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    Pressure Advance Calibration

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    Tuning and tweaking
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    • MOundefined
      MO
      last edited by

      0_1539512448390_IMG_20181014_201508.jpg 0_1539512462594_IMG_20181014_201558.jpg

      Here you can see exactly where I've disabled pressure advance.

      What's the lowest jerk you'd recommend to go with that doesn't cause issues with pressure advance?

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      • LHelgeundefined
        LHelge
        last edited by

        Also made a python-script to generate test pattern for pressure advance. I use a cylinder printed in spiral vase mode instead. Plan to extend it into generating a square pattern and Marlin-like pattern as well. Feel fre to try it out if you like. Just rename it to pa_cal.py and run the command:
        python pa_cal.py > pa_cal.gcode
        0_1539520976520_pa_cal.py.txt

        Anyone running into problems with Jerk on a Zesty Nimble above 0.1 when tuning pressure advance?

        garisundefined 1 Reply Last reply Reply Quote 0
        • deckingmanundefined
          deckingman
          last edited by

          I don't want to appear to be spamming these forums but I've just posted something elsewhere that has a bearing on this thread. https://forum.duet3d.com/topic/7276/high-speed-high-volume-flow-rate-printing

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

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          • LHelgeundefined
            LHelge
            last edited by

            A g-code question...

            Will these produce a different result for calibrating pressure advance, if so, which of them is best?

            G1 X0 Y0 F12000
            G1 F4200
            G1 X0 Y30 Ex.xx
            G1 X0 Y60 Ex.xx F1200
            G1 X0 Y90 Ex.xx F4200

            or

            G1 X0 Y0 F12000
            G1 F4200
            G1 X0 Y30 Ex.xx F4200
            G1 F1200
            G1 X0 Y60 Ex.xx
            G1 F4200
            G1 X0 Y90 Ex.xx

            Will the top one be interpreted as a linear interpolation of the feedrate between the start and end point and the second one be a more instant acceleration?

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            • dc42undefined
              dc42 administrators
              last edited by

              They should both be treated exactly the same.

              Duet WiFi hardware designer and firmware engineer
              Please do not ask me for Duet support via PM or email, use the forum
              http://www.escher3d.com, https://miscsolutions.wordpress.com

              LHelgeundefined 1 Reply Last reply Reply Quote 0
              • LHelgeundefined
                LHelge @dc42
                last edited by

                @dc42 That's a little bit contradictory to what's written in the feedrate part of G0/G1 documentation: https://duet3d.dozuki.com/Wiki/Gcode#Section_G0_G1_Move

                G1 F1500
                G1 X50 Y25.3 E22.4
                In the above example, we set the feedrate to 1500mm/minute on line 1, then move to 50mm on the X axis and 25.3mm on the Y axis while extruding 22.4mm of filament between the two points.
                G1 F1500
                G1 X50 Y25.3 E22.4 F3000
                However, in the above example, we set a feedrate of 1500mm/minute on line 1, then do the move described above accelerating to a feedrate of 3000 mm/minute as it does so. The extrusion will accelerate along with the X and Y movement, so everything stays synchronized.
                Feedrate is treated as simply another variable (like X, Y, Z, and E) to be linearly interpolated. This gives complete control over the acceleration and deceleration of the printer head in such a way that ensures that everything moves smoothly together, and the right volume of material is extruded at all points. The feedrate specified may not be reached due to a lower feedrate limit being configured, or the move being too short for the axis to accelerate and decelerate in time.

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                • RCarlyleundefined
                  RCarlyle @dc42
                  last edited by

                  @dc42 Why doesn’t RRF use jerk in those cases?

                  dc42undefined 1 Reply Last reply Reply Quote 0
                  • dc42undefined
                    dc42 administrators @RCarlyle
                    last edited by

                    @rcarlyle said in Pressure Advance Calibration:

                    @dc42 Why doesn’t RRF use jerk in those cases?

                    The reason for having jerk is so that if you print a curve made up form short line segments, the print head can maintain a constant speed. Without jerk it would have to stop at the boundaries between segments, to avoid an instantaneous change in X or Y speed due to the small change in direction. Jerk is not required in other cases.

                    Duet WiFi hardware designer and firmware engineer
                    Please do not ask me for Duet support via PM or email, use the forum
                    http://www.escher3d.com, https://miscsolutions.wordpress.com

                    RCarlyleundefined 1 Reply Last reply Reply Quote 1
                    • RCarlyleundefined
                      RCarlyle @dc42
                      last edited by

                      @dc42 If jerk benefits you on extrusion-extrusion corners, it will also benefit you on starts, stops, travel-extrusion changes, and extrusion-travel changes. From a speed standpoint at least, if not so much blobbing. I could see it affecting performance of coast, as well, if you slow to a stop between the last print segment and the start of the first coast segment. What’s the downside of doing it?

                      Another question if you don’t mind, since we’re talking about it... does RRF ever use different entry/exit speeds at the same corner? For example, if two colinear segments have different feedrates, will RRF decel all the way to the new speed, or will it jerk at the corner?

                      deckingmanundefined 1 Reply Last reply Reply Quote 0
                      • deckingmanundefined
                        deckingman @RCarlyle
                        last edited by

                        @rcarlyle said in Pressure Advance Calibration:

                        @dc42 If jerk benefits you on extrusion-extrusion corners, .....................

                        But does it offer a benefit? My take on it is that "jerk" or instantaneous speed change is just horrible (as it's name implies). As David has pointed out, it is necessary for segmented curves otherwise the print head would have to decelerate to a complete stop at the end of every segment, before starting the next so the time to complete a segmented arc move would be just too long. The same could be said for any situation where there are a series of very small moves. The only "benefit" is that it saves time in those situations but in terms of motion control, it's just horrible. I just think of "jerk" as a necessary evil that we have to put up with.

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

                        DigitalVisionundefined 1 Reply Last reply Reply Quote 0
                        • dc42undefined
                          dc42 administrators
                          last edited by dc42

                          Using XY jerk on starts/stops etc. would require intentional extruder jerk too. In order to apply pressure advance to a move that requires extruder jerk, it would be necessary to instantaneously advance or retract the filament by the appropriate amount (the amount of extruder jerk required multiplied by the pressure advance time). That's even more impossible than changing the speed instantaneously.

                          Duet WiFi hardware designer and firmware engineer
                          Please do not ask me for Duet support via PM or email, use the forum
                          http://www.escher3d.com, https://miscsolutions.wordpress.com

                          RCarlyleundefined 1 Reply Last reply Reply Quote 0
                          • DigitalVisionundefined
                            DigitalVision @deckingman
                            last edited by

                            @deckingman said in Pressure Advance Calibration:

                            I just think of "jerk" as a necessary evil that we have to put up with.

                            Based on some experimentation I theorize that some "jerk" might actually be advantageous and make the motion smoother. Consider the motion platform a dynamic system (difference in actual vs commanded nozzle position). This can be modeled as a spring-damper system, which models e.g. ringing artifacts. "jerk" effectively puts some pre-tension on the spring in this system which in some of my tests seems to lead to a more faithful adherence to a linear acceleration profile, and in turn make the assumption pressure advance is based on more accurate.

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                            • RCarlyleundefined
                              RCarlyle @dc42
                              last edited by

                              @dc42 other firmwares use extruder jerk with no issues. High E jerk is commonly used to improve retraction performance with geared 3mm filament extruders. Sailfish even allows small instant position jumps with E axis pressure advance. It works great!

                              Remember, there’s a “torsion spring” torque/error relationship between the stepper driver’s coil energization angle and the physical rotor angle. A position jump of a few microsteps just rapidly changes the instantaneous torque on the rotor. There’s absolutely nothing wrong with doing that on an extruder which is highly damped. In fact, I would say it’s highly desirable since it lets you unload the built-up pressure/compression a lot faster.

                              Imagine an extruder pushing right up near stall (one full step of load angle) and you want to retract. Why would you gradually accelerate through the ”dead travel” of >1 full step of coil energization angle between peak forward torque and applying reverse torque?

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                              • dc42undefined
                                dc42 administrators
                                last edited by

                                @RCarlyle, I understand what you are saying, especially in regard to using extruder jerk or position jump to increase initial retraction speed. Maybe I will implement that. However, the movement in 3D printer firmware seems to be in the opposite direction, towards using S-curve acceleration. Once we have S-curve acceleration it becomes possible to compensate for elasticity in the motion system.

                                Duet WiFi hardware designer and firmware engineer
                                Please do not ask me for Duet support via PM or email, use the forum
                                http://www.escher3d.com, https://miscsolutions.wordpress.com

                                RCarlyleundefined 1 Reply Last reply Reply Quote 0
                                • Synapsisundefined
                                  Synapsis
                                  last edited by

                                  After going from rods to lineat Hiwin guides I tried your test print. This is the results I get.
                                  I'm no expert so I would like to know what could be done to better the print.
                                  alt text

                                  Thanks for the info and any help

                                  DigitalVisionundefined 1 Reply Last reply Reply Quote 0
                                  • RCarlyleundefined
                                    RCarlyle @dc42
                                    last edited by

                                    @dc42 what’s the thinking on how you improve elasticity compensation with S-curve motion? (Not disagreeing; just don’t know what the approach is)

                                    dc42undefined 1 Reply Last reply Reply Quote 0
                                    • DigitalVisionundefined
                                      DigitalVision @Synapsis
                                      last edited by

                                      @synapsis, it's a bit hard to see from that photo, but here's roughly what I think you see:

                                      0_1540052464500_test_print.jpg

                                      It's a bit weird that the four different bands look so different from each other. What are your jerk and acceleration settings? What type of extruder?

                                      For the right setting here, I'd try the orange arrow location. Above it pressure advance is too aggressive leading to under-extrusion during deceleration.

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                                      • dc42undefined
                                        dc42 administrators @RCarlyle
                                        last edited by dc42

                                        @rcarlyle said in Pressure Advance Calibration:

                                        @dc42 what’s the thinking on how you improve elasticity compensation with S-curve motion? (Not disagreeing; just don’t know what the approach is)

                                        My thinking is this. Let's assume a Cartesian printer for now. In order to accelerate (say) the X axis, the motor and belt have to impart a force to the head given by F=ma. But the belt and the motor are springy, so in order to really impart force F to the mass of the print carriage, the motor must move by an additional amount S=ma/k where k is the spring constant. If the acceleration changes abruptly, then this requires instantaneous changes in motor position, which are impossible; but if we use S-curve acceleration then the required changes in motor position are gradual and should be achievable.

                                        Duet WiFi hardware designer and firmware engineer
                                        Please do not ask me for Duet support via PM or email, use the forum
                                        http://www.escher3d.com, https://miscsolutions.wordpress.com

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                                        • Synapsisundefined
                                          Synapsis
                                          last edited by

                                          Thanks for the reply. my setting at the moment are
                                          M92 X160 Y160 Z3200 E837 ; Set axis steps/mm Set extruder steps/mm
                                          M350 X16 Y16 Z16 E16 I1 ; Set 16x microstepping with interpolation
                                          M566 X500 Y500 Z150 E500 ; Maximum jerk speeds mm/minute
                                          M201 X1200 Y1200 Z18 E800 ; Accelerations (mm/s^2)
                                          M203 X12000 Y12000 Z120 E12000 ; Maximum speeds (mm/min)
                                          M906 X1200 Y1200 Z1800 E1200 I30 ; Set motor currents (mA)extruder current just enough will click not strip

                                          I have a Titan Aero and have set my pressure advance to the settings that are in the first post. Opps just seen that in fact there are 0 and not 0.074 I try and take a better picture in day light.

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                                          • DigitalVisionundefined
                                            DigitalVision
                                            last edited by

                                            Here are measurements from three different step functions in extrusion rates:

                                            0_1540059384616_85c38dae-b378-4696-a943-91d89c25ebb5-image.png

                                            And here they are individually:

                                            0_1540058967869_b0c174c6-e88f-4796-9d51-0df9f0d30e5b-image.png

                                            I'd not completely trust these measurements. There is some hysteresis in the measurement jig that makes these values not perfectly repeatable (probably varies by up to ±5 % when remeasured), and the lower speed extrusions suffer from some extrusion pulsing that makes a width measurement more questionable. That being said – the general shape and trends should be fairly accurately captured.

                                            Let's try to create a simple model: Assume there is a property p that roughly corresponds to what one could call 'pressure' inside of the hot end, and let's assume the rate of change of this property with time corresponds to

                                            p'(t) = i(t) - o(t),

                                            where i is the input rate (feed rate) of filament into the hot end, and o is the output or extrusion rate of molten plastic from the nozzle. We then assume that the extrusion rate is a function of this property p: o = f(p). The simplest possible function is a simple linear correspondence:

                                            o = k*p

                                            for some k. Substituting this function into the change rate p'(t) gives:

                                            p'(t) = i(t) - k*p(t),

                                            Which gives a simple differential equation. Fitting this to the data to find the model parameter k yields the following results:

                                            0_1540060832165_5f6e6cad-44a4-4fb6-aa8c-a4eab1e16b9a-image.png

                                            The optimal model parameter found was k = 6.507 (the unit for i and o is mm^3/s). While this extremely simple model is obviously not a perfect fit (there seems to be some second order lagging extrusion), it still seems to explain most of the observed behavior.

                                            @dc42, would this model be similar or even identical to the one currently used?

                                            brunofportoundefined 1 Reply Last reply Reply Quote 0
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