Mechanical couplings are used to transmit the actuator force to the articulated structure. Yeh et. al [9] described a system using a sliding shuttle and push-rod to rotate structures out of the plane. In the mechanical coupled structure as shown in figure 6, one end of a pushing rod is connected to the shuttle by a revolute joint. The level arm is connected to the pushing rod and the fixed supporting surface by revolute joints. The level arm can be rotated for a large range, while the only actuated motion is the planar motion of the shuttle.
Figure 6: The side view of the unloaded level arm, coupled with a pushing rod
and shuttle.
Again this structure contains a loop: inertia-frame 
pushing-rod level-arm inertia-frame.
Besides the tricks used in the stepper motor simulation, we do the
following to avoid the looped forward dynamics: (1) create a bogus
link with extremely large mass;
(2) constrain the bogus link to the level arm by a revolute
joint; (3) turn off the gravity; (4) apply torques to the level arm and
the pushing rod, using impulse control laws, to simulate the gravity
effect on the pushing rod and the level arm. These tricks will make
the bogus link serve as a fixed end without a loop in the forward dynamics.