Constraints

Length constraints

RaiSim currently has three types of length constraints:

All length constraints offer three stretch types: STRETCH_RESISTANT_ONLY, COMPRESSION_RESISTANT_ONLY, BOTH. The first two are unilateral constraints (i.e., acting only in one direction) and BOTH is a bilateral constraint (i.e., acting in both directions). The stretch types are explained in StiffLengthConstraint.

You can find a short example in examples/src/server/length_constraints_newtons_cradle.cpp. This code simulates the following behavior:

How constraints are solved

RaiSim supports two different enforcement paths for constraints:

StiffLengthConstraint (hard): converted into a contact problem and solved by the contact solver as an impulse constraint. This behaves like a hard constraint and is handled alongside collisions.
CompliantLengthConstraint / CustomLengthConstraint (soft): applied as explicit forces each step (spring or user-defined tension). These do not go through the contact solver and therefore behave more like soft constraints.

Pin constraints for closed-loop systems are also solved by the contact solver (as equality constraints), but they are specified in the URDF under <constraints> rather than through the length-constraint API.

API

LengthConstraint

class LengthConstraint : public raisim::Constraints

Subclassed by raisim::CompliantLengthConstraint, raisim::CustomLengthConstraint, raisim::StiffLengthConstraint

Public Functions

void update(contact::ContactProblems &contact_problems): update internal variables (called by World::integrate1())

double getLength() const

Returns:: the length of the wire

inline double getVisualizationWidth()

get wire width for visualization

Returns:: width

inline void setVisualizationWidth(double width)

set visualization width

Parameters:: width – width

double getDistance() const

Returns:: the distance between the two mounting points

const Vec<3> &getP1() const

Returns:: the first attachment point in the World frame

const Vec<3> &getP2() const

Returns:: the second attachment point in the World frame

Object *getBody1() const

Returns:: the first object to which the wire is attached

Object *getBody2() const

Returns:: the second object to which the wire is attached

const Vec<3> &getNorm() const

Returns:: the direction of the normal (i.e., p2-p1 normalized)

size_t getLocalIdx1() const

Returns:: the local index of object1

size_t getLocalIdx2() const

Returns:: the local index of object2

double getStretch() const

Returns:: the stretch length (i.e., constraint violation)

inline void setName(const std::string &name)

set name of the wire

Parameters:: name – the constraint’s name

inline const std::string &getName() const

get name of the wire

Returns:: the wire name

inline void setStretchType(StretchType type)

set stretch type of the wire Check http://raisim.com/sections/StiffLengthConstraint.html

Parameters:: type – the wire stretch type. Available types: STRETCH_RESISTANT_ONLY, COMPRESSION_RESISTANT_ONLY, BOTH

inline StretchType getStretchType() const

get the constraint’s stretch type

Returns:: the wire stretch type

inline WireType getWireType() const

get wire type

Returns:: wire type

const Vec<3> &getOb1MountPos() const

get pos on ob1 where the wire is attached

Returns:: mount position 1

const Vec<3> &getOb2MountPos() const

get pos on ob2 where the wire is attached

Returns:: mount position 2

Public Members

bool isActive: True when the constraint is active in the solver.