Linear Dashpot
 
 
 
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Create panel (Helpers) reactor L Dashpot

Animation menu reactor Create Object Linear Dashpot

reactor toolbar (Create Linear Dashpot)

Dashpots typically server to cushion impact. An example of a real-world linear dashpot is the hydraulic cylinder in an automobile shock absorber. The Linear Dashpot constraint lets you constrain two rigid bodies together in the simulation, or to constrain one body to a position in world space. It behaves like a heavily damped spring with zero rest length. You can specify the strength and damping, and whether collisions between the attached bodies are disabled.

reactor lets you specify a dashpot attachment point in each body's local space. During the simulation, the dashpot exerts impulses on the attached bodies in an attempt to make these points match up in world space, thus keeping the bodies in the same positions relative to each other. The bodies are still free to rotate around the attachment point.

You do not need to add a Linear Dashpot explicitly to a simulation, as all valid constraints in a scene are added to the simulation by default. A two-bodied dashpot is valid if it has two rigid bodies attached, while a single-bodied dashpot is valid if it has one rigid body attached. When not selected, invalid dashpots are colored red in the viewport.

Procedures

To create a Linear Dashpot:

To attach objects to the Linear Dashpot:

  1. Create the Linear Dashpot and the objects to connect using it.
  2. On the linear dashpot’s Properties rollout, click the Child pick button, and then select the object to use as the child object in one of the viewports. By default, this attaches the linear dashpot to the body at the body’s pivot point.
  3. If you do not want to assign a second body to the dashpot, turn off the Parent check box, if necessary.
  4. If the dashpot is two-bodied, repeat step 2, using the Parent pick button to set Parent for the Linear Dashpot.

To create the constraint and attach objects in a single step:

To change the attachment positions for the bodies:

  1. With the dashpot selected, open the Modify panel.
  2. In the modifier stack, open the dashpot’s sub-object list.
  3. Highlight the sub-object corresponding to the object in whose space you want to move the attachment point: Parent Space or Child Space. If the dashpot is single-bodied, Parent Space allows you to move the world attachment position.
  4. The appropriate point becomes active in the viewport and can be moved using the Move tool. The attachment point maintains its position relative to its corresponding object; for example, if you move the child object, its space, and hence its attachment point, moves with it. The attachment point doesn't have to actually be on either object.
  5. To reset the attachment point to the child object’s pivot, reattach the objects to the dashpot, or use the Align Spaces To Child Body option.

To align the parent and child constraint spaces:

  1. Ensure that the constraint is selected in one of the viewports.
  2. In the Align Spaces To group, click the button for your chosen alignment. You can find out more about the alignment options in the Working With Constraint Spaces section.

Interface

Modifier Stack

Child Space

At this sub-object level, you can select and move the dashpot attachment point for the child body. When you assign a child body to the dashpot, the Child Space attachment point is set to the child object’s pivot point. If you alter the position of the Child Space attachment point, it maintains its new position relative to the child during the simulation.

Parent Space

At this sub object level, you can select and move the dashpot attachment point for the parent body. When you assign a parent object to the dashpot, the Parent Space attachment point is set to the parent object’s pivot point. If you alter the position of the Parent Space attachment point, it maintains its new position relative to the parent during the simulation. If the dashpot is single-bodied, then the Parent Space's attachment point represents the point in the world where the dashpot will be attached.

Properties rollout

Parent

When on, the dashpot has two bodies and you can designate an object to be the parent body. When off, the dashpot is single-bodied and the point occupied by the Parent Space sub-object is the parent.

You assign the parent object by clicking this button and then selecting a rigid body from any viewport. Thereafter the button displays the name of the parent body.

Child

Displays the name of the second rigid body attached to the dashpot. You assign the child object by clicking this button and then selecting a rigid body from any viewport.

Align Spaces To

Use these options to align the bodies' local constraint spaces. You can find out more about each option in the Working With Constraint Spaces section.

Lock Relative Transform

When on, the relative transform between the child and parent constraint spaces is locked: If you move either space in the viewport, the other space moves along with it.

Strength

Governs the size of the impulse the dashpot applies to each attached body, taking into account the distance between the attachment points. The strength value is mass-dependent. For example, a Strength value of 10 generates a different behavior when connecting two 5 kg bodies than when connecting two 50 kg bodies. Values greater than or equal to 0 are valid for this property. Default=1.

Damping

Determines how quickly the oscillation of the linear dashpot settles down. Damping governs the impulse applied to the connected bodies due to the relative velocities of their dashpot connection points. Valid damping values are those greater than or equal to 0 and less than or equal to 100000. Default=0.1.

TipTypically, a damping value of 1/10th of the stiffness yields good results.
Allow Interpenetration

When on, collisions are disabled between the dashpot’s objects, so that they can pass through each other during the simulation. Defaults=off.

Disabled

When on, the dashpot is not added to the simulation.

Reset Default Values

Click this to set Strength, Damping, and Disabled to their respective default values.