# Subsystem: Planar

Model ElementA Subsystem_Planar element aggregates a collection of Reference_2DCluster elements.

## Description

A Reference_2DCluster specifies that a given set of bodies are constrained to move in one of the three principal planes: XY, YZ, or ZX. Such bodies have only three degrees of freedom: Two corresponding to translations in the plane, and one corresponding to rotation along a vector 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

<Subsystem_Planar
num_2d_cluster = "integer" >
integer integer integer integer integer
...     ...     ...     ...     ...
...     ...     ...     ...     ...
integer integer integer integer
</Subsystem_Planar>

## Attributes

num_2d_cluster
The number of Reference_2DCluster elements to be activated by the Subsystem_Planar element. This parameter is followed by the list of IDs of Reference_2DCluster elements.

## Example

Consider an air-hockey table. Assume that you want to model the motion of the pucks as they glide 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 the 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 the ID 1 to be activated. If you want 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.