# Reference: 2DCluster

Model ElementA Reference_2DCluster specifies that a given set of bodies be constrained to move in one of the three principal planes: XY, YZ, or ZX.

## Description

Such bodies have only three degrees of freedom: Two corresponding to translations in the plane, and one corresponding to rotation about an axis normal to the plane. The bodies can be of type Body_Rigid or Body_Point. Common uses for planar bodies include belt and chain drives and general planar mechanisms.

## Format

<Reference_2DCluster
id                  = "integer"
planar_type         = { "XY" | "YX" | "YZ" | "ZY" | "ZX" | "XZ" }
num_body            = "integer">
integer integer integer integer

integer integer integer integer
</Reference_2DCluster>

## Attributes

id
Element identification number (integer>0). This number is unique among all Reference_2DCluster elements.
planar_type
Select one of the three global planes XY, YZ, or ZX. All members of the 2D cluster are constrained to move in this plane.
num_body
The number of bodies in the 2D cluster. This argument is followed by the list of IDs of all the bodies.

## Example

Consider an air-hockey table. Assume that you interested in modeling the motion of the pucks as they slide on the table on an air-cushion and collide with each other and the table edges. Do not consider the case where they bounce off the table. In such a situation, it is advantageous to model the pucks as planar bodies.

Defining a body to be planar is a two step process:
1. Define a Reference_2DCluster element as shown below. Essentially, you are specifying that the pucks, whose IDs are 30101 30301 30401, belong to a 2D cluster with ID 1 whose three members are constrained to move in the YZ plane.
<Reference_2DCluster
id                  = "1"
planar_type         = "YZ"
num_body            = "3">
30101 30301 30401
</Reference_2DCluster>
2. Refer to the Reference_2DCluster element defined above in a Subsystem_Planar element as shown below:
<Subsystem_Planar
num_2d_cluster      = "1">
1
</Subsystem_Planar> 

This element specifies one 2D cluster with an ID of 1 to be activated. If you would like to see how the model behaves if the bodies are not constrained to move in the plane, simply delete or comment out the Subsystem_Planar element.