# Universal

Joint with two revolute joint primitives

Joints

## Description

The Universal block represents a composite joint with two rotational degrees of freedom (DoFs) as two revolute primitives. There are no constraints among the primitives.

 Warning   A joint with two revolute primitives becomes singular if the two rotation axes become parallel ("gimbal lock"). The simulation stops with an error in this case.

### Satisfying Joint Requirements

A Joint block represents the relative degrees of freedom between two bodies, not the bodies themselves.

You must connect any Joint block to two and only two Body blocks, the base and the follower. All Joints have two connector ports for these connections, defining the direction of joint motion (base to follower). You connect each side of the Joint block to these Body blocks at a Body coordinate system (CS) port.

You specify the joint primitive axes, if any, in the Joint dialog.

#### Assembly Restrictions on Assembled Joints

This Joint block is assembled and places restrictions on the connected Body CSs.

• If the Joint has no prismatic primitives, the origins of the connected Body CSs on either side of the Joint must be spatially collocated points, to within assembly tolerances.

• If the Joint has one or more prismatic primitives, the origins of the connected Body CSs must lie in the span of the prismatic axes:

Number of Prismatic PrimitivesSpan of Primitive Axes
OneAlong the primitive axis
TwoIn the plane of the primitive axes
ThreeAnywhere in three-dimensional space

## 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 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 Universal 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, Universal Base and Follower Body Connector Ports.

The base Body is automatically connected to the first joint primitive `R1` in the primitive list in Parameters.

Current follower

When you connect the follower (F) connector port on the Universal 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, Universal Base and Follower Body Connector Ports.

The follower Body is automatically connected to the last joint primitive `R2` 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 motion of revolute primitives is specified in angular units.

Universal Base and Follower Body Connector Ports

## Parameters

Switch between the Axes and Advanced tabs.

### Axes Tab

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

Name - Primitive

The primitive list states the names and types of joint primitives that make up the Universal block: revolute primitives `R1`, `R2`.

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:

• 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, the two revolute axes cannot 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 tab 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.