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    Pressure Advance Calibration

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    Tuning and tweaking
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    • deckingmanundefined
      deckingman @Phaedrux
      last edited by

      @phaedrux Yes I think it's best to tune everything else first - speeds, temperatures, acceleration, jerk etc until you get the best quality you can. Only then start playing around with pressure advance and other things. And only ever change one thing at a time (I'm not a fan of Taguchi methods). There are just too many interactions going on I've found. Oh, and make copious notes along the way.

      Ian
      https://somei3deas.wordpress.com/
      https://www.youtube.com/@deckingman

      1 Reply Last reply Reply Quote 0
      • bartolomeusundefined
        bartolomeus
        last edited by

        For python noobs like me: I uploaded the script into a browser based python implementaition, and it outputs the gcode: http://www.skulpt.org/#

        1 Reply Last reply Reply Quote 2
        • bartolomeusundefined
          bartolomeus
          last edited by

          I just printed with this script two times, one with PA from 0 to 0.2, and one with PA from 0.1 to 0.3.
          During the print I entered M572 a few times to see what the actual settings are, and I could see the values change. At the end of the second print the returned value was: M572 - Extruder pressure advance: 0.298, 0.000

          First: what is the second output: 0.000?
          Second: I saw nothing in the print....really nothing. No gaps tearing or anything.

          My setup: CoreXY, E3d, Bondtech BMG, 650mm capricorn, Firmware 2.02RC1

          wilrikerundefined 1 Reply Last reply Reply Quote 0
          • wilrikerundefined
            wilriker @bartolomeus
            last edited by

            @bartolomeus It is not gaps or tearing that you are looking for but the value where there is a consistent extrusion width.

            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

            bartolomeusundefined 1 Reply Last reply Reply Quote 0
            • bartolomeusundefined
              bartolomeus @wilriker
              last edited by

              @wilriker ok, but I still see nothing. All layers are identical, al columns in the print turned out exactly the same. Nothing like the pictures above.

              Phaedruxundefined wilrikerundefined 2 Replies Last reply Reply Quote 0
              • Phaedruxundefined
                Phaedrux Moderator @bartolomeus
                last edited by

                @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?

                Z-Bot CoreXY Build | Thingiverse Profile

                bartolomeusundefined 1 Reply Last reply Reply Quote 0
                • wilrikerundefined
                  wilriker @bartolomeus
                  last edited by

                  @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.

                  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

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                  • bartolomeusundefined
                    bartolomeus @Phaedrux
                    last edited by

                    @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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                    • bartolomeusundefined
                      bartolomeus
                      last edited by

                      0_1536956172493_IMG_20180914_221237.jpg

                      wilrikerundefined 1 Reply Last reply Reply Quote 0
                      • wilrikerundefined
                        wilriker @bartolomeus
                        last edited by

                        @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

                        bartolomeusundefined 1 Reply Last reply Reply Quote 0
                        • bartolomeusundefined
                          bartolomeus @wilriker
                          last edited by

                          @wilriker I'll give higher a go tomorrow.

                          My config: 0_1536956692480_config.g

                          wilrikerundefined 1 Reply Last reply Reply Quote 0
                          • wilrikerundefined
                            wilriker @bartolomeus
                            last edited by

                            @bartolomeus You don't have a M584 command 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.

                            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

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                            • bartolomeusundefined
                              bartolomeus
                              last edited by

                              One thing I should add: in the script I changed min speed to 20.

                              1 Reply Last reply Reply Quote 0
                              • bartolomeusundefined
                                bartolomeus
                                last edited by

                                Ok, so I had to increase PA to 1 to see a result. I really need to switch to direct drive.

                                1 Reply Last reply Reply Quote 0
                                • fmaundefined
                                  fma
                                  last edited by

                                  Thanks to this great script! I ran it, and found 0.1s, which is the value I was using.

                                  Frédéric

                                  1 Reply Last reply Reply Quote 0
                                  • brunofportoundefined
                                    brunofporto
                                    last edited by

                                    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?

                                    bartolomeusundefined 1 Reply Last reply Reply Quote 0
                                    • RCarlyleundefined
                                      RCarlyle
                                      last edited by

                                      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:

                                      1. 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.
                                      2. Higher extrusion rate = shorter residence time in the hot zone = less time for heat to transfer = cooler extrusion = more viscosity.
                                      3. 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.

                                      DigitalVisionundefined 1 Reply Last reply Reply Quote 3
                                      • bartolomeusundefined
                                        bartolomeus @brunofporto
                                        last edited by

                                        @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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                                        • DigitalVisionundefined
                                          DigitalVision @RCarlyle
                                          last edited by DigitalVision

                                          @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:

                                          1. 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.
                                          2. Run this script to find the pressure advance setting that minimizes with changes during the transition period.
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                                          • DigitalVisionundefined
                                            DigitalVision
                                            last edited by

                                            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.

                                            0_1539061215335_866b3134-bb52-4888-b4e5-5b166e54cd8c-image.png

                                            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.

                                            deckingmanundefined 1 Reply Last reply Reply Quote 0
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