Pressure Advance Calibration
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@bartolomeus 650mm Bowden tube is long. Perhaps even higher values are needed.
The second value reported as zero may be from a second extruder that's defined but unused?
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@bartolomeus That is interesting. My latest test can be seen here.
Regarding the second value: this is the one for the second extruder. If you did not explicitly tell RRF that there is only one (don't remember the exact command, it's the one where you assign motors to axes) then I guess it has the second extruder configured by default.
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@phaedrux higher than 0.3? I went through my config to be sure, but there's no second extruder configured. I'll go over it again and then post my config.
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@bartolomeus So you definitely need higher values. All layers are identical but all are identically bad. In my image at about a little above the middle layer that is how it should look like. One consistent extrusion width from left to right. No bumps.
Manuel
Duet 3 6HC (v0.6) with RPi 4B on a custom Cartesian
with probably always latest firmware/DWC (incl. betas or self-compiled)
My Tool Collection -
@wilriker I'll give higher a go tomorrow.
My config: 0_1536956692480_config.g
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@bartolomeus You don't have a
M584command in your config to tell RRF that there is only one extruder (besides other things, check the documentation). That's probably why you get the value for the second (implicit) extruder. -
One thing I should add: in the script I changed min speed to 20.
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Ok, so I had to increase PA to 1 to see a result. I really need to switch to direct drive.
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Thanks to this great script! I ran it, and found 0.1s, which is the value I was using.
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Thanks for the great script!!!
But I could not use it to set the pressure advance as all my layers look almost the same... Very very thick at the slower part and ok at the high speed parts. Against the light I can see some difference but nothing that can direct me to a decision.
The only thing I changed was the fast speed to 80mm/s and the center position. Maybe I am over extruding to start with?
Thanks!
What else should I check?
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I'm confused by how y'all are trying to use this script / tuning technique. Are people trying to use pressure advance to eliminate the difference in extrusion flow between 100mm/s and 5mm/s? That should be done with non-linear extrusion, not pressure advance.
All hot ends extrude relatively less material (deviate from linearity between E axis travel and extruded filament volume) as the flow rate starts to approach the hot end's extrusion limit. This limit is largely a function of the nozzle size, and of the hot end's ability to transfer heat to the filament:
- PTFE-lined hot ends allow less heat flux and thus deviate from linearity at lower flow rates / print speeds. Insufficient heat flux = cooler extrusion = more viscosity.
- Higher extrusion rate = shorter residence time in the hot zone = less time for heat to transfer = cooler extrusion = more viscosity.
- Higher flow rate = more pressure drop at the nozzle.
etc.
There's a flow range where extrusion is very close to linear, and once you break out of that range, flow starts to deviate a lot. Basically as the back-pressure at the nozzle gets higher, the compressive force at the extruder gets higher, and the extruder drive loses travel because the hob bites squeeze closer together.
So, it's totally expected that the hot end will extrude less material at a very high print speed, and you SHOULD see differences at 100mm/s vs 5mm/s. That's why the non-linear extrusion feature was made -- to offset that.
Hot ends ALSO also have time lag / afterflow issues that specifically cause blobbing at corners, which is a largely unrelated issue. Flow lag is a transient effect that is only visible close to the speed change. This is primarily a matter of cumulative elasticity or "wind up" of all the components between the extruder motor and the nozzle:
- Torque/error response of the stepper motor
- Elastic shear of the drive hob bite zone
- Compression of the filament between the drive hob and melt pool
- Stretch of the bowden tube
- Bulk compression of the melt pool
(There there's more but I don't want to get into it right now.)
All this elasticity means the extruder system has to "wind up" when it starts extruding to build up full extrusion force (effectively losing some filament volume), and then has to "unwind" when it stops extruding (getting the lost filament volume back as afterflow). Pressure advance is supposed to compensate for that.
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@brunofporto I had the same thing on my printer, with a long bowden (65cm) tube. I had to set a very high pressure advance to see any result. I think I needed about 0.9 to 1. Not a very desirable value, so I decided to switch to direct drive.
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@rcarlyle I think you are right. This approach assumes that nonlinear extrusion calibration is performed. In my case, I measured constant (linear) extrusion rates in free air from 1~12 mm^3/s, and extrusion volume only starts dropping off nonlinearly beyond that. 100 mm/sec @ 0.2 mm layer, 0.4 mm width is only 8 mm^3/s. But it is very likely that extrusion rates during extrusion differs from free-air extrusion though. Higher back-pressure, but maybe elasticity also helps drag plastic out when the nozzle is moving.
What I look for here is the z layer where the thickness is as close to uniform as possible across the transition zone (high-to-low, and low-to-high speed). The fact that the lag effect is so significant that it lasts during the entire duration of the high and low speed sections is somewhat surprising, but seems to be the case here. You may be right in that this means that there are two different effects being affected by a single parameter.
An alternative (better) approach would be to:
- Run the same fast/slow extrusion while varying the commanded extrusion width for the fast section. Repeat this twice for two different speeds, e.g. 10 vs 30, and 10 vs 100 mm/s. Find the z height for the two where the layer thickness is equal (in the center of the extrusion segment) and use that to calibrate a nonlinear extrusion model for that particular layer thickness.
- Run this script to find the pressure advance setting that minimizes with changes during the transition period.
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I reviewed the results I posted above and the reason I saw a color gradient on the short extrusion segments is because of over- and under-extrusion due to pressure lag. Basically, with insufficient pressure advance the short segment tends to become one long elongated blob. If one modifies the script to increase the length of the short segment (
pattern_width) the center of the slow segments are not thicker than the centers of the fast segments when measured with a caliper on my tests on two different printers. That is not to say that others may see nonlinear extrusion effects already at 8mm^3/s. If one is concerned about this, I'd recommend increasing the length of the slow speed segment.
So the method seems valid. I can also report that print quality is improves significantly: this approach virtually eliminates corner and infill blobbing, and under-extrusion during fast sparse infill moves.
Also worth noting is that the single parameter pressure advance model doesn't completely eliminate all acceleration dependent effects – there seems to be some nonlinearities left. There is also some asymmetry in respect to acceleration and deceleration that isn't modelled completely.
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@digitalvision said in Pressure Advance Calibration:
.......................................Also worth noting is that the single parameter pressure advance model doesn't completely eliminate all acceleration dependent effects – there seems to be some nonlinearities left. There is also some asymmetry in respect to acceleration and deceleration that isn't modelled completely.
Which all comes back to what @RCarlyle has been saying and what I have been banging on about for quite some time, in that whilst this technique may give improvements, it's using pressure advance to compensate for non-pressure related problems.
The lag time between an XY move accelerating up to speed and the extruded filament accelerating up to the same speed, is not the same as nozzle pressure build up.
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@deckingman said in Pressure Advance Calibration:
Which all comes back to what @RCarlyle has been saying and what I have been banging on about for quite some time, in that whilst this technique may give improvements, it's using pressure advance to compensate for non-pressure related problems.
I'm not sure I follow. This test is all about pressure related compensation. The script ramps extrusion up and down between two extremes and allows selecting a setting that gives an as uniform result as possible.
The lag time between an XY move accelerating up to speed and the extruded filament accelerating up to the same speed, is not the same as nozzle pressure build up.
The fact that the slope of the extrusion ramps are tied to xy acceleration is not necessarily wrong. That's exactly the ramps you will encounter during printing.
Here's a mental model: If you change the minimum speed in the script to exactly match your max instantaneous speed change ("jerk") setting, and shrink the length of the short segments (
pattern_width) to something insubstantial like 0.1–0.5 mm, the extrusion profile generated here will perfectly match the one you get if you print for example an outline of a cube, or do linear infill.The short segments are stretched out here is because it exacerbates the effect. It allows all (most) of the excessive pressure to bleed off, and separates the effects of overpressure during deceleration from the underpressure during acceleration. The purpose is to make the effect more apparent in order to make tuning easier. I don't think this is adding a systematic error to the process – but I may be wrong.
The thickness of the short segments is (generally) not related to nonlinear extrusion. It s a long continuous blob – exacerbated by the profile.
If you look at the last image I posted, the extruder moves left-to-right on the front outline of the print. On the bottom part of the print you see severe underextrusion during acceleration (gaps in the print) that correspond to what you would see when printing sparse linear infill. This is the opposite of pressure build-up – but also corrected for by the pressure advance parameter.
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@digitalvision said in Pressure Advance Calibration:
At the risk of seeming to labour the point, you said......Also worth noting is that the single parameter pressure advance model doesn't completely eliminate all acceleration dependent effects – there seems to be some nonlinearities left. There is also some asymmetry in respect to acceleration and deceleration that isn't modelled completely.
Of course pressure compensation doesn't eliminate all acceleration dependent effects. For the simple reason that there are factors other than pressure that are at work during the acceleration and deceleration phases.
Pressure build up in the hot end is both a function of how fast you try to cram filament into it and also the time that you maintain that filament speed (within limits). The pressure build up continues way beyond the acceleration phase and well into the constant speed phase. So if you only calibrate it during the acceleration and deceleration phases, then when you start to do long moves at highish speeds, you'll find that more compensation may be needed.
But hey, I'm not going to labour the point. If you are happy doing it that way, then that's fine. I have a different way of calibrating pressure compensation which works very well indeed for me (edit - and there is no asymmetric requirement for the start of a move and the end of a move) . I just get nervous when people suggest that changes should be made to the way something in firmware works (in this case pressure advance) to suit their particular test method. In a nutshell, "it ain't broke (for me) so please don'y fix it".
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The Sailfish "JKN Advance" scheme (from, like, 2012) uses two factors, one primary pressure advance factor to pull in corner blobs, and a secondary factor which deals with skew between accel and decel on opposite sides of the corner. I'm not aware of any other pressure advance schemes using two factors like this.
I think we all know pressure advance isn't an ideal or perfectly accurate model... it's just a good fudge factor that helps correct some issues.
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I have an upcoming blog post that includes pressure advance in the testing, and I think it will surprise quite a few people.
