Linking Retraction to Linear/Non Linear Extrusion Correction?
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I wasn't completely sure on how the k factor was applied so I searched and found this:
actual_extrusion_speed = requested_extrusion_speed + (K * current_extruder_acceleration)
If you apply non linear extrution, the acceleration would ramp up slowly to accommodate for the multiplier and stay at the requested feed rate when hitting the maximum speed. (requested x multiplier from non linear extrution). If setting up the pressure advance before the non linear multiplier it would mean the result of the formula above is increasing with the multiplier and the advance would turn out to high.
Therefor I would think the non linear extrution needs to be set up first. To create an accurate extrution at all nozzle pressures.
Then set up the pressure advance to control the nozzle pressure when starting and stopping.
In theory you would then only need a tiny bit of retraction to pull the filament out of the melt chamber to stop oozing.Any thoughts?
Jeff
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Linear and non linear extrusion essentially correct for the same thing. A difference between the actual length of filament and that was requested, and that difference is linked to the speed by either a simple linear correction or with a non-linear component. What it required for a specific machine will be related to hotend design, material, nozzle diameter, and temperature setting.
I'd definitely look to set that up before pressure advance but after finding a temperature setting.
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@twigz said in Linking Retraction to Linear/Non Linear Extrusion Correction?:
Therefor I would think the non linear extrution needs to be set up first. To create an accurate extrution at all nozzle pressures.
Then set up the pressure advance to control the nozzle pressure when starting and stopping.
In theory you would then only need a tiny bit of retraction to pull the filament out of the melt chamber to stop oozing.Any thoughts?
Jeff
That's pretty well what I said towards the end of my post above. Quote from that post....:
"Anyway, I suggest you set non-linear extrusion by checking to see if there is a difference between measured filament extrusion at normal extruder flow rates and at normal extruder move length, with and without a hot end. Then once you have that set, check and adjust pressure advance. Then when you have set, check and adjust the retraction speed and amount."
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I landed here after trying pressure advance calibration , successfully printed the test but saw no change at all due to changing pressure advance values in the print , a comment made by Rcarlyle ( https://forum.duet3d.com/topic/6698/pressure-advance-calibration/44# ) led me here trying to work out why it didn't work.
Reading posts both here and elsewhere and doing some other searchs , I did not see any conversations on why filament feed rate is not PID controlled . Any reason why ( besides not looking hard enough :)) ? It looks to me like the control strategy for feeding filament is just proportional ( linear ) with the change in speeds while printing I would have thought that filament speed would increase to overcome the offset error to just proportional control from what ever the ideal speed is ?
For me I visualize the nozzle traveling over a smooth surface with a constant gap and that gap being filled by plastic and acting as a valve to plastic behind it , the result is it gets pushed wide at slow speeds as new plastic emerges and stretched out thinner at high speed with the new plastic emerging keeping up, the speed of movement is like a valve opening , fast wide open valve , slow almost closed .
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The filament is not PID controlled because on the majority of systems there is no feedback.
The laser monitor appeared to be a bit fussy on filament type to be usable for that. I had hoped that the magnetic based filament monitor would be able to tune the Extruder drive and would potentially be able to cope with the effect of the different back pressures that you allude to caused by different layer thicknesses, track widths, temperatures, and whether the deposited material is bridge, low density infill, etc. In truth there is too many variables to account for to do a full dead reconing correction style calibration. Decent filament monitoring is essential, and I suspect what will eventually drive me needing to unpgrade my d0.6 / 0.8.5 systems to Duet 2.
Edit: The magnetic filament monitor may or may not do that. It's only just been released and I've been too absorbed in machine builds to check it out.
Edit 2: link to blurb follows. On a flash read it doesn't appear to be linked to extrusion correction. Hopefully that will change soon if possible.
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Do we need feedback ?
Filament demand isn't varied due to another downstream process or disturbance like cooling ( changing ambient air temp) with the hotend or heatbed. The printer just pushes it out linearly ( Proportionally) based on print speed which we know and control before hand , that is a type of feedforward system . The gain of the system as a whole and just proportional control would be acceptable for a usual speed you print at . Once you go out side that ideal speed you get filament feed speed offset errors , that is where the I and D come in to get it back to the ideal extrusion speed to get correct width at a particular speed.
Maybe I should have asked straight off . Is Linear extrusion correction Feed forward?
Are the A and B variables the I and D from PID control?From that Extrusion Advance test I get at 20mm/sec 1.4mm and at 100 .5mm line widths using a .3mm nozzle I don't think a small change in filament diameter will have much effect on those widths as much as the change in speed.
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@digid thought you needed feedback for a PID loop yes, or you'd have no idea of how far from target you are.
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"A proportional–integral–derivative controller (PID controller. or three-term controller) is a control loop mechanism employing feedback that is widely used in industrial control systems"
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Thanks Doc , appreciate your time .
I found a blog that describes feedback and feedforward . Realise now that PID only works with feedback . Feedforward uses something else entirely . Kind of like what non-linear does it just uses an equation based on input disturbance and output result .
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No problem. I'm hoping the magnetic sensor is used for setting up the extrusion correction at some point but I suspect due to the data rate and resolution required for on-the-fly correction it will be limited to calibrating a fixed correction rather than correcting on the fly.
