  Pneumatic artificial muscles are a quite interesting type of actuators which
have a very high power-to-weight and power-to-volume ratio. However, their
efficient use requires very accurate control methods which can take into
account their complex dynamic, which is highly nonlinear. This paper consider a
model of two-degree-of-freedom platform whose attitude is determined by three
pneumatic muscles controlled by servovalves, which mimics a simplified version
of a Stewart platform. For this testbed, a model-based control approach is
proposed, based on accurate first principle modeling of the muscles and the
platform and on a static model for the servovalve. The employed control method
is the so-called flatness-based control introduced by Fliess. The paper first
recalls the basics of this control technique and then it shows how it can be
applied to the proposed experimental platform; being flatness-based control an
open-loop kind of control, a proportional-integral controller is added on top
of it in order to add robustness with respect to modelling errors and external
perturbations. At the end of the paper, the effectiveness of the proposed
approach is shown by means of experimental results. A clear improvement of the
tracking performance is visible compared to a simple proportional-integral
controller.
