The NADS-1 motion system provides motion cues to the driver using a 9 degree-of-freedom (DOF) motion system that responds in real-time to the driver's inputs, with significant complexity in its control, operation, and maintenance. The system diagram below illustrates the major subsystems that work together in real-time to present the simulated environment to the driver. The motion system is physically comprised of the motion control system (washout and DOF control) and the motion subsystem (the machine elements).

Block diagram of the NADS-1 subsystems (Click on the image for a larger version)

The motion control system uses a washout algorithm to transform the 3 specific force and 3 rotational velocity commands generated by the vehicle dynamics system into acceleration commands for each of the motion system's 9 degrees of freedom. The DOF controller uses a three-variable control topology to provide good control bandwidth fidelity at both low and high frequencies by using acceleration and displacement feedback signals, and constructing a velocity feedback channel from these. Four additional high-frequency vibration actuators under the cab provide road-surface-specific vibration cues depending on the surface type, and custom vibration command files can be used to emulate different road surfaces.

There is a continual effort to characterize, improve, and monitor the performance of the motion subsystem, utilizing qualitative and quantitative daily operational tests. The qualitative tests involve daily 'test-drives' by Simulator Operations staff to ensure that all the subsystems operate together as intended, i.e. motion, visuals, and cab. Quantitative tests involve the use of specific file playback files to drive the DOFs individually and in coordinated motion. The systems response is analyzed, and performance measures such as step response, roughness, and backlash are calculated. These measures are tracked over time to allow identification of possible trends indicating a change in system characteristics. Transfer functions for each DOF are also routinely generated, to ensure the flattest response possible for the bandwidth in question.

The motion system supports several active areas of research:

Selected references:
Two mitigation strategies for motion system limits,
Schwarz, C., Driving Simulation Conference-North America, 2005

Motion characteristics of the National Advanced Driving Simulator,
Schwarz, C., Gates, T., Papelis, Y., Driving Simulation
Conference-North America, 2003