Assuming that the calibration is correct for each diameter, is there any impact to performance or quality by using a different bobbin size on the motors? Its more of a theory question than anything I suppose.

Since I'm using a wound string (as opposed to the beaded counter weighted approach), the only thing I can see is with an exceptionally small bobbin diameter. As more string is taken up, the diameter will change. Especially true when using a heavy gauge string.

I'm wondering if the native step angle of the motor comes in to play at some point. Curious more than anything. If no one's explored it to any great length, I might give it a go.

Josh

Bear in mind the string doesn't need to hang directly off the bobbin - so you could have two huge bobbins in the middle, right next to the arduino (short motor wires = good), and have the string going out to a couple of little hooks or pulleys at the very top corners.

The DRBO I wrote about on the blog the other day has this approach - works good as long as the gondola is heavy enough and the thread is light enough.

The thread-buildup issue is the reason why I switched to beaded cord back in the day - It was primarily an issue with repeatability though, rather than outright precision. Every time I wound the thread in and out, it would wind up in a slightly different pattern and I could never get to the same point twice. But you're right, that was with really small bobbins (sewing bobbins), and by all accounts, using something bigger yields very good results. The simplicity of using thread (in terms of routing) is extremely tempting.

Big sprockets/bobbins: Upside - faster top speed, thread buildup is proportionally less significant. Downside - less torque, and lower resolution output.