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 Post subject: extreme digital rotary encoder
PostPosted: Wed Mar 02, 2011 10:49 am 

Joined: Wed Mar 02, 2011 9:14 am
Posts: 2
I am in possession of a glass disc that has around its circumference a series of bars that alternate opaque/transparent. It is like a 360 degree barcode, but you need a microscope to see the bars. This component is from a digital theodolite, so no doubt it can resolve angles to an arcminute or smaller. I want to turn this into a digital rotary encoder which could then be used with a rotary table for fabrication, scanning, telescope positioning, for whatever you may need to measure very small or very accurate angles.

I know the basic concept: shine light through the disc, convert to pulses, count pulses, convert to angles. I am guessing the microcontroller folks have the counting/angle conversion totally covered. I don't have much microcontroller experience so I would be looking for guidance there, but I don't foresee this as the primary hurdle.

The primary hurdle I see is the detector design. I know we need a light source such as an led or laser, and a detector such as a photodiode or phototransistor. How to couple such large components to a microscopic grating seems to be the rub. You could probably use a lens and a pinhole. might work. Worth trying first I suppose based on ease of implementation. Second problem, how to differentiate positive vs negative rotation. Perhaps not a design goal for the beta version, but definitely required. So the counting software has to count up and down. I foresee errors accumulating with each back and forth transition.

Anyone have any experience with the kind of encoding I am describing or have a better idea for the detector?
Anyone interested in working on this project?

-tom


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 Post subject: Re: extreme digital rotary encoder
PostPosted: Wed Mar 02, 2011 11:52 am 
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Joined: Sun Feb 14, 2010 9:29 pm
Posts: 84
Location: Downtown/Campus - Madison, WI
This is how inkjet printers run with closed loop feedback and DC motors (which is why tearing apart printers isn't as productive as it was about 5 years ago when they all had stepper motors in them). We've got a box full of resolver wheels and more importantly the optical pickups that you need to do quadrature counting from your resolver wheel. You need IR emitter/receivers at very specific geometry to figure out which way the wheel is turning and to only see the effects of a single slit at a time. You could more than likely create a simple single slit counting system from a lense and IR tx/rx as you outlined as well, but I think it might be faster to start with a vetted commercial part from a printer - you're welcome to dig through the box.

Doing the intrepretation on a microcontroller is pretty simple - http://www.arduino.cc/playground/Main/RotaryEncoders The only issue you run into is if you're interrupting the processor more often than it can complete its loop to increment the counter, and if you have a really high resolution resolver wheel and spin it at a good clip you might have to head down to lower level C code or a dedicated IC - http://groups.csail.mit.edu/drl/courses/cs54-2001s/pdf/hctl2000.pdf


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 Post subject: Re: extreme digital rotary encoder
PostPosted: Thu Mar 03, 2011 7:58 pm 

Joined: Sat Jan 15, 2011 1:42 pm
Posts: 1
I agree, you are going to have a lot of frustration adapting optics to that encoder wheel than it's worth.

I would suggest either digging through the assembled encoders at S67 or searching Ebay for a deal. In the gobs of time I have spent hunting for deals on encoders on Ebay, Renco encoders always seem to come up with the best prices. Also, add the word "lot" (as in lot of sensors, encoders, flying monkeys, etc...) when searching for parts like this - often times someone tries to offload three at a time and no one bites even though the price is low. I hope this helps.


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