Figure 9. Effects of parametric excitation on the dynamics of a simple “drift and act” controller.
a) Graphical representation of a simple realization of the feedback function that produces a limit cycle oscillation in (2) in the absence of parametric excitation and noisy perturbations, where 
and 
, 
, 
, and 
. The displacement from the upright position, 
, grows when 
and decreases when 
. b) Periodic parametric excitation is turned on at the 
. The effect is to decrease the amplitude of the limit cycle oscillation. Parameters are 
and 
.