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SimTK::UnilateralContact Class Referenceabstract

(Experimental – API will change – use at your own risk) A unilateral contact constraint uses a single holonomic (position) constraint equation to prevent motion in one direction while leaving it unrestricted in the other direction. More...

+ Inheritance diagram for SimTK::UnilateralContact:

Public Member Functions

 UnilateralContact (int sign=1)
 The base class constructor allows specification of the sign convention to be used with this constraint. More...
 
virtual ~UnilateralContact ()
 
Real getSignConvention () const
 Report the sign convention (1 or -1) supplied at construction. More...
 
virtual bool disable (State &state) const =0
 Disable the normal and friction constraints if they were enabled. More...
 
virtual bool enable (State &state) const =0
 Enable the normal and friction constraints if they were disabled. More...
 
virtual bool isEnabled (const State &state) const =0
 Return true if this contact is enabled. More...
 
virtual Vec3 whereToDisplay (const State &state) const =0
 This returns a point in the ground frame at which you might want to say the constraint is "located", for purposes of display only. More...
 
virtual Real calcEffectiveCOR (const State &state, Real defaultCaptureSpeed, Real defaultMinCORSpeed, Real impactSpeed) const =0
 Returns the effective coefficient of restitution (COR) for this contact, given an impact speed (a nonnegative scalar). More...
 
virtual Real getPerr (const State &state) const =0
 Return the position error for the contact constraint (usually a signed distance function). More...
 
virtual Real getVerr (const State &state) const =0
 Return the time derivative of the contact constraint position error. More...
 
virtual Real getAerr (const State &state) const =0
 Return the time derivative of the contact constraint velocity error. More...
 
virtual bool isProximal (const State &state, Real ptol) const
 Given the position constraint tolerance currently in use, is this contact close enough to contacting that we should treat it as though it is in contact? Normally we just see if sign*perr <= tol, but individual contacts can override this if they want to do some scaling. More...
 
virtual MultiplierIndex getContactMultiplierIndex (const State &state) const =0
 Return the multiplier index Simbody assigned for the unilateral contact constraint (for contact, this is the normal constraint). More...
 
virtual bool hasFriction (const State &state) const
 Returns true if there is a friction constraint associated with this contact constraint. More...
 
virtual Real calcEffectiveCOF (const State &state, Real defaultTransitionSpeed, Real slipSpeed) const
 Returns the effective coefficient of friction mu for this contact, given a relative slip speed (a nonnegative scalar). More...
 
virtual Vec2 getSlipVelocity (const State &state) const
 
virtual void getFrictionMultiplierIndices (const State &state, MultiplierIndex &ix_x, MultiplierIndex &ix_y) const
 If hasFriction(), this method returns the multipliers used for the x- and y-direction friction constraints. More...
 
virtual Vec3 getPositionInfo (const State &state) const
 TODO: kludge needed because we're misusing existing constraints. More...
 
virtual void setInstanceParameter (State &state, const Vec3 &pos) const
 TODO: kludge to set instance parameters on internal constraints; this should be the same Vec3 you got from getPositionInfo(). More...
 
void setMyIndex (UnilateralContactIndex cx)
 
UnilateralContactIndex getMyIndex () const
 

Detailed Description

(Experimental – API will change – use at your own risk) A unilateral contact constraint uses a single holonomic (position) constraint equation to prevent motion in one direction while leaving it unrestricted in the other direction.

Examples are surface-surface contact, joint stops, and inextensible ropes. These constraints are subject to violent impacts that are treated with a coefficient of restitution that may be state dependent.

Some unilateral contacts may be associated with one or more friction elements that are dependent on the normal force generated by the contact. Whenever the unilateral contact is inactive (meaning its associated multiplier is zero), its associated friction elements are also inactive.

There are two possible sign conventions, depending on the underlying Constraint element definition. The default (sign=1) is to consider the constraint position error (perr) to be a signed distance function, meaning that perr>=0 is valid and perr<0 is a violation. Similarly verr>=0 is separation while verr<0 is approach or penetration velocity, and aerr>=0 is separation acceleration. The corresponding force should also be >= 0, but since constraint multipliers have the opposite sign from applied forces that means the inequality restricting the multiplier is lambda<=0; lambda>0 would produce an attractive force. That would suck, so is not allowed. In the opposite sign convention (sign=-1), the inequalities that must be satisfied are perr,verr,aerr<=0 and lambda>=0. So the constraints to be enforced are:

    sign*perr >= 0 (always)
    sign*verr >= 0 (if perr==0)
    if perr==verr==0 then:
        sign*aerr >= 0 && -sign*lambda >= 0 && aerr*lambda==0

In practice we enforce constraints up to a tolerance, so the zeroes above are not enforced exactly.

Constructor & Destructor Documentation

SimTK::UnilateralContact::UnilateralContact ( int  sign = 1)
inlineexplicit

The base class constructor allows specification of the sign convention to be used with this constraint.

The sign convention cannot be changed later. See the class documentation for more information.

virtual SimTK::UnilateralContact::~UnilateralContact ( )
inlinevirtual

Member Function Documentation

Real SimTK::UnilateralContact::getSignConvention ( ) const
inline

Report the sign convention (1 or -1) supplied at construction.

virtual bool SimTK::UnilateralContact::disable ( State state) const
pure virtual

Disable the normal and friction constraints if they were enabled.

Return true if we actually had to disable something.

Implemented in SimTK::EdgeEdgeContact, SimTK::SphereSphereContact, SimTK::SpherePlaneContact, SimTK::PointPlaneContact, SimTK::PointPlaneFrictionlessContact, SimTK::Rope, SimTK::HardStopLower, and SimTK::HardStopUpper.

virtual bool SimTK::UnilateralContact::enable ( State state) const
pure virtual

Enable the normal and friction constraints if they were disabled.

Return true if we actually had to enable something.

Implemented in SimTK::EdgeEdgeContact, SimTK::SphereSphereContact, SimTK::SpherePlaneContact, SimTK::PointPlaneContact, SimTK::PointPlaneFrictionlessContact, SimTK::Rope, SimTK::HardStopLower, and SimTK::HardStopUpper.

virtual bool SimTK::UnilateralContact::isEnabled ( const State state) const
pure virtual
virtual Vec3 SimTK::UnilateralContact::whereToDisplay ( const State state) const
pure virtual

This returns a point in the ground frame at which you might want to say the constraint is "located", for purposes of display only.

Implemented in SimTK::EdgeEdgeContact, SimTK::SphereSphereContact, SimTK::SpherePlaneContact, SimTK::PointPlaneContact, SimTK::PointPlaneFrictionlessContact, SimTK::Rope, SimTK::HardStopLower, and SimTK::HardStopUpper.

virtual Real SimTK::UnilateralContact::calcEffectiveCOR ( const State state,
Real  defaultCaptureSpeed,
Real  defaultMinCORSpeed,
Real  impactSpeed 
) const
pure virtual

Returns the effective coefficient of restitution (COR) for this contact, given an impact speed (a nonnegative scalar).

For a given pair of contacting materials this is typically a function of just the impact speed, but it could also depend on the time and configuration in state, which should be realized through Stage::Position. The given default impact speed thresholds (also nonnegative) are used to calculate the COR unless this Contact overrides those.

Implemented in SimTK::EdgeEdgeContact, SimTK::SphereSphereContact, SimTK::SpherePlaneContact, SimTK::PointPlaneContact, SimTK::PointPlaneFrictionlessContact, SimTK::Rope, SimTK::HardStopLower, and SimTK::HardStopUpper.

virtual Real SimTK::UnilateralContact::getPerr ( const State state) const
pure virtual

Return the position error for the contact constraint (usually a signed distance function).

You have to apply the sign convention to interpret this properly.

Implemented in SimTK::EdgeEdgeContact, SimTK::SphereSphereContact, SimTK::SpherePlaneContact, SimTK::PointPlaneContact, SimTK::PointPlaneFrictionlessContact, SimTK::Rope, SimTK::HardStopLower, and SimTK::HardStopUpper.

virtual Real SimTK::UnilateralContact::getVerr ( const State state) const
pure virtual

Return the time derivative of the contact constraint position error.

You have to apply the sign convention to interpret this properly.

Implemented in SimTK::EdgeEdgeContact, SimTK::SphereSphereContact, SimTK::SpherePlaneContact, SimTK::PointPlaneContact, SimTK::PointPlaneFrictionlessContact, SimTK::Rope, SimTK::HardStopLower, and SimTK::HardStopUpper.

virtual Real SimTK::UnilateralContact::getAerr ( const State state) const
pure virtual

Return the time derivative of the contact constraint velocity error.

You have to apply the sign convention to interpret this properly.

Implemented in SimTK::EdgeEdgeContact, SimTK::SphereSphereContact, SimTK::SpherePlaneContact, SimTK::PointPlaneContact, SimTK::PointPlaneFrictionlessContact, SimTK::Rope, SimTK::HardStopLower, and SimTK::HardStopUpper.

virtual bool SimTK::UnilateralContact::isProximal ( const State state,
Real  ptol 
) const
inlinevirtual

Given the position constraint tolerance currently in use, is this contact close enough to contacting that we should treat it as though it is in contact? Normally we just see if sign*perr <= tol, but individual contacts can override this if they want to do some scaling.

virtual MultiplierIndex SimTK::UnilateralContact::getContactMultiplierIndex ( const State state) const
pure virtual

Return the multiplier index Simbody assigned for the unilateral contact constraint (for contact, this is the normal constraint).

If the constraint is not enabled, there is no multiplier and the returned index will be invalid.

Implemented in SimTK::EdgeEdgeContact, SimTK::SphereSphereContact, SimTK::SpherePlaneContact, SimTK::PointPlaneContact, SimTK::PointPlaneFrictionlessContact, SimTK::Rope, SimTK::HardStopLower, and SimTK::HardStopUpper.

virtual bool SimTK::UnilateralContact::hasFriction ( const State state) const
inlinevirtual

Returns true if there is a friction constraint associated with this contact constraint.

If so, calcEffectiveCOF() must be overridden.

Reimplemented in SimTK::EdgeEdgeContact, SimTK::SphereSphereContact, SimTK::SpherePlaneContact, and SimTK::PointPlaneContact.

virtual Real SimTK::UnilateralContact::calcEffectiveCOF ( const State state,
Real  defaultTransitionSpeed,
Real  slipSpeed 
) const
inlinevirtual

Returns the effective coefficient of friction mu for this contact, given a relative slip speed (a nonnegative scalar).

For a given pair of contacting materials this is typically a function of just the slip speed, but it could also depend on the time and configuration in state, which should be realized through Stage::Position. The given default slip-to-roll transition speed threshold (also nonnegative) is used to calculate mu unless this Contact overrides it with its own transition speed.

Reimplemented in SimTK::EdgeEdgeContact, SimTK::SphereSphereContact, SimTK::SpherePlaneContact, and SimTK::PointPlaneContact.

virtual Vec2 SimTK::UnilateralContact::getSlipVelocity ( const State state) const
inlinevirtual
virtual void SimTK::UnilateralContact::getFrictionMultiplierIndices ( const State state,
MultiplierIndex ix_x,
MultiplierIndex ix_y 
) const
inlinevirtual

If hasFriction(), this method returns the multipliers used for the x- and y-direction friction constraints.

If no friction, or if this constraint is disabled, the returned values are invalid.

Reimplemented in SimTK::EdgeEdgeContact, SimTK::SphereSphereContact, SimTK::SpherePlaneContact, and SimTK::PointPlaneContact.

virtual Vec3 SimTK::UnilateralContact::getPositionInfo ( const State state) const
inlinevirtual

TODO: kludge needed because we're misusing existing constraints.

This must be called while Stage::Position is valid.

virtual void SimTK::UnilateralContact::setInstanceParameter ( State state,
const Vec3 pos 
) const
inlinevirtual

TODO: kludge to set instance parameters on internal constraints; this should be the same Vec3 you got from getPositionInfo().

void SimTK::UnilateralContact::setMyIndex ( UnilateralContactIndex  cx)
inline
UnilateralContactIndex SimTK::UnilateralContact::getMyIndex ( ) const
inline

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