.ModelicaTest.MultiBody.FourbarVariants.UniversalSpherical .ModelicaTest.MultiBody.FourbarVariants.UniversalSphericalThis is a fourth version of the "four-bar" mechanism. In this case the three revolute joints on the left top-side and the two revolute joints on the right top side have been replaced by the joint UniversalSpherical that is a rod with a spherical and a universal joint on two sides. This joint is defined by 1 constraint stating that the distance between the two spherical joints is constant. Using this joint in a kinematic loop reduces the sizes of non-linear algebraic equations. For this loop, only one non-linear algebraic system of equations of order 1 remains.
The essential difference to joint SphericalSpherical is that the orientation of the rod can be computed by removing one degree of freedom of one of the spherical joints (i.e., replacing it by a universal joint). Usually, the eigenrotation of the connecting rod is of no technical interest and by this approximation it is constrained to move in a somewhat arbitrary way. This allows to have an additional connector, frame_ia, to be fixed on the rod, where other objects can be attached. In this example, the coordinate system of frame_ia is visualized.
Another nice feature is that the length of the connecting rod can be automatically calculated during initialization. In order to do this, another initialization condition has to be given. In this example, the initial value of the distance of the prismatic joint j2 has been fixed (via the "Initialization" menu) and the length parameter of joint "UniversalSpherical" is computed during initialization since parameter computeLength = true is set in the joint parameter menu (this sets "fixed=false" on parameter "length").