Abstract: The objective of the proposed research is to develop modeling, identification and control methods for a two-axis pendulum on wheels, referred to as a wheeled balancing system or WBS, and to demonstrate these methods experimentally.  The WBS is a mobile platform with an inverted pendulum payload, and it may be viewed as a combination of a reaction wheel pendulum (RWP) and a wheeled inverted pendulum (WIP).  The platform freely rotates along the pitch-axis, and the payload freely rotates along the roll-axis.  Motors actuate the two wheels in ground contact to stabilize pitch motions, drive yaw-axis rotations, and impose linear translations.  The reaction principle is used to stabilize roll-axis payload motions, by driving a motor with an inertia load, connected at the non-pivoting pendulum end.  Encoders are mounted on the rotor shaft of each motor and on the pendulum pivot to measure angular positions.  Inertial sensors are mounted near the WBS center-of-mass to infer the angular velocities and accelerations of the platform, and pendulum payload.  Additional disturbances affecting the system include motor cogging torque, quantization in the position measurements, and analog noise in the current and inertial sensor measurements.  The first control objective is to maintain upright stabilization, and the second objective is to keep the pendulum payload upright regardless of the platform-base roll-angle.