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April 2000 Tech Review

Motion Simulation

By Jerry Isdale

When I covered motion platforms last year, I lamented the lack of good reference articles on motion simulation. This year I am pleased to report there were two informative and well written articles on motion systems presented at IEEE VR2000 in late March. Hauro Noma, Toshiaki Sugihara and Tsutomu Miyasato of ATR Media Integration and Communications Research Labs (Japan) presented "Development of Ground Surface Simulator for Tel-E-Merge System". Zen-Chung Shih and Yuh-Sen Jaw (National Chiao Tung Univ. Tawan, R.O.C.) and Mei-Ling Hsu (National Center of High Performance Computing Tawan, R.O.C.) presented "Virtual Roller Coaster". Both articles are in the conference proceedings which should be available in the IEEE Digital Library soon. I will discuss these articles a bit in a moment, but first we will review some motion platform background and check up on some systems I mentioned last year.

Human motion sensing is based primarily on the visual and vestibular systems with some contribution by forces on the rest of the body. One important observation has been that impulse or changes to acceleration is often more important than sustained acceleration, particularly in combination with visual cues. This leads to the extensive use of "onset cues" or short bursts of acceleration followed by a gentle reduction in force known as "washout". This technique allows a device with a relatively small range of motion to work effectively. The forces on the rider are the sum of gravity and the platform accelerations. Some sustained forces can be achieved using gravity on a tilted platform. When a sustained force is desired, a centrifuge or very large linear device is required.

Motion Platforms are often characterized by the range of motion, load capacity, degrees of freedom (DOF) , and types of actuators used to move the platform. Single DOF linear motion systems have been used as a 'seat shaker' to give some vibration sense, although acoustic systems are more often used for this purpose. A 6 DOF motion platform provides limited linear (x/y/z )and rotational (roll/pitch/yaw ) motion. A common configuration for these systems is the "Stewart Platform" or "hexapod", originally described by D. Stewart (UK Institution of Mechanical Engineers Proceedings 1965-66 Vol. 180, Pt1, No 15). This is a frame with six or more extendable legs (actuators) connecting a fixed base to a moveable platform. An "inverted hexapod" secures the upper platform to a frame and allows the lower base to be moved. The robotics and nanotechnology folks have published a fair bit on these devices, although most of the information is on positioning control and not motion simulation.

The most common types of actuators are pneumatic, electrical, and hydraulic. Hydraulic systems are more powerful and accurate but also more costly. Pneumatic and Hydraulic systems require compressors that may be noisy. Electric systems are quiet but not nearly as responsive.

Last year I reported that the US National Advanced Driving Simulator (NADS) would soon be the world's largest motion platform, surpassing NASA's Vertical Motion Simulator. Well NADS is still under construction and now scheduled to be operational in October, 2000. The system being built at University of Iowa by the National Highway Traffic Safety Administration (NHTSA), combines a large hexapod platform with a 64ft x 64ft X-Y motion table. The platform will enable entire vehicles to be placed inside the projection dome on top of the hexapod platform. This should provide unrivaled fidelity in land vehicle simulation.

There were two homebrew flight simulator projects with motion platforms reported in last year's column. These two systems by Klaus Schmidinger in Germany and John Dunkley in Australia are still active although without a lot of progress. Klaus ran into some construction problems but recently got things back on track. John retired from the Dept of Defense this past year and managed to do a a considerable amount of research into motion platforms for his simulator. He also built a large shed (aka The Hanger) to in his backyard to house the simulator.

Now back to the VR2000 articles

The ATR article (7 pages) is an excellent design study over three generations of walking motion simulators. ATLAS (ATR Locomotion Interface for Active Self Motion) uses a motorized treadmill on a 3DOF (roll, pitch, yaw) motion platform. An infrared vision subsystem tracking the user's feet helps estimate true walking speed and direction changes. The motion platform can cancel direction changes with an appropriate yaw rotation. The platform can also simulate inclined terrain with roll and pitch rotation. The ALF (Alive Floor) system simulates a terrain surface using a matrix of equilateral triangle plates. The plates are grouped into hexagonal drive units, with a vertical actuator below the common vertex and hinges on the peripheral edge. As the actuator travels through its 100mm stroke it raises or lowers the center vertex of the six tiles in the hexagon, creating bumps and dents in the terrain. The Ground Surface Simulator (GSS) combines a treadmill with an surface simulator to provide a more complete simulation. The supporting stage of the belt is divided into six sub-stages that can move independently up to 6cm vertically. This provides simulation of bumps and inclines, however the GSS is not mounted on a rotation platform so it can not handle the turning simulation of ATLAS. The ATR article also provides a good survey of related walking motion simulation projects.

The ATR locomotion projects are part of the larger "Tel-E-Merge" project. The goal of which is to merge a remotely located person (a tele-visitor) into a tel-inviters space. The Tele-visitor would use a locomotion simulator equipped VR system, while a robot (AIR: ATR Imaging Robot) would provide her avatar at the remote location. The thesis is that when people walk together, there is a large amount of unconscious synchronization and interaction. Simulating the walking and providing a robotic avatar will enhance the tele-presence experience. I think it is very intriguing if a bit spooky.

The Virtual Roller Coaster article (8 pages) is a very different writing style than the ATR article. It provides a mathematical discussion of the physics of roller coasters and some details of the control system used. The authors discuss the motion platform options, including the venerable stewart platform (hexapod), but chose a novel design that provides two rotational axis (roll and pitch). The platform is a chair designed to fit in a relatively small space, allow modular component replacement, and provide low cost, easy construction and maintenance. I have not reviewed the math, but the article gives several good references on design and control of stewart platforms.