Local_CS Frenet-Serret wrong axes direction.

[Tato3d]

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.

Frenet-Serret wrong axes direction

local CS Frenet NBT.png (214.85 KiB) Viewed 2300 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.

correct directions of the axes

Frenet NBT handmade.png (214.41 KiB) Viewed 2300 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)

[Kunda1]

Please add you About info to the original post (please don't screenshot the dialog). thank you.

[UR_]

Screenshot 002.png (10.98 KiB) Viewed 2265 times

Only NBT ...

[Tato3d]

Please add you About info to the original post (please don't screenshot the dialog). thank you.

I'm sorry. I forgot .

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)

[UR_]

Any explanations about this?

[DeepSOIC]

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_]

... 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 (20.43 KiB) Viewed 2036 times

[Tato3d]

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!!!!!!!

[galou_breizh]

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

Gaël