Seat Mechanism

Mechanism Browser

Overview of the Mechanism Browser.

Access the Mechanism Browser from the menu bar by clicking Tools > Mechanism.

Restriction: The Mechanism Browser is available in the LS-DYNA and Radioss user profiles.

You can undo and redo actions made in the Mechanism Browser using the Undo and Redo commands on the Restore toolbar.

The Mechanism Browser consists of two panes. The first pane displays the mechanism structure which consists of joints, bodies, and constraints. The second pane displays the Entity Editor.

The following data is displayed in the Mechanism Browser:

Column	Description
Entity	Lists the mechanism, joints, bodies, and constraints in your model.
ID	Displays the mechanism, joints, bodies, and constraints IDs.
Type	Displays the joint or constraints type.
Color	Displays the joint and body entity colors. Body color is different from the component color set-up in the Model Browser. The body color is activated when a body is in Review mode.
Lock level	Displays the lock level of a joint (green = free joint, yellow = locked joint).

Entity Editor

The Entity Editor is used to assign, modify and quickly view the attributes defined inside Mechanism Browser entities.

For example, once you create a new body or select a body in the browser, the Entity Editor opens and displays the bodies corresponding attributes, which you can view and modify.

From the Entity Editor, you can also create or edit an entity assigned to the selected body by right-clicking on the entity assignment field and selecting Create or Edit, respectively.

Context Menu

Option	Available for	Description
Actuate Mechanism	Mechanism	Gives the ability to articulate a mechanism by defining one of the following actions: Translate a node Rotate a body Move a node to a target location
Check Mechanism	Mechanism	Enables you to check the validity of a mechanism, looking for common nodes, empty bodies, and redundant joints.
Collapse	All entities	Closes all of the folders for the selected mechanism, so that only the mechanism item displays.
Collapse All	In Mechanism Browser	Closes all of the folders in the browser, so that only the mechanism items display.
Column Visibility	Mechanism and in Mechanism Browser	Enables you to hide or show the ID, Type, Color, Lock, Common nodes, Empty Body and Redundant Joint columns in the browser.
Create	Mechanism and in Mechanism Browser	Create a new mechanism, body, or constraint. You can only create a body or constraint by right-clicking on a mechanism in the browser.
Delete	All entities	Deletes the selected entity. Shortcut: To delete a selected entity in the Mechanism Browser, press Delete.
De-Link Dummy	Mechanism	When a dummy has been linked to a mechanism, this option is activated in order to remove the link between the dummy and the mechanism.
Expand	All entities	Opens the selected folder, exposing every entity nested at every level.
Expand All	In Mechanism Browser	Opens all of the folders in the browser, exposing every entity nested at every level.
Export Position	Mechanism	Opens the Export Position dialog, from which you can export a saved position of the selected mechanism into a solver file containing the corresponding solver transformation cards.
Hide	All entities	Turns off the entity in the graphics area. This selection affects each item’s local display control, that is, it will make the icon become ghosted indicating the display state is off.
Isolate	All entities	Displays only the selected entities, and turns off all other entities of the same type.
Link to Dummy	Mechanism	In case a dummy is present in the session, this option is activated in order to link the dummy, for example, to a seat mechanism.
Lock/Unlock Joint	Joint	Block (Lock) all degrees of freedom of the selected joint or make them free again (Unlock).
Move	Joint	Opens the Entity Editor to define the positioning values of the selected joint and activate the joint manipulator in the display that can be used to interactively move the joint.
Move Mechanism To	Mechanism	Moves the selected mechanism to its initial position or other positions, which have been previously saved.
Review	All entities	Invokes Review mode, which displays selected entities irrespective of their display state, masked, active state (Entity State Browser), but not outside of the spherical clipping (if enabled). By selecting a body, it will be displayed with its body color while other entities are grayed out and displayed with transparency. By selecting a joint or a mechanism, the joint marker with the bodies links between will be displayed.
Reset Review	All entities	Resets the review of the previously selected entity.
Rename	All entities	Rename an entity in the Name field. The new name must be unique. All instances of the renamed entity will update automatically. You can cancel the rename operation by pressing Esc.
Save Position as	Mechanism	Save the displayed mechanism position as a new position or replace an already existing position.
Show	All entities	Displays the selected entity in the graphics area. The entities icon changes to bold indicating that the display state is on. Using the Show option on a mechanism or joint will display all nested entities inside.
XRef Entities	All entities	Opens the References Browser and displays the relationship of the selected entities to other entities in the model in a hierarchical tree structure. Any single entity or multiple entities can be selected.

Supported Entities

A Mechanism (

) is the root of the hierarchy in the Mechanism Browser. To create a Mechanism, right click in the Mechanism Browser and select Create > Mechanism from the context menu. A mechanism is defined by the following entities:

Bodies () define a kinematic assembly made of FE parts or nodes, which can be selected in the Entity Editor. To create a body, right-click on the Mechanism and select Create > Body from the context menu.
Joints () define the kinematic relationship between two bodies (for the following joint types: Ball, Cylinder, Revolute, Slider) or three bodies (for the DoubleSlider joint). To create a joint, select two bodies to connect in the browser, then right-click and select Connect from the context menu. For the DoubleSlider joint, the third body has to be defined in the Entity Editor.
Constraints () define kinematical constraints on a body at a specified node or point location.

Supported Keywords Exported in Solver Deck

The mechanism information is embedded in the input deck using keywords following the /END (for Radioss) or *END (for LS-DYNA).

For Radioss, a mechanism is defined between the /MECHANISM_START and /MECHANISM_END keywords, and is composed of:

/ASSEMBLY defines a body with the following attributes:
- Assembly ID
- Assembly name
- Number of part sets and related set IDs
- Number of parts and related part IDs
- Number of node sets and related set IDs
- Locked degrees of freedom
- Optional local coordinate system
/CONNECTION defines a joint between assemblies with the following attributes:
- Connection name
- Assembly ID #1
- Assembly ID #2
- Connection position (defined by Node IDs or connection position coordinates)
- Joint limits in + and - directions
- Current joint distance/angle value
- Lock level
- Assembly ID #3 (for the Double Slider only)
- Scale factors for relative displacement between Assembly #3 and Assembly #1 and #2
/POSITION defines, for a stored position, the position information for the mechanism’s assemblies with the following attributes:
- Position name
- Assemblies position matrixes
  Note: The Reference and Initial positions should never be deleted inside the mechanism.
/CHILD_DUMMY defines the coupling between a mechanism and a dummy with the following attributes:
- Name
- Master assembly ID of the mechanism
- Dummy ID
- Number of child assemblies of the dummy
- Degrees of freedom linking master to child
- Child assembly IDs of the dummy

The following connection keywords can be defined, depending on the type of joint created in the Mechanism Browser:

/CONNECTION_PIN defines a ball joint
/CONNECTION_HINGE defines a revolute joint
/CONNECTION_LINE defines a cylindrical joint, or a double slider joint if assembly ID #3 is defined or a slider joint has the #HM_SLIDER_JOINT comment defined before the name of the connection

For LS-DYNA, the same keywords are used but their syntax begins with * and not /.

Creating a Mechanism

This task explains how to create a mechanism.

In HyperMesh 2019 there are two ways to create a mechanism: Manual and Auto Generate. In this task, you will create a mechanism using the Auto Generate method.

Using the Manual approach, you create the mechanism manually by defining the bodies and joints.
Using the Auto Generate approach, HyperMesh will automatically create bodies and joints based on a selection of components and elements of the FE model.

In the Mechanism Browser, right-click and select Create > Mechanism > Auto Generate.

When you select Auto Generate, you will see the Mechanism Builder dialog box.

To enable an extraction of the mechanism, each field in the dialog must be defined or selected. Set the options according to the Mechanism Builder Options table.

Mechanism Builder Options Table

Use this table to set options for the Mechanism Builder.

Mechanism Builder Options
Option	Action
Mechanism Type	Select the type of mechanism, either Seat or General. Your selection will affect other options in the dialog box.
Mechanism Name	Type the name you want to give to the mechanism after the extraction is completed.
Entities	Select the FE components that define the mechanism. You can choose Components, Mechanisms, Assemblies, or Sets.
Node Selection on Lower Track	When the Mechanism Type is set to Seat, you can select nodes on the lower track components of the seat.
Node Selection on Upper Track	When the Mechanism Type is set to Seat, you can select nodes on the upper track components of the seat.
Sliding Vector on Upper Track	When the Mechanism Type is set to Seat, you can specify the translation direction of the joint defined for the seat track.
Seat Track with Roller Balls	When the Mechanism Type is set to Seat, you can specify whether the seat track contains roller balls. If you check this option, HyperMesh will automatically create a double slider joint for the seat track, and an additional selector for the seat track roller will appear.
Track Roller Balls	When the Mechanism Type is set to Seat, and Seat Track with Roller Balls is checked, this option allows you to select the roller ball components that will be defined as 3rd body in the double slider joint.
Joint Elements	Select the FE elements (springs or beams) that have to be created as joints in the mechanism extraction process.
Excluded Elements	Select the FE elements that should be excluded from the extraction process of the mechanism. For example, nodal rigid bodies that are used to link components in the FE model but that are not physical conditions, can be selected here and will not be considered in the extraction process.
Excluded Contacts	Select to exclude tied contacts that may be defined on the FE model but don't represent physical connections. This excludes them from the extraction logic.
Detect Holes	Select to activate the hole detection parameters. This allows you to detect joint positions by looking for concentric holes in components.
Relative Search Distance	When Detect Holes is checked, this option defines the distance of search for concentric holes that define a joint location.
Components for Hole Search	When Detect Holes is checked, this option allows you to select the components on which the hole detection must be performed.
Preferred Revolute Joint Axis	Type the vector that defines the revolute joint axis. This will overwrite the direction of the joint elements.
Angular Tolerance	Type the angular tolerance into this field. The joint axis within this angle will be overwritten by the Preferred Revolute Joint Axis value.

Position Mechanisms and Joints

Overview of how to modify the position of mechanism and joints.

Modify the Position of Mechanisms

In the Mechanism Browser, right-click on a mechanism and select Actuate Mechanism from the context menu, then choose from one of the following:
- Choose Translate to open the Entity Editor and select a Node/Constraint, give a Direction of motion, and define the Magnitude of the translation to apply.
  The Magnitude can also be modified, using the up and down arrow buttons. Moreover, you can control the increments of the operations by changing the Increment value.
  
  Figure 5.
- Choose Rotate to open the Entity Editor, select a Body, define a Rotation axis, a rotation center (Base point), and provide the Magnitude of the rotation to apply to the body.
  The Magnitude can also be modified, using the up and down arrow buttons. Moreover, you can control the increments of the operations by changing the Increment value.
  
  Figure 6.
- Choose Target Point to open the Entity Editor, select a Node and define its Target location. Click Move to calculate the position of each body to bring the selected node to the target location or the nearest location, depending on joints limit values.
  This option can be applied on a set of nodes and targets, by setting Multiple pairs to Yes.
  
  Figure 7.
Click Save to save the achieved position.
A name will be appended to the New position name field.

Modify the Position of Joints

In the Mechanism Browser, right-click on a joint and select Move from the context menu.
Modify the position of the joint.
- In the graphics area, click-and-drag the manipulator.
  
  Figure 8.
- In the Entity Editor, modify the Current angle of the joint.
  
  Figure 9.
Change the Increment value to control the increments of the operation.

Link a Dummy to a Mechanism

In a vehicle FE model, quickly and efficiently set kinematic systems such as seats, steering wheels and pedals in different positions using the Mechanism Browser.

A dummy can then be positioned on the seat and may have its hands placed on the steering wheel and feet on the pedals. To couple a dummy to a seat mechanism the degree of freedom of one or more bodies of the dummy (“child” bodies) must be linked to a body of the seat mechanism (“master” body). The motion of the master body will drive the motion of the child bodies; however, the motion of the master body may be limited by the angle limits of the dummy joints.

In the Mechanism Browser, right-click on the seat mechanism and select Link to Dummy from the context menu.
In the Dummies dialog, select a dummy to link to the seat mechanism and click Next.
In the Mechanism bodies (Master body) dialog, select the master body from the seat and click Next.

Note: For this use case, it is common to select a seat cushion.
In the Dummy bodies (child bodies) dialog, select the dummy slave bodies and the linked degrees of freedom and click Close.

Note: For this use case, it is common to link to the lower torso of the dummy in degrees of freedom Tx, Ty, and Tz (selected per default).
To fix some of the bodies of the dummy, for example, hands and feet, define constraints on those bodies by selecting Create > Constraint > Point Node or Point Location from the context menu.
Position the seat mechanism.
The position of the dummies limbs will be automatically updated.