Quote:
Originally Posted by AdamHeard
I would say reduced instead of completely lost. [...and what are you actually measuring?]
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True, but from an instrumentationalist's perspective, if it won't give me the same value every time I return to the same point, I can't use the solution.
What is our project? This is a Fall R&D project.
We wish to gain the capacity to accurately servo control a 24" sliding joint to say ~7-8 bits (or better) position resolution over its length (~4-10+ steps/inch) , with a cheap, easily made, compact feedback device.
Rack and pinion pots won't easily fit there, and optical slides could get dirty. Without a rotational joint present, cheap pots won't work either.
Something equivalent to a 25" string pot (into our Analog Inputs) seems ideal, but it:
A) Needs to be CHEAP to make (no $100 commercial string pots!), and
B) Preferably can be assembled MOSTLY with COTS hardware store items, with as little machining as possible.
Ideally, the sensor should be made with Hand Tools only, HT + Dremel Only, or at least as few specialized machine tools as possible. That makes the solution available to ALL teams, regardless of budget, or machine tool capacity.
We've tried dissecting and hacking on some key chain retrievers to add a 10-turn pot, but that still requires machining several parts. The basic KCR drums allow string overlap too, making it unsuitable for reliably hovering an axis, or even returning it to the same position.
I was
hoping that someone here already solved this generic problem with COTS hardware store items. (Has anyone???)
BTW: Custom PCBs and soldered on arrays of lots of VERY cheap sensors are still included as a candidate here, as long as the total cost for the entire sensor array device would be in the $25 or better range.
Now, I
like the idea about turning a measuring tape into an incremental encoder! That's the kind of divergent thinking I'm seeking here! However it would be NICE not to be forced to run the axis to one end at boot-up just to home/sync the position counter. Not all applications may easily allow us to do that. Something that gives us an absolute position at bootup is preferred.
How about something like:
A) Having a pinch roller pair and a 10-turn pot watch a COTS tape measure go in and out? (Friction/slip may be a problem. Can that be overcome?)
B) Punching/drilling holes in a COTS tape measure, and have a crude drum sprocket and 10-turn pot follow it? (No slip, but requires more fab time to mod the tape, and make the pin drum.)
Some linear devices:
C) Use a Mylar CNC 8-channel punched tape, punch codes into it, lay it out, and watch it optically. (Not all teams could easily replicate this one.)
D) Laser Print an encoder strip with multichannel absolute Grey code, watch it optically. (Cheap and doable by all teams, but may have mounting issues in some instances.)
Both C & D require more space, and may have dirt issues (which is why we keep finding ourselves gravitating back to string pots techniques)...
Thoughts?
(Hey Al - do you know of a cheap printer head monitoring system, that'll do 25"??? It has to be COTS, so we could use it in a contest.)
- Keith