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Each closed kinematic loop must contain at least one joint block without motion inputs or computed actuation force/torque. This condition applies even if one of the joints acts as a virtual joint, e.g., the bushing joint in the Prescribe Two-Bar Motion example. The joint without motion inputs or automatically computed actuation forces/torques can still accept actuation forces/torques from input.
In models not meeting this condition, you can replace a rigid connection line between two Solid blocks with a Weld Joint block. Since the Weld Joint block represents a rigid connection, this approach leaves the model dynamics unchanged. The advantage of this approach lies in its ability to satisfy the SimMechanics™ closed-loop requirement without altering model dynamics.
Spherical joint primitives provide no motion actuation parameters. You can prescribe the actuation torque acting on the spherical primitive, but not its desired trajectory. For models requiring motion prescription for three concurrent rotational degrees of freedom, use joint blocks with three revolute primitives instead. These blocks include Gimbal Joint, Bearing Joint, and Bushing Joint.
Redundant actuation, in which the end effector trajectory of a high-degree-of-freedom linkage is prescribed, is not allowed. Such linkages possess more degrees of freedom than are necessary to uniquely position the end effector and, as such, have no single solution. Models that have more degrees of freedom with automatically computed actuation forces/torques than with prescribed motion inputs cause simulation errors.
In models with motion input, the assembled state achieved by updating the block diagram (Ctrl+D) does not generally match the initial simulation state at time zero (Ctrl+T). This discrepancy is visible in Mechanics Explorer, where it can cause a sudden state change at time zero when simulating a model after updating it. It is also reflected in Model Report, whose initial state data does not generally apply to the simulation time zero when a model has motion inputs.
The number of degrees of freedom with prescribed trajectories must equal the number of degrees of freedom with automatically computed force or torque. In models not meeting this condition, simulation fails with an error.