Project Chrono: Links

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>();
   chrono_types::make_shared
   std::shared_ptr< T > make_shared(Args &&... args)
   Replacement for make_shared guaranteed to use operator new rather than placement new in order to avoi...
   Definition ChTypes.h:66
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