Drake
Joint< T > Class Template Referenceabstract

A Joint models the kinematical relationship which characterizes the possible relative motion between two bodies. More...

#include <multibody/multibody_tree/joints/joint.h>

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## Classes

struct  BluePrint
(Advanced) Structure containing all the information needed to build the MultibodyTree implementation for a Joint. More...

struct  JointImplementation
(Advanced) A Joint is implemented in terms of MultibodyTree elements such as bodies, mobilizers, force elements and constraints. More...

## Public Member Functions

Joint (const std::string &name, const Frame< T > &frame_on_parent, const Frame< T > &frame_on_child)
Creates a joint between two Frame objects which imposes a given kinematic relation between frame F attached on the parent body P and frame M attached on the child body B. More...

virtual ~Joint ()

const std::string & get_name () const
Returns the name of this joint. More...

const Body< T > & get_parent_body () const
Returns a const reference to the parent body P. More...

const Body< T > & get_child_body () const
Returns a const reference to the child body B. More...

const Frame< T > & get_frame_on_parent () const
Returns a const reference to the frame F attached on the parent body P. More...

const Frame< T > & get_frame_on_child () const
Returns a const reference to the frame M attached on the child body B. More...

int num_dofs () const
Returns the number of degrees of freedom for this joint. More...

void AddInOneForce (const systems::Context< T > &context, int joint_dof, const T &joint_tau, MultibodyForces< T > *forces) const
Adds into forces a force along the one of the joint's degrees of freedom indicated by index joint_dof. More...

Does not allow copy, move, or assignment
Joint (const Joint &)=delete

Jointoperator= (const Joint &)=delete

Joint (Joint &&)=delete

Jointoperator= (Joint &&)=delete

## Protected Member Functions

virtual int do_get_num_dofs () const =0
Returns the number of degrees of freedom for this joint. More...

virtual void DoAddInOneForce (const systems::Context< T > &context, int joint_dof, const T &joint_tau, MultibodyForces< T > *forces) const =0
Adds into forces a force along the one of the joint's degrees of freedom given by joint_dof. More...

void DoSetTopology (const MultibodyTreeTopology &)

virtual std::unique_ptr< BluePrintMakeImplementationBlueprint () const =0
This method must be implemented by derived classes in order to provide JointImplementationBuilder a BluePrint of their internal implementation JointImplementation. More...

const JointImplementationget_implementation () const
Returns a const reference to the internal implementation of this joint. More...

Methods to make a clone templated on different scalar types.

Clones this Joint (templated on T) to a joint templated on double.

virtual std::unique_ptr< Joint< double > > DoCloneToScalar (const MultibodyTree< double > &tree_clone) const =0

virtual std::unique_ptr< Joint< AutoDiffXd > > DoCloneToScalar (const MultibodyTree< AutoDiffXd > &tree_clone) const =0
Clones this Joint (templated on T) to a joint templated on AutoDiffXd. More...

## Friends

template<typename >
class Joint

class internal::JointImplementationBuilder< T >

## Detailed Description

### template<typename T> class drake::multibody::Joint< T >

A Joint models the kinematical relationship which characterizes the possible relative motion between two bodies.

The two bodies connected by a Joint object are referred to as the parent and child bodies. Although the terms parent and child are sometimes used synonymously to describe the relationship between inboard and outboard bodies in multibody models, this usage is wholly unrelated and implies nothing about the inboard-outboard relationship between the bodies. A Joint is a model of a physical kinematic constraint between two bodies, a constraint that in the real physical system does not even allude to the ordering of the bodies.

In Drake we define a frame F rigidly attached to the parent body P with pose X_PF and a frame M rigidly attached to the child body B with pose X_BM. A Joint object specifies a kinematic relation between frames F and M, which in turn imposes a kinematic relation between bodies P and B.

Typical joints include the ball joint, to allow unrestricted rotations about a given point, the revolute joint, that constraints two bodies to rotate about a given common axis, etc.

Consider the following example to build a simple pendulum system:

MultibodyTree<double> model;
// ... Code here to setup quantities below as mass, com, etc. ...
const Body<double>& pendulum =
// We will connect the pendulum body to the world using a RevoluteJoint.
// In this simple case the parent body P is the model's world body and frame
// F IS the world frame.
// Additionally, we need to specify the pose of frame M on the pendulum's
// body frame B.
// Say we declared and initialized X_BM...
const RevoluteJoint<double>& elbow =
"Elbow", /* joint name */
model.get_world_body(), /* parent body */
{}, /* frame F IS the world frame W */
pendulum, /* child body, the pendulum */
X_BM, /* pose of frame M in the body frame B */
Vector3d::UnitZ()); /* revolute axis in this case */
Warning
Do not ever attempt to instantiate and manipulate Joint objects on the stack; it will fail. Add joints to your model using the provided API MultibodyTree::AddJoint() as in the example above.
Template Parameters
 T The scalar type. Must be a valid Eigen scalar.

## Constructor & Destructor Documentation

 Joint ( const Joint< T > & )
delete
 Joint ( Joint< T > && )
delete
 Joint ( const std::string & name, const Frame< T > & frame_on_parent, const Frame< T > & frame_on_child )
inline

Creates a joint between two Frame objects which imposes a given kinematic relation between frame F attached on the parent body P and frame M attached on the child body B.

See this class's documentation for further details.

Parameters
 [in] name A string with a name identifying this joint. [in] frame_on_parent The frame F attached on the parent body connected by this joint. [in] frame_on_child The frame M attached on the child body connected by this joint.
 virtual ~Joint ( )
inlinevirtual

## Member Function Documentation

 void AddInOneForce ( const systems::Context< T > & context, int joint_dof, const T & joint_tau, MultibodyForces< T > * forces ) const
inline

Adds into forces a force along the one of the joint's degrees of freedom indicated by index joint_dof.

The meaning for this degree of freedom and even its dimensional units depend on the specific joint sub-class. For a RevoluteJoint for instance, joint_dof can only be 0 since revolute joints's motion subspace only has one degree of freedom, while the units of joint_tau are those of torque (N⋅m in the MKS system of units). For multi-dof joints please refer to the documentation provided by specific joint sub-classes regarding the meaning of joint_dof.

Parameters
 [in] context The context storing the state and parameters for the model to which this joint belongs. [in] joint_dof Index specifying one of the degress of freedom for this joint. The index must be in the range 0 <= joint_dof < num_dofs() or otherwise this method will abort. [in] joint_tau Generalized force corresponding to the degree of freedom indicated by joint_dof to be added into forces. [out] forces On return, this method will add force joint_tau for the degree of freedom joint_dof into the output forces. This method aborts if forces is nullptr or if forces doest not have the right sizes to accommodate a set of forces for the model to which this joint belongs.

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 virtual int do_get_num_dofs ( ) const
protectedpure virtual

Returns the number of degrees of freedom for this joint.

Implementation to the NVI num_dofs() that must be implemented by all subclasses. E.g., this method should return one for a revolute joint and it should return three for a ball joint.

 virtual void DoAddInOneForce ( const systems::Context< T > & context, int joint_dof, const T & joint_tau, MultibodyForces< T > * forces ) const
protectedpure virtual

Adds into forces a force along the one of the joint's degrees of freedom given by joint_dof.

How forces is added to a MultibodyTree model depends on the underlying implementation of a particular joint and therefore specific Joint subclasses must provide a definition for this method. For instance, a revolute joint could be modeled with a single generalized coordinate for the angular rotation (implemented through a RevoluteMobilizer) or it could be modeled using a constraint that only allows rotation about the joint's axis but that constrains the motion in the other five degrees of freedom. This method is only called by the public NVI AddInOneForce() and therefore input arguments were checked to be valid.

The public NVI AddInOneForce() for details.
 virtual std::unique_ptr > DoCloneToScalar ( const MultibodyTree< double > & tree_clone ) const
protectedpure virtual
 virtual std::unique_ptr > DoCloneToScalar ( const MultibodyTree< AutoDiffXd > & tree_clone ) const
protectedpure virtual

Clones this Joint (templated on T) to a joint templated on AutoDiffXd.

 void DoSetTopology ( const MultibodyTreeTopology & )
inlineprotected
 const Body& get_child_body ( ) const
inline

Returns a const reference to the child body B.

 const Frame& get_frame_on_child ( ) const
inline

Returns a const reference to the frame M attached on the child body B.

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 const Frame& get_frame_on_parent ( ) const
inline

Returns a const reference to the frame F attached on the parent body P.

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 const JointImplementation& get_implementation ( ) const
inlineprotected

Returns a const reference to the internal implementation of this joint.

Warning
The MultibodyTree model must have already been finalized, or this method will abort.
 const std::string& get_name ( ) const
inline

Returns the name of this joint.

 const Body& get_parent_body ( ) const
inline

Returns a const reference to the parent body P.

 virtual std::unique_ptr MakeImplementationBlueprint ( ) const
protectedpure virtual

This method must be implemented by derived classes in order to provide JointImplementationBuilder a BluePrint of their internal implementation JointImplementation.

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 int num_dofs ( ) const
inline

Returns the number of degrees of freedom for this joint.

E.g., one for a revolute joint and three for a ball joint.

 Joint& operator= ( const Joint< T > & )
delete
 Joint& operator= ( Joint< T > && )
delete

## Friends And Related Function Documentation

 friend class internal::JointImplementationBuilder< T >
friend
 friend class Joint
friend

The documentation for this class was generated from the following file:
• multibody/multibody_tree/joints/joint.h