SimMechanics

Joint Sensor

Measure the motion and force/torque of a joint primitive

Library

Sensors & Actuators

Description

The Joint Sensor block measures the position, velocity, and/or acceleration of a joint primitive in a Joint block.

The Joint Sensor measures the motion along/about the joint axis (or about the pivot point for a spherical primitive) 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 motion.

Depending on the joint primitive being sensed, you measure one of these motion types:

• Translational motion for a prismatic joint primitive, in terms of linear position, velocity, and/or acceleration
• Rotational motion for a revolute joint primitive, in terms of angular position, velocity, and/or acceleration
• Spherical motion for a spherical joint primitive, in terms of a quaternion, quaternion derivative, and/or quaternion second derivative

The input is the connector port connected to the Joint being sensed. The outport is a set of Simulink signals or one bundled Simulink signal of the position, velocity, and/or acceleration of the joint primitive.

A Joint Sensor block measures one joint primitive at a time:

• A primitive Joint (Prismatic or Revolute) has only one primitive within the Joint to sense.
• A composite Joint has multiple joint primitives within, and you must choose which primitive to sense with the Joint Sensor.

A body's orientation rotation matrix R relates vector components measured in the body CS and in the inertial World CS by [R]v_b = v_s. The column vector v_b lists the vector v's three components measured in the body CS. The column vector v_s lists the vector v's three components measured in the World CS.

Dialog Box and Parameters

The dialog box has one active area, Measurements.

Measurements

Connected to primitive

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

If the Joint Sensor is not connected to a Joint block, this menu shows only Unknown.

The Measurements pane you see in the Joint Sensor dialog depends on the type of joint primitive to which you connect the Joint Sensor.

Prismatic joint primitive (P)

The bundled Simulink output signal for a prismatic primitive has these measurements ordered in a row vector. Unselected components are removed from the vector signal:

Position

Velocity

Acceleration

Computed Force

Reaction Torque (3-vector)

Reaction Force (3-vector)

Revolute joint primitive (R)

Note    The absolute angle of revolute motion is mapped on to the interval (-180o, +180o] degrees or (-,+] radians.

The bundled Simulink output signal for a revolute primitive has these measurements ordered in a row vector. Unselected components are removed from the vector signal:

Angle

Angular Velocity

Angular Acceleration

Computed Torque

Reaction Force (3-vector)

Reaction Torque (3-vector)

Spherical joint primitive (S)

The bundled Simulink output signal for a spherical primitive has these quaternion measurements ordered into a larger row vector. Unselected components are removed from the vector signal:

Quaternion (4-vector)

Quaternion, derivative (4-vector)

Quaternion, second derivative (4-vector)

Reaction Torque (3-vector)

Reaction Force (3-vector)

Reactions measured on

Choose the Body on which the reaction force and/or torque vector(s) is (are) measured, Base or Follower. The default is Base.

With respect to coordinate system

In the pull-down menu, choose the coordinate system in which the reaction torque and/or force vector(s) is (are) measured: either the Local (Body CS) to which the Sensor is connected or the default Absolute (World).

In the Absolute case, the force and torque vectors have components measured relative to the inertial World CS axes. In the Local case, the same force and torque signals are premultiplied by the inverse orientation rotation matrix R^-1 = R^T for the Body selected in Reactions measured on.

Output selected parameters as one signal

Select this check box to convert all the output signals into a single bundled signal. The default is selected. If you unselect it, the Joint Sensor block will grow as many Simulink outports as there are active signals selected, in the same order top to bottom, in the dialog box.

If the check box is selected, the Simulink signal out has all the active signals ordered into a single row vector. The order and type of the signal components depend on the joint primitive, as listed in the Simulink signal tables above.

Example

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

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

See Also

Body Sensor, Constraint & Driver Sensor, Joint Actuator, Joint Initial Condition Actuator, Joint Stiction Actuator, Mechanical Branching Bar, Prismatic, Revolute, Spherical

See Representing Body Positions and Orientations and Modeling Sensors.

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

Joint Initial Condition Actuator

Joint Spring & Damper