# 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.

## Comments

1. You may refer to more than one 2D cluster in one Subsystem_Planar element.
2. If you have different collections of bodies constrained to different planes, use one 2-D cluster for each collection.
3. In general, the 2D cluster element improves simulation speed because it eliminates out-of-plane translational and in-plane rotational states of the bodies. The exact speed increase depends on the details of the model.
4. To see how much the planar bodies improve simulation speed, simply comment out the Subsystem_Planar element. You may leave the Reference_2DCluster element in the MotionSolve XML input file.