  Motion planning for a general 2-trailer system poses a hard problem for any
motion planning algorithm and previous methods have lacked any completeness or
optimality guarantees. In this work we present a lattice-based motion planning
framework for a general 2-trailer system that is resolution complete and
resolution optimal. The solution will satisfy both differential and obstacle
imposed constraints and is intended as a driver support system to automatically
plan complicated maneuvers in backward and forward motion. The proposed
framework relies on a precomputing step that is performed offline to generate a
finite set of kinematically feasible motion primitives. These motion primitives
are then used to create a regular state lattice that can be searched for a
solution using standard graph-search algorithms. To make this graph-search
problem tractable for real-time applications a novel parametrization of the
reachable state space is proposed where each motion primitive moves the system
from and to a selected set of circular equilibrium configurations. The approach
is evaluated over three different scenarios and impressive real-time
performance is achieved.
