E acceleration has to be limited to to E jerk/PA

oliof

@dc42 dropped this absolute truth bomb in discord today:

E acceleration has to be limited to the value of E jerk divided by pressure advance

I really would like to have this enshrined in the documentation.

Also, it would be great if the firmware warned on acceleration exceeding this limit, and allowing for an E acceleration setting that autocomputes accel by dividing jerk by PA.

ctilley79

@oliof

Can you give an example with typical jerk settings for a geared direct drive extruder motor?

oliof

@ctilley79 my Orbiter 1.5 on an EVA3 print head (roughly 60mm between feeder gear and melt zone) I used jerk 300 at a pa of 0.02 to 0.03, so 10,000 to 15,000 max accel with the limits above (my setting was at 6,000 accel, so well within limits).

ctilley79

@oliof Gotcha. I have my E jerk set ridiculously high at like 3000, so I'd never hit any limit with acceleration. It was my prior understanding is that for PA, you need high jerk values for E. I guess I need to reevaluate that.

oliof

@ctilley79 my jerk may be too low. Your understanding is correct.

oliof

@ctilley79 look, @Phaedrux dropped this other thing the other day: E Jerk can be very high, between 3000 and 6000:

https://forum.duet3d.com/topic/33948/very-low-acceleration-during-printing/6?_=1698255872645

droftarts

@oliof I’ll check with @dc42 and update the advice on M566 jerk for extruders, and acceleration. In the old days before PA it was usually set quite low, to avoid skipping or stripping the filament during advance or retract, but this seems to have changed with PA, with a knock on affect on extruder acceleration.

Ian

oliof

@droftarts thanks! Please don't forget to also update the configurator to provide sane values. I know it errs on the side of conservative values, but it should not be completely outside the ballpark mentioned in documentation (machine specific values notwithstanding).

gloomyandy

@oliof I know this topic is old, but I have a quick question about the assumed units in this discussion.

Normally jerk is given in mm/min but acceleration in mm/s/s. In the example above @oliof has a jerk of 300 (which I assume is 300mm/min) and with a PA of 0.02 this allows for an acceleration of up to 15000. However doesn't the jerk value need to be converted to mm/s before that calculation? If you do that you get a much lower acceleration limit of only 250mm/s/s! Even with the higher jerk values suggest above of 3000mm/min the limit is "only" 2500mm/s/s.

So my questions are:

Are the jerk values quoted above in mm/min?
should the jerk value be converted to mm/s before dividing by PA to give the acceleration limit?

oliof

@gloomyandy the values I quote are as used in my config.g, so yes there is a unit confusion error here. Which would put massive limits on e acceleration (although it is not entirely clear to me what happens if E acceleration exceeds the limit of E jerk/PA).

EDIT: OR the limits are to be computed as above and the unit confusion is irrelevant. One for @dc42 to clarify.