Chapter #6: Determining Distance Using Frequency Sweep
Chapter #6: Determining Distance Using Frequency Sweep
What’s a Frequency Sweep? In general, a frequency sweep is what you do when checking your favorite radio stations. Set the station for one frequency, and check the output. If you don’t like the song that’s playing, change the frequency and check the output again.
Activity #1: Testing the Frequency Sweep
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Figure 6.1: The left axis of the graph compares IR frequency to the relative sensitivity of the IR detector. The right side of the graph shows how the relative sensitivity of the IR detector relates to distance detection. As detector sensitivity decreases with the increase in frequency, the object must be closer for the IR signal to be detected. Why closer? When the detectors are made less sensitive by sending higher frequencies, it’s like giving them darker and darker lenses to look through. Just as a flashlight beam appears brighter when reflected off an object that’s closer to you, IR reflected off a closer object appears brighter to the IR detectors.
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The Boe-Bot can be programmed to send different IR frequencies, and to check for object detection at each frequency. By keeping track of the frequencies for which the IR detector reported an object, its distance can be determined. The left axis of the graph in Figure 6.1 shows how the sensitivity of the IR detector’s electronic filter decreases as it receives frequencies greater than 38.5 kHz. The filter essentially causes the IR detector to become less able to detect IR at these frequencies. Another way to think about it is that you have to move an object closer if you want it to be detected at a less sensitive frequency. Since the detector is less sensitive, it will take brighter IR (or a closer object) to make the detector see the signal.
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