Infrared detection, characterized by its robust anti-interference capability and all-weather operability, finds extensive application in infrared optical systems, particularly in military domains such as tracking, reconnaissance, and surveillance. Advancing technology has escalated the requirements for infrared optical systems, necessitating the implementation of variable F-numbers for enhanced performance through F-number matching. Ultrasonic motors, offering distinct advantages such as compact size, simple structure, and absence of electromagnetic interference, present a compelling alternative to electromagnetically driven variable apertures. Despite the introduction of several ultrasonic motor-driven variable apertures, research emphasis has primarily been on structural design and performance evaluation, underscoring the critical need for control methodology studies. The inherent challenges of limited assembly space, non-linear friction dynamics among aperture blades, non-linear relationships between rotation angle and aperture size, and the output non-linearity of ultrasonic motors, introduce complexities in the control of ultrasonic motor-driven variable apertures. Thus, the investigation of effective control methods for ultrasonic motor-driven variable apertures is imperative to meet the requirements of miniaturization and precise aperture control.This study proposes an innovative open-loop control approach for ultrasonic motor-driven variable apertures, grounded in a grey box model. A traveling wave type ultrasonic motor-driven variable aperture was conceptualized and developed, featuring an adjustable aperture diameter spanning from 4 mm to 60 mm. A dynamic control model for the ultrasonic motor-driven variable aperture was formulated (Eq.11). Leveraging image recognition technology, aperture change data was meticulously collected, and the parameters were identified through the grey box model (Tab.1). To substantiate the efficacy of the open-loop control method based on the grey box model, a series of experimental validations were carried out (Fig.11-Fig.12).The grey box identification model exhibited a fitting degree of 97.06% during the aperture opening phase, with error margins confined within ±0.15° (Fig.7). When the aperture was in the closing phase, the model showed a fitting degree of 92.49%, with errors tightly bounded within ±0.1° (Fig.8). These metrics underscore the model's effectiveness in accurately representing the system's dynamic characteristics. When subjected to aperture changes from 10-20 mm, experimental validation revealed a maximum error of -0.11965° and a minimum error of -0.02304°, confined within ±0.12° (Fig.11). During aperture transitions from 20-10 mm, the maximum error was recorded at 0.12707°, with the minimum at -0.02183°, all within ±0.13° (Fig.12). The average absolute error was measured at 0.077° during aperture opening and 0.079° during closing, corroborating the feasibility and precision of the ultrasonic motor-driven variable aperture control method.An open-loop control strategy for ultrasonic motor-driven variable apertures, underpinned by a grey box model, has been successfully proposed. The method adeptly tackles the multifaceted challenges posed by limited assembly space, the non-linear dynamics of aperture blade friction, and the output non-linearity of ultrasonic motors. A grey box control model was established, and aperture change data was meticulously collected using image recognition techniques. The grey box model's parameter identification capability was validated, achieving a fitting degree of 97.06% during the aperture opening phase and 92.49% during the closing phase. Experimental results demonstrated negligible error values during aperture changes, attesting to the control method's efficacy in achieving precise control of ultrasonic motor-driven variable apertures without the reliance on additional sensors. This approach not only simplifies system architecture but also enhances response speed, making it a compelling solution for advanced infrared optical systems.