Explicit force-based control strategies for compliant robot-assisted ultrasound imaging

In this paper, we present a robotic ultrasound system that implements explicit contact force control and force-based full probe orientation optimization to achieve stable, responsive, and high-quality ultrasound imaging. Traditional robotic ultrasound systems often rely on implicit force control methods, such as admittance control, which are limited by their underlying motion-control loops. In contrast, our approach directly regulates contact force and moment at the end-effector, enabling rapid adaptation to heterogeneous tissue properties and dynamic environments. We benchmark the proposed explicit force controller against a state-of-the-art integral adaptive admittance controller, demonstrating a significant reduction in phase lag from 121° to 5.76° at force tracking frequencies exceeding typical respiratory rates. In a second set of benchmarking experiments, compared to the admittance-based method, the explicit force controller achieves a reduction in average force tracking error of 77.7 % when the phantom is moving toward the transducer, and 72.6 % when the phantom is moving away. We integrate the explicit force and moment controller into a six-DoF haptic framework that renders physically-grounded interaction forces to the operator while the robot autonomously regulates contact force and optimizes probe alignment based on acoustic coupling. Validation across static and dynamic scans, as well as under external perturbations, shows that the system consistently maintains target force and moment profiles, aligns the probe with local surface normals, and adapts to changing contact conditions. Experimental results demonstrate that explicit force-based control improves ultrasound image quality, as quantified by confidence maps, compared to a manual haptic scan. These findings support the use of explicit force and moment control as an effective approach for robotic ultrasound imaging.