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.
To create a Linear Dashpot:
- Choose one of the above options, and
then click in any viewport to add the dashpot.
NoteThe icon’s position
has no effect on the constraint’s behavior.
To attach objects to the Linear Dashpot:
- Create the Linear Dashpot and the objects
to connect using it.
- 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.
- If you do not want to assign a second
body to the dashpot, turn off the Parent check box, if necessary.
- If the dashpot is two-bodied, repeat
step 2, using the Parent pick button to set Parent for the Linear
To create the constraint and attach objects
in a single step:
To change the attachment positions for
- With the dashpot selected, open the Modify panel.
- In the modifier stack, open the dashpot’s
- 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.
- 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.
- 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:
- Ensure that the constraint is selected
in one of the viewports.
- 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.
- 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.
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.
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
- 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.
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.
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.
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.