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Author Topic: Homing
benden
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Posts: 10
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Post Homing
on: May 11, 2012, 15:06
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Hi

Just thought I'd post some info on my drawbot build and the homing routine I've added.

Blog http://arckarts.wordpress.com
Code http://code.google.com/p/benden-drawbot/
Video http://www.youtube.com/watch?v=i25TfoVmgmU

Bob
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sandy
Administrator
Posts: 1392
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sandy
Post Re: Homing
on: May 11, 2012, 17:31
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That homing routine is brilliant Bob - got to get this working on my own machine! The vid is great, thanks for sharing it, fills in the blanks very nicely.

Can I ask what kind of motors, and what voltage power supply you use?

Sandy Noble

benden
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Posts: 10
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Post Re: Homing
on: May 11, 2012, 21:29
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Hi Sandy

I'm using SanyoDenki 103H5208-10U41 NEMA 17 steppers from eBay, $11.99 ea. 200 steps/rev, 5.3 Ohm. As regards power supply, I'm driving them from the Arduino 5V rail with a 9V wallwart for the main power and it all seems to work OK.

Bob

sandy
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Posts: 1392
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sandy
Post Re: Homing
on: May 11, 2012, 23:15
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Champion, thanks. So you get decent torque from the the motors running on a 5v supply? I'd have thought you'd need higher for a non-counterweighty system. Perhaps my motors are just wusses!

sn

benden
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Post Re: Homing
on: May 12, 2012, 13:48
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Hi

No problems with torque or missed steps, only issue I had was with the homing routine when I'm 'pulling' the de-energised motor and I had to reduce the speed. I also use the same approach on my MkI system with different steppers and a larger diameter bobbin. I've steered clear of the counterweight system myself as it always appeared more complex mechanically 🙂

sandy
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Posts: 1392
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sandy
Post Re: Homing
on: May 12, 2012, 18:50
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Don't blame you, it's not complex in itself, but it just needs more care, it's less general-purpose. More stuff to go wrong. The reason for it is for positional accuracy, but this could be got by using big enough diameter spools too. The down-side to using spools is the change in drum diameter when the thread layers-up. The change in diameter is particularly significant on very small spools. You know, this was a problem for me on my first machine when I was using sewing bobbins and a pulley-type system where the thread ran through an eyelet on the gondola, and then back to an anchor point near the spool. So it had to reel in twice as much thread as a single line's-worth for any particular move. I'm quite jealous of the simplicity of the thready, spooly machines, so I'm definitely going to look to that in the future, and I don't see why the standard sprockets couldn't also be used for it.. I was asking about the voltage because I know it'll need a lot more _grip_ to bear the full weight of the gondola. On the other hand, it'll have a lot more traction on the spool. Hm!

benden
Newbie
Posts: 10
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Post Re: Homing
on: May 12, 2012, 19:37
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I was also concerned about the accuracy as well with the cord build up on the bobbins. However, in practice, it doesn't seem to be an issue. I just took a few measurements on the MkII
Pulley dia, empty = 13.2mm
Pulley dia, at normal operation = 14.8 mm
Cord dia ~0.3mm
Traversing from top right to bottom left for my normal paper 11x17"/279x432mm takes ~10 revs. Calculated error at final position from beginning is ~1.9mm. Over this distance it uses ~2 layers. In practice, I can't notice it.
The bobbins on the Mk1 were 34mm dia and consequently even smaller error
Bob

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