# Analysis of a Customized Clutch Joint Designed for the Safety Management of an Ultrasound Robot
## 1 Introduction  
Arm-based robotic ultrasound systems have great potential to assist extra-corporeal ultrasound (US) in small clinics and hospitals, battlefields, or remote locations. This is motivated by the deficiencies of on-site manual manipulation of hand-held probes, such as the unavailability of enough skilledsonographers for performing US examinations [1; 2]. Even with skilled personnel, difficulties have also been identified in accurately holding the probe still for long periods of time using human hands when US is used for diagnosis or interventional procedure guidance, which can cause stress-related musculoskeletal complaints to the operator [3; 4]. Moreover, with US-guided robotic surgery more widely performed [5], the small surgical field would potentially require another intelligent robot to hold and manipulate a US probe, rather than allowing human operators access to the surgical filed. Because of these motivations, motorizing extra-corporeal ultrasound has always been a research interest within medical robotics and ultrasound communities since the late 1990s [6]. Over the last 20 years, many systems with different mechanical configurations have been proposed, as reviewed in detail in [6; 7]. The use of these robotic systems for fetal and abdominal examinations is one of the biggest research directions as they are widely used types of US scans and could include scanning of many anatomies. Examples of systems include the robotic tele-echography (TER) system proposed by [PERSON] et al., which utilized a cable-driven mechanism and was clinically tested for the abdominal aortic scan [8; 9; 10]. Another well-known example is a partially motorized US robot with three degrees-of-freedom (DOFs) resulting from projects funded by the European Space Agency (ESA) [11; 12; 13; 14]. The system was tested on both fetal and general abdominal scans. Additionally, many US robots were also tested for other applications such as in carotid artery, cardiology, and lower limb examinations. Examples of research include the works proposed by [PERSON] et al. [15; 16] and [PERSON] et al. [17; 18; 19] for image-servoing-based automated remote carotid artery examinations, the work proposed by [PERSON] et al. for robotic-assisted multi-modality image registration developed for guiding cardiac interventional procedures [20], the work proposed by [PERSON] et al. for evenly spaced US scans along the length of blood vessels in lower limb as described in [21], and the work proposed by [PERSON] et al. for the design and control approaches of a customized US robot to diagnose shoulder diseases [22]. In more recent years, new generations of lightweight commercially available robots have been more widely employed to explore the potentials of robotic US examination and procedure guidance. Example works include the use of an UR5 robot (Universal Robots, Odense, Denmark) from [PERSON] et al. [23], [PERSON] et al. [24], [PERSON] et al. [25], and [PERSON] et al. [26], as well as the use of a Kuka robot (Kuka Robotics, Augsburg, Germany) from [PERSON] et al. [27]. These works focus on system-level integration by making use of the matured functions of commercial robots. Application-driven advanced control methods and robotic-related image processing algorithms are the typical topics of these works. In contrast to the research carried out in the early days, the fundamental robot development itself is no longer the main focus of the research. Among these research studies on the extra-corporeal US robot, the safe use of the robot on patients has always been an important consideration. In contrast to arm-based robots used in industry, it is impossible to keep the robot away from people as these robots are intended to directly interact with the patient's body. For US robots, controlling and limiting the contact force that the end-effector (i.e., the US probe) can apply onto the patient's body is crucial as inadequate forces could result in low-quality images while excessive forces could be uncomfortable for patients and cause potential injuries.