Experiment on closed-loop subspace model identification of an unstable underactuated system

We report an experimental result on closed-loop subspace model identification of a cart-inverted pendulum system. Such a system as mentioned here is known as a typical unstable underactuated system, which is impossible of open-loop identification. Furthermore, any conventional SISO identification is not applicable to this system since this system is of single input and two outputs. Therefore, in this case, closed-loop subspace model identification is the only way of identification. Bode diagrams will show that there is fairly good agreement between an identified model obtained from closed-loop subspace identification and an initial model used for a stabilizing feedback controller at the frequency important for control of this inverted pendulum.