It turns out the most reliable way for Linuxcnc to control a hardware stepgen
(like is used in a Mesa card (like your 6i25) is to treat it like a sudo servo
using a PID loop to close a position loop between Linuxcnc and the hardware
stepgenerator. Supposedly this helps to cancel out some problems caused by
variance latency times. Since this isn't a true servo loop, there is no tuning
of the PID numbers, they are precisely calculated from known variables, so you
do not need to mess with the default settings for the PID.
The hardware stepgen is capable of working with pure position commands also
(without using the PID). But smarter minds than me figured out a long time ago
that the other way works slightly better. So I'd suggest leaving it be.
It also has the advantage of, if you decide to add real position feedback for a
true closed loop system, every thing you need is already set up in the config.
All you need to do is replace the stepgen's position feedback with the real
postiong feedback, then retune the PID loop.
Todd Zuercher
P. Graham Dunn Inc.
630 Henry Street
Dalton, Ohio 44618
Phone: (330)828-2105ext. 2031
-----Original Message-----
From: Ed W <li...@wildgooses.com>
Sent: Wednesday, February 26, 2020 12:58 PM
To: Enhanced Machine Controller (EMC) <emc-users@lists.sourceforge.net>
Subject: [Emc-users] Making progress
[EXTERNAL EMAIL] Be sure links are safe.
Hi, I've made good progress converting my little OmioCNC X6 from Mach3 to
LinuxCNC. I think I owe a writeup at some point, but I still have a few
glitches before I'm ready to do that.
First thing which I'm worrying about is that having run pncconf, it generates
an output for my open loop steppers which involves a PID type setup, eg here is
my X axis from the ini
[AXIS_X]
MAX_VELOCITY = 100.0
MAX_ACCELERATION = 750.0
MIN_LIMIT = -0.01
MAX_LIMIT = 355.0
[JOINT_0]
TYPE = LINEAR
HOME = 0.0
FERROR = 1.0
MIN_FERROR = 0.01
MAX_VELOCITY = 100.0
MAX_ACCELERATION = 750.0
# The values below should be 25% larger than MAX_VELOCITY and
MAX_ACCELERATION
# If using BACKLASH compensation STEPGEN_MAXACCEL should be 100% larger.
STEPGEN_MAXVEL = 125.00
STEPGEN_MAXACCEL = 937.50
P = 1000.0
I = 0.0
D = 0.0
FF0 = 0.0
FF1 = 1.0
FF2 = 0.0
BIAS = 0.0
DEADBAND = 0.0
MAX_OUTPUT = 0.0
# these are in nanoseconds
DIRSETUP = 10000
DIRHOLD = 10000
STEPLEN = 5000
STEPSPACE = 5000
STEP_SCALE = 320.0
MIN_LIMIT = -0.01
MAX_LIMIT = 355.0
HOME_OFFSET = -5.000000
HOME_SEARCH_VEL = -33.333333
HOME_LATCH_VEL = 0.500000
HOME_FINAL_VEL = 33.333333
HOME_USE_INDEX = NO
HOME_IGNORE_LIMITS = YES
HOME_SEQUENCE = 1
The corresponding .hal (again by pncconf) is as follows:
#*******************
# AXIS X JOINT 0
#*******************
setp pid.x.Pgain [JOINT_0]P
setp pid.x.Igain [JOINT_0]I
setp pid.x.Dgain [JOINT_0]D
setp pid.x.bias [JOINT_0]BIAS
setp pid.x.FF0 [JOINT_0]FF0
setp pid.x.FF1 [JOINT_0]FF1
setp pid.x.FF2 [JOINT_0]FF2
setp pid.x.deadband [JOINT_0]DEADBAND
setp pid.x.maxoutput [JOINT_0]MAX_OUTPUT
setp pid.x.error-previous-target true
setp pid.x.maxerror .0005
net x-index-enable <=> pid.x.index-enable
net x-enable => pid.x.enable
net x-pos-cmd => pid.x.command
net x-pos-fb => pid.x.feedback
net x-output <= pid.x.output
# Step Gen signals/setup
setp hm2_5i25.0.stepgen.00.dirsetup [JOINT_0]DIRSETUP
setp hm2_5i25.0.stepgen.00.dirhold [JOINT_0]DIRHOLD
setp hm2_5i25.0.stepgen.00.steplen [JOINT_0]STEPLEN
setp hm2_5i25.0.stepgen.00.stepspace [JOINT_0]STEPSPACE
setp hm2_5i25.0.stepgen.00.position-scale [JOINT_0]STEP_SCALE
setp hm2_5i25.0.stepgen.00.step_type 0
setp hm2_5i25.0.stepgen.00.control-type 1
setp hm2_5i25.0.stepgen.00.maxaccel [JOINT_0]STEPGEN_MAXACCEL
setp hm2_5i25.0.stepgen.00.maxvel [JOINT_0]STEPGEN_MAXVEL
setp hm2_5i25.0.stepgen.00.step.invert_output true
setp hm2_5i25.0.stepgen.00.direction.invert_output true
# ---closedloop stepper signals---
net x-pos-cmd <= joint.0.motor-pos-cmd
net x-vel-cmd <= joint.0.vel-cmd
net x-output <= hm2_5i25.0.stepgen.00.velocity-cmd
net x-pos-fb <= hm2_5i25.0.stepgen.00.position-fb
net x-pos-fb => joint.0.motor-pos-fb
net x-enable <= joint.0.amp-enable-out
net x-enable => hm2_5i25.0.stepgen.00.enable
# ---setup home / limit switch signals---
net min-home-x => joint.0.home-sw-in
net min-home-x => joint.0.neg-lim-sw-in
net x-pos-limit => joint.0.pos-lim-sw-in
What would someone experienced recommend I do here? This is a simple openloop
stepper machine, no feedback or position sensors. Just a 6i25 to a basic
breakout board, going to a chinese 556 stepper driver clone and onto some small
steppers on a 6040 type of machine?
Is there any disadvantage (or advantage) to leaving all the PID and closed loop
stuff in here?
Thanks
Ed W
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