This information is part of the Modelica Standard Library maintained by the Modelica Association.

All 3-dimensional force and torque elements should be based on this superclass. This model defines frame_a and frame_b, computes the relative translation and rotation between the two frames and calculates the cut-force and cut-torque at frame_a by a force and torque balance from the cut-force and cut-torque at frame_b. As a result, in a subclass, only the relationship between the cut-force and cut-torque at frame_b has to be defined as a function of the following relative quantities:

  r_rel_b[3]: Position vector from origin of frame_a to origin
              of frame_b, resolved in frame_b
  R_rel     : Relative orientation object to rotate from frame_a to frame_b

Assume that force f = {100,0,0} should be applied on the body to which this force element is attached at frame_b, then the definition should be:

   model Constant_x_Force
      extends Modelica.Mechanics.MultiBody.Interfaces.PartialForce;
   equation
      frame_b.f = {-100, 0, 0};
      frame_b.t = zeros(3);
   end Constant_x_Force;

Note, that frame_b.f and frame_b.t are flow variables and therefore the negative value of frame_b.f and frame_b.t is acting at the part to which this force element is connected.