Revolute Joint

Joint with one revolute primitive

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Libraries:
Simscape / Multibody / Joints

Description

The Revolute Joint block models a joint that has one rotational degree of freedom.

The joint constrains the motion of two arbitrary frames that connect to the base and follower frames of the joint to pure rotation about a common axis. The axis of rotation is aligned with the z-axis of the joint base frame. The base and follower frames have a common origin and z-axis, and the follower frame rotates about the z-axis, 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

Single Pendulum in Simulink and Simscape Multibody

A single pendulum modeled using Simulink® input/output blocks and using Simscape™ Multibody™. The initial angle for the joint is defined by a MATLAB® variable. The annotations on the integrator block show the initial angle of the joint with respect to the world frame.

• Open Model

Ports

Frame

B — Base frame

frame

Base frame of the joint block.

F — Follower frame

frame

Follower frame of the joint block.

Input

t — Actuation torque

physical signal

Physical signal input port that accepts the actuation torque for the joint primitive. The signal provides the value of the torque that applies on both the base and follower frames of the joint primitive.

Dependencies

To enable this port, under Z Revolute Primitive (Rz) > Actuation, set Torque to Provided by Input.

q — Motion profile

physical signal

Physical signal input port that accepts the motion profile for the joint primitive. The signal provides the rotation of the follower frame with respect to the base frame about the joint primitive axis. Note that the signal must also contain the first and second derivatives of the rotation.

Dependencies

To enable this port, under Z Revolute Primitive (Rz) > Actuation, set Motion to Provided by 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, disengaged when the input signal is -1, and locked 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 Locked	The joint position retains the current value and remains constant after the transition.	The joint velocity becomes zero and remains constant after the transition.
Normal to Disengaged	The joint position retains the current value but can change in any direction after the transition.	The joint velocity retains the current value but can change in any direction after the transition.
Locked to Normal	The joint position retains the current value but can change in the directions aligned with the joint degrees of freedom (DOFs) after the transition.	The joint velocity remains at zero but can change in the directions aligned with the joint DOFs after the transition.
Locked to Disengaged	The joint position retains the current value 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	For the directions aligned with the joint DOFs, the joint positions initially take values calculated by using Newton's method and can change thereafter. In the constrained directions, the joint positions become zero and remain constant after the transition.	For the directions aligned with the joint DOFs, the joint velocities initially take values calculated by using Newton's method and can change thereafter. In the constrained directions, the joint velocities become zero and remain constant after the transition.
Disengaged to Locked	For the directions aligned with the joint DOFs, the joint positions initially take values calculated by using Newton's method and remain constant after the transition. In the constrained directions, the joint positions become zero and remain constant after the transition.	The joint velocity becomes zero and remains constant after the transition.

Dependencies

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

Output

q — Position of joint primitive

physical signal

Physical signal port that outputs the position of the joint primitive. The value is the rotation angle of the follower frame with respect to the base frame.

Dependencies

To enable this port, under Z Revolute Primitive (Rz) > Sensing, select Position.

w — First derivative of position of joint primitive

physical signal

Physical signal port that outputs the first derivative of position of the joint primitive.

Dependencies

To enable this port, under Z Revolute Primitive (Rz) > Sensing, select Velocity.

b — Second derivative of position of joint primitive

physical signal

Physical signal port that outputs the second derivative of position of the joint primitive.

Dependencies

To enable this port, under Z Revolute Primitive (Rz) > Sensing, select Acceleration.

t — Actuator torque acting on joint primitive

physical signal

Physical signal port that outputs the actuator torque acting on the joint primitive.

Dependencies

To enable this port, under Z Revolute Primitive (Rz) > Sensing, select Actuator Torque.

tll — Lower-limit torque

physical signal

Physical signal port that outputs the lower-limit torque. The block applies the torque when the joint primitive position exceeds the lower bound of the free region. The torque 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 Revolute Primitive (Rz) > Sensing, select Lower-Limit Torque.

tul — Upper-limit torque

physical signal

Physical signal port that outputs the upper-limit torque. The block applies the torque when the joint primitive position exceeds the upper bound of the free region. The torque 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 Revolute Primitive (Rz) > Sensing, select Upper-Limit Torque.

fc — Constraint force

physical signal

Physical signal port that outputs constraint force that acts in the joint. The force maintains the translational constraints of the joint. See Measure Joint Constraint Forces for more information.

Dependencies

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

tc — Constraint torque

physical signal

Physical signal port that outputs constraint torque that acts in the joint. The torque maintains the rotational constraints of the joint. See Force and Torque Sensing for more information.

Dependencies

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

ft — Total force

physical signal

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 force includes actuation, internal, limit and constraint forces. See Force and Torque Sensing for more information.

Dependencies

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

tt — Total torque

physical signal

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. See Force and Torque Sensing for more information.

Dependencies

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

Parameters

Z Revolute Primitive (Rz)

State Targets > Specify Position Target — Whether to specify position target

off (default) | on

Select this parameter to enable parameters that specify the position target of the joint primitive.

State Targets > Specify Position Target > Priority — Priority level of position target

High (desired) (default) | Low (approximate)

Priority level of the position target, specified as High (desired) or Low (approximate).

Dependencies

To enable this parameter, under Z Revolute Primitive (Rz) > State Targets, select Specify Position Target.

State Targets > Specify Position Target > Value — Angle of position target

0 deg (default) | scalar with a unit of angle

Angle to specify the position target, specified as a scalar with a unit of angle.

Dependencies

To enable this parameter, under Z Revolute Primitive (Rz) > State Targets, select Specify Position Target.

State Targets > Specify Velocity Target — Whether to specify velocity target

off (default) | on

Select this parameter to enable parameters for that specify the velocity target of the joint primitive.

State Targets > Specify Velocity Target > Priority — Priority level of velocity target

High (desired) (default) | Low (approximate)

Priority level of the velocity target, specified as High (desired) or Low (approximate).

Dependencies

To enable this parameter, under Z Revolute Primitive (Rz) > State Targets, select Specify Velocity Target.

State Targets > Specify Velocity Target > Value — Velocity target of joint primitive

0 deg/s (default) | scalar with unit of angular velocity

Velocity target of the joint primitive, specified as a scalar with a unit of angular velocity.

Dependencies

To enable this parameter, under Z Revolute Primitive (Rz) > State Targets, select Specify Velocity Target.

Internal Mechanics > Equilibrium Position — Position where internal torque is zero

0 deg (default) | scalar with unit of angle

Position where the spring torque is zero, specified as a scalar with a unit of angle. The value specifies the rotation angle of the follower frame with respect to the base frame.

Internal Mechanics > Spring Stiffness — Stiffness of force law

0 N*m/deg (default) | scalar with unit of stiffness

Stiffness of the internal spring-damper force law for the joint primitive, specified as a scalar with a unit of stiffness.

Internal Mechanics > Damping Coefficient — Damping coefficient of force law

0 N*m/(deg/s) (default) | scalar with unit of damping coefficient

Damping coefficient of the internal spring-damper force law for the joint primitive, specified as a scalar with a unit of damping coefficient.

Limits > Specify Lower Limit — Whether to specify lower position limit

off (default) | on

Select this parameter to enable parameters that specify the lower limit of the joint primitive.

Limits > Specify Lower Limit > Bound — Lower bound of free region

-90 deg (default) | scalar with unit of angle

Lower bound for the free region of the joint primitive, specified as a scalar with a unit of angle.

Dependencies

To enable this parameter, under Z Revolute Primitive (Rz) > Limits, select Specify Lower Limit.

Limits > Specify Lower Limit > Spring Stiffness — Stiffness of spring at lower bound

1e4 N*m/deg (default) | scalar with unit of stiffness

Stiffness of the spring at lower bound, specified as a scalar with a unit of stiffness.

Dependencies

To enable this parameter, under Z Revolute Primitive (Rz) > Limits, select Specify Lower Limit.

Limits > Specify Lower Limit > Damping Coefficient — Damping coefficient at lower bound

10 N*m/(deg/s) (default) | scalar with unit of damping coefficient

Damping coefficient at lower bound, specified as a scalar with a unit of damping coefficient.

Dependencies

To enable this parameter, under Z Revolute Primitive (Rz) > Limits, select Specify Lower Limit.

Limits > Specify Lower Limit > Transition Region Width — Region to smooth spring and damper forces

0.1 deg (default) | scalar with unit of angle

Region to smooth the spring and damper forces, specified as a scalar with a unit of angle.

The forces get full value once the penetration reaches the width of the transition region. The smaller the region, the sharper the onset of forces and the smaller the time-step required of the solver. In the tradeoff between simulation accuracy and simulation speed, reducing the transition region improves accuracy while expanding it improves speed.

Dependencies

To enable this parameter, under Z Revolute Primitive (Rz) > Limits, select Specify Lower Limit.

Limits > Specify Upper Limit — Whether to specify upper position limit

off (default) | on

Select this parameter to enable parameters for specifying the upper limit of the joint primitive.

Limits > Specify Upper Limit > Bound — Upper bound of free region

90 deg (default) | scalar with unit of angle

Upper bound for the free region of the joint primitive, specified as a scalar with a unit of angle.

Dependencies

To enable this parameter, under Z Revolute Primitive (Rz) > Limits, select Specify Upper Limit.

Limits > Specify Upper Limit > Spring Stiffness — Stiffness of spring at upper bound

1e4 N*m/deg (default) | scalar with unit of stiffness

Stiffness of the spring at upper bound, specified as a scalar with a unit of stiffness.

Dependencies

To enable this parameter, under Z Revolute Primitive (Rz) > Limits, select Specify Upper Limit.

Limits > Specify Upper Limit > Damping Coefficient — Damping coefficient at upper bound

10 N*m/(deg/s) (default) | scalar with unit of damping coefficient

Damping coefficient at upper bound, specified as a scalar with a unit of damping coefficient.

Dependencies

To enable this parameter, under Z Revolute Primitive (Rz) > Limits, select Specify Upper Limit.

Limits > Specify Upper Limit > Transition Region Width — Region to smooth spring and damper forces

0.1 deg (default) | scalar with unit of angle

Region to smooth the spring and damper forces, specified as a scalar with a unit of angle.

The forces get full value once the penetration reaches the width of the transition region. The smaller the region, the sharper the onset of forces and the smaller the time-step required of the solver. In the tradeoff between simulation accuracy and simulation speed, reducing the transition region improves accuracy while expanding it improves speed.

Dependencies

To enable this parameter, under Z Revolute Primitive (Rz) > Limits, select Specify Upper Limit.

Actuation > Torque — Option to provide actuation torque

None (default) | Provided by Input | Automatically Computed

Option to provide the actuation torque for the joint primitive, specified as None, Provided by Input, or Automatically Computed.

Actuation Torque Setting	Description
None	No actuation torque.
Provided by Input	Expose the input port t.
Automatically computed	The torque required to satisfy the motion inputs to the mechanism. The block computes the torque automatically. Note that if you set this parameter to Automatically computed, it doesn’t mean that you must use Provided by Input for the Motion parameter for the same joint primitive. The automatically computed torque could be to satisfy a motion input somewhere else in the mechanism.

Actuation > Motion — Option to provide actuation motion

Automatically Computed (default) | Provided by Input

Option to provide the actuation motion for the joint primitive, specified as Automatically Computed or Provided by Input.

Actuation Torque Setting	Description
Automatically computed	The block computes and applies the joint primitive motion based on model dynamics.
Provided by Input	Expose the input port q.

Sensing > Position — Whether to sense joint primitive position

off (default) | on

Select this parameter to enable the output port q.

Sensing > Velocity — Whether to sense joint primitive velocity

off (default) | on

Select this parameter to enable the port w.

Sensing > Acceleration — Whether to sense joint primitive acceleration

off (default) | on

Select this parameter to enable the port b.

Sensing > Lower-Limit Torque — Whether sense to lower-limit torque

off (default) | on

Select this parameter to enable the port tll.

Sensing > Upper-Limit Torque — Whether sense to upper-limit torque

off (default) | on

Select this parameter to enable the port tul.

Mode Configuration

Mode — Joint mode

Normal (default) | Locked | Disengaged | Provided by Input

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

Mode	Description
Locked	Locked mode constrains all the degrees of freedom (DOFs) for the joint. The locked joint maintains its initial assembly position with zero velocity during the simulation. The joint block can sense forces or torques in accordance with the settings of the Internal Mechanics, Limits, and Actuation parameters.
Normal	Normal mode enables the DOFs and the constraints of the joint 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.