Homing with absolute encoders
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09 May 2021 14:18 - 09 May 2021 17:12 #208374
by PKM
Homing with absolute encoders was created by PKM
Hello!
I am building a machine with absolute encoders.
So I would like to understand the homing.
linuxcnc.org/docs/html/config/ini-homing...ome_absolute_encoder says:
But why set the current position to [JOINT_n]HOME_OFFSET while it's reasonable to add some pre-defined offset to the motor position instead?
I.e., how is it different from an incremental encoder?
I am building a machine with absolute encoders.
So I would like to understand the homing.
linuxcnc.org/docs/html/config/ini-homing...ome_absolute_encoder says:
Say, I run the machine, encoder positions are read from the drives and passed to joint.N.motor-pos-fbWhen a request is made to home the joint, the current joint position is set to the [JOINT_n]HOME_OFFSET value.
But why set the current position to [JOINT_n]HOME_OFFSET while it's reasonable to add some pre-defined offset to the motor position instead?
I.e., how is it different from an incremental encoder?
Last edit: 09 May 2021 17:12 by PKM.
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10 May 2021 22:48 #208504
by andypugh
Replied by andypugh on topic Homing with absolute encoders
I don't think that is actually what happens. ie, I suspect that the documentation is wrong.
I don't have actual absolute encoders, but I do have HOME_ABSOLUTE_ENCODER set because there is a way to fake it with single-turn absolute and the position.txt file.
And, what happens, is that the axis position becomes the encoder output position.
I don't have actual absolute encoders, but I do have HOME_ABSOLUTE_ENCODER set because there is a way to fake it with single-turn absolute and the position.txt file.
And, what happens, is that the axis position becomes the encoder output position.
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23 May 2021 16:16 - 23 May 2021 17:30 #209920
by PKM
Replied by PKM on topic Homing with absolute encoders
I have some troubles with this setting.
Either it doesn't work as supposed or I don't understand something.
I tested at the machine with actual absolute encoders, but it works the same in sim.
Say, edit a [JOINT_0] section in axis_mm config like this
HOME = 10.00
HOME_OFFSET = 10.00
HOME_ABSOLUTE_ENCODER = 2
Then start the machine, home, close the machine. Repeat a few times. Note the axes values. Correspondingly joint.0.motor-offset is decreasing by 10 each time, why?
Either it doesn't work as supposed or I don't understand something.
I tested at the machine with actual absolute encoders, but it works the same in sim.
Say, edit a [JOINT_0] section in axis_mm config like this
HOME = 10.00
HOME_OFFSET = 10.00
HOME_ABSOLUTE_ENCODER = 2
Then start the machine, home, close the machine. Repeat a few times. Note the axes values. Correspondingly joint.0.motor-offset is decreasing by 10 each time, why?
Last edit: 23 May 2021 17:30 by PKM.
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23 May 2021 19:54 #209947
by andypugh
Replied by andypugh on topic Homing with absolute encoders
I think that HOME_OFFSET needs to be zero.
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25 May 2021 10:23 #210211
by PKM
Replied by PKM on topic Homing with absolute encoders
Not working well either.
Each start of the machine (even doing nothing at all) the joint 0 position gets increased by joint.0.motor-pos-fb value.
Each start of the machine (even doing nothing at all) the joint 0 position gets increased by joint.0.motor-pos-fb value.
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25 May 2021 11:02 #210214
by andypugh
Replied by andypugh on topic Homing with absolute encoders
github.com/LinuxCNC/linuxcnc/blob/master...motion/homing.c#L649
Goes straight to
github.com/LinuxCNC/linuxcnc/blob/master...motion/homing.c#L988
Which looks like (with an offset of zero) it should be doing the right thing.
github.com/LinuxCNC/linuxcnc/blob/2e75b0...otion/control.c#L376
Is where joint->pos_fb is updated. I haven't found anywhere in the code that joint->motor_offset is given a non-zero value. There is a command, but as far as I can see it is never called.
github.com/LinuxCNC/linuxcnc/blob/2e75b0...ask/taskintf.cc#L208
Goes straight to
github.com/LinuxCNC/linuxcnc/blob/master...motion/homing.c#L988
Which looks like (with an offset of zero) it should be doing the right thing.
github.com/LinuxCNC/linuxcnc/blob/2e75b0...otion/control.c#L376
Is where joint->pos_fb is updated. I haven't found anywhere in the code that joint->motor_offset is given a non-zero value. There is a command, but as far as I can see it is never called.
github.com/LinuxCNC/linuxcnc/blob/2e75b0...ask/taskintf.cc#L208
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25 May 2021 13:35 - 25 May 2021 13:35 #210223
by PKM
Replied by PKM on topic Homing with absolute encoders
This looks like a suspect
github.com/LinuxCNC/linuxcnc/blob/2e58fb...motion/homing.c#L997
I compiled from source, so I can try to change something...
github.com/LinuxCNC/linuxcnc/blob/2e58fb...motion/homing.c#L997
I compiled from source, so I can try to change something...
Last edit: 25 May 2021 13:35 by PKM.
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25 May 2021 13:57 #210225
by PKM
Replied by PKM on topic Homing with absolute encoders
Oh.
I mentioned that the position changed even without homing.
So it must be somewhere else, right?
I mentioned that the position changed even without homing.
So it must be somewhere else, right?
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25 May 2021 15:22 #210232
by andypugh
That line shouldn't execute if HOME_ABSOLUTE_ENCODER is set.
Replied by andypugh on topic Homing with absolute encoders
This looks like a suspecthttps://github.com/LinuxCNC/linuxcnc/blob/2e58fb153ca8197d5a2149313fbc7cb300ca0b2f/src/emc/motion/homing.c#L997
That line shouldn't execute if HOME_ABSOLUTE_ENCODER is set.
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25 May 2021 16:03 - 25 May 2021 16:04 #210236
by PKM
Replied by PKM on topic Homing with absolute encoders
I understand that it should not...
Anyways, it changes even without homing.
Is the offset kept somewhere when the machine is off? The problem might be there
Anyways, it changes even without homing.
Is the offset kept somewhere when the machine is off? The problem might be there
Last edit: 25 May 2021 16:04 by PKM.
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