SpatialVelocity< T > Class Template Reference

Detailed Description

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

This class represents a spatial velocity V (also called a twist) and has 6 elements with an angular (rotational) velocity ω (3-element vector) on top of a translational (linear) velocity v (3-element vector).

Spatial velocity represents the rotational and translational motion of a frame B with respect to a measured-in frame M. This class assumes that both the angular velocity ω and translational velocity v are expressed in the same expressed-in frame E. This class only stores 6 elements (namely ω and v) and does not store the underlying frames B, M, E. The user is responsible for explicitly tracking the underlying frames with monogram notation. For example, V_MB_E denotes frame B's spatial velocity measured in frame M, expressed in frame E and contains ω_MB_E (B's angular velocity measured in M, expressed in E) and v_MBo_E (Bo's translational velocity measured in M, expressed in E), where Bo is frame B's origin point. For an offset frame Bp, the monogram notation V_MBp_E denotes the spatial velocity of frame Bp measured in M, expressed in E. Details on spatial vectors and monogram notation are in sections Spatial Vectors and Multibody Quantities.

Template Parameters

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

#include <drake/multibody/math/spatial_velocity.h>

Public Member Functions
	SpatialVelocity ()
	Default constructor.
	SpatialVelocity (const Eigen::Ref< const Vector3< T > > &w, const Eigen::Ref< const Vector3< T > > &v)
	Constructs a spatial velocity V from an angular velocity ω and a translational velocity v.
template<typename Derived>
	SpatialVelocity (const Eigen::MatrixBase< Derived > &V)
	Constructs a spatial velocity V from an Eigen expression that represents a 6-element vector, i.e., two 3-element vectors, namely an angular velocity ω and a translational velocity v.
void	ShiftInPlace (const Vector3< T > &offset)
	In-place shift of a SpatialVelocity from a frame B to a frame C, where both B and C are fixed to the same frame or rigid body.
SpatialVelocity< T >	Shift (const Vector3< T > &offset) const
	Shifts a SpatialVelocity from a frame B to a frame C, where both B and C are fixed to the same frame or rigid body.
SpatialVelocity< T >	ComposeWithMovingFrameVelocity (const Vector3< T > &position_of_moving_frame, const SpatialVelocity< T > &velocity_of_moving_frame) const
	Compose this spatial velocity (measured in some frame M) with the spatial velocity of another frame to form the 𝐨𝐭𝐡𝐞𝐫 frame's spatial velocity measured in frame M.
T	dot (const SpatialForce< T > &force) const
	Calculates the power generated by a spatial force.
T	dot (const SpatialMomentum< T > &momentum) const
	Calculates twice (2x) a body B's kinetic energy measured in a frame M.
Implements CopyConstructible, CopyAssignable, MoveConstructible, MoveAssignable
	SpatialVelocity (const SpatialVelocity &)=default
SpatialVelocity &	operator= (const SpatialVelocity &)=default
	SpatialVelocity (SpatialVelocity &&)=default
SpatialVelocity &	operator= (SpatialVelocity &&)=default
Public Member Functions inherited from SpatialVector< SpatialVelocity, T >
int	size () const
	For 3D (three-dimensional) analysis, the total size of the concatenated rotational vector (3 elements) and translational vector (3 elements) is six (6), which is known at compile time.
const T &	operator[] (int i) const
	Const access to the i-th element of this spatial vector.
const Vector3< T > &	rotational () const
	Const access to the rotational component of this spatial vector.
const Vector3< T > &	translational () const
	Const access to the translational component of this spatial vector.
const T *	data () const
	Returns a (const) bare pointer to the underlying data.
T *	mutable_data ()
	Returns a (mutable) bare pointer to the underlying data.
std::tuple< const T, const T >	GetMaximumAbsoluteDifferences (const SpatialQuantity &other) const
	Returns the maximum absolute values of the differences in the rotational and translational components of this and other (i.e., the infinity norms of the difference in rotational and translational components).
decltype(T()< T())	IsNearlyEqualWithinAbsoluteTolerance (const SpatialQuantity &other, double rotational_tolerance, double translational_tolerance) const
	Compares the rotational and translational parts of this and other to check if they are the same to within specified absolute differences.
decltype(T()< T())	IsApprox (const SpatialQuantity &other, double tolerance=2 *std::numeric_limits< double >::epsilon()) const
	Determines whether all six corresponding elements of two spatial vectors are equal to each other to within a specified tolerance epsilon.
void	SetNaN ()
	Sets all the elements in this SpatialVector to NaN.
SpatialQuantity &	SetZero ()
	Sets both the rotational and translational components of this SpatialVector to zero.
CoeffsEigenType &	get_coeffs ()
	Returns a mutable reference to the underlying storage.
SpatialQuantity	operator- () const
	Unary minus operator.
SpatialQuantity &	operator+= (const SpatialQuantity &V)
	Addition assignment operator.
SpatialQuantity &	operator-= (const SpatialQuantity &V)
	Subtraction assignment operator.
SpatialQuantity &	operator*= (const T &s)
	Multiplication assignment operator.
	SpatialVector (const SpatialVector &)=default
SpatialVector &	operator= (const SpatialVector &)=default

Related Symbols
(Note that these are not member symbols.)
template<typename T>
SpatialVelocity< T >	operator+ (const SpatialVelocity< T > &V1_E, const SpatialVelocity< T > &V2_E)
	Adds two spatial velocities by simply adding their 6 underlying elements.
template<typename T>
SpatialVelocity< T >	operator- (const SpatialVelocity< T > &V1_E, const SpatialVelocity< T > &V2_E)
	Subtracts spatial velocities by simply subtracting their 6 underlying elements.
Related Symbols inherited from SpatialVector< SpatialVelocity, T >
std::ostream &	operator<< (std::ostream &o, const SpatialVector< SpatialQuantity, T > &V)
	Stream insertion operator to write SpatialVector objects into a std::ostream.

Additional Inherited Members
Public Types inherited from SpatialVector< SpatialVelocity, T >
enum
	Sizes for spatial quantities and its components in 3D (three dimensions). More...
using	SpatialQuantity
	The more specialized spatial vector class templated on the scalar type T.
using	CoeffsEigenType
	The type of the underlying in-memory representation using an Eigen vector.
Static Public Member Functions inherited from SpatialVector< SpatialVelocity, T >
static SpatialQuantity	Zero ()
	Factory to create a zero spatial vector, i.e., a SpatialVector whose rotational and translational components are both zero.

Constructor & Destructor Documentation

SpatialVelocity() [1/5]

template<typename T>

SpatialVelocity ( const SpatialVelocity< T > & )

default

SpatialVelocity() [2/5]

template<typename T>

SpatialVelocity ( SpatialVelocity< T > && )

default

SpatialVelocity() [3/5]

template<typename T>

SpatialVelocity

(

)

Default constructor.

In Release builds, all 6 elements of a newly constructed spatial velocity are uninitialized (for speed). In Debug builds, the 6 elements are set to NaN so that invalid operations on an uninitialized spatial velocity fail fast (fast bug detection).

SpatialVelocity() [4/5]

template<typename T>

SpatialVelocity	(	const Eigen::Ref< const Vector3< T > > &	w,
		const Eigen::Ref< const Vector3< T > > &	v )

Constructs a spatial velocity V from an angular velocity ω and a translational velocity v.

SpatialVelocity() [5/5]

template<typename T>

template<typename Derived>

SpatialVelocity ( const Eigen::MatrixBase< Derived > & V )

explicit

Constructs a spatial velocity V from an Eigen expression that represents a 6-element vector, i.e., two 3-element vectors, namely an angular velocity ω and a translational velocity v.

This constructor will assert the size of V is six (6) either at compile-time for fixed sized Eigen expressions or at run-time for dynamic sized Eigen expressions.

Member Function Documentation

ComposeWithMovingFrameVelocity()

template<typename T>

SpatialVelocity< T > ComposeWithMovingFrameVelocity	(	const Vector3< T > &	position_of_moving_frame,
		const SpatialVelocity< T > &	velocity_of_moving_frame ) const

Compose this spatial velocity (measured in some frame M) with the spatial velocity of another frame to form the 𝐨𝐭𝐡𝐞𝐫 frame's spatial velocity measured in frame M.

Herein, this is the spatial velocity of a frame (designated B) in frame M and the 𝐨𝐭𝐡𝐞𝐫 frame is designated C.

Parameters

[in]	position_of_moving_frame	which is the position vector p_BoCo_E (from frame B's origin Bo to frame C's origin Co), expressed in a frame E. p_BoCo_E must have the same expressed-in frame E as this, where this is V_MB_E (frame B's spatial velocity measured in M, expressed in E).
[in]	velocity_of_moving_frame	which is V_BC_E, frame C's spatial velocity measured in frame B, expressed in frame E.

Return values

V_MC_E

frame C's spatial velocity measured in frame M, expressed in frame E.

Note

The returned spatial velocity V_MC_E contains an angular velocity ω_MC_E and translational velocity v_MCo_E that are calculated as:

 ω_MC_E  = ω_MB_E + ω_BC_E
 v_MCo_E = v_MBo_E + ω_MB_E x p_BoCo_E + v_BCo_E

If frame C is rigidly fixed to frame B, V_BC_E = 0 and this method produces a Shift() operation (albeit inefficiently). In other words, use Shift() if velocity_of_moving_frame = 0.

See also

SpatialAcceleration::ComposeWithMovingFrameAcceleration().

dot() [1/2]

template<typename T>

T dot

(

const SpatialForce< T > &

force

)

const

Calculates the power generated by a spatial force.

For an arbitrary frame B, calculates the dot-product of this = V_MB_E (frame B's spatial velocity measured in a frame M, expressed in a frame E) with F_B_E (frame B's spatial force, expressed in frame E).

Parameters

[in]

force

which is F_B_E, frame B's spatial force, expressed in the same frame E as this = V_MB_E.

Returns

Power of spatial force F_B_E in frame M, i.e., F_B_E ⋅ V_MB_E.

Note

Just as equating force 𝐅 to mass * acceleration as 𝐅 = m𝐚 relies on acceleration 𝐚 being measured in a world frame (also called an inertial or Newtonian frame), equating power = dK/dt (where K is kinetic energy) relies on K being measured in a world frame. Hence, it is unusual to use this method unless frame M is the world frame W.

Although the spatial vectors F_B_E and V_MB_E must have the same expressed-in frame E, the returned scalar is independent of frame E.

dot() [2/2]

template<typename T>

T dot

(

const SpatialMomentum< T > &

momentum

)

const

Calculates twice (2x) a body B's kinetic energy measured in a frame M.

For any frame (e.g., an offset frame) Bp that is fixed to a rigid body B, calculates the dot-product of this = V_MBp_E (frame Bp's spatial velocity measured in frame M, expressed in frame E) with L_MBp_E (body B's spatial momentum measured in frame M, about Bp's origin, expressed in frame E).

Parameters

[in]

momentum

which is L_MBp_E, body B's spatial momentum measured in frame M, about frame Bp's origin, expressed in the same frame E as this = V_MBp_E.

Returns

2*K_MB, twice (2x) body B's kinetic energy measured in frame M.

Note

In general, kinetic energy calculations are only useful when frame M is a world frame (also called a Newtonian or inertial frame). Hence, it is unusual to use this method unless frame M is the world frame W.

Although the spatial vectors V_MBp_E and L_MBp_E must have the same expressed-in frame E, the resulting scalar K_MB is independent of frame E.

As shown below, K_MB can be calculated from any frame Bp fixed on B, including body B's center of mass frame Bcm. This is due to how spatial momentum and spatial velocity shift from Bcm to Bp. For more information, see SpatialMomentum::Shift() and SpatialVelocity::Shift().

  K_MB = 1/2 (L_MBp · V_MBp) = 1/2 (L_MBcm · V_MBcm)

operator=() [1/2]

template<typename T>

SpatialVelocity & operator= ( const SpatialVelocity< T > & )

default

operator=() [2/2]

template<typename T>

SpatialVelocity & operator= ( SpatialVelocity< T > && )

default

Shift()

template<typename T>

SpatialVelocity< T > Shift

(

const Vector3< T > &

offset

)

const

Shifts a SpatialVelocity from a frame B to a frame C, where both B and C are fixed to the same frame or rigid body.

Parameters

[in]

offset

which is the position vector p_BoCo_E from frame B's origin to frame C's origin, expressed in frame E. p_BoCo_E must have the same expressed-in frame E as this spatial velocity (this = V_MB_E).

Return values

V_MC_E

which is frame C's spatial velocity measured in frame M, expressed in frame E.

Note

Shift() differs from ShiftInPlace() in that Shift() does not modify this whereas ShiftInPlace() does modify this.

See also

ShiftInPlace() for more information and how V_MC_E is calculated.

ShiftInPlace()

template<typename T>

void ShiftInPlace

(

const Vector3< T > &

offset

)

In-place shift of a SpatialVelocity from a frame B to a frame C, where both B and C are fixed to the same frame or rigid body.

On entry, this is V_MB_E (frame B's spatial velocity measured in a frame M and expressed in a frame E). On return this is modified to V_MC_E (frame C's spatial velocity measured in frame M and expressed in frame E). The components of V_MC_E are calculated as:

 ω_MC_E = ω_MB_E                (angular velocity of `this` is unchanged).
 v_MC_E = v_MB_E + ω_MB_E x p_BoCo_E     (translational velocity changes).

Parameters

[in]

offset

which is the position vector p_BoCo_E from frame B's origin to frame C's origin, expressed in frame E. p_BoCo_E must have the same expressed-in frame E as this spatial velocity.

See also

Shift() to shift spatial velocity without modifying this.

Friends And Related Symbol Documentation

operator+()

template<typename T>

SpatialVelocity< T > operator+ ( const SpatialVelocity< T > & V1_E, const SpatialVelocity< T > & V2_E )

related

Adds two spatial velocities by simply adding their 6 underlying elements.

Parameters

[in]	V1_E	spatial velocity expressed in the same frame E as V2_E.
[in]	V2_E	spatial velocity expressed in the same frame E as V1_E.

Note

The general utility of this operator+() function is questionable and it should only be used if you are sure it makes sense. One use case is for calculating the spatial velocity V_MC of a frame C measured in a frame M when frame C is moving on a frame B and one has pre-calculated V_MBc (frame Bc's spatial velocity measured in frame M, where frame Bc is instantaneously coincident with frame C). For this use case, the operator+ function returns V_MC_E = V_MBc_E + V_BC_E, where the precalculated V_MBc_E is equal to V_MBo_E.Shift(p_BoCo_E).

See also

Shift(), ShiftInPlace(), and ComposeWithMovingFrameVelocity().

operator-()

template<typename T>

SpatialVelocity< T > operator- ( const SpatialVelocity< T > & V1_E, const SpatialVelocity< T > & V2_E )

related

Subtracts spatial velocities by simply subtracting their 6 underlying elements.

Parameters

[in]	V1_E	spatial velocity expressed in the same frame E as V2_E.
[in]	V2_E	spatial velocity expressed in the same frame E as V1_E.

Note

The general utility of this operator-() function is questionable and it should only be used if you are sure it makes sense. One use case is for calculating relative spatial velocity, e.g., a frame C's spatial velocity relative to a frame B, measure in a frame M. This use case calculates V_M_BC_E = V_MC_E - V_MB_E, which contains ω_BC (C's angular velocity measured in B) and v_M_BoCo (Co's velocity relative to Bo, measured in M), where both ω_BC and v_M_BoCo are expressed in frame E.

 ω_BC  = ω_MC - ω_MB
 v_M_BoCo = v_MCo - v_MBo = DtM(p_BoCo)

where DtM(p_BoCo) is the time-derivative in frame M of p_BoCo (position vector from Bo to Co). A second use case has to do with a frame C that is moving on a frame B and calculates frame C's spatial velocity measured in frame B. It assumes you have pre-calculated V_MBc (frame Bc's spatial velocity measured in frame M, where frame Bc is fixed to B and instantaneously coincident with frame C. This use case returns V_BC_E = V_MC_E - V_MBc_E, where the precalculated V_MBc_E is equal to V_MBo_E.Shift(p_BoBc_E).

See also

Shift(), ShiftInPlace(), and ComposeWithMovingFrameVelocity().

---

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

• drake/multibody/math/spatial_force.h
• drake/multibody/math/spatial_velocity.h