Each motion base provides six axes of motion through electrohydraulic cylinders controlled by servovalves and fitted with magnetostrictive linear displacement transducers for closed-loop position and motion control. As with the Star Trek attraction, motion bases have six cylinders arranged into a tripod configuration. In fact, motion bases for both the Atlantis and Star Trek attractions were designed and built by McFadden Systems, Santa Fe Springs, Calif.

Each cylinder is controlled by a servovalve mounted to it. This design configuration minimizes the amount of fluid between the servovalve and cylinder, which, in turn, maximizes system response by limiting fluid compliance between the servovalve and cylinder.

An accumulator serves each pair of cylinders to ensure that flow and pressure can be maintained by each cylinder. Accumulators also are relied upon by the hydraulic power unit (HPU) to store energy for peak flow demands. This allows using a smaller pump because the accumulators can be recharged during periods of low demand and release their stored energy when the system needs more flow than the pump can provide.

A third use of accumulators is for storing emergency power. Visitors board each motion base from a moveable loading/unloading platform. Before a ride sequence can begin, a pair of hydraulic cylinders pivots each loading platform out of the way. This prevents a collision from damaging the motion base or platform.

If, for example, an electrical power outage would occur, the HPU might not be able to power the cylinders into position to unload the motion bases. To prevent this from happening, hydraulic circuits for the platforms contain accumulators to store enough energy to fully actuate the cylinders, even if a complete electrical power outage occurs.

Application challenges

One of the challenges for the motion program, according to Brian Robison, general manager of Race for Atlantis, was the wide range of effects. For example, because some of the simulated adventure takes place in water, much of the motion has to give visitors the sensation of floating. This means when motion changes from up to down, and vise versa, there cannot be any jerking or stick-slip operation in the transition between directional changes. At other times, the motion base must behave like a chariot racing down a cobblestone street. This calls for sharp jolts and a rumble of wheels against the road surface.

Robison cited control signal management as another challenge. The main program takes up 1716 GB of hard drive space. The projector itself, with a pair of 15-kW, water-cooled xenon arc lamps, requires its own computer to monitor operation and generate the cues for the headsets, motion bases, safety systems, and other subsystems.

Search leads to simple solution

One of the tradeoffs often considered for motion base applications is how much motion is actually required. After all, if a visual presentation is large enough to encompass a viewer's entire field of vision, he or she will usually get a sense of motion even while standing on a stationary platform. Also, as motion bases incorporate more axes of motion, more complicated and sophisticate control equipment is needed to keep a motion sequence in synchronization not only with the visual presentation, but with itself. For instance, a 6-axis motion base arranged in a tripod configuration requires all six cylinders to move in close synchronization with each other, even for simple movements.

If motion is not in sync with the visual program, participants may experience motion sickness or become aware of the control error. In either case, visitors to such an attraction may leave unsatisfied. If motion of individual cylinders is out of sync, not only will the quality of the motion suffer, but the out-of-balance forces can apply excessive stresses to the motion base chassis.

In contrast to the motion bases for Star Trek and Atlantis, In Search of the Obelisk use only three axes of motion surge (34 in.), heave (18 in.), and sway (34 in.). John Lichtsteiner, technical manager, rides and attractions at the Luxor, explained that controlling only three axes of motion is much less complicated than controlling six. Furthermore, because each axis actuates independently, control programs are simpler.

For example, to execute a horizontal movement (sway) with a 6-axis motion base configured in a tripod arrangement, piston rods on all six cylinders would have to move. Some would extend, some would retract, and not all would stroke at the same speed. With a 3-axis base configured for linear motion, movement in any single axis requires only one cylinder to actuate. So the same horizontal move would require only the sway cylinder to actuate.

Lichtsteiner also explained that using only three axes of motion does not necessarily reduce the quality of perceived motion. He said the roll axis can be experienced simply by having the visual image pivot one way while the motion base moves horizontally in sync with it. He added that with linear motions, all riders experience the same movement. If a 6-axis motion platform rolls to the right, visitors near the center of the platform will simply tilt, whereas those near the perimeter will tilt and mover vertically. With linear motions, all visitors experience the same sensations no matter where they are seated.