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Can't get backlash compensation to work

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23 Mar 2024 16:10 #296583 by ltcnc
Can't get backlash compensation to work was created by ltcnc
I have a new build using the 7i96s + 7i85s and closed loop stepper motors. I've worked through pncconf and am able to drive my motors and am getting encoder feedback. I'm trying to take a few thou (0.025-0.075mm) of backlash out of various axes by testing in Axis, but no matter what I set the backlash setting to on each axis, I still get the same backlash reading from my dial indicator. (Even when I set 1mm+ of backlash.) I've ensured STEPGEN_MAXACCEL is 2x MAX_ACCELERATION for each axis. Anyone know what I'm doing wrong?

One more note: pncconf did automatically assign the stepgen position-fb signals to the pos-fb net of each axis, which I had to comment out just to get Axis to open the configuration without throwing an error.

HAL file:
# Generated by PNCconf at Thu Mar 21 17:28:52 2024
# Using LinuxCNC version:  Master (2.9)
# If you make changes to this file, they will be
# overwritten when you run PNCconf again

loadrt [KINS]KINEMATICS
loadrt [EMCMOT]EMCMOT servo_period_nsec=[EMCMOT]SERVO_PERIOD num_joints=[KINS]JOINTS
loadrt hostmot2
loadrt hm2_eth board_ip="10.10.10.10" config="num_encoders=5 num_pwmgens=1 num_stepgens=9 sserial_port_0=0xxxxxxx" 
setp    hm2_7i96s.0.pwmgen.pwm_frequency 20000
setp    hm2_7i96s.0.pwmgen.pdm_frequency 6000000
setp    hm2_7i96s.0.watchdog.timeout_ns 5000000
loadrt pid names=pid.x,pid.y,pid.z,pid.s

addf hm2_7i96s.0.read          servo-thread
addf motion-command-handler   servo-thread
addf motion-controller        servo-thread
addf pid.x.do-pid-calcs       servo-thread
addf pid.y.do-pid-calcs       servo-thread
addf pid.z.do-pid-calcs       servo-thread
addf pid.s.do-pid-calcs       servo-thread
addf hm2_7i96s.0.write         servo-thread
setp hm2_7i96s.0.dpll.01.timer-us -50
setp hm2_7i96s.0.stepgen.timer-number 1

# external output signals


# external input signals

# --- MIN-X ---
net min-x     <=  hm2_7i96s.0.inm.00.input-00

# --- MAX-X ---
net max-x     <=  hm2_7i96s.0.inm.00.input-01

# --- MIN-Y ---
net min-y     <=  hm2_7i96s.0.inm.00.input-02

# --- MAX-Y ---
net max-y     <=  hm2_7i96s.0.inm.00.input-03

# --- MIN-Z ---
net min-z     <=  hm2_7i96s.0.inm.00.input-04

# --- MAX-Z ---
net max-z     <=  hm2_7i96s.0.inm.00.input-05


#*******************
#  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
# This setting is to limit bogus stepgen
# velocity corrections caused by position
# feedback sample time jitter.
setp   pid.x.maxerror 0.012700

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_7i96s.0.stepgen.00.dirsetup        [JOINT_0]DIRSETUP
setp   hm2_7i96s.0.stepgen.00.dirhold         [JOINT_0]DIRHOLD
setp   hm2_7i96s.0.stepgen.00.steplen         [JOINT_0]STEPLEN
setp   hm2_7i96s.0.stepgen.00.stepspace       [JOINT_0]STEPSPACE
setp   hm2_7i96s.0.stepgen.00.position-scale  [JOINT_0]STEP_SCALE
setp   hm2_7i96s.0.stepgen.00.step_type        0
setp   hm2_7i96s.0.stepgen.00.control-type     1
setp   hm2_7i96s.0.stepgen.00.maxaccel         [JOINT_0]STEPGEN_MAXACCEL
setp   hm2_7i96s.0.stepgen.00.maxvel           [JOINT_0]STEPGEN_MAXVEL

# ---closedloop stepper signals---

net x-pos-cmd    <= joint.0.motor-pos-cmd
net x-vel-cmd    <= joint.0.vel-cmd
net x-output     => hm2_7i96s.0.stepgen.00.velocity-cmd
# net x-pos-fb     <= hm2_7i96s.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_7i96s.0.stepgen.00.enable

# ---Encoder feedback signals/setup---

setp    hm2_7i96s.0.encoder.00.counter-mode 0
setp    hm2_7i96s.0.encoder.00.filter 1
setp    hm2_7i96s.0.encoder.00.index-invert 0
setp    hm2_7i96s.0.encoder.00.index-mask 0
setp    hm2_7i96s.0.encoder.00.index-mask-invert 0
setp    hm2_7i96s.0.encoder.00.scale  [JOINT_0]ENCODER_SCALE

net x-pos-fb               <=  hm2_7i96s.0.encoder.00.position
net x-vel-fb               <=  hm2_7i96s.0.encoder.00.velocity
net x-pos-fb               =>  joint.0.motor-pos-fb
net x-index-enable    joint.0.index-enable  <=>  hm2_7i96s.0.encoder.00.index-enable
net x-pos-rawcounts        <=  hm2_7i96s.0.encoder.00.rawcounts

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

net x-home-sw     =>  joint.0.home-sw-in
net min-x     =>  joint.0.neg-lim-sw-in
net max-x     =>  joint.0.pos-lim-sw-in

#*******************
#  AXIS Y JOINT 1
#*******************

setp   pid.y.Pgain     [JOINT_1]P
setp   pid.y.Igain     [JOINT_1]I
setp   pid.y.Dgain     [JOINT_1]D
setp   pid.y.bias      [JOINT_1]BIAS
setp   pid.y.FF0       [JOINT_1]FF0
setp   pid.y.FF1       [JOINT_1]FF1
setp   pid.y.FF2       [JOINT_1]FF2
setp   pid.y.deadband  [JOINT_1]DEADBAND
setp   pid.y.maxoutput [JOINT_1]MAX_OUTPUT
setp   pid.y.error-previous-target true
# This setting is to limit bogus stepgen
# velocity corrections caused by position
# feedback sample time jitter.
setp   pid.y.maxerror 0.012700

net y-index-enable  =>  pid.y.index-enable
net y-enable        =>  pid.y.enable
net y-pos-cmd       =>  pid.y.command
net y-pos-fb        =>  pid.y.feedback
net y-output        <=  pid.y.output

# Step Gen signals/setup

setp   hm2_7i96s.0.stepgen.01.dirsetup        [JOINT_1]DIRSETUP
setp   hm2_7i96s.0.stepgen.01.dirhold         [JOINT_1]DIRHOLD
setp   hm2_7i96s.0.stepgen.01.steplen         [JOINT_1]STEPLEN
setp   hm2_7i96s.0.stepgen.01.stepspace       [JOINT_1]STEPSPACE
setp   hm2_7i96s.0.stepgen.01.position-scale  [JOINT_1]STEP_SCALE
setp   hm2_7i96s.0.stepgen.01.step_type        0
setp   hm2_7i96s.0.stepgen.01.control-type     1
setp   hm2_7i96s.0.stepgen.01.maxaccel         [JOINT_1]STEPGEN_MAXACCEL
setp   hm2_7i96s.0.stepgen.01.maxvel           [JOINT_1]STEPGEN_MAXVEL

# ---closedloop stepper signals---

net y-pos-cmd    <= joint.1.motor-pos-cmd
net y-vel-cmd    <= joint.1.vel-cmd
net y-output     => hm2_7i96s.0.stepgen.01.velocity-cmd
# net y-pos-fb     <= hm2_7i96s.0.stepgen.01.position-fb
net y-pos-fb     => joint.1.motor-pos-fb
net y-enable     <= joint.1.amp-enable-out
net y-enable     => hm2_7i96s.0.stepgen.01.enable

# ---Encoder feedback signals/setup---

setp    hm2_7i96s.0.encoder.01.counter-mode 0
setp    hm2_7i96s.0.encoder.01.filter 1
setp    hm2_7i96s.0.encoder.01.index-invert 0
setp    hm2_7i96s.0.encoder.01.index-mask 0
setp    hm2_7i96s.0.encoder.01.index-mask-invert 0
setp    hm2_7i96s.0.encoder.01.scale  [JOINT_1]ENCODER_SCALE

net y-pos-fb               <=  hm2_7i96s.0.encoder.01.position
net y-vel-fb               <=  hm2_7i96s.0.encoder.01.velocity
net y-pos-fb               =>  joint.1.motor-pos-fb
net y-index-enable    joint.1.index-enable  <=>  hm2_7i96s.0.encoder.01.index-enable
net y-pos-rawcounts        <=  hm2_7i96s.0.encoder.01.rawcounts

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

net y-home-sw     =>  joint.1.home-sw-in
net min-y     =>  joint.1.neg-lim-sw-in
net max-y     =>  joint.1.pos-lim-sw-in

#*******************
#  AXIS Z JOINT 2
#*******************

setp   pid.z.Pgain     [JOINT_2]P
setp   pid.z.Igain     [JOINT_2]I
setp   pid.z.Dgain     [JOINT_2]D
setp   pid.z.bias      [JOINT_2]BIAS
setp   pid.z.FF0       [JOINT_2]FF0
setp   pid.z.FF1       [JOINT_2]FF1
setp   pid.z.FF2       [JOINT_2]FF2
setp   pid.z.deadband  [JOINT_2]DEADBAND
setp   pid.z.maxoutput [JOINT_2]MAX_OUTPUT
setp   pid.z.error-previous-target true
# This setting is to limit bogus stepgen
# velocity corrections caused by position
# feedback sample time jitter.
setp   pid.z.maxerror 0.012700

net z-index-enable  =>  pid.z.index-enable
net z-enable        =>  pid.z.enable
net z-pos-cmd       =>  pid.z.command
net z-pos-fb        =>  pid.z.feedback
net z-output        <=  pid.z.output

# Step Gen signals/setup

setp   hm2_7i96s.0.stepgen.02.dirsetup        [JOINT_2]DIRSETUP
setp   hm2_7i96s.0.stepgen.02.dirhold         [JOINT_2]DIRHOLD
setp   hm2_7i96s.0.stepgen.02.steplen         [JOINT_2]STEPLEN
setp   hm2_7i96s.0.stepgen.02.stepspace       [JOINT_2]STEPSPACE
setp   hm2_7i96s.0.stepgen.02.position-scale  [JOINT_2]STEP_SCALE
setp   hm2_7i96s.0.stepgen.02.step_type        0
setp   hm2_7i96s.0.stepgen.02.control-type     1
setp   hm2_7i96s.0.stepgen.02.maxaccel         [JOINT_2]STEPGEN_MAXACCEL
setp   hm2_7i96s.0.stepgen.02.maxvel           [JOINT_2]STEPGEN_MAXVEL

# ---closedloop stepper signals---

net z-pos-cmd    <= joint.2.motor-pos-cmd
net z-vel-cmd    <= joint.2.vel-cmd
net z-output     => hm2_7i96s.0.stepgen.02.velocity-cmd
# net z-pos-fb     <= hm2_7i96s.0.stepgen.02.position-fb
net z-pos-fb     => joint.2.motor-pos-fb
net z-enable     <= joint.2.amp-enable-out
net z-enable     => hm2_7i96s.0.stepgen.02.enable

# ---Encoder feedback signals/setup---

setp    hm2_7i96s.0.encoder.02.counter-mode 0
setp    hm2_7i96s.0.encoder.02.filter 1
setp    hm2_7i96s.0.encoder.02.index-invert 0
setp    hm2_7i96s.0.encoder.02.index-mask 0
setp    hm2_7i96s.0.encoder.02.index-mask-invert 0
setp    hm2_7i96s.0.encoder.02.scale  [JOINT_2]ENCODER_SCALE

net z-pos-fb               <=  hm2_7i96s.0.encoder.02.position
net z-vel-fb               <=  hm2_7i96s.0.encoder.02.velocity
net z-pos-fb               =>  joint.2.motor-pos-fb
net z-index-enable    joint.2.index-enable  <=>  hm2_7i96s.0.encoder.02.index-enable
net z-pos-rawcounts        <=  hm2_7i96s.0.encoder.02.rawcounts

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

net z-home-sw     =>  joint.2.home-sw-in
net min-z     =>  joint.2.neg-lim-sw-in
net max-z     =>  joint.2.pos-lim-sw-in


#******************************
# connect miscellaneous signals
#******************************

#  ---HALUI signals---

net axis-select-x  halui.axis.x.select
net jog-x-pos      halui.axis.x.plus
net jog-x-neg      halui.axis.x.minus
net jog-x-analog   halui.axis.x.analog
net x-is-homed     halui.joint.0.is-homed
net axis-select-y  halui.axis.y.select
net jog-y-pos      halui.axis.y.plus
net jog-y-neg      halui.axis.y.minus
net jog-y-analog   halui.axis.y.analog
net y-is-homed     halui.joint.1.is-homed
net axis-select-z  halui.axis.z.select
net jog-z-pos      halui.axis.z.plus
net jog-z-neg      halui.axis.z.minus
net jog-z-analog   halui.axis.z.analog
net z-is-homed     halui.joint.2.is-homed
net jog-selected-pos      halui.axis.selected.plus
net jog-selected-neg      halui.axis.selected.minus
net spindle-manual-cw     halui.spindle.0.forward
net spindle-manual-ccw    halui.spindle.0.reverse
net spindle-manual-stop   halui.spindle.0.stop
net machine-is-on         halui.machine.is-on
net jog-speed             halui.axis.jog-speed
net MDI-mode              halui.mode.is-mdi

#  ---coolant signals---

net coolant-mist      <=  iocontrol.0.coolant-mist
net coolant-flood     <=  iocontrol.0.coolant-flood

#  ---probe signal---

net probe-in     =>  motion.probe-input

#  ---motion control signals---

net in-position               <=  motion.in-position
net machine-is-enabled        <=  motion.motion-enabled

#  ---digital in / out signals---

#  ---estop signals---

net estop-out     <=  iocontrol.0.user-enable-out
net estop-out     =>  iocontrol.0.emc-enable-in

#  ---manual tool change signals---

net tool-change-request    <= iocontrol.0.tool-change
net tool-change-confirmed  => iocontrol.0.tool-changed
net tool-number            <= iocontrol.0.tool-prep-number

#  ---Use external manual tool change dialog---

loadusr -W hal_manualtoolchange
net tool-change-request    =>  hal_manualtoolchange.change
net tool-change-confirmed  <=  hal_manualtoolchange.changed
net tool-number            =>  hal_manualtoolchange.number

#  ---ignore tool prepare requests---
net tool-prepare-loopback   iocontrol.0.tool-prepare      =>  iocontrol.0.tool-prepared

INI file:
# Generated by PNCconf at Thu Mar 21 17:28:52 2024
# Using LinuxCNC version:  Master (2.9)
# If you make changes to this file, they will be
# overwritten when you run PNCconf again

[EMC]
MACHINE = my_LinuxCNC_test4
DEBUG = 0
VERSION = 1.1

[DISPLAY]
DISPLAY = axis
POSITION_OFFSET = RELATIVE
POSITION_FEEDBACK = ACTUAL
MAX_FEED_OVERRIDE = 2.000000
MAX_SPINDLE_OVERRIDE = 1.000000
MIN_SPINDLE_OVERRIDE = 0.500000
INTRO_GRAPHIC = linuxcnc.gif
INTRO_TIME = 5
PROGRAM_PREFIX = /home/lukecnc/linuxcnc/nc_files
INCREMENTS = 50mm 10mm 5mm 1mm .5mm .1mm .05mm .01mm .005mm
POSITION_FEEDBACK = ACTUAL
DEFAULT_LINEAR_VELOCITY = 6.000000
MAX_LINEAR_VELOCITY = 25.000000
MIN_LINEAR_VELOCITY = 0.500000
DEFAULT_ANGULAR_VELOCITY = 12.000000
MAX_ANGULAR_VELOCITY = 180.000000
MIN_ANGULAR_VELOCITY = 1.666667
EDITOR = gedit
GEOMETRY = xyz
CYCLE_TIME = 100

[FILTER]
PROGRAM_EXTENSION = .png,.gif,.jpg Greyscale Depth Image
PROGRAM_EXTENSION = .py Python Script
png = image-to-gcode
gif = image-to-gcode
jpg = image-to-gcode
py = python

[TASK]
TASK = milltask
CYCLE_TIME = 0.010

[RS274NGC]
PARAMETER_FILE = linuxcnc.var
RS274NGC_STARTUP_CODE = G21 G40 G90 G94 G97 G64 P0.025

[EMCMOT]
EMCMOT = motmod
COMM_TIMEOUT = 1.0
SERVO_PERIOD = 1000000

[HMOT]
# **** This is for info only ****
CARD0=hm2_7i96s.0

[HAL]
HALUI = halui
HALFILE = my_LinuxCNC_test4.hal
HALFILE = custom.hal
POSTGUI_HALFILE = custom_postgui.hal
SHUTDOWN = shutdown.hal

[HALUI]

[KINS]
JOINTS = 3
KINEMATICS = trivkins coordinates=XYZ

[TRAJ]
COORDINATES =  XYZ
LINEAR_UNITS = mm
ANGULAR_UNITS = degree
DEFAULT_LINEAR_VELOCITY = 3.75
MAX_LINEAR_VELOCITY = 37.50

[EMCIO]
EMCIO = io
CYCLE_TIME = 0.100
TOOL_TABLE = tool.tbl
TOOL_CHANGE_QUILL_UP = 1

#******************************************
[AXIS_X]
MAX_VELOCITY = 25.0
MAX_ACCELERATION = 100.0
MIN_LIMIT = -160.0
MAX_LIMIT = 240.0

[JOINT_0]
TYPE = LINEAR
HOME = 0.0
FERROR = 10.0
MIN_FERROR = 0.025
MAX_VELOCITY = 25.0
MAX_ACCELERATION = 100.0
# 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 = 31.25
STEPGEN_MAXACCEL = 200.0
P = 250.0
I = 1.0
D = 0.0
FF0 = 0.0
FF1 = 1.0
FF2 = 0.0
BIAS = 0.0
DEADBAND = 0.025
MAX_OUTPUT = 0.0
ENCODER_SCALE = -800.0
# these are in nanoseconds
DIRSETUP   = 5000
DIRHOLD    = 5000
STEPLEN    = 2000
STEPSPACE  = 2000
STEP_SCALE = -160.0
MIN_LIMIT = -160.0
MAX_LIMIT = 240.0
HOME_OFFSET = 0.0
HOME_SEQUENCE = 1
#******************************************

#******************************************
[AXIS_Y]
MAX_VELOCITY = 25.0
MAX_ACCELERATION = 250.0
MIN_LIMIT = -50.0
MAX_LIMIT = 50.0

[JOINT_1]
TYPE = LINEAR
HOME = 0.0
FERROR = 10.0
MIN_FERROR = 1.0
MAX_VELOCITY = 25.0
MAX_ACCELERATION = 250.0
# 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 = 31.25
STEPGEN_MAXACCEL = 312.50
P = 250.0
I = 0.1
D = 0.0
FF0 = 0.0
FF1 = 1.0
FF2 = 0.0
BIAS = 0.0
DEADBAND = 0.05
MAX_OUTPUT = 0.0
ENCODER_SCALE = 800.0
# these are in nanoseconds
DIRSETUP   = 5000
DIRHOLD    = 5000
STEPLEN    = 2000
STEPSPACE  = 2000
STEP_SCALE = 160.0
MIN_LIMIT = -50.0
MAX_LIMIT = 50.0
HOME_OFFSET = 0.0
HOME_SEQUENCE = 2
#******************************************

#******************************************
[AXIS_Z]
MAX_VELOCITY = 25.0
MAX_ACCELERATION = 150.0
MIN_LIMIT = -250.0
MAX_LIMIT = 0.00

[JOINT_2]
TYPE = LINEAR
HOME = 0.0
FERROR = 10.0
MIN_FERROR = 0.025
MAX_VELOCITY = 25.0
MAX_ACCELERATION = 150.0
# 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 = 46.88
STEPGEN_MAXACCEL = 300.0
P = 250.0
I = 0.5
D = 0.0
FF0 = 0.0
FF1 = 1.0
FF2 = 0.0
BIAS = 0.0
DEADBAND = 0.05
MAX_OUTPUT = 0.0
ENCODER_SCALE = -800.0
# these are in nanoseconds
DIRSETUP   = 5000
DIRHOLD    = 5000
STEPLEN    = 2000
STEPSPACE  = 2000
STEP_SCALE = -160.0
MIN_LIMIT = -250.0
MAX_LIMIT = 0.00
HOME_OFFSET = 0.0
HOME_SEQUENCE = 0
#******************************************

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23 Mar 2024 18:26 - 23 Mar 2024 18:35 #296588 by PCW
Replied by PCW on topic Can't get backlash compensation to work
1. I don't see any backlash settings in the ini file.
2. Your PID deadzone is about as big as your backlash, can you reduce this?
Last edit: 23 Mar 2024 18:35 by PCW.

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23 Mar 2024 20:17 #296593 by ltcnc
Replied by ltcnc on topic Can't get backlash compensation to work
Derp. Even after reading a bunch of forum posts, I was still (mistakenly) thinking that DEADBAND was the setting for backlash compensation. Now I see that it's BACKLASH, and after another test it looks to be working!

Thanks again for the help!

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23 Mar 2024 23:25 #296602 by tommylight
Replied by tommylight on topic Can't get backlash compensation to work
Each joint that requires backlash compensation must have
BACKLASH = 0.2 #this is an example value
and can also have
BACKLASH_VELOCITY = 10 # the speed to move during compensation, if this is not present the joint/axis will try to move with the max velocity it can do. Value for metric machine, 600mm/m.
-
Should double check this, it is from memory and was way to long ago.

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  3. CNC Machines
  5. Milling Machines
  7. Can't get backlash compensation to work
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