Towards a “Brain-Guided” Cognitive Architecture - Bioinspired Approaches for Human-Centric Technologies - page 212

An example is whole-body reaching while standing. A biomimetic approach,
based on PMP, for synergy formation of whole-body movements in humanoid
robots is described by Morasso et al. ( 2010 ). It is supposed to combine a double
task: (1) a focal task (reaching or approaching as much as possible a target in 3D
space) and (2) a postural task (keeping the vertical projection of the center of mass
inside the support base of the standing body). The synergy formation mechanism
uses two force fields applied to the body schema: one linked to the hands for the
focal part and the other linked to the pelvis for the postural part, thus implementing
a hip strategy of stabilization. Remarkably, the simulated patterns generated by the
model are consistent with distinctive aspects of human behavior for this kind of
task, namely, the synchronized velocity peaks of the reaching hand and the forward
shift of the center of mass. However, this PMP-based mechanism is massless and is
not yet a control system because it does not provide specific stabilization signals of
the inverted pendulum which, at least approximately, represents the standing body.
The intrinsic instability of the inverted pendulum model is due to the fact that the
rate of growth of the gravity-related toppling torque is greater than the stiffness of
the critical joint involved in the stabilization of the standing body, namely, the
ankle. Therefore, a controller is needed for providing ankle torque control signals
that stabilize the inverted pendulum. A continuous-time PD feedback controller
applied to the ankle does not work because the delay of the feedback signals (sway
angle and sway speed) becomes itself a source of instability. However, such delay-
induced instability can be avoided by means of an intermittent controller (Asai
et al. 2009 ), which closes the loop according to a decision mechanism based on the
analysis of the trajectories of the inverted pendulum in the phase space: this
mechanism achieves bounded stability, consistent with the recorded sway move-
ments of the standing body, in a robust way. A recent paper (Morasso et al. 2013 )
demonstrates the feasibility of extending the intermittent controller from quiet
standing to dynamic standing. It integrates in a bidirectional manner the PMP
synergy formation mechanism, which generates time-varying reference joint rota-
tions, with the intermittent controller which switches on/off the feedback control
law according to the current state of the pendulum. In other situations, as in
grabbing/pushing in which there is a physical interaction, the control part of the
synergy might be more concerned with a modulation of the end-effector stiffness, in
order to take into account task-dependent features like fragility of the manipulated
objects. In any case, stiffness modulation requires, as a prerequisite, the selection
and real-time adjustment of appropriate body postures that can be naturally pro-
vided by the animated body schema.