Control of parallel robots for flight simulators

Birlea AngelaGr. 1542Content

The concept of parallel manipulators...3Structure and modeling of parallel manipulators....4Flight simulators..6The beginnings of flight simulators (1909-1960)6Flight simulators applications..9Training pilots..9Engineering applications10Full Flight Simulators..10Flight simulators using parallel robots11Control of parallel robots used in flight simulators13Achievements..14Conclusions....22Bibliography...23

1. The concept of parallel manipulators

Parallel robots are closed-loop mechanisms presenting very good performances in terms of accuracy, velocity, rigidity and ability to manipulate large loads. They have been used in a large number of applications ranging from astronomy to flight simulators and are becoming increasingly popular in the field of machine-tool industry.A parallel manipulator is a mechanical system that uses several computer-controlled serial chains to support a single platform, or end-effector. Perhaps, the best known parallel manipulator is formed from six linear actuators that support a movable base for devices such as flight simulators. This device is called a Stewart platform or the Gough-Stewart platform in recognition of the engineers who first designed and used them.Also known as parallel robots, these systems are articulated robots that use similar mechanisms for the movement of either the robot on its base, or one or more manipulator arms. Their 'parallel' distinction, as opposed to a serial manipulator, is that the end effector (or 'hand') of this linkage (or 'arm') is connected to its base by a number of (usually three or six) separate and independent linkages working in parallel. 'Parallel' is used here in the topological sense, rather than the geometrical; these linkages act together, but it is not implied that they are aligned as parallel lines.A parallel manipulator is designed so that each chain is usually short, simple and can thus be rigid against unwanted movement, compared to a serial manipulator. Errors in one chain's positioning are averaged in conjunction with the others, rather than being cumulative. Each actuator must still move within its own degree of freedom, as for a serial robot; however in the parallel robot the off-axis flexibility of a joint is also constrained by the effect of the other chains. It is this closed-loop stiffness that makes the overall parallel manipulator stiff relative to its components, unlike the serial chain that becomes progressively less rigid with more components.This mutual stiffening also permits simple construction: Stewart platform hexapods chains use prismatic joint linear actuators between any-axis universal ball joints. The ball joints are passive: simply free to move, without actuators or brakes; their position is constrained solely by the other chains. Delta robots have base-mounted rotary actuators that move a light, stiff, parallelogram arm. The effector is mounted between the tips of three of these arms and again, it may be mounted with simple ball-joints. Static representation of a parallel robot is often akin to that of a pin-jointed truss: the links and their actuators feel only tension or compression, without any bending or torque, which again reduces the effects of any flexibility to off-axis forces.A further advantage of the parallel manipulator is that the heavy actuators may often be centrally mounted on a single base platform, the movement of the arm taking place through struts and joints alone. This reduction in mass along the arm permits a lighter arm construction, thus lighter actuators and faster movements. This centralization of mass also reduces the robot's overall moment of inertia, which may be an advantage for a mobile or walking robot.All these features result in manipulators with a wide range of motion capability. As their speed of action is often constrained by their rigidity rather than sheer power, they can be fast-acting, in comparison to serial manipulators.The area of motion simulation, especially that of flight simulators is currently the main commercial application of parallel mechanisms. These simulators, albeit very popular and providing very realistic cues, have several notable disadvantages including a restricted workspace (mainly with respect to rotation), prohibitive cost, limited operation and they require high maintenance. Moreover, the oils contained in the actuators can be an environmental problem for some people.

2. Structure and modeling of parallel manipulatorsFor each link of a manipulator, the connection degree is the number of rigid bodies attached to this link by a joint.Simple kinematic chains are defined as being those in which each member possesses a connection degree that is less than or equal to 2. A closed loop kinematic chain is obtained when one of the links, but not the base, possesses a connection degree greater than or equal to 3. Some of the problems occurring with serial robots can be solved mechanically by distributing the load on the links, which means linking the end-effector to the ground by a set of chains that each supports only a fraction of the total load. The use of closed-loop kinematic chains for manipulators thus seems to be quite interesting; actually this option had already been explored even before the term robot had been coined. Some theoretical problems linked to this type of structure were mentioned as early as 1645 by Christopher Wren, then in 1813 by Cauchy, in 1867 by Lebesgue and in 1897 by Bicard. One of the main theoretical problems in this field is called the spherical motion problem.A positioning device for a simulator imposes constraints which may be quite different from those necessary in a robotic system. In the later case, accuracy may be most important, while the amplitude of motion is less so. Dynamics is also important for tasks involving a contact between the robot and its surrounding, as in grinding of surface following; or for tasks where execution speed is crucial.Another important concept is the compliance of the robot.Fully parallel robots with m degrees of freedom possess m chains supporting the end-effector. If these chains are identical, Grubler formula for three-dimensional mechanisms may be written as

The use of this strictly combinatorial formula can sometimes lead to mistakes because it does not take into consideration the geometric relations between the joints. Many parallel robots have been proposed, but an important question should be addressed: may we design a synthesis approach that deals with any kinematic performance other than the number of degrees of freedom of the robot?For parallel robots, it seems that a simple design rule such as that derived for 3R and Cartesian robots cannot be established, as their performance will be highly dependent on their dimensioning. Consequently, structural synthesis cannot be separated from dimensional synthesis and we put forward the following conjecture: a parallel robot with well-designed dimensions will exhibit overall better performance compared to another parallel robot whose structure seems to be more appropriate but whose dimensions have been poorly chosen.An important component of the simulators software is the aircraft dynamics mathematical model, which can calculate the aircrafts response to control inputs at various speeds and orientations. Then the output signal is transmitted to other subsystems to generate visual scenery, sound, motion and force feedback feeling for trainees. The aircraft dynamics model involves the principles of mathematical modeling of the aerodynamic, flight control, propulsion, ground handling and environmental characteristics of the aircraft. The simulation of a complete mission, however, from take-off to landing, needs the nonlinear equations of motion, covering the full flight envelope.The design and development of a full flight dynamics engine needs a flight test or purchasing a data package, which is temporarily unavailable for us. Therefore it is of critical importance to select an existing set of products which would provide the features we want.

3. Flight simulatorsA flight simulator is a device that artificially re-creates aircraft flight and various aspects of the flight environment. This includes the equations that govern how aircraft fly, how they react to applications of their controls and other aircraft systems, and how they react to external environmental factors such as air density, turbulence, cloud, precipitation, etc. Flight simulation is used for a variety of reasons, including flight training (mainly of pilots), the design and development of the aircraft itself, and research into aircraft characteristics and control handling qualities.Depending on their purpose, flight simulations employ various types of hardware, modeling detail and realism. They can range from PC laptop-based models of aircraft systems to simple replica cockpits for familiarization purposes to more complex cockpit simulations with some working controls and systems to highly detailed cockpit replications with all controls and aircraft systems and wide-field outside-world visual systems, all mounted on six degrees-of-freedom (DOF) motion platforms which move in response to pilot control movements and external aerodynamic factors.

3.1. The beginnings of flight simulators (1909-1960)3.1.1. Before World War IThe first known flight simulation device was to help pilots fly the Antoinette monoplane. Whereas the earlier Wright designs used levers for pitch and roll control, the Antoinette used two wheels mounted left and right of the pilot, one for pitch and one for roll. Although the pitch wheel operated in a natural sense, the roll wheel did not (this had to wait until the "invention" of the centrally mounted control column or "stick" or "joystick").A training rig was developed in 1909 to help the pilot operate the control wheels before the aircraft was flown. This consisted of a seat mounted in a half-barrel and the two wheels. The whole unit was pivoted so that assistants outside could pitch and roll the device in accordance with the pilot's use of the wheels, using long wooden rods attached to the barrel structure. A full-size model of the "Antoinette Barrel Trainer" is in the foyer of the Airbus Training Centre at Toulouse, France.

3.1.2 World War I (1914-1918)A number of pilot training devices were developed during World War I. Some, like the earlier Antoinette trainer of 1909, were for teaching pilots how to operate the flight controls. Examples include a 1915 UK trainer with a "rocking" cockpit described by H.G. Anderson, moving cockpit trainers by Lender and Heidelberg in France (patented in 1917), and the U.S."Ruggles Orientator" by W.G. Ruggles, patented in 1917.Air Gunnery. Another area of training was for air gunnery handled by the pilot or a specialist air gunner. Firing at a moving target requires aiming ahead of the target (which involves the so-called lead angle) to allow for the time the bullets require to reach the vicinity of the target. This is sometimes also called "deflection shooting" and requires skill and practice. During World War I, some ground-based simulators were developed to teach this skill to new pilots.

3.1.3. The 1920s and 1930sThe best-known early flight simulation device was the Link Trainer, produced by Edwin Link in Binghamton, New York, USA, which he started building in 1927. He later patented his design, which was first available for sale in 1929. The Link Trainer was a basic metal frame flight simulator usually painted in its well-known blue color. Some of these early war era flight simulators still exist, but it is becoming increasingly difficult to find working examples.The Link family firm in Binghamton manufactured keyboard organs, and Ed Link was therefore familiar with such components as leather bellows and reed switches. He was also an amateur pilot, but dissatisfied with the amount of real flight training that was available, he decided to build a ground-based device to provide such training without the restrictions of weather and the availability of aircraft and flight instructors. His design had a pneumatic motion platform driven by inflatable bellows which provided pitch and roll cues. An electric motor rotated the platform, providing yaw cues. A generic replica cockpit with working instruments was mounted on the motion platform. When the cockpit was covered, pilots could practice flying by instruments in a safe environment. The motion platform gave the pilot cues as to real angular motion in pitch (nose up and down), roll (wing up or down) and yaw (nose left and right).Initially, aviation flight schools showed little interest in the "Link Trainer". Link also demonstrated his trainer to the U.S. Army Air Force (USAAF), but with no result. However, the situation changed in 1934 when the Army Air Force was given a government contract to fly the postal mail. This included having to fly in bad weather as well as good, for which the USAAF had not previously carried out much training. During the first weeks of the mail service, nearly a dozen Army pilots were killed. The Army Air Force hierarchy remembered Ed Link and his trainer. Link flew in to meet them at Newark Field in New Jersey, and they were impressed by his ability to arrive on a day with poor visibility, due to practice on his training device. The result was that the USAAF purchased six Link Trainers, and this can be said to mark the start of the world flight simulation industry.The company Link Aviation Devices Inc was then formed, and other sales followed including to the UK Royal Air Force and, ironically in view of the Pearl Harbor attack on 7 December 1941, to the Imperial Japanese Naval Air Arm.

3.1.4. World War II (1939-45)The principal pilot trainer used during World War II was the Link Trainer. Some 10,000 were produced to train 500,000 new pilots from allied nations, many in the USA and Canada because many pilots were trained in those countries before returning to Europe or the Pacific to fly combat missions. Almost all US Army Air Force pilots were trained in a Link Trainer.During World War II, other ground-based flight training devices were produced. For instance, in 1943 a fixed-base aircraft-specific trainer for the British Halifax bomber was produced at the RAF Station at Silloth in the north of England. This consisted of a mock-up of the front fuselage of the Halifax, the pilot's flight controls being simulated through an analogue system that gave artificial resistance ("feel") when the pilot moved the controls. Another name for this device was the "Silloth Trainer".A different type of World War II trainer was used for navigating at night by the stars. The Celestial Navigation Trainer of 1941 was 13.7 m (45 ft) high and capable of accommodating the navigation team of a bomber crew. It enabled sextants to be used for taking "star shots" from a projected display of the night sky.3.1.5. 1945 to the 1960sIn 1948, Curtiss-Wright delivered a trainer for the Boeing 377 Stratocruiser transport aircraft to Pan American. This was the first complete aircraft-specific cockpit trainer owned by an airline. There was no motion or visual system, but the cockpit was closely replicated and the controls functioned and produced responses on the cockpit instruments. The device provided training to flight crews in checks, drills and basic flight procedures.In 1954, United Airlines bought four flight simulators at a cost of $3 million dollars from Curtiss-Wright that were similar to the earlier models, with the addition of visuals, sound and movement. This was the first of today's modern flight simulators for commercial aircraft. With the advent of jet airliners such as the UK Comet and U.S. Boeing 707 and DC-8, simulators were produced to train for checks and drills, and to avoid using the actual aircraft for familiarization exercises that could be carried out in the simulator. An example was the simulator for the Comet 4, which had a three-axis motion platform on which the forward section of a Comet fuselage was mounted. It was produced by the Redifon Company of Aylesbury, UK.

3.2. Flight simulators applications

3.2.1. Training pilots

Flight simulation is used extensively in the aviation industry to train pilots and other flight crew for both civil and military aircraft. It is also used to train maintenance engineers in aircraft systems, and has applications in aircraft design and development, in aviation, and in other fields of research.Several different devices are utilized in modern flight training. These range from simple Part-Task Trainers (PTTs) that cover one or more aircraft systems to Full Flight Simulators (FFS) with comprehensive aerodynamic and systems modeling. This spectrum encompasses a wide range of fidelity as to physical cockpit characteristics and quality of software models, as well as various implementations of sound, motion, and visual sensory cues.In comparison to training in an actual aircraft, simulation-based training allows for the training of maneuvers or situations that may be impractical (or even dangerous) to perform in the aircraft, while keeping the pilot and instructor in a relatively low-risk environment on the ground. For example, electrical system failures, instrument failures, hydraulic system failures, environmental system failures, and even flight control failures can be simulated without risk to the pilots or aircraft.

3.2.2. Engineering applications

Flight simulators for engineering applications have the following purposes:

Developing and testing flight hardware. Simulation (emulation) and stimulation techniques can be used, the latter involving feeding real hardware with artificially generated or real signals (stimulated) in order to verify its operation. Such signals can be electrical, RF, sonar, etc., depending on the equipment to be tested. Developing and testing flight software. It is much safer to develop critical flight software on simulators or using simulation techniques than with actual aircraft in flight. Developing and testing aircraft systems. For electrical, hydraulic, and flight control systems, full-size engineering rigs, sometimes called 'iron birds', are used during the development of the aircraft and its systems.

3.3. Full Flight SimulatorsFull Flight Simulator (FFS) is a term used by National (civil) Aviation Authorities (NAA) for a high technical level of Flight simulator. Such authorities include the Federal Aviation Administration (FAA) in the United States and the European Aviation Safety Agency (EASA).There are currently four levels of Full Flight Simulator, levels A - D, level D being the highest standard and being eligible for Zero Flight Time (ZFT) training of civil pilots when converting from one airliner type to another. In about 2012, these FFS levels will be changed as a result of work by an International Working Group chaired by the UK Royal Aeronautical Society Flight Simulation Group (RAeS FSG), which rationalized 27 previous categories of flight training device into 7 international ones. This work has been accepted by ICAO and is published under ICAO document 9625 Issue 3. The new Type 7 Full Flight Simulator will be the old Level D with enhancements in a number of areas including motion, visual and Communications/Air Traffic simulations.A Level D/Type 7 simulator simulates all aircraft systems that are accessible from the flight deck and are critical to training. For instance, accurate force feedback for the pilot's flight controls is provided through a simulator system called "control loading", and other systems such as avionics, communications and "glass cockpit" displays are also simulated.This standard of simulator is used both for Initial and Recurrent training for Commercial Air Transport (CAT) aircraft. Initial training is for conversion to a new aircraft type, and Recurrent training is that which all commercial pilots must carry out at regular intervals (such as every six months) in order to retain their qualification to fly "fare-paying passengers" in Commercial Air Transport (CAT) aircraft, loosely "airliners".A Level D/Type 7 FFS also provides motion feedback to the crew through a motion platform upon which the simulator cabin is mounted, The motion platform must produce accelerations in all of the six degrees of freedom (6-DoF) that can be experienced by a body that is free to move in space, using a principle called Acceleration Onset Cueing, generally using the Stewart platform design.

3.4. Flight simulators using parallel robots

A Full flight simulator (FFS) duplicates relevant aspects of the aircraft and its environment, including motion. This is typically accomplished by placing a replica cockpit and visual system on a motion platform. A six degrees-of-freedom (DOF) motion platform using six jacks is the modern standard, and is required for the so-called Level D flight simulator standard of civil aviation regulatory authorities such as FAA in the USA and EASA in Europe. Since the travel of the motion system is limited, a principle called 'acceleration onset cueing' is used. This simulates initial accelerations well, and then returns the motion system to a neutral position at a rate below the pilot's sensory threshold in order to prevent the motion system from reaching its limits of travel.Flight simulator motion platforms used to use hydraulic jacks, but electric and electric-pneumatic jacks are now common. The latter do not need hydraulic motor rooms and other complications of hydraulic systems and can be designed to give lower latencies (transport delays) compared to hydraulic systems. Level D flight simulators are used at training centers such as those provided by Airbus, FlightSafety International, CAE, Boeing Training and Flight Services (ex-Alteon) and at the training centers of large airlines. In the military, motion platforms are commonly used for large multi-engined aircraft and also for helicopters, except where a training device is designed for rapid deployment to another training base or to a combat zone.Statistically significant assessments of skill transfer based on training on a simulator and leading to handling an actual aircraft are difficult to make, particularly where motion cues are concerned. Large samples of pilot opinion are required and many subjective opinions tend to be aired, particularly by pilots not used to making objective assessments and responding to a structured test schedule. However, it is generally agreed that a motion-based simulation gives the pilot closer fidelity to flight control operations and aircraft responses to control inputs and external forces. This is described as "handling fidelity", which can be assessed by test flight standards such as the numerical Cooper-Harper rating scale for handling qualities. Generally, motion-based aircraft simulation feels like being in an aircraft rather than in a static procedural trainer. In a re-structuring of civil flight training device characteristics and terminology that will take place in about 2012, Level D Full flight simulator will be re-designated as ICAO Type 7 and will have improved specifications for both motion and visual systems. This is a result of a rationalization of worldwide civil flight training devices in which 27 previous categories have been reduced to seven.

Stewart platform

High-end commercial and military flight simulators have large field-of-view (FoV) image generation and display systems of high resolution. All civil Full Flight Simulators (FFS) and many military simulators for large aircraft and helicopters also have motion platforms for cues of real motion. Platform motions complement the visual cues and are particularly important when visual cues are poor such as at night or in reduced visibility or, in cloud, non-existent. The majority of simulators with motion platforms use variants of the six-cylinder Stewart platform to generate cues of initial acceleration. These platforms are also known as Hexapods (literally "six feet") and use an operating principle known as Acceleration onset cueing. Motion bases using modern hexapod platforms can provide about +/- 35 degrees of the three rotations pitch, roll and yaw, and about one meter of the three linear movements heave, sway and surge.

3.5. Control of parallel robots used in flight simulatorsDynamic modeling and controlling are two fundamental issues of robotics. Dynamics of redundant parallel manipulators is much more complicated than that of serial robots due to multiple closed chains and redundant actuation. A new scheme to compute the inverse dynamics is proposed using Lagrange-DAlemberts principle. The new strategy is more computationally efficient for real-time control. Independent joint control and model based control methods represented for redundant parallel manipulators are formulated, i.e. PD control, augmented PD control and computed torque control. The stability analysis of the proposed controllers is presented. A two DOF planar parallel manipulator with redundant actuation was built as a test bed to investigate the dynamics and control of redundant closed-chain mechanisms. The dynamic model was formulated using the proposed scheme in the research. The unknown parameters of the system are obtained by both experimental measures and identification methods. To verify the estimated dynamic model and to evaluate the proposed control schemes, the control algorithms were implemented on the mechanism.There is little research about the dynamics and control of redundantly actuated parallel manipulators. The work done on the dynamic modeling and controlling will be valuable attempts in the field.Actuation redundancy makes the design of the parallel structure more complicated. It introduces not only high computational requirements in calculating the dynamic equations, but also high challenges in controlling the closed-chain system. In principle, it would be possible to reduce these disadvantages by driving only the minimum necessary number of actuators. Hence, it should be clear that redundant actuation is desirable and advantageous.

3.6. AchievementsSimulation replicates controls and aircraft systems and wide-field outside-world visual systems, all mounted on six degree-of-freedom (DOF) motion platforms moving in response to pilot control movements and external aerodynamic factors. Simulation implements sensory clues such as sound, motion, and visual systems.Compared to training in real aircraft, simulation based training allows for the training of maneuvers or situations that may be impractical (or even dangerous) to perform in the aircraft, while keeping the pilot and instructor in a relatively low-risk environment on the ground. For example, electrical system failures, instrument failures, hydraulic system failures, environmental system failures, and even flight control failures can be simulated without risk to the pilots or an aircraft.

Map of the simulators locations in Europe

3.6.1. Airbus A320-200 Full Flight Simulator

The Airbus A320-200 Full Flight Simulator located in Vilnius (LT). STD Qualification Level- JAR-FSTD A, Level D. Certifcates are issued by CAA of Lithuania.

3.6.2. Airbus A333-202 Full Flight Simulator

The Airbus A333-202 Full Flight Simulator located in Mardid (ISP). STD Qualification Level- JAR-FSTD A, Level D. Certifcates are issued by CAA of Spain.

3.6.3. Airbus A340-300/600 Full Flight Simulator

The Airbus A340-300/600 Full Flight Simulator located in Madrid (ISP). STD Qualification Level- JAR-FSTD A, Level D. Certifcates are issued by CAA of Spain.

3.6.4. ATR 42-300 Full Flight Simulator

The ATR 42-300 Full Flight Simulator located in Brussels (BE). STD Qualification Level- JAR-FSTD A, Level C. Certifcates are issued by CAA of Belgium.

3.6.5. ATR 72-500 Full Flight Simulator (FFS)The ATR 72-500 Full Flight Simulator (FFS) located in Madrid (Spain) is IAW JAR - FSTD A requirements Level DG Simulator for ATR 72 aircraft training. Certificates are issued by Spanish CAA.

3.6.6. Boeing 737 CL Full Flight Simulator (FFS)The Boeing 737 CL Full Flight Simulator (FFS) located at the Baltic Aviation Academy Training Centre in Vilnius (Lihuania) is IAW JAR - FSTD A requirements Level CG simulator for B737-300, B737400 and B737-500 aircraft training. CAA of Lithuania has issued respective certificates.

3.6.7. Boeing 737NG Full Flight Simulator (FFS)The Boeing 737NG Full Flight Simulator (FFS) located in Gatwick (UK) is IAW JAR - FSTD A requirements Level D Simulator. Certificate issued by UK CAA.

3.6.8. Boeing 747-400 Full Flight Simulator (FFS)

The Boeing 747-400 Full Flight Simulator (FFS) located in London (UK) is IAW JAR - FSTD A requirements Level D Simulator for B747400 aircraft training. Certificate issued by UK CAA.

3.6.9. Boeing 757-200 Full Flight Simulator (FFS)

The Boeing 757-200 Full Flight Simulator (FFS) located in Madrid (Spain) is IAW JAR - FSTD A requirements Level DG Simulator for B757-200 aircraft training. Certificates are issued by Spanish DGAC E-1A-018.

3.6.10. Bombardier CRJ-100/200 Full Flight Simulator (FFS)

The Bombardier CRJ-100/200 Full Flight Simulator (FFS) located in Madrid (Spain), is IAW JAR - FSTD A requirements Level D. Certificates are issued by Spanish DGAC.

3.6.11. SAAB 340 Full Flight Simulator (FFS)

The SAAB 340 Full Flight Simulator (FFS) located in Minneapolis (USA) is IAW JAR - FSTD A requirements Level D Simulator for SAAB 340 aircraft training.

4. ConclusionsThe use of flight simulators for initial and recurrent pilot training by major airlines is universal, and its effectiveness is well recognized. However, the availability of flight simulator resources is too limited to meet huge training needs of all airlines due to high purchase cost, deployment cost and per hour usage cost of traditional flight simulator. Airlines are seeking affordable flight simulators, which can cover certain training tasks, to be a kind of substitution for traditional flight simulators. To meet these demands, some efforts have been made to develop low cost flight simulator and similar training devices.

5. Bibliography

1. J.P. Merlet, Parallel Robots 2. http://en.wikipedia.org3. http://www.balticaa.com/en/facilities-and-services/full-flight-simulators-ffs/4. www.waset.org/journals/waset/v36/v36-116.pdf5. citeseerx.ist.psu.edu23