PASSIVE_RIGID_BODY

In order to define such motion as generally as possible, a number of parameters have to be set in Imposed motions.

PASSIVE_RIGID_BODY motion defines the standard 6-DOF motion and constrains motion in a number of ways, and also includes linear and torsional spring forces that act on the body. PASSIVE_RIGID_BODY offers parameters that can be set for this type of motion. Some of the commands are simple and can be considered self-explanatory, for example, body_mass, init_CoM, init_vel, but others require clarification.

Depending on the complexity of the case and the data you have available, you may need to specify 0, 1, or 2 additional coordinate systems.

The Cartesian coordinate system, which is commonly referred to as a global coordinate system or global reference frame, is the default coordinate system in which the code is operating and does not need a definition. It is assumed that it coincides with the inertial frame of reference.

Where the “x y z” coordinates represent the unit vectors expressed in global coordinates. The suffix “i” at the end of the command stands for “inertial”, to indicate which coordinates to use when defining the unit vector components.

The three coordinate systems and their respective commands are shown in Figure 1.

An example of a hexagonal body that is rotated by 45 degrees around the z axis in Figure 2. Since it is easier in this case to specify the moment of inertia with only diagonal terms, you will define a principal axes coordinate system.

The exact same principle can be applied to the constraint coordinate system if you, for example, want to constrain the motion of the body in the direction that is under 45 degrees with respect to the global x or y axes.

By default the origin of the constraint coordinate system is located at the center of mass of the body. That is to say that if the body is to rotate, it will rotate around its center of mass. However, there are a number of situations where this behavior is not suitable. One example is a simulation of a hinge. In a situation like that, where you want the rigid body to rotate around a specific point, you need to shift the coordinate center of the constraint coordinate systems. For this purpose you can use the prbcon_pt_i command. Along with the new coordinate center you need to set the rotational constraints for the new “hinge point”. This is done using prbcon_ax_hinge_c command, where it is a vector that says which rotational motions are locked (x, y or z axis in constraint coordinate system). By setting all three values to 0 you will essentially define a ball-joint. The simplified 2D setup of a hinge is shown in Figure 3.

Linear or torsional springs can also be defined. To illustrate the basic concept of such a setup, refer to Figure 4. The example shows the setup of a body which is hanging on a spring. The initial location of the body does not correspond to the equilibrium point of the spring, that is to say that you have a pre-deformation of the spring (prbcon_linspr_p_c) when starting the simulation. Along with this you of need to set the stiffness coefficient of the spring (prbcon_angspr_k_c). Between these two parameters you can define a force that is initially acting on the spring. Same principles apply for torsional springs, except that the deformation is expressed in [rad], and the stiffness of the spring in [Nm/rad].

There is also an option to set upper and lower coordinate (angle) bounds for the body. In order to do so you must use prbcon_linlim_pls_candprbcon_linlim_mns_c (or prbcon_anglim_pls_candprbcon_anglim_mns_c for angular limits) to define positive and negative displacement, respectively. These commands are always defined with respect to the init_CoM.

With passive rigid body motion it is also possible to prescribe constant force or torque acting on a body by using prbcon_cnstfrc_c or prbcon_cnsttrq_c.

Alternatively, you can prescribe either constant or varying linear or angular velocities to each of the bodies by using prbcon_linvel_* and prbcon_angvel_* sets of commands.