DIY encoder for digitizing

[Cliff_J]

Graham, like a simple groove on the large drum to match the angle could be machined to keep the wire in perfect alignment? Or is it even simplier and the shortest distance (and automatic path of the wire) between the two drums would force the wire to follow the path automatically. Seems the latter would be true, but trying to think if a lateral tracking error on the wire could cause issues. Seems not but more eyeballs can't hurt...

Cliff

[cbcnc]

Alright, I'm finding this thread awfully interesting. I don't know if I am interested in building a digitizing arm but I do have some thoughts.
You are talking about getting the resolution down to 50000 ppr or 0.0072°/step. Then you are talking about winding cable around a pully to move the encoder.
If I assume a 1/2" dia. pully then a movement of .0000314" will be one count. (.5 * pi / 50000 = .0000314" ) Or if I assume a 1" dia pully then one count = .0000628"
That is an awful lot to ask of any material let alone vinyl coated SS cable. I think that the vinyl coating will compress and decompress many times that amount. Plain SS cable would be a little better but then you are are also talking about thermal effects on the cable too.
I like the idea of cable wrapping around a pully. I'm not sure it is the best solution though. But, if you get several wraps around it then it will form it's own natural helix. As long as the static point that holds the free cable is behind the wrap point it should just fall into place against the previous wraps.
I don't mean to rain on your parade but a mechanical solution like this needs to be in some tightly constrained conditions to get that kind of resolution.

Chris

[Graham S]

Cliff, it doesn't have to have a groove although it could, the important part is that the end point on the large pulley is positioned correctly. The whole point of laying the wire on the drum at an angle is that it automatically tracks the helix as it moves.

Chris, you are quite right this kind of accuracy is not going to be easy to get. The stainless cable is not vinyl coated but more impregnated. You can hardly tell it is there, better than the likely surface roughness of any pulley I make, but that's another story. The fact is that a precise ratio is not required just a ratio, the important thing is the lack of backlash and linear response, backlash will come from stetch in the cable (hopefully non, eek). Many of the other effects will be balanced on each side and should not cause a problem assuming a preload of tension. The balance nature of the design is probably the best thing it has going for it.

My mega clever mate also suggested the problem of dust on the roller, yep this could be getting tricky.

As I have said it isn't ideal and might not work but I don't see an alternative, a homebrew high res encoder is not going to happen because no one will sit down and do it and to be honest I don't think they would suceed if they did. (just MHO, please prove me wrong).

Graham

p.s. My intention has always been to build an arm small enough to use the 10000ppr encoders, this is for the mad Faro arm copiers

[lerman]

100,000 lines is only .2 minutes of arc (12 seconds). http://easyweb.easynet.co.uk/~chrish/worms.htm describes how to make a worm gear with a periodic error of around 5 seconds of arc, so that's in the right ballpark. If you drove the worm with a servo so as to keep a mark aligned with the arm, you should be able to use an encoder on the worm as a readout.

Ken

[MaxLogan]

Guys...

Digital Absolute 16bit......(65,536 ppr)

[Tagaotse]

How about this.
Get 8 of these tilt sensors (http://www.spectronsensors.com/datas...S-117-1104.pdf) and place them in a circle facing outward at 45 degrees to each other. Tie into them with a PIC and keep track of current angle through software(firmware?).
0.02 arc degrees = 18,000 cpr
0.005 arc degrees = 72,000 cpr
Is this possible?