Drake
JointActuator< T > Class Template Referencefinal

Detailed Description

template<typename T> class drake::multibody::JointActuator< T >

The JointActuator class is mostly a simple bookkeeping structure to represent an actuator acting on a given Joint.

It helps to flag whether a given Joint is actuated or not so that MultibodyTree clients can apply forces on actuated joints through their actuators, see AddInOneForce().

Template Parameters
 T The scalar type, which must be one of the default scalars.

#include <drake/multibody/tree/multibody_tree.h>

Public Member Functions

JointActuator (const std::string &name, const Joint< T > &joint, double effort_limit=std::numeric_limits< double >::infinity())
Creates an actuator for joint with the given name. More...

const std::string & name () const
Returns the name of the actuator. More...

const Joint< T > & joint () const
Returns a reference to the joint actuated by this JointActuator. More...

void AddInOneForce (const systems::Context< T > &context, int joint_dof, const T &tau, MultibodyForces< T > *forces) const
Adds into forces a force along one of the degrees of freedom of the Joint actuated by this actuator. More...

const Eigen::Ref< const VectorX< T > > get_actuation_vector (const VectorX< T > &u) const
Gets the actuation values for this actuator from the actuation vector u for the entire model. More...

void set_actuation_vector (const Eigen::Ref< const VectorX< T >> &u_instance, EigenPtr< VectorX< T >> u) const
Given the actuation values u_instance for this actuator, this method sets the actuation vector u for the entire MultibodyTree model to which this actuator belongs to. More...

double effort_limit () const
Returns the actuator effort limit. More...

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

JointActuatoroperator= (const JointActuator &)=delete

JointActuator (JointActuator &&)=delete

JointActuatoroperator= (JointActuator &&)=delete

Reflected Inertia

The JointActuator class offers the ability to model the effects of reflected inertia for revolute and prismatic joints via an approximation commonly used in robotics (see [Featherstone, 2008]). Reflected inertia is an approximate method for accounting for the mass/inertia contributions to kinetic energy and equations of motion for effects of a spinning motor's rotor (and any additional shafts/gears) inside a gearmotor. This "quick and dirty" approximation does not depend on the internals of the gearbox (e.g., whether there is a single stage or multistage gear, whether there are additional shafts/gears, or the relative position or orientation of the outboard shaft with respect to the motor rotor's shaft. Note: Reflected inertia does not account for mass/inertia of the body attached to the gearmotor's output shaft (labeled S in the figure below). The value of reflected inertia is a function of the gear ratio and the rotor inertia of the modeled motor. JointActuator exposes these two values as parameters (see SetRotorInertia() and SetGearRatio()). Reflected inertia has units of kg for prismatic joints and units of kg⋅m² for revolute joints. A zero value indicates reflected inertia is not modeled. This value is used as part of an approximation of a rotor's inertial effects in a geared motor. It should be noted that the approximation is reasonable for high gear ratios but less so for small gear ratios (see [Featherstone, 2008], Chapter 9.6 on gears).

Actuated revolute joints

For an actuator driving a revolute joint, the reflected inertia can be estimated from the rotational inertia Iᵣ of the actuator's rotor about its axis of rotation and from the dimensionless gear ratio ρ. To define gear ratio ρ, consider a gear-motor combination that has a stator/rigid case A, motor rotor R, and output shaft B. Rotor R spins relative to stator A with angular speed wR and shaft B spins relative to stator A with angular speed wB. The gear ratio is defined ρ ≝ wR / wB. Typically, ρ >> 1. For this gear-motor, reflected inertia Iᵣᵢ = ρ² ⋅ Iᵣ.

Actuated prismatic joints

To define the gear ratio ρ for prismatic joints, consider a gear-motor combination that has a stator/rigid case A, motor rotor R, and translating output shaft B. Rotor R spins relative to stator A with angular speed wR and shaft B translates relative to stator A with translational speed vB. The gear ratio is defined ρ ≝ wR / vB (units of 1/m). Typically, ρ >> 1. For the gear-motor here, reflected inertia Iᵣᵢ = ρ² ⋅ Iᵣ (units of kg).

double default_rotor_inertia () const
Gets the default value for this actuator's rotor inertia. More...

double default_gear_ratio () const
Gets the default value for this actuator's gear ratio. More...

void set_default_rotor_inertia (double rotor_inertia)
Sets the default value for this actuator's rotor inertia. More...

void set_default_gear_ratio (double gear_ratio)
Sets the default value for this actuator's gear ratio. More...

double default_reflected_inertia () const
Returns the default value for this actuator's reflected inertia. More...

const T & rotor_inertia (const systems::Context< T > &context) const
Returns the associated rotor inertia value for this actuator, stored in context. More...

const T & gear_ratio (const systems::Context< T > &context) const
Returns the associated gear ratio value for this actuator, stored in context. More...

void SetRotorInertia (systems::Context< T > *context, const T &rotor_inertia) const
Sets the associated rotor inertia value for this actuator in context. More...

void SetGearRatio (systems::Context< T > *context, const T &gear_ratio) const
Sets the associated gear ratio value for this actuator in context. More...

calc_reflected_inertia (const systems::Context< T > &context) const
Calculates the reflected inertia value for this actuator in context. More...

Public Member Functions inherited from MultibodyElement< JointActuator, T, JointActuatorIndex >
virtual ~MultibodyElement ()

JointActuatorIndex index () const
Returns this element's unique index. More...

ModelInstanceIndex model_instance () const
Returns the ModelInstanceIndex of the model instance to which this element belongs. More...

const MultibodyPlantDeferred & GetParentPlant () const
Returns the MultibodyPlant that owns this MultibodyElement. More...

void DeclareParameters (internal::MultibodyTreeSystem< T > *tree_system)
Declares MultibodyTreeSystem Parameters at MultibodyTreeSystem::Finalize() time. More...

MultibodyElement (const MultibodyElement &)=delete

MultibodyElement (MultibodyElement &&)=delete

MultibodyElementoperator= (const MultibodyElement &)=delete

MultibodyElementoperator= (MultibodyElement &&)=delete

Friends

template<typename U >
class JointActuator

Protected Member Functions inherited from MultibodyElement< JointActuator, T, JointActuatorIndex >
MultibodyElement ()
Default constructor made protected so that sub-classes can still declare their default constructors if they need to. More...

MultibodyElement (ModelInstanceIndex model_instance)
Constructor which allows specifying a model instance. More...

const internal::MultibodyTree< T > & get_parent_tree () const
Returns a constant reference to the parent MultibodyTree that owns this element. More...

const internal::MultibodyTreeSystem< T > & GetParentTreeSystem () const
Returns a constant reference to the parent MultibodyTreeSystem that owns the parent MultibodyTree that owns this element. More...

void SetTopology (const internal::MultibodyTreeTopology &tree)
Gives MultibodyElement-derived objects the opportunity to retrieve their topology after MultibodyTree::Finalize() is invoked. More...

systems::NumericParameterIndex DeclareNumericParameter (internal::MultibodyTreeSystem< T > *tree_system, const systems::BasicVector< T > &model_vector)
To be used by MultibodyElement-derived objects when declaring parameters in their implementation of DoDeclareParameters(). More...

systems::AbstractParameterIndex DeclareAbstractParameter (internal::MultibodyTreeSystem< T > *tree_system, const AbstractValue &model_value)
To be used by MultibodyElement-derived objects when declaring parameters in their implementation of DoDeclareParameters(). More...

◆ JointActuator() [1/3]

 JointActuator ( const JointActuator< T > & )
delete

◆ JointActuator() [2/3]

 JointActuator ( JointActuator< T > && )
delete

◆ JointActuator() [3/3]

 JointActuator ( const std::string & name, const Joint< T > & joint, double effort_limit = std::numeric_limits< double >::infinity() )

Creates an actuator for joint with the given name.

The name must be unique within the given multibody model. This is enforced by MultibodyPlant::AddJointActuator().

Parameters
 [in] name A string with a name identifying this actuator. [in] joint The joint that the created actuator will act on. [in] effort_limit The maximum effort for the actuator. It must be strictly positive, otherwise an std::exception is thrown. If +∞, the actuator has no limit, which is the default. The effort limit has physical units in accordance to the joint type it actuates. For instance, it will have units of N⋅m (torque) for revolute joints while it will have units of N (force) for prismatic joints.

Member Function Documentation

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

Adds into forces a force along one of the degrees of freedom of the Joint actuated by this actuator.

The meaning for this degree of freedom, sign conventions and even its dimensional units depend on the specific joint sub-class being actuated. 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 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 degrees of freedom for this joint. The index must be in the range 0 <= joint_dof < num_velocities() or otherwise this method will throw an exception. [in] joint_tau Generalized force corresponding to the degree of freedom indicated by joint_dof to be added into forces. Refere to the specific Joint sub-class documentation for details on the meaning and units for this degree of freedom. [out] forces On return, this method will add force 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 actuator belongs.

◆ calc_reflected_inertia()

 T calc_reflected_inertia ( const systems::Context< T > & context ) const

Calculates the reflected inertia value for this actuator in context.

◆ default_gear_ratio()

 double default_gear_ratio ( ) const

Gets the default value for this actuator's gear ratio.

◆ default_reflected_inertia()

 double default_reflected_inertia ( ) const

Returns the default value for this actuator's reflected inertia.

It is calculated as ρ²⋅Iᵣ, where ρ is the default gear ratio and Iᵣ is the default rotor inertia for this actuator. See reflected_inertia.

◆ default_rotor_inertia()

 double default_rotor_inertia ( ) const

Gets the default value for this actuator's rotor inertia.

◆ effort_limit()

 double effort_limit ( ) const

Returns the actuator effort limit.

◆ gear_ratio()

 const T& gear_ratio ( const systems::Context< T > & context ) const

Returns the associated gear ratio value for this actuator, stored in context.

◆ get_actuation_vector()

 const Eigen::Ref > get_actuation_vector ( const VectorX< T > & u ) const

Gets the actuation values for this actuator from the actuation vector u for the entire model.

Returns
a reference to a nv-dimensional vector, where nv is the number of velocity variables of joint().

◆ joint()

 const Joint& joint ( ) const

Returns a reference to the joint actuated by this JointActuator.

◆ name()

 const std::string& name ( ) const

Returns the name of the actuator.

◆ operator=() [1/2]

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

◆ operator=() [2/2]

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

◆ rotor_inertia()

 const T& rotor_inertia ( const systems::Context< T > & context ) const

Returns the associated rotor inertia value for this actuator, stored in context.

◆ set_actuation_vector()

 void set_actuation_vector ( const Eigen::Ref< const VectorX< T >> & u_instance, EigenPtr< VectorX< T >> u ) const

Given the actuation values u_instance for this actuator, this method sets the actuation vector u for the entire MultibodyTree model to which this actuator belongs to.

Parameters
 [in] u_instance Actuation values for this actuator. It must be of size equal to the number of degrees of freedom of the actuated Joint, see Joint::num_velocities(). For units and sign conventions refer to the specific Joint sub-class documentation. [out] u The vector containing the actuation values for the entire MultibodyTree model to which this actuator belongs to.
Exceptions
 std::exception if u_instance.size() != this->joint().num_velocities(). std::exception if u is nullptr. std::exception if u.size() != this->get_parent_tree().num_actuated_dofs().

◆ set_default_gear_ratio()

 void set_default_gear_ratio ( double gear_ratio )

Sets the default value for this actuator's gear ratio.

◆ set_default_rotor_inertia()

 void set_default_rotor_inertia ( double rotor_inertia )

Sets the default value for this actuator's rotor inertia.

◆ SetGearRatio()

 void SetGearRatio ( systems::Context< T > * context, const T & gear_ratio ) const

Sets the associated gear ratio value for this actuator in context.

◆ SetRotorInertia()

 void SetRotorInertia ( systems::Context< T > * context, const T & rotor_inertia ) const

Sets the associated rotor inertia value for this actuator in context.

◆ JointActuator

 friend class JointActuator
friend

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