Optimization of travelling wave ultrasonic motors using a three-dimensional analysis of the contact mechanism at the stator-rotor interface

The present paper investigates optimization rules and new design methodolog ies dealing with the contact mechanics in rotative travelling wave ultrason ic motors (TWUM). The proposed approaches focus on the design of the rotor, including the friction layer that is usually deposited onto its lower surf ace, while the starer is supposed to be preliminary designed. Contact aspec ts such as the transmission of mechanical power as well as the wear mechani sm of the friction layer are investigated, according to the analysis of the stator-rotor contact mechanics in both hoop and radial directions. Conside ring a classical wear criterion in a preliminary step, a contact ratio, tha t allows the mechanical power to be optimized, is pointed out in the hoop d irection. In a further step, the contact conditions in the radial direction are improved through the elastic fitting of the stator and rotor radial de flexions, therefore allowing the material's wear to be decreased. Some expe rimental tests, that have been recently performed, give a comparison of wea r marks, which occur onto optimal and non-optimal rotor geometries. A first mechanism synthesis is finally proposed in such a way to allow the mechani cal architecture of the rotor (including the friction layer) to be automati cally designed according to a given set of mechanical constraints. (C) 1999 Conditions scientifiques et medicales Elsevier SAS.