Pressure Advance killing extruder motor

Edgars Batna

@garis The firmware jerk actually increases when you enable PA in the current implementation. It is computed as

pa_jerk = jerk / pa_time

so, in your case the firmware attempts to wiggle the motor 50 times faster than you've set it. At 8 microsteps (assuming TMC 2660) without interpolation you get loads of resonance fast. Plus at 40:1 it's a stretch to get it spinning as fast as 30mm/s with all the wiggling going on.

Have you checked if the EMF is too high on https://reprapfirmware.org/ ?

Alexander Mundy

I have Nimbles which use 30:1 and solved the jerk problem by using these 3.6 low inertia disc magnet steppers:

https://www.portescap.com/products/disc-magnet-motors/p430-disc-magnet-high-speed-step-motor

They are extremely expensive new but I lucked into a sweet deal on used ones which didn't break the bank.

Here is an 80mm/s retraction test that shows how fast that stepper can change direction:

https://youtu.be/sBAlXLfjx78

garis

Perhaps I haven't used a sufficiently high magnifying glass to understand what defects I should be observing or tolerating. My printing experience is limited and just now I am reprinting a few parts to upgrade the originals I printed for my Hypercube now that it is working.

As I understand it during the jerk phase the driver is asking the stepper to accelerate as fast as possible limited by the the motor current, up to the jerk setting leading to resonance in the more controlled velocity ramp during the acceleration phase. For the XY motors this is of concern but for the extruder I could imagine some/much of this resonance would be absorbed by the plastic, and perhaps some under extrusion if steps were missed. For a stepper with 2,600 steps that is likely to be of minor concern???

I am using a pancake stepper with lower inertia and it seems to work fine. (My original stepper was a recycled cheap one that did not move - it just growled at me - clearly the EMF was way too high on that one.)

The general print quality with PA of 0.02 was better than no PA. Using retraction speeds of 30 mm/s seems to be the sweet spot. The other variable is the print jerk with
the extruder being the slave but with the extruder able to limit the print speed - I am using XY jerk of 1,200 but higher values up to 2,000 + also seem to mechanically work ok, although print quality may be another matter.

I also don't know how far to go down this burrow given it seems to work ok and others don't seem have a problem - another limiting factor is the rotational flexibility of the nimble cable with the high performance magnetic motors just mentioned - curious about them.

Alexander Mundy

I don't have any data on the rotational flex of the Nimble cable. What I do know is that with the recommended Nema 14 and or the recommended Nema 17 pancake I could only increase extruder jerk to 60 and coupled with pressure advance made for really slow prints. With the disc steppers I can go to "normal" jerk and acceleration settings. Out of town so I can't see what they are right now.

gnydick

This post is deleted!

ayudtee

I can't explain the mystery as to why PA is causing this problem, very strange that everything works when it is turned off. PA is such a good feature, its worth trying to get it to work. I am recommending that you consider increasing your extruder motor current. 400mA seems low to me, as I am running 800mA on my Ormerod 2 and 700mA on my Delta.

deckingman

@ayudtee The motor current is what the stepper motor can tolerate and a good rule of thumb is to 85% of the rated maximum. So increasing the current to 800mA would not be a good idea if the Op's motors are rated at (say) 500 mA.

gnydick

@deckingman they're definitely rated for more. I think the logic is, at 40:1 gearing, anything much more than 400-500mA doesn't work well. I've tried it, it's not happy.

deckingman

@gnydick said in Pressure Advance killing extruder motor:

@deckingman they're definitely rated for more. I think the logic is, at 40:1 gearing, anything much more than 400-500mA doesn't work well. I've tried it, it's not happy.

Fair enough - there is nothing in this thread to state what the rated current of your motors are so I had to revet to using a crystal ball.

Referring back to my earlier post of 30th March @ 05:51, I might have some ideas but you haven't answered the questions I asked.

Alexander Mundy

It was explained to me this way. At such high steps per time frame required with high gear ratio the current doesn't have much time to decay and so higher current exasperates the situation. With 3.6 degree steppers I run normal current levels but couldn't with 1.8 degree steppers.

dc42

With such a high gearing ratio, you don't need much torque but you do need low inertia and high speed. So I suggest choosing a motor with low inductance, modest torque, and a high ratio of holding torque to rotor inertia. Run it close to its rated current (e.g. 85%).

Edgars Batna

@garis said in Pressure Advance killing extruder motor:

Perhaps I haven't used a sufficiently high magnifying glass to understand what defects I should be observing or tolerating. My printing experience is limited and just now I am reprinting a few parts to upgrade the originals I printed for my Hypercube now that it is working.

You need to use a magnifying crystal ball, preferably with Christmas decorations within the ball.

For the XY motors this is of concern but for the extruder I could imagine some/much of this resonance would be absorbed by the plastic, and perhaps some under extrusion if steps were missed. For a stepper with 2,600 steps that is likely to be of minor concern???

At 40:1 there's basically no resistance whatsoever on the motor end to absorb anything, so there is just the rotor.

gnydick

@dc42 thanks. Can you give an example of what qualifies as those suggestions? low inductance, modest, torque, high ratio of holding torque to rotor inertia?

garis

@dc42 My pancake has more than enough torque @ 30:1. What is the reason to use high current?

dc42

@garis said in Pressure Advance killing extruder motor:

@dc42 My pancake has more than enough torque @ 30:1. What is the reason to use high current?

The reason to use a motor current close to the rated maximum is to provide enough torque to overcome the inertia of the rotor, because high gearing means you need high acceleration.