PRIMER 22.1

Automake: Creating Mechanisms Automatically

As well as creating mechanisms manually PRIMER 11 onwards can calculate mechanisms automatically from the structure of your model.

How it works

(1) Select an initial "seed" node, the Start node

In this example, designed to create a mechanism from the front suspension of a vehicle model, the user has picked a node on the front tyre, shown here with a cross.

(2) Press Calculate

PRIMER will "grow" the first assembly, here the red wheel, outwards from that seed node until no more connected structure can be found, at which point the first assembly has been defined.

It then looks for potential connections, in the form of joints (*CONSTRAINED JOINT) or discrete elements (*ELEMENT DISCRETE, or *ELEMENT BEAM type 6) on that assembly.

If any are found then the 2nd node on these forms a seed node on a new assembly, and the growth processes is repeated. If a valid assembly can be created then PRIMER uses the joint or discrete element to make a connection.

This "grow assembly", "look for connections", "make new assembly on the other side" process is repeated until no more eligible structure can be found, at which point the mechanism is complete. In this example it can be seen that five assemblies have been created:

Initially selected wheel (red)
Wishbone (green)
Subframe (blue)

During this process PRIMER provides visual feedback by colouring each assembly as it is created, with the rest of the model being made transparent, as shown in the example here. (These are the default settings, to alter them see " Controlling how things are drawn " below.)

You can use Position to try out positioning this mechanism, and if you are happy with it you can Save it in the database.

In an ideal world this will produce a mechanism that models exactly what the model will do during the analysis, and sometimes it does, but in most cases you will need to adjust the "growth" procedure to achieve the result you want. The various controls available to you are described below.

Once you have used Calculate for the first time the Automake panel will become populated with the details of the mechanism you have created. You can adjust the settings here and repeat the Calculate process as many times as you like until you achieve the results you require.

Assemblies and Connections list what has been created.

These buttons are colour coded to match the display in the graphics window, and if you hover the mouse over a button the relevant item will be highlighted and labelled graphically, making it easy to identify items.

To edit an assembly or connection click on its row button, and this will map the relevant editing panel as shown above. Automatically calculated mechanisms are always "scratch" definitions until they are saved, so you can edit items without fear of affecting any other mechanisms saved in your model.

"Locking" assemblies and connections using

Normally each new Calculate operation will delete all assemblies and connections made previously and restart from scratch, meaning that any edits you have made will be lost. If you want to keep any item during this process then "lock" it using this button, and it will be retained. A locked assembly or connection will show a closed padlock coloured in red, in addition if you lock a connection then it also implicitly locks any assemblies to which it is attached, and their padlock symbols will turn blue to reflect this.

Note :

"Locking" an assembly in this context does not restrain it in any way during positioning. To do this edit it by clicking on its row, and select the combination of restraints that you require.
In the same way "Locking" a connection here does not "clamp together" its assemblies in any way. To do this edit the connection and select the "Locked" button on that panel.

Merge... "Merging" assemblies

Sometimes pieces of structure are split up into more assemblies than necessary, and while it would be possible to adjust the attributes of connections to correct this it is easier just to merge the offending assemblies directly. Merge... does this for you in the following way:

You are given a list of all the assemblies in the current mechanism-to-be.
You must select two or more of these
All the selected assemblies will be merged into the first one.

The merge process does the following:

All part sets, parts and node sets in assemblies #2 to #n are added to assembly 1.
All assemblies #2 to #n are deleted.
All connections between a pair of assemblies both in the list #1 to #n are deleted.
All connections referencing an assembly #2 to #n has this connection reassigned to #1.
Assembly #1 is locked to prevent any later passes deleting and/or modifying it.

Assemble from... controls the items used to create assemblies.

Assembly growth will only occur across the item types selected below.

Elem parts . (Solid, shell, beam and thick shell parts)	Will grow via nodal connectivity only for deformable parts, ie growth only occurs across elements connected by common nodes. For rigid parts growth extends to all elements in the part regardless of connectivity.
Nodal RBs	Nodal Rigid Bodies grow across all nodes in the body, which add their node set to the assembly.
RB Extra no	Extra nodes on rigid bodies grow to all such nodes. Their nodes are not added to the assembly since these will be picked up anyway by virtue of the parent part being used.
RB Merges	Rigid body merges (*CONSTRAINED RIGID BODIES) grow to the "other" part, either constrained or lead part, including it in the assembly
More constrained	Maps a sub panel listing other *CONSTRAINED items that can cause connectivity, for example welds. The relevant parts, nodes or elements get added to the assembly.
Tied conts	Tied contacts, which includes spotweld contacts, can be used to track across tied nodes. This is off by default since it can result in "too much growth" unless controlled.

Controlling what is used for assemblies via "Exclusion".

By default all these categories (except tied contacts) are turned on, and hence eligible for assembly growth. However you can turn any category off at two levels:

Turn off the whole category by deselecting its button.
Turn off individual coponents within it by using the [ Exc ] "exclusion" button.

Exclusion allows you to deselect individual parts, constrained items and contacts so that they do not contribute to growth in a Calculate operation, and this can be used to limit what is considered.

Selective exclusion can be changed at any time, and used in combination with "locking" of assemblies you might exclude some items, eg contacts, to grow initial assemblies, then lock these and re-enable growth across the excluded items for subsequent Calculate operations.

Connect via joints

Controlling which joints make connections

The folowing subset of joints (*CONSTRAINED JOINT) types are used to make connections between assemblies

Joint type	Mechanism connection type created.
SPHERICAL	Pin joint at the location of N1
REVOLUTE	Hinge joint from N2 to N4
CYLINDRICAL	Line joint from N2 to N4, no restraint on sliding or rotation
UNIVERSAL	Pin joint at the location of N1
TRANSLATIONAL	Line joint from N2 to N4, sliding unlimited but rotation forbidden.
Other types	Are currently not considered

Exclude... may be used as above to exclude any joint from being eligible to make a connection.

Connect via Element Discrete

Controlling which discrete elements (springs and dampers defined by *ELEMENT DISCRETE) make connections.

"Short" discrete elements, usually springs, are sometimes used to model flexible connections, so this option allows you to make mechanism connections using them.

Discrete element type	Length	Mechanism type created
2 noded springs and dampers	<= Shorter than value	Pin joint at average location of N1 and N2
1 noded springs and dampers	n/a	Fully restrained point on parent assembly at location of N1

Exclude... may be used as above to exclude any element from being eligible to make a connection.

Connect via Element Beam

Controlling which discrete beam elements (*ELEMENT BEAM using section type ELFORM = 6) make connections.

"Short" discrete beam elements using section formulation ELFORM = 6, often of zero length, are also often used to make flexible connections between components in a model.

Beam element type	Length	Mechanism type created
2 noded beams of element formulation (ELFORM) 6	<= Shorter than value	Pin joint at average location of N1 and N2
other beam types	n/a	Are not considered

Exclude... may be used as above to exclude any element from being eligible to make a connection.

Controlling how things are drawn

Generally a mechanism forms only a small part of a larger model, and it can sometimes be hard to see what is happening. To try to relieve this problem the Automake function temporarily changes the visual attributes of the plot, breaking the model down into two categories:

"Model"	Everything in the model that is not in the mechanism By default this is drawn in transparent grey.
"Mechanism"	Those parts of the model in the mechanism. By default this is drawn "normally" (ie opaque) in the colours of the various mechanism assemblies.

You can change these settings at any time, and the graphics will update immediately. Swap all swaps over all model and mechanism settings, and can be useful when trying to see what is not in the mechanism.

When you finally exit from Automake the original model colours and other graphics attributes will be restored automatically.

Other commands

Position performing a test mechanism positioning operation.

Once you have created some assemblies and connections you can use Position at any time to try positioning them. This will launch the mechanism positioner on you current definition.

Warning : Within Automake your mechanism is always a "scratch" definition, so anything you do with it will not affect items stored permanently in the database. However an exception to this is that if you perform a positioning operation and then Accept rather than Reject the results any changes you have made to nodal coordinates and rigid body attributes will become permanent .

Reset All resets all settings in the Automake panel.

If your mechanism has gone horribly wrong, or you simply want to start again with a new one, then Reset all will reset to default the following:

All existing mechanisms and connections are deleted, whether or not they are "locked".
All exclusion lists for assemblies and connections are deleted.

Save stores your mechanism in the database.

If you are happy with your mechanism you can store it permanently in the database with Save . This will also exit from the Automake panel, and restore all model colours and visual attributes.

[X] To exit without saving.

If you want to abandon Automake without saving your results simply use the [X] button at the top right of the panel. Colours and visual attributes will be restored, but no mechanism data will be saved.

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