Placement/fr

Description
Le Positionnement (placement en anglais) est la fonction qu'utilise FreeCAD pour spécifier l'emplacement et la position (orientation) d'un objet dans l'espace.

La fonction Positionnement peut être accessible de plusieurs manières :
 * via un script Python à partir de la console, et de son API.




 * ou dans la fenêtre Vue combinée → Propriétés → Données → Placement → ,
 * ou par le menu  Édition → Positionnement....

Puis la boîte à outils de Tâches Placement s'affiche.



Angle, Axes et Position
Placement = [Angle, Axis, Position]







Angle, Axis and Position
Placement = [Angle, Axis, Position]

Placement : fixe la Position de l'objet dans l'espace et décrit son orientation comme une seule rotation autour d'un axe. Angle = r : est un scalaire qui indique la quantité de rotation de l'objet sur l'axe. La saisie se fait en degrés, mais en interne la valeur est stockée en radians.

Axis = (ax,ay,az) is a vector describing an axis of rotation (See Note about axis of rotation). Examples are: (1,0,0)      ==> about X axis (0,1,0)      ==> about Y axis (0,0,1)      ==> about Z axis (0.71,0.71,0) ==> about the line y=x Position = (x,y,z) is a Vector describing the world coordinates of the reference point of the object. Note that in scripts, Placement.Base is used to denote the Position component of a placement. The Property Editor calls this value "Position".

Position and Yaw, Pitch and Roll
Placement = [Position, Yaw-Pitch-Roll]

The second form of Placement fixes an object's location in space with a Position (as in the first form), but describes it's orientation using Yaw, Pitch and Roll angles (Yaw, Pitch, Roll). These angles are sometimes referred to as Euler angles or Tait-Bryan angles (Euler angles). Yaw, Pitch and Roll are common aviation terms for a body's orientation (or attitude).

Position = (x,y,z) is a Vector describing the world coordinates of the reference point of the object.

Yaw-Pitch-Roll = (y,p,r) is a tuple that specifies the attitude of the object. Values for y,p,r  specify degrees of rotation about each of the z,y,x axis (see note).

Yaw is the rotation about the Z axis, that is to say a rotation from left to right. (The yaw angle is the Psi ψ).

Pitch is rotation about the Y axis, that is to say nose-up and nose-down. (The Pitch angle is the Phi φ).

Roll is rotation about the X axis, that is to say wing up and down. (The Roll angle is the Thêta θ).

Matrix
Placement = Matrix

La troisième forme de Placement, décrit la position de l'objet et l'orientation avec une matrice de transformation affine de 4 x 4 (Transformation Affine).

Matrice =

((r11,r12,r13,t1),   (r21,r22,r23,t2),    (r31,r32,r33,t3),    (0,0,0,1)), with rij specifying rotation and ti specifying translation.

The Placement Dialog
The Placement Dialog is invoked from the Edit menu. It is used to precisely rotate/translate objects. It is also used when we need to create a sketch on a "non standard" plane or change a sketch's orientation to a new plane. The Translation section adjusts the objects location in space. The Center section adjusts the rotational axis to one that does not pass through the object's reference point. The Rotation section adjusts the rotational angle(s) and the method of specifying those angles. The Apply incremental changes to object placement tick box is useful when translations/rotations are to be made relative the object's current position/attitude, rather than to the original position/attitude. Ticking this box resets the dialog's input fields to zero, but does not change the object's orientation or location. Subsequent entries do change the orientation/location, but are applied from the object's current position.

Examples
Rotations about a single axis:

Before Rotation (top view)

After Rotation about Z (top view)

After Rotation about y=x (right view)

Rotation with offset centre point:

Before Rotation (top view)

After Rotation about Z (top view)

Rotation using Euler angles:

Before Rotation

After Rotation

Placement.Base vs Shape Definition
Placement is not the only way to position a shape in space. Note the Python console in this image:



Both cubes have the same value for Placement, but are in different locations! This is because the 2 shapes are defined by different vertices. For the 2 shapes in the above illustration:

>>> ev = App.ActiveDocument.Extrude.Shape.Vertexes >>> for v in ev: print v.X,",",v.Y,",",v.Z ...   30.0,30.0,0.0 30.0,30.0,10.0 40.0,30.0,0.0  40.0,30.0,10.0  40.0,40.0,0.0  40.0,40.0,10.0  30.0,40.0,0.0  30.0,40.0,10.0  >>> e1v = App.ActiveDocument.Extrude001.Shape.Vertexes >>> for v in e1v: print v.X,",",v.Y,",",v.Z ...   0.0,10.0,0.0 0.0,10.0,10.0 10.0,10.0,0.0  10.0,10.0,10.0  10.0,0.0,0.0  10.0,0.0,10.0  0.0,0.0,0.0  0.0,0.0,10.0  >>> The Vertices (or Vectors) that define the shape use the Placement.Base attribute as their origin. So if you want to move a shape 10 units along the X axis, you could add 10 to the X coordinates of all the Vertices or you could set Placement.Position to (10,0,0).

Using "Center" to Control Axis of Rotation
By default, the axis of rotation isn't really the x/y/z axis. It is a line parallel to the selected axis, but passing through the reference point (Placement.Base) of the object to be rotated. This can be changed by using the Center fields in the Placement dialog or, in scripts, by using the Center parameter of the FreeCAD.Placement constructor.

Par exemple, supposons que nous ayons un cube (ci-dessous) positionné aux coordonnées (20,20,10).



Nous voulons faire tourner le cubee autour de son propre axe vertical (c'est à dire local Z), tout en gardant la même position. Nous pouvons facilement y parvenir en spécifiant une valeur de centre égale aux coordonnées du point central du cube (25,25,15).

Dans un script ,nous aurons : Same script with the file example RotateCoG2.fcstd (discussion on the forum)

Limitations
Depuis la version 0.13, la mise à jour des propriétés de positionnement dans l'onglet Données a été désactivée pour les objets créés dans l'atelier PartDesign, à l'exception de l'esquisse initiale du solide créé.

C'est pourquoi la mise en place d'un solide créé dans PartDesign à partir d'une esquisse ne peut être modifiée qu'en ajustant les paramètres de la construction initiale de l'esquisse (la première esquisse) du solide créé.

La fonctionnalité de placement sera éventuellement traitée dans l'atelier d'assemblage, à venir dans une version ultérieure.

En savoir plus

 * Le tutoriel: Aéroplane traite largement des mécanismes de modifications d'un objet.
 * Une explication détaillée sur la fonction Placement peut être trouvée ici Tasks_Placement.