Joint Initial Condition Actuator (SimMechanics)

Apply initial positions and velocities to the primitives of a Joint before starting simulation

Library

Sensors & Actuators

Description

The Joint Initial Condition Actuator block supplies the prismatic and revolute joint primitives of a Joint block with initial value data. The initial values are the positions and velocities of the joint primitives and fully specify the initial state of motion (initial kinematic state) of those primitives.

When you build your machine, the geometric configuration implicitly specifies the initial positions/angles of bodies relative to one another and to World. The Ground, Body, and Joint layout only specifies initial coordinates (degrees of freedom), not their corresponding velocities, however. Starting a simulation in this state implicitly sets all initial velocities to zero. You can set the full initial kinematic state, both positions and velocities, of joint primitives by using Joint Initial Condition Actuator blocks.

You can set initial positions and velocities for two primitive types:

Translational initial conditions for a prismatic primitive, in terms of linear position and velocity
Rotational initial conditions for a revolute primitive, in terms of angular position and velocity

This block can actuate one, some, or all of the prismatic and revolute primitives of a Joint.

The Joint Initial Condition Actuator applies the initial state along/about the joint axis in the reference coordinate system (CS) specified for that joint primitive in the Joint's dialog box. The Joint connects a base and a follower Body. The base-follower sequence determines the sense of the actuation signal.

The output is the connector port you connect to the Joint block whose initial conditions you want to set. You set the initial linear and/or angular positions and velocities in the block's dialog box, so there is no input signal.

You cannot actuate a Spherical or spherical primitive with a Joint Initial Condition Actuator.

Dialog Box and Parameters

The dialog box has one active area, Actuation.

Actuation

The menu choices are available for every primitive in the Joint to which the Joint Initial Condition Actuator is connected. If you connect the Actuator with its dialog open, the primitive list is automatically updated to reflect the connected Joint's primitives.

Enable: Select this check box if you want to actuate the primitive with initial conditions. The default is unselected.
Primitive: Displays the name of the primitive within the Joint. Not an active field.
Position: Enter a value for the initial position of the primitive, either prismatic or revolute. The default is 0.
Units: In the pull-down menu, select units for the initial position. The defaults are m (meters) for prismatic primitives and rad (radians) for revolute primitives.
Velocity: Enter a value for the initial velocity of the primitive, either prismatic or revolute. The default is 0.
Units: In the pull-down menu, select units for the initial velocity. The defaults are m/s (meters/second) for prismatic primitives and rad/s (radians/second) for revolute primitives.

You cannot connect a Joint Initial Condition Actuator to a spherical primitive. If the connected Joint has a spherical primitive, the entry for that primitive is grayed out:

Example

Here is a Joint Initial Condition Actuator connected to a Custom Joint, which connects two Bodies:

You must add an Actuator port (connector port) to the Joint block to connect the Joint Initial Condition Actuator to it. The base (B)-follower (F) Body sequence on the two sides of the Joint determines the sense of the Joint Initial Condition Actuator data.

See Using JICA Blocks for setting general initial conditions (positions and velocities) of DoFs in a machine.

See Creating Joints.

In Simulink, see the Signal Routing Library and the Sources Library.

Joint Actuator Joint Sensor

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