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 @ref chrono::ChLink "ChLink" class various sets of links are derived. The most noticeable are those derived from:

@ref chrono::ChLinkMate "ChLinkMate": more efficient, however they do not implement limits and only few of them can have an imposed motion;
@ref chrono::ChLinkLock "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;
@ref chrono::ChLinkMotor "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 @ref chrono::ChMarker "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](@ref solvers);

ChLink Quick Reference

ConDOF

Task

Description

Class

Fix

Fix position and rotation

@ref chrono::ChLinkMateFix "ChLinkMateFix" @ref chrono::ChLinkLockLock "ChLinkLockLock"

6|3

Bushing

Linear compliance + optional spherical joint

@ref chrono::ChLinkBushing "ChLinkBushing"

Revolute

Allows rotation along Z axis

@ref chrono::ChLinkMateRevolute "ChLinkMateRevolute" @ref chrono::ChLinkLockRevolute "ChLinkLockRevolute" @ref chrono::ChLinkRevolute "ChLinkRevolute"

Prismatic

Allows translation along Z axis

@ref chrono::ChLinkMatePrismatic "ChLinkMatePrismatic" @ref chrono::ChLinkLockPrismatic "ChLinkLockPrismatic"

Universal

Universal joint (along X and Y axes)

@ref chrono::ChLinkUniversal "ChLinkUniversal"

Revolute+Prismatic

Allow translation along X axis and rotation along Z axis

@ref chrono::ChLinkLockRevolutePrismatic "ChLinkLockRevolutePrismatic"

Oldham

Oldham joint; does not fix axes position

@ref chrono::ChLinkLockOldham "ChLinkLockOldham"

Cylindrical

Z Axes are collinear

@ref chrono::ChLinkMateCylindrical "ChLinkMateCylindrical" @ref chrono::ChLinkLockCylindrical "ChLinkLockCylindrical"

Spherical

Fix translations

@ref chrono::ChLinkMateSpherical "ChLinkMateSpherical" @ref chrono::ChLinkLockSpherical "ChLinkLockSpherical"

Planar

XY planes are coplanar

@ref chrono::ChLinkLockPlanar "ChLinkLockPlanar" @ref chrono::ChLinkMatePlanar "ChLinkMatePlanar"

Aligned

Fix rotations

@ref chrono::ChLinkLockAlign "ChLinkLockAlign"

Revolute+Spherical

Fix distance wrt rotation axis (Z axis of revolute)

@ref chrono::ChLinkRevoluteSpherical "ChLinkRevoluteSpherical"

Revolute+Align

Allow translation respect to a rotating frame

@ref chrono::ChLinkRevoluteTranslational "ChLinkRevoluteTranslational"

Point on a Plane

Z translations are blocked

@ref chrono::ChLinkMateDistanceZ "ChLinkMateDistanceZ" @ref chrono::ChLinkLockPointPlane "ChLinkLockPointPlane"

Point on a Line

Point belongs to a given line; free to rotate

@ref chrono::ChLinkLockPointLine "ChLinkLockPointLine" @ref chrono::ChLinkLockPointSpline "ChLinkLockPointSpline"

Parallel

Z axes are parallel

@ref chrono::ChLinkMateParallel "ChLinkMateParallel" @ref chrono::ChLinkLockParallel "ChLinkLockParallel"

Orthogonal

X and Y axes are kept orthogonal; Z axes are aligned

@ref chrono::ChLinkMateOrthogonal "ChLinkMateOrthogonal" @ref chrono::ChLinkLockPerpend "ChLinkLockPerpend"

Distance

Polar distance is fixed

@ref chrono::ChLinkDistance "ChLinkDistance"

Rack-Pinion

Couple rotation of the pinion Z axis with rack X axis

@ref chrono::ChLinkMateRackPinion "ChLinkMateRackPinion"

Pulley

Couple rotation over Z axes; pulley-specific features

@ref chrono::ChLinkLockPulley "ChLinkLockPulley"

Gear

Couple rotation over Z axes; gear-specific features

@ref chrono::ChLinkLockGear "ChLinkLockGear"

Free

No constraints

@ref chrono::ChLinkLockFree "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 @ref chrono::ChBody "SetFixed()" method; this will also eliminate the state from the system.

Actuators

ConDOF

MotDOF

Task

Description

Class

0|3|5

Linear Actuator

Applies linear force|speed|position between frames; optionally adds none|prismatic|spherical joints to its ends can be paired with 1D @ref chrono::ChShaft "ChShaft"

@ref chrono::ChLinkMotorLinear "ChLinkMotorLinear" and derived

0|3|5

Rotating Actuator

@ref chrono::ChLinkMotorRotation "ChLinkMotorRotation" and derived

Linear Spring+Damper

Spring+Damper depending to frame distance; also with custom force

@ref chrono::ChLinkTSDA "ChLinkTSDA"

Rotational Spring+Damper

Spring+Damper depending to frame rotation along Z axis; also with custom force

@ref chrono::ChLinkRSDA "ChLinkRSDA"

Also @ref chrono::ChLinkLockLock "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:

Create one of the links of the @ref chrono::ChLink "ChLink" derived class (e.g. @ref chrono::ChLinkLockSpherical "ChLinkLockSpherical")
```
auto mylink =  chrono_types::make_shared<ChLinkLockSpherical>();
```

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)
                            )
                   );

Add the link to a ChSystem
```
my_system.Add(mylink);
```
Optional: set link properties

Links

ChLink Quick Reference

Actuators

Usage

Examples