# Analysis of a Customized Clutch Joint Designed for the Safety Management of an Ultrasound Robot
## 3 Results and Discussion
### Implementation and Practical Experiments  
Based on the proposed design, we developed and manufactured a clutch joint which has been successfully used in the US robot and tested in a volunteer study (study title: Investigating Robotic Abdominal Ultrasound Imaging, Study reference: HR-17/18-5412, approved by the King's College London local ethics committee, 4 April 2018). It was based on the exact design configuration shown in the previous sections. In our practical tests during the volunteer study with the setup shown in Figure 9, the clutch was triggered when a large amount of force was identified by the clinician. A manual re-engagement was then performed to reset the clutch. This verified the successful working of the clutch mechanism in a real scenario, although it only happened for a limited time and there was no report of pain from the volunteers. Figure 8: Triggering force variations at different postures of the US robot with the probe positioned at (**a**) the lower abdominal area and (**b**) the upper abdominal area, based on the joint configurations obtained from the inverse kinematics of the US robot using the geometry of a fetal phantom. To further verify the correct working of the proposed clutch and quantify the practical performance of the design, we measured the real triggering force at the US probe end when the robot was in its neutral position as the triggering force at other postures could then be estimated based on the results shown and discussed in Section 3.1. The experimental setup for the measurement is shown in Figure 10 with the robot positioned onto an experimental stage. A force sensor (Mini 40, ATI Technologies, Markham, ON, Canada) was clamped to the stage with spacers in between to locate the sensor in contact with the US probe end when the robot was in its neutral position. Two plastic plates were assembled for the mounting and measuring sides of the force sensor. During the experiment, the robot was driven downwards with increasing pressure against the force sensor. The contact vertical forces were recorded in software and the triggering values were identified when the measured force started to decrease after continuously increasing. This can be seen from an example measurement as shown in Figure 11. The triggering point can also be observed by visually inspecting the movement of \(\mathrm{J}_{5}\). Figure 10: Experimental setup for the practical measurement of the triggering force using a force sensor with the robot positioned in its neutral position. Figure 9: Implementation of the customized clutch based on the exact design configuration shown in the previous sections and use of the clutch joint in the pre-clinical volunteer study. The measurement was repeated 30 times with the triggering force recorded for each sequence. The clutch was manually reset for each measurement after being triggered. The mean and standard deviation values of the measured triggering forces were then calculated. This was found to be \(21.98\pm 0.96\) N (mean \(\pm\) standard deviation) when the clutch cover was pushed in to its extreme position. The assembly of the clutch cover was done using a bolt. The bolt could be slightly adjusted to fine tune the triggering force by changing the preloaded force of the spring. The adjustable range was found to be between 17 N and 24 N. The measured forces, although slightly differing from the theoretical calculated value (19.96 N at the neutral position of the robot as shown in Figure 8 for the robot configuration 1 described in Figures 4 and 5), are still very close and fall into the similar allowable range taking the preload impact into consideration. As the variation of the triggering force is limited, the similar resulting triggering force range, as identified in Figure 8, is expected for the real performance of the clutch after being assembled on the US robot. The small standard deviation of the measured triggering force indicates a relatively reliable performance of the proposed clutch with an acceptable repeatability.