Local_CS Frenet-Serret wrong axes direction.

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Tato3d
Posts: 35
Joined: Wed Apr 17, 2019 9:08 am

Local_CS Frenet-Serret wrong axes direction.

Post by Tato3d » Sun Dec 29, 2019 9:08 am

Hello!

I'm having a look at the create new coordinate system tool in Part Desing workbench and I think that for the Frenet attachments the direction of the tangential, normal and binormal axes is not correct.

If I remember correctly these axes have the following functions:

Tangential = motion direction of the particle (map path parameter value 0 to 1) along the path (Spline).

Normal = curvature direction of the particle, always looking at the center of the osculating circle. Not to be confused with the normal of a plane.

Binormal = twist direction of the particle, perpendicular direction to the normal and tangential axes.


In the Local_CS Frenet NBT image the directions of the tangential (Z), normal (N) and binormal (Y) axes are not correct.
local CS Frenet NBT.png
Frenet-Serret wrong axes direction
local CS Frenet NBT.png (214.85 KiB) Viewed 564 times
I leave also what I suppose must be the correct directions of the axes in the Handmade Frenet NBT image, a file with a simulation (Frenet NB motion) and a little program (movimiento.FCmacro) for you to have a look at it.
Frenet NBT handmade.png
correct directions of the axes
Frenet NBT handmade.png (214.41 KiB) Viewed 564 times
Sorry for my poor english.

Edit to add FreeCAD info.

OS: Ubuntu 18.04.3 LTS
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.18.4.
Build type: Release
Python version: 3.6.8
Qt version: 5.9.5
Coin version: 4.0.0a
OCC version: 7.3.0
Locale: Spanish/Spain (es_ES)
Attachments
movimiento.FCMacro
python macro
(238 Bytes) Downloaded 8 times
Frenet NB motion.FCStd
Frenet tangential motion
(20.51 KiB) Downloaded 9 times
Last edited by Tato3d on Sat Jan 04, 2020 6:38 pm, edited 1 time in total.
Si eres nuevo en Part Desing y no te aclaras con los planos de referencia te dejo unos tutoriales que tengo en youtube por aqui.
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Kunda1
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Re: Local_CS Frenet-Serret wrong axes direction.

Post by Kunda1 » Sun Dec 29, 2019 2:08 pm

Please add you About info to the original post (please don't screenshot the dialog). thank you.
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UR_
Posts: 1175
Joined: Tue Jan 03, 2017 8:42 pm

Re: Local_CS Frenet-Serret wrong axes direction.

Post by UR_ » Sun Dec 29, 2019 4:11 pm

Screenshot 002.png
Screenshot 002.png (10.98 KiB) Viewed 529 times

Only NBT ... :relieved:
Tato3d
Posts: 35
Joined: Wed Apr 17, 2019 9:08 am

Re: Local_CS Frenet-Serret wrong axes direction.

Post by Tato3d » Sun Dec 29, 2019 5:30 pm

Kunda1 wrote:
Sun Dec 29, 2019 2:08 pm
Please add you About info to the original post (please don't screenshot the dialog). thank you.
I'm sorry. I forgot :oops: .

OS: Ubuntu 18.04.3 LTS
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.18.4.
Build type: Release
Python version: 3.6.8
Qt version: 5.9.5
Coin version: 4.0.0a
OCC version: 7.3.0
Locale: Spanish/Spain (es_ES)
Si eres nuevo en Part Desing y no te aclaras con los planos de referencia te dejo unos tutoriales que tengo en youtube por aqui.
UR_
Posts: 1175
Joined: Tue Jan 03, 2017 8:42 pm

Re: Local_CS Frenet-Serret wrong axes direction.

Post by UR_ » Sun Jan 05, 2020 10:03 am

DeepSOIC wrote: :bell:
Any explanations about this?
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DeepSOIC
Posts: 7479
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Location: Saint-Petersburg, Russia

Re: Local_CS Frenet-Serret wrong axes direction.

Post by DeepSOIC » Mon Jan 06, 2020 7:40 pm

I designed attachment modes to be used directly as sketch attachment. These flipped axes usually gave more convenient sketching orientations. Sorry about this mess. Fixing it now is quite tricky, as just fixing the code will break projects where this attachment mode is used.
UR_
Posts: 1175
Joined: Tue Jan 03, 2017 8:42 pm

Re: Local_CS Frenet-Serret wrong axes direction.

Post by UR_ » Tue Jan 07, 2020 8:47 am

DeepSOIC wrote:
Mon Jan 06, 2020 7:40 pm
... as just fixing the code will break projects ...

What about introducing three new mathematical correct modes
e.g. mNBT, mTNB, mTBN
and preserving sketcher existing convenience modes NB, TN, TB.

Therefore user doesn't break old files, but has a possibility to choose an appropriate attachment mode, if mathematical correctness is preferred.
Also he can use old "sketcher convenience modes" in new projects too

Screenshot 004.png
Screenshot 004.png (20.43 KiB) Viewed 300 times
Tato3d
Posts: 35
Joined: Wed Apr 17, 2019 9:08 am

Re: Local_CS Frenet-Serret wrong axes direction.

Post by Tato3d » Tue Jan 07, 2020 8:09 pm

DeepSOIC wrote:
Mon Jan 06, 2020 7:40 pm
I designed attachment modes to be used directly as sketch attachment. These flipped axes usually gave more convenient sketching orientations. Sorry about this mess. Fixing it now is quite tricky, as just fixing the code will break projects where this attachment mode is used.
Okay, now I know because the tangential and normal axes are reversed.
This also happens with normal to edge attachment.

I understand that in order not to break compativity it will stay that way.

Thank you for the explanation!!!!!!!
Si eres nuevo en Part Desing y no te aclaras con los planos de referencia te dejo unos tutoriales que tengo en youtube por aqui.
galou_breizh
Posts: 312
Joined: Wed Sep 15, 2010 9:38 am

Re: Local_CS Frenet-Serret wrong axes direction.

Post by galou_breizh » Fri Jan 10, 2020 11:30 am

Just my two cents. FreeCAD is still alpha-software and breaking changes were announced.

Gaël
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