shaft encoder/absolute positioning

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randybardwell
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shaft encoder/absolute positioning

Post by randybardwell » Sun Oct 23, 2005 8:55 am

can anyone steer me in the right direction for inexpensive shaft encoder to go with a stepper motor positioning system? I also need some information relative to how to code feedback from shaft encoders to control absolute positioning....books, articles, etc.<p>any help is much appreciated...

josmith
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Re: shaft encoder/absolute positioning

Post by josmith » Sun Oct 23, 2005 7:10 pm

The cheapest shaft encoder available is inside a serial mouse.Two for a buck in the dollar store. The encoding is well documented,just do a search.
Industrial grade encoders cost three to five hundred dollars. The output is a square wave that needs to be processed.
Ablolute positioning implies that the system never needs to be zeroed. The only way that can be done is to keep the encoder alive with a battery when the system is off.

dyarker
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Re: shaft encoder/absolute positioning

Post by dyarker » Sun Oct 23, 2005 11:07 pm

How many degrees of rotation?<p>If 360 degrees, or less, a Gray code disk (see http://www.nist.gov/dads/HTML/graycode.html) will provide absolute position info.<p>Single sensor and two sensor quadrature codes always need position ref after reset/power cycle.<p>Cheers,<p>added: a better link http://www.qsl.net/oe5jfl/encoder.htm<p>[ October 24, 2005: Message edited by: Dale Y ]</p>
Dale Y

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Sambuchi
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Re: shaft encoder/absolute positioning

Post by Sambuchi » Mon Nov 07, 2005 10:16 am

I made one from a mouse.. heres some of the data that i collected .. may help you out

http://www.unf.edu/~sama0004/Projects/e ... r_home.htm

grant fair
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Re: shaft encoder/absolute positioning

Post by grant fair » Sat Nov 12, 2005 9:09 pm

Imagine a strip of paper with a fixed number of spaces, say 200, on it. Imagine each space has either a hole, or a solid, so the strip looks like this:

00XX00XX00XX00XX

If you read each space on the strip with a simple optoelectronic device, moving the strip one space along, forward or reverse, between reads, you will get some useful information sometimes. Other times you won't be able to interpret it.


If you add a second strip underneath the first, so you have:

00XX00XX00XX00XXO
X00XX00XX00XX00XX

And if you read both strips, and use simple logic to remember the last position and compare it to the next, with a bit of logic you can tell if you have gone forward or back. If you count the transitions you will also know how many spaces you have moved the strips.

Now put the two strips around the circumference of a disk. With your two opto reading devices you can keep track or number of spaces moved, and the direction. Put the disk on your stepper shaft, and if the stepper is driving a lead screw, you can work out the position of a nut on the leadscrew.

A simply DIY rotary encoder coded this way is described in Handbook for Parallel Port Design (Paperback) by James Barbarello. If you can borrow it from your library you can see if this helps. It includes some simple Quick Basic code for the parallel port to read the encoder disk. You could make a disc with more resolution, and rewrite the code.

He had a couple of related articles in the now defunct Electronics Now or Poptronix, but their online index is also long-gone, so locating them might be a challenge.

C&H sales also has some surplus encoders. The HEWLETT PACKARD ENCODER #QEDS-7148 sells for $15.00 and used to include information on how to rewire it so it gives the output in quadrature.

Grant
Grant

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