SimMechanics™

Bushing

Represent composite joint with three translational and three rotational DoFs

Library

Joints

Description

The Bushing block represents a composite joint with three translational degrees of freedom (DoFs) as three prismatic primitives and three rotational DoFs as three revolute primitives. There are no constraints among the primitives. Unlike Six-DoF, Bushing represents the rotational DoFs as three revolutes, rather than as one spherical.

Warning A joint with three revolute primitives becomes singular if two or three of the rotation axes become parallel ("gimbal lock"). A joint with two or three prismatic primitives becomes singular if two or three of the translation axes become parallel. The simulation stops with errors in these cases.

A joint with three revolute primitives must be configured in the initial state with the three revolute primitive axes mutually orthogonal. There are no restrictions on the primitive axes once the simulation starts, except to prevent any two of the primitive axes from becoming parallel.

You must connect each side of the Joint block to a Body block at a Body coordinate system (CS) point. The Bushing block is assembled: the origins of these Body CSs must lie along the primitive axes, and the Body CS origins on either side of the Joint must be spatially collocated points, to within assembly tolerances.

You must connect any Joint block to two and only two Body blocks, and Joints have a default of two connector ports for connecting to base and follower Bodies.

A Joint block represents only the abstract relative motion of two bodies, not the bodies themselves. You must specify reference CSs to define the directions of the joint axes.

Dialog Box and Parameters

The dialog has two active areas, Connection parameters and Parameters.

Connection Parameters

The base (B)-follower (F) Body sequence determines the sense of positive motion. Positive translation is the follower moving in the direction of the translation axis. Positive rotation is the follower moving around the rotational axis following the right-hand rule.

Current base

When you connect the base (B) connector port on the Bushing block to a Body CS Port on a Body, this parameter is automatically reset to the name of this Body CS. See the following figure, Bushing Base and Follower Body Connector Ports.

The base Body is automatically connected to the first joint primitive P1 in the primitive list in Parameters.

Current follower

When you connect the follower (F) connector port on the Bushing block to a Body CS Port on a Body, this parameter is automatically reset to the name of this Body CS. See the following figure, Bushing Base and Follower Body Connector Ports.

The follower Body is automatically connected to the last joint primitive R3 in the primitive list in Parameters.

Number of sensor/actuator ports

Using this spinner menu, you can set the number of extra connector ports needed for connecting Joint Actuator and Joint Sensor blocks to this Joint. The default is 0.

The motions of prismatic and revolute primitives are specified in linear and angular units, respectively.

Bushing Base and Follower Body Connector Ports

Parameters

Toggle between the Axes and Advanced panels with the tabs.

The entries on the Axes pane are required. Each DoF primitive in Bushing has an entry line. These lines specify the direction of the axes of action of the DoFs that the Bushing represents.

Name - Primitive

The primitive list states the names and types of joint primitives that make up the Bushing block: prismatic primitives P1, P2, P3, and revolute primitives R1, R2, R3.

Axis of Action [x y z]

Enter here as a three-component vector the directional axes defining the allowed motions of these primitives and their corresponding DoFs:

Prismatic: axis of translation
Revolute: axis of rotation

The default vectors are shown in the dialog above. The axis is a directed vector whose overall sign matters.

To prevent singularities and simulation errors, no two of the revolute axes and no two of the prismatic axes can be parallel.

Reference CS

Using the pull-down menu, choose the coordinate system (World, the base Body CS, or the follower Body CS) whose coordinate axes the vector axis of action is oriented with respect to. This CS also determines the absolute meaning of forces/torques and motion along/about the joint axis. The default is World.

The Advanced pane is optional. You use it to control the way SimMechanics™ simulation interprets the topology of your schematic diagram.

Mark as the preferred cut joint

In a closed loop, the simulation internally and automatically cuts one and only one joint.

If you want this particular joint to be weighted preferentially for cutting during the simulation, select the check box. The default is not selected.