Drake

This class is used to represent rotational inertias for unit mass bodies. More...
#include <multibody/multibody_tree/unit_inertia.h>
Public Member Functions  
UnitInertia ()  
Default UnitInertia constructor sets all entries to NaN for quick detection of uninitialized values. More...  
UnitInertia (const T &Ixx, const T &Iyy, const T &Izz)  
Creates a unit inertia with moments of inertia Ixx , Iyy , Izz , and with each product of inertia set to zero. More...  
UnitInertia (const T &Ixx, const T &Iyy, const T &Izz, const T &Ixy, const T &Ixz, const T &Iyz)  
Creates a unit inertia with moments of inertia Ixx , Iyy , Izz , and with products of inertia Ixy , Ixz , Iyz . More...  
UnitInertia (const RotationalInertia< T > &I)  
Constructs a UnitInertia from a RotationalInertia. More...  
template<typename Scalar >  
UnitInertia< Scalar >  cast () const 
Returns a new UnitInertia object templated on Scalar initialized from the value of this unit inertia. More...  
UnitInertia< T > &  SetFromRotationalInertia (const RotationalInertia< T > &I, const T &mass) 
Sets this unit inertia from a generally nonunit inertia I corresponding to a body with a given mass . More...  
UnitInertia< T > &  ReExpressInPlace (const Matrix3< T > &R_FE) 
Reexpress a unit inertia in a different frame, performing the operation in place and modifying the original object. More...  
UnitInertia< T >  ReExpress (const Matrix3< T > &R_FE) const 
Given this unit inertia G_BP_E of a body B about a point P and expressed in frame E, this method computes the same unit inertia reexpressed in another frame F as G_BP_F = R_FE * G_BP_E * (R_FE)ᵀ . More...  
UnitInertia< T > &  ShiftFromCenterOfMassInPlace (const Vector3< T > &p_BcQ_E) 
For a central unit inertia G_Bcm_E computed about a body's center of mass (or centroid) Bcm and expressed in a frame E, this method shifts this inertia using the parallel axis theorem to be computed about a point Q. More...  
UnitInertia< T >  ShiftFromCenterOfMass (const Vector3< T > &p_BcQ_E) const __attribute__((warn_unused_result)) 
Shifts this central unit inertia to a different point, and returns the result. More...  
UnitInertia< T > &  ShiftToCenterOfMassInPlace (const Vector3< T > &p_QBcm_E) 
For the unit inertia G_BQ_E of a body or composite body B computed about a point Q and expressed in a frame E, this method shifts this inertia using the parallel axis theorem to be computed about the center of mass Bcm of B. More...  
UnitInertia< T >  ShiftToCenterOfMass (const Vector3< T > &p_QBcm_E) const __attribute__((warn_unused_result)) 
For the unit inertia G_BQ_E of a body or composite body B computed about a point Q and expressed in a frame E, this method shifts this inertia using the parallel axis theorem to be computed about the center of mass Bcm of B. More...  
Implements CopyConstructible, CopyAssignable, MoveConstructible, MoveAssignable  
UnitInertia (const UnitInertia &)=default  
UnitInertia &  operator= (const UnitInertia &)=default 
UnitInertia (UnitInertia &&)=default  
UnitInertia &  operator= (UnitInertia &&)=default 
Disable operators that may result in nonunit inertias.  
UnitInertia< T > &  operator+= (const RotationalInertia< T > &)=delete 
UnitInertia< T > &  operator= (const RotationalInertia< T > &)=delete 
UnitInertia< T > &  operator*= (const T &)=delete 
UnitInertia< T > &  operator/= (const T &)=delete 
Public Member Functions inherited from RotationalInertia< T >  
RotationalInertia ()  
Constructs a rotational inertia that has all its moments/products of inertia equal to NaN (helps quickly detect uninitialized values). More...  
RotationalInertia (const T &Ixx, const T &Iyy, const T &Izz)  
Creates a rotational inertia with moments of inertia Ixx , Iyy , Izz , and with each product of inertia set to zero. More...  
RotationalInertia (const T &Ixx, const T &Iyy, const T &Izz, const T &Ixy, const T &Ixz, const T &Iyz)  
Creates a rotational inertia with moments of inertia Ixx , Iyy , Izz , and with products of inertia Ixy , Ixz , Iyz . More...  
RotationalInertia (const T &mass, const Vector3< T > &p_PQ_E)  
Constructs a rotational inertia for a particle Q of mass mass , whose position vector from aboutpoint P is p_PQ_E (E is expressedin frame). More...  
int  rows () const 
For consistency with Eigen's API, the rows() method returns 3. More...  
int  cols () const 
For consistency with Eigen's API, the cols() method returns 3. More...  
Vector3< T >  get_moments () const 
Returns 3element vector with moments of inertia [Ixx, Iyy, Izz]. More...  
Vector3< T >  get_products () const 
Returns 3element vector with products of inertia [Ixy, Ixz, Iyz]. More...  
T  Trace () const 
Returns a rotational inertia's trace (i.e., Ixx + Iyy + Izz, the sum of the diagonal elements of the inertia matrix). More...  
T  CalcMaximumPossibleMomentOfInertia () const 
Returns the maximum possible moment of inertia for this rotational inertia aboutpoint P for any expressedin frame E. More...  
const T &  operator() (int i, int j) const 
Const access to the (i, j) element of this rotational inertia. More...  
Matrix3< T >  CopyToFullMatrix3 () const 
Gets a full 3x3 matrix copy of this rotational inertia. More...  
bool  IsNearlyEqualTo (const RotationalInertia &other, double precision) const 
Compares this rotational inertia to other rotional inertia within the specified precision (which is a dimensionless number specifying the relative precision to which the comparison is performed). More...  
RotationalInertia< T > &  operator+= (const RotationalInertia< T > &I_BP_E) 
Adds a rotational inertia I_BP_E to this rotational inertia. More...  
RotationalInertia< T >  operator+ (const RotationalInertia< T > &I_BP_E) const 
Adds a rotational inertia I_BP_E to this rotational inertia. More...  
RotationalInertia< T > &  operator= (const RotationalInertia< T > &I_BP_E) 
Subtracts a rotational inertia I_BP_E from this rotational inertia. More...  
RotationalInertia< T >  operator (const RotationalInertia< T > &I_BP_E) const 
Subtracts a rotational inertia I_BP_E from this rotational inertia. More...  
RotationalInertia< T > &  operator*= (const T &nonnegative_scalar) 
Multiplies this rotational inertia by a nonnegative scalar (>= 0). More...  
RotationalInertia< T >  operator* (const T &nonnegative_scalar) const 
Multiplies this rotational inertia by a nonnegative scalar (>= 0). More...  
Vector3< T >  operator* (const Vector3< T > &w_E) const 
Multiplies this rotational inertia aboutpoint P, expressedin frame E by the vector w_E (which must also have the same expressedin frame E). More...  
RotationalInertia< T > &  operator/= (const T &positive_scalar) 
Divides this rotational inertia by a positive scalar (> 0). More...  
RotationalInertia< T >  operator/ (const T &positive_scalar) const 
Divides this rotational inertia by a positive scalar(> 0). More...  
void  SetToNaN () 
Sets this rotational inertia so all its elements are equal to NaN. More...  
void  SetZero () 
Sets this rotational inertia so all its moments/products of inertia are zero, e.g., for convenient initialization before a computation or for inertia calculations involving a particle (pointmass). More...  
bool  IsNaN () const 
Returns true if any moment/product in this rotational inertia is NaN. More...  
template<typename Scalar >  
RotationalInertia< Scalar >  cast () const 
Returns a new RotationalInertia object templated on Scalar initialized from the values of this rotational inertia's entries. More...  
Vector3< double >  CalcPrincipalMomentsOfInertia () const 
This method takes this rotational inertia aboutpoint P, expressedin frame E, and computes its principal moments of inertia aboutpoint P, but expressedin a frame aligned with the principal axes. More...  
bool  CouldBePhysicallyValid () const 
Performs several necessary checks to verify whether this rotational inertia could be physically valid, including: More...  
RotationalInertia< T > &  ReExpressInPlace (const Matrix3< T > &R_AE) 
Reexpresses this rotational inertia I_BP_E to I_BP_A . More...  
RotationalInertia< T >  ReExpress (const Matrix3< T > &R_AE) const __attribute__((warn_unused_result)) 
Reexpresses this rotational inertia I_BP_E to I_BP_A i.e., reexpresses body B's rotational inertia from frame E to frame A. More...  
RotationalInertia (const RotationalInertia &)=default  
RotationalInertia &  operator= (const RotationalInertia &)=default 
RotationalInertia (RotationalInertia &&)=default  
RotationalInertia &  operator= (RotationalInertia &&)=default 
RotationalInertia< T > &  ShiftFromCenterOfMassInPlace (const T &mass, const Vector3< T > &p_BcmQ_E) 
Shifts this rotational inertia for a body (or composite body) B from aboutpoint Bcm (B's center of mass) to aboutpoint Q. More...  
RotationalInertia< T >  ShiftFromCenterOfMass (const T &mass, const Vector3< T > &p_BcmQ_E) const __attribute__((warn_unused_result)) 
Calculates the rotational inertia that results from shifting this rotational inertia for a body (or composite body) B from aboutpoint Bcm (B's center of mass) to aboutpoint Q. More...  
RotationalInertia< T > &  ShiftToCenterOfMassInPlace (const T &mass, const Vector3< T > &p_QBcm_E) 
Shifts this rotational inertia for a body (or composite body) B from aboutpoint Q to aboutpoint Bcm (B's center of mass). More...  
RotationalInertia< T >  ShiftToCenterOfMass (const T &mass, const Vector3< T > &p_QBcm_E) const __attribute__((warn_unused_result)) 
Calculates the rotational inertia that results from shifting this rotational inertia for a body (or composite body) B from aboutpoint Q to aboutpoint Bcm (B's center of mass). More...  
RotationalInertia< T > &  ShiftToThenAwayFromCenterOfMassInPlace (const T &mass, const Vector3< T > &p_PBcm_E, const Vector3< T > &p_QBcm_E) 
Shifts this rotational inertia for a body (or composite body) B from aboutpoint P to aboutpoint Q via Bcm (B's center of mass). More...  
RotationalInertia< T >  ShiftToThenAwayFromCenterOfMass (const T &mass, const Vector3< T > &p_PBcm_E, const Vector3< T > &p_QBcm_E) const __attribute__((warn_unused_result)) 
Calculates the rotational inertia that results from shifting this rotational inertia for a body (or composite body) B from aboutpoint P to aboutpoint Q via Bcm (B's center of mass). More...  
Static Public Member Functions  
Unit inertia for common 3D objects  
The following methods assist in the construction of UnitInertia instances for common 3D objects such as boxes, spheres, rods and others. This method computes a UnitInertia for body with unit mass, typically around its centroid, and in a frame aligned with its principal axes. To construct general UnitInertia objects use these methods along with ShiftFromCenterOfMassInPlace() to move the point about which the inertia is computed and use ReExpress() to express in a different frame. A nonunit RotationalInertia is obtained by multiplying the generated UnitInertia by a nonunit mass value.  
static UnitInertia< T >  PointMass (const Vector3< T > &p_FQ) 
Construct a unit inertia for a point mass of unit mass located at point Q, whose location in a frame F is given by the position vector p_FQ (that is, p_FoQ_F). More...  
static UnitInertia< T >  SolidSphere (const T &r) 
Computes the unit inertia for a unitmass solid sphere of uniform density and radius r taken about its center. More...  
static UnitInertia< T >  HollowSphere (const T &r) 
Computes the unit inertia for a unitmass hollow sphere of radius r consisting of an infinitesimally thin shell of uniform density. More...  
static UnitInertia< T >  SolidBox (const T &Lx, const T &Ly, const T &Lz) 
Computes the unit inertia for a unitmass solid box of uniform density taken about its geometric center. More...  
static UnitInertia< T >  SolidCube (const T &L) 
Computes the unit inertia for a unitmass solid cube (a box with equalsized sides) of uniform density taken about its geometric center. More...  
static UnitInertia< T >  SolidCylinder (const T &r, const T &L, const Vector3< T > &b_E=Vector3< T >::UnitZ()) 
Computes the unit inertia for a unitmass cylinder B, of uniform density, having its axis of revolution along input vector b_E . More...  
static UnitInertia< T >  SolidCylinderAboutEnd (const T &r, const T &L) 
Computes the unit inertia for a unitmass cylinder of uniform density oriented along the zaxis computed about a point at the center of its base. More...  
static UnitInertia< T >  AxiallySymmetric (const T &J, const T &K, const Vector3< T > &b_E) 
Returns the unit inertia for a unitmass body B for which there exists a line L passing through the body's center of mass Bcm having the property that the body's moment of inertia about all lines perpendicular to L are equal. More...  
static UnitInertia< T >  StraightLine (const T &K, const Vector3< T > &b_E) 
Computes the unit inertia for a body B of unitmass uniformly distributed along a straight, finite, line L with direction b_E and with moment of inertia K about any axis perpendicular to this line. More...  
static UnitInertia< T >  ThinRod (const T &L, const Vector3< T > &b_E) 
Computes the unit inertia for a unit mass rod B of length L, about its center of mass, with its mass uniformly distributed along a line parallel to vector b_E . More...  
static UnitInertia< T >  TriaxiallySymmetric (const T &I_triaxial) 
Constructs a unit inertia with equal moments of inertia along its diagonal and with each product of inertia set to zero. More...  
Static Public Member Functions inherited from RotationalInertia< T >  
static RotationalInertia< T >  TriaxiallySymmetric (const T &I_triaxial) 
Constructs a rotational inertia with equal moments of inertia along its diagonal and with each product of inertia set to zero. More...  
Additional Inherited Members  
Protected Member Functions inherited from RotationalInertia< T >  
RotationalInertia< T > &  MinusEqualsUnchecked (const RotationalInertia< T > &I_BP_E) 
Subtracts a rotational inertia I_BP_E from this rotational inertia. More...  
Related Functions inherited from RotationalInertia< T >  
template<typename T >  
std::ostream &  operator<< (std::ostream &o, const RotationalInertia< T > &I) 
Insertion operator to write RotationalInertia's into a std::ostream . More...  
This class is used to represent rotational inertias for unit mass bodies.
Therefore, unlike RotationalInertia whose units are kg⋅m², the units of a UnitInertia are those of length squared. A unit inertia is a useful concept to represent the geometric distribution of mass in a body regardless of the actual value of the body mass. The rotational inertia of a body can therefore be obtained by multiplying its unit inertia by its mass. Unit inertia matrices can also be called gyration matrices and therefore we choose to represent them in source code notation with the capital letter G. In contrast, the capital letter I is used to represent nonunit mass rotational inertias. This class restricts the set of allowed operations on a unit inertia to ensure the unitmass invariant. For instance, multiplication by a scalar can only return a general RotationalInertia but not a UnitInertia.
T  The underlying scalar type. Must be a valid Eigen scalar. 
Instantiated templates for the following kinds of T's are provided:
They are already available to link against in the containing library.

default 

default 

inline 
Default UnitInertia constructor sets all entries to NaN for quick detection of uninitialized values.

inline 
Creates a unit inertia with moments of inertia Ixx
, Iyy
, Izz
, and with each product of inertia set to zero.
In debug builds, throws std::logic_error if unit inertia constructed from these arguments violates RotationalInertia::CouldBePhysicallyValid().

inline 
Creates a unit inertia with moments of inertia Ixx
, Iyy
, Izz
, and with products of inertia Ixy
, Ixz
, Iyz
.
In debug builds, throws std::logic_error if unit inertia constructed from these arguments violates RotationalInertia::CouldBePhysicallyValid().

inlineexplicit 
Constructs a UnitInertia from a RotationalInertia.
This constructor has no way to verify that the input rotational inertia actually is a unit inertia. But the construction will nevertheless succeed, and the values of the input rotational inertia will henceforth be considered a valid unit inertia. It is the responsibility of the user to pass a valid unit inertia.

inlinestatic 
Returns the unit inertia for a unitmass body B for which there exists a line L passing through the body's center of mass Bcm
having the property that the body's moment of inertia about all lines perpendicular to L are equal.
Examples of bodies with an axially symmetric inertia include axisymmetric objects such as cylinders and cones. Other commonly occurring geometries with this property are, for instance, propellers with 3+ evenly spaced blades. Given a unit vector b defining the symmetry line L, the moment of inertia J about this line L and the moment of inertia K about any line perpendicular to L, the axially symmetric unit inertia G is computed as:
G = K * Id + (J  K) * b ⊗ b
where Id
is the identity matrix and ⊗ denotes the tensor product operator. See Mitiguy, P., 2016. Advanced Dynamics & Motion Simulation.
std::runtime_error 

[in]  J  Unit inertia about axis b. 
[in]  K  Unit inertia about any axis perpendicular to b. 
[in]  b_E  Vector defining the symmetry axis, expressed in a frame E. b_E can have a norm different from one; however, it will be normalized before using it. Therefore its norm is ignored and only its direction is used. 
G_Bcm_E  An axially symmetric unit inertia about body B's center of mass, expressed in the same frame E as the input unit vector b_E . 

inline 
Returns a new UnitInertia object templated on Scalar
initialized from the value of this
unit inertia.
Scalar  The scalar type on which the new unit inertia will be templated. 
UnitInertia<From>::cast<To>()
creates a new UnitInertia<To>
from a UnitInertia<From>
but only if type To
is constructible from type From
. As an example of this, UnitInertia<double>::cast<AutoDiffXd>()
is valid since AutoDiffXd a(1.0)
is valid. However, UnitInertia<AutoDiffXd>::cast<double>()
is not.

inlinestatic 
Computes the unit inertia for a unitmass hollow sphere of radius r
consisting of an infinitesimally thin shell of uniform density.
The unit inertia is taken about the center of the sphere.

delete 

delete 

delete 

delete 

default 

default 

inlinestatic 
Construct a unit inertia for a point mass of unit mass located at point Q, whose location in a frame F is given by the position vector p_FQ
(that is, p_FoQ_F).
The unit inertia G_QFo_F
of point mass Q about the origin Fo
of frame F and expressed in F for this unit mass point equals the square of the cross product matrix of p_FQ
. In coordinatefree form:
\[ G^{Q/F_o} = (^Fp^Q_\times)^2 = (^Fp^Q_\times)^T \, ^Fp^Q_\times = ^Fp^Q_\times \, ^Fp^Q_\times \]
where \( ^Fp^Q_\times \) is the cross product matrix of vector \( ^Fp^Q \). In source code the above expression is written as:
G_QFo_F = px_FQ² = px_FQᵀ * px_FQ = px_FQ * px_FQ
where px_FQ
denotes the cross product matrix of the position vector p_FQ
(expressed in F) such that the cross product with another vector a
can be obtained as px.cross(a) = px * a
. The cross product matrix px
is skewsymmetric. The square of the cross product matrix is a symmetric matrix with nonnegative diagonals and obeys the triangle inequality. Matrix px²
can be used to compute the triple vector product as p x (p x a) = p.cross(p.cross(a)) = px² * a
.

inline 
Given this
unit inertia G_BP_E
of a body B about a point P and expressed in frame E, this method computes the same unit inertia reexpressed in another frame F as G_BP_F = R_FE * G_BP_E * (R_FE)ᵀ
.
[in]  R_FE  Rotation matrix from the basis of frame E to the basis of frame F. 
G_BP_F  The same unit inertia for body B about point P but now reexpressed in frameF. 
R_FE
represents a valid rotation or not. It is the responsibility of users to provide valid rotation matrices.

inline 
Reexpress a unit inertia in a different frame, performing the operation in place and modifying the original object.

inline 
Sets this
unit inertia from a generally nonunit inertia I corresponding to a body with a given mass
.
mass
is not strictly positive.

inline 
Shifts this central unit inertia to a different point, and returns the result.
See ShiftFromCenterOfMassInPlace() for details.
[in]  p_BcmQ_E  A vector from the body's centroid Bcm to point Q expressed in the same frame E in which this inertia is expressed. 
G_BQ_E  This same unit inertia taken about a point Q instead of the centroid Bcm . 

inline 
For a central unit inertia G_Bcm_E
computed about a body's center of mass (or centroid) Bcm
and expressed in a frame E, this method shifts this inertia using the parallel axis theorem to be computed about a point Q.
This operation is performed in place, modifying the original object which is no longer a central inertia.
[in]  p_BcmQ_E  A vector from the body's centroid Bcm to point Q expressed in the same frame E in which this inertia is expressed. 
this
unit inertia, which has now been taken about point Q so can be written as G_BQ_E
.

inline 
For the unit inertia G_BQ_E
of a body or composite body B computed about a point Q and expressed in a frame E, this method shifts this inertia using the parallel axis theorem to be computed about the center of mass Bcm
of B.
See ShiftToCenterOfMassInPlace() for details.
[in]  p_QBcm_E  A position vector from the about point Q to the body's centroid Bcm expressed in the same frame E in which this inertia is expressed. 
G_Bcm_E  This same unit which has now been taken about point Bcm so that it can be written as G_BBcm_E , or G_Bcm_E . 

inline 
For the unit inertia G_BQ_E
of a body or composite body B computed about a point Q and expressed in a frame E, this method shifts this inertia using the parallel axis theorem to be computed about the center of mass Bcm
of B.
This operation is performed in place, modifying the original object.
[in]  p_QBcm_E  A position vector from the about point Q to the body's centroid Bcm expressed in the same frame E in which this inertia is expressed. 
this
unit inertia, which has now been taken about point Bcm
so can be written as G_BBcm_E
, or G_Bcm_E
.Bcm
about point Q as: G_Bcm_E = G_BQ_E  G_BcmQ_E = G_BQ_E  px_QBcm_E² Therefore the resulting inertia could have negative moments of inertia if the unit inertia of the unit mass at point Bcm
is larger than G_BQ_E
. Use with care.

inlinestatic 
Computes the unit inertia for a unitmass solid box of uniform density taken about its geometric center.
If one length is zero the inertia corresponds to that of a thin rectangular sheet. If two lengths are zero the inertia corresponds to that of a thin rod in the remaining direction.
[in]  Lx  The length of the box edge in the principal xaxis. 
[in]  Ly  The length of the box edge in the principal yaxis. 
[in]  Lz  The length of the box edge in the principal zaxis. 

inlinestatic 
Computes the unit inertia for a unitmass solid cube (a box with equalsized sides) of uniform density taken about its geometric center.
[in]  L  The length of each of the cube's sides. 

inlinestatic 
Computes the unit inertia for a unitmass cylinder B, of uniform density, having its axis of revolution along input vector b_E
.
The resulting unit inertia is computed about the cylinder's center of mass Bcm
and is expressed in the same frame E as the input axis of revolution b_E
.
[in]  r  The radius of the cylinder, it must be nonnegative. 
[in]  L  The length of the cylinder, it must be nonnegative. 
[in]  b_E  Vector defining the axis of revolution of the cylinder, expressed in a frame E. b_E can have a norm different from one; however, it will be normalized before using it. Therefore its norm is ignored and only its direction is used. It defaults to Vector3<T>::UnitZ() . 
G_Bcm_E  The unit inertia for a solid cylinder B, of uniform density, with axis of revolution along b_E , computed about the cylinder's center of mass Bcm , and expressed in the same frame E as the input axis of rotation b_E . 
std::runtime_error 


inlinestatic 
Computes the unit inertia for a unitmass cylinder of uniform density oriented along the zaxis computed about a point at the center of its base.
[in]  r  The radius of the cylinder. 
[in]  L  The length of the cylinder. 

inlinestatic 
Computes the unit inertia for a unitmass solid sphere of uniform density and radius r
taken about its center.

inlinestatic 
Computes the unit inertia for a body B of unitmass uniformly distributed along a straight, finite, line L with direction b_E
and with moment of inertia K about any axis perpendicular to this line.
Since the mass of the body is uniformly distributed on this line L, its center of mass is located right at the center. As an example, consider the inertia of a thin rod for which its transversal dimensions can be neglected, see ThinRod().
This method aborts if K is not positive.
[in]  K  Unit inertia about any axis perpendicular to the line. 
[in]  b_E  Vector defining the direction of the line, expressed in a frame E. b_E can have a norm different from one. Its norm is ignored and only its direction is needed. 
G_Bcm_E  The unit inertia for a body B of unit mass uniformly distributed along a straight line L, about its center of mass Bcm which is located at the center of the line, expressed in the same frame E as the input unit vector b_E . 

inlinestatic 
Computes the unit inertia for a unit mass rod B of length L, about its center of mass, with its mass uniformly distributed along a line parallel to vector b_E
.
This method aborts if L is not positive.
[in]  L  The length of the rod. It must be positive. 
[in]  b_E  Vector defining the axis of the rod, expressed in a frame E. b_E can have a norm different from one. Its norm is ignored and only its direction is needed. 
G_Bcm_E  The unit inertia of the rod B about its center of mass Bcm , expressed in the same frame E as the input unit vector b_E . 

inlinestatic 
Constructs a unit inertia with equal moments of inertia along its diagonal and with each product of inertia set to zero.
This factory is useful for the unit inertia of a uniformdensity sphere or cube. In debug builds, throws std::logic_error if I_triaxial is negative/NaN.