Contact model of a ring-type traveling wave ultrasonic motor

Traveling wave ultrasonic motors are driven by frictional force in the contact interface between the stator and the rotor. The operational characteristics of ultrasonic motors are determined by the contact mechanics. A quasi-analytical contact model of a ring-type traveling wave ultrasonic motor applied to the Canon autofocus lens drive system is presented here. First, the circumferential contact length and the contact pressure between the stator and the rotor are calculated using finite element analysis (FEA). Next, the tangential motion of the contact interface is calculated using a closed-form analytical method. The stick-slip behavior at the contact interface is taken into account, and the tangential stress and stick-slip distributions are obtained. Good correspondence between the theoretical and experimentally measured torque-speed characteristics is observed.