Page 158 - Designing Autonomous Mobile Robots : Inside the Mindo f an Intellegent Machine

Navigation as a Filtering Process

Uncertainty is most easily calculated relative to the vehicle’s frame of reference. The
reasons for keeping uncertainty in a relative reference are several. Most importantly,
errors will occur for different reasons along the vehicle’s lateral and longitudinal
axes. Heading errors will occur for different reasons yet, and it will induce lateral
error as the robot drives.

Figure 10.2. Uncertainty diagnostic display
(Courtesy of Cybermotion, Inc.)

Figure 10.2 shows a diagnostic display for monitoring uncertainty. The ellipse indi-
cates the lateral and longitudinal uncertainty, while the intersecting lines indicate
the heading uncertainty. In this case, the uncertainty represents the absolute bound-
ary for correction data that is received. Data outside of this boundary will be ignored,
but this does not mean that a correction of slightly less than this magnitude would
be used to correct the position estimates directly. More will be said about this in the
coming chapters.

Azimuth (heading) uncertainty
1
A robot that is not moving generally accumulates no error . Heading uncertainty is
therefore caused primarily by the acts of turning or driving. Like other forms of un-
certainty, this one has both an inherent platform component and a surface component.

1 Actually, it can falsely accumulate uncertainty if an encoder repeatedly registers forward and
backward ticks as the result of Servo-hunting or edge jitter on an encoder. Therefore, it is a good
policy to incorporate hysteresis on all such signal inputs.

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