# Analysis of a Customized Clutch Joint Designed for the Safety Management of an Ultrasound Robot
## 1 Introduction
Currently, the safety management method to limit the force that a US robot can apply mostly relies on force sensing and software-based algorithms, where a commercial or a customized force sensor is employed and works with various force control laws. This can be done by controlling the force based on the expert's haptic inputs within a certain range and limits the force to a maximum allowable threshold. In the works proposed by [PERSON] et al. [13; 14], large-scale clinical trials of a US robot used for fetal and abdominal US examinations were introduced. The weight of the passive holder applied a force that ensured basic contact between the patient and the probe. The expert could apply additional force, which was limited to 15 N. In a subsequent study demonstrated by [PERSON] et al. [28], the maximum allowable force was set to 20 N. In the study proposed by [PERSON] et al. [29], 20 N was identified as the higher threshold as well for the cardiac, abdominal, and renal US examinations. In addition to the force control relying on expert experiences and a maximum allowable threshold, studies use metrics of US image quality as feedback to estimate the optimal force during the US acquisition, which has also become a new direction with the recent advancement of the online processing capability of US images. In a more recent study proposed by [PERSON] et al. [30], the applied force is optimized based on the image quality and can increase up to 20 N for the abdominal aortic scan. Additionally, in the recent study proposed by [PERSON] et al. [31], although the mean value of the applied force was found to be 8.2 N, the maximum force during abdominal US examination varied widely, reaching 36.5 N. There is also evidence suggesting that the required level of force is related to the body mass index (BMI) of the patient. Although the area of sensor-based force control for US robots has been extensively discussed throughout the last decades, these control schemes have to rely on force sensing and software-based algorithms. This causes concern to both clinicians and patients that the potential failure of sensors, electrical systems, or software could lead to patient injuries, especially when US robots are practically used in remote locations where there are no engineers who can identify and deal with any technical failure. This could also be one of the reasons that only a very limited number of robotic US systems have been successfully translated clinically and commercialized. One of the most successful examples is the 3-DOF MELODY robot (MELODY, AdEchoTech, Naveil, France), resulting from ESA funded projects, which significantly relies on the on-site operator's control of safety by manually holding the passive holder. Although US robots do not usually require a highly accurate control of force similar to the level of many surgical robots, we believe they still need some special force management features that could guarantee safety when they are used remotely without much technical support. Therefore, we believe US robots destined for clinical translation should include easily manageable mechanical safety features in addition to force sensing and software control. We consider this as a special requirement which is logically reasonable but which was missing from previous research. More generally for a commercial mechatronic system where management of force is crucial, the use of a torque-limiting joint would always be an ideal choice, although this is rarely covered in the research on US robots. This feature can be achieved by using various devices, from basic shear pins to friction and ball detent units, some with pneumatic or electrical controls. Although commercial torque-limiting devices exist in the industry, they are rarely used in the existing US robots. A possible reason is that these devices are usually expensive, un-customized, or too heavy to be included in a US robot where the aim is usually to be lightweight and inexpensive to deploy widely to remote clinics.