screens that collected data together. These, with small print and large amounts of information compressed on screens, must be seen more clearly than analog instruments. Getting the stick out of the way would allow easy views of all the forward-mounted instruments and place both the pilot's hands near to the side-mounted switches. Also, there would be symmetry in the cockpit. The pilot would have the control stick in his right hand, throttle in his left, both festooned with switches and buttons. In combat, the pilot would sit reclined, both hands able to toggle all needed functions, looking out

the window with a head-up display showing critical air data.


A side-stick could be installed much more simply with a fly-by-wire control system than with a mechanical system. There were two schools of thought on which type to install. The first type was a force-sensing side-stick. The pilot, wanting to climb, pulled back on the stick, like a conventional one, but it did not move. Instead, force sensors would translate the pressure applied by the pilot into a voltage. This signal would then be transmitted to the computer. This type of side-stick often caused the "Popeye" effect on the pilots. Like the cartoon character Popeye the Sailor Man, they experienced their forearm growing larger as they subconsciously did isometrics with the stick in violent maneuvering. The other type was a displacement stick. This side-stick did move, but not much.

 

Impact and Legacy of NASA's Digital Fly-By-Wire Project

The research program achieved important goals: it proved that active control could be accomplished with digital systems and that multiple computers could be synchronized and provide a greater measure of flight safety. It also demonstrated many other ideas, such as adaptive control laws, sensor analytic redundancy, and new methods of flight testing digital systems remotely.

 

 

The Certification of Commercial Fly-By-Wire Airliners

Early in the 1980s, Boeing introduced two airliners with advanced avionics: the 757 and the 767. Even though the company had built a prototype cargo airplane using a fly-by-wire and hydromechanical system in the mid-1970s, neither of the two new airliners had digital

technology in their controls.

It was left to the relatively young Airbus to make the leap into the future and challenge Boeing's domination of the narrow-body airliner market with an advanced airplane using a flight-control architecture different from the American prototypes. Conscious of potential resistance by certifying agencies, pilots, and the flying public, Airbus engineers devised a scheme that is both redundant and a reduced-functionality backup at the same time. The objective was to introduce as much diversity in the system as possible (and thus avoid the dreaded generic software defect that could bring down all the computers) yet provide

functional redundancy.


There are two separate control systems in Airbus fly-by-wire aircraft (which now include the models A318, A319, A320, A321, A330, and A340). These work together to provide highly optimized handling in the pitch and roll axes. (The yaw axis still uses a mechanical system.) One is called the ELAC (Elevator Aileron Computers) and the other the SEC (Spoiler Elevator Computers). Thomson-CSF built the ELAC system, using two different computers, one designed in Paris, the other in Toulouse, by different teams not in contact during development. In addition, the SEC, built by SFENA, is triply redundant. In order to achieve even higher reliability, the software is written in different languages, such as assembler unique to the

processor, PL/M, and Pascal.


If one or the other system fails completely, the remaining one becomes the backup. It is quite possible to fly an Airbus using spoilers for ailerons, and also without the spoilers, but the elevator control is built into both systems because that is still the primary control surface for pitch, and any other arrangement would not do. This rather Byzantine system is claimed to have a reliability of about one failure in 10 trillion operations, the highest ever achieved.


A 320 primary flight controls

  • Ailerons for roll control

  • Elevators for pitch control

  • Rudder for yaw control

A 320 secondary flight controls

  • Trimmable horizontal stabilizer for longitudinal trim

  • Spoilers for roll control, speed brake and lift dumping

  • Flaps and slats for lift augmenting during low speed configuration

 


 

Fly by wire system: block diagram

 

  • Electrical inputs coming from pilot or autopilot are processed by computers

  • Electrical orders are signalled to the servocontrols that are hydraulically powered

  • Feedback from jack/surface and airplane response are both used by the system


 

 

 

 

 

 

A 321 primary flight control computers: EFCS laws distribution

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

COMPUTER

 

 

LAW/FUNCTION implemented

 

 

CONTROL of

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

ELAC

 

 

1-

 

PITCH NORMAL LAW

 

 

Two elevators

 

 

 

 

 

2-

 

PITCH ALTERNATE LAW

 

 

One THS

 

 

 

 

 

3-

 

PITCH DIRECT LAW

 

 

Two ailerons

 

 

 

 

 

4-

 

LATERAL NORMAL LAW

 

 

 

 

 

 

 

 

5-

 

LATERAL DIRECT LAW

 

 

 

 

 

 

 

 

6-

 

LAF NORMAL LAW

 

 

 

 

 

 

 

 

7-

 

AILERON DROOP FUNCTION

 

 

 

 

 

 

 

 

8-

 

ABNORMAL ATTITUDE LAW (*)

 

 

 

 

 

 

 

 

9-

 

A/P ORDERS ACQUISITION

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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