QTPlasmaC Z axis "following error" (but it works in AXIS)

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12 Aug 2021 12:28 - 12 Aug 2021 14:27 #217604 by fernandosf
I have a  basic machine config working on AXIS interface. Machine home itself ok. It's a gantry type (XYYZ).

Then I have the same config (.hal and .ini files) running on QTPlasmaC.

I can home X and Y with success on QTPlasmaC.
For some reason, When I try to home Z on QTPlasmaC I get errors like :

[QTvcp.QTVCP.QT_ACTION][INFO] Homing Joint: 3 (qt_action.py:96)
joint 3 following error
emc/task/taskintf.cc 947: Error on joint 3, command number 125



I tried to check configs between AXIS and QTPlasmaC, but could not find differences.

I found [url=http://www.vdwalle.com/Norte/LinuxCNC%20Following%20Error.htm](this page) which says:

Following Error and Stepper Driven Machines

The concept of a following error is funny when talking about stepper motors. Since they are an open loop system, there is no position feedback to let you know if you actually are out of range.

following errors usually have 2 causes on steppers
1. too little ferror or min_ferror
2. the RT pulsing can't keep up with the speed needed
2.a. BASE_PERIOD set incorrectly or (requested step rate is not possible at this BASE_PERIOD)
2.b. max velocity too high
2.c. max acceleration too high

Problem is it works in AXIS!
Could someone please take a look on my files or advise?[/url]
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Last edit: 12 Aug 2021 14:27 by fernandosf.

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12 Aug 2021 12:31 - 12 Aug 2021 14:27 #217605 by fernandosf

File Attachment:

File Name: axis-homing-ok.tar.gz
File Size:7 KB
 

File Attachment:

File Name: qtplasmac-...k.tar.gz
File Size:11 KB


 
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Last edit: 12 Aug 2021 14:27 by fernandosf.

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12 Aug 2021 13:05 #217608 by tommylight
In your QTPlasmaC .ini file
--
[AXIS_Z]
MAX_VELOCITY = 25
MAX_ACCELERATION = 40

--
should be
--
[AXIS_Z]
MAX_VELOCITY = 50
MAX_ACCELERATION = 60

--
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12 Aug 2021 14:05 - 12 Aug 2021 14:28 #217615 by fernandosf
Thanks, will try that.
But I completely ignore why that should be.

I'm not in front of the machine now, but I had been reducing max_vel and max_accel of Z trying to make it work without success.

So now I end up with something like this:

[AXIS_Z]
MAX_VELOCITY     = 25
MAX_ACCELERATION = 40
...
[JOINT_3]
MAX_VELOCITY     = 25
MAX_ACCELERATION = 30
STEPGEN_MAXACCEL = 50
...
HOME_SEARCH_VEL =  22
HOME_LATCH_VEL  =  5
HOME_FINAL_VEL  =  30
...


I can think of HOME_FINAL_VEL been 30 might be the culprit.
But again, it works in AXIS, I don't know what would be different in QTPlasmaC
Last edit: 12 Aug 2021 14:28 by fernandosf.

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12 Aug 2021 21:37 - 12 Aug 2021 21:39 #217638 by snowgoer540
The steplen and stepspace in your .hal files are not the same for both setups. I am showing Joint 3 as an example, but each Joint has the same issue.

Axis, Joint 3:
setp stepgen.3.steplen 2750
setp stepgen.3.stepspace 0

QtPlasmaC, Joint 3:
setp stepgen.3.steplen 5000
setp stepgen.3.stepspace 5000

I am not an expert with STEPGEN (here's the manual linuxcnc.org/docs/devel/html/man/man9/stepgen.9.html) but it looks to me like the periods for QtPlasmaC are much longer, and are perhaps the source of your issue.

That's what I see as being different though.
Last edit: 12 Aug 2021 21:39 by snowgoer540. Reason: accidentally had a old link to the 2.5 version of stepgen
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12 Aug 2021 23:17 - 14 Aug 2021 08:13 #217642 by phillc54
Also the [AXIS_?] MAX_VEL & MAX_ACC need to be twice the corresponding [JOINT_?] value for all axes.
Last edit: 14 Aug 2021 08:13 by phillc54.
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13 Aug 2021 13:48 - 13 Aug 2021 13:51 #217678 by fernandosf
Thank you all, I will correct those configs (make then the same) and report back with tests on AXIS and QTPC.

I have set lower values for Z MAX_VEL & MAX_ACCEL and got the Z working again without errors.

@snowgoer540:  I think AXIS config is using double-stepping, maybe it explains why higher velocities are been possible
Last edit: 13 Aug 2021 13:51 by fernandosf. Reason: add last paragraph
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14 Aug 2021 20:36 #217769 by fernandosf
I have correct that diff and followed those rules about VEL & ACCEL and everything is consistent now.

Thank you all!
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