This paper presents a stabilization control of inherently unstable systems in their nature. An inverted pendulum which is a typical, unstable mechanical system, is considered and stabilized by moving a weight horizontally through a feedback control. A stabilization problem in this system coresponds to that in the postural control of a man.
It is shown that, so far as the dry friction at the bearings etc. is negligible, this inverted pendulum is stabilizable when the linearized system is controlable and observable. At the experiment, the output, the weight position r and the pendulum angle θ, and the estimated state (r, θ) by a minimum order observer or an approximate differentiator are fed back. Experimental results show that stabilization was posible in spite of the existence of the dry friction at the bearings etc..