Drake
PrismaticJoint< T > Class Template Referencefinal

This Joint allows two bodies to translate relative to one another along a common axis. More...

#include <drake/multibody/multibody_tree/joints/prismatic_joint.h>

## Public Types

template<typename Scalar >
using Context = systems::Context< Scalar >

## Public Member Functions

PrismaticJoint (const std::string &name, const Frame< T > &frame_on_parent, const Frame< T > &frame_on_child, const Vector3< double > &axis, double lower_limit=-std::numeric_limits< double >::infinity(), double upper_limit=std::numeric_limits< double >::infinity(), double damping=0)
Constructor to create a prismatic joint between two bodies so that frame F attached to the parent body P and frame M attached to the child body B, translate relatively to one another along a common axis. More...

const Vector3< double > & translation_axis () const
Returns the axis of translation for this joint as a unit vector. More...

double damping () const
Returns this joint's damping constant in N⋅s/m. More...

double lower_limit () const
Returns the lower limit for this joint in meters. More...

double upper_limit () const
Returns the upper limit for this joint in meters. More...

void AddInForce (const systems::Context< T > &context, const T &force, MultibodyForces< T > *multibody_forces) const
Adds into multibody_forces a given force, in Newtons, for this joint that is to be applied along the joint's axis. More...

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

PrismaticJointoperator= (const PrismaticJoint &)=delete

PrismaticJoint (PrismaticJoint &&)=delete

PrismaticJointoperator= (PrismaticJoint &&)=delete

Context-dependent value access

These methods require the provided context to be an instance of MultibodyTreeContext.

Failure to do so leads to a std::logic_error.

const Tget_translation (const Context< T > &context) const
Gets the translation distance of this mobilizer from context. More...

const PrismaticJoint< T > & set_translation (Context< T > *context, const T &translation) const
Sets context so that the generalized coordinate corresponding to the translation distance of this joint equals translation. More...

const Tget_translation_rate (const Context< T > &context) const
Gets the rate of change, in meters per second, of this joint's translation distance (see get_translation()) from context. More...

const PrismaticJoint< T > & set_translation_rate (Context< T > *context, const T &translation_dot) const
Sets the rate of change, in meters per second, of this joint's translation distance to translation_dot. More...

Public Member Functions inherited from Joint< T >
Joint (const std::string &name, const Frame< T > &frame_on_parent, const Frame< T > &frame_on_child, const VectorX< double > &lower_limits, const VectorX< double > &upper_limits)
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 & name () const
Returns the name of this joint. More...

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

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

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

const Frame< T > & 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...

int velocity_start () const
Returns the index to the first generalized velocity for this joint within the vector v of generalized velocities for the full multibody system. More...

int num_velocities () const
Returns the number of generalized velocities describing this joint. More...

int position_start () const
Returns the index to the first generalized position for this joint within the vector q of generalized positions for the full multibody system. More...

int num_positions () const
Returns the number of generalized positions describing this joint. More...

const VectorX< double > & lower_limits () const
Returns a vector of size num_positions() storing the lower limits for each generalized position for this joint. More...

const VectorX< double > & upper_limits () const
Returns a vector of size num_positions() storing the upper limits for each generalized position for this joint. More...

const TGetOnePosition (const systems::Context< T > &context) const
Returns the position coordinate for joints with a single degree of freedom. More...

const TGetOneVelocity (const systems::Context< T > &context) const
Returns the velocity coordinate for joints with a single degree of freedom. 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...

void AddInDamping (const systems::Context< T > &context, MultibodyForces< T > *forces) const
Adds into forces the force due to damping within this joint. More...

Joint (const Joint &)=delete

Jointoperator= (const Joint &)=delete

Joint (Joint &&)=delete

Jointoperator= (Joint &&)=delete

## Protected Member Functions

void DoAddInOneForce (const systems::Context< T > &, int joint_dof, const T &joint_tau, MultibodyForces< T > *forces) const final
Joint<T> virtual override called through public NVI, Joint::AddInForce(). More...

void DoAddInDamping (const systems::Context< T > &context, MultibodyForces< T > *forces) const override
Joint<T> override called through public NVI, Joint::AddInDamping(). More...

Protected Member Functions inherited from Joint< T >
void DoSetTopology (const MultibodyTreeTopology &)

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

## Friends

template<typename >
class PrismaticJoint

class JointTester

## Detailed Description

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

This Joint allows two bodies to translate relative to one another along a common axis.

That is, given a frame F attached to the parent body P and a frame M attached to the child body B (see the Joint class's documentation), this Joint allows frames F and M to translate with respect to each other along an axis â. The translation distance is defined positive when child body B translates along the direction of â. Axis â is constant and has the same measures in both frames F and M, that is, â_F = â_M.

Template Parameters
 T The scalar type. Must be a valid Eigen scalar.

Instantiated templates for the following kinds of T's are provided:

• double
• AutoDiffXd

They are already available to link against in the containing library. No other values for T are currently supported.

## ◆ Context

 using Context = systems::Context

## ◆ PrismaticJoint() [1/3]

 PrismaticJoint ( const PrismaticJoint< T > & )
delete

## ◆ PrismaticJoint() [2/3]

 PrismaticJoint ( PrismaticJoint< T > && )
delete

## ◆ PrismaticJoint() [3/3]

 PrismaticJoint ( const std::string & name, const Frame< T > & frame_on_parent, const Frame< T > & frame_on_child, const Vector3< double > & axis, double lower_limit = -std::numeric_limits::infinity(), double upper_limit = std::numeric_limits::infinity(), double damping = 0 )
inline

Constructor to create a prismatic joint between two bodies so that frame F attached to the parent body P and frame M attached to the child body B, translate relatively to one another along a common axis.

See this class's documentation for further details on the definition of these frames and translation distance. The first three arguments to this constructor are those of the Joint class constructor. See the Joint class's documentation for details. The additional parameter axis is:

Parameters
 [in] axis A vector in ℝ³ specifying the translation axis for this joint. Given that frame M only translates with respect to F and there is no relative rotation, the measures of axis in either frame F or M are exactly the same, that is, axis_F = axis_M. This vector can have any length, only the direction is used. [in] lower_limit Lower limit, in meters, for the translation coordinate (see get_translation()). [in] upper_limit Upper limit, in meters, for the translation coordinate (see get_translation()). [in] damping Viscous damping coefficient, in N⋅s/m, used to model losses within the joint. The damping force (in N) is modeled as f = -damping⋅v, i.e. opposing motion, with v the translational speed for this joint (see get_translation_rate()).
Exceptions
 std::exception if the L2 norm of axis is less than the square root of machine epsilon. std::exception if damping is negative. std::exception if lower_limit > upper_limit.

## Member Function Documentation

 void AddInForce ( const systems::Context< T > & context, const T & force, MultibodyForces< T > * multibody_forces ) const
inline

Adds into multibody_forces a given force, in Newtons, for this joint that is to be applied along the joint's axis.

The force is defined to be positive in the direction along this joint's axis. That is, a positive force causes a positive translational acceleration along the joint's axis.

## ◆ damping()

 double damping ( ) const
inline

Returns this joint's damping constant in N⋅s/m.

 void DoAddInDamping ( const systems::Context< T > & context, MultibodyForces< T > * forces ) const
inlineoverrideprotectedvirtual

Joint<T> override called through public NVI, Joint::AddInDamping().

Therefore arguments were already checked to be valid. This method adds into forces a dissipative force according to the viscous law f = -d⋅v, with d the damping coefficient (see damping()).

Reimplemented from Joint< T >.

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

Joint<T> virtual override called through public NVI, Joint::AddInForce().

Therefore arguments were already checked to be valid. For a PrismaticJoint, we must always have joint_dof = 0 since there is only a single degree of freedom (num_velocities() == 1). joint_tau is the linear force applied along the joint's axis, on the body declared as child (according to the prismatic joint's constructor) at the origin of the child frame (which is coincident with the origin of the parent frame at all times).

Implements Joint< T >.

## ◆ get_translation()

 const T& get_translation ( const Context< T > & context ) const
inline

Gets the translation distance of this mobilizer from context.

Parameters
 [in] context The context of the MultibodyTree this joint belongs to.
Returns
The translation coordinate of this joint read from context.

## ◆ get_translation_rate()

 const T& get_translation_rate ( const Context< T > & context ) const
inline

Gets the rate of change, in meters per second, of this joint's translation distance (see get_translation()) from context.

Parameters
 [in] context The context of the MultibodyTree this joint belongs to.
Returns
The rate of change of this joint's translation read from context.

## ◆ lower_limit()

 double lower_limit ( ) const
inline

Returns the lower limit for this joint in meters.

## ◆ operator=() [1/2]

 PrismaticJoint& operator= ( PrismaticJoint< T > && )
delete

## ◆ operator=() [2/2]

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

## ◆ set_translation()

 const PrismaticJoint& set_translation ( Context< T > * context, const T & translation ) const
inline

Sets context so that the generalized coordinate corresponding to the translation distance of this joint equals translation.

Parameters
 [in] context The context of the MultibodyTree this joint belongs to. [in] translation The desired translation in meters to be stored in context.
Returns
a constant reference to this joint.

## ◆ set_translation_rate()

 const PrismaticJoint& set_translation_rate ( Context< T > * context, const T & translation_dot ) const
inline

Sets the rate of change, in meters per second, of this joint's translation distance to translation_dot.

The new rate of change translation_dot gets stored in context.

Parameters
 [in] context The context of the MultibodyTree this joint belongs to. [in] translation_dot The desired rate of change of this joints's translation in meters per second.
Returns
a constant reference to this joint.

## ◆ translation_axis()

 const Vector3& translation_axis ( ) const
inline

Returns the axis of translation for this joint as a unit vector.

Since the measures of this axis in either frame F or M are the same (see this class's documentation for frames's definitions) then, axis = axis_F = axis_M.

## ◆ upper_limit()

 double upper_limit ( ) const
inline

Returns the upper limit for this joint in meters.

## ◆ JointTester

 friend class JointTester
friend

## ◆ PrismaticJoint

 friend class PrismaticJoint
friend

The documentation for this class was generated from the following files: