Prismatic Joint
Joint that allows relative motion along single axis
Libraries:
Simscape /
Multibody /
Joints
Description
The Prismatic Joint block models a connection that limits the relative motion between two frames to one translational degree of freedom. This type of motion is common in hydraulic or pneumatic cylinders.
The Prismatic Joint block has only one prismatic primitive, which is along the z-axis of the base frame. During simulation, the z-axes of the base and follower frames align, and the follower frame moves with respect to the base frame along the z-axis, as shown in the image.
Meanwhile, the x and y axes of the follower frame remain parallel to the x and y axes of the base frame, respectively.
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
How to Build a Multibody in Simulink
Highlights key concepts and recommended steps for building a mechanical model using Simscape™ Multibody™. A simple design problem has been chosen to serve this purpose. The following section describes the design problem and subsequent sections discuss how to solve it.
Ports
Frame
Input
Output
Physical signal port that outputs the position of the primitive. The value is the displacement of the follower frame with respect to the base frame.
Dependencies
To enable this port, under Z Prismatic Primitive (Pz) > Sensing, select Position.
Physical signal port that outputs the velocity of the primitive.
Dependencies
To enable this port, under Z Prismatic Primitive (Pz) > Sensing, select Velocity.
Physical signal port that outputs the acceleration of the primitive.
Dependencies
To enable this port, under Z Prismatic Primitive (Pz) > Sensing, select Acceleration.
Physical signal port that outputs the actuator force acting on the joint primitive.
Dependencies
To enable this port, under Z Prismatic Primitive (Pz) > Sensing, select Actuator Force.
Physical signal port that outputs the lower-limit force. The block applies the force when the joint primitive position is less than the lower bound of the free region. The force applies to both the base and follower frames of the joint primitive to accelerate the position back to the free region.
Dependencies
To enable this port, under Z Prismatic Primitive (Pz) > Sensing, select Lower-Limit Force.
Physical signal port that outputs the upper-limit force. The block applies the force when the joint primitive position exceeds the upper bound of the free region. The force applies to both the base and follower frames of the joint primitive to accelerate the position back to the free region.
Dependencies
To enable this port, under Z Prismatic Primitive (Pz) > Sensing, select Upper-Limit Force.
Physical signal port that outputs the constraint force that acts in the joint. The force maintains the translational constraints of the joint. For more information, see Measure Joint Constraint Forces.
Dependencies
To enable this port, under Composite Force/Torque Sensing, select Constraint Force.
Physical signal port that outputs the constraint torque that acts in the joint. The torque maintains the rotational constraints of the joint. For more information, see Force and Torque Sensing.
Dependencies
To enable this port, under Composite Force/Torque Sensing, select Constraint Torque.
Physical signal port that outputs the total force that acts in the joint. The total force is the sum of the forces transmitted from one frame to the other through the joint. The figure shows a crank-slider piston modeled using a Prismatic Joint block.
The total force that acts at the joint includes the actuator force (FA), internal force (FI), and constraint forces (FC). For more information, see Force and Torque Sensing.
Dependencies
To enable this port, under Composite Force/Torque Sensing, select Total Force.
Physical signal port that outputs the total torque that acts in the joint. The total torque is the sum of the torques transmitted from one frame to the other through the joint. The torque includes actuation, internal, limit, and constraint torques. For more information, see Force and Torque Sensing.
Dependencies
To enable this port, under Composite Force/Torque Sensing, select Total Torque.
