OPTIMAL-CONTROL OF ANKLE JOINT MOMENT - TOWARD UNSUPPORTED STANDING IN PARAPLEGIA

This paper considers part of the problem of how to provide unsupported standing for paraplegics by feedback control. In this work our overal l objective is to stabilize the subject by stimulation only of his ank le joints while the other joints are braced, Here, we investigate the problem of ankle joint moment control. The ankle plantarflexion muscle s are first identified with pseudorandom binary sequence (PRBS) signal s, periodic sinusoidal signals, and twitches. The muscle is modeled in Hammerstein form as a static recruitment nonlinearity followed by a l inear transfer function. A linear-quadratic-Gaussian (LQG)-optimal con troller design procedure for ankle joint moment was proposed based on the polynomial equation formulation, The approach was verified by expe riments in the special Wobbler apparatus with a neurologically intact subject, and these experimental results are reported. The controller s tructure is formulated in such a way that there are only two scalar de sign parameters, each of which has a clear physical interpretation. Th is facilitates fast controller synthesis and tuning in the laboratory environment. Experimental results show the effects of the controller t uning parameters: the control weighting and the observer response time , which determine closed-loop properties. Using these two parameters t he tradeoff between disturbance rejection and measurement noise sensit ivity can be straightforwardly balanced while maintaining a desired sp eed of tracking. The experimentally measured reference tracking, distu rbance rejection, and noise sensitivity are good and agree with theore tical expectations.