## 1 Introduction  
The field of precision motion control has witnessed a significant advancement with the development of inertial linear piezoelectric actuators (ILPAs), proposed in the 1980s, leveraging the inverse piezoelectric effect to convert electrical energy to mechanical displacement and working based on the vibration motion against frictional interaction. Due to their high resolution, fast response, and compact structure, they have been widely used in various applications such as camera modules [1; 2; 3], micro-robotics [4; 5], biomedical [6; 7] aerospace, etc. [8; 9; 10]. Based on their operating frequency, ILPAs can be categorized into non-resonant-type inertial linear piezoelectric actuators and inertial linear ultrasonic motors (ILUMs) [11]. Utilizing the deformation generated by the lead zirconate titanate piezoelectric ceramic (PZT) stacks under adjustable voltage and frequency, non-resonant-type ILPAs are capable of outputting displacement. However, the output performance of these actuators has been shown to be limited by the deformation of the PZT stacks, so some non-resonant-type ILPAs are usually proposed with an integrated structure [12; 13; 14] and configurations that are used for amplifying displacement in addition to the necessary clamping structures [15; 16]. In addition, their large thrust force and high velocity cause difficulty in actualization as a result of their low driving frequency [17]. ILUMs exhibit the merits of higher positioning accuracy and displacement resolution on account of their high driving frequency [18], small stepping pitch [19; 20], and various exciting signals [21]. Compared to non-resonant-type ILPAs, the output performance of ILUMs is improved significantly since they are driven at a resonant frequency under high voltage, and therefore more complicated driving methods [21] and strict driving frequency control feeding methods are required to avoid operation failures. As for research on the driving mechanisms, signal, friction, and structure are common design methods [11]. However, the complexity of the control method is not conducive to the overall integration of ILUMs, so it is necessary to arrange the structure of the motor while retaining the characteristics of the resonant driving method. Several types of ILUMs operating based on the resonant mode have been proposed. However, from a structural perspective, the direct exposure of the edges of PZTs in motors presents a significant design shortcoming that makes the structure fragile [17]. Furthermore, the elastomer to which the PZTs are bonded lacks the necessary constraints, resulting in an unstable and unreliable structure [1,2]. This not only leads to the activation of undesired excitation modes but also significantly degrades the output performance. Meanwhile, the improper arrangement of PZTs and an unstable structure can complicate and hinder the processing and assembly procedures from an engineering application standpoint. Therefore, these drawbacks suggest a need for structural innovation and design improvements to enhance ILUMs' stability, reliability, and performance. In this paper, an inertial linear ultrasonic motor with a double-stator structure (ILUM-D) was proposed based on the bending mode, focusing on the process of designing and analyzing a novel structure that leads to higher performance and requires only a single signal for an effective output. The double-stator structure improves the output performance of the motor while allowing for easy installation and a wide range of engineering applications. A finite element model of the prototype has been proposed to obtain the mechanical characteristics for further structure optimization, demonstrating the performance advantages of the ILUM-D over an inertial linear ultrasonic motor with a single-stator structure (ILUM-S). Several experiments were conducted to verify the feasibility and property of the prototype. In the meantime, the experimental results were also compared with those of the analytical modeling and the finite element analyses.