FLOAT PIN FOR FOLLOWING ERRORS?

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7 years 11 months ago #120679 by COlger81
What following error would you expect to see roughly at 150 IPM from a decent mill? .030" (.762mm) seems pretty extreme and i think i have some tuning to do.

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7 years 11 months ago #120681 by dgarrett
joint.N.f-error-lim is a hal output pin from the motion module
Code:
$ man motion ... joint.N.f−error−lim OUT FLOAT The following error limit ...
linuxcnc.org/docs/master/html/man/man9/motion.9.html


Relevant ini settings:
Code:
2.13. [JOINT_<num>]Section ... MIN_FERROR = 0.010 - This is the value in machine units by which the axis is permitted to deviate from commanded position at very low speeds. If MIN_FERROR is smaller than FERROR, the two produce a ramp of error trip points. You could think of this as a graph where one dimension is speed and the other is permitted following error. As speed increases the amount of following error also increases toward the FERROR value. FERROR = 1.0 - FERROR is the maximum allowable following error, in machine units. If the difference between commanded and sensed position exceeds this amount, the controller disables servo calculations, sets all the outputs to 0.0, and disables the amplifiers. If MIN_FERROR is present in the .ini file, velocity-proportional following errors are used. Here, the maximum allowable following error is proportional to the speed, with FERROR applying to the rapid rate set by [TRAJ]MAX_VELOCITY, and proportionally smaller following errors for slower speeds. The maximum allowable following error will always be greater than MIN_FERROR. This prevents small following errors for stationary axes from inadvertently aborting motion. Small following errors will always be present due to vibration, etc.
Ref: linuxcnc.org/docs/master/html/config/ini-config.html

Note: Above docs are for the master branch (2.8~pre)

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7 years 11 months ago #120682 by andypugh

COlger81 wrote: What following error would you expect to see roughly at 150 IPM from a decent mill? .030" (.762mm) seems pretty extreme and i think i have some tuning to do.


It depends a lot on the machine, and also how the acceleration and velocity limits in the controller compare to the actual physical limits of the machine.

Do you have reason to believe that things were better before switching to 2.8, or had you just not really looked in detail?
(Assuming I have not got my posters mixed up).

What happens if you lower the acceleration limits of the joints, does the f-error reduce?

I-gain might help.

When cruising at high speed and with a large f-error, is the amplifier anywhere near saturated?

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7 years 11 months ago #120688 by COlger81
Lowering accelerations had no effect on the following errors. I am using the halscope right now. I think it's just a tuning issue.

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7 years 11 months ago #120697 by COlger81
Andy;
I was getting too aggressive on the drives so i changed my plan. I tuned the drives to Fadal manual spec. Now i'm going through PID tuning in software. Halscope looks pretty linear as far as following errors go. I just need to stiffen them up to reduce following errors. Any starting suggestions? I lowered max velocity to 30 and I'm at P of 120 currently.

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7 years 11 months ago #120698 by PCW
Replied by PCW on topic FLOAT PIN FOR FOLLOWING ERRORS?
Are the drives running in velocity mode?
If so the main tuning is via FF1

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7 years 11 months ago #120703 by COlger81
Yes, they are in velocity mode. I currently have P=400 and FF1=1.152;

2.88volts at 150 ipm
10volts at 520.833
520.833/60=8.681
10/8.681=1.152

Does this look correct?

(I'm getting .007" following error at 150 IPM)

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7 years 11 months ago #120704 by PCW
Replied by PCW on topic FLOAT PIN FOR FOLLOWING ERRORS?
Can you post a halscope plot of following error + commanded velocity?

Note that even if you cannot get the calibrate popup to work,
you can set the PID parameters "live" via halcmd. You can also do this with
halshow, but halcmd is easier since it has the consoles command history and line editing features

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7 years 11 months ago #120705 by COlger81

Attachment not found

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7 years 11 months ago #120706 by PCW
Replied by PCW on topic FLOAT PIN FOR FOLLOWING ERRORS?
That looks like almost exclusively FF1 tweaking needed:
( halcmd setp pid.N.ff1 something, make a 10% change, is it better or worse? )

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