The Design of a High-Performance All-Composite Flight Simulator Motion Platform

Sunjoo K. Advani' , Michel van Tooren2 and Stefaan de ~ i n t e ?

International Centre for Research in Simulation, Motion and Navigation Technologies SIMONA

Delft University of Technology Kluyverweg I, 2629 HS Delft

The Netherlands

Keywords: flight simulators, simulator motion systems, hardware standardization, simulator flight decks, high performance motion systems, composite materials

Abstract The design principles used for the development of a flight simulator motion platform built entirely from non-metallic composite materials are discussed in this paper. The goals of this research simulator, namely to improve simulation techniques such as math modelling, and to identify human motion perception processes, require that the moving elements be kept both low in mass and high in rigidity. These factors contribute significantly to the maximum level of fidelity which can be achieved by a six- degrees-of-freedom motion system. The configuration developed applies advanced materials, and also new techniques for the integration of all on-board systems to reduce their contributions to the system mass properties. The flight-deck shell, made from aramidlcarbon-fibre laminates, features a large frameless windscreen and an interior capable of hosting a number of research workstations representing transport aircraft, helicopters and road vehicles. This strategy allows the interiors, including flight instruments and controls of specific vehicle types to be integrated within the standardized structural volume. The light-weight collimation display system is supported by an aramid-carbon space frame construction attached to the flight deck. With the motion system directly attached to the flight-deck shell, the inertial properties are kept to a minimum. 1 Assistant Professor, SIMONA Program Manager,

Member AlAA 2 8 3 Project Engineer

Copyright @ 1995 by S.K. Advani. Published by the American Institute of Aeronautics and Astronautics, Inc.

with permission

This simulator, with a gross moving mass of 2853 kilograms and a minimum natural frequency of 17.5 Hertz, provides a high- performance modular design concept suitable for the next generation of cost-effective simulators. The first prototype, now under construction, is expected to be operational in 1996. When used in conjunction with new motion control strategies also under development, this simulator shall be used to establish new standards for motion cueing in flight simulators. This research is conducted under the framework of the lnternational Centre for Research in Simulation, Motion and Navigation Technologies SIMONA of the Delft University of Technology.

Introduction The essential purpose of piloted flight simulators is to reproduce the physical (vestibular, visual, aural) and environmental (aircraft systems, instruments) cues as closely as possible to those encountered in flight. These inputs result from control activity by the pilot, and external disturbances. The pilot-vehicle interaction forms a closed loop in which the pilot receives information about the state, and change in state of the vehicle, through human sensory organs. Visual information is obtained through the foveal and central vision, while motion cues are derived from the visual and the vestibular system. These stimuli are processed by the brain, which can be thought of as to compare all the signals to an internal model, developed through the encountering of perception and

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action, particularly (but not exclusively) in flying related tasks. In order for the response of two "similarn systems to match (hence, implying the fidelity of the simulation), the input signals must correspond with the aircraft specific forces and angular accelerations' at the pilot head position. Then, the true value of training can be maximized.

In human control tasks, visual information requires longer processing time than the vestibular cues. In other words, the onset of motion is primarily detected by the vestibular system, while long-term changes in motion are

2 sensed predominately by the eyes . This requires that the initial response of the simulator motion system matches that of the vehicle being simulated. This "onset cue" often includes high- frequency components to which the human operator responds during control tasks. Ideally, the latency of the simulator (above that of the vehicle dynamics) should be minimal to yield a favourable phase lag over the entire frequency range of the vehicle motion. In practice, this is not easily achieved, mainly due to the limitations in the mechanical hardware and motion control software which induce the sensation of motions to the pilots. Host processor speed and vehicle model complexity also limit the update rate of the state vectors.

The present study focuses on the "motion platform" alone, comprised of the load-bearing structural frame and occupants area (cockpit and instructor station), and suggests means to improve its inherent properties through the application of composite materials in a light- weight, rigid, load bearing structure.

Motivation The representation of smooth accelerations, and sharp vibrations are both requirements in flight simulation. The latter requires stability in the control feedback system, which includes the motion platform. As increasingly complex mathematical models can be cycled through higher performance digital host computers, attention focuses on the motion system itself in order to maintain on overall high level of physical performance. High-frequency ground contact dynamics, flight through turbulence with the presence of structural flexibilities and vibrations are all issues which influence aircraft handling qualities. Representing these in training

simulators with a high level of fidelity will only be possible when the overall system, including the motion system and platform can manage these phenomena.

The SIMONA Research Simulator is used to demonstrate new technologies in manned vehicle simulation, as well as for human erception and man-machine interface research ' 4. For research into human factors, if is critical

that the desired motion Cues are not polluted by parasitic noises. Therefore, every measure has been taken to develop a high-performance motion system for SIMONA, and also to demonstrate forthcoming technological trends in flight simulation.

Motion System Frequency Response and its Value on Simulation From the point of view of the simulator designer, it is the specific forces and angular accelerations which need be presented as motion cues to the pilot (rather than absolute magnitudes of acceleration) . Due to the high-frequency nature of the human vestibular sensors however, the quality of the responses, particularly their temporal characteristics such as delay and phase, should be reproduced with little deviation from reality. The critical requirement of a motion system is therefore to provide a given acceleration at the pilot position such that the onset of this motion is presented with a minimal delay. This should be made possible over a wide frequency range, well beyond those fundamentally associated with the vehicle dynamics.

The pilot position experiences frequencies through the audible range, however for the purposes of motion systems, one should try to address a cut-off frequency of approximately 15 Hz for most vehicles. It can be shown from flight tests that aircraft encounter excitations with frequency components in excess of 15 Hz. These can lead to sharp accelerations at the pilot position.

Current Standards: Simulator Performance Current simulator requirements, such as those established by the United States Federal Aviation Administration, stipulate that the motion response should precede all others, and that the total latency not exceed 150 milliseconds5 (for Level-D training simulators). It is the aim of the

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present research to significantly reduce this figure, and maintain a very high level of fidelity with the required responses.

Current Techniques: Large Motion Plaffoms Commercially-available flight simulators of today fail to take full advantage of potential design concepts, including the use of composite materials in the primary load-bearing structures. This is usually due to the relatively large-scale approach taken to the packaging of on-board equipment, requiring a large volume and leading to a substantial mass of the platform. In the case of traditional flight training simulator designs, there in fact would be little advantage to the use of composite materials, other than for some mass reduction, ease of manufacture, and modernized appearances to non-critical elements (fairings, display system elements, etc.). The wholesale use of composites, necessitated by the need to improve the overall performance of the simulator motion system, represents more than a replacement of steel by non-metallics, and thus requires a paradigm shift in design and production techniques.

Motion Platfonn Inertial Prowflies and their Effect on Controllability The motion platform inertial properties should afford a minimal level of required compensation from the mechanical motion system and its control feedback system. It can be shown that the dynamic load variations on actuators can vary by an order of magnitude6, and these variations can limit the eventual performance of the entire system. Therefore, a practical means of reducing the effective actuator loads should be sought. The eventual solution should also provide a platform with a stiffness greater than or equal to the control system frequency range, in order that the maximum performance is achieved. Note also that the actuators themselves should have inherent natural frequencies in this range.

At first glance, it may appear that larger simulator payload masses require only additional power from the actuators to achieve the same level of performance as a low-mass alternative. During a parametric study to investigate the sensitivity of the moving platform to variations in payload mass properties, it was shown that power alone cannot compensate for dynamic performance, especially when high

frequencies are desired. In this study, the modelled response of a simulator platform resulting from step accelerations in the surge (X) direction were shown, while the total platform mass was varied between 2000 and 18000 kilograms. (Note that a typical full flight simulator may have a mass up to 18000 kg). With increased mass, a decrease in the system natural frequency, and increase in damping, were noted as would be expected, results from increasing payload. Most notably, the onset of the motion is delayed; as payloads increase, the desired acceleration level is delayed significantly.

Secondly, in the same study, variations to the centre of mass location on the surge response showed a simultaneous decrease in both the damping and the natural frequency. This rate of decrease appears can be attributed to the virtual mass (and not the real mass), caused by the dynamic load on the actuators. Unlike a true increase in the gross moving mass, the increase in the vertical offset in the centre of gravity increases a moment coupling effect, hence increasing the load on the actuators.

Reductions in the platform mass and vertical location of the centre of mass can better the inherent characteristics of the motion system. The system natural frequency in particular appears to benefit directly from these properties.

Motion Platform Stiffness and its Effect on Hiqh- Frequency Stability A hydraulic actuator control scheme making use of robust pressure-based control can be shown to improve the bandwidth of the servo system8. This concept incorporates a feedback system which requires an actuator velocity feedback signal (in addition to the conventional position and acceleration signals). Actuator stability is achieved through careful selection of the servo valve, reducing the lengths of transmission lines between valve and actuator orifices, and measuring the pressure differential at the actuator orifices, instead of at the servo orifices. The fourth stability requirement to this approach calls for high stiffness in the motion platform, so that the high-gain feedback system does not encounter high-frequency oscillations.

The aforementioned technique also calls for a multi-variable motion system control scheme,

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which compensates for total platform dynamics. Once more, the amount of compensation can be reduced by minimizing the dynamic load exposed to the actuatorsg. Note that this aforementioned scheme, developed for SIMONA, will be integrated with the foregoing work.

Towards High-Performance Motion Platforms As a result of all of the above, in order to maximize the motion performance of the entire simulator, it is essential to maximize the natural properties of the payload-carrying motion platform. Namely, the mass distribution of the gross moving load, and stiffness of the platform system should be optimized.

A motion platform must sustain the occupants and equipment, and transfer the loads from the motion system actuators to the simulator cab. The occupants may include the pilot(s), instructors or experiment controller, and observers. The equipment payload will usually consist of the aircraft or aircraft-like instruments, displays, the controls and loading devices, electronic equipment required for the simulation, computer systems, and the visual display system. Modern simulators often employ wide- angle collimating display systems which use off- axis projectors mounted above the cab, presenting the image onto a spherical-section mirror, and via a rear projection screen. A result of this solution however is a high contribution to the mass moment of inertia, while requiring a stiff and often heavy mounting structure.

Traditionally, simulators are constructed in a layered fashion, beginning with a fairly rigid platform structure mounted atop the motion system gimbals. This platform then supports the cockpit replica with its interior, instruments, displays and controls. Behind the cockpit, an instructor cabin is placed where the instructor station and observers' chairs are mounted. Above this cabin, the display system projectors are mounted. The mirror structure is mounted directly to the platform at floor height.

Composite-Materials "Shuttle": An Integrated Motion Plafform Concept The candidate design of the SIMONA Research Simulator SRS integrated platform/flight deck, also called the "Shuttle", was developed through an iterative process involving the interior and

structural requirements, as well as the production issues. The SRS is not intended for training; therefore a number of the on-board systems can be simplified or neglected, reducing the total volume and payload mass requirements7.

In efforts to reduce significantly the moment of inertia of the platform, a solution has been developed whereby the internal volume requirements are first defined, the constraints of the motion and visual display system established (so that no structure blocks the light beams or interferes with the motion system in any actuator position), and a structural shell placed around the occupants and internal equipment. An emphasis was placed on reducing the mass while maintaining sufficient stiffness of the entire system. Finite element analyses showed that a minimum natural frequency of 17.5 Hz could be achieved for the required configuration; a higher stiffness requiring increasingly more material, thereby raising the mass as well as cost. Lowering the centre of gravity was also considered a crucial design objective.

The resulting natural frequencies of the platform structural dynamics exceed those of the rigid and relevant aeroelastic aircraft dynamics. Furthermore, the small cockpit will place the pilot very close to the centroid of the upper gimbals of the motion system. In this way, maximum use of the pure pitch and roll capabilities of the motion system are provided to the pilot. It is also possible to create a large window free of any reinforcing frames, while still meeting the aforementioned structural stiffness requirements. This allows a variety of cockpit interiors to be installed, and cosmetic window frames inserted if necessary.

Structural Analysis The design of a simulator motion platform is not a strength-driven process; instead the stiffness and, particularly in this case, the stiffness-to- weight considerations predominate. Design iterations were checked with a Finite Element Analysis to determine the structural natural frequency. The desired 20 Hertz minima could have been achieved, however with severe mass penalties. As a result, a 17.5 Hertz theoretical natural frequency was accepted. The final design is shown in Figure 1.

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The stress distribution around the motion system gimbals required careful attention, to distribute the load in to the shuttle skin and beam structures. The mechanical gimbals attach to the curved shuttle structure via large flat mounting plates. A localized flat is present in the negative mold, and the flat surface is laminated into the shuttle outer surface. The gimbal loads are then carried from the plates to adjoining structural beams located internally.

The major load-bearing contribution in the SRS shuttle is due to the visual display system, particularly the projectors. Typical masses for these devices, including their amplifier electronics boxes, range from 150 to 250 kilograms. Three are required for the SRS to give a continuous collimated field of view of 40" by 180". A carbon-epoxy sandwich projector platform, supported by a space truss of carbon tubes, is mounted above the shuttle. Circumferal beams in the shuttle transfer the projector load directly to the motion system gimbal plates. A similar space truss also supports the back projection screen and mirror cell of the visual display system.

Shuttle Construction The SRS Shuttle, Figure 2, was constructed in a female mold using hand lay-up techniques. Low temperature curing epoxies were selected due to their favourable cost over pre-impregnated autoclave-cured materials. The shell is comprised of a sandwich structure of aramid/Nomex/Aramid. Stiffening frames, longerons and floor beams are formed by taminating pre-shaped foam blocks with carbon strips. Cut-outs in the floor beams allow the passage of hydraulic lines (for control loading), electrical cabling, and air conditioning piping. Reinforced hard points house blind nuts for future installations including control loading devices, instrument panel frame supports, and floor panels. These panels, constructed from a carbon/Nomex/carbon sandwich, are like tiles which bolt to the floor frames. The crew seats, throttle pedestal, and instructor station are subsequently attached to these panels.

Interior Desian Intended for fundamental research into simulation techniques, human perception and man-machine interface design, the SRS interior was developed with a maximum of flexibility in

mind. Note that the operator area volumes of a number of vehicles, including large transport aircraft, helicopters and automobiles, were used to define the minimum interior volume of the SRS flight deck. This would allow the capability to emulate a number of vehicle categories, as well as vehicle types.

Another requirement is the ability to reconfigure the interior within two to four man-days. With the aid of a full-scale wooden mock-up, the interiors could be prototyped and checked for suitable clearance. With this mock-up, the volumes of controls, control loading devices, loudspeakers, cabling, ducting, etc. could be initially checked. In a later stage, Computer Aided Design packages were used to accurately define the interiors and confirm sufficient physical clearances and visual lines of sight.

Figure 3 shows the forward section of the SRS interior with the transport airplane research equipment installed. Four CRT glass cockpit displays are shown, and control columns represent the Boeing 747 (left) and the Fokker 100 (right), to evaluate human interfaces with these. The seats shown are from the McDonnell Douglas MD-11.

In the aft section of the simulator, a small space is reserved for a compact Experiment Control Station. This will allow one operator to control simulation runs and guide experiments from inside the flight deck. These functions will also be available from the main control room.

Displa v System Interface The aramidlcarbon fibre cockpit is equipped with carbon-fibre outrigger rods which accept the visual display system structure, Figure 4. These rods are joined at their ends by aluminium "atom" balls which accept threaded aluminium cones, bonded to the carbon tubes.

Light-tighting is necessary in these types of displays to prevent ambient light from interfering with the display system optics. In the case of the SRS, a black sail cloth is attached to the periphery of the mirror structure, back projection screen and projector forward faces. This is attached to the outrigger tubes to prevent flutter.

Wide-angle cross-cockpit viewable collimation displays such as that used on the SRS provide a

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major contribution to the inertial properties of the gross moving load. Traditionally, a thin aluminized mylar film, supported inside a spherical section structure and tightened by a partial vacuum, provides the reflective surface. When viewed from within the cockpit, a collimated impression of the image displayed on the back projection screen is presented, yielding an optical quality resembling flight operations. The vibration characteristics of these thin films in the presence of high-frequency motion inputs is of considerable interest in the regime of high- bandwidth motion cues, and will be investigated in the SRS. As part of a long-term development program, SIMONA is considering new light- weight solid mirrors as alternatives to the standard aluminized mylar mirror due to these resonance and mass concerns. The mass breakdown of the SRS is given in Table 1.

Conclusions The application of composite materials in the integrated design of the SIMONA Research Simulator is expected to yield a new standard of motion performance. Results to date show that a reduced mass contributes to improved controllability of the motion platform, and that high stiffness of the gross moving load allows the application of pressure feedback control and robust control strategies while maintaining system stability throughout the operational bandwidth. Moreover, in combination with the control schemes proposed, it is expected that high bandwidth motion cueing will be presented to the pilots in his simulator.

When completed in 1996, the functional cockpit of the SRS will allow fundamental research into human perception processes, interactions with displays, flight control concepts, and flight dynamics models. Experiments, such as investigations into the human perception systems, will not be polluted parasitic errors, thereby yielding new opportunities for future work in this area. High-frequency math models of aircraft dynamics will be demonstrated in this responsive simulator system.

The integrated SRS system is shown in Figure 5, the development of which has in itself been an exercise in applying all facets of simulator technology, and new applications of materials sciences, control theories and design integration.

Acknowledgemenl The Netherlands Ministry of Education, Science and Culture, and the SIMONA Industrial and Scientific Partners are thanked for their financial support of the SIMONA programme.

Development of the SIMONA Research Simulator, managed by the principal author of this paper, is carried out by the staff and students of the Delft University of Technology. The SIMONA "Shuttle" motion platform was developed in a collaboration involving the Discipline Group for Stability and Control, and the Structures and Materials Laboratory of the Faculty of Aerospace Engineering. It was built by Polymarin b.v., and with additional financial assistance from the AKZO Corporation.

The SIMONA motion system is developed jointly by the Faculty of Aerospace Engineering and the Faculty of Mechanical Engineering who, together with the Faculty of Electrical Engineering, form the core team of SIMONA.

References 1. Hosrnan, R.J.A.W. and Steen, H. van der, "False Cue Detection Thresholds in Flight Simulation". AIAA-93-35-CP. From AlAA Flight Simulation Technologies Conference, Monterey, August, 1993.

2. Vaart, J.C. van der, "Modelling of Perception and Action in Compensatory Manual Control Tasks". Ph.D. thesis, Delft University of Technology, December 1992.

3. Advani, S.K., "The Development of SIMONA: A Simulator Facility for Advanced Research into Simulation Techniques, Motion System Control and Navigation Systems Technologies". AIAA- 93-3574-CP. From AlAA Flight Simulation Technologies Conference, Monterey, August, 1993.

4. Advani, S.K., et al, "What Optical Cues do Pilots Use to Initiate the Landing Flare? Results from a Simulator Experiment". AIAA-93-3568- CP, From AlAA Flight Simulation Technologies Conference, Monterey, August, 1993.

5. Advisory Circular l20-4OC, United States Department of Transportation, Federal Aviation Administration, Draft, January, 1995.

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6. Levi, R.W., and Hayashigawa, L., "Specification Considerations for a Small Motion-Base". From A I M Flight Simulation Technologies Conference, Atlanta, 1988.

7. Advani, S.K., and Verbeek, R.J., "The Influence of Platform Inertial Properties on Simulator Motion System Performance". AIAA- 94-3418-CP. From AlAA Flight Simulation Technologies Conference, Scottsdale, August, 1994.

8. Schothorst, G. van, Teerhuis, P.C., Weiden, A.J.J. van der, "Stability Analysis of a Hydraulic Servo-System Including Transmission Line Effects". From Proceedings of International Conference on Automation, Robotics and Computer Vision, Singapore, November 1994.

9. Advani, S.K. and Mulder, J.A., "Achieving High-Fidelity Motion Cues in Flight Simulation". From Proceedings of the AGARD Flight Vehicle Integration Panel Symposium 'Flight Simulation - Where are the Challenges'. Braunschweig, Germany, May 1995.

Item I Component Mass (kg) Shuttle structure 1 815

I Structural fittings, fasteners ( 495 I

Table 1. Component mass breakdown of SlMONA Research Simulator gross moving payload

Space truss structure, light tighting

Figure 1. Motion Platform "shuttle" of SlMONA Research Simulator. Shell is an aramid-epoxy laminate, while structural frames use carbon-epoxy laminates.

Interior, electronics racks, controls Two pilots, two observers (80 kg each) Total

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463 320 2853

Figure 2. Completed SRS shuttle shell, prior to finishing and mounting. Photo courtesy Polymarin b.v.

Figure 3. interior of airplane a controls a

28 1

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then carried directly to motion system aimbals.

Figure 5. Completed (artist's impression). F of flight compartment attachments.

SIMONA Research Simulator dote relatively low placement with respect to motion system

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