  The Festo air muscle is today one of the most known commercial version of the
so-called McKibben pneumatic artificial muscle. A major advantage of handmade
McKibben muscles, as well as its commercial versions, lies in the possibility
it offers of realizing antagonist muscle actuator on the model of the
biceps-triceps system. If pressures are independently controlled in each
artificial muscle, it is then possible to define a position-stiffness control
of the antagonist actuator by analogy with natural neural control of antagonist
skeletal muscles. Such a control however requires a knowledge model of the
actuator making possible a stiffness estimation provided by control pressures,
while position closed-loop control is facilitated by a feedforward model of
this highly nonlinear actuation device. We discuss this issue in the particular
case of the antagonist Festo air muscle actuator, and we propose a simplified
static actuator model derived from the classic static theoretical model of the
McKibben artificial muscle, simple enough for a future MIMO position-stiffness
controller be able to integrate it. If the proposed model highlights the
ability of the antagonist Festo muscle actuator to mimic the stiffness neural
control by the mean of the sum of pressures inside artificial muscles, it also
highlights the difficulty to derive a simple but accurate model of the static
force produced by Festo muscle in the full control range of pressure. The
reported results suggest that it would be particularly interesting to derive
advantage from numerous studies about Festo muscle modelling to go further in
order to find such an inverse actuator model including an accurate estimation
of the actuator stiffness.
