Where am I? Sensors and Methods for Mobile Robot Positioning
Where am I? Sensors and Methods for Mobile Robot Positioning
Where am I? Sensors and Methods for Mobile Robot Positioning
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Chapter 4: <strong>Sensors</strong> <strong>for</strong> Map-Based <strong>Positioning</strong> 123<br />
Note also the spurious data points between edges (e.g., between C <strong>and</strong> D). These may be<br />
attributed to two potential causes:<br />
&<br />
&<br />
The “ghost-in-the-machine problem,” in which crosstalk directly between the transmitter <strong>and</strong><br />
receiver occurs even when no light is returned. Ad<strong>am</strong>s' solution involves circuit neutralization <strong>and</strong><br />
proper grounding procedures.<br />
The “be<strong>am</strong>width problem,” which is caused by the finite transmitted width of the light be<strong>am</strong>. This<br />
problem shows itself in <strong>for</strong>m of range points lying between the edges of two objects located at<br />
different distances from the lidar. To overcome this problem Ad<strong>am</strong>s designed a software filter<br />
capable of finding <strong>and</strong> rejecting erroneous range readings. Figure 4.33 shows the lidar map after<br />
applying the software filter.<br />
Figure 4.33: Resulting lidar map after applying a software filter.<br />
a. “Good” data that successfully passed the software filter; R <strong>and</strong> S are “bad” points that “slipped<br />
through.”<br />
b. Rejected erroneous data points. Point M (<strong>and</strong> all other square data points) was rejected because<br />
the <strong>am</strong>plitude of the received signal was too low to pass the filter threshold.<br />
(Reproduced with permission from [Ad<strong>am</strong>s <strong>and</strong> Probert, 1995].)<br />
4.3 Frequency Modulation<br />
A closely related alternative to the <strong>am</strong>plitude-modulated phase-shift-measurement ranging scheme<br />
is frequency-modulated (FM) radar. This technique involves transmission of a continuous electromagnetic<br />
wave modulated by a periodic triangular signal that adjusts the carrier frequency above <strong>and</strong><br />
below the mean frequency f 0 as shown in Figure 4.34. The transmitter emits a signal that varies in<br />
frequency as a linear function of time:
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