Weld Joint

Joint with zero primitives

• 

Libraries:
Simscape / Multibody / Joints

Description

This block represents a joint with zero degrees of freedom. It contains no joint primitives. Base and follower frames, each connected to a separate rigid body, are coincident for all time. The block dialog box provides sensing options for constraint and total forces and torques.

Joint Degrees of Freedom

Faults

Using mode faults, you can change the joint modes during a simulation without modifying the model design. The fault injection overrides the mode setting. For example, if a joint has the Mode parameter set to Locked and the Fault behavior parameter set to Disengaged, the joint becomes disengaged.

To add a mode fault to a joint block, click on the joint block, in the Simscape Block tab, and the Faults section, click Fault > Add Fault. Alternatively, you can click the joint block, hover over the ellipsis to open the action bar, and click the Add a fault on the block icon . You can add multiple faults to a joint block, but the joint block can have only one active fault during a simulation.

As you add faults, in the Property Inspector, under the Fault section, specify the behavior and the trigger type of the fault. To define the fault behavior, click the link next to the Fault Behavior. This joint supports Normal or Disengaged mode. The joint blocks support these trigger types: Always on, Timed, Manual, and Conditional. For more details of these trigger types, see Set Fault Triggers. To trigger a conditional fault, you can use Simulink signals, Simscape language blocks, and MATLAB workspace variables. To set the active fault for a block, use the Fault Table. For more details, see Access the Fault Table and Fault Dashboard.

To enable fault simulation, in the Simscape Block tab and the Faults section, turn on the Fault Simulation button. The fault simulation is on when the button is green and the status is on. The simulation logs the trigger status data. To view the data, use the Simulation Data Inspector. Also, you can see the fault status and a summary of the triggered faults in the Fault Dashboard. To open the Fault Dashboard, in the Simscape Block tab, click Faults > Fault Dashboard.

To create and modify faults, you can also use Simscape™ and Simulink^® fault functions. For more details, see the function section of the Simulink Fault Controls and Simscape Faults Interface.

Ports

Frame

B — Base frame

frame

Base frame of the joint block.

F — Follower frame

frame

Follower frame of the joint block.

Input

Mode Configuration

mode — Joint mode control

physical signal

Input port that controls the mode of the joint. The signal must be a unitless scalar. The joint mode is normal when the input signal is 0 and disengaged when the input signal is -1. You can change the mode at any time during the simulation.

The table shows how the position and velocity of the joint change during transitions between modes.

Transitions	Position	Velocity
Normal to Disengaged	The joint position remains at zero but can change in any direction after the transition.	The joint velocity remains at zero but can change in any direction after the transition.
Disengaged to Normal	The joint positions become zero and remain constant after the transition.	The joint velocities become zero and remain constant after the transition.

Dependencies

To enable this port, under Mode Configuration, set Mode to Provided by Input.

Output

Composite Force/Torque Sensing

fc — Constraint force, N

physical signal

Physical signal port that outputs the constraint forces that act across the joint. These forces maintain the translational constraints of the joint. The output has a 3-by-1 vector format and represents the force components along the x, y, and z axes of the resolution frame. For more information, see Force and Torque Sensing.

Dependencies

To enable this port, under Composite Force/Torque Sensing, select Constraint Force.

tc — Constraint torque, N*m

physical signal

Physical signal port that outputs the constraint torques that act across the joint. These torques maintain the rotational constraints of the joint. The output has a 3-by-1 vector format and represents the torque components about the x, y, and z axes of the resolution frame. For more information, see Force and Torque Sensing.

Dependencies

To enable this port, under Composite Force/Torque Sensing, select Constraint Torque.

ft — Total force, N

physical signal

Physical signal port that outputs the total force that acts across the joint. The total force is the sum of the of all forces transmitted between the connected frames through the joint. The output has a 3-by-1 vector format and represents the force components along the x, y, and z axes of the resolution frame.

To demonstrate the total force acting on a joint, the figure shows a model using a Prismatic Joint block.

The total force includes the actuator force (F_A), internal force (F_I), and constraint forces (F_C). For more information, see Force and Torque Sensing.

Dependencies

To enable this port, under Composite Force/Torque Sensing, select Total Force.

tt — Total torque, N*m

physical signal

Physical signal port that outputs the total torque that acts across the joint. The total torque is the sum of all torques transmitted between the connected frames through the joint. The torque includes the actuation, internal, limit, and constraint torques. The output has a 3-by-1 vector format and represents the torque components about the x, y, and z axes of the resolution frame. For more information, see Force and Torque Sensing.

Dependencies

To enable this port, under Composite Force/Torque Sensing, select Total Torque.

Parameters

To edit block parameters interactively, use the Property Inspector. From the Simulink Toolstrip, on the Simulation tab, in the Prepare gallery, select Property Inspector.

Mode Configuration

Mode — Joint mode

Normal (default) | Disengaged | Provided by Input

Joint mode for the simulation, specified as one of these values:

Mode	Description
Normal	Normal mode enables the joint constraints work as intended during the simulation.
Disengaged	Disengaged mode releases the joint from all constraints throughout the simulation. The settings for Internal Mechanics, Limits, and Actuation parameters do not affect the disengaged joint. All output ports output zero.
Provided by Input	The Provided by Input option allows you to specify the joint mode by using an input signal. For more information, see the port mode in the Input section.

Composite Force/Torque Sensing

Direction — Measurement direction

Follower on Base (default) | Base on Follower

Measurement direction, specified as one of these values:

• Follower on Base — The block senses the force and torque that the follower frame exerts on the base frame.

• Base on Follower — The block senses the force and torque that the base frame exerts on the follower frame.

This parameter affects only the output signals under the Composite Force/Torque Sensing section. Reversing the direction changes the sign of the measurements. For more information, see Force and Torque Measurement Direction.

Resolution Frame — Frame used to resolve measurements

Base (default) | Follower

Frame used to resolve the measurements, specified as one of these values:

• Base — The block resolves the measurements in the coordinates of the base frame.

• Follower — The block resolves the measurements in the coordinates of the follower frame.

This parameter affects only the output signals under the Composite Force/Torque Sensing section.

Extended Capabilities

C/C++ Code Generation
Generate C and C++ code using Simulink® Coder™.

Version History

Introduced in R2012a

R2025a: Model mode faults

You can now model mode faults for all the joint blocks. Using the faults, you can change the joint modes during a simulation without changing the model design. For more information, see the Faults section in the description.