DistanceConstraintParams Class Reference

Detailed Description

Parameters for a distance constraint.

A distance constraint is modeled as a holonomic constraint. Distance constraints can be "soft", and are implemented as a spring force, f: f = -k⋅(d(q) - d₀) - c⋅ḋ(q), where d₀ is a fixed length, k a stiffness parameter in N/m and c a damping parameter in N⋅s/m. We use d(q) to denote the Euclidean distance between two points P and Q, rigidly affixed to bodies A and B respectively, as a function of the configuration of the model q. This constraint reduces to d(q) = d₀ in the limit to infinite stiffness and it behaves as a linear spring damper for finite values of stiffness and damping.

Warning

A distance constraint is the wrong modeling choice if the distance needs to go through zero. To constrain two points to be coincident we need a 3-dof ball constraint, the 1-dof distance constraint is singular in this case. Therefore we require the distance parameter to be strictly positive.

#include <drake/multibody/plant/distance_constraint_params.h>

Public Member Functions
	DistanceConstraintParams ()=default
	Construction to an invalid set of parameters only meant to facilitate use with STL containers. More...
	DistanceConstraintParams (BodyIndex bodyA, const Vector3< double > &p_AP, BodyIndex bodyB, const Vector3< double > &p_BQ, double distance, double stiffness, double damping)
	Constructor for a set of valid parameters. More...
BodyIndex	bodyA () const
	Index of body A. More...
BodyIndex	bodyB () const
	Index of body B. More...
const Vector3< double > &	p_AP () const
	Position of point P in body A's frame. More...
const Vector3< double > &	p_BQ () const
	Position of point Q in body B's frame. More...
double	distance () const
	The fixed length of the constraint. More...
double	stiffness () const
	Constraint stiffness. More...
double	damping () const
	Constraint damping. More...
Implements CopyConstructible, CopyAssignable, MoveConstructible, MoveAssignable
	DistanceConstraintParams (const DistanceConstraintParams &)=default
DistanceConstraintParams &	operator= (const DistanceConstraintParams &)=default
	DistanceConstraintParams (DistanceConstraintParams &&)=default
DistanceConstraintParams &	operator= (DistanceConstraintParams &&)=default

Constructor & Destructor Documentation

DistanceConstraintParams() [1/4]

DistanceConstraintParams ( const DistanceConstraintParams &  )

default

DistanceConstraintParams() [2/4]

DistanceConstraintParams ( DistanceConstraintParams &&  )

default

DistanceConstraintParams() [3/4]

DistanceConstraintParams ( )

default

Construction to an invalid set of parameters only meant to facilitate use with STL containers.

DistanceConstraintParams() [4/4]

DistanceConstraintParams	(	BodyIndex	bodyA,
		const Vector3< double > &	p_AP,
		BodyIndex	bodyB,
		const Vector3< double > &	p_BQ,
		double	distance,
		double	stiffness,
		double	damping
	)

Constructor for a set of valid parameters.

Parameters

[in]	bodyA	Index of body A in the MultibodyPlant.
[in]	bodyB	Index of body B in the MultibodyPlant.
[in]	p_AP	Position of a point P in A's body frame.
[in]	p_BQ	Position of a point Q in Q's body frame.
[in]	distance	The fixed length of this constraint.
[in]	stiffness	If finite, this constraint models a massless elastic rod of length distance with the given stiffness and damping. "Infinite" is a valid value if a hard constraint is intended. See the class documentation for details.
[in]	damping	When stiffness is finite, this value parametrizes the damping model of the elastic massless rod. See the class documentation for details.

Warning

This class does not have a way to check whether body indices do correspond to valid indices in a MultibodyPlant. Use MultibodyPlant APIs to safely register distance constraints between valid bodies and to later retrieve their valid set of parameters.

Exceptions

std::exception	if BodyIndex::is_valid() is false for either body.
std::exception	if bodies A and B are the same body.
std::exception	if distance is not strictly positive.
std::exception	if stiffness is not strictly positive.
std::exception	if damping is not positive or zero.

Member Function Documentation

bodyA()

BodyIndex bodyA

(

)

const

Index of body A.

bodyB()

BodyIndex bodyB

(

)

const

Index of body B.

damping()

double damping

(

)

const

Constraint damping.

See class documentation.

distance()

double distance

(

)

const

The fixed length of the constraint.

operator=() [1/2]

DistanceConstraintParams& operator= ( DistanceConstraintParams &&  )

default

operator=() [2/2]

DistanceConstraintParams& operator= ( const DistanceConstraintParams &  )

default

p_AP()

const Vector3<double>& p_AP

(

)

const

Position of point P in body A's frame.

p_BQ()

const Vector3<double>& p_BQ

(

)

const

Position of point Q in body B's frame.

stiffness()

double stiffness

(

)

const

Constraint stiffness.

See class documentation.

---

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

• drake/multibody/plant/distance_constraint_params.h