Foot rotation indicator (FRI) point: a new gait planning tool to evaluate postural stability of biped robots

  • Goswami A
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The focus of the paper is the problem of foot rotation in biped
robots during the single support phase. Foot rotation is an indication
of postural instability and should be carefully treated in a dynamically
stable walk and avoided altogether in a statically stable walk. We
introduce the foot rotation indicator (FRI) point which is a point on
the foot/ground contact surface where the net ground reaction force
would have to act to keep the foot stationary. To ensure no foot
rotation, the FRI point must remain within the convex hull of the foot
support area. In contrast with the ground projection of the center of
mass (GCoM), which is a static criterion, the FRI point incorporates the
robot dynamics. As opposed to the center of pressure (CoP)-better known
as the zero moment point (ZMP) in the robotics literature-which may not
leave the support area, the FRI point may. Due to these important
properties the FRI point helps not only to monitor the state of static
stability of a biped robot during the entire gait cycle, but indicates
the stability robustness or the severity of instability of the gait as
well. In response to a recent need the paper also resolves the
misconceptions surrounding the CoP/ZMP equivalence

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  • A. Goswami

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