Prismatic Constraint
 
 
 
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Create panel (Helpers) reactor Prismatic

Animation menu reactor Create Object Prismatic Constraint

reactor toolbar (Create Prismatic Constraint)

The Prismatic constraint serves as a constraint between two rigid bodies, or a rigid body and the world, that allows its bodies to move relative to each other along one axis only. Rotations, as well as the remaining two translation axes, are fixed. For example, you could use a Prismatic constraint when creating a forklift truck.

A Prismatic constraint is simulated if it has the correct number of rigid bodies attached and is included in a valid Constraint Solver. When not selected, an invalid prismatic constraint is red in the viewport.

Procedures

To create a Prismatic constraint:

To attach objects to the prismatic constraint:

  1. Create the Prismatic constraint and the objects to connect using it.
  2. On the constraint’s Properties rollout, click the Child pick button and then, in one of the viewports, select the object to use as the child.

    This attaches the constraint to the body at the body’s pivot point. The constraint's sliding axis is aligned to the Z axis of the object.

  3. If you do not want to assign a second body to the prismatic constraint, turn off the Parent check box, if necessary.
  4. If the constraint is to be two-bodied, make sure Parent is on and then use the Parent pick button to set the parent for the constraint.

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

To change the Prismatic constraint positions and rotations for the bodies:

  1. With the constraint selected, open the Modify panel.
  2. In the modifier stack, open the constraint’s sub-object list.
  3. Access the sub-object level corresponding to the object whose attachment point you want to move: Child Space or Parent Space. If the constraint is single-bodied, Parent Space represents the world attachment position and rotation of the constraint axes.
  4. The space, represented by a pair of axes, the larger of which represents the sliding axis, becomes active in the viewport and can be moved or rotated using the Move or Rotate tool, respectively. The space maintains its position and rotation relative to its corresponding object. For example, if you move or rotate the child body, Child Space moves with it.
  5. To realign the constraint space to the child body, reattach the object to the Prismatic constraint, or choose the Align To Child Body option.

To limit the movement of the child body along the sliding axis:

  1. With the constraint selected, open the Properties rollout.
  2. Turn on the Limited check box.

    Both Limit parameters (Min and Max) are set by default to 0.0. This means that from a situation where the sub-objects are fully aligned for the objects, the child can move 0 units in either direction along the shared sliding axis relative to the parent.

    The limits are displayed as a line in Parent Space.

  3. Change the Min Limit and Max Limit settings as necessary.

Interface

Modifier Stack

Parent Space

At this sub-object level, you can move and rotate the representation of the Prismatic constraint for the parent body. The constraint axes are defined in the parent body's space, which means that if you alter the position or rotation of the parent, the space moves as well and maintains its offset rotation and translation with respect to the parent. When you assign a parent body to the constraint, Parent Space is also aligned with the child body's local space, with the constraint axis going through the child body's pivot point. If the constraint is single-bodied, then Parent Space represents the world orientation and position of the constraint.

Child Space

At this sub-object level, you can move and rotate the representation of the Prismatic constraint for the child body. The constraint axes are defined in the child body's space, which means that if you alter the position or rotation of the child, the space moves as well and maintains its offset rotation and translation with respect to the child. When you assign a child body to the constraint, the Child Space axis is positioned so that it goes through the child body pivot point, and the axes are aligned with the child body's local space: The prismatic sliding axis is aligned with the child's Z axis.

Properties rollout

Parent

When on, the constraint has two bodies and you can designate an object to be the parent body. When off, the constraint 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 constraint. 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 Working With Constraint Spaces.

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 and rotates along with it, and vice-versa.

Strength group

Strength/Tau

Govern the impulses applied to the constraint's bodies in order to maintain the constraint, and so how strongly the constraint works to restrict their movement. You can find out more about these parameters in Strength and Tau.

Limited group

Limited

When on, limits the movement of the child object along the sliding axis; the child object can move relative to the parent only within the range specified by the Min Limit and Max Limit values. You can also specify a Friction value for a limited prismatic constraint.

Min Limit

The minimum distance the constraint allows between the two spaces. For example, if Min Limit is –5.0 then the child body then the child object cannot move more than 5 units backwards relative to the parent along the sliding axis.

Max Limit

The maximum distance the constraint allows between the two spaces. For example, if Max Limit is 5, then the child object will not be able to move more than 5 units forward relative to the parent along the sliding axis.

Friction

The level of friction applied to the objects as they try to move along the sliding axis. It is applied only to limited Prismatic constraints.

Breakable group

Breakable

When on, the constraint is breakable. If its breakable limits are exceeded during simulation, it ceases to exert impulses on the attached objects. You can find out more about breakable constraints in Breakable Constraints.

Display group

Size

Lets you change the size of the constraint space display in the viewport.

Reset Default Values

Returns the constraint's parameters to their default values.