The Klipper tuning guide suggests how to tune pressure_advance parameter. However, there is an additional parameter that affects Pressure Advance - the smooth time. Typically it is recommended to leave it at default value, and changing it does not noticeably affect the extruder performance. However, I recently came across the case where I noticed the effect of the smooth_time on the print quality.
I’ve been recently printing with a Matrix Light extruder with Volcano hotend and a bit larger nozzles - 0.5 and 0.6 mm. I noticed some small defects around corners that looked like they could be related to PA (though the PA coefficient was tuned using the tower). So I tried to see if smooth_time has any effect on it, so I printed a couple of tests to check that. Next, come the tests with 0.5 mm nozzle.
First, I printed 4 small square models with smooth_time = 0.04 (default), 0.02, 0.01 and 0.005 with 0 infill and 2 perimeters. This was printed with PA=0.034 that I tuned using the default smooth_time = 0.04. Here’s how one of the corners of the model (the same corner) looks like (counter-clockwise from the top left corner - 0.04, 0.02, 0.01, 0.005):
It is not easy to capture, but one can probably see that the corner of 0.04 model is a bit more ‘smoothed’ and there is under-extrusion, not exactly at the corner, but slightly before it. I see it that the PA starts to ‘kick in’ a bit too early due to smooth_time being a bit too large for the extruder, and so the filament flow starts to decrease before the toolhead actually starts to slow down, hence the underextrusion. On 0.02 model it is already a bit better, and 0.01 and 0.005 models are about the same, and there is a tiny hole between the two wall on those 2 last models, but it is pretty much precisely at the corner, unlike the other two models (0.04 and 0.02).
Here’s the same corner from the side (0.04, 0.02, 0.01 and 0.005 left to right):
The 0.04 model has more smoothing in the corner, and there is even a small ‘dent’ in the middle of that smooth corner, which is quite visible under certain angles. 0.02 model is a lot better, and 0.01 and 0.005 are pretty much the same, they are good without major visible defects.
I then also re-printed the PA tuning tower with smooth_time = 0.01, here’s how it looks like in comparison with 0.04 (left, a bit unfinished is 0.04, right is 0.01):
I noticed that the PA gets a bit larger with smaller smooth_time = 0.01 - it is ~0.04-0.044 instead of 0.034. One can also notice a small dent ~1 cm from the corner. This is where the acceleration starts to change. It goes through almost the entire 0.04 model, but starts much later on 0.01 model. This is, I think, due to the fact that PA gets a bit out of sync with the actual motion with smooth_time = 0.04, and so we start to see over- and under-extrusions even before we exceed the optimal PA value.
So, my take is that on the extruders with a very small PA value (and/or very short filament path) it might be beneficial to adjust the smooth_time parameter to visibly improve the quality of the prints. However, I am unsure about the exact tuning procedure: TUNING_TOWER is not necessarily helpful here, as it is quite beneficial to look at the corner of a model from the top to see how the filament lines from different walls go together at the corner. Changing smooth_time on different layers does not permit that inspection. And it might be difficult to see the right smooth_time value by looking at the model (e.g. PA tuning tower) from the side, as the defect, if present, is fairly faint.
@koconnor FYI, I know you did some experiments with the smooth_time previously, and you had some draft proposal how to tune it.