Gas station without pumps

2017 October 22

A beacon detector for mechatronics

Filed under: Robotics — gasstationwithoutpumps @ 22:04
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I’ve not been blogging much lately, because so much of my time has been taken up with taking the Mechatronics course at UCSC.  I’d love to blog about what I’m doing in the course, but they use the same assignments every year, and I don’t want my solutions to replace student ones.

I will post a picture of the beacon-detector board that I soldered up for Lab 2, as reverse-engineering the design from the photo would be harder than doing the design from scratch:

This is a two-chip solution, using eight op amps. It is definitely overkill for the problem of detecting an IR LED flashing at 2kHz, while rejecting ones flashing at 1.5kHz or 2.5kHz. The Molex connector for power is a bit of a pain, as its pitch is wider than 0.1″. I ended up having to drill extra holes in the PermaProtoboard to accommodate it.

I did this design with very conventional approaches (though not the ones that the course was encouraging) and got a filter whose measured characteristics closely matched the theoretical ones over a wide frequency range:

The active filter does a good job of responding to 2 kHz while rejecting other frequencies. I initially had made the Q higher (that is, made the peak narrower), but the filter started oscillating.
The increasing response at frequencies above 10kHz is expected from an active filter—but the problem is not significant here.

I also characterized the filter including the infrared emission and reception:

The infrared signal here was strong enough to cause clipping around the central peak, which reduced somewhat the discrimination between the desired 2kHz and the undesired 1.5kHz and 2.5kHz. This plot is from an earlier version of the design—the one that had some oscillation.

After building this conventional solution, which takes 8 op amps, I also built (just on a breadboard) a 3-op-amp solution, which I have not yet tested in the lab, only at home with different beacon sources. I don’t plan to solder it up, though, as I plan to use the digital beacon detector that I built over the summer, which has been tested and seems more reliable than the analog ones (as well as giving 8 channels of detection, rather than just one).

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