How to tune Stall Detection?
I am currently trying to tune stall detection on my cartesian printer.
So I started by adding [c]M915 X Y Z S0 F0 R1[/c] to the config.g
First thing I learned is that stall detection obviously does not work when moving the axis via Web Control. It seems to be only enabled when printing a g-code file. Is this correct? If so, why? This makes tuning the values very difficult.
So I did some prints and changed the S value for each axis each time a stall detection was thrown. I suppose driver 0 is X, driver 1 is Y and 2 is Z.
Btw: Is there a way toch see the axis for each driver number?
I ended up with S12 for X and S16 for Y. But Z will never throw a false, even with S-10 which is supposed to be the lowest suggested value.
So what is best practice to tune these values and get the correct values for all three axes?
Stall detection is only enabled when printing a GCode file. It's not really possible to test it any other way, because you need the variety of moves that you get in a print to test it properly.
The mapping of axes to drivers is determined by the M584 command. Send M584 with no parameters to show the existing parameters.
If your Z axis never trigger stall detection, it probably means that your Z axis speed is configured such the the back emf of the motor is greater than the driver supply voltage at that speed. See https://duet3d.com/wiki/Choosing_stepper_motors#How_to_work_out_the_power_supply_voltage_you_need (there is a link to a spreadsheet at the end).
So this means, Z is simply to fast?
Use the link in my previous post to work out whether Z is too fast for stall detection to work, or not.
Unfortunately I do not know all the required information because there is no chart with all specifications available for my stepper motors. These are Anet 42SHDC3025-24B motors.
Phase inductance is the one that is missing.
I entered all values I know (left phase inductance as it was) and got these results:
[c]Speed at which torque starts to drop (mm/sec), low slip angle 22,3
Step pulse frequency at this speed and x16 microstepping 8,9
Speed at which torque starts to drop (mm/sec), high slip angle 29,7
Step pulse frequency at this speed and x16 microstepping 11,9
So the maximum speed for stall detection should be 22,3 mm/s, right?
My Z axis speed is 16.7mm/s but I never got a false stall detection with M915 Z S-10 F0 R1.
Is this possible? I would like to make sure it works.
You really do need the phase inductance. To estimate it, search a good range of stepper motors (e.g. the ones at stepperonline.com) for another Nema 17 motor with the same length and a similar phase resistance and rated current.
Well I tried to find similar stepper motors or other sources for the required information. After several hours I gave up now. I wrote the Anet support an email but to be honest, I do net expect to get a useful answer.
So I could reduce the speed and try to find a speed where I get false stall reports, reduce the speed even more (additional -2mm/s or so) and tune the S value again. Would this work to give me precise stall detection?
What I did now is:
I switched to using two stepper drivers (Z and E1) for my Z axis.
I added axes U and V for Z1 and Z2. I zero them when Z is homed so homing does not throw errors.
Changed [c]M915 Z S-10 F0 R1[/c] to [c]M915 P2:4 S-10 F0 R1[/c] just to be sure.
What I expected:
Using two drivers instead of one with the motors cinnected in series should double the maximum speed for stall detection. Is this correct?
Even at the very low speed of 5mm/s no false stall detections occur.
I am stuck now
Now I wrote a gcode script that moves Z up and down with 5mm/s over and over again.
No stalls detected.
When I reduce the current of the motors to 20mA, it stopps moving up (obviously), but still no stalls are detected.
M950 S was -50 during this test!
Well, no other suggestions?
I need to make this work!
Are you definitely running 1.20 or 1.21RC series firmware, on a Duet WiFi or Duet Ethernet?
What are your Z steps/mm?
It's 400 steps/mm
From comparing other motors, I estimate the inductance of that motor to be about 8 to 10mH. What maximum Z speed at full torque does that give?
I am not 100% sure, if all the other values I have are correct.
I took them from this site: http://www.mytechnologie.de/2017/05/22/projekt-3d-drucker-die-motoren/
I suppose by "maximum Z speed at full torque" you mean "Speed at which torque starts to drop (mm/sec), high slip angle".
For 8mH this gives me 32,7mm/s and for 10mH it is 27,9mm/s.
No additional suggestions?
This feature was one of the main reasons for buying the Duet
Is it just the Z motor that you can't get stall detection working on? Does it work on the X and Y motors? How about U?
It works on X and Y. I tuned the values until I got no more false detections. S22 seems to be the value to go on these axes.
I tried [c]M915 Z S-50 F0 R1[/c] and [c]M915 P2:4 S-50 F0 R1[/c] for Z but never got stall detections on drives other than 0 and 1 (X and Y).
I have added this to my list of pending investigations. I should get to it before the end of this coming week.
Great, thank you!