Contents I. Introduction
Harmonic drive (HD) is a novel mechanical transmission invented in the 1950s [1], and it has been widely applied in the field of robotics and aerospace owing to its huge reduction ratio and extremely compact structure [2], [3]. The gear engagement of the HD is achieved by the elastic deformation of the flexspline (FS) [4], [5], and this operation principle inevitably induces some awful transmission attributes, including kinematic error, friction, and hysteresis stiffness [6]–[9]. Among them, the friction is responsible for most energy loss and unsteady output speed of the HD [10], [11], and its suppression has always been a challenge. Although researchers have proposed numerous model-based control methods to compensate the friction loss of the HD [12]–[14], the mechanism of the friction in the HD is still not clear, especially for low speed operation [4], [5], which limits the accuracy of existing friction models of the HD. Therefore, it is necessary to reveal the actual friction characteristics of the HD under low speed condition.
