Subject:
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Re: Barcodes & error detection
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Newsgroups:
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lugnet.robotics
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Date:
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Wed, 7 Sep 2005 23:34:13 GMT
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Viewed:
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988 times
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In lugnet.robotics, Brian Davis wrote:
> Some of you at BrickFest may have seen Gus, a small robotic forklift that was
> one of my GBC modules. Gus had two problems at BrickFest. First, while the
> codereading task was 98% accurate, that still means on average once every 16
> circuits of the course Gus would mis-read a barcode, resulting in it doing..
> something inappropriate (leaving a crate on the table, pushing over or
> destroying a structure, or in at least one instance making inappropriate
> advances on a member of the audience). Second, after a while the line-following
> got rough, resulting in 180 degree turns on the line.
> To increase reliability I wanted to add error-detection. Suggestions? Right
> now the codes are made of bits (black=1 white=0) with each bit preceeded by a
> highly reflective strip (reflective thin tape used to repair flexible ducts -
> great stuff, especially if they'd ever produce it without printing). Since each
> bit is preceeded by a reflective flag, I can use variable length 'words' (i.e. -
> '0' is distinguishable from '00').
> It turns out the errors seem to only be of the form of a '1' (black)
> mistakenly read as a '0' (white). Currently I can check by making sure any code
> read has "even parity" (an even number of '1's), and not using the odd parity
> codes. But that means of the 30 codes it can currently read (up to 4 bits long)
> I can only use 15 (the even parity ones).
> Question (finally): given the unique nature of the errors (only mistakes '1's
> for '0's, never the converse) is there any better error-detecting (or, even
> better yet, error-*correcting*) coding system I can use? There's plenty of
> options, but I want something that minimizes the number of complexity of the
> barcodes (I've got to make them by hand... no, I've not made them from LEGO
> tiles *yet*, it's just tape on cardboard).
>
> And hey, while we're on the subject... any suggestion for tasks Gus should
> try to accomplish? Loading/unloading crates on a conveyor and wharehousing
> crates, as well as detecting when it has a full or empty crate, are either
> implemented or planned. When I can figure out how to position the train
> accurately enough I'll work on loading crates on the train.
Right off hand I don't know any checking mechanism that works best with false
readings of one polarity.
Do you know anything about Error Correction Codes? ECC is used to detect and
correct single bit errors, it can also detect double bit errors.
If you have four bits of barcode, then you would need two extra bits of ECC.
I'll have to dig up some documentation on the polynomials for four bits of data,
but it can't be too complicated. I travel on Thursday, but I'll try to get it
to you Thursday afternoon.
I'll have to scratch out some stuff and see what I can come up with.
Kevin
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Message is in Reply To:
 | | Barcodes & error detection
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| Some of you at BrickFest may have seen Gus, a small robotic forklift that was one of my GBC modules. Gus had two problems at BrickFest. First, while the codereading task was 98% accurate, that still means on average once every 16 circuits of the (...) (19 years ago, 7-Sep-05, to lugnet.robotics)
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