Stepper motors and linear scale closed loop setup?

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19 Nov 2020 09:45 #189766 by Muzzer
Nice setup, Chris! The 7i76 doesn't have those ribbon cable headers that you see on the 7i76e, so it seems I will need to connect the 7i85 back to the 5i25. What I didn't recall was that the 5i25 has a second D25 header.

I will see about getting my hands on one of those adaptors mentioned. I will then have 2 of the screened D25 cables from the 5i25, one per daughterboard.

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21 Nov 2020 12:44 #189970 by Muzzer
Received a 26 pin header to DB25 cable today - just need the cable to arrive now. The bummer is that the case for my mini-ITX board only has one slot opening, so I will bring the ribbon cable through and remove the plate. Not the end of the world.

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09 Apr 2021 14:42 #205435 by liviuzarnesti
Hello, please tell me what resolution have your linear scale 5um or 1um, I use 7I76 and 7I89.I want to buy linear scale and I need some help.Thank you

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09 Apr 2021 18:56 #205457 by Muzzer
I have 5um scales on both X and Z axes. The Z axis is a generic Chinese glass scale (from HXX in my case) and the X axis is one of the miniature magnetic scale that fits inside the cross slide (invisible!). In practice, it means that you may be able to control to within perhaps +/-10um realistically. If you try to position your axis better than the scale resolution, it will grumble and hunt around the set point. Part of the PID setup is the "deadband" parameter which I set to 4.5um - I don't know the science here but that seemed like a good idea and sure enough it cleans up the control nicely.

I might have gone for a higher resolution but I already had the Z axis scale and besides, the machine probably isn't capable of much better than 10um or so, even in closed loop, as it's a bit old and worn.

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11 Apr 2021 06:35 - 05 Dec 2021 18:18 #205601 by Joco
I’m using 5um scales as well. They work well and I can get to within 0.02mm on my target dimensions. While 1um scales would have been ideal I already had the 5um ones from when the mill was manual with DRO.

If I didn’t have the scales already and I didn’t mind the extra money the I would get the 1um. More because I tend to over engineer than them being necessary.

The other thing to consider is the inherent accuracy of your machine. If it can hold 0.005mm precision then 1um scales would probably be worth it. But if realistic accuracy is 0.01 to 0.02mm then 5um seems to do the business.

Cheers - J.
Last edit: 05 Dec 2021 18:18 by Joco. Reason: Fixed typo, meant 1um not 5 re “if didnt already have scales” comment

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29 Nov 2021 23:46 #227959 by bobi4ko23
Hello, I have a MESA 7i96 card connected with 7i85 extension. I got the pins working all right, and the x and y axis working to my satisfaction (they use open loop steppers and i close the loop using linear encoder scales). However, i have a different motor mounted on the z-axis (the motor is a closed loop stepper, i think the proper term is hybrid stepper, i.e it sends the number of steps it made back to the driver). It was working fine but after connecting the Z-axis linear encoder scale i got following error. I have been tinkering with the PID parameters for 2 hours but nothing solves the issue.

The problems im facing:
1.I order the axis to move 5 mm and it moves 5.005 and i get following error.
2. I order the axis to move 20 mm and it moves around 11 (not exact same number each time) and i get following error.

The PID parameters i used are:
Ferror = 40.0                  (Ferror and min_ferror i just set so that it doesn't show the error but to no prevail)
Min_ferror = 10.0
P = 5.0
I = 0
D =  0.02
FF0 = 0
FF1 = 0.95
FF2 = 0
BIAS = 0
DEADBAND = 0
MAX_OUTPUT = 0

Any advice in the right direction is very appreciated.

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30 Nov 2021 00:11 - 30 Nov 2021 00:12 #227962 by PCW
FF1 really should be 1.0000 and if it doesn't work well at that setting
its suggests a scaling error between the step/dir and the encoder feedback

Have you verified that the system works properly with local feedback?
(no encoder) I think its best to start with a pure step/dir system and make
sure it operates properly before changing the feedback source to an encoder.
(for a pure step/dir system, P should be 1000 and FF1 should be 1.0000)
Last edit: 30 Nov 2021 00:12 by PCW.

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30 Nov 2021 00:16 - 30 Nov 2021 00:19 #227963 by bobi4ko23
Yes, I have verified the system worked perfectly fine before adding the feedback loop from the linear scales. The P and FF1 values were not tampered with (default as pncconf configures them)

What does this mean "scaling error between the step/dir and the encoder feedback"?
Last edit: 30 Nov 2021 00:19 by bobi4ko23. Reason: additional information

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30 Nov 2021 00:20 #227964 by PCW
Just wondering why FF1 was not exactly 1.000

What magnitude of following error do you have?
Can you post a halscope plot of the following error vs velocity?

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30 Nov 2021 00:25 #227965 by bobi4ko23
FF1 was not 1.00 because i thought the deacceleration and lowered speed (slow come up to the input value) would help it not overshoot. Should i return it to 1.00?

I'm a bit of a rookie, so can you point me to where i can understand how to use halscope?
I will try to get back to you with this information ASAP.

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