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Author Topic: How to best reduce pen wobble?
chunk23
Newbie
Posts: 3
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Post How to best reduce pen wobble?
on: June 3, 2015, 03:53
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So I've had my machine assembled for a few days now but I'm struggling with one main issue - spiral/curves.

Straight images come out pretty dang sharp.
Image

But I'm having tons of issue with very uneven, very wobbly spirals and bends.
Image

I'm using a gondola design I found on Thingiverse that uses d-batteries as counterweights, so the gondola is quite heavy.
Image

At present I'm using a mega2560 with a motorshield v1 set to interleave at 400 steps per rev, 1 multiplier (nema 17 200 motors).

Is this simply an issue that I'm not going to be able to overcome without switching to microstepping? Or is this more a physical issue from maybe a too-heavy gondola, bad balance, too high motorspeed (dropped this to 400 on one print and didn't seem to help), or that the sprockets are too far from the board?

Any thoughts on the matter would be appreciated. Thanks!
kongorilla
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Posts: 362
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kongorilla
Post Re: How to best reduce pen wobble?
on: June 3, 2015, 05:58
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Howdie Chunk23!
I'll leave the microstepping issue to Sandy, he's more qualified to discuss that.

Though that gondola is somewhat based on one of my designs, there are some changes to the design that compromise it a bit, in my opinion. Foremost, I don't like the weight of the gondola being so far below the pen. It leads to wobble -- the tail wags the dog too much.

My original design that Carlo based his gondola on (the one you printed) can be seen here: https://www.flickr.com/photos/91789568@N00/10636536794/in/album-72157629448247128/

Since that rendering was done, I switched to using a big metal washer that goes around the cylinder that holds the pen. Putting the weight where the pen is held is a better solution, I've found.

If your cord hangs parallel to the drawing board, your sprockets are the correct distance from the board.

If, with the motors off, your gondola hangs near the middle of the area in which you'll be drawing, or slightly higher than that, you have the weight balanced correctly. I suspect the washers I use are much lighter than a battery of that size, though.

Looking at your photo -- if your pen is down on the paper, I'd say its point is poking out too far from the "stabilizer" (the round plate). To stabilize wobble, the plate should be closer to the paper.

Hope those tips help a bit!
chunk23
Newbie
Posts: 3
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Post Re: How to best reduce pen wobble?
on: June 3, 2015, 20:14
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Very helpful. I'll try making the few small changes first and see how that helps (primarily the pen distance).

Do you have an .stl file for your gondola design? I had suspected that the length of the gondola would be an issue.

sandy
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Posts: 1317
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sandy
Post Re: How to best reduce pen wobble?
on: June 3, 2015, 20:20
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Well even the straight lines in the pen width test have a wobble to it, any chance you're just moving too fast? Or accelerating too fast?

sn
kongorilla
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Posts: 362
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kongorilla
Post Re: How to best reduce pen wobble?
on: June 4, 2015, 22:29
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@Chunk 23 - I never released the STLs because I never considered the design finished. The gondola I use now is a bit slicker than that old design, but essentially the same. If you take the battery off yours, and tape some coins to your servo mount, you'd be close to my original. Or drill a hole for a bolt to hold some washers. The benefit of washers or coins is you can easily adjust the weight of the gondola by adding or removing them.

Are the "wings" rotating smoothly? I use PVC pipe instead of a printed cylinder because I was afraid a printed part wouldn't be smooth enough.

As Sandy observed, the straight lines in that middle image show some shaking that looks like something other than gondola issues. (I'm assuming you're using smooth paper). Are the motors running smoothly? If the current they're being fed isn't in a given range the steps can be a bit rough. And yes, I'd try microstepping.

chunk23
Newbie
Posts: 3
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Post Re: How to best reduce pen wobble?
on: June 6, 2015, 01:25
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Good eye, it appears that the power supply is inadequate, will amend that as soon as possible.

I have switched to a lighter, simpler gondola, but if the power is the true issue, this likely does not matter. Both gondolas I've used so far don't seem to have rotation issues, although the new one is a bit better balanced.
http://www.thingiverse.com/thing:372244

It's my understanding that the motorshield V1 is not well-suited for microstepping, is that correct? If so, I'll upgrade to the polarshield.

I appreciate both of your assistance on this, thank you.
sandy
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sandy
Post Re: How to best reduce pen wobble?
on: June 6, 2015, 23:07
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I'm not a fan of the microstepping modes in the adafruit shields, but it is a bit of an irrational fear. Its noisy, squeaky, it disturbs me.

I think I had a bad experience with software controlled microstepping motor shields when I was a child.

Since you've got it already, I'd give it a shot with the microstepping on the adafruit shield. It _will_ work, just it'll sound weird, and you can report back whether it makes an improvement or not 🙂

The problems I see with the adafruit shields is that the power is uncontrollable, except by changing the input voltage. If you can change your input voltage (variable voltage power supply) then you will be in a better position to experiment - and a lot cheaper than a polarshield!

Sn
dickholmer
Beginner
Posts: 39
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Post Re: How to best reduce pen wobble?
on: June 13, 2015, 09:42
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hi,
one thing from my side regarding the wobbling issue mentioned above. i do have pretty heavy weights pretty far below my gondola; i use bottles with water. and this bottle really swings a lot if i go fast. the thing is that i do have the pen 100% in the center of the gondola, so the swinging does not effect the drawing at all. on the picture of your gondola the pen seems to be above the center. if it now starts to swing the pen will move / wobble.

cheers thomas
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