A composite system model for a wheeled mobile robot is proposed. The overall system can be thought of as a composite of two subsystems: one is the vehicle with m degrees of freedom, and the other is the robot arm with n degrees of freedom. The interconnections between them are interactive forces. A simplified decentralized controller for the two subsystems is proposed and the resulting closed-loop error systems are modelled as a composite of two error subsystems. The interconnections between the error subsystems are mismatches on interactive forces. Conditions are derived for stability when the interconnections are not completely known. An interesting conclusion is that we can choose arbitrarily large weighting gains to decrease the trajectory errors without increasing the magnitude of the physical control torques.