# Analysis of a Customized Clutch Joint Designed for the Safety Management of an Ultrasound Robot
## 2 Materials and Methods  
### System Overview  
In our previous research, a robotic US system with a 5-DOF lightweight wrist unit [32] for holding and locally adjusting the probe (J\({}_{4}\), J\({}_{5}\), J\({}_{6}\), J\({}_{7}\), and J\({}_{8}\)), a 2-DOF two-bar arm-based set of parallel link mechanisms (J\({}_{2}\), J\({}_{3}\)), and a 1-DOF rotational axis for global positioning (J\({}_{1}\)) was proposed and implemented (Figure 1). The proposed US robot was lightweight and had a smaller footprint compared to most of the commercial robotic arms used in the industry. As a result of this design, the total weight of the 5-DOF wrist was less than 2 kg including the US probe, and the length of the unit was about 25 cm. Kinematically, J\({}_{1}\), J\({}_{2}\), and J\({}_{3}\) were used for global positioning of a US probe in a 3-DOF space. The next joints in the 5-DOF wrist, which were positioned on top of the patient's abdomen, were used to accurately adjust the US probe's orientation. J\({}_{4}\) could rotate the next structures 360 degrees to allow the US probe to point towards different sides of the scanning area, such as the top, bottom, and sides of the abdomen. J\({}_{5}\) was used to tilt the probe down to align with the surface of the scanning area. The last three orthogonal revolute joints (J\({}_{6}\), J\({}_{7}\), and J\({}_{8}\)) were used to control the tilting and axial rotation of the probe, allowing fine adjustments of the probe in a local area.  
In this design, the proposed clutch was implemented in J\({}_{5}\) as this is the joint intended to tilt down to align the probe with the surface. This joint also had adequate space that could incorporate mechanical structures of the clutch. Moreover, the effective distance from the US probe end to the rotational center of J\({}_{5}\) only varied in a limited range at different postures of the robot. Therefore, the proposed clutch could potentially limit the force applied within a safe range while at the same time ensuring that enough required force was applied at different robot postures due to the limited changes of this effective distance from the US probe end to J\({}_{5}\). Detailed studies of the kinematic impacts will be presented in the following sections. For the clutch mechanism, four ball-spring pairs are inserted into the clutch detent holes in Link 5 with the two round clutch covers pushing the spring into the clutch mechanism for preloading. When the clutch is engaged, the assembled balls in Link 5, pushed by the preloaded spring, should be tightly locked inside the detent holes of the driven spur gears on Link 4 (Figure 2a). Therefore, Links 4 and 5 are connected through the engaged spring-ball pairs and the movements of the gears actuate Link 5, which forms the driving mechanism of Joint 5. However, when excessive force is exerted at the end-effector (i.e., the US probe), the clutch is triggered, which stops the torque transmission from Link 4 to Link 5, once the relative movement between the gears and Link 5 completely pushes the balls out from the detent holes (Figure 2b). If the joint continues to actuate without any intervention from the operator after the clutch is triggered, the balls re-engage and move into the next holes with the motors still running. However, the clutch is triggered again with the balls moving out from the new detent holes and the maximum force that the US probe can apply during this process is always limited to a safe threshold mechanically. Between the process of engagement and disengagement, Link 5 remains in its position with the next structures lying on top of the patient's body. In this case, the weight of the next mechanical structures results in a pressure on the patient, but this is less than 15 N and is considered to be safe.  
Figure 1: Schematic representation of the newly developed robotic ultrasound (US) system working on a fetal phantom model, with its joints labelled. The clutch design is shown in more detail in the enlarged view with the clutch mechanism and its components illustrated.