Compensating for extruder lag August 31, 2014 02:40PM |
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Re: Compensating for extruder lag August 31, 2014 03:17PM |
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dc42
2. Is there anyone reading this who has a subscription that allows them to download this article free of charge: [manufacturingscience.asmedigitalcollection.asme.org]..
[forums.reprap.org]Quote
Calendar
...Research article: "liquefier dynamics in fused deposition modeling" gives this model, the article is based on a bigger thesis with loots of information on what happens to the plastic in the extruder..
Re: Compensating for extruder lag August 31, 2014 03:18PM |
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Re: Compensating for extruder lag August 31, 2014 03:46PM |
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ormerod168
Could be "Calendar" from this short thread has access to the article
[forums.reprap.org]Quote
Calendar
...Research article: "liquefier dynamics in fused deposition modeling" gives this model, the article is based on a bigger thesis with loots of information on what happens to the plastic in the extruder..
Re: Compensating for extruder lag August 31, 2014 03:51PM |
Registered: 10 years ago Posts: 14,672 |
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zombiepantslol
My guess is that there is still an issue in the Move code, because some extruder moves are not performed too well. I added a work-around for an extruder issue a while ago, but it isn't perfect, it just conceals the actual bug. Especially during top infills, the selected extruder should accelerate and decelerate the same way the print head does, but I feel its feedrate is quite constant.
Re: Compensating for extruder lag August 31, 2014 04:52PM |
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Re: Compensating for extruder lag September 01, 2014 02:03AM |
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Re: Compensating for extruder lag September 01, 2014 04:07AM |
Registered: 9 years ago Posts: 35 |
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dc42
2. Is there anyone reading this who has a subscription that allows them to download this article free of charge: [manufacturingscience.asmedigitalcollection.asme.org]
Re: Compensating for extruder lag September 01, 2014 04:26AM |
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Re: Compensating for extruder lag September 01, 2014 05:40AM |
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dc42
I've thought about this some more and I think the main issue may be the springiness of the filament in the Bowden tube rather than the fluid dynamics in the nozzle. I've worked out a mathematical model for the effect this has, and the adjustment needed to the extruder drive step rate to counteract it. One consequence is that the end of a deceleration, it may sometimes be necessary to reverse the direction of the steps, i.e. retract filament instead of feed it.
I hope to try this out in the firmware when I find time. It's not trivial because of the need to retract as well as extrude filament, but it's probably easier than my original plan (introducing a delay between extruder movement and XYZ movement).
What I would really like to know is whether direct drive (non-Bowden) extruders suffer from the same effects (e.g. excess filament deposited at the ends of top infill runs). If my hypothesis is correct, they shouldn't, or at least the effect should be greatly reduced.
Re: Compensating for extruder lag September 01, 2014 06:35AM |
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Re: Compensating for extruder lag September 01, 2014 06:46AM |
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Re: Compensating for extruder lag September 01, 2014 07:35AM |
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Re: Compensating for extruder lag September 01, 2014 08:11AM |
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dc42
VortyZA, what you are describing is hysteresis in the Bowden filament feed. This is an effect I had not considered, and separate from the springiness issue that I have been looking at. My hope is that during normal extrusion where filament is being extruded continually but at varying rates, hysteresis can be ignored, because there is always a positive force on the filament and so the filament is always at the top of the tube. But hysteresis obviously does increase the amount of retraction needed at the end of a print or during a tool change, when releasing the force on the filament is not enough to prevent ooze and it is necessary to retract the filament from the nozzle.
Another contribution to hysteresis is when the tongue that locks the brass Bowden end into the extruder drive is not a tight fit, and the brass end moves up and down a little as filament is extruded and retracted.
Re: Compensating for extruder lag September 01, 2014 08:48AM |
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Re: Compensating for extruder lag September 01, 2014 10:17AM |
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VortyZA
That being said, I'm not sure if that's what is being shown in your picture - are you talking about the extra extrusion whenever the hot end changes direction?
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VortyZA
I haven't looked at the G-code, but does the extrusion change speed at those points? If so, then could it not be hysteresis causing some of that?
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VortyZA
If you were to have a single straight line, would it start thin, then end with a thicker bit? That would probably imply that it's something to do with the springiness in the tube/filament/hysteresis - building up pressure, then constant pressure, then releasing the pressure at the end.
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VortyZA
If you made a V, then surely the point of the V would be slightly thicker as the filament is overlapping on the same spot? Compounding that, there would be a deceleration and acceleration at the point, while the extruder is unlikely to change speed, so there would be even more excess material.
Re: Compensating for extruder lag September 01, 2014 01:40PM |
Registered: 10 years ago Posts: 176 |
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dc42
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VortyZA
If you made a V, then surely the point of the V would be slightly thicker as the filament is overlapping on the same spot? Compounding that, there would be a deceleration and acceleration at the point, while the extruder is unlikely to change speed, so there would be even more excess material.
In theory I don't think there should be any overlapping, unless the filament takes a short cut when the direction changes.
Re: Compensating for extruder lag September 01, 2014 01:56PM |
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Re: Compensating for extruder lag September 01, 2014 02:39PM |
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Re: Compensating for extruder lag September 01, 2014 02:53PM |
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Re: Compensating for extruder lag September 02, 2014 04:29PM |
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KimBrown
Hi DC, I'm still not convinced that the acceleration of the Feeder isn't just a fraction high, and / or at the end of a movement, the de-acceleration of the feeder isn't high enough to allow for the slight dribble from the nozzle at the end of a run. I'm going to guess that acceleration and de-acceleration rates are treated the same, which in a perfect world would be all well and good. If it was possible to add a de-acceleration rate for the feeder to the M201 line I think that would help (or have it as a seperate M??? setting so it got ignored by other firmware). The value for the de-acceleration would only have to be a fraction greater than the acceleration I think. Also I think the spring in the PTFE tube doesn't help things as it adds a tiny squirt of plastic at the end of a run.
Which reminds me, the backlash in the feeder gears can't be helping my machine, so I'd best print a new set later.
Kim