Design and analysis of an actuated seat for cerebral palsy research

The focus of this work was the design and analysis of an actuated seat for use in cerebral palsy research. A conceptual design was produced for the seat which would allow rotations to occur on the pitch and roll axes. The dynamics of the seat were formulated using the Lagrange mechanics and the non-linear equations of motion were derived. Using these equations along with analysis tools for multi-link robot systems, different position-control schemes such as PD or Passivity-based control were explored and the stability of the closed-loop system was analyzed. Finally, a passivity-based control scheme was selected and implemented, and the response of the physical seat using this control was compared to the simulated response. It was found that passivity control produced excellent tracking of a sinusoidal trajectory in simulation, and matched very closely with the experimental response. Next, it was desired to find a clinical method for determining the postural frequency response of subjects who were seated on the actuated seat. Several test subjects were selected, and the roll axis of the seat was used to provide pseudorandom position perturbations to the subject's lower body. Subjects then attempted to maintain a vertical orientation of their upper back by using trunk movements. Optical sensors were used to measure trunk displacements and velocities during this task. These data were analyzed using system identification techniques to compute the frequency response of the subjects.