General info for all loads

How a load acts

A load acts through a marker: the marker locates it - a point of a body, a node, a coordinate - and provides the Jacobian that takes the load to the coordinates of the body or node. For a force \(\LU{0}{\fv}\) at a position marker this is the virtual work \(\delta W = \delta \LU{0}{\pv}\tp \LU{0}{\fv}\), which gives the generalized forces

\[ \Qm = \LU{0}{\Jm_{pos}}\tp\, \LU{0}{\fv}, \quad \LU{0}{\Jm_{pos}} = \frac{\partial \LU{0}{\pv}}{\partial \qv} ; \]

a torque uses the rotation Jacobian \(\LU{0}{\Jm_{rot}} = \partial \LU{0}{\tomega} / \partial \dot\qv\) the same way. The Jacobians are global, so a load given in a local frame is transformed into the global frame first. Each load page gives its formula.

Global and body-fixed loads

A force or a torque is given in the global frame, or with bodyFixed = True in the frame of its marker, where it turns with the body or node - a follower load. A body-fixed load needs a marker with an orientation: MarkerBodyRigid on a body, MarkerNodeRigid on a node; a point node has no orientation and takes only global loads.

Loads that change in time

A load is constant, unless

• a user function gives it: loadVectorUserFunction(mbs, t, loadVector) or loadUserFunction(mbs, t, load), called at every evaluation with the current time, replaces the value of the load;

• a preStepUserFunction of mbs changes it before every step, with mbs.SetLoadParameter.

Loads in static computations

The static solver increases the loads over its load steps (staticSolver.numberOfLoadSteps): every load is multiplied by the load factor of the current step, which reaches one in the last step. A load with a user function is not multiplied: its value is used as the function returns it, and the function gets the quasi-time of the load step (staticSolver.loadStepDuration), which it has to use itself to increase the load.

Creating loads

mbs.CreateForce and mbs.CreateTorque add the marker and the load in one call - for a force a MarkerBodyPosition, or a MarkerBodyRigid if it is body-fixed; for a torque a MarkerBodyRigid - unless a marker is given instead of a body; and the gravity argument of CreateMassPoint and CreateRigidBody adds a MarkerBodyMass with a LoadMassProportional.

Drawing

A load on a marker with a position is drawn if loads.show and its own show are True: an arrow at the marker, of length loads.defaultSize - a fraction of openGL.advanced.initialMaxSceneSize if -1 -, or, with loads.fixedLoadSize False, of the length of the load times loads.loadSizeFactor; lines with loads.drawSimplified, otherwise a 3D arrow of radius loads.defaultRadius (general.axesTiling), in loads.defaultColor. With loads.drawWithUserFunction, the value of the user function is drawn - of a symbolic one always, of a Python one only with general.useMultiThreadedRendering False, as the render thread cannot call Python. loads.showNumbers writes its number. A load on a coordinate draws nothing.

The settings: loads.show, loads.defaultSize, loads.defaultRadius, loads.defaultColor, loads.drawSimplified, loads.fixedLoadSize, loads.loadSizeFactor, loads.drawWithUserFunction, loads.showNumbers, general.axesTiling, general.useMultiThreadedRendering, openGL.advanced.initialMaxSceneSize.