More strange pressure advance behaviour

deckingman

I'm wondering if there is any point in me raising this issue yet again but here goes...........

So on another forum, I have now established that although slic3r generates uneven segment sizes, seemingly at random, the extrusion rate itself does not vary. To illustrate:

Here is a sample of "problem" gcode.....

G1 X246.350 Y203.493 E0.06750
G1 X245.357 Y203.301 E0.06755
G1 X244.373 Y203.062 E0.06765
G1 X242.926 Y202.622 E0.10103
G1 X241.976 Y202.271 E0.06763
G1 X241.044 Y201.877 E0.06763
G1 X239.687 Y201.208 E0.10104
G1 X238.807 Y200.709 E0.06763
G1 X237.951 Y200.170 E0.06756
G1 X237.122 Y199.592 E0.06754

Calculating the move distance from the XY coordinates we get these segment lengths...

1.0113916155476
1.01260900647782
1.5124182622542
1.01276897661806
1.01185967406551
1.51294745447422
1.01163283853384
1.0115616639632
1.01060625369129

Dividing the extrusion amount by the segment lengths we get the following extrusion rates.....

0.066789163526362
0.066807622258179
0.066800304202502
0.066777321937563
0.066837331038475
0.066783548695757
0.066852317781633
0.066787821649257
0.06683117163911

Therefore the theory that it is changes in the extrusion rate that causes pressure advance to trigger, doesn't explain the behaviour I am seeing (unless changes of extrusion rate to the 4th decimal place are considered significant).

To summarise.......

A value of 0.4 or 0.5 pressure advance is required by my printer/hot end to cure over/under extrusion at the start and end of longish moves at anything other than slow print speed. If I try to print cylinders, circles or arcs using that amount of pressure advance, (with a single extruder) it will sometimes trigger, seemingly at random, multiple times during an arc move. This is visible and has been captured on video and a link to that video has been provided (as have the stl files, the gcode files, and the configuration files as well as M122 diagnostic reports). This multiple triggering of pressure advance causes the print head to falter and jerk leading to ruined prints.

Coincidentally, the gcode generated by slic3r randomly has uneven segment sizes for any given arc, but the extrusion rate itself does not vary. I can't help think that might be a clue as to the cause - we have seemingly random changes to segment length, and we also have seemingly random triggering of pressure advance......

Also, this behaviour (with a single extruder), was not apparent prior to firmware updates around January this year. Although since July of last year, I have had similar problems when using multiple extruders.

Over the last 12 months, I have changed just about every setting that has been suggested or that I can think of. I've updated the firmware multiple times. I've generated multiple reports, made numerous videos, and carried out extensive testing in order to provide as much information as possible. Yet still the problem persists with no sign of any cure being forthcoming. Getting just a tad fed up now.............

dc42

This is on my list to look at for the 2.01 release.

dc42

Hi Ian, please can you provide the links to a GCode file generated by slic3r that exhibits the problem shown in your video, and your config.g file.

EDIT: I've just found a link to https://drive.google.com/drive/folders/0B_MwtHtQR_Zvd1BSd1RKSFYwbTQ in an older post of yours. Do those correspond to the video that you posted?

deckingman

@dc42
Hi David. Sorry that it took a while but yes, those files are the ones and that folder is still on my google drive and shared. My config.g has changed somewhat since then due to changing the Titan extruders for Bondtech ones and one or two other changes that I've made.

For info, I've raised the uneven segment length as an issue on slic3r GitHub but I doubt if anything will come of it as it doesn't seem to affect print quality (as long as pressure advance isn't used).

dc42

Thanks. Are you able to reproduce the problem using a cylinder sliced in vase mode?

deckingman

@dc42 said in More strange pressure advance behaviour:

Thanks. Are you able to reproduce the problem using a cylinder sliced in vase mode?

I don't think I've ever tried that but I'll give it a go to see what happens. I have no time to do it this weekend though. However, I'm taking a bit of break from my day job next week, so I should have a fair amount of time to do some more testing from Tuesday onwards.

deckingman

I've had a quick look at this. I can't use vase mode on those test cylinders because they are multiple. So I've created a tapered cylinder which will give me circles of differing radii. I've sliced it firstly with zero infill, 3 perimeters and no top, and secondly in spiral vase mode (single perimeter, no top, constantly varying Z). Looking through the gcode files, both look like they have unequal segment lengths for any given circle. I'll print them both with highish pressure advance enabled (probably won't manage it over the weekend but I'll do me best) and report back. I'll do them with a single extruder to start with, and again using multiple extruders.

Knaudler

hello

no problems any more with BondTech extruder (800mA on stepper - cr10)
with 0.4 Pressure advance (700mm Bowden)
(on very small things 0.1-0.2)
4mm retract with 60mm/sec

(maybe the normal extruder loses steps ?)
bondtech has 3:1 ? so there is more torque

Firmware Version: 2.0(RTOS) (2018-06-05b3)
WiFi Server Version: 1.19.2 (for better Wifi)
Web Interface Version: 1.21.1

cura or S3D, it works

on anet a8 bondtech works great because of more torque
(original Board)
the stepper has always lost steps

deckingman

@dc42

David, I've found something really significant. Please read this post fully.

In summary, and contrary to what we've both been thinking, increasing extruder micro stepping from 16x to 256x cures the problem (or at least it's a usable work-around)!

That is to say, printing the version of the tapered cylinder mentioned above, which has 3 perimeters but no infill, with 256x micro-stepping for the extruders, pressure advance happens just once at the start/end of a circle, as I would expect. Printing the spiral vase version of the same object, after the first 3 base layers, pressure advance doesn't appear to be applied ever (that's also fine by me as print speed is slow under those conditions).

However, if I set micro-stepping to 16x, (which is what I have always used until a short time ago) both gcode files print appallingly badly. That is to say the observable behaviour is as if pressure advance is being applied multiple times throughout any given circle and the print head seems to pause and wait for that to complete before it moves on the next segment. This applies to the spiral vase version of the file, as well as the version with 3 perimeters. Running M122 while this appallingly bad behaviour is happening, doesn't report anything untoward that I can see.

Printing either of those files at 16x micro-stepping but no pressure advance also results in good prints with no pauses or jerky motion. It's a combination of 16x micro-stepping plus 0.5 pressure advance that causes the problems.

Both files show significantly unequal segment sizes. I created the stl from an OpenScad file using $fn set to 300. This causes Slic3R to "do it's own thing" and change the number of facets to something else (64 IIRC). That may or may not be the cause of the unequal length segments.

Possibly the unequal size segments cause pressure advance to trigger but only with low micro-stepping. Is that possible? Does it make any sense to you?

I did some further testing with extruder micro-stepping set to 32x and 64x. In each case , the observable problem was less severe. i.e 32x was better than 16x and 64x was better than 32x but still not good.

Finally, I did an extreme test using 3 extruders (all the above prints were done with a single extruder) with 256x micro-steppping and pushed the speed up to 300%. The objects were sliced at 60mm/sec so I was printing at somewhere up to 180mm/sec. All with pressure advance set to 0.5. No problem! Although under those conditions with 0.5 pressure advance, the extruders do crazy things but there was no hesitation or jerky behaviour of the print head.

I've created a folder within that shared folder that I have already linked to called "AsOf140718" and put both the gcode files in there, as well as my latest config.g.

Finally, I'm still on older firmware -
Firmware Electronics: Duet Ethernet 1.0 + DueX5
Firmware Version: 1.20.1RC2 (2018-01-01)
Web Interface Version: 1.20

Hope this all helps shed some light on what the heck is happening.

Ian

dc42

Thanks Ian, that really helps. I have a couple of theories that I need to follow up about why low microstepping and high pressure advance together might cause the problem.

deckingman

@dc42

David, I've just tried printing one of those files again with lower pressure advance. The problem is present but not as severe even when using a value of 0.1 but with a value of 0.05 there is no observable issue (might just be too mild to be noticeable). To be clear, that's using a single extruder at 16x micro-stepping. So it's not necessarily high pressure advance but the problem is more sever the higher the pressure advance value.

What's bugging me is that it isn't being widely reported. It's not like it's difficult to spot - it's really bad and ruins the print. Quite noticeable even with pressure advance set to 0.1.

dc42

When you say it's with a single extruder, do you really mean that, or do you have the other extruders set to a very low percentage in the mix? I think it may make a big difference.

deckingman

@dc42 said in More strange pressure advance behaviour:

When you say it's with a single extruder, do you really mean that, or do you have the other extruders set to a very low percentage in the mix? I think it may make a big difference.

Dam. My bad. I thought I had mixing ratios set to unity but I've just checked and somewhere along the line I'd uploaded a config with mixing ratios of 0.96:0.02:0.02. That's what I was using. When I get time, I'll test again with unity mixing ratio. Sorry....

deckingman

OK. Tested with mixing ratio of 1.00:0.00:0.00 and the result was the same as with a mixing ratio of 0.96:0.02:0.02. That is to say very jerky head movement and by observing the extruder, the appearance that pressure advance is being applied multiple times around the circle. It is observable (if you watch the extruder gear) at lower pressure advance values like 0.1 but much less severe that at higher values. That's with 16x extruder microstepping. I can toggle the fault on and off by changing the extruder micro-stepping "on the fly" from the gcode console. So entering this M350 E16:16:16:16:16 will cause the fault to happen, and entering this M350 E256:256:256:256:256 will cure the fault.

deckingman

Trying to understand why this problem isn't being more widely reported. It's probably a unique combination of various factors but in an attempt to eliminate one of them, I defined Tool 0 simple as M563 P0 D0 H1 and commented out all the lines to do with mixing etc. Printing the tapered cylinder with 16x micro-stepping for the extruder and 0.5 pressure advance exhibits the same bad behaviour as having the tool defined to use multiple extruders but with the mixing ratio set to 1.00:0.00:0.00, so it isn't that. Oh well.....

wilriker

@deckingman I guess that a value of 0.5 for pressure advance is even high for a long bowden setup as far as I can tell. So maybe that's the reason why it is rarely ever reported.

deckingman

@wilriker said in More strange pressure advance behaviour:

@deckingman I guess that a value of 0.5 for pressure advance is even high for a long bowden setup as far as I can tell. So maybe that's the reason why it is rarely ever reported.

The thing is, I can see the same behaviour with lower values although at 0.1 or less, most people probably wouldn't notice unless they looked really closely. 0.3 or more is quite alarming though so I'm sure there must be some other reason why I see this behaviour. There are a lot of other things that are a bit unique to my printer. Two A and two B motors, 5 colour Diamond hot end, 0.5mm nozzle, big build volume, the use of Slic3R which generates strange looking segments sizes for arcs, - the list goes on. It's strange why upping the extruder steps per mm to 256x from 16x cures the problem though.

deckingman

Following on from this thread https://forum.duet3d.com/topic/6090/problem-printing-circles/26 I tried re-slicing the object with "detect thin walls" disabled and "resolution" set to 0.1. The problem persists with extruder micro-stepping set to 16x, so no change. The gcode file still shows large variations in segment size which may be acting as trigger for whatever is causing the issue?

OBELIKS

Check this one also, at the end: https://forum.duet3d.com/topic/6078/core-xy-movement-issues/57
He swapped M92 and M350 in config.g

deckingman

@obeliks said in More strange pressure advance behaviour:

Check this one also, at the end: https://forum.duet3d.com/topic/6078/core-xy-movement-issues/57
He swapped M92 and M350 in config.g

Thanks for trying but yes I've been trying to help that OP also so I'm well aware.

Edit. But just for the hell of it, I made those same changes (even thought it's not correct) and it didn't help my issue.