3. A normal pendulum subject to a moving pivot point such as a load lifted by a crane, has a peaked response at the pendulum radian frequency of omega_p = sqrt {g/ell} . To prevent uncontrolled swinging, the frequency spectrum of the pivot motion should be suppressed near omega_p . The inverted pendulum requires the same band stop filter to achieve stability.
Note that, as a consequence of the null angle modulation strategy, the position feedback is positive, that is, a sudden command to move right will produce an initial cart motion to the left followed by a move right to rebalance the pendulum. The interaction of the pendulum instability and the positive position feedback instability to produce a stable system is a feature that makes the mathematical analysis an interesting and challenging problem.