Modeling, control, and analysis of a robotic assist device

This paper presents a model-based approach to control design for an existing lower-limb robotic assist device, the portable powered ankle-foot orthosis (PPAFO). This approach seeks to address two key limitations of the PPAFO caused by the use of solenoid valves: slow system response and inefficient actuation during assistance. System limitations were addressed using a proportional valve coupled with a modified control approach. The two different system configurations were compared in simulation and on an experimental test fixture during motion and torque control tasks. Root mean square (RMS) trajectory tracking error was used to evaluate system performance, while system efficiency was assessed by measuring pneumatic fuel consumed during each task. The proportional valve system reduced RMS tracking error by as much as 91%, and increased efficiency by as much as 95% over the solenoid valve system.