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

Joint where frame_a rotates around axis n_a which is fixed in frame_a and frame_b rotates around axis n_b which is fixed in frame_b. The two frames coincide when "revolute_a.phi=0" and "revolute_b.phi=0". This joint has the following potential states;

• The relative angle phi_a = revolute_a.phi [rad] around axis n_a,
• the relative angle phi_b = revolute_b.phi [rad] around axis n_b,
• the relative angular velocity w_a (= der(phi_a)) and
• the relative angular velocity w_b (= der(phi_b)).

They are used as candidates for automatic selection of states from the tool. This may be enforced by setting "stateSelect=StateSelect.always" in the Advanced menu. The states are usually selected automatically. In certain situations, especially when closed kinematic loops are present, it might be slightly more efficient, when using the "StateSelect.always" setting.

In the following figure the animation of a universal joint is shown. The light blue coordinate system is frame_a and the dark blue coordinate system is frame_b of the joint (here: n_a = {0,0,1}, n_b = {0,1,0}, phi_a.start = 90^o, phi_b.start = 45^o).