On the kinematic error in harmonic drive gears

Harmonic drive gears are widely used in space applications, robotics, and p recision positioning systems because of their attractive attributes includi ng near-zero backlash, high speed reduction ratio, compact size, and small weight. On the other hand, they possesses an inherent periodic positioning error known as kinematic error responsible for transmission performance deg radation. No definite understanding of the mechanism of kinematic error as well as its characterization is available in the literature. In this paper, we report analytical and experimental results on kinematic error using a d edicated research Harmonic Drive Test Apparatus. We first show that the err or referred to in the literature as kinematic error actually consists of a basic component, representing "pure" kinematic error, colored with a second component resulting from inherent torsional flexibility in the harmonic dr ive gear. The latter component explains the source of variability in publis hed kinematic error profiles. The decomposition of the kinematic error into a basic component and a flexibility related component is demonstrated expe rimentally as well as analytically by matching a mathematical model to expe rimental data. We also characterize the dependence of the kinematic error o n inertial load, gear assembly, and rotational speed. The results of this p aper offer a new perspective in the understanding of the mechanism of kinem atic error and will be valuable in the mechanical design of harmonic drive gears as well as in the dynamic modeling and precision control of harmonic drive systems.