Simbody
3.6

This constraint represents a bilateral connection between a sphere on one body and a sphere on another. More...
Public Member Functions  
Construction  
Methods in this section refer both to constructors, and to methods that can be used to set or change construction (Topologystage) parameters; these specify the values assigned by default to the corresponding state variables. Note:
The default parameters can be overridden in any given State, and modified without affecting Topology; see the "Runtime Changes" section below.  
SphereOnSphereContact (MobilizedBody &mobod_F, const Vec3 &defaultCenterOnF, Real defaultRadiusOnF, MobilizedBody &mobod_B, const Vec3 &defaultCenterOnB, Real defaultRadiusOnB, bool enforceRolling)  
Construct a sphereonsphere constraint as described in the Constraint::SphereOnSphereContact class documentation. More...  
SphereOnSphereContact ()  
Default constructor creates an empty handle that can be used to reference any SphereOnSphereContact Constraint. More...  
const MobilizedBody &  getMobilizedBodyF () const 
Return a reference to the first MobilizedBody to which a sphere is attached. More...  
const MobilizedBody &  getMobilizedBodyB () const 
Return a reference to the second MobilizedBody to which a sphere is attached. More...  
bool  isEnforcingRolling () const 
Report whether this Constraint was constructed to generate rolling constraints (otherwise it is frictionless). More...  
SphereOnSphereContact &  setDefaultCenterOnF (const Vec3 &defaultCenter) 
Replace the default center point for the sphere attached to the first body, mobod_F, that was supplied on construction. More...  
SphereOnSphereContact &  setDefaultRadiusOnF (Real defaultRadius) 
Replace the default radius for the sphere attached to the first body, mobod_F, that was supplied on construction. More...  
SphereOnSphereContact &  setDefaultCenterOnB (const Vec3 &defaultCenter) 
Replace the default center point for the sphere attached to the second body, mobod_B, that was supplied on construction. More...  
SphereOnSphereContact &  setDefaultRadiusOnB (Real defaultRadius) 
Replace the default radius for the sphere attached to the second body, mobod_B, that was supplied on construction. More...  
const Vec3 &  getDefaultCenterOnF () const 
Return the default center point for the sphere attached to the first body, mobod_F, as set during construction or by the most recent call to setDefaultCenterOnF(). More...  
Real  getDefaultRadiusOnF () const 
Return the default radius for the sphere attached to the first body, mobod_F, as set during construction or by the most recent call to setDefaultRadiusF(). More...  
const Vec3 &  getDefaultCenterOnB () const 
Return the default center point for the sphere attached to the second body, mobod_B, as set during construction or by the most recent call to setDefaultCenterOnB(). More...  
Real  getDefaultRadiusOnB () const 
Return the default radius for the sphere attached to the second body, mobod_B, as set during construction or by the most recent call to setDefaultRadiusB(). More...  
Runtime Changes  
These refer to Positionstage discrete state variables that determine the sphere parameters to be used to calculate constraint forces from a given State object. If these are not set explicitly, the parameters are set to those provided in the constructor or via the corresponding setDefault...() methods. Note:
You can also modify and examine the default parameters; see the "Construction" section above.  
const SphereOnSphereContact &  setCenterOnF (State &state, const Vec3 &sphereCenter) const 
Modify the location of the sphere on the first body, mobod_F, in this state by providing a new vector p_FSf giving the sphere center location Sf relative to body F's body frame origin Fo, and expressed in the F frame. More...  
const SphereOnSphereContact &  setRadiusOnF (State &state, Real sphereRadius) const 
Modify the radius of the sphere on the first body, mobod_F, in this state. More...  
const SphereOnSphereContact &  setCenterOnB (State &state, const Vec3 &sphereCenter) const 
Modify the location of the sphere on the second body, mobod_B, in this state by providing a new vector p_BSb giving the sphere center location Sb relative to body B's body frame origin Bo, and expressed in the B frame. More...  
const SphereOnSphereContact &  setRadiusOnB (State &state, Real sphereRadius) const 
Modify the radius of the sphere on the second body, mobod_B, in this state. More...  
const Vec3 &  getCenterOnF (const State &state) const 
Return the position of the center point of the sphere attached to the first body, mobod_F, as currently set in the given state. More...  
Real  getRadiusOnF (const State &state) const 
Return the radius of the sphere attached to the first body, mobod_F, as currently set in the given state. More...  
const Vec3 &  getCenterOnB (const State &state) const 
Return the position of the center point of the sphere attached to the second body, mobod_B, as currently set in the given state. More...  
Real  getRadiusOnB (const State &state) const 
Return the radius of the sphere attached to the first body, mobod_F, as currently set in the given state. More...  
Computations  
Methods here provide access to values already calculated by this constraint element, and provide operators you can call to calculate related values.  
Real  getPositionError (const State &state) const 
The returned position error can be viewed as the signed distance between the spheres. More...  
Vec3  getVelocityErrors (const State &state) const 
The returned velocity error vector has the time derivative of the quantity returned by getPositionError() in its z coordinate, and violation of the rolling constraints in its x and y coordinates. More...  
Vec3  getAccelerationErrors (const State &state) const 
This vector is the time derivative of the value returned by getVelocityError(). More...  
Vec3  getMultipliers (const State &state) const 
These are the Lagrange multipliers required to enforce the constraint equations generated here. More...  
Vec3  findForceOnSphereBInG (const State &state) const 
Return the force vector currently being applied by this constraint to the contact point Co on the sphere attached to body B (the second body in the constructor), expressed in the Ground frame. More...  
Transform  findContactFrameInG (const State &state) const 
Return the instantaneous contact frame C in the Ground frame. More...  
Real  findSeparation (const State &state) const 
Calculate the separation distance or penetration depth of the two spheres. More...  
Public Member Functions inherited from SimTK::Constraint  
Constraint ()  
Default constructor creates an empty Constraint handle that can be used to reference any Constraint. More...  
Constraint (ConstraintImpl *r)  
For internal use: construct a new Constraint handle referencing a particular implementation object. More...  
void  disable (State &) const 
Disable this Constraint, effectively removing it from the system. More...  
void  enable (State &) const 
Enable this Constraint, without necessarily satisfying it. More...  
bool  isDisabled (const State &) const 
Test whether this constraint is currently disabled in the supplied State. More...  
bool  isDisabledByDefault () const 
Test whether this Constraint is disabled by default in which case it must be explicitly enabled before it will take effect. More...  
void  setDisabledByDefault (bool shouldBeDisabled) 
Normally Constraints are enabled when defined and can be disabled later. More...  
operator ConstraintIndex () const  
This is an implicit conversion from Constraint to ConstraintIndex when needed. More...  
const SimbodyMatterSubsystem &  getMatterSubsystem () const 
Get a const reference to the matter subsystem that contains this Constraint. More...  
SimbodyMatterSubsystem &  updMatterSubsystem () 
Assuming you have writable access to this Constraint, get a writable reference to the containing matter subsystem. More...  
ConstraintIndex  getConstraintIndex () const 
Get the ConstraintIndex that was assigned to this Constraint when it was added to the matter subsystem. More...  
bool  isInSubsystem () const 
Test whether this Constraint is contained within a matter subsystem. More...  
bool  isInSameSubsystem (const MobilizedBody &mobod) const 
Test whether the supplied MobilizedBody is in the same matter subsystem as this Constraint. More...  
int  getNumConstrainedBodies () const 
Return the number of unique bodies directly restricted by this constraint. More...  
const MobilizedBody &  getMobilizedBodyFromConstrainedBody (ConstrainedBodyIndex consBodyIx) const 
Return a const reference to the actual MobilizedBody corresponding to one of the Constrained Bodies included in the count returned by getNumConstrainedBodies(). More...  
const MobilizedBody &  getAncestorMobilizedBody () const 
Return a const reference to the actual MobilizedBody which is serving as the Ancestor body for the constrained bodies in this Constraint. More...  
int  getNumConstrainedMobilizers () const 
Return the number of unique mobilizers directly restricted by this Constraint. More...  
const MobilizedBody &  getMobilizedBodyFromConstrainedMobilizer (ConstrainedMobilizerIndex consMobilizerIx) const 
Return a const reference to the actual MobilizedBody corresponding to one of the Constrained Mobilizers included in the count returned by getNumConstrainedMobilizers(). More...  
const SimbodyMatterSubtree &  getSubtree () const 
Return a subtree object indicating which parts of the multibody tree are potentially affected by this Constraint. More...  
int  getNumConstrainedQ (const State &, ConstrainedMobilizerIndex) const 
Return the number of constrainable generalized coordinates q associated with a particular constrained mobilizer. More...  
int  getNumConstrainedU (const State &, ConstrainedMobilizerIndex) const 
Return the number of constrainable mobilities u associated with a particular constrained mobilizer. More...  
ConstrainedUIndex  getConstrainedUIndex (const State &, ConstrainedMobilizerIndex, MobilizerUIndex which) const 
Return the index into the constrained mobilities u array corresponding to a particular mobility of the indicated ConstrainedMobilizer. More...  
ConstrainedQIndex  getConstrainedQIndex (const State &, ConstrainedMobilizerIndex, MobilizerQIndex which) const 
Return the index into the constrained coordinates q array corresponding to a particular coordinate of the indicated ConstrainedMobilizer. More...  
int  getNumConstrainedQ (const State &) const 
Return the sum of the number of coordinates q associated with each of the constrained mobilizers. More...  
int  getNumConstrainedU (const State &) const 
Return the sum of the number of mobilities u associated with each of the constrained mobilizers. More...  
QIndex  getQIndexOfConstrainedQ (const State &state, ConstrainedQIndex consQIndex) const 
Map one of this Constraint's constrained q's to the corresponding index within the matter subsystem's whole q vector. More...  
UIndex  getUIndexOfConstrainedU (const State &state, ConstrainedUIndex consUIndex) const 
Map one of this Constraint's constrained U's (or mobilities) to the corresponding index within the matter subsystem's whole u vector. More...  
void  getNumConstraintEquationsInUse (const State &state, int &mp, int &mv, int &ma) const 
Find out how many holonomic (position), nonholonomic (velocity), and accelerationonly constraint equations are currently being generated by this Constraint. More...  
void  getIndexOfMultipliersInUse (const State &state, MultiplierIndex &px0, MultiplierIndex &vx0, MultiplierIndex &ax0) const 
Return the start of the blocks of multipliers (or acceleration errors) assigned to this Constraint. More...  
void  setMyPartInConstraintSpaceVector (const State &state, const Vector &myPart, Vector &constraintSpace) const 
Set the part of a complete constraintspace vector that belongs to this constraint. More...  
void  getMyPartFromConstraintSpaceVector (const State &state, const Vector &constraintSpace, Vector &myPart) const 
Get the part of a complete constraintspace vector that belongs to this constraint. More...  
Vector  getPositionErrorsAsVector (const State &) const 
Get a Vector containing the position errors. More...  
Vector  calcPositionErrorFromQ (const State &, const Vector &q) const 
Matrix  calcPositionConstraintMatrixP (const State &) const 
Matrix  calcPositionConstraintMatrixPt (const State &) const 
Matrix  calcPositionConstraintMatrixPNInv (const State &) const 
void  calcConstraintForcesFromMultipliers (const State &, const Vector &lambda, Vector_< SpatialVec > &bodyForcesInA, Vector &mobilityForces) const 
This operator calculates this constraint's body and mobility forces given the complete set of multipliers lambda for this Constraint. More...  
Vector  getVelocityErrorsAsVector (const State &) const 
Get a Vector containing the velocity errors. More...  
Vector  calcVelocityErrorFromU (const State &, const Vector &u) const 
Matrix  calcVelocityConstraintMatrixV (const State &) const 
Matrix  calcVelocityConstraintMatrixVt (const State &) const 
Vector  getAccelerationErrorsAsVector (const State &) const 
Get a Vector containing the acceleration errors. More...  
Vector  calcAccelerationErrorFromUDot (const State &, const Vector &udot) const 
Vector  getMultipliersAsVector (const State &) const 
Get a Vector containing the Lagrange multipliers. More...  
void  getConstraintForcesAsVectors (const State &state, Vector_< SpatialVec > &bodyForcesInG, Vector &mobilityForces) const 
Given a State realized through Acceleration stage, return the forces that were applied to the system by this Constraint, with body forces expressed in Ground. More...  
Vector_< SpatialVec >  getConstrainedBodyForcesAsVector (const State &state) const 
For convenience, returns constrained body forces as the function return. More...  
Vector  getConstrainedMobilityForcesAsVector (const State &state) const 
For convenience, returns constrained mobility forces as the function return. More...  
Real  calcPower (const State &state) const 
Calculate the power being applied by this Constraint to the system. More...  
Matrix  calcAccelerationConstraintMatrixA (const State &) const 
Matrix  calcAccelerationConstraintMatrixAt (const State &) const 
void  setIsConditional (bool isConditional) 
(Advanced) Mark this constraint as one that is only conditionally active. More...  
bool  isConditional () const 
(Advanced) Get the value of the isConditional flag. More...  
Public Member Functions inherited from SimTK::PIMPLHandle< Constraint, ConstraintImpl, true >  
bool  isEmptyHandle () const 
Returns true if this handle is empty, that is, does not refer to any implementation object. More...  
bool  isOwnerHandle () const 
Returns true if this handle is the owner of the implementation object to which it refers. More...  
bool  isSameHandle (const Constraint &other) const 
Determine whether the supplied handle is the same object as "this" PIMPLHandle. More...  
void  disown (Constraint &newOwner) 
Give up ownership of the implementation to an empty handle. More...  
PIMPLHandle &  referenceAssign (const Constraint &source) 
"Copy" assignment but with shallow (pointer) semantics. More...  
PIMPLHandle &  copyAssign (const Constraint &source) 
This is real copy assignment, with ordinary C++ object ("value") semantics. More...  
void  clearHandle () 
Make this an empty handle, deleting the implementation object if this handle is the owner of it. More...  
const ConstraintImpl &  getImpl () const 
Get a const reference to the implementation associated with this Handle. More...  
ConstraintImpl &  updImpl () 
Get a writable reference to the implementation associated with this Handle. More...  
int  getImplHandleCount () const 
Return the number of handles the implementation believes are referencing it. More...  
Additional Inherited Members  
Public Types inherited from SimTK::Constraint  
typedef Rod  ConstantDistance 
Synonym for Rod constraint. More...  
typedef Ball  CoincidentPoints 
Synonym for Ball constraint. More...  
typedef Ball  Spherical 
Synonym for Ball constraint. More...  
typedef Weld  CoincidentFrames 
Public Types inherited from SimTK::PIMPLHandle< Constraint, ConstraintImpl, true >  
typedef PIMPLHandle< Constraint, ConstraintImpl, PTR >  HandleBase 
typedef HandleBase  ParentHandle 
Protected Member Functions inherited from SimTK::PIMPLHandle< Constraint, ConstraintImpl, true >  
PIMPLHandle ()  
The default constructor makes this an empty handle. More...  
PIMPLHandle (ConstraintImpl *p)  
This provides consruction of a handle referencing an existing implementation object. More...  
PIMPLHandle (const PIMPLHandle &source)  
The copy constructor makes either a deep (value) or shallow (reference) copy of the supplied source PIMPL object, based on whether this is a "pointer
semantics" (PTR=true) or "object (value) semantics" (PTR=false, default) class. More...  
~PIMPLHandle ()  
Note that the destructor is nonvirtual. More...  
PIMPLHandle &  operator= (const PIMPLHandle &source) 
Copy assignment makes the current handle either a deep (value) or shallow (reference) copy of the supplied source PIMPL object, based on whether this is a "pointer sematics" (PTR=true) or "object (value) semantics" (PTR=false, default) class. More...  
void  setImpl (ConstraintImpl *p) 
Set the implementation for this empty handle. More...  
bool  hasSameImplementation (const Constraint &other) const 
Determine whether the supplied handle is a reference to the same implementation object as is referenced by "this" PIMPLHandle. More...  
This constraint represents a bilateral connection between a sphere on one body and a sphere on another.
On construction you may choose whether the connection enforces rolling; otherwise the spheres will slip against each other. There is always one position (holonomic) constraint equation: the sphere surfaces must touch at some point. That leaves the spheres with five unconstrained degrees of freedom (dofs) to take on any relative orientation and to be touching at any point on their surfaces. Optionally, there are also two velocity (nonholonomic) constraint equations that prevent relative slip between the spheres and thus enforce rolling. In that case there can be five position dofs but only three unconstrained velocity dofs.
Note that this is a bilateral, unconditional connection and will push or pull as necessary to keep the spheres in contact. If rolling is being enforced then whatever tangential forces are necessary to keep the spheres rolling will be generated, regardless of the normal force. These constraints can form the basis for unilateral contact with Coulomb friction, but additional conditions must be added to determine when they are active.
There are two mobilized bodies involved, we'll call them F and B. Either body can move or either can be Ground; however, we will orient the signs in our calculations so that we treat F as though it were "fixed"; for example, the contact normal will be considered to point from F towards B, as though F were Ground. F has a sphere attached to it with center point Sf and radius rf. B has a sphere attached to it with center Sb and radius rb. The line between the centers is p_SfSb, oriented from body F's sphere center to body B's sphere center. The contact normal will be a unit vector aligned with p_SfSb.
Let d=p_SfSb be the separation between the sphere centers. We define the contact point Co to be located on the centertocenter line so that it divides the line into two segments whose lengths are proportional to the size of the sphere nearest each segment:
Co = Sf + (rf/(rf+rb)) * p_SfSb
If the contact constraint is satisfied (so that the spheres are touching), then d=rf+rb and Co will be located at the point of contact, at a distance rf from Sf and rb from Sb. Otherwise Co will be located along the center line with the error distributed in proportion to the radii. Since position constraints cannot be satisifed perfectly, this ensures that the fractional torque error produced by applying tangential forces at a point away from the sphere surfaces is the same for each sphere.
For any given pose, we will define a contact frame C with origin Co whose z axis Cz is aligned with the centertocenter line, pointing from Co towards Sb. If Sf and Sb are coincident (as can happen prior to assembly), Cz will be chosen arbitrarily. The x and y axes Cx and Cy define the contact plane. They are always somewhat arbitrary except for being perpendicular to Cz; don't expect them to change smoothly. However, once chosen for a new pose, Cx and Cy are held constant during subsequent velocity and acceleration calculations using the same pose. Cx and Cy are used to parameterize the slip velocity and the tangential forces; the two tangential multipliers are measure numbers in the Cx and Cy directions. The instantaneous contact frame in Ground is available as the transform X_GC.
The contact constraints here are enforced by a normal multiplier acting along Cz, and optionally two tangential multipliers acting along Cx and Cy respectively. Together these three multipliers can be interpreted as a force expressed in frame C, and applied equal and opposite to bodies F and B at their material points coincident with contact point Co.
The assembly condition is the same as the position constraint: the surfaces of the spheres must meet at some point. There is no assembly condition for the tangential constraints since they do not restrict the allowable pose during assembly.
SimTK::Constraint::SphereOnSphereContact::SphereOnSphereContact  (  MobilizedBody &  mobod_F, 
const Vec3 &  defaultCenterOnF,  
Real  defaultRadiusOnF,  
MobilizedBody &  mobod_B,  
const Vec3 &  defaultCenterOnB,  
Real  defaultRadiusOnB,  
bool  enforceRolling  
) 
Construct a sphereonsphere constraint as described in the Constraint::SphereOnSphereContact class documentation.
mobod_F  The first MobilizedBody object to which a contacting sphere is attached. We'll call it F, the "fixed" body just to orient the contact; actually either or both bodies can be moving. 
defaultCenterOnF  This is the location of the sphere center on mobod_F, given as a vector from the F frame origin Fo to the sphere center Sf, expressed in F. This is the center location that will be present in a default State; you can modify it later. 
defaultRadiusOnF  The radius rf of the sphere attached to mobod_F. This is the value that will be present in a default State; you can modify it later. 
mobod_B  The second MobilizedBody object to which a second contacting sphere is attached. We'll call this mobilized body B. It can be Ground or a moving body. 
defaultCenterOnB  This is the location of the sphere center on mobod_B, given as a vector from the B frame origin Bo to the sphere center Sb, expressed in B. This is the center location that will be present in a default State; you can modify it later. 
defaultRadiusOnB  The radius rb of the sphere attached to mobod_B. This is the value that will be present in a default State; you can modify it later. 
enforceRolling  Whether to generate tangential forces to make the sphere roll on the plane. Otherwise only a normal force is generated and the sphere is free to slip. 
Default constructor creates an empty handle that can be used to reference any SphereOnSphereContact Constraint.
const MobilizedBody& SimTK::Constraint::SphereOnSphereContact::getMobilizedBodyF  (  )  const 
Return a reference to the first MobilizedBody to which a sphere is attached.
This refers to the mobod_F that was given in the constructor and cannot be changed after construction.
const MobilizedBody& SimTK::Constraint::SphereOnSphereContact::getMobilizedBodyB  (  )  const 
Return a reference to the second MobilizedBody to which a sphere is attached.
This refers to the mobod_B that was given in the constructor and cannot be changed after construction.
bool SimTK::Constraint::SphereOnSphereContact::isEnforcingRolling  (  )  const 
Report whether this Constraint was constructed to generate rolling constraints (otherwise it is frictionless).
This cannot be changed after construction.
SphereOnSphereContact& SimTK::Constraint::SphereOnSphereContact::setDefaultCenterOnF  (  const Vec3 &  defaultCenter  ) 
Replace the default center point for the sphere attached to the first body, mobod_F, that was supplied on construction.
This is a topological change; you'll have to call realizeTopology() again if you call this method.
SphereOnSphereContact& SimTK::Constraint::SphereOnSphereContact::setDefaultRadiusOnF  (  Real  defaultRadius  ) 
Replace the default radius for the sphere attached to the first body, mobod_F, that was supplied on construction.
This is a topological change; you'll have to call realizeTopology() again if you call this method.
SphereOnSphereContact& SimTK::Constraint::SphereOnSphereContact::setDefaultCenterOnB  (  const Vec3 &  defaultCenter  ) 
Replace the default center point for the sphere attached to the second body, mobod_B, that was supplied on construction.
This is a topological change; you'll have to call realizeTopology() again if you call this method.
SphereOnSphereContact& SimTK::Constraint::SphereOnSphereContact::setDefaultRadiusOnB  (  Real  defaultRadius  ) 
Replace the default radius for the sphere attached to the second body, mobod_B, that was supplied on construction.
This is a topological change; you'll have to call realizeTopology() again if you call this method.
const Vec3& SimTK::Constraint::SphereOnSphereContact::getDefaultCenterOnF  (  )  const 
Return the default center point for the sphere attached to the first body, mobod_F, as set during construction or by the most recent call to setDefaultCenterOnF().
Real SimTK::Constraint::SphereOnSphereContact::getDefaultRadiusOnF  (  )  const 
Return the default radius for the sphere attached to the first body, mobod_F, as set during construction or by the most recent call to setDefaultRadiusF().
const Vec3& SimTK::Constraint::SphereOnSphereContact::getDefaultCenterOnB  (  )  const 
Return the default center point for the sphere attached to the second body, mobod_B, as set during construction or by the most recent call to setDefaultCenterOnB().
Real SimTK::Constraint::SphereOnSphereContact::getDefaultRadiusOnB  (  )  const 
Return the default radius for the sphere attached to the second body, mobod_B, as set during construction or by the most recent call to setDefaultRadiusB().
const SphereOnSphereContact& SimTK::Constraint::SphereOnSphereContact::setCenterOnF  (  State &  state, 
const Vec3 &  sphereCenter  
)  const 
Modify the location of the sphere on the first body, mobod_F, in this state by providing a new vector p_FSf giving the sphere center location Sf relative to body F's body frame origin Fo, and expressed in the F frame.
This overrides the defaultCenterOnF in the given state, whose Stage::Position is invalidated.
const SphereOnSphereContact& SimTK::Constraint::SphereOnSphereContact::setRadiusOnF  (  State &  state, 
Real  sphereRadius  
)  const 
Modify the radius of the sphere on the first body, mobod_F, in this state.
This overrides the defaultRadiusOnF in the given state, whose Stage::Position is invalidated.
const SphereOnSphereContact& SimTK::Constraint::SphereOnSphereContact::setCenterOnB  (  State &  state, 
const Vec3 &  sphereCenter  
)  const 
Modify the location of the sphere on the second body, mobod_B, in this state by providing a new vector p_BSb giving the sphere center location Sb relative to body B's body frame origin Bo, and expressed in the B frame.
This overrides the defaultCenterOnB in the given state, whose Stage::Position is invalidated.
const SphereOnSphereContact& SimTK::Constraint::SphereOnSphereContact::setRadiusOnB  (  State &  state, 
Real  sphereRadius  
)  const 
Modify the radius of the sphere on the second body, mobod_B, in this state.
This overrides the defaultRadiusOnB in the given state, whose Stage::Position is invalidated.
Return the position of the center point of the sphere attached to the first body, mobod_F, as currently set in the given state.
The value is returned as a vector p_FSf from body F's origin Fo to its sphere center Sf, expressed in the F frame.
Return the radius of the sphere attached to the first body, mobod_F, as currently set in the given state.
Return the position of the center point of the sphere attached to the second body, mobod_B, as currently set in the given state.
The value is returned as a vector p_BSb from body B's origin Bo to its sphere center Sb, expressed in the B frame.
Return the radius of the sphere attached to the first body, mobod_F, as currently set in the given state.
The returned position error can be viewed as the signed distance between the spheres.
It is positive when the spheres are separated and measures their closest approach distance. It is negative when the spheres are interpenetrating and measures the penetration depth. The given state must have already been realized through Stage::Position.
The returned velocity error vector has the time derivative of the quantity returned by getPositionError() in its z coordinate, and violation of the rolling constraints in its x and y coordinates.
If rolling is not being enforced then the x and y components are returned zero; they will not contain the slip velocity in that case since any slip velocity is acceptable. Note that the returned vector is expressed in the instantaneous contact frame C, considered as fixed on body F. That is, this is the velocity of the contact point on body B's sphere in the F frame, expressed in C. The given state must have already been realized through Stage::Velocity.
This vector is the time derivative of the value returned by getVelocityError().
Note that this is different than the acceleration of the point of sphere B at the contact point because the contact point moves with respect to that sphere. The given state must have already been realized through Stage::Acceleration.
These are the Lagrange multipliers required to enforce the constraint equations generated here.
For this Constraint it has units of force, but recall that the sign convention for multipliers is the opposite of that for applied forces. Thus the returned value is the negative of the force being applied to sphere B at the contact point, expressed in the contact frame C. The x,y coordinates are the tangential force used to enforce rolling (or zero if rolling is not being enforced), and the z coordinate is the force needed to enforce contact. Since this is an unconditional, bilateral constraint the multipliers may have any sign and magnitude. The given state must already be realized to Stage::Acceleration.
Return the force vector currently being applied by this constraint to the contact point Co on the sphere attached to body B (the second body in the constructor), expressed in the Ground frame.
An equal and opposite force is applied to the sphere attached to body F, to its material point that is at that same location in space. This is zero if the constraint is not currently enabled. Tangential forces are generated only when rolling is being enforced, but since this result is in the Ground frame all the vector measure numbers may be nonzero regardless. The given state must already be realized to Stage::Acceleration.
Transform SimTK::Constraint::SphereOnSphereContact::findContactFrameInG  (  const State &  state  )  const 
Return the instantaneous contact frame C in the Ground frame.
The actual contact point is the origin Co of the returned frame, which is placed at a point along the centertocenter line segment as described in the class documentation for this SphereOnSphereContact class. Note that this does not imply that the normal constraint is satisifed; point Co could be far from the sphere surfaces or embedded beneath them. The z direction of this frame is the contact normal; it lies along the centertocenter direction unless the centers are coincident, in which case it is arbitrary. The xy directions Cx and Cy are always somewhat arbitrary except for being perpendicular to z; don't expect them to change smoothly. This method calculates a valid value even if this constraint is currently disabled. The given state must already be realized to Stage::Position.
Calculate the separation distance or penetration depth of the two spheres.
It is positive when the spheres are separated and measures their closest approach distance. It is negative when the spheres are interpenetrating and measures the penetration depth. This calculates a valid value even if this constraint is currently disabled. The given state must be realized to Stage::Position.