joint 2 following error

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02 Nov 2013 06:14 #40499 by garymcrobertpdx
A new problem has appeared, I am experiencing joint 2 following errors every
ten or twelve minutes about 3/4 of the way through drilling 37 holes causing
a machine shut down.

I am using a Mesa 5i25 board & step motors the configuration via Pncconfig
and mostly unaltered The ini for the axis looks like this.

#********************
# Axis Z
#********************
[AXIS_2]
TYPE = LINEAR
HOME = 0.0
FERROR = 0.005
MIN_FERROR = 0.0005
MAX_VELOCITY = 1.622
MAX_ACCELERATION = 3.0
# these are in nanoseconds
DIRSETUP = 200000
DIRHOLD = 1000
STEPLEN = 1000
STEPSPACE = 1000
STEP_SCALE = 20000.0
MIN_LIMIT = -10.0
MAX_LIMIT = 10.0
HOME_OFFSET = 0.000000
HOME_SEARCH_VEL = 1.000000
HOME_LATCH_VEL = 0.100000
HOME_FINAL_VEL = 0.000000
HOME_USE_INDEX = NO
HOME_SEQUENCE = 0

The HAL looks like this:
#*******************
# AXIS Z
#*******************

# Step Gen signals/setup

setp hm2_5i25.0.stepgen.07.dirsetup [AXIS_2]DIRSETUP
setp hm2_5i25.0.stepgen.07.dirhold [AXIS_2]DIRHOLD
setp hm2_5i25.0.stepgen.07.steplen [AXIS_2]STEPLEN
setp hm2_5i25.0.stepgen.07.stepspace [AXIS_2]STEPSPACE
setp hm2_5i25.0.stepgen.07.position-scale [AXIS_2]STEP_SCALE
setp hm2_5i25.0.stepgen.07.step_type 0
setp hm2_5i25.0.stepgen.07.control-type 0
setp hm2_5i25.0.stepgen.07.maxaccel 3.1
setp hm2_5i25.0.stepgen.07.maxvel 2.1

net z-pos-fb axis.2.motor-pos-fb <= hm2_5i25.0.stepgen.07.position-fb
net z-pos-cmd axis.2.motor-pos-cmd => hm2_5i25.0.stepgen.07.position-cmd
net z-enable axis.2.amp-enable-out => hm2_5i25.0.stepgen.07.enable

# ---setup home / limit switch signals---

net home-z => axis.2.home-sw-in
net z-neg-limit => axis.2.neg-lim-sw-in
net z-pos-limit => axis.2.pos-lim-sw-in


Tried increasing FERROR = 0.005 to 0.010 this had no effect.

By the book [STEPLEN = 1000 + STEPSPACE = 1000 ] = 0.000002 sec / step

[1 / STEP_SCALE = 20000.0 ] = 0.000050 inch / step
[1 / 0.000002 sec / step] = 500,000 step / sec

0.000050 inch / step * 500,000 step / sec = 25 inches / second
Way faster than maxvel requires

Any ideas why this is happing and suggestions for a fix?

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02 Nov 2013 08:13 #40501 by aike
Replied by aike on topic joint 2 following error
increase FERROR till 1,0

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02 Nov 2013 09:27 #40502 by PCW
Replied by PCW on topic joint 2 following error
This looks quite strange:
DIRSETUP = 200000

Can your drives really have a 200 usec dir setup time?

Also this doesn't look right:

MAX_ACCELERATION = 3.0
and
setp hm2_5i25.0.stepgen.07.maxaccel 3.1

stepgen maxaccel is suggested to be 1.25 * machine maxaccel

so setp hm2_5i25.0.stepgen.07.maxaccel 3.75

may fix the problem

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02 Nov 2013 10:12 #40503 by garymcrobertpdx
OK I will try tweaking the values. Is the wizard prone to generating
incorrect velocity and or acceleration values? Since I did not change
any of these values you see them as they were created by Pncconf.

Thanks

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03 Nov 2013 02:37 #40515 by cmorley
Replied by cmorley on topic joint 2 following error
What version of linuxcnc are you using?
I would suggest the latest version. 2.5.3

Chris M

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03 Nov 2013 05:14 #40517 by garymcrobertpdx
I adjusted setp hm2_5i25.0.stepgen.07.maxaccel to = 3.75
This improved things but still had a following error during rigid
tapping.

I Then set FERROR = 0.010 and MIN_FERROR = 0.001
so far so good no further errors.

I am using 2.6.0 pre it offers a remapping option which I need
for auto tool change.


Thanks much for the help

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03 Nov 2013 22:45 #40519 by PCW
Replied by PCW on topic joint 2 following error
Did you change the
DIRSETUP = 200000
line?

What are you drive specs?

I dont see how that can be anywhere near right

(there are probably only a couple step drives in the world with such
long setup/hold times, Old Parker/Compumotor drives come to mind)

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04 Nov 2013 00:58 #40520 by aike
Replied by aike on topic joint 2 following error
If your PID adjusted with errors, you will get FERROR too.
If your P is too small. the error will occures.

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04 Nov 2013 06:50 #40528 by andypugh
Replied by andypugh on topic joint 2 following error

If your PID adjusted with errors, you will get FERROR too.
If your P is too small. the error will occures.


This is a stepper system, I think. So no PID.

With a stepper system two things cause f-error

1) Stepgen velocity and stepgen accel not having enough "headroom" over the axis valuse
2) The base thread rate not actually being able to make all the requested pulses.

I am guessing 2 here. What is your base thread rate, and what is your max vel and scale?

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16 Nov 2013 09:54 #40850 by garymcrobertpdx
After correcting the hal and ini velocity and accel values
the follow errors stoped with the exception of when a G33.1
is used. The problem appears to be due to sudden spindle
velocity changes during low RPMs tapping.

I am replacing the spindle motor with a real servo motor
this should produce a more uniform rotation of the spindle
and thus the Z axis should be less chalanged to follow
the spindel encoder.

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