Loads

Chrono offers different options to apply loads to objects.

As the name suggests, objects that can carry a load are those inheriting from the @ref chrono::ChLoadable "ChLoadable" class; for example @ref chrono::ChBody "ChBody", finite elements and nodes or @ref chrono::ChShaft "ChShaft"s.

Loads can be applied to these objects through different approaches:

• @ref chrono::ChForce "ChForce":

  • available only for @ref chrono::ChBody "ChBody"

  • applies either a force or a torque with respect to some pre-defined frame (body or world);

• @ref chrono::ChLoad "ChLoad" + @ref chrono::ChLoader "ChLoader":

  • available for any @ref chrono::ChLoadable "ChLoadable" inherited object;

  • the specific implementation must be provided through a @ref chrono::ChLoader "ChLoader" object;

  • it is more tightly coupled with the Chrono system;

  • requires the computation of the generalized loads;

• loads inheriting from @ref chrono::ChLoadCustom "ChLoadCustom"|@ref chrono::ChLoadCustomMultiple "ChLoadCustomMultiple":

  • they are applied to either one or multiple @ref chrono::ChLoadable "ChLoadable" objects;

  • it is the preferred choice for loads applied to pair of objects;

  • multiple pre-defined classes are available, simplifying the writing of the generalized loads.

While not being considered explicitely in the previous list, also the @ref chrono::ChLinkTSDA "ChLinkTSDA"|@ref chrono::ChLinkRSDA "ChLinkRSDA" can do similar jobs, also including the Jacobian for the generalized forces or also some advanced internal dynamic, by providing custom functors (@ref chrono::ChLinkTSDA::ForceFunctor "ChLinkTSDA::ForceFunctor"|@ref chrono::ChLinkRSDA::TorqueFunctor "ChLinkRSDA::TorqueFunctor") or even proper ODEs (only @ref chrono::ChLinkTSDA::ODE "ChLinkTSDA::ODE").

Other simplified approaches, limited to ChBody, allow to accumulate forces, by using @ref chrono::ChBody::AccumulateForce() "AccumulateForce()" and @ref chrono::ChBody::AccumulateTorque() "AccumulateTorque()".

The overall contributions of forces to a given ChBody can be retrieved through @ref chrono::ChBody::GetAppliedForce() "GetAppliedForce()" and similarly for torques.

For FEA nodes, similary to the @ref chrono::ChForce "ChForce" for @ref chrono::ChBody "ChBody", it is possible to add a force directly to the node through @ref chrono::fea::ChNodeFEAxyz::SetForce() "ChNodeFEAxyz::SetForce()". However, in this case the options are even more limited, since the force is expressed as a simple @ref chrono::ChVector3 "ChVector3", thus always assumed constant and expressed in absolute frame. The @ref chrono::fea::ChNodeFEAxyzrot "ChNodeFEAxyzrot" class implements also @ref chrono::fea::ChNodeFEAxyzrot::SetTorque() "ChNodeFEAxyzrot::SetTorque()".

Some more peculiar class has been excluded from this list: please look at @ref chrono::ChLoadBase "ChLoadBase" to have a full perspective on the load classes in Chrono.

ChForce

The @ref chrono::ChForce "ChForce" can be applied directly to a @ref chrono::ChBody "ChBody" by calling:

  auto force = chrono_types::make_shared<ChForce>();
  body->AddForce(force)

  // AFTER calling AddForce
  force->SetMforce(10);

Please mind that:

• use body->AddForce(force) and not force->SetBody(body): the latter it is not sufficient since the ChForce wouldn't be considered by the ChSystem;

• always call ChForce methods after having called body->AddForce(force)

The application point, direction, position and modulus can be set either through constant values or through [ChFunctions](@ref ChFunction_objects). The reference system can be either relative to the body or absolute, but cannot be set to a generic frame.

ChLoad and inherited

These sets of loads allows for the maximum freedom and coupling with the Chrono system.

Contrary to @ref chrono::ChForce "ChForce", these other _ChLoad_s requires the introduction of a @ref chrono::ChLoadContainer "ChLoadContainer" in order to be added to the system. For example:

  auto load_container = chrono_types::make_shared<ChLoadContainer>();
  sys.Add(load_container);

  auto load_bb = chrono_types::make_shared<ChLoadBodyBodyTorque>(bodyA, bodyB, ChVector3d(0,10.0,0), false);
  load_container->Add(load_bb);

For the case of the @ref chrono::ChLoad "ChLoad", the user is asked to either provide one of the pre-defined @ref chrono::ChLoader "ChLoader" objects or to write its own. Please refer to the documentation of each single ChLoader to understand their use. This method considers the load applied to a single object.

A similar set of loads includes those inheriting from @ref chrono::ChLoadCustom "ChLoadCustom"|@ref chrono::ChLoadCustomMultiple "ChLoadCustomMultiple": while the features are similar compared to @ref chrono::ChLoad "ChLoad" type and also their usage might have big overlaps, they usually offere a wider set of pre-defined classes that might match the user needs.

These more advanced approaches allow for a tighter coupling with the Chrono system, allowing to introduce also entire stiffness matrix blocks (see @ref chrono::ChLoadBodyBodyBushingGeneric "ChLoadBodyBodyBushingGeneric" and others), providing Jacobians and much more. This come with the added price of having to implement some additional code.