The three outer legs connect the base and the platform through a passive spherical and universal joint and a driven translational joint.
Usually, only one spherical or universal joint is used to replicate the shoulder in the designs of humanoids and constant joint limits are imposed.
An exact solution can be obtained by explicitly considering the kinematic model of the universal joint.
However, their assumption that there is no axial rotation of the strut when a universal joint is used for the base joint is only an approximation.
The passive link is fixed on the middle platform at one end, and connected to the end-effector platform by a universal joint at the other end.
The four limbs consist of an actuated linear slide, a passive spatial joint, a fixed-length strut and a passive universal joint, respectively.
Another example is the universal joint.
This led to the failure of a universal joint, which ruptured the main fuel tank.
