driving frequency and amplitude, the mean torque of the force of inertia depends only on the angle of the pendulum’s deflection from the direction of the pivot’s vibration.
In the absence of gravity the inertial torque gives a clear physical explanation for existence of the two equivalent stable equilibrium positions that correspond to the two preferable orientations of the pendulum’s rod along the direction of the pivot’s vibration. With gravity, the inverted pendulum is stable with respect to small deviations from this position provided the mean torque of the force of inertia is greater than the torque of the force of gravity that tends to tip the pendulum down. This
occurs when the following condition is fulfilled: a2 ω2 > 2gl, or (a/l)(ω/ω0 ) > √2 (see,
e.g., [20]). However, this is only an approximate criterion for dynamic stability of the
inverted pendulum, which is valid at small amplitudes of forced vibrations of the pivot
(a ฟ l). Below we establish a more precise criterion [see equation (12)].
To provide the dynamic stabilization of the inverted pendulum within some finite
interval of the angles of deflection from the vertical position, the product of the normalized driving amplitude and the normalized driving frequency must be greater
than √2 by a finite value. We note that the explanation of the physical reason for the
dynamic stabilization of the inverted pendulum in [20] is free from the restriction of small angles. In particular, for given values of the driving frequency ω and amplitude a, this approach allows us to find the maximal admissible angular deflection from the inverted vertical position θmax below which the pendulum tends to return to this position, even when θmax is almost as large as π/2: cos θmax = 2gl/(a2 ω2 ). Being deflected from the vertical position by an angle that does not exceed θmax , the pendulum will execute relatively slow oscillations about this inverted position. This motion is executed both under the mean torque of the force of inertia and the force of gravity, and can be described by a slow-varying function ψ(t) satisfying the following approximate differential equation (if friction is ignored):