Design and Evaluation of a Compliant Quasi Direct Drive End-effector for Safe Robotic Ultrasound Imaging

TL;DR

The paper addresses patient safety and compliant interaction in robotic ultrasound by introducing a single-DOF end-effector powered by a quasi-direct-drive actuator to deliver both passive compliance and high-bandwidth active force regulation. It couples this end-effector to a versatile mechatronics platform and an ex vivo abdominal motion simulator to rigorously evaluate force tracking under static, breathing, and sudden-movement conditions, demonstrating significantly improved RMS force tracking (e.g., 0.83 N vs 4.70 N for a robot arm under dynamic motion). Key contributions include the QDD-based end-effector design with a timing-belt transmission, a dual-host CAN-based control architecture, and a moving-tissue testbed that enables realistic assessment of compliant ultrasound systems. The results suggest substantial potential for safer, more consistent robotic ultrasound in clinical settings, with future work aimed at material upgrades and broader multi-axis, higher-speed scanning capabilities.

Abstract

Robot-assisted ultrasound scanning promises to advance autonomous and accessible medical imaging. However, ensuring patient safety and compliant human-robot interaction (HRI) during probe contact poses a significant challenge. Most existing systems either have high mechanical stiffness or are compliant but lack sufficient force and precision. This paper presents a novel single-degree-of-freedom end-effector for safe and accurate robotic ultrasound imaging, using a quasi-direct drive actuator to achieve both passive mechanical compliance and precise active force regulation, even during motion. The end-effector demonstrates an effective force control bandwidth of 100 Hz and can apply forces ranging from 2.5N to 15N. To validate the end-effector's performance, we developed a novel ex vivo actuating platform, enabling compliance testing of the end-effector on simulated abdominal breathing and sudden patient movements. Experiments demonstrate that the end-effector can maintain consistent probe contact during simulated respiratory motion at 2.5N, 5N, 10N, and 15N, with an average force tracking RMS error of 0.83N compared to 4.70N on a UR3e robot arm using conventional force control. This system represents the first compliant ultrasound end-effector tested on a tissue platform simulating dynamic movement. The proposed solution provides a novel approach for designing and evaluating compliant robotic ultrasound systems, advancing the path for more compliant and patient-friendly robotic ultrasound systems in clinical settings.

Figures (5)

• Figure 1: CAD model of the compliant robotic US end-effector
• Figure 2: Experiment setup and force tracking results on porcine tissue with dynamic movement. A: experiment setup of the end-effector attached to the UR3e robot arm; B: Force tracking comparison plot between end-effector and robot arm only conditions under sudden movement while targeting 5N; C: Force tracking plots of the end-effector targeting 2.5N, 5N, 10N, and 15N during simulated breathing motion lasting 8 seconds; D: Force tracking plots of the robot arm targeting 2.5N, 5N, 10N, and 15N during simulated breathing motion lasting 8 seconds
• Figure 3: Mechatronics architecture of the end-effector. The mechatronics setup also powers the ex vivo motion simulator.
• Figure 4: Controller architecture for both the end-effector and the robot arm. Only one controller is active during their respective experiments. The end-effector uses current-based force control, while the robot arm uses position-based force control.
• Figure 5: Force tracking distribution box plots, showing concatenated data from three 8-second measurements (total of 24 seconds) at each target force. A: no motion, end-effector on porcine tissue; B: simulated motion, end-effector on tissue phantom; C: simulated motion, end-effector on porcine tissue; D: simulated motion, robot arm on porcine tissue.