Links

A Chrono body can be constrained in its relative motion with respect to a different body or ground. This is achieved by using ChLink classes.
From the ChLink class various sets of links are derived. The most noticeable are those derived from:

• ChLinkMate: more efficient, however they do not implement limits and only few of them can have an imposed motion;
• ChLinkLock: more general, the relative motion can be limited within boundaries, forces and relative displacements can easily be retrieved; it is possible to specify a constraint over points that are moving respect to the body reference frame;
• ChLinkMotor: ChLinkMate derived joints with included actuation.

Thus, some of the ChLinkMate and ChLinkLock derived classes may overlap. The latter being more flexible, the first more efficient. Because of this, the ChLinkMate version should be preferred, in general.

Some general information are fundamental to effectively use these classes:

• links often refer to a pair of ChMarker, but such markers can be automatically added to bodies during link initialization; see below;
• each link has a reference/master frame; reaction forces and axis directions are computed with respect to this reference marker;
• it is worth to set the initial position of the markers/bodies to a feasible position;
• it is generally useful to set the solver parameters to finer values when constraints are present; see Solvers;

ChLink Quick Reference

ConDOF	Task	Description	Class
6	Fix	Fix position and rotation	ChLinkMateFix ChLinkLockLock
6|3	Bushing	Linear compliance + optional spherical joint	ChLinkBushing
5	Revolute	Allows rotation along Z axis	ChLinkMateRevolute ChLinkLockRevolute ChLinkRevolute
5	Prismatic	Allows translation along Z axis	ChLinkMatePrismatic ChLinkLockPrismatic
4	Universal	Universal joint (along X and Y axes)	ChLinkUniversal
4	Revolute+Prismatic	Allow translation along X axis and rotation along Z axis	ChLinkLockRevolutePrismatic
4	Oldham	Oldham joint; does not fix axes position	ChLinkLockOldham
4	Cylindrical	Z Axes are collinear	ChLinkMateCylindrical ChLinkLockCylindrical
3	Spherical	Fix translations	ChLinkMateSpherical ChLinkLockSpherical
3	Planar	XY planes are coplanar	ChLinkLockPlanar ChLinkMatePlanar
3	Aligned	Fix rotations	ChLinkLockAlign
2	Revolute+Spherical	Fix distance along Z axis; free rotations	ChLinkMateDistanceZ ChLinkRevoluteSpherical
2	Revolute+Align	Allow translation respect to a rotating frame	ChLinkRevoluteTranslational
2	Point on a Plane	Z translations are blocked	ChLinkLockPointPlane
2	Point on a Line	Point belongs to a given line; free to rotate	ChLinkLockPointLine ChLinkLockPointSpline
2	Parallel	Z axes are parallel	ChLinkMateParallel ChLinkLockParallel
2	Orthogonal	X and Y axes are kept orthogonal; Z axes are aligned	ChLinkMateOrthogonal ChLinkLockPerpend
1	Distance	Polar distance is fixed	ChLinkDistance
1	Rack-Pinion	Couple rotation of the pinion Z axis with rack X axis	ChLinkMateRackPinion
1	Pulley	Couple rotation over Z axes; pulley-specific features	ChLinkLockPulley
1	Gear	Couple rotation over Z axes; gear-specific features	ChLinkLockGear
0	Free	No constraints	ChLinkLockFree

Additionally, if a body has to be fixed to the global frame and reaction forces are of no interest, it is possible to use the SetFixed() method; this will also eliminate the state from the system.

Actuators

ConDOF	MotDOF	Task	Description	Class
0|3|5	1	Linear Actuator	Applies linear force|speed|position between frames; optionally adds none|prismatic|spherical joints to its ends can be paired with 1D ChShaft	ChLinkMotorLinear and derived
0|3|5	1	Rotating Actuator	Applies torque|speed|position between frames; optionally adds none|revolute|cylindrical|Oldham joints to its ends can be paired with 1D ChShaft	ChLinkMotorRotation and derived
0	1	Linear Spring+Damper	Spring+Damper depending to frame distance; also with custom force	ChLinkTSDA
0	1	Rotational Spring+Damper	Spring+Damper depending to frame rotation along Z axis; also with custom force	ChLinkRSDA

Also ChLinkLockLock can be used to impose a motion between bodies.

Usage

In many cases there is no need to explicitly create two markers. Use in the case the ChLink::Initialize() function, passing the two bodies and the position of the constraint. This function will automatically create the two markers and it will add them to the bodies.

Alternatively, one can create the two markers by explicitly, add them to the two bodies, and then call Initialize() by passing the two markers.

In general, the process involves the following steps:

1. Create one of the links of the ChLink derived class (e.g. ChLinkLockSpherical)

   auto mylink =  chrono_types::make_shared<ChLinkLockSpherical>();

2. Use mylink->Initialize(…) to connect two bodies; different links may accept different arguments. Refer to the documentation of the specific link for further information.

   mylink->Initialize(pendulumBody,   // the 1st body to connect
                      floorBody,      // the 2nd body to connect
                      ChFramed(ChVector3d(1, 0, 0),
                               QuatFromAngleAxis(-CH_PI / 2, VECT_X)
                               )
                      );

3. Add the link to a ChSystem

   my_system.Add(mylink);

4. Optional: set link properties

Examples

See also:

• demo_MBS_fourbar
• demo_MBS_suspension