April
2004, Issue 165
Mini
Rover 7
Electronic Compassing fo Mobile Robotics
by
Joseph Miller
BASIC
ROBOT NAVIGATION
Dead
reckoning (DR) is the fundamental navigation method
used in mobile robotics. It is a method of mathematically
tracking your present position by measuring speed and
direction traveled at regular intervals, or distance
and direction at any convenient interval. The latter
is easier to do on most wheeled mobile robots if you
use wheel encoders. This is also referred to as odometry.
The
basic tools for DR in mobile robotics are a compass
and an odometer. In more complex robots with advanced
navigation resources, DR is still used to navigate between
absolute position fixes. Position tracking by DR is
used to get to and from fixed locations, help create
maps of surroundings, and keep track of position when
moving around unexpected obstacles.
Position
errors have a tendency to accumulate over time when
navigating with DR. A robot happily hobbling down a
path can easily drift off the path because of the imperfect
traction of its drive wheels or limbs. Furthermore,
the terrain may be uneven, inconsistent, obstacle-ridden,
or even move, as is the case for aquatic vehicles. A
robot could simply correct its steering to stay on course
(or on bearing), but that little off-course excursion
could add a small position error. The magnitude of the
position error is the integral of time, distance, and
off-course angle traveled while the robot is off course.
What is really required is a new bearing calculation
to increase the robot’s chances to reach the desired
target.
