In this paper, we present a model of a contact interface with projections onthe surface of the rotor of a traveling wave ultrasonic motor.The projections are introduced to enlarge the actual contact area between thestator and the rotor. The friction force is assumed to be proportional to therelative velocity between the rotor and the stator, which is different fromthe traditional model of using the Coulomb friction law. The relation betweenthe preload and the output torque is computed. There is a certain preload thatwould result in a maximum output torque. This optimal preload, which gives thehighest output torque, is larger when the projection density is higher.
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