Tutorial KinematicSkeleton

    From FreeCAD Documentation
    Other languages:
    Freecad.svg Tutorial
    Topic
    Assembly3, a kinematic skeleton
    Level
    Basic knowledge of Assembly3 and Sketcher tools is helpful
    Time to complete
    40 minutes
    Authors
    FBXL5
    FreeCAD version
    0.20 and later
    Example files
    None
    See also
    Tutorial KinematicAssembly, Tutorial KinematicController

    Introduction

    This tutorial is about how to set up a simple 2D mechanism and attach 3D objects, mainly with the tools from the external Assembly3 workbench icon.svg Assembly3 Workbench.

    This tutorial does not use the skeleton sketch principle (see Assembly3 Create-Skeleton-Sketch on GitHub).

    Instead we will use PartDesign Body.svg PartDesign Bodies containing only one Workbench Sketcher.svg Sketch each, to build a 2D mechanism, a multi sketch skeleton.

    The dimensions, and the colours as well, are taken from the SolveSpace tutorial which is referred to on the Assembly3 GitHub page (see above).

    Multi sketch skeleton

    This so-called multi sketch skeleton consists of several individual PartDesign Body.svg Bodies and an Assembly New Assembly.svg Assembly container. To be able to attach further objects each body is put into a separate Assembly container.

    2D Body objects

    The bodies, and their sketches, that are used in this assembly:

    • A base plate (green)
    • A crank (blue)
    • Two movable plates (red and grey)
    • Four connecting rods (white, yellow, purple, and brown)

    Assembly3 SketchSkeleton-01.png

    All eight sketches individually coloured and manually positioned by moving their parent bodies

    The shape can deviate from that of the real part, but the position of the joint defining geometry must be accurate.

    Assembly containers

    Parent assembly

    To fix or control the positions of all bodies we need an Assembly New Assembly.svg Assembly object. It adds an assembly branch to the Tree view.

    Sub-assemblies

    Repeat above action to create an Assembly object for each Body and drag the Body into its Parts container. Then fix the Body to its Assembly:

    1. Activate the Assembly object (double-click).
    2. Select a circle/arc belonging the Body object.
    3. Press the Assembly ConstraintLock.svg Create "Locked" constraint button to fix the Body in its sub-assembly.

    The Crank-Assembly, for example, should look like this:

    Assembly3 SketchSkeleton-25.png

    The Crank's sub-assembly branch in the Tree view and the Crank with its locked Element in the 3D view

    Assembly tree

    In the Tree view drag all sub-assembly branches into the Parts container of the parent Assembly object.

    Assembly3 SketchSkeleton-26.png

    Assembly branch in the Tree view

    Now they are ready to be arranged.

    Fixed base plate

    First we need a fixed part. To fix the Base completely we would usually select a face, but in this case a circle will do as well.

    1. Select a circle of the Base.
    2. Press the Assembly ConstraintLock.svg Create "Locked" constraint button to fix the Base.

    Assembly3 SketchSkeleton-02.png Button right.svg Assembly3 SketchSkeleton-03.png

    Selected circle → Fixed Base with the created Element object and element coordinate system (ECS) displayed (green)

    Joints

    For hinge-like joints we select one circle of each sketch and use the Assembly ConstraintCoincidence.svg Plane Coincidence constraint. It not only sets both Element's XY planes coplanar, but sets their Z axes colinear, too.

    1. Select a circle of each object to connect.
    2. Press the Assembly ConstraintCoincidence.svg Create "Plane Coincidence" constraint button.

    Base - Crank

    Assembly3 SketchSkeleton-04.png Button right.svg Assembly3 SketchSkeleton-05.png

    Circles on Base and Crank selected → Relocated Crank with the created Element objects and ECSs displayed (green)

    Base - Upper Plate

    Assembly3 SketchSkeleton-06.png Button right.svg Assembly3 SketchSkeleton-07.png

    Circles on Base and Upper Plate selected → Relocated Upper Plate

    Previously created joints can be identified by their constraint representations (red).

    Crank - Rod 1

    Assembly3 SketchSkeleton-08.png Button right.svg Assembly3 SketchSkeleton-09.png

    Circles on Crank and Rod 1 selected → Relocated Rod 1 and tilted Crank

    Upper Plate - Rod 1

    The last link in this kinematic chain connects two Elements whose Z directions are already defined and a Assembly ConstraintPointOnLine.svg Point on line constraint is all we need.

    1. Select a circle of each object to connect.
    2. Press the Assembly ConstraintPointOnLine.svg Create "PointOnLine" constraint button.

    Assembly3 SketchSkeleton-10.png Button right.svg Assembly3 SketchSkeleton-11.png

    Circles on Upper Plate and Rod 1 selected → Relocated Rod 1, and tilted Crank and Upper Plate

    If the Z axes of three elements or joints are parallel and lie on the same virtual plane, the solver may fail to rearrange them in a following step because it is unable to decide in which direction the middle joint should be rotated. This can occur for the Rod 1 element, the Crank - Rod 1 joint, and the Base - Crank joint we have here. If this happens we need to help the solver and rotate one object (e.g. the Crank) manually using the Assembly AxialMove.svg Axial move tool.

    Upper Plate - Rod 2

    Another kinematic (sub-)chain starts with Assembly ConstraintCoincidence.svg Plane Coincidence constraints.

    Assembly3 SketchSkeleton-12.png Button right.svg Assembly3 SketchSkeleton-13.png

    Circles on Upper Plate (or Base) and Rod 2 selected → Relocated Rod 2

    Rod 2 - Lower Plate

    Assembly3 SketchSkeleton-14.png Button right.svg Assembly3 SketchSkeleton-15.png

    Circles on Rod 2 and Lower Plate selected → Relocated Lower Plate and tilted Rod 2

    Upper Plate - Rod 3

    Assembly3 SketchSkeleton-16.png Button right.svg Assembly3 SketchSkeleton-17.png

    Circles on Upper Plate and Rod 3 selected → Relocated Rod 3 and rearranged upper kinematic sub-chain

    Lower Plate - Rod 3

    And this kinematic (sub-)chain ends with a Assembly ConstraintPointOnLine.svg Point on line constraint, too.

    Assembly3 SketchSkeleton-18.png Button right.svg Assembly3 SketchSkeleton-19.png

    Circles on Lower Plate and Rod 3 selected → Relocated Rod 3 and rearranged ukinematic sub-chains

    To connect both kinematic sub-chains we use Rod 4 with a Assembly ConstraintCoincidence.svg Plane Coincidence constraint on one end and a Assembly ConstraintPointOnLine.svg Point on line constraint on the other.

    Crank - Rod 4

    Assembly3 SketchSkeleton-20.png Button right.svg Assembly3 SketchSkeleton-21.png

    Circles on Crank and Rod 4 selected → Relocated Rod 4

    Lower Plate - Rod 4

    Assembly3 SketchSkeleton-22.png Button right.svg Assembly3 SketchSkeleton-23.png

    Circles on Lower Plate and Rod 4 selected → Relocated Rod 4 and final layout of kinematic assembly

    Actuator

    Since Assembly3 doesn't provide any means to control kinematic assemblies, we need external assistance such as this kinematic controller. To use this controller we need to mark one constraint's label with the suffix "Driver" to make it a driving constraint. It may be separated by a "." or "-" for clarity, as the controller will only check if the label ends with "Driver".

    We therefore change the label of the Base-Crank joint to Base-Crank.Driver.

    Finished skeleton

    The finished kinematic assembly with deactivated representation of Elements and Constraints should look like this:

    Assembly3 SketchSkeleton-24.png

    Finished assembly in the Tree view and the 3D view

    Assembly3 SketchSkeleton-27.gif

    GIF animation made from an image sequence from this kinematic controller

    Attaching 3D geometry

    My expectations about attaching a new object to a base object belonging to a kinematic assembly were something like:

    • Put the new object into the base object's Parts container.
    • Position the new object in relation to the base object.
    • Fix the relative offset and orientation using the Attachment constraint.

    But that would have been too easy.


    The Assembly ConstraintAttachment.svg Assembly3 ConstraintAttachment tool, like any Assembly3 constraint tool, relies on the use of Element objects and their element coordinate systems (ECSs) for positioning tasks.

    And so attaching objects is just another way of adding objects to a (sub-)assembly.


    Let's attach Rod 4-3D to Rod 4 for example:

    The objects have a different orientation and the 3D object should have an offset from the 2D object.

    1. Put the new object into the base object's Parts container.
    2. Select two corresponding circles or arcs.
    3. Press the Assembly ConstraintAttachment.svg Create "Attachment" constraint button.
    Assembly3 SketchSkeleton-28.png Button right.svg Assembly3 SketchSkeleton-29.png Button right.svg Assembly3 SketchSkeleton-30.png

    Rod 4 (locked) and Rod 4-3D → Selected arcs → Relocated Rod 4-3D (both ECSs are in the same place with identical orientation)

    It is now plain to see that the Assembly ConstraintAttachment.svg Assembly3 ConstraintAttachment tool ignores the offset and orientation between both objects.

    However the position is already defined as we wanted and so we only need to adapt the angle manually and define the desired offset:

    • Set the DataOffset, Angle of the first Element in the Attachment container to match the orientation.
    • Set the DataOffset, Position, z of the same Element to apply an offset.

    In case we set the properties of the second Element, the movement of angle and offset would go in the opposite direction.

    Assembly3 SketchSkeleton-30.png Button right.svg Assembly3 SketchSkeleton-31.png Button right.svg Assembly3 SketchSkeleton-32.png

    As attached → Angle adapted → Offset defined

    If there is a 3D object attached to each 2D object, it could look like this:

    Assembly3 SketchSkeleton-33.gif

    Notes

    The section Attaching 3D geometry just scratches the surface of extending a sub-assembly, and other constraints or combinations of constraints may be more suitable than the attachment constraint.

    It is important to move such a kinematic assembly in tiny steps or the solver will give up and fail. It is almost impossible to use Assembly Move.svg Move part or Assembly AxialMove.svg Axial move for this task.

    The Assembly ConstraintCoincidence.svg Assembly3_ConstraintCoincidence constraint is used to drive the kinematic assembly, its property DataAngle (enabled by the property DataLock Angle) accepts positive or negative floating point numbers greater than 360 and so could do several full turns.