A method for fore-aft stabilization of a vehicle for motion in a specified
direction over an underlying surface. The vehicle has at least one
forward wheel and at least one aft wheel, and the forward wheel is
characterized by a force normal to the instantaneous direction of motion
of the vehicle. A motor actuator drives each aft wheel, and a controller
governs the motor actuator or motor actuators in such a manner as to
dynamically stabilize the vehicle, according to a uniform control law,
when the forward wheel is in contact with the underlying surface or not.
A torque is applied to the aft wheel on the basis of vehicle pitch or the
force on the forward wheel normal to the direction of motion.
Additionally, a periodic rotational modulation may be applied to the aft
wheel, and a stabilizing torque provided based on a detected response,
either of vehicle pitch or of normal force on the front wheel. Left and
right motor actuators may independently control left and right aft wheels
to continue turns as governed by user steering, whether or not forward
wheels are in contact with the ground.