Page 299 - Introduction to Autonomous Mobile Robots

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o 4 Chapter 6
obstacle points
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o 2 l 2
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M
forklift

Figure 6.16
i
l
Distances resulting from the curvature , when the robot rotates around M (from [128]).
i c
The global dynamic window approach promises real-time, dynamic constraints, global
thinking, and minimal free obstacle avoidance at high speed. An implementation has been
demonstrated with an omnidirectional robot using a 450 MHz on-board PC. This system
produced a cycle frequency of about 15 Hz when the occupancy grid was 30 × 30 m with
a 5 cm resolution. Average robot speed in the tests was greater than 1 m/s.

6.2.2.6 The Schlegel approach to obstacle avoidance
Schlegel [128] presents an approach that considers the dynamics as well as the actual shape
of the robot. The approach is adopted for raw laser data measurements and sensor fusion
using a Cartesian grid to represent the obstacles in the environment. Real-time performance
is achieved by use of precalculated lookup tables.
As with previous methods we have described, the basic assumption is that a robot moves
i
in trajectories built up by circular arcs, defined as curvatures . Given a certain curvature
c
l
i Schlegel calculates the distance to collision between a single obstacle point xy,[ ] in
c i
the Cartesian grid and the robot, depicted in figure 6.16. Since the robot is allowed to be
any shape this calculation is time consuming and the result is therefore precalculated and
stored in a lookup table.
For example, the search space window V s is defined for a differential-drive robot to be
all the possible speeds of the left and right wheels, v v, . The dynamic constraints of the
r l
robot are taken into account by refining V s to only those values which are reachable within
the next timestep, given the present robot motion. Finally, an objective function chooses
the best speed and direction by trading off goal direction, speed, and distance until colli-
sion.
During testing Schlegel used a wavefront path planner. Two robot chassis were used,
one with synchro-drive kinematics and one with tricycle kinematics. The tricycle-drive