So far, we have studied the dynamics modeling of systems with non-geared, direct-drive hinge actuators. However, many systems have geared joints. When the gear ratios are large, the motor inertias have significant effect on the system dynam- ics, since the equations of motion contain motor inertia terms that are magnified by the square of the gear ratio. For such systems, it is important that gear and actuator dynamics be included in the dynamics model of the manipulator. Reliance on sim- plified and approximate models can lead to errors in control design, with consequent degradation in system stability and performance.