@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.