Direct Sensing of Thrust and Velocity for a Quadrotor Rotor Array

Trajectory tracking of micro UAVs requires accurate thrust and velocity estimates. High-quality motion control of quadrotors typically employs accurate high-speed optical sensing indoors. However, these measurements are not available outdoors, and GPS is unreliable and inadequate for performancemotion control. We have developed small embedded force sensors that mount between a micro UAV's airframe and motors-forces generated by the rotors are measured as they are transmitted to the rigid body of the aircraft. The direct force measurements can be directly integrated with a model of the mapping between aircraft motion and induced rotor forces. From this, the translational velocities of the rotors relative to the local wind can be measured, and thus the overall velocity of the aircraft can be inferred. We have assembled a proof-of-concept quadrotor platform with all four rotors fitted with force sensors, and demonstrate accurate measurement of a quadrotor's lateral velocity during prescribed motions using the onboard force sensors. The vertical velocity measurement was found to be insensitive.