Previously, two-wheel wheelchair systems have been receiving attention since they have high mobility better than conventional four-wheel wheelchair systems. However, in two-wheel wheelchair systems, pitch angle must be stabilized by actuated wheels since two-wheel wheelchair systems are underactuated systems. Furthermore, stabilization control needs to be achieved under parameter variation or disturbances considering safety of human. In this paper, two kinds of disturbance observer (DOB) based controllers are compared in terms of stability of inertia variation. One is Lyapunov controller based on pitch angle disturbance observer with DOB. The other is Lyapunov controller based on synthesized pitch angle disturbance observer. Root locus is conducted to analyze stability of inertia variation of these systems. Simulation and experimental results verify the accuracy of this analysis.
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