Universal Joint
Joint that allows for transfer of rotary motion between unaligned shafts
Libraries:
Simscape /
Multibody /
Joints
Description
The Universal Joint block models a coupling between two spinning shafts. The two shafts align with the z-axes of the base and follower frames and rotate at the same average angular velocity. However, the relative velocities of the shafts fluctuate if the shafts are not aligned. The fluctuation increases as the bend angle increases. This joint constrains the origins of the two frames to be coincident.
The joint has two revolute primitives that represent two sequential rotations. The rotations specify the orientation of the follower frame with respect to the base frame. The first rotation is about the x-axis of the follower frame, and the second rotation is about the y-axis of the follower frame generated after the first rotation, as shown in the image.
To specify the target of the initial state for a joint primitive, use the parameters under State Targets. The targets are specified in the base frame. You can also set the priority levels for the targets. If the joint is not able to satisfy all the state targets, the priority level determines which targets to satisfy first and how closely to satisfy them. For an example, see the Guiding Assembly section of How Multibody Assembly Works.
To model damping and the spring behavior for a joint primitive, use the parameters under Internal Mechanics. Use the Damping Coefficient parameter to model energy dissipation and the Spring Stiffness parameter to model energy storage. Joint springs attempt to displace the joint primitive from its equilibrium position, and joint dampers act as energy dissipation elements. The springs and dampers are strictly linear.
To specify the limits of a joint primitive, use the parameters under Limits. The lower and upper bounds define the width of the free region. The block applies a force to accelerate the joint position back to the free region when the position exceeds the bounds. The block uses a smoothed spring-damper method to compute the force. For more information about the smoothed spring-damper method, see the Description section of the Spatial Contact Force block.
The Force, Torque, and Motion parameters in the Actuation section control the motion of the joint primitives during simulation. For more information, see Specifying Joint Actuation Inputs. Additionally, the joint block has ports that output sensing data, such as position, velocity, acceleration, force, and torque, that you can use to perform analytical tasks on a model. For more information, see Sensing and Force and Torque Sensing.
To specify the joint mode configuration, use the Mode parameter. For more details, see Mode Configuration under the Ports and Parameters sections.
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 Locked, 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.
Examples
Full Vehicle on Four Post Testrig
Models a passenger vehicle on a four-post testrig. The posts move up and down to replicate the vertical movement of the wheels as it travels along a road. The simulation results and animation show the response of the vehicle body and suspension as it is subjected to the motions from the testrig. The roll and pitch of the vehicle body can be observed, and by varying the inputs wheel hop frequencies can be determined. The vehicle model can be configured to use different suspension types for the front suspension with different linkage combinations.
Modeling Constant Velocity Joints - Power Take-Off Shaft
A Power Take-Off (PTO) shaft, a device for transferring power from tractor engines to auxiliary equipment such as soil tillers and wood chippers. The model includes two PTO subsystems identical in every sense but their joints. One contains universal (U) joints, the other constant-velocity (CV) joints.
