Nonlinear defaults correction of an integrated position sensor for ultrasonic motors

In a previous paper [1997] we presented a method for angular positioning of a traveling wave type ultrasonic motor (TWUM). In this method, the position is estimated from the phase of the mechanical vibrations of the rotor. Experimental results showed some small nonlinear errors (a few degrees) on the phase value. In this paper an analysis of these errors and an improvement of the position measurement are presented. Laser analysis of the vibrations on the rotor and the stator of the TWUM, together with a precise modeling and simulation of the whole system taking into account the viscoelastic losses in the stator, lead to a better knowledge of the position errors. Two different methods leading to an accurate position measurement are described