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Follow Error - Open Loop Stepper Motor

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21 Jan 2021 23:38 - 21 Jan 2021 23:39 #196161 by Project_Hopeless
Follow Error - Open Loop Stepper Motor was created by Project_Hopeless
I'm in the process of upgrading my stepper motors for more speed. I have already set up the joint and cartesian speed limits in LinuxCNC. I did not change any accel/deccel params.

The stepper motors are open loop. I'm running LinuxCNC 2.8 on Buster over ethernet to a Mesa 7i96e card.

When I jog an axis from the key board I almost immediately get a "follow error". The actual speed is no where near the max speed of even the old setup max.

any thoughts?
Last edit: 21 Jan 2021 23:39 by Project_Hopeless.

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21 Jan 2021 23:41 #196162 by cmorley
Replied by cmorley on topic Follow Error - Open Loop Stepper Motor
please post your config files - I'd guess the PID values are not right.

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22 Jan 2021 00:36 #196165 by Project_Hopeless
Replied by Project_Hopeless on topic Follow Error - Open Loop Stepper Motor

please post your config files - I'd guess the PID values are not right.

To be honest I just used the defaults. This is a DIY machine at the low end of the speed spectrum.
# This file was created with the 7i96 Wizard on Jun 27 2020 16:56:08
# Changes to most things are ok and will be read by the wizard

[EMC]
VERSION = 1.1
MACHINE = borg
DEBUG = 0x00000000

[HOSTMOT2]
DRIVER = hm2_eth
IPADDRESS = "10.10.10.10"
BOARD = 7i96
STEPGENS = 4
ENCODERS = 0
SSERIAL_PORT = 1

[DISPLAY]
DISPLAY = axis
POSITION_OFFSET = RELATIVE
POSITION_FEEDBACK = COMMANDED
MAX_FEED_OVERRIDE = 1.2
CYCLE_TIME = 0.1
INTRO_GRAPHIC = emc2.gif
INTRO_TIME = 0
OPEN_FILE = ""
PROGRAM_PREFIX = /home/anto/linuxcnc/nc_files
PYVCP = borg.xml

[KINS]
KINEMATICS = trivkins coordinates=XYYZ kinstype=BOTH
JOINTS = 4

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

[RS274NGC]
PARAMETER_FILE = borg.var

[EMCMOT]
EMCMOT = motmod
SERVO_PERIOD = 2000000

[TASK]
TASK = milltask
CYCLE_TIME = 0.010

[TRAJ]
COORDINATES = XYYZ
LINEAR_UNITS = metric
ANGULAR_UNITS = degree
MAX_LINEAR_VELOCITY = 50

[HAL]
HALFILE = borg.hal
HALFILE = io.hal
HALFILE = custom.hal
POSTGUI_HALFILE = postgui.hal
HALUI = halui

[HALUI]
MDI_COMMAND = o<ztouch> call
MDI_COMMAND = o<sheetload> call
MDI_COMMAND = o<lubestroke> call
MDI_COMMAND = o<toolchg> call
MDI_COMMAND = o<tool_setter_test> call

[AXIS_X]
MIN_LIMIT = -.01
MAX_LIMIT = 1205
MAX_VELOCITY = 27.5 <is now 59.7>
MAX_ACCELERATION = 50

[AXIS_Y]
MIN_LIMIT = -.01
MAX_LIMIT = 1230
MAX_VELOCITY = 27.5 <is now 59.7>
MAX_ACCELERATION = 50

[AXIS_Z]
MIN_LIMIT = -165
MAX_LIMIT = .01
MAX_VELOCITY = 22 <is now 47.7>
MAX_ACCELERATION = 50

[JOINT_0]
AXIS = X
MIN_LIMIT = -.01
MAX_LIMIT = 1205
MAX_VELOCITY = 27.5 <is now 59.7>
MAX_ACCELERATION = 50
TYPE = LINEAR
SCALE = 80
STEPGEN_MAX_VEL = 33.0
STEPGEN_MAX_ACC = 60.0
FERROR = 0.0051
MIN_FERROR = 0.0025
DIRSETUP = 10000
DIRHOLD = 10000
STEPLEN = 5000
STEPSPACE = 5000
DEADBAND = 0
P = 1000
I = 0
D = 0
FF0 = 0
FF1 = 1
FF2 = 0.00013
BIAS = 0
MAX_OUTPUT = 0
MAX_ERROR = 0.0127
HOME = 0
HOME_OFFSET = -1
HOME_SEARCH_VEL = -10
HOME_LATCH_VEL = 1
HOME_SEQUENCE = 2
HOME_USE_INDEX = False
HOME_IGNORE_LIMITS = True

[JOINT_1]
AXIS = Y
MIN_LIMIT = -.01
MAX_LIMIT = 1250
MAX_VELOCITY = 27.5 <is now 59.7>
MAX_ACCELERATION = 50
TYPE = LINEAR
SCALE = 80
STEPGEN_MAX_VEL = 33.0
STEPGEN_MAX_ACC = 60.0
FERROR = 0.0051
MIN_FERROR = 0.0025
DIRSETUP = 10000
DIRHOLD = 10000
STEPLEN = 5000
STEPSPACE = 5000
DEADBAND = 0
P = 1000
I = 0
D = 0
FF0 = 0
FF1 = 1
FF2 = 0.00013
BIAS = 0
MAX_OUTPUT = 0
MAX_ERROR = 0.0127
HOME = 0
HOME_OFFSET = -1
HOME_SEARCH_VEL = -10
HOME_LATCH_VEL = 1
HOME_SEQUENCE = 1
HOME_USE_INDEX = False
HOME_IGNORE_LIMITS = True

[JOINT_2]
AXIS = Y
MIN_LIMIT = -.01
MAX_LIMIT = 1250
MAX_VELOCITY = 27.5 <is now 59.7>
MAX_ACCELERATION = 50
TYPE = LINEAR
SCALE = 80
STEPGEN_MAX_VEL = 33.0
STEPGEN_MAX_ACC = 60.0
FERROR = 0.0051
MIN_FERROR = 0.0025
DIRSETUP = 10000
DIRHOLD = 10000
STEPLEN = 5000
STEPSPACE = 5000
DEADBAND = 0
P = 1000
I = 0
D = 0
FF0 = 0
FF1 = 1
FF2 = 0.00013
BIAS = 0
MAX_OUTPUT = 0
MAX_ERROR = 0.0127
HOME = 0
HOME_OFFSET = -0.644
HOME_SEARCH_VEL = -10
HOME_LATCH_VEL = 1
HOME_SEQUENCE = 1
HOME_USE_INDEX = False
HOME_IGNORE_LIMITS = True

[JOINT_3]
AXIS = Z
MIN_LIMIT = -165
MAX_LIMIT = .01
MAX_VELOCITY = 22 <is now 47.7>
MAX_ACCELERATION = 50
TYPE = LINEAR
SCALE = -100
STEPGEN_MAX_VEL = 26.4
STEPGEN_MAX_ACC = 60.0
FERROR = 0.0051
MIN_FERROR = 0.0025
DIRSETUP = 10000
DIRHOLD = 10000
STEPLEN = 5000
STEPSPACE = 5000
DEADBAND = 0
P = 1000
I = 0
D = 0
FF0 = 0
FF1 = 1
FF2 = 0.00013
BIAS = 0
MAX_OUTPUT = 0
MAX_ERROR = 0.0127
HOME = 0
HOME_OFFSET = .25
HOME_SEARCH_VEL = 10
HOME_LATCH_VEL = -2
HOME_SEQUENCE = 0
HOME_USE_INDEX = False
HOME_IGNORE_LIMITS = True

[SPINDLE]
SPINDLE_TYPE = False
SCALE = 
PWM_FREQUENCY = 20000
MAX_RPM = 
MIN_RPM = 
DEADBAND = 
P = 
I = 
D = 
FF0 = 
FF1 = 
FF2 = 
BIAS = 
MAX_ERROR = 

[INPUTS]
# DO NOT change the input text
INPUT_0 = Min Limit & Home
INPUT_JOINT_0 = 0
INPUT_1 = Max Limit
INPUT_JOINT_1 = 0
INPUT_2 = Min Limit & Home
INPUT_JOINT_2 = 1
INPUT_3 = Max Limit
INPUT_JOINT_3 = 1
INPUT_4 = Min Limit & Home
INPUT_JOINT_4 = 2
INPUT_5 = Max Limit
INPUT_JOINT_5 = 2
INPUT_6 = Max Limit & Home
INPUT_JOINT_6 = 3
INPUT_7 = Min Limit
INPUT_JOINT_7 = 3
INPUT_8 = Select
INPUT_JOINT_8 = False
INPUT_9 = Probe
INPUT_JOINT_9 = False
INPUT_10 = E-Stop In
INPUT_JOINT_10 = False

[OUTPUTS]
# DO NOT change the output text
OUTPUT_0 = Spindle On
OUTPUT_1 = Select
OUTPUT_2 = Select
OUTPUT_3 = Select
OUTPUT_4 = Select

[OPTIONS]
MANUAL_TOOL_CHANGE = False
HALUI = True
PYVCP = True
GLADEVCP = True
LADDER = True

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22 Jan 2021 01:39 #196173 by PCW
Replied by PCW on topic Follow Error - Open Loop Stepper Motor
The FERROR limits are too small

Change these:

FERROR = 0.0051
MIN_FERROR = 0.0025

to

FERROR = 0.02
MIN_FERROR = 0.01
The following user(s) said Thank You: rodw

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22 Jan 2021 20:03 - 22 Jan 2021 20:04 #196267 by Project_Hopeless
Replied by Project_Hopeless on topic Follow Error - Open Loop Stepper Motor

The FERROR limits are too small

Change these:

FERROR = 0.0051
MIN_FERROR = 0.0025

to

FERROR = 0.02
MIN_FERROR = 0.01

Ok, so I made the changes and I still get the error. So I created a dummy config using pncConfig and compared to my original *.ini file.

pncConfig generated a comment that the STEPGEN MAX_VEL and MAX_ACC should be 25% higher. In my file it is actually less than. Could that be the root cause?
-.01
MAX_LIMIT = 1205
MAX_VELOCITY = 27.5 <is now 59.7>
MAX_ACCELERATION = 50
TYPE = LINEAR
SCALE = 80
STEPGEN_MAX_VEL = 33.0
STEPGEN_MAX_ACC = 60.0
FERROR = 0.0051
MIN_FERROR = 0.0025
DIRSETUP = 10000
DIRHOLD = 10000
STEPLEN = 5000
STEPSPACE = 5000
DEADBAND = 0
P = 1000
I = 0
D = 0
FF0 = 0
FF1 = 1
FF2 = 0.00013
BIAS = 0
MAX_OUTPUT = 0
MAX_ERROR = 0.0127
HOME = 0
HOME_OFFSET = -1
HOME_SEARCH_VEL = -10
HOME_LATCH_VEL = 1
HOME_SEQUENCE = 2
HOME_USE_INDEX = False
HOME_IGNORE_LIMITS = True
Last edit: 22 Jan 2021 20:04 by Project_Hopeless.

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22 Jan 2021 20:31 #196271 by PCW
Replied by PCW on topic Follow Error - Open Loop Stepper Motor
I think the remaining issue is that the P term is 1000
but should be 500 for a 2 ms (20000000 ns) servo thread (
in addition to widening the ferror limits )

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22 Jan 2021 21:00 #196274 by rodw
Replied by rodw on topic Follow Error - Open Loop Stepper Motor

pncConfig generated a comment that the STEPGEN MAX_VEL and MAX_ACC should be 25% higher. In my file it is actually less than. Could that be the root cause?


It would be important to fix these too. I have found relaxing thee a bit can remove following errors in the past so if they are too short, its bound to cause probems
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22 Jan 2021 21:14 #196281 by PCW
Replied by PCW on topic Follow Error - Open Loop Stepper Motor
I should add that the P value should be 1/Servo_thread_period
pncconf does this but if you change the servo thread period
you need to update the P values.

The effect of the P term is is this:

A P value of 1/Servo_thread_period means that any small deviation
in position found is corrected by the next servo period.
Lower P values are possible but do reduce the pulse stream
accuracy somewhat, larger values will result in ringing and overshoot
and values larger than 2/Servo_thread_period will result in oscillations
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22 Jan 2021 21:48 #196285 by Project_Hopeless
Replied by Project_Hopeless on topic Follow Error - Open Loop Stepper Motor
All is good now!

The problem was STEPGEN_MAX_VEL and STEPGEN_MAX_ACC values set less than the MAX_ values.
The following user(s) said Thank You: rodw, thefabricator03

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