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Joint Actuator

Time-dependent force, torque, or motion input to a joint

Library

Sensors & Actuators

Description

A joint between two bodies represents relative degrees of freedom (DoFs) between the bodies. The Joint Actuator block actuates a Joint block connected between two Bodies with one of these signals:

  • A generalized force:

    • Force for translational motion along a prismatic joint primitive

    • Torque for rotational motion about a revolute joint primitive

  • A motion:

    • Translational motion for a prismatic joint primitive, in terms of linear position, velocity, and acceleration.

    • Rotational motion for a revolute joint primitive, in terms of angular position, velocity, and acceleration.

The generalized force or the motion is a function of time specified by a Simulink® input signal, which can include a signal feedback from a Sensor block.

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

The inport is the Simulink input signal. The output is the connector port you connect to the Joint block you want to actuate. A Joint Actuator block actuates one joint primitive at a time:

  • A primitive Joint (Prismatic or Revolute) has only one primitive within the Joint to actuate.

  • A composite Joint has multiple joint primitives within, and you must choose which of those primitives to actuate with the Joint Actuator.

You cannot connect a Joint Actuator to a Spherical or spherical primitive.

    Caution   You cannot simultaneously actuate a joint primitive with Joint Actuator motion actuation and with a Joint Initial Condition Actuator.

Joint Motion Actuation Requires Acceleration, Velocity, and Position Input Signals

  • If the motion is translational, the block requires three input signals, corresponding to position, velocity, and acceleration.

    The velocity signal must be the derivative of the position signal, and the acceleration the derivative of the velocity.

  • If the motion is rotational, the block requires three input signals, corresponding to angle, angular velocity, and angular acceleration.

    The angular velocity signal must be the derivative of the angle signal, and the angular acceleration the derivative of the angular velocity.

Dialog Box and Parameters

The dialog has one active area, Actuation. The full block parameters are not displayed unless you connect it to a specific Joint block.

Actuation

Connected to primitive

In the pull-down menu, choose the joint primitive within the Joint that you want to actuate with the Joint Actuator. A primitive Joint block has only one joint primitive.

You cannot connect a Joint Actuator to a spherical primitive.

If the Joint Actuator is not connected to a Joint block, this menu is blank.

Actuate with

In the pull-down menu, choose one of two types of actuation, Generalized Forces or Motion.

Generalized Forces

This option interprets the actuation signal as a force or a torque between the Bodies connected by the Joint block you are actuating. Choose units depending on whether you are actuating a prismatic or revolute primitive.

Applied force units

In the pull-down menu, choose units for the actuation force. The default is N (newtons).

The Simulink input is a 1-component signal.

Applied torque units

In the pull-down menu, choose units for the actuation torque. The default is N*m (newton-meters).

The Simulink input is a 1-component signal.

Motion

This option interprets the actuation signal as a motion of the joint primitive to which you connect the Joint Actuator. Choose units depending on whether you are actuating a prismatic or revolute primitive. The Simulink input is a bundled 3-component signal with components in the order shown in the dialog.

Position units

In the pull-down menu, choose units for the linear position motion actuation. The default is m (meters).

Velocity units

In the pull-down menu, choose units for the linear velocity motion actuation. The default is m/s (meters/second).

Acceleration units

In the pull-down menu, choose units for the linear acceleration motion actuation. The default is m/s2 (meters/second2).

Angular units

In the pull-down menu, choose units for the angular motion actuation. The default is deg (degrees).

Angular velocity units

In the pull-down menu, choose units for the angular velocity motion actuation . The default is deg/s (degrees/second).

Angular acceleration units

In the pull-down menu, choose units for the angular acceleration motion actuation. The default is deg/s2 (degrees/second2).

Example

Here is a Joint Actuator connected to a Prismatic that connects two Bodies:

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

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