I. Introduction
Serving as the additional linear axis of the machine tool, the main function of the fast tool servo (FTS) is to position the cutting tool to objective locations in a fast and accurate manner [1]. A typical FTS consists of three key component units, namely, the actuator unit, motion translation unit, and motion control unit, while its main performance indices include the motion degree-of-freedom (DOF), motion stroke, positioning resolution, working bandwidth, etc. [2]. After several decades of exploration, the working performances of FTS have progressed to an unprecedented level with impressive instances including nanometric FTS with ultrafast trajectory tracking [3], FTS with working bandwidth up to ultrasonic [4] and electromagnetic–piezoelectric hybrid-driven triaxial FTS [5]. The machining capability of FTS is also extending to cover the full length scales from millimeter to nanometer [6] and wider ranges of machinable materials, including difficult-to-cut metals [7] and brittle crystals [8].