Robust Stability of Human Balance Keeping

Despite its apparent simplicity, the nature of the control mechanisms that allow humans to stand up is still an object of great research interest [1-9]. In a recent paper [1], a PID controller with a sensory delay is proposed to model the central nervous system for the control of balance keeping; and based on experiment data it is suggested that ageing people with poor balance keeping ability can be explained by a reduced derivative gain in the PID controller. Using the models presented in [1], this paper studies a further topic: robust stability of human balance keeping. Computation methods to find the ranges of the controller parameters for which the closed loop system is stable, and with a given stability phase or gain margin, are presented. An example is used in the paper to demonstrate the application of these methods.