Page 229 - Designing Autonomous Mobile Robots : Inside the Mindo f an Intellegent Machine

Chapter 14

the agent does not compensate for the movement of the robot during this period, then
the correction will be in error by that amount.
In cases where the sensor collects an array of readings, each reading must be com-
pensated for the movement made since the collection started. Figure 14.2 shows the
angular distortion that can result from failing to compensate for motion that occurred
within the sweep of a lidar.

Apparent wall

Actual wall

First range reading

Last range reading
B

A

Figure 14.2. Angular distortion from data latency

In this case, the lidar is scanning at 0.3-second intervals (typical for high-resolution
lidar), and the robot is moving at about 20 km/hr (13 miles per hour). Only 90 de-
grees of the 180-degree scan fall upon the wall to be mapped, meaning that the time
difference between the first reading and the last reading is .15 seconds. At this rate
the robot travels between points A and B or about .83 meters (3 feet) during the
time the beam is on the wall. If the range readings are not compensated individually
for movement, the measured azimuth of the wall will be in error by 6.7 degrees! At
an order of magnitude slower speed, the distortion would still be unacceptable. Worse
yet, our fuzzy navigation techniques are unlikely to filter out these errors because they
represent a constant bias at any given speed and not random noise—and this is only
the angular error!

212