• SetFree[dropcon, options] builds a FreeSystem object containing the mathematical equations required to find equilibrium conditions of the current model. The dropcon argument is used to specify the constraint(s) that are dropped to make the system underconstrained.
• The format of dropcon is identical to that used by the Constraints function.
• The FreeSystem object is solved with SolveFree.
• The following options can be given:
• If InitialGuess or InitialCondition is unspecified, SetFree uses the current model initial guesses, as obtained from LastSolve.
• InitialCondition can also be specified in SolveFree.
• If Solution->Static is given, the model must have some nonzero applied loads for a static equilibrium position to exist.
• If Solution->Kinematic is given, the model must have velocity-dependent loads applied, or no equilibrium velocity can exist.
• If Solution->Dynamic is given, the model must have nonzero masses applied to at least one moving body with SetBodies.
• See also: SetCouple.
Load the Modeler2D package and define a simple model.
Here we build a FreeSystem object that can be used to simulate the underconstrained motion of the help model when the constraint that controls the rotation of the crank has been removed. Before calling SetFree, a velocity solution is generated with SolveMech so that initial conditions for the location and velocity of each body will be available.
Because the help model has only one body with nonzero mass and one applied load (the link body has a 10-unit mass and a 10-unit applied load in the X direction), the instantaneous free acceleration is somewhat predictable. The X acceleration of the link body, X3dd, is equal to 1.0.
Here we integrate the free acceleration of the help model with respect to time for four seconds.
Here is a plot of the X position of the link. The link is initially decelerating until its X coordinate reaches a local minimum, and then it accelerates the other way.