NEW METHODS FOR INCREASED FIDELITY AND SAFETY OF SIMULATION MOTION PLATFORMS

Desirable performance characteristics of motion platforms used for simulation include high bandwidth, low phase lags and minimal cross-axis coupling. Because people occupy simulators, the need for safety cannot be over-emphasized. Using standard feedback control techniques there is a trade-off between performance and stability, where performance is usually sacrificed for greater stability and safety. Recent advances in real-time computational power coupled with in-depth knowledge of the dynamic and hydraulic characteristics of motion platforms have been used to build a new system architecture. This architecture increases the performance of multidegree of freedom motion platforms without sacrificing safety. The new system includes a model based feedforward path that determines the required servovalve opening for each actuator to achieve the desired motion. A lightly tuned feedback path runs in parallel. Because the feedback path is so softly tuned, control stability is not a concern. This paper explains the basic system strategy and presents data obtained from a recently built six degree of freedom Stewart platform with a table and payload mass of 2500 Ibm. Results show a linear phase in all six degrees of freedom associated with a pure time delay between 9 and 14 ms and less than +/2 db magnitude variation to 10 Hz. In addition, the new method improved the overturning moment disturbance rejection when compared to standard feedback techniques. The pitch response due to a Y input was improved by nearly three times and the roll response due to an X input was almost eliminated showing an improvement of twenty two times. ©1998 MTS Systems Corporation. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission. Introduction Motion platforms are used in simulators to provide motion cues to the occupant. Realistic cues require the platform to reproduce the desired motion with minimum phase lag, high bandwidth and little cross-talk between degrees of freedom. Late, spurious or ill formed motion cues degrade the effectiveness of the simulation and can result in simulator sickness.' In a typical simulator, the desired motion is derived from a vehicle dynamics model using input from the occupant. Because the occupant is in the loop, the motion platform must produce the desired motion with little delay using no look ahead information. In addition, the platform must be completely safe to the occupant. Motion platforms typically use standard feedback techniques which create error signals between the desired and achieved motion and multiply these signals by gains to create commands for the actuating devices. Higher gains result in increased performance but a lower stability margin. Complex multi-degree of freedom platforms contain highly non-linear dynamics which vary as a function of position. Feedback control loops must be tuned softly, resulting in low bandwidth, in order to ensure stability in all positions. The control strategy presented here is a technique that combines the computing power of today's processors with detailed nonlinear dynamic knowledge of the motion platform's structural and hydraulic elements. This control system includes a model based feedforward path that determines the required servovalve opening for each actuator to achieve the desired motion. A lightly tuned feedback path runs in parallel, only to keep the motion platform from drifting from the desired position. Because the feedback path is so softly tuned, control stability is not