Exterior vs. interior rotors in robotic brushless motors

Many robotic actuators must generate high torques and accelerations. Exterior-rotor brushless motors seem better suited for these tasks than conventional interior rotor brushless motors. Models of these two motor types were used to calculate benchmarks and evaluate motor performance in simulated tasks for different motor geometries. Exterior rotor motors had higher motor constants and efficiency, but lower rated power compared with interior-rotor motors. They required less energy to perform typical robotic tasks. Four exterior-rotor motors were fabricated and compared with manufacturing data from the best existing brushless DC motors. The exterior-rotor motors had more than a 250% increase in stall torque density compared with the best known brushless DC motor (RoboDrive) and more than a 740% increase in stall torque density compared with more common motors such as those produced by MicroMo and Maxon. These exterior-rotor motors appear promising for robotic applications.